Two-Dimensional Layered Heterojunctions for Photoelectrocatalysis

Two-dimensional (2D) layered nanomaterial heterostructures, arising from the combination of 2D materials with other low-dimensional species, feature a large surface area to volume ratio, which provides a high density of active sites for catalytic applications and for (photo)electrocatalysis (PEC). Meanwhile, their electronic band structure and high electrical conductivity enable efficient charge transfer (CT) between the active material and the substrate, which is essential for catalytic activity. In recent years, researchers have demonstrated the potential of a range of 2D material interfaces, such as graphene, graphitic carbon nitride (g-C3N4), metal chalcogenides (MCs), and MXenes, for (photo)electrocatalytic applications. For instance, MCs such as MoS2 and WS2 have shown excellent catalytic activity for hydrogen evolution, while graphene and MXenes have been used for the reduction of carbon dioxide to higher value chemicals. However, despite their great potential, there are still major challenges that need to be addressed to fully realize the potential of 2D materials for PEC. For example, their stability under harsh reaction conditions, as well as their scalability for large-scale production are important factors to be considered. Generating heterojunctions (HJs) by combining 2D layered structures with other nanomaterials is a promising method to improve the photoelectrocatalytic properties of the former. In this review, we inspect thoroughly the recent literature, to demonstrate the significant potential that arises from utilizing 2D layered heterostructures in PEC processes across a broad spectrum of applications, from energy conversion and storage to environmental remediation. With the ongoing research and development, it is likely that the potential of these materials will be fully expressed in the near future.

Design of molybdenum phosphide @ nitrogen-doped nickel-cobalt phosphide heterostructures for boosting electrocatalytic overall water splitting

Transition metal phosphides (TMPs) are one of the most promising alternatives to noble metal electrocatalysts, but so far their activity and stability still fall short of expectations. Here, we prepare nitrogen-doped nickel-cobalt phosphide (N-NiCoP) and molybdenum phosphide (MoP) heterostructures engineered on nickel foam (NF) with nanosheet structure by high-temperature annealing and low-temperature phosphorylation. Notably, heteroatomic N doping and heterostructures construction are achieved together through a simple co-pyrolysis method. The distinctive composition can synergistically promote the electron transfer, lower the reaction barriers, thus improving the catalytic performance. Therefore, the modified MoP@N-NiCoP requires low overpotentials of 43 mV and 232 mV to reach 10 mA cm^-2 current density for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) accompanied by satisfactory stability in 1 M KOH. Density functional theory (DFT) calculations reveal the electron coupling and synergistic interfacial effects at the heterogeneous interface. This study provides a new strategy for heterogeneous electrocatalysts with elemental doping to promote hydrogen applications.

Revealing the enhanced photoelectrochemical water oxidation activity of Fe-based metal-organic polymer-modified BiVO4 photoanode

Metal-organic polymers (MOPs) can enhance the photoelectrochemical (PEC) water oxidation performance of BiVO₄ photoanodes, but their PEC mechanisms have yet to be comprehended. In this work, we constructed an active and stable composite photoelectrode by overlaying a uniform MOP on the BiVO₄ surface using Fe²⁺ as the metal ions and 2,5-dihydroxyterephthalic acid (DHTA) as ligand. Such modification on the BiVO₄ surface yielded a core-shell structure that could effectively enhance the PEC water oxidation activity of the BiVO₄ photoanode. Our intensity-modulated photocurrent spectroscopy analysis revealed that the MOP overlayer could concurrently reduce the surface charge recombination rate constant (k_sr) and enhance the charge transfer rate constant (k_tr), thus accelerating water oxidation activity. These phenomena can be ascribed to the passivation of the surface that inhibits the recombination of the charge carrier and the MOP catalytic layer that improves the hole transfer. Our rate law analysis also demonstrated that the MOP coverage shifted the reaction order of the BiVO₄ photoanode from the third-order to the first-order, resulting in a more favorable rate-determining step where only one hole accumulation is required to overcome water oxidation. This work provides new insights into the reaction mechanism of MOP-modified semiconductor photoanodes.