Synthesis, Assembly, and Electrochromic Properties of Uniform

Aug 22, 2008 - Xuqiang Ji , Min Ma , Ruixiang Ge , Xiang Ren , Hui Wang , Jingquan Liu , Zhiang Liu , Abdullah M. Asiri , and Xuping Sun. Inorganic Ch...
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J. Phys. Chem. C 2008, 112, 14306–14312

Synthesis, Assembly, and Electrochromic Properties of Uniform Crystalline WO3 Nanorods Jinmin Wang, Eugene Khoo, Pooi See Lee,* and Jan Ma School of Materials Science and Engineering, Nanyang Technological UniVersity, Singapore 639798, Singapore ReceiVed: May 7, 2008; ReVised Manuscript ReceiVed: July 11, 2008

We report the synthesis of uniform crystalline WO3 nanorods and their assembly without any surfactants. WO3 nanorods have been synthesized by using a facile hydrothermal process without employing lithium ions and sulfates. Transparent WO3 nanorod film has been prepared by assembled coating of the as-synthesized WO3 nanorods suspension onto ITO coated glass. It is found that the assembly was not affected by the surface properties of substrates, but dependent on the drying rate of the film, the concentration of suspension, and the aspect ratio of nanorods. The assembly process is proposed to follow an aggregation-deposition mechanism. The resulting WO3 nanorod film exhibits high electrochromic stability and comparable color display, contrast, and coloration/bleaching response. 1. Introduction Tungsten oxide (WO3) is of great interest due to its applications in electrochromic,1 photocatalytic,2 photoluminescent,3 and gas sensing materials.4 Electrochromic materials can reversibly and persistently change their optical properties by the application of an electrical voltage, which can be used as energy-saving smart windows to tune the indoors light by changing the windows’ colors.5 It is well-known that tungsten oxide has numerous structural polymorphs,6 in which the hexagonal structure has been widely studied as an intercalation host for obtaining tungsten bronzes MxWO3 (M ) H+, Li+, Na+, K+, etc.) with blue color because of its special tunnel structure.7,8 The electrochromic applications of WO3 may be utilized in high contrast displays, energy-saving smart windows, antiglare mirrors, active camouflage, or reduced visibility windshields.9 Electrochromic smart windows are effective and prospective candidates for energy-saving devices. They can be used to tune the indoors light by changing the windows’ color with an application of voltage and are able to combine two features that are often thought of as incompatible: energy efficiency (as a result of the curtailing of air conditioning) and indoor comfort (due to less glare and thermal discomfort).5 Most of the electrochromic investigations of WO3 focused on its amorphous films due to their fast switching.10 Unfortunately, the amorphous WO3 has poor structural and chemical stability, resulting in the poor electrochromic stability of amorphous WO3 thin films.10 Compared to the amorphous structure, crystalline WO3 is much more stable due to the denser structure and slower dissolution rate in electrolytes. However, crystalline bulk WO3 particles usually have slower switching responses. To overcome this drawback, nanostructured WO3 with large specific surface area is expected to improve the switching characteristic of crystalline WO3 by increasing the active surface in the electrochemical process. Among the various nanostructures, one-dimensional (1D) nanostructures have attracted great interest for dimensionality and size which have been regarded as significant factors that may bring novel and excellent properties.11 Recently, 1D WO3 * Author to whom correspondence should be addressed. E-mail: pslee@ ntu.edu.sg. Phone: 65-67906661. Fax: 65-67909081.

