SYNTHESIS AND STUDY OF ZINC OXIDE NANOSTRUCTURES AND FILMS

SYNTHESIS AND STUDY OF ZINC OXIDE NANOSTRUCTURES AND FILMS. WEE RUI QI (B. Eng. (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF MATERIALS SCIENCE NATIONAL UNIVERSITY OF SINGAPORE 2012 Acknowledgements I would like take this opportunity to express my heartfelt gratitude to my supervisor Prof. Gong Hao for his continuous encouragement and guidance. I sincerely appreciate the time and effort he provided regardless of his busy schedule. He taught me how to express my ideas clearly and how to construct frameworks to solve challenging problems. I genuinely thank Dr. Yang Weifeng for his patience in guiding me in my approach towards research work. I would also like to thank my fellow group members which include Dr. Wang Yu, Miss Sun Jian, Miss Tang Chunhua, and Mr Yin Xuesong for the fruitful discussions, suggestions, and support over the past two years. I would like to thank Dr. Chen Rui from Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University (NTU) for his help in photoluminescence measurements. I thank the technical staff of the Department of Material Science and Engineering, National University of Singapore (NUS) for their continuous technical support. I would like to thank DuPont Apollo and Singapore EDB for their financial support, and National University of Singapore (NUS) for giving me an opportunity to pursue my interest in research as a graduate student. Lastly, I would like to give special thanks to my loved ones for their unconditional understanding and support during this period of time. i Tables of Contents Acknowledgements …………………………………………………………............. i Table of Contents ……………………………………………………………………ii Summary …………………………………………………………………………....vi List of Tables ……………………………….………………………………….......viii List of Figures ……………………………………………………………………....ix List of Publications ………………………………………………………………..xiv Chapter 1: Introduction…………………………………………………..…………1 1.1 Background information ……………………………………………………….…1 1.1.1 Introduction to Nanostructured ZnO Properties and Applications ……………………………………………………..……1 1.1.2 Synthesis Methods for Nanostructured Materials and ZnO……..….…4 1.1.2.1 Solvothermal/Hydrothermal methods……………………....…5 1.1.2.2 Sol-Gel………………………………………………..…….….7 1.1.2.3 Microwave-Assisted Synthesis………………………….….…8 1.1.2.4 Nano-Lithography……………………………………..…..…10 1.1.2.5 Vapor-Phase Synthesis…………………………………….…12 1.1.2.6 Direct Oxidation by Air ………………………………….…..13 1.1.3 Challenges Identified ………………………………………………..14 1.2 Outline of Thesis……………………………………………………...……...16 1.3 References……………………………………………………………………17 Chapter 2: Synthesis and Characterization…………………………………...….25 ii 2.1 Fabrication of Samples…………………………………………………………..25 2.1.1 Sputtering of Zn and ZnO Thin Films…………………………...…….25 2.1.2 Heating in Furnace……………………………………………………..27 2.1.3 Hydrothermal Synthesis………………………………………………..28 2.1.3.1 Background of Hydrothermal Synthesis………………….….28 2.1.3.2 Experimental Setup….……………………………………….30 2.1.3.3 Chemistry Behind Hydrothermal Synthesis………………....30 2.2 Characterization Techniques………………………………………………..……34 2.2.1 Surface Profiler……………………………………………………...…34 2.2.2 X-ray Diffraction (XRD)…… ………………………………………...35 2.2.3 Scanning Electron Microscopy (SEM)… ……………………….…….37 2.2.4 Transmission Electron Microscopy (TEM) …………………………...38 2.2.5 Photoluminescence (PL)… ……………………………………………41 2.2.6 Vibrating Sample Magnetometer (VSM).. ……………………………42 2.3 References………………………………………………………………………..43 Chapter 3: Investigation on Origins of Black Zinc Oxide……………………….46 3.1 Introduction….……..……..……..……..……..……..……..……..……..……….46 3.2 Results and Discussion….……..……..……..……..……..……..……..….…..…47 3.2.1 Structural Features and Surface Morphology…………..…….....…..…48 3.2.2 Photoluminescence Properties ………..……..……..……..………...…55 3.2.3 Magnetic Properties……..……..……..……..……..……..……...…….59 3.3 Conclusions………..……..……..……..……..……..……..……..……..………..61 3.4 References……..……..……..……..……..……..……..……..……..……..……..62 iii Chapter 4: Growth of ZnO Nanostructured Films by Zn films in NaCl Solution……………………………………………………………………………...65 4.1 Introduction……………………………………………….……………………...65 4.2 Results and Discussion …….…………………….….…….…….……………....66 4.2.1 Surface Morphology and Structural Features……….……...………..67 4.2.2 Optical Properties………..….…….…….……….…….…….……….70 4.2.3 Investigation of ZnO Growth Mechanism……….….………….……73 4.3 Conclusions……….…….…….……….…….…….……….…….…….………...81 4.4 References…….…….…….…….…….…….…….…….…….…….…….……...82 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia…...84 5.1 Introduction………………………………………………………………………84 5.2 Results and Discussion…………………………………………………………..85 5.2.1 Synthesis of ZnO and its Properties………………………...………….87 5.2.1.1 Synthesis of ZnO by Sputtering…………………………..….87 5.2.1.2 Synthesis of ZnO on Different Substrates…………………...88 5.2.1.3 Synthesis of ZnO on Bare Silicon Substrates for 4 to 24 hours……………………………………………………..…...92 5.2.2 Structural Properties and Composition of Ga Incorporated ZnO……...95 5.2.3 Influence of Ga on Morphology…………………………………….....98 5.2.4 Influence of Ga on Optical Properties………………………………..101 5.2.5 Influence of Ga on GZO Growth Mechanism………………………..102 5.3 Conclusions……………………………………………………………………..109 5.4 References……………….……….……….……….……….……….…………..111 iv Chapter 6: Conclusions and Future Work…….……….……….………...……..114 6.1 Conclusions…….……….……….…………….…………….…………….……114 6.2 Future Work…….…………….…………………………………………..….....118 v Summary Zinc oxide (ZnO) is a promising candidate for many applications. Nanostructured ZnO has been gaining a strong foothold as they vastly improve ZnO properties. In this project, nanostructured ZnO and its related compounds are synthesized with sputtering, furnace and hydrothermal methods. Characterization is done with X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), photoluminescence (PL), and vibrating sample magnetometer (VSM). The film thickness is done with surface profilometer. This M. Eng work determines if black ZnO (pure ZnO) exists while investigating properties of the resultant annealed zinc films. The findings disputed this claim. TEM suggested a Zn/ZnO layered structure. The enhanced ultraviolet (UV) emission in ZnO films is attributed to a low annealing temperature of 100 oC as its structure is retained. Zn is able to enhance UV emissions for ZnO film annealed at 200 oC. Zn is also responsible for ferromagnetism in annealed ZnO films. In addition, the synthesis of ZnO by a new method with aqueous sodium chloride is succeeded. Films with network of circular pores to a film with nanowirelike network with bigger pores were obtained. The rise and subsequent decline in green emission could be related to the morphology change over time since ZnO films are obtained within 3 hours of heating. Further investigation demonstrates the importance of nanostructured Zn films in oxidation by aqueous NaCl solution. The model behind pitting corrosion is responsible for nanostructured ZnO films in this study. Finally, this work studied the effect of substrates, heating durations and Ga addition on ZnO by hydrothermal methods. The substrates did not have much vi significant role in affecting morphologies or optical properties. The thickness of assynthesized powder grown on substrates can be tuned with heating durations from 4 to 24 hours. It is found that GZO is obtained when 10 and 20 at % of Ga was added in, while ZnGa2O4 is obtained with 30 to 50 at % of Ga introduced. The amount of Ga used for GZO is much larger than typical chemical methods as usually GZO can only tolerate less than 10 at % Ga. Without any Ga introduction, rods with diameter 1-1.5 µm and length 10-12 µm were grown and arranged in a neat floral arrangement. With 10 at % Ga, hexagonal discs littered with vertically protruding spike-like rods were formed. This is a unique morphology which has not yet been reported. PL spectra showed that the visible emission centers shifted to shorter wavelengths from 2.11 to 2.57 eV with 0, 10 and 20 at % Ga in GZO, suggesting that the Ga dopants contributed to the defects in ZnO. With ZnGa2O4, blue emissions emerged as well though they were blue-shifted drastically to 2.66-2.73 eV. In summary, these studies have sprouted interesting ideas towards nanostructured ZnO, and provided room for further investigations. However, this will be left to the other group members to explore these prospects. vii List of Tables Table 2.1 Flow on the development of hydrothermal synthesis over time………......29 Table 3.1 Ratio of the height of the peak intensities of ZnO (101) and Zn (101) in Zn film before annealing, ZnO films annealed at 100 oC for 15 h, 200 oC for 214 h, and 400 oC for 6 h………………………………………………………………………...50 Table 4.1 Tabulations for an estimation of moles of Zn present in Zn films………..78 Table 4.2 Tabulations of experimental results……………………………………….80 viii List of Figures Fig. 1.1 Stick and ball representation of ZnO crystal structures: (a) cubic rocksalt, (b) cubic zinc blende, and (c) hexagonal wurtzite. The shaded gray and black spheres denote Zn and O atoms, respectively……………………………………………..…...2 Fig. 1.2 (a) A schematic of a DSC based on the ZnO branched nanorod array, (b) Photocurrent–voltage curves of the DSSCs based on the ZnO branched nanorod array, its corresponding primary nanorod array, and the nanowire arrays, and (c) Performance characteristics of the DSCs based on different nanostructures….…..….4 Fig. 1.3 SEM images of ZnO nanorods synthesized in Q. Yu et al with (a) no H3BO3, and (b) 0.03 mol/L of H3BO3 concentration……………………………………..……6 Fig. 1.4 SEM images of (a) 15% fluorine-doped ZnO, (b) Zn0.9432Mn0.0568O nanostructured thin films obtained from sol-gel method.. ……………………..……..8 Fig. 1.5 Illustration of comparison between conventional and microwave-assisted heating. ………………………………………………………………………..……....9 Fig. 1.6 Two nanoimprint lithography schemes developed as (a) thermal imprinting process, (b) UV imprinting process, and (c) soft imprinting process.. ……………...11 Fig. 1.7 SEM images of the patterned ZnO film, area with (a) array pattern, and (b) line pattern, obtained by soft lithography….………………………………………...12 Fig. 1.8 SEM images of (a) cross-section of growth of nanowires on Si substrate in A. K. Srivastava, (b) GZO nanorods with 1 wt% Ga on sapphire substrate, both obtained by RF magnetron sputtering in Young et al…………………………………...…..…13 Fig. 1.9 SEM images of (a) ZnO nano-needles on ZnO/Zn/ZnO multilayer structure annealed at 300-400 oC in S. Kumar et al, and (b) annealed dense ZnO film-like nanobelts obtained under plasma power of 70 W in G. X. Li et al…………………..14 ix Fig. 2.1 Schematic of physical sputtering process………………………………...…26 Fig. 2.2 Setup of the RF magnetron sputtering system used in the experiments….....27 Fig. 2.3 Experimental setup of furnace used in the experiments. ………………...…28 Fig. 2.4 Experimental setup of Teflon-liner and outer stainless steel casting used in the experiments…..…………………………………………………………………..30 Fig. 2.5 Diagram showing pressure as a function of temperature for pure water, with the filling factor (% degree of fill) of the autoclave. The critical temperature (Tcr = 374.1 °C) and pressure (ρ = 221.2 bar) are indicated….…………………………….31 Fig. 2.6 Viscosity of water as a function of density and temperature…………….….32 Fig. 2.7 Dielectric constant of water plotted against as a function of pressure and temperature….……………………………………………………………………….33 Fig. 2.8 Diagram showing the percent of Zn(II) present in the labelled form at each pH. Only species that were present at a ratio of greater than 10% in the pH range 2– 13.5 are displayed….………………………………………………………………...34 Fig. 2.9 Illustration of Bragg’s law………..………………………………………....36 Fig. 2.10 Schematic of the XRD measurement.……………………………………..37 Fig. 2.11 Schematic diagram of a working SEM………………………………….....38 Fig. 2.12 Schematic diagram of the central process by which images and diffraction patterns are formed within the objective lens of the TEM…………………………..41 Fig. 2.13 Illustration of VSM……………………………………………………...…43 Fig. 3.1 XRD spectra of films under different conditions of (a) Zn film before annealing, (b) ZnO films after annealed at 100 oC for 15 h, (c) 200 oC for 214 h, and (d) 400 oC for 6 h…………………………………………………………………….50 x Fig. 3.2 Morphology and visual appearances (insets) of ZnO films under different conditions with (a) SEM of Zn film before annealing, (b) SEM of ZnO films after annealed at 100 oC for 15 h, (c) 200 oC for 214 h, (d) 400 oC for 6 h, and (e) HR-TEM of ZnO film annealed at 200 oC for 214 h……………………………………...……53 Fig. 3.3 Schematic diagram of mechanism to obtain Zn/ZnO layered film………....55 Fig. 3.4 PL spectra of films under different conditions of (a) Zn film before annealing, ZnO films after annealed at 100 oC for 15 h, 200 oC for 214 h, and 400 oC for 6 h, and (b) Inset: PL spectrum magnification of Zn film before annealing……………...….58 Fig. 3.5 Illustration of mechanism behind UV and green emissions in Zn/ZnO layered film………………………………………………………………………….……..…59 Fig. 3.6 M-H curves by VSM at room temperature of films annealed under different conditions of (a) Zn film before annealing, ZnO films after annealed at (b) 100 oC for 15 h, (c) 200 oC for 214 h, and (d) 400 oC for 6 h.. …………………………………61 Fig. 4.1 SEM of ZnO nanostructured films (a) before heating, after heating at 170 oC for (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, and (f) 15 h……………………………….……69 Fig. 4.2 XRD of ZnO nanostructured films (a) before heating, after heating at 170 oC for (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, and (f) 15 h……………………………….……70 Fig. 4.3 PL spectra of ZnO nanostructured films (a) before heating, after heating at 170 oC for (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, and (f) 15 h.…………………………...72 Fig. 4.4 TEM of different parts of ZnO nanostructured films after heating at 170 oC for 15 h in (a) one section, (b) SAED of the section, (c) a nanowire with HRTEM as inset, and (d) branched section of a nanowire…………………………………...…..74 Fig. 4.5 An illustration of growth mechanism for nanostructured ZnO films……....77 xi Fig. 4.6 XRD of powders on a Si wafer with pre-fixed Zn:NaCl ratio of (a) 1:1, (b) 10:1, (c) 1:10………………………………………………………………………..80 Fig. 5.1 (a) XRD pattern, (b) SEM image, (c) PL spectrum for ZnO-seeded glass substrates heated for 4 hours, and (d) cross-sectional SEM image for ZnO-seeded glass substrate heated for 24 hours…………………………………………………..88 Fig. 5.2 XRD patterns of ZnO growing on (a) ZnO-seeded glass, (b) glass, (c) silicon substrates for 4 hours.……………………………………………………...…….…. 89 Fig. 5.3 SEM images of ZnO growing on (a) ZnO-seeded glass, (b) glass, (c) silicon substrates for 4 hours.…………………………………………………………..…... 90 Fig. 5.4 PL spectra of ZnO grown on (a) ZnO-seed glass, (b) glass, (c) silicon substrates for 4 hours.……………………………………………………………..... 91 Fig. 5.5 XRD patterns of ZnO grown on silicon substrates for (a) 4, (b) 8, (c) 12, and (d) 24 hours.……………………………………………………………………..….. 92 Fig. 5.6 SEM of ZnO grown on silicon substrates for (a) 4, (b) 8, (c) 12, (d) 24 hours, and (e) Plot of ZnO film thickness to heating duration.…………………………..... 94 Fig. 5.7 PL spectra of ZnO grown on silicon substrates for (a) 4, (b) 8, (c) 12, and (d) 24 hours.……………………………………………………………………………...95 Fig. 5.8 XRD patterns of as-synthesized GZO powder with (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 at % of Ga:Zn ratio in the starting precursors. ………….……......96 Fig. 5.9 ICP of as-synthesized powder with (a) 10, (b) 20, (c) 30, (d) 40, and (e) 50 at % of Ga:Zn in the starting precursors.………………………………….………………97 Fig. 5.10 SEM images of as-synthesized powder with (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 at % Ga at the start…………………………………………………..100 xii Fig. 5.11 PL spectra of as-synthesized GZO powders with (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 at % Ga at the start……………………………………………….102 Fig. 5.12 SEM mapping of (a) electron image, (b) Zn, (c) Ga, and (d) O at one section of as-synthesized powder with 10 at % Ga at the start……………………………..104 Fig. 5.13 (a) TEM, (b) SAED of TEM, (c) HRTEM, and (d) SAED of HRTEM for as-synthesized powder with 10 at % of Ga/Zn at the start………………………….106 Fig. 5.14 XRD patterns of as-synthesized powders with (a) 0, (b) 10, (c) 20 at % Ga, and (d) Williamson-Hall Plot for as-synthesized GZO powder with 10 at % Ga/Zn at the start.…………………………………………………………………………......107 xiii List of Publications 1) R. Q. Wee, H. Gong, W. F. Yang, R. Chen, H. D. Sun, C. F. Wang, A.Y. S. Lee, “Growth of Zinc Oxide Nanorods in Different Directions by a Simple Chemical Method”, International Conference of Young Researchers on Advanced Materials (ICYRAM) 2012, Singapore. Abstract is accepted as oral presentation. 2) R. Q. Wee, W. F. Yang, R. Chen, H. D. Sun, C. F. Wang, A. Y. S. Lee, and H. Gong, “Development of ZnO Nanostructured Films via Sodium Chloride Solution and Investigation of Its Growth Mechanism and Optical Properties”. Accepted in Journal of the American Ceramic Society for publication. 3) R. Q. Wee, H. Gong, W. F. Yang, R. Chen, H. D. Sun, “On Black ZnO Films and Light Emission Properties”. Submitted to Journal of Physics D for publication. xiv Chapter 1: Introduction Wee Rui Qi Chapter 1: Introduction 1.1 Background Information 1.1.1 Introduction to Nanostructured ZnO Properties and Applications Zinc oxide (ZnO) has attracted tremendous attention having good electrical properties. Transparent conducting oxides (TCOs),1 organic light-emitting diodes,2 gas sensors,3 field-emitters,4 photocatalysts,5 antireflection coatings,6 dye-sensitized solar cells,7 ferromagnetic materials,8 and even pyroelectric generators9 are some which made use of this property. Pure ZnO is an n-type semiconductor with a wide band gap of 3.4 eV.10 Round nanorods, hexagonal rods, flower-like and coral-reef like ZnO are found on substrates such as Si, cotton, nylon, FTO, and ITO.11-16 Most of the group-II-VI binary compound semiconductors crystallize in either cubic zinc-blende or hexagonal wurtzite structure where each anion is surrounded by four cations at the corners of a tetrahedron, and vice versa. This tetrahedral coordination is typical of sp3 covalent bonding, but these materials also have a substantial ionic character. ZnO is a II-VI compound semiconductor. Its ionicity resides at the borderline between covalent and ionic semiconductor. Wurtzite, zinc blende, and rocksalt are the crystal structures in ZnO, as schematically shown in Fig. 1.1. Wurtzite is the most common phase as it is thermodynamically stable phase at ambient conditions. The zinc-blende ZnO structure can be stabilized only by growth on cubic substrates, while the rocksalt (NaCl) structure are obtained at relatively high pressures.17,18 1 Chapter 1: Introduction Wee Rui Qi Fig. 1.1 Stick and ball representation of ZnO crystal structures: (a) cubic rocksalt, (b) cubic zinc blende, and (c) hexagonal wurtzite. The shaded gray and black spheres denote Zn and O atoms, respectively.17 The optical properties in ZnO constitute one of the basic properties often examined in ZnO as light emissions in the ultraviolet (UV) and green light regions are commonly observed. It is reported by Jin et al. that those defect-related luminescences are caused by radiative transitions between shallow donors (related to oxygen vacancies) and deep acceptors (Zn vacancies).19 The acceptor level (Zn vacancy) is located 2.5 eV below the conduction band edge, while the donor level is known as shallow as 0.05–0.19 eV. For UV light emission, it is due to recombination of electrons and holes. Red, orange, yellow blue emissions are also reported though these are less common.20,21 Metal capping of ZnO noble metals or infusion of metal nanoparticles into ZnO -based structures is one way to control optical properties. Elements currently under study include Ag, Au, Al and Pt 22-24. With much development in nanostructured ZnO, there is no doubt that it will bring forth exciting improvements with the incorporation of ZnO nanostructures in devices. However, even different morphology can influence its potential in improving device performance differently. In M. Raula et al, Friedel-Crafts acylation reaction of anthracene with benzoyl chloride, a typical test for photocatalytic activity, is carried 2 Chapter 1: Introduction Wee Rui Qi out. It is found that the yields of flower-like ZnO nanostructures were higher than their spherical nano-counterparts, showing greater potential