An-Xiang Yin

Facile Synthesis for Ordered Mesoporous γ-Aluminas with High Thermal Stability

The facile synthesis of highly ordered mesoporous aluminas with high thermal stability and tunable pore sizes is systematically investigated. The general synthesis strategy is based on a sol-gel process associated with nonionic block copolymer as templates in ethanol solvent. Small-angle XRD, TEM, and nitrogen adsorption and desorption results show that these mesoporous aluminas possess a highly ordered 2D hexagonal mesostructure, which is resistant to high temperature up to 1000 degrees C. Ordered mesoporous structures with tunable pore sizes are obtained with various precursors, different acids as pH adjustors, and different block copolymers as templates. These mesoporous aluminas have large surface areas (ca. 400 m2/g), pore volumes (ca. 0.70 cm3/g), and narrow pore-size distributions. The influence of the complexation ability of anions and hydro-carboxylic acid, acid volatility, and other important synthesis conditions are discussed in detail. Utilizing this simple strategy, we also obtained partly ordered mesoporous alumina with hydrous aluminum nitrate as the precursor. FTIR pyridine adsorption measurements indicate that a large amount of Lewis acid sites exist in these mesoporous aluminas. These materials are expected to be good candidates in catalysis due to the uniform pore structures, large surface areas, tunable pore sizes, and large amounts of surface Lewis acid sites. Loaded with ruthenium, the representative mesoporous alumina exhibits reactant size selectivity in hydrogenation of acetone, D-glucose, and D-(+)-cellobiose as a test reaction, indicating the potential applications in shape-selective catalysis.

Shape-selective synthesis and facet-dependent enhanced electrocatalytic activity and durability of monodisperse sub-10 nm Pt-Pd tetrahedrons and cubes

Monodisperse single-crystalline sub-10 nm Pt-Pd nanotetrahedrons (NTs) and nanocubes (NCs) were synthesized with high shape selectivity via one-pot hydrothermal routes with small ions as efficient facet-selective agents. These alloy nanocrystals showed facet-dependent enhanced electrocatalytic activity and durability for methanol electrooxidations with commercial Pt/C catalyst as a reference. The (100)-facet-enclosed Pt-Pd NCs demonstrated a higher activity, whereas the (111)-facet-enclosed Pt-Pd NTs exhibited a better durability.

Colloidal synthesis and blue based multicolor upconversion emissions of size and composition controlled monodisperse hexagonal NaYF4 : Yb,Tm nanocrystals

Monodisperse beta-NaYF4:Yb,Tm nanocrystals with controlled size (25-150 nm), shape (sphere, hexagonal prism, and hexagonal plate), and composition (Yb: 20-40%, Tm: 0.2-5%) were synthesized from the thermolysis of metal trifluoroacetates in hot surfactant solutions. The upconversion (UC) of near-infrared light (980 nm) to ultra-violet (360 nm), blue (450 and 475 nm), red (650 and 695 nm) and infrared (800 nm) light in the beta-NaYF4:Yb,Tm nanocrystals has been studied by UC spectroscopy. Both the total intensity of UC emissions and the relative intensities of emissions at different wavelengths have shown a strong dependence on different particle sizes and different Tm3+ and Yb3+ concentrations. As a result, different overall output colors of UC emissions can be achieved by altering sizes and Yb3+/Tm3+ doping concentrations of the beta-NaYF4:Yb,Tm nanocrystals. The intensity-power curves of a series of samples have proved that emissions at 360 and 450 nm can be ascribed to four-photon process (1D2 to 3H6 and 1D2 to 3H4, respectively), while emissions at 475 and 650 nm are three-photon processes (1G4 to 3H6 and 1G4 to 3H4, respectively) and emissions at 695 and 800 nm are two-photon ones (3F2 to 3H6 and 3F4 to 3H6, respectively). A UC saturation effect would occur under a certain excitation intensity of the 980 nm CW diode laser for the as-obtained beta-NaYF4:Yb,Tm nanocrystals, leading to the decrease of the slopes of the I-P curves. The results of our study also revealed that the successive transfer model instead of the cooperative sensitization model can be applied to explain the UC behaviors of the beta-NaYF4:Yb,Tm nanocrystals. Further, an unexpected stronger emissions of four-photon process at 360 and 450 nm for approximately 50 nm beta-NaYF4:Yb,Tm nanocrystals than those for the bigger (approximately 150 nm) nanocrystals was observed and explained in terms of the effects of crystallite size, surface-to-volume ratio and homogeneity of the doping cations.

