Hydrogen (H2) is one of the most important clean and renewable energy sources for future energy sustainability. Nowadays, photocatalytic and electrocatalytic hydrogen evolution reactions (HERs) from water splitting are considered as two of the most efficient methods to convert sustainable energy to the clean energy carrier, H2. Catalysts based on transition metal dichalcogenides (TMDs) are recognized as greatly promising substitutes for noble-metal-based catalysts for HER. The photocatalytic and electrocatalytic activities of TMD nanosheets for the HER can be further improved after hybridization with many kinds of nanomaterials, such as metals, oxides, sulfides, and carbon materials, through different methods including the in situ reduction method, the hot-injection method, the heating-up method, the hydro(solvo)thermal method, chemical vapor deposition (CVD), and thermal annealing. Here, recent progress in photocatalytic and electrocatalytic HERs using 2D TMD-based composites as catalysts is discussed.

2D Transition‐Metal‐Dichalcogenide‐Nanosheet‐Based Composites for Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions

Artificial Z-scheme photocatalyst can not only reduce the recombination of photogeneraged electron–holepairs, but also retain prominent redox ability. In this study, direct solid-state dual Z-scheme WO3/g-C3N4/Bi2O3 photocatalyst was successfully synthesized by one step co-calcination stratage using tungstic acid, melamine and bismuth (III) nitrate pentahydrate as the precursors. Surface, morphological, and structural properties of the resulting materials were comprehensive characterized by XRD, XPS, SEM, TEM, UV–vis diffuse reflection spectroscopy, BET surface areas, photoluminescence and ESR analysis. The WO3/g-C3N4/Bi2O3 composite exhibited superior photocatalytic activities for tetracycline degradation than that of pure g-C3N4, WO3, Bi2O3 and their binary composites under visible light irradiation. The enhanced photocatalytic performance of WO3/g-C3N4/Bi2O3 composite can be ascribed to improved visible light absorption, increased surface area and enhanced separation efficiency of photo-generated electron-hole pairs. In addition, the photocatalyst exhibits high stability and reusability. On the basis of the results, a novel direct solid-state dual Z-scheme photocatalytic mechanism was also proposed.

In-situ synthesis of direct solid-state dual Z-scheme WO3/g-C3N4/Bi2O3 photocatalyst for the degradation of refractory pollutant

2D Nanomaterials, with unique structural and electronic features, have shown enormous potential toward photocatalysis fields. However, the photocatalytic behavior of pristine 2D photocatalysts are still unsatisfactory, and far below the requirements of practical applications. In this regard, surface defect engineering can serve as an effective means to tune photoelectric parameters of 2D photocatalysts through tailoring the local surface microstructure, electronic structure, and carrier concentration. In this review, recent progress in the design of surface defects with the classified anion vacancy, cation vacancy, vacancy associates, pits, distortions, and disorder on 2D photocatalysts to boost the photocatalytic performance is summarized. The strategies for controlling defects formation and technique to distinguish various surface defects are presented. The crucial roles of surface defects for photocatalysis performance optimization are proposed and advancement of defective 2D photocatalysts toward versatile applications such as water oxidation, hydrogen production, CO2 reduction, nitrogen fixation, organic synthesis, and pollutants removal are discussed. Surface defect modulated 2D photocatalysts thus represent a powerful configuration for further development toward photocatalysis.

Surface Defect Engineering in 2D Nanomaterials for Photocatalysis

Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

Review on nanoscale Bi-based photocatalysts

Semiconductor-based photocatalytic H2 generation as a direct approach of converting solar energy to fuel is attractive for tackling the global energy and environmental issues but still suffers from low efficiency. Here, we report a MoS2/CdS nanohybrid as a noble-metal-free efficient visible-light driven photocatalyst, which has the unique nanosheets-on-nanorod heterostructure with partially crystalline MoS2 nanosheets intimately but discretely growing on single-crystalline CdS nanorod. This heterostructure not only facilitates the charge separation and transfer owing to the formed heterojunction, shorter radial transfer path, and fewer defects in single-crystalline nanorod, thus effectively reducing the charge recombination, but also provides plenty of active sites for hydrogen evolution reaction due to partially crystalline structure of MoS2 as well as enough room for hole extraction. As a result, the MoS2/CdS nanosheets-on-nanorod exhibits a state-of-the-art H2 evolution rate of 49.80 mmol g–1 h–1 and a...

MoS2/CdS Nanosheets-on-Nanorod Heterostructure for Highly Efficient Photocatalytic H2 Generation under Visible Light Irradiation

The effect of defect on the photocatalytic and photoelectric performance of BiPO4 has been revealed. The bulk defects of BiPO4 such as bismuth vacancies (VBi), oxygen vacancies (VO) and so on were formed during the ball-milling process. These bulk defects of BiPO4 inhibited the separation of photo-generated charges greatly and thus reduced photocatalytic activity. Most of the bulk defects were repaired and the photocatalytic activity of BiPO4 was also recovered mostly via calcination and reflux. Reflux could repair the bulk defects of BiPO4 much better than calcination. The mechanism of the photocatalytic degradation of pollutants over BiPO4 was not changed by defects, and the main oxidation active species of BiPO4 is photo-generated hole.

/pdf/photocatalytic-performance-of-bipo4-nanorods-adjusted-via-3zrqa8gcgg.pdf

Photocatalytic performance of BiPO4 nanorods adjusted via defects

At present, composite photocatalysts containing MoS2 as a co-catalyst and a subjective semiconductor material are usually obtained via various complex reduction methods using NH4MoS4 or Na2MoO4 as a precursor. In this work, a simple method was proposed to synthesize MoS2–TiO2 composite photocatalysts via mechanochemistry using MoS2 as a direct precursor. 4.0% MoS2–TiO2 after ball-milling at 300 rpm for 2 h possessed the maximum photocatalytic activity for H2 evolution. The rate of H2 evolution was up to 150.7 μmol h−1, which was 48.6 times higher than that of pure TiO2. The MoS2–TiO2 composite possesses stable photocatalytic performance for H2 evolution. Photoelectrochemical measurements confirmed the electronic interaction between TiO2 and MoS2. The photo-generated electrons on the conduction band of TiO2 could easily transfer to the MoS2 co-catalyst, which promoted efficient charge separation and improved the photocatalytic performance.

Photocatalytic H2 evolution on MoS2–TiO2 catalysts synthesized via mechanochemistry

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Photocatalytic performance of BiPO4 nanorods adjusted via defects

Photocatalytic H2 evolution on MoS2–TiO2 catalysts synthesized via mechanochemistry