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Photocatalytic reduction of CO 2 is a considerable method to alleviate global warming and energy shortage. In this work, CoAl-LDH/Ni 1 Cu NP nitrogen-doped carbon composites (CA/Ni 1 Cu NP N-C) can efficiently reduce CO 2 into CO, CH 4 , and C 2 H 6 under visible light. In situ characterization techniques and theoretical calculations were employed to investigate the CO 2 evolution pathway and photocatalytic mechanism. The synergistic interaction of Cu nanoparticles and Ni single-atom sites accelerated the photogenerated charge separation and facilitated the adsorption properties of key intermediates. As a result, the yields of CH 4 and C 2 H 6 on CA/Ni 1 Cu NP N-C were achieved up to 35.245 and 25.328 μmol g−1 h−1 with electron selectivity of 40.24% and 50.61%, respectively. This work is instructive for using metal nanoparticles to optimize single-atom catalysts to promote their catalytic activity. [Display omitted] • CoAl-LDH/Ni 1 Cu NP N-C single-atom photocatalysts were synthesized. • The yields of CH 4 and C 2 H 6 were 35.245 (40.24%) and 25.328 (50.61%) μmol g−1 h−1. • The charge transfer mechanism and CO 2 evolution path were investigated. • Synergistic interaction of Cu NP and Ni 1 sites optimized the photocatalytic mechanism. [ABSTRACT FROM AUTHOR]

Graphite carbon nitride (g‐C3N4), as a non‐metallic photocatalyst, possesses special band structure, excellent stability and unique surface properties, which has become a focus of attention for photocatalytic application. However, pure g‐C3N4 is subject to the limit of visible light response, small specific surface area and rapid recombination of photogenerated carriers, which leads to its low photocatalytic efficiency. Due to the unique electronic structure, various quantum dots (QDs) can be applied to modify g‐C3N4 to improve the photocatalytic activity. Thus, we highlight these QDs modified g‐C3N4‐based composites by emphasizing their synthesis, their physicochemical properties as well as their photocatalytic applications in CO2 reduction, hydrogen production and pollutants degradation in this review. At last, critical summaries and some perspectives on the challenges and directions in this research field are also presented. [ABSTRACT FROM AUTHOR]

• S-scheme Bi-BiOCl/MgIn 2 S 4 marigold microflower heterostructure. • 6 % Bi-BiOCl/MgIn 2 S 4 sample behaved the optimal photocatalytic performance for CO 2 photoreduction into CH 4. • Metal Bi acted as the bridge connecting the BiOCl and MgIn 2 S 4 to facilitate the charge transfer. Step-scheme (S-scheme) Bi-BiOCl/MgIn 2 S 4 heterostructure (BBOC/MIS) was successfully fabricated via NaBH 4 reduction and facile in-situ hydrothermal process and applied in photocatalytic CO 2 reduction. A part of BBOC ultrathin nanosheets processed by ultrasonication exfoliation was grown on the surfaces of MIS marigold microflower. Another part of BBOC and MIS exhibited as the core and shell of BBOC/MIS, respectively. The morphology, band alignment, optical and electronic characteristics of photocatalysts were comprehensively studied via diverse characterization methods. The S-scheme BBOC/MIS heterojunction possessed superior photocatalytic performance and well reusability to primarily convert CO 2 into CH 4 with high efficiency under the simulated solar light irradiation without using any sacrificial agent in water. Experimental results displayed that the 6% BBOC/MIS sample behaved the optimal photocatalytic performance in photocatalytic reduction of CO 2 , and its maximum CH 4 production rate could reach up to 4.29 μmol g−1 h−1, which was about 7.56 times as high as that of unadulterated MIS. The enhanced photocatalytic performance of BBOC/MIS could be attributed to the improved visible light response and highly-efficient separation of the photogenerated electron-hole pairs between BBOC and MIS through the S-scheme heterojunction interface. In addition, the metal Bi acted as the bridge connecting the BOC and MIS to facilitate the charge transfer, which greatly promoted the CO 2 photocatalytic reduction to CH 4. [ABSTRACT FROM AUTHOR]

