Aqueous synthesis of tin- and indium-doped WO3 films via evaporation-driven deposition and their electrochromic properties

M-doped WO3 (M = Sn or In) films were prepared from aqueous coating solutions via evaporation-driven deposition during low-speed dip coating. Sn- and In-doping were easily achieved by controlling the chemical composition of simple coating solutions containing only metal salts and water. The crystallinity of the WO3, Sn-doped WO3, and In-doped WO3 films varied with heating temperature, where amorphous and crystalline films were obtained by heating at 200 and 500 °C, respectively. All the amorphous and crystalline films showed an electrochromic response, but good photoelectrochemical stability was observed only for the crystalline samples heated at 500 °C. The crystalline In–WO3 films exhibited a faster electrochromic color change than the WO3 or Sn–WO3 films, and good cycle stability for the electrochromic response in the visible wavelength region.


Introduction
Electrochromic lm materials are able to reversibly change their optical properties by applying an electrical voltage, and have thus raised much attention for various practical applications, such as in smart windows, display devices, and sensor materials. [1][2][3][4][5] Tungsten oxide (WO 3 ) is a typically used cathodic electrochromic material because of its relatively fast color change and good stability. [5][6][7][8][9][10][11] Two-terminal electrochromic devices based on cathodic WO 3 electrodes and appropriate anodic materials such as dimethyl ferrocene, 12 hydroquinone, 13 tetrathiafulvalene 14 (soluble anodic species), and NiO, 15,16 polyaniline 17 (insoluble anodic species) have been widely studied for practical applications. The electrochromic reaction of WO 3 can be represented as: where the electrochemical insertion of cations (M + ¼ H + , Li + , or Na + ) provides a reversible color change from transparent to blue.
The electrochromic performance of WO 3 materials, including the response speed, the durability, and the degree of color change, is known to strongly depend on the crystallinity of the WO 3 phase. 5,6,[18][19][20][21][22][23][24] The electrochromic behavior of amorphous WO 3 is reported to be related to the polaron transition between W 4+ , W 5+ , and W 6+ ions, 18,19 and the coloration efficiency of amorphous materials is generally better than crystalline materials. 5,6,[20][21][22][23][24] However, amorphous WO 3 lms oen dissolve in acidic electrolyte solutions, resulting in a low cycle stability. 5,6,[20][21][22][23][24] The electrochromism of crystalline WO 3 can be discussed on the basis of the variation in electron density with enhanced electron scattering resulting from the intercalation of cations. Although crystalline WO 3 lms exhibit a high electrochemical stability and an optical modulation over a wide range of wavelengths, including the infrared region, 6 the electrochromic response speed is relatively low. 5,6,20,21 Therefore, controlling the crystallinity of the WO 3 lm material is essential for improving the electrochromic performance.
Doping with metal ions (such as Ni and Ti, among others) has been widely investigated as another strategy for making high-performance WO 3 electrochromic materials. [24][25][26][27] Bathe et al. investigated the inuence of Ti doping on the electrochromic properties of WO 3 thin lms prepared by pulsed spray pyrolysis, and suggested that doping WO 3 with Ti induced a phase transformation from monoclinic to amorphous and a rough surface morphology, resulting in improvement in the cycle stability, charge storage capacity, and reversibility of the lms. 24 Cai et al. prepared Ti-doped WO 3 lms with a hierarchical star-like structure by a hydrothermal method, where the star-like structure had a low charge-transfer resistance and ion diffusion resistance, leading to fast switching speed and high coloration efficiency. 26 Zhou reported the preparation of Nidoped WO 3 lms by a hydrothermal method, where the doping caused a distortion of the WO 3 crystal structure and the formation of vertically aligned nanorods, which enhanced the optical modulation, coloration efficiency, and cycle stability. 27 Such doping strategy has been also applied to other electrode materials than WO 3 , such as NiO. Kim et al. reported that the Cu doping into NiO lms resulted in the high performance electrochromic supercapacitors. 28 These results suggest that doping should allow us to achieve improvement in the electrochromic performance of WO 3 materials.
