Self-Assembled Vanadium Oxide Nanoflakes for p-Type Ammonia Sensors at Room Temperature

VO₂(B), VO₂(M), and V₂O₅ are the most famous compounds in the vanadium oxide family. Here, their gas-sensing properties were investigated and compared. VO₂(B) nanoflakes were first self-assembled via a hydrothermal method, and then VO₂(M) and V₂O₅ nanoflakes were obtained after a heat-phase transformation in nitrogen and air, respectively. Their microstructures were evaluated using X-ray diffraction and scanning and transmission electron microscopies, respectively. Gas sensing measurements indicated that VO₂(M) nanoflakes were gas-insensitive, while both VO₂(B) and V₂O₅ nanoflakes were highly selective to ammonia at room temperature. As ammonia sensors, both VO₂(B) and V₂O₅ nanoflakes showed abnormal p-type sensing characteristics, although vanadium oxides are generally considered as n-type semiconductors. Moreover, V₂O₅ nanoflakes exhibited superior ammonia sensing performance compared to VO₂(B) nanoflakes, with one order of magnitude higher sensitivity, a shorter response time of 14⁻22 s, and a shorter recovery time of 14⁻20 s. These characteristics showed the excellent potential of V₂O₅ nanostructures as ammonia sensors.