Yujing Li

Stabilization of high-performance oxygen reduction reaction Pt electrocatalyst supported on reduced graphene oxide/carbon black composite.

Oxygen reduction reaction (ORR) catalyst supported by hybrid composite materials is prepared by well-mixing carbon black (CB) with Pt-loaded reduced graphene oxide (RGO). With the insertion of CB particles between RGO sheets, stacking of RGO can be effectively prevented, promoting diffusion of oxygen molecules through the RGO sheets and enhancing the ORR electrocatalytic activity. The accelerated durability test (ADT) demonstrates that the hybrid supporting material can dramatically enhance the durability of the catalyst and retain the electrochemical surface area (ECSA) of Pt: the final ECSA of the Pt nanocrystal on the hybrid support after 20 000 ADT cycles is retained at >95%, much higher than the commercially available catalyst. We suggest that the unique 2D profile of the RGO functions as a barrier, preventing leaching of Pt into the electrolyte, and the CB in the vicinity acts as active sites to recapture/renucleate the dissolved Pt species. We furthermore demonstrate that the working mechanism can be applied to the commercial Pt/C product to greatly enhance its durability.

Platinum nanocrystals selectively shaped using facet-specific peptide sequences

The properties of a nanocrystal are heavily influenced by its shape. Shape control of a colloidal nanocrystal is believed to be a kinetic process, with high-energy facets growing faster then vanishing, leading to nanocrystals enclosed by low-energy facets. Identifying a surfactant that can specifically bind to a particular crystal facet is critical, but has proved challenging to date. Biomolecules have exquisite specific molecular recognition properties that can be explored for the precise engineering of nanostructured materials. Here, we report the use of facet-specific peptide sequences as regulating agents for the predictable synthesis of platinum nanocrystals with selectively exposed crystal surfaces and particular shapes. The formation of platinum nanocubes and nanotetrahedrons are demonstrated with Pt-{100} and Pt-{111} binding peptides, respectively. Our studies unambiguously demonstrate the abilities of facet-selective binding peptides in determining nanocrystal shape, representing a critical step forward in the use of biomolecules for programmable synthesis of nanostructures.

Plasmonic Modulation of the Upconversion Fluorescence in NaYF4:Yb/Tm Hexaplate Nanocrystals using Gold Nanoparticles or Nanoshells

Automatic upgrade: attachment of gold nanoparticles (NPs) onto upconversion nanocrystals (NCs) results in plasmonic interactions that lead to a significant enhancement of upconversion emission of more than 2.5. Conversely, formation of a gold shell greatly suppresses the NC emission because of considerable scattering of excitation irradiation (see picture; a=NC before seed attachment; b, c=NC with attached Au NPs; c=NC with Au shell; scale bar=50 nm).

Electrically conductive and optically active porous silicon nanowires.

We report the synthesis of vertical silicon nanowire array through a two-step metal-assisted chemical etching of highly doped n-type silicon (100) wafers in a solution of hydrofluoric acid and hydrogen peroxide. The morphology of the as-grown silicon nanowires is tunable from solid nonporous nanowires, nonporous/nanoporous core/shell nanowires, to entirely nanoporous nanowires by controlling the hydrogen peroxide concentration in the etching solution. The porous silicon nanowires retain the single crystalline structure and crystallographic orientation of the starting silicon wafer and are electrically conductive and optically active with visible photoluminescence. The combination of electronic and optical properties in the porous silicon nanowires may provide a platform for novel optoelectronic devices for energy harvesting, conversion, and biosensing.

Synthesis of PtPd Bimetal Nanocrystals with Controllable Shape, Composition, and Their Tunable Catalytic Properties

We report a facile synthetic strategy to single-crystalline PtPd nanocrystals with controllable shapes and tunable compositions. In the developed synthesis, the molar ratio of the starting precursors determines the composition in the final PtPd nanocrystals, while the halides function as the shape-directing agent to induce the formation of PtPd nanocrystals with cubic or octahedral/tetrahedral morphology. These obtained PtPd nanocrystals exhibit high activity in the hydrogenation of nitrobenzene, and their performance is highly shape- and composition-dependent with Pt in ∼50% showing the optimum activity and the {100}-facet-enclosed PtPd nanocrystals demonstrating a higher activity than the {111}-facet-bounded PtPd nanocrystals.

