Oxygen-Doped Red Carbon Nitride: Enhanced Charge Separation and Light Absorption for Robust CO2 Photoreduction.

Utilizing artificial photosynthesis for the conversion of CO2 into value-added fuels has been recognized as a promising strategy for the ever-increasing energy crisis and the greenhouse effect. Herein, the element doping engineering of red spherical g-C3N4 having oxygen bonded with compositional carbon (C-O-C) for CO2 photoreduction has been explored to address this challenge. The C-O bond was formed by hydrothermal treatment with dicyandiamide and 1,3,5-trichlorotriazine. The experimental and DFT results displayed the optimum oxygen substitution sites and demonstrated that the oxygen doping greatly improved the light utilization efficiency, CO2 affinity, and charge carrier transfer, which enhanced photoreduction efficiency of CO2. The evolution rates of CO (47.2 µmol g-1) and CH4 (9.1 µmol g-1) using O-CN were much higher than that of bulk-CN without a cocatalyst. The main reason was the contribution of the O 2p orbital to the conduction band (CB) and valence band of O-CN, which effectively reduced the electron mass, facilitating electron/hole separation and enhancing its fluidity. Furthermore, the Fermi level also shifted to the bottom of the CB, leading to higher electron density, which further improved the CO2 reduction ability. Our study marks an important step for developing high-performance photocatalysts for reduction of CO2.

Pd Nanosheet-Decorated 2D/2D g-C3N4/WO3·H2O S-Scheme Photocatalyst for High Selective Photoreduction of CO2 to CO.

The development of the global economy in recent years, environmental problems, greenhouse effect, and so forth have been of concern for countries all over the world. The key for solving the greenhouse effect is the reduction of CO2. With the development of photocatalytic reduction of CO2, hybrid photocatalytic nanostructures composed of noble metals and plasmonic semiconductors are being widely studied. In this work, S-scheme photocatalysts with a g-C3N4/WO3·H2O/Pd heterostructure was constructed by introducing ultrathin Pd nanosheets into the optimized 2D/2D g-C3N4/WO3·H2O binary system. The S-scheme charge transfer generated by the matched band gap of g-C3N4 and WO3·H2O can effectually improve the electron transfer rate and the redox ability of photogenerated carriers. The introduction of Pd nanosheets can inject a large number of hot electrons into the semiconductor on the basis of the S-scheme heterojunction to participate in the reaction. The S-scheme electron transfer method is used to improve the utilization rate of thermionic electrons and achieve the effect of widening the near-infrared-light absorption area of the composite material. Moreover, the reaction was carried out in water without the addition of any sacrificial agent, which can better reflect the green environmental protection of the experiment. This investigation will promote the broad-spectrum application of new and environment-friendly thermoelectron-assisted S-scheme photocatalysts, and on this basis, the possible reaction mechanism is discussed.

Tailored Linker Defects in UiO-67 with High Ligand-to-Metal Charge Transfer toward Efficient Photoreduction of CO2.

Defect engineering can be used as a potential tool to activate metal-organic frameworks by regulating the pore structure, electronic properties, and catalytic activity. Herein, linker defects were effectively controlled by adjusting the amount of formic acid, and UiO-67 with different CO2 reduction capabilities was obtained. Among them, UiO-67-200 had the highest ability to selectively reduce CO2 to CO (12.29 µmol g-1 h-1). On the one hand, the results based on time-resolved photoluminescence decay curves and photochemical experiments revealed that UiO-67-200 had the highest charge separation efficiency. On the other hand, the linker defects affected the band structure of UiO-67 by changing the lowest unoccupied molecular orbital (LUMO) based on the density functional theory and UV-vis spectra. Hence, the proper linker defects enhanced the ligand-to-metal charge transfer process by promoting the transfer of electrons between the highest occupied molecular orbital and LUMO. Additionally, in situ Fourier transform infrared spectra and 13CO2 labeling experiments also indicated that COOH* was an important intermediate for CO formation and that CO originated from the photoreduction of CO2.

Construction of Heterogenous S-C-S MoS2/SnS2/r-GO Heterojunction for Efficient CO2 Photoreduction.

