The significance of photocatalysts is unquestionable, and scientists are devoted to improving their photocatalytic efficiency. To solve the high recombination rates of photogenerated electron-hole pairs and their low reduction and oxidation abilities in a single photocatalyst, heterojunction manipulation is urgently required. Two mainstream heterojunctions-type-II and Z-scheme heterojunctionshave been widely acknowledged. However, we soberly reflect the charge-transfer mechanism from many perspectives and are finally aware of the fundamental challenges they face. To ensure a correct understanding, it is necessary to share our analysis with others. Moreover, step-scheme (S-scheme) heterojunctions, consisting of a reduction photocatalyst and an oxidation photocatalyst with staggered band structure, are introduced to avoid misinterpretation. The differences in the charge-transfer mechanism between S-scheme, type-II, and Z-scheme heterojunctions are highlighted. Finally, limitations and the future research direction of S-scheme photocatalysts are discussed.

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strategies in addressing the low-efficiency issue will be discussed.Photocatalysis mainly deals with electron and energy transfer processes. Prior to the discussion, it is necessary to learn the basic behaviors of the excited state of a molecule, which can enhance the understanding on electron transfer and energy dissipation in semiconductors. Generally, the first process is the absorption of a photon by a molecule (in the time scale of femtoseconds (fs)), where the ground state is lifted energetically to the first excited singlet state. Subsequently, a simplified Jablonski diagram is reconsidered, [2] as shown in Figure 1. The charge carriers generated in one molecule experience several possible processes with varying possibilities: [3] a) Vibrational relaxation (VR): it is relaxation of excited state electrons to the lowest energy level which generally occurs in picoseconds (ps). It can happen from each excited state to each non-excited state including the ground state. This VR results in loss of energy because excessive vibrational energy is converted into heat. b) Fluorescence: After the relaxation to the lowest vibrational level, the excited molecule can finally get back to the ground state by emitting a photon. This is named as fluorescence, which occurs in a relatively long time, ranging from ps to nanoseconds (ns). c) Internal conversion (IC): it is a crossover process in which an electronically excited molecule moves from one electronic state to a lower one of the same multiplicity (singlet-to-singlet or triplet-to-triplet states) and can be measured from ps to fs. [4] d) Intersystem crossing (ISC): A transition from one electronic state to another one with a different spin multiplicity is called ISC. e) Phosphorescence: After the molecule transitions through ISC to the triplet state, further deactivation occurs through phosphorescence. And its lifetime ranges from one millisecond (ms) to hundreds of seconds. Apart from these, other processes such as vibrational cooling are also possible, which are not illustrated here.The above-mentioned processes in a single molecule can be used as a reference when discussing a semiconductor photocatalyst. Analogously, electrons are excited to the conduction band (CB) after absorption. Afterward, they will undergo several decay processes or they will finally migrate to the surface and participate in a specific redox reaction. These decay processes are briefly introduced here by comparing with those in a molecule (Figure 1): a) Relaxation of electrons to the lowest CB energy states. b) Radiative recombination of electrons and holes via emitted as fluorescence; c) Non-radiative decay, also referred to as Photocatalysis is a green technology to use ubiquitous and intermittent sunlight. The emerging S-scheme heterojunction has demonstrated its superiority in photocatalysis. This article covers the state-of-the-art progress and provides new insights into its general designing criteria. It starts with the challenges confronted by single photocatalyst from the perspective of...

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became a Professor at Wuhan University of Technology. His current research interests include semiconductor photocatalysis, photocatalytic hydrogen production, CO 2 reduction to hydrocarbon fuels, and so on.such as noble metals, non-noble metals, metal oxides (e.g., CuO and NiO), [28,40] metal hydroxides, metal sulfides, metal phosphides (e.g., Co 2 P and Ni 2 P) [53,54] and carbonaceous materials. By contrast, some transition metal oxides (e.g., MnO x , CoO x , and Fe 2 O 3 ) [12,22,64] and phosphide (e.g., Co-Pi) [65] are used as oxidation cocatalysts for photocatalytic oxygen evolution reaction and photocatalytic degradation of pollutants.

