Semiconductor photocatalysts have received much attention in recent years due to their great potentials for the development of renewable energy technologies, as well as for environmental protection and remediation. The effective harvesting of solar energy and suppression of charge carrier recombination are two key aspects in photocatalysis. The formation of heterostructured photocatalysts is a promising strategy to improve photocatalytic activity, which is superior to that of their single component photocatalysts. This Feature Article concisely summarizes and highlights the state-of-the-art progress of semiconductor/semiconductor heterostructured photocatalysts with diverse models, including type-I and type-II heterojunctions, Z-scheme system, p–n heterojunctions, and homojunction band alignments, which were explored for effective improvement of photocatalytic activity through increase of the visible-light absorption, promotion of separation, and transportation of the photoinduced charge carries, and enhancement of the photocatalytic stability.

Photocatalytic reduction of CO₂ into hydrocarbon fuels, an artificial photosynthesis, is based on the simulation of natural photosynthesis in green plants, whereby O₂ and carbohydrates are produced from H₂O and CO₂ using sunlight as an energy source. It couples the reductive half-reaction of CO₂ fixation with a matched oxidative half-reaction such as water oxidation, to achieve a carbon neutral cycle, which is like killing two birds with one stone in terms of saving the environment and supplying future energy. The present review provides an overview and highlights recent state-of-the-art accomplishments of overcoming the drawback of low photoconversion efficiency and selectivity through the design of highly active photocatalysts from the point of adsorption of reactants, charge separation and transport, light harvesting, and CO₂ activation. It specifically includes: i) band-structure engineering, ii) nanostructuralization, iii) surface oxygen vacancy engineering, iv) macro-/meso-/microporous structuralization, v) exposed facet engineering, vi) co-catalysts, vii) the development of a Z-scheme system. The challenges and prospects for future development of this field are also present.

In the past few decades, some novel low-cost nanostructured devices have been explored for converting solar energy into electrical or chemical energy, such as organic photovoltaic cells, photoelectrochemical solar cells, and solar water splitting cells. Generally, higher light absorption and/or charge separation efficiency are considered as the main reasons for improved performance in a nanostructured device versus a planar structure. However, quantitative analysis and definite experimental evidence remain elusive. Here, using BiVO₄ as an example, comparable samples with porous and dense structures have been prepared by a simple method. The porous and dense films are assembled into a solid-electrolyte bulk and planar heterojunction, respectively. Some quantitative results are obtained by decoupling photon absorption, interfacial charge transfer, and charge separation processes. These results suggest that higher charge separation efficiency is mainly responsible for enhanced performance in a solid-electrolyte bulk heterojunction. Moreover, we also present visualized evidence to show higher charge separation efficiency comes from a shorter photo-generated hole diffusion distance in a bulk heterojunction. These results can deepen understanding charge transfer in a bulk heterojunction and offer guidance to design a more efficient low-cost device for solar conversion and storage.

As global energy demand continues to grow, the need to find a carbon-neutral and sustainable energy source for future generations has become imperative. An especially attractive solution is to store solar energy in the form of chemical fuel via artificial photosynthesis to convert carbon dioxide into hydrocarbons. An artificial photosynthesis system is introduced based on a zinc gallogermanate solid solution photocatalyst that can convert the carbon dioxide and water into methane. The solid solution of cubic spinel ZnGa₂O₄ and pseudocubic inverse spinel Zn₂GeO₄ is successfully synthesized by hydrothermal ion exchange reaction. Introducing Zn₂GeO₄ into ZnGa₂O₄ can effectively narrow band gap by the upshift of valence band edge from the enhanced p-d (O2p-Zn3d) repulsion effect by incorporating s and p orbitals of Ge, and the downshift of conduction band edge by introducing the low-energy s orbital of Ge. The zinc gallogermanate solid solution has a light-hole effective mass, which is beneficial to improving hole mobility, and thus enhancing the ability of photocatalyst in water oxidation to provide protons for CO₂ photoreduction. As a result of band gap narrowing and high hole mobility, the zinc gallogermanate solid solution exhibits high activity in converting CO₂ and H₂O into CH₄ and O₂.

