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shell nanorod arrays
Solar Energy Materials & Solar Cells 117 (2013) 231–238
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Efficient electrochromic materials based on TiO2@WO3 core/shell nanorod arrays G.F. Cai a, D. Zhou a, Q.Q. Xiong a, J.H. Zhang a, X.L. Wang a,b, C.D. Gu a,b, J.P. Tu a,b,n a State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou 310027, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 March 2013 Received in revised form 17 May 2013 Accepted 23 May 2013
TiO2@WO3 core/shell nanorod arrays are prepared by the combination of hydrothermal and electrodeposition method. The array films show remarkable enhancement of the electrochromic properties. In particular, a significant optical modulation (57.2% at 750 nm, 70.3% at 1800 nm and 38.4% at 10 μm), fast switching speed (2.4 s and 1.6 s), high coloration efficiency (67.5 cm2 C−1 at 750 nm) and excellent cycling performance (65.1% after 10,000 cycles) are achieved for the core/shell nanorod arrays. The improved electrochromic properties are mainly attributed to the core/shell structure and the porous space among the nanorod array, which makes the ion diffusion become easier and it also gives larger surface area for charge-transfer reactions. The data present great promise for the TiO2@WO3 core/shell nanorod arrays as practical electrochromic materials. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electrochromism Titanium dioxide Tungsten trioxide Core/shell structure Nanorod array
1. Introduction Today, it is common for people to spend about 90% of their time inside buildings and vehicles, and more and more energy is used to maintain the indoor environment at a level that is both comfortable and healthy [1,2]. Hence, it is important to design a material capable of providing high energy efficiency and indoor comfort simultaneously. Electrochromic materials can change their optical properties persistently and reversibly in the presence of a small electric difference. When thin films of such materials are integrated in devices, they become possible to modulate the transmittance, reflectance, absorptance, and emitance between widely separated extrema [3–7], which endow them to be effective candidates in various applications, such as large area information displays, rearview mirrors for automobiles, thermal control of spacecraft and military camouflage [8–16]. Until recently, most of the applications have been based on the optical changes in the visible spectrum whereas the infrared (IR) region is still rare [17–20]. Electrochromic materials can be also used in the IR region [21–23]. It is widely accepted that the electrochromic phenomena of some inorganic materials is attributed to the injection/extraction of electrons and
n Corresponding author at: State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail addresses: [email protected], [email protected] (J.P. Tu).
0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.05.049
cations [24–27]. The kinetics and magnitude of ion insertion and the electrochromic reaction strongly depend on the diffusion length of ions and the available surface area, which is because the ratedetermining steps of ion intercalation and release are under diffusion control and are limited to a very thin surface layer of the host materials [11,28]. Therefore, it is important to design a material with unique architecture and proper crystal structure to obtain fast insertion kinetics and enhanced durability. Recently, synthesis of aligned single-crystalline TiO2 nanorod or nanowire films has attracted much attention because these nanorods or nanowires offer direct electrical pathways for electrons and can increase the electron transport rate [29,30]. In addition, the nanorods or nanowires provide the vertically aligned nanostructure with a high surface area which allows the electrolyte to penetrate and shorten the proton diffusion paths within the bulk of TiO2. However, the electrochromic performances of pure TiO2 need to be improved. Tungsten trioxide (WO3) has been identified as one of the most promising inorganic electrochromic materials [31–34]. There has been great interest in the addition of WO3 to TiO2 because the addition of adequate WO3 in TiO2 can improve the electrochromic performances of TiO2 [35–37]. However, to the best of our knowledge, there are no reports dedicated to electrochromic performance of TiO2@WO3 core/shell nanorod arrays. In this present work, the TiO2@WO3 core/shell nanorod arrays were prepared on a fluorine-doped tin oxide (FTO)-coated glass, and the electrochromic performances were investigated not only in the visible spectrum but also in the IR region. We show a facile
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route to cover the TiO2 nanorods with amorphous WO3 and demonstrate that these core/shell nanorod arrays have significantly enhanced electrochromic characteristics.
2. Experimental 2.1. Preparation of TiO2 nanorod array Rutile TiO2 nanorod arrays were prepared on a FTO glass substrate by a previously reported hydrothermal method [29]. In a typical synthesis, 30 ml of concentrated hydrochloric acid (Sinopharm) was diluted with 30 ml de-ionized water, and mixed with 1 ml titanium n-butoxide (Sinopharm) in a 100 ml beaker. This clear solution mixture and a clean FTO glass substrate were transferred to a Teflon-lined stainless steel autoclave (100 ml in volume), where the FTO substrate was submerged in the solution and placed at an angle against the wall of the Teflon-liner with the conducting side facing down. The sealed autoclave was heated in a vacuum oven at 150 1C for 4 h, and then cooled down to room temperature under flowing water. The sample was taken out, rinsed extensively with de-ionized water and dried in vacuum at 60 1C. A TiO2 nanorod array film was uniformly coated on the FTO glass substrate. 2.2. Preparation of TiO2@WO3 core/shell nanorod array TiO2@WO3 core/shell nanorod arrays were obtained by depositing a WO3 thin layer onto the surface of TiO2 nanorods by cathodic electrodeposition. The deposition solution was prepared by dissolving Na2WO4 (Sinopharm) salt in de-ionized water (concentration: 12.5 mM). Then hydrogen peroxide (Sinopharm, 30%) was added to the solution maintaining a concentration ratio of 3 with sodium tungstate, according to the literatures [38,39]. The pH value of the resulting solution was adjusted down to 1.2 by adding perchloric acid (Sinopharm). The FTO glass coated with TiO2 nanorod array was used as the deposition electrode. The electrodeposition was performed with a CHI660D electrochemical workshop at room temperature and in a three-electrode cell. The reference electrode was an Ag/AgCl electrode and the counter electrode was a 2 2 cm2 platinum foil. The electrodeposition process was conducted at a potential of −0.7 V (vs. Ag/AgCl) for 200 s, 400 s and 600 s, and named as TiO2@200WO3, TiO2@400WO3 and TiO2@600WO3 for the corresponding samples, respectively. Argon was bubbled for 5 min in the cell prior to the deposition. After electrodeposition, the samples were thoroughly washed with methanol and water and finally dried in air. For comparison, a pure WO3 film on an FTO substrate was also prepared by electrodeposition for 400 s with the same parameters.