nanostructures have been synthesized by both chemical and physical methods.12 Dillon et al.13 has synthesized crystalline WO3 nanoparticles and nanorods by hot-wire chemical-vapor deposition and fabricated a film by electrophoresis deposition process at a high voltage of 300 V. The film exhibited greater charge density (32 mC cm-2 mg-1) for H+ ions intercalation. Liao et al.14 has prepared crystalline WO3-x nanowires based electrochromic devices by thermal evaporation of WO3 powder at 950 °C. The electrochromic device shows a contrast of 34.5% and can be cycled up to 1000 cycles. Considering the low-cost synthesis of WO3 nanorods and preparation of large-area smart windows, chemical methods have more advantages than physical methods in large-scale synthesis, simple operation, and inexpensive apparatus. Yao et al.8 synthesized WO3 nanowires by a hydrothermal process with lithium tungstate, lithium sulfate, and hydrochloric acid solution as the precursor. They found lithium sulfate was necessary for the growth of WO3 nanowires and other sulfates can also be used to synthesize WO3 nanowires or nanorods. Essentially, sulfates seem necessary for the formation of 1D WO3 nanostructures. Following the synthesis of 1D nanostructures, directed assembly of them on specific locations of a substrate is another challenge in the field of nanotechnology.15 Flow-assisted alignment,16 Langmuir-Blodgett technique,17 evaporation of liquidcrystalline phase,18 and spin-coating19 have been applied to the assemblies of nanorods and nanowires. However, only smalldiameter nanorods (usually less than or around 10 nm) are suitable to be assembled by these reported assembly methods due to the large driving forces between small nanorods. Unfortunately, small nanorods are easy to agglomerate, resulting in the formation of separated small-area assembly. Moreover, small nanorods must be modified and dispersed by surfactants to form stable suspensions before an assembly process. The introduction of surfactants may cause some degradation in physical properties of the nanorods. On the other hand, the removal of surfactants may destroy the assembly structures. Thus, it is more attractive to identify a large-area assembly approach without using surfactants. Recently, the blowingbubble method20 has been used to assemble 1D nanostructures and shows attractive applications in large-area field-effect

10.1021/jp804035r CCC: $40.75  2008 American Chemical Society Published on Web 08/22/2008

Synthesis, Assembly, and EC Properties of WO3 transistors. However, more effort is required to improve the uniformity of the assembled film. The conventional electrodeposition of WO3 films was restricted by the shapes of the substrates, the expensive equipment, and the poor crystallinity of WO3. For the large-area assembly of WO3 films, these disadvantages will be more obvious. So it is still a challenge to identify a simple, low-cost process to prepare large-area nanocrystalline WO3 films with uniform dimensionality, size, and good crystallinity.21 To our best knowledge, the preparation of an electrochromic film by assembling wet-chemically synthesized WO3 nanorods and realization of its electrochromic functionality have not been demonstrated to date. In this work, we develop a facile wetchemical process to synthesize high-quality WO3 nanorods without conventional sulfates as capping agents. Besides, a new surfactant-less large-area compact assembly of WO3 nanorods is reported. It is found that common salt (NaCl) is a good capping agent for the growth of WO3 nanorods. A transparent electrochromic film was prepared by an oriented aggregationdeposition process of WO3 nanorods on tin-doped indium oxide (ITO) coated glass. Interestingly, a large-area assembly (spread all over the film in a centimeter scale) configuration of WO3 nanorods was successfully realized and the resulting device exhibited enhanced electrochromic properties. 2. Experimental Section 2.1. Synthesis of WO3 Nanorods. WO3 nanorods were synthesized by a hydrothermal process. Na2WO4 · 2H2O, HCl, and NaCl aqueous solution were used as the precursor. In a typical synthesis, 0.825 g of Na2WO4 · 2H2O and 0.290 g of NaCl were dissolved in 19 mL of deionized water. Subsequently, 3 M HCl was slowly dropped into the solution with stirring until the pH value of the solution reached 2.0. Then, the solution was transferred into a Teflon-lined 45 mL capacity autoclave. Hydrothermal reaction was carried out at 180 °C for 24 h in an oven. After the autoclave cooled to room temperature, a white product was obtained. At last, WO3 nanorods were synthesized after repeatedly washing the product with deionized water. 2.2. Assembly of WO3 Nanorods on ITO Coated Glass. The as-synthesized WO3 nanorods were dispersed in deionized water by vigorous ultrasonic treatment to form a stable suspension with a WO3 concentration of 2 mg mL-1. Transparent and conductive ITO coated glass with dimensions of 50 mm × 7 mm × 0.7 mm was used as the substrate and washed by deionized water, acetone, isopropanol (IPA), and deionized water, respectively. 0.3 mL of suspension was dropped onto the surface of cleaned ITO glass. Transparent film was prepared after drying at room temperature for 48 h. 2.3. Characterization. The phase of the product was identified by X-ray powder diffraction (XRD, Shimadzu), using Cu KR (λ ) 0.15406 nm) radiation at 50 kV and 50 mA in a 2θ range from 10° to 80° at room temperature. The morphologies of the as-synthesized WO3 nanorods and the nanorods coated film were characterized by field-emission scanning electron microscopy (FESEM), using an accelerating voltage of 5 kV and beam current of ∼12 µA. The FESEM sample was prepared by sputtering Pt coating, using a sputtering machine at a beam current of 20 mA for 45 s. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) images, and selected area electron diffraction (SAED) pattern of the WO3 nanorods were obtained on a JEM-2010 microscope, using an accelerating voltage of 200 kV. Electrochromic properties of the WO3 nanorods coated film were measured by a three-electrode electrochemical cell with 1.0 M