A novel, in situ simultaneous reduction-hydrolysis technique (SRH) is developed for fabrication of TiO₂--graphene hybrid nanosheets in a binary ethylenediamine (En)/H₂O solvent. The SRH technique is based on the mechanism of the simultaneous reduction of graphene oxide (GO) into graphene by En and the formation of TiO₂ nanoparticles through hydrolysis of titanium (IV) (ammonium lactato) dihydroxybis, subsequently in situ loading onto graphene through chemical bonds (Ti–O–C bond) to form 2D sandwich-like nanostructure. The dispersion of TiO₂ hinders the collapse and restacking of exfoliated sheets of graphene during reduction process. In contrast with prevenient G-TiO₂ nanocomposites, abundant Ti³⁺ is detected on the surface of TiO₂ of the present hybrid, caused by reducing agent En. The Ti³⁺ sites on the surface can serve as sites for trapping photogenerated electrons to prevent recombination of electron–hole pairs. The high photocatalytic activity of G-TiO₂ hybrid is confirmed by photocatalytic conversion of CO₂ to valuable hydrocarbons (CH₄ and C₂H₆) in the presence of water vapor. The synergistic effect of the surface-Ti³⁺ sites and graphene favors the generation of C₂H₆, and the yield of the C₂H₆ increases with the content of incorporated graphene. The work may open a new doorway for new significant application of graphene for selectively catalytic C–C coupling reaction

Graphene-semiconductor nanocomposites, considered as a kind of most promising photocatalysts, have shown remarkable performance and drawn significant attention in the field of photo-driven chemical conversion using solar energy, due to the unique physicochemical properties of graphene. The photocatalytic enhancement of graphene-based nanocomposites is caused by the reduction of the recombination of electron-hole pairs, the extension of the light absorption range, increase of absorption of light intensity, enhancement of surface active sites, and improvement of chemical stability of photocatalysts. Recent progress in the photocatalysis development of graphene-based nanocomposites is highlighted and evaluated, focusing on the mechanism of graphene-enhanced photocatalytic activity, the understanding of electron transport, and the applications of graphene-based photocatalysts on water splitting, degradation or oxidization of organic contaminants, photoreduction of CO₂ into renewable fuels, toxic elimination of heavy metal ions, and antibacterial applications.

To realize practical applications of the photocatalysis technique, it is necessary to synthesize semiconductor photocatalysts with specific facets that induce high reactive activities and high reactive selectivity. However, a challenge lies in the synthesis of metal oxides containing more than one type of metal with specific facets. Usually, surfactants are used to control the crystal morphology, which induces surface contamination for the final products. Here, using the GaOOH nanoplate as precursor, ZnGa₂O₄ nanocubes with exposed {100} facets are synthesized by an hydrothermal ion-exchange route without requiring the introduction of morphology controlling agents. These ZnGa₂O₄ nanocubes exhibit improved performance in the photoreduction of CO₂ into CH₄ or water splitting into hydrogen. Theoretical calculations indicates that the light-hole effective mass on the {100} facets of ZnGa₂O₄ corresponds to the high hole mobility, which contributes to the efficient water oxidation to offer the protons for promoting CO₂ photoreduction into hydrocarbon fuels.

The photoreduction of water to hydrogen represents a promising method for generating sustainable clean fuel. The molecular processes of this photoreduction require an effective light absorber, such as the ruthenium polybipyridine complex, to collect and convert the solar energy into a usable chemical form. In the search for a highly active and stable photosensitizer (PS), iridium complexes are attractive because of their desirable photophysical characteristics. Herein, a series of homoleptic tris-cyclometalated iridium complexes, based on different 2-phenylpyridine ligands, were utilized as PSs in photocatalytic systems for hydrogen production with [Rh(dtb-bpy)₃](PF₆)₃ (dtb-bpy=4,4′-di-tert-butyl-2,2′-dipyridyl) serving as the water reduction catalyst (WRC) and triethanolamine (TEOA) as the electron donor. The photophysical and electrochemical properties of these complexes were systematically investigated. The excited state of neutral iridium complexes (PS*) could not be quenched by using TEOA as an electron donor, but they could be quenched by using [Rh(dtb-bpy)₃](PF₆)₃ as an electron acceptor, indicating that the PS* quenching pathway in catalytic reactions was most likely an oxidative quenching process. A set of long-lived and highly active systems for hydrogen evolution were obtained in Ir^III–Rh^III–TEOA systems. These systems maintained their activity for more than 72 h with visible-light irradiation, and the total turnover number was up to 3040. Comparative studies indicated that the photocatalytic performance of these homoleptic tris-cyclometalated iridium compounds was superior to that of the cationic iridium complex [Ir(ppy)₂(bpy)](PF₆) (ppy=2-phenylpyridine, bpy=2,2′-dipyridyl) (4), which was used as a reference. The significant increase in the photocatalytic efficiencies was in part attributed to the higher photostability of the neutral Ir^III complexes. This assumption was supported by their different coordination modes, photophysical, and electrochemical properties.

