Catalytic Nanostructures Research Articles

Kinetics of optically excited charge carriers at the GaN surface: Influence of catalytic Pt nanostructures

In this work, we use GaN with different deposited Pt nanostructures as a controllable model system to investigate the kinetics of photo-generated charge carriers in hybrid photocatalysts. We combine conductance and contact potential difference measurements to investigate the influence of Pt on the processes involved in the capture and decay of photo-generated charge carriers at and close to the GaN surface. We found that in the presence of Pt nanostructures the photo-excitation processes are similar to those found in Pt free GaN. However, in GaN with Pt nanostructures, photo-generated holes are preferentially trapped in surface states of the GaN covered with Pt and/or in electronic states of the Pt and lead to an accumulation of positive charge there, whereas negative charge is accumulated in localized states in a shallow defect band of the GaN covered with Pt. This preferential accumulation of photo-generated electrons close to the surface is responsible for a dramatic acceleration of the turn-off charge transfer kinetics and a stronger dependence of the surface photovoltage on light intensity when compared to a Pt free GaN surface. Our study shows that in hybrid photocatalysts, the metal nanostructures induce a spatially inhomogeneous surface band bending of the semiconductor that promotes a lateral drift of photogenerated charges towards the catalytic nanostructures.

ChemInform Abstract: Molecular Catalysis Science: Nanoparticle Synthesis and Instrument Development for Studies under Reaction Conditions

AbstractReview: 77 refs.

Molecular catalysis science: Nanoparticle synthesis and instrument development for studies under reaction conditions

The synthesis of architecturally designed catalytic nanostructures and their in situ characterization under reaction conditions enable the development of catalysts with improved stability, reactivity, and product selectivity. Throughout this review paper, we will explore three recent reports that demonstrate the invaluable synergetic impact of combining synthesis, catalysis, and in situ spectroscopy for catalysts development. In the first example, product selectivity in 1,3 butadiene hydrogenation reaction was tuned by employing size-selected Pt nanoparticles as catalysts. SFG vibrational spectroscopy measurements uncovered the mechanism that induced the size-dependent selectivity. The important role of metal/metal-oxide interface during CO oxidation reaction is demonstrated in the second part of this review paper. In situ synchrotron-sourced X-ray spectroscopy correlated between the oxidation state of the metal-oxide support and its impact on the catalytic reactivity of supported Pt nanoparticles. In the third example, dendrimer-encapsulated Au nanoparticles were used as catalyst for cascade reactions, which were previously catalyzed by homogeneous catalysts. Reactants into product evolution and the oxidation state of catalytically active Au nanoparticles within the flow reactor were mapped with micrometer-sized IR and X-ray beams. These three examples demonstrate the important role of colloidal synthesis and in situ spectroscopy measurements for in-depth analysis of structure–reactivity correlations.

Energy Conversion Processes in Catalytic Electrolyte-Free Metal-Oxide Nanostructures

Over the past decade, a wide range of investigation has been carried out toward the development of high performance energy conversion devices. An interesting trend in the modern energy conversion research is an increased interest in electrolyte-free device concepts. In this presentation, we introduce a new type of device architecture based on an electrolyte-free catalytic metal-oxide nanostructure for conversion of chemical energy to electrical energy. In this device, a reaction-induced current is generated upon exposure of hydrogen and oxygen gases on a catalytic nanomesh metal (e.g. Pt, Rh) and a mesoporous oxide layer (TiO2). Interestingly, the reaction current kinetics revealed a new second mode of the reaction in addition to the usual catalytic oxidation of hydrogen to water. This mode induces a negative stationary reaction current at room temperature conditions. This production of negative current cannot be explained with any known mechanisms i.e thermal and hot electron. Instead, protons spillover from Pt to TiO2 substrate resulted in higher energy conversion efficiency compared to metal single-crystal systems. The number of charge carriers per one hydrogen molecule oxidized at Pt/TiO2 surface may reach 0.04 at room temperature, which is much higher than the values reported in pervious studies.

