The issues of global warming and fossil fuel shortage have increased the demand for clean and renewable energy. Many researchers are investigating strategies to produce hydrogen and reduce CO₂ by using solar power. Two-dimensional (2D) materials, such as graphene, graphene derivatives, and transition metal dichalcogenides (TMDs), have been extensively used owing to their extraordinary electronic and optical properties. In this review, we investigate the recent developments in 2D materials for photocatalytic applications involving the hydrogen evolution reaction and CO₂ reduction. The synthesis methods and the photocatalytic properties of TMDs and graphene-based 2D materials are thoroughly discussed. Moreover, a summary of the recently developed 2D nanostructures and devices for solar hydrogen production and CO₂ reduction is presented, and it is revealed that the use of 2D catalyst materials has great potential for commercialization in the near future to help overcome the energy crisis.

Low dimensional transition metal carbide and nitride (MXenes) have been emerging as frontier materials for energy storage and conversion. Ti₃C₂T_x was the first MXenes that discovered and soon become the most widely investigated among the MXenes family. Interestingly, Ti₃C₂T_x exhibits ultrahigh catalytic activity towards the hydrogen evolution reaction. In addition, Ti₃C₂T_x is electronically conductive, and its optical bandgap is tunable in the visible region, making it become one of the most promising candidates for the photocatalytic hydrogen evolution reaction (HER). In this review, we provide comprehensive strategies for the utilization of Ti₃C₂T_x as a catalyst for improving solar-driven HER, including surface functional groups engineering, structural modification, and cocatalyst coupling. In addition, the reaming obstacle for using these materials in a practical system is evaluated. Finally, the direction for the future development of these materials featuring high photocatalytic activity toward HER is discussed.

A highly efficient and facile sensing system made of N-doped graphene quantum dots (N-GQDs)/poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT–PSS) was fabricated by drop coating on interdigitated Au electrodes with high uniformity over a large area of silicon substrate, and was used to sense the vapors of volatile organic compounds (VOCs) for detection at room temperature. An innovative method was developed for the synthesis of N-GQDs through hydrothermal processing of citric acid. The N-GQDs obtained have a N to C atomic ratio of 0.058 and a diameter of 2–7 nm. The nanocomposite sensing system exhibits highly sensitive, selective, rapid, and reversible responses for the detection of 1–1000 ppm VOCs. The N-GQDs/PEDOT–PSS nanocomposite gas sensor has an enhanced methanol sensing property of 13 times the value for pristine PEDOT–PSS in a low concentration of 50 ppm methanol. Moreover, the gas sensor based on the nanocomposite has a fast response (ca. 12 s) and recovery (ca. 32 s) behavior, excellent room temperature selectivity and stability. The methanol-sensing mechanisms of the N-GQDs/PEDOT–PSS nanocomposite gas sensor, based on direct charge transfer and a swelling process, are highlighted.

Transition metal dichalcogenides (TMDs) are promising two-dimensional (2D) materials, and MoS₂ has been specifically utilized in electronic devices and integrated circuits. However, the direct synthesis of MoS₂ on traditional semiconductors, such as silicon, remains challenging due to the hydrophobic surface of nonoxide wafers (e.g., Si, GaAs, and InP). Herein, a novel, facile, reliable, and one-step method for the direct synthesis of single-crystal MoS₂ on a p-Si wafer via hybrid thermolysis is proposed. To demonstrate the applicability of the proposed method, a MoS₂/p-Si heterojunction was fabricated and used for solar-driven hydrogen production. The as-fabricated n-MoS₂/p-Si heterojunction exhibited a benchmark current density of −13.5 ± 1 mA/cm² at 0 V and an onset potential of +0.02 V. This method reliably and efficiently produced high-quality MoS₂ crystals on a wafer scale and is sufficiently simple to overcome the challenges associated with previous approaches. The method developed herein represents a tremendous advancement in the fabrication of 2D electronic devices.