hydrothermally synthesized wo3 nanowire arrays with highly improved

Hydrothermally synthesized WO 3 nanowire arrays with highly improved electrochromic performance† Jun Zhang, Jiang-ping Tu, * Xin-hui Xia, Xiu-li Wang and Chang-dong Gu Received 14th December 2010, Accepted 11th February 2011 DOI: 10.1039/c0jm04361c A hexagonal WO 3 nanowire array film is obtained using a template-free hydrothermal method by adding ammonium sulfate as a capping agent. The WO 3 nanowires grown vertically on a FTO-coated glass substrate are woven together at the surface of the film, forming well-aligned arrays at the bottom part and a porous surface morphology. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) reveal that each nanowire is a hexagonal single crystal and their long axes are oriented toward the [0001] direction. Due to the highly porous surface, good contact with the conductive substrate and large tunnels of the hexagonal-structured WO 3 , a fast switching speed of 7.6 and 4.2 s for coloration and bleaching, respectively, and a high coloration efficiency of 102.8 cm 2 C À1 are achieved for the WO 3 nanowire array film. 1 Introduction Tungsten trioxide (WO 3 ) attracts extensive attention because of its distinctive physical and chemical properties, making it suit- able for applications in electrochromic (EC) devices, 1–4 photo- catalysis 5,6 and gas sensing. 7,8 EC devices are able to change their optical properties (coloring/bleaching) reversibly by alternating the polarity of the applied small voltage. Compared to other transition metal oxides, WO 3 is the most widely studied EC material due to its multiple oxidation states, high coloration efficiency and good cyclic stability. The interest in EC WO 3 has increased over the last decade due to its application in solar light control and energy saving in modern buildings, which always have a large area made up of windows. Generally, WO 3 film is fabricated on a transparent conductive substrate to form a working electrode in an EC device. The approaches to fabricating WO 3 films including sputtering, 9,10 pulsed laser deposition, 11,12 sol–gel, 13–16 electrode- position, 17 anodic oxidation, 18,19 thermal evaporation, 20 chemical vapor deposition (CVD), 21 hot-wire CVD 22 and electrophoresis deposition (EPD). 23 However, each of these methods has one or more characteristic drawbacks, including being highly energetic, being vacuum dependent, or requiring exotic and often dangerous reagents. The hydrothermal method is a facile, dominant tool for the synthesis of crystalline oxides with a high surface area and unique morphology. The significant advantages of this less complicated method are controllable size, growth at low temperature and cost-effectiveness. 24 The hydrothermal method has been used to synthesize one-dimensional (1D) WO 3 nanowires or nanorods with Li 2 SO 4 ,(NH 4 ) 2 SO 4 , or NaCl as the capping agent. 25–29 Their EC properties have been investigated in coatings fabricated from 1D WO 3 nanowires through drop-assembly 25,26 or elec- trophoretic deposition (EPD). 30 Although EC performance was enhanced for some aspects of these WO 3 nanowire films, two drawbacks affected the response (coloring/bleaching) speed and the coloration efficien cy: (a) the poor contact between the WO 3 nanowires (especially the upper parts of the films) and the transparent conductive substrate; (b) the active surface area of the nanostructures was not sufficien tly utilized due to the compact stacking. Growing the nanowires vertically onto the substrate to form the WO 3 arrays (so each nanowire has good contact with both the substrate and the electrolyte) would probably be a promising ap proach to resolve this problem. It has been reported that WO 3 nanowire arrays can be deposited on metal surfaces, such as Mo and W, using a thermal evaporation method or CVD, 31–34 while