New preparation method of gold nanoparticles on SiO2

It is shown that adsorption of the [Au(en)(2)](3+) cationic complex can be successfully employed for the deposition of gold nanoparticles (1.5 to 3 nm) onto SiO2 with high metal loading, good dispersion, and small Au particle size. When the solution pH increases ( from 3.8 to 10.5), the Au loading in the Au/SiO2 samples increases proportionally ( from 0.2 to 5.5 wt %), and the average gold particle size also increases ( from 1.5 to 2.4 nm). These effects are explained by the increase in the amount of negatively charged sites present on the SiO2 surface, namely, when the solution pH increases, a higher number of [Au(en)(2)](3+) species can be adsorbed. Extending the adsorption time from 2 to 16 h gives rise to an increase in the gold loading from 3.3 to 4.0 wt % and in the average particle size from 1.8 to 2.9 nm. Different morphologies of gold nanoparticles are present as a function of the particle size. Particles with a size of 3-5 nm show defective structure, some of them having a multiple twinning particle (MTP) structure. At the same time, nanoparticles with an average size of ca. 2 nm exhibit defect-free structure with well-distinguishable {111} family planes. TEM and HAADF observations revealed that Au particles do not agglomerate on the SiO2 support: gold is present on the surface of SiO2 only as small particles. Density functional theory calculations were employed to study the mechanisms of [Au(en)(2)](3+) adsorption, where neutral and negatively charged silica surfaces were simulated by neutral cluster Si4O10H4 and negatively charged cluster Si4O10H3, respectively. The calculation results are totally consistent with the suggestion that the deposition of gold takes place according to a cationic adsorption mechanism.