Effect of W‒O bonding on gas sensitivity of nanocrystalline Bi2WO6 and WO3

Abstract

It is known that adsorptive capacity and surface reactivity of metal oxides depends on chemical composition, which influences the characteristics of metal‒oxygen bonds, e.g. degree of covalency, effective atomic charges, bond energy and bond length. Tungsten oxide and bismuth tungstate have perovskite-related structures and close semiconductor properties, but different chemical composition. The presence of bismuth in Bi2WO6 results in distinct W‒O bond length and bond energy, respective to WO3, which may be one of the factors controlling the surface reactivity, as well as the occurrence of bismuth-related surface sites. Tungsten oxide is a renowned material for heterogeneous catalysts, photocatalysts, and sensors. Bismuth tungstate has been extensively studied as a photocatalytic material, and an interest in sensor applications of this compound emerged recently. Yet, there is a lack of comparative and systematic studies of the electronic structure and sensing properties of tungsten oxide and bismuth tungstate. Such a study would be promising for the elucidation of the role of W‒O bonds in controlling the sensing behavior to different analyte gases. In this work, the comparative study of electronic and sensing properties of Bi2WO6 and WO3 was performed. Band structures, charge distribution and metal-oxygen bonds energies were calculated by first principles quantum chemical approach. Nanocrystalline Bi2WO6, WO3 and composite Bi2WO6 + WO3 were synthesized. Bismuth tungstate showed an improved sensitivity to volatile organic compounds (ethanol, formaldehyde, acetone, benzene) and poor sensitivity to nitrogen dioxide, in contrast to WO3. Based on the experimental and computational data, the effects of W‒O bond energy and charge distribution on the sensitivity to target gases of bismuth tungstate and tungsten oxide were rationalized.