Hydrogen generation from the direct splitting of water by photocatalysis is regarded as a promising and renewable solution for the energy crisis. The key to realize this reaction is to find an efficient and robust photocatalyst that ideally makes use of the energy from sunlight. Recently, due to the attractive properties such as appropriate band structure, ultrahigh specific surface area, and more exposed active sites, two-dimensional (2D) photocatalysts have attracted significant attention for photocatalytic water splitting. This Review attempts to summarize recent progress in the fabrication and applications of 2D photocatalysts including graphene-based photocatalysts, 2D oxides, 2D chalcogenides, 2D carbon nitride, and some other emerging 2D materials for water splitting. The construction strategies and characterization techniques for 2D/2D photocatalysts are summarized. Particular attention has been paid to the role of 2D/2D interfaces in these 2D photocatalysts as the interfaces and heterojunctions are critical for facilitating charge separation and improving photocatalysis efficiency. We also critically discuss their stability as photocatalysts for water splitting. Finally, we highlight the ongoing challenges and opportunities for the future development of 2D photocatalysts in this exciting and still emerging area of research.

The separation of photogenerated carriers and photocatalytic hydrogen production efficiency was greatly enhanced by the 2D/2D heterojunction of Ti3C2/g-C3N4.

Hydrogen production through facile photocatalytic water splitting is regarded as a promising strategy to solve global energy problems. Transition-metal carbides (MXenes) have recently drawn attention as potential co-catalyst candidates for photocatalysts. Here, we report niobium pentoxide/carbon/niobium carbide (MXene) hybrid materials (Nb O /C/Nb C) as photocatalysts for hydrogen evolution from water splitting. The Nb O /C/Nb C composites were synthesized by one-step CO oxidation of Nb CT . Nb O grew homogeneously on Nb C after mild oxidation, during which some amorphous carbon was also formed. With an optimized oxidation time of 1.0 h, Nb O /C/Nb C showed the highest hydrogen generation rate (7.81 μmol h g ), a value that was four times higher than that of pure Nb O . The enhanced performance of Nb O /C/Nb C was attributed to intimate contact between Nb O and conductive Nb C and the separation of photogenerated charge carriers at the Nb O /Nb C interface; the results presented herein show that transition-metal carbide are promising co-catalysts for photocatalytic hydrogen production.

The CuO–Fe2O3–ZrO2/HZSM-5 bifunctional catalyst was prepared and used for the direct synthesis of dimethyl ether (DME) from CO2 and H2. The results revealed that doping the CuO–Fe2O3 catalyst with ZrO2 might increase the specific surface area and change the chemical combination state of CuO by decreasing the outer-shell electron density of Cu via an obvious change in the interaction between CuO and Fe2O3. Addition of ZrO2 to the catalyst strongly affects the hydrocarbon selectivity. When using the CuO–Fe2O3–ZrO2/HZSM-5 bifunctional catalyst system, both the conversion of CO2 and the yield of DME were much higher than those obtained using CuO–Fe2O3/HZSM-5 as catalysts. Reactions carried out at 260 °C and 3.0 MPa with a gaseous hourly space velocity of 1500 mL·gcat –1·h–1 using CuO–Fe2O3–ZrO2/HZSM-5 with 1.0 wt % ZrO2 as the hydrogenation catalyst provided a 28.4% conversion of CO2 with 64.5% selectivity for DME.