Synthesis of one dimensional sno2 nanostructure via hydrothermal method for gas sensor application

MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY - VU XUAN HIEN SYNTHESIS OF ONE-DIMENSIONAL SnO2 NANOSTRUCTURE VIA HYDROTHERMAL METHOD FOR GAS SENSOR APPLICATION MAJOR: ENGINEERING PHYSICS MASTER OF SCIENCE THESIS ENGINEERING PHYSICS SUPERVISOR: Dr DANG DUC VUONG HANOI - 2011 BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI - VŨ XUÂN HIỀN CHẾ TẠO VẬT LIỆU NANO SnO2 CẤU TRÚC MỘT CHIỀU ỨNG DỤNG LÀM CẢM BIẾN KHÍ CHUYÊN NGÀNH: VẬT LÝ KỸ THUẬT LUẬN VĂN THẠC SĨ KHOA HỌC VẬT LÝ KỸ THUẬT HƯỚNG DẪN KHOA HỌC: TS ĐẶNG ĐỨC VƯỢNG HÀ NỘI - 2011 ACKNOWLEDGEMENTS First, it’s pleasure to send my sincere thanks to teachers and brothers at Electronic Materials Department, School of Engineering Physics about the supports during my study and research Especially, I want to give my thank Dr Dang Duc Vuong, who enthusiastically guided me throughout the course to complete this thesis I also thank the members of the sensor group by the valuable suggestions and enthusiastic support during the implementation of the Dissertation Finally, during the course, I also received many supports and instructions of internal as well as external laboratories, such as Structural Analysis Laboratory School of Engineering Physics and Electron Microscopy Laboratory - National Institute of Hygiene and Epidemiology Again I sincerely thank about it Hanoi, July 8th 2011 Author Vu Xuan Hien GUARANTEE I hereby declare that this is my own research Results described in the thesis are true and never published in any works before Author Vu Xuan Hien SUMMARY OF THESIS Thesis title: “Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application” Author: Vu Xuan Hien Course: 2009 Supervisor: Dr Dang Duc Vuong Content: SnO2 is a special n-type metal oxide semiconductor which possesses many feature properties, namely low cost, high gas sensitivity, good chemical resistance and fast electron mobility Therefore, many scientists have paid special attention to this material, especially in Nano-scale Many studies in preparing different morphologies of SnO2 by various methods have been successfully carried out Today, the rod-like structure of tin dioxide nanomaterial has been attracting many concerns of scientists because of its huge potential application in manufacturing gas sensor, optical devices, dye-sensitized solar cell and transparent conducting electrodes Among numerous methods, hydrothermal treatment is an interesting approach which can support mass synthesize nanomaterials uniformly and cheaply at low temperature Therefore it has been widely choosing for synthesizing SnO2 nanorods, recently Purpose of this work is to investigate the optimum process of synthesizing SnO nanorods under hydrothermal treatment as well as propose a possible formation mechanism for the growth Also, the thesis studies about LPG and ethanol sensing properties of SnO2 nanorods in order to orientate the use of SnO2 nanomaterial in gas sensor application The thesis focuses on synthesizing SnO2 nanorods in powder form using hydrothermal method The material is characterized by FE-SEM, TEM, XRD to investigate the structure and morphology In addition, the study about LPG and ethanol sensing properties of as-prepared material is also conducted Summary: Base on the experimental results, key conclusions of the thesis can be described as follow:  SnO2 nanorods (5-7 nm diameters and 10-30 nm lengths), nanoflowers (Constructed of nanorods with the diameter is from 40 to approximately 200 nm and the length is from 150 nm to roughly µm) and microspheres (from several nanometers to approximately micrometers diameter) were successfully synthesized by a low temperature (below 200 oC) process using hydrothermal method Also, the striking feature of the process comes from the uniform and high density all over the powders of the as-prepared materials  In the formation period, hypothetically, the spheres were made up by the isotropic aggregation (in case the hydrothermal temperature, “T” is below 190 oC or hydrothermal time “t” is under 20 hours) of SnO2 crystals, nuclei and clusters, whereas the rods and flowers were together constructed via crystallization process (T ≥ 190 oC and t ≥ 20 hours) The XRD, FE-SEM and TEM results indicate that the