On the mechanism of photoluminescence quenching in tin dioxide nanowires by NO2 adsorption

The recent observation of selective photoluminescence (PL) quenching in tin dioxide (SnO2) nanowires (NWs) upon adsorption of nitrogen dioxide (NO2) molecules triggered much interest on possible applications of SnO2 nanostructures as selective optochemical transducers for gas sensing. Understanding the peculiar gas-nanostructure interaction mechanisms lying behind this phenomenon may be of great interest in order to improve the selectivity of solid-state gas sensing devices. With this aim, we studied the luminescence features of SnO2 NWs in controlled adsorption conditions by means of continuous wave-and time-resolved PL techniques. We show that, under assumption of a Langmuir-like adsorption of gas molecules on the nanostructures surface, the decrease of PL intensity is linearly proportional to surface density of adsorbed molecules, while the recombination rates of excited states are not significantly affected by the interaction with NO2. These findings support a picture in which NO2 molecules act as 'static quenchers', suppressing emitting centres of SnO2 in an amount proportional to the number of adsorbed molecules. A simple model based on the above mechanism and allowing good fitting of the data is described and discussed. The possible indirect or direct role of oxygen vacancy states in SnO2 luminescence is finally discussed.