as catalysts.5 In Y. X. Wang et al, ZnO nanoflowers showed an improved ability on the photocatalytic degradation of 4-cholrophenol (4-CP) in aqueous solution under UV radiation than that of ZnO nanorods. From Fig. 1.2, the 4-CP in aqueous solutions can be almost completely eliminated by ZnO nanoflowers while ZnO nanorods show ~80% degradation of 4-CP after illuminated by UV light for 120 min.25 In Y. Zhang et al, brush-like hierarchical ZnO nanostructures showed greater response to ethanol compared with ZnO nanowires. This could be due to the enhanced oxygen vacancy defects observed from PL.26 In C. X. Wang et al, ZnO nanoflower films have better dye loading than ZnO nanorod films. This translated in an overall conversion efficiency of 1.37 % for the dye-sensitized solar cell (DSSC) with ZnO nanoflowers, making it higher than DSSC with nanorods.7 In X. M. Fang et al, branched ZnO architecture with nanorods in a 3D array overrode its nanowire and nanorod counterparts in conversion efficiency in DSSC. The reason behind was the increased surface area in branched 3D array which increased dye absorption.27 3 Chapter 1: Introduction Wee Rui Qi Fig. 1.2 (a) A schematic of a DSSC based on the ZnO branched nanorod array, (b) Photocurrent–voltage curves of the DSSCs based on the ZnO branched nanorod array, its corresponding primary nanorod array, and the nanowire arrays, and (c) Performance characteristics of the DSCs based on different nanostructures.27 1.1.2 Synthesis Methods for Nanostructured Materials and ZnO As seen from Section 1.1.3, there is no fixed preference on morphology required for improved device performance. Experiments have to be fine-tuned with different nanostructures in order to gauge its comparison. There is therefore a need to have a precise control over the synthesis of ZnO nanostructures before progressing to research on device performance. Chemical synthesis of nano-materials may be conducted in solid, liquid, or gaseous state. This section highlights some of the common synthesis methods for nanostructures and ZnO. 4 Chapter 1: Introduction Wee Rui Qi 1.1.2.1 Solvothermal/Hydrothermal methods In solvothermal techniques, the reaction mixture is heated above the boiling point of the solvent in an autoclave or other closed system and the sample is exposed to steam at high pressures. The reactions may be carried out in water or in any other solvent (e.g. methanol, ethanol, polyol). When water is used as a solvent, the process is described as hydrothermal. Compared with synthesis routes at atmospheric pressure, the increased reaction temperature in the solvothermal technique may lead to an accelerated crystal growth accompanied by a narrow particle size distribution and better crystallinity.28 Typically, an aqueous solution of Zn salts such as zinc nitrate hexahydrate Zn(NO3)2·6H2O, zinc sulfate heptahydrate ZnSO4·7H2O and zinc acetate dehydrate (Zn(CHCOO)2·2H2O mixed with ammonia or ammonia precursors is mixed before introducing into the Teflon-lined container. Heating is usually carried out at low temperatures below 110 oC, where 30 oC is known to produce ZnO in Zhao et al.29,30 The ammonia provides a steady source of hydroxide ions to form zinc hydroxide, which later undergoes a condensation reaction to form ZnO.5,31 Therefore, the molar ratio of Zn salts and hydroxide ions present is usually closely monitored as it is well known for hydroxide ions in shape alteration of ZnO.5,30,32,33 Hydroxide sources include ammonia, NaOH and hexamethylenetetramine C6H12N4 (HMT). Parameters such as cooling rate, heating temperatures and durations are also known to affect synthesis though they are less studied.30,34 In some cases, surfactants are also introduced. The externally added surfactants or capping agents were adsorbed preferentially on some crystal planes of the growing particles that ultimately alter the growth kinetics and the relative stability of the 5 Chapter 1: Introduction Wee Rui Qi crystal faces and hence either promote or inhibit crystal growth in some particular crystal planes, resulting in the formation of anisotropic ZnO nanostructures. In M. Raula et al, the introduction of sodium ascorbate resulted in flower-like ZnO.5 Further experiments by changing the concentration of precursors and the shape-directing agent showed that intermediate morphologies include spherical/quasi-spherical and spindle shaped nanostructures. Ethylene diaminetetra acetic acid (EDTA) and cetyltrimethylammonium bromide (CTAB) are other surfactants known to be added in.7,31,33 Though addition of metal salts can alter shape configurations too, these metal salts are better known in altering ZnO properties so that the modified properties can better fulfill the requirements of the applications. Antimony chloride (SbCl3), silver nitrate Ag(NO3), aluminum chloride hexahydrate (AlCl3·6H2O), cobalt nitrate (Co(NO3)2·6H2O), boric acid (H3BO3), manganese acetate, and indium chloride (InCl3) were some of the dopant salts found in hydrothermal synthesis of dopedZnO.35-40 The influence of H3BO3 on ZnO morphology is given in Fig. 1.3.38 Fig. 1.3 SEM images of ZnO nanorods synthesized in Q. Yu et al with (a) no H3BO3, and (b) 0.03 mol/L of H3BO3 concentration.38 6 Chapter 1: Introduction Wee Rui Qi The growth of nanostructured ZnO films requires an additional step; that is, to grow firmly on the substrate. However, to grow such nanostructured films, the substrates are usually first coated with a thin layer of ZnO before the nanostructured ZnO growth can proceed.29,30,32,41 Known as the seeding layer, it is said that this ZnOseeded layer allows the nucleation step to be bypassed. Growth can take place immediately since the interfacial energy between the crystal nuclei and the substrate is effectively lowered. It is also reported that types of substrates can affect ZnO morphology though the number of studies done are very sparse.15 1.1.2.2 Sol-Gel Sol–gel processes are another wet chemical synthesis commonly used for nanostructures such as powders, films, fibers, and monoliths.28 Typical sol–gel process involves hydrolysis and condensation of metal alkoxides and metal salts such as chlorides, nitrates and acetates. In metal alkoxides M(OR)x, the synthesis involves the reaction of metal species (a metal, metal hydroxide, metal oxide, or metal halide) with an alcohol. Metal alkoxides are good precursors because they readily undergo hydrolysis that replaces an alkoxide with a hydroxide group from water and a free alcohol is formed. Hydrolysis occurred when heated over time, allowing the sol to progress further in its reaction. Condensation (polymerization) occurred, leading to gel formation. The two hydrolyzed fragments then join together during condensation to release either an alcohol or water. In nanostructured ZnO, zinc acetate dehydrate Zn(CH3COO)2.H2O is usually dissolved in an alcohol along with a stabilizer, monoethanolamine (C2H7NO, MEA) 7 Chapter 1: Introduction Wee Rui Qi Alcohols include 2-methoxethanol (C3H8O2) and isopropanol. To get doped ZnO, dopant salts are also introduced. Examples include antimony chloride (SbCl3), ammonium fluoride (FNH4), aluminum chloride hexahydrate (AlCl3.6H2O), copper acetate (Cu(CH3COO)2), manganese acetate dihydrate and (Mn(CH3COO)2.2H2O. The influence of FNH4 on ZnO morphology is given in Fig. 1.4. The mixture is then heated to yield a clear and homogeneous solution before cooling to room temperatures. The solution is usually spin coated multiple times before annealing to obtain a film. Nanorods, nanofibers, nanoparticulate films have been obtained by solgel.42-48 Fig. 1.4 SEM images of (a) 15% fluorine-doped ZnO, (b) Zn0.9432Mn0.0568O nanostructured thin films obtained from sol-gel method.43,46 1.1.2.3 Microwave-Assisted Synthesis Compared with the conventional heating, microwave heating can heat up the reaction system rapidly due to its unique characteristics, resulting in high reaction rate, short reaction time, enhanced reaction selectivity, energy saving, and is environmentally friendly as there are no byproducts of combustion.14 It is usually used in conjunction with other synthesis methods.28 Fig. 1.5 gives the comparison between conventional and microwave-assisted heating. It is observed that a uniform heating is achieved with microwave. The 8 Chapter 1: Introduction Wee Rui Qi microwave electromagnetic fields can greatly enhance the reaction/diffusion, which increase the crystal growth rate during processing of various materials. This reduced synthesis time and cut costs.49 In addition, the very high temperatures and pressures of collapsing gas bubbles led to thermal dissociation of water vapor into ·OH and ·H radicals, allowing for quicker reactions.50 Fig. 1.5 Illustration of comparison between conventional and microwave-assisted heating.49 In nanostructured ZnO, microwave-assisted synthesis is usually used as a complement for other steps during fabrication.51-54 In K. D. Bhatte et al, formation of nanocrystalline ZnO was carried out using microwave irradiation and by using 1,3propanediol as a solvent and zinc acetate as a precursor The mixture is transferred into a Teflon-liner tube and kept in a microwave oven for heating.51 In J. F. Huang et al, Zn(NO3)2·6H2O and NaOH are placed in Teflon-liner before keeping in temperature cum pressure-controlled microwave hydrothermal system. Nanorods and nanowires were obtained.52 9 Chapter 1: Introduction Wee Rui Qi 1.1.2.4 Nano-Lithography Template-assisted fabrications are also used for nanostructured films. A solution is then deposited on the template and formed the desired nanostructures. In electron beam (e-beam) lithography, a beam of electrons is emitted in a patterned fashion across a surface covered with a film (called the resist).55 Pattern transfer to underlying substrates usually occurred by reactive-ion etching (RIE). The advantage of e-beam lithography is that the wavelength of a 100 keV electron at 4 pm is much smaller than the wavelength of photons at 193-436 nm used in conventional lithography. This allows creation of nanostructures as dimensions of features cannot be smaller due to diffraction limit of light. Nanoimprint lithography is an upcoming method of fabricating nanometer scale patterns.56 It is simpler and faster than electron beam lithography while achieving nano-sized features. Patterns are created by mechanical deformation of imprint resist rather than electron beam. Three schemes are illustrated in Fig. 1.6. The differences lie in the steps before the mold is separate from the substrate after patterning, and before etching occurs for pattern to be transferred onto underlying substrate. Thermal imprinting makes use of a high viscosity spin-coated layer is applied on substrate before the patterned template comes in contact.57 The temperature of the spin-on material is raised above its Tg while applying a high pressure to the stack of the mold and substrate to conform them. In UVnanoimprinting, a low-viscosity UV-curable material is used. Discrete drops of lowviscosity UV-curable material are first dispensed between the mask and the substrate to induce the filling of the mask features. UV curing is then carried out to solidify the resist. In soft-lithography, the mold is generally made with a very flexible material 10 Chapter 1: Introduction Wee Rui Qi such as polydimethylsiloxane or PDMS.58 This enables patterning with the use of flexible low-cost molds instead of rigid molds such as silicon or fused silica used in imprint lithography. “Ink” is applied on the raised features of mold where its pattern was transferred on the resist upon stamping. Fig. 1.6 Two nanoimprint lithography schemes developed as (a) thermal imprinting process, (b) UV imprinting process, and (c) soft imprinting process.56 Nanostructured ZnO has been obtained by the as-mentioned types of lithography.59-64 Fig. 1.7 gives patterned ZnO film obtained by soft lithography. The patterned resist was obtained before depositing ZnO solution by spin-coatng, sol-gel or precipitation. In Y. Leprince-Wang et al. and J. K. Hwang et al where both soft and UV lithography are involved, PDMS stamp is first used before curing and subsequent deposition of ZnO solution.63,64 Nanowire-arrays, nano-pillar and nano-ribs were some unique patterns obtained with help of lithography. 11 Chapter 1: Introduction Wee Rui Qi Fig. 1.7 SEM images of the patterned ZnO film, area with (a) array pattern, and (b) line pattern, obtained by soft lithography.62 1.1.2.5 Vapor-Phase Synthesis Atoms or molecules are deposited onto surfaces to form coatings or thin films ranging in thickness from one atomic layer (~0.3 nm) to hundreds of micrometers. Vapor deposition can be categorized into either physical (PVD) or chemical (CVD). The main differences lie in the method used for deposition. In PVD, the coating method involves purely physical processes such as high temperature vacuum evaporation with subsequent condensation, or plasma sputter bombardment. PVD includes electron beam evaporation, pulsed laser deposition and sputtering.65-68 However in CVD, a chemical reaction at the surface is involved. Zn, ZnO or zinc sulphide (ZnS) powder was usually introduced as source material in tube furnace in CVD. ZnO films are obtained at 500-1100 oC usually either in N2 or argon gas flow. O2 gas is sometimes added too.69-73 ZnO nanostructures can also be obtained without the use of high temperature. Though sputtering is more commonly known for thin film deposition, nanostructures have been obtained.74-78 In A. K. Srivastava, nanorods and nanowires were obtained from sputtering ZnO target on Si substrates.77 In Young et al, vertically arrayed Ga-doped ZnO nanorods were grown on sapphire substrate during RF magnetron sputtering by 12 Chapter 1: Introduction Wee Rui Qi ZnO targets pre-mixed with Ga (Fig. 1.8).78 Ga dopants promoted nanorod growth by inducing island growth in the initial stage. Fig. 1.8 SEM images of (a) cross-section of growth of nanowires on Si substrate in A. K. Srivastava, (b) GZO nanorods with 1 wt% Ga on sapphire substrate, both obtained by RF magnetron sputtering in Young et al.77,78 1.1.2.6 Direct Oxidation by Air It is possible ZnO nanostructures to form through annealing in air.79-81 In S. Kumar et al, annealing of ZnO/Zn/ZnO multilayer structure at 300-400 oC after sputtering led to the formation of ZnO nano-needles on the surface.79 The ultra-thin Zn layer was found to be the self-catalytic agent to nucleate the growth of the ZnO nano-needles. This is reasonable as nanowires and nanobelts were observed after oxidation of sputtered zinc films at 350 oC obtained with target RF powers of 50–100 W in G. X. Li et al (Fig. 1.9).80 In Parkansky et al, ZnO nanorods are obtained after annealing of ZnO films at 300 oC.81 13 Chapter 1: Introduction Wee Rui Qi Fig. 1.9 SEM images of (a) ZnO nano-needles on ZnO/Zn/ZnO multilayer structure annealed at 300-400 oC in S. Kumar et al, and (b) annealed dense ZnO film-like nanobelts obtained under plasma power of 70 W in G. X. Li et al.79,80 1.1.3 Challenges Identified With the importance of ZnO and nanostructures, there is a continued interest to study ZnO and address existing disputes regarding it. Some challenges pertaining ZnO which further understanding is sought after, and work has been carried out in Chapters 3, 4, and 5. In Tian et al, ZnO films with pyramids with extremely sharp tips on its surface, are being suspected to be the cause behind black appearance.82 The fabrication however, was done purely on a zinc block. It is difficult to distinguish whether the remnant zinc after hydrothermal treatment or the ZnO pyramids morphology, is accountable to the appearance of black color. As black coatings are said to be most effective in suppressing reflections from the transparent conducting oxide (TCO), it is important to analyse its origin.83-85 This controversy surrounding the origins of black ZnO triggered an interest to study in detail here. Chapter 3 seeks to have a clearer understanding behind origins of black ZnO by depositing Zn films on clear glass substrate instead, before oxidized gradually into ZnO. Using molten salts with Zn has been one of the methods to obtain ZnO. It is peculiar that there is no known report on having aqueous sodium chloride (NaCl) to 14 Chapter 1: Introduction Wee Rui Qi obtain ZnO even though aqueous solutions are widely known in synthesis of ZnO.86-90 For instance, A.N. Baranov et.al. reported the synthesis of ZnO nanorods by adjusting the ratio of zinc precursor to sodium chloride powder of 1:10 prepared by freezedrying followed by ball milling before heating up to 800 oC.86 It is well known that zinc undergoes corrosion in the presence of humidity or seawater. Thus, it may be possible that sodium chloride can play a role in the formation of ZnO. It is hypothesized that controlled etching by solution method can be achieved to obtain ZnO nanostructures. Chapter 4 reports the successful synthesis of ZnO with aqueous sodium chloride as well as the manipulation of nanostructures with heating durations. In hydrothermal synthesis, an aqueous solution of Zn salts is usually mixed with ammonia or ammonia precursors, is used as discussed earlier. The ammonia provides a steady source of hydroxide ions to form zinc hydroxide, which later undergoes a condensation reaction to form ZnO. However, to grow specifically nanostructured films, the substrates are usually first coated with a thin layer of ZnO before the nanostructured ZnO growth is carried out. It is reported that the types of substrates can affect ZnO morphology though the number of studies done are very sparse.15 Moreover, there is no systematic study whereby different substrates underwent the same set of synthesis conditions. In addition, gallium is of interest due to its ease of Ga3+ ions as a substitution for Zn2+ ions without much lattice distortion. It is often used to improve electrical properties. Few reports existed in obtaining GZO via hydrothermal methods.91,92 As ammonia is commonly used to form a complex with Zn precursors for ZnO synthesis, it is proposed that GZO can be obtained by this method even with large amount of Ga. 15 Chapter 1: Introduction Wee Rui Qi It is hoped that interesting morphologies and properties will be obtained as well. The first section in Chapter 5 examines the effect of substrates and heating durations on ZnO synthesis while the second section investigates the influence of Ga on ZnO morphology and properties. 1.2 Outline of Thesis The thesis comprises of six chapters whereby Chapters 3 to 5 constituted of studies done. In these chapters, detailed background information is given to allow readers a comprehensive insight on the existing works done. Chapter 1 provides the background information on ZnO, and the importance of nanostructures and nanostructured ZnO which has been discussed in this chapter. Chapter 2 contains mainly details of synthesis methods for our studies as well as the characterization techniques. Chapter 3 seeks to have a clearer understanding behind origins of black ZnO by depositing Zn films on clear glass substrate instead, before oxidized gradually into ZnO. Chapter 4 reports the successful synthesis of ZnO with aqueous sodium chloride as well as the manipulation of nanostructures with heating durations. The first section in Chapter 5 examines the effect of substrates and heating durations on ZnO synthesis while the second section investigates the influence of Ga on ZnO morphology and properties. To sum up the works done, Chapter 6 gave a summary of the works done and also suggested some future works related to ZnO. 16 Chapter 1: Introduction Wee Rui Qi 1.3 References 1 N. R. Shiju and V. V. Guliants, Appl. Catal., A 356, 1 (2009). 2 P. K. Nayak, J. Kim, S. Chung, J. Jeong, C. Lee, and Y. Hong, J. Phys. D: Appl. Phys. 42, 139801 (2009). 3 H.-S. Hong and G. S. 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Sci. 249, 71 (2005). 42 S. Ilican, Y. Caglar, M. Caglar, F. Yakuphanoglu, and J. Cui, Physica E 41, 96 (2008). 43 S. Ilican, Y. Caglar, M. Caglar, and F. Yakuphanoglu, Appl. Surf. Sci. 255, 2353 (2008). 44 Y. S. Kim and W. P. Tai, Appl. Surf. Sci. 253, 4911 (2007). 45 D. Wang, J. Zhou, and G. Liu, J. Alloys Compd. 487, 545 (2009). 46 U. N. Maiti, P. K. Ghosh, S. Nandy, and K. K. Chattopadhyay, Physica B: Condensed Matter. 387, 103 (2007). 47 M. H. Habibi and M. K. Sardashti, J. Nanomat. 2008, 1 (2008). 