Illuminating CO2 reduction on frustrated Lewis pair surfaces: investigating the role of surface hydroxides and oxygen vacancies on nanocrystalline In2O(3-x)(OH)y.

Designing catalytic nanostructures that can thermochemically or photochemically convert gaseous carbon dioxide into carbon based fuels is a significant challenge which requires a keen understanding of the chemistry of reactants, intermediates and products on surfaces. In this context, it has recently been reported that the reverse water gas shift reaction (RWGS), whereby carbon dioxide is reduced to carbon monoxide and water, CO2 + H2 → CO + H2O, can be catalysed by hydroxylated indium oxide nanocrystals, denoted In2O(3-x)(OH)y, more readily in the light than in the dark. The surface hydroxide groups and oxygen vacancies on In2O(3-x)(OH)y were both shown to assist this reaction. While this advance provides a first step toward the rational design and optimization of a single-component gas-phase CO2 reduction catalyst for solar fuels generation, the precise role of the hydroxide groups and oxygen vacancies in facilitating the reaction on In2O(3-x)(OH)y nanocrystals has not been resolved. In the work reported herein, for the first time we present in situ spectroscopic and kinetic observations, complemented by density functional theory analysis, that together provide mechanistic information into the surface reaction chemistry responsible for the thermochemical and photochemical RWGS reaction. Specifically, we demonstrate photochemical CO2 reduction at a rate of 150 μmol gcat(-1) hour(-1), which is four times better than the reduction rate in the dark, and propose a reaction mechanism whereby a surface active site of In2O(3-x)(OH)y, composed of a Lewis base hydroxide adjacent to a Lewis acid indium, together with an oxygen vacancy, assists the adsorption and heterolytic dissociation of H2 that enables the adsorption and reaction of CO2 to form CO and H2O as products. This mechanism, which has its analogue in molecular frustrated Lewis pair (FLP) chemistry and catalysis of CO2 and H2, is supported by preliminary kinetic investigations. The results of this study emphasize the importance of engineering the surfaces of nanostructures to facilitate gas-phase thermochemical and photochemical carbon dioxide reduction reactions to energy rich fuels at technologically significant rates.

Mapping electroactivity at individual catalytic nanostructures using high-resolution scanning electrochemical-scanning ion conductance microcopy.

Combined scanning electrochemical-scanning ion conductance microcopy (SECM-SICM) has been used to map the electroactivity of surfaces decorated with individual features at the 100-150 nm scale. Dual channel capillary probes consisting of an open SICM barrel, and a solid carbon SECM electrode enabled correlation of surface activity with accurate topographical information. Measurements were validated by approach curve analysis and imaging of model systems in feedback and substrate generation-tip collection modes and then applied to the examination of two nanostructured test substrates. First, electronically isolated gold nanodisk arrays were imaged using a simple electrochemical redox mediator, in which a clear positive feedback signal was observed at the SECM electrode, and the topographical channel compared well with AFM imaging. Second, platinum nanosphere ensembles were mapped using platinum-modified carbon probes to detect oxygen consumption in a redox competition mode, demonstrating the means to study electrocatalytic processes at individual nanoparticles. This work demonstrates the value of high-resolution SECM-SICM for low-current amperometric imaging of nanosystems, and is a step toward quantitative measurement of electrokinetics at the single particle level.

Recyclable l-Proline Functional Nanoreactors with Temperature-Tuned Activity Based on Core–Shell Nanogels

Recyclable core-shell (CS) nanogels based on l-proline-containing hydrophobic cores with a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) shell have been synthesized via a seeded precipitation polymerization process. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) were used to verify the successful addition of the shell and investigate the thermoresponsive properties of the nanostructures. The catalytic activity of the nanogels was assessed in a model asymmetric aldol reaction, where an enhancement was observed with increasing temperature, attributed to the hydrophobic nature of the PNIPAM shell. However, when a nanogel was synthesized with core-shell morphology based on a gradient of cross-linking density in the corona (GS), a dramatic drop in activity was observed at elevated temperatures: the collapse of the outer, lightly cross-linked, "corona" polymer chains appears to block access to the catalytic core. High activity and enantioselectivity were maintained in a number of recovery and reuse cycles, highlighting the recycling potential of these catalytic nanostructures.

