Phosphonic Acids on Atomically-Defined Co3O4(111): Adsorption in the Electrochemical Environment

WAIDHAS, F.; MONTERO, M. A.; BERTRAM, M.; SCHWARZ, M.; BRUMMEL, O. ; SCHUSCHKE, C.; HOHNER, C.; LIBUDA, J.

Conferencia; 118th General Assembly of the German Bunsen Society for Physical Chemistry; 2019

Phosphonates are increasingly studied as anchor groups in organic/inorganic hybrid materials. Here, we employed the surface science approach in a comparative study to investigate the interactions of phenylphosphonic acid (PPA) on well-ordered Co3O4(111) thin films on Ir(100) both under ultra-high vacuum (UHV)1 and under electrochemical conditions. On the one hand, we prepared Co3O4(111) thin films in UHV and followed PPA deposition by infrared reflection-absorption spectroscopy (IRAS). On the other hand, we transferred the atomically-defined Co3O4(111) thin films into the liquid environment (pH 10, ammonia buffer) under ultraclean conditions2 and analyzed the PPA deposition from solution by electrochemical infrared reflection-absorption spectroscopy (EC-IRRAS).In UHV, we deposited PPA at 380 K. At sub-monolayer coverages PPA binds to the surface as fully deprotonated chelating tridentate. By increasing the coverage to a monolayer, the tridentate binding is replaced by partially protonated bidentates. Upon deposition in the electrochemical environment at pH 10, where Co3O4(111) is stable, PPA shows a potential-dependent behavior. This behavior is strongly influenced by the limited amount of electrolyte in thin-layer configuration and the ammonia-to-PPA ratio. At a high PPA-to-ammonia ratio, an increase of the potential leads to surface oxidation which results in a pH drop in the thin layer. The pH drop leads to the formation of singly protonated PPA at the Co3O4(111) surface. At low PPA-to-ammonia ratios PPA adsorbs stably in its completely deprotonated form and slightly changes its orientation at the surface upon increasing the potential.1 Schuschke et al. J. Phys. Chem. Lett. 2018, 9, 1937-1943.2 Faisal et al. Rev. Sci. Instr. 2018, 89, 114101.