Monodispersed CuO octahedral crystals were successfully synthesized using a low-temperature co-precipitation method. Zinc doping in CuO created surface defects that enhanced oxygen adsorption on the surface crucial for gas sensing applications. Pure and Zn-doped CuO sensor films were realized using the doctor blade method. The sensor films showed selective response towards a low concentration of NO₂ at a lower operating temperature of 150 °C. Doping with Zn causes the resistance of the sensor film to decrease due to the enhancement of charge carriers with an analogous improvement in the sensor response. The observed decrease in sensor resistance agreed well with the findings of the work function studies. Zinc doping resulted in an increase in work function by 180 meV which, after NO₂ exposure, was found to increase by a further 130 meV, attributed to the oxidizing behavior of the test gas.

NiO thin films prepared by RF sputtering have been investigated for possible gas sensing application. The sensor films having thickness of around 30 nm were extremely smooth exhibiting a surface roughness of ∼0.7 Å. These films exhibited a highly sensitive and selective response towards ${\mathrm{H}}_2\mathrm{S}$ at an operating temperature of 300°C.

A 2 × 3 sensor array based on pure, Au and CuO modified (two of each kind) ${\mathrm{WO}}_3$ films was fabricated for e-nose application. Effect of surface modification has been investigated and correlated with the observed sensor response values. The sensor array was able to detect ${\mathrm{H}}_2\mathrm{S},{\mathrm{NH}}_3$ and NO selectively based on the sensor response pattern exhibited by the sensor array. ${\mathrm{WO}}_3$ sensor film modified with CuO exhibited a maximum sensitivity towards ${\mathrm{H}}_2\mathrm{S}$ (S = 92, 10 ppm, 250°C) while pure ${\mathrm{WO}}_3$ films exhibited better sensing characteristics towards NO (S=2.3, 10 ppm, 300°C) and ${\mathrm{NH}}_3$ (S=4.5, 50 ppm, 300°C). The shape of the curve remains unaltered by the change in the concentration of the gas. The shape and position of the combined response curve could be used to determine an unknown gas both qualitatively and quantitatively.

Gas sensing properties of pure and surface modified ZnO nanowires towards H ₂ S gas have been investigated. The hydrothermally grown ZnO nanowires network has been selectively modified with various sensitizers using RF sputtering to generate a 2×7 sensor array. After fabrication, the sensor array device has been investigated for its gas sensing properties. The optimal temperature for H ₂ S sensing has been observed to be 300°C. The maximum sensor response of S=52 was obtained for Au _5.7at% modified ZnO NWs with response and recovery times of 22 sec and 52 min, respectively. Concentration profile at 300°C revealed the same pattern with change in amplitude by changing H ₂ S concentration. The sensor array was able to detect H ₂ S both qualitatively and quantitatively based on the graphical pattern and position of the pattern, respectively.

Ammonia sensing properties of pure and Al modified WO ₃ thin films prepared using RF sputtering have been investigated. Both the sensor films exhibited a maximum response at an operating temperature of 250°C. Modification with Al has been demonstrated to improve the response kinetics in particular the response time (6 min) in comparison to that of pure WO ₃ (8 min) towards NH ₃ (100 ppm). Interestingly, both the sensor films showed an increase in resistance i.e., a p-type behavior upon exposure to NH ₃ .