Effects of Particle Size of Au Added to CuO-ZnO Composite on Gas Sensing Properties

Abstract

CuO-ZnO composites with or without ultrafine Au particles were fabricated by infiltrating cupric solution with or without gold cyanide into a porous ZnO matrix and firing. The particle size of Au was controlled within 5-100nm by changing heating process of the cupric solution and gold cyanide concentration. The rectifying current (I)-voltage (V) characteristics and gas sensitivity largely depended on the Au particle size. Evident nonlinear I-V characteristics, due to p-n junction of CuO and ZnO semiconductors, were not observed in composites with Au particles. When the Au particle size was about 10nm, the gas sensitivity of the composite was substantially improved; the current was increased about 20 times by an introduction of CO and H2 gas (4000ppm) at 150°C in the bias region within ±6V. However, in composites with Au particles smaller than 10nm and larger than 20nm, current changes by introduction of the gases were only below 10% of the current in air. The interface capacitance of the composites also showed changes by the gases depending on the Au particle size, which is similar to those in gas sensitivity. It was found that the addition of Au particles with controlled size is effective to improve gas sensitivity in CuO-ZnO composites.