Photocatalytic reduction of CO2 by semiconductors is of great significance in generating value-added fuels. Here, we construct a novel S-C-S heterojunction constituted of MoS2/SnS2/r-GO by a simple solvothermal method. The prepared MoS2/SnS2/r-GO showed significant photoexcitation of photosensitive oxygen (ROS) by electron spin resonance spectroscopy, demonstrating that superoxide radicals (•O2-), pores, and hydroxyl radicals (•OH) are the main active species. The constructed S-C-S heterojunction has a multilevel electron transport mechanism and synergistic effect, which provides the possibility of producing more organic fuel. The photocatalytic materials were characterized by XRD, XPS, SEM, TEM, PL, etc. As a result, the atomic layer MoS2/SnS2/r-GO heterojunction exhibited a CO formation rate of 68.53 µmol g-1 h-1 and a CH4 formation rate of 50.55 µmol g-1 h-1, respectively. This work opens up new prospects for the formation of heterojunctions of chalcogenide transition-metal sulfides.

Construction of Au-modified CN-based donor-acceptor system coupled with dual photothermal effects for efficient photoreduction of carbon dioxide.

Conversion of CO2 into high value-added fuels through the photothermal effect is an effective approach for utilizing solar energy. In this study, we prepared the CN-based photocatalyst Py-CTN-Au with both donor-acceptor (D-A) system and dual photothermal effects using a simple two-step method involving calcination and photo-deposition. Real-time monitoring with a thermal imaging camera revealed that Py-CTN-Au0.5 achieved a maximum stable temperature of 180 °C, which was approximately 1.2 times higher than that of Py-CTN (155 °C) and 1.9 times higher than that of g-CN (95 °C) under the same reaction conditions. Under the optimized reaction conditions, Py-CTN-Au0.5 exhibited a CO release rate of 30.59 umol g-1 after 4 h of reaction, which was 7.3 times higher than that of pure g-CN (4.18 umol g-1). The D-A system not only facilitated the separation and transformation of charge carriers but also induced a photothermal effect to accelerate the photoreduction of CO2. Additionally, the cocatalyst Au nanoparticles (Au NPs) further enhanced the charge carrier dynamics and photothermal effect by increasing the built-in electric field intensity and localized surface plasmon resonance (LSPR) effect, respectively. The dual photothermal effects resulting from the non-radiative photon conversion of the D-A structure and the Au NPs LSPR effect, along with the enhanced charge carrier dynamics, catalyzed the efficient photoreduction of CO2. DFT simulations were used to confirm the effect of D-A system and Au NPs. In-situ FTIR results demonstrated that the synergistic photothermal effect promoted the formation of the key intermediate species COOH*, which is beneficial for the photocatalytic reduction of CO2. This study provides valuable insights into the multiple photothermal synergistic effects in photocatalytic reactions.

Facile synthesis of MOF-808/RGO-based 3D macroscopic aerogel for enhanced photoreduction CO2.

Three-dimensional (3D) macroscopic aerogels have emerged as a critical component in the realm of photocatalysis. Maximizing the integration of materials can result in enhanced efficiency and selectivity in photocatalytic processes. In this investigation, we fabricated MOF-808/reduced graphene oxide (RGO) 3D macroscopic aerogel composite materials employing the techniques of hydrothermal synthesis and freeze-drying. The results revealed that the macroscopic aerogel material exhibited the highest performance in CO2 reduction to CO, particularly when the concentration of RGO was maintained at 5 mg mL-1. In addition, we synthesized powder materials of MR-5 composite photocatalysts and conducted a comparative analysis in terms of photocatalytic CO2 reduction performance and electron transfer efficiency. The results showthat the macroscopic aerogel material boasts a high specific surface area, an abundant internal pore structure, and increased active sites. These attributes collectively enhance light energy utilization, and electron transfer rates, thereby, improving photothermal and photoelectric conversion efficiencies. Furthermore, we conducted in-situ FT-IR measurements and found that the M/R-5 aerogel exhibited the best CO2 adsorption capacity under a CO2 flow rate of 10 mL min-1. The density functional theory results demonstrate the correlation between the formation pathway of the product and the charge transfer pathway. This study provides useful ideas for realizing photocatalytic CO2 reduction of macroscopic aerogel materials in gas-solid reaction mode.

Fabricated ZnO@ZnIn2S4 S-scheme heterojunction photocatalyst for enhanced electron-transfer and CO2 reduction.

Step-scheme (S-scheme) heterojunctions can efficiently promote the separation of photogenerated carriers while maintaining the strong oxidation/reduction ability of photocatalysts; thus, research attention on S-scheme heterojunctions is increasing year by year. In this study, the S-scheme ZnO@ZnIn2S4 (ZnO@ZIS) heterojunction was prepared successfully. Then, electron spin resonance (ESR) characterization was applied to prove the successful construction of the S-scheme heterojunction. Photoluminescence (PL), time-resolved photoluminescence (TRPL), and photoelectrochemical experiments have demonstrated efficient interfacial charge transport in ZnO@ZIS. Finally, the mechanism of CO2 activation and electron transport was investigated by in situ Fourier transform infrared spectroscopy (FT-IR) and discrete Fourier transform (DFT) calculation analysis. The 40-ZnO@ZIS composite showed the best activity under light, and its CO and CH4 yields reached 39.76 and 3.92 µmol∙g-1∙h-1, respectively. This study provides a solution for optimizing the photocatalytic reduction activity of semiconductor photocatalysts by constructing S-scheme heterojunction materials to improve the CO2 reduction capacity.