• 20 % BiO 2- x/g-C 3 N 4 composites exhibited the best photocatalytic activity. • BiO 2-x narrowed the band gap and expanded the visible light range. • The separation and migration of photogenerated carriers are promoted. Photoreduction CO 2 to hydrocarbons are the sustainable and green methods to solve energy shortages and environmental pollution problems. Therefore, the development of efficient and environment-friendly photocatalysts remains a huge challenge, while coupling co-catalyst on traditional photocatalysts is a potential approach for improving photocatalytic activity. Here, a novel BiO 2-x co-catalyst coupled g-C 3 N 4 photocatalyst was prepared by a simple hydrothermal approach for the photocatalytic reduction of CO 2. The experimental results proved that BiO 2-x /g-C 3 N 4 composites played a vital part for promoting the photocatalytic capability. The 20 % BiO 2-x /g-C 3 N 4 composites exhibited the best photocatalytic capability under the simulated visible-light. A maximum yield of CO (42.29 μmol/gcat) and CH 4 (7.2 μmol/gcat) were obtained, which were 3.8 and 2.5 times that over pure g-C 3 N 4 , and its apparent quantum yield (AQY) was 1.5 % at 420 nm. According to the characterization results, the significantly increased photocatalytic activity could be ascribed to larger specific surface area, stronger visible-light absorption properties and more effective separation and transfer of photogenerated electron-hole pairs. Our finding provides a certain reference for the development and application of novel co-catalyst. [ABSTRACT FROM AUTHOR]

The MoS 2 /TiO 2 heterojunction composites photocatalysts exhibits excellent photocatalytic performance, and they were resoundingly synthesized by a one-step hydrothermal experimental method combined with mild calcination (300 °C) under an argon atmosphere. Under the visible light irradiation, the photocatalytic activity of MoS 2 /TiO 2 heterojunction composites were tested for photocatalytic reduction of CO 2 and it exhibited superior performance and stability. The consequences show that the pure MoS 2 demonstrates a poor photocatalytic activity because of its high electron-hole recombination, and the maximum yields of CO and CH 4 on 10% MoS 2 /TiO 2 heterojunction composite are 268.97 μmol/g.cat and 49.93μmol/g.cat, respectively, which are about 5.33 times and 16.26 times of pure TiO 2 (P25). The improved photocatalytic activity of 10% MoS 2 /TiO 2 heterojunction composite could be put down to the high-efficiency separation of photogenerated electron-hole pairs, strong response to visible light and the narrowed band gap. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Eu could facilitate the separation and transfer of photogenerated electron-hole pairs. • The Eu/C 3 N 4 composites have superior photocatalytic performance in both CO 2 reduction and H 2 evolution. • In-situ FTIR analysis indicates that the HCOOH is a major intermediate during the CO 2 conversion progress. • Massive photogenerated carriers existed in Eu/g-C 3 N 4 are favorable to the CO 2 photoconversion to CH 4. Abstract The high-efficiency photocatalytic CO 2 reduction and H 2 evaluation performance are quite attractive and desired for clean energy utilization and environmental pollution alleviation. Herein, the novel visible-light activated Eu (III) doped g-C 3 N 4 composites with different contents (2%, 4%, 6%, 8% and 10%) were successfully synthesized for the first time. The Eu (III) species acted as the Lewis acids could trap photoexcited electrons with superior ability. The photocatalytic activity of Eu/g-C 3 N 4 catalysts exhibited superior performance and stability in both CO 2 photoreduction and H 2 production with the presence of water under the visible light irradiation. A maximum CH 4 and H 2 yields of 22.8 μmol/(h·g-cat) and 78.1 μmol/(h·g-cat), respectively, had been obtained on 8% Eu/g-C 3 N 4 catalyst, which was about 2.2 and 7.3 times as high as the pure g-C 3 N 4. In situ FTIR analysis discloses that HCOOH is a major intermediate during the CO 2 conversion progress. The Mott–Schottky method also indicated massive photogenetated carriers existed in Eu/g-C 3 N 4 catalysts that in favor of the CO 2 conversion to CH 4. The origin of such remarkable photocatalytic activities could be explained as follows: (i) Eu3+ species captured photogenerated electrons and inhibited the recombination of photogenerated electron-hole pairs; (ii) the doped Eu3+ narrowed the band gap and enhanced the visible-light absorption capacity; (iii) the introduction Eu3+ increased the activated specific surface area. This work may provide meaningful guidelines for rare earth mental application on improving the performance of photocatalyst. [ABSTRACT FROM AUTHOR]

