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 (V-p) 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 (E-ads) of the different species on both surfaces was studied by means of DFT-vdW calculations. All phosphate species have tri-dentate binding. The |E-ads| 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, C-3 nu and C-4 nu. 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.