Enhanced degradation of polyethylene terephthalate plastics by CdS/CeO2 heterojunction photocatalyst activated peroxymonosulfate.

Waste plastics have posed enormous to the environment, but their recycling, especially polyethylene terephthalate plastics, was still a huge challenge. Here, CdS/CeO2 was used as the photocatalyst to promote the degradation of PET-12 plastics by activating peroxymonosulfate (PMS) synergistic photocatalytic system. The results showed that 10 % CdS/CeO2 had the best performance under the illumination condition, and the weight loss rate of PET-12 could reach 93.92 % after adding 3 mM PMS. The effects of important parameters (PMS dose and co-existing anions) on PET-12 degradation were systematically studied, and the excellent performance of the photocatalytic-activated PMS system was verified by comparison experiments. SO4•- contributed the most to the degradation performance of PET-12 plastics, which was demonstrated by electron paramagnetic resonance (EPR) and free radical quenching experiments. Furthermore, the results of GC showed that the gas products including CO, and CH4. This indicated that the mineralized products could be further reduced to hydrocarbon fuel under the action of the photocatalyst. This job supplied a new idea for the photocatalytic treatment of waste microplastics in the water, which will help recycle waste plastics and recycle carbon resources.

Boosting mineralized organic pollutants by using a sulfur-vacancy CdS photocatalyst.

Photocatalytic mineralization of organic pollutions and simultaneously converting CO2 to CO (tetracycline → CO2 → CO) represents a fascinating way to solve the environmental and energy crisis. This work demonstrates the excellent mineralization and CO2 reduction performance of S-vacancy CdS and reveals the high efficiency of the carbon self-recycling two-in-one photocatalytic system.

Constructing Schottky junctions via Pd nanosheets on DUT-67 surfaces to accelerate charge transfer.

The separation, transfer and recombination of charge often affect the rate of photocatalytic reduction of CO2. Schottky junctions can promote the rapid separation of space charge. Therefore, in this paper, Pd nanosheets were grown on the surface of DUT-67 by a hydrothermal method, and a Schottky junction was constructed between DUT-67 and Pd. Under the action of the Schottky junction, the CO yield of 0.3-Pd/DUT-67 reached 12.15 µmol/g/h, which was 17 times higher than that of DUT-67. Efficient charge transfer was demonstrated in photochemical experiments. The large specific surface area and the increased light utilization rate also contributed to the increase in the CO2 reduction efficiency. In addition, the mechanism of Pd/DUT-67 photocatalytic reduction of CO2 was proposed.

Fabricating intramolecular donor-acceptor system via covalent bonding of carbazole to carbon nitride for excellent photocatalytic performance towards CO2 conversion.

Photocatalytic conversion of CO2 into hydrocarbon fuels is an ideal technology of mitigating greenhouse effect caused by excessive emission of CO2. However, the high recombination rate of electron-hole pairs and limited charge carriers transport speed constrained the catalytic performance of many semiconductor catalysts. In this contribution, a series of carbon nitride (g-CN) samples with intramolecular donor-acceptor (D-A) system were successfully prepared by introducing organic donor into their structures. Characterization results confirmed that carbazole was successful connected to the structure of g-CN via chemical bond. The formation of intramolecular D-A system greatly enlarged the light response region of g-CN-xDbc. In addition, a new charge transfer transition mode was formed in g-CN-0.01Dbc due to the incorporation carbazole, which enable it to use light with energy lower than the intrinsic absorption of g-CN. Meanwhile, the D-A structure led to the spatial separation of electrons and holes in g-CN-xDbc and significantly decreased the recombination rate of electron-hole pairs. The g-CN-0.01Dbc presented the best catalytic performance and the CO evolution rate was 9.6 times higher than that of g-CN. Moreover, the reaction was performed in water without any additive, which made it green and sustainable. DFT simulation confirmed the D-A structure and charge carrier migration direction in the prepared samples.

Enhanced light utilization efficiency and fast charge transfer for excellent CO2 photoreduction activity by constructing defect structures in carbon nitride.

