Effect of Li doping on the structural, optical and formaldehyde sensing properties of In2O3 thin films

Chemical spray pyrolysis technique is utilized to deposit undoped and Li-doped indium oxide thin films on cleaned glass substrates. The effect of Li doping on the structural, optical, electrical and formaldehyde sensing characteristics on In2O3 has been studied. All the thin film samples were characterized by X-ray diffraction for structural and UV–vis spectroscopy for optical analyses, respectively. The doping of Li into In2O3 reduces the value of the average crystallite size and the optical transmittance in the visible region. However, a blue shift is observed in the optical band gap in the Li-doped films. The value of the direct optical band gap is found to increase from 3.35 eV in the undoped sample to 3.59 eV in the 2 at.% Li:In2O3 thin film. Further, the undoped and the Li-doped films have been tested for formaldehyde vapor to check their suitability to detect the presence of the same in air. We observed that the 2 at.% Li:In2O3 film exhibits the highest sensitivity for formaldehyde at a low concentration of 60 ppm and at an operating temperature of 300 °C. The sensing mechanism of formaldehyde vapor has also been presented.