Influence of the surface morphology and structure on the gas-sorption properties of SiO$_2$CuO$_x$

Thin films of SiO$_2$CuO$_x$ nanocomposite materials were synthesized by the sol-gel method upon deposition of solutions containing 1, 3, 5, and 7 wt% Cu. The scanning electron microscopy examination of the surface morphology revealed that a change in the copper concentration in the initial solution has an influence on the size and amount of crater-like pores formed in the amorphous silicon dioxide matrix and on the localization of copper crystallites on the surface of the films. X-ray absorption near-edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS) investigations showed that the structure of crystallites is predominantly formed by divalent copper oxide (CuO). However, an increase in the copper concentration in the initial solution leads to a systematic increase in the content of the Cu$_2$O phase. At copper concentrations of 1 and 7 wt%, the surface layers are most likely characterized by the formation of several divalent copper oxides (Cu(OH)$_2$, CuO, CuSiO$_3$), which results in the deterioration of the gas-sensitive characteristics of the material. It was established that the optimum set of parameters (the presence of pores, localization of crystallites, copper phase composition in the crystallites) for the best gas-sensitive characteristics of SiO$_2$CuO$_x$ composite films is observed at copper concentrations of 3 and 5 wt% in the initial solution.