PtCu and PtPdCu Concave Nanocubes with High-Index Facets and Superior Electrocatalytic Activity

Pt-Cu nanostructures: Pt-Cu nanocubes (NCs), concave nanocubes (CNCs), and Pt-Pd-Cu CNCs with high-index facets (HIFs) were prepared through progressive galvanic replacements in a one-pot hydrothermal approach. The HIF-enclosed CNCs showed superior activities to (100)-enclosed NC catalysts for methanol oxidations owing to the modification of both the surface electronic structures and the surface atomic arrangements (see figure).

Thermally Stable Pt/CeO2 Hetero-Nanocomposites with High Catalytic Activity

Pt/CeO(2) hetero-nanocomposites were prepared from Pt/CeO(2)@SiO(2) obtained by a microemulsion-mediated method. Facilitated by the earlier calcination under the protection of a silica shell, the as-formed Pt/CeO(2) hetero-nanocomposites exhibit a good thermal stability, which can preserve their pristine properties after subsequent calcination at even 450 degrees C. The thermally stable Pt/CeO(2) hetero-nanocomposites possess the characteristics of small particle size, low aggregation, and maximized Pt/CeO(2) interfaces and thus exhibit high catalytic activity in CO oxidation.

Ru Nanocrystals with Shape-Dependent Surface-Enhanced Raman Spectra and Catalytic Properties: Controlled Synthesis and DFT Calculations

Despite its multidisciplinary interests and technological importance, the shape control of Ru nanocrystals still remains a great challenge. In this article, we demonstrated a facile hydrothermal approach toward the controlled synthesis of Ru nanocrystals with the assistance of first-principles calculations. For the first time, Ru triangular and irregular nanoplates as well as capped columns with tunable sizes were prepared with high shape selectivity. In consistency with the experimental observations and density functional theory (DFT) calculations confirmed that both the intrinsic characteristics of Ru crystals and the adsorption of certain reaction species were responsible for the shape control of Ru nanocrystals. Ultrathin Ru nanoplates exposed a large portion of (0001) facets due to the lower surface energy of Ru(0001). The selective adsorption of oxalate species on Ru(10-10) would retard the growth of the side planes of the Ru nanocrystals, while the gradual thermolysis of the oxalate species would eliminate their adsorption effects, leading to the shape evolution of Ru nanocrystals from prisms to capped columns. The surface-enhanced Raman spectra (SERS) signals of these Ru nanocrystals with 4-mercaptopyridine as molecular probes showed an enhancement sequence of capped columns > triangle nanoplates > nanospheres, probably due to the sharp corners and edges in the capped columns and nanoplates as well as the shrunk interparticle distance in their assemblies. CO-selective methanation tests on these Ru nanocrystals indicated that the nanoplates and nanospheres had comparable activities, but the former has much better CO selectivity than the latter.

Time--temperature indicator for perishable products based on kinetically programmable Ag overgrowth on Au nanorods.

Food safety is a constant concern for humans. Besides adulteration and contamination, another major threat comes from the spontaneous spoilage of perishable products, which is basically inevitable and highly dependent on the temperature history during the custody chain. For advanced quality control and assessment, time-temperature indicators (TTIs) can be deployed to document the temperature history. However, the use of TTIs is currently limited by either relatively high cost or poor programmability. Here we describe a general, kinetically programmable, and cost-efficient TTI protocol constructed from plasmonic nanocrystals. We present proof-of-principle demonstrations that our TTI can be specifically tailored and thus used to track perishables, dynamically mimic the deteriorative processes therein, and indicate product quality through sharp-contrast multicolor changes. The flexible programmability of our TTI, combined with its substantially low cost and low toxicity, promises a general applicability to each single packaged item of a plethora of perishable products.

Chemoaffinity-Mediated Synthesis of NaRES2-Based Nanocrystals as Versatile Nano-Building Blocks and Durable Nano-Pigments

In this article, we present a chemoaffinity-mediated synthetic strategy toward trivalent rare earth (RE) sulfides-based nanocrystals with poor affinity between cation and anion (i.e., RE(3+) and S(2-)). With the affinity mediation among multiple constituents based on hard and soft acid and base theory, we synthesized a series of monodisperse NaRES(2) nanocrystals (RE = La to Lu, Y). The revelation of the nanocrystal growth mechanism from both experimental evidence and crystal structure modeling has enabled a robust control over the sizes and morphologies of the nanocrystals. This principle of chemoaffinity has also promised the synthesis of well-defined but even more complex RE-based hetero-nanostructures (i.e. NaLaS(2)-Au, Au@NaLaS(2), NaLaS(2)@Ag(2)S, Au@NaLaS(2)@Ag(2)S) with tunable optical properties. Furthermore, this synthetic method has yielded durable NaCeS(2)-based red nano-pigments under ambient conditions, with superior brightness and permeability in polydimethylsiloxane.

