Aloysius Soon

Single-Atom Catalyst of Platinum Supported on Titanium Nitride for Selective Electrochemical Reactions

As a catalyst, single-atom platinum may provide an ideal structure for platinum minimization. Herein, a single-atom catalyst of platinum supported on titanium nitride nanoparticles were successfully prepared with the aid of chlorine ligands. Unlike platinum nanoparticles, the single-atom active sites predominantly produced hydrogen peroxide in the electrochemical oxygen reduction with the highest mass activity reported so far. The electrocatalytic oxidation of small organic molecules, such as formic acid and methanol, also exhibited unique selectivity on the single-atom platinum catalyst. A lack of platinum ensemble sites changed the reaction pathway for the oxygen-reduction reaction toward a two-electron pathway and formic acid oxidation toward direct dehydrogenation, and also induced no activity for the methanol oxidation. This work demonstrates that single-atom platinum can be an efficient electrocatalyst with high mass activity and unique selectivity.

Stability and morphology of cerium oxide surfaces in an oxidizing environment: A first-principles investigation

We present density functional theory investigations of the bulk properties of cerium oxides (CeO2 and Ce2O3) and the three low index surfaces of CeO2, namely, (100), (110), and (111). For the surfaces, we consider various terminations including surface defects. Using the approach of “ab initio atomistic thermodynamics,” we find that the most stable surface structure considered is the stoichiometric (111) surface under “oxygen-rich” conditions, while for a more reducing environment, the same (111) surface, but with subsurface oxygen vacancies, is found to be the most stable one, and for a highly reducing environment, the (111) Ce-terminated surface becomes energetically favored. Interestingly, this latter surface exhibits a significant reconstruction in that it becomes oxygen terminated and the upper layers resemble the Ce2O3(0001) surface. This structure could represent a precursor to the phase transition of CeO2 to Ce2O3.

The role of titanium nitride supports for single-atom platinum-based catalysts in fuel cell technology

As a first step towards a microscopic understanding of single-Pt atom-dispersed catalysts on non-conventional TiN supports, we present density-functional theory (DFT) calculations to investigate the adsorption properties of Pt atoms on the pristine TiN(100) surface, as well as the dominant influence of surface defects on the thermodynamic stability of platinized TiN. Optimized atomic geometries, energetics, and analysis of the electronic structure of the Pt/TiN system are reported for various surface coverages of Pt. We find that atomic Pt does not bind preferably to the clean TiN surface, but under typical PEM fuel cell operating conditions, i.e. strongly oxidizing conditions, TiN surface vacancies play a crucial role in anchoring the Pt atom for its catalytic function. Whilst considering the energetic stability of the Pt/TiN structures under varying N conditions, embedding Pt at the surface N-vacancy site is found to be the most favorable under N-lean conditions. Thus, the system of embedding Pt at the surface N-vacancy sites on TiN(100) surfaces could be promising catalysts for PEM fuel cells.

Shape effects of cuprous oxide particles on stability in water and photocatalytic water splitting

Cuprous oxide (Cu2O) has received much attention as a photocatalyst due to its direct band gap structure, small band gap energy, non-toxicity, and abundance. However, Cu2O usually suffers from poor stability because the oxidation state of copper is easily changed. In this work, Cu2O particles of three different shapes were prepared with distinct surface structures: cubes with (100) facets, octahedra with (111) facets, and rhombic dodecahedra with (110) facets. Their shape stability was estimated in deionized water with or without light irradiation. The Cu2O(100) facets were selectively deformed under dark conditions, as expected from density functional theory calculations. The rhombic dodecahedra showed the most violent degradation under light irradiation, with many large thorns appearing on the surface. When water splitting was attempted using the shaped Cu2O particles, the rhombic dodecahedra produced the most hydrogen, whereas the cubes produced none. Oxygen was not measured because the holes generated upon light absorption were used to oxidize the Cu2O surface to CuO. A conformal TiIrOx overlayer was successfully formed on the rhombic dodecahedral Cu2O particles, and the coated particles presented overall water splitting producing both hydrogen and oxygen. They also showed significantly improved stability over repeated water splitting reactions relative to bare Cu2O particles or TiOx-coated Cu2O particles.