48 Y. Q. Huang, M. Liu, Y. Zeng, C. Li, D. Xia, and S. Liu, Mater. Sci. Eng., B 86, 232 (2001). 49 H. Cheng, J. Cheng, Y. Zhang, and Q. M. Wang, J. Cryst. Growth 299, 34 (2007). 50 51 P. Riesz and T. Kondo, Free Radical Biol. Med. 13, 247 (1992). ). K. D. Bhatte, D. N. Sawant, R. A. Watile, and B. M. Bhanage, Mater. Lett. 69, 66 (2012). 52 J. Huang, C. Xia, L. Cao, and X. Zeng, Mater. Sci. Eng., B 150, 187 (2008). 53 Y. Du, C. Hao, and G. Wang, Mater. Lett. 62, 30 (2008). 54 R. Al-Gaashani, S. Radiman, N. Tabet, and A. R. Daud, Mater. Chem. Phys. 125, 846 (2011). 55 D.M. Tennant, A.R. Bleier, in Comprehensive Nanoscience and Technology; Vol. 4, edited by D. L. Andrews, G. D. Scholes, G. P. Wiederrecht (Elsevier, 2011), p. 35. 20 Chapter 1: Introduction 56 Wee Rui Qi S.V. Sreenivasan, J. Choi, P. Schumaker, F. Xu, in Comprehensive Nanoscience and Technology; Vol. 4, edited by D. L. Andrews, G. D. Scholes, G. P. Wiederrecht (Elsevier, 2011), p. 83. 57 S. Y. Chou, P. R. Krauss, and P. J. Renstrom, J. Vac. Sci. Technol. B 14, 6 (1996). 58 Y. Xia and G. M. Whitesides, Annu. Rev. Mater. Sci. 28, 153 (1998). 59 S. Donthu, Z. Pan, B. Myers, G. Shekhawat, N. Wu, and V. Dravid, Nano Lett. 5, 9 1710 (2005). 60 M. H. Jung and H. Lee, Nanoscale Res. Lett. 6, 159 (2011). 61 C. Y. Kuan, J. M. Chou, I. C. Leu, and M. H. Hon, J. Am. Ceram. Soc. 91, 3160 (2008). 62 S. Sepulveda-Guzman, B. Reeja-Jayan, E. De la Rosa, U. Ortiz-Mendez, C. Reyes-Betanzo, R. Cruz-Silva, and M. Jose-Yacaman, Appl. Surf. Sci. 256, 3386 (2010). 63 Y. Leprince-Wang, S. Bouchaib, T. Brouri, M. Capo-Chichi, K. Laurent, J. Leopoldes, S. Tusseau-Nenez, L. Lei, and Y. Chen, Mater. Sci. Eng., B 170, 107 (2010). 64 J. K. Hwang, S. Cho, E. K. Seo, J. M. Myoung, and M. M. Sung, ACS Appl. Mater and Interfaces, 1, 2843 (2009). 65 W. Kern and K. K. Schuegraf, in Handbook of Thin Film Deposition Processes and Techniques (2)- Principles, Methods, Equipment and Applications; Ch. 1, edited by K. Seshan (William Andrew Inc., 2001), p. 11. 21 Chapter 1: Introduction 66 Wee Rui Qi S. Rossnagel, in Handbook of Thin Film Deposition Processes and Techniques (2)- Principles, Methods, Equipment and Applications; Ch. 8, edited by K. Seshan (William Andrew Inc., 2001), p. 319. 67 H. O. Pierson, in Handbook of Chemical Vapor Deposition (CVD)- Principles, Technology, and Applications; Ch. 2, 2 ed., edited by H. O. Pierson (William Andrew Inc., 1999). 68 P. M. Martin, in Handbook of Deposition Technologies for Films and Coatings- Science, Applications and Technology; Ch. 1, 3 ed., edited by P. M. Martin (Elsevier, 2010), p. 1. 69 Y. Liu, Z. Chen, Z. Kang, I. Bello, X. Fan, I. Shafiq, W. Zhang, and S. T. Lee, J. Phys. Chem. C 112, 9214 (2008). 70 S. Kar, B. N. Pal, S. Chaudhuri, and D. Chakravorty, J. Phys. Chem. B 110, 4605 (2006). 71 A. Umar, C. Ribeiro, A. Al-Hajry, Y. Masuda, and Y. B. Hahn, J. Phys. Chem. C 113, 14715 (2009). 72 J. Li, Q. Zhang, H. Peng, H. O. Everitt, L. Qin and J. Liu, J. Phys. Chem. C 113, 3950 (2009). 73 B. J. Chen, X. W. Sun, C. X. Xu, and B. K. Tay, Physica E 21, 103 (2004). 74 Y. R. Park, E. K. Kim, D. Jung, T. S. Park, and Y. S. Kim, Appl. Surf. Sci. 254, 2250 (2008). 75 H. Gong, J. Q. Hu, J. H. Wang, C. H. Ong, and F. R. Zhu, Sens. Actuators, B. 115, 247 (2006). 22 Chapter 1: Introduction 76 Wee Rui Qi A. K. Srivastava, Praveen, M. Arora, S. K. Gupta, B. R. Chakraborty, S. Chandra, S. Toyoda, and H. Bahadur, J. Mater. Sci and Technol. 26, 986 (2010). 77 A. K. Srivastava, B. R. Chakraborty, and S. Chandra, J. Nanomater. 2009, 1 (2009). 78 Y. Y. Kim, B. H. Kong, and H. K. Cho, J. Cryst. Growth 330, 17 (2011). 79 S. Kumar, V. Gupta, and K. Sreenivas, Nanotechnol. 16, 1167 (2005). 80 G. Li, B. Wang, Y. Liu, T. Tan, X. Song, and H. Yan, Appl. Surf. Sci. 255, 3112 (2008). 81 N. Parkansky, G. Shalev, B. Alterkop, S. Goldsmith, R. L. Boxman, Z. Barkay, L. Glikman, H. Wulff, and M. Quaas, Surf. and Coatings Technol. 201, 2844 (2006). 82 Y. Tian, C. Hu, Y. Xiong, B. Wan, C. Xia, X. He, and Hong Liu, J. Phys. Chem. C 114, 10265 (2010). 83 D. Murias, C. Reyes-Betanzo, M. Moreno, A. Torres, A. Itzmoyotl, R. Ambrosio, M. Soriano, J. Lucas, and P. R. i. Cabarrocas, Mater. Sci. Eng., B (2012). 84 A. Y. Vorobyev and C. Guo, Appl. Surf. Sci. 257, 7291 (2011). 85 Y. Xia, B. Liu, J. Liu, Z. Shen, and C. Li, Sol. Energy 85, 1574 (2011). 86 A. N. Baranov, C. H. Chang, O. A. Shlyakhtin, G. N. Panin, T. W. Kang, and Y.-J. Oh, Nanotechnol. 15, 1613 (2004). 87 C. P. Fah, J. Xue, and J. Wang, J. Am. Ceram. Soc. 85, 1, 273 (2002). 88 A. N. Baranov, G. N. Panin, T. W. Kang, and Y. J. Oh, Nanotechnol. 16, 1918 (2005). 23 Chapter 1: Introduction Wee Rui Qi 89 L. Shen, L. Guo, N. Bao, and K. Yanagisawa, Chem. Lett. 32, 826 (2003). 90 C. Xu, G. Xu, Y. Liu, and G. Wang, Solid State Commun. 122, 175 (2002). 91 R. Jothi Ramalingam and G. S. Chung, Mater. Lett. 68, 247 (2012). 92 S. Cimitan, S. Albonetti, L. Forni, F. Peri, and D. Lazzari, J. Colloid Interface Sci. 329, 73 (2009). 24 Chapter 2: Synthesis and Characterization Wee Rui Qi Chapter 2: Synthesis and Characterization 2.1 Fabrication of Samples 2.1.1 Sputtering of Zn and ZnO Thin Films A brief introduction to sputtering will be given here as sputtering was used to produce a thin layer of zinc (Zn) used in Chapters Three and Four, and zinc oxide (ZnO) used in Chapter Five. Physical vapor deposition (PVD) is a general term used to describe any of a variety of methods to deposit thin solid films by the condensation of a vaporized form of the solid material onto various surfaces. PVD involved physical ejection of atoms or molecules, followed by condensation onto a substrate. Nucleation of these atoms occurred on substrate which resulted in sample growth. This process is known as reactive deposition as the vapor-phase material consists of ions or plasma and is often chemically reacted with gases introduced into the vapor during growth.1-4 Sputtering is a well-known technique to deposit thin films on substrates. It falls under PVD category along with electron beam evaporation, thermal evaporation and pulsed laser deposition (PLD). The technique is based on physical ion bombardment of a source material, also known as the target. The incident energetic particles resulted from plasma, which was produced when a huge voltage passes through gas molecules. During ion bombardment, collisions between the incident energetic particles, and/or resultant recoil atoms, with surface atoms caused the atoms to be ejected from the solid target. This is illustrated in Fig. 2.1. It condensed on the substrate where film growth then occurred. Sputter yield γ is defined as the ratio between the number of sputter-ejected atoms and the number of incident projectiles, 25 Chapter 2: Synthesis and Characterization Wee Rui Qi and is determined by the energy and mass of the ions at the target. However, the sputtering rate of the target depended on the total power. Magnetron source is one common approach for growing thin films by sputter deposition where positive ions present in the plasma of a magnetically enhanced glow discharge will bombard the target. The target can be powered in different ways, ranging from direct current (DC) for conductive targets to radio frequency (RF) for non-conductive targets. Since sputtering is a purely physical process, a reactive gas is added to the plasma in order to change the chemistry of the sample growth. Fig. 2.1 Schematic of physical sputtering process.2 Theoretically, a cathode and an anode are positioned opposed to each other in a vacuum chamber. The vacuum chamber is typically pumped by a combination of turbomolecular and rotary pumps, although a diffusion pump is still often used. After pumping to a base pressure of the order of 1×10−4 Pa (~7.5 ×10−7 Torr) or lower, a noble gas (usually argon) is introduced into the vacuum chamber. A working pressure between 1 and 10 Pa (~7.5 ×10−3 – 7.5 ×10−2 Torr) is reached. When a high voltage difference in the range of 2000 V is applied between cathode and anode, a glow discharge is ignited. Material deposition then began.4 In the magnetron sputtering system used in these experiments, radio frequency (RF) is used as source of power as non-conductive target is implored. Fig. 2.2 shows 26 Chapter 2: Synthesis and Characterization Wee Rui Qi the experimental setup of RF sputtering system used for the experiments. Prior to sputtering, the substrates are first washed with deionized (DI) water, ethanol, acetone, and with DI water as the last step. The system is however, pumped to ~ 4 x 10-5 Torr or above before sputtering started. This was slightly lower than the theoretical pressure mentioned earlier. The power and time for sputtering are varied to control the morphology and properties of the sputtered films. Typically, 60 W was usually used in experiments. The temperature during sputtering was kept at room temperature. Air is introduced into vacuum chamber before replacing or removing substrates. Fig. 2.2 Setup of the RF magnetron sputtering system used in the experiments. 2.1.2 Heating in Furnace The annealing of Zn films in Chapter Three and the hydrothermal processes discussed in Chapters Four and Five, both occurred in a furnace. The model DZF6030A from Yiheng Technical Co. Ltd is implored. The samples are removed with the help of gloves, and cooled to room temperature in fume-hood. The furnace is temperature-controlled in ambient air. Fig. 2.3 gives an experimental setup of the furnace used. 27 Chapter 2: Synthesis and Characterization Wee Rui Qi Fig. 2.3 Experimental setup of furnace used in the experiments. 2.1.3 Hydrothermal Synthesis 2.1.3.1 Background of Hydrothermal Synthesis Hydrothermal synthesis is a usual technique used for nanostructured ZnO. This technique is used extensively in Chapter Four and Five where the bulk of experiments are carried out with this. Model 4744 General Purpose Acid Digestion Vessel with Teflon-lined insert from Parr Instrument Company with capacity of 45 mL is used. Sir Roderick Murchison, a British Geologist, was the first to use the term hydrothermal in the mid-19th century.5 The main objective was to create or simulate the natural conditions existing in the earth’s crust in the laboratory. This early interest in hydrothermal research stemmed from the quest in obtaining the synthesis of a particular mineral or in concocting compounds similar to natural minerals. In 1839, the German chemist Robert Wilhelm Bunsen contained aqueous solutions in thickwalled glass tubes at temperatures above 200 ◦C and at pressures above 100 bars. The crystals of barium carbonate and strontium carbonate that he formed under these conditions marked the first use of hydrothermal aqueous or other solvents as a 28 Chapter 2: Synthesis and Characterization Wee Rui Qi reaction media.6 However, the first publication on hydrothermal synthesis only appeared in 1845. This was a report outlining the successful synthesis of tiny quartz crystals upon transformation of freshly precipitated silicic acid in Papin’s digestor by Schafthaul. Since then, there had been tremendous development on hydrothermal technology. The materials of interest have also progressed beyond minerals. Table 2.1 gives a flow on development of hydrothermal synthesis over time.5 Table 2.1 Flow on the development of hydrothermal synthesis over time.5 29 Chapter 2: Synthesis and Characterization Wee Rui Qi 2.1.3.2 Experimental Setup Since hydrothermal synthesis required high temperature and pressure under prolonged duration, the use of autoclaves is usually required. In modern laboratories, autoclaves with Teflon inserts are used for temperatures below 200 °C and 200 bar as seen in Fig. 2.4. The autoclave material must be inert with respect to the solvent. To prevent corrosion of the internal cavity of the autoclave, protective inserts are generally used. Teflon makes an ideal container under these conditions since, in contrast to glass and quartz, it is inert to both hydrofluoric acid and alkaline media. However, above 200 °C the creep behavior becomes a problem. For synthesizing under more extreme conditions of up to 1100 °C and 3 kbar, commercial equipment is available with the name TZM-Apparatus.5 Fig. 2.4 Experimental setup of Teflon-liner and outer stainless steel casting used in the experiments. 2.1.3.3 Chemistry Behind Hydrothermal Synthesis Water is an environmentally safe material and cheaper than other solvents, and it can act as a catalyst for transformation of desired materials by tuning temperature and pressure. Since water is the solvent for most reactions, it is 30 Chapter 2: Synthesis and Characterization Wee Rui Qi imperative to understand its phase-temperature diagram to provide a conducive but yet safe environment. Safety is of paramount importance as the pressure will rise very steeply above 100 °C when fill volumes are in the 80-90 % range. Fig. 2.5 gives the phase-temperature (PT) diagram of water.7 Fig. 2.5 Diagram showing pressure as a function of temperature for pure water, with the filling factor (% degree of fill) of the autoclave. The critical temperature (Tcr = 374.1 °C) and pressure (ρ = 221.2 bar) are indicated.5 To understand why hydrothermal synthesis is well favoured, the chemistry behind reactions in hydrothermal synthesis is looked into. Diffusion is important in chemical reactions.8 Arrhenius equation in Equation 2.1 gives the dependence of the rate constant k of chemical reactions with temperature. 𝑘 = 𝐴𝑒 −𝐸𝑎� 𝑅𝑇 (2.1) 31 Chapter 2: Synthesis and Characterization Wee Rui Qi This means that the large the reaction barrier height, the larger the temperature effect on the reaction. A reaction pathway which was previously not available in ambient water due to a high barrier can now be made accessible thermally in supercritical water. At very high PT conditions (1000 oC and 100 kbar), water is completely dissociated into H3O+ and OH-, behaving like molten salt. The viscosity of solvent is inversely proportionate to diffusion and hence, the rate constant k. When density ρ is larger than 0.8 g/cm3 in Fig. 2.6, a drastic decrease is observed whereby viscosity is correlated with rising temperatures. Fig. 2.6 Viscosity of water as a function of density and temperature.8 Meanwhile, the solubility of solute in solvent is also affected under hydrothermal conditions.8 The relative dielectric of solvent and solute can be 32 Chapter 2: Synthesis and Characterization Wee Rui Qi interpreted as a measure of its polarity. The dielectric constant of water at ambient conditions in room temperature is ~80.1. With a temperature increase, there is a reduction in dielectric constant of water under hydrothermal conditions seen in Fig. 2.7. Therefore, the solubility of non-polar species increases whereas solubility of ionic and polar species decreases. Fig. 2.7 Dielectric constant of water plotted against as a function of pressure and temperature.8 Particularly for ZnO, the presence of NH4+ ions in the solution produced a marked increase in the rate of growth in the prismatic faces.9 Much investigation has been done on chemistry behind effect by NH4+ concentration. In M. N. R. Ashfold et al., the chemistry behind hydrothermal synthesis was examined.10 With hexamethylenetetramine (HMT) zinc nitrate Zn(NO)3, the continual decomposition of HMT ensured a continuous supply of ammonium ions NH4+ and formaldehyde CH2O in the reaction solution over time as seen in Equation 2.2. As pKa was interpreted as 4.01, a high concentration of Zn ions (Zn2+) existed as seen in Fig. 2.8. Precipitation 33 Chapter 2: Synthesis and Characterization Wee Rui Qi of ZnO ensued from the aqueous solution with hydroxide (OH-) ions from either Equations 2.3 or 2.4. With thermodynamic calculations, zinc hydroxide (Zn(OH)2) will precipitate out first as it is less stable. However, the rise in OH- ions produced by Equation 2.2 leads to an increase in pH of solution. Combined with a decrease of Zn2+ ion concentration by Equation (4), Zn(OH)2 becomes thermodynamically unstable and will dissolve to form ZnO subsequently.10 C6H12N4 + 10H2O ⇌ 6CH2O + 4NH4+ + 4OH− (2.2) Zn2+ + 2OH− ⇌ Zn(OH)2 ⇌ ZnO + H2O (2.4) Zn2+ + 2OH− ⇌ ZnO + H2O (2.3) Fig. 2.8 Diagram showing the percent of Zn(II) present in the labelled form at each pH. Only species that were present at a ratio of greater than 10% in the pH range 2– 13.5 are displayed.10 2.2 Characterization Techniques 2.2.1 Surface Profiler The surface profilometer is from Tencor with model Alpha-Step 500. The Alpha-Step 500 is equipped with a standard stylus of 12.5 micron radius. Only 34 Chapter 2: Synthesis and Characterization Wee Rui Qi sputtered films are measured here. The profilometer is a non-destructive step instrument. It is typically used to measure the film thickness whereby a diamond tipped stylus is utilized to measure the depth of the thin films, ranging from micrometer to nanometer scale. During measurement, the tipped stylus is in direct contact and scanned across the surface for a specified distance and contact force. The profilometer can measure small surface variations in the vertical stylus displacement as a function of position. The height position or information from the sample surface is picked up by the diamond stylus to generate an analog signal. This signal is then converted into a digital signal for analysis and display. The resolution of the measurement is dependent on the radius of the stylus and the geometries of the features. 2.2.2 X-ray Diffraction (XRD) X-ray diffraction (XRD) spectra are obtained with BRUKER AXS (model D8 ADVANCE, CuKα1 where λ=0.154056 nm). For powdered samples, a layer of powders are deposited onto clean substrates. For non-powder samples, measurement is carried out directly. The samples are scanned from diffraction angle 2θ of 30 ° to 90 ° in Chapter 3 while a range of 25 ° to 90 ° is used for Chapters 4 and 5. The scan step used is 0.02 ° for all experiments. For the detailed study of XRD done in Chapter 5, the scan step used is 0.0025 o. The scan data is acquired from BRUKER AXS software XRD commander and analyzed by Eva. The qualitative identification is achieved by comparing the X-ray diffraction patterns obtained from the thin film samples with an internationally recognized database JCPDS (International Centre for Diffraction Data).11,12 35 Chapter 2: Synthesis and Characterization Wee Rui Qi X-ray crystallography is a method of determining the arrangement of atoms within a crystal. During an X-ray diffraction measurement, a crystal is mounted on a goniometer and gradually rotated while being bombarded with X-rays continuously. When a beam of X-rays strikes a crystal in the sample, it causes the beam of light to spread into many specific directions. A diffraction pattern of regularly spaced spots known as reflections are produced. These diffraction peaks are given as a function of diffraction angle 2θ. The diffraction peaks are a result of constructive interference of waves scattered from the atoms or ions composing a crystal. As x rays diffract from the periodic arrangement of atoms in a crystal, Bragg’s law in Equation 2.5 and Fig. 2.9 can be used to describe the diffraction, where n is an integer, λ is the wavelength of incident wave, d is the spacing between the planes in the atomic lattice, and θ is the angle between the incident ray and the scattering planes. λ = 2dhkl sin θ (2.5) Fig. 2.9 Illustration of Bragg’s law.12 During measurement, the x-ray source (i.e., the x-ray tube target), the specimen, and the receiving slit are kept on a common circle, called the focusing circle. The sample is usually rotated to ensure a systematic measurement. The sample 36 Chapter 2: Synthesis and Characterization Wee Rui Qi is moved through an angle of θ while the detector is scanned through an angle of 2θ. This θ-2θ motion implies that the radius of the focusing circle is continually changing throughout a diffraction pattern scan. This is a common configuration for powder diffraction. Fig. 2.10 gives a schematic of the measurement. Fig. 2.10 Schematic of the XRD measurement.11 2.2.3 Scanning Electron Microscopy (SEM) A scanning electron microscope (SEM) is a type of electron microscope that images a sample by scanning it with a beam of electrons in a raster scan pattern. A ZEISS system is implored here while energy-dispersive X-ray spectroscopy (EDX) analysis is used to determine the sample composition. For powdered samples, a layer of powders are deposited onto clean substrates. For non-powder samples, measurement is carried out directly. During