Enhanced optical and electrical gas sensing response of sol–gel based NiO–Au and ZnO–Au nanostructured thin films

NiO and ZnO thin films of about 40–50 nm thickness with embedded Au nanoparticles have been synthesized with a simple and reliable sol–gel procedure. The nanocomposites films are crystalline and porous and they show optical absorptions in the visible range according to Au nanoparticles concentration. These films have been tested as optical and electrical sensors for pollutant gases detection. A fast and reversible response has been detected for hydrogen, CO and NO2. Au nanoparticles have been found to improve the optical sensing properties of both NiO and ZnO films over the Au surface plasmon resonance peak wavelength range, but also to enhance the ZnO optical response in the near UV range, where Au nanoparticles are optically inactive. Moreover, combining the observed shift in the surface plasmon resonance peak and the different semiconductive type of the two oxides, it has been proved that reducing gases inject electrons into the oxide and then afterward the charge variation is detected by Au nanoparticles. Electrical tests confirm the n-type behavior of ZnO and p-type behavior of NiO, and show good performances at lower temperatures. Moreover, an enhancing effect of Au nanoparticles in the overall sensing performances is observed also in electrical tests.