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Metal-organic frameworks (MOFs) materials have become a research forefront in the field of photocatalytic CO2 reduction attributed to their ultra-high specific surface area, adjustable structure and abundant catalytic active sites. Particularly, MOFs can be facilely tuned to match CO2 photoreduction by means of post-modification of metal nodes, organic linkers with specific functions, and combined with other active materials. Herein, we highlight the recent advances in the construction strategy of MOF-based photocatalytic materials for CO2 reduction. Some systematic modification strategies on MOFs-based photocatalysts are also been discussed, such as modification of metal sites and organic ligands, construction of heterojunction, introduction of single/diatoms and strain engineering. Finally, the future developments directions of MOF-based photocatalysis in the field of CO2 reduction are presented.

In recent years, the use of quantum dots (QDs) cocatalysts to improve the hydrogen evolution activity from the water splitting of photocatalysts has become a popular research topic. Herein, we successfully prepared a novel 0 dimension/2 dimension (0D/2D) heterojunction nanocomposite (denoted Ag2S quantum dots (QDs)/g-C3N4) with excellent photocatalytic performance by anchoring the Ag2S QDs cocatalyst on the surface of g-C3N4 through a self-assembly strategy. Ag2S QDs with an average particle size of approximately 5.8 nm were uniformly and tightly modified on g-C3N4. The Ag2S QDs/g-C3N4 composite with 0.5 wt% Ag2S QDs loading achieved the highest hydrogen evolution rate of 471.1 μmol·g−1·h−1 with an apparent quantum efficiency (AQE) of 1.48% at 405 nm. Such remarkable hydrogen evolution activity far exceeded that of undoped g-C3N4 and Ag2S nanoparticles (NPs)/g-C3N4. Moreover, it was 2.04 times the activity of Pt/g-C3N4 with Pt as the cocatalyst. The enhanced photocatalytic performance was attributed to the energy band broadening of Ag2S QDs caused by the quantum size effect and the convenient and effective charge transfer between g-C3N4 and Ag2S QDs cocatalysts. The mechanism underlying the enhanced photocatalytic H2 evolution activity was further proposed. This study demonstrates that semiconductor-based quantum dots are strong candidates for excellent cocatalysts in photocatalysis.

With photoconductor being transferred and separated, interface contact plays a crucial role in developing composites photocatalyst. In the present work, 2D crystalline g-C3N4 (called as CCN) with 2D TiO2 nanosheets (called as TO) obtained in situ oxidized single-layered Ti3C2Tx MXene is designed by an electrostatic self-assembly technology. This CCN/TO nanosheets system with a few TiO2 nanosheets distributed to the surface is not only prolonging lifetime of photoelectron but also stimulating photogenerated carriers transferred in contact interface. The electron transfer mechanism of CCN/TO is further proved by Pt photo-deposition method. Therein, the optimal CCN-TO-0.6 exhibits excellent performance of H2 generation compared with single photocatalyst of CCN. The result shows that the crystalline g-C3N4 photocatalysts introduced TiO2 with interfacial effect favorably reduce H+ to H2 and enhance photocatalytic activity.

Bi-composed TiO2 hollow microspheres with enhanced visible-light photocatalytic activity are prepared by an impregnation–calcination method.

