The effect of small addition of copper on the growth process, structure, surface charge and adsorption properties of ZnO films in the pyrolysis of dithiocarbamates

Abstract

The development of composite materials based on copper and zinc oxides is one of the main trends in low-temperature catalysis, sensor technology, and optoelectronics. Of particular interest are thin-film coatings where outer faces of the ZnO crystals decorated with clusters of copper oxides of different valence, the sharing of electronic processes within such structures, leads to the unique properties of these materials. Herein, we synthesized textured ZnO/ZnS, ZnO/ZnS:Cu and ZnO:Cu films using the atmosphere pressure spray pyrolysis technique with identical organic precursor for copper and zinc ions, dithiocarbamate (DTC). The most perfect films were obtained in the presence of 0.2% copper at 220 °C (surface reaction rate limited regime) on Si(111)/SiO₂ substrates. Textured ZnO:Cu films are characterized by an increased growth rate, a more pronounced structure, superior photoelectric properties, and “unusual” chemical functionality. The growth of a composite material is considered on the basis of the tip-growth model, where catalytically active copper clusters are located on the outer polar surface of growing ZnO nanorod arrays. Segregation of the copper on the external polar face explained by the Jahn–Teller effect leads to the “jittering” in the coordination sphere of an ion in a highly symmetric environment of the ZnO crystal lattice. On the surface, the degeneracy of the electronic levels is removed, and Cu²⁺ acts as an “adaptive” surface passivator with a high structural variability of the coordination polyhedron. An increase in the growth rate of ZnO columns along the longitudinal direction of the hexagonal wurtzite structure was explained by the catalytic action of copper compounds, which stimulate the transformation of the sulfur-containing precursor into zinc oxide. This model is confirmed by the results of acoustoelectric measurements, which indicate a positive charge of the outer boundary of ZnO:Cu and the features of its interaction with gaseous analytes typical for copper-containing surfaces. The established mechanism of catalytic enhanced metallorganic – atmospheric pressure – direct liquid injection – chemical vapor deposition method utilizing a universal DTC precursor and one-step low-temperature spray pyrolysis process opens the way for the low-cost production of high-quality large-scale ZnO:Cu composite materials on various substrates of arbitrary shape and structure.