Chemical Composition and Surface Roughness of AlOx-Controlled Activity of Pt/AlOx Thin Film Catalysts for Methanol Oxidation Reaction

Chemical composition and surface roughness of AlOx films were facilely controlled by manipulating O2/Ar ratio during aluminum oxide sputter-deposition process. Pt/AlOx catalyst synthesized with O2/Ar ratio of 2 % exhibits the highest roughness (8.55 nm) and lowest O/Al ratio (0.11), which showed the best catalytic activity with a methanol conversion rate of 92.5 % and a remarkable temperature rise of 12 °C induced by exothermic reaction at 100 °C. It was demonstrated that Pt/AlOx catalysts had significant higher activity for methanol oxidation when the AlOx was nonstoichiometric compared with its stoichiometric counterpart (Al2O3). The outstanding catalytic performance and novel nanostructure of the deposited Pt/AlOx thin film catalyst endow it with great potential for developing next-generation energy conversion devices and combustible gas sensors.

Ion‐Responsive Hemin–G‐Quadruplexes for Switchable DNAzyme and Enzyme Functions

Programmed nucleic acid sequences undergo K(+) ion-induced self-assembly into G-quadruplexes and separation of the supramolecular structures by the elimination of K(+) ions by crown ether or cryptand ion-receptors. This process allows the switchable formation and dissociation of the respective G-quadruplexes. The different G-quadruplex structures bind hemin, and the resulting hemin-G-quadruplex structures reveal horseradish peroxidase DNAzyme catalytic activities. The following K(+) ion/receptor switchable systems are described: 1) The K(+) -induced self-assembly of the Mg(2+) -dependent DNAzyme subunits into a catalytic nanostructure using the assembly of G-quadruplexes as bridging unit. 2) The K(+) -induced stabilization of the anti-thrombin G-quadruplex nanostructure that inhibits the hydrolytic functions of thrombin. 3) The K(+) -induced opening of DNA tweezers through the stabilization of G-quadruplexes on the "tweezers' arms" and the release of a strand bridging the tweezers into a closed structure. In all of the systems reversible, switchable, functions are demonstrated. For all systems two different signals are used to follow the switchable functions (fluorescence and the catalytic functions of the derived hemin-G-quadruplex DNAzyme).

Platinum catalysts promoted by In doped SnO2 support for methanol electrooxidation in alkaline electrolyte

Composite metal oxides InxSnO2 are prepared with a simple hydrothermal process and used as functionalized support of Pt catalyst toward methanol electrooxidation reaction (MOR). The catalytic activity of Pt is strongly dependent on the composition of the support. Introduction of a small amount of In into SnO2 support exhibits much higher promoting effect to the Pt catalytic properties as compared with Pt/SnO2 and commercial Pt/C catalysts. The mass-specific activity (MSA) and intrinsic activity (IA) of Pt in Pt/In0.1SnO2 is 3.0 and 4.3 times that of Pt/C, respectively. Changes in Pt electronic structure arising from the interaction between Pt and the support are responsible for this improvement. Our findings clearly suggest that the composite metal oxides InxSnO2 can not only act as the catalyst support but also act as an effective promoter to Pt toward MOR, which would be promising in designing new catalysts that can replace the traditional catalytic nanostructure.

Separation of hot electron current component induced by hydrogen oxidation on resistively heated Pt/n-GaP Schottky nanostructures

Studies of chemically induced hot electron flow over Schottky barriers in catalytic planar nanostructures provide a direct insight into underlying charge transfer processes involved in chemical energy dissipation at solid surfaces. A systematic approach is described here to separate the hot electron and thermal current contributions to the total generated current based on in-situ resistive heating of cathode nanolayer of the Schottky structure. The method is applicable at high pressures in the gas phase. Analysis of the current induced by H2 oxidation to H2O on Pt/n-GaP nanostructure is performed for surface temperatures in the range of 453–513 K, and 120 Torr oxyhydrogen environment with 15 Torr H2. All the current components grow monotonously with temperature, while relative fraction of the hot electron current decreases with temperature from 85 to 52%.