measurement, the sample is put outside the focusing field and then tilted towards the detector. The electron gun is either a tungsten hairpin or a lanthanum hexaboride single crystal. Typically the beam diameter is of the order of 10 nm with a beam energy Eo of 10-30 keV is focused by condensing lenses into a very fine focal spot of ~5.0 nm on a conducting sample surface. The electrons will interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography and composition. Fig. 2.11 gives a schematic view. 37 Chapter 2: Synthesis and Characterization Wee Rui Qi Two main modes of signals are used for the samples. Secondary electrons (SE) consist of ejection of electrons from the sample after interaction with primary beam electrons. With energies of typically 1-10 eV, secondary electrons can be excited within the specimen by a single primary electron at any time. However, only those which are excited within ~10 nm of the surface can escape into the vacuum and be detected. Backscattered electrons (BSE) are high-energy electrons that come from primary electron beam. The primary electrons interacted with nuclei atoms in the sample and are reflected from it. Fig. 2.11 Schematic diagram of a working SEM.13 2.2.4 Transmission Electron Microscopy (TEM) Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra-thin specimen. As it passes through, it interacts with the sample and forms an image. The image is magnified and 38 Chapter 2: Synthesis and Characterization Wee Rui Qi focused onto an imaging device, such as a fluorescent screen, or to be detected by a sensor such as a CCD camera. In the experiments, TEM model JEOL 2010F and JEOL 3010F operated at 200 keV. The samples are prepared by depositing minimal powder into a vial of ethanol before quick sonication. A drop of solution is then dripped on copper grid. High resolution transmission electron spectroscopy (HRTEM) is carried out to determine the lattice structure on the sample. Selected-area electron diffraction (SAED) is done to investigate the diffraction pattern as well. Typically, high-energy (~ 100-400 keV) electrons are used. Amorphous materials, such as polymers and glasses, are rarely studied because of the difficulties of interpreting the images and diffraction patterns and the susceptibility of such materials to damage by the electron beam. One advantage TEM has over a light microscope is its resolution. The resolution of a microscope (d, the smallest observable distance between two points in an image) is controlled by the wavelength (λ) of the radiation used. The value of λ for 100 keV electrons is 0.0037 nm, where λ for visible light is ~0.5 µm for visible light. Therefore, the resolution limit of the TEM is, theoretically, several orders of magnitude smaller than that of the light microscope. The whole instrument is first evacuated to ≤ 10 −6 Pa to prevent electron scatter by ambient conditions. During measurements, electrons are produced either from a lanthanum hexaboride single crystal heated to ~2000 K or by a field-emission gun (FEG), which consists of a sharp tungsten needle (radius ~ 100 nm). An intense electric field (>107 Vcm−1) is applied. Electrons emitted from the source and are accelerated through ~ 100-400 kV and enter the illumination system. The illumination system produces a demagnified image of the source at the plane of the specimen by using two or more electromagnetic condenser lenses to restrict the electron paths. In 39 Chapter 2: Synthesis and Characterization Wee Rui Qi combination with limiting diaphragms (apertures) within the lens, an approximately parallel, coherent electron beam of ~ 2-60 µm diameter is produced at the specimen or, if analysis is to be performed, fine electron probes down to ~0.1 nm are obtainable from a FEG. In TEM, useful specimens can range in thickness from ~ 20nm to 0.5µm. Electrons travelling straight through the specimen constitute the direct beam, while scattered beams of electrons travel off the optic axis. Electrons scattered in specific directions are focused at specific points in the back-focal plane to form the diffraction pattern. The specimen is in the object plane, the diffraction pattern is formed in the back-focal plane (distance f from the lens) and the various images are formed in the image plane of the lens at a magnification given by the ratio of the image and object distances, v/u. To form images, electrons emerging from any point in the specimen are recombined to a point in the image plane, thus forming images. Fig. 2.12 gives an illustration. The restricting apertures can be used to select the area of the sample where measurement is taken. The image may be recorded on electron-sensitive film in the camera chamber or viewed on a TV/computer screen via a digital CCD camera, usually situated in the camera or viewing chamber. Similarly, if the backfocal plane is imaged, the electron diffraction pattern appears on the screen. 40 Chapter 2: Synthesis and Characterization Wee Rui Qi Fig. 2.12 Schematic diagram of the central process by which images and diffraction patterns are formed within the objective lens of the TEM.14 2.2.5 Photoluminescence (PL) Photoluminescence signals (PL) result from radiative recombination processes in optically excited semiconductors. This is one of many forms of luminescence (light emission) and is distinguished by photoexcitation (excitation by photons). The period between absorption and emission is typically extremely short, in the order of 10 nanoseconds. For experiments here, an in-house photoluminescence (PL) system with laser excitation of 325 nm with power of 10 mW and laser spot 1 mm is used to examine light emitting properties at room temperature. For powdered samples, a layer of powders are deposited onto clean substrates. For non-powder samples, measurement is carried out directly. Optical excitation, with photons of energy larger than the bandgap, generates excess carriers in semiconductor materials. These carriers recombine both radiatively 41 Chapter 2: Synthesis and Characterization Wee Rui Qi and nonradiatively. The emitted light (photoluminescence) is caused by band-to-band optical transitions which involved radiative centers in the gap. Disregarding quantum effects, band-to-band transitions involve nearband- edge states in conduction and valence bands occurred during photoluminescence.15 2.2.6 Vibrating Sample Magnetometer (VSM) Vibrating sample magnetometer (VSM) is used to determine magnetic properties of the samples at room temperature. The sample is wrapped with carbon tape around the plastic holder prior to measurement. For experiments, VSM used is a product from Lake Shore at a sensitivity of 1 x 10-6 emu. VSM involves a large-scale applied field, using an electromagnet to immerse an entire small sample in a nearly uniform field. The field is swept slowly (usually less than mTs−1) and the sample is moved (vibrated) in the applied field. The magnetic sources associated with a sample's geometry and magnetization are able to internal magnetostatic fields and therefore, care is required to ensure symmetry in sample sizes. The inductive pick-up coils are placed in close proximity to the sample and are marked as X inside the applied field in Fig. 2.13. The motion of sample results in induced voltage. This means that induced voltage is proportional to the sample’s magnetic moment. Assumption is such that the external fields are wholly proportional to the net internal magnetization, which is affected by the sample geometry, though other factors may influence too. When the measurement is completed, the hysteresis curve of the sample is obtained.16 42 Chapter 2: Synthesis and Characterization Wee Rui Qi Fig. 2.13 Illustration of VSM.16 2.3 References 1 W. Kern, K. K. Schuegraf, in Handbook of Thin Film Deposition Processes and Techniques (2)- Principles, Methods, Equipment and Applications; Ch. 1, edited by K. Seshan (William Andrew Inc., 2001), p. 11. 2 S. Rossnagel, in Handbook of Thin Film Deposition Processes and Techniques (2)- Principles, Methods, Equipment and Applications; Ch. 8, edited by K. Seshan (William Andrew Inc., 2001), p. 319. 3 P. M. Martin, in Handbook of Deposition Technologies for Films and Coatings (Third Edition)- Science, Applications and Technology; Ch. 1, 3 ed., edited by P. M. Martin (Elsevier, 2010), p. 1. 4 D. Depla, S. Mahieu, J.E. Greene, in Handbook of Deposition Technologies for Films and Coatings (Third Edition)- Science, Applications and Technology; Ch. 5, 3 ed., edited by P. M. Martin (Elsevier, 2010), p. 253. 43 Chapter 2: Synthesis and Characterization 5 Wee Rui Qi K. Byrappa and M. Yoshimura, in Handbook of Hydrothermal Technology- A Technology for Crystal Growth and Materials Processing; Ch. 2, edited by K. Byrappa and M. Yoshimura (William Andrew Inc., 2001), p. 53. 6 R. A. Laudise, in 50 years Progress in Crystal Growth- A Reprint Collection; edited by R. A. Laudise (Elsevier Science, 2004), p. 185. 7 D. O’Hare, in Encyclopedia of Materials: Science and Technology; edited by K. H. J. Buschow, R. W. Cahn, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan, and P. Veyssière (Elsevier 2001), p. 3989. 8 K. Byrappa and M. Yoshimura, in Handbook of Hydrothermal Technology- A Technology for Crystal Growth and Materials Processing; Ch. 4, edited by K. Byrappa, M. Yoshimura (William Andrew Inc., 2001), p. 161. 9 E. D. Kolb and R. A. Laudise, J. Am. Ceram. Soc. 49, 302 (1966). 10 M. N. R. Ashfold, R. P. Doherty, N. G. Ndifor-Angwafor, D. J. Riley, Y. Sun, Thin Solid Films, 515, 8679 (2007). 11 S.T. Misture, R.L. Snyder, in Encyclopedia of Materials: Science and Technology; edited by by K. H. J. Buschow, R. W. Cahn, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan, and P. Veyssière (Elsevier 2001), p. 9799. 12 A.K. Chatterjee, in Handbook of Analytical Techniques in Concrete Science and Technology - Principles, Techniques, and Applications; Ch. 8, edited by V.S. Ramachandran and James J. Beaudoin (William Andrew Inc., 2001), p. 275. 13 O. C. Wells, in Encyclopedia of Materials: Science and Technology; edited by by K. H. J. Buschow, R. W. Cahn, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan, and P. Veyssière (Elsevier 2001), p. 8265. 44 Chapter 2: Synthesis and Characterization 14 Wee Rui Qi D. B. Williams, in Encyclopedia of Materials: Science and Technology; edited by by K. H. J. Buschow, R. W. Cahn, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan, and P. Veyssière (Elsevier 2001), p. 2577. 15 S. K. Krawczyk, in Encyclopedia of Materials: Science and Technology; edited by by K. H. J. Buschow, R. W. Cahn, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan, and P. Veyssière (Elsevier 2001), p. 8397. 16 M. W. Muller, in Encyclopedia of Materials: Science and Technology; edited by by K. H. J. Buschow, R. W. Cahn, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan, and P. Veyssière (Elsevier 2001), p. 1. 45 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi Chapter 3: Investigation on Origins of Black Zinc Oxide 3.1 Introduction As transparent conducting oxide, it is essential to have a good antireflective property on the TCO to increase light coupling into devices. Vertical rods, nanowires and porous structures are some surface morphologies found to be effective as ZnO antireflection coatings.1-4 However, black coatings are said to be most effective in suppressing reflections from the TCOs. This is observed in black silicon where structuring its surface morphology leads to low reflectivity and therefore, a black appearance.5-7 Recently, black indium tin oxide (ITO) and ZnO have been reported. For black ITO, it is obtained by increasing plasma power as described in Ma et al while black ITO nanopowers are obtained by solution method investigated in Wang et al.8,9 In Tian et al, ZnO films with pyramids with extremely sharp tips on its surface, are being suspected to be the cause behind black appearance.10 The fabrication however, was done purely on a Zn block. It is difficult to distinguish whether the remnant Zn after hydrothermal treatment or the ZnO pyramids morphology, is accountable to the appearance of black color. Black zinc coatings exist by the inclusion of fine metallic particles dispersed in a dielectric material11 through a chromating process to Zn plates, these black Zn coatings are found in surgical instruments and machinery compartments to improve its visibility. It is therefore intriguing to investigate the reason behind black ZnO. As the potential of ZnO is vast, there has been much research to enhance its properties. Zn is often used to enhance ZnO properties. Zn-ZnO core shell structures produced violet and blue emissions.12,13 Zn coated ZnO nanowires resulted in the 46 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi large enhancement of UV and green emissions.14 In Yi et al, Zn nanoclusters embedded in ZnO nanowires turned out to be responsible for ferromagnetism.15 Nonetheless, there is still a lack of studies on the successful introduction of Zn into ZnO and effect of properties. The introduction of zinc by incomplete oxidation of Zn to ZnO could solve this. Zn is embedded in ZnO during the fabrication process.16,17 Zn/ZnO nanowires and core-shell structures have been fabricated by laser ablation, thermal evaporation, electrodepositon and chemical vapor deposition.12,13,15,18-21 Partially oxidized Zn films are yet to be studied, though obtaining ZnO films from Zn films followed by subsequent annealing treatments are well-publicized.17,22-28 In this work, ZnO films with varying amount of Zn are fabricated from Zn films. The aim of this study is to determine whether black ZnO with purely ZnO exists, while understanding properties of the resultant annealed films. Different amount of Zn in ZnO films is controlled by varying the annealing conditions. The physical appearance, structural, optical and magnetic properties are examined with XRD, SEM, TEM, PL and VSM. 3.2 Results and Discussion Zn films were first sputtered onto cleaned glass substrate, at 60 W for 3 h at ~4 x 10-5 Torr with argon gas. A film thickness of 3.63 μm was obtained as checked using a surface profiler. To oxidize Zn into ZnO, post-sputtering heat treatment is performed on the films by placing the sample on a hot plate with temperature control in ambient conditions. The conditions were 100 oC for 15 h, 200 oC for 214 h, 400 oC for 6 h. X-ray diffraction (XRD) was done on the samples with CuKα1 radiation to determine the phases while scanning electron microscopy (SEM) was carried out to 47 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi examine morphology of the films. For XRD, the following JCPDS cards were used: 01-1244 (Zn), 36-1451 (ZnO), 44-1349 (SiO2), and 27-1402 (Si). High resolution transmission electron spectroscopy (HR-TEM) is carried out to determine the particle structure on the film. A photoluminescence (PL) system with laser excitation of 325 nm with power of 10 mW and laser spot 1 mm was used to examine light emitting properties at room temperature. Lastly, vibrating sample magnetometer (VSM) was used to determine hysteresis loops (M-H curves) (of the samples at room temperature. Zn and ZnO films are referred to as before and after annealing of the films respectively. 3.2.1 Structural Features and Surface Morphology XRD spectra of the films are given in Fig. 3.1. Both Zn and ZnO peaks were present in film before annealing. The presence of ZnO before any annealing treatment is attributed to the chamber base ambience that contained humid air due to machine fault. Thus, oxygen was already introduced during sputtering of Zn film. The amount of Zn present decreased gradually with increasing annealing temperature as seen from Table 3.1. There was no oxidation of Zn into ZnO after the Zn film was annealed at 100 oC as similar XRD spectra are observed in Figs. 3.1(a)-(b). To obtain varying Zn in ZnO, partial to complete oxidation are carried out from 200 oC to 400 oC as the intensity of Zn (101) peak at 43.4 o decreased gradually with an increased annealing temperature. The reason why annealing at 200 oC is carried out for 214 h instead of 615 h for 100 oC and 400 oC because it was hoped that a complete oxidation of Zn will occur at a low temperature of 200 oC over a long period of time. However, no apparent change in visible appearance over a long heating suggested otherwise. This is confirmed by XRD shown in Fig. 3.1(c). 48 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi To give a rough idea of how the elemental composition of the film changed with different annealing conditions, the ratio of Zn/ZnO peak intensities is calculated from XRD patterns in Fig. 3.1. The ratio comparison of selected peaks is used since the absolute amount of Zn present cannot be calculated from an XRD spectrum alone. Chosen for their strong peak intensities in ZnO and Zn respectively, the peak intensities of ZnO (101) and Zn (101) are compared in Table 3.1. The ratio of ZnO (101)/Zn (101) peak intensities remained at 2:6 before annealing, and after annealed at 100 oC. However, the ratio of ZnO (101)/Zn (101) peak intensities increased to 7:4 when annealed at 200 oC, and then 5:0 at 400 oC. This confirms that Zn content in ZnO films can be controlled by annealing. However, more study is required to determine the exact composition of Zn versus ZnO in ZnO films. 49 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi Fig. 3.1 XRD spectra of films under different conditions of (a) Zn film before annealing, (b) ZnO films after annealed at 100 oC for 15 h, (c) 200 oC for 214 h, and (d) 400 oC for 6 h. Table 3.1 Ratio of the height of the peak intensities of ZnO (101) and Zn (101) in Zn film before annealing, ZnO films annealed at 100 oC for 15 h, 200 oC for 214 h, and 400 oC for 6 h. To further support the claim that black ZnO contains Zn, the physical appearance of the films before and after annealing is compared in Fig. 3.2. Before annealing, Zn film is black. After annealing temperatures of 100 oC and 200 oC, the 50 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi resultant annealed films remained black or grey. When the zinc films are annealed at 400 oC, the ZnO film became opaque-white. In Figs. 3.1(a)-(b), Zn peaks existed in annealed films. In Fig. 3.1(d), a complete oxidation to ZnO has taken place. This suggests that the intensity of black appearance is directly related to the amount of Zn in the annealed films, whereby the darkest ZnO films contained the most Zn content. The control of black intensity in ZnO films suggests a simple method of fabrication other than chromating zinc plates. Two factors can contribute to black films. Firstly, black appearance could be due to metallic Zn particles embedded in ZnO films. It is well known that when the particle size of a metal, for example Au, becomes in nanometer range, the color becomes black. Another factor is the porous network within the ZnO films. Light is absorbed due to the nanostructure where the irregular surface of the films is brought by oxidation of particles. However, as porous structure is observed in all films, this is unlikely to be a contributing factor. Therefore, the black appearance is likely to be due to Zn, and that black ZnO films which consisted purely of ZnO does not exist. The morphology of the films before and after annealing at various temperatures and time is examined with SEM in Fig. 3.2. Before annealing, a mix of distinct elongated and irregular-shaped particles of size 100-200 nm was observed. The sputtered Zn film was quite porous compare to normal ones which were dense, which is again explainable by humidity presence during sputtering. This morphology remained when ZnO films are annealed at 100 oC and 200 oC though some sections of the particles coalesced due to sintering. At an annealing temperature of 400 oC, the particles became more spherical with a diameter of 100-200 nm. There was also a porous centre in each ring-like particle. 