In this work, Bi 7 O 9 I 3 /Cd 0.5 Zn 0.5 S QDs/WO 3−x ternary heterojunction with full spectrum response was prepared and applied in the photodegradation of phenol. The results of photocatalytic experiments showed that the prepared Bi 7 O 9 I 3 /Cd 0.5 Zn 0.5 S QDs/WO 3−x heterojunction had the highest performance under full spectrum, visible and near-infrared light irradiation, and the reaction rates were 4.02, 3.75 and 5.71 times that of pure Bi 7 O 9 I 3 , respectively. This performance improvement results from the construction of double S-scheme heterojunction and the full-spectrum response, which had effective charge distribution and migration as well as an excellent response to sunlight. In addition,·OH and·O 2 - took the lead role in the process of photodegradation. Combined with the free radical-trapping experiment, ESR experiment and DFT theoretical calculation, the photocatalytic mechanism of double S-scheme system was proposed. This work is contributory to designing novel photocatalytic materials with dual S-scheme heterojunction and the efficient purification of wastewater in environmental remediation. [Display omitted] • Bi 7 O 9 I 3 /Cd 0.5 Zn 0.5 S QDs/WO 3−x ternary heterojunction with full spectrum response was prepared. • The degradation efficiency of BCW-2 heterojunction to phenol reached 100% under full-spectrum irradiation within 90 min. • ·OH and·O 2 - took the lead role in the process of photodegradation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The formation of N–C–S–Ni bonds and N–C–O–Ni–S bonds in NiS/SO-PCN composites provided the rapid charge transfer pathways. • The loading of NiS on O-PCN could extend the light absorption to the near-infrared region for photocatalytic hydrogen evolution. • The reduced metallic Ni0 from Ni2+ species acted as the charge transfer mediators to constitute the Z-scheme mechanism. In this study, both narrow band-gap semiconductor NiS and S and O defects modification were introduced into the design of PCN photocatalyst to enhance its photocatalytic performance. The O and S defects, on one hand, provided the higher specific area and more charge separation sites, and constituted the dual fast charge transfer pathways with NiS through the N–C–S–Ni bonds and N–C–O–Ni–S bonds, which greatly increased the carrier transfer kinetics at the interface and restricted the carrier recombination. On the other hand, the midgap level derived from S and O defects also enhanced the absorption of visible light and accommodated more excited electrons. Furthermore, Z-scheme mode could form as Ni2+ species were reduced to metallic Ni0 to acts as charge transfer mediator under irradiation. As such, a maximum hydrogen evolution rate of 1239.3 μmol/g/h was achieved over 5%NiS/SO-Melon composite, nearly 31.4 times the rate of pristine PCN. [ABSTRACT FROM AUTHOR]