anisotropic of SnO2 flowers is higher than nanorods and is followed by SnO2 microspheres This may result in the better selectivity toward ethanol of SnO2 nanoflowers while comparing to other morphologies In addition, the gas testing result again proves the enhancement of LPG and ethanol sensing properties for SnO2 nanorods to nanoparticles Main results of the thesis have been used to write a paper entitled “Synthesis of SnO2 micro-spheres, nano-rods and nano-flowers via simple hydrothermal route” which has been accepted by Physica E Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application CONTENT LIST OF ABBREVIATIONS III LIST OF TABLES V LIST OF FIGURES VI PREFACE I INTRODUCTION SnO2 material and its applications 1.1 Physical and chemical properties 1.1.1 Physical properties 1.1.2 Chemical properties 1.2 Typical applications .4 1.2.1 Transparent conductor 1.2.2 Oxidation catalyst 1.2.3 Solid state gas sensor Tin dioxide in the Nano-world 11 2.1 Overview of nanomaterials 11 2.2 One-dimensional SnO2 nanostructure 14 2.3 Methods for synthesis of nanomaterials 16 2.3.1 Physical vapor deposition 17 2.3.2 Chemical vapor deposition 18 2.3.3 Hydrothermal synthesis 19 2.4 Methods for synthesis of SnO2 nanorods 22 2.4.1 Vapor-Liquid-Solid .22 2.4.2 Hard template 24 2.4.3 High-pressure pulsed laser deposition 25 2.4.4 Molten-salt 26 2.4.5 Hydrothermal treatment 28 Master thesis I Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application II EXPERIMENT .31 Synthesis of SnO2 nanorods 31 Influence of technical parameters .32 Coating thick film SnO2 nanomaterial 33 Characterization techniques of SnO2 nanomaterial 33 4.1 X-Ray diffraction (XRD) 33 4.2 Scanning Electron Microscopy (SEM) 34 4.3 Transmission Electron microscopy (TEM) 36 4.4 Gas sensing properties 36 III RESULTS AND DISCUSSION 39 Prepare SnO2 nanorods in large scale .39 Influence of hydrothermal time 40 Influence of tin (IV) chloride weight 45 Influence of hydrothermal temperature .47 Gas sensing characteristics 49 CONCLUSION 53 FUTURE PLAN 54 REFERENCES 56 Master thesis II Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application LIST OF ABBREVIATIONS  IUPAC: International Union of Pure and Applied Chemistry  XRD: X-ray Diffraction  SEM: Scanning electron microscope  FE-SEM: Field Emission Scanning Electron Microscopy  EDX or EDS: Energy-dispersive X-ray spectroscopy  TEM: Transmission electron microscopy  AFM: Atomic force microscopy  TCOs: Transparent conducting films  CNTs: Carbon nanotubes  VLS: Vapor-liquid-solid  PVD: Physical vapor deposition  CVD: Chemical vapor deposition  APCVD: Atmospheric pressure CVD  LPCVD: Low-pressure CVD  UHVCVD: Ultrahigh vacuum CVD  AACVD: Aerosol assisted CVD  DLICVD: Direct liquid injection CVD  MPCVD: Microwave plasma-assisted CVD  PECVD: Plasma-Enhanced CVD  RPECVD: Remote plasma-enhanced CVD  ALCVD: Atomic layer CVD  CCVD: Combustion Chemical Vapor Deposition  HWCVD: Hot wire CVD  MOCVD: Metal-organic chemical vapor deposition  HPCVD: Hybrid Physical-Chemical Vapor Deposition Master thesis III Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application  RTCVD: Rapid thermal CVD  VPE: Vapor phase Epitaxy  R.F.: Radio Frequency  PAA: Porous Anodic Membrane  PLD: Pulsed Laser Deposition  CTAB: Cetyltrimethyl Ammonium Bromide  PEG: Polyethylene Glycol  Acc Voltage: Accumulation Voltage  JCPDS: Joint Committee on Powder Diffraction Master thesis IV Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application the different diameter micro-spheres were received in the condition Interestingly, as the treated temperature was raised to 140 oC, the FE-SEM image shows that the micro-spheres seem to be broken or possess ununiformed shape The reason comes from the rapid movement of the seeds and nuclei in the condition of high treatment temperature and pressure where the crystallization does not occur In this situation, the movement energy is higher than the energy of Van der Waals forces or chemical bonding The aggregation, therefore, does not take place optimally making (112) (301) (002) (310) (211) (220) Intensity (a.u.) (200) (111) (110) (101) ununiformed spheres a a, SnO2 nano-rods b, SnO2 nano-flowers c, SnO2 micro-spheres b c 20 30 40 2 50 60 70 Figure 35 XRD pattern of SnO2 condensed micro-spheres (c), nano-flowers (b) and nano-rods (a) The XRD spectrums of