Enbo Zhu

High-performance transition metal–doped Pt3Ni octahedra for oxygen reduction reaction

Molybdenum doping drives high activity Platinum (Pt) is an effective catalyst of the oxygen reduction reaction in fuel cells but is scarce. One approach to extend Pt availability is to alloy it with more abundant metals such as nickel (Ni). Although these catalysts can be highly active, they are often not durable because of Ni loss. Huang et al. show that doping the surface of octahedral Pt3Ni nanocrystals with molybdenum not only leads to high activity (∼80 times that of a commercial catalyst) but enhances their stability. Science, this issue p. 1230 Molybdenum-doped platinum-nickel nanocrystal catalysts exhibit high activity and durability for a key fuel cell reaction. Bimetallic platinum-nickel (Pt-Ni) nanostructures represent an emerging class of electrocatalysts for oxygen reduction reaction (ORR) in fuel cells, but practical applications have been limited by catalytic activity and durability. We surface-doped Pt3Ni octahedra supported on carbon with transition metals, termed M‐Pt3Ni/C, where M is vanadium, chromium, manganese, iron, cobalt, molybdenum (Mo), tungsten, or rhenium. The Mo‐Pt3Ni/C showed the best ORR performance, with a specific activity of 10.3 mA/cm2 and mass activity of 6.98 A/mgPt, which are 81- and 73‐fold enhancements compared with the commercial Pt/C catalyst (0.127 mA/cm2 and 0.096 A/mgPt). Theoretical calculations suggest that Mo prefers subsurface positions near the particle edges in vacuum and surface vertex/edge sites in oxidizing conditions, where it enhances both the performance and the stability of the Pt3Ni catalyst.

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.

Ultrafine jagged platinum nanowires enable ultrahigh mass activity for the oxygen reduction reaction

An activity lift for platinum Platinum is an excellent but expensive catalyst for the oxygen reduction reaction (ORR), which is critical for fuel cells. Alloying platinum with other metals can create shells of platinum on cores of less expensive metals, which increases its surface exposure, and compressive strain in the layer can also boost its activity (see the Perspective by Stephens et al.). Bu et al. produced nanoplates—platinum-lead cores covered with platinum shells—that were in tensile strain. These nanoplates had high and stable ORR activity, which theory suggests arises from the strain optimizing the platinum-oxygen bond strength. Li et al. optimized both the amount of surface-exposed platinum and the specific activity. They made nanowires with a nickel oxide core and a platinum shell, annealed them to the metal alloy, and then leached out the nickel to form a rough surface. The mass activity was about double the best reported values from previous studies. Science, this issue p. 1410, p. 1414; see also p. 1378 Improving the platinum (Pt) mass activity for the oxygen reduction reaction (ORR) requires optimization of both the specific activity and the electrochemically active surface area (ECSA). We found that solution-synthesized Pt/NiO core/shell nanowires can be converted into PtNi alloy nanowires through a thermal annealing process and then transformed into jagged Pt nanowires via electrochemical dealloying. The jagged nanowires exhibit an ECSA of 118 square meters per gram of Pt and a specific activity of 11.5 milliamperes per square centimeter for ORR (at 0.9 volts versus reversible hydrogen electrode), yielding a mass activity of 13.6 amperes per milligram of Pt, nearly double previously reported best values. Reactive molecular dynamics simulations suggest that highly stressed, undercoordinated rhombus-rich surface configurations of the jagged nanowires enhance ORR activity versus more relaxed surfaces.

A Facile Strategy to Pt3Ni Nanocrystals with Highly Porous Features as an Enhanced Oxygen Reduction Reaction Catalyst

A facile strategy to Pt3Ni nanocrystals with highly porous features is developed. The integration of a high surface area and rich step/edge atoms endows the nanocrystals with an impressive oxygen reduction reaction (ORR) specific activity and mass activity. These nanocrystals are more stable in ORR and show a small activity change after 6000 potential sweeps. This is a promising material for practical electrocatalytic applications.

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

Toward barrier free contact to molybdenum disulfide using graphene electrodes.

Two-dimensional layered semiconductors such as molybdenum disulfide (MoS2) have attracted tremendous interest as a new class of electronic materials. However, there are considerable challenges in making reliable contacts to these atomically thin materials. Here we present a new strategy by using graphene as the back electrodes to achieve ohmic contact to MoS2. With a finite density of states, the Fermi level of graphene can be readily tuned by a gate potential to enable a nearly perfect band alignment with MoS2. We demonstrate for the first time a transparent contact to MoS2 with zero contact barrier and linear output behavior at cryogenic temperatures (down to 1.9 K) for both monolayer and multilayer MoS2. Benefiting from the barrier-free transparent contacts, we show that a metal-insulator transition can be observed in a two-terminal MoS2 device, a phenomenon that could be easily masked by Schottky barriers found in conventional metal-contacted MoS2 devices. With further passivation by boron nitride (BN) encapsulation, we demonstrate a record-high extrinsic (two-terminal) field effect mobility up to 1300 cm(2)/(V s) in MoS2 at low temperature.

