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The shortage of fossil fuels and the disastrous pollution of the environment have led to an increasing interest in artificial photosynthesis. The photocatalytic conversion of CO 2 into solar fuel is believed to be one of the best methods to overcome both the energy crisis and environmental problems. It is of significant importance to efficiently manage the surface reactions and the photo-generated charge carriers to maximize the activity and selectivity of semiconductor photocatalysts for photoconversion of CO 2 and H 2 O to solar fuel. To date, a variety of strategies have been developed to boost their photocatalytic activity and selectivity for CO 2 photoreduction. Based on the analysis of limited factors in improving the photocatalytic efficiency and selectivity, this review attempts to summarize these strategies and their corresponding design principles, including increased visible-light excitation, promoted charge transfer and separation, enhanced adsorption and activation of CO 2 , accelerated CO 2 reduction kinetics and suppressed undesirable reaction. Furthermore, we not only provide a summary of the recent progress in the rational design and fabrication of highly active and selective photocatalysts for the photoreduction of CO 2 , but also offer some fundamental insights into designing highly efficient photocatalysts for water splitting or pollutant degradation. INTRODUCTIONThe shortage of the energy supply and the problem of disastrous environmental pollution have been recognized as two main challenges in the near future [1]. It is a better way to efficiently and inexpensively convert solar energy into chemical fuels by developing an artificial photosynthetic (APS) system because solar fuels are high density energy carriers with long-term storage capacity. The most important and challenging reactions in APS-the photocatalytic water splitting into H 2 and O 2 (water reduction and oxidation) [2][3][4] and photoreduction of CO 2 to solar fuel, such as CH 4 and CH 3 OH [5,6] have been extensively studied since the photocatalytic water splitting on TiO 2 electrodes was discovered by Honda and Fujishima in 1972 [7]. The photocatalytic reduction of CO 2 by means of solar energy has attracted growing attention in the recent years, which 1 State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China 2 College of Science, South China Agricultural University, Guangzhou 510642, China 3 Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia * Corresponding author (email: jiaguoyu@yahoo.com) is also believed to be one of the best methods to overcome both global warming and energy crisis [8]. However, it is also generally thought that photocatalytic CO 2 reduction is a more complex and difficult process than H 2 production due to preferential H 2 production and low selectivity for the carbon species produced [9,10]. The progress achieved in the photoreduction of CO 2 is still far behind that in wate...

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The construction of exceptionally robust and high-quality semiconductor-cocatalyst heterojunctions remains a grand challenge toward highly efficient and durable solar-to-fuel conversion. Herein, novel graphitic carbon nitride (g-CN) nanosheets decorated with multifunctional metallic Ni interface layers and amorphous NiS cocatalysts were fabricated via a facile three-step process: the loading of Ni(OH) nanosheets, high-temperature H reduction, and further deposition of amorphous NiS nanosheets. The results demonstrated that both robust metallic Ni interface layers and amorphous NiS can be utilized as electron cocatalysts to markedly boost the visible-light H evolution over g-CN semiconductor. The optimized g-CN-based photocatalyst containing 0.5 wt % Ni and 1.0 wt % NiS presented the highest hydrogen evolution of 515 μmol g h, which was about 2.8 and 4.6 times as much as those obtained on binary g-CN-1.0%NiS and g-CN-0.5%Ni, respectively. Apparently, the metallic Ni interface layers play multifunctional roles in enhancing the visible-light H evolution, which could first collect the photogenerated electrons from g-CN, and then accelerate the surface H-evolution reaction kinetics over amorphous NiS cocatalysts. More interestingly, the synergetic effects of metallic Ni and amorphous NiS dual-layer electron cocatalysts could also improve the TEOA-oxidation capacity through upshifting the VB levels of g-CN. Comparatively speaking, the multifunctional metallic Ni layers are dominantly favorable for separating and transferring photoexcited charge carriers from g-CN to amorphous NiS cocatalysts owing to the formation of Schottky junctions, whereas the amorphous NiS nanosheets are mainly advantageous for decreasing the thermodynamic overpotentials for surface H-evolution reactions. It is hoped that the implantation of multifunctional metallic interface layers can provide a versatile approach to enhance the photocatalytic H generation over different semiconductor-cocatalyst heterojunctions.

