Hongpan Rong

Ultrathin rhodium nanosheets

Despite significant advances in the fabrication and applications of graphene-like materials, it remains a challenge to prepare single-layered metallic materials, which have great potential applications in physics, chemistry and material science. Here we report the fabrication of poly(vinylpyrrolidone)-supported single-layered rhodium nanosheets using a facile solvothermal method. Atomic force microscope shows that the thickness of a rhodium nanosheet is <4 Å. Electron diffraction and X-ray absorption spectroscopy measurements suggest that the rhodium nanosheets are composed of planar single-atom-layered sheets of rhodium. Density functional theory studies reveal that the single-layered Rh nanosheet involves a δ-bonding framework, which stabilizes the single-layered structure together with the poly(vinylpyrrolidone) ligands. The poly(vinylpyrrolidone)-supported single-layered rhodium nanosheet represents a class of metallic two-dimensional structures that might inspire further fundamental advances in physics, chemistry and material science.

Single-crystalline octahedral Au-Ag nanoframes.

We report the formation of single-crystalline octahedral Au-Ag nanoframes by a modified galvanic replacement reaction. Upon sequential addition of AgNO(3), CuCl, and HAuCl(4) to octadecylamine solution, truncated polyhedral silver nanoparticles formed first and then changed into octahedral Au-Ag nanoframes, without requiring a conventional Ag removal step with additional oxidation etchant. The nanoframes have 12 sides, and all of the eight {111} faces are empty. The side grows along the [110] direction, and the diameter is less than 10 nm. The selective gold deposition on the high-energy (110) surface, the diffusion, and the selective redeposition of Au and Ag atoms are the key reasons for the formation of octahedral nanoframes.

Oleylamine-mediated shape evolution of palladium nanocrystals.

The properties of noble metal nanocrystals (NCs) are shaperelated, especially for Au, Ag, Pd, and Pt. Nonspherical Au and Ag NCs show distinct surface plasmon resonance, and the catalytic activity of Pd and Pt NCs can be dictated by exposing different lattice planes and increasing the number of corners and edges. 6] Therefore, morphology control of noble metal NCs is critically important. Various synthetic strategies have been developed to construct nonspherical noble metal NCs in hydrophilic solvents such as water, ethylene glycol, and DMF. As a complement to the prevalent hydrophilic synthesis, shape control of noble metals in hydrophobic organic solvents is also necessary. Advantages of noble metal NCs made in hydrophobic solvents include narrow size distribution, tunable surface chemistry, and good compatibility with organic functional groups. First, monodisperse NCs can easily selfassemble into highly ordered patterns, and thus facilitate fabrication of devices. Moreover, monodispersity is of great importance for the study of size-related properties. Second, in hydrophobic synthesis, the choice of surfactants is greatly expanded to carboxylic acids, alkane thiols, alkyl phosphines, alkyl amines, and so on, which will greatly enrich the surface chemistry of NCs and further influence their catalytic, optical, and magnetic properties. For example, charge transport by Au and Ag NCs can be regulated by the length of alkane thiol molecules capping their surface. Third, noble metal NCs can be modified with various functional organic groups in hydrophobic solvents to give advanced hybrid materials. For instance, by incorporating azobenzene groups in the ligands, hydrophobic Au NCs become capable of writing self-erasing images. Several groups have reported the synthesis of nonspherical hydrophobic noble metal NCs. Examples include Au nanowires obtained by reducing polymeric strands of Au– oleylamine (OAm) complex, 30] Pt nanocubes, multipods, and Rh tetrahedra prepared through gas-assisted (e.g., H2, CO) synthesis, and Ag nanocubes obtained by oxidative etching. 36] Despite this success, systematic shape control of noble metal NCs in hydrophobic systems remains a challenging task, and the surfactant effects behind shape evolution need to be further identified. Herein we report an OAm-based system for synthesis of Pd NCs in toluene; icosa-, deca-, octa-, tetrahedral, and triangular platelike Pd NCs were systematically prepared. The synthesis follows a threestep “intermediates formation–nucleation–growth” process, during which OAm plays a key role in the shape evolution of Pd NCs by mediating the counterbalance between crystal strain and surface energy. The electrochemical activities of asobtained Pd NCs were investigated in the oxidation of formic acid. Palladium NCs were typically synthesized by stirring a mixture of OAm, formaldehyde, [Pd(acac)2] (acac = acetylacetonate), and toluene at room temperature in air, heating it in an autoclave at 100 8C for 8 h, and allowing it to slowly cool to ambient temperature (Scheme S1, Supporting Information). Figure 1 shows TEM images of the as-obtained Pd NCs. Generally, the products are all uniform and exclusively bound by well-defined {111} facets. Icosahedra have an edge length of 8.8 0.6 nm. Their icosahedral shape can be identified by the hexagonal projections and the threefold twinned structure observed in Figure 1c. Decahedra have an edge length of 10.3 0.8 nm. Their decahedral shape can be easily confirmed by the pentagonal profiles and fivefold twinned structure observed in Figure 1 f. Octahedra with an edge length of 6.6 0.5 nm can spontaneously assemble into large-scale 2D arrays on a copper grid (Figure 1g). Continuous lattice fringes in the high-resolution (HR) TEM image (Figure 1 i) confirm its single-crystalline nature. When oriented along [001], [110], and [111], octahedra exhibited square, rhombic, and hexagonal projections, respectively (Figure S2, Supporting Information). Tetrahedra or truncated tetrahedra with an edge length of 12.9 1.4 nm can also form patterned arrays on copper grid (Figure 1 j). Although a triangular profile is observed in the TEM image, high-angle annular dark-field scanning TEM (HAADF-STEM) studies unveil their tetrahedral shape (Figure S3, Supporting Information). The triangular plates have an edge length of 17.3 4.1 nm and a thickness of 6.6 1.0 nm. Figure 1n shows a monolayer of patterned Pd triangular plates. We found that the as-obtained Pd triangular plates could form highly ordered 3D assemblies in two different ways by controlling their concentration (Figure S4, Supporting Information). To elucidate the mechanism of this synthetic system, the roles of reagents were investigated in detail. First, we found OAm would coordinate to [Pd(acac)2] to form different intermediates under room temperature stirring. Figure 2a displays the evolution of UV/Vis spectra of a toluene solution containing [Pd(acac)2] and OAm. A strong adsorption peak around 329 nm was observed for pure [Pd(acac)2]. After addition of 0.02 mL of OAm, the peak at 329 nm decreased while another adsorption peak at about 315 nm emerged, which can be attributed to the formation of [Pd(acac)x[*] Z. Niu, Dr. Q. Peng, M. Gong, H. Rong, Prof. Y. Li Department of Chemistry Tsinghua University, Beijing, 100084 (P. R. China) Fax: (+ 86)10-6278-8765 E-mail: ydli@mail.tsinghua.edu.cn

