Raman study of the interactions between highly ordered pyrolytic graphite (HOPG) and polyoxometalates: The effects of acid concentration

Heteropoly acids (HPAs) have found wide applications, such as in catalysis, energy conversion and storage, analytical chemistry, clinical medicine, materials science and others, but their use is limited due to their low surface area and high solubility in water. One of the possible ways to overcome these obstacles is to use height specific surface area supports for HPAs, such as carbon nanomaterials. Raman spectroscopy was applied for a studying the interaction between HPAs and highly ordered pyrolytic graphite (HOPG) as a model of a support. HOPG was exposed to two different HPAs: 12-tungsto-phosphoric acid and 12-molybodphosphoric acid, at different concentrations. It was noticed that 12-molybodphosphoric acid had stronger effects on the HOPG structure causing a weak doping and an increase of structural disorder. It was supposed that HOPG interacts with especially external oxygen atoms of 12-molybodphosphoric acid. Atomic force microscopy showed that the surface roughness of HOPG treated with 12-molybodphosphoric acid increases with increasing acid concentration, while in the case of HOPG exposed to 12-tungsto-phosphoric acid, the surface roughness concentration independent. The growth trend in the measured surface roughness (RMS) was in the agreement with the changes in the intensity ratio I-D/I-G obtained from the Raman spectra of the HOPG samples treated with 12-molybdophosphoric acid.