In this work, we doped amorphous and crystalline WO 3 lms with tin (Sn 4+ ) and indium (In 3+ ) ions, and investigated the effect of the metal ion doping on the electrochromic properties of the WO 3 lms. The addition of Sn 4+ or In 3+ ions was expected to improve the durability of WO 3 electrochromic lms because these ions are electrochemically more stable than W 6+ ions in aqueous media. Moreover, doping with low-valence metal ions should induce oxygen vacancies in the WO 3 crystal lattice, which could inuence the electron density in the WO 3 phase, affecting the electrochromic performance. We recently suggested a low-speed dip-coating technique as a novel coating technique for making metal oxide thin lms from organicadditive-free aqueous solutions. [29][30][31] Fig. 1 shows a schematic illustration of the lm deposition during low-speed dip coating. The evaporation-driven deposition of solutes during dip coating with extremely low substrate withdrawal speeds would hinder the aqueous solution from gathering to form droplets, achieving a homogeneous deposition of a lm layer on the substrate. The evaporation-driven deposition process does not need any organic additives for modifying the wettability of the aqueous coating solutions and, thus, pure inorganic precursor layers can be obtained on a glass substrate. Such inorganic precursors, thus obtained, would enable simple control of the crystallinity of WO 3 lms from amorphous to crystalline by varying the heating temperature. Moreover, the aqueous route is thought to allow for the doping of WO 3 materials with various metal ions because water can dissolve many different metal salts. Here, we prepared amorphous and crystalline M-doped WO 3 lms (M ¼ Sn or In) by a low-speed dip-coating technique with simple aqueous solutions containing only (NH 4 ) 10 -W 12 O 41 $5H 2 O, In(NO 3 ) 3 $3H 2 O and SnCl 4 $5H 2 O, and evaluated their electrochromic properties. The effect of Sn-and In-doping on the electrochromic performance of WO 3 lms was studied by measuring the cyclic voltammogram and optical modulation during electrochromic reactions.

Experimental
Preparation of M-doped WO 3 (M ¼ Sn or In) thin lms by lowspeed dip coating HCl aqueous solutions of pH 1.1-1.5 and NH 3 aqueous solutions of pH 11.5 were prepared as the solvents for coating solutions by diluting with puried water ca. 36.0 mass% hydrochloric acid (Wako Pure Chemical Industries, Osaka, Japan) and ca. 10 mass% ammonia solutions (Wako Pure Chemical Industries, Osaka, Japan), respectively. Table 1 shows the compositions of the coating solutions for WO 3 3 11.5 (NH 3 ) 5.0 6.0 -35 0.085 In-WO 3 1.1 (HCl) 5.0 -6.0 38 0.12 a The mole ratio was analyzed by X-ray photoelectron spectroscopy (XPS). WO 3 and M-doped WO 3 (M ¼ Sn or In) precursor lms were deposited on uorine-doped tin oxide (FTO) glass substrates (20 mm Â 40 mm Â 1.0 mm) by a low-speed dip-coating technique. Low-speed dip coating was performed using a dip-coater (Portable Dip Coater DT-0001, SDI, Kyoto, Japan) in a thermostatic oven, where the substrates were withdrawn at 0.05 cm min À1 . The coating temperature, i.e., the temperature of the substrates, solutions, and atmosphere, was kept at 25 C (for Indoped WO 3 lms) or 40 C (for WO 3 and Sn-doped WO 3 lms), where the solutions and substrates were heated at the prescribed temperature for 10 min in the thermostatic oven before the dip coating. The precursor lms were heated in air at 200 C for 24 h or at 500 C for 0.5 h, where the precursor lms were directly transferred to an electric furnace held at the prescribed temperature. Hereaer, the Sn-and In-doped WO 3 lms are denoted as Sn-and In-WO 3 lms.

Characterization
Microscopic observation of the thin lm samples was carried out using an optical microscope (KH-1300, HiROX, Tokyo, Japan). The microstructure of the thin lms was observed using a eld emission scanning electron microscope (FE-SEM; Model JSM-6500F, JEOL, Tokyo, Japan). The crystalline phases were identied using an X-ray diffractometer (Model Rint-Ultima III, Rigaku, Tokyo, Japan) with CuKa radiation operated at 40 kV and 40 mA at an incident angle of 0.5 . The chemical compositions of the product lms were obtained using an X-ray photoelectron spectrometer (XPS; PHI5000 Versa Probe, ULVAC-PHI, Chigasaki, Japan) with a monochromatic AlKa Xray source. The XPS analysis was done for the lm samples coated on silica glass substrates, because FTO substrates containing Sn 4+ ions inhibited the analysis of Sn-WO 3 lms. To counter the surface charging, a charge neutralizer was used during the collection of the spectra.