• The B-TiO 2 /CuInS 2 Z-scheme heterojunction with visible light response was prepared. • B-TiO 2 /CuInS 2 could convert CO 2 into CO and CH 4 with the yields 21.39 and 4.83 µmol g−1 h−1. • Both boron doping and CuInS 2 sensitization can effectively improve the utilization of visible light. • The synergistic effect of B doping Z-scheme heterojunction optimized the separation and migration of photogenerated carriers. Photocatalytic reduction CO 2 into valuable solar fuels is a promising strategy to alleviate the greenhouse effect and the energy crisis on Earth. In this work, a B-TiO 2 /CuInS 2 photocatalyst has been synthesized through a simple hydrothermal-calcination method for efficient reducing CO 2 into CO and CH 4. Boron doping and sensitization of CuInS 2 ensure the excellent visible light response capacity of the composite catalyst. The synergistic effect between oxygen vacancies and Z-scheme heterojunction also endow the photocatalyst with strong carrier separation capability. Consequently, the optimized B-TO/CIS-2.5 sample exhibited the best CO (21.39 μmol g−1 h−1) and CH 4 (4.83 μmol g−1 h−1) production, approximately 9 and 14 times higher than that of TO and B-TO, respectively. This study provides a reliable approach for developing photocatalysts with boosted CO 2 reduction activity and high stability. [ABSTRACT FROM AUTHOR]

[Display omitted] • The CoAl-LDH/CGCNN-Metal NPs ternary photocatalysts with visible light response were prepared. • CoAl-LDH/CGCNN-Au NPs could convert CO 2 into CO, CH 4 , C 2 H 4 , and C 2 H 6 with the yields of 5.93, 17.68, 1.25 and 2.15 µmol g−1h−1. • The synergistic effect between S-scheme heterojunction and metal NPs could improve the light absorption capacity and the separation of photogenerated electron-hole pairs. Converting CO 2 into high-value products by photocatalysis can alleviate the global greenhouse effect and energy crisis. In this work, CoAl-LDH/CGCNN-Metal NPs (AC-M) ternary photocatalysts were synthesized by calcination, hydrothermal, and ultrasonic self-assembly methods. In pure water, the best sample AC-Au could convert CO 2 into CO, CH 4 , C 2 H 4 , and C 2 H 6 with the yields of 5.93, 17.68, 1.25 and 2.15 µmol g−1h−1, respectively, and exhibited good stability within 5 cycles (30 h). The synergistic effect between S-scheme heterojunction and metal NPs could broaden the composite's photo-response range and promote the separation of photogenerated carriers. Furthermore, the metal NPs could also serve as the electron-rich centers to improve the chemisorption of CO 2 and convert it into C 1 and C 2 products under visible light. This work can provide meaningful guidance for studying the efficient reduction of CO 2 to C 1 and C 2 products by S-scheme heterojunction/metal NPs ternary photocatalytic system. [ABSTRACT FROM AUTHOR]

A ball-flower like NiO/g-C 3 N 4 heterojunction composite was synthesized via a hydrothermal deposition method combined with subsequent calcination route. The NiO/g-C 3 N 4 heterojunction exhibited a superior performance in CO 2 photoreduction. A maximum CO yields of 4.17 μmol/(h g-cat) had been obtained on 40% NiO/g-C 3 N 4 composite, which was 2.5 and 7.6 times as high as the pure g-C 3 N 4 and NiO respectively. The promotion mechanism could be ascribed to the perfect band matching and efficient internal charge transfer within the p-n junction, which results in high-efficiency separation of photogenerated electron-hole pairs, strong visible-light response and high specific surface area. [ABSTRACT FROM AUTHOR]

A series of flaked Mg/g-C 3 N 4 catalysts were synthesized via an in-situ hydrothermal deposition method and measured their photocatalytic activity for CO 2 reduction. The characterization results indicated that the obtained Mg/pCN catalysts had high surface area and massive active sites for CO 2 absorption. The introduction of Mg species had a positive effect to extend the spectral response property and increased the separation rate of photogenerated electron-holes pairs. Mg-doped g-C 3 N 4 catalysts showed much higher photocatalytic performance than pCN. The maximum CH 4 and CO yields of 17.09 and 4.13 μmol/g-cat, respectively, had been obtained for CO 2 reduction after 6 h reaction. [ABSTRACT FROM AUTHOR]