preparing WO 3 nano- wire arrays through a template-free solvent path still remains a challenge. In our previous work, 35 thick films of WO 3 nanowire arrays were successfully fabricated on an alumina plate and tungsten foil, and they showed fast reversible wettability change between superhydrophilicity and superhydrophobicity. Very recently, photoelectrochemical properties of WO 3 nanowire/ nanoflake arrays have been investigated by Grimes et al. 36 In this work, we make an attempt to grow WO 3 thin films directly on FTO-coated glass by a hydrothermal method. Sulfate-assisted, hydrothermally synthesized, porous WO 3 nanowire array film is State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering, Zhejiang University, Hangzhou, 310027, China. E-mail: tujp@zju.edu.cn; Fax: +86 571 7952573; Tel: +86 571 87952856 † Electronic supplementary information (ESI) available: Figs. S1 and S2 and Table S1. See DOI: 10.1039/c0jm04361c 5492 | J. Mater. Chem., 2011, 21, 5492–5498 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Journal of Materials Chemistry Cite this: J. Mater. Chem., 2011, 21, 5492 www.rsc.org/materials PAPER Downloaded by Sungkyunkwan University on 13 June 2011 Published on 24 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04361C View Online expected to have a high surface area and an enhanced EC performance. 2 Experimental 2.1 Chemical materials All solvents and chemicals were of analytical grade and were used without further purification. FTO-coated glass was purchased from Nippon Sheet Glass (Japan). Sodium tungstate, ammo- nium sulfate and hydrochloric acid (35%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Lithium perchlorate (anhydrous) and propylene carbonate (PC) were purchased from Aladdin Chemistry Co., Ltd (China). All aqueous solutions were freshly prepared with de-ionized water. 2.2 Preparation of WO 3 films AWO 3 seed layer was prepared on FTO-coated glass through a sol–gel method. Firstly, FTO-coated glass (15 mm  20 mm in size) was washed with acetone, then ethanol, and finally de- ionized water in an ultrasonic bath for 10 min per wash. The WO 3 sol was prepared according to a literature method, 37 then the sol was cast onto the FTO-coated glass through spin-coating technology, followed by annealing at 400  C for 30 min to form a seed layer (see ESI†, Fig. S1). WO 3 nanowire arrays were synthesized by a sulfate-assisted hydrothermal method. Briefly, 3.29 g sodium tungstate powder was dissolved in 76 ml de-ionized water, and a 3 M HCl aqueous solution was used to adjust the pH value to 2.0. Afterwards, ammonium sulfate (2.64 g) was added to the reaction precursor to control the morphology of the WO 3 product. After stirring for 1 h, the clear solution obtained was transferred into a Teflon- lined stainless autoclave. The FTO-coated glass with the WO 3 seed layer was placed vertically in the autoclave, and then the autoclave was sealed and heated at 180  C for 4 h. The obtained film with a thickness of 1.5 mm was washed and dried in a vacuum oven at 60  C for 12 h. For comparison, a WO 3 film with equal thickness was prepared by the same process without the addition of ammonium sulfate. 2.3 Characterizations X-ray diffraction (XRD) measurements were conducted with a PANalytical X’Pert PRO diffractometer, Cu-Ka radiation (l ¼ 1.54056  A). The 2q range was 10–80  with a step of 0.02  and a scanning speed of 2.4  min À1 . The morphology of the films was characterized by a field emission scanning electron microscope (FESEM, Hitachi S-4800) and a high resolution transmission electron microscope (HRTEM, Philips CM200 UT, operated at 160 kV). The Brunauer–Emmett–Teller (BET) surface area of the WO 3 nanowire array film was studied using nitrogen adsorption at 77 K using an Autosorb-1-C analyzer (Quantachrome). Electrochemical measurements were carried out on an electro- chemical workstation (CHI660C, Shanghai Chenhua Instru- ments, Inc.) using a conventional three-electrode test cell. The working electrode was the WO 3 film on FTO glass. An Ag/AgCl electrode and Pt foil were used as reference and counter elec- trodes, respectively. