Theoretical Improvement of Photocatalytic Activity of Two-dimensional GaSe, GaS and Their Heterostructures for Water Splitting via Doping and Strain
DescriptionPhotocatalytic water splitting is a green and fast way to producing hydrogen ― a promising renewable fuel for reducing our dependency on environmentally polluting fossil fuels. The overall reaction involves two essential electrochemical processes: (i) hydrogen evolution reaction (HER) at photocathode to reduce H2O into H2and (ii) oxygen evolution reaction (OER) at photoanode to oxidize H2O into O2. Currently, a major challenge of making hydrogen production economically viable is the high cost of novel photocatalytic semiconductors used as these photochemical electrodes. Besides, the most advanced materials for water splitting used nowadays may perform efficiently as either a photoanode or a photocathode, but not both! Gallium chalcogenides (GaSe and GaS) are rising as best alternative materials suitable for both photooxidation and photoreduction of water because of their widely tunable electronic properties plausibly via chemical doping and mechanical strain and their flexible two-dimensional (2D) large surface areas, maximizing solar radiation harvesting. It is known that the surfaces of 2D GaSe and GaS are actually inert and hence show low water splitting activity. However, it is less known that how exactly and to what extent doping and strain can modify their electron properties and improve their surface reaction thermodynamics. This proposal aims to establish a theoretical understanding of the underlying molecular principles of the effects of doping and strain on the overall catalytic performance of the 2D GaSe and GaS based on density functional theories. The photocatalytic efficiency of both HER and OER relies on materials with (1) suitable electron properties, described by their bandgaps for optimal solar-light absorption, and band-edge positions relative to the respective redox potentials for H2and O2generations and (2) active surface reaction thermodynamics, described by the adsorption Gibbs free energies of reaction intermediates. Some selected nonmetals (e.g. C, N, O, P and As), which are commonly used as dopants to modify chemical properties of 2D materials, will be considered to substitute Se and S atoms in GaSe and GaS. The effects of varying dopant and strain strength on the aforementioned surface properties will be schematically examined theoretically and better understood. Through the proposed research, we anticipate theoretical developments of some single best-integrated GaSe/GaS heterostructures for HER and OER simultaneously. The present theoretical designs will be of fundamental importance as references for future experimental fabrications of efficient materials for photocatalytic water splitting to produce hydrogen fuel.
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