Electrophoretic deposition (EPD) of WO3 nanorods for electrochromic application

Electrophoretic deposition (EPD) was applied in coating hydrothermally synthesized crystalline tungsten oxide (WO3) nanorods onto ITO glass for electrochromic application. Nanorods suspension of 10 mg/cm3 was used in the EPD with optimum electric field of 5–6 V/cm. Saturation in WO3 deposited amount at electric field >7 V/cm was observed during constant voltage EPD. This could be attributed to the oxide layer shielding effect on the electric field induced electrophoresis. Constant current EPD from 0.2 mA/cm2 to 1.4 mA/cm2 was also performed for the WO3 nanorods. The deposited amount of nanorods was found to be proportional to the current density from 0.2 mA/cm2 to 0.8 mA/cm2 under constant deposition duration. However, the deposited amount decreased at current density >0.8 mA/cm2. This could be due to the high deposition rate that resulted in poor adhesion and hence nanorods peel off during the substrate removal. It was noted that the EPD of nanorods followed a linear relationship in I vs. t−1/2 plot according to Cottrell equation, which implied that the reaction was a diffusion controlled process. The EPD coated substrate was tested in 1 M LiClO4/propylene carbonate (PC) electrolyte for electrochromic studies. The porous WO3 nanorods layer exhibited optical modulation ΔT700 nm of 40%, moderate coloration time tc70% of 28.8 s and improved bleaching time tb70% of 4.5 s, which could be due to the porous oxide layer with large surface area that facilitates the ion insertion/extraction and the electrolyte penetration in the oxide layer shortens the ionic diffusion length of Li.