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Figure 1. XRD pattern of the hydrothermal product.

lithium perchlorate (LiClO4) in propylene carbonate (PC) as the electrolyte. The measurements were carried out with use of the Autolab Potentiostat (PGSTAT302). The WO3 nanorod film was vertically inserted into the electrolyte and acted as the working electrode, Pt wire acted as the counter electrode, and the Ag/ AgCl electrode acted as the reference electrode. Cyclic voltammetry (CV) curves were measured between +1.0 and -1.0 V with a sweep rate of 0.1 V s-1 and recorded for the 1st, 1000th, 2000th, and 3000th cycle, respectively. The transmittance spectra were in situ measured by a UV-vis spectrophotometer (UV-2501 PC SHIMADZU) in standard cuvettes in the spectral region between 300 and 900 nm by applying voltages of -1.0, -2.0, -3.0, and +3.0 V, respectively. Coloration/ bleaching switching characteristics of the film were in situ recorded by UV-vis spectrophotometer with an absorbance wavelength of 632.8 nm between -3.0 and 3.0 V. 3. Results and Discussion 3.1. Structural Characterization and Growth Mechanism of WO3 Nanorods. Figure 1 shows the XRD pattern of the product. All of the peaks can be indexed to the hexagonal phase of WO3 structure (JCPDS 85-2460) and no nonstoichiometric tungsten oxides (WO3-x) and tungsten oxide hydrates (WO3 · xH2O) were detected, indicating pure WO3 has been obtained. Strong diffraction peaks also indicate the good crystallinity of the hydrothermal product. It should be noted that the (0002) diffraction peak can hardly be found in the XRD pattern. It is well-known that only (000l) diffraction peaks can be found in the patterns of many vertically aligned nanowire arrays with hexagonal structures.11a Thus, it can be inferred that the as-synthesized product possesses preferential growth direction and is almost completely horizontally aligned on the substrate. TEM images of the product are shown in Figure 2. It can be seen that uniform nanorods with diameters of ∼100 nm and lengths of ∼2 µm were successfully synthesized, corresponding to an aspect ratios of ∼20:1. The surfaces of the nanorods are very coarse, which increase their surface areas. To study the structural characteristics of the nanorods, a SAED pattern (inset in Figure 2A) was taken from a single nanorod. Regular diffraction spots imply the nanorod is a single crystal. The growth direction of the nanorod is the (0002) direction (c-axis). A HRTEM image of the single nanorod is shown in Figure 2C. Clear lattice fringes can be seen, indicating a good crystallinity of the as-synthesized hexagonal WO3 nanorods. The lattice spacing of 0.38 nm between adjacent lattice planes perpendicular to the growth direction of the nanorod corresponds to the d-spacing of (0002) planes, which indicates that the nanorod grow along the (0002) direction (c-axis). This result is consistent with SAED data.

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Figure 2. (A) Low-magnification TEM image, (B) higher-magnification TEM image showing the coarse surface of WO3 nanorod, and (C) HRTEM images of the as-synthesized WO3 nanorods. The inset in Figure 2B shows the SAED pattern taken from the single nanorod.

Figure 3. FESEM images of the WO3 nanorod film at (A) low magnification and (B) higher magnification. The inset in Figure 3A shows a high-magnification FESEM image showing the highly oriented compact assembly of WO3 nanorods.