The active site amount of photocatalysts, being the key factors in photocatalytic reactions, directly affects the photocatalytic performance of the photocatalyst. Pristine graphitic carbon nitride (g-C3N4) exhibits moderate photocatalytic activity due to insufficient active sites. In this study, cyano-modified porous g-C3N4 nanosheets (MCN-0.5) were synthesized through molecular self-assembly and alkali-assisted strategies. The cyano group acted as the active site of the photocatalytic reaction, because the good electron-withdrawing property of the cyano group promoted carrier separation. Benefiting from the effect of the active sites, MCN-0.5 exhibited significantly enhanced photocatalytic activity for CO2 reduction under visible light irradiation. Notably, the photocatalytic activity of MCN-0.5 was significantly reduced when the cyano groups were removed by hydrochloric acid (HCl) treatment, further verifying the role of cyano groups as active sites. The photoreduction of Pt nanoparticles provided an intuitive indication that the introduction of cyano groups provided more active sites for the photocatalytic reaction. Furthermore, the controlled experiments showed that g-C3N4 grafted with cyano groups using melamine as the precursor exhibited enhanced photocatalytic activity, which proved the versatility of the strategy for enhancing the activity of g-C3N4 via cyano group modification. In situ diffuse reflectance infrared Fourier transform spectroscopy and theoretical calculations were used to investigate the mechanism of enhanced photocatalytic activity for CO2 reduction by cyano-modified g-C3N4. This work provides a promising route for promoting efficient solar energy conversion by designing active sites in photocatalysts.

Graphitic carbon nitride (g-C3N4) has emerged as a remarkably promising photocatalyst for addressing environmental and energy issues; however, it exhibits only moderate photocatalytic activity because of its low specific surface area and high recombination of carriers. Preparation of crystalline g-C3N4 by the molten salt method has proven to be an effective method to improve the photocatalytic activity. However, crystalline g-C3N4 prepared by the conventional molten salt method exhibits a less regular morphology. Herein, highly crystalline g-C3N4 hollow spheres (CCNHS) were successfully prepared by the molten salt method using cyanuric acid-melamine as a precursor. The higher crystallization of the CCNHS samples not only repaired the structural defects at the surface of the CCNHS samples but also established a built-in electric field between heptazine-based g-C3N4 and triazine-based g-C3N4. The hollow structure improved the level of light energy utilization and increased the number of active sites for photocatalytic reactions. Because of the above characteristics, the as-prepared CCNHS samples simultaneously realized photocatalytic hydrogen evolution with the degradation of the plasticizer bisphenol A. This research offers a new perspective on the structural optimization of supramolecular self-assembly.

In recent years, solar desalination and water evaporation/purification with various artificial architectures have drawn significant attention. Herein, we introduce a rational design structure for efficient solar water evaporation and purification, focusing on the balance between water transportation and thermal insulation. Natural wood after a simple flame treatment on the surface was utilized as the solar absorber, with a high solar absorbance (∼90 %), good hydrophilicity, and excellent heat localization abilities. Besides, a thermal insulator (polystyrene foam) was used to further reduce the thermal loss, and the optimized ratio between the water path and thermal insulator was obtained. A set of floating foam-flamed-wood (F-F-wood) devices were fabricated with a high evaporation rate of 3.92 kg m−2 h−1, exhibiting photothermal purification abilities from seawater and wastewater containing organic dyes and heavy metals. This study sheds light on the rational design of scalable and low-cost devices for solar water evaporation and purification.

The solar-driven catalytic conversion of CO2 to useful chemical fuels is regarded as an environmentally friendly approach to reduce the consumption of fossil fuels and mitigate the greenhouse effect. However, it is highly intriguing and challenging to promote the selectivity and efficiency of visible-light-responsive photocatalysts that favor the adsorption of CO2 in photoreduction processes. In this work, three-dimensional hierarchical Cd0.8Zn0.2S flowers (C8Z2S-F) with ultrathin petals were successfully synthesized through an in-situ self-assembly growth process using sodium citrate as a morphology director. The flower-like Cd0.8Zn0.2S solid solution exhibited remarkable photocatalytic performance in the reduction of CO2, generating CO up to 41.4 μmol g−1 under visible-light illumination for 3 h; this was nearly three times greater than that of Cd0.8Zn0.2S nanoparticles (C8Z2S-NP) (14.7 μmol g−1). Particularly, a comparably high selectivity of 89.9% for the conversion of CO2 to CO, with a turnover number of 39.6, was obtained from the solar-driven C8Z2S-F system in the absence of any co-catalyst or sacrificial agent. Terahertz time-domain spectroscopy indicated that the introduction of flower structures enhanced the light-harvesting capacity of C8Z2S-F. The in situ diffuse reflectance infrared Fourier transform spectroscopy unveiled the existence of surface-adsorbed species and the conversion of photoreduction intermediates during the photocatalytic process. Empirical characterizations and predictions of the photocatalytic mechanism demonstrated that the flower-like Cd0.8Zn0.2S solid solution possessed desirable CO2 adsorption properties and an enhanced charge-transfer capability, thus providing a highly effective photocatalytic reduction of CO2.

