Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

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Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective

The search for highly effective and environmentally safe photocatalysts for water splitting and photovoltaic solar cells is essential for renewable solar energy conversion and storage. Based on first principles calculations, we show that novel 2D $\beta$-PdX$_2$ (X = S, Te) monolayer possesses excellent stabilities and great potentials in solar energy conversion applications. Comprehensive studies show that the $\beta$-PdX$_2$ monolayer exhibits semiconductor characteristics with an indirect gap, suitable band alignment, efficient carrier separation, and high solar to hydrogen (STH) efficiencies, supporting its good photoelectronic performance. The surface catalytic and adsorption/intercalation energies calculation reveals that the photogenerated holes have adequate driving forces to render hydrogen reduction half-reactions to proceed spontaneously and the ability to cover and incorporate water molecules on $\beta$-PdX$_2$ monolayer. Besides, the $\beta$-PdX$_2$ monolayer is promising donor material for excitonic solar cells with high photovoltaic performance. More importantly, due to suitable donor band gap and small conduction band offset in the proposed type-II heterostructure, the calculated power conversion efficiencies (PCE) is calculated up to ~23% ($\beta$-PdX$_2$/WTe$_2$), ~21% ($\beta$-PdX$_2$/ MoTe$_2$) and ~18% ($\beta$-PdTe2/$\beta$-PdX$_2$), making it a promising candidate for solar energy conversion applications. 

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Two-dimensional β-PdX<sub>2</sub> (X = S, Te) monolayers for efficient solar energy conversion applications

Highly-anisotropic carrier transport and optical properties of two-dimensional titanium trisulfide

Phosphorene has offered an additional advantage for developing new optoelectronic devices due to its anisotropic and high carrier mobility. However, its instability in air causes a rapid degradation of the performance of the device. Thus, improving the stability of phosphorene while maintaining its original properties has become the key to the development of high-performance electronic devices. Herein, we propose that the formation of two-dimensional (2D) P-rich P-S compounds could achieve this goal. First-principles swarm-structural searches revealed two previously unkonwn P3S and P2S monolayers. The P3S monolayer, consisting of n-bicyclo-P6 units along the armchair direction, exhibits anisotropic and wide band gap characteristics. Interestingly, its carrier mobility reaches 1.11 × 104 cm2 V-1 s-1 and is much higher than in phosphorene. Its electronic band gap and optical absorption coefficients in the ultraviolet region reach 2.71 eV and 105 cm-1, respectively. Additionally, the P3S monolayer has a high structural stability and resistance to air oxidation.

Wide Band Gap P3S Monolayer with Anisotropic and Ultrahigh Carrier Mobility.

Two-dimensional materials promise great potential for photochemical water splitting due to the abundant active sites and large surface area, but few of the known materials meet the rigorous requirements. In this work, we systematically investigate structural, electronic, and optical properties of an experimentally unexplored 2D material, i.e., gold telluride iodide (AuTeI) monolayer using density functional theory and Bethe–Salpeter equation approaches. Bulk AuTeI is a layered material and was realized in experiments a few decades ago. However, its bandgap is relatively small for water splitting. We find the exfoliation of monolayer AuTeI from the bulk phase is highly favorable, and 2D AuTeI is dynamically stable. The bandgap of 2D AuTeI becomes larger due to the quantum confinement effect. Importantly, the edge positions of the conduction band minimum and valence band maximum of 2D AuTeI perfectly fit the water oxidation and reduction potentials, enabling it a promising photocatalyst for water splitting. Additionally, the exciton binding energy of 2D AuTeI is calculated to be 0.35 eV, suggesting efficient electron-hole separation. Our results highlight a new and experimentally accessible 2D material for potential application in photocatalytic water splitting.

https://www.mdpi.com/2079-4991/12/11/1915/pdf?version=1654576493

Versatile Gold Telluride Iodide Monolayer as a Potential Photocatalyst for Water Splitting

Taking into account the high conductivity and stability of carbon materials, such as graphene, and the strong polar covalent bonding character of main-group compounds, we explore potential 2D materials in the C-S binary system through first-principles structure search calculations. Herein, a hitherto unknown semiconducting C3S monolayer is identified, consisting of well-known n-biphenyl and S atom linked benzenes, exhibiting an obvious direction-dependent atomic arrangement. Thus, it exhibits anisotropic mechanical properties and carrier mobility. Its electron mobility reaches 2.14 × 104 cm2 V-1 s-1 in the b direction, along which n-biphenyl units are arranged, and is much higher than that in the well-used MoS2 monolayer and black phosphorus. Meanwhile, the C3S monolayer has high optical absorption coefficients (105 cm-1), high thermal and dynamical stabilities, and a moderate ability to split water. All these desirable properties make the C3S monolayer a promising candidate for applications in novel optoelectronic devices.

Meng Tang

Papers

Anisotropic and High-Mobility C3S Monolayer as a Photocatalyst for Water Splitting.

Wide Band Gap P3S Monolayer with Anisotropic and Ultrahigh Carrier Mobility.