Identification of CO2 adsorption sites on MgO nanosheets by solid-state nuclear magnetic resonance spectroscopy.

The detailed information on the surface structure and binding sites of oxide nanomaterials is crucial to understand the adsorption and catalytic processes and thus the key to develop better materials for related applications. However, experimental methods to reveal this information remain scarce. Here we show that 17O solid-state nuclear magnetic resonance (NMR) spectroscopy can be used to identify specific surface sites active for CO2 adsorption on MgO nanosheets. Two 3-coordinated bare surface oxygen sites, resonating at 39 and 42 ppm, are observed, but only the latter is involved in CO2 adsorption. Double resonance NMR and density functional theory (DFT) calculations results prove that the difference between the two species is the close proximity to H, and CO2 does not bind to the oxygen ions with a shorter O···H distance of approx. 3.0 Å. Extensions of this approach to explore adsorption processes on other oxide materials can be readily envisaged.

17O Solid-State NMR Studies of Ta2O5 Nanorods.

17O solid-state NMR spectroscopy was used to study the structure of Ta2O5 nanorods for the first time. Although the observations of oxygen ions in the "bulk" part of the Ta2O5 nanorods can be achieved with conventional high-temperature enrichment with 17O2, low-temperature isotopic labeling with H2 17O generated samples whose surfaces are selectively enriched, leading to surface-only detection of oxygen species. By applying 17O-1H double-resonance NMR techniques and 1H NMR spectroscopy, surface hydroxyl species and adsorbed water can also be studied. The results form the basis for further understanding of the structure-property relationship of Ta2O5 nanomaterials.

Probing Interactions of γ-Alumina with Water via Multinuclear Solid-State NMR Spectroscopy.

Interaction of γ-alumina with water are important in controlling its structure and catalytic properties. We apply solid-state multinuclear NMR spectroscopy to investigate this interaction by monitoring 1H and 17O spectra in real-time. Surface-selective detection is made possible by adsorbing 17O-enriched water on γ-alumina nanorods. Structural evolution on the surface was selectively probed by 1H/17O double resonance NMR and 27Al NMR at ultrahigh 35.2 T magnetic field. Formation of hydroxyl species on the surface of nanorods is rapid upon the exposure of water, which involves low coordinated aluminum ions with doubly bridging and isolated hydroxyl species being generated first. Fast exchange occurs between oxygen atoms in the water molecules and bare surface sites, indicating high reactivity of these oxygen species. These results provide new insights into the structure and dynamics on the surface of γ-alumina and the methods applied here can be extended to study the interaction of other oxides with water.

Polar surface structure of oxide nanocrystals revealed with solid-state NMR spectroscopy.

Compared to nanomaterials exposing nonpolar facets, polar-faceted nanocrystals often exhibit unexpected and interesting properties. The electrostatic instability arising from the intrinsic dipole moments of polar facets, however, leads to different surface configurations in many cases, making it challenging to extract detailed structural information and develop structure-property relations. The widely used electron microscopy techniques are limited because the volumes sampled may not be representative, and they provide little chemical bonding information with low contrast of light elements. With ceria nanocubes exposing (100) facets as an example, here we show that the polar surface structure of oxide nanocrystals can be investigated by applying 17O and 1H solid-state NMR spectroscopy and dynamic nuclear polarization, combined with DFT calculations. Both CeO4-termination reconstructions and hydroxyls are present for surface polarity compensation and their concentrations can be quantified. These results open up new possibilities for investigating the structure and properties of oxide nanostructures with polar facets.

Shape transformation of {hk0}-faceted Pt nanocrystals from a tetrahexahedron into a truncated ditetragonal prism.

Shape transformation of Pt nanocrystals from a {730}-bounded tetrahexahedron into a {310}-bounded truncated ditetragonal prism was achieved using the electrochemical square-wave potential method. The transformation process and mechanism were revealed. This study provides new insight into the nanocrystal growth habit.

Hydrogen adsorption-mediated synthesis of concave Pt nanocubes and their enhanced electrocatalytic activity.

Concave nanocubes are enclosed by high-index facets and have negative curvature; they are expected to have enhanced reactivity, as compared to nanocubes with flat surfaces. Herein, we propose and demonstrate a new strategy for the synthesis of concave Pt nanocubes with {hk0} high-index facets, by using a hydrogen adsorption-mediated electrochemical square-wave potential method. It was found that Pt atoms prefer to deposit on edge sites rather than terrace sites on Pt surfaces with intensive hydrogen adsorption, resulting in the formation of concave structures. The as-prepared concave Pt nanocubes exhibit enhanced catalytic activity and stability towards oxidation of ethanol and formic acid in acidic solutions, compared to commercial Pt/C catalysts.

Electrochemically Seed-Mediated Synthesis of Sub-10 nm Tetrahexahedral Pt Nanocrystals Supported on Graphene with Improved Catalytic Performance.

Controlling the surface structure of Pt nanocrystals (NCs), especially creating high-index facets with abundant active step sites, is an effective approach to enhance catalytic performances. However, the available high-index faceted Pt NCs have large particle sizes, which severely impedes their practical applications. In this study, we reported a new electrochemically seed-mediated method, by which sub-10 nm tetrahexahedral Pt NCs (THH Pt NCs) enclosed with {210} high-index facets supported on graphene were synthesized. Pt nanoparticles of ∼3 nm in size as high-density crystal seeds play a key role in the small-sized control. The obtained THH Pt NCs exhibited a higher mass activity than commercial Pt/C catalyst for ethanol electrooxidation. We further demonstrated that this method is also valid for reshaping commercial Pt/C, to create high-index facets on surfaces and thus to improve both mass activity and stability.