Film thickness was measured using a contact probe surface prolometer (SE-3500K31, Kosaka Laboratory, Tokyo, Japan). A part of the thin lm was scraped off with a surgical knife immediately aer the lm deposition, and the level difference between the coated part and the scraped part was measured aer heat treatment.

Measurement of electrochromic properties
Electrochromic properties of the WO 3 and M-doped WO 3 lms were evaluated at room temperature in a three-electrode cell using a potentiostat (HZ-7000, Hokuto Denko, Osaka, Japan) consisting of the lm sample, a platinized Pt electrode, and a saturated calomel electrode (SCE) as the working, counter, and reference electrodes, respectively. An aqueous solution of 1 M H 2 SO 4 was used as the supporting electrolyte.
Cyclic voltammetry (CV) was performed on the WO 3 and Mdoped WO 3 lms at a scan rate of 10 mV s À1 between À1.0 and 2.0 V vs. the SCE.
The optical modulation induced by electrochromic reactions was evaluated by an in situ optical absorption measurement. The three-electrode cell with circular silica windows (1.77 cm 2 ) as a light path for an in situ UV-Vis-NIR absorption measurement was connected to a potentiostat, and set in the optical spectrometer (V-570, JASCO, Tokyo, Japan). The coloring and bleaching of the WO 3 and M-doped WO 3 lms was carried out at À0.75 and 1.5 V vs. the SCE, respectively. The application of voltage was stopped at 5-(coloring) or 100-(bleaching) second intervals, and then optical absorption spectra were measured at wavelengths of 300-1300 nm. An FTO glass substrate was used as the reference for the optical measurement. Coloring and bleaching times were dened as the times when the variation in the optical absorption spectra stopped moving upon application of a voltage.
The cycle stability was tested by repeating the electrochromic color change. The coloring and bleaching cycles were repeated 50 times, where the coloring and bleaching of the lms were performed by a voltage application of À0.75 V vs. the SCE for 20 s and 1.5 V vs. the SCE for 200 s, respectively. Aer 1 and 50 cycles, the optical absorption spectra of the lm samples were measured.  coating, and then heated at 200 C for 24 h or at 500 C for 0.5 h for the thermal conversion to WO 3 , Sn-WO 3 , and In-WO 3 lms. Fig. 2 shows optical micrographs of the WO 3 , Sn-WO 3 , and In-WO 3 lms heated at 500 C. All the product lms were free from cracks and had a high level of transparency. In this work, we prepared Sn-WO 3 and In-WO 3 lms with a wide range of M/W mole ratios (M ¼ Sn or In) between 0.050 and 0.20. However, the Sn-WO 3 lms became cloudy when the Sn/W mole ratio was over 0.10, which may be because of the generation of SnO 2 phase in the lms. Therefore, the M/W mole ratio was xed at 0.10 and these results are discussed hereaer. The thickness and the M/W (M ¼ Sn or In) mole ratio of the product lms heated at 500 C are shown in Table 1. The lm thickness was ca. 35 nm irrespective of whether doping was with Sn 4+ or In 3+ ions. The M/W mole ratios (M ¼ Sn or In) in the Sn-WO 3 and In-WO 3 lms, as evaluated by XPS, were 0.085 and 0.12, respectively, which closely agree with the compositions of the coating solutions (0.10) (the XPS spectra are shown in ESI Fig. S1 †). In this work, low-concentration (NH 4 ) 10 W 12 O 41 aqueous solutions were used as the coating solutions ([(NH 4 ) 10 W 12 O 41 $5H 2 O] ¼ 5.00 or 10.0 mM) because tungsten salts are poorly soluble in many different solvents containing water and alcohols. However, homogeneous coating on the whole substrates was achieved with the low-concentration solutions via one-time coating. In the case of low-speed dip coating, the solvent preferentially evaporates at the edge of the meniscus, and the coating solutions are then locally concentrated there, resulting in the deposition of solutes on the substrates (Fig. 1). [29][30][31] The evaporation-induced concentration enabled us to make homogeneous WO 3 coating layers even from low-concentration solutions. Moreover, in this process, Sn-or In-doping was easily succeeded by the addition of tin or indium salts into the coating solutions.

