Citrate adsorption on Pt{h k l} electrodes and its role in the formation of shaped Pt nanoparticles

The adsorption of citrate anions on Pt{h k l} single crystal electrodes and Pt nanoparticles was investigated using cyclic voltammetry (CV). Citrate is found to behave as a simple, specifically adsorbing anion, stronger in its surface interaction with platinum than either (bi-) sulphate or OH but displaying no tendency to decompose at 293 K into molecular fragments such as chemisorbed carbon monoxide. No preference for a particular adsorption site was exhibited by citrate either, as would be expected from its reported ability to control the shape of growing Pt nanoparticles. Rather, citrate is readily removed from platinum surfaces either by exposure to aqueous sodium hydroxide (it acts as a non-specifically adsorbing anion at high pH) or potential excursions to potentials <0.15 V vs. Pd/H. Hydrogen reduction of a platinum precursor (K2PtCl4) at 293 K both in the presence and absence of citrate anions leads to almost identical nanoparticle structures being formed as measured by CV. It is concluded that at 293 K in the presence of hydrogen, citrate anions play little part in controlling nanoparticle shape. In contrast, heating the platinum precursor at 373 K using citrate as both reducing agent and stabilizer facilitated formation of Pt nanoparticles containing a slightly greater proportion of Pt{1 1 1} terrace sites and a relative diminution in the surface density of Pt{1 0 0} terraces. It proved impossible to clean these Pt nanoparticles via potential cycling/hydrogen evolution indicating that a much more tenaciously held intermediate is formed during the reduction process in the absence of hydrogen and it is speculated that it is this intermediate that facilitates shape selectivity during platinum nanoparticle synthesis in accordance with previous studies investigating gold nanoparticle formation using citrate.