Photocatalytic water-splitting for hydrogen generation by sunlight provides a new route to address the energy and environmental problems. In recent years, tremendous efforts have been devoted to design highly efficient water-splitting photocatalysts. Adequate light absorption, effective photogenerated carrier separation, and sufficiently large overpotentials for water redox are crucial in achieving high solar-to-hydrogen (STH) efficiency. These parameters thus strongly influence the design of novel photocatalytic materials. Two-dimensional (2D) photocatalysts have flourished because of the large specific surface area ratio, short carrier migration distance compared to bulk photocatalysts, enormous design flexibility via van der Waals heterostrucutre (HS) engineering and many other unique capabilities that meet the criteria for high-efficiency STH conversion. In this review, we summarize the recent developments of 2D materials and HSs for water-splitting applications from a theoretical perspective. Specifically, we first discuss a number of 2D materials and HSs employed for water-splitting. We review various strategies of material designs to modulate and enhance the photocatalytic performance via improving light harvesting and carrier separation, such as the introduction of defects, dopants and the application of strain, external electric field, rotation angles, ferroelectric switching. We then discuss the methods to evaluate hydrogen evolution reaction, oxygen evolution reaction, and STH efficiency. Finally, the opportunities and challenges of designing 2D materials and HSs for water-splitting are presented.

Thermal cracking of a high density hydrocarbon fuel, JP-10 (exo-tetrahydrodicyclopentadiene), was studied on a batch reactor under different pressures. The effluent was cooled and collected at room temperature and atmospheric pressure. The gaseous and liquid components were quantitatively determined by gas chromatography and gas chromatography−mass spectrometry, respectively. The conversion of JP-10 has relatively low value at atmospheric pressure and increases under pressure. With an increase of the pressure, the relative content of ethene or propene decreases and that of methane, ethane, or propane increases simultaneously. In the liquid products, cyclopentane, cyclopentene, 1,3-cyclopentadiene, and cis-bicyclo[3.3.0]oct-2-ene are found to be major components. Substituted cyclopentene, benzene, toluene, and naphthalene are also observed under high pressures and temperatures. A probable mechanism of the thermal cracking of JP-10 is proposed to explain the product distribution. The process of isomerization might be dominating for liquid product formation during the thermal cracking under elevated pressure.

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Coking of three model compounds of hydrocarbon fuelsn-heptane, cyclohexane, and tricyclo[5.2.1.0 2.6 ]decane (JP-10)sduring their thermal cracking processes under supercritical condition (873.15 K, 4.1 MPa) has been investigated. The product distributions of the thermal cracking are analyzed by gas chromatography-mass spectrometry (GC-MS). The morphology and microstructures of the cokes are characterized by the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The results show that chemical structures play important roles in the thermal stability and coking property of the fuels. The thermal cracking conversion of n-heptane is highest, and the coke yield of JP-10 is highest under the same conditions. It is interestingly observed that the morphologies of the cokes produced from the thermal cracking of three fuels are quite different, which from n-heptane, cyclohexane, and JP-10 are in the forms of carbon nanofilaments, carbon nanotubes, and irregular carbon particles, respectively.