Title: Understanding Atomic-Scale Compositions, Structures, and Properties of Semi-Crystalline Inorganic Nanomaterials
Surface interactions at nanostructured interfaces determine the crystallization, adsorption, optoelectronic, and/or catalytic reaction properties of solid-state materials such as heterogeneous catalysts, battery materials, nanocrystalline semiconductors, and cementitious solids. However, the atomic-scale compositions and molecular interactions that yield the macroscopic properties are often challenging to characterize by conventional scattering or spectroscopic techniques. Here, recently-developed surface characterization techniques are applied to understand and correlate the structures, compositions, interactions, and macroscopic properties of partially- or non-ordered heterogeneous inorganic materials. Analyses using dynamic nuclear polarization (DNP) enhanced solid-state NMR spectroscopy, with complementary X-ray diffraction (XRD), electron microscopy, and surface forces measurements, elucidate the types, distributions, and interactions of dilute non-stoichiometric surface and heteroatom active sites. These insights are correlated to the macroscopic catalytic or optoelectronic properties of technological materials such as zeolite catalysts, semiconductor nanoparticles, and carbon-based electrocatalysts.
Results and analyses will be presented for two systems: aluminosilicate zeolite catalysts, where catalytic adsorption and reaction properties arise from non-stoichiometric framework heteroatom sites; and the historic pigment Maya Blue, an inorganic-organic hybrid material developed and utilized in pre-Columbian Mesoamerica that exhibits remarkable durability and longevity. For nanoporous aluminosilicate zeolite catalysts, the analyses establish the types and distributions of Al heteroatom active sites, which are correlated to the macroscopic catalytic reaction (hydrocarbon conversion) properties. Notably, DNP-NMR techniques yield large enhancements in NMR signal sensitivity (ca. ×100), which enables measurement of highly resolved two-dimensional (2D) NMR correlation spectra that were previously infeasible to acquire. These 2D NMR spectra establish the site-specific interactions of different framework heteroatom sites and molecular guest species, including reactant molecules and dilute adsorbed catalytic reaction byproducts. Specific types of surface defects are found to act as important hydrocarbon adsorption sites at early stages of catalyst deactivation, providing useful insights into the deactivation processes of zeolite materials with applications for hydrocarbon conversion and pollution removal. The methods are extended to the Maya Blue pigment, a complex of organic indigo dye molecules (<1 wt%) and the nanoporous clay palygorskite that retains its vibrant blue-turquoise color even after aging, weathering, thermal degradation, or exposure to concentrated acid or chemical solvents. Detailed solid-state NMR, electron microscopy, and XRD analyses of Maya Blue determine the types, adsorption environments, dynamics, and framework interactions of dilute occluded indigo molecules within the palygorskite matrix that account for the stability of the complex. The methods, analyses, and results presented here for two challenging and interesting heterogeneous systems are expected to be of broad importance in understanding the molecular-level origins of the macroscopic properties of diverse heterogeneous inorganic materials with important technological applications.