51 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi It is hypothesized that the resultant annealed films consisted of core-shell particles with Zn as core and ZnO as shell since the exterior of each Zn particle can oxidized first before its interior due to its exposure to both heat and oxygen. The porous nature of ZnO films made it easy for oxygen to be in contact with each Zn particle. To examine the composition makeup of individual particle, particles from ZnO film annealed at 200 oC are scraped off onto TEM copper grid with holey carbon. Fig. 3.2(e) shows the image obtained from HR-TEM, which showed the ZnO nanograins. For section A in the image, the lattice spacing between each plane indicated the presence of (100) planes in ZnO. In addition, intersecting parallel planes at 120 o corresponding to two of the three dimensions of unit cell of ZnO are seen in section B. 52 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi Fig. 3.2 Morphology and visual appearances (insets) of ZnO films under different conditions with (a) SEM of Zn film before annealing, (b) SEM of ZnO films after annealed at 100 oC for 15 h, (c) 200 oC for 214 h, (d) 400 oC for 6 h, and (e) HR-TEM of ZnO film annealed at 200 oC for 214 h. This suggests the existence of a Zn/ZnO layered film. Zn peaks present in Fig. 3.1(a) by XRD suggests the possibility of Zn residing in the bottom layer, which was 53 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi nearer to glass substrate. The formation of ZnO films can be interpreted as follow. Through heating, the surface which was in contact with oxygen in air first oxidized. As heating progressed, this conversion to ZnO proceeded to deeper layers of the film. The inner-most layer remained least affected by oxidation though nano-scale interface roughness was created by heating which contributed to the dark appearance of film. When heated at 400 oC for 6 hours, this inner-most layer finally oxidized to ZnO, giving rise to a complete oxidation of ZnO film. This explains why ZnO films retained a dark appearance for annealing temperatures below 400 oC. Since the change in interface between the two layers was likely to be gradual, it is difficult to distinguish the exact occurrence of interface in the sample. It is thus unclear if this bottom layer constituted of solely Zn or a mixture of Zn and ZnO. Fig. 3.3 illustrates the mechanism. From this mechanism, it is demonstrated that simple heating can induce a layer of oxide at its metal-air interface. This is a useful alternative method to physical methods such as sputtering and pulsed laser deposition for growth of ZnO with good adhesion on surfaces. This method also supersedes most typical chemical methods since good adhesion is obtained. In this method, ZnO molecules have to first attach on the surface chemically before growth takes place. With extensive surface irregularities, any chemical attachment can be difficult to take place, resulting in poor oxide coverage. 54 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi Fig. 3.3 Schematic diagram of mechanism to obtain Zn/ZnO layered film. 3.2.2 Photoluminescence Properties The PL spectroscopy is employed to study the optical properties of ZnO nanostructures. Direct comparison for emission intensity is made as the surface area of all films is the same. Before annealing, there were two weak and broad emission peaks at 2.2 eV and 3.26 eV. Strong emissions at 2.2 eV and 3.26 eV are seen in Figs. 3.4(a)-(b) when ZnO films are annealed at 100 oC, 200 oC and 400 oC. The origins of these green and UV emissions are explained earlier. UV and green emissions for ZnO films increased with annealing temperatures, with the lowest for annealing 55 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi temperature of 100 oC. However, UV emission is significantly enhanced when ZnO film is annealed at 200 oC. Firstly, neither UV nor green emission was seen in Zn films before annealing. This is so even though both Zn and ZnO were present before annealing. This clearly indicates a modification of optical properties in ZnO purely by annealing as UV and green emissions became stronger after zinc film is annealed at 100 oC. This is not unusual as annealing at 900 o C enhanced strong blue-green and orange-red luminescence for Zn-incorporated ZnO films obtained by electrodeposition in Lee et al.20 However, this is the first time a low annealing temperature of 100 oC has a profound effect on luminescence enhancement in Zn-ZnO structures. This is useful for applications that cannot withstand high temperatures. Transitions taken place before annealing of Zn films were mostly nonradiative rather than radiative. During annealing, the oxygen present in the environment came into contact with Zn film through its porous structure and boundaries. Oxygen vacancies (Vo+ and Vo++) formed during sputtering are filled and reduced, which increased emission for annealed ZnO films. This is why all annealed ZnO films exhibited strong UV emissions. Defects were also formed during annealing. Green emission surfaced in all annealed ZnO films. This is attributed to the formation of defects since green emissions are known to be caused by surface defects. At annealing temperature of 100 oC, Zn had more time to move around for its optimum stoichiometric Zn-O positions at that temperature. However, the heating time was insufficient for its optimum position, leading to defect formation and green light emission. At a higher annealing temperature of 400 oC, Zn is rapidly converted to ZnO.16,27 Due to the hasty ZnO formation, a high concentration of defects occurred. 56 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi Defects such as Zn interstitials and antisite defects (OZn) may form, which contributed to green light emissions seen in Fig. 3.4(a).29 It is of particular interest that UV emission intensified significantly in ZnO film annealed at 200 oC. The peak intensification is caused by resonant coupling between surface plasmons in Zn and emission of ZnO. This is not unfound as this phenomenon has been reported elsewhere.18 In Tang et al, UV emission intensification seen in Zn/ZnO nanowires is due to Zn nanoparticles infused in ZnO.18 This model as illustrated in Fig. 3.5, applies here since both Zn and ZnO are involved. The work function of Zn is 4.33 eV while it is 5.2 eV in ZnO, forming an Ohmic contact between Zn and ZnO. When both Fermi levels aligned, the bending of energy bands allowed electrons to move from Zn to ZnO. The absorption energy by electrons in Zn occurred at 380 nm.30 This can be matched with UV emissions in ZnO, which is also the reason behind UV enhancement by silver in ZnO.31 Thus, coupling between surface plasmons in Zn and emission of ZnO can occur. The electrons are transferred to conduction band of ZnO, giving rise to a higher electron density. This increase in electron-hole recombination rate enhances UV light emission in ZnO. The energy for surface plasmons is also provided by the defect-related emissions in ZnO in visible light region, leading to its suppression. However, not all defect-related emissions contributed to surface plasmons. Moreover, electrons at conduction band in ZnO can first transferred from conduction band to defect level before proceeding to valence band, giving rise to the green emissions.14 As Zn contributes to surface plasmons, it is imperative for annealed films to contain sufficient Zn in order for a peak enhancement. However, sufficient ZnO is still required as Zn can only enhance UV and green emissions. This probably explains why no UV peak enhancement is observed in both ZnO films annealed at 100 oC and 57 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi 400 oC. When annealed at 100 oC, the film contained insufficient ZnO; whereby the film no longer contained any Zn after annealed at 400 oC. Fig. 3.4 PL spectra of films under different conditions of (a) Zn film before annealing, ZnO films after annealed at 100 oC for 15 h, 200 oC for 214 h, and 400 oC for 6 h, and (b) Inset: PL spectrum magnification of Zn film before annealing. 58 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi Fig. 3.5 Illustration of mechanism behind UV and green emissions in Zn/ZnO layered film. 3.2.3 Magnetic Properties VSM is a usual technique to investigate the magnetic properties in materials. In these measurements, glass substrates which the films laid on and the tape were already taken into account of. In addition, the magnetic response of the machine in the background is also subtracted from the presented results. Fig. 3.6 show the M-H curves obtained for ZnO films before and after annealing at 100 oC, 200 oC and 400 o C. There existed a hysteresis loop for ZnO films before and after annealing at 100 oC and 200 oC, and ferromagnetism ceased when annealing occurs at 400 oC. This trend suggests that Zn was responsible for the hysteresis loops since there was no more hysteresis loop for ZnO film annealed at 400 oC. It is unlikely that any impurities or contamination during sample fabrication contributed to ferromagnetic behavior as otherwise, ferromagnetic behavior would be observed in all samples. It is 59 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi also implausible that ferromagnetism arose from the interaction of different phases of ZnO since only wurtzite phase is observed in Fig. 3.1(d). Defect-mediated magnetism is unlikely since no ferromagnetic behavior was exhibited in ZnO film annealed at 400 oC though defects still resided as seen in Fig. 3.3(a). Zn is therefore a contributing factor in ferromagnetic ZnO films. This is not unusual as the phenomenon has been reported in Yi et al.15 In Yi et al, Zn nanowires are oxidized to ZnO over different durations. At 10 h, ferromagnetism peaked, which ceased when annealed at 40 h. Experiments showed that ferromagnetism was due Zn clusters formed in ZnO nanowires. The structure is verified with XANES spectra which showed a deviation from curve fitting for both Zn foil and ZnO. This is likely to occur in this study as ferromagnetism subsided when no Zn was observed in ZnO films annealed at 400 oC. Interfacial effects between Zn and ZnO might play a role since both Zn and ZnO e are not ferromagnetic individually. It is interesting to note that there might be a relation between luminescence and ferromagnetic properties. When ZnO film is annealed at 100 oC and 200 oC, both UV enhancement and ferromagnetic behavior are observed. However, further detailed study is required to fully understand the relationship behind these properties. 60 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi Fig. 3.6 M-H curves by VSM at room temperature of films annealed under different conditions of (a) Zn film before annealing, ZnO films after annealed at (b) 100 oC for 15 h, (c) 200 oC for 214 h, and (d) 400 oC for 6 h. 3.3 Conclusions In summary, we have investigated if black ZnO with purely ZnO existed. Ratios of ZnO (101)/Zn (101) peak intensities of 2:6, 2:6, 7:4 and 5:0 are attained for ZnO films at annealing conditions of 100 oC for 15 h, 200 oC for 214 h and 400 oC for 6 h respectively. From visible appearance and XRD spectra, Zn was found to be present in black ZnO films. The black appearance disappeared when the film no longer contained metallic Zn particles. The investigation therefore disputed the claims of black ZnO made purely from ZnO. We also examined the structural, optical and magnetic properties of these ZnO films. The influence of annealing and Zn are also studied. From the SEM images, elongated and irregular-shaped particles obtained are transformed into spherical particles after ZnO films are annealed at 400 oC for 6 h. 61 Chapter 3: Investigation on Origins of Black Zinc Oxide Wee Rui Qi TEM revealed ZnO nanostructures. It is demonstrated that simple heating can induce a layer of oxide at its metal-air interface. This is a useful alternative to physical methods to obtain growth of ZnO with good adhesion on surfaces. It is hypothesized that annealing of Zn films resulted in Zn/ZnO layered films. A low annealing temperature of 100 oC has a profound effect by enhancing UV emission in ZnO films. Reverting back to the effect of Zn on ZnO, UV enhancement observed at ZnO film annealed at 200 oC strongly suggested the effect of Zn in ZnO. Zn also causes ferromagnetic behavior in all films except for the film annealed at 400 oC. 3.4 References 1 J. Y. Chen and K. W. Sun, Sol. Energy Mater. Sol. Cells 94, 930 (2010). 2 Y.–J. Lee, D. S. Ruby, D. W. Peters, B. B. McKenzie, and J. W. P. Hsu, Nano Lett. 8, 1501 (2008). 3 E. Osorio, R. Urteaga, L. N. Acquaroli, G. García-Salgado, H. Juaréz, and R. R. Koropecki, Sol. Energy Mater. Sol. Cells 95, 3069 (2011). 4 X. M. Liu and J. H. He, J. Phys. Chem. C, 113, 148 (2009). 5 D. Murias, C. Reyes-Betanzo, M. Moreno, A. Torres, A. Itzmoyotl, R. Ambrosio, M. Soriano, J. Lucas, and P. R. i. Cabarrocas, Mater. Sci. Eng, B (2012). 6 A. Y. Vorobyev and C. Guo, Appl. Surf. Sci. 257, 7291 (2011). 7 Y. Xia, B. Liu, J. Liu, Z. Shen, and C. Li, Solar Energy 85, 1574 (2011). 8 H. B. Ma, J. -S. Cho, and C. -H. Park, Surf. Coat. Technol. 153, 131 (2002). 9 H. W. Wang, X. J. Xu, J. R. Zhang, and C. Z. Li, J. Mater. Sci. Technol. 26, 1037 (2010). 62 Chapter 3: Investigation on Origins of Black Zinc Oxide 10 Wee Rui Qi Y. S. Tian, C. G. Hu, Y. F. Xiong, B. Y. Wan, C. H. Xia, X. S. He, and H. Liu, J. Phys. Chem. C 114, 10265 (2010). 11 M. Nikolova, O. Harizanov, P. Steftchev, I. Kristev, and S.Rashkov, Surf. Coat. Technol. 34, 501 (1988). 12 H. Zeng, W. Cai, J. Hu, G. Duan, P. Liu, and Y. Li, Appl. Phys. Lett. 88, 171910 (2006). 13 W. S. Khan, C. Cao, Z. Chen, and G. Nabi, Mater. Chem. Phys. 124, 493 (2010). 14 Y. J. Fang, J. Sha, Z. L. Wang, Y. T. Wan, W. W. Xia, and Y. W. Wang, Appl. Phys. Lett. 98, 033103 (2011). 15 J. B. Yi, H. Pan, J. Y. Lin, J. Ding, Y. P. Feng, S. Thongmee, T. Liu, H. Gong, and L. Wang, Adv. Mater. 20, 1170 (2008). 16 W. Gao, Z.W. Li, R. Harikisun, and S.-S. Chang, Mater. Lett. 57, 1435 (2003). 17 Z. W. Li, W. Gao, and R. J. Reeves, Surf. Coat. Technol. 198, 319 (2005). 18 W. Tang, D. Huang, L. Wu, C. Zhao, L. Xu, H. Gao, X. Zhang, and W. Wang, CrystEngComm 13, 2336 (2011). 19 X. Zhang, J. Dai, C. Lam, H. Wang, P. Webley, Q. Li, and H. Ong, Acta Mater 55, 5039 (2007). 20 M.-K. Lee and H.-F. Tu, Jpn. J. Appl. Phys. 47, 980 (2008). 21 H. B. Zeng, Z. G. Li, W. P. Cai, B. Q. Cao, P. S. Liu, and S. K. Yang, J. Phys. Chem. B 111, 14311 (2007). 22 M. Bouderbala, S. Hamzaoui, M. Adnane, T. Sahraoui, and M. Zerdali, Thin Solid Films 517, 1572 (2009). 23 M. Cui, X. Wu, L. Zhuge, and Y. Meng, Vacuum 81, 899 (2007). 63 Chapter 3: Investigation on Origins of Black Zinc Oxide 24 Wee Rui Qi T. Hiramatsu, M. Furuta, T. Matsuda, C. Li, and T. Hirao, Appl. Surf. Sci. 257, 5480 (2011). 25 L. Wu, Y. Wu, X. Pan, and F. Kong, Opt. Mater. 28, 418 (2006). 26 O. Martínez, V. Hortelano, J. Jiménez, J. L. Plaza, S. de Dios, J. Olvera, E. Diéguez, R. Fath, J. G. Lozano, T. Ben, D. González, and J. Mass, J. Alloys Compd. 509, 5400 (2011). 27 S. Ren, Y. F. Bai, J. Chen, S. Z. Deng, N. S. Xu, Q. B. Wu, and S. Yang, Mater. Lett. 61, 666 (2007). 28 J. Zhao, L. Hu, Z. Wang, Y. Zhao, X. Liang, and M. Wang, Appl. Surf. Sci. 229, 311 (2004). 29 Z. Li and W. Gao, Thin Solid Films 515, 3323 (2007). 30 K. Aslan, M.J. R. Previte, Y. X. Zhang, and C. D. Geddes, J. Phys. Chem. C 112, 18368 (2008). 31 X. H. Xiao, F. Ren, X. D. Zhou, T. C. Peng, W. Wu, X. N. Peng, X. F. Yu, and C. Z. Jiang, Appl. Phys. Lett. 97, 071909 (2010). 64 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi Chapter 4: Growth of ZnO Nanostructured Films by Zn films in NaCl Solution 4.1 Introduction It is well known that Zn undergoes corrosion in the presence of humidity or seawater. Selective corrosion on Zn takes place, inducing interesting features on ZnO while simultaneously oxidizing Zn. We therefore proposed that sodium chloride (NaCl) solution might be helpful in forming ZnO nanostructures. Current studies showed that Zn with NaCl powder has successfully produced ZnO. One synthesis method is to obtain ZnO from a Zn-NaCl mixture. It involved the use of Zn precursors with NaCl powder to synthesize ZnO.1-5 In A.N. Baranov et al., the ratio of Zn precursor to NaCl powder of 1:10 is prepared by freeze-drying followed by ball milling before heating up to 800 oC to synthesize ZnO nanorods. High temperatures were required as synthesis of ZnO was not observed below 500 oC.1 NaCl assisted by preventing agglomeration of the Zn precursor during mechanical action of ball milling. Water molecules were not involved in chemical reactions as they had been removed.3,4 Large amount of NaCl was needed for synthesizing small amount of ZnO via decomposition of Zn precursor when Zn powder was used. Moreover, only ZnO nanowires are reported.6 Despite numerous studies on ZnO via NaCl, using NaCl solution to obtain ZnO appears relatively new. The idea of corroding Zn to obtain ZnO nanostructures is reasonable as this has been exemplified by Mouanga et al. where Zn foils were immersed in NaCl solution for days.7 Apart from the prolonged duration required, atmospheric carbon dioxide was also introduced into ZnO nanostructures which may degrade the quality of ZnO. Tian et al and Yan et al also attained ZnO pyramids and 65 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi nanorods by subjecting Zn foils at 170 oC and 120 oC respectively.8,9 However, there is a lack of understanding of using corrosion mechanism on Zn in these studies. Thus, more can be done to explore the use of NaCl solution for ZnO nanostructures. In this paper, a new strategy to obtain ZnO by using NaCl solution on glass is presented. The glass which has been pre-coated with a layer of Zn, acted as Zn source. It is hoped that ZnO will grow directly on Zn film as this will eliminate the need to apply as-synthesized powder onto substrate. Moreover, as NaCl exists in solution form, the fabrication temperature will be much lower since NaCl crystals did not have to be in molten state. This greatly reduces the amount of NaCl required relative to Zn precursor since water is the main medium. Moreover, since only NaCl and water are used, this fabrication process is environmental friendly and is replicable in large-scale production. The morphology, structure and optical properties of the obtained product are investigated after synthesis. 4.2 Results and Discussion Zn films were first sputtered onto cleaned glass substrate, at 60 W for 3 h at ~4x 10-5 Torr with argon gas. A film thickness of 3.63 μm was obtained as checked using a surface profiler. The Zn on glass was placed in a Teflon container with a wellmixed solution of 0.116 g of NaCl and 20 mL water. It was then heated at 170 oC from 3 to 15 hours. The samples were replaced after cooled to room temperature. The samples were characterized with SEM, XRD, TEM and PL. X-ray diffraction (XRD) was done on the samples with CuKα1 radiation to determine the phases while scanning electron microscopy (SEM) was carried out to examine morphology of the films. In XRD, the following JCPDS cards were used: 01-1244 (Zn), 36-1451 (ZnO), 66 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi 20-1435 (Zn(OH)2), 16-0850 (ZnCl2), 44-1349 (SiO2), and 27-1402 (Si). High resolution transmission electron spectroscopy (HR-TEM) was carried out to determine the particle structure on the film. A photoluminescence (PL) system with laser excitation of 325 nm with power of 10 mW and laser spot 1 mm was used to examine light emitting properties at room temperature. 