Results and discussion
XRD patterns of the WO 3 , Sn-WO 3 , and In-WO 3 lms heated at 200 and 500 C are shown in Fig. 3 and 4, respectively. Any diffraction peaks other than those due to FTO substrates were not observed for the WO 3 , Sn-WO 3 , or In-WO 3 lms heated at 200 C (Fig. 3), which indicates that the lms heated at this temperature consisted of an amorphous phase. Diffraction patterns attributable to the monoclinic WO 3 phase appeared for all the lms heated at 500 C (Fig. 4), where SnO 2 or In 2 O 3 phases were not detected in the Sn-WO 3 or In-WO 3 lms, respectively. The peak shi due to the substitution of Sn 4+ or In 3+ ions for W 6+ ions was not clearly observed in the XRD patterns of the Sn-WO 3 or In-WO 3 lms, while the diffraction peaks of the (002), (020), and (200) planes of monoclinic WO 3 weakened and broadened upon the addition of Sn 4+ and In 3+ ions, which suggests that Sn-or Indoping could result in deformation of the WO 3 lattice.

Electrochromic properties of WO 3 and M-doped WO 3 (M ¼ Sn or In) lms
The electrochromic properties were evaluated for the amorphous and crystalline WO 3 , Sn-WO 3 , and In-WO 3 lms heated at 200 and 500 C, respectively, where the electrochemical measurements were performed in an aqueous electrolyte of 1 M H 2 SO 4 . In this case, the coloring and bleaching of WO 3 lms progressed with the intercalation and deintercalation of H + ions. The coloring of WO 3 lms is the following reduction reaction: 5 WO 3 + H + + e À / HWO 3 (deep blue color).
The bleaching of the lms then follows the oxidation reaction: HWO 3 / WO 3 + H + + e À (light yellow color).    5 shows cyclic voltammograms obtained for the amorphous and crystalline product lms. Cathodic peaks from the reduction reaction with H + ions were detected between À0.65 and À1.0 V vs. the SCE, irrespective of the heating temperature or the metal ion dopant used (Fig. 5), where the coloring of the lms to deep blue was visually conrmed for all the lms (see Fig. 6a). The cathodic response was almost unchanged with the variation in the crystallinity and the addition of Sn 4+ or In 3+ ions (Fig. 5). The anodic response due to the oxidation reaction was observed between À0.50 and 0.30 V vs. the SCE in the CV curves for all the lms, and the bleaching of the blue color started there (Fig. 6b). The anodic peaks in the CV curves broadened with a decrease in heating temperature (Fig. 5), and the amorphous WO 3 , Sn-WO 3 , and In-WO 3 lms heated at 200 C were partially dissolved aer the coloring and bleaching cycle (Fig. 7a). Such dissolution of the lms was not observed for the crystalline lms heated at 500 C (Fig. 7b). These results could indicate that the electrochemical stability of the amorphous lms was relatively low, and the Sn-and In-doping did not improve the durability of the amorphous WO 3 lms. Therefore, hereaer, we mainly focused on the effect of the Sn-and In-doping on the electrochromic response of the crystalline WO 3 , Sn-WO 3 , and In-WO 3 lms heated at 500 C.
