Imaging Structure Sensitive Catalysis on Different Shape-Controlled Platinum Nanoparticles

Challenges in nanoelectrochemical and nanomechanical studies of individual anisotropic gold nanoparticles

The characterization of nanoparticles and the correlation of physical properties such as size and shape to their (electro)chemical properties is an emerging field, which may facilitate future optimization and tuning of devices involving nanoparticles. This requires the investigation of individual particles rather than obtaining averaged information on large ensembles. Here, we present atomic force – scanning electrochemical microscopy (AFM-SECM) measurements of soft conductive PDMS substrates modified with gold nanostars (i.e., multibranched Au nanoparticles) in peak force tapping mode, which next to the electrochemical characterization provides information on the adhesion, deformation properties, and Young's modulus of the sample. AFM-SECM probes with integrated nanodisc electrodes (radii < 50 nm) have been used for these measurements. Most studies attempting to map individual nanoparticles have to date been performed at spherical nanoparticles, rather than highly active asymmetric gold nanoparticles. Consequently, this study discusses challenges during the nanocharacterization of individual anisotropic gold nanostars.

Cuboctahedral vs. octahedral platinum nanoclusters: insights into the shape-dependent catalytic activity for fuel cell applications

The shape of a catalyst plays an important role in any catalytic reaction.

Imaging Electrogenerated Chemiluminescence at Single Gold Nanowire Electrodes

We report electrogenerated chemiluminescence (ECL) generated at single gold nanowire electrodes supported on tin-doped indium oxide. Unlike other single nanoparticle electrochemical characterization techniques, ECL provides a massively parallel direct readout of electrochemical activity on individual nanoparticle electrodes without the need for extrinsic illumination or a scanning electrochemical probe. While ECL is not observed from as-purchased nanowires due to the surfactant layer, by removing the layer and coating the nanowires with a polymer blend, ECL from single nanowire electrodes is readily measured. With an increase in polymer thickness, an increase in ECL image quality and reproducibility over multiple redox cycles is observed. The polymer coating also provides a strategy for stabilizing gold nanoparticle electrodes against complete surface oxidation in aqueous environments.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

Rotating disk slurry electrodeposition of platinum at Y-zeolite/carbon Vulcan XC-72R for methanol oxidation in alkaline media

Platinum was electrodeposited onto Y-zeolite and Y-zeolite (Y)/Vulcan XC-72R (V) to produce Pt/Y/V and Pt/Y catalysts using a rotating disk slurry electrode technique.

Shape-directional growth of Pt and Pd nanoparticles

The design and synthesis of shape-directed nanoscale noble metal particles have attracted much attention due to their enhanced catalytic properties and the opportunities to study fundamental aspects of nanoscale systems. As such, numerous methods have been developed to synthesize crystals with tunable shapes, sizes, and facets by adding foreign species that promote or restrict growth on specific sites. Many hypotheses regarding how and why certain species direct growth have been put forward, however there has been no consensus on a unifying mechanism of nanocrystal growth. Herein, we develop and demonstrate the capabilities of a mathematical growth model for predicting metal nanoparticle shapes by studying a well known procedure that employs AgNO3 to produce {111} faceted Pt nanocrystals. The insight gained about the role of auxiliary species is then utilized to predict the shape of Pd nanocrystals and to corroborate other shape-directing syntheses reported in literature. The fundamental understanding obtained herein by combining modeling with experimentation is a step toward computationally guided syntheses and, in principle, applicable to predictive design of the growth of crystalline solids at all length scales (nano to bulk).

Oxygen reduction reaction and peroxide generation on shape-controlled and polycrystalline platinum nanoparticles in acidic and alkaline electrolytes

Shape-controlled Pt nanoparticles (cubic, tetrahedral, and cuboctahedral) are synthesized using stabilizers and capping agents. The nanoparticles are cleaned thoroughly and electrochemically characterized in acidic (0.5 M H2SO4 and 0.1 M HClO4) and alkaline (0.1 M NaOH) electrolytes, and their features are compared to that of polycrystalline Pt. Even with less than 100% shape-selectivity and with the truncation at the edges and corners as shown by the ex-situ TEM analysis, the voltammetric features of the shape-controlled nanoparticles correlate very well with that of the respective single-crystal surfaces, particularly the voltammograms of shape-controlled nanoparticles of relatively larger size. Shape-controlled nanoparticles of smaller size show somewhat higher contributions from the other orientations as well because of the unavoidable contribution from the truncation at the edges and corners. The Cu stripping voltammograms qualitatively correlate with the TEM analysis and the voltammograms. The fractions of low-index crystallographic orientations are estimated through the irreversible adsorption of Ge and Bi. Pt-nanocubes with dominant {100} facets are the most active toward oxygen reduction reaction (ORR) in strongly adsorbing H2SO4 electrolytes, while Pt-tetrahedral with dominant {111} facets is the most active in 0.1 M HClO4 and 0.1 M NaOH electrolytes. The difference in ORR activity is attributed to both the structure-sensitivity of the catalyst and the inhibiting effect of the anions present in the electrolytes. Moreover, the percentage of peroxide generation is 1.5-5% in weakly adsorbing (0.1 M HClO4) electrolytes and 5-12% in strongly adsorbing (0.5 M H2SO4 and 0.1 M NaOH) electrolytes.

