facile synthesis of zno micro-nanostructures with controllable morphology and

Applied Surface Science 261 (2012) 759– 763 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Facile synthesis of ZnO micro-nanostructures with controllable morphology and their applications in dye-sensitized solar cells Yi Zhou a,∗ , Dang Li a , Xiangchao Zhang b , Jianlin Chen a , Shiying Zhang b a Department of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410114, China b Department of Science and Technology, Changsha University, Changsha 410003, China a r t i c l e i n f o Article history: Received 11 May 2012 Received in revised form 31 July 2012 Accepted 31 July 2012 Available online 31 August 2012 Keywords: ZnO Micro-nanostructures Urchin Dye sensitization solar cell Photoelectric properties a b s t r a c t Different morphologies of ZnO micro-nanostructures were successfully prepared by hydrothermal method at relatively mild conditions using ammonia to adjust the pH of the reaction system. The samples were characterized by X-ray powder diffraction, scanning electron microscopy, optical reflectance spec- tra, and photocurrent–voltage curve. The results demonstrated that the morphologies of ZnO changed from “wire” to “flower”, “urchin” and “wire” with increase in the pH of the reaction system due to the increased concentration of ammonia. The diffused reflectance spectra illustrated that the reflectance of denser urchin-like ZnO was low at 18% in the visible region. When the as-synthesized ZnO micro- nanostructures were used as the anode of the dye sensitization solar cell, the denser urchin-like ZnO exhibited the best photoelectric properties. The short circuit current (J sc ), open circuit voltage (V oc ), and conversion efficiency (Á) were 6.50 mA/cm 2 , 0.682 V, and 1.92%, respectively. © 2012 Elsevier B.V. All rights reserved. 1. Introduction As an important low-cost semiconductor functional mate- rial with large band gap (3.37 eV) and large excitation binding energy (60 meV) [1], zinc oxide (ZnO) is recognized as one of the most promising materials for optoelectronic applications. Micro-nanostructured ZnO has drawn considerable attention due to its unique electrical, mechanical, and optical properties, in addition to its applications in numerous fields, such as solar cells [2], gas sensors [3,4], piezoelectric materials [5], pho- tonic crystals [6], and optoelectric devices [7]. Previous studies demonstrated that the properties of ZnO are closely related to the size and shape of the structures. For example, tetra- pod ZnO nanostructures exhibit strong UV emission [8] and needle-like ZnO arrays exhibit strong blue light emission [9]. Thus, studying the morphology of micro-nanostructured ZnO is important. ZnO micro-nanostructures have been synthesized with various methods, such as chemical vapour deposition [10], template-based method [11], laser ablation [12], spray pyrolysis technique [13], hydrothermal method [14], and electrodeposition method [15]. Researchers have prepared micro-nanostructured ZnO with dif- ferent morphologies using these different methods. For example, Polsongkram et al. [14] prepared ZnO nanorods by hydrothermal ∗ Corresponding author. Tel.: +86 731 85258328; fax: +86 731 85258328. E-mail addresses: zhouyihn@163.com, zhouyihn@yahoo.com.cn (Y. Zhou). method. Chen et al. [16] synthesized ZnO nanotubes by a sonochemical method at low temperature. Liu and Zeng [17] fabricated ZnO dandelions by a modified Kirkendall process. Jana et al. [18] prepared water lily-type ZnO flowers by a simple solution method, and Elias et al. [19] prepared hollow urchin-like ZnO thin films by electrochemical deposition. How- ever, majority of these studies are limited to the research of one kind of morphology. Few studies have reported on ZnO micro-nanostructures with different morphologies using a single method. In the present study, several types of ZnO micro-nanostructures with different morphologies and photoelectric properties were prepared by a simple hydrothermal method at relatively mild conditions. The concentration of ammonia, which can adjust the pH of the reaction system, was controlled. Subsequently, the influence of pH on the photoelectric properties of the ZnO micro-nanostructures was investigated by studying the photocurrent–voltage (I–V) characteristics of the dye-sensitized solar cell (DSSC). 2. Experimental 2.1. Materials All chemicals were of analytical reagent grade and used with- out further purification. All aqueous solutions were prepared using double distilled water. 