Differentiated interactions in phosphate solutions: Comparing Ag(111) and Ag(100) surfaces

The electrochemical behavior of phosphate species on Ag(111) and Ag(100) single crystal surfaces was studied by experimental techniques and theoretical calculations. Characteristic cathodic/anodic potential peaks (Vp) appear in the current density-potential curves (j-V) for H2PO4−, H3PO4, and in the case of PO43− for concentrations lower than 0.1 M, where the interaction is stronger for the Ag(100) electrode. This demonstrate that the adsorption is sensitive to the surface structure. The adsorption energy (Eads) of the different species on both surfaces was studied by means of DFT-vdW calculations. All phosphate species have tri-dentate binding. The |Eads| increased for the H3PO4; H2PO4−; HPO42− and PO43− sequence, and the tetrahedral structures are compressed. Although the same trend was observed on both crystalline surfaces, the films have the symmetry of the substrate, C3v and C4v. The influence of hydroxide co-adsorption was studied for different [HPO42−]/[PO43−] ratios in solution. The strength of adsorption obtained from experimental and theoretical calculations show an excellent qualitative correlation.