Qing Hua

Morphological evolution of Cu2O nanocrystals in an acid solution: stability of different crystal planes.

The morphological evolution of uniform Cu(2)O nanocrystals with different morphologies in a weak acetic acid solution (pH = 3.5) has been studied for cubic, octahedral, rhombic dodecahedral, {100} truncated octahedral, and {110} truncated octahedral nanocrystals. Cu(2)O nanocrystals undergo oxidative dissolution in weak acid solution, but their morphological changes depend on the exposed crystal planes. We found that the stability of Cu(2)O crystal planes in weak acid solution follows the order of {100} ≫ {111} > {110} and determines how the morphology of Cu(2)O nanocrystals evolves. The stable {100} crystal planes remain, and new {100} facets form at the expense of the less stable {111} and {110} crystal planes on the surface of Cu(2)O nanocrystals. Density functional theory calculations reveal that the Cu-O bond on Cu(2)O(100) surface has the shortest bond length. These results clearly exemplify that the morphology of inorganic crystals will evolve with the change of local chemical environment, shedding light on fundamentally understanding the morphological evolution of natural minerals and providing novel insights into the geomimetic synthesis of inorganic materials in the laboratory.

Size-dependent interaction of the poly(N-vinyl-2-pyrrolidone) capping ligand with Pd nanocrystals.

Pd nanocrystals were prepared by the reduction of a H(2)PdCl(4) aqueous solution with C(2)H(4) in the presence of different amounts of poly(N-vinyl-2-pyrrolidone) (PVP). Their average size decreases monotonically as the PVP monomer/Pd molar ratio increases up to 1.0 and then does not vary much at higher PVP monomer/Pd molar ratios. Infrared spectroscopy and X-ray photoelectron spectroscopy results reveal the interesting size-dependent interaction of PVP molecules with Pd nanocrystals. For fine Pd nanocrystals capped with a large number of PVP molecules, each PVP molecule chemisorbs with its oxygen atom in the ring; for large Pd nanocrystals capped by a small number of PVP molecules, each PVP molecule chemisorbs with both the oxygen atom and nitrogen atom in the ring, which obviously affects the structure of chemisorbed PVP molecules and even results in the breaking of involved C-N bonds of some chemisorbed PVP molecules. Charge transfer always occurs from a chemisorbed PVP ligand to Pd nanocrystals. These results provide novel insights into the PVP-metal nanocrystal interaction, which are of great importance in the fundamental understanding of surface-mediated properties of PVP-capped metal nanocrystals.

Crystal-Plane-Controlled Surface Restructuring and Catalytic Performance of Oxide Nanocrystals†

Catalysts forheterogeneous catalytic reactions operate under pressures upto several hundred atmospheres and at temperatures up toseveral hundred degrees Celsius, so that the catalyst nano-particles caneasily undergosurfacerestructuringtoadopt thethermodynamically most stable structure. Thus, it is crucial toexplore the restructuring process of catalyst surfaces underthe reaction conditions to understand catalytic processes atthe microscopic level; moreover, it is desirable to tune thecatalytic performance of the catalyst nanoparticles by con-trolling the surface restructuring process in reactive atmos-pheres.The direct study of catalyst nanoparticles is challengingbecause of the complexity of their structures. Model catalystssuch as single crystals and vicinal surfaces have beenextensively investigated to understand the surface restructur-ing phenomenon, and these studies have provided deepinsights. Strongly chemisorbed adsorbates are well known toinduce structural changes to metal surfaces.

