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 ΔT_{700 nm} of 40%, moderate coloration time t_c^{70 %} of 28.8 s and improved bleaching time t_b^{70 %} 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.