High-κ oxide nanoribbons as gate dielectrics for high mobility top-gated graphene transistors

Deposition of high-κ dielectrics onto graphene is of significant challenge due to the difficulties of nucleating high quality oxide on pristine graphene without introducing defects into the monolayer of carbon lattice. Previous efforts to deposit high-κ dielectrics on graphene often resulted in significant degradation in carrier mobility. Here we report an entirely new strategy to integrate high quality high-κ dielectrics with graphene by first synthesizing freestanding high-κ oxide nanoribbons at high temperature and then transferring them onto graphene at room temperature. We show that single crystalline Al2O3 nanoribbons can be synthesized with excellent dielectric properties. Using such nanoribbons as the gate dielectrics, we have demonstrated top-gated graphene transistors with the highest carrier mobility (up to 23,600 cm2/V·s) reported to date, and a more than 10-fold increase in transconductance compared to the back-gated devices. This method opens a new avenue to integrate high-κ dielectrics on graphene with the preservation of the pristine nature of graphene and high carrier mobility, representing an important step forward to high-performance graphene electronics.

Tailoring Molecular Specificity Toward a Crystal Facet: a Lesson From Biorecognition Toward Pt{111}

Surfactants with preferential adsorption to certain crystal facets have been widely employed to manipulate morphologies of colloidal nanocrystals, while mechanisms regarding the origin of facet selectivity remain an enigma. Similar questions exist in biomimetic syntheses concerning biomolecular recognition to materials and crystal surfaces. Here we present mechanistic studies on the molecular origin of the recognition toward platinum {111} facet. By manipulating the conformations and chemical compositions of a platinum {111} facet specific peptide, phenylalanine is identified as the dominant motif to differentiate {111} from other facets. The discovered recognition motif is extended to convert nonspecific peptides into {111} specific peptides. Further extension of this mechanism allows the rational design of small organic molecules that demonstrate preferential adsorption to the {111} facets of both platinum and rhodium nanocrystals. This work represents an advance in understanding the organic-inorganic interfacial interactions in colloidal systems and paves the way to rational and predictable nanostructure modulations for many applications.

Morphology-controlled synthesis of platinum nanocrystals with specific peptides.