Al13@Pt42 core-shell cluster for oxygen reduction reaction

To increase Pt utilization for oxygen reduction reaction (ORR) in fuel cells, reducing particle sizes of Pt is a valid way. However, poisoning or surface oxidation limits the smallest size of Pt particles at 2.6 nm with a low utility of 20%. Here, using density functional theory calculations, we develop a core-shell Al13@Pt42 cluster as a catalyst for ORR. Benefit from alloying with Al in this cluster, the covalent Pt-Al bonding effectively activates the Pt atoms at the edge sites, enabling its high utility up to 70%. Valuably, the adsorption energy of O is located at the optimal range with 0.0-0.4 eV weaker than Pt(111), while OH-poisoning does not observed. Moreover, ORR comes from O2 dissociation mechanism where the rate-limiting step is located at OH formation from O and H with a barrier of 0.59 eV, comparable with 0.50 eV of OH formation from O and H2O on Pt(111).

From single crystal surfaces to single atoms: investigating active sites in electrocatalysis

Electrocatalytic processes will undoubtedly be at the heart of energising future transportation and technology with the added importance of being able to create the necessary fuels required to do so in an environmentally friendly and cost effective manner. For this to be successful two almost mutually exclusive surface properties need to be reconciled, namely producing highly active/reactive surface sites that exhibit long term stability. This article reviews the various approaches which have been undertaken to study the elusive nature of these active sites on metal surfaces which are considered as adatoms or clusters of adatoms with low coordination number. This includes the pioneering studies at extended well defined stepped single crystal surfaces using cyclic voltammetry up to the highly sophisticated in situ electrochemical imaging techniques used to study chemically synthesised nanomaterials. By combining the information attained from single crystal surfaces, individual nanoparticles of defined size and shape, density functional theory calculations and new concepts such as mesoporous multimetallic thin films and single atom electrocatalysts new insights into the design and fabrication of materials with highly active but stable active sites can be achieved. The area of electrocatalysis is therefore not only a fascinating and exciting field in terms of realistic technological and economical benefits but also from the fundamental understanding that can be acquired by studying such an array of interesting materials.

Platinum nanoparticles modified with alkylamine derivatives as an active and stable catalyst for oxygen reduction reaction

Platinum nanoparticles (NPs) protected with octylamine (OA) as well as partially replaced by controlled ratios of alkylamine with pyrene group (PA) were successfully synthesized by a two-phase liquid reduction method of Pt(IV). The NPs without any pretreatment to remove the introduced organic-protected agents have been well-characterized and applied as supported catalysts on carbon black for oxygen reduction reaction (ORR). The modification of the Pt NP surface with OA and PA significantly improved the electrocatalytic activity such as area specific and mass specific activities, whose values increased by an increase in PA ratios. The potential cycle test demonstrates that the PA modification of Pt NPs enhances the stability of the catalyst and sustains high area and mass specific activities of ORR.

Ion effects at electrode/solid polymer electrolyte membrane interfaces

The differential capacity and the potential distribution at electrode/solid polymer electrolyte membrane/solution interfaces are calculated through an analytical approach. The model considers coions' and counterions' permeation through the membrane from the solvent phase and the ions' partitioning equilibrium at the SPEM/solution interface. The latter effects are included by incorporating the Donnan equilibrium, the steric hindrance, the solvation energy change when ions move from water to membrane pores and ion electrostatic interactions. It is shown that capacitance maxima in capacitance-potential curves may appear because of the acid-base dissociation process inside the membrane and the change in the ions' total interaction energy with the applied potential. For low dielectric constants inside membrane pores, εp, sharp peaks can be obtained. These peaks broaden, decrease in magnitude and shift to positive potentials once εp is increased. Finally, model predictions are discussed in light of recent experimental data obtained on Nafion® covered Pt(111) electrodes, providing a theoretical framework for the qualitative electroanalysis of these systems.