Self-Assembled Catalytic DNA Nanostructures for Synthesis of Para-directed Polyaniline

Templated synthesis has been considered as an efficient approach to produce polyaniline (PANI) nanostructures. The features of DNA molecules enable a DNA template to be an intriguing template for fabrication of emeraldine PANI. In this work, we assembled HRP-mimicking DNAzyme with different artificial DNA nanostructures, aiming to manipulate the molecular structures and morphologies of PANI nanostructures through the controlled DNA self-assembly. UV-vis absorption spectra were used to investigate the molecular structures of PANI and monitor kinetic growth of PANI. It was found that PANI was well-doped at neutral pH and the redox behaviors of the resultant PANI were dependent on the charge density of the template, which was controlled by the template configurations. CD spectra indicated that the PANI threaded tightly around the helical DNA backbone, resulting in the right handedness of PANI. These reveal the formation of the emeraldine form of PANI that was doped by the DNA. The morphologies of the resultant PANI were studied by AFM and SEM. It was concluded from the imaging and spectroscopic kinetic results that PANI grew preferably from the DNAzyme sites and then expanded over the template to form 1D PANI nanostructures. The strategy of the DNAzyme-DNA template assembly brings several advantages in the synthesis of para-coupling PANI, including the region-selective growth of PANI, facilitating the formation of a para-coupling structure and facile regulation. We believe this study contributes significantly to the fabrication of doped PANI nanopatterns with controlled complexity, and the development of DNA nanotechnology.

Catalytic polymeric nanoreactors: more than a solid supported catalyst

Polymeric nanostructures can be synthesized where the catalytic motif is covalently attached within the core domain and protected from the environment by a polymeric shell. Such nanoreactors can be easily recycled, and have shown unique properties when catalyzing reactions under pseudohomogeneous conditions. Many examples of how these catalytic nanostructures can act as nanosized reaction vessels have been reported in the literature. This prospective will focus on the exclusive features observed for these catalytic systems and highlight their potential as enzyme mimics, as well as the importance of further studies to unveil their full potential.

Catalytic Pt-on-Au Nanostructures: Why Pt Becomes More Active on Smaller Au Particles

Platinum is a widely used precious metal in many catalytic nanostructures. Engineering the surface electronic structure of Pt-containing bi- or multimetallic nanostructure to enhance both the intrinsic activity and dispersion of Pt has remained a challenge. By constructing Pt-on-Au (Pt^Au) nanostructures using a series of monodisperse Au nanoparticles in the size range of 2-14 nm, we disclose herein a new approach to steadily change both properties of Pt in electrocatalysis with downsizing of the Au nanoparticles. A combined tuning of Pt dispersion and its surface electronic structure is shown as a consequence of the changes in the size and valence-band structure of Au, which leads to significantly enhanced Pt mass-activity on the small Au nanoparticles. Fully dispersed Pt entities on the smallest Au nanoparticles (2 nm) exhibit the highest mass-activity to date towards formic acid electrooxidation, being 2 orders of magnitude (75-300 folds) higher than conventional Pt/C catalyst. Fundamental relationships correlating the Pt intrinsic activity in Pt^Au nanostructures with the experimentally determined surface electronic structures (d-band center energies) of the Pt entities and their underlying Au nanoparticles are established.

A DFT and HRTEM Study on MoS2/Co: Locating Promoters in Catalytic Nanostructures

ABSTRACTLocating cobalt promoters on catalytically MoS2 structures is a challenging task to achieve; this is due to the size on those MoS2 nanostructures. Previous reports in the literature indicate that specific locations for Co in MoS2 slabs are (1010)-plane creating either a sulfur-Co or Molybdenum-Co termination edge, due to lower energy required for the permutation Mo, S and Co to occur. We present results obtained from Density Functional Theory study done on the interface between MoS2 and Co9S8 crystal structures; the interface show an interesting thiocubane cluster and it is suspected to be the responsible for Mo-S-Co bonding to exist, along with HDS reaction. In order to understand electronic properties on thiocubane Density of States and Mulliken Population Analysis calculations were implemented using Cambridge Serial Total Energy Package (CASTEP). Results indicate a strong electron donation from Co to Mo through intermediate sulfur atom bonded to both metals while an enhanced metallic character is also found.

