Visible Light-Induced Room-Temperature Formaldehyde Gas Sensor Based on Porous Three-Dimensional ZnO Nanorod Clusters with Rich Oxygen Vacancies.

Oxygen vacancy (V) is a kind of primary point defect that extensively exists in semiconductor metal oxides (SMOs). Owing to some of its inherent qualities, an artificial manipulation of V content in one material has evolved into a hot research field, which is deemed to be capable of modulating band structures and surface characteristics of SMOs. Specific to the gas-sensing area, V engineering of sensing materials has become an effective means in enhancing sensor response and inducing light-enhanced sensing. In this work, a high-efficiency microwave hydrothermal treatment was utilized to prepare a V-rich ZnO sample without additional reagents. The X-ray photoelectron spectroscopy test revealed a significant increase in V proportion, which was from 9.21% in commercial ZnO to 36.27% in synthesized V-rich ZnO possessing three-dimensional and air-permeable microstructures. The subsequent UV-vis-NIR absorption and photoluminescence spectroscopy indicated an extension absorption in the visible region and band gap reduction of V-rich ZnO. It turned out that the V-rich ZnO-based sensor exhibited a considerable response of 63% toward 1 ppm HCHO at room temperature (RT, 25 °C) under visible light irradiation. Particularly, the response/recovery time was only 32/20 s for 1 ppm HCHO and further shortened to 10/5 s for 10 ppm HCHO, which was an excellent performance and comparable to most sensors working at high temperatures. The results in this work strongly suggested the availability of V engineering and also provided a meaningful candidate for researchers to develop high-performance RT sensors detecting volatile organic compounds.