The development of low-cost semiconductor photocatalysts for highly efficient and durable photocatalytic H2 evolution under visible light is very challenging. In this study, we combine low-cost metallic Ni3C cocatalysts with twin nanocrystal Zn0.5Cd0.5S (ZCS) solid solution homojunctions for an efficient visible-light-driven H2 production by a simple approach. As-synthesized Zn0.5Cd0.5S-1% Ni3C (ZCS-1) heterojunction/homojunction nanohybrid exhibited the highest photocatalytic H2-evolution rate of 783 μmol h-1 under visible light, which is 2.88 times higher than that of pristine twin nanocrystal ZCS solid solution. The apparent quantum efficiencies of ZCS and ZCS-1 are measured to be 6.13% and 19.25% at 420 nm, respectively. Specifically, the homojunctions between the zinc blende and wurtzite segments in twin nanocrystal ZCS solid solution can significantly improve the light absorption and separation of photogenerated electron-hole pairs. Furthermore, the heterojunction between ZCS and metallic Ni3C NP cocatalysts can efficiently trap excited electrons from ZCS solid solution and enhance the H2-evolution kinetics at the surface for improving catalytic activity. This study demonstrates a unique one-step strategy for constructing heterojunction/homojunction hybrid nanostructures for a more efficient photocatalytic H2 evolution compared to other noble metal photocatalytic systems.

The current semiconductor photocatalysts only exhibit unsatisfactory photocatalytic efficiency due to the easy recombination of photogenerated carriers. Regulating the separation and transfer of photogenerated carriers and reducing the recombination of photogenerated carriers are essential for improving the catalytic efficiency of photocatalysts. The recombination of photogenerated carriers is produced by the attraction of Coulombic force. Consequently, reducing the recombination of photogenerated carriers requires an additional driving force to counteract the attraction of Coulombic force. However, exploring the mechanism of photogenerated carrier regulation from the perspective of the driving force of photogenerated carrier separation and transfer has received only sporadic attention. This review first starts with the fundamental mechanism of photogenerated carrier recombination and the impairment of photogenerated carrier recombination to photocatalytic performance. Then it focuses on several main strategies for regulating the behavior of photogenerated carriers from the aspect of the driving force for the separation and transfer of photogenerated carriers, including heterojunction design, construction of an intramolecular donor–acceptor (D–A) system, exciton regulation, and electron spin regulation. Subsequently, advanced characterization techniques and the latest developments of theoretical calculations in the separation and migration of photogenerated carriers have been introduced. At the end, we summarize the current challenges of photogenerated carrier regulation and put forward the prospects for future development.

Photoexcited dynamic modulation, maximizing the effective utilization of photoinduced electron-hole pairs, dominates the multiple electrons-involving reduction pathways for terminal CH

Owing to their unique physicochemical, optical and electrical properties, two-dimensional (2D) MoS2 cocatalysts have been widely applied in designing and developing highly efficient composite photocatalysts for hydrogen generation under suitable light irradiation. In this review, we first elaborated on the fundamental aspects of 2D MoS2 cocatalysts to include the structural design principles, synthesis strategies, strengths and challenges. Subsequently, we thoroughly highlighted and discussed the modification strategies of 2D MoS2 H2-evolution cocatalysts, including doping heteroatoms (e.g. metals, non-metals, and co-doping), designing interfacial coupling morphologies, controlling the physical properties (e.g. thickness, size, structural defects or pores), exposing the reactive facets or edge sites, constructing cocatalyst heterojunctions, engineering the interfacial bonds and confinement effects. In the future, the forefront challenges in understanding and in precise controlling of the active sites at molecular level or atomic level should be carefully studied, while various potential mechanisms of photogenerated-electrons interactions should be proposed. The applications of MoS2 cocatalyst in the overall water splitting are also expected. This review may offer new inspiration for designing and constructing novel and efficient MoS2-based composite photocatalysts for highly efficient photocatalytic hydrogen evolution.