The growth mechanism of the WO3 nanorods can be explained according to the following reactions:

Na2WO4 + 2HCl f H2WO4 + 2NaCl

(1)

H2WO4 f WO3 (crystal nucleus) + H2O

(2)

WO3 (crystal nucleus) f WO3 nanorods

(3)

First, H2WO4 was formed after HCl solution was added into Na2WO4 solution. Nucleation started and the WO3 crystal nucleus was obtained when the reaction temperature exceeded the decomposition temperature of H2WO4. Analogous to hexagonal zinc oxide, the hexagonal WO3 crystal can be regarded as a polar crystal with ((0001) polar planes.22 The growth rate of different crystal planes varies, which causes the anisotropy crystal growth. These polar crystals tend to preferentially grow along their polar directions (c-axes) at low growth rates, resulting in the formation of 1D structures.23 Besides, some capping agents can selectively adsorb onto certain crystal planes to accelerate the preferential growth. The growth mechanism of WO3 nanowires and nanorods synthesized in the existence of lithium sulfate has been explained as the capping action of lithium sulfate.8 In our present work, it is believed that NaCl acts as the capping agent and selectively adsorbed onto the crystal planes parallel to the c-axes of WO3 crystal nucleus. A slow polar growth controls the 1D growth of WO3 nanorods due to the low reaction temperature. The capping action and polar growth alternatively control the growth process until the hydrothermal system markedly cools down. At the same time,

smaller WO3 nanorods dissolve and act as growth species of larger nanorods due to the well-known “Ostwald ripening” mechanism,24 resulting in the formation of the WO3 nanorods with uniform size and shape. 3.2. Assembly of WO3 Nanorods. Transparent electrochromic film was prepared using the as-synthesized WO3 nanorods at room temperature. Figure 3 shows the FESEM images of WO3 nanorod film. Interestingly, a large-area assembly of WO3 nanorods was obtained and assembled systematically into a layer-by-layer stacking configuration. The multilayered nanorod superstructures are very homogeneous and composed of highly compact nanorods, occasionally disrupted by high angle domain boundaries. Higher-resolution FESEM images (Figure 3B and inset in Figure 3A) show ordered anisotropic orientation resembles 3D-nematic configuration with side-by-side alignment of nanorods. Some interstices can also be found among the ends of nanorods. The assembly is homogeneous and spreads all over the film, which indicates a centimeter-scale assembly has been obtained. More FESEM images with different magnifications of the as-prepared WO3 nanorod film are shown in Figure S1, Supporting Information. 3.3. Electrochromic Properties of WO3 Nanorod Film. The observed electrochromic phenomena of the as-prepared WO3 nanorod film were shown in Figure 4A-E. It can be seen that the as-prepared WO3 nanorod film is almost colorless and the Pt wire placed behind the film can be seen easily (Figure 4A), which indicates the film is transparent enough. When applying a voltage of -1.0 V, the film displayed a green color

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Figure 4. Color changes of the WO3 nanorod film at different voltages with 1.0 M lithium perchlorate (LiClO4) in propylene carbonate (PC) as the electrolyte: (A) the as-prepared film, (B) colored at -1.0 V, (C) colored at -2.0 V, (D) colored at -3.0 V, (E) bleached at 3.0 V, (F) corresponding UV-vis spectra, and (G) switching time characteristics between the colored and bleached states for the WO3 nanorod film measured at (3.0 V with an absorbance wavelength of 632.8 nm.

immediately. The film became blue when the applied voltage was -2.0 V. Increasing the voltage to -3.0 V, the film displayed a deep blue color. The production of a color change that persists with little or no additional input of power is the unique energysaving memory effect of electrochromic films, which is absent in other devices such as liquid crystal displays (LCDs) that require a constant input of power to display their colors. The colored WO3 nanorod film can retain its colors for several days after the applied voltages are removed. That is, once it is charged within a very short time (several seconds or minutes), the WO3 nanorod film will provide long-time color displays for several days, so significant energy-savings can be realized. The bleaching ability is also of interest for active modulated smart displays or windows. When a voltage of +3.0 V was applied to the deep blue film, its color was bleached quickly and a transparent film was obtained again. Figure 4F shows the in situ transmittance spectra of the WO3 nanorod film recorded at -1.0, -2.0 -3.0, and +3.0 V. There are three strong and wide transmittance peaks at ∼533, 512, and 448 nm in the transmittance curves measured at -1.0, -2.0, and -3.0 V, respectively, which implies that the nanorod film can display green, green-blue, and blue color at different voltages. This is consistent with the observations from the electrochromic phenomena. The maximum transmittance wavelengths have obvious blue-shifts and the transmittance intensities decrease with the increase of the applied negative voltages within the spectra of 500-900 nm. All the transmittance peaks show that their corresponding maximum absorbance wavelengths should be in the range of 600-700 nm. Here, a wavelength of 632.8 nm used in many literatures reported for amorphous WO3 electrochromic films10 was selected as the maximum absorbance wavelength of the as-prepared WO3 nanorod film. The maximum optical modulation (% T) of coloration/bleaching at 632.8 nm was found to be ∼66% after applying a voltage of -3.0 V, which lies within the range of the reported values of amorphous WO3 film prepared by electrodeposition,23 sol-gel,10a,b and sputtering processes.10c The

