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 (101̅ 0) 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(101̅ 0) 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.

Selectivity and mechanism of thermal decomposition of β-diketones on ZnO powder

“…All ZnO powder samples were prepared as described earlier [20]. A sample of approximately 30 mg of ZnO powder (99.99% purity, Alfa Aesar) was pressed onto a tungsten mesh under a pressure of ∼4 tons using a hydraulic press.…”

“…This approach has been successfully used earlier to obtain energy landscapes and spectroscopic observables based on cluster calculations [20, 25, 33–35]. As mentioned above, the majority of ZnO powder surface is ZnO(101̄0), and in these studies it is represented by a Zn 20 O 20 cluster model.…”

“…Several reports also explicitly target chemical vapor deposition (CVD) and atomic layer deposition (ALD) of transition metal catalysts on oxide materials [18–20], where decomposition reactions of β-diketonates are complex and can cause carbon, oxygen or fluorine contamination on the surface at elevated temperatures.…”

Deposition of copper from Cu(i) and Cu(ii) precursors onto HOPG surface: Role of surface defects and choice of a precursor

“…First, the position of this peak is consistent with the fluorine still being attached to carbon rather than with the formation of Cu− −F species. 22,52 Thus, it is likely that the source of the remaining fluorine is the − −CF 3 groups of the hfac. Second, the Cu/F ratio is calculated to be 1:1 again for Cu(hfac) 2 while that for Cu(hfac)VTMS is found to be approximately 4:1.…”

“…For example, Cu(hfac)VTMS (copper(i) hexafluoroacetylacetonate vinyl trimethylsilane) and Cu(hfac) 2 (copper(ii) hexafluoroacetylacetonate) are common precursors for ALD or CVD methods and are used to produce either coppercontaining nanoparticles or continuous thin films. [18][19][20][21][22] Due to the structural differences of the two precursors, different reactivities and therefore surface morphologies of the structures produced are expected, as has been observed on surfaces such as ZnO, 22 silica, and silicon. 18,20,21 In addition, since the VTMS moiety from Cu(hfac)VTMS is expected to desorb from the substrate immediately upon adsorption, as was observed on several substrates even at room temperature, [20][21][22][23][24][25][26][27] Cu(hfac) is actually expected to be the reactive adsorbate (Figure 1(a)) that will interact with the surface for deposition based on this precursor.…”