Crystal‐Plane‐Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen

The selective oxidation of propylene with O2 to propylene oxide and acrolein is of great interest and importance. We report the crystal-plane-controlled selectivity of uniform capping-ligand-free Cu2 O octahedra, cubes, and rhombic dodecahedra in catalyzing propylene oxidation with O2 : Cu2 O octahedra exposing {111} crystal planes are most selective for acrolein; Cu2 O cubes exposing {100} crystal planes are most selective for CO2 ; Cu2 O rhombic dodecahedra exposing {110} crystal planes are most selective for propylene oxide. One-coordinated Cu on Cu2 O(111), three-coordinated O on Cu2 O(110), and two-coordinated O on Cu2 O(100) were identified as the catalytically active sites for the production of acrolein, propylene oxide, and CO2 , respectively. These results reveal that crystal-plane engineering of oxide catalysts could be a useful strategy for developing selective catalysts and for gaining fundamental understanding of complex heterogeneous catalytic reactions at the molecular level.

Compositions, Structures, and Catalytic Activities of CeO2@Cu2O Nanocomposites Prepared by the Template-Assisted Method

CeO2@Cu2O nanocomposites were prepared from Cu2O cubes and octahedra by the template-assisted method involving the liquid (Ce(IV))-solid (Cu2O) interfacial reaction. Their compositions, structures, and catalytic activities in CO oxidation were studied in detail. Under the same reaction conditions, CeO2@Cu2O nanocomposites prepared from cubic and octahedral Cu2O templates exhibit different compositions and structures. With an increasing amount of Ce(IV) reactant, a smooth CeO2-CuOx shell develops on the surface of Cu2O cubes and eventually void cubic core/multishell Cu2O/CeO2-CuOx nanocomposites form; however, a rough CeO2-CuOx shell develops on the surface of Cu2O octahedra, and eventually hollow octahedral CeO2-CuOx nanocages form. The formation of different compositions and structures of CeO2@Cu2O nanocomposites was correlated with the different exposed crystal planes and surface reactivities of Cu2O cubes and octahedra. The catalytic activity of CeO2@Cu2O nanocomposites in CO oxidation depends on their compositions and structures. The most active CeO2@Cu2O nanocomposites become active at 70 °C and achieve a 100% CO conversion at 170 °C. These results broaden the versatility of Cu2O nanocrystals as the sacrificial template for the fabrication of novel nanocomposites with core/shell and hollow nanostructures and exemplify the morphology effect of Cu2O nanocrystals in liquid-solid interfacial reactions with respect to the composition, structure, and properties of nanocomposites prepared by the template-assisted method.

Catalytically active structures of SiO2-supported Au nanoparticles in low-temperature CO oxidation

Various Au/SiO2 catalysts have been prepared by the deposition–precipitation method followed by calcination in air or reduction in H2. The structures of supported Au nanoparticles were characterized in detail by XRD, TEM, XPS, in situ XANES and operando DRIFTS of CO chemisorption, and their catalytic activity in CO oxidation was evaluated. Calcined in air, the gold precursor decomposes into Au(I) species at low temperatures and further to Au(0) at elevated temperatures, forming supported Au nanoparticles mostly larger than 4.5 nm. Reduced in H2, the gold precursor can be facilely reduced to Au(0) at low temperatures, forming supported Au nanoparticles with different size distributions depending on the reduction temperature. Supported Au nanoparticles around 3–4.5 nm with both abundant low-coordinated Au atoms and bulk Au-like electronic structure effectively chemisorb CO and catalyze CO oxidation at room temperature (RT). Larger supported Au nanoparticles with bulk Au-like electronic structure but few low-coordinated Au atoms do not chemisorb CO and catalyze CO oxidation at RT, and finer supported Au nanoparticles with abundant low-coordinated Au atoms but bulk Au-unlike electronic structure also do not chemisorb CO and catalyze CO oxidation at RT. These results provide solid and comprehensive experimental evidence that supported Au nanoparticles with both abundant low-coordinated Au atoms and bulk Au-like electronic structure are the catalytic active structures for catalyzing CO oxidation at RT without the involvement of oxide supports. The density of low-coordinated Au atoms increases with the decrease of their size, but their electronic structure eventually deviates from bulk Au-like electronic structure; therefore, the catalytic activity of SiO2-supported Au nanoparticles in low-temperature CO oxidation inevitably exhibits a volcano-shaped dependence on their size with the optimum size between 3 and 4.5 nm.