A rational design of carbon-supported dispersive Pt-based octahedra as efficient oxygen reduction reaction catalysts

Bimetallic PtNi nanocrystals represent an emerging class of newly discovered electrocatalysts which are expected to exhibit exciting oxygen reduction reaction (ORR) activity. Colloidal syntheses have been proven to be suitable for controlling PtNi nanocrystals with well-defined morphologies and tunable compositions with the use of capping agents or ligands. However, these colloidal PtNi nanocrystals have inherent limitations associated with the ligand-covered surfaces, which not only limit the free access of surface active sites but also hinder electron transport between the catalyst and the support, leading to deteriorated ORR performance. Herein, we report a facile one-pot strategy to synthesize highly dispersive PtNi octahedra directly on various carbon materials without using any bulky capping agents, which enhances the surface exposure of the PtNi octahedra and their catalytic activity over ORR while largely reduces the preparation costs. The obtained octahedral PtNi/C catalysts have high ORR activities of 2.53 mA cm−2 and 1.62 A mgPt−1 at 0.9 V versus RHE, which are far better than those of commercial Pt/C catalysts (0.131 mA cm−2 and 0.092 A mgPt−1, all the ORR measurements were performed at room temperature in O2-purged 0.1 M HClO4 solutions at a sweep rate of 10 mV s−1). This strategy has been extended to fabricate trimetallic PtNiCo octahedra on carbon black with further enhanced activities up to 3.88 mA cm−2 and 2.33 A mgPt−1 at 0.9 V versus RHE. The octahedral PtNiCo/C catalyst is also more stable than the commercial Pt/C under the ORR conditions and shows small activity change after 6000 potential sweeps. The work demonstrates that the carbon-supported Pt-based materials reported herein are promising material candidates with enhanced performances for practical electrocatalytic applications.

Near-Infrared Plasmonic-Enhanced Solar Energy Harvest for Highly Efficient Photocatalytic Reactions

We report a highly efficient photocatalyst comprised of Cu7S4@Pd heteronanostructures with plasmonic absorption in the near-infrared (NIR)-range. Our results indicated that the strong NIR plasmonic absorption of Cu7S4@Pd facilitated hot carrier transfer from Cu7S4 to Pd, which subsequently promoted the catalytic reactions on Pd metallic surface. We confirmed such enhancement mechanism could effectively boost the sunlight utilization in a wide range of photocatalytic reactions, including the Suzuki coupling reaction, hydrogenation of nitrobenzene, and oxidation of benzyl alcohol. Even under irradiation at 1500 nm with low power density (0.45 W/cm(2)), these heteronanostructures demonstrated excellent catalytic activities. Under solar illumination with power density as low as 40 mW/cm(2), nearly 80-100% of conversion was achieved within 2 h for all three types of organic reactions. Furthermore, recycling experiments showed the Cu7S4@Pd were stable and could retain their structures and high activity after five cycles. The reported synthetic protocol can be easily extended to other Cu7S4@M (M = Pt, Ag, Au) catalysts, offering a new solution to design and fabricate highly effective photocatalysts with broad material choices for efficient conversion of solar energy to chemical energy in an environmentally friendly manner.

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 and Phase Controlled Construction of Pt–Ni Nanostructures for Efficient Electrocatalysis

Highly open metallic nanoframes represent an emerging class of new nanostructures for advanced catalytic applications due to their fancy outline and largely increased accessible surface area. However, to date, the creation of bimetallic nanoframes with tunable structure remains a challenge. Herein, we develop a simple yet efficient chemical method that allows the preparation of highly composition segregated Pt-Ni nanocrystals with controllable shape and high yield. The selective use of dodecyltrimethylammonium chloride (DTAC) and control of oleylamine (OM)/oleic acid (OA) ratio are critical to the controllable creation of highly composition segregated Pt-Ni nanocrystals. While DTAC mediates the compositional anisotropic growth, the OM/OA ratio controls the shapes of the obtained highly composition segregated Pt-Ni nanocrystals. To the best of our knowledge, this is the first report on composition segregated tetrahexahedral Pt-Ni NCs. Importantly, by simply treating the highly composition segregated Pt-Ni nanocrystals with acetic acid overnight, those solid Pt-Ni nanocrystals can be readily transformed into highly open Pt-Ni nanoframes with hardly changed shape and size. The resulting highly open Pt-Ni nanoframes are high-performance electrocatalysts for both oxygen reduction reaction and alcohol oxidations, which are far better than those of commercial Pt/C catalyst. Our results reported herein suggest that enhanced catalysts can be developed by engineering the structure/composition of the nanocrystals.