Photocatalytic CO2 reduction is an effective way to simultaneously mitigate the greenhouse effect and the energy crisis. Herein, CdS hollow spheres, on which monolayer nitrogen‐doped graphene is in situ grown by chemical vapor deposition, are applied for realizing effective photocatalytic CO2 reduction. The constructed photocatalyst possesses a hollow interior for strengthening light absorption, a thin shell for shortening the electron migration distance, tight adhesion for facilitating separation and transfer of carriers, and a monolayer nitrogen‐doped graphene surface for adsorbing and activating CO2 molecules. Achieving seamless contact between a photocatalyst and a cocatalyst, which provides a pollution‐free and large‐area transport interface for carriers, is an effective strategy for improving the photocatalytic CO2 reduction performance. Therefore, the yield of CO and CH4, as dominating products, can be increased by four and five times than that of pristine CdS hollow spheres, respectively. This work emphasizes the importance of contact interface regulation between the photocatalyst and the cocatalyst and provides new ideas for the seamless and large‐area contact of heterojunctions.

Recently, a novel step-scheme (S-scheme) heterojunction was proposed and has attracted researchers' attention. [23][24][25][26][27][28][29][30][31][32][33][34][35][36] Usually, S-scheme heterojunction consists of reduction photocatalyst (RP) and oxidation photocatalyst (OP). Besides, the directional migration of free electrons will lead to band bending and internal electric field (IEF) at their interface owing to the work function difference. Notably, under the influence of IEF, the photogenerated electrons of OP with weak reduction ability can recombine with the photogenerated holes of RP with weak oxidation ability; while those with strong redox abilities are preserved. Therefore, reasonable construction of TiO 2 -based S-scheme heterojunction is of great significance to improve photocatalytic reaction performance. Apart from the charge carrier separation, the morphology of photocatalysts is another important factor to influence the photocatalytic performance. [15,17,19,37,38] Photocatalysts with hollow structures have attracted great attention owing to manifold advantages including larger specific surface area, abundant active sites, shortened diffusion distance as well as improved light reflection and scattering. [37,[39][40][41][42][43][44] Therefore, the design of hollow S-scheme heterojunction photocatalyst is of vital importance to enhance photocatalytic performance.ZnIn 2 S 4 , as a typical reduction photocatalyst, stands out for its layered structure, narrow bandgap, suitable redox potentials, and good chemical stability. And it has been used for various photocatalytic applications including hydrogen production, CO 2 reduction, and organic degradation. [45][46][47][48][49][50][51] Unfortunately, pristine ZnIn 2 S 4 photocatalyst shows low photocatalytic efficiency owing to the fast recombination of photogenerated charge carriers. [45][46][47][48][49] Considering the suitable match of band gap of ZnIn 2 S 4 and TiO 2 for S-scheme heterojunction, [27,52] we construct the hollow TiO 2 @ZnIn 2 S 4 core-shell structure. Up to now, to the best of our knowledge, it has never been reported.Herein, we grow ZnIn 2 S 4 nanosheets on the outer surface of TiO 2 hollow spheres by in situ chemical bath deposition reaction. This rational design is not only able to provide large specific surface areas and abundant reaction sites for PCR reaction, but also can effectively suppress the recombination of useful photogenerated electrons and holes. As a result, the optimized TiO 2 @ZnIn 2 S 4 heterojunction exhibits high PCR performance, and the total CO 2 photoreduction conversion rates (the sum yield of CO, CH 3 OH and CH 4 ) are obviously higher than those of blank ZnIn 2 S 4 , TiO 2 , and ex situ prepared TiO 2 -ZnIn 2 S 4 composite. Finally, S-scheme mechanism is also thoroughly analyzed and discussed in this work.Reasonable design of efficient hierarchical photocatalysts has gained significant attention. Herein, a step-scheme (S-scheme) core-shell TiO 2 @ZnIn 2 S 4 heterojunction is designed for photocatalytic CO 2 redu...

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