Some reflections on the understanding of the oxygen reduction reaction at Pt(111)

The oxygen reduction reaction (ORR) is a pivotal process in electrochemistry. Unfortunately, after decades of intensive research, a fundamental knowledge about its reaction mechanism is still lacking. In this paper, a global and critical view on the most important experimental and theoretical results regarding the ORR on Pt(111) and its vicinal surfaces, in both acidic and alkaline media, is taken. Phenomena such as the ORR surface structure sensitivity and the lack of a reduction current at high potentials are discussed in the light of the surface oxidation and disordering processes and the possible relevance of the hydrogen peroxide reduction and oxidation reactions in the ORR mechanism. The necessity to build precise and realistic reaction models, which are deducted from reliable experimental results that need to be carefully taken under strict working conditions is shown. Therefore, progress in the understanding of this important reaction on a molecular level, and the choice of the right approach for the design of the electrocatalysts for fuel-cell cathodes is only possible through a cooperative approach between theory and experiments.

25th anniversary article: galvanic replacement: a simple and versatile route to hollow nanostructures with tunable and well-controlled properties

This article provides a progress report on the use of galvanic replacement for generating complex hollow nanostructures with tunable and well-controlled properties. We begin with a brief account of the mechanistic understanding of galvanic replacement, specifically focused on its ability to engineer the properties of metal nanostructures in terms of size, composition, structure, shape, and morphology. We then discuss a number of important concepts involved in galvanic replacement, including the facet selectivity involved in the dissolution and deposition of metals, the impacts of alloying and dealloying on the structure and morphology of the final products, and methods for promoting or preventing a galvanic replacement reaction. We also illustrate how the capability of galvanic replacement can be enhanced to fabricate nanomaterials with complex structures and/or compositions by coupling with other processes such as co-reduction and the Kirkendall effect. Finally, we highlight the use of such novel metal nanostructures fabricated via galvanic replacement for applications ranging from catalysis to plasmonics and biomedical research, and conclude with remarks on prospective future directions.

Shape-controlled nanostructures in heterogeneous catalysis

Nanotechnologies have provided new methods for the preparation of nanomaterials with well-defined sizes and shapes, and many of those procedures have been recently implemented for applications in heterogeneous catalysis. The control of nanoparticle shape in particular offers the promise of a better definition of catalytic activity and selectivity through the optimization of the structure of the catalytic active site. This extension of new nanoparticle synthetic procedures to catalysis is in its early stages, but has shown some promising leads already. Here, we survey the major issues associated with this nanotechnology-catalysis synergy. First, we discuss new possibilities associated with distinguishing between the effects originating from nanoparticle size versus those originating from nanoparticle shape. Next, we survey the information available to date on the use of well-shaped metal and non-metal nanoparticles as active phases to control the surface atom ensembles that define the catalytic site in different catalytic applications. We follow with a brief review of the use of well-defined porous materials for the control of the shape of the space around that catalytic site. A specific example is provided to illustrate how new selective catalysts based on shape-defined nanoparticles can be designed from first principles by using fundamental mechanistic information on the reaction of interest obtained from surface-science experiments and quantum-mechanics calculations. Finally, we conclude with some thoughts on the state of the field in terms of the advances already made, the future potentials, and the possible limitations to be overcome.

Shape-dependent electrocatalytic activity of monodispersed palladium nanocrystals toward formic acid oxidation

The catalytic activity of different-shaped and monodispersed palladium nanocrystals, including cubes, octahedra and rhombic dodecahedra, toward the electrochemical oxidation of formic acid has been systematically evaluated in both HClO4 and H2SO4 solutions. Notably, the cubic palladium nanocrystals wholly exposed with {100} facets exhibit the highest activity, while the rhombic dodecahedra with {110} facets show the lowest electrocatalytic performance. Furthermore, compared with HClO4 electrolyte, the catalytic activity is found to be obviously lower in H2SO4 solution likely due to the competitive adsorption of SO4(2-) ions and formic acid on the surface of Pd nanocrystals.