optical modulations decrease at lower applied voltages and are 47% at -2.0 V and 37% at -1.0 V, respectively. Figure 4G shows the in situ coloration/bleaching characteristic of the as-prepared WO3 nanorod film recorded at the absorbance wavelength of 632.8 nm. The coloration and bleaching times are extracted as the time required for 50%, 70%, and 90% changes in the whole transmittance modulation at 632.8 nm. The coloration time of 50% changes tc(50%) is found to be 13 s while bleaching time tb(50%) is 8 s. Both coloration and bleaching of the films for 70% modulation are slowed down and the times increased to tc(70%) ) 38 s and tb(70%) ) 42 s. The coloration/ bleaching times are comparable to amorphous structures,10 which makes the crystalline WO3 nanorod film suitable for electrochromic devices. The fast switching times should be attributed to the large active surface of the as-synthesized WO3 nanorods. The switching time further increased up to tc(90%) ) 272 s and tb(90%) ) 364 s for 90% modulation. This indicates considerable optical modulation (50%) can be obtained within a short time but higher modulation (90%) will require longer charging and bleaching time. It is well-known that the electrochromic mechanism of WO3 in Li+ electrolytes can be expressed as: intercalation

WO3 + xe- + xLi+ {\} LixWO3

(4)

deintercalation

The intercalation/deintercalation of electrons and chargebalancing Li+ ions results in the coloration/bleaching switching.5,9 The Li+ intercalation capacities of the WO3 nanorod film extracted from the coloration parts of the switching steps and are calculated according to the intercalated charge per unit area and weight. The average intercalated charge of each step is ∼133 mC cm-2 mg-1, which is much higher than the typically reported data10b,13,25 (∼32 mC cm-2 mg-1). Cyclic voltammetry (CV) curves were recorded for the 1st, 1000th, 2000th, and 3000th cycle at room temperature and shown in Figure 5A. Only a slight reduction in the current

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Figure 5. (A) CV curves of the WO3 nanorods coated film at the 1st, 1000th, 2000th, and 3000th cycles and measured in 1.0 M LiClO4/PC solution with a sweep rate of 0.1 V s-1 and (B) capacity retention of the WO3 nanorods coated film after different CV cycles.

Figure 6. FESEM images of the WO3 nanorod film prepared (A) at a high drying rate (dried at 100 °C), (B) at a low drying rate (dried at room temperature), (C) with a low-concentration (0.1 mg mL-1) WO3 nanorod suspension, and (D) on Cu substrate.

densities within the first 1000 cycles is observed and the current densities are almost constant after the 1000th cycle. The intercalation and deintercalation charges extracted for different cycles decrease at the beginning and become almost constant after 1000 cycles (Figure 5B), which is consistent with the CV results. Both the CV curves and charge retention curves indicate the high stability of the as-prepared crystalline WO3 nanorod electrochromic film. The electrochromic behavior of the WO3 nanorod film is correlated to its microstructure. The nanorod film is composed of large-area assembled WO3 nanorods with large specific surface areas due to their coarse surfaces and some interstices among the oriented aligned nanorod domains. This kind of microstructure allows sufficient Li+ ions to be intercalated into WO3 nanorods, resulting in deep blue color with high contrast. The displayed colors of the WO3 nanorod film can be modulated by changing the applied voltages to tailor an appropriate amount of Li+ ions intercalated in WO3 nanorods. The crystalline structure of the as-synthesized WO3 nanorods greatly enhances the electrochromic stability of the film. The coarse surfaces of nanorods and interstices among nanorod domains increase the

