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

Abstract

Heteropoly acids (HPAs) found wide applications, such as in catalysis, energy conversion and storage, analytical chemistry, clinical medicine, materials science and others, but because of their low surface area and high solubility in water their use is limited. One of the possible ways to overcome these obstacles is to use height specific surface area support for HPAs, such as carbon nanomaterials. Raman spectroscopy was applied for studying study interaction between HPAs and highly ordered pyrolytic graphite (HOPG) as a model of a support. HOPG was exposed to two different HPAs: 12-tungstophosphoric acid and 12-molybodphosphoric acid, at different concentrations. It was noticed that 12-molybodphosphoric acid has stronger effects on HOPG structure causing a weak doping and increase of structural disorder. It is supposed that HOPG interacts with especially external oxygen atoms of 12-molybodphosphoric acid. Atomic force microscopy showed that surface roughness of HOPG treated with 12-molybodphosphoric acid increases with increase of acid concentration, while in the case of HOPG exposed to 12-tungstophosphoric acid the surface roughness is not concentration dependent. The growth trend in measured surface roughness (RMS) is in the agreement with the changes in the intensity ratio ID/IG obtained from Raman spectra of HOPG samples treated with 12-molybdophosphoric acid.