Tungsten Oxide Thin Films: Gas Sensing And Photocatalysis Properties

Tungsten trioxide and its hydrates are typical n-type transition metal oxide semiconductors, belonging to the d0 metal oxide. Due to their good properties in the fields of gas sensing and photochemical water splitting, such materials attract much attention in recent years. This article mainly focuses on the study of the hydrothermal synthesis of tungsten trioxide and its hydrates as well as their properties. The morphology and microstructures of the materials were characterized by using several techniques like XRD, SEM and TEM. Property studies were mainly focused on the NO2 of gas sensing and visible light driven photoelectrochemical water splitting. The main research contents of this paper are as follows:1. Studies on the preparation and gas sensing properties of tungsten trioxide monohydrate micro-flake films. Experimental studies showed that using nitric acid as acid source and sodium tungstate as tungsten source and without utilizing any morphology control agents, films comprising flake-like monohydrated WO3 crystals on FTO substrates could be deposited directly by hydrothermal method. XRD, SEM and TEM results reveal that the monohydrated WO3 crystals belong to orthogonal phase and are single crystalline, possessing a decanedron shape with two big square bottom sides and ten small trapezoidal lateral sides. The monohydrated WO3 micro-flake films exhibit good gas sensing properties to nitrogen dioxide at room temperature with a detection limit of 20 ppm and good reproducibility.2. Studies on the deposition of flake-like tungsten trioxide on FTO substrates and its visible light driven photoelectrochemical water splitting. In the preparation, the inorganic acid and sodium tungsten were used as the starting materials. Many conditions that would affect the photoelectrochemical water splitting performance were studied, including the preparation methods of materials, the categories of the inorganic acids and the electrolytes. The experiments revealed that the higher the concentration of alkaline electrolyte solution, the higher the performance of the photoelectrochemical water splitting. When nitric acid was used as the acid source, the tungsten oxide films were composed of small flower-like clusters comprising short and narrow sheets. It was showed that after photo-etching, the tungsten trioxide generated pores and could much enhance the photoelectrochemical water splitting performance, which was probably due to the more active sites providing by the pore structure. The photocurrent density of WO3 photoanode could reach as high as 0.7 mA/cm2 in 0.1 M PBS solution at 1.23 V vs. RHE. The IPCE experiments illustrated that in PBS electrolyte solution(pH=7) the WO3 films could get an IPCE of 20% to 340 nm of light at a potential of 1.2 V(vs. RHE).