Theoretical prediction of the electronic structure, optical properties and photocatalytic performance of type-I SiS/GeC and type-II SiS/ZnO heterostructures

Nowadays, it would be ideal to develop high-performance photovoltaic devices as well as highly efficient photocatalysts for the production of hydrogen via photocatalytic water splitting, which is a feasible and sustainable energy source for addressing the challenges related to environmental pollution and a shortage of energy. In this work, we employ first-principles calculations to investigate the electronic structure, optical properties and photocatalytic performance of novel SiS/GeC and SiS/ZnO heterostructures. Our results indicate that both the SiS/GeC and SiS/ZnO heterostructures are structurally and thermodynamically stable at room temperature, suggesting that they are promising materials for experimental implementation. The formation of SiS/GeC and SiS/ZnO heterostructures gives rise to reduction of the band gaps as compared to the constituent monolayers, enhancing the optical absorption. Furthermore, the SiS/GeC heterostructure possesses a type-I straddling gap with a direct band gap, while the SiS/ZnO heterostructure forms a type-II band alignment with indirect band gap. Moreover, a red-shift (blue-shift) has been observed in SiS/GeC (SiS/ZnO) heterostructures as compared with the constituent monolayers, enhancing the efficient separation of photogenerated electron-hole pairs, thereby making them promising candidates for optoelectronic applications and solar energy conversion. More interestingly, significant charge transfers at the interfaces of SiS-ZnO heterostructures, have improved the adsorption of H, and the Gibbs free energy Δ_H* becomes close to zero, which is optimal for the hydrogen evolution reaction to produce hydrogen. The findings pave the path for the practical realization of these heterostructures for potential applications in photovoltaics and photocatalysis of water splitting.

Challenges of modeling nanostructured materials for photocatalytic water splitting

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano-photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

Sn-doped hematite modified by CaMn2O4 nanowire with high donor density and enhanced conductivity for photocatalytic water oxidation

Herein, we report a novel nanocomposite consisting of n-type Sn-doped hematite and p-type CaMn₂O₄ nanowire (CaMn₂O₄/α-Fe₂O₃). The nanocomposite was characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), ultraviolet-visible absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), which showed that nanospindle-like Sn-doped hematite and CaMn₂O₄ nanowire contact intimately in the nanocomposite, resulting in efficient charge transfer and separation. Photoelectrochemical results reveal that the nanocomposite possesses higher donor density, enhanced conductivity and lower overpotential for dioxygen evolution. In addition, the nanocomposite demonstrates high photocatalytic activity for water oxidation to produce oxygen in a photoelectrochemical cell. The amount of O₂ evolved from the optimized photoanode of the photoelectrochemical cell was 1.98 μmol in 2 h of simulated sunlight irradiation. This work demonstrates a facile synthesis of a novel nanocomposite as anode material for photocatalytic water oxidation to produce O₂.