DFT Simulations of Water Adsorption and Activation on Low-Index α-Ga2O3Surfaces

Looking Outside the Square: The Growth, Structure, and Resilient Two-Dimensional Surface Electron Gas of Square SnO2 Nanotubes

Nanotechnology has delivered an amazing range of new materials such as nanowires, tubes, ribbons, belts, cages, flowers, and sheets. However, these are usually circular, cylindrical, or hexagonal in nature, while nanostructures with square geometries are comparatively rare. Here, a highly scalable method is reported for producing vertically aligned Sb-doped SnO₂ nanotubes with perfectly-square geometries on Au nanoparticle covered m-plane sapphire using mist chemical vapor deposition. Their inclination can be varied using r- and a-plane sapphire, while unaligned square nanotubes of the same high structural quality can be grown on silicon and quartz. X-ray diffraction measurements and transmission electron microscopy show that they adopt the rutile structure growing in the [001] direction with (110) sidewalls, while synchrotron X-ray photoelectron spectroscopy reveals the presence of an unusually strong and thermally resilient 2D surface electron gas. This is created by donor-like states produced by the hydroxylation of the surface and is sustained at temperatures above 400 °C by the formation of in-plane oxygen vacancies. This persistent high surface electron density is expected to prove useful in gas sensing and catalytic applications of these remarkable structures. To illustrate their device potential, square SnO₂ nanotube Schottky diodes and field effect transistors with excellent performance characteristics are fabricated.

Theoretical screening of single atom doping on β-Ga2O3 (100) for photoelectrochemical water splitting with high activity and low limiting potential

Photoelectrochemical (PEC) water splitting combined with renewable energy is an appealing approach for solar energy conversion and storage. Monoclinic gallium oxide (β-Ga₂O₃) has been identified as a promising photoelectrode for PEC because of its good electrical conductivity and chemical and thermal stability. However, the wide bandgap (around 4.8 eV) and the recombination of photogenerated electrons and holes inside β-Ga₂O₃ limit its performance. Doping β-Ga₂O₃ is a practical strategy to enhance photocatalytic activity, but studies on doped β-Ga₂O₃ based photoelectrodes are lacking. In this study, we evaluate the doping effect of ten different dopants for β-Ga₂O₃ photoelectrode at the atomic level using density functional theory calculations. In addition, the oxygen evolution performance is evaluated on doped structures as it is considered the bottleneck reaction in water slitting on the anode of the PEC cell. Our results suggest that rhodium doping is optimal as it demonstrated the lowest overpotential for oxygen evolution reaction. We performed further electronic structure analysis, indicating the narrower bandgap and enhanced photogenerated electron-hole transfer comparing with β-Ga₂O₃ are the main reasons for the improved performance after Rh doping. This study demonstrates that doping is an attractive strategy for the development of efficient Ga₂O₃-based photoanodes and it will be of great importance in helping the design of other semiconductor-based photoelectrodes for practical application.

First principles study of Schottky barriers at Ga2O3(100)/metal interfaces

A low Schottky barrier height (SBH) of metal–semiconductor contact is essential for achieving high performance electronic devices. Based on first principles calculations, we have comprehensively investigated the interfacial properties of β-Ga₂O₃ (100) with different metals including Mg, Ni, Cu, Pd and Pt. SBHs have been calculated via layered partial density of states (PDOS) and validated by visual wavefunctions. The results surprisingly show that Mg contact possesses the lowest SBH of 0.23 eV, while other SBHs range from 1.06 eV for Ni, 1.17 eV for Pd and 1.27 eV for Cu to 1.39 eV for Pt. This shows that SBHs of β-Ga₂O₃ are not fully dependent on metal work functions due to a Fermi level pinning effect. The tunneling barrier was also calculated via electrostatic potential with a 72.85% tunneling probability of the Mg/Ga₂O₃ interface. The present study will provide an insight into characteristics of Ga₂O₃/metal interfaces and give guidance for metal choice for Ga₂O₃ electronic devices.

A low Schottky barrier height (SBH) of metal–semiconductor contact is essential for achieving high performance electronic devices.

