controlled synthesis of 1d zno nanostructures via hydrothermal

Controlled synthesis of 1D ZnO nanostructures via hydrothermal process Di Liu, Yanfang Liu, Ruilong Zong, Xiaojuan Bai, Yongfa Zhu * Department of Chemistry, Tsinghua University, Beijing 100084, PR China 1. Introduction ZnO is a potentially useful semiconductor with a direct band- gap of 3.37 eV and can be used as photocatalyst to destroy the organic pollutants. In some cases, ZnO may exhibit a better efficiency than TiO 2 in photo-catalytic degradation for some dyes [1]. Because the electrical, optical and some other properties of nanocrystals were governed by the sizes and shapes [2], much works have been done to fabricate ZnO nanostructures of controlled shape and size. Generally, in comparison to 0D nanostructures, 1D nanostructures have less grain boundary, surface defect, dislocation etc., thus leading to more effective carrier transport [3]. Therefore, the methods to fabricate one- dimensional (1D) ZnO such as nanowires and nanorods have received the most attention, for example, the high-temperature physical evaporation [4], the template-induced method [5–7], hydrothermal synthesis [8–11], reverse micelle [12], biominerali- zation method [13], precursors induced solution phase method [14], direct calcination of zinc acetate [15] were used. Among all these methods, the simple solution synthesis, by thermal treatment of the reactant in different solvents, may be the most simple and effective way to prepare sufficiently crystallized materials at relatively low temperatures [16–18]. Therefore, it is necessary to elucidate the relationship between reaction condi- tions and ZnO morphologies and explore an effective and simple route to control the morphology of ZnO for practical applications. This work focuses on the synthesis of 1D ZnO with the hexagonal system wurtzite-type structure. A wide range of ZnO nanostructures with various shapes were obtained by adjusting synthetic parameters such as precursor concentration, reaction time, solvent, reaction temperature, annexing agent, alkali source. The mechanisms of reaction of the formation of ZnO with various morphology and structure are investigated respectively. 2. Materials and methods 2.1. Chemicals All chemicals (analytical-grade reagents) were purchased from Beijing Chemicals Co. Ltd. and used without further purification. 2.2. The preparation of ZnO micro/nanostructures ZnO nanostructures were synthesized by using a simple hydrothermal method. A certain amount of zinc nitrate hexahy- drate (Zn(NO 3 ) 2 Á6H 2 O) and some corresponding amount of alkali were mixed with solvent composed of distilled water and ethanol (v/v = 1:2). It is noteworthy that the solvents for samples in Fig. 3 are water-methanol (v/v = 1:2), water-alcohol (v/v = 1:2), water-n- butyl alcohol (v/v = 1:2) respectively. The mixture was stirred for two hours. Then a certain amount of annexing agent (CTAB/ PEG400) and were transferred into the mixture under stirring. Materials Research Bulletin 49 (2014) 665–671 A R T I C L E I N F O Article history: Received 13 December 2012 Received in revised form 19 August 2013 Accepted 29 September 2013 Available online 17 October 2013 Keywords: A. Nanostructures A. Semiconductors B. Chemical synthesis B. Crystal growth A B S T R A C T ZnO nanostructures with various morphologies and sizes were successfully prepared via a hydrothermal method. The influence of precursor concentration, reaction time, solvent, reaction temperature, annexing agent and alkali source on the morphology and structure was elucidated systematically. The preferential growth of ZnO crystal along C-axis direction was obvious when the precursor concentration was low. A balance between growth and dissolve existed during the process of forming ZnO. The solvent of water-methanol would make for the generation of products with a big size and regular morphology. The adsorption of CTA + from the additive of cetyltrimethyl ammonium bromide (CTAB) and hydroxyl groups from polyethylene glycol (PEG400) on ZnO would slow down the preferential growth rate along C-axis direction, leading to rod-like products with smaller aspect ratios. When the concentration of NaOH was increased to a certain degree, the growth rate of ZnO was slower than the decomposition rate, leading to appearance of irregular grain-like products. ß 2013 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +86 10 62787601; fax: +86 10 62787601. E-mail address: zhuyf@mail.tsinghua.edu.cn (Y. Zhu). Contents lists available at ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.09.046 Next, the solution mixture was pretreated under an ultrasonic water bath for 30 min and then was hydrothermally treated at a given temperature for certain time in a Teflon-lined autoclave. After the reaction, the white precipitate was collected and washed several times with distilled water and pure ethanol. Finally, ZnO were obtained after dried at 80 8C for 8 h. A series of condition-dependent experiments were carried out to investigate the growth process of ZnO. Ultrasonic pretreatment can promote the generation of a certain number of active nucleus, so all experiments in this work adopted the ultrasonic pretreat- ment operation so as to accelerate reaction process [11,19]. The reaction parameters for each series were summarized in Table 1. 2.3. Characterizations The morphologies and dimensions of the as-synthesized ZnO nanostructures were observed with transmission electron micros- copy (TEM) by a JEOL JEM 2100 electron microscope operated at an accelerating voltage of 100 kV. 2.4. Photocatalytic experiments The photocatalytic performance of the as-prepared samples was evaluated by photocatalytic degradation of methylene blue (MB) under UV light irradiation. The samples (25 mg) were dispersed in the 50 ml MB aqueous solution (3 Â 10 À5 M). The mixed suspension was magnetically stirred for 0.5 h in the dark to reach an adsorption–desorption equilibrium. Under ambient conditions and stirring, the mixed suspensions were exposed to UV irradiation produced by a 300 W high pressure Hg lamp with the main wave crest at 365 nm. At certain time intervals, 2 ml of the mixed suspension were extracted and centrifugated to remove the photocatalyst. The degradation process was monitored by measuring the absorption of MB in filtrate at 664 nm using UV–vis absorption spectrometer. 3. Results and discussion ZnO nanostructures with various sizes and morphologies were obtained with different precursor concentration, reaction time, solvent, reaction temperature, annexing agent, alkali source etc. The detailed effects of these conditions are shown in the following paragraphs. Determining these effects will be helpful to further extend the scope of the hydrothermal synthesis method employed in this work. 3.1. Effects of precursor concentration Fig. 1 shows the TEM images of the ZnO prepared with different precursor concentrations. When the precursor concentration was high (>0.0167 M), the product had a flake-like structure (Fig. 1a and b). Long spindle-like products formed when the precursor concentration decreased below 0.0067 M (Fig. 1c and d). The diameters were in the range of 180–370 nm and 140–300 nm, and the aspect ratios were in the range of 6–13 and 12–15 for the products prepared at 0.0067 M and 0.001 M, respectively. It is considered that the crystal prefers lateral growth when the zinc source is abundant, leading to the appearance of flake-like structure. The formation of the spindle-like structure can be attributed to the reactant shortage and subsequent limited growth during the late period of the reaction. Because the preferential growth of ZnO crystal is along C-axis direction, the (0 0 0 1) crystal plane which has a rapid growth rate would be covered gradually, forming a cuspate shape structure. With the concentration further reduced, diameter of rods decreased and the aspect ratio increased. The above results show that when the precursor concentration is low, the preferential growth of ZnO crystal along C-axis direction is obvious. At this time, because of the low concentration of reactants, the reaction rate is slow and the distinction of the probability that growth units combine to each crystal plane is also significant. Then the crystal grain presents a polar growth trend, leading to 1D morphology. Our proposition is consistent with the previous conclusion [20]. At the same time, due to the strict monomer diffusion, the crystal growth rate is restricted, thus the product size is much smaller. 