Effects of withdrawal speeds on the structural, morphological, electrical, and optical properties of CuO thin films synthesized by dip-coating for CO2 gas sensing

Copper oxide (CuO) thin films have been deposited on glass substrates by a facile sol-gel dip-coating technique with varying withdrawal speeds from 0.73 to 4.17 mm/s. The variation of film thickness manifested by dip-coating withdrawal speeds was investigated in detail to investigate its effect on the structural, morphological, optoelectrical, and wettability properties of CuO thin films for carbon dioxide (CO2) gas-sensing applications. The crystallinity, as well as phase purity of dip-coated CuO, were confirmed by both X-ray diffraction (XRD) and Raman spectral analyses. The surface morphology of the films characterized by the scanning electron microscopy (SEM) revealed that pore density decreases with the increase of withdrawal speeds and grain size is found to increase with the increase of film thickness corroborating the XRD results. The optical bandgap of dip-coated CuO films was estimated in the range of 1.47 - 1.52 eV from the UV-VIS-NIR transmission data and it is found to decrease with the increment of Urbach tail states accompanied by the increase of film thickness. The ratio of the electrical and optical conductivity of CuO films is found to decrease with increasing withdrawal speeds due to the variation of carrier concentration. Among all the studied films, the sample deposited to a 0.73 mm/s withdrawal speed exhibited the highest crystallinity, porous morphology, highest pore density, optoelectrical conductivity as well as water contact angle, and therefore maximum gas sensing response of CO2 vapor in the air recorded at room temperature.