Underpotential deposition at single crystal surfaces of Au, Pt, Ag and other materials

Green Synthesis of Smart Metal/Polymer Nanocomposite Particles and Their Tuneable Catalytic Activities

Herein we report a simple and green synthesis of smart Au and Ag@Au nanocomposite particles using poly(N-isopropylacrylamide)/polyethyleneimine (PNIPAm/PEI) core-shell microgels as dual reductant and templates in an aqueous system. The nanocomposite particles were synthesized through a spontaneous reduction of tetrachloroauric (III) acid to gold nanoparticles at room temperature, and in situ encapsulation and stabilization of the resultant gold nanoparticles (AuNPs) with amine-rich PEI shells. The preformed gold nanoparticles then acted as seed nanoparticles for further generation of Ag@Au bimetallic nanoparticles within the microgel templates at 60 °C. These nanocomposite particles were characterized by TEM, AFM, XPS, UV-vis spectroscopy, zeta-potential, and particle size analysis. The synergistic effects of the smart nanocomposite particles were studied via the reduction of p-nitrophenol to p-aminophenol. The catalytic performance of the bimetallic Ag@Au nanocomposite particles was 25-fold higher than that of the monometallic Au nanoparticles. Finally, the controllable catalytic activities of the Au@PNIPAm/PEI nanocomposite particles were demonstrated via tuning the solution pH and temperature.

Nanoarchitectured Nb2O5 hollow, Nb2O5@carbon and NbO2@carbon Core-Shell Microspheres for Ultrahigh-Rate Intercalation Pseudocapacitors

Li-ion intercalation materials with extremely high rate capability will blur the distinction between batteries and supercapacitors. We construct a series of nanoarchitectured intercalation materials including orthorhombic (o-) Nb₂O₅ hollow microspheres, o-Nb₂O₅@carbon core-shell microspheres and tetragonal (t-) NbO₂@carbon core-shell microspheres, through a one-pot hydrothermal method with different post-treatments. These nanoarchitectured materials consist of small nanocrystals with highly exposed active surface, and all of them demonstrate good Li⁺ intercalation pseudocapacitive properties. In particular, o-Nb₂O₅ hollow microspheres can deliver the specific capacitance of 488.3 F g⁻¹, and good rate performance of 126.7 F g⁻¹ at 50 A g⁻¹. The o-Nb₂O₅@carbon core-shell microspheres show enhanced specific capacitance of 502.2 F g⁻¹ and much improved rate performance (213.4 F g⁻¹ at 50 A g⁻¹). Furthermore, we demonstrate for the first time, t-NbO₂ exhibits much higher rate capability than o-Nb₂O₅. For discharging time as fast as 5.9 s (50 A g⁻¹), it still exhibits a very high specific capacitance of 245.8 F g⁻¹, which is 65.2% retention of the initial capacitance (377.0 F g⁻¹ at 1 A g⁻¹). The unprecedented rate capability is an intrinsic feature of t-NbO₂, which may be due to the conductive lithiated compounds.

A simple synthesis of urchin-like Pt–Ni bimetallic nanostructures as enhanced electrocatalysts for the oxygen reduction reaction

Pt–Ni nanostructures have superior oxygen reduction reaction activities.

Electrical double layer effects on ion transfer reactions

The effects of the electrical double layer on ion transfer reactions and their voltammetry are investigatedviaGibbs energy surfaces.

Surface Interrogation Scanning Electrochemical Microscopy of Ni(1-x)Fe(x)OOH (0 < x < 0.27) Oxygen Evolving Catalyst: Kinetics of the "fast" Iron Sites

Nickel-iron mixed metal oxyhydroxides have attracted significant attention as an oxygen evolution reaction (OER) catalyst for solar fuel renewable energy applications. Here, we performed surface-selective and time-dependent redox titrations to directly measure the surface OER kinetics of Ni(IV) and Fe(IV) in NiOOH, FeOOH, and Ni(1-x)Fe(x)OOH (0 < x < 0.27) electrodes. Most importantly, two types of surface sites exhibiting "fast" and "slow" kinetics were found, where the fraction of "fast" sites in Ni(1-x)Fe(x)OOH matched the iron atom content in the film. This finding provides experimental support to the theory-proposed model of active sites in Ni(1-x)Fe(x)OOH. The OER rate constant of the "fast" site was 1.70 s(-1) per atom.

