Synthesis, field emission and humidity sensing characteristics of monoclinic VO2 nanostructures

VO2 (M1) nanostructures were synthesized via hydrothermal and heat-transformation processes using NH4VO3 as source and oxalic acid as reductant. Different morphologies of nanorods, nanoflakes, nanoflowers, and nanospheres were obtained by adjusting the concentration of oxalic acid. Their microstructures were evaluated using X-ray diffraction and scanning and transmission electron microscopies, respectively. FE properties measurement of these nanostructures showed that the hollow nanospheres have best field emission performance with turn-on field of ∼4 V/μm and threshold field of ∼11.4 V/μm. Due to the high surface-to-volume ratio, nanoflowers were chosen to test their humidity sensing properties, demonstrating that the sensor based on VO2 (M1) nanoflowers have fast response speed with response time of ∼13 s and recovery time of ∼5 s. This indicates that VO2 (M1) nanostructures are promising candidate materials for the application in high performance humidity sensors.