specific surface area of the film and accelerate the intercalation/ deintercalation of Li+ ions, resulting in fast color/bleaching switching. The enhanced electrochromic properties of the WO3 nanorod film should be attributed to its unique assembly configuration and crystalline structure of WO3 nanorods. 3.4. Assembly Mechanism of WO3 Nanorod Film. To understand the assembly mechanism of the as-prepared WO3 nanorod film, the preparation conditions of the nanorod films were changed to study the influence to their morphologies. The corresponding FESEM images were shown in Figure 6. It is found that the drying rate can markedly influence the morphologies of the WO3 nanorod film. When the film was dried in an oven at 100 °C (higher drying rate than that at room temperature) for 2 h, a swirl-like assembly of WO3 nanorods was formed (Figure 6A) instead of the straight oriented assembly that was formed by drying at room temperature (Figure 6B). When a low-concentration (0.1 mg mL-1) suspension was used to coat the ITO glass, no assembly structure was found after drying. Only randomly oriented WO3 nanorods were deposited on ITO glass and more pores were obtained (Figure 6C). The in-plane oriented assembly structure was also obtained when the WO3

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J. Phys. Chem. C, Vol. 112, No. 37, 2008 14311 aspect ratio, it can be imagined that the parallel compact alignment of nanowires will become difficult due to the steric hindrance among the nanowires. 4. Conclusions

Figure 7. Schematic illustration for the assembly mechanism of WO3 nanorods on ITO coated glass showing (A) drying and deposition of WO3 nanorods, (B) further deposition and assembly, and (C) highly compact assembly of WO3 nanorods after complete removal of water from the suspension.

nanorods were coated on Cu substrate (Figure 6D), which implies that the assembly process does not depend on substrates but on the characteristics of the as-synthesized WO3 nanorods. On the basis of the above observations and analysis, an assembly mechanism of the WO3 nanorods is proposed, as shown in the schematic illustration of Figure 7. Because of the large specific surface area of the as-synthesized WO3 nanorods and large surface tension of water, the WO3 nanorods can be completely surrounded and strongly attracted by water molecules and dragged into the water phase to form stable suspension with monodisperse nanorods. When the suspension was dried, some WO3 nanorods were randomly deposited onto ITO coated glass with the evaporation of water, resulting in the formation of randomly aligned WO3 nanorod thin film (Figure 7A). With a further evaporation of water, more WO3 nanorods were deposited layer by layer. Due to the large surface area of the assynthesized WO3 nanorods, they were easily aggregated together side by side (Figure 7B) through the action of capillary force and van der Waals attraction between adjacent nanorods to decrease the surface energy and to reach a stable entropy state. Moreover, the large surface tension of water accelerated the aggregation of WO3 nanorods with the evaporation of water. After water molecules among WO3 nanorods were completely removed, horizontally assembled WO3 nanorod film was obtained (Figure 7C). So the assembly of the WO3 nanorod film can be regard as an aggregation-deposition mechanism. This mechanism is related to the characteristics of the deposited species and the aggregation rate instead of the substrates. At a low aggregation rate (dried at room temperature), van der Waals attraction controlled the assembly process, resulting in the perfect straight horizontal side-by-side alignment of WO3 nanorods. However, at a high aggregation rate (dried at 100 °C), the assembly was driven by the surface tension and rigorous water flow, resulting in the formation of swirl-like nanorod film. Sun and Sirringhaus18,26 found a similar assembly in ZnO nanostructures driven by surface tension and fluid flow. However, ZnO nanorods must be modified by surfactant to obtain a stable suspension and the assembly was driven by the surface tension of surfactant solution. Herein, no surfactant was used in the preparation of WO3 nanorod suspension, which indicates that the as-synthesized WO3 nanorods are easily dispersed in water and their assembly is driven by capillary force and van der Waals attraction, surface tension of water, and water flow. Besides, the rod-like shape and the aspect ratio are important for the assembly behavior. If the nanostructures are curvy and long enough, such as curving nanowires with a large

