Optical Gas Sensing Properties of ZnO Nanowires

Abstract

New breakthroughs in preparation technology have given further impulse for developing chemical sensing devices based on nanostructured ZnO. Due to their peculiar characteristics and size effects, these materials often show novel physical properties. In this work, we studied the optical properties of ZnO nanowires, synthesized via evaporation-condensation (EC) process, by room temperature photoluminescence (PL) and Time Resolved PL analysis. ZnO PL spectrum shows an UV peak, attributed to free exciton, and a broad visible band, strongly depending on the preparation conditions in the range from green to yellow. UV photoluminescence of ZnO nanowires was studied in gaseous atmosphere, targeting mainly NO₂ sensing applications. Continuous wave PL shows that a reversible modification is obtained upon exposure to low concentration of NO₂. The effect of interfering species like ethanol and RH% has also been studied. Time-resolved PL shows small modifications of recombination rates due to introduction of NO₂ not proportional to simultaneous changes in PL intensity. This behaviour supports the idea that NO₂ molecules, chemisorbed on the ZnO surface, lead to suppression of radiative states.