Semiconductor TiO2–Ga2O3 thin film gas sensors derived from particulate sol–gel route

Abstract

Nanostructured and mesoporous TiO2–Ga2O3 thin films with various Ti:Ga atomic ratios were prepared by a new straightforward particulate sol–gel route. Titanium isopropoxide and gallium (III) nitrate hydrate were used as precursors, and hydroxypropyl cellulose (HPC) was used as a polymeric fugitive agent (PFA) in order to increase the specific surface area (SSA). XRD and TEM analysis of the powders revealed that the Ga2O3 formed from the nitrate precursor retarded anatase-to-rutile transformation, crystallization and crystal growth. The average crystallite size of pure TiO2 powder annealed at 600–1000°C were in the range 4–10nm; the values that could be decreased to 2–6nm for TiO2–Ga2O3 powders. Furthermore, one of the highest SSA was obtained by introducing Ga2O3 into TiO2, being 305m2g−1 for TG11 (Ti:Ga=50:50 atomic ratio) binary oxide annealed at 600°C. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO and NO2 gases at low operating temperature of 200°C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption. TG11 sensor showed the highest response towards all CO and NO2 concentrations operated at 200°C. The response magnitude of 13.7 and 4.3 with response times of 30s and 108s were achieved for TG11 sensor towards 400ppm CO and 10ppm NO2, respectively. Furthermore, calibration curves revealed that TiO2–Ga2O3 sensors follow the power law (S=A[gas]B) (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two types of gases, and they have excellent capability for the detection of low gas concentrations (25ppm CO and 0.5ppm NO2). The maximum response of TiO2–Ga2O3 sensors towards CO and NO2 was measured at 450 and 400°C, respectively. The sensor response decreased with increasing film annealing temperature owing to sintering of the particles. The response magnitude and response time of the sensors obtained in this work is superior to TiO2-based sensors reported in previous studies.