Transmetalation Process as a Route for Preparation of Zinc-Oxide-Supported Copper Nanoparticles

Abstract

Supported nanoparticulate materials have a variety of uses, from energy storage to catalysis. In preparing such materials, precision control can often be achieved by applying chemical deposition methods. However, ligand removal following the initial deposition presents a substantial challenge because of potential surface contamination. Traditional approaches normally include multistep processing and require a substantial thermal budget. Using transmetalation chemistry, it is possible to circumvent both disadvantages and prepare chemically reactive copper nanoparticles supported on a commercially available ZnO powder material by metalorganic vapor copper deposition followed by very mild annealing to 350 K. The self-limiting copper deposition reaction is used to demonstrate the utility of this approach for hexafluoroacetylacetonate-copper-vinyltrimethylsilane, Cu(hfac)VTMS, reacting with ZnO. The low-temperature transmetalation is confirmed by a combination of spectroscopic studies. Model density functional theory calculations are consistent with a thermodynamic driving force for the process.

Figure 1

FT-IR spectra of Cu(hfac)VTMS reaction with the ZnO powder surface at room temperature: (a) without predosing water and (b) following predosing water. (c) hfacH adsorption on ZnO powder at room temperature.

Figure 2

Temperature-dependent infrared spectra following the transformation of the material prepared by the exposure of Cu(hfac)VTMS onto the water-predosed ZnO powder surface. All the spectra are collected at room temperature, following a very brief annealing to the temperatures indicated.

Figure 3

Negative ion ToF-SIMS spectra of Cu(hfac)VTMS exposure on (a) water-predosed ZnO powder surface and following by (b) annealing at 350 K, (c) hfac exposure on ZnO powder and (d) ZnO powder.

Figure 4

Zoom-in of the negative ion ToF-SIMS spectra of Cu(hfac)VTMS exposure on water-predosed ZnO powder surface before (b) and after (a) annealing at 350 K, (c) hfac exposure on ZnO powder and (d) ZnO powder. The proposed species indicated in the figure are only examples and do not represent the complete list of species with similar m/z signatures.

Figure 5

Temperature dependent F 1s XPS of water-predosed ZnO powder following Cu(hfac)VTMS deposition and brief annealing to temperatures indicated.

Figure 6

Cu 2p XPS spectra of the (a) Cu(hfac)VTMS deposited on water-predosed ZnO powder and (b) the same surface recorded following a brief annealing to 350 K.

Figure 7

hfac ligand migration (transmetalation) during copper deposition followed by a brief annealing to 350 K.

Figure 8

Summary of SEM investigations of (a) pure ZnO powder as received, (b) formation of copper nanoparticles following exposure of the water-predosed ZnO powder to Cu(hfac)VTMS at room temperature, and (c) sample in (b) briefly annealed to 350 K.

Figure 9

Possible adsorbed forms of hfac on Cu/ZnO(101̄0) represented by a cluster model based on dissociation of hfacH on model surfaces with corresponding energy changes. The third structure (right) is calculated without copper atoms on the ZnO cluster.