Nanostructured WO3 for Electrochromic and Photocatalytic Applications

Abstract

Transition metal oxide semiconductors are known to be a smart category of materials with extensive demand for large scale production at lower costs. Facts in the present technologies remains with tunable and altering material properties that make them efficient for wide ranging applications such as smart windows, sensors, photoelectrics, solar cells, photocatalysis and etc. Their physical and chemical properties are influenced by the invariant surface structures affecting the surface energy and bonding that can be controlled by various synthesis approaches. Thus, in perspective of energy storage system, ion holding capacity and efficient use of solar energy, WO3 and ZnO are the most researched materials due to its simple and viable construction with effective visible light harvesting ability other than TiO2. In the present research work, selective synthesis of two different transition metal oxide semiconductor nanostructures namely, WO3 and ZnO has been developed for assessment of electrochromic and photocatalytic application. Four different classes of morphologies i.e., spherical, rod, cuboid and fiber WO3 nanoparticles are prepared through co-precipitation and hydrothermal techniques. Low temperature co-precipitation is an excellent approach to prepare spherical WO3 nanoparticles and rod shaped nanoparticles. Hydrogen peroxide concentration and temperature directs the phase, crystallinity and highest surface area of monoclinic spherical nanoparticles in comparison to CTAB directed rod shaped particles. Hydrothermal process favours in confined growth of nanocuboids and nanofibers under particular reaction conditions. In the present study, fluoroboric acid (HBF4) and sodium chloride (NaCl) has been chosen as structure directing reagents (SDR). Molar concentrations of SDR, time and temperature have prime importance to control the morphology and phase during the hydrothermal reactions. Cuboid morphology has an intermediate metastable hexagonal phase which is subsequently transformed to monoclinic phase at specific processing condition. A stable hexagonal phase is recorded for nanofibers with predominant (001) plane. Phase and morphology has no significant effect on the band gap energy. On the other hand, quasi-fibrous zinc oxide (ZnO) is prepared using oxalic acid fuel with zinc nitrate oxidizer through solution-combustion synthesis method.

Commercially viable methods such as drop-coating and dip-coating of nanostructured WO3 onto ITO glass substrates have been explored in light of efficient electrochromism performance. High surface continuity without any cracks and flaws after drying exhibits high current density for dip-coated electrodes. WO3 nanofiber electrode with micro thin uniformity exhibits high current density in comparison to electrode fabricated for other nanostructures. An excellent electrochromic property is observed for WO3 nanofiber coated ITO than WO3 nanocuboids due to high structural openness and tunneling zone to hold ions through its hexagonal crystal structure. However, pure WO3 shows negligible photocatalytic activity towards the organic dyes. The photocatalytic degradation of dyes becomes effective upon coupling with ZnO. Monoclinic WO3 nanocuboid shows enhanced photocatalysis in presence of both UV and visible light. Combustion synthesized quasi-fibrous ZnO enhances the photocatalytic performance than commercial ZnO. In addition, the quasi-fibrous ZnO coupled with WO3 has better photocatalytic efficacy in comparison to individual quasi-fibrous ZnO only. High photocatalytic activity is achieved for methylene blue and orange G dye solution with 10 wt% loading of monoclinic WO3 nanocuboids in ZnO matrix under visible light irradiation due to suppressed rate of electron-hole recombination. The hexagonal WO3 nanofibers and nanocuboids WO3 coupled with ZnO is found as an invaluable source for electrochromic and photocatalytic application, respectively