nanorods (S.9), nanoflowers (S.4) and microspheres (S.11) are shown in figure 35 It can be easily observed that all the strong reflection peaks of nanoflowers in the XRD patterns are indexed to tetragonal rutile structure SnO2 The lattice constants of a = b = 0.483 nm and c = 0.372 nm, which correspond with the data of SnO2 powders recorded in the JCPDS document (Powder Diffraction File Compiled by the Joint Committee on Powder Diffraction, 1985, Card No 03-1116) It can be also seen that there are no other impurities in the analyzed samples As has been predicted above, the spectrum of SnO2 microspheres proclaims the amorphous phase Master thesis 48 Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application It can be observed in the pattern that the peaks of nanorods are shaper and stronger comparing with nanoflowers This outcome can be understood as nanorods are anisotropic growth whereas nanoflowers are grown not only by a single direction which is proved by the pyramidal head they have Gas sensing characteristics SnO2 nanorods 14 SnO2 nanorods 14 SnO2 nanoflowers SnO2 nanoflowers SnO2 microspheres SnO2 microspheres 12 Response (S = Ra / Rg) Response (S = Ra / Rg) 12 780 ppm C2H5OH 10 10000ppm LPG 10 0 260 280 300 320 340 360 380 400 260 o 280 300 320 340 360 380 400 o Temperature ( C) Temperature ( C) Figure 36 Influence of sensor response to operating temperature for microspheres, nanorods and nanoflowers at 780 ppm C2H5OH and 10000 ppm LPG C2H5OH (780 ppm) and LPG (10000 ppm) have been used for sensing characterization purpose The range of operating temperature for this experience is chosen from 260 to 400 oC Figure 36 presents the influence of sensor response to operating temperature for nanorods (S.9), nanoflowers (S.4) and microspheres (S.11) According to the left chart, the sensor response of sample S.9 and S.4 elevate from 260 oC to 360 oC, peak at 370 oC (just over 10 for S.4 and just under 14 for S.9) and suddenly drop to approximately for S.9 and 7.9 for S.4 at 400 oC, whereas the response of S.11 slightly fluctuates around 2.9 from 260 oC to 400 oC For LPG sensing results, it can be easily seen that samples S.11 and S.4 seem to be constant while exposing to LPG SnO2 nanorod (S.9), inversely, introduces good response to LPG (7.8 at 300 oC) and hit a peak (approximately 14) at 370 oC Therefore, the sensitivity of SnO2 nanorods is better than SnO2 nanoparticles because for the particle-like, the highest response reached nearly at 300 oC and roughly at 370 oC [20] Master thesis 49 Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application SnO2 nanorods 14 14 Target gas: LPG SnO2 nanoflowers SnO2 microspheres 12 Target gas: Ethanol 10 Response (S = Ra / Rg) Response (S = Ra / Rg) 12 o T = 370 C o T = 370 C SnO2 nanorods 10 SnO2 nanoflowers SnO2 microspheres 100 200 300 400 500 600 700 800 2000 4000 6000 8000 10000 Concentration (ppm) Concentration (ppm) Figure 37 Influence of sensor response to gases concentration for microspheres, nanorods and nanoflowers at 370 oC The most triking feature of these results is the selectivity of SnO2 nanoflowers to C2H5OH because when exposing to 10000 ppm LPG, the response of flower sample hits the highest value of 1.9 at 370 oC whilst that value for 780 ppm C2H5OH is up to 10 o Response (S = Ra / Rg) 12 14 o T = 260 C o T = 330 C o T = 390 C T = 300 C o T = 370 C o T = 400 C 12 Response (S = Ra / Rg) 14 Target gas: Ethanol 10 10 Target gas: LPG T = 260 T = 370 T = 300 T = 390 T = 330 T = 400 100 200 300 400 500 600 700 2000 800 Concentration (ppm) 4000 6000 8000 10000 Concentration (ppm) Figure 38 Influence of sensor response to gases concentration for nanorods at different temperatures A survey of sensor response to gases concentration is carried out The chosen ranges are from 150 ppm to 780 ppm for C2H5OH and from 2000 ppm to 10000 ppm for LPG For this test, optimum temperature (370 oC) is used and results is drawn as line charts in figure 37 The responses for sample S.9 reach the bottom of roughly 4.3 at 150 ppm C2H5OH and 10.2 at 2000 ppm LPG Contrast to the response of rod-like and flower-like, SnO2 microspheres vary insignificantly to the Master thesis 50 Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application increment of ethanol as well as LPG concentration Since the Sphere-like and flower-like