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) tests were performed in a propylene carbonate (PC) solution of 1 M LiClO 4 . CV measurements were carried out at a scanning rate of 50 mV s À1 between À1.0–1.0 V at room temperature. EIS tests were carried out with a superimposed 5 mV sinusoidal voltage in the frequency range 100 kHz to 0.01 Hz. The EIS results obtained experimentally were analyzed using a non-linear least squares fitting program EQUIVCRT. CA tests were con- ducted under a square-wave voltage of Æ2.0 V with a pulse width of 100 s. The optical properties were recorded using a UV-vis spectrometer (Shimadzu UV-240). 3 Results and discussion 3.1 Structure and morphology Fig. 1 shows the SEM images of the WO 3 films prepared from a tungstate acid sol using a simple hydrothermal method with and without the addition of ammonium sulfate. The plan-view SEM images of the films prepared in the presence of ammonium sulfate show a macroporous surface morphology where nano- wires are interwoven together (Figs 1a and b). From the sectional view and back view images of the films, it is found that the well- aligned WO 3 nanowires grow vertically on the substrate (Figs 1c and d). The nanowires first grow in a parallel manner from the seed layer, and then as the nanowires are lengthened, they change their direction gradually. It is reasonable to form such a morphology under the hydrothermal conditions of high Fig. 1 SEM images of WO 3 films prepared in the presence of ammonium sulfate: (a) a plan view of the front, (b) a magnified view, (c) a cross- sectional view, (d) a view of the back; without ammonium sulfate: (e) a plan view, and (f) a cross-sectional view. This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21, 5492–5498 | 5493 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 24 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04361C View Online temperature and high pressure. The WO 3 film grown in the solution without ammonium sulfate is composed of bulk micro- bricks (Figs 1e and f) and has a similar surface morphology to that reported by Jiao et al. 37 The XRD patterns of the FTO- coated glass, the WO 3 micro-brick film and the WO 3 nanowire array film are shown in Fig. 2. From the XRD patterns, it is confirmed that the nanowire array film is a pure phase of hexagonal WO 3 corresponding to JCPDS No. 85-2459. A strongly preferential growth direction along the c-axis [0001] can be found. The synthesis in the absence of ammonium sulfate yielded an orthorhombic phase of WO 3 hydrate with the XRD pattern corresponding to JCPDS No. 87-1203. It is found that the WO 3 nanowires have a BET surface area of 116.5 m 2 g À1 (see Fig. S2†). Fig. 3 shows the TEM images of the WO 3 nanowires, which are scraped from the substrate and ultrasound-treated in alcohol. The nanowires have a typical length of $1.5 mm with diameters of 20–40 nm. The selected area electron diffraction (SAED) pattern and the HRTEM image show that each nano- wire is a single hexagonal crystal that preferentially grows along the [0001] direction, which is in accordance with the results of the XRD characterization. The precipitation of WO 3 from a tungstate ion solution using concentrated acid is a well-known synthetic route described as follows: 38,39 WO 4 2À +2H + + nH 2 O # H 2 WO 4 $nH 2 O (1) H 2 WO 4 $nH 2 O # WO 3 +(n + 1)H 2 O (2) First, tungstate acid hydrate is synthesized from sodium tungstate in acidic solution at room temperature to form a WO 3 sol. Then WO 3 nucleates to form primary particles from the precursor under 180  C hydrothermal conditions. In the presence of an appropriate amount of ammonium sulfate in the solution, WO 3 primary particles aggregate along the [0001] direction of the hex-WO 3 unit cell via self-assembly, because sulfate ions prefer- entially adsorb on the faces parallel to the c-axis of the WO 3 nanocrystal and thus 1D single crystal nanowires are formed. 