A Crucial Step to Platinum Nanocrystals with Special Surfaces:  Control of Aquo/Chloro Ligand Exchange in Aqueous PtCl₆²⁻Solution

Platinum (Pt) nanocrystals with special surfaces attract much attention in the field of chemistry because the surface structure plays a vital role for the activity and selectivity of Pt catalyst. The surface evolution of Pt nanocrystals is closely related with facet growth, which depends on not only the oxidizing property of Pt precursor but also the competitiveness of precursor adsorption on the nanocrystal surface. Besides precursor/protector ratio, the charge of precursor also influences its competitive adsorption on Pt nanocrystals since metal nanocrystals smaller than 10 nm have a strong tendency to keep electroneutrality. By using blue or violet light, the control of aquo/chloro ligand exchange of PtCl 6 2- is attempted with a 1.5 × 10 -4 mol·dm -3 K 2 PtCl 6 aqueous solution at room temperature. First derivative spectra of UV absorption, X-ray photoelectron spectra (XPS), and 195 Pt nuclear magnetic resonance (NMR) spectra reveal that the aquo/chloro ligand exchange between PtCl 6 2- and H 2 O is dominant; hydroxyl/chloro exchange and the reduction is not significant under the investigated conditions. The influence of visible light on the stability of PtCl 6 2- in aqueous solution is remarkably dependent on the wavelength, which is associated with the spin-forbidden (465 nm) or spin-allowed (483 nm) d−d electron transition of PtCl 6 2- . The measurements of open-circuit potential (OCP) and d i /d E − E plots suggest that the oxidizing property of the studied K 2 PtCl 6 solution is enhanced due to the aquo/chloro ligand exchange of PtCl 6 2- . The electroneutral product of the aquo/chloro ligand exchange, PtCl 4 (H 2 O) 2, can be easily adsorbed on Pt nanocrystals and probably be more reducible under H 2 reduction than the parent PtCl 6 2- ions, which effects a faster growth of Pt{111} and results in a selective formation of cubic Pt nanocrystals enclosed with {100} facets.