reflectance reference (R≈99% in IR region). Each spectrum was recorded ex-situ (after the samples taken out of the threecompartment system, instantly rinsed and wiped off from the remaining persistent water). The cyclic voltammetry (CV) and chronoamperometry (CA) measurements were carried out in a three-compartment system containing 0.5 M H2SO4 as electrolyte, Ag/AgCl as a reference electrode and Pt foil as the counter electrode. CV measurements of the films were performed using a CHI660D electrochemical workshop at a scan rate of 20 mv s−1 between −0.7 and 1.0 V at room temperature (257 1 1C). Electrochemical impedance spectrum (EIS) tests were conducted on this electrochemical workstation with a superimposed 5 mV sinusoidal voltage in the frequency range of 0.01 Hz–100 kHz.
3. Results and discussion 3.1. Structure and morphology To determine the crystal structure and possible phase changes during the electrodeposition process, XRD patterns are collected from the films before and after electrodeposition for various deposition times (Fig. 1). From pattern a, after subtracting the diffraction peaks of FTO glass, all the diffraction peaks are indexed to the tetragonal rutile TiO2 (JCPDS no. 03-1122). Compared to the standard JCPDS card, the peak centered at 2θ degree of 62.71 corresponding to the (002) diffraction is significantly enhanced, which indicates that the film is highly oriented in the [001] direction on the substrate. From patterns b–d, the films only display an additional broadened peak around 2θ≈251 compared to the FTO glass substrate with TiO2 nanorod array. It indicates an amorphous WO3 in presence, which is in accordance with the previous report [39]. In addition, a marginal reduction in peak intensity clearly reveals an increase of WO3 in the films with the deposition time. From pattern e, the WO3 film only displays an additional broadened peak, indicating an amorphous characteristic. It is well known from the literature survey that the electrochromic performance of metal oxide is closely related to its level of crystallization [11,40,41]. For instance, crystalline WO3 film exhibited inferior performance to amorphous one. However, the amorphous WO3 has less cyclic stability [42]. Therefor, synthesis of the TiO2@WO3 core/shell nanorod array is an interesting way to overcome these deficiencies. Crystalline TiO2 nanorod as core will have a good durability and amorphous WO3 as shell will exhibit good electrochromic performance.
2.3. Characterization The structural and morphological characterizations of the films were carried out using X-ray diffraction (XRD, RIGAKU D/MAX 2550/PC with Cu Kα radiation), X-ray photoelectron spectroscopy (XPS, AXIS UTLTRADLD equipped with a dual Mg Kα-Al Kα anode for photo excitation) and field emission scanning electron microscopy (FESEM, Hitachi SU-70). For the transmission electron microscopy (TEM, JEOL JEM-200CX) analysis, the films were scratched from the FTO substrate and re-dispersed in ethanol solution. The transmission spectra of electrochromic films in the fully colored and fully bleached states were measured over the wavelength range from 400 to 2500 nm with a SHIMADZU UV3600 spectrophotometer. The IR reflectance spectra of the films were measured using an IR spectrophotometer TENSOR27 in the wavelength range from 2.5 to 25 μm. A gold mirror was used as a
Fig. 1. XRD patterns of (a) TiO2 nanorod, (b) TiO2@200WO3, (c) TiO2@400WO3, (d) TiO2@600WO3 array, and (e) WO3 film.
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The high magnification TEM (HRTEM) image collected from the individual nanorod shows that it is completely crystalline along its entire length and reveals clear lattice fringes with interplanar spacings of 0.32 and 0.29 nm, which are consistent with the d-spacings of (110) and (001) planes of rutile TiO2 (Fig. 4b). In addition, the [110] axis is perpendicular to the nanorod side walls, and the corresponding sharp FFT pattern of the nanorod is examined along the [110] zone axis. These data confirm the single-crystalline structure and demonstrate that the TiO2 nanorod grows along the [001] direction. After electrodeposited for 200 s, the TiO2 nanorod array is uniformly covered by amorphous WO3 (Fig. 4c). The high magnification TEM image collected for a representative TiO2@200WO3 nanorod shows that an amorphous WO3 layer is uniformly coated on the entire nanorod (Fig. 4d). The TEM image also reveals the amorphous nature of the WO3 shell. 3.2. Electrochemical and electrochromic properties
Fig. 2. (a) XPS survey spectra of TiO2 nanorod array and TiO2@200WO3 core/shell nanorod array and (b) detail of the W4f peak for TiO2@200WO3 core/shell nanorod array.
XPS measurements were performed to investigate the chemical composition and oxidation state of WO3 on the TiO2 nanorods. Fig. 2a shows XPS survey spectra of pure TiO2 and TiO2@200WO3. The pure TiO2 only contains Ti, O, some traces of C and Sn. However, the TiO2@200WO3 contains W, O and some traces of C. The disappearance of Ti and Sn in this spectrum indicates that the TiO2 nanorods are covered by the amorphous WO3. Fig. 2b shows the high resolution XPS spectrum of the W4f peak. There are spin– orbit doublets in this spectrum corresponding to W4f7/2, W4f5/2 and W5p3/2 peaks which are located at 35.5 eV, 37.8 eV and 41.5 eV, respectively, which match well with those reported in the literature for WO3 [36,43]. The surface and cross-sectional SEM images of the films before and after electrodeposition for various deposition times are illustrated in Fig. 3. Before electrodeposition, the entire surface of the FTO substrate is covered uniformly with TiO2 nanorods with a rectangular cross section (Fig. 3a and b). The TiO2 nanorods are nearly perpendicular to the FTO substrate with diameters of 80– 100 nm and lengths of 700 nm. After electrodeposited, amorphous WO3 nanoparticles are conformably coated onto the surface of TiO2 nanorods (Fig. 3c–e). The morphology and the alignment of TiO2 nanorods are preserved after coating. The TiO2@WO3 nanorods are changed to cylinder in shape with rounded top facets and the lengths are about the same as the TiO2 nanorods. The WO3 film electrodeposited on the FTO substrate has a thickness of 350 nm. Furthermore, the film presents compact and smooth surface, and a fine-grained structure. The structural characteristics of TiO2 nanorod and TiO2@200WO3 film are further investigated by TEM, as shown in Fig. 4. From the low magnification image (Fig. 4a), the TiO2 film is composed of uniform nanorod array.