shows the weaker gas sensing properties, nanorods have been chosen for a test about the influence of sensor response to gases concentration at different working temperatures (figure 38) In general, the variation of the sensor response seems to be a nearly function to the concentration of gases It can be also seen that the optimum working temperature is 370 oC which is followed by 330 oC, 300 oC and 390 oC for ethanol test For LPG test, the optimum temperature is 370 oC following by 390 oC, 330 oC and 400 oC SnO2 nanorods 2000 ppm 0.37% LPG 4x10 Target gas: LPG SnO2 nanoflowers T = 370 C 3750 ppm 5000 ppm 7500 ppm 10 10000 ppm 20 40 60 80 100 120 o T = 370 C 3,5x10 Resistance () Resistance () 10 3x10 3750 ppm 5000 ppm 7500 ppm 2,5x10 10000 ppm 2x10 140 160 20 40 60 80 Time (second) 100 120 140 160 180 200 220 Time (second) 2000 ppm 0.37% LPG Target gas: LPG 2000 0.37% ppm LPG o SnO2 microspheres 1,2x10 Target gas: LPG o T = 370 C Resistance () 10 3750 ppm 8x10 5000 ppm 7500 ppm 10000 ppm 6x10 20 40 60 80 100 120 140 160 180 200 Time (second) Figure 39 Response-recovery curves of SnO2 nanorods, nanoflowers and microspheres to LPG at 370 oC The response-recovery curves of SnO2 nanorods (S.9), nanoflowers (S.4) and microspheres (S.11) while exposing to LPG are shown in figure 39 As can be seen in the figure, the samples have good response and recover behavior as LPG is pulled in and out because the air resistances (Ra) seem to be constant before and Master thesis 51 Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application after the appearance of LPG Besides, the response time of the rods is approximately seconds and that is followed by the response time of the flowers (just under 25 seconds) and spheres (just above 30 seconds) In additions, the rods possess the fastest recovery time (nearly 48 seconds) whilst comparing with the flowers (just above 70 seconds) and spheres (just above 100 seconds) In short, the gas sensing experience once again proves the enhancement of SnO2 nanorods, nanoflowers to SnO2 nanoparticles Interestingly, the ethanol and LPG testing results indicates that SnO2 nanoflowers sample reacts selectively to ethanol whereas the small rods owning good transducer function presents better sensitivity while comparing with SnO2 nanoparticles Master thesis 52 Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application CONCLUSION In summary, SnO2 nanorods (5-7 nm diameters and 10-30 nm lengths), nanoflowers (Constructed of nanorods with the diameter is from 40 to approximately 200 nm and the length is from 150 nm to roughly µm) and microspheres (from several nanometers to approximately micrometers diameter) were successfully synthesized by a low temperature (below 200 oC) process using hydrothermal method Also, the striking feature of the process comes from the uniform and high density all over the powders of the as-prepared materials Besides the synthesis results, the influence of hydrothermal time, temperature and SnCl4.5H2O weight to the formation of SnO2 nanomaterial has been conducted In the formation period, hypothetically, the spheres were made up by the isotropic aggregation (in case the hydrothermal temperature, “T” is below 190 oC or hydrothermal time “t” is under 20 hours) of SnO2 crystals, nuclei and clusters, whereas the rods and flowers were together constructed via crystallization process (T ≥ 190 oC and t ≥ 20 hours) The XRD, FE-SEM and TEM results indicate that the anisotropic of SnO2 flowers is higher than nanorods and is followed by SnO2 microspheres This may result in the better selectivity toward ethanol of SnO2 nanoflowers while comparing to other morphologies In addition, the gas testing result again proves the enhancement of LPG and ethanol sensing properties for SnO2 nanorods to nanoparticles Main results of the thesis have been used to write a paper entitled “Synthesis of SnO2 micro-spheres, nano-rods and nano-flowers via simple hydrothermal route” which has been accepted by Physica E Master thesis 53 Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application FUTURE PLAN The studies of SnO2 nanorods using hydrothermal treatment have been worth, recently because hydrothermal seem to be the best method for mass synthesis 1-D structure or nanomaterial at low temperature with good uniform throughout the sample