Adv. Mater. 2010, 22, 1921–1925 2010 WILEY-VCH Verlag G T IO N The physical and chemical properties of nanocrystals (NCs) strongly depend on their sizes and morphologies. One notable example is that noble metal NCs of various shapes have been reported to demonstrate different catalytic properties as a result of the distinct crystallographic facets displayed on the NC surfaces, which have been proven to exhibit distinct catalytic properties by affecting the molecular adsorption and desorption processes in a reaction. For example, Pt NCs have attracted enormous interest due to their excellent catalytic activity in a wide range of reactions. It has been reported that the Pt NC morphology has significant effect on the oxygen reduction (ORR) activity due to different adsorption rates of sulfate ions on different low index facets such as {100}, {110} and {111} faces; and that Pt NCs of distinct morphologies exhibit different specific electrochemical surface areas (ECSAs) that affect the ORR efficiency. Various synthetic strategies have been developed to obtain Pt NCs with different morphologies. Using low molecular surfactants or polymers as stabilizing agents and H2 or NaBH4 as the reducing agent, Pt NCs of different morphologies such as cubes and tetrahedrons have been synthesized at room temperature. Pt NCs of novel shapes with high index facets have also been demonstrated with excellent catalytic activities. More recently biomacromolecules, including proteins, peptides, RNAs and DNAs that have specific binding ability to target inorganic materials, have been demonstrated to have the capability to direct the formation of inorganic NCs. Notably, peptide sequence selected from a random peptide library has been shown to have specific binding ability to target materials, thus can perform as capping agent in the NC synthesis. The synthesis of Ag, Pd and some semiconductor NCs have been studied by using specific peptides as the capping agents, although the degree of synthetic control in biomimetic approaches has yet to be improved. Our group has recently demonstrated peptideregulated synthesis of ultrasmall Pt NCs with atomic layer control. Here we demonstrate the synthesis of water dispersable Pt NCs with controllable multipod structure under mild reaction conditions. The stabilizing agent used was a peptide selected with phage display technique against Pt wires. Using potassium tetrachloroplatinate (K2PtCl4) as the precursor and sodium borohydride (NaBH4) as the reducing agent, the reaction was conducted in an aqueous solution at room temperature. We note that multipod Pt NCs had been reported before using chemical capping agents, and at much higher temperatures. ] In our synthetic process, the addition of NaBH4 was achieved by slow injection through a computer controlled syringe pump, so that the growth kinetics of NCs was controllable and observable. The as-synthesized Pt NCs displayed a uniform multipod-like morphology with high yield. It was also observed that the pod length could be tuned by altering the reaction kinetics. Electrochemical characterizations were also carried out to evaluate the electrocatalytic activity of the as-synthesized multipod Pt NC and compared with commercial Pt black. The peptide Ac-Thr-Leu-His-Val-Ser-Ser-Tyr-CONH2 (termed BP7A, MW: 846.93) was selected from a Ph.D. 7 Library after the 3rd round selection against Pt wire, and synthesized using solid state peptide synthesizer with N-terminal acylation and C-terminal amidation. The integrity of the BP7A peptide was also tested (See the Supporting Information, Fig. S1 and S2). The highly uniform multipod Pt NCs were synthesized by slowly injecting NaBH4 solution into the K2PtCl4/BP7A mixed solution with a syringe pump at controllable rates, while at the same time keeping the reaction solution strongly stirred. The color of the solution evolved from clear to light brown and finally to dark black. The molar ratio between precursor K2PtCl4 and peptide was changed to achieve different morphologies. Figure 1a–d shows the as-synthesized Pt NCs with different molar ratios between K2PtCl4 and peptide (i.e., by varying the peptide concentration while keeping the K2PtCl4 concentration constant). The transmission electron microscopy (TEM) samples were prepared after 1 h of reaction. The control reaction without adding peptide is shown in Figure S3. The typical crystals obtained without peptide added are polycrystalline and with hyperbranched morphologies with an average size around 20 nm. Negative controls with non-relevant peptides were demonstrated to exhibit no effect on Pt NC growth. With the introduction of BP7A peptide, typical crystals observed have a multipod morphology with the average size down to 10 nm, as shown in Figure 1a. The term multipod is used because bi-pod, tri-pod and tetra-pod NCs coexist. With the increasing peptide concentrations (Fig. 1, a–c, 22.5–100mgmL ), it was found that the length of the pods decreased from ca. 6 nm to ca. 1 nm. At sufficiently high peptide concentrations (e.g., 250mgmL , Fig. 1d) multipod NCs ceased to exist, instead, the typical Pt NCs at high peptide concentrations displayed a nearly spherical shape with an average size around 2.5 nm. It was also observed that the dispersity the Pt NCs in water improved with increasing peptide concentration. Lattice analysis with high-resolution TEM (HRTEM) further revealed the single crystal nature of the multipod Pt NCs, and that most of the pods grew along h111i direction with the d spacing of

Specific Peptide Regulated Synthesis of Ultrasmall Platinum Nanocrystals

We demonstrate the rational synthesis of monodisperse ultrasmall platinum (Pt) nanocrsytals (NCs), in aqueous solution at room temperature, with specifically selected peptide molecules. The specific Pt-binding peptide, selected using a phage display technique, can function as a stabilizer to regulate Pt crystal nucleation and growth and, therefore, control both the morphology and size of the final Pt NCs. Uniform near-spherical Pt NCs with sizes ranging from 1.73 to 3.54 nm were achieved with a very narrow size distribution. The peptide-stabilized Pt NCs can be dispersed well in water for months. It was also demonstrated that the strong binding of peptides to the Pt NC surface is reversible by either pH modulation or peptide photolysis.

Composition tuning the upconversion emission in NaYF4:Yb/Tm hexaplate nanocrystals

Single crystal hexagonal NaYF4:Yb/Tm nanocrystals have been synthesized with uniform size, morphology and controlled chemical composition. Spectroscopic studies show that these nanocrystals exhibit strong energy upconversion emission when excited with a 980 nm diode laser, with two primary emission peaks centered around 452 nm and 476 nm. Importantly, the overall and relative emission intensity at these wavelengths can be readily tuned by controlling the concentration of the trivalent rare earth element dopants at the beginning of the synthesis which has been confirmed by EDX for the first time. Through systematic studies, the optimum rare earth ion doping concentration can be determined for the strongest emission intensity at the selected peak(s). Confocal microscopy studies show that the upconversion emission from individual NCs can be readily visualized. These studies demonstrate a rational approach for fine tuning the upconversion properties in rare-earth doped nanostructures and can broadly impact areas ranging from energy harvesting, energy conversion to biomedical imaging and therapeutics.