Herein, a detailed analysis was carried out using high-field (19.9 T) ²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR) on three specially prepared aluminum oxide samples where the γ-, δ-, and θ-Al₂O₃ phases are dominantly expressed through careful control of the synthesis conditions. Specifically, two-dimensional (2D) multiquantum (MQ) MAS ²⁷Al was used to obtain high spectral resolution, which provided a guide for analyzing quantitative 1D ²⁷Al NMR spectra. Six aluminum sites were resolved in the 2D MQ MAS NMR spectra, and seven aluminum sites were required to fit the 1D spectra. A set of octahedral and tetrahedral peaks with well-defined quadrupolar line shapes was observed in the θ-phase dominant sample and was unambiguously assigned to the θ-Al₂O₃ phase. The distinct line shapes related to the θ-Al₂O₃ phase provided an opportunity for effectively deconvoluting the more complex spectrum obtained from the δ-Al₂O₃ dominant sample, allowing the peaks/quadrupolar parameters related to the δ-Al₂O₃ phase to be extracted. The results show that the δ-Al₂O₃ phase contains three distinct Al_O sites and three distinct Al_T sites. This detailed Al site structural information offers a powerful way of analyzing the most complex γ-Al₂O₃ spectrum. It is found that the γ-Al₂O₃ phase consists of Al sites with local structures similar to those found in the δ-Al₂O₃ and θ-Al₂O₃ phases albeit with less ordering. Spin-lattice relaxation time measurement further confirms the disordering of the lattice. Collectively, this study uniquely assigns ²⁷Al features in transition aluminas, offering a simplified method to quantify complex mixtures of aluminum sites in transition alumina samples.

A major challenge limiting the practical applications of nanomaterials is that the activities of nanostructures (NSs) increase with reduced size, often sacrificing their stability in the chemical environment. Under oxidative conditions, NSs with smaller sizes and higher defect densities are commonly expected to oxidize more easily, since high-concentration defects can facilitate oxidation by enhancing the reactivity with O₂ and providing a fast channel for oxygen incorporation. Here, using FeO NSs as an example, we show to the contrary, that reducing the size of active NSs can drastically increase their oxidation resistance. A maximum oxidation resistance is found for FeO NSs with dimensions below 3.2 nm. Rather than being determined by the structure or electronic properties of active sites, the enhanced oxidation resistance originates from the size-dependent structural dynamics of FeO NSs in O₂. We find this dynamic size effect to govern the chemical properties of active NSs.

Geometric or electronic confinement of guests inside nanoporous hosts promises to deliver unusual catalytic or opto-electronic functionality from existing materials but is challenging to obtain particularly using metastable hosts, such as metal-organic frameworks (MOFs). Reagents (e.g. precursor) may be too large for impregnation and synthesis conditions may also destroy the hosts. Here we use thermodynamic Pourbaix diagrams (favorable redox and pH conditions) to describe a general method for metal-compound guest synthesis by rationally selecting reaction agents and conditions. Specifically we demonstrate a MOF-confined RuO₂ catalyst (RuO₂@MOF-808-P) with exceptionally high catalytic CO oxidation below 150 °C as compared to the conventionally made SiO₂-supported RuO₂ (RuO₂/SiO₂). This can be caused by weaker interactions between CO/O and the MOF-encapsulated RuO₂ surface thus avoiding adsorption-induced catalytic surface passivation. We further describe applications of the Pourbaix-enabled guest synthesis (PEGS) strategy with tutorial examples for the general synthesis of arbitrary guests (e.g. metals, oxides, hydroxides, sulfides).

Topological insulators (TIs) are quantum materials with topologically protected surface states surrounding an insulating bulk. However, defect-induced bulk conduction often dominates transport properties in most TI materials, obscuring the Dirac surface states. In order to realize intrinsic topological insulating properties, it is thus of great significance to identify the spatial distribution of defects, understand their formation mechanism, and finally control or eliminate their influence. Here, the electronic heterogeneity in polyol-synthesized Bi2Se3 and chemical vapor deposition-grown Sb2Te3 nanocrystals is systematically investigated by multimodal atomic-to-mesoscale resolution imaging. In particular, by combining the Drude response sensitivity of infrared scattering-type scanning near-field optical microscopy with the work-function specificity of mirror electron microscopy, characteristic mesoscopic patterns are identified, which are related to carrier concentration modulation originating from the formation of defects during the crystal growth process. This correlative imaging and modeling approach thus provides the desired guidance for optimization of growth parameters, crucial for preparing TI nanomaterials to display their intrinsic exotic Dirac properties.