Ultrathin 2D Photocatalysts: Electronic‐Structure Tailoring,Hybridization, and Applications

As a sustainable technology, semiconductor photocatalysis has attracted considerable interest in the past several decades owing to the potential to relieve or resolve energy and environmental‐pollution issues. By virtue of their unique structural and electronic properties, emerging ultrathin 2D materials with appropriate band structure show enormous potential to achieve efficient photocatalytic performance. Here, the state‐of‐the‐art progress on ultrathin 2D photocatalysts is reviewed and a critical appraisal of the classification, controllable synthesis, and formation mechanism of ultrathin 2D photocatalysts is presented. Then, different strategies to tailor the electronic structure of ultrathin 2D photocatalysts are summarized, including component tuning, thickness tuning, doping, and defect engineering. Hybridization with the introduction of a foreign component and maintaining the ultrathin 2D structure is presented to further boost the photocatalytic performance, such as quantum dots/2D materials, single atoms/2D materials, molecular/2D materials, and 2D–2D stacking materials. More importantly, the advancement of versatile photocatalytic applications of ultrathin 2D photocatalysts in the fields of water oxidation, hydrogen evolution, CO₂ reduction, nitrogen fixation, organic syntheses, and removal pollutants is discussed. Finally, the future opportunities and challenges regarding ultrathin 2D photocatalysts to bring about new opportunities for future research in the field of photocatalysis are also presented.     

Hybridized 2D Nanomaterials Toward Highly Efficient Photocatalysis for Degrading Pollutants: Current Status and Future Perspectives

Organic pollutants including industrial dyes and chemicals and agricultural waste have become a major environmental issue in recent years. As an alternative to simple adsorption, photocatalytic decontamination is an efficient and energy‐saving technology to eliminate these pollutants from water environment, utilizing the energy of external light, and unique function of photocatalysts. Having a large specific surface area, numerous active sites, and varied band structures, 2D nanosheets have exhibited promising applications as an efficient photocatalyst for degrading organic pollutants, particularly hybridization with other functional components. The novel hybridization of 2D nanomaterials with various functional species is summarized systematically with emphasis on their enhanced photocatalytic activities and outstanding performances in environmental remediation. First, the mechanism of photocatalytic degradation is given for discussing the advantages/shortcomings of regular 2D materials and identifying the importance of constructing hybrid 2D photocatalysts. An overview of several types of intensively investigated 2D nanomaterials (i.e., graphene, g‐C₃N₄, MoS₂, WO₃, Bi₂O₃, and BiOX) is then given to indicate their hybridized methodologies, synergistic effect, and improved applications in decontamination of organic dyes and other pollutants. Finally, future research directions are rationally suggested based on the current challenges.     

Two-dimensional photocatalyst design: A critical review of recent experimental and computational advances

In recent years, two-dimensional (2D) semiconductor photocatalysts have been widely applied in water splitting, CO₂ reduction, N₂ fixation, as well as many other important photoreactions. Photocatalysts in the form of 2D nanosheet possess many inherent advantages over traditional 3D nanopowder photocatalysts, including improved light absorption characteristics, shorter electron and hole migration paths to the photocatalysts’ surface (thus minimizing undesirable electron-hole pair recombination), and abundant surface defects which allow band gap modulation and facilitate charge transfer from the semiconductor to adsorbates. When synergistically exploited and optimized, these advantages can impart 2D photocatalysts with remarkable activities relative to their 3D counterparts. Accordingly, a wide range of experimental approaches is now being explored for the synthesis of 2D photocatalysts, with computational methods increasingly being used for identification of promising new 2D photocatalytic materials. Herein, we critically review recent literatures related to 2D photocatalyst development and design. Particular emphasis is placed on 2D photocatalyst synthesis and the importance of computational studies for the fundamental understanding of 2D photocatalyst electronic structure, band gap structure, charge carrier mobility and reaction pathways. We also explore the practical challenges of using 2D photocatalysts, such as their difficulty to synthesize in large quantity and also their characterization. The overarching aim of this review is to provide a snapshot of recