UC Santa Barbara

Many important material properties are determined by the surface layer (0.5-100 nm); these include optoelectronic properties like conductivity, absorptivity, and reflectance as well as physiochemical properties such as molecular adsorption, hydrophobicity, and surface diffusivity. Furthermore, the crystallization or assembly processes of technologically-important nanomaterials are mediated by surface interactions and influence the surface compositions and properties of the resultant material, including zeolites, colloidal semiconductors, carbon-based electrocatalysts, and mesoporous inorganic oxides. Catalytic reaction properties of diverse porous heterogeneous materials are determined by molecular interactions of adsorbates, reactants, or product species at pore or exterior surface sites.

Despite the broad importance of surface interactions in determining material properties, fundamental questions remain regarding the physiochemical interactions at surfaces that determine crystallization, adsorption, optoelectronic, and/or reaction properties. This is because such properties often depend on dilute surface moieties, defect species, and/or molecular adsorbates that occupy distributions that are partially- or non-ordered and are therefore challenging or impossible to characterize by conventional scattering techniques. Developing atomic-level insights into the types, interactions, and distributions of such dilute non-ordered species is crucial to elucidate the molecular-level origins of the properties and synthesis pathways of materials such as zeolites, semiconductor nanoparticles, and mesoporous electrocatalysts. By understanding the crystallization, synthesis, and assembly processes of these materials, as well as the resulting structures and active species, the resulting insights can be applied to develop new synthetic or post-synthetic treatments to generate more effective, stable, and/or active materials with desirable properties.

The objective of this dissertation is to measure, understand, and correlate the atomic-scale compositions, structures, and properties of heterogenous materials with diverse applications, including heterogeneous catalysis and solid-state lighting. Recently-developed solid-state nuclear magnetic resonance (NMR) techniques with complementary X-ray diffraction and electron microscopy analyses are applied to elucidate the structures and compositions of dilute surface, defect, or heteroatom species, which are correlated to the macroscopic properties of interest. These techniques are applied to analyze diverse heterogeneous inorganic nanomaterials, including aluminosilicate zeolites, cementitious solids, precious-metal-free electrocatalysts, and nanocrystalline semiconductors. Though the material systems vary in composition and application, in each case the important optical, electronic, and/or catalytic properties arise from dilute partially- or non-ordered defect or heteroatom species in a semi-crystalline lattice. The overall unifying themes are: (1) analysis of order and disorder in semi-crystalline inorganic solids using state-of-the-art diffraction and spectroscopic characterization techniques; (2) determining the distributions and structures of non-stoichiometric species, particularly at surfaces and interfaces; and (3) correlating atomic-level structures and compositions with macroscopic material properties. The insights provided are of broad importance and relevance to diverse material systems of technological interest for sustainable energy storage, conversion, and utilization.