4.2.1 Surface Morphology and Structural Features Fig. 4.1(a) shows SEM image of the Zn film sputtered on glass at a low base vacuum of ~4 x 10-5 Torr. A mix of distinct elongated and irregular-shaped particles of size 100-200 nm is observed. The porous sputtered Zn film was due to the presence of humid air in the low base vacuum chamber, in which oxygen and water vapour were present during the process of sputtering Zn film. Figs. 4.1(b)-(f) show the surface morphology of nanostructured ZnO after heating from 3 to 15 hours. After 3 hours of heating, a network with circular pores of diameter 0.5-1 µm emerged. The network remained after 6 hours of heating though the circular pores appeared to have a more uniform diameter compared to after heating for 3 hours. With 9 hours of heating, the circular pores enlarged, causing a network of nanowires with diameters of 100 nm to 200 nm. There was little change of the nanowires after 9 hours as the morphology remained similar for the Zn films heated for 12 and 15 hours. It should be noted that spheres with diameter 100 nm to 200 nm were seen in some areas of the network of nanowires with 15 hours. The network of nanowires also appeared more spacious than the nanostructured ZnO after heating for 9 and 12 hours. Fig. 4.2(a) shows the XRD spectrum of the Zn film before heating. Peaks of both Zn and ZnO were present, indicating that the resultant sputtered film contained both 67 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi Zn and ZnO before any heat treatment. This again, was due to the presence of oxygen during sputtering. Figs. 4.2(b)-(f) show the XRD spectra of nanostructured ZnO after heating from 3 to 15 hours. All peaks could be indexed to ZnO. From Fig. 4.2(b), 3 hours of heating were sufficient to fully oxidize Zn to ZnO. No peaks of NaCl and zinc chloride (ZnCl2) were observed in any of the samples, though two peaks due to zinc hydroxide were identified after the sputtered Zn film was heated for 6 hours. A pH value of 6 for the solution kept unchanged after heating reactions. It is proven that water acted as oxidizer while NaCl remained in solution form instead of reacting with Zn. It is evident that NaCl played the role of a catalyst as different morphologies of ZnO films are observed with different heating time. 68 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi Fig. 4.1 SEM of ZnO nanostructured films (a) before heating, after heating at 170 oC for (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, and (f) 15 h. 69 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi Fig. 4.2 XRD of ZnO nanostructured films (a) before heating, after heating at 170 oC for (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, and (f) 15 h. 4.2.2 Optical Properties The PL spectroscopy has been employed to study the optical properties of ZnO nanostructures. Before heating, there are two weak and broad emission peaks at 2.2 eV and 3.26 eV. This is attributed to the UV light emission due to recombination of electrons and holes. An additional peak appears at ~2.2 eV, which is defect-related 70 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi luminescence caused by radiative transitions between shallow donors (related to oxygen vacancies) and deep acceptors (Zn vacancies) in ZnO. A peak of ~3.15 eV appeared for all samples seen in Figs. 4.3(a)-(f). Ultraviolet emission is observed for all samples. When the Zn film underwent heating for 3 hours, there was no peak at green light wavelength. However, a hump is observed after heating for 6 hours, which increased after 9 and 12 hours of heating. This hump subsequently disappeared after 15 hours of heating. As heating duration was the only experimental variable, it was responsible for the rise and subsequent decline of green emissions of the films. Surprisingly, there was no fixed elimination or enhancement of the emissions as heating duration increased. It was widely reported that annealing gave rise to a consistent rise or decline in green emissions.10 The observations in Fig. 4.3(a)-(e) clearly showed the effect of morphology changes derived from the different heating durations. It is suggested that morphology changes led to defect formation and elimination which manifested as a rise and decline in green emissions. From Figs. 4.3(b)-(c), there was little change in PL luminescence when heating time was between 3 and 6 hours. This reinforces that a longer heating time did not lead to more defects forming since there was no change in morphology. From Figs. 4.1(c)-(d), the transition of circular pore structure to a network of nanowires occurred between 6 and 9 hours of heating. The abrupt change in morphology over this period of time led to defect formation, which caused green emission. Subsequently with more heating, green emission disappeared after heating for 15 hours. The minimal change in morphology from 9 to 15 hours of heating suggested that elimination of defects was due to a longer heating time. These 71 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi preliminary findings therefore proposed that morphology changes affected photoluminescence properties in addition to heating durations. Fig. 4.3 PL spectra of ZnO nanostructured films (a) before heating, after heating at 170 oC for (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, and (f) 15 h. 72 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi 4.2.3 Investigation of ZnO Growth Mechanism From Fig. 4.2(f), it appears that the nanowires are interconnected. TEM is used to further examine this sample. Particles from ZnO film after heating for 15 hours are scraped off onto TEM copper grid with holey carbon. Figs. 4.4(a)-(d) show the TEM images obtained at different sections of the sample. Fig. 4.4(a) shows the HR-TEM while Fig. 4.4(b) gives its selected area diffraction pattern (SAED). A lattice spacing of 0.247 nm belonging to ZnO (101) is obtained in Fig. 4.4(a). This is further supported by its SAED which can be indexed to ZnO, as shown in Fig. 4.4(b). The TEM results supported Fig. 4.2(f) where only ZnO formed. From Fig. 4.4(c), it is observed that a single nanowire can have a diameter of ~50 nm. This suggests that the diameter of a single nanowire could actually be much smaller than the 50-200 nm as predicted from SEM in Fig. 4.1(f). It is also observed that the end of the single nanowire was rough as clearly seen in its HR-TEM inset. The rough surface of nanowire probably explains why these individual nanowires are linked to each other by an interconnected network in Fig. 4.4(d). This accounted for the interconnected network even as the network morphed from a mix of distinct elongated and irregularshaped particles to having circular porosity and lastly, to nanowires. 73 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi Fig. 4.4 TEM of different parts of ZnO nanostructured films after heating at 170 oC for 15 h in (a) one section, (b) SAED, (c) a nanowire with HRTEM as inset, and (d) branched section of a nanowire. The strategy for growth mechanism present in this paper is interesting as it demonstrated the use of salt solution in controlling ZnO nanostructures. The total amount of Zn present was kept constant, and thus could not be responsible for varying ZnO nanostructures. As heating duration was the only variable in this set of experiments, it was the determinant behind the emergence of different ZnO nanostructures. The proposed mechanisms parallel to those present in corrosion as Zn 74 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi is often used a sacrificial anode as cathodic protection.11,12 Corrosion is usually seen as something undesirable and thus, most studies involved investigating corrosion to minimize corrosion preserving Zn or Zn alloy.13,14 The corrosion on Zn alloys was due to pitting corrosion, where the gaps between pits were no more than few micrometers wide. Pitting has the ability to oxidize Zn and offered non-uniform corrosion rates on Zn films. This mechanism is illustrated in Fig. 4.5. The Zn substrate immersed in NaCl solution can be seen as local cathode and anode which are both contained in the same environment. Known as anode dissolution, the areas of Zn in the pores is oxidized in water to form Zn2+ in Equation 4.1. Meanwhile, the dissolved oxygen in water reacts with water to form OH- on the protruding surface as seen in Equation 4.2.15 Anodic reaction: Zn (s)  Zn2+ (aq) + 2e- (4.1) Cathodic reaction: O2 (dis) + 4e- + 2H2O  4OH- (aq) (4.2) Zn2+ ions then react with water to form zinc hydroxide in acidic conditions. Zinc hydroxide will then dehydrate to ZnO. A thin layer of ZnO formed on the glass surface, giving rise to a ZnO seed layer. This paved way for rapid growth of as more ZnO deposited on the glass substrate, thus forming the nanostructured film. 2Zn2+ + 2H2O Zn(OH)2 + 2H+ (4.3) The presence of Cl- ions in the solution accelerated this corrosion mechanism. As Zn2+ ions remained entrenched in the deep trench provided by pores within the Zn 75 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi film surface, this led to the trapping of Cl- ions from NaCl solution in the same area in order to maintain charge neutrality in the localized region. This encouraged the Zn2+ ions to react with the water, thus forming zinc hydroxide in Equation 4.3.16 The production of H+ ions will release Cl- ions. This acidic condition thus led to an autocatalytic effect as Cl- ions encouraged the further oxidation of Zn to Zn2+ within the pores. As OH- ions produced by Equation 4.2 remained unused, Zn(OH)2 would eventually decompose to ZnO when being heated up sufficiently. Meanwhile, the protective layer of ZnO on Zn film is slowly dissolved, making even bulk Zn in Zn film susceptible to corrosion. The reaction is as follow. ZnO + 2OH- + H2O  Zn (OH)42- (4.4) Upon dehydration, Zn (OH)42- would contribute to the re-forming of an oxide film thicker than observed on Zn surface previously. Without NaCl as catalyst, it is likely that only reactions in Equations 4.1 and 4.2 occurred. If NaCl took part in the chemical reactions, ZnCl2 would form too. However, reactions in Equations 4.1-4.3 occurred instead. From Fig. 4.2(b) by XRD, it is seen that 3 hours were sufficient to fully oxidize Zn to ZnO. However, the change in morphology of ZnO films continued with a longer heating time though there is no change in its structural composition. This suggests that the nutrient for ZnO growth on glass substrate indeed came from Zn2+ ions in solution. No NaCl or ZnCl2 peaks were identified in Figs. 4.2(b)-(f) by XRD, and the pH value of the solution before and after reaction remained unchanged. These suggested that pitting corrosion that has an autocatalytic nature can be the growth mechanism of ZnO nanostructures and NaCl as catalyst in this part of study. 76 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi Fig. 4.5 An illustration of growth mechanism for nanostructured ZnO films. To further validate this proposed mechanism, a series of experiments where Zn powder is first mixed with NaCl solution and then subjected to at 170 oC for 15 hours is carried out. There were no permanent pits as Zn powder was loose. The resultant precipitates are then characterized with XRD. Figs. 4.6(a)-(d) showed the XRD spectra of the samples on a Si wafer support, and Tables 4.1 and 4.2 showed the number of moles of Zn and NaCl used for each reaction. To observe the effect of Zn amount, different amount of Zn with same amount of NaCl was first carried out in experiment 1 and 2. To mimic the condition with little Zn since only 3.567 x 10-6 mol of Zn was present in experiments with Zn films, experiment 3 was carried out whereby the amount of NaCl added is multiplied by 10. 77 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi It is interesting to notice that ZnO, Zn(OH)2 and ZnCl2 peaks exist in the XRD patterns in Fig. 4.6, obviously different from the case of the porous Zn film for which almost only pure ZnO was obtained (Fig. 4.2). It should be pointed out that ZnO, Zn(OH)2 and ZnCl2 were formed for polished Zn pieces immersed in NaCl solution at room temperature for a few days, while only ZnO was formed for unpolished Zn foils immersed in NaCl solution at 170 oC and 120 oC. 8,9 In experiment 1, a resultant pH of 9 was observed along with a mixture of ZnCl2, Zn(OH)2 and ZnO. When more Zn powder was added in experiment 2, pH increases to 11. When large amount of NaCl was used in experiment 3, a mixture of ZnCl2, Zn(OH)2 and ZnO were produced while the pH decreased to 6 (Fig. 4.6). NaCl reacted with Zn2+ ions and did not merely acted as a catalyst in these three experiments. It was likely that dehydration of Zn(OH)2 did not occur as the assynthesized powders comprised mostly of ZnCl2. Table 4.1 Tabulations for an estimation of moles of Zn present in Zn films. 78 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi It is concluded that reactions by Equations 4.1 and 4.2 have occurred. Due to good circulation of solution between loose Zn powder, ZnCl2 produced will neither be trapped in the pit nor forced to react with H2O because of the entrapment. Thus, ZnO formation was reduced greatly. Instead, Zn2+ ions reacted directly with OH- ions produced, to form Zn(OH)2. The OH- ions produced from the reduction of water with dissolved oxygen in Equation 4.2 were not neutralized. This was in line with our observations as an increasing pH is observed with more Zn powder introduced at the start. The oxidation of Zn had to be counter-balanced with reduction of water in dissolved oxygen to form hydroxide ions. In addition, Cl- ions from NaCl solution did not act as catalyst here but instead, as a reactant with Zn2+ ions to produce ZnCl2. This is why when more NaCl was added in experiment 3, the pH drops to 6 since more ZnCl2 were formed. 79 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi Fig. 4.6 XRD of powders on a Si wafer with pre-fixed Zn:NaCl ratio of (a) 1:1, (b) 10:1, and (c) 1:10. Table 4.2 Tabulations of experimental results. 80 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi 4.3 Conclusions We successfully obtained ZnO nanostructures by immersing Zn films in NaCl solution at 170 oC from 3 to 15 hours in hydrothermal conditions. The morphology of ZnO structure can be tuned by changing the duration of time during heating. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) indicated that ZnO films with different morphologies are obtained: from a film with network of circular pores to a film with nanowire-like network with bigger pores. This is particularly useful in mesoporous films’ applications. This paper proved that NaCl solution can simultaneously aid in the oxidation of Zn films while modifying surface morphologies of resultant ZnO films. Further studies by transmission electron microscopy (TEM) revealed the existence of an interconnected network through linking of individual nanowires. Photoluminescence (PL) properties depend on morphologies resulting from heating durations for the samples. Ultraviolet emission is observed for all ZnO films. Green emission surfaced after heating for 6 hours, which subsequently disappeared after 15 hours of heating. It is suggested that the morphology change over different heating durations led to the rise and subsequent decline of green emissions. The growth mechanism behind Zn oxidation and morphology changes here is investigated with the aid of experimental method. To determine the effect of Zn, Zn powder with varying amount of NaCl is carried out under hydrothermal conditions. A mixture of ZnCl2, Zn(OH)2 and ZnO obtained therefore illustrated the importance of nanostructured Zn films in oxidation by aqueous NaCl solution. It is also found that the model behind pitting corrosion can be used to describe the morphology change in ZnO films in this chapter. 81 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution Wee Rui Qi 4.4 References 1 A. N. Baranov, C. H. Chang, O. A. Shlyakhtin, G. N. Panin, T. W. Kang, and Y.-J. Oh, Nanotechnol. 15, 1613 (2004). 2 C. P. Fah, J. M. Xue, and J. Wang, J. Am. Ceram. Soc. 85, 273 (2002). 3 A. N. Baranov, G. N. Panin, T. W. Kang, and Y.-J. Oh, Nanotechnol. 16, 1918 (2005). 4 L. Shen, L. Guo, N. Bao, and K. Yanagisawa, Chem. Lett. 32, 826 (2003). 5 C. K. Xu, G. D. Xu, Y. K. Liu, and G. H. Wang, Solid State Commun. 122, 175 (2002). 6 J. Yang, W. Wang, Y. Ma, D. Z. Wang, D. Steeves, B. Kimball, and Z. F. Ren, Journal of Nanosci. and Nanotechnol. 6, 2196 (2006). 7 M. Mouanga, P. Berçot, and J. Y. Rauch, Corros. Sci. 52, 3984 (2010). 8 Y. S.Tian, C. G. Hu, Y. F. Xiong, B. Y. Wan, C. H. Xia, X. S. He, and H. Liu. J. Phys. Chem. C 114, 10265–10269 (2010). 9 C. Yan and D. Xue, J. Cryst. Growth 310, 1836 (2008). 10 J. Lim, K. Shin, H. W. Kim, and C. Lee, Mater. Sci. Eng., B 107, 301 (2004). 11 C. Rousseau, F. Baraud, L. Leleyter, and O. Gil, J. Hazard. Mater. 167, 953 (2009). 12 A. K. Neufeld, I. S. Cole, A. M. Bond, and S. A. Furman, Corros. Sci. 44, 555 (2002). 13 C. A. Huang, C. K. Lin, and Y. H. Yeh, Corros. Sci. 52, 1326 (2010). 14 J.-C. Lin, K.-C. Peng, I.-G. Peng, and S.-L. Lee, Thin Solid Films 517, 4777 (2009). 82 Chapter 4: Growth of ZnO Nanostructured Films by Zn Films in NaCl Solution 15 Wee Rui Qi D. A. Jones, Principles and Prevention of Corrosion, Prentice Hall, Second Edition, Chapter 7, pp 209-215 (2005). 16 M. G. Fontana Corrosion Engineering, McGraw-Hill, New York, Chapter 3, pp. 66-67 (1987). 83 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia 5.1 Introduction The introduction of metals such as Group-III elements (B, Al, Ga, In) affects ZnO morphologies and properties.1-3 Gallium is of interest due to its ease of Ga3+ ions as a substitution for Zn2+ ions without much lattice distortion. The ionic radius of Ga3+ (0.62 Å) is smaller than that of Zn2+ (0.74 Å), while the covalent band length of Ga–O (1.92 Å) is slightly shorter than that of Zn–O (1.97 Å).4,5 Thus, this minimized the disruption in structure when Zn2+ ions are replaced with Ga3+. Moreover, gallium is less reactive and more resistive to oxidation, which is an advantage over another common dopant: aluminum.6,7 Typical methods of gallium doped zinc oxide (GZO) include sputtering,8 pulsed laser deposition,9 spray pyrolysis,10 thermal evaporation11 using Ga or Ga2O3 as gallium source, while methods involving sol-gel6,12 and hydrothermal13,14 usually include metal precursors where gallium is provided through gallium nitrate nonahydrate and gallium chloride. Chemical methods provided an easy and cheap synthesis method to obtain GZO. They can easily be scaled up for large-area production. There are several studies involving sol-gel or spin coating methods but they involved repetitive steps and small amount of Ga in their synthesis process.12,15,16 Till date, there are few reports in obtaining GZO via hydrothermal methods. In Ramalingam et al, seeded layer of GZO thin films was first prepared by spin coating in presence of a polymer. The films were then allowed to grow in Teflon container with Zn(CH3CO2)2 and Ga(NO3)3·xH2O at 90 oC for 15 hours.17 In Cimitan et al, solution with Zn(CH3CO2)2 84 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi and Ga(NO3)3·xH2O were heated to 150 oC where pressure is maintained at 2 bar for 15 min before lowering to room temperature.1 However, ZnGa2O4 is usually synthesized when large amount of Ga is used instead.18-20 As ammonia is commonly used to form a complex with Zn precursors for ZnO synthesis, it is proposed that GZO can be obtained by this method even with large amount of Ga.21,22 It is hoped that interesting morphologies and properties will be obtained as well. As the effect of each parameter is specific to each experimental condition, it will be prudent to study conditions in detail before examining the effect of adding metal elements which is the focus in later part of the study. In the first part of this study, ZnO is synthesized with three different substrates and heating durations. Silicon, glass and ZnO-seeded glass substrates are chosen as they are conventional substrates used for study. The aim is to allow a better understanding of ZnO synthesis. This detailed study will pave the way for second part of the study where effect of Ga is expected to influence the synthesis of ZnO significantly. In the second part of study, we seek to obtain Ga incorporated ZnO. GZO with Ga introduced. In both studies, the structural, morphological and optical properties are characterized for the assynthesized powders. 5.2 Results and Discussion In the first part of study, zinc acetate Zn(CH3CO2)2 was first mixed in 0.03 dm3 of deionized (DI) water. The mass of zinc acetate was fixed at 0.165 g. The solution was stirred quickly till fully dissolved after adding 1 mL of ammonia (24 wt %). It was then transferred carefully into a Teflon container with capacity of 45 85 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi mL before heating at 60 oC for 4-24 hours, after which it was left to cool at room temperature for 1 hour before removing the substrates. The substrates were then gently rinsed with DI water and dried with N2. Substrates introduced during synthesis are cleaned silicon, glass and ZnO-seed glass obtained by sputtering at 60 W for 3 hours. In the second part of the study, the heating duration was fixed at 4 hours. Gallium nitrate Ga(NO3)3.xH2O was added in the precursor solution. The mass of Zn(CH3CO2)2 was fixed at 0.165 g while mass for Ga(NO3)3.xH2O was determined by the different molar ratio to be examined in this study. The synthesis method was repeated as previous. In addition, the powder was dried before annealing in air at 150 o C for 1 hour. The molar ratio under study was 0, 10, 20, 30, 40 and 50 at % of Ga in comparison to amount of Zn in zinc acetate added, which was fixed. X-ray diffraction (XRD) was performed on the samples with CuKα1 radiation to determine the phases while scanning electron microscopy (SEM) was carried out to examine morphology of the films. In XRD, the following JCPDS cards were used: 36-1451 (ZnO), 20-1435 (Zn(OH)2), 50-0448 (ZnGa2O), 44-1349 (SiO2), and 271402 (Si). High resolution transmission electron spectroscopy (HR-TEM) was carried out to determine the particle structure on the film. A photoluminescence (PL) system with laser excitation of 325 nm with power of 10 mW and laser spot 1 mm was used to examine light emitting properties at room temperature. 