Room-temperature nitrogen-dioxide sensors based on ZnO1−x coatings deposited by solution precursor plasma spray

- Solution precursor plasma spray was used to deposit porous ZnO1−x coatings.
- ZnO1−x coatings exhibited good responses to NO2 under light illumination.
- Light wavelength and humidity had important influences on sensor performance.

The stoichiometric ZnO has a bandgap greater than 2.6 eV and has no or weak response to visible light, which greatly suppresses its applications. We report here that a process of solution precursor plasma spray (SPPS) can effectively narrow the bandgap and extend the absorption of ZnO to visible light region, which mainly results from highly concentrated oxygen vacancies generated during SPPS process. By photoluminescence spectroscopy, electron paramagnetic resonance and X-ray photoelectron spectroscopy, we found that a large number of oxygen vacancies were implanted into ZnO1−x prepared by SPPS. The generation of highly concentrated oxygen vacancies were mainly attributed to reducing atmosphere as well as fast heating and cooling process inherently provided by SPPS. The findings of this work create a new way for developing narrow bandgap ZnO1−x. We present that the SPPS ZnO1−x coatings can be utilized directly as sensitive materials under visible-light illumination. The oxygen vacancies have considerable influence on its optical and electrical properties. The ZnO1−x coatings exhibited an obvious absorption covering the whole visible-light region and its bandgap was calculated to be 2.15 eV which was much narrower than that of stoichiometric ZnO (3.37 eV). The sensors based on ZnO1−x coatings showed significant responses to NO2 at room temperature. In addition, the sensor response increased linearly with NO2 concentration. The enhanced sensor properties were attributed to the rich oxygen vacancies and special coating microstructure provided by SPPS.