Role of Oxygen Vacancies on Oxygen Evolution Reaction Activity: β-Ga₂O₃as a Case Study

Abstract

Neutral oxygen vacancies (Ovʼs) in semiconductor oxides give rise to excess electrons that have the potential to affect the binding of adsorbates to the surface through surface-to-adsorbate charge transfer, and, as a result, to alter the overpotential (OP) of reactions on oxygen-deficient materials compared to stoichiometric materials. We report a systematic computational investigation of the effects of Ovʼs on the oxygen evolution reaction (OER) overpotential for β-Ga2O3, a d 10 semiconductor that has been shown to exhibit high activity for water splitting. We investigated 18 β-Ga2O3 surfaces/slabs, with and without Ovʼs and observed a clear dependence of OER activity on Ovʼs. A general finding emerged that the excess electrons associated with Ovʼs are found to participate in charge transfer to OER intermediates, making their bonds to the surface more ionic and stronger, depending on the amount of charge transfer. The OER reaction step free energies are significantly affected and the ensuing overpotentials are altered. The amount of charge transfer varies with the types of intermediates (OH*, dangling O*, surface-bound peroxo O*, and dangling OOH*), their open valencies, and their electronegativity. The work function and the position of the gap states of the excess electrons in the band gap are found to be useful descriptors of whether and how much Ov-induced charge transfer may occur and affect the overpotential. However, it was also found that the chemical environment of the O atom where the vacancy was created, may have a negating effect on the general observation. Specifically some Ov structures underwent a strong relaxation to form Ga–Ga bonds, trapping the vacancy electrons, and preventing them to engage in charge transfer. Oxygen vacancies are common defects in photocatalyst materials so that our investigation can provide guiding principles for designing efficient photocatalysts.

Enhanced Electrocatalytic Activity in GaSe and InSe Nanosheets: The Role of Surface Oxides

“…The presence of oxygen vacancies additionally decreases energy cost of OER in both environments to 0.5 and 2.0 eV, respectively. Our results are consistent with previous theoretical [49] and experimental [50] results for water-gas shift reactions catalyzed by defective Ga 2 O 3 . Calculations also demonstrate that increasing the amount of oxygen vacancies decreases the efficiency of defective Ga 2 O 3 surface for OER ( Figure S16d, Supporting Information).…”

Defect-Engineered MoO2 Nanostructures as an Efficient Electrocatalyst for Oxygen Evolution Reaction

“…In the OER process, Ru, Ir-based compounds are considered to be the benchmark catalyst. − However, these compounds are highly scarce and excessively expensive; as a result, the broad-scale OER application gets hampered. , Thus, researchers have already started working extensively on the growth of different kinds of OER catalysts, which will be economical and superior in water-splitting to replace these noble metals. − Further, to improve the electrocatalytic performance, it is very imperative to enhance the conductivity of the catalysts as low electrical conductivity yields modest reaction kinetics. In recent times, vacancy creation in the catalysts is one of the leading techniques for increasing the conductivity, and the role of oxygen vacancies in electrocatalytic performance has got tremendous importance. − In the semiconductor oxide materials, oxygen vacancy (O V ) delivers excess electrons and improves the charge-transfer conductivity, and as a result, the overpotential of OER decreases and increases the OER activity. − , …”

Surface Oxygen Vacancy Formation Energy Calculations in 34 Orientations of β-Ga2O3 and θ-Al2O3

“…β-Ga2O3 also demonstrates catalytic activity. For example, β-Ga2O3 is experimentally found to split water 55 , and computational studies indicate that β-Ga2O3 is active in CO2 hydrogenation 56 and that excess electrons from neutral O vacancies affect the activation energy for oxygen evolution reaction intermediates 57 . Al2O3 has many polymorphs including α (corundum), θ, δ, κ, γ, χ, and η [58][59] .…”