With the development in photocatalysis field, photocatalysts have received increasing attention due to their important role in environmental pollution and energy crisis. As a nonmetallic photocatalyst, graphitic carbon nitride (g-C3N4) has been widely recognized because of its excellent optical properties, low cost, and environment friendliness. In the g-C3N4 intrinsic frameworks, carbon atom tends to be the reducing active site, while nitrogen atom tends to be the oxidizing active site and reducing active site according to the difference of electronegativity. However, the quantity and quality of these active sites are affected by many factors, including C N covalent bonds, surface properties, etc. Active sites play an important role in photocatalysis; however, this role is not detailed in most reports. In this review, we proposed the following possible mechanisms of active sites in improving the photocatalytic activity of traditional g-C3N4 based on its intrinsic: morphology regulation, carrier migration, surface active treatment, and substrate adsorption. The following factors affecting the active sites of g-C3N4, including basal engineering and hybrid engineering, were also investigated. The roles of these active sites in improving the photocatalytic activity of g-C3N4-based photocatalytic materials, including morphology regulation, surface treatment, heteroatom doping, and interfacial interaction, were also expounded. Current challenges and future development of g-C3N4-based photocatalysts that are rich in active surface sites were also discussed. This review provides an in-depth understanding of g-C3N4-based photocatalysts.

MXene-based materials are considered to be promising candidates for electrodes in energy storage field owing to their features of two-dimensional layered structure, good conductivity, and chemical stability. Carbon-coated Fe3O4 nanoparticles (C@Fe3O4 NPs)-decorated Ti3C2 MXene composites were synthesized via a facile method for the first time. The prepared C@Fe3O4 NPs/Ti3C2 MXene composite was characterized by various techniques (XRD, SEM, TEM, TGA, and XPS), and the results showed that C@Fe3O4 NPs (diameter: 60 nm, layer thickness: 10 nm) were homogeneously distributed on the surfaces of Ti3C2 MXene interlayers while the accordion-like structure maintained. It is noted that the specific surface areas dramatically increased from 7.8 m2/g to 37.0 m2/g after introducing the nanoparticles. The C@Fe3O4 NPs/Ti3C2 MXene products exhibited a specific capacity of 231.5 mAh g−1 after 200 cycles under 100 mA/g when serving as anode materials for Li-ion batteries, which was markedly higher than that of pure Ti3C2 MXene (107.8 mAh g−1). This study presents and investigates a novel kind of MXene-based composite, which has the potential as electrode materials for energy storage.

Single metal atom photocatalysts have received widespread attention due to the rational use of metal resources and maximum atom utilization efficiency. Especially N-rich amorphous g-C3N4 is always ...

Amine-functionalized graphitic carbon nitride (g-C3N4) decorated with Au nanoparticles (CN/Au) was prepared by N2 plasma treatment of g-C3N4 powders impregnated with HAuCl4·3H2O. Well-dispersed Au nanoparticles with a small particle size were deposited on g-C3N4 nanosheets. In addition, the amino group was introduced into the CN/Au system. Without the addition of cocatalyst and sacrificial agent, CN/Au exhibited enhanced photocatalytic activity for CO2 reduction under visible-light irradiation. CO and CH4 evolution rates of CN/Au reached 28.3 and 1.3 μmol·h−1·g−1, which were 7.6 and 2.6 times higher than those of pristine g-C3N4 (CN-0), respectively. The enhanced activity can be explained by these factors. (1) The introduced amino group improved the adsorption capacity of CN/Au for CO2; (2) the hot electrons generated by Au nanoparticles activated the surrounding electrons through energy transfer and caused local temperature to rise, increasing the efficiency of the photoreduction reaction of CO2; (3) the Schottky junction between Au and g-C3N4 promoted the migration of electrons from g-C3N4 to Au nanoparticles, suppressing the recombination of the carriers. Time-of-flight secondary ion mass spectrometry confirmed the introduction of amino groups, and solid-state 13C nuclear magnetic resonance spectra provided a support for inferring the position of the amino group.

