Understanding mechanistic pathways to radiolytic hydrogen generation by metal oxyhydroxide nanomaterials is challenging because of the difficulties of distinguishing key locations of OH bond scission, from structural interiors to hydroxylated surfaces to physi-sorbed water molecules. In this work, we exploited the interface-selectivity of vibrational sum frequency generation (VSFG) to isolate surface versus bulk hydroxyl groups for gibbsite and boehmite nanoplatelets before and after 60Co irradiation at dose levels of approximately 7.0 and 29.6 Mrad. While high-resolution microscopy revealed no effects on particle bulk and surface structures, VSFG results clearly indicated up to 83% and 94% radiation-induced surface OH bond scission for gibbsite and boehmite, respectively, a substantially higher proportion than observed for interior OH groups by IR and Raman spectroscopy. Rehydration of the irradiated surfaces showed incomplete recovery of the surface hydroxyl groups with differences in the re-hydration behaviour between gibbsite and boehmite. Using ab initio molecular simulations, we performed a detailed analysis of the water-boehmite (010) and water-gibbsite (001) interfaces interactions and evaluated the effect of hydrogen surface vacancy on the strength of the hydrogen bonds. A defect energetic analysis indicates that H2 and H2O are the energetically most favorable products species to form from radiation induced surface H and OH defects. By performing nudged elastic band (NEB) calculations, we give an atomistic level description of the proton transfer mechanisms, as could be involved in rehydration and dehydration processes, at the water-boehmite (010) and water-gibbsite (001) interfaces.

Radiation-induced interfacial hydroxyl transformation on boehmite and gibbsite

ACS Fall 2020 Virtual Meeting

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