In order to investigate the ion insertion and electrochromic properties of the films, CV measurements were recorded in the potential region of −0.7–1.0 V at a scan rate of 20 mv s−1. Fig. 5 compares the CV curves of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films carried out in 0.5 M H2SO4 solution. The shapes of the CV curves are similar for all the electrodes. However, the TiO2@WO3 core/shell nanorod arrays show significantly large current densities (high exchanged charge densities) compared to that of TiO2 and WO3 films, which reflects the fact that proton insertion/extraction into the host lattice is facilitated at a given applied potential. Furthermore, the onset potential of the cathodic current for the TiO2@WO3 core/shell nanorod arrays is strongly shifted in the positive direction compared to the TiO2 nanorod; that is, insertion can be achieved at a considerably low applied voltage. When the WO3 films were cathodically polarized in H2SO4, a blue color was observed, indicating the intercalation of H+ into the WO3 layer. When the blue films were anodically polarized, they were bleached, corresponding to the deintercalation process. The area of the hysteresis curves and the position of anodic and cathodic peaks are closely related to the electrochemical processes occurring in the electrochromic films. It is observed that with increasing the electrodeposition time, the anodic and cathodic currents of the film electrodes shift to higher and lower potentials, respectively. The increase in anodic and cathodic currents indicates that the amount of protons and electrons incorporated into the film increases, implying that the film electrode contains more active material with the duration of electrodeposition. To quantitatively compare the electrochromic properties of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films, the visible and near-infrared transmittance and IR reflectance spectra were measured after the film electrodes had been subjected to CV testing for 10 cycles in 0.5 M H2SO4. Fig. 6a shows the visible and near-infrared transmittance spectra of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films in colored and bleached states, which are applied at −0.7 V and 1.0 V (vs. Ag/AgCl) for 30 s, respectively. Considering the transmittance modulations (ΔT¼ Tb−Tc, where Tb and Tc denote transmittance in bleached and colored states, respectively) of the TiO2 array being negligible (Fig. 6b), the WO3 film possesses large transmittance modulation in the wavelength range of the 600–1800 nm including the contribution of FTO substrates, but the transmittance modulation is small over the wavelength 2500 nm. The three TiO2@WO3 core/shell nanorod array films have very similar transmittance modulation behaviors, that is, showing a doublet characteristic and possessing larger transmittance modulation than that of the WO3 film in the wavelength range of the 400–1100 nm and over 1800 nm. The modulation ranges of the transmittance for the films at 750 and 1800 nm are given in Table 1. Compared to the
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Fig. 3. SEM images of (a, b) TiO2 nanorod array, (c) TiO2@200WO3, (d) TiO2@400WO3, (e) TiO2@600WO3 core/shell nanorod array and (f) WO3 film (cross sectional view presented in the inset).
TiO2 and WO3 films, the TiO2@WO3 core/shell nanorod arrays yield much larger values in transmittance modulations; ΔT are 4.3% and 3.1% for the TiO2 nanorod array, 31.6% and 35.5% for the WO3 film. However, ΔT are 40.3% and 70.3% for TiO2@200WO3 core/shell nanorod array film, 52.2% and 46.3% for TiO2@400WO3 core/shell nanorod array film, 57.2% and 38.2% for TiO2@600WO3core/shell nanorod array film. As a general trend, an increase in electrodeposition time leads to an increase of the optical modulation in the visible spectrum region. However, the optical modulation of the core/shell nanorod array films decreases in the near-infrared region at the same time. In addition, the transparency of the TiO2@WO3 core/shell nanorod arrays decreases in its colored state with the electrodeposition duration. This is because as the electrodeposition time increases, the amount of WO3 increases. IR reflectance and corresponding reflectance modulations (ΔR ¼Rb−Rc, where Rb and Rc denote reflectance in bleached and colored states, respectively) of colored and bleached TiO2, WO3 and TiO2@WO3core/shell nanorod array films in the wavelength range of 2.5–25 μm are shown in Fig. 7. As shown in Fig. 7b, the reflectance modulations of the TiO2@WO3 core/shell nanorod arrays are improved remarkably compared to the TiO2 array and the WO3 film in this present work. In addition, the WO3 film only
possesses large transmittance modulation in the wavelength range of the 3–8 μm, while the TiO2@WO3 core/shell nanorod array films exhibit large transmittance modulation in wider wavelength. The modulation ranges of the reflectance at 10 μm are depicted in Table 1. Considering the ΔR of the TiO2 array and pure WO3 film being negligible, the modulation range of the reflectance is 36.7%, 38.4% and 17.8% at 10 μm for the TiO2@200WO3, TiO2@400WO3 and TiO2@600WO3 core/shell nanorod array films, respectively. Switching speed from one state to another state is a very important factor in practical applications of electrochromic systems. The switching characteristics of the TiO2, WO3 and TiO2@WO3 core/shell nanorod array films are investigated by CA and the corresponding in situ transmittance at 750 nm, as shown in Fig. 8. The tests were performed by stepping the voltages between −0.7 V and 1.0 V (in this case, the switching time is defined as the one required for a system to reach 90% of its full modulation). The values of tc and tb for all the films are given in Table 1 (tc and tb denotes the coloration time and the bleaching time, respectively). Generally, inorganic materials show a relatively slow response time due to the low conductivity and the resultant slow electron transfer. For the TiO2 nanorod array film, the switching speed for fully coloration and bleaching is faster
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Fig. 4. TEM images of (a, b) TiO2 nanorod (inset) corresponding FFT pattern of a TiO2 nanorod and (c, d) TiO2@200WO3 core/shell nanorod array.