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.160 760 Y. Zhou et al. / Applied Surface Science 261 (2012) 759– 763 2.2. Preparation of ZnO film ZnO micro-nanostructures were synthesized using a hydro- thermal method. The procedure was as follows: 1.18 g zinc nitrate hexahydrate (Zn(NO 3 ) 2 ·6H 2 O) and 0.56 g hexamethylenetetramine (C 6 H 12 N 4 ) were added to 40 mL double distilled water under strong magnetic stirring at room temperature to obtain a transparent and homogeneous solution. NH 3 (25%) was then dropped into the solu- tion at 60 drops/min to change the pH from 7 to 11. The solution was kept at room temperature for 0.5 h under vigorous stirring to obtain the precursor. The ZnO micro-nanostructures used in this work were grown on FTO-coated glass substrates. First, the FTO-coated glass substrates were successively cleaned in an ultrasonic bath with acetone, ethanol, and double distilled water for 15 min to remove dust and prevent surface contamination. The FTO-coated glass substrates were then dipped in 0.5 mol/L ZnCl 2 aqueous solution for 5 min at room temperature. Finally, the substrates were pulled upward by a hoist with constant speed, and then dried in the air to obtain functionalized FTO-coated glass substrates [20]. The precursor solution and the functionalized substrates were transferred to a Teflon-sealed autoclave. Then the reaction was kept at 90 ◦ C for 9 h to synthesize the ZnO micro-nanostructures. After deposition, the samples were cleaned several times with double distilled water and then dried in the air. 2.3. Construction of dye-sensitized solar cell The construction of DSSC has been reported in a previous research [21]. 2.4. Characterization The samples were characterized using scanning electron microscopy (SEM, JEOL JSM-6700F) and X-ray diffraction patterns were recorded on an X-ray diffraction system (SIEMENS D5000). The diffused reflectance spectra were measured by an IPCE tester (Solar Cell Scan 100, Beijing Zhuo Li Han Guang). The I–V charac- teristics were measured using a computer-controlled digital source meter (Keithley, Model 2400) under the illumination of a Newport solar simulator (AM 1.5, 100 mW/cm 2 ). 3. Results and discussion 3.1. Morphology and structural analyses Fig. 1 depicts the SEM images of micro-nanostructured ZnO grown under different pH, namely, 7, 8, 9, 10 and 11. The growth temperature and time were 90 ◦ C and 9 h, respectively. As shown in Fig. 1, the morphology of the as-grown micro-nanostructured ZnO was closely related to the pH of the precursor solution. Fig. 1a indicates that ZnO nanowires formed on the substrate when the applied pH was 7. The dense ZnO nanowires with hexagonal structure were vertically well-aligned and uniformly distributed on the substrate. The average diameters of the ZnO nanowires were approximately 30–50 nm; the length–diameter ratios were approximately 6–10. The sample prepared with pH = 8 resulted in the formation of flower-like ZnO whose petals were approximately 500–700 nm in length and 300–400 nm in width (Fig. 1b). When the pH was increased to 9, urchin-like ZnO were formed. Fig. 1c reveals that the urchin-like ZnO was comprised of nanorods, which had similar centers and were approximately 5–6 ␮m in length and 300–500 nm in width. Notably, urchin-like ZnO also formed when the applied pH was controlled at 10 (Fig. 1d). However, this urchin-like ZnO was comprised of needle-like ZnO nanowires, and the sizes and amounts of ZnO nanowires were also different from those in Fig. 1c. When the applied pH was increased to 11, the urchin-like morphologies disappeared and changed to ZnO nanowires with poor orientations. The average diameters of these ZnO nanowires were approximately 50–80 nm, and their average lengths were approximately 500–600 nm. Fig. 1f and g is the lower magnification SEM images of Fig. 1c and d, respectively. The urchin structures were lined by a single layer on the substrate, and all the ZnO nanowires were relatively homoge- neous. Fig. 1h reveals the side view images of micro-nanostructured ZnO at pH = 10. As shown in Fig. 1h, the film thicknesses were about 2 ␮m. Urchin-like ZnO micro-nanostructures were formed on FTO- coated glass substrates by a hydrothermal method. The formation process can be expressed as follows [22]: (CH) 6 N 4 + 6H 2 O → 6HCHO + NH 3 (1) NH 3 + H 2 O → NH 4 + + OH − (2) Zn 2+ + NH 3 → Zn(NH 3 ) 4 2+ (3) Zn 2+ + 4OH − → Zn(OH) 4 2− (4) Zn(NH 3 ) 4 2+ + 2OH − → ZnO + 4NH 3 + H 2 O (5) Zn(OH) 4 2− → ZnO + H 2 O + 2OH − (6) Based on the growth habits of ZnO crystals in aqueous