Splitting water with viruses

A bacteriophage can be used as a template for assembling catalytic nanostructures for the light-driven oxidation of water molecules.

Electrochemical Synthesis of Nanostructured Palladium of Different Morphology Directly on Gold Substrate through a Cyclic Deposition/Dissolution Route

Uniform nanostructured palladium (Pd) has been successfully electrodeposited directly on gold substrate through a facile template-free approach, which involved a cyclic electrochemical deposition/dissolution of Pd. Under the regulation of potential scan and surfactant, standing Pd nanoplates and nanotrees were prepared. Improved and superior electrocatalytic activities toward the oxidation of ethanol were observed from these electrodes coated with nanostructured Pd. Furthermore, the uniform Pd nanostructures can be conveniently deposited onto a flexible substrate with a gold precoating, thus allowing fabrication of miniaturized flexible devices. This approach provides a simple route for the synthesis of highly catalytic Pd nanostructures with an excellent electrical contact to a substrate, which is important for the overall device performance. The critical effects exerted by the electrochemical conditions and surfactant will also be discussed.

Phase transitions and the nucleation of catalytic nanostructures under the action of chemical, physical, and mechanical factors

Basic concepts of first-order phase transitions are formulated, and the state of the art in this field is considered using nanoparticle nucleation and growth as examples. General equations are presented to describe the nanoparticle size distribution function; the average nanoparticle radius; and the density, morphology, and composition of the new phase. Examples are given to illustrate the catalytic effect of nanoparticles on the kinetics of first-order phase transitions in multicomponent systems. The effect of the acidity (pH) of the medium on phase transitions in solutions and the effect of mechanical stress on the nucleation, evolution, and properties of quantum dots are considered.

Integrated plasma synthesis of efficient catalytic nanostructures for fuel cell electrodes

A single plasma process involving three consecutive steps has been developed for producinghigh gas flow catalytic nanostructures on the electrodes of proton exchange membrane(PEM) fuel cells (FC).Using a high density helicon radio frequency (13.56 MHz) plasma, nickel is sputtered onto aporous carbon support. Changing the background gas from argon to methane/hydrogen allowed2 µm long, 37 nm diameter carbon nanofibres (CNFs) to be grown by diffusion through thenickel clusters in a ‘tip growth’ mechanism at the relatively low temperature of400 °C. The third step involves plasma sputtering of platinum onto the CNFs, resulting innanoclusters (3–8 nm) being formed on the periphery of the CNFs. Four FC cathodes weresynthesized on carbon paper and PTFE/carbon loaded cloth (known as gas diffusionlayer, GDL), both with and without CNFs, with the Pt/CNFs nanostructuresgrown on PTFE/carbon loaded cloth having the best FC performances. Comparedwith conventional FCs, the efficiency of sputtered platinum in the Pt/CNF basedcathode is much higher than in a chemically deposited system over the entirerange of operating current. This indicates that combination of different, simple,plasma techniques is an effective method for preparing highly efficient catalystlayers.

Micromachining of Novel SiC on Si Structures for Device and Sensor Applications

In this paper the multifariousness of SiC/Si heterostructures for device and sensor applications will be demonstrated. 3C-SiC based microelectromechanical resonator beams (MEMS) with different geometries actuated by the magnetomotive effect operating under ambient conditions were fabricated. The resonant frequency reaches values up to 2 MHz. The applications of these resonators are the measurement of the viscosity of liquids or mass detection. Furthermore, photonic devices in the form of SiC/Si infrared gratings for wavelength and polarization filters in infrared spectra are processed. SiC wear protection for a dosing system with the possibility to dose nano- or picoliter droplets of water based liquids as well as SiC nanomasking for catalytic agent nanostructures are demonstrated.