Defect structure is one of the crucial factors for enhancing the catalytic activities of photocatalysts. However, rational design and construction of defect structures in catalysts to meet the aim of enhancing photocatalytic performance in a simple and cost-effective way is still a challenge. In this contribution, we report a strategy to construct defect structures in graphitic carbon nitride (g-CN) by simple copolymerizing of urea with polyethyleneimine (PEI). Among the prepared catalysts, u-0.05PEI presents the best photocatalytic activity for CO2 reduction, with CO and CH4 yields of 32.86 and 1.68 µmol g-1 in 4 h, which is about 3.2 and 2.5 times higher than that of g-CN, respectively. Characterization results show that both C and N defects are formed in the newly prepared catalysts. The C defects on the surface of u-xPEI result in the formation of more amino groups which are beneficial for CO2 adsorption. Meanwhile, the N defects inside the samples lead to the generation of midgap states between the valance band and conduction band of u-xPEI. The midgap states greatly enlarge the light absorption extent, and enable the use of light with energy lower than the intrinsic absorption of g-CN in the photoreduction of CO2. As confirmed by DRS, EPR, PL analysis, the excellent catalytic activity of u-0.05PEI is mainly attributed to the remarkably improved light utilization efficiency and fast charge transfer. Moreover, the reaction is performed in water without any additive or organic solvent, which makes it environmentally friendly.

Chemical fixation of CO2 into cyclic carbonates catalyzed by bimetal mixed MOFs: the role of the interaction between Co and Zn.

Bimetal mixed MOFs of [CoZn][(BDC)(DABCO)0.5] (CZ-BDO), [CoNi][(BDC)(DABCO)0.5] (CN-BDO), and [NiZn][(BDC)(DABCO)0.5] (NZ-BDO) were prepared under solvothermal conditions and further employed as highly active accelerants for converting carbon dioxide into cyclic carbonates. The characteristics of the bimetal compounds were revealed via various techniques, including ICP-OES, XRD, FT-IR, Raman, XPS, SEM, EDS maps, N2 adsorption, TG-DTG, and CO2/NH3-TPD. The catalytic results revealed that CZ-BDO is superior to the other samples for obtaining a satisfactory chloropropene carbonate (CPC) yield. The excellent catalytic activity may be owing to the presence of a solid solution within the Co and Zn bimetal sample, which provides synergistic catalysis in the carbon dioxide cycloaddition. In addition, the synergistic catalysis was further confirmed by the NH3-TPD profiles, whereby the amount of CZ-BDO basic sites was obviously enhanced compared to the other samples. Furthermore, DFT calculations were also performed to reveal the synergistic catalysis between Co and Zn for the coupling reaction. Additionally, when the coupling reaction was carried out at 100 °C for 5 h in the presence of 0.5 wt% epichlorohydrin (ECH) as a catalyst at 3.0 MPa, 99.31% conversion of ECH and 97.05% yield of CPC were obtained over the optimal CZ-BDO sample. Moreover, the bimetal sample can also efficiently convert other epoxides into the corresponding cyclic carbonates.

Fabricated rGO-modified Ag2S nanoparticles/g-C3N4 nanosheets photocatalyst for enhancing photocatalytic activity.

Photocatalytic technology provides a new strategy for the treatment of water pollution and energy crisis. Developing photocatalytic materials with high efficiency and stable visible light response has always been the direction of scientific researchers in the photocatalytic field. In this paper, we designed and prepared an efficient and stable rGO-modifited type-I Ag2S/g-C3N4 heterojunction photocatalyst (rGO/Ag2S/CN). The TEM (Transmission electron microscope) technology shows that the morphology of rGO/Ag2S/CN is a sandwich-like structure. UV-Vis DRS (UV-visible diffuse-reflectance spectrum) shows that the loading of Ag2S NPs and the modification of rGO effectively enhance the light response performance of nanocomposite materials in visible light. PL (Photoluminescence) and PEC (Photoelectrochemical) results prove that the photogenerated carriers transport and separation efficiency of rGO/Ag2S/CN have been improved. The photodegradation of RhB and the photoreduction of CO2 results confirmed that this ternary nanocomposite has great photocatalytic activity. The photodegradation efficiency of the RhB aqueous solution is about 98.5% after 30 min under the visible light irradiation. The photoreduction experiments showed that the yields of CO and CH4 are 178.05 µmol/g and 121.11 µmol/g, respectively after 8 h under the UV light irradiation. The results of cyclic photocatalytic experiments and the XRD pattern after those processes further revealed that the modification of rGO not only enhanced the photocatalytic performance of the nanocomposite photocatalyst, but also effectively improved the stability of Ag2S in the photocatalytic process. The possible photocatalytic reaction mechanisms were discussed in detail.