Theoretical advances in two-dimensional materials for photocatalytic water splitting

Currently, the energy and environment associated problems become more and more serious. Hydrogen is a kind of clean and renewable resource. Hydrogen generation through photocatalytic water splitting is a feasible and efficient route to resolve these problems. In general, the photocatalytic water splitting process is composed of three pivotal steps: Light harvest, carrier separation and transportation, and hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER). Improving the efficiencies of these three steps are keys to enhance the solar-to-hydrogen (STH) efficiency, which stimulates the development of high-performance photocatalysts. Typically, for two-dimensional (2D) materials, the photogenerated electrons and holes produce directly from the surfaces of 2D materials with atomic thickness. In addition, compared with their bulk counterparts, 2D materials with an extremely large specific surface area, rather high carrier mobility, and controllable interfaces offer a large number of advantages for the photocatalysts exploration. Thus, search and design of 2D photocatalysts for water splitting has been a hot topic. Density functional theory (DFT) based first-principles calculations is a powerful tool for materials design and has been widely used in exploring 2D photocatalysts for water splitting. Here, recent advances in the design of 2D materials for photocatalytic water splitting are presented from a theoretical perspective. In the initial stage, a great of new 2D photocatalysts for water splitting are expected to be discovered. Mechanical and chemical assisted exfoliation of layered materials is straightforward approaches to get access to single-layer and few-layer 2D materials. By using DFT calculations, many potential 2D photocatalysts are screened out for water splitting. In addition, molecular designing and chemical regulating are alternative approaches for exploring new 2D photocatalysts for water splitting. The light harvest is the initial process of photocatalytic water splitting, and increasing the light adsorption is one of keys to enhance the STH efficiency. Generally, there are two ways for enhancing the light harvest of 2D materials. One strategy is designing and constructing new 2D materials for photocatalytic water splitting. The other one is improving the light harvest by regulating 2D materials that have been reported previously. Both of the two strategies are employed for developing 2D visible-light-driven covalent organic frameworks for photocatalytic water splitting. Improving the separation of the photo-generated electrons and holes is another key to enhance the STH efficiency. The development of (type-II and Z-scheme) heterojunctions provides a feasible way to design high performance photocatalysts for water splitting due to the benefit for inhibiting electron-hole recombination. That is because that the HER and OER occur on different materials. By using first-principles calculations, a variety of 2D materials based heterojunctions with an expectation of designing highly efficient photocatalytic 2D materials for water splitting have been developed. The STH efficiency is determined by the product of the efficiencies of all the steps for photocatalytic water splitting. Thus, the successful enhancement of energy conversion efficiency calls for the simultaneous improvement for the light harvest and electron-hole separation. A new mechanism for photocatalytic water splitting reveals that the vertical intrinsic electric field in 2D material not only accelerates the carrier separation, but also breaks the conventional limitation of 1.23 eV for band gap of photocatalysts, leading to enlarged light absorption, even to infrared region. Referring to this new mechanism, some 2D materials with vertical electric field have been theoretically proved to be potential photocatalysts for water splitting. In additional, future opportunities and challenges in theoretical design of 2D photocatalysts toward water splitting are also included.