Investigation into the catalytic activity of porous platinum nanostructures

The catalytic activity of porous platinum nanostructures, viz. platinum nanonets (PtNNs) and platinum nanoballs (PtNBs), synthesized by radiolysis were studied using two model reactions (i) electron transfer reaction between hexacyanoferrate (III) and sodium thiosulfate and (ii) the reduction of p-nitrophenol by sodium borohydride to p-aminophenol. The kinetic investigations were carried out for the platinum nanostructure-catalyzed reactions at different temperatures. The pseudofirst-order rate constant for the electron transfer reaction between hexacyanoferrate (III) and sodium thiosulfate catalyzed by PtNNs and PtNBs at 293 K are (9.1 ± 0.7) × 10(-3) min(-1) and (16.9 ± 0.6) × 10(-3) min(-1), respectively. For the PtNN- and PtNB-catalyzed reduction of p-nitrophenol to p-aminophenol by sodium borohydride, the pseudofirst-order rate constant was (8.4 ± 0.3) × 10(-2) min(-1) and (12.6 ± 2.5) × 10(-2) min(-1), respectively. The accessible surface area of the PtNNs and PtNBs determined before the reaction are 99 and 110 m(2)/g, respectively. These nanostructures exhibit significantly higher catalytic activity, consistent with the largest accessible surface area reported so far for the solid platinum nanoparticles. The equilibrium of the reactants on the surface of the platinum nanostructures played an important role in the induction time (t0) observed in the reaction. A possible role of structural modifications of PtNBs catalyzed the reaction leading to change in the accessible surface area of PtNBs is being explored to explain the nonlinear behavior in the kinetic curve. The activation energy of the PtNN- and PtNB-catalyzed reduction of p-nitrophenol are 26 and 6.4 kJ/mol, respectively. These observations open up new challenges in the field of material science to design and synthesize platinum nanostructures which could withstand such reaction conditions.

Polyallylamine-directed green synthesis of platinum nanocubes. Shape and electronic effect codependent enhanced electrocatalytic activity

The synthesis of Pt nanocrystals with controlled size and morphology has drawn enormous interest due to their particular catalytic activity. We present a facile and green hydrothermal method for synthesizing monodisperse Pt nanocubes (Pt-NCs) with polyallylamine hydrochloride (PAH) as a complex-forming agent, capping agent and facet-selective agent, and formaldehyde as a reductant. The formation mechanism, particle size and surface composition of the Pt-NCs were investigated by Ultraviolet and visible spectroscopy (UV-vis), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), etc. In the proposed PAH-K(2)PtCl(4)-HCHO synthesis system, the raw material could be reutilized to re-synthesize the Pt-NCs, and the particle size of the Pt-NCs could be readily controlled by the reduction rate of the Pt(II) species in the Pt(II)-PAH complex. After UV/Ozone and electrochemical cleaning, the residual PAH on the Pt-NC surfaces still strongly influenced the d-band centre of Pt due to the strong N-Pt interaction. The as-prepared 6 nm Pt-NCs showed superior electrocatalytic activity (mass activity and specific activity) and stability towards the oxygen reduction reaction (ORR) in both H(2)SO(4) and HClO(4) electrolytes compared to the commercial E-TEK Pt black, owing to the combination of the facets effect and electronic effect.

Template-free synthesis of uniform single-crystal hollow cerium dioxide nanocubes and their catalytic activity

Monodisperse single-crystal hollow cerium dioxide nanocubes with exposed (001) facets have been synthesized with the assistance of ammonium chloride (NH4Cl) and polyvinylpyrrolidone (PVP) in a water and ethanol system. A series of experiments indicate that ammonium ion plays an important role in the formation of the hollow structure and PVP plays a key role in the formation of a cubic shape. The hollow cerium dioxide nanocubes exhibit excellent CO catalytic oxidation activity.

New insights into the oxygen reduction reaction mechanism on Pt(111): a detailed electrochemical study

The oxygen reduction reaction (ORR) is undoubtedly the most important fuel-cell cathodic reaction. In this work, a detailed electrochemical analysis of the ORR on Pt(111) in nonadsorbing electrolytes was performed, which included the high-potential region Eup =1.15 V while ensuring the electrode surface structure stability. Our results suggest that the reduction of a soluble intermediate species formed during the ORR is the rate-determining step in the whole reaction mechanism. This species does not undergo any other electrochemical reaction at E>0.9 V and may accumulate close to the electrode surface. Together with dissolved O₂, this intermediate may modify the oxide-growth dynamics on Pt(111). Hence, both species interact with the electrode surface through complex catalytic networks. Under certain experimental conditions, oxygenated species from the oxidation of Pt(111) may enhance the overall ORR current. These results propose an alternative to explain the current state of the art for this fundamental process.