Solution-based synthesis of anisotropic metal chalcogenide nanocrystals and their applications

This article reviews recent advances in solution phase synthesis to generate 1-D and 2-D anisotropic metal chalcogenide (MC) nanostructures with a focus on using different growth mechanisms to control the shapes of the MCs. Four different synthetic approaches have been reviewed: naturally favoured growth due to its intrinsically anisotropic crystal structure, modified anisotropic growth by changing surface energies or utilizing organic templates, oriented attachment of small nanocrystal building blocks to form nanowires or nanosheets, and chemical transformation from existing nanostructures into new species. We discuss current understanding of the thermodynamic and kinetic aspects associated with the mechanisms of forming these anisotropic MC nanostructures. We provide examples of representative applications of anisotropic chalcogenide nanomaterials that are expected to be practically meaningful in the near future. The applications include electrodes for lithium ion batteries, photodetectors, thermoelectric devices, and solar cells. A brief review of other potential applications (oxygen reduction reaction, localized surface plasmon resonance, topological insulator, superconductor) is provided as well. This review ends with discussions on the challenges to be investigated thoroughly in the solution-based synthesis of anisotropic nanomaterials, which includes surface energy control, correcting the nucleation & growth mechanism, removal of the organic surfactant, kinetic study on the chemical transformation, scale-up of production, and eco-friendly synthesis.

Identification of critical stacking faults in thin-film CdTe solar cells

Cadmium telluride (CdTe) is a p-type semiconductor used in thin-film solar cells. To achieve high light-to-electricity conversion, annealing in the presence of CdCl2 is essential, but the underlying mechanism is still under debate. Recent evidence suggests that a reduction in the high density of stacking faults in the CdTe grains is a key process that occurs during the chemical treatment. A range of stacking faults, including intrinsic, extrinsic, and twin boundary, are computationally investigated to identify the extended defects that limit performance. The low-energy faults are found to be electrically benign, while a number of higher energy faults, consistent with atomic-resolution micrographs, are predicted to be hole traps with fluctuations in the local electrostatic potential. It is expected that stacking faults will also be important for other thin-film photovoltaic technologies.

Playing with dimensions: rational design for heteroepitaxial p-n junctions.

A design for a heteroepitaxial junction by the way of one-dimensional wurzite on a two-dimensional spinel structure in a low-temperature solution process was introduced, and its capability was confirmed by successful fabrication of a diode consisting of p-type cobalt oxide (Co(3)O(4)) nanoplate/n-type zinc oxide (ZnO) nanorods, showing reasonable electrical performance. During thermal decomposition, the 30° rotated lattice orientation of Co(3)O(4) nanoplates from the orientation of β-Co(OH)(2) nanoplates was directly observed using high-resolution transmission electron microscopy. The epitaxial relations and the surface stress-induced ZnO nanowire growth on Co(3)O(4) were well supported using the first-principles calculations. Over the large area, (0001) preferred oriented ZnO nanorods epitaxially grown on the (111) plane of Co(3)O(4) nanoplates were experimentally obtained. Using this epitaxial p-n junction, a diode was fabricated. The ideality factor, turn-on voltage, and rectifying ratio of the diode were measured to be 2.38, 2.5 V and 10(4), respectively.

Nonstoichiometric nucleation and growth of multicomponent nanocrystals in solution.

