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Single-atom functionalization of transition-metal dichalcogenide (TMD) nanosheets is a powerful strategy to tune the optical, magnetic, and catalytic properties of two-dimensional materials. In this work, we demonstrate a simple solution-phase method to generate nucleophilic sulfide sites on colloidal WS2nanosheets that subsequently serve as ligands for Ni single atoms. These materials can be controllably functionalized with varying amounts of Ni on the surface ranging from 9% to 47% coverage with respect to W. High-resolution scanning transmission electron microscopy coupled to electron energy loss spectroscopy and X-ray absorption spectroscopy indicate that adsorbed Ni species bind as single atoms at low coverage and a mixture of single atoms and multimetallic clusters at high coverage. The Ni single atoms adsorbed on WS2show altered electronic properties, and both the electronic perturbation and isolated atom geometry play a role in enhancing the intrinsic catalytic activity of Ni-WS2samples for the electrochemical oxygen evolution reaction.

Silica supported Pd and Pd–In catalysts with different In : Pd atomic ratios and similar particle size (∼2 nm) were tested for ethane dehydrogenation at 600 °C. For a monometallic Pd catalyst, at 15% conversion, the dehydrogenation selectivity and initial turnover rate (TOR, per surface Pd site) were 53% and 0.03 s−1, respectively. Addition of In to Pd increased the dehydrogenation selectivity to near 100% and the initial TOR to 0.26 s−1. Carbon monoxide IR, in situsynchrotron XAS and XRD analysis showed that for Pd–In catalysts with increasing In loading, different bimetallic structures were formed: at low In loading a fraction of the nanoparticle surface was transformed into PdIn intermetallic compound (IMC, also known as intermetallic alloy) with a cubic CsCl structure; at higher In loading, a Pd-core/PdIn-shell structure was formed and at high In loading the nanoparticles were pure PdIn IMC. While a Pd metal surface binds CO predominantly in a bridge fashion, the PdIn IMC predominantly binds CO linearly. Formation of the PdIn IMC structure on the catalyst surface geometrically isolates the Pd catalytic sites by non-catalytic, metallic In neighbors, which is suggested to be responsible for the high olefin selectivity. Concomitant electronic effect due to Pd–In bond formation likely leads to the increase in TOR. Though multiple IMC structures with different atomic ratios are possible for the Pd–In binary system, only a cubic PdIn IMC with CsCl structure was observed, implying a kinetically controlled solid state IMC formation mechanism.

We investigate the aerobic oxidation of high-pressure, high-temperature nanodiamonds (5–50 nm dimensions) using a combination of carbon and oxygen K-edge X-ray absorption, wavelength-dependent X-ray photoelectron, and vibrational spectroscopies. Oxidation at 575 °C for 2 h eliminates graphitic carbon contamination (>98%) and produces nanocrystals with hydroxyl functionalized surfaces as well as a minor component (<5%) of carboxylic anhydrides. The low graphitic carbon content and the high crystallinity of HPHT are evident from Raman spectra acquired using visible wavelength excitation (λexcit= 633 nm) as well as carbon K-edge X-ray absorption spectra where the signature of a core–hole exciton is observed. Both spectroscopic features are similar to those of chemical vapor deposited (CVD) diamond but differ significantly from the spectra of detonation nanodiamond. The importance of these findings to the functionalization of nanodiamond surfaces for biological labeling applications is discussed.

Well-controlled synthesis of nanocrystals is necessary to unambiguously correlate the structural properties of nanocrystals with the catalytic properties. The most common low-index surfaces are (111) and (100). Therefore, model materials with {111} and {100} facets are highly desirable, in order to understand the catalytic properties of (111) and (100) surfaces for various structure-sensitive reactions. We report a solution-phase synthesis using metal carbonyls as additives. This synthetic method produces highly monodisperse Pt octahedra and icosahedra as the model of Pt{111}, Pt cubes as the model of Pt{100}, respectively. Several other morphologies, such as truncated cubes, cuboctahedra, spheres, tetrapods, star-shaped octapods, multipods, and hyper-branched structure, are produced, as well. A bifunctional role of metal carbonyl in the synthesis is identified: zerovalent transition metal decomposed from metal carbonyl acts as a shape-directing agent, while CO provides the reducing power. These high-quality shape-controlled Pt nanocrystals are suitable for model catalyst studies.