Kinetically Controlling Surface Structure to Construct Defect‐Rich Intermetallic Nanocrystals: Effective and Stable Catalysts

Kinetic control of surface defects is achieved, and cubic, concave cubic, and defect-rich cubic intermetallic Pt3 Sn nanocrystals are prepared for the electro-oxidation of formic acid. The generality of this kinetic approach is demonstrated by the fabrication of Pt-Mn nanocrystals with different surface defects. The defect-rich nanocrystals exhibit high catalytic activity and stability concurrently, indicating their potential application in fuel cells.

Structure Evolution and Associated Catalytic Properties of PtSn Bimetallic Nanoparticles

Bimetallic nanoparticles (NPs) often show new catalytic properties that are different from those of the parent metals. Carefully exploring the structures of bimetallic NPs is a prerequisite for understanding the structure-associated properties. Herein, binary PtSn NPs with tunable composition are prepared in a controllable manner. X-ray characterizations reveal that their structures evolve from SnO2-x -patched PtSn alloys to SnO2-x -patched Pt clusters when more tin is incorporated. An obvious composition-dependent catalytic performance is observed for the hydrogenation of α,β-unsaturated aldehydes: the selectivity to unsaturated alcohol increases substantially at high tin content, whereas the reaction rate follows a volcano shape. Furthermore, Pt sites are responsible for hydrogen dissociation, whereas oxygen vacancy (Ovac ) sites, provided by SnO2-x , drastically enhance the adsorption of carbonyl group.

PtAl truncated octahedron nanocrystals for improved formic acid electrooxidation

We successfully obtained truncated octahedron PtAl alloy nanocrystals (∼10 nm) via a facile one-pot strategy. The PtAl nanocrystals exhibited enhanced activity in formic acid electrooxidation compared to commercial Pt/C, which could be ascribed to the large density of defects on the surface as well as the synergy between Pt and Al.