Two-dimensional polar materials for photocatalytic water splitting

As one of the most charming and attractive technologies, photocatalytic overall water-splitting using solar energy is known as a feasible and efficient route to alleviate energy crisis and environmental pollution. Since the seminal work of photocatalytic decomposition of H₂O into H₂ and O₂ on a Pt/TiO₂ electrode by Fujishima and Honda in 1972, photocatalytic water splitting has been persistently investigated, achieving many remarkable advancements. In principle, photocatalytic water splitting reaction mainly involves the following fundamental processes: Absorption of photons induces electron transition and forms electron-hole pairs; then the photogenerated electron-hole pairs separate and migrate to the surface of the photocatalyst; finally, redox reaction occurs on the surface. Therefore, to develop high-performance photocatalysts, several criteria must be fulfilled, including a moderate band gap that should be larger than 1.23 eV but lower than 3.0 eV to maximize absorption and utilization solar light, the ability of rapid separation and transfer of carriers, and the suitable band edge alignment to straddle the redox potentials of water. To this end, numerous efforts have been devoted to searching for photocatalytic candidates. In recent years, two-dimensional (2D) materials, e.g., g-C₃N₄, phosphorene and transition-metal dichalcogenides, have been identified as the potential photocatalysts candidates for water splitting. Compared with the bulk materials, 2D materials hold excellent ability of optical adsorption, short carrier migration distance, high carrier mobility, large specific surface area, and abundant active sites. Therefore, the 2D materials are desirable for high-performance photocatalysts. Nevertheless, most of the 2D photocatalysts for water splitting have the structural symmetry along the out-of-plane direction, which mainly harvests the ultraviolet light and limits the efficiency of solar energy utilization. Some new materials and mechanisms for photocatalytic water-splitting are urgently needed. Interestingly, the asymmetric 2D materials with the intrinsic dipole moment, termed as 2D polar materials, show exciting prospects for the photocatalytic applications with high efficiency. The intrinsic dipole moment in 2D polar materials not only accelerates the separation of photogenerated carriers but also is good for relieving the conventional restriction of 1.23 eV for band gap of photocatalysts, leading to a widened light absorption region (from visible to near-infrared light). Thus, the solar energy utilization efficiency will be substantially enhanced. Motivated by this mechanism, some 2D polar photocatalysts are proposed, such as MXY (M=W, Mo, Cr, Pt; X, Y=S, Se, Te, X¹Y), M₂XY (M=Ga, In; X, Y=S, Se, Te, X¹Y), and M₂X₃ (M=Al, Ga, In; X=S, Se, Te). In this review, we outline the recent progress in material discovery and photocatalytic characteristics of the different types of 2D polar photocatalysts, including Janus materials and ferroelectrics. The underlying mechanisms for polarization-promoted photocatalysis are discussed to better understand the role of 2D polar materials in photocatalysis. After the presentation of the experimental efforts on 2D polar materials, we conclude with a discussion of the emerging challenges and new perspective for future research and development on 2D polar photocatalysts. We hope that this review, with a specific focus on 2D polar photocatalysts, would help readers gain deep understandings on 2D polar material-based photocatalysts and in turn stimulate further both theoretical and experimental efforts.