Surface oxygen-dependent water cluster growth in carbon nanospaces with GCMC simulation-aided in situ SAXS

The surface oxygen concentration of pitch-based activated carbon fibers (ACF) was controlled by heat treatment in Ar and H-2. The surface oxygen-to-carbon ratio was determined by X-ray photoelectron spectroscopy; the O/C ratios of the as-received ACF, the ACF treated in Ar, and the ACF treated in H-2 were 0.07, 0.03, and 0.02, respectively. The adsorbed states of water in the nanopores of the three ACFs were investigated by density fluctuation analysis from small-angle X-ray scattering and grand canonical Monte Carlo (GCMC) simulation. Although the adsorption isotherms of the three ACFs at 303 K have almost similar features, their density fluctuation profiles with filling differ. The density fluctuations in the course of adsorption showed that the cluster size of the adsorbed water increased with the decrease in the number of surface oxygens. Radial distributions calculated from the snapshots of the GCMC simulation suggested that the water structure has a short-range order with hydrogen bonding. The island type growth of clusters on the surface oxygens served as evidence for the ACF with surface oxygen.