Origin of improved sensitivity of nanocrystalline InVO4 to NO2

Metal vanadates are attracting an increasing interest as photocatalysts, photovoltaics, electrolytes and sensors. Mixed-metal oxide semiconductors are promising materials for gas sensors. Indium vanadate InVO4 is derivative from indium and vanadium(V) oxides, which are well known sensing materials. However, the sensing behavior, active sites and sensing mechanism of InVO4 were not studied. In this work, InVO4 nanoparticles are synthesized hydrothermally, and In2O3 and V2O5 are obtained as reference materials. Surface acidity and surface oxygen species are determined by temperature-programmed techniques with probe molecules, XPS and FTIR. Gas sensitivity of the materials to noxious inorganic and volatile organic compounds is investigated. InVO4 demonstrates superior sensitivity to NO2 at moderate temperature 150 °C with good stability at 0–60 % relative humidity (at 27 °C). Using in situ infrared (DRIFT) spectroscopy, it is shown that NO2 was adsorbed on InVO4 in different forms, including nitrosyl species. It enables deep NO2 reduction at the surface of InVO4, in addition to the reaction of nitrates and nitrites formation observed on In2O3. Based on the results of temperature programmed reduction, the origin of higher surface reactivity and sensitivity to NO2 is deduced to be a larger number of oxygen vacancies in InVO4, in comparison to simple oxides.