27 Although WO 3 nanorods can be synthesized with NaCl as the capping agent, 25,26 it is likely that only the sulfate ions lead to the formation of 1D hex-WO 3 nanostructures with a high aspect ratio. 27,28 On the other hand, ammonium ions play a crucial role in the formation of the hex-WO 3 structure. 28,40 Fig. 4 summarizes the formation process of WO 3 nanowires and micro-bricks under different hydrothermal conditions. 3.2 Electrochemical and electrochromic properties Fig. 5 shows the cyclic voltammograms (CVs) of the WO 3 films prepared with (a) and without ammonium sulfate (b), i.e. micro- brick film and nanowire array film, respectively. As can be seen from the CVs, the current density of the micro-brick film first increases then becomes stable, indicating an activation process. Fig. 2 XRD patterns of FTO and WO 3 films prepared with or without ammonium sulfate. Fig. 3 TEM images of nanowires (insets of (b) are the SEAD (bottom left) and HRTEM (top right) images). Fig. 4 A schematic illustration of the formation process of the WO 3 nanowires and micro-bricks under different hydrothermal conditions. 5494 | J. Mater. Chem., 2011, 21, 5492–5498 This journal is ª The Royal Society of Chemistry 2011 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 24 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04361C View Online Contrastingly, the nanowire array film shows no activation stage, and its current density decreases slowly from the first cycle to the 1000th cycle. This difference could be attributed to an effect of the morphology, that the nanowire array film with a porous surface facilitates Li + ion intercalation/deintercalation, similar to our previous works. 41,42 The evolution of the current density in the CVs of both films also indicates that the nanowire array film degenerated slightly faster than the micro-brick film. The porous surface is responsible for the degeneration of the current density, because the porous morphology causes it to dissolve into the solution faster than that of the dense morphology. The electrochromic properties of WO 3 films were measured after the film electrodes had been subjected to CV testing for 10 cycles in 1 M LiClO 4 /PC solution. Fig. 6 shows the UV-vis transmittance spectra of WO 3 films in colored and bleached states, which are applied at À2.0 V and 2.0 V (vs. Ag/AgCl) for 100 s, respectively. The color of the WO 3 films changes from deep blue (colored state) to transparent (bleached state) reversibly. This process is in accordance with intercalation (deintercalation) of the Li + into (out from) the WO 3 films: WO 3 þ xLi þ þ xe À # Li x WO 3 Transparent Blue (3) It is clearly seen that the WO 3 nanowire array film shows a larger transmittance modulation than the micro-brick film. The modulation range of the transmittance of the WO 3 nanowire array film is high, up to 58% at 633 nm, while the micro-brick film only exhibits 45% at 633 nm. Fig. 7 shows the digital photo- graphs of the WO 3 nanowire array film at different stages. It can be seen from the digital photos that the color is uniform and can be controlled by polarization at different voltages. The switching characteristics of the WO 3 films are investigated by chronoamperometry and the corresponding in situ trans- mittance at 633 nm (Fig. 8). The chronoamperometry was per- formed on the WO 3 films between À2.0 and +2.0 V. The coloration and bleaching times are defined as the time required for a 90% change in the full transmittance modulation at 633 nm. For the WO 3 nanowire