We evaluated the response speed of the electrochromic optical modulation for the crystalline WO 3 , Sn-WO 3 , and In-WO 3 lms, where the UV-Vis-NIR absorption spectra were measured when a voltage was applied. The coloring and bleaching of the lms were carried out at À0.75 and 1.5 V vs. the SCE, respectively. Fig. 8 shows the variation in the UV-Vis-NIR absorption spectra of the crystalline WO 3 ( Fig. 8a and b), Sn-WO 3 (Fig. 8c and d), and In-WO 3 (Fig. 8e and f) lms upon application of a voltage, and Table 2 shows the reaction time that was needed to nish the coloring and breaching of the lms. The transmittance of the as-prepared crystalline WO 3 , Sn-WO 3 , and In-WO 3 lms was over 80% at wavelengths of 500-1300 nm (Fig. 8). The transmittance of the lms decreased with the coloring by applying À0.75 V vs. the SCE. The coloring of the lms concluded within 20 s for all the crystalline lms, where the transmittance of WO 3 , Sn-WO 3 , and In-WO 3 lms at wavelengths greater than 600 nm was reduced to ca. 30, 20, and 40%, respectively (Fig. 8a, c, e and Table 2). The bleaching of the lms was achieved by applying 1.5 V vs. the SCE. The crystalline WO 3 and Sn-WO 3 lms needed around 800 s for bleaching, where the transmittance was not completely returned to that of the as-prepared state (Fig. 8b, d and Table 2). However, the blue color of the In-WO 3 lms rapidly bleached within 200 s, and the   transparency almost completely returned to the as-prepared state ( Fig. 8f and Table 2). Although the initial transmittance of colored In-WO 3 lms was relatively high (ca. 40%) (Fig. 8e), the response of the In-WO 3 lms was obviously faster than those of the WO 3 and Sn-WO 3 lms. To our knowledge, the improvement in the electrochromic properties of WO 3 materials by doping with In 3+ ions has not been reported previously.
The response speed of electrochromic materials is inuenced by several factors, such as the porosity, crystallinity, and electron conductivity, among other factors. Because electrochemical reactions occur on the surface of electrode materials, the microstructures of the WO 3 lms can affect the electrochromic properties. However, there were no signicant differences in the surface structures, as observed by FE-SEM, between the WO 3 , Sn-WO 3 , and In-WO 3 lms (ESI Fig. S2 †). Disorder in the WO 3 crystal lattice is also known to improve the electrochromic response because the resulting broadened channels can allow for smoother insertion of cations (such as H + and Li + ). Many researchers have reported that metal ion doping deformed the monoclinic structure of WO 3 materials, providing faster electrochromic reactions. 24,26,27 In the present work, Snand In-doping caused deformation of the WO 3 lattice (Fig. 4). Thus, the improvement of response speed by In-doping was thought to be inuenced by the deformation of the WO 3 lattice (Fig. 4). However, the Sn-doping did not provide such a fast electrochromic response (Fig. 8c, d and Table 2), despite the lattice deformation. The difference in the effect on the electrochromic properties between the Sn-and In-doped materials  Finally, we evaluated the cycle stability of crystalline In-WO 3 lms by repeating the coloring and bleaching. Fig. 9 shows UV-Vis-NIR absorption spectra of the crystalline In-WO 3 lms aer 1 and 50 electrochromic cycles. The degree of coloring was maintained in the visible and near-infrared regions even aer 50 cycles (Fig. 9). On the other hand, the transparency of the bleached states did not completely return to the as-prepared state in the near-infrared range over 800 nm aer 50 cycles (Fig. 9). However, the bleached lms aer 50 cycles exhibited sufficiently high transmittance (over 80%) in the visible wavelength range between 400 and 800 nm (Fig. 9). These results show that the crystalline In-WO 3 lms have good cycle stability for use in electrochromic devices working at visible wavelengths.
Conclusions WO 3 and M-doped WO 3 (M ¼ Sn or In) electrochromic lms were obtained from aqueous solutions containing (NH 4 ) 10 -W 12 O 41 $5H 2 O, In(NO 3 ) 3 $3H 2 O and SnCl 4 $5H 2 O by a low-speed dip-coating technique. Evaporation-driven deposition during low-speed dip coating enabled us to make homogeneous coating layers, even from low-concentration aqueous solutions.
The Sn-and In-doping were easily achieved by controlling the chemical compositions of the solutions. The crystallinity of the WO 3 , Sn-WO 3 , and In-WO 3 lms was controlled by varying the heating temperature. The crystalline lms heated at 500 C showed high electrochemical stability, while the amorphous lms obtained by heating at 200 C dissolved aer the electrochromic reactions. The crystalline In-WO 3 lms exhibited a faster coloring and bleaching response than did the WO 3 and Sn-WO 3 lms, and this may be because of lattice deformation of the monoclinic WO 3 phase and the change in valence states of the elements resulting from the In-doping. The crystalline In-WO 3 lms showed good cycle stability at visible wavelengths and are thus expected to be able to be applied to electrochromic devices.

Conflicts of interest
There are no conicts to declare.