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.

Formation of inorganic liquid gallium particle-manganese oxide composites

Gallium (Ga) is a low melting point post-transition metal that, under mild mechanical agitation, can form micron and submicron-sized particles with combined fluid-like and metallic properties. In this work, an inorganic network of Ga liquid metal particles was synthesised via spontaneous formation of manganese (Mn) oxide species on their liquid metallic surfaces forming an all-inorganic composite. The micron-sized Ga particles formed by sonication were connected together by Mn oxide nanostructures spontaneously established from the reduction of a Mn salt in aqueous solution slightly above the melting point of Ga. The formed Mn oxide nanostructures were found to coalesce from the surface of the Ga particles into a continuous inorganic network. The morphology of the composites could be altered by varying the Mn salt concentration and by performing post-treatment annealing. The composites presented a shell of various Mn oxide nanostructures including wrinkled sheets, rods and nanoneedles, around spherical liquid Ga particles, and a liquid metal core. The photoelectric and optical properties of the composites were thoroughly characterised, which revealed decreasing bandgaps and valence band edge characteristics as a function of increased Mn oxide coverage. The photoluminescence properties of the composites could be also engineered by increasing the Mn oxide coverage. The all-inorganic liquid Ga composite could be formed via a straightforward reduction reaction of a Mn-rich salt at the surface of liquid Ga particles with tunable surface properties for future optoelectronic applications.

Anisotropic dependence of radiation from excitons in Ga2O3/MoS2 heterostructure

The anisotropic dependence of radiation arising from exciton recombination in the Ga₂O₃/MoS₂ heterostructure is investigated, using density functional theory and the Bethe-Salpeter equation. The wurtzite (WZ) and zinc blende (ZB) structures of the Ga₂O₃ monolayer with ferroelectric (FE) properties are assembled with a MoS₂ monolayer. Projected band structure, charge transfer and life time of excitons are discussed, to analyze which transition may be important to the creation of excitons from the electron-hole pair. A general formula of the angle-dependent intensities of radiation is derived. The characteristics of angle-dependent intensities that are closely related to the dipole moment of excitons are discussed, from the viewpoint of in-plane and out-of-plane polarizations. These predictions on radiation of the Ga₂O₃/MoS₂ heterostructure should guide exciton dynamics in low dimensional systems and rational design of optoelectronic devices.

Predicting the Raman spectra of ferroelectric phases in two-dimensional Ga2O3 monolayer

We investigate the vibrational properties and Raman spectra of the two-dimensional Ga₂O₃ monolayer, using density functional theory. Two ferroelectric (FE) phases of the Ga₂O₃ monolayer with wurtzite (WZ) and zinc blende (ZB) structures (FE-WZ and FE-ZB, respectively) are considered. The Raman tensor and angle-dependent Raman intensities of two major Raman peaks (A11 and A21) in both FE-WZ (497, and 779 cm^-1) and FE-ZB (481, and 772 cm^-1) Ga₂O₃ monolayers, are calculated for the polarization of scattered light, parallel and perpendicular to that of the incident light. The characteristics of angle-dependent Raman intensities are analyzed. The average non-resonant Raman spectra of the minor peaks in FE-WZ (E¹) and FE-BZ (E¹ and E²) are compared with those of major peaks A11 and A21. These predictions of the Raman spectra of the Ga₂O₃ monolayer may guide the rational design of two-dimensional optical devices.

Critical Thermodynamic Conditions for the Formation of p-Type β-Ga2O3 with Cu Doping

As a promising third-generation semiconductor, β-Ga2O3 is facing bottleneck for its p-type doping. We investigated the electronic structures and the stability of various Cu doped structures of β-Ga2O3. We found that Cu atoms substituting Ga atoms result in p-type conductivity. We derived the temperature and absolute oxygen partial pressure dependent formation energies of various doped structures based on first principles calculation with dipole correction. Then, the critical thermodynamic condition for forming the abovementioned substitutional structure was obtained.

Modulation of electronic and optical properties by surface vacancies in low-dimensional β-Ga2O3

Calculations using the Heyd-Scuseria-Ernzerhof screened hybrid functional reveal the detailed influence that surface vacancies have on the electronic and optical properties of low-dimensional (LD) β-Ga2O3. Vacancies manifest subtle changes to the electronic characteristics as oxygen states predominate the valence band at the surface. Dielectric functions at the surface are found to increase with vacancies and defects. A broad impact on optical properties, such as absorption coefficients, reflectivity, refractive indices, and electron loss, is seen with increased vacancy defects. Both visible and infrared regions show direct correlation with vacancies while there is a marked decrease in the deep ultraviolet (UV) region. These calculations on the β-Ga2O3 model system may guide the rational design of two-dimensional optical devices with minimized van der Waals forces.

Photocatalytic hydrogen production from water splitting with N-doped β-Ga2O3 and visible light

Based on the first principles calculations, the feasibility of the photocatalytic hydrogen production from water splitting driven by N-doped β-Ga₂O₃ in the visible light is investigated. The formation energy and dynamics properties are used to examine the stability of the doped structures. The absolute positions of the band energy edges are obtained and compared to the redox potentials of the hydrogen production reaction. Moreover, we calculate the carrier lifetime and mobility for both electron and hole of all the considered structures. The optical absorption is also calculated for each structure. The results show that the 5.00 at.% N-doped β-Ga₂O₃ has the satisfactory band energy edges, obvious difference of mobilities between electron and hole, and significant enhancement of absorption in visible light range, indicating it is a promising photocatalytic material to catalyze hydrogen production from water splitting under the irradiation of the visible light.

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).

Electronic structure and optical property of metal-doped Ga2O3: a first principles study

A long list of main group and transition metals, even some lanthanides, have been examined based on first principles studies, to search for potential p-type dopants for β-Ga₂O₃.