The development of photocatalytic technology can not only reduce CO 2 emissions, but also realize the effective utilization of solar energy, thus alleviating the global energy crisis. In this work, BiOIO 3 /g-C 3 N 4 Z-scheme heterojunction modified by Ti 3 C 2 nanosheets was designed and fabricated through hydrothermal method and electrostatic self-assembly strategy. The g-C 3 N 4 /BiOIO 3 /Ti 3 C 2 (4 wt%) photocatalytic system exhibited the optimal photocatalytic efficiency, reaching 5.88 and 1.55 μ mol/g/h for CO and CH 4 , respectively, and the CO production rate was 6.6 folds as high as that of bare g-C 3 N 4. Various characterization techniques were utilized to research the physicochemical properties of as-prepared photocatalysts. Z-scheme heterojunction formed between g-C 3 N 4 and BiOIO 3 and the decorating of 2D Ti 3 C 2 co-catalyst were responsible for the boosted photocatalytic behavior. Z-scheme heterojunction could prevent the recombination of charge carriers, the co-catalyst Ti 3 C 2 acted as electron sink could achieve quick shift of photo-induced electrons and supply numerous active sites for photocatalytic reaction. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Direct Z-scheme FeWO 4 /NiAl-LDH heterostructure was fabricated. • The 10%FWLDH possessed the optimal photocatalytic CO yield. • The effective separation of the light-induced e-/h+ pairs was available on FWLDH composite. In recent years, photocatalytic reduction of carbon dioxide technology has been considered as an effective approach to moderate the energy crisis. In order to achieve efficient reduction of CO 2 , direct Z-scheme heterostructure composed of NiAl-LDH flower-like sphere and FeWO 4 nanoflakes (FWLDH) were obtained through a facile hydrothermal strategy. The prepared FWLDH photocatalysts revealed outstanding performance for CO 2 photoreduction to CO under the irradiation of visible light. The 10%FWLDH possessed the optimal photocatalytic CO yield (5 μmol· g−1·h−1), reaching up to 2.4 folds higher than that of pristine LDH (2.1 μmol·g−1·h−1). In addition, the CO selectivity was more than 93% over hydrogen evolution from the side reaction of water reduction. Finally, EPR characterization indicated that the electron transfer mechanism of FWLDH catalyst conformed to the Z-scheme pattern. The effective separation of the light-induced electron-hole pairs and the enhancement of the photoelectron reduction ability at the NiAl-LDH conduction band were the reasons for the prominent improvement of the photocatalytic performance of NiAl-LDH. This research could provide a novel perspective for designing photocatalyst with high efficiency for CO 2 photoreduction. [ABSTRACT FROM AUTHOR]

• 30% In 2 S 3 /NiAl-LDH exhibited the best photocatalytic performance. • Efficient separation of photogenerated electron-hole pairs. • A perfect band matching and efficient internal charge transfer within the n-n junction. A novel ball-flower In 2 S 3 /NiAl-LDH heterojunction photocatalyst was synthesized via a simple one-step hydrothermal approach for the first time and applied for CO 2 photocatalytic reduction. The In 2 S 3 /LDH composites exhibited excellent photocatalytic performance and stability in CO 2 photoreduction in water under the irradiation of simulated solar light. Experimental results indicated that the 30% In 2 S 3 /NiAl-LDH heterojunction photocatalyst exhibited the best photocatalytic performance. And the maximum CH 4 yields of 36.1 μmol/(h·g-cat) under 7 h visible light irradiation was about 2.5 and 3.5 times of unadulterated LDH and In 2 S 3. The enhanced photocatalytic performance of 30% In 2 S 3 /NiAl-LDH catalyst was due to the highly-efficient separation of photogenerated electron-hole pairs, its larger specific surface area and stronger visible-light absorption capacity. [ABSTRACT FROM AUTHOR]