Tailoring vanadium pentoxide nanoparticles for humidity sensing: impact of microwave annealing

Microwave annealing technology is gaining importance for processing metal oxides owing to its faster reaction time and volumetric heating. However, the utilization of this technique for producing vanadium oxide has not been explored. This study investigates the impact of both conventional annealing and microwave annealing on the crystal structure, light absorption, defect formation and humidity sensing performance of V 2 O 5 nanoparticles. V 2 O 5 was synthesized using the polyol method, involving the thermolysis of vanadyl ethylene glycol followed by annealing in oxygen atmosphere at 400 °C, 500 °C and 600 °C. The formation of layered, orthorhombic and stable phase of V 2 O 5 nanoparticles was confirmed using X-ray diffraction and Raman spectroscopy analyses. Field emission scanning microscopy showed the development of sheet-like morphology with average particle sizes of 99 ± 40 nm and 104 ± 51 nm for conventional annealing and microwave annealing, respectively. Annealing at elevated temperatures induced grain growth and facilitated oxygen diffusion, leading to the formation of oxygen vacancies. This was confirmed by optical studies, which revealed a reduction in the bandgap and the presence of defect states within the band. Relatively, microwave annealing resulted in fewer oxygen vacancies due to rapid heating, as evidenced by electron paramagnetic resonance studies and X-ray photoelectron spectroscopy. Moreover, the samples were evaluated for humidity sensing capabilities. The superior sensitivity of 48% at a higher relative humidity (97%) was observed for M5 sample that can be attributed to the smaller particle size facilitating more active sites, which makes it suitable for humidity sensing applications. Graphical abstract