The ability to assemble nanoscale functional building blocks is a useful and modular way for scientists to design valuable materials with specific physical and chemical properties. Chemists expect multicomponent, heterostructured nanocrystals to show unique electrical, thermal, and optical properties not seen in homogeneous, single-phase nanocrystals. Although researchers have made remarkable advances in heterogeneous nucleation and growth, design of synthetic conditions for obtaining nanocrystals with a target composition and shape is still a big challenge. There are several outstanding issues that chemists need to address before they can successfully carry out the design-based synthesis of multicomponent nanocrystals. For instance, small changes in the reaction parameters, such as the precursor, solvent, surfactant, reducing agent, and the reaction temperature, often result in changes in the structure and chemical composition of the final product. Although scientists do not fully understand the mechanisms underlying the nucleation and growth processes involved in the synthesis of these multicomponent nanocrystals, recent progress in understanding of the thermodynamic and kinetic factors have improved our control over their final structure and chemical composition. In this Account, we summarize our recent advances in understanding of the nucleation and growth mechanisms involved in the solution-based synthesis of multicomponent nanocrystals. We also discuss the various challenges encountered in their synthesis, emphasizing what still needs special consideration. We first discuss the three different nucleation paths from a thermodynamics perspective: amorphous nucleation, crystalline nucleation, and two-step nucleation. Amorphous nucleation and two-step nucleation involve the generation of nonstoichiometric nuclei. We initiate this process mainly by introducing an imbalance in the concentrations of the reduced elements. When the nonstoichiometric nuclei grow, we can add secondary elements to the growing nonstoichiometric nuclei. This leads to either the physical deposition or atomic mixture formation through the diffusion and rearrangement of constituents. The processes of mixture formation and the physical deposition of the secondary constituent element also compete and determine the shape and chemical composition of the final product. If the free energy change by mixture formation is positive (ΔGAB ≥ 0), physical deposition takes place predominantly, and the spreading coefficient (S) determines the structure of the nanocrystals. However, when mixture formation is highly spontaneous (ΔGAB < -ξ), the chemical composition of the final product is usually stoichiometric, and its shape then depends on the size of the primary nanocrystals. When the mixture formation and physical deposition are in competition (-ξ ≤ ΔGAB < 0), as commonly seen for many nanoalloy systems, both the chemical composition and the structure are determined by the size of the primary nanocrystals as well as the degree of mixture formation at the interface of the constituent components. Finally, we discuss the challenges and caveats that one needs to take into account when synthesizing multicomponent nanocrystals.

Bridging the temperature and pressure gaps: close-packed transition metal surfaces in an oxygen environment

An understanding of the interaction of atoms and molecules with solid surfaces on the microscopic level is of crucial importance to many, if not most, modern high-tech materials applications. Obtaining such accurate, quantitative information has traditionally been the realm of surface science experiments, carried out under ultra-high vacuum conditions. Over recent years scientists have realized the importance of obtaining such knowledge also under the high pressure and temperature conditions under which many industrial processes take place, e.g. heterogeneous catalysis, since the material under these conditions may be quite different to that under the conditions of typical surface science experiments. Theoretical studies too have been aimed at bridging the so-called pressure and temperature gaps, and great strides have been made in recent years, often in conjunction with experiment. Here we review recent progress in the understanding of the hexagonal close-packed surfaces of late transition and noble metals in an oxygen environment, which is of relevance to many heterogeneous catalytic reactions. In many cases it is found that, on exposure to high oxygen pressures and elevated temperatures, thin oxide-like structures form which may or may not be stable, and which may have little similarity to the bulk oxides, and thus possess unique chemical and physical properties.

Anharmonicity in the High-Temperature Cmcm Phase of SnSe: Soft Modes and Three-Phonon Interactions.

The layered semiconductor SnSe is one of the highest-performing thermoelectric materials known. We demonstrate, through a first-principles lattice-dynamics study, that the high-temperature Cmcm phase is a dynamic average over lower-symmetry minima separated by very small energetic barriers. Compared to the low-temperature Pnma phase, the Cmcm phase displays a phonon softening and enhanced three-phonon scattering, leading to an anharmonic damping of the low-frequency modes and hence the thermal transport. We develop a renormalization scheme to quantify the effect of the soft modes on the calculated properties, and confirm that the anharmonicity is an inherent feature of the Cmcm phase. These results suggest a design concept for thermal insulators and thermoelectric materials, based on displacive instabilities, and highlight the power of lattice-dynamics calculations for materials characterization.