Fig. 5. The 10th CV curves of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films.
than that of the TiO2@WO3 core/shell ones, indicating that the single-crystalline TiO2 nanorods have high reaction kinetics. While the WO3 film shows the slowest switching speed. Coloration efficiency (CE) is defined as the ratio of change of optical density (ΔOD) of the film in its colored (Tc) and bleached (Tb) state at a certain wavelength and corresponding injected (or ejected) charge density (Q) per unit area. Both OD and CE present the ability of optical modulation during the coloration–bleaching process but the latter one is under the considering of energy consumption. A high CE provides large optical modulation with a small charge insertion or extraction. It can be calculated from the following formulas [44,45]: CEðλÞ ¼
ΔODðλÞ Q
ð1Þ
Fig. 6. (a) Visible and near-infrared transmittance spectra of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films (solid curves, bleached state; dashed curves, colored state) and (b) corresponding in transmittance modulations.
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Table 1 Various parameters of the WO3 film, TiO2 nanorod and TiO2@WO3 core/shell nanorod arrays. Samples
Optical modulation range (%)
Pure WO3 Pure TiO2 TiO2@200WO3 TiO2@400WO3 TiO2@600WO3
Switching speed (s)
750 nm
1800 nm
10 μm
tb
tc
31.6 4.3 40.3 52.2 57.2
35.5 3.1 70.3 46.3 38.2
6.9 0.6 38.4 37.2 17.8
5.0 0.6 2.4 3.2 6.4
5.2 3.6 4.4 4.6 5.4
Fig. 8. (a) CA with voltage interval from −0.7 V (30 s) to 1.0 V (30 s) and (b) corresponding in situ transmittance curves at 750 nm wavelength.
Fig. 7. (a) IR reflectance of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films in the wavelength range of 2.5–25 μm (solid curves, bleached state; dashed curves, colored state) and (b) corresponding in reflectance modulations.
ΔODðλÞ ¼ log
Tb Tc
ð2Þ
Fig. 9 shows the plots of OD at a wavelength of 750 nm vs. the inserted charge density at a potential of −0.7 V. The CE value is found to be 30.9 cm2 C−1 at 750 nm for the WO3 film, 54.1, 67.5 and 43.8 cm2 C−1 at 750 nm for the TiO2@200WO3, TiO2@400WO3 and TiO2@600WO3 core/shell nanorod array films, respectively. The CE value of 67.5 cm2 C−1 at 750 nm for the TiO2@400WO3 core/shell nanorod array, which is comparable to that of the macroporous WO3 thin film prepared by the template-assisted sol–gel method [46], but much higher than that of pure WO3 film, others obtained by anodic oxidation [47] and spray deposited [48]. Combining the results of CA and in situ transmittance vs. time shown in Fig. 8a and b, it can be concluded that, for the TiO2@WO3 core/shell nanorod array electrodeposited for 400 s, a major optical modulation is completed in a short time after voltage switching.
To further understand the electrochemical behavior of the TiO2 and TiO2@WO3 core/shell nanorod array films, EIS measurements were conducted by applying an AC voltage of 5 mV in a frequency range of 0.01 Hz–100 kHz at their bleached state (about 0.33 V vs. Ag/AgCl). Fig. 10 shows the Nyquist plots of the TiO2, WO3 and TiO2@WO3core/shell nanorod array films. All the EIS spectra consist of a semicircle in high-frequency region and a straight line in low frequency region. The high-frequency semicircle is ascribed to the charge-transfer impedance on the electrode/electrolyte interface, while the straight line in low frequency region corresponds to the ion diffusion process within the electrode. The spectra indicate that the film electrodes switch from the H+ diffusion controlled regime to the charge transfer controlled regime as the frequency increases. It is well accepted that a larger semicircle means a larger charge-transfer resistance and a lower slope signifies a lower ion-diffusion rate [49,50]. Apparently, the TiO2 nanorod array exhibits smaller semicircle than the TiO2@WO3 ones, but the angles of the inclined lines are almost the same. In addition, the semicircle is larger with the increase of electrodeposition duration for the TiO2@WO3 core/shell nanorod arrays. Furthermore, the TiO2@400WO3 core/shell nanorod array exhibits lower line slope than the WO3 film, indicating that the core/shell nanorod array has a lower charge transfer resistance than the WO3 film. This result is in accordance with fast switching time. The durability of pure WO3 and TiO2@400WO3 core/shell nanorod array film was evaluated by CA measurements and corresponding insitu transmittance at 750 nm. Fig. 11 shows the transmittance discrepancies of the pure WO3 and the TiO2@400WO3 core/shell nanorod array film during a 10,000-cycle test with cyclically applied voltage range of −0.7 to 1.0 V for interaction/extraction of H+ ions. The pure WO3 film only sustains a transmittance modulation of
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Fig. 9. Variation of the in situ optical density (OD) vs. charge density for (a) WO3, (b) TiO2@200WO3, (c) TiO2@400WO3 and (d) TiO2@600WO3 array films.
Fig. 11. The durability test of the WO3 and TiO2@WO3 core/shell nanorod array films for 10,000 cycles at 750 nm wavelength.
Fig. 10. Nyquist plots of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films.
about 28.1% after subjected for 1200 cycles. However, the TiO2@400WO3 core/shell nanorod array film sustains a transmittance modulation of about 65.1% even after subjected for 10,000 cycles, indicating that the TiO2@WO3 core/shell nanorod array film possesses quite good cycling durability. The data present great promise for the core/shell nanorod arrays as practical electrochromic materials.
properties with high coloration efficiency and large optical modulation. In addition, the TiO2@WO3 core/shell nanorod array films also exhibit excellent cycling performance. The improved electrochromic properties are mainly due to the core/shell structure and the porous space among the nanorod array, which makes the ion diffusion become easier and it also gives larger surface area for charge-transfer reactions. These nanostuctured electrodes can find applications, not only in electrochromic devices but also for photoelectrodes in photocatalytic devices or photoelectrochemical solar cells.
4. Conclusions Acknowledgments The TiO2@WO3 core/shell nanorod arrays have been successfully prepared on a FTO-coated glass. Compared to the TiO2 nanorod array and WO3 films, the core/shell nanorod arrays exhibit highly enhanced ion insertion and electrochromic
This work was supported by Key Science and Technology Innovation Team of Zhejiang Province (2010R50013) and a support program of the Ministry of Education of China.