Owing to the maximum atom-utilization efficiency and excellent catalytic properties, Au single-atom catalysts (SACs) have been extensively studied in various catalytic systems. However, the performance of Au SACs in CO2 reduction has seldom been investigated. Herein, Au single atoms on amino-group-modified graphitic carbon nitride (U-ACN) was successfully synthesized through a mild and eco-friendly urea reduction method. U-ACN showed a remarkable performance for CO2 reduction, with CO and CH4 yields 1.97 and 4.15 times higher than those of pure graphitic carbon nitride over 2.5 h visible-light irradiation. The excellent catalytic activity of U-ACN derived from the introduction of Au single atoms, which lowered the energy barrier of CH4 formation, narrowed the band gap, and hindered the recombination of charge carriers. In addition, U-ACN showed improved CO2 affinity owing to the amino groups in the catalysts introduced by urea.

Crystalline carbon nitride (CCN) prepared by a molten-salt method is attracting increased attention because of its promising properties and excellent photocatalytic activity. In this work, we further improve the crystallinity of CCN through synthesis by the molten-salt method under the action of aqueous hydrochloric acid (HCl) solution. Our results showed that the cryst allinity of the as-prepared samples increased with increasing HCl concentration and reached the maximum value at 0.1 mol L−1. This can be attributed to the removal of some potassium ions (K+) from the terminal amino groups of CCN by the aqueous HCl solution, which results in a release of the polymerization sites. As a result, the crystallinity of the as-prepared samples further increased. Moreover, the obtained 0.1 highly crystalline carbon nitride (0.1HCCN; treated with 0.1 mol L−1 aqueous HCl solution) exhibited an excellent photocatalytic hydrogen evolution of 683.54 µmol h−1 g−1 and a quantum efficiency of 6.6% at 420 nm with triethanolamine as the sacrificial agent. This photocatalytic hydrogen evolution was 2 and 10 times higher than those of CCN and bulk carbon nitride, respectively. The enhanced photocatalytic activity was attributed to the improved crystallinity and intercalation of K+ into the xHCCN interlayer. The improved crystallinity can decrease the number of surface defects and hydrogen bonds in the as-prepared sample, thereby increasing the mobility of the photoinduced carriers and reducing the recombination sites of the electron–hole pairs. The K+ intercalated into the xHCCN interlayer also promoted the transfer of the photoinduced electrons because these ions can increase the electronic delocalization and extend the π-conjugated systems. This study may provide new insights into the further development of the molten-salt method.

Here, nanosheet-assembled hierarchical flower-like g-C3N4 (CMN) was prepared through a molecular self-assembly and ethanol molecular insertion strategy. CMN possesses enhanced CO2 photoreduction capability due to the enhanced adsorption capacity for CO2, increased specific surface area, and improved carrier diffusion.

