Chemical Titrations and Temperature-Programmed Desorption Study of the Surface Chemistry of Graphene Oxide and 12-Tungstophosphoric Acid Nanocomposite

The surface chemistry of graphene oxide (GO) and its nanocomposite with 12-tungstophosphoric acid (WPA) (up to 50 wt.% WPA) was studied both in aqueous suspension and in the solid state. The titrations revealed the formation of the composite already in the suspension and that WPA influences GO’s functionalities and their conversion (-COOR to -COOH). There is a loading of WPA (>20 wt.%) beyond which the WPA dominates the chemical character of the GO/WPA suspension. Part of the nanocomposite titrated with NaOH was processed into a powdered form and compared with an annealed sample (450 °C, Ar atmosphere). An FTIR analysis revealed the removal of functional groups in both titrated and thermally annealed samples. Annealing did not induce structural changes in WPA within the composite, whereas titration led to noticeable modifications of WPA-related bands. The TPD measurements revealed that the extent of functional group removal by titration was lower compared to annealing. The zeta-potential measurements demonstrated improved stability of the nanocomposite as the WPA content increased. Methylene blue adsorption experiments showed that the presence of oxygen functional groups and WPA on the GO enhances adsorption performance compared to pristine GO. Titration improved the adsorption capacity of the composites, whereas annealing completely suppressed their adsorption properties.