array film, the coloration time t c is found to be 7.6 s, and the bleaching time t b is 4.2 s. However, for the WO 3 micro-brick film, the coloration time t c is 46.1 s, and the bleaching time t b is 38.8 s. The switching speed of the WO 3 nanowire array film is faster than the micro-brick one. Further- more, it is faster than the compact assembled nanorod films prepared by drop-assembly, which need 272 and 364 s for a 90% Fig. 5 Cyclic voltammograms of the WO 3 films prepared with (a) and without (b) ammonium sulfate. Fig. 6 The UV-vis transmittance spectrum of the WO 3 films in the colored and bleached states (solid square: nanowire array film; open circle: micro-brick film). Fig. 7 Digital photographs of the WO 3 nanowire array film at different stages: (a) as-prepared; (b) colored at À1.0 V for 30 s; (c) colored at À2.0 V for 30 s; (d) bleached at 2.0 V for 30 s. This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21, 5492–5498 | 5495 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 24 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04361C View Online modulation in coloration and bleaching, respectively. 25 The fast switching speed of the WO 3 nanowire array film is due to the large active surface area of a highly porous structure, good contact between the nanowires and the substrate, and large tunnels in the hexagonal WO 3 . Coloration efficiency (CE), which is defined as the change in optical density (OD) per unit of charge (Q) inserted into (or extracted from) the EC films, is a characteristic parameter for comparing different EC materials. It can be calculated from the following formulas: CE ¼ DOD/DQ (4) DQ ¼ log(T b /T c ) (5) where T b and T c refer to the transmittance of the film in its bleached and colored states, respectively. A high value of CE indicates that the EC film exhibits a large optical modulation with a small charge inserted (or extracted). Fig. 9 shows the plots of OD at a wavelength of 633 nm versus the inserted charge density at a potential of À2.0 V. Under the biasing of potential, the OD tends to a constant value with an increase in charge density after a short time. The CE is extracted as the slope of the line fits to the linear region of the curve. The calculated CE values are 24.5 and 102.8 cm 2 C À1 , for the micro-brick film and nanowire array film, respectively. Combining the results of Fig. 8 Chronoamperometry curves and the corresponding in situ transmittance at 633 nm for the nanowire array film (a, b) and micro- brick film (c, d). Fig. 9 The variation of the in situ optical density (OD) vs. the charge density for the WO 3 nanowire array film (a) and micro-brick film (b). The OD was measured at 633 nm at a potential of À2.0 V. 5496 | J. Mater. Chem., 2011, 21, 5492–5498 This journal is ª The Royal Society of Chemistry 2011 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 24 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04361C View Online chronoamperometry and in situ transmittance vs. time shown in Figs 8a and b, it can be concluded that, for the WO 3 nanowire array film, a major optical modulation is completed in a short time after voltage switching. However, for the micro-brick film it takes a longer time and more charge to accomplish the saturation of OD. Compared to the hydrothermally-prepared micro-brick film in this work (24.5 cm 2 C À1 ) and a previously reported work (38.2 cm 2 C À1 ), 37 the CE value of the WO 3 nanowire array film is much higher. The obtained CE value is also higher than that for the compact films assembled by nanowires or nanorods, 26 while it is comparable with the thin film derived from nanorods prepared by a solvothermal method (89–141 cm 2 C À1 ). 