Efficient and selective photocatalytic CO

Single-atom photocatalysis has been demonstrated as a novel strategy to promote heterogeneous reactions. There is a diversity of monoatomic metal species with specific functions; however, integrating representative merits into dual-single-atoms and regulating cooperative photocatalysis remain a pressing challenge. For dual-single-atom catalysts, enhanced photocatalytic activity would be realized through integrating bifunctional properties and tuning the synergistic effect. Herein, dual-single-atoms supported on conjugated porous carbon nitride polymer are developed for effective photocatalytic CO

In recent years, the use of quantum dots (QDs) cocatalysts to improve the hydrogen evolution activity from the water splitting of photocatalysts has become a popular research topic. Herein, we successfully prepared a novel 0 dimension/2 dimension (0D/2D) heterojunction nanocomposite (denoted Ag

Strong demand for affordable clean energy to support applications ranging from conventional energy supply to space propulsion places spotlight on advanced energy generation using photovoltaic and wind power. Yet, the intermittent nature of solar and wind sources drives the search for energy storage solutions that would permit the needed level of resilience and support further growth in the use of renewable sources of power. Hydrogen generation using sunlight is a promising pathway to decouple demand from supply. Herein, we show how exposure to reactive Ar-H2, Ar-H2-N2, and Ar-O2 plasma environments can notably enhance surface properties of photocatalytic TiO2 nanosheets used in advanced energy generation systems. Treatment using Ar-H2 plasmas produced highly hydrogenated, surface-disordered TiO2 nanosheets with oxygen vacancies, whereas exposure to Ar-H2-N2 plasmas resulted in N doping. Surprisingly, Ar-O2 plasma treatment did not change surface properties of TiO2. Optical emission spectroscopy was used to monitor transient species to further understand surface modification in plasma. Direct measurements demonstrated that among thus-produced samples, hydrogenated TiO2 nanosheets exhibit the highest photocatalytic H2-generation activity under visible-light irradiation, which is also greater than the activity of pure, untreated nanosheets. The mechanism of enhancing the visible-light photocatalytic H2-generation activity on hydrogenated TiO2 nanosheets is also proposed. The level of surface disorder and oxygen vacancies plays an important role in enhancing visible-light absorption and reducing the recombination of photogenerated electrons and holes.

Transition-metal (TM)-ion-doped TiO2 materials are of great importance for photocatalysis- and photoelectrochemical (PEC)-related applications. We introduced a facile, low-cost, and scalable doping method of TM ions (Cr, Co, Cu, Fe, Mn, etc.) into TiO2 nanotubes (NTs), while maintaining their high-ordered tubular structures (with ∼120 nm outside diameter). Both crystallization and doping processes were simultaneously accomplished in aqueous solution at a temperature as low as ∼90 °C, and the fastest doping process could be accomplished in 30 min for Fe doping. Besides, the surface areas of the doped TiO2 NTs were increased to 129.0 m2/g, and their absorption ranges could be expanded from 380 to >500 nm. This study shed light on a facile method for doping TM ions that is extendable to other semiconductors in the field of PEC water splitting and could improve their efficiencies as well.

Ultraviolet (UV) photochromism is observed from natural light in amorphous TiO2 nanotube arrays (NTAs) for the first time. Surface color of the NTAs film would change from light yellow to dark brown eventually, either under ultraviolet bulb irradiation or basking under natural sunlight. This photochromism is attributed to the appearance of Ti3 + ions in the TiO2 NTAs after UV illumination. Furthermore, a UV radiation cumulative dosimeter is designed and fabricated, consisting of a photochromic film and a colorimetric card, which convert invisible ultraviolet rays into visible color changes. This device helps people to understand intuitively how much UV radiation has been received in total from surrounding environments, which is of great importance for skin safety and public health.

Photocatalysis is believed to be one of the best methods to realize sustainable H2 production. However, achieving this through heterogeneous photocatalysis still remains a great challenge owing to the absence of active sites, sluggish surface reaction kinetics, insufficient charge separation, and a high thermodynamic barrier. Therefore, cocatalysts are necessary and of great significance in boosting photocatalytic H2 generation. This review will focus on the promising and appealing low-cost Ni-based H2-generation cocatalysts as the alternatives for the high-cost and low-abundance noble metal cocatalysts. Special emphasis has been placed on the design principle, modification strategies for further enhancing the activity and stability of Ni-based cocatalysts, and identification of the exact active sites and surface reaction mechanisms. Particularly, four types of modification strategies based on increased light harvesting, enhanced charge separation, strengthened interface interaction, and improved electrocatalytic activity have been thoroughly discussed and compared in detail. This review may open a new avenue for designing highly active and durable Ni-based cocatalysts for photocatalytic H2 generation.