Self-organizing domino-like superlattices through stereochemical recognition match at the organic-inorganic interface in solution.

Nature produces various self-assembled architectures of different scales and with unique functions spontaneously through delicate control of building blocks at a molecular level. In the laboratory, chemical self-assembly of molecules or nanocrystals is considered to be one of the most practical approaches to build up various functional nanostructures, as well as plenty of potential nanodevices through the “bottom-up” approach. Many unique collective properties could be realized by the self-assembly of various nano building blocks with different symmetries into ordered superstructures. General principles of self-assembly can be summarized in two rules: 1) Like prefers like, and 2) self-assembly is driven by total energy minimization. Further studies on natural biological superstructures or functional minerals reveal that local stereochemical recognition at the organic–inorganic interface plays an important role in the formation of selfassembled organic–inorganic hybrid structures, directed by specific macromolecules with specialized stereochemical configurations. Hybrid superstructures self-assemble spontaneously through the cooperation and recognition of both organic and inorganic species at the organic–inorganic interface. In contrast, present trials for chemical self-assembly of nanocrystals are rigid and tedious. Commonly employed methods are “post-synthesis” assembly procedures, in which external forces are still needed to form the assembled structures through usual procedures, such as evaporation-induced self-assembly (EISA) and Langmuir–Blodgett (LB) compressing, instead of a simultaneous and spontaneous route. The key factors considered in these procedures are mainly focused on the level of “particle”, such as the size (several nanometers), shape, and electrostatic properties of the nanocrystals. While the stereochemical recognition of the capping surfactant molecules at the molecular level (several hundred picometers), as well as their configurations at the organic–inorganic interface, are still not well understood, although it has been proved to be a fundamental factor for the spontaneous self-assembly in biochemical or biomimetic systems and might also play an important role in the spontaneous self-assembly of nanocrystals in solutions. As is known, alkaline earth halides are widely used in optoelectronic and microelectronic devices or photostimulated luminescence storage devices. Here, we demonstrate the wet-chemical preparation of close-packed and orderedaligned, luminescent, domino-like superlattices of ultrathin nanoplates of alkaline earth halides (BaFCl, SrFCl, BaFBr). Assisted by a molecular mechanics simulation, the stereochemical recognition match of different chemical species (mainly alkaline earth cations and long-chain surfactants) at the organic–inorganic interface is ascribed to be the decisive factor for the solution-based spontaneous self-assembly of the nanoplate superlattices. The as-formed micelle structure of long-chain surfactant molecules of oleic acid (OA) and oleylamine (OM) acts as both the shape-directing agent (ShDA) for the ultrathin nanoplates and the structure-directing agent (StDA) for the self-assembled 1D superlattices. Structures and compositions of the self-assembled domino-like superlattices were confirmed by means of transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy dispersive X-ray spectroscopy (EDS) analysis, and X-ray diffraction (XRD) measurements. TEM images (Figure 1 and Figure S1 in the Supporting Information) reveal that these highly compact and ordered superlattices (SP) of ultrathin BaFCl nanoplates, obtained from face-to-face formation, can reach a length of tens of micrometers or even longer. These uniform rectangular nanoplates have a thickness of (2.3 0.4) nm and an edge length of (79.8 4.5) nm. Interestingly, the interplate distances are generally no more than 2 nm (Figure 1 and Figure S2 in the Supporting Information), fitting the thickness of only one monolayer of the OA/OM molecules and thus suggesting that a penetrating stereostructure would be adopted by these interplate surface ligands. HRTEM images (Figure 1 d and f) show that these uniform nanoplates, preferring to grow faster along the <100> direction, are single crystals bound by (110), (1̄10) and (001) facets, and that they tend to [a] A.-X. Yin, Prof. Y.-W. Zhang, Prof. C.-H. Yan Beijing National Laboratory for Molecular Sciences State Key Laboratory of Rare Earth Materials Chemistry and Applications PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry College of Chemistry and Molecular Engineering Peking University, Beijing 100871 (P. R. China) Fax: (+86) 10-6275-4179 E-mail : ywzhang@pku.edu.cn yan@pku.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101005.