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Bond regulation not only makes the interlayer spacing larger, but also gives MoSe2+x a double reaction mechanism combining intercalation and conversion reactions. Consequently, the capacity and energy density of MoSe2+x can be greatly improved.

Global warming and energy shortage are two major stumbling blocks on the road of human social progress, and have gradually aroused a sense of crisis. Artificial photosynthesis is a two-pronged solution to both problems. However, due to the harsh reaction conditions and low efficiency, traditional semiconductors cannot achieve this. Two-dimensional (2D) materials with larger specific surface areas, lower carrier migration distances, more active surface atoms, and higher elastic strain tolerance play a critical role in the solar to chemical energy conversion scheme, and provide a novel methodology for the synthesis of fine chemicals. This review details the principles of photocatalytic reduction of CO2, and highlights the reduction pathways and product selectivity via experimental methods and theoretical calculations. The state-of-the-art achievements of 2D materials in the field of photocatalytic reduction of CO2 are summarized, mainly including material structure, characteristics, and modification strategies to improve the performance of CO2 reduction. And the research on the combination of 2D materials and single atoms is emphasized. Moreover, bottlenecks and challenges in the design and application of 2D materials, as well as prospects of the future development direction, will be highlighted in order to seek new breakthroughs by exploring new materials design solutions.

The ultrathin thickness and lateral morphology of a two dimensional (2D) MoS2 nanosheet contribute to its high surface-to-volume ratio and short diffusion path, rendering it a brilliant electrode material for lithium-ion batteries (LIBs). However, the low conductivity and easy restacking character of the pure MoS2 nanosheet during extended cycling result in severe capacity fading and poor cycling performance. In this work, we developed an attractive strategy by using a metal-doping method to engineer chemical, physical and electronic properties of MoS2, achieving an outstanding performance in LIBs. The computational results show that V–Mo–S has semimetallic properties. Semimetallic vanadium molybdenum sulfide nanoarrays (V–Mo–S NAs) were prepared to overcome the low conductivity of semiconducting MoS2 and thus further optimize its performance in LIBs. A reversible capacity as high as 1047 mA h g−1 was achieved at 1000 mA g−1. It also displayed an excellent stability even after 700 cycles. This fascinating study may pave a way for utilizing semimetallic material-based nanomaterials for batteries.

Studies on the removal of As(III) by Fe-based materials have been carried out for decades, but the time-consuming process and low removal capacity are obstacles for large-scale practical applications. Here, a rapid and efficient technique was proposed for the removal of As(III) using Cu-doped Fe@Fe2O3 core–shell nanoparticles (CFF) synthesized by a facile two-step reduction method and aging process, which realized a thorough removal of As(III) from smelting wastewater at neutral pH within 30 min. The copper doped in CFF not only provided two extra oxygen reduction pathways to enhance the molecular oxygen activation, but also improved the electron transfer ability and removal efficiency of As(III). The existence of copper contributed to the rapid oxidization and adsorption of As(III), and the removal rate increased nearly 10-times in the aerobic system. Meanwhile, the proposed Cu-doped Fe@Fe2O3 core–shell nanoparticles and shifted oxygen reduction pathway could be easily scaled up for other transition metals, such as Ni. Molecular dynamics (MD) simulations based on the large-scale atomic/molecular massively parallel simulator (LAMMPS) were also employed to investigate the formation process of CFF. Furthermore, the removal efficiency of arsenic in smelting wastewater remained to be 90% after 6 times of cycling. Therefore, the distinctive oxidation activities of CFF hold great promise for applications in arsenic removal.

Molybdenum disulfide (MoS2) is a promising non-precious-metal catalyst, but its performance is limited by its density of active sites and poor electrical transport. Here, we report the design and preparation of cracked monolayer 1T MoS2 with a porous structure through the ultrasonication enhanced lithium intercalation of hydrothermally synthesized MoS2 nanosheets. The unique resulting catalyst can have more active sites introduced via the formation of porosity within the monolayer nanosheet, and the electrical transport ability can be increased through the change in electronic states from semiconducting in the 2H phase to metallic in the 1T phase. As is expected, the cracked monolayer 1T MoS2 exhibited good durability and an excellent hydrogen evolution reaction performance with a low overpotential (at 10 mA cm−2) of 156 mV (V vs. RHE) in acid media and a small Tafel slope of 42.7 mV dec−1. This work will provide an intriguing and effective approach to designing electrocatalysts based on MoS2 or other layered materials with enhanced HER performance.

It is challenging to prepare monolayer MoS2 with activated basal planes in a simple and efficient way. In this study, an interlayer spacing expanded counterpart, ammonia-intercalated MoS2, was obtained by a simple hydrothermal reaction of ammonium molybdate and elemental sulfur in hydrazine monohydrate solution. Then, the ammonia-intercalated MoS2 could be easily exfoliated by ultrasonication to get monolayer MoS2. Importantly, this monolayer MoS2 possessed rich S vacancies. The produced MoS2 demonstrated a proliferated active site density as well as low-loss electrical transport for efficient electrochemical hydrogen production from water. As expected, the monolayer MoS2 with S vacancies exhibited an excellent electrocatalytic hydrogen evolution reaction performance with a low overpotential (at 10 mA cm−2) of 160 mV (V vs. RHE) in acid media and a small Tafel slope of 54.9 mV dec−1. Furthermore, the catalyst displayed a good long-term stability and chemical stability during the electrochemical hydrogen production process. Computational studies prove that the S vacancies enabled the inert basal planes by introducing localized donor states into the bandgap and lowered the hydrogen adsorption free energy. This study could open new opportunities for the rational design and a better understanding of structure–property relationships of MoS2-based catalysts for water splitting or other applications.

Three-dimensional (3D) network-like graphitic carbon nitride nanobelts (g-C3N4 NBs) were facilely achieved by the hydrothermal treatment of bulk g-C3N4 in a medium strong oxalic acid solution (1 M, pH 0.89). The positions of the conduction band (CB) and valence band (VB) were upraised from −0.90 and +1.86 eV for bulk g-C3N4 to −0.92 and +1.92 eV for g-C3N4 NB networks with enhanced redox ability, respectively. With an optimized Pt loading of 3%, the g-C3N4 NB networks showed excellent visible-light photocatalytic H2 production activity (1360 μmol g−1 h−1), which was 10.9 times higher than that of optimized 2% Pt@bulk g-C3N4 (124.7 μmol g−1 h−1) using triethanolamine as a sacrificial agent. Furthermore, Pt@g-C3N4 NBs exhibited a considerable rate of H2 evolution of 33.3 μmol g−1 h−1, much higher than 1.79 μmol g−1 h−1 for Pt@bulk g-C3N4 in distilled water without any sacrificial agents, revealing a great potential for photocatalytic overall water splitting. This outstanding performance not only originates from its unique 3D nanostructure and prolonged electron lifetime, but also from the electronic structure modulation and improved redox capacities of the CB and VB. The pH effect of hydrothermal conditions on the g-C3N4 molecular structure, chemical elements, optical properties and catalytic performance is also expounded. This study demonstrates a facile and environmentally friendly strategy to design highly efficient g-C3N4 catalysts for potential applications in solar energy driven photocatalytic water splitting.