In summary, crystalline WO3 nanorods with uniform size and shape were synthesized by a facile wet-chemical method without conventional sulfates as capping agents. NaCl was found to be a good capping agent for the growth of high-quality WO3 nanorods. Transparent electrochromic film was successfully prepared by an oriented aggregation-deposition process of WO3 nanorods on ITO coated glass. Large-area assembly configuration of WO3 nanorods was successfully realized without any surfactant at room temperature. An “aggregation-deposition” mechanism is proposed for the assembly process. Capillary force, van der Waals attraction, surface tension of water, and water flow control the dynamics process of WO3 nanorods assembly. It was found that the assembly was not affected by the substrates, but dependent on the concentration of suspension, the aspect ratio of nanorods, and the drying rate of the film. The developed assembly process provides a simple and effective surfactant-less, low-temperature solution approach to large-area assembly of crystalline 1D nanostructures due to no restriction from the sizes and shapes of substrates or deposition equipment. This process can be extended to assemble other nanorods with large active specific surface area. The as-prepared electrochromic device exhibits satisfactory integrated electrochromic properties: high electrochromic stability (>3000 CV cycles), high contrast (∼66%), comparable color display (from colorless to green, blue and deep blue), and coloration/bleaching response (∼8 s). These properties of the assembled WO3 nanorod film endow its promising practical applications in large-area smart windows. Acknowledgment. We thank Mr. Ting Sun for technical assistance on TEM characterizations. Supporting Information Available: Additional FESEM images with different magnifications of the as-prepared WO3 nanorod film. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Santato, C.; Odziemkowski, M.; Ulmann, M.; Auustynski, J. J. Am. Chem. Soc. 2001, 123, 10639. (2) Baeck, S.-H.; Choi, K.-S.; Jaramillo, T. F.; Stucky, G. D.; McFarland, E. W. AdV. Mater. 2003, 15, 1269. (3) Feng, M.; Pan, A. L.; Zhang, H. R.; Li, Z. A.; Liu, F.; Liu, H. W.; Shi, D. X.; Zou, B. S.; Gao, H. J. Appl. Phys. Lett. 2005, 86, 141901. (4) Rossinyol, E.; Prim, A.; Pellicer, E.; Arbiol, J.; Herna´ndez-Ramı´rez, F.; Peiro´, F.; Cornet, A.; Morante, J. R.; Solovyov, L. A.; Tian, B.; Bo, T.; Zhao, D. AdV. Funct. Mater. 2007, 17, 1801. (5) (a) Granqvist, C. G. Handbook of Inorganic Electrochromic Materials; Elsevier: Amsterdam, The Netherlands, 1995. (b) Niklasson, G. A.; Granqvist, C. G. J. Mater. Chem. 2007, 17, 127. (6) Wang, G.; Ji, Y.; Huang, X. R.; Yang, X. Q.; Gouma, P.-I.; Dudley, M. J. Phys. Chem. B 2006, 110, 23777. (7) Reis, K. P.; Ramanan, A.; Whittingham, M. S. Chem. Mater. 1990, 2, 219. (8) Gu, Z. J.; Li, H. Q.; Zhai, T. Y.; Yang, W. S.; Xia, Y. Y.; Ma, Y.; Yao, J. N. J. Solid State Chem. 2007, 180, 98. (9) Granqvist, C. G. Sol. Energy Mater. Sol. Cells 2000, 60, 201. (10) (a) Deepa, M.; Joshi, A. G.; Srivastava, A. K.; Shivaprasad, S. M.; Agnihotry, S. A. J. Electrochem. Soc. 2006, 153, C365. (b) Deepa, M.; Saxena, T. K.; Singh, D. P.; Sood, K. N.; Agnihotry, S. A. Electrochim. Acta 2006, 51, 1974. (c) Subrahmanyam, A.; Karuppasamy, A. Sol. Energy Mater. Sol. Cells 2007, 91, 266. (11) (a) Wang, J. M.; Gao, L. J. Mater. Chem. 2003, 13, 2551. (b) Wang, B.; Zhu, L. F.; Yang, Y. H.; Xu, N. S.; Yang, G. W. J. Phys. Chem. C 2008, 112, 6643. (c) Cheng, B. C.; Qu, S. C.; Zhou, H. Y.; Wang, Z. G. J. Phys. Chem. B 2006, 110, 15749. (d) Zhang, S.-Y.; Fang, C.-X.; Wei, W.;

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