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The paper demonstrated cost-effective one-step hydrothermal process to synthesize TiO2 microrods modified with Cr2O3 nanoparticles. The crystal structure, microstructure and chemical composition of the obtained materials were characterized by XRD, SEM, TEM and EDS analysis. Subsequently, for gas sensor tests the sensors were fabricated onto a ceramic tube having a designed pair of Au electrodes with four Pt wires. The results indicate that the sensors based on the optimal content of Cr2O3 nanoparticles modified TiO2 microrods possess excellent gas-sensing performance to acetone. Specifically, the sensor based on optimal 3% Cr2O3 modified TiO2 microrods exhibited high response (15), fast response/recovery time (5/28 s), outstanding selectivity (against 100 ppm ethanol, methanol and formaldehyde) and good stability (1-40 days) towards 100 ppm acetone at 300 °C. Relying on the analysis, we infer that performance can be controlled by varying the Cr2O3 contents. A plausible sensing mechanism is also discussed to explain the reasons for the enhanced acetone sensing with Cr2O3-TiO2 heterostructures. [ABSTRACT FROM AUTHOR]

• Synthesis of high quality thermally reduced Graphene oxide coatings. • Excellent humidity sensing characteristics of the GO sheets. • An n- to p-type conductivity inversion was observed under humidity. • The conductivity inversion phenomenon is elucidated with basic characterization. Graphene oxide (GO) is regarded as one of the potential candidates for gas and humidity sensor applications owing to its excellent water adsorption capabilities. However, there is a lack of understanding the response of GO to humidity. Some authors believe the water adsorption leads to p-type response while other think it to be n-type. The present work demonstrates the mechanism of water adsorption and n- to p-type response inversion of GO to humidity. The GO suspension was synthesised by modified Hummers' method and then drop-casted onto the substrates having already patterned gold interdigital electrodes (IDE). The sensors can detect wide humidity range (5–95 %RH) with a low voltage 0.1 V leading to low power consumption <10−5 W. It is also found that rGO samples annealed at temperature below 150 °C show n-type (e.g. R air /R RH ~ 1.29 × 105 for rGO-40) while the samples annealed above this temperature demonstrate p-type humidity response (e.g. R RH /R air ~ 40 for rGO-150). The static test response time is 8 s, while recovery time is 13 s with excellent long-term stability and RH accuracy value (±4% RH). Further characterization are performed to elucidate the physio-chemical model of the sensing mechanism as well as n- to p-type conductivity inversion under humidity. [ABSTRACT FROM AUTHOR]