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Efficient electrochromic materials based on TiO2@WO3 core/shell nanorod arrays G.F. Cai a, D. Zhou a, Q.Q. Xiong a, J.H. Zhang a, X.L. Wang a,b, C.D. Gu a,b, J.P. Tu a,b,n a State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou 310027, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 March 2013 Received in revised form 17 May 2013 Accepted 23 May 2013
TiO2@WO3 core/shell nanorod arrays are prepared by the combination of hydrothermal and electrodeposition method. The array films show remarkable enhancement of the electrochromic properties. In particular, a significant optical modulation (57.2% at 750 nm, 70.3% at 1800 nm and 38.4% at 10 μm), fast switching speed (2.4 s and 1.6 s), high coloration efficiency (67.5 cm2 C−1 at 750 nm) and excellent cycling performance (65.1% after 10,000 cycles) are achieved for the core/shell nanorod arrays. The improved electrochromic properties are mainly attributed to the core/shell structure and the porous space among the nanorod array, which makes the ion diffusion become easier and it also gives larger surface area for charge-transfer reactions. The data present great promise for the TiO2@WO3 core/shell nanorod arrays as practical electrochromic materials. & 2013 Elsevier B.V. All rights reserved.
Keywords: Electrochromism Titanium dioxide Tungsten trioxide Core/shell structure Nanorod array
1. Introduction Today, it is common for people to spend about 90% of their time inside buildings and vehicles, and more and more energy is used to maintain the indoor environment at a level that is both comfortable and healthy [1,2]. Hence, it is important to design a material capable of providing high energy efficiency and indoor comfort simultaneously. Electrochromic materials can change their optical properties persistently and reversibly in the presence of a small electric difference. When thin films of such materials are integrated in devices, they become possible to modulate the transmittance, reflectance, absorptance, and emitance between widely separated extrema [3–7], which endow them to be effective candidates in various applications, such as large area information displays, rearview mirrors for automobiles, thermal control of spacecraft and military camouflage [8–16]. Until recently, most of the applications have been based on the optical changes in the visible spectrum whereas the infrared (IR) region is still rare [17–20]. Electrochromic materials can be also used in the IR region [21–23]. It is widely accepted that the electrochromic phenomena of some inorganic materials is attributed to the injection/extraction of electrons and
n Corresponding author at: State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail addresses: [email protected], [email protected] (J.P. Tu).
0927-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.solmat.2013.05.049
cations [24–27]. The kinetics and magnitude of ion insertion and the electrochromic reaction strongly depend on the diffusion length of ions and the available surface area, which is because the ratedetermining steps of ion intercalation and release are under diffusion control and are limited to a very thin surface layer of the host materials [11,28]. Therefore, it is important to design a material with unique architecture and proper crystal structure to obtain fast insertion kinetics and enhanced durability. Recently, synthesis of aligned single-crystalline TiO2 nanorod or nanowire films has attracted much attention because these nanorods or nanowires offer direct electrical pathways for electrons and can increase the electron transport rate [29,30]. In addition, the nanorods or nanowires provide the vertically aligned nanostructure with a high surface area which allows the electrolyte to penetrate and shorten the proton diffusion paths within the bulk of TiO2. However, the electrochromic performances of pure TiO2 need to be improved. Tungsten trioxide (WO3) has been identified as one of the most promising inorganic electrochromic materials [31–34]. There has been great interest in the addition of WO3 to TiO2 because the addition of adequate WO3 in TiO2 can improve the electrochromic performances of TiO2 [35–37]. However, to the best of our knowledge, there are no reports dedicated to electrochromic performance of TiO2@WO3 core/shell nanorod arrays. In this present work, the TiO2@WO3 core/shell nanorod arrays were prepared on a fluorine-doped tin oxide (FTO)-coated glass, and the electrochromic performances were investigated not only in the visible spectrum but also in the IR region. We show a facile
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route to cover the TiO2 nanorods with amorphous WO3 and demonstrate that these core/shell nanorod arrays have significantly enhanced electrochromic characteristics.
2. Experimental 2.1. Preparation of TiO2 nanorod array Rutile TiO2 nanorod arrays were prepared on a FTO glass substrate by a previously reported hydrothermal method [29]. In a typical synthesis, 30 ml of concentrated hydrochloric acid (Sinopharm) was diluted with 30 ml de-ionized water, and mixed with 1 ml titanium n-butoxide (Sinopharm) in a 100 ml beaker. This clear solution mixture and a clean FTO glass substrate were transferred to a Teflon-lined stainless steel autoclave (100 ml in volume), where the FTO substrate was submerged in the solution and placed at an angle against the wall of the Teflon-liner with the conducting side facing down. The sealed autoclave was heated in a vacuum oven at 150 1C for 4 h, and then cooled down to room temperature under flowing water. The sample was taken out, rinsed extensively with de-ionized water and dried in vacuum at 60 1C. A TiO2 nanorod array film was uniformly coated on the FTO glass substrate. 2.2. Preparation of TiO2@WO3 core/shell nanorod array TiO2@WO3 core/shell nanorod arrays were obtained by depositing a WO3 thin layer onto the surface of TiO2 nanorods by cathodic electrodeposition. The deposition solution was prepared by dissolving Na2WO4 (Sinopharm) salt in de-ionized water (concentration: 12.5 mM). Then hydrogen peroxide (Sinopharm, 30%) was added to the solution maintaining a concentration ratio of 3 with sodium tungstate, according to the literatures [38,39]. The pH value of the resulting solution was adjusted down to 1.2 by adding perchloric acid (Sinopharm). The FTO glass coated with TiO2 nanorod array was used as the deposition electrode. The electrodeposition was performed with a CHI660D electrochemical workshop at room temperature and in a three-electrode cell. The reference electrode was an Ag/AgCl electrode and the counter electrode was a 2 2 cm2 platinum foil. The electrodeposition process was conducted at a potential of −0.7 V (vs. Ag/AgCl) for 200 s, 400 s and 600 s, and named as TiO2@200WO3, TiO2@400WO3 and TiO2@600WO3 for the corresponding samples, respectively. Argon was bubbled for 5 min in the cell prior to the deposition. After electrodeposition, the samples were thoroughly washed with methanol and water and finally dried in air. For comparison, a pure WO3 film on an FTO substrate was also prepared by electrodeposition for 400 s with the same parameters.
reflectance reference (R≈99% in IR region). Each spectrum was recorded ex-situ (after the samples taken out of the threecompartment system, instantly rinsed and wiped off from the remaining persistent water). The cyclic voltammetry (CV) and chronoamperometry (CA) measurements were carried out in a three-compartment system containing 0.5 M H2SO4 as electrolyte, Ag/AgCl as a reference electrode and Pt foil as the counter electrode. CV measurements of the films were performed using a CHI660D electrochemical workshop at a scan rate of 20 mv s−1 between −0.7 and 1.0 V at room temperature (257 1 1C). Electrochemical impedance spectrum (EIS) tests were conducted on this electrochemical workstation with a superimposed 5 mV sinusoidal voltage in the frequency range of 0.01 Hz–100 kHz.
