Improving CO2 sensing and p-n conductivity transition under UV light by chemo-resistive sensor based on ZnO nanoparticles

Abstract

Chemo-resistive gas sensors for CO2 detection were fabricated using ZnO nanoparticles synthesized by the hydrothermal method. The CO2 sensors were activated by thermal and/or UV light exposure. While the resistance changes of sensors exposed to CO2 at high temperatures (150–300 °C) exhibited noise, those activated by UV light remained stable, indicating that UV light can improve CO2 sensing. Good response, short response time (<10 s), stability, a linear calibration curve, and high sensitivity were obtained under UV light. Therefore, the UV-activated sensor could be applied in practical environments for CO2 monitoring. Furthermore, the measurements for CO2 also showed that the p-n transition was observed with changes in temperature. The CO2 sensor acted as p-type at RT, while it acted as n-type at 80 °C. The p-type conductivity of ZnO at RT, which can be attributed to Zn vacancies acting as acceptors, was confirmed through PL and EDX spectra, as well as Hall Effect measurements. It should be noted that the p-n transition in sensors based on undoped materials under UV irradiation at low temperatures (<100 °C) has been less reported. The n-p transition could offer intriguing opportunities to customize the electronic properties of the nanostructured devices.