Electrochromic Enhancement of WO₃-TiO₂Composite Films Produced by Electrochemical Anodization

Abstract

An enhancement of the electrochromic properties of WO 3 thin films by the incorporation of TiO 2 is presented. The composite films are obtained by anodizing co-sputtered tungsten / titanium on conductive glass substrates in 1 M H 2 SO 4 solution containing 0.3 wt% NaF. As the titanium content is increased from 0 to 15 at.% a morphology evolution occurs, from nano-porous to nano-flakes and finally to nano-blocks interweaved with nano-pores. The nano-flakes, obtained at 10 at% Ti, proved to be the most conducive structure for the insertion and removal of Li + ions such that this composition exhibits transmittance regulation abilities of 58.5%, 72% and 77.7% at 550 nm, 632.8 nm and 800 nm respectively, which are more than a 25% improvement at all wavelengths over a pure WO 3 film formed in the same way. The WO 3 /TiO 2 film with addition of 10 at.% Ti also has faster coloration/bleaching times, especially in the critical near infrared region with values of 10 s / 64 s at 800 nm compared with 32 s / 90 s of a pure WO 3 thin film, as well as improved stability losing less than 5% of its capacity after 1000 switching cycles. and electrodeposition. 11,12 However, the electrochemical anodization method adopted in this work provides a simple and economic way of synthesizing targeted morphologies such as nanoporous or nanotube structures. Unfortunately there are still drawbacks when compared to vacuum deposited films, for instance the electrochemically prepared WO 3 has poorer reversibility and stability as well as a shorter optical modulation range. This has led to interest in mixed metal oxide composite systems to compensate for these eficiencies. [13][14][15][16][17][18][19] To date, there are many reports on the doping or mixing other oxides with WO 3 to improve its electrochromic properties, with oxides of barium, 20 cobalt, 21 nickel, 22 molybdenum 23 and most successfully titanium.24 Even a small quantity of the guest oxide has a profound effect on the host's optical properties. Hashimoto and Matsuoka first reported in 1991 that an amorphous WO 3 -TiO 2 film, produced by electron beam deposition method, exhibits a longer life-time than a pure tungsten oxide film. 24 Subsequently, in 1993, Gottsche et al. 25 compared the electrochromic properties of mixed WO 3 -TiO 2 thin films fabricated by sputtering and sol-gel techniques. These authors point out that the addition of TiO 2 with molar percentages ranging from 10% to 15% reduced the crystallinity of WO 3 host and led to significant structural changes. Later, Wang and Hu successfully composed a TiO 2 -doped WO 3 film by spin coating and found that the Ti-addition stabilized the peroxotungstic acid in the spin coating solutions. 26 In 2005, Patil et al. 27 prepared WO 3 -TiO 2 films via a spray pyrolysis method and discussed the improved electrochromic reversibility and coloration efficiency.However, in these earlier works the coloration efficiency has not improved significantly to the level where it can be used in a practical smart window. Indeed, in some cases cert...

Electrical, optical and electrochromic properties of Ti:WO3 thin films deposited by the pulsed chemical spray technique

“…The electrochromic performance of the spray pyrolysed Ti:WO 3 thin films improves significantly on Ti doping in a similar way as that previously reported in the literature [45][46][47]. The durability of the electrochromic behavior is increased when WO 3 films are doped with Ti atoms.…”

Sodium‐Salt‐Promoted Growth of Self‐Supported Copper Oxides with Comparative Supercapacitive Properties

“…The obtained data are displayed in Figure A, and it is apparent that all three samples have very similar impedance profiles: a semicircle in the high‐frequency range, and an up‐sloping curve in the low‐frequency range bending towards to the x ‐axis. Such a profile is commonly observed in supercapacitors and is usually modeled with the equivalent circuit shown in Figure B, where: R S implies the series resistance which is usually composed of several components including resistance of the NaOH electrolyte, contact resistance, as well as the resistance of the electrode; R CT is the resistance of charge transfer process from the electrolyte to the film; CPE dl represents a constant phase element originating from the double layer capacitance; R L stands for the leakage resistance of the supercapacitor; W S is the a finite length Warburg impedance (short circuit) associated with the diffusion of ions inside the film; and CPE in is the constant phase element showing the capacitance rising from insertion of the ions in the surface region of the copper oxide film …”

“…Such a profile is commonly observed in supercapacitors and is usually modeled with the equivalent circuit shown in Figure 7B [67,68] where: R S implies the series resistance which is usually composed of several components including resistance of the NaOH electrolyte, contact resistance, as well as the resistance of the electrode; R CT is the resistance of charge transfer process from the electrolyte to the film; CPE dl represents a constant phase element originating from the double layer capacitance; R L stands for the leakage resistance of the supercapacitor; W S is the a finite length Warburg impedance (short circuit) associated with the diffusion of ions inside the film; and CPE in is the constant phase element showing the capacitance rising from insertion of the ions in the surface region of the copper oxide film. [69] All the fitted data of these elements, along with parameters indicating the high quality of the fits, are listed in Table 1. It can be seen that the time constants (t) of the Warburg elements are of the order of 10 s, from which the diffusion length (L) can be calculated [Eq.…”

Honey-Comb Structured WO3/TiO2Thin Films with Improved Electrochromic Properties

“…25 The RF power to the tungsten and titanium targets were 90 W and 15 W giving sputtering rates of about 6 nm min −1 and 1 nm min −1 respectively. As there are losses during the sputtering process, the actual atomic ratio can only be determined by EDS after co-sputtering.…”