3. Results and discussion 3.1. Structure and morphology To determine the crystal structure and possible phase changes during the electrodeposition process, XRD patterns are collected from the films before and after electrodeposition for various deposition times (Fig. 1). From pattern a, after subtracting the diffraction peaks of FTO glass, all the diffraction peaks are indexed to the tetragonal rutile TiO2 (JCPDS no. 03-1122). Compared to the standard JCPDS card, the peak centered at 2θ degree of 62.71 corresponding to the (002) diffraction is significantly enhanced, which indicates that the film is highly oriented in the [001] direction on the substrate. From patterns b–d, the films only display an additional broadened peak around 2θ≈251 compared to the FTO glass substrate with TiO2 nanorod array. It indicates an amorphous WO3 in presence, which is in accordance with the previous report [39]. In addition, a marginal reduction in peak intensity clearly reveals an increase of WO3 in the films with the deposition time. From pattern e, the WO3 film only displays an additional broadened peak, indicating an amorphous characteristic. It is well known from the literature survey that the electrochromic performance of metal oxide is closely related to its level of crystallization [11,40,41]. For instance, crystalline WO3 film exhibited inferior performance to amorphous one. However, the amorphous WO3 has less cyclic stability [42]. Therefor, synthesis of the TiO2@WO3 core/shell nanorod array is an interesting way to overcome these deficiencies. Crystalline TiO2 nanorod as core will have a good durability and amorphous WO3 as shell will exhibit good electrochromic performance.
2.3. Characterization The structural and morphological characterizations of the films were carried out using X-ray diffraction (XRD, RIGAKU D/MAX 2550/PC with Cu Kα radiation), X-ray photoelectron spectroscopy (XPS, AXIS UTLTRADLD equipped with a dual Mg Kα-Al Kα anode for photo excitation) and field emission scanning electron microscopy (FESEM, Hitachi SU-70). For the transmission electron microscopy (TEM, JEOL JEM-200CX) analysis, the films were scratched from the FTO substrate and re-dispersed in ethanol solution. The transmission spectra of electrochromic films in the fully colored and fully bleached states were measured over the wavelength range from 400 to 2500 nm with a SHIMADZU UV3600 spectrophotometer. The IR reflectance spectra of the films were measured using an IR spectrophotometer TENSOR27 in the wavelength range from 2.5 to 25 μm. A gold mirror was used as a
Fig. 1. XRD patterns of (a) TiO2 nanorod, (b) TiO2@200WO3, (c) TiO2@400WO3, (d) TiO2@600WO3 array, and (e) WO3 film.
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The high magnification TEM (HRTEM) image collected from the individual nanorod shows that it is completely crystalline along its entire length and reveals clear lattice fringes with interplanar spacings of 0.32 and 0.29 nm, which are consistent with the d-spacings of (110) and (001) planes of rutile TiO2 (Fig. 4b). In addition, the [110] axis is perpendicular to the nanorod side walls, and the corresponding sharp FFT pattern of the nanorod is examined along the [110] zone axis. These data confirm the single-crystalline structure and demonstrate that the TiO2 nanorod grows along the [001] direction. After electrodeposited for 200 s, the TiO2 nanorod array is uniformly covered by amorphous WO3 (Fig. 4c). The high magnification TEM image collected for a representative TiO2@200WO3 nanorod shows that an amorphous WO3 layer is uniformly coated on the entire nanorod (Fig. 4d). The TEM image also reveals the amorphous nature of the WO3 shell. 3.2. Electrochemical and electrochromic properties
Fig. 2. (a) XPS survey spectra of TiO2 nanorod array and TiO2@200WO3 core/shell nanorod array and (b) detail of the W4f peak for TiO2@200WO3 core/shell nanorod array.
XPS measurements were performed to investigate the chemical composition and oxidation state of WO3 on the TiO2 nanorods. Fig. 2a shows XPS survey spectra of pure TiO2 and TiO2@200WO3. The pure TiO2 only contains Ti, O, some traces of C and Sn. However, the TiO2@200WO3 contains W, O and some traces of C. The disappearance of Ti and Sn in this spectrum indicates that the TiO2 nanorods are covered by the amorphous WO3. Fig. 2b shows the high resolution XPS spectrum of the W4f peak. There are spin– orbit doublets in this spectrum corresponding to W4f7/2, W4f5/2 and W5p3/2 peaks which are located at 35.5 eV, 37.8 eV and 41.5 eV, respectively, which match well with those reported in the literature for WO3 [36,43]. The surface and cross-sectional SEM images of the films before and after electrodeposition for various deposition times are illustrated in Fig. 3. Before electrodeposition, the entire surface of the FTO substrate is covered uniformly with TiO2 nanorods with a rectangular cross section (Fig. 3a and b). The TiO2 nanorods are nearly perpendicular to the FTO substrate with diameters of 80– 100 nm and lengths of 700 nm. After electrodeposited, amorphous WO3 nanoparticles are conformably coated onto the surface of TiO2 nanorods (Fig. 3c–e). The morphology and the alignment of TiO2 nanorods are preserved after coating. The TiO2@WO3 nanorods are changed to cylinder in shape with rounded top facets and the lengths are about the same as the TiO2 nanorods. The WO3 film electrodeposited on the FTO substrate has a thickness of 350 nm. Furthermore, the film presents compact and smooth surface, and a fine-grained structure. The structural characteristics of TiO2 nanorod and TiO2@200WO3 film are further investigated by TEM, as shown in Fig. 4. From the low magnification image (Fig. 4a), the TiO2 film is composed of uniform nanorod array.