43 To further understand the electrochemical behavior of the as- prepared film electrodes, EIS measurements were conducted by applying an AC voltage of 5 mV in a frequency range of 10 mHz to 10 kHz at their bleached state (about 0.33 V vs. Ag/AgCl). As shown in Fig. 10, the plots of the nanowire array and micro-brick films show two semicircles in the high frequency and medium frequency ranges, respectively. Fig. 11a presents the equivalent circuit for the nanowire array and micro-brick films to simulate the experimental EIS plots, 44–46 and the fitting curves are drawn in Figs 10a and b, correspondingly. R e designates the solution resistance; R sl (i) and C sl (i)(i ¼ 1, 2) denote the migration of lithium ions and the capacity of the layer, respectively. R ct and C dl represent the charge-transfer resistance and a double-layer capacitance. Z W is the Warberg impedance. The detailed kinetic steps involved in Li + intercalation into WO 3 films are illustrated schematically in Figs 11b and c. These parameters can be calculated using ZView software (see Table S1†). It is found that the nanowire array film shows much lower R sl and Z W than the micro-brick one, indicating that the porous and well-aligned structure is more favorable for charge transfer and Li + ion diffusion than the compact structure, resulting in higher reac- tivity and reaction kinetics. 4 Conclusion WO 3 thin films were prepared by a hydrothermal method on FTO-coated glass. Nanowire arrays of WO 3 were synthesized successfully by adding an appropriate amount of ammonium sulfate as the capping agent into the hydrothermal solution. Comparing with the micro-brick structured WO 3 film, the highly porous and well-aligned WO 3 nanowire array film exhibited a much better electrochromic performance. The WO 3 nanowire array film showed a transmittance modulation of up to 58% at 633 nm and the coloration efficiency was calculated to be 102.8 cm 2 C À1 , while the WO 3 micro-brick film only gave values of 45% and 24.5 cm 2 C À1 at 633 nm, respectively. The WO 3 nanowire array film also showed faster coloration and bleaching Fig. 10 EIS plots of the nanowire array film (a) and micro-brick film (b). Insets are an enlargement of the high frequency range. Fig. 11 The equivalent circuit used for fitting the experimental impedance data (a); schematic presentations of the kinetic steps involved in Li + intercalation into the WO 3 nanowire array film (b) and micro-brick film (c). This journal is ª The Royal Society of Chemistry 2011 J. Mater. Chem., 2011, 21, 5492–5498 | 5497 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 24 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04361C View Online speed during voltage switching. The CV and EIS measurements revealed that higher reactivity and reaction kinetics were obtained for the nanowire arrays. The highly improved electro- chromic performance of the WO 3 nanowire array film enables it to be a promising prospect for application in electrochromic devices. In addition, the WO 3 nanowire arrays will also have applications in field-emission devices, gas sensors and photo- catalysis. References 1 C. R. Granqvist, Nat. Mater., 2006, 5, 89. 2 G. A. Niklasson and C. G. Granqvist, J. Mater. Chem., 2007, 17, 127. 3 D. T. Gillaspie, R. C. Tenent and A. C. Dillon, J. Mater. Chem., 2010, 20, 9585. 4 J. Zhang, J. P. Tu, X. H. Xia, Y. Qiao and Y. Lu, Sol. Energy Mater. Sol. Cells, 2009, 93, 1840. 5 M. Shibuya and M. Miyauchi, Adv. Mater., 2009, 21, 1373. 6 M. Sadakane, K. Sasaki, H. Kunioku, B. Ohtani, R. Abe and W. Ueda, J. Mater. Chem., 2010, 20, 1811. 7 L. F. Zhu, J. C. She, J. Y. Luo, S. Z. Deng, J. Chen and N. S. Xu, J. Phys. Chem. C, 2010, 114, 15504. 8 E. K. Heidari, C. Zamani, E. Marzbanrad, B. Raissi and S. Nazarpour, Sens. Actuators, B, 2010, 146, 165. 9 Y. Yamada, K. Tabata and T. Yashima, Sol. Energy Mater. Sol. Cells, 2007, 91, 29. 10 A. Karuppasamy and A. Subrahmanyam, J. Appl. Phys., 2007, 101, 113522. 