Crystal‐Plane‐Controlled Surface Chemistry and Catalytic Performance of Surfactant‐Free Cu2O Nanocrystals

Surfactant-free Cu2 O nanocrystals, including cubes exposing {100} crystal planes, octahedra exposing {111} crystal planes, and rhombic dodecahedra exposing {110} crystal planes, were used as model catalysts to study the effect of the crystal plane on the surface chemistry and catalytic performance for CO oxidation of Cu2 O nanocrystals. The catalytic performance follows the order of octahedra rhombic dodecahedra>cubes; this suggests that Cu2 O(111) is most active in catalyzing CO oxidation among Cu2 O (111), (110), and (100) surfaces. CO temperature-programmed reduction results demonstrate that Cu2 O octahedra are the most easily reduced of the Cu2 O cubes, octahedra, and rhombic dodecahedra. Diffuse reflectance FTIR spectra show that CO chemisorption on Cu2 O nanocrystals depends on their shape and the chemisorption temperature. CO chemisorption is strongest on rhombic dodecahedra at 30°C, but at 150°C on octahedra. Both the reducibility and chemisorption ability of various Cu2 O nanocrystals toward CO are consistent with their catalytic performance in CO oxidation. The observed surface chemistry and catalytic performance in CO oxidation of various Cu2 O nanocrystals can be well correlated with their exposed crystal plane and surface composition/structure. Cu2 O octahedra expose the {111} crystal plane with coordinated, unsaturated Cu(I) sites, and thus, are most active in chemisorbing CO and catalyzing CO oxidation. These results nicely demonstrate the crystal-plane-controlled surface chemistry and catalytic performance of oxide catalysts.

Influence and Removal of Capping Ligands on Catalytic Colloidal Nanoparticles

Size- and morphology-controlled catalytic colloidal nanoparticles are emerging as novel catalysts for heterogeneous catalysis, but capping ligands adsorbed on the nanoparticle surfaces inherited from the colloidal synthesis always stand as a problem. This perspective highlights recent progress on the influences and removal of capping ligands on catalytic colloidal nanoparticles. Depending on the system, capping ligands can act as either a poison or a promoter for the capped nanoparticles and the underlying mechanisms will be discussed. Various methods for the removal of capping ligands on catalytic colloidal nanoparticles are summarized with an emphasis on a novel controlled oxidation treatment that we recently developed.Graphical Abstract

Chemical etching induced shape change of magnetite microcrystals

We observed an interesting chemical etching induced shape change of magnetite microcrystals. Treatment in a NaBH4 aqueous solution was found to be capable of transforming the shape of magnetite microcrystals from octahedra to plates, meanwhile, a small amount of magnetite was reduced to form an Fe–B amorphous alloy. The observed shape change process proceeded massively in ambient conditions and involved no surfactant. The structures of magnetite microcrystals were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy with selected area electron diffraction. The magnetite microplates are single crystals with the flat top surface parallel to the (111) plane of their fcc crystal structure. We observed a similar shape change process for Fe2O3, Co3O4, and Co2O3 microcrystals, but not for Pb3O4 microcrystals. A probable mechanism for the shape change of magnetite microcrystals induced by treatment in NaBH4 aqueous solution was proposed on the basis of the experimental results.

Reduction of Cu2O nanocrystals: reactant-dependent influence of capping ligands and coupling between adjacent crystal planes

A systematic study of the reduction behavior of various types of uniform Cu2O nanocrystals reveals that the surface blocking effect exerted by the capping ligand on the reduction behavior of the Cu2O nanocrystals depends not only on the type of capping ligand but also on the reactant and that coupling between adjacent crystal planes occurs for Cu2O nanocrystals exposing two types of crystal planes during the reduction reactions and exerts great influence on the reduction kinetics of the involved crystal planes.