Solar driven catalysis on semiconductors to produce clean chemical fuels, such as hydrogen, is widely considered as a promising route to mitigate environmental issues caused by the combustion of fossil fuels and to meet increasing worldwide demands for energy. The major limiting factors affecting the efficiency of solar fuel synthesis include; (i) light absorption, (ii) charge separation and transport and (iii) surface chemical reaction; therefore substantial efforts have been put into solving these problems. In particular, the loading of co-catalysts or secondary semiconductors that can act as either electron or hole acceptors for improved charge separation is a promising strategy, leading to the adaptation of a junction architecture. Research related to semiconductor junction photocatalysts has developed very rapidly and there are a few comprehensive reviews in which the strategy is discussed (A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253–278, K. Li, D. Martin, and J. Tang, Chinese Journal of Catalysis, 2011, 32, 879–890, R. Marschall, Advanced Functional Materials, 2014, 24, 2421–2440). This critical review seeks to give an overview of the concept of heterojunction construction and more importantly, the current state-of-the art for the efficient, visible-light driven junction water splitting photo(electro)catalysts reported over the past ten years. For water splitting, these include BiVO₄, Fe₂O₃, Cu₂O and C₃N₄, which have attracted increasing attention. Experimental observations of the proposed charge transfer mechanism across the semiconductor/semiconductor/metal junctions and the resultant activity enhancement are discussed. In parallel, recent successes in the theoretical modelling of semiconductor electronic structures at interfaces and how these explain the functionality of the junction structures is highlighted.

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low‐cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g‐C₃N₄) from a low‐cost precursor, urea, is reported. The g‐C₃N₄ exhibits an extraordinary hydrogen‐evolution rate (ca. 20 000 μmol h⁻¹ g⁻¹ under full arc), which leads to a high turnover number (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5 % under visible light, which is nearly an order of magnitude higher than that observed for any other existing g‐C₃N₄ photocatalysts. Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen‐evolution rate is significantly enhanced.

Graphene-based materials have generated tremendous interest in a wide range of research activities. A wide variety of graphene related materials have been synthesised for potential applications in electronics, energy storage, catalysis, and gas sorption, storage, separation and sensing. Recently, gas sorption, storage and separation in porous nanocarbons and metal–organic frameworks have received increasing attention. In particular, the tuneable porosity, surface area and functionality of the lightweight and stable graphene-based materials open up great scope for those applications. Such structural features can be achieved by the design and control of the synthesis routes. Here, we highlight recent progresses and challenges in the syntheses of graphene-based materials with hierarchical pore structures, tuneable high surface area, chemical doping and surface functionalization for gas (H2, CH4, CO2, N2, NH3, NO2, H2S, SO2, etc.) sorption, storage and separation

Contemporary science is witnessing a rapid expansion of the two-dimensional (2D) materials family, each member possessing intriguing emergent properties of fundamental and practical importance. Using the particle-swarm optimization method in combination with first-principles density functional theory calculations, here we predict a new category of 2D monolayers named tellurene, composed of the metalloid element Te, with stable $1\mathrm{T}\text{−}\mathrm{Mo}{\mathrm{S}}_2$-like ($\alpha \text{−}\mathrm{Te}$), and metastable tetragonal ($\beta \text{−}\mathrm{Te}$) and $2\mathrm{H}\text{−}\mathrm{Mo}{\mathrm{S}}_2$-like ($\gamma \text{−}\mathrm{Te}$) structures. The underlying formation mechanism is inherently rooted in the multivalent nature of Te, with the central-layer Te behaving more metal-like (e.g., Mo), and the two outer layers more semiconductorlike (e.g., S). We also show that the $\alpha \text{−}\mathrm{Te}$ phase can be spontaneously obtained from the magic thicknesses divisible by three layers truncated along the [001] direction of the trigonal structure of bulk Te, and both the $\alpha$- and $\beta \text{−}\mathrm{Te}$ phases possess electron and hole mobilities much higher than ${\mathrm{MoS}}_2$. Furthermore, we present preliminary but convincing experimental evidence for the layering behavior of Te on HOPG substrates, and predict the importance of multivalency in the layering behavior of Se. These findings effectively extend the realm of 2D materials to group-VI elements.

Bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are highly desirable for rechargeable metal–air batteries and regenerative fuel cells. However, the commercial oxygen electrocatalysts (mainly noble metal based) can only exhibit either ORR or OER activity and also suffer from inherent cost and stability issues. It remains challenging to achieve efficient ORR and OER bifunctionality on a single catalyst. Metal-free structures offer relatively large scope for this bifunctionality to be engineered within one catalyst, together with improved cost-effectiveness and durability. Herein, by closely coupled computational design and experimental development, highly effective bifunctionality was achieved in a phosphorus and nitrogen co-doped graphene framework (PNGF) – with both ORR and OER activities reaching the theoretical limits of metal-free catalysts, superior to their noble metal counterparts in both (bi)functionality and durability. In particular, with the identification of active P–N sites for OER and N-doped sites for ORR, we successfully intensified these sites by one-pot synthesis to tailor the PNGF. The resulting catalyst achieved an ORR potential of 0.845 V vs. RHE at 3 mA cm⁻² and an OER potential of 1.55 V vs. RHE at 10 mA cm⁻². Its combined ORR and OER overpotential of 705 mV is much lower than those previously reported for metal-free bifunctional catalysts.