A facile method for preparation of uniformly decorated-spherical SnO₂ by CuO nanoparticles for highly responsive toluene detection at high temperature

We reported a facile preparation of a uniform decoration of spherical n-type SnO₂ by p-type CuO nanoparticles as well as their utilization for enhanced performance on toluene gas detection. CuO nanoparticles and spherical SnO₂ were synthesized by a facile non-hydrolytic solvothermal reaction, which could easily control their morphology. A uniform CuO nanoparticles decoration onto spherical SnO₂ was achieved by a simple sonication and vigorous stirring at room temperature. We revealed organic solvents used in the oxide synthesis had a considerable influence on its surface charge that was beneficial for a uniformly electrostatic self-decoration between positively charged p-type CuO nanoparticles and negatively charged n-type spherical SnO₂. Interestingly, CuO was partially reduced to Cu metal during high concentration of toluene exposure destroying p-n contact and developing new metal-semiconductor contact so-called ohmic junction, resulting in extraordinarily responsive and selective to toluene gas at 400 °C as compared to a single p- CuO and n- SnO₂. It was also found that the amount of particle decoration had an influence on sensor response and resistance. The optimum amount of CuO nanoparticle decoration was 0.1 mmol on 0.5 mmol SnO₂. The response (S = R_a/R_g) and selectivity of CuO/SnO₂ based material toward the exposure of 75 ppm toluene had reached to such high as 540 and 5, respectively. The effect of p-n heterojunction and metal-semiconductor contact on the gas sensing mechanism of p-type CuO/n-type SnO₂ was discussed. Furthermore, by decorating with CuO nanoparticles, CuO/SnO₂ morphology was well-maintained after gas sensing evaluation demonstrated its excellency for high temperature toluene gas sensor application.