Role of molybdenum substitutional dopants on H2S-sensing enhancement of flame-spray-made SnO2 nanoparticulate thick films

In this work, Mo-doped SnO2 nanoparticulate sensing films were fabricated by flame spray pyrolysis (FSP) and spin-coating processes with varying Mo-doping concentrations (0-2 wt%) and numbers of spin-coating cycles (1-5). Structural characterizations by electron microscopy and X-ray analysis suggested that Mo atoms were substitutionally doped in polycrystalline SnO2 nanoparticles at low Mo concentrations (<1 wt%) but then segregated as secondary MoO3 crystallites at high Mo levels (1-2 wt%). In addition, the incorporation of Mo resulted in the reduction of size and the increase of surface area of SnO2 nanoparticles. The gas-sensing properties of sensors were investigated towards H2S, NO2, NH3, H2 and CO at the working temperature ranging from 150 °C to 350 °C. The results showed that the moderate Mo-doping level of 0.5 wt% and the high number of spin-coating cycles of 4 led to the optimal enhancement of H2S response. The optimal Mo concentration could be correlated to the highest doping level that did not induce secondary MoO3 crystallites. In particular, the 0.5 wt% Mo-doped SnO2 sensor prepared with 4 spin-coating cycles exhibited a high response of ∼105 and a short response time of ∼5 s-10 ppm H2S at an optimal working temperature of 250 °C. Furthermore, the optimal sensor displayed good H2S selectivity against NO2, NH3, H2 and CO. Therefore, the flame-spray-made Mo-doped SnO2 thick film sensor is a promising candidate for sensitive and selective detection of H2S at a threshold limit value (TLV) of lower than 10 ppm and may be useful for general industrial applications.