In order to investigate the ion insertion and electrochromic properties of the films, CV measurements were recorded in the potential region of −0.7–1.0 V at a scan rate of 20 mv s−1. Fig. 5 compares the CV curves of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films carried out in 0.5 M H2SO4 solution. The shapes of the CV curves are similar for all the electrodes. However, the TiO2@WO3 core/shell nanorod arrays show significantly large current densities (high exchanged charge densities) compared to that of TiO2 and WO3 films, which reflects the fact that proton insertion/extraction into the host lattice is facilitated at a given applied potential. Furthermore, the onset potential of the cathodic current for the TiO2@WO3 core/shell nanorod arrays is strongly shifted in the positive direction compared to the TiO2 nanorod; that is, insertion can be achieved at a considerably low applied voltage. When the WO3 films were cathodically polarized in H2SO4, a blue color was observed, indicating the intercalation of H+ into the WO3 layer. When the blue films were anodically polarized, they were bleached, corresponding to the deintercalation process. The area of the hysteresis curves and the position of anodic and cathodic peaks are closely related to the electrochemical processes occurring in the electrochromic films. It is observed that with increasing the electrodeposition time, the anodic and cathodic currents of the film electrodes shift to higher and lower potentials, respectively. The increase in anodic and cathodic currents indicates that the amount of protons and electrons incorporated into the film increases, implying that the film electrode contains more active material with the duration of electrodeposition. To quantitatively compare the electrochromic properties of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films, the visible and near-infrared transmittance and IR reflectance spectra were measured after the film electrodes had been subjected to CV testing for 10 cycles in 0.5 M H2SO4. Fig. 6a shows the visible and near-infrared transmittance spectra of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films in colored and bleached states, which are applied at −0.7 V and 1.0 V (vs. Ag/AgCl) for 30 s, respectively. Considering the transmittance modulations (ΔT¼ Tb−Tc, where Tb and Tc denote transmittance in bleached and colored states, respectively) of the TiO2 array being negligible (Fig. 6b), the WO3 film possesses large transmittance modulation in the wavelength range of the 600–1800 nm including the contribution of FTO substrates, but the transmittance modulation is small over the wavelength 2500 nm. The three TiO2@WO3 core/shell nanorod array films have very similar transmittance modulation behaviors, that is, showing a doublet characteristic and possessing larger transmittance modulation than that of the WO3 film in the wavelength range of the 400–1100 nm and over 1800 nm. The modulation ranges of the transmittance for the films at 750 and 1800 nm are given in Table 1. Compared to the
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Fig. 3. SEM images of (a, b) TiO2 nanorod array, (c) TiO2@200WO3, (d) TiO2@400WO3, (e) TiO2@600WO3 core/shell nanorod array and (f) WO3 film (cross sectional view presented in the inset).
TiO2 and WO3 films, the TiO2@WO3 core/shell nanorod arrays yield much larger values in transmittance modulations; ΔT are 4.3% and 3.1% for the TiO2 nanorod array, 31.6% and 35.5% for the WO3 film. However, ΔT are 40.3% and 70.3% for TiO2@200WO3 core/shell nanorod array film, 52.2% and 46.3% for TiO2@400WO3 core/shell nanorod array film, 57.2% and 38.2% for TiO2@600WO3core/shell nanorod array film. As a general trend, an increase in electrodeposition time leads to an increase of the optical modulation in the visible spectrum region. However, the optical modulation of the core/shell nanorod array films decreases in the near-infrared region at the same time. In addition, the transparency of the TiO2@WO3 core/shell nanorod arrays decreases in its colored state with the electrodeposition duration. This is because as the electrodeposition time increases, the amount of WO3 increases. IR reflectance and corresponding reflectance modulations (ΔR ¼Rb−Rc, where Rb and Rc denote reflectance in bleached and colored states, respectively) of colored and bleached TiO2, WO3 and TiO2@WO3core/shell nanorod array films in the wavelength range of 2.5–25 μm are shown in Fig. 7. As shown in Fig. 7b, the reflectance modulations of the TiO2@WO3 core/shell nanorod arrays are improved remarkably compared to the TiO2 array and the WO3 film in this present work. In addition, the WO3 film only
possesses large transmittance modulation in the wavelength range of the 3–8 μm, while the TiO2@WO3 core/shell nanorod array films exhibit large transmittance modulation in wider wavelength. The modulation ranges of the reflectance at 10 μm are depicted in Table 1. Considering the ΔR of the TiO2 array and pure WO3 film being negligible, the modulation range of the reflectance is 36.7%, 38.4% and 17.8% at 10 μm for the TiO2@200WO3, TiO2@400WO3 and TiO2@600WO3 core/shell nanorod array films, respectively. Switching speed from one state to another state is a very important factor in practical applications of electrochromic systems. The switching characteristics of the TiO2, WO3 and TiO2@WO3 core/shell nanorod array films are investigated by CA and the corresponding in situ transmittance at 750 nm, as shown in Fig. 8. The tests were performed by stepping the voltages between −0.7 V and 1.0 V (in this case, the switching time is defined as the one required for a system to reach 90% of its full modulation). The values of tc and tb for all the films are given in Table 1 (tc and tb denotes the coloration time and the bleaching time, respectively). Generally, inorganic materials show a relatively slow response time due to the low conductivity and the resultant slow electron transfer. For the TiO2 nanorod array film, the switching speed for fully coloration and bleaching is faster
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Fig. 4. TEM images of (a, b) TiO2 nanorod (inset) corresponding FFT pattern of a TiO2 nanorod and (c, d) TiO2@200WO3 core/shell nanorod array.
