Lingyan Ruan

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.

Biomimetic Synthesis of an Ultrathin Platinum Nanowire Network with a High Twin Density for Enhanced Electrocatalytic Activity and Durability

Fuel cells have attracted much research interest as they are promising candidates for providing clean energy. A fuel cell catalyzes reactions between a fuel (e.g., hydrogen or alcohols) at the anode and the oxidant (molecular oxygen) at the cathode, converting chemical energy into electrical power. One of the most critical challenges for fuel-cell applications is the sluggish reduction kinetics of the oxygen reduction reaction (ORR) at the cathode. So far, platinum and Ptbased nanomaterials are recognized as the most effective electrocatalysts for the ORR. 2] Current state-of-the-art electrocatalysts rely almost exclusively on Pt black or Pt nanoparticles (2–5 nm) dispersed onto a carbon black support (Pt/C). However, the practical large-scale commercialization of fuel cells is still a great challenge because of the loss of electrochemical surface area (ECSA) and the decrease of catalytic activity over time. 4] Another important issue of fuel-cell applications is the anodic reaction, that is, the oxidation of hydrogen or alcohols (for example, methanol or ethanol). The direct methanol fuel cell is particularly attractive due to its high volumetric energy density and its ease of storage and transport compared to the hydrogen fuel cell. Similar to the cathode, Pt-based nanomaterials are currently used as the most efficient electrocatalysts for the methanol oxidation reaction (MOR), which unfortunately suffers from the same problems including poor reaction kinetics and poisoning. Therefore, the development of electrocatalysts with improved catalytic activity and durability is highly desirable but remains a significant challenge. The control on nanomaterial structures provides a sensitive knob to tune the properties and improve the functions. Nanostructured Pt with various morphologies has been extensively exploited in the search to improve the electrocatalytic performance. To this end, one-dimensional (1D) nanostructures, such as nanowires, represent an important research direction because 1D nanostructures possess unique advantages compared to their zero-dimensional (0D) counterparts. For example, the structural anisotropy can slow down the ripening process; and the display of long segments of low-index crystalline planes along the nanowire is particularly beneficial for the ORR. 15, 16] As a result, various methods including both top-down and bottom-up have been developed to synthesize Pt 1D nanostructures. Most prior studies reported the production of single-crystalline Pt nanowires. 21, 23–25] Meanwhile, twinning of materials has been shown to greatly affect the physical and chemical material properties, including various surface adsorption, heterogeneous catalytic, and electrocatalytic processes. While nanotwins have been widely employed to enhance mechanical properties, very limited effort has been devoted to engineering of twin defects in nanomaterials for improved catalysis and electrocatalysis. 31] Herein, we report an ultrathin Pt multiple-twinned nanowire network (MTNN) as an efficient electrocatalyst, which exhibits a higher ECSA and much improved activity toward both ORR and MOR when compared to current state-of-theart commercial Pt/C electrocatalysts. Furthermore, it shows significantly improved durability with much less decay of ECSA at prolonged reaction times. The unique feature of the nanowire in our studies is its high density of twin planes, which have been proposed to play a significant role in the promotion of the electrocatalytic performance. 29] The Pt MTNN was synthesized by biomimetic synthesis using a specific Pt-binding peptide (amino acid sequence AcTLHVSSY-CONH2, named BP7A, identified through phage display). Peptide-directed biomimetic syntheses have emerged as a new synthetic route and demonstrated the potential to maneuver nanomaterial structures and functions in a controllable manner under mild environmental-benign conditions. Particularly, peptide BP7A has been demonstrated to lead to exclusive formation of twinned Pt nanoparticles, which is uncommon in conventional syntheses. Here, we further exploit its potential for the synthesis of 1D Pt nanowire with dense twin defects. Although nanowires with twin defects have been observed before, the occurrence of twinning appeared relatively random and rare. This is the first report on synthesis of Pt nanowires that are ultrathin yet show a high twin population. The synthesis was carried out at room temperature in aqueous solution using the peptide BP7A as the surfactant [*] L. Ruan, E. Zhu, Y. Chen, X. Huang, Prof. Y. Huang Department of Materials Science and Engineering University of California, Los Angeles Los Angeles, CA 90095 (USA) E-mail: yhuang@seas.ucla.edu

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.

High density catalytic hot spots in ultrafine wavy nanowires.

Structural defects/grain boundaries in metallic materials can exhibit unusual chemical reactivity and play important roles in catalysis. Bulk polycrystalline materials possess many structural defects, which is, however, usually inaccessible to solution reactants and hardly useful for practical catalytic reactions. Typical metallic nanocrystals usually exhibit well-defined crystalline structure with few defects/grain boundaries. Here, we report the design of ultrafine wavy nanowires (WNWs) with a high density of accessible structural defects/grain boundaries as highly active catalytic hot spots. We show that rhodium WNWs can be readily synthesized with controllable number of structural defects and demonstrate the number of structural defects can fundamentally determine their catalytic activity in selective oxidation of benzyl alcohol by O2, with the catalytic activity increasing with the number of structural defects. X-ray photoelectron spectroscopy (XPS) and cyclic voltammograms (CVs) studies demonstrate that the structural defects can significantly alter the chemical state of the Rh WNWs to modulate their catalytic activity. Lastly, our systematic studies further demonstrate that the concept of defect engineering in WNWs for improved catalytic performance is general and can be readily extended to other similar systems, including palladium and iridium WNWs.