3.2. Effects of reaction time TEM images of ZnO obtained with different reaction time are shown in Fig. 2. There exist some short columnar products at the initial stage of the reaction (1 h). Their average diameters are in the range of 180–300 nm. It’s important to note that some hexagon thin slices formed within 1 h. An hour later, the hexagon thin slices began to deform, the short columnar structure coarsened and lengthened at the same time. The aspect ratio increased a little since it grew faster in the length direction than in the diameter. The hexagon thin slice seemed to dissolve away after 5 h. Then the hexagon slices gradually disappeared when the reaction time is prolonged and the diameters of nanorods increased as the reaction time increased from 5 h to 16 h. Based on Fig. 2, it is concluded that there exists a balance of growth-dissolve during the formation process of ZnO. It is considered that the crystal prefers lateral growth at the initial stage of the reaction due to the abundant zinc source, leading to the appearance of the complete hexagon section slice. As reaction time increases, the hexagon thin slices gradually dissolve and then disappear, the short columnar structure further coarsens, whereas the aspect ratio slowly increases. 3.3. Effects of solvent All products shown in Fig. 3 present flake-like shape when the adding amount of additive (PEG400) was set at 5 ml and the precursor concentration was 0.067 M. ZnO prepared in the solvent of water-methanol has a relatively regular morphology which is close to rectangle shape with the shorter length of about 500 nm. However, some small irregular pieces with average length of several dozens of nanometers and smaller length–width ratios were obtained when the solvent changed. These observations demonstrate that solvent plays an important role in the morphology of product. This may be caused by the different Table 1 The experimental conditions of ZnO nanostructure preparation. Designation n(Zn 2+ ):n(HMT) C(Zn 2+ ) Solvent (V H2O :V ethanol ) Reaction temperature (8C) Reaction time (h) Samples-Fig1 1:1 1:2 100 24 Samples-Fig2 1:1 0.0067 M 1:2 100 Samples-Fig3 1:1 0.067 M 150 24 Samples-Fig4 1:1 0.0067 M 1:2 16 Samples-Fig5/6 1:1 0.0067 M 1:2 100 16 Samples-Fig7 0.050 M 1:2 150 16 D. Liu et al. / Materials Research Bulletin 49 (2014) 665–671 666 Fig. 1. TEM images of ZnO with different precursor concentrations: (a) 0.067 M; (b) 0.0167 M; (c) 0.0067 M and (d) 0.001 M. Fig. 2. TEM images of ZnO samples with different reaction time: (a) and (b) 1 h; (c) and (d) 2 h; (e) 5 h; (f) 8 h; (g) 12 h and (h) 16 h. D. Liu et al. / Materials Research Bulletin 49 (2014) 665–671 667 properties of each solvent such as polarity of solvent, saturated vapor pressure, adhesion coefficient, dissolving capacity to salt or metal ions, solvability and so forth [18]. Firstly, the solvation of PEG400 is obvious when water-methanol is used as solvent because of its strong polarity. When the PEG400 molecules are surrounded by solvent molecules, their adsorption on the surface of ZnO grain will decrease. In addition, the solute is difficult to disperse, thus large sheet-like products with complete morphol- ogies are easy to form in relatively concentrated solute. Secondly, methanol has a low boiling point, and the reaction temperature in this experiment has reached its boiling point. As a result, the collision and coalescence of nucleuses are significant in the violently boiling solution, leading to product with large size. On the contrary, ethanol and n-butyl alcohol both have lower polarity and higher boiling points compared to methanol, thus at the initial stage of nucleation when the temperature is below the boiling point, the collision and coalescence of nucleuses and the solvation of PEG400 are not strong and the dispersion of solute is good, thus well-scattered small pieces were obtained. 3.4. Effects of reaction temperature In Fig. 4, the aspect ratio of ZnO obtained at different reaction temperature has little change. However, the aspect ratio of product decreases and the quantity of long spindle-like product decreases at lower temperature, which indicates that the growth rate along C-axis direction weakens. It is also can be observed that when the reaction temperature is reduced, the hydrolysis velocity (that is the rate of releasing OH À ) of hexamethylenetetramine (HMT) slows down, the property of crystal and growth integrity of product both weaken. 