Enhanced NO 2 gas sensing performance of bare and Pd-loaded SnO 2 thick film sensors under UV-light irradiation at room temperature

NO2 gas sensing performance of bare and Pd-loaded SnO2 thick film sensors were measured under UV-light irradiation, using a UV-LED of 365 nm wavelength and light intensities ranging from 0 to 137 mW cm−2 at room temperature (30 °C). Template free conventional and microwave-assisted hydrothermal methods were applied to synthesize SnO2 powder with different specific surface areas and morphologies (rod-shape and nanoparticle). Two different Pd-loaded SnO2 (0.03 and 0.1 wt%) powders were also prepared by a simple wet-impregnation method. XRD, FE-SEM/SEM, XPS, BET surface area and BJH pore size distribution measured with N2 adsorption isotherms were utilized to characterize structural and morphological characteristics of all the samples. The results clearly confirmed that the presence of a low amount of Pd could effectively enhance the response value to 5 ppm NO2 gas and shorten the recovery time under the UV-light irradiation so that the sensor loaded with 0.1 wt% Pd (0.1Pd_MH1_400 sensor) showed the largest improvement in response value (almost 11 times) and recovery time (around 27 s) compared with the bare SnO2 sensor (MH1_400) at an UV-light intensity of 79 mW cm−2. This enhancement is most likely due to the role of Pd in facilitating the sensing reactions via producing additional NO2 adsorption sites on the SnO2 surface. Moreover, in the present study, the SnO2 sensor which was fabricated from as-synthesized, i.e. uncalcined, powder (MH1) showed the highest response (over 3000) compared with other sensors under an UV-light intensity of 7 mW cm−2 with a recovery time of about 48 s. The drastic decrease in the resistance in air of this sensor under the UV-light irradiation may be a possible reason for the highest response value. It was also revealed that high NO2 response could be achieved only at the optimum conditions of both the number of the NO2 adsorption sites (optimum specific surface area) and the electron density in the bulk under the UV-light irradiation.