solu- tions, urchin-like ZnO micro-nanostructures can be obtained only when the pH of the bulk solution are controlled at certain values. When the pH is low, the concentrations of OH − and NH 3 in the precursor solution are correspondingly low, leading to the small amount of Zn(OH) 4 2− and Zn(NH 3 ) 4 2+ , which are insufficient to form the nuclei. Therefore, ZnO nanowires form on the substrate when the applied pH is controlled at 7 (Fig. 1a). The amount of Zn(OH) 4 2− and Zn(NH 3 ) 4 2+ increases with the pH. When the pH is above 8, Zn(OH) 4 2− and Zn(NH 3 ) 4 2+ will gather and decompose to ZnO nuclei at the beginning of the reaction. The growth units of Zn(OH) 4 2− and Zn(NH 3 ) 4 2+ are then adsorbed on the nuclei due to intermolecular absorption forces, such as van der Waals interactions, and finally grow to nanowires in all directions to form three-dimensional urchin-like ZnO. The SEM micrographs in Fig. 1 reveal that the morphologies of three-dimensional ZnO are different under different conditions. As the pH increases, the mor- phologies of ZnO change from “flower” (Fig. 1b), to sparse “sea urchin” (Fig. 1c) and denser “sea urchin” (Fig. 1d). The pH has an important function during the formation of the ZnO micro- nanostructures. This can be explained as follows. Ammonia can easily separate from the solution when the pH is high, which results in an increase in the air pressure of the Teflon-sealed autoclave. This will influence the growth of ZnO micro-nanostructures, lead- ing to change in the morphology of ZnO. The pH of the reaction solution increases with the addition of ammonia. At the same time, the growth units are more likely to come in contact with the nuclei. This phenomenon can be propitious for the formation of Zn(OH) 4 2− and Zn(NH 3 ) 4 2+ , finally leading to an increase in the amount of nanowires on the nuclei. However, when the pH increases to a certain degree, the air pressure in the Teflon-sealed autoclave will increase to a greater degree; thus, Zn(OH) 4 2− and Zn(NH 3 ) 4 2+ are unable to gather to form the initial ZnO nuclei. Therefore, the pre- cursor solution is generated for the single and independent ZnO nanowires, hindering them from forming flower- or urchin-like ZnO. The concentration of ammonia, the pH, and the air pressure of the Teflon-sealed autoclave has an important function in the above transformation processes of ZnO morphologies. However, the specific reaction mechanism requires further research. Y. Zhou et al. / Applied Surface Science 261 (2012) 759– 763 761 Fig. 1. SEM images of the micro-nanostructured ZnO under different pH: (a) 7; (b) 8; (c and f) 9; (d and g) 10; and (e) 11. Lower magnification SEM images (f) and (g). Side view images of micro-nanostructured ZnO at pH = 10 (h). 3.2. XRD patterns Fig. 2 shows the XRD spectra of the micro-nanostructured ZnO under different pH. All diffraction peaks can be indexed to a hexag- onal wurtzite phase of ZnO, in agreement with the standard card (JCPDS 78-2486). No characteristic peaks of any impurities, except polycrystalline SnO 2 (from the FTO substrate), were detected in the pattern, confirming that the obtained products are pure ZnO. The characteristic peaks were high in intensity and narrow, which indicated that ZnO micro-nanostructure had high crystallinity. The intensities of the diffraction peaks of micro-nanostructured ZnO were obviously different, indicating that the pH had effect on the crystallinity of grown ZnO micro-nanostructure. 3.3. Optical reflection spectra analyses Fig. 3 shows the optical reflection spectra of the micro- nanostructured ZnO under different pH. Fig. 3 indicates that the light scattering of the ZnO micro-nanostructure is closely related to its morphology. The surface areas of denser urchin- like structure, sparse urchin-like structure, vertically well-aligned nanowire structure, disorderly nanowire structure and flower-like structure were 350, 290, 223, 147, 96 m 2 /g, respectively. And due to the different surface areas of the different morphologies, the order of intensity of the ZnO micro-nanostructure light scatter- ing is as follows: denser urchin-like structure < sparse urchin-like structure < vertically well-aligned nanowire structure < disorderly nanowire structure < flower-like structure. Among these ZnO micro-nanostructures, the reflectance of the denser urchin-like ZnO is the lowest at approximately 18% in the visible region. The reflectance of the flower structure is the highest. This could attribute to the size [23] and the morphology [24] of ZnO, which