In this report, the CdS nanorods/g-C 3 N 4 heterojunctions loaded by noble-metal-free NiS cocatalyst were firstly fabricated by in situ hydrothermal method. The as-synthesized heterostructured photocatalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution 10 transmission electron microscopy, UV-visible spectroscopy, nitrogen absorption, photoluminescence (PL) spectra, transient photocurrent responses and electrochemical impedance spectroscopy (EIS) measurements. Their photocatalytic activity for hydrogen production was evaluated using an aqueous solution containing triethanolamine under visible light (λ≥ 420 nm). The results clearly demonstrated that the ternary hybridization of NiS cocatalyst, 1D CdS nanorods and 2D g-C 3 N 4 nanosheets is a 15 promising strategy to achieve highly efficient visible-light-driven photocatalytic H 2 evolution. Among all the photocatalysts employed, the ternary hybrid g-C 3 N 4 -CdS-9%NiS composite materials show the best photocatalytic performance with a H 2 -production rate of 2563 umol h -1 g -1 , which is 1582 times higherthan that of the pristine g-C 3 N 4 . The enhanced photocatalytic activity was ascribed to the combined effects of NiS cocatalyst loading and the formation of the intimate nanoheterojunctions between 1D CdS nanorods 20 and 2D g-C 3 N 4 nanosheets, which was favorable for promoting charge transfer, improving separation efficiency of photoinduced electron-hole pairs from bulk to interfaces and accelerating the surface H 2evolutioth kinetics. This work would not only provide a promising photocatalyst candidate for applications in visible-light H 2 generation, but also offer a new insight into the construction of highly efficient and stable g-C 3 N 4 -based hybrid semiconductor nanocomposites for diverse photocatalytic 25 applications.

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In this work, robust nanocarbons, including graphite (G), carbon nanotube (CNT), reduced graphene oxide (rGO), carbon black (CB), and acetylene black (AB), have been successfully coupled into the interfaces between g-C3N4 and NiS using a facile precipitation method. The results demonstrated that nanocarbons played trifunctional roles in boosting the photocatalytic H2 evolution over g-C3N4, which can not only act as effective H2-evolution co-catalysts but can also serve as conductive electron bridges to collect photogenerated electrons and boost the H2-evolution kinetics over the NiS co-catalysts. More interestingly, the nanocarbons can also result in the downshift of valence band of g-C3N4, thus facilitating the fast oxidation of triethanolamine and charge-carrier separation. Particularly, in all five ternary multiheterostructured systems, the g-C3N4-0.5%CB-1.0%NiS (weight ratio) and g-C3N4-0.5%AB-1.0%NiS photocatalysts exhibited the highest H2-evolution rates of 366.4 and 297.7 μmol g–1 h–1, which are 3.17 and 2.57 times higher than that of g-C3N4-1.0%NiS, respectively. Apparently, the significantly enhanced H2-evolution activity of multiheterostructured g-C3N4/carbon/NiS composite photocatalysts can be mainly ascribed to the trifunctional nanocarbons, which serve as the conductive electron bridges rather than the general co-catalysts. More importantly, it is revealed that the amorphous carbons with higher electrical conductivity and weaker electrocatalytic H2-evolution activity are more suitable interfacial bridges between g-C3N4 and NiS co-catalysts for maximizing the H2 generation. This work may give a new mechanistic insight into the development of multiheterostructured g-C3N4-based composite photocatalysts using the combination of trifunctional nanocarbon bridges and earth-abundant co-catalysts/semiconductors for various photocatalytic applications.