Cr-Doped Nanocrystalline TiO₂-Cr₂O₃ Nanocomposites with p-p Heterojunction as a Stable Gas-Sensitive Material

Nanocrystalline TiO₂ is a perspective semiconductor gas-sensing material due to its long-term stability of performance, but it is limited in application because of high electrical resistance. In this paper, a gas-sensing nanocomposite material with p-p heterojunction is introduced based on p-conducting Cr-doped TiO₂ in combination with p-conducting Cr₂O₃. Materials were synthesized via a single-step flame spray pyrolysis (FSP) technique and comprehensively studied by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) specific surface area analysis, transition electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and Raman spectroscopy. Gas sensor performance in direct current (DC) mode was studied toward a number of gasses (H₂, CO, CH₄, NO₂, H₂S, NH₃) as well as volatile organic compounds (VOCs) (acetone, methanol, and formaldehyde) in dry and humid conditions. The long-term stability of the obtained materials' gas sensor performance was evaluated alongside with an ex situ study of structural evolution. High sensitivity toward oxygenated VOCs and a lower detection limit below ppm level with a limited influence of humidity were shown. The long-term gas sensor performance stability of the obtained materials and its connection to the defect structure of doped TiO₂ is demonstrated.