play important roles for controlling the light scattering. In addition, the graph indicates that the entire ultraviolet absorption spectrum edge is approximately 380 nm, which is in agreement with the direct wide band gap (3.37 eV) of ZnO. 3.4. Application of ZnO micro-nanostructure in DSSC Fig. 4 compares the I–V characteristics of the DSSC based on ZnO with different micro-nanostructures. The corresponding values are summarized in Table 1, which demonstrates photoelectro- chemical characteristics, such as current density at short circuit (J sc ), voltage at open circuit (V oc ), fill factor (FF), and efficiency 762 Y. Zhou et al. / Applied Surface Science 261 (2012) 759– 763 Fig. 2. XRD spectra of the micro-nanostructured ZnO under different pH: (a) 7; (b) 11; (c) 8; (d) 9; and (e) 10. Fig. 3. Optical reflection spectra of the micro-nanostructured ZnO under different pH: (a) 8; (b) 11; (c) 7; (d) 9; and (e) 10. Fig. 4. Photocurrent–voltage characteristics of ZnO with different micro- nanostructure-based DSSCs. (a) Denser urchin-like ZnO; (b) sparse urchin-like ZnO; (c) orderly ZnO nanowire; (d) flower-like ZnO; and (e) disorderly ZnO nanowire. Table 1 Photovoltaic parameters of micro-nanostructured ZnO with different micro- nanostructures. Film type J sc (mA/cm 2 ) V oc (V) FF (%) Á (%) Orderly ZnO nanowire 4.48 0.598 36.7 0.98 Disorderly ZnO nanowire 3.13 0.526 27.3 0.45 Flower-like ZnO 3.93 0.537 33.2 0.70 Sparse urchin-like ZnO 4.82 0.654 38.5 1.21 Denser urchin-like ZnO 6.50 0.682 43.4 1.92 of power conversion (Á) in different samples. Table 1 indicates that the photoelectrochemical characteristics of the DSSC-based denser urchin-like ZnO are high, reaching a maximum value of 1.92%. Notably, the denser urchin-like structure is beneficial for the transfer of electrolytes, and this specific structure can increase the production of carriers and photoelectric activity due to its larger surface area and increased activity centers for absorbing dye. In the urchin-like structure, the needle-like ZnO nanowires have the same center extended to the surroundings. This special structure can effectively improve the efficiency of electron transmission, decreases the transmission path of charge in the electrode materi- als and the recombination of carriers. As a result, the photoelectric activity of ZnO increases. Table 1 further shows that the ZnO nanowires were formed on the substrate when the applied pH was controlled at 7 and 11, but the photoelectric parameters of these two different nanowires varied enormously. Due to their excellent orientation, the ZnO nanowires prepared at pH = 7 can provide an effective transmis- sion path for electrons, reduce the recombination of carriers, and increase the photoelectric activity of ZnO. 4. Conclusions Several kinds of ZnO micro-nanostructures with different mor- phologies and photoelectric properties have been prepared by a simple hydrothermal method at relatively mild conditions. The pH is essential in the growth of ZnO, and results in the mor- phology of ZnO changing from “wire” to “flower”, “urchin” and “wire” with the addition of different amounts of ammonia. Due to the different surface areas of the various morphologies, the order of intensity of the ZnO micro-nanostructure light scatter- ing is as follows: denser urchin-like structure < sparse urchin-like Y. Zhou et al. / Applied Surface Science 261 (2012) 759– 763 763 structure < vertically well-aligned nanowire structure < disorderly nanowire structure < flower-like structure. Among the foregoing, the reflectivity of the denser urchin-like structure was the lowest at 18%. When the obtained ZnO micro-nanostructures were used as the anode of the DSSC, the photoelectrochemical characteristics of the DSSC based on ZnO with different micro-nanostructures vary. The denser urchin-like ZnO micro-nanostructures display excellent photoelectric properties due to their larger surface area, increased activity centers, and more effective transmission paths. Acknowledgments This work was supported by the National Natural Science Foundation of China (grant no. 21171027). The authors are also grateful to the aid provided by the Science and Technology Inno- vative Research Team in Higher Educational Institutions of Hunan Province. References [1] J.R. Jennings, L.M. Peter, A reappraisal of the electron diffusion length in solid- state dye-sensitized solar cells, J. Phys. Chem. C 111 (2007) 16100–16104. [2] B.N. Pawar, G. Cai, D. Hama, R.S. Mane, T. Ganesh, A. Ghule, R. 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