Fig. 5. The 10th CV curves of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films.
than that of the TiO2@WO3 core/shell ones, indicating that the single-crystalline TiO2 nanorods have high reaction kinetics. While the WO3 film shows the slowest switching speed. Coloration efficiency (CE) is defined as the ratio of change of optical density (ΔOD) of the film in its colored (Tc) and bleached (Tb) state at a certain wavelength and corresponding injected (or ejected) charge density (Q) per unit area. Both OD and CE present the ability of optical modulation during the coloration–bleaching process but the latter one is under the considering of energy consumption. A high CE provides large optical modulation with a small charge insertion or extraction. It can be calculated from the following formulas [44,45]: CEðλÞ ¼
ΔODðλÞ Q
ð1Þ
Fig. 6. (a) Visible and near-infrared transmittance spectra of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films (solid curves, bleached state; dashed curves, colored state) and (b) corresponding in transmittance modulations.
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Table 1 Various parameters of the WO3 film, TiO2 nanorod and TiO2@WO3 core/shell nanorod arrays. Samples
Optical modulation range (%)
Pure WO3 Pure TiO2 TiO2@200WO3 TiO2@400WO3 TiO2@600WO3
Switching speed (s)
750 nm
1800 nm
10 μm
tb
tc
31.6 4.3 40.3 52.2 57.2
35.5 3.1 70.3 46.3 38.2
6.9 0.6 38.4 37.2 17.8
5.0 0.6 2.4 3.2 6.4
5.2 3.6 4.4 4.6 5.4
Fig. 8. (a) CA with voltage interval from −0.7 V (30 s) to 1.0 V (30 s) and (b) corresponding in situ transmittance curves at 750 nm wavelength.
Fig. 7. (a) IR reflectance of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films in the wavelength range of 2.5–25 μm (solid curves, bleached state; dashed curves, colored state) and (b) corresponding in reflectance modulations.
ΔODðλÞ ¼ log
Tb Tc
ð2Þ
Fig. 9 shows the plots of OD at a wavelength of 750 nm vs. the inserted charge density at a potential of −0.7 V. The CE value is found to be 30.9 cm2 C−1 at 750 nm for the WO3 film, 54.1, 67.5 and 43.8 cm2 C−1 at 750 nm for the TiO2@200WO3, TiO2@400WO3 and TiO2@600WO3 core/shell nanorod array films, respectively. The CE value of 67.5 cm2 C−1 at 750 nm for the TiO2@400WO3 core/shell nanorod array, which is comparable to that of the macroporous WO3 thin film prepared by the template-assisted sol–gel method [46], but much higher than that of pure WO3 film, others obtained by anodic oxidation [47] and spray deposited [48]. Combining the results of CA and in situ transmittance vs. time shown in Fig. 8a and b, it can be concluded that, for the TiO2@WO3 core/shell nanorod array electrodeposited for 400 s, a major optical modulation is completed in a short time after voltage switching.
To further understand the electrochemical behavior of the TiO2 and TiO2@WO3 core/shell nanorod array films, EIS measurements were conducted by applying an AC voltage of 5 mV in a frequency range of 0.01 Hz–100 kHz at their bleached state (about 0.33 V vs. Ag/AgCl). Fig. 10 shows the Nyquist plots of the TiO2, WO3 and TiO2@WO3core/shell nanorod array films. All the EIS spectra consist of a semicircle in high-frequency region and a straight line in low frequency region. The high-frequency semicircle is ascribed to the charge-transfer impedance on the electrode/electrolyte interface, while the straight line in low frequency region corresponds to the ion diffusion process within the electrode. The spectra indicate that the film electrodes switch from the H+ diffusion controlled regime to the charge transfer controlled regime as the frequency increases. It is well accepted that a larger semicircle means a larger charge-transfer resistance and a lower slope signifies a lower ion-diffusion rate [49,50]. Apparently, the TiO2 nanorod array exhibits smaller semicircle than the TiO2@WO3 ones, but the angles of the inclined lines are almost the same. In addition, the semicircle is larger with the increase of electrodeposition duration for the TiO2@WO3 core/shell nanorod arrays. Furthermore, the TiO2@400WO3 core/shell nanorod array exhibits lower line slope than the WO3 film, indicating that the core/shell nanorod array has a lower charge transfer resistance than the WO3 film. This result is in accordance with fast switching time. The durability of pure WO3 and TiO2@400WO3 core/shell nanorod array film was evaluated by CA measurements and corresponding insitu transmittance at 750 nm. Fig. 11 shows the transmittance discrepancies of the pure WO3 and the TiO2@400WO3 core/shell nanorod array film during a 10,000-cycle test with cyclically applied voltage range of −0.7 to 1.0 V for interaction/extraction of H+ ions. The pure WO3 film only sustains a transmittance modulation of
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Fig. 9. Variation of the in situ optical density (OD) vs. charge density for (a) WO3, (b) TiO2@200WO3, (c) TiO2@400WO3 and (d) TiO2@600WO3 array films.
Fig. 11. The durability test of the WO3 and TiO2@WO3 core/shell nanorod array films for 10,000 cycles at 750 nm wavelength.
Fig. 10. Nyquist plots of TiO2, WO3 and TiO2@WO3 core/shell nanorod array films.
about 28.1% after subjected for 1200 cycles. However, the TiO2@400WO3 core/shell nanorod array film sustains a transmittance modulation of about 65.1% even after subjected for 10,000 cycles, indicating that the TiO2@WO3 core/shell nanorod array film possesses quite good cycling durability. The data present great promise for the core/shell nanorod arrays as practical electrochromic materials.
properties with high coloration efficiency and large optical modulation. In addition, the TiO2@WO3 core/shell nanorod array films also exhibit excellent cycling performance. The improved electrochromic properties are mainly due to the core/shell structure and the porous space among the nanorod array, which makes the ion diffusion become easier and it also gives larger surface area for charge-transfer reactions. These nanostuctured electrodes can find applications, not only in electrochromic devices but also for photoelectrodes in photocatalytic devices or photoelectrochemical solar cells.
4. Conclusions Acknowledgments The TiO2@WO3 core/shell nanorod arrays have been successfully prepared on a FTO-coated glass. Compared to the TiO2 nanorod array and WO3 films, the core/shell nanorod arrays exhibit highly enhanced ion insertion and electrochromic
This work was supported by Key Science and Technology Innovation Team of Zhejiang Province (2010R50013) and a support program of the Ministry of Education of China.
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