86 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi 5.2.1 Synthesis of ZnO and its Properties 5.2.1.1 Synthesis of ZnO by Sputtering The properties of ZnO-seeded glass are examined with XRD, SEM and PL. Fig. 5.1(a)-(c) give data obtained for ZnO-seeded glass heated for 4 hours. In addition, the cross-sectional SEM for ZnO-seeded glass heated for 24 hours is given in Fig. 5.1(d) as the image taken for 4 hours was not clear. From Fig. 5.1(d), vertically upright nanorods with diameter 100-200 nm and height of 3.5-4 mm were observed. The peaks positions in the XRD pattern were identified to be ZnO. The SEM image showed that the film consisted of fine particles with ~100 nm as diameter. In its PL spectrum, there is a strong ultraviolet (UV) emission peak at 2.99 eV which is attributed to excitonic recombination of the near-band-edge emission.23 The strong intensity suggested that the ZnO obtained by sputtering is of good crystal quality. 87 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Fig. 5.1 (a) XRD pattern, (b) SEM image, (c) PL spectrum for ZnO-seeded glass substrates heated for 4 hours, and (d) cross-sectional SEM image for ZnO-seeded glass substrate heated for 24 hours. 5.2.1.2 Synthesis of ZnO on Different Substrates The as-synthesized powders after hydrothermal method were characterized with XRD, SEM and PL. Other than looking into the effect of substrates, samples with 4 and 24 hours of heating were chosen to examine the effect of heating duration. To examine the structure, XRD was done on the samples after hydrothermal conditions. XRD spectra of the three substrates at 4 hours were given in Fig. 5.2. The evolving of multiple peaks to (002) peak clearly indicated that ZnO with preferred [002] orientation were formed when ZnO-seeded glasses were used, regardless of the synthesis time. When other substrates were used, the structure remained 88 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi polycrystalline with different crystal orientations. This confirmed that a seeded layer of ZnO was required to induce growth of highly oriented ZnO. Fig. 5.2 XRD patterns of ZnO growing on (a) ZnO-seeded glass, (b) glass, (c) silicon substrates for 4 hours. SEM was carried out on the samples for the morphological features. Fig. 5.3(a) revealed the formation of vertically upright ZnO nanorods on ZnO-seeded glass substrates where they were about ~100-200 nm in diameter. The nanorods obtained in 4 hours were ~2.5-3 mm and tapered. An obvious change in morphology occurred when glass and silicon substrate were used. In Figs. 5.3(b)-(c), rods with diameter ~11.5 mm were formed with 4 hours of heating for both Si and glass substrates. The rods were arranged in a floral pattern whereby the height of each rod was ~10-12 mm. 89 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi As there were growth on both silicon and glass substrates, there was doubt whether precipitation actually occurred and deposited on top of the substrates. However, rods in floral arrangement were observed at side of the substrate when cross sectional SEM was done. This proved that the existence of strong adhesion of the rods to the substrates. It also clearly demonstrated the direct growth of ZnO on bare silicon and glass substrates by aqueous method, whereby it was usually obtained by methods such as pulsed laser deposition and thermal evaporation.24-26 In addition, cross sectional SEM was made on the samples to give indication on controlling thickness with time. Other than ZnO-seeded glass substrates, it appeared that the substrates did not have much significant role in affecting morphologies. Fig. 5.3 SEM images of ZnO growing on (a) ZnO-seeded glass, (b) glass, (c) silicon substrates for 4 hours. 90 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi PL spectroscopy was carried out on the samples in Fig. 5.4. All samples displayed diminished UV emissions at ~3.1 eV but strong green emissions at ~2.10 eV comparatively. It appeared that there was a substrate effect on the optical properties as there was a slight hump at ~3.1 eV when glass substrate is used instead of ZnOseed glass substrate. However, it was later found out that the emission from glass substrate itself though it was unclear what caused its emissions at that wavelength. The strong green emissions proved that plenty of defects were introduced during hydrothermal synthesis. Similarly, the substrates did not affect optical properties for ZnO. Fig. 5.4 PL spectra of ZnO grown on (a) ZnO-seed glass, (b) glass, (c) silicon substrates for 4 hours. 91 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi 5.2.1.3 Synthesis of ZnO on Bare Silicon Substrates for 4 to 24 hours Fig. 5.5 gave the XRD spectra of as-synthesized powders on silicon substrates heated from 4 to 24 hours. There was no distinct change in structure as the peak positions and intensities remained the same. Fig. 5.5 XRD patterns of ZnO grown on silicon substrates for (a) 4, (b) 8, (c) 12, and (d) 24 hours. Morphologies of the as-synthesized powders grown on silicon substrates were examined in Figs. 5.6(a)-(d). The rods with diameter ~1-1.5 mm and in floral arrangement were seen for all heating durations, further verifying the lack of structural change with heating durations. Cross sectional SEM was done on the samples to give indication on controlling thickness with time. It was assumed that 92 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi ZnO growth on silicon and glass substrates was similar since the substrates did not appear to play a significant role in morphology. Fig. 5.6(e) showed that though the thickness increased with heating duration, its growth rate decreased substantially after 8 hours of heating. This further verified that increasing heating time served to increase thickness, and would not alter the morphology and structure much. Similarly, the PL spectra (Fig. 5.7) showed similarities amongst all samples. It appeared if that defects introduced were uniformed regardless of heating durations as all spectra spotted high green emissions. It therefore suggests that the film thickness of assynthesized powder can be tuned with heating durations from 4 to 24 hours. 93 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Fig. 5.6 SEM of ZnO grown on silicon substrates for (a) 4, (b) 8, (c) 12, (d) 24 hours, and (e) Plot of ZnO film thickness to heating duration. 94 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Fig. 5.7 PL spectra of ZnO grown on silicon substrates for (a) 4, (b) 8, (c) 12, and (d) 24 hours. 5.2.2 Structural Properties and Composition of Ga Incorporated ZnO XRD was performed on the as-synthesized Ga incorporated ZnO (GZO) powders for structural characterization. The initial and finals molar ratios of Ga:Zn refer to those in the initial precursor solution and the synthesized GZO powder, respectively. In Fig. 5.8, ZnO peaks were observed in XRD patterns for samples with starting Ga:Zn molar ratios of 0, 10 and 20 at %, while ZnGa2O4 and ZnO peaks were observed in initial Ga:Zn molar ratios of 30, 40 and 50 at %. The peak positions marked in blue belong to the silicon substrate where GZO was deposited on. To give an understanding on elemental composition after being synthesized, the starting and final Ga:Zn molar ratio was plotted in Fig. 5.9. The final Ga:Zn molar ratio remained similar even though its starting Ga:Zn molar ratio increased from 10 to 20 at %. 95 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi However, more Ga is found in synthesized GZO powder than that in the original precursor, for samples with Ga:Zn initial ratio equal and greater than 30 at %. Fig. 5.8 XRD patterns of as-synthesized GZO powder with (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 at % of Ga:Zn ratio in the starting precursors. 96 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Fig. 5.9 ICP of as-synthesized powder with (a) 10, (b) 20, (c) 30, (d) 40, and (e) 50 at % of Ga:Zn in the starting precursors. It was obvious that the major transition in structural change occurred between 20 and 30 at % of Ga/Zn. The transition in structure was further proven in Fig. 5.9 by ICP where an abrupt decrease in Ga:Zn molar ratio of as-synthesized powder was observed between 20 and 30 at % of Ga/Zn in its starting powder. It is evident that certain degree of ionic substitution has taken place. Before this structural transition, Ga doping was present in ZnO due to the minute change in the final Ga:Zn molar ratio. However, doping ceased after this structural transition as there were substantial amount of Zn and Ga present. It was remarkable that ZnO structure was maintained with the introduction of relatively large amount of Ga as less than 5 at % of Ga/Zn was known to be able to 97 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi cause the formation of ZnGa2O4.27 In this study, doping of Ga in ZnO was sustained at 20 at % Ga. During synthesis, Ga and Zn precursors were well-mixed in asprepared solution. It was likely for Ga to be embedded as it was well known for ZnNH3 complex to form with Zn precursors and ammonia.23,28 As heating progressed, ZnO formed together with Ga trapped within its structure. This could explain the relatively high amount of Ga doped in ZnO before structural transition to ZnGa2O4. From Figs. 5.8(d)-(f), it appeared that either ZnGa2O4 or Ga2O3 could form with 30 to 50 at % Ga due to their peak positions being uncannily close to each other. However, it is more likely for ZnGa2O4 to form in this study due to the large amount of Zn present when characterized by ICP. This was also observed in Hirano’s work where ZnGa2O4 was synthesized when Ga:Zn molar ratio was 0.47 to 0.61 after converting from the given ZnO/Ga2O3 ratio.19 From this study, it was found that the annealing temperature of 150 oC was able to eliminate hydroxide peaks due to Zn(OH)2. Therefore, it was also reasonable to rule out Ga(OH)3 even though it shares a peak position at ~76.4 o. 5.2.3 Influence of Ga on Morphology The morphology of the as-synthesized powder was examined with SEM in Fig. 5.10. Without any Ga introduction, rods with diameter 1-1.5 µm and length 10-12 µm were grown and arranged in a neat floral arrangement. The atomic % Ga stated is defined as the amount of Ga in the initial precursor solution. With 10 at % Ga, hexagonal discs littered with vertically protruding spike-like rods are formed. Fig. 5.10(b) indicates that the diameter and thickness of hexagonal discs were ~7 µm and 98 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi ~0.4 µm respectively, while the length of its rods was ~0.7 µm. When amount of Ga was increased to 20 at %, there was an obvious lack of rod formation though some sites featured very short snubs which could be due to the initial growth of the rods. The diameter of hexagonal discs diminished to ~4.5 µm though the morphology retained. Its thickness also decreased to ~0.2 µm. Upon further increase of Ga to 30 at %, a continuous network of hexagonal discs appeared whereby the outline was less distinctive. Moreover, the vertically protruding rods disappeared entirely. This morphology was repeated for 40 at % and 50 at % Ga. 99 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Fig. 5.10 SEM images of as-synthesized powder with (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 at % Ga at the start. The formation of hexagonal discs with vertically protruding rods is a unique morphology. The study showed that 10 at % Ga was the only combination that promoted this distinctive morphology. Upon further increase in Ga addition, the change in dimensions and morphologies were obtained. This was previously not 100 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi reported as only bare hexagonal features in ZnO have been disclosed.29-32 This marked the first time that vertical rods grown on hexagonal discs were observed. Since the vertical rods on hexagonal discs can promote a larger surface area, it is likely that it would boost its potential as photocatalyst or gas-sensors. 5.2.4 Influence of Ga in ZnO on Optical Properties The PL spectra were used to examine the optical properties of the assynthesized powder in Fig. 5.11. From Fig. 5.11(a), distinct peaks at UV and green emissions are observed. These are attributed to be band-edge and defect-related respectively.23 However, the PL spectra in Figs. 5.11(b)-(f) exhibited an emission over a broad range of wavelengths. The distinct change in PL spectra once 10 at % Ga was added in, was testimony to effect of Ga in ZnO properties. In GZO, the visible emission centers shifted to shorter wavelengths from 2.11 to 2.57 eV with 0, 10 and 20 at % Ga, suggesting that the Ga dopants contributed to the defects in ZnO.5 With ZnGa2O4, blue emissions emerged as well though they were blue-shifted drastically to 2.66-2.73 eV. It is obvious that emissions were not due to the band gap as band gap of ZnGa2O4 is ~5 eV.33 The addition of Ga contributed to defects in ZnGa2O4, which gave rise to visible emissions. This was verified by Kim et al whereby blue emissions were due to the regular Ga-O octahedron which allowed charge transfer between Ga3+ and Zn2+ to take place.34,35 It was reasonable for distortion to incur due to shift in Ga:Zn molar ratio, causing minor shift in visible emission. 101 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Fig. 5.11 PL spectra of as-synthesized GZO powders with (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 at % Ga at the start. 5.2.5 Influence of Ga on GZO Growth Mechanism It was of interest on the powder with 10 at % Ga at the start as it was the lone sample with hexagonal discs littered with vertically protruding spike-like rods. It was 102 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi suspected that a delicate balance between Zn and Ga resulted in the growth of this morphology. To shed light on its formation at this elemental composition, elemental mapping by SEM on resultant powder by 10 at % Ga was done. Specific compositions at various sections were calculated in Fig. 5.12(a). It was clear that different sites showed different elemental composition. The molar ratio of Ga:Zn was 0.52 at position 1 where the surface of hexagonal discs was bare. This decreased sharply on the hexagonal discs with vertically outright rods, whereby the ratio was 0.19 on the disc while it was 0.09 on its rods. Elemental mapping by SEM in Figs. 5.12(b)-(d) displayed the elemental distribution in a section of the as-synthesized powder. The rods grown on hexagonal discs contained more Zn than the discs themselves. Ga was concentrated in bare hexagonal discs. This accounted for Ga in final Ga:Zn molar ratio of 0.114 in the as-synthesized powder. 103 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Fig. 5.12 SEM mapping of (a) electron image, (b) Zn, (c) Ga, and (d) O at one section of as-synthesized powder with 10 at % Ga at the start. It was initially expected that these sites would have less elemental distribution of Zn in the rods compared to its surrounding due to presence of space between the rods. Since the opposite occurred, this hereby confirmed that zinc was concentrated in the rods grown on hexagonal discs. In addition, the high concentration of Ga in hexagonal discs suggested that Ga could act as a catalyst for secondary growth of ZnO nanorods from its surface. This has been observed in growth of Ga2O3 nanowires where Ga behaved as catalyst.36 This proved that it was possible for Ga to encourage growth of vertical rods. 104 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi TEM characterization is carried out in Fig. 5.13 on as-synthesized powder with 10 % Ga at the start. At low magnification, it is found that the hexagonal disc was made of single crystal with a lattice spacing of d = 0.279 nm which corresponded to ZnO (100). The diameter of this disc was smaller than measured in Fig. 5.10(b), which was reasonable as there would be small deviations in particle size. Upon higher magnification in Figs. 5.13(c)-(d) whereby the focus was on the edge of the hexagonal disc, areas with dislocations were observed. This suggested that the assynthesized powder contained plenty of strain. Its lattice spacing of d = 0.278 nm further confirmed the presence of ZnO. However, it was discovered from SAED in Fig. 5.13(d) that the hexagonal discs actually consisted of polycrystalline ZnO as well. It is postulated that these signals were due to the vertical upright rods grown from the disc surface, where they positioned in such a way that TEM could not detect their presence. Another possibility is that the polycrystalline characteristics were due to detached rods since their presence were observed from SEM in Fig. 5.10(b). The rods were free to stick on the disc surface without any orientation, thus contributing to the polycrystalline nature in SAED. Since strain was present in the hexagonal disc, it is postulated that strain caused a secondary growth where the rods grew from the hexagonal discs after primary growth of the discs was completed. 105 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi Fig. 5.13 (a) TEM, (b) SAED of TEM, (c) HRTEM, and (d) SAED of HRTEM for as-synthesized powder with 10 at % of Ga/Zn at the start. The presence of dislocations is not sufficient to prove that its responsibility for the morphology in the samples since strain presence is common in many materials. Moreover, substrate clearly did not contribute to strain here since the as-synthesized powder were deposited onto the silicon substrates rather than grown directly on them.37 To further verify the effect by strain, an additional study with XRD with step size of 0.0025 o was carried out on as-synthesized powder with 0, 10 and 20 at % Ga 106 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi at the start in Figs. 5.14 (a)-(c). The samples were chosen since it was thought that structural transition occurred between 20 and 30 at % Ga. The peak positions at ~31.8 o , ~34.5 o and ~36.3 o verified that ZnO was indeed present in all these samples. By comparing the original signals with respect to the substrate signal of 33.08 o in 0 at % Ga, the shifted signals in 10 % and 20 % Ga powders were marked in red. Using ZnO (100) as an example, its peak position increased gradually from 31.88 o, 31.89 o and then to 31.91 o with Ga increase. Therefore, strain such as dislocations, was the origin behind lattice shrinkage. As dislocations were detected from XRD study, it was possible for them to contribute to luminescent properties of the as-synthesized powders.38 Fig. 5.14 XRD patterns of as-synthesized powders with (a) 0, (b) 10, (c) 20 at % Ga, and (d) Williamson-Hall Plot for as-synthesized GZO powder with 10 at % Ga/Zn at the start. 