3.5. Effects of annexing agent Scheme 1 is a schematic procedure to prepare ZnO nanos- tructures in the presence of annexing agent. As shown in Scheme 1, CTA + and hydroxyl groups coming from the additive of CTAB and PEG400 respectively can adsorb on the surface of ZnO crystal grain through the electrostatic attraction effect with the growth units or zinc ion in solution. However, based on the results reported before, the growth rate of each crystal plane of ZnO is different [21]. The more active the crystal plane is, the more growth units or zinc ions it would grasp. As a result, the clapping role acted by CTAB or PEG400 molecules realizes the regulation of crystal growth. Figs. 5 and 6 shows TEM micrographs of the ZnO crystals grown in the aqueous solutions with certain amount of additive. As shown in Fig. 5, all products prepared with or without the additive of CTAB exhibit rod-like structure. It’s worth noting that the aspect ratio of the product prepared free of additive is about 6–13. However, the aspect ratio decreases to 2–5 after adding some amount of CTAB. At the same time, when the adding amount is more than 0.2 mmol, the diameters of the products increased to 1 m m. CTAB is a cationic surfactant which can be completely hydrolyzed in water or ethanol. The cation generated from hydrolysis has a tetragonal structure with a hydrophobic long chain tail, therefore there is an ion pair of the growth elementary Zn(OH) 4 2À and CTA + . Thus it is Fig. 4. TEM images of ZnO prepared at different reaction temperature: (a) and (b) 100 8C; (c) and (d) 80 8C. Fig. 3. TEM images of ZnO prepared in different solvent: (a) water-methanol; (b) water-alcohol; (c) water-n-butyl alcohol. D. Liu et al. / Materials Research Bulletin 49 (2014) 665–671 668 considered that the surfactant molecules, which can be adsorbed on the surface of zinc oxide during the crystallization process, have the following two roles: structural directing agent and protectant for preventing product from gathering. CTAB may also act as the transmission medium or the modifying agent for the small crystal nucleus in the initial solution. The surfactant molecules adsorbed on the surface of crystal nucleus can be seen as links for small crystal nucleus gathering into crystal nucleus cluster [22]. As can be seen from Fig. 5(b–e), CTAB plays a role of limiting domain, and the CTA + is much easier to adsorb on the active (0 0 0 1) crystal plane due to the electrostatic interaction as shown in Scheme 1(a). Thus the preferential growth rate along C-axis direction is weakened to some extent, leading to relatively uniform growth rates of the crystal planes. Fig. 6 shows that the diameters of products are in the range of 150–800 nm when the adding amount of PEG400 changes from Scheme 1. Schematic illustration of the proposed formation mechanisms of ZnO. Fig. 5. TEM images ZnO prepared with different adding volume of CTAB: (a) 0; (b) 0.02 mmol; (c) 0.1 mmol; (d) 0.2 mmol and (e) 0.4 mmol. D. Liu et al. / Materials Research Bulletin 49 (2014) 665–671 669 0 ml to 2 ml. However, when the adding amount is increased to 5 ml, the diameter of the product increases to several microns and the aspect ratio decreases obviously. It is also found that with the adding amount of PEG400 increased, long spindle-like structure gradually reduces, quite to the opposite, long columnar structure increases. As an organic polymer with a long non-polar carbon chain, PEG is often used as surfactant to control the growth of nanocrystals. The atom O in the entire long PEG400 molecule has coordination abilities with metal ions, thus it has a relatively strong electrostatic attraction to Zn 2+ . Then the adsorption of PEG400 on the surface of ZnO crystal grain would make the activity of ZnO particles greatly reduced [9]. The corresponding illustration can be seen in Scheme 1(b). It can be seen in general that PEG400 has a similar function as CTAB which is different only in the ways of adsorption because of their different molecular structures. Solvation effect caused by pure water will weaken the adsorption of PEG400 molecules on the surface of ZnO crystal grain. Thus, water-ethanol (V:V = 1:2) was selected as solvent to avoid the solvation effect. 3.6. Effects of alkali source TEM micrographs of ZnO prepared with different concentration of NaOH are shown in Fig. 7.With the molar ratio of Zn 2+ to NaOH Fig. 6. TEM images of ZnO with different adding volume of PEG400: (a) 0; (b) 0.2 ml; (c) 1 ml; (d) 2 ml and (e) 5 ml. Fig. 7. TEM images of ZnO prepared with different concentration of NaOH: (a) 0.25 M (Zn 2+ :NaOH = 1:5); (b) 0.4 M (Zn 2+ :NaOH = 1:8) and (c) 0.6 M (Zn 2+ :NaOH = 1:12). D. Liu et al. / Materials Research Bulletin 49 (2014) 665–671 670 increased from 1:5 to 1:12, the morphology of product turns gradually from rod-like to grain-like shape. Based on the crystal growth characteristics of ZnO, the product presents column-like morphology. When the pH rises, the supersaturation of solution which can affect the size of critical nucleus is bigger, thus making the size of crystal nucleus much smaller at the stage of nucleation and leading to the formation of products with small size. ZnO is a kind of amphoteric oxide, so when the concentration of NaOH is high, the growth rate of ZnO may be smaller than the decomposi- tion rate [23]. Thus the products prepared with high concentration of NaOH present irregular grain-like structure due to the larger decomposition rate of (0 0 0 1) crystal plane. The growth of crystal experiences a process of dissolution-crystallization. 