Synthesis of Platinum Single-Twinned Right Bipyramid and {111}-Bipyramid through Targeted Control over Both Nucleation and Growth Using Specific Peptides

Shape-controlled synthesis requires rigorous kinetic control over both nucleation and growth. For platinum (Pt), to date it is still challenging to generate twinned seeds in nucleation in a controllable fashion. Here, we report that a specific Pt binding peptide BP7A is able to mediate and stabilize single-twinned seeds formation at the nucleation stage under mild conditions. Importantly, it targets the control over nucleation directly. Combining with control over growth, we further demonstrate the rational design and synthesis of single-twinned structures, right bipyramid and {111}-bipyramid, by incorporating targeted facet stabilization over {100} facet and {111} facet, respectively. To the best of our knowledge, this is the first report on the successful synthesis of single-twinned bipyramids for Pt nanocrystals (NCs) with high yields. The work here demonstrates the power of biomolecules in recognizing and mediating inorganic nanomaterials synthesis, guiding the formation of material structures that are otherwise unconventional, and hence presenting one step further toward predictable and programmable biomimetic synthesis.

Integration of molecular and enzymatic catalysts on graphene for biomimetic generation of antithrombotic species

The integration of multiple synergistic catalytic systems can enable the creation of biocompatible enzymatic mimics for cascading reactions under physiologically relevant conditions. Here we report the design of a graphene-haemin-glucose oxidase conjugate as a tandem catalyst, in which graphene functions as a unique support to integrate molecular catalyst haemin and enzymatic catalyst glucose oxidase for biomimetic generation of antithrombotic species. Monomeric haemin can be conjugated with graphene through π-π interactions to function as an effective catalyst for the oxidation of endogenous L-arginine by hydrogen peroxide. Furthermore, glucose oxidase can be covalently linked onto graphene for local generation of hydrogen peroxide through the oxidation of blood glucose. Thus, the integrated graphene-haemin-glucose oxidase catalysts can readily enable the continuous generation of nitroxyl, an antithrombotic species, from physiologically abundant glucose and L-arginine. Finally, we demonstrate that the conjugates can be embedded within polyurethane to create a long-lasting antithrombotic coating for blood-contacting biomedical devices.

A Rational Biomimetic Approach to Structure Defect Generation in Colloidal Nanocrystals

Controlling the morphology of nanocrystals (NCs) is of paramount importance for both fundamental studies and practical applications. The morphology of NCs is determined by the seed structure and the following facet growth. While means for directing facet formation in NC growth have been extensively studied, rational strategies for the production of NCs bearing structure defects in seeds have been much less explored. Here, we report mechanistic investigations of high density twin formation induced by specific peptides in platinum (Pt) NC growth, on the basis of which we derive principles that can serve as guidelines for the rational design of molecular surfactants to introduce high yield twinning in noble metal NC syntheses. Two synergistic factors are identified in producing twinned Pt NCs with the peptide: (1) the altered reduction kinetics and crystal growth pathway as a result of the complex formation between the histidine residue on the peptide and Pt ions, and (2) the preferential stabilization of {111} planes upon the formation of twinned seeds. We further apply the discovered principles to the design of small organic molecules bearing similar binding motifs as ligands/surfactants to create single and multiple twinned Pd and Rh NCs. Our studies demonstrate the rich information derived from biomimetic synthesis and the broad applicability of biomimetic principles to NC synthesis for diverse property tailoring.

Molecular ligand modulation of palladium nanocatalysts for highly efficient and robust heterogeneous oxidation of cyclohexenone to phenol

Molecular ligand modulation of Pd nanoparticle catalysts achieves exceptional activity and stability. Metallic nanoparticles are emerging as an exciting class of heterogeneous catalysts with the potential advantages of exceptional activity, stability, recyclability, and easier separation than homogeneous catalysts. The traditional colloid nanoparticle syntheses usually involve strong surface binding ligands that could passivate the surface active sites and result in poor catalytic activity. The subsequent removal of surface ligands could reactivate the surface but often leads to metal ion leaching and/or severe Ostwald ripening with diminished catalytic activity or poor stability. Molecular ligand engineering represents a powerful strategy for the design of homogeneous molecular catalysts but is insufficiently explored for nanoparticle catalysts to date. We report a systematic investigation on molecular ligand modulation of palladium (Pd) nanoparticle catalysts. Our studies show that β-functional groups of butyric acid ligand on Pd nanoparticles can significantly modulate the catalytic reaction process to modify the catalytic activity and stability for important aerobic reactions. With a β-hydroxybutyric acid ligand, the Pd nanoparticle catalysts exhibit exceptional catalytic activity and stability with an unsaturated turnover number (TON) >3000 for dehydrogenative oxidation of cyclohexenone to phenol, greatly exceeding that of homogeneous Pd(II) catalysts (TON, ~30). This study presents a systematic investigation of molecular ligand modulation of nanoparticle catalysts and could open up a new pathway toward the design and construction of highly efficient and robust heterogeneous catalysts through molecular ligand engineering.