Photocatalytic water splitting, CO2 reduction, and pollutant degradation have emerged as promising strategies to remedy the existing environmental and energy crises. However, grafting of expensive and less abundant noble-metal cocatalysts on photocatalyst materials is a mandatory practice to achieve enhanced photocatalytic performance owing to the ability of the cocatalysts to extract electrons efficiently from the photocatalyst and enable rapid/enhanced catalytic reaction. Hence, developing highly efficient, inexpensive, and noble-metal-free cocatalysts composed of earth-abundant elements is considered as a noteworthy step toward considering photocatalysis as a more economical strategy. Recently, MXenes (two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides) have shown huge potential as alternatives for noble-metal cocatalysts. MXenes have several excellent properties, including atomically thin 2D morphology, metallic electrical conductivity, hydrophilic surface, and high specific surface area. In addition, they exhibit Gibbs free energy of intermediate H atom adsorption as close to zero and less than that of a commercial Pt-based cocatalyst, a Fermi level position above the H2 generation potential, and an excellent ability to capture and activate CO2 molecules. Therefore, there is a growing interest in MXene-based photocatalyst materials for various photocatalytic events. In this review, we focus on the recent advances in the synthesis of MXenes with 2D and 0D morphologies, the stability of MXenes, and MXene-based photocatalysts for H2 evolution, CO2 reduction, and pollutant degradation. The existing challenges and the possible future directions to enhance the photocatalytic performance of MXene-based photocatalysts are also discussed.

A Review on MXene Synthesis, Stability, and Photocatalytic Applications.

Abstract Converting water into hydrogen fuel and oxidizing benzyl alcohol to benzaldehyde simultaneously under visible light illumination is of great significance, but the fast recombination of photogenerated carriers in photocatalysts seriously decreases the conversion efficiency. Herein, a novel dual-functional 0D Cd 0.5 Zn 0.5 S/2D Ti 3 C 2 hybrid was fabricated by a solvothermally in-situ generated assembling method. The Cd 0.5 Zn 0.5 S nano-spheres with a fluffy surface completely and uniformly covered the ultrathin Ti 3 C 2 nanosheets, leading to the increased Schottky barrier (SB) sites due to a large contact area, which could accelerate the electron-hole separation and improve the light utilization. The optimized Cd 0.5 Zn 0.5 S/Ti 3 C 2 hybrid simultaneously presents a hydrogen evolution rate of 5.3 mmol/(g·h) and a benzaldehyde production rate of 29.3 mmol/(g·h), which are ∼3.2 and 2 times higher than those of pristine Cd 0.5 Zn 0.5 S, respectively. Both the multiple experimental measurements and the density functional theory (DFT) calculations further demonstrate the tight connection between Cd 0.5 Zn 0.5 S and Ti 3 C 2 , formation of Schottky junction, and efficient photogenerated electron—hole separation. This paper suggests a dual-functional composite catalyst for photocatalytic hydrogen evolution and benzaldehyde production, and provides a new strategy for preventing the photogenerated electrons and holes from recombining by constructing a 0D/2D heterojunction with increased SB sites. 

Efficient photocatalytic hydrogen evolution coupled with benzaldehyde production over 0D Cd0.5Zn0.5S/2D Ti3C2 Schottky heterojunction

Photothermal functional material and structure for photothermal catalytic CO2 reduction: Recent advance, application and prospect

Cocatalysts loading is an effective strategy to boost photocatalytic CO2 reduction reaction (CO2RR). The cocatalysts based on non-noble metal compounds have an obvious advantage in balancing the cost and efficiency in the scale-up application. The anions in non-noble metal cocatalysts have a distinctive influence on the photocatalytic CO2RR. This review focuses on the impact of different anions in non-noble metal compounds, including oxide-, sulphide-, phosphide-, carbide- and nitride-based cocatalysts. These materials have been demonstrated to improve the charge carrier separation, light harvesting, CO2 adsorption ability and act as active sites for reduction reactions. The recent progresses of non-noble metal-based cocatalysts for photocatalytic CO2RR, together with possible challenges and opportunities have been discussed. 

Junyue Li

Papers

A surface-alkalinized Ti3C2 MXene as an efficient cocatalyst for enhanced photocatalytic CO2 reduction over ZnO