Hydrothermal assisted growth of vertically aligned platelet like structures of WO3 films on transparent conducting FTO substrate for electrochromic performance

Abstract

Controlled growth of nanostructured metal oxide films growing on the conducting substrates with good adhesion in a single step process via hydrothermal method is still challenging. In the present work, WO3 platelets were deposited on transparent conducting fluorine doped tin oxide substrates by single step hydrothermal process via tailoring the reaction time (8 h, 12 h and 16 h) and reaction temperature (120 °C and 180 °C). X-ray diffraction analysis showed that the WO3 thin films grown at 120 °C for 8 h and 12 h reaction time comprised with mixed phases of hexagonal and monoclinic system, whereas 16 h of reaction time produced single phase monoclinic system. However the WO3 films deposited at 180 °C for 8 h, 12 h and 16 h reaction time acquired monoclinic system. Raman spectrum showed the stretching and bending modes of WO3. The WO3 films deposited at 120 °C and 180 °C for different reaction time 8 h, 12 h and 16 h exhibited various surface morphology such as buds like, bricks like, platelets like and sheets with platelets like structures. Optical studies showed that the transmittance of WO3 film is decreased with increase in the reaction temperature. The WO3 film deposited at 120 °C for 12 h reaction time acquired transmittance of about 86% at 463 nm whereas the film deposited at 180 °C for 12 h reaction time showed transmittance of about 75% at 465 nm. The direct and indirect optical band gap of the deposited WO3 films varies in the range 2.5 eV– 2.9 eV and 2.6–2.75 eV respectively with varying growth parameters. The electrical properties of WO3 film deposited at 120 °C for 12 h showed relatively low resistivity (1.8 × 10−3 Ωcm) and high carrier concentration (9.3 × 1019 cm−3). The electrochromic studies carried out for the WO3 film synthesized at 120 °C for 16 h exhibits high peak current density, diffusion coefficient and fast switching kinetics with respect to other deposited films.