One-dimensional Fe₂V₄O₁₃ nanoribbons 10–20 μm long and 20–30 nm thick growing directly on a stainless-steel mesh (SSM) have been successfully obtained by a simple and facile hydrothermal reaction without any templates or surfactants. The SSM served as not only the Fe source but also the substrate for the deposition of vanadium and oxide elements. The Fe₂V₄O₁₃ nanoribbon as an easily reused photocatalyst shows great potential for the efficient photoreduction of CO₂ into renewable hydrocarbon fuel (CH₄) and for the removal of organics from air under visible-light illumination.

The development of an efficient and stable artificial photosensitizer for visible-light-driven hydrogen production is highly desirable. Herein, a new series of charge-neutral, heteroleptic tricyclometalated iridium(III) complexes, [Ir(thpy)₂(bt)] (1–4; thpy=2,2′-thienylpyridine, bt=2-phenylbenzothiazole and its derivatives), were systematically synthesized and their structural, photophysical, and electrochemical properties were established. Three solid-state structures were studied by X-ray crystallographic analysis. This design offers the unique opportunity to drive the metal-to-ligand charge-transfer (MLCT) band to longer wavelengths for these iridium complexes. We describe new molecular platforms that are based on these neutral iridium complexes for the production of hydrogen through visible-light-induced photocatalysis over an extended period of time in the presence of [Co(bpy)₃]²⁺ and triethanolamine (TEOA). The maximum amount of hydrogen was obtained under constant irradiation over 72 h and the system could regenerate its activity upon the addition of cobalt-based catalysts when hydrogen evolution ceased. Our results demonstrated that the dissociation of the [Co(bpy)₃]²⁺ catalyst contributed to the loss of catalytic activity and limited the long-term catalytic performance of the systems. The properties of the neutral complexes are compared in detail to those of two known non-neutral bpy-type complexes, [Ir(thpy)₂(dtb-bpy)]⁺ (5) and [Ir(ppy)₂(dtb-bpy)]⁺ (6; ppy=2-phenylpyridine, dtb-bpy=4,4′-di-tert-butyl-2,2′-dipyridyl). This work is expected to contribute toward the development of long-lasting solar hydrogen-production systems.

Robust hollow spheres consisting of molecular-scale alternating titania (Ti_0.91O₂) nanosheets and graphene (G) nanosheets are successfully fabricated by a layer-by-layer assembly technique with polymer beads as sacrificial templates using a microwave irradiation technique to simultaneously remove the template and reduce graphene oxide into graphene. The molecular scale, 2D contact of Ti_0.91O₂ nanosheets and G nanosheets in the hollow spheres is distinctly different from the prevenient G-based TiO₂ nanocomposites prepared by simple integration of TiO₂ and G nanosheets. The nine times increase of the photocatalytic activity of G-Ti_0.91O₂ hollow spheres relative to commercial P25 TiO₂ is confirmed with photoreduction of CO₂ into renewable fuels (CO and CH₄). The large enhancement in the photocatalytic activity benefits from: 1) the ultrathin nature of Ti_0.91O₂ nanosheets allowing charge carriers to move rapidly onto the surface to participate in the photoreduction reaction; 2) the sufficiently compact stacking of ultrathin Ti_0.91O₂ nanosheets with G nanosheets allowing the photogenerated electron to transfer fast from the Ti_0.91O₂ nanosheets to G to enhance lifetime of the charge carriers; and 3) the hollow structure potentially acting as a photon trap-well to allow the multiscattering of incident light for the enhancement of light absorption.

The energetic and electronic properties of N/V-doped and N-V-codoped anatase TiO₂ (101) surfaces are investigated by first-principles calculations, with the aim to elucidate the relationship between the electronic structure and the photocatalytic performance of N-V-codoped TiO₂. Several substitutional and interstitial configurations for the N and/or V impurities in the bulk phase and on the surface are studied, and the relative stability of different doping configurations is compared by the impurity formation energy. Systematic calculations reveal that N and V impurities can be encapsulated by TiO₂ to form stable structures as a result of strong N-V interactions both in the bulk and the surface model. Through analyzing and comparing the electronic structures of different doping systems, the synergistic doping effects are discussed in detail. Based on these discussions, we suggest that N_OV_Ti codoping cannot only narrow the band gap of anatase TiO₂, but also forms impurity states, which are propitious for the separation of photoexcited electron–hole pairs. In the case of N_OV_Ti-codoped TiO₂ (101) surfaces, this phenomenon is especially prominent. Finally, a feasible synthesis route for N_OV_Ti codoping into anatase TiO₂ is proposed.