Structure and morphology of shape-controlled Pd nanocrystals

Pd nanocrystals were produced with uniform truncated-cube shape and a narrow size distribution, yielding controlled surface area fractions from low Miller index ({100}, {110}, {111}) crystalline facets. Details on the structure and morphology of the nanocrystals were obtained by combining X-ray powder diffraction line profile analysis, high-resolution transmission electron microscopy and surface electrochemistry based on Cu underpotential deposition.

Pt monolayer coating on complex network substrate with high catalytic activity for the hydrogen evolution reaction

A deposition process has been developed to fabricate a complete-monolayer Pt coating on a large-surface-area three-dimensional (3D) Ni foam substrate using a buffer layer (Ag or Au) strategy. The quartz crystal microbalance, current density analysis, cyclic voltammetry integration, and X-ray photoelectron spectroscopy results show that the monolayer deposition process accomplishes full coverage on the substrate and the deposition can be controlled to a single atomic layer thickness. To our knowledge, this is the first report on a complete-monolayer Pt coating on a 3D bulk substrate with complex fine structures; all prior literature reported on submonolayer or incomplete-monolayer coating. A thin underlayer of Ag or Au is found to be necessary to cover a very reactive Ni substrate to ensure complete-monolayer Pt coverage; otherwise, only an incomplete monolayer is formed. Moreover, the Pt monolayer is found to work as well as a thick Pt film for catalytic reactions. This development may pave a way to fabricating a high-activity Pt catalyst with minimal Pt usage.

A theoretical and experimental approach for correlating nanoparticle structure and electrocatalytic activity

The objective of the research described in this Account is the development of high-throughput computational-based screening methods for discovery of catalyst candidates and subsequent experimental validation using appropriate catalytic nanoparticles. Dendrimer-encapsulated nanoparticles (DENs), which are well-defined 1-2 nm diameter metal nanoparticles, fulfill the role of model electrocatalysts. Effective comparison of theory and experiment requires that the theoretical and experimental models map onto one another perfectly. We use novel synthetic methods, advanced characterization techniques, and density functional theory (DFT) calculations to approach this ideal. For example, well-defined core@shell DENs can be synthesized by electrochemical underpotential deposition (UPD), and the observed deposition potentials can be compared to those calculated by DFT. Theory is also used to learn more about structure than can be determined by analytical characterization alone. For example, density functional theory molecular dynamics (DFT-MD) was used to show that the core@shell configuration of Au@Pt DENs undergoes a surface reconstruction that dramatically affects its electrocatalytic properties. A separate Pd@Pt DENs study also revealed reorganization, in this case a core-shell inversion to a Pt@Pd structure. Understanding these types of structural changes is critical to building correlations between structure and catalytic function. Indeed, the second principal focus of the work described here is correlating structure and catalytic function through the combined use of theory and experiment. For example, the Au@Pt DENs system described earlier is used for the oxygen reduction reaction (ORR) as well as for the electro-oxidation of formic acid. The surface reorganization predicted by theory enhances our understanding of the catalytic measurements. In the case of formic acid oxidation, the deformed nanoparticle structure leads to reduced CO binding energy and therefore improved oxidation activity. The final catalytic study we present is an instance of theory correctly predicting (in advance of the experiments) the structure of an effective DEN electrocatalyst. Specifically, DFT was used to determine the optimal composition of the alloy-core in AuPd@Pt DENs for the ORR. This prediction was subsequently confirmed experimentally. This study highlights the major theme of our research: the progression of using theory to rationalize experimental results to the more advanced goal of using theory to predict catalyst function a priori. We still have a long way to go before theory will be the principal means of catalyst discovery, but this Account begins to shed some light on the path that may lead in that direction.