Simultaneously probing the electronic structure and morphology of materials at the nanometer or atomic scale while a chemical reaction proceeds is significant for understanding the underlying reaction mechanisms and optimizing a materials design. This is especially important in the study of nanoparticle catalysts, yet such experiments have rarely been achieved. Utilizing an environmental transmission electron microscope equipped with a differentially pumped gas cell, we are able to conduct nanoscopic imaging and electron energy loss spectroscopy in situfor cobalt catalysts under reaction conditions. Studies reveal quantitative correlation of the cobalt valence states with the particles’ nanoporous structures. The in situexperiments were performed on nanoporous cobalt particles coated with silica, while a 15 mTorr hydrogen environment was maintained at various temperatures (300–600 °C). When the nanoporous particles were reduced, the valence state changed from cobalt oxide to metallic cobalt and concurrent structural coarsening was observed. In situmapping of the valence state and the corresponding nanoporous structures allows quantitative analysis necessary for understanding and improving the mass activity and lifetime of cobalt-based catalysts, for example, for Fischer–Tropsch synthesis that converts carbon monoxide and hydrogen into fuels, and uncovering the catalyst optimization mechanisms.

Formic acid is a promising chemical fuel for fuel cell applications. However, due to the dominance of the indirect reaction pathway and strong poisoning effects, the development of direct formic acid fuel cells has been impeded by the low activity of existing electrocatalysts at desirable operating voltage. We report the first synthesis of Pt3Pb nanocrystals through solution phase synthesis and show they are highly efficient formic acid oxidation electrocatalysts. The activity can be further improved by manipulating the Pt3Pb–Pt core–shell structure. Combined experimental and theoretical studies suggest that the high activity from Pt3Pb and the Pt–Pb core–shell nanocrystals results from the elimination of CO poisoning and decreased barriers for the dehydrogenation steps. Therefore, the Pt3Pb and Pt–Pb core–shell nanocrystals can improve the performance of direct formic acid fuel cells at desired operating voltage to enable their practical application.

Nonpolar AlGaN/GaN heterostructures have the potential to supplant polar heterostructures in infrared optoelectronic devices due to their theoretical advantages stemming from the absence of built‐in polarization fields along nonpolar directions of the nitride wurtzite lattice. However, development of nonpolar m‐plane infrared devices in a broad spectral range has been hampered, so far, by challenges to grow homogeneous high Al‐composition AlGaN on m‐plane GaN. AlxGa1‐xN layers with 0.6< x <0.8 are found to be kinetically unstable under metal‐rich growth conditions by plasma‐assisted molecular‐beam epitaxy. After reviewing recent progress in the field, this paper focuses on the effect of the structure of m‐plane AlxGa1−xN/GaN (x <0.6) superlattices on near‐infrared intersubband absorption. Even at these intermediate Al‐compositions, the effective growth rate of AlGaN is drastically reduced, and the AlGaN‐GaN interface roughness is unexpectedly high. Consequently, accurate determination of layer thicknesses and alloy composition necessitates structural characterization by a combination of scanning transmission electron microscopy and high‐resolution X‐ray diffraction. The energy and linewidth of near‐infrared intersubband transitions are also significantly affected by this unusual growth behavior. The experimental results for intersubband absorption of m‐ and c‐plane superlattices are compared to each other and with numerical calculations, and the main reasons for discrepancies are discussed. Nonpolar m‐plane AlGaN/GaN heterostructures are potential materials for infrared optoelectronics. It has been challenging, though, to grow homogeneous high‐Al content m‐plane alloys. Recent progress toward measuring and theoretically modeling infrared intersubband absorption in m‐plane AlGaN/GaN superlattices is discussed. Current factors limiting their performance are explored in comparison with similar c‐plane heterostructures.