107 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi As explained previously, the ionic radius of Ga3+ is similar than Zn2+ ions 4,5 . This facilitated the easy substitution of Zn with Ga and vice versa. As amount of Ga introduced at the start increased, there will be more Ga to replace Zn in the structure. Since ionic radius of Ga3+ is slightly smaller than Zn2+ ions, this explained the subsequent lattice shrinkage. A further illustration on presence of strain was done by tabulating the Williamson-Hall (WH) plot for powder with 10% Ga at the start in Fig. 5.14(d). The WH plot featured a relationship between Bragg angles θB with strain ε and crystallite size via a quadratic equation. The average size and strain can be calculated from the fitted linear line when the WH plot is based on Equation 5.1 where β, θB, t, and ε represent full width at half maximum (FWHM) of the XRD peak, Bragg’s angle, wavelength of X-ray (0.154056 nm), average particle size (diameter), and strain, respectively. In the equation, slope gives 4ε while its intercept is related to average particle size t.39,40 𝛽 cos θ𝐵 = 0.9 𝜆 𝑡 + 4𝜀 sinθ𝐵 (5.1) Clearly, there is no linear relationship in the WH plot in Fig. 5.14(d). The obtained slope was therefore not related to strain ε. It was expected as the WH plot was based on an assumption of isotropic strain.41 Hexagonal discs with vertically protruded rods were present, indicating that this assumption probably did not hold. 108 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi However, it can be concluded that there was indeed strain ε as there was an upward trend from the plot. 5.3 Conclusions For the first part of study in this chapter, a growth of ZnO was demonstrated directly on substrates comprising of silicon, glass and ZnO-seeded glass substrates. Rods in floral arrangement were obtained on silicon and glass substrates while vertically upright nanorods were obtained for the ZnO-seeded substrates. It was also demonstrated that direct growth of ZnO occurred on bare silicon and glass substrates by aqueous method, whereby it was usually obtained by methods such as pulsed laser deposition and thermal evaporation. Other than ZnO-seeded glass substrates, it appeared that the substrates did not have much significant role in affecting morphologies. All as-synthesized powder displayed diminished UV emissions at ~3.1 eV but strong green emissions at ~2.10 eV comparatively. Similarly, the substrates did not affect optical properties for ZnO. It was verified that the thickness of assynthesized powder film can be tuned with heating durations from 4 to 24 hours. In the second part of study, GZO and ZnGa2O4 were prepared by hydrothermal method with zinc acetate Zn(CH3CO2)2, gallium nitrate Ga(NO3)3.xH2O and ammonia. Characterization was made with XRD, SEM, TEM and PL. The atomic % of Ga/Zn stated is defined as the amount of Ga in the initial precursor solution. It is found that GZO is obtained when 10 and 20 at % of Ga was added in, while ZnGa2O4 is obtained with 30 to 50 at % of Ga introduced. The amount of Ga used for GZO is much larger than typical chemical methods in which 109 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi GZO can only tolerate less than 10 at % Ga. The influence of Ga on ZnO morphology is found. Without any Ga introduction, rods with diameter 1-1.5 µm and length 10-12 µm were grown and arranged in a neat floral arrangement. With 10 at % Ga, hexagonal discs littered with vertically protruding spike-like rods were formed. This is a unique morphology which has not yet been reported, and only bare hexagonal features in ZnO have been reported. Thus, this provides potential as photocatalyst or gas-sensors since the vertical rods on hexagonal disc promoted a larger surface area. It is probable for Ga to encourage secondary growth of vertical rods where elemental mapping by SEM showed large amount of Ga in the hexagonal discs. When amount of Ga is increased to 20 at %, there is an obvious lack of rod formation though some sites feature very short snubs which can be due to the initial growth of the rods. It is proven with detailed XRD and Williamson-Hall Plot that lattice shrinkage due to ionic substitution occurred during doping. With further increase of Ga to 30 at %, a continuous network of hexagonal discs appeared whereby the outline was less distinctive. Moreover, the vertically protruding rods disappeared entirely. This morphology was repeated for 40 at % and 50 at % Ga. The influence of Ga in optical properties in ZnO is also apparent. PL spectra showed that the visible emission centers shifted to shorter wavelengths from 2.11 to 2.57 eV with 0, 10 and 20 at % Ga in GZO, suggesting that the Ga dopants contributed to the defects in ZnO. With ZnGa2O4, blue emissions emerged as well though they were blue-shifted drastically to 2.66-2.73 eV. 110 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia Wee Rui Qi 5.4 References 1 S. Cimitan, S. Albonetti, L. Forni, F. Peri, and D. Lazzari, J. Colloid Interface Sci. 329, 73 (2009). 2 J. Kobayashi, N. Ohashi, H. Sekiwa, I. Sakaguchi, M. Miyamoto, Y. Wada, Y. Adachi, K. Matsumoto, and H. Haneda, J. Cryst. Growth 311, 4408 (2009). 3 M. Izakia, and J. Katayama, J. Electrochem. Soc. 147, 1, 210 (2000). 4 V. Assunção, E. Fortunato, A. Marques, A. Gonçalves, I. Ferreira, H. Águas, and R. Martins, Thin Solid Films 442, 102 (2003). 5 C.-Y. Tsay, C.-W. Wu, C.-M. Lei, F.-S. Chen, and C.-K. Lin, Thin Solid Films 519, 1516 (2010). 6 A. Kalaivanan, S. Perumal, N. Neelakanda Pillai, and K. R. Murali, Mater. Sci. Semicond. Process. 14, 94 (2011). 7 F. Wu, L. Fang, Y. J. Pan, K. Zhou, Q. L. Huang, and C. Y. Kong, Physica E 43, 228 (2010). 8 P. K. Song, M. Watanabe, M. Kon, A. Mitsui, Y.Shigesato, Thin Solid Films 411, 82 (2002). 9 M. Yan, H. T. Zhang, E. J. Widjaja, and R. P. H. Chang, J. Appl. Phys. 94, 5240 (2003). 10 K. T. R. Reddy, H. Gopalaswamy, P. J. Reddy, R.W. Miles, J. Cryst. Growth 210 516 (2000). 11 S. Y. Bae, C. W. Na, J. H. Kang, J. Park, J. Phys. Chem. B 109, 2526 (2005). 12 V. Fathollahi and M. Mohammadpour Amini, Mater. Lett. 50, 235 (2001). 13 H. Q. Le, S. K. Lim, G. K. L. Goh, S. J. Chua, and J. Ong, J. Electrochem. Soc. 157, H796 (2010). 14 H. Wang, S. Baek, J. Song, J. Lee, and S. Lim, Nanotechnol. 19, 075607 (2008). 15 G. K. Paul and S. K. Sen, Mater. Lett. 57, 742 (2002). 111 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia 16 Wee Rui Qi P. K. Nayak, J. Kim, S. Chung, J. Jeong, C. Lee, and Y. Hong, J. Phys. D: Appl. Phys. 42, 139801 (2009). 17 R. Jothi Ramalingam and G. S. Chung, Mater. Lett. 68, 247 (2012). 18 D. P. Dutta, R. Ghildiyal, and A. K. Tyagi, J. Phys. Chem. C 113, 16954 (2009). 19 M. Hirano, J. Mater. Chem. 10, 469 (2000). 20 Y.-H. Zheng, C.-Q. Chen, Y.-Y, Zhan, X.-Y, Lin, Q. Zheng, K.-M. Wei, J.-F. Zhu, and Y.-J. Zhu, Inorg. Chem. 46, 16 (2007) 21 Q. Ahsanulhaq, A. Umar, and Y. B. Hahn, Nanotechnol. 18, 115603 (2007). 22 K. Yu, Z. Jin, X. Liu, J. Zhao, and J. Feng, Appl. Surf. Sci. 253, 4072 (2007). 23 L. Wu, Y. Wu, X. Pan, and F. Kong, Opt. Mater. 28, 418 (2006). 24 L. Feng, A. Liu, M. Liu, Y. Ma, J. Wei, and B. Man, J. Alloys Compd. 492, 427 (2010). 25 Y. Sun, G. M. Fuge, and M. N. R. Ashfold, Chem. Phys. Lett. 396, 21 (2004). 26 H. I. Abdulgafour, F. K.Yam, Z. Hassan, K. Al-Heuseen, and M. J. Jawad, J. Alloys Compd. 509, 5627 (2011). 27 A. de Souza Gonçalves, S. Antonio Marques de Lima, M. Rosaly Davolos, S. Gutierrez Antônio, and C. de Oliveira Paiva-Santos, J. Solid State Chem. 179, 1330 (2006). 28 A. Wei, X. W. Sun, C. X. Xu, Z. L. Dong, M. B. Yu, and W. Huang, Appl. Phys. Lett. 88, 213102 (2006). 29 X L. Cao, H. B. Zeng, M. Wang, X. J. Xu, M. Fang, S. l. Ji, L. D. Zhang, J. Phys. Chem. C, 112, 14 (2008). 30 R. C. Pawar, J. S. Shaikh, A. A. Babar, P. M. Dhere, and P. S. Patil, Sol. Energy 85, 1119 (2011). 31 D. J. Gargas, M. C. Moore, A. Ni, S.-W. Chang, Z. Y. Zhang, S.-L. Chuang, P. D. Yang, ACS Nano 4, 6. 112 Chapter 5: Chemical Synthesis Using Zinc and Metal Salts with Ammonia 32 Wee Rui Qi M. Wang, S. H. Hahn, J. S. Kim, J. S. Chung, E. J. Kim, and K.-K. Koo, J. Cryst. Growth 310, 1213 (2008). 33 T. Omata, N. Ueda, K. Ueda, and H. Kawazoe, Appl. Phys. Lett. 64, 1077 (1994). 34 J. S. Kim, H. I. Kang, W. N. Kim, J. I. Kim, J. C. Choi, H. L. Park, G. C. Kim, T. W. Kim, Y. H. Hwang, S. I. Mho, M. C. Jung, and M. Han, Appl. Phys. Lett. 82, 2029 (2003). 35 J. S. Kim, H. L. Park, C. M. Chon, H. S. Moon, and T. W. Kim, Solid State Commun. 129, 163 (2004). 36 L. Xu, Y. Su, Q. T. Zhou, S. Li, Y. Q. Chen, Y. Feng, Cryst. Growth Des. 7, 4 (2007). 37 R. Ghosh, D. Basak, and S. Fujihara, J. Appl. Phys. 96, 2689 (2004). 38 A. Stroud and J. H. You, J. Cryst. Growth 340, 92 (2012). 39 P. K. Giri, S. Bhattacharyya, D. K. Singh, R. Kesavamoorthy, B. K. Panigrahi, and K. G. M. Nair, J. Appl. Phys. 102, 093515 (2007). 40 G. K. Williamson and W. H. Hall, Acta Metall. 1, (1953). 41 A. Khorsand Zak, W. H. Abd. Majid, M. E. Abrishami, R. Yousefi, Solid State Sci. 13, 251 (2011). 113 Chapter 6: Conclusions and Future Work Wee Rui Qi Chapter 6: Conclusions and Future Work 6.1 Conclusions Zinc oxide (ZnO) is a promising candidate for many applications. Nanostructured ZnO has been gaining a strong foothold as they vastly improve ZnO properties. In this project, nanostructured ZnO and its related compounds are synthesized with sputtering, furnace and hydrothermal methods. Sample characterization is performed with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Photoluminescence (PL), and Vibrating sample magnetometry (VSM). The film thickness is estimated with a surface profilometer. The results are presented in Chapters 3-5. Chapter 3 determines if black ZnO (pure ZnO) exists while investigating properties of the resultant annealed zinc films. Zinc (Zn) films underwent annealing at various conditions. The ratios of ZnO (101)/Zn (101) peak intensities of 2:6, 2:6, 7:4 and 5:0 are attained for ZnO films at annealing conditions of 100 oC for 15 h, 200 o C for 214 h and 400 oC for 6 h respectively. Visual inspection and XRD spectra showed presence of Zn in black ZnO films. The black appearance disappeared when the film no longer contained metallic Zn particles. This demonstrates that black ZnO made purely from ZnO does not exist, clarifying our doubt on the literature claimed black ZnO film. The influence of annealing and Zn are also studied. Elongated and irregularshaped particles are obtained and can be transformed into spherical particles after ZnO films are annealed at 400 oC for 6 h. A layer of oxide is found at its film’s metalair interface, and the presence of a Zn/ZnO layered structure is found. The effect of 114 Chapter 6: Conclusions and Future Work Wee Rui Qi annealing is found not only confined to the morphology and structure but also ZnO properties. The enhanced ultraviolet (UV) emission in ZnO films is found and attributed to a low annealing, which is useful for applications that cannot withstand high temperatures. Reverting back to the effect of Zn on ZnO, Zn is capable of enhancing UV emissions and is also suggested that Zn is responsible for ferromagnetism in annealed Zn films. Chapter 4 reports the successful synthesis of ZnO by a new method with aqueous sodium chloride. ZnO nanostructures are successfully obtained by immersing zinc films in sodium chloride (NaCl) solution at 170 oC from 3 to 15 hours in hydrothermal conditions. This is a novel method in obtaining ZnO nanostructures as a reduced amount of sodium chloride is sufficed for oxidation of Zn. The morphology of ZnO structure is tuned by changing the duration of time during heating. SEM and XRD indicated that ZnO films with different morphologies are obtained: from a film with network of circular pores to a film with nanowire-like network with bigger pores. This has potential as mesoporous films. At the same time, NaCl solution can simultaneously oxidized Zn films. Further studies by TEM revealed the existence of an interconnected network through linking of individual nanowires. From PL, UV emission is observed for all ZnO films. Green emission surfaced after heating for 6 hours, which subsequently disappeared after 15 hours of heating. It is suggested that the morphology change over different heating durations led to the rise and subsequent decline of green emissions as ZnO films are already obtained within 3 hours of heating. To determine the effect of Zn, Zn powder with varying amount of NaCl is carried out under hydrothermal conditions. A mixture of ZnCl2, Zn(OH)2 and ZnO 115 Chapter 6: Conclusions and Future Work Wee Rui Qi obtained therefore illustrated the importance of nanostructured Zn films in oxidation by aqueous NaCl solution. The model behind pitting corrosion is found to be responsible for nanostructured ZnO films in this study. Chapter 5 examines the effect of substrates, heating durations and Ga addition on ZnO by hydrothermal methods. For the first part of study, growth of ZnO was demonstrated directly on substrates comprising of silicon, glass and ZnO-seeded glass substrates. Rods in floral arrangement were obtained on silicon and glass substrates while vertically upright nanorods were obtained for the ZnO-seeded substrates. It was also demonstrated that direct growth of ZnO occurred on bare silicon and glass substrates by aqueous method whereby a seeded layer of ZnO is usually required. However, it appeared that the substrates did not have much significant role in affecting morphologies or optical properties. All as-synthesized powder displayed diminished UV emissions at ~3.1 eV but strong green emissions at ~2.10 eV comparatively. It was confirmed that thickness of as-synthesized powder can be tuned with heating durations from 4 to 24 hours. In the second part of study, GZO and ZnGa2O4 were prepared by hydrothermal method with zinc acetate Zn(CH3CO2)2, gallium nitrate Ga(NO3)3.xH2O and ammonia. The atomic % Ga stated is defined as the amount of Ga in the initial precursor solution. It is found that GZO is obtained when 10 and 20 at % of Ga was added in, while ZnGa2O4 is obtained with 30 to 50 at % of Ga introduced. The amount of Ga used for GZO is much larger than typical chemical methods as usually GZO can only tolerate less than 10 at % Ga. The influence of Ga on ZnO morphology is apparent. Without any Ga introduction, rods with diameter 11.5 µm and length 10-12 µm were grown and arranged in a neat floral arrangement. 116 Chapter 6: Conclusions and Future Work Wee Rui Qi With 10 at % Ga, hexagonal discs littered with vertically protruding spike-like rods were formed. This is a unique morphology which has not yet been reported. This provides tremendous potential as photocatalyst or gas-sensors since the vertical rods on hexagonal disc promoted a larger surface area. It is probable for Ga to encourage secondary growth of vertical rods where elemental mapping by SEM showed large amount of Ga in the hexagonal discs. When amount of Ga is increased to 20 at %, there is an obvious lack of rod formation though some sites feature very short snubs which can be due to the initial growth of the rods. It is proven with detailed XRD and Williamson-Hall Plot that lattice shrinkage due to ionic substitution occurred during doping of Ga. With further increase of Ga to 30 at %, a continuous network of hexagonal discs appeared whereby the outline was less distinctive. Moreover, the vertically protruding rods disappeared entirely. This morphology was repeated for 40 at % and 50 at % Ga. The influence of Ga in optical properties in ZnO is also apparent. PL spectra showed that the visible emission centers shifted to shorter wavelengths from 2.11 to 2.57 eV with 0, 10 and 20 at % Ga in GZO, suggesting that the Ga dopants contributed to the defects in ZnO. With ZnGa2O4, blue emissions emerged as well though they were blue-shifted drastically to 2.66-2.73 eV. In summary, nanostructured ZnO particles and films are found achievable by using different ways in this research work. Structural, optical and magnetic properties of these nanostructures and films are studied, and interesting results are obtained, analyzed and discussed. A better understanding of the formation and properties of nanostructured ZnO is achieved. 117 Chapter 6: Conclusions and Future Work Wee Rui Qi 6.2 Future Work The challenges regarding nanostructured ZnO were identified and addressed. It has facilitated important information on the origins of black ZnO, executed the relevance of aqueous NaCl solution, as well as explored synthesis parameters to influence ZnO nanostructures. These studies have sprouted interesting ideas towards nanostructured ZnO, and provided room for further investigations. For instance, it is worthwhile to study the effect of a mixture of aqueous NaCl and Ga salts on synthesis and properties of nanostructured ZnO as it has been achieved separately in Chapters 4 and 5. However, this will be left to the other group members to explore this prospect. 118 [...]... Illustration of VSM…………………………………………………… …43 Fig 3.1 XRD spectra of films under different conditions of (a) Zn film before annealing, (b) ZnO films after annealed at 100 oC for 15 h, (c) 200 oC for 214 h, and (d) 400 oC for 6 h…………………………………………………………………….50 x Fig 3.2 Morphology and visual appearances (insets) of ZnO films under different conditions with (a) SEM of Zn film before annealing, (b) SEM of ZnO films. .. types of substrates can affect ZnO morphology though the number of studies done are very sparse.15 1.1.2.2 Sol-Gel Sol–gel processes are another wet chemical synthesis commonly used for nanostructures such as powders, films, fibers, and monoliths.28 Typical sol–gel process involves hydrolysis and condensation of metal alkoxides and metal salts such as chlorides, nitrates and acetates In metal alkoxides... al.29,30 The ammonia provides a steady source of hydroxide ions to form zinc hydroxide, which later undergoes a condensation reaction to form ZnO.5,31 Therefore, the molar ratio of Zn salts and hydroxide ions present is usually closely monitored as it is well known for hydroxide ions in shape alteration of ZnO.5,30,32,33 Hydroxide sources include ammonia, NaOH and hexamethylenetetramine C6H12N4 (HMT) Parameters... h, and (f) 15 h……………………………….……69 Fig 4.2 XRD of ZnO nanostructured films (a) before heating, after heating at 170 oC for (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, and (f) 15 h……………………………….……70 Fig 4.3 PL spectra of ZnO nanostructured films (a) before heating, after heating at 170 oC for (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, and (f) 15 h.………………………… 72 Fig 4.4 TEM of different parts of ZnO nanostructured films. .. Chen, H D Sun, C F Wang, A Y S Lee, and H Gong, “Development of ZnO Nanostructured Films via Sodium Chloride Solution and Investigation of Its Growth Mechanism and Optical Properties” Accepted in Journal of the American Ceramic Society for publication 3) R Q Wee, H Gong, W F Yang, R Chen, H D Sun, “On Black ZnO Films and Light Emission Properties” Submitted to Journal of Physics D for publication xiv... fine-tuned with different nanostructures in order to gauge its comparison There is therefore a need to have a precise control over the synthesis of ZnO nanostructures before progressing to research on device performance Chemical synthesis of nano-materials may be conducted in solid, liquid, or gaseous state This section highlights some of the common synthesis methods for nanostructures and ZnO 4 Chapter 1:... factor (% degree of fill) of the autoclave The critical temperature (Tcr = 374.1 °C) and pressure (ρ = 221.2 bar) are indicated….…………………………….31 Fig 2.6 Viscosity of water as a function of density and temperature…………….….32 Fig 2.7 Dielectric constant of water plotted against as a function of pressure and temperature….……………………………………………………………………….33 Fig 2.8 Diagram showing the percent of Zn(II) present... (d) 400 oC for 6 h, and (e) HR-TEM of ZnO film annealed at 200 oC for 214 h…………………………………… ……53 Fig 3.3 Schematic diagram of mechanism to obtain Zn/ZnO layered film……… 55 Fig 3.4 PL spectra of films under different conditions of (a) Zn film before annealing, ZnO films after annealed at 100 oC for 15 h, 200 oC for 214 h, and 400 oC for 6 h, and (b) Inset: PL spectrum magnification of Zn film before annealing……………... Jagadish, in Zinc Oxide Bulk, Thin Films and Nanostructures – Processing, Properties and Applications; Ch 1, edited by C Jagadish and S Pearton (Elsevier, 2006), p 1 19 B J Jin, S H Bae, S Y Lee, S Im, Mater Sci Eng., B 71, 301 (2000) 20 F Manjón, Solid State Commun 128, 35 (2003) 21 S Ghosh, G G Khan, B Das, and K Mandal, J Appl Phys 109, 123927 (2011) 22 J B You, X W Zhang, Y M Fan, Z G Yin, P F Cai, and. .. and indium chloride (InCl3) were some of the dopant salts found in hydrothermal synthesis of dopedZnO.35-40 The influence of H3BO3 on ZnO morphology is given in Fig 1.3.38 Fig 1.3 SEM images of ZnO nanorods synthesized in Q Yu et al with (a) no H3BO3, and (b) 0.03 mol/L of H3BO3 concentration.38 6 Chapter 1: Introduction Wee Rui Qi The growth of nanostructured ZnO films requires an additional step; that ... chemical synthesis commonly used for nanostructures such as powders, films, fibers, and monoliths.28 Typical sol–gel process involves hydrolysis and condensation of metal alkoxides and metal... 73 (2009) 24 Chapter 2: Synthesis and Characterization Wee Rui Qi Chapter 2: Synthesis and Characterization 2.1 Fabrication of Samples 2.1.1 Sputtering of Zn and ZnO Thin Films A brief introduction... thin layer of zinc (Zn) used in Chapters Three and Four, and zinc oxide (ZnO) used in Chapter Five Physical vapor deposition (PVD) is a general term used to describe any of a variety of methods