Topochemical Speciation of Acidic Sites in Graphene Oxide via Tailored FTIR Probes

An infrared (IR) probe method was proposed to study the acid-base properties of the graphene oxides (GOs) to distinguish Lewis (L) and Brønsted-Lowry (B) adsorption sites at the GO surfaces. The method was validated on commercial and laboratory-synthesized Hummers GO samples (HGO). Upon binding of IR-active probe molecules, newly appearing bands and significant shifts in positions of the existing peaks made it possible to identify L- and B-sites. Characteristic bands were observed at 1540 cm^-1 (B), 1485 cm^-1 (B + L), and 1440 cm^-1 (L). It was demonstrated that Brønsted bands appear as new bands absent in the original spectra of the probe or HGO, while Lewis bands manifest themselves as a shift (mean, ±5 cm^-1) and broadening of existing probe bands. Band assignments were supported by the quantum-chemistry simulations of IR probe spectra, along with a comparative analysis of the IR spectra for similar systems. The preparation of the test samples needs to include a preliminary dialysis purification step to remove traces of potential probe-binding species, e.g., Mn²⁺. Vapor-phase saturation of HGO with nine selected organic probes unveiled pyridine as the most suitable due to clear band resolution and selective interaction. The effect of probe deuteration and steric hindrance caused by the complex molecular structure of the probe was examined. IR probe testing was complemented by isopiestic sorption measurements. Collectively, the results demonstrate that IR-probe approaches were made to estimate the concentration of adsorption sites on the GO surface.