Surface Hydration and Hydroxyl Configurations of Gibbsite and Boehmite Nanoplates

The discontinuation of the bulk structure at the interface between metal oxide particles and water leads to altered bonding characteristics and unique facet-dependent molecular environments. Surface hydration and hydroxylation add further complexity to the interface, details that for metal (oxy)hydroxides are especially difficult to isolate from the background signal of bulk structural hydroxyls. Here, we probe for the first time the surface hydroxyl structures and the effect of hydration on basal surfaces of gibbsite (α-Al(OH)₃) and boehmite (γ-AlOOH) nanoplates under ambient conditions, using the interface-sensitive technique vibrational sum frequency generation (VSFG) spectroscopy. VSFG spectra of the hydroxyl stretching modes at the interfaces with adsorbed water layers compared directly to Raman and infrared spectra of the bulk modes show that while gibbsite surface frequencies were sharp and nearly identical to those in the bulk, boehmite surface hydroxyls displayed a very different broad spectrum of states. Ab initio molecular dynamics simulations of both basal surfaces with and without hydration waters reveal that gibbsite surface hydroxyls interact only weakly with overlying hydration waters remaining essentially unperturbed, whereas those on boehmite can interact more strongly facilitated by higher configurational degrees of freedom at the interface and there is more extensive H-bonding on boehmite surface under ambient conditions. The findings clearly unveil substantial differences in the hydrated interfacial dynamics of these two otherwise similar materials, with implications for their interfacial chemistry, wettability, and rheology.