The importance of charge redistribution during electrochemical reactions: a density functional theory study of silver orthophosphate (Ag3PO4)

The charge redistribution during oxygen evolution reaction relates to the electrochemical activity as shown for Ag₃PO₄ structures.

Highly symmetrical, 24-faceted, concave BiVO4 polyhedron bounded by multiple high-index facets for prominent photocatalytic O2 evolution under visible light

A highly symmetrical, 24-faceted, concave BiVO₄ polyhedron bounded by multiple high-index facets was designed to exhibit prominent photocatalytic O₂ evolution under visible light.

Polyhedral 30-Faceted BiVO4 Microcrystals Predominantly Enclosed by High-Index Planes Promoting Photocatalytic Water-Splitting Activity

Unprecedented 30-faceted BiVO₄ polyhedra predominantly surrounded by {132}, {321}, and {121} high-index facets are fabricated through the engineering of high-index surfaces by a trace amount of Au nanoparticles. The growth of high-index facets results in a 3-5 fold enhancement of O₂ evolution from photocatalytic water splitting by the BiVO₄ polyhedron, relative to its low-index counterparts. Theory calculations reveal that water dissociation is more energetically favorable on the high-index surfaces than on the low-index (010), (110), and (101) surfaces, which is accompanied by a notable reduction in the overpotential (0.77-1.14 V) for the oxygen evolution reaction. The apparent quantum efficiency of O₂ generation without an external electron supply reaches 18.3% under 430 nm light irradiation, which is an order of magnitude higher than that of the catalysts reported hitherto.

Monoclinic Ga2O3 (100) surface as a robust photocatalyst for water-splitting

The β-Ga₂O₃ (100) surface, with or without defects, as a robust photocatalyst for water decomposition was studied on the basis of density functional theory (DFT).

Enhanced electrochemical water oxidation: the impact of nanoclusters and nanocavities

Hematite surfaces with a nanocavity are more active for OER than surfaces with nanoclusters.

First principles calculations on oxygen vacant hydrated α-MnO2 for activating water oxidation and its self-healing mechanism

Understanding the mechanism behind water oxidation is the prime requirement for designing better catalysts for electrochemical energy devices. In this work, we demonstrate by employing first principles calculations that an initial step of water oxidation is observed to be associated with the dissociation of water dimers into hydronium and hydroxide ions, in the tunnel of a hydrated α-MnO2 compound with an oxygen vacancy. The former ion is intercalated within the network, while the latter ion occupies the oxygen vacant site and interacts strongly with the Mn atoms. Based on our calculations, the factor responsible for this dissociation of water molecules is observed to be the presence of mixed charge states of Mn atoms in the triangular lattice. Further, the coulombic attraction of a hydronium ion with a water molecule leads to the formation of a Zundel cation in the tunnel, while by dehydrogenating the adsorbed hydroxide ion, the self-healing property of the compound is achieved along with another hydronium ion as a reaction product. These cations can be exchanged with Li(+) ions. Thus, the protonic moieties formed in the tunnel of α-MnO2 leads to niche applications in the field of fuel cells and lithium ion batteries.

Modeling and Simulations in Photoelectrochemical Water Oxidation: From Single Level to Multiscale Modeling

This review summarizes recent developments, challenges, and strategies in the field of modeling and simulations of photoelectrochemical (PEC) water oxidation. We focus on water splitting by metal-oxide semiconductors and discuss topics such as theoretical calculations of light absorption, band gap/band edge, charge transport, and electrochemical reactions at the electrode-electrolyte interface. In particular, we review the mechanisms of the oxygen evolution reaction, strategies to lower overpotential, and computational methods applied to PEC systems with particular focus on multiscale modeling. The current challenges in modeling PEC interfaces and their processes are summarized. At the end, we propose a new multiscale modeling approach to simulate the PEC interface under conditions most similar to those of experiments. This approach will contribute to identifying the limitations at PEC interfaces. Its generic nature allows its application to a number of electrochemical systems.

Computational study of α-M2O3 (M = Al, Ga): surface properties, water adsorption and oxidation

DFT calculations reveal the most stable phases of water monolayer on α-M₂O₃(0001) (M = Al, Ga) in the (Δμ_O, Δμ_H2O) space.