3.7. Photocatalytic activity It is well known that ZnO has been used as a semiconductor photocatalyst for the degradation of pollutants. The photocatalytic activity of samples in Fig. 1 in the degradation of well-known organic dye MB was presented in Fig. 8. The photocatalytic properties of samples in Fig. 1, from Fig. 1a to d, show a trend which is decreased at first and then increased. From the comparison between Fig. 1a and b, it can be seen that the former has a better crystalline performance which also means that it would have less bulk defects. The bulk defects are usually seen as recombination centers of photo-generated electrons and holes. Thus, Fig. 1b is supposed to have a higher probability of the recombination of the photo-generated electron/hole pairs in comparison to sample a, resulting in a lower photocatalytic activity. Fig. 1c and d with smaller crystal sizes and larger specific surface area were obtained by further reduce the concentration of reactants. The large surface area supplies more active sites to adsorb MB, and then facilitates the diffusion and mass transportation of MB molecules and hydroxyl radicals during the photochemical reaction. Moreover, the structures of nanoscale favor the movement or transfer of electrons and holes generated inside the crystal to the surface [24], which also helps to enhance the photocatalytic activity of Fig. 1c and d to some degree. The photocatalytic activity of ZnO is also strongly dependent on the surface orientation of the nanocrystals which could result in orientation-dependent charge-transfer processes [25,26]. Thus, as for the hexagonal ZnO nanorods (Fig. 1c and d), their regular surface orientation with larger emergences of {1 0 1 0} and {0 0 0 1} surfaces would be propitious to the separation of the photo-induced electrons and holes, leading to relatively higher photocatalytic efficiencies. However, in comparison to Fig. 1c and d has a smaller size and a larger specific surface area which result in a higher photocatalytic performance. 4. Conclusions Summarizing, ZnO nanostructures with various well-defined morphologies, such as columnar-, long spindle-, flake- and grain- like samples have been successfully synthesized by adjusting synthetic parameters. A possible mechanism of the growth process of the ZnO nanostructures under different conditions has been elucidated systematically. The controllable fabrication also pro- vides a chance for studying the morphology-dependent properties of ZnO micro/nanocrystals. Besides, a growth mechanism of the influence of surfactants on the growth of crystal was discussed in detail. Acknowledgments This work was partly supported by the Chinese National Science Foundation (20925725, 21373121) and National Basic Research Program of China (2013CB632403) and National High Technology Research and Development Program of China (2012AA062701) and Special Project on Innovative Method from the Ministry of Science and Technology of China (2009IM030500). References [1] B. Dindar, S. Icli, J. 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Photocatalytic degradation of MB in the presence of (A) Sample-Fig. 1a, (B) Sample-Fig. 1b, (C) Sample-Fig. 1c, (D) Sample-Fig. 1d. C0 and C represent initial MB concentration and evolution of MB concentration during photodegradation, respectively. D. Liu et al. / Materials Research Bulletin 49 (2014) 665–671 671 . Controlled synthesis of 1D ZnO nanostructures via hydrothermal process Di Liu, Yanfang Liu, Ruilong Zong, Xiaojuan Bai, Yongfa Zhu * Department of Chemistry, . The preparation of ZnO micro /nanostructures ZnO nanostructures were synthesized by using a simple hydrothermal method. A certain amount of zinc nitrate hexahy- drate . applications. This work focuses on the synthesis of 1D ZnO with the hexagonal system wurtzite-type structure. A wide range of ZnO nanostructures with various shapes were