  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.