Besides, the formation mechanism of nanomaterial during hydrothermal process is a debate now so researches about that problem are playing an important role not only in human knowledge but also in practical This study concentrates on the synthesis and discussion about the possible growth mechanism of SnO2 nanorods as well as nanoflowers and microspheres However, it is necessary to carry out another experiment using the literature I propose a study named “Control the growth position of SnO2 nanorods/nanowires using hydrothermal treatment method” which base on the formation mechanism to grow SnO2 nanorods or nanowires at desire positions on a substrate Figure 40 Proposed route In order to grow SnO2 nanorods at desire position on a substrate, I propose a solution which combines three mentioned routes above using sol suspension (or powder) of SnO2 nanoparticles, SnCl4.5H2O, NaOH, CTAB, silicon substrate and pure ethanol as precursors A typical process can be illustrated in figure 40 The route depends mainly on the second route with support of CTAB The possible growth mechanism of SnO2 nanorods can be introduced as follow: First, SnO2 crystals are formulated by two follow reactions: SnCl4 + 6NaOH → Na2Sn(OH)6 + 4NaCl and Na2Sn(OH)6 → SnO2 + 2NaOH + 2H2O In the conditions of high temperature and high pressure of hydrothermal process, the crystals collide with others to form bigger crystals or clusters Once the dimensions of clusters are large Master thesis 54 Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application enough (probably nm, equal to Debye length of SnO2) to be stable, those clusters become nuclei It is the nucleation stage Second, while the hydrothermal temperature is high enough to make the vapor pressure of the mixture become super saturation, a “driving force” which leads the SnO2 crystals to land on priority faces of nuclei may appears in situ For SnO 2, the priority faces are illustrated as follow: (110) < (100) < (101) < (001) [2] Pure ethanol, in this process, is used to increase average vapor pressure in the same conditions while comparing with deionized water In addition, pure ethanol does not react with such precursors mentioned above If the process is taken place normally, nevertheless, many vacancy defects, which may occur inside the structure, result in the break of the rods during the crystallization Therefore, CTAB is used to decrease not only the surface tension of the mixture leading the landing process of SnO2 crystals onto nuclei much easier but also the defects so as to create longer rods Figure 41 Process of depositing SnO2 nanoparticles on a substrate In short, it is possibility to grow SnO2 nanorods or nanowires at desire positions once SnO2 nuclei can be located at those positions Indeed, by using spin coating technique to deposit SnO2 nanoparticles onto a substrate (silicon or aluminum oxide substrate) and photolithography technique to create a mask, the SnO2 nanoparticles can be put on substrate at definite locations (figure 41) Master thesis 55 Vu Xuan Hien Synthesis of one-dimensional SnO2 nanostructure via hydrothermal method for gas sensor application REFERENCES [1] Barsan N., Schweizer-Berberich M., Gopel W (1999), Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: a status report, Fresenius J Anal Chem 365, pp 287-304 [2] Batzill M., Diebold U (2005), The surface and materials science of tin oxide, Progress in Surface Science 79, pp 47-154 [3] Brattain W.H., Bardeen J (1953), Surface properties of germanium, Bell Syst Tech J 32, pp 1-41 [4] Centi G., Cornils B., Hermann W.A., Schlögel R., Wong C.H (2000), Catalysis from A to Z, Wiley-VCH, Weinheim [5] Chappel S., Zaban A 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Vu Xuan Hien Synthesis of one- dimensional SnO2 nanostructure via hydrothermal method for gas sensor application this method 2.4 Methods for synthesis of SnO2 nanorods As it is mentioned above,... Hien Synthesis of one- dimensional SnO2 nanostructure via hydrothermal method for gas sensor application II EXPERIMENT Synthesis of SnO2 nanorods Figure 23 Process for preparing SnO2 nanorods by hydrothermal. .. Xuan Hien Synthesis of one- dimensional SnO2 nanostructure via hydrothermal method for gas sensor application Figure 18 Process of synthesizing SnO2 nanorods via molten salt method  Mix SnO2 powder