Layer-by-layer evolution of structure, strain, and activity for the oxygen evolution reaction in graphene-templated Pt monolayers

In this study, we explore the dimensional aspect of structure-driven surface properties of metal monolayers grown on a graphene/Au template. Here, surface limited redox replacement (SLRR) is used to provide precise layer-by-layer growth of Pt monolayers on graphene. We find that after a few iterations of SLRR, fully wetted 4-5 monolayer Pt films can be grown on graphene. Incorporating graphene at the Pt-Au interface modifies the growth mechanism, charge transfers, equilibrium interatomic distances, and associated strain of the synthesized Pt monolayers. We find that a single layer of sandwiched graphene is able to induce a 3.5% compressive strain on the Pt adlayer grown on it, and as a result, catalytic activity is increased due to a greater areal density of the Pt layers beyond face-centered-cubic close packing. At the same time, the sandwiched graphene does not obstruct vicinity effects of near-surface electron exchange between the substrate Au and adlayers Pt. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) techniques are used to examine charge mediation across the Pt-graphene-Au junction and the local atomic arrangement as a function of the Pt adlayer dimension. Cyclic voltammetry (CV) and the oxygen reduction reaction (ORR) are used as probes to examine the electrochemically active area of Pt monolayers and catalyst activity, respectively. Results show that the inserted graphene monolayer results in increased activity for the Pt due to a graphene-induced compressive strain, as well as a higher resistance against loss of the catalytically active Pt surface.

Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction

Ultrathin Pd films with one to five atomic layers were decorated on nanoporous gold by underpotential deposition and galvanic displacement.

Solution based, on chip direct growth of three-dimensionally wrinkled gold nanoparticles for a SERS active substrate

This work reports a solution-based method for on chip growth of SERS active wrinkled gold nanoparticles.

A facile electrochemical route to the preparation of uniform and monoatomic copper shells for gold nanoparticles

Monoatomic copper shells are electrodeposited onto gold nanoparticles without the presence of a solid substrate.

Spectroelectrochemistry of Silver Deposition on Single Gold Nanocrystals

We report the electrodeposition of metallic silver onto gold nanostars adsorbed to ITO electrodes. The electrochemical process was studied at the single particle level by correlated in situ dark field spectroscopy and scanning electron microscopy (SEM). Underpotential deposition avoids bulk silver formation on the ITO substrates. SEM proves that deposition occurs on all surfaces of the gold nanostars when polyvinylpyrrolidone (PVP) is stabilizing the nanostars or preferentially at the nanostar tips when the ligand is removed. The surface plasmon resonance blue-shifts by more than 100 nm following the formation of a 5 nm Ag film on PVP stabilized gold nanostars, moving the scattered color from the near-infrared to red or orange. The spectral shifts can be accurately modeled using finite element simulations. These results demonstrate that the morphology and composition of individual bimetallic nanocrystals can be engineered electrochemically.

CO₂ electroreduction at bare and Cu-decorated Pd pseudomorphic layers: catalyst tuning by controlled and indirect supporting onto Au(111)

We report here the results of electrochemical studies on CO2 electroreduction at multilayered catalyst composed of the monatomic layer of copper covering palladium overlayers (0.8-10 monolayers) deposited on the well-defined Au(111) surface. These multilayered systems were obtained by successive underpotential deposition steps: Pd on Au(111) as well as Cu on Pd/Au(111). Low index orientation of Au substrate was chosen to compare Pd overlayers with bulk Pd(111), which is known to reduce CO2 to CO adsorbates in acidic solutions. The process of CO2 electroreduction was studied by using classical transient electrochemical methods. Catalytic activity of bare Pd layers was investigated in acidic and neutral solutions. In the latter case, much higher activity of Pd overlayers was observed. The results showed that the palladium layer thickness significantly changed the catalytic activities of both bare Pd overlayers and the one Cu monolayer covered electrodes toward CO2 electroreduction. Results show that catalytic activity can be finely tuned by using the multilayered near-surface-alloy approach.

