First-Principles Calculation of Pt Surface Energies in an Electrochemical Environment: Thermodynamic Driving Forces for Surface Faceting and Nanoparticle Reconstruction.

Platinum is a widely used catalyst in aqueous and electrochemical environments. The size and shape of Pt nanoparticles and the faceting (and roughness) of extended Pt surfaces change during use in these environments due to dissolution, growth, and reconstruction. Further, many Pt nanoparticle synthesis techniques are carried out in an aqueous environment. The surface structures formed are impacted by the relative surface energies of the low index facets in these environments. Density functional theory is used to calculate the surface energy of the low index facets of platinum as a function of electrochemical potential and coverage of adsorbed hydrogen, hydroxide, oxygen, and the formation of surface oxide in an aqueous environment. Whereas Pt(111) is the lowest energy bare surface in vacuum, the strong adsorption of hydrogen to Pt(100) at low potentials and of hydroxide to Pt(110) and oxygen to Pt(100) at high potentials drives these surfaces to be more stable in an electrochemical environment. We experimentally conditioned a polycrystalline platinum electrode by cycling the potential and find a growth in the total area as well as in the fraction of 110 and 100 sites, which are lower in energy at potentials where dissolved Pt is deposited or surface oxide is reduced. Further, we find that the lower surface energy of Pt(100) at low potentials may play a role in the growth of tetrahexahedral nanoparticles seen on square wave cycling of spherical Pt nanoparticles. Wulff constructions are presented as a function of Pt electrode potential.