Quantum-sized ZnO nanoparticles: Synthesis, characterization and sensing properties for NO₂

Quantum-sized ZnO nanoparticles have been synthesized at room temperature by a mild sol-gel process using tetraethylorthosilicate (TEOS) as the capping agent to control the particle growth of ZnO. The crystal structure, particle size and optical properties have been investigated by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), photoluminescence (PL) spectra and Raman spectra, respectively. The results show that the ZnO nanoparticles exhibit hexagonal wurtzite structure and the average crystallite size is 5.7 nm which is a little less than TEM results. It has been testified by room-temperature PL spectra that the TEOS capped the surface of ZnO nanoparticles and obviously reduced grain size, as an emission at 520 nm almost disappeared and a new peak with an anomalous blue shift as great as 9 nm, appeared for the TEOS capped ZnO. The sensing tests indicate that the ZnO based sensors not only show a high response to NO₂ but also exhibit high selectivity over CO and CH₄ at a low operating temperature of 290 °C. The response increases with NO₂ concentration and decreases with calcination temperature, and is in agreement with Raman and XRD results.