Formation of Copper Nanoparticles on ZnO Powder by a Surface-Limited Reaction

Abstract

Formation of copper nanoparticles on zinc oxide (ZnO) powder is studied using a common chemical vapor deposition precursor, copper hexafluoroacetylacetonate vinyl trimethyl silane, Cu(hfac)(VTMS). This process is investigated by high-vacuum Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The growth was found to be promoted by exposing ZnO powder to the gas-phase water, and the intensity of the hydroxyl groups stretching signatures decreases after the powder is exposed to the copper deposition precursor. Vibrational spectroscopy results support the reaction on both polar and (101̅0) surfaces of the powder, and XPS confirms that the copper deposition takes place and identifies Cu(I) species as the main copper species on the surface of ZnO powder. The mechanism of the reaction includes the elimination of hfac ligand that reacts with surface hydrogen present in hydroxyl groups, and this surface-limited process stops when the surface runs out of available hydrogen. SEM is used to visualize the formation of copper-containing nanoparticles on ZnO(101̅0) and ZnO(0001̅) surfaces and defects. The mechanism for the initial stages of the deposition is proposed based on the computational investigation consistent with the experimental results. This general approach can be used to design a range of copper catalysts supported on ZnO with a high degree of control over the amount of copper deposited and the desired size distribution of the nanoparticles produced.