In this paper, we fabricated p-Co3O4/n-TiO2 heterostructures and investigated their gas sensing properties. The structural and morphological characterization were performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy analysis (XPS). The electrical properties of the heterostructure were studied within the temperature range from 293 K to 423 K. Changes in electrical properties and sensing behavior against reducing and oxidizing gases were attributed to the formation of p–n heterojunctions at the Co3O4 and TiO2 interface. In comparison with sensing performed with pristine TiO2 nanotubes (NTs), a significant improvement in H2 sensing at 200 °C was observed, while the sensing response against NO2 decreased for the heterostructures. Additionally, a response against toluene gas, in contrast to pristine TiO2 NTs, appeared in the Co3O4/TiO2 heterostructure samples.

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Pristine and WO 3 decorated TiO 2 nanorods (NRs) were synthesised to investigate n-n-type heterojunction gas sensing properties. TiO 2 NRs were fabricated via hydrothermal method on fluorine-doped tin oxide coated glass (FTO) substrates. Then, tungsten was sputtered on the TiO 2 NRs and thermally oxidised to obtain WO 3 nanoparticles. The heterostructure was characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy. Fabricated sensor devices were exposed to VOCs such as toluene, xylene, acetone and ethanol, and humidity at different operation temperatures. Experimental results demonstrated that the heterostructure has better sensor response toward ethanol at 200 °C. Enhanced sensing properties are attributed to the heterojunction formation by decorating TiO 2 NRs with WO 3 .

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Pristine and chromium (Cr) doped WO3 nanoflakes (NFs) with various concentrations were successfully fabricated by a facile hydrothermal technique on alumina (Al2O3) substrates. The structural, optical, and morphological properties of these NFs were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Visible diffusion reflectance spectroscopy (UV-Vis DRS), and X-ray photoelectron spectroscopy (XPS). Gas sensor tests were performed against various volatile organic compounds (VOCs) such as ethanol, xylene, toluene, and isopropanol gases in the temperature range between 50˚C and 250˚C. According to the results of these tests, the isopropanol sensing ability of WO3 NFs is enhanced with Cr-doping due to the increasing active adsorption sites on the surface and improved surface reactions with the decreasing band gap energy. The highest isopropanol sensing response which was calculated to be 77.1 has been obtained by nominally 2% Cr doping at the optimal operating temperature of 150˚C.