11 M. J. Wang, G. J. Fang, L. Y. Yuan, H. H. Huang, Z. H. Sun, N. S. Liu, S. H. Xia and X. Z. Zhao, Nanotechnology, 2009, 20, 185304. 12 M. Ranjbar, N. T. Garavand, S. M. Mahdavi and A. I. Zad, Sol. Energy Mater. Sol. Cells, 2010, 94, 201. 13 W. Wang, Y. X. Pang and S. N. B. Hodgson, J. Mater. Chem., 2010, 20, 8591. 14 M. Epifani, T. Andreu, J. Arbiol, R. Diaz, P. Siciliano and J. R. Morante, Chem. Mater., 2009, 21, 5215. 15 S. Balaji, Y. Djaoued, A. S. Albert, R. Z. Ferguson and R. Bruning, Chem. Mater., 2009, 21, 1381. 16 T. Brezesinski, D. Fattakhova-Rohlfing, S. Sallard, M. Antonietti and B. M. Smarsly, Small, 2006, 2, 1203. 17 Y. H. Pang, Q. Chen, X. F. Shen, L. L. Tang and H. Qian, Thin Solid Films, 2010, 518, 1920. 18 J. Zhang, X. L. Wang, X. H. Xia, C. D. Gu, Z. J. Zhao and J. P. Tu, Electrochim. Acta, 2010, 55, 6953. 19 Y. C. Nah, A. Ghicov, D. Kim and P. Schmuki, Electrochem. Commun., 2008, 10, 1777. 20 K. J. Patel, C. J. Panchal, M. S. Desai and P. K. Mehta, Mater. Chem. Phys., 2010, 124, 884. 21 Y. S. Lin, S. S. Wu and T. H. Tsai, Sol. Energy Mater. Sol. Cells, 2010, 94, 2283. 22 C. M. White, D. T. Gillaspie, E. Whitney, S. H. Lee and A. C. Dillon, Thin Solid Films, 2009, 517, 3596. 23 S. H. Lee, R. Deshpande, P. A. Parilla, K. M. Jones, B. To, A. H. Mahan and A. C. Dillon, Adv. Mater., 2006, 18, 763. 24 S. Rajagopal, D. Nataraj, D. Mangalaraj, Y. Djaoued, J. Robichaud and O. Y. Khyzhun, Nanoscale Res. Lett., 2009, 4, 1335. 25 J. M. Wang, E. Khoo, P. S. Lee and J. Ma, J. Phys. Chem. C, 2008, 112, 14306. 26 J. M. Wang, E. Khoo, P. S. Lee and J. Ma, J. Phys. Chem. C, 2009, 113, 9655. 27 Z. J. Gu, T. Y. Zhai, B. F. Gao, X. H. Sheng, Y. B. Wang, H. B. Fu, Y. Ma and J. N. Yao, J. Phys. Chem. B, 2006, 110, 23829. 28 A. Phuruangrat, D. J. Ham, S. J. Hong, S. Thongtem and J. S. Lee, J. Mater. Chem., 2010, 20, 1683. 29 Z. J. Gu, Y. Ma, W. S. Yang, G. J. Zhang and J. N. Yao, Chem. Commun., 2005, 3597. 30 E. Khoo, P. S. Lee and J. Ma, J. Eur. Ceram. Soc., 2010, 30, 1139. 31 J. Zhou, Y. Ding, S. Z. Deng, L. Gong, N. S. Xu and Z. L. Wang, Adv. Mater., 2005, 17, 2107. 32 X. P. Wang, B. Q. Yang, H. X. Zhang and P. X. Feng, Nanoscale Res. Lett., 2007, 2, 405. 33 B. B. Cao, J. J. Chen, X. J. Tang and W. L. Zhou, J. Mater. Chem., 2009, 19, 2323. 34 Y. Shingaya, T. Nakayama and M. Aono, Sci. Technol. Adv. Mater., 2004, 5, 647. 35 C. D. Gu, J. Zhang and J. P. Tu, J. Colloid Interface Sci., 2010, 352, 573. 36 J. Su, X. Feng, J. D. Sloppy, L. Guo and C. A. Grimes, Nano Lett., 2011, 11, 203. 37 Z. H. Jiao, X. W. Sun, J. M. Wang, L. Ke and H. V. Demir, J. Phys. D: Appl. Phys., 2010, 43, 285501. 38 X. X. Li, G. Y. Zhang, F. Y. Cheng, B. Guo and J. Chen, J. Electrochem. Soc., 2006, 153, H133. 39 Z. J. Gu, H. Q. Li, T. Y. Zhai, W. S. Yang, Y. Y. Xia, Y. Ma and J. N. Yao, J. Solid State Chem., 2007, 180, 98. 40 J. H. Ha, P. Muralidharan and D. K. Kim, J. Alloys Compd., 2009, 475, 446. 41 X. H. Xia, J. P. Tu, J. Zhang, X. L. Wang, W. K. Zhang and H. Huang, ACS Appl. Mater. Interfaces, 2010, 2, 186. 42 X. H. Xia, J. P. Tu, J. Zhang, X. H. Huang, X. L. Wang and X. B. Zhao, Sol. Energy Mater. Sol. Cells, 2010, 94, 386. 43 S. Y. Park, J. M. Lee, C. Noh and S. U. Son, J. Mater. Chem., 2009, 19, 7959. 44 M. D. Levi and D. Aurbach, J. Phys. Chem. B, 2004, 108, 11693. 45 C. A. C. Sequeira and D. M. F. Santos, J. Electrochem. Soc., 2010, 157, J202. 46 G. Orsini and V. Tricoli, J. Mater. Chem., 2010, 20, 6299. 5498 | J. Mater. Chem., 2011, 21, 5492–5498 This journal is ª The Royal Society of Chemistry 2011 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 24 February 2011 on http://pubs.rsc.org | doi:10.1039/C0JM04361C View Online . Hydrothermally synthesized WO 3 nanowire arrays with highly improved electrochromic performance† Jun Zhang,. reaction kinetics were obtained for the nanowire arrays. The highly improved electro- chromic performance of the WO 3 nanowire array film enables it to be a