Adsorption and Thermal Stability of Phenylphosphonic Acid on Cerium Oxides

This paper reports on a study of the adsorption and thermal stability of phenylphosphonic acid (PPA) adsorbed by physical vapor deposition on the surfaces of epitaxial cerium oxide films of different structure, stoichiometry and composition. Advanced analytical methods based on photoelectron spectroscopy combined with DFT calculations showed that the binding of PPA to cerium oxide is through the phosphonate group, while the decomposition temperature is defined by the nature of the oxide. Tridentate PPA species are present on all substrates (CeO₂, CeO_1.7, Ce₂O₃, and Ce₆WO₁₂), indicating a strong affinity of PPA for cerium oxide. The presence of vacancies in the oxide influences the molecular orientation. The phenyl ring of the PPA tilts about 10° more toward the surface plane of the reduced cerium oxides compared to CeO₂, which is attributed to the adsorption of phosphonate groups on Ce⁴⁺ and Ce³⁺ cations. The PPA adlayer is more stable on the surfaces with higher concentrations of oxygen vacancies and/or Ce³⁺ cations, increasing the temperature to initiate cleavage of the P-C bond from 225 °C for PPA/CeO₂ to 350 °C for the other systems. The PPA decomposition is signaled by the desorption of carbonaceous species above a critical temperature, while the phosphorus species remain stable even after annealing at 450 °C for all the cerium oxides. Overall, the results provide a comprehensive understanding of the binding of PPA to cerium oxides, allowing further development of functionalization strategies for inorganic materials by phosphonic acids.