MXene Ti3C2/TiO2 hybrids were successfully synthesized through a simple calcination of F-terminated Ti3C2. The resultant Ti3C2/TiO2 composite photocatalysts retained a 2D multilayer structure like MXene Ti3C2, and TiO2 exhibited a truncated octahedral bipyramidal structure with exposed (001) facets under the participation of fluorine ions. The residual Ti3C2 could act as a co-catalyst to enhance the photocatalytic H2 production activity by capturing photogenerated electrons from TiO2 because of its electron reservoir feature and suitable Fermi level. The (101)–(001) surface heterojunction of the truncated octahedral bipyramidal TiO2 further accelerated the separation of photogenerated carriers. As a result, the Ti3C2/TiO2 hybrids with calcining F-terminated Ti3C2 exhibited photocatalytic hydrogen production that is twofold higher than that of Ti3C2/TiO2 hybrids with calcining OH-terminated Ti3C2. This work presented a new strategy to prepare MXene Ti3C2/TiO2 hybrids for photoconversion applications.

Introducing oxygen vacancies and Ti 3+ into TiO2 is thought to be an effective method of increasing the utilization of sunlight and promoting the efficiency of electron transfer . In this work, novel carbon and nitrogen self-doped TiO 2 hollow spheres (CNTH) with oxygen vacancies and Ti3+ were synthesized by a one-pot hydrothermal strategy in H2O C2H5OH HF H2O2 mixed solution with TiCN as a precursor of anatase TiO2 and self-doping source of N and C. The presence of oxygen vacancies and Ti3+ in the resulting CNTH photocatalyst was supported by electron paramagnetic resonance (EPR) and X-ray absorption near-edge structure spectroscopy (XANES). X-ray photoelectron spectroscopy (XPS) analysis showed that the C and N atoms were successfully self-doped into TiO 2 lattice, which resulted in the CNTH materials exhibiting the enhanced absorption in the UV–visible region with a clear red shift at the absorption edges. The electronic structure of CNTH was examined by first-principle density functional theory (DFT) calculations, which further confirmed the optical properties of CNTH for the enhanced visible-light absorption properties and the red-shift phenomenon of the absorption edges. The resulting CNTH photocatalyst showed enhanced visible-light photocatalytic activity for H2 production. The enhanced activity of CNTH was due to the introduction of C and N self-doping and the existence of oxygen vacancies and Ti3+. The former can promote the photocatalytic reactions of the photogeneration electrons and holes on CNTH under visible-light irradiation, while the latter create more active sites and prolong the life time of electrons and holes by their donor states.

Recently, the emerging two-dimensional material MXene enjoys a high reputation due to its fascinating characteristics and shines in various research fields. Among them, MXene has received increasing attention and favor from the photocatalysis community due to its regular planar structure, outstanding metal conductivity, surface adjustable chemical properties, abundant derivatives, and excellent optical and thermal properties. There is no doubt that the introduction of MXene has endowed the photocatalytic system with extraordinary performance and has made an important impact in the field of advanced catalysis and chemical technology. Herein, on the one hand, we summarize the synthetic route of MXenes and MXene-derived 0D quantum dots (MQDs), and propose the corresponding improved synthetic strategies for the defects of MXenes obtained by the current synthetic methods. On the other hand, recent progress in MXene-based photocatalysts has been reviewed to provide a comprehensive dissection of their designs and applications. It is highly anticipated that these will present researchers with a one-stop point for the state-of-the-art development of MXene-based photocatalysts. Finally, the future opportunities and challenges of MXenes in the flourishing field of photocatalysis are also discussed.