Effects of anions on the electrodeposition of cobalt on pt(111) electrode

Voltammetry and in-situ scanning tunneling microscopy (STM) were used to examine electrodeposition of cobalt (Co) on a stationary Pt(111) electrode in 0.1 M K2SO4 + 1 mM H2SO4 + 10 mM CoSO4 (or the sulfate solution) without and with 10 mM chloride (the chloride solution). Under- and overpotential deposition (UPD and OPD) of Co resulted in reduction peaks at -0.52 and -0.74 V (vs Ag/AgCl), respectively. Hydrogen evolution was the major obstruction to Co(2+) reduction, which limited the efficiency of Co deposition at ∼63% in both solutions. UPD of Co resulted in a highly ordered honeycomb structure in the sulfate solution, whereas that formed in the chloride solution was clearly disordered. Multilayer Co deposit formed by OPD at -0.74 V in the sulfate medium was crystalline, forming moiré structures for the first eight layers, followed by pyramids made of stacked triangles. These results suggested face-centered cubic stacking of the Co deposit. Co film produced in the chloride solution was also layered, except perimeters of Co layers were mostly rugged. Distinct screw dislocations and spiral defects were seen in the Co thin films produced in both solutions.

Hydrothermal synthesis of Pt-Ag alloy nano-octahedra and their enhanced electrocatalytic activity for the methanol oxidation reaction

The high-quality Pt48Ag52 alloy nano-octahedra are synthesized via one-pot hydrothermal method. The catalytic growth of Ag(0) atoms on Pt nuclei and selective oxidative etching on the Ag(0) atoms contribute to the formation of alloy nano-octahedra. Pt48Ag52 alloy nano-octahedra show excellent electrocatalytic activity and durability for the methanol oxidation reaction (MOR).

Kinetically controlled nucleation of silver on surfactant-free gold seeds

We report on the heterogeneous nucleation of Ag on Au seeds using a surfactant-free synthesis where nanoparticle aggregation is nullified through the immobilization of bare Au seeds on the surface of a substrate. Requiring only silver nitrate, ascorbic acid, and Au seeds, the synthesis is facile and, from a mechanistic standpoint, far less convoluted than conventional protocols. The results reveal that, even in the absence of surfactants, highly anisotropic growth modes are achieved which result in a lone Ag structure emanating from a single (100) Au facet. Consistent with surfactant-based protocols is the ability to vary the product of the reaction by varying the reaction rate. It allows for kinetic control which is able to direct the reaction toward either a bimetallic heterodimer or a core-shell configuration. The observed growth modes cannot be explained in terms of those proposed for surfactant-based growth modes where surfactants, surface diffusion, and/or collision patterns are used to rationalize the reaction product. We, instead, propose a growth mode reliant on the formation of a space charge region around each seed consisting of a double layer of ions, where the integrity of the layer is dependent upon the facets expressed by the seed, the rate at which the reduced ions are being deposited, and the pH of the solution. Our work reveals the rich nature of surfactant-free heteroepitaxial growth modes as well as the utility of the substrate-based platform in defining growth pathways.

Learning from nature: introducing an epiphyte-host relationship in the synthesis of alloy nanoparticles by co-reduction methods

This communication reports an epiphytic co-reduction method which can overcome the common tendency of sequential deposition in the synthesis of alloy nanoparticles. In this method the reduction of one of the metals (the epiphyte-metal) is only turned-on and rendered more facile by the in situ generated fresh surfaces of the other metal (the host-metal).

Subcellular neural probes from single-crystal gold nanowires

Size reduction of neural electrodes is essential for improving the functionality of neuroprosthetic devices, developing potent therapies for neurological and neurodegenerative diseases, and long-term brain–computer interfaces. Typical neural electrodes are micromanufactured devices with dimensions ranging from tens to hundreds of micrometers. Their further miniaturization is necessary to reduce local tissue damage and chronic immunological reactions of the brain. Here we report the neural electrode with subcellular dimensions based on single-crystalline gold nanowires (NWs) with a diameter of ∼100 nm. Unique mechanical and electrical properties of defect-free gold NWs enabled their implantation and recording of single neuron-activities in a live mouse brain despite a ∼50× reduction of the size compared to the closest analogues. Reduction of electrode dimensions enabled recording of neural activity with improved spatial resolution and differentiation of brain activity in response to different social situations for mice. The successful localization of the epileptic seizure center was also achieved using a multielectrode probe as a demonstration of the diagnostics potential of NW electrodes. This study demonstrated the realism of single-neuron recording using subcellular-sized electrodes that may be considered a pivotal point for use in diverse studies of chronic brain diseases.