Structural and Interfacial H2O/OH Species: Two Case Studies on Their Roles in Host Structural Stability
Structural H2O/OH is widely found in hydrous materials such as zeolites, clays, hydrates, and many other zeolite-like (nanoporous) systems, where the H2O/OH species can be considered either as an integral part of the crystal or as guest species. In particular, structurally bound H2O molecules in natural zeolites can occur in both ways and they can exert strong influence on the host framework configurations through host-guest interactions. Similarly, interfacial H2O/OH and its layering within ~2-3 molecular layers of the sorbing substrate reveal variations in structure and dynamics depending on the nature of the interface. The structural ordering and time scales for the diffusional motions of interfacial H2O/OH species locally and over larger length scales are of fundamental importance in many chemical and biological reactions.
Here, we present two independent case studies that exemplify these different types of H2O, structurally bound H2O in NAT-topology zeolites and surface-bound H2O/OH species on SnO2 nanocrystals. In the case of NAT-zeolites, the guest H2O molecules (albeit strongly bound) directly communicate with both the external environment and the host NAT-framework, which influences the framework T - P (H2O) structure and stability. Indeed, differences in external P (H2O) (i.e., relative humidity) give rise to very different thermal transformation paths observed from X-ray structural analysis. This finding suggests that the way the structure behaves during thermal dehydration depends on the coupling force between the guest dynamics and the flexibility of the host. In the case of SnO2 nanocrystals, the structure of a few layers of adsorbed H2O/OH on SnO2 has been elucidated by neutron total scattering methods, coupled with molecular dynamic simulations.
These highly structured interfacial H2O/OH layers, in registry with the substrate structure, not only stabilize the host phase as nanoparticles but also induce particle-size-dependent structural modifications. The minimum concentration of OH groups required to prevent rapid growth of nanoparticles during thermal dehydration corresponds to ~0.7 monolayer coverage. Dynamically, at around this minimum coverage, OH groups yield unique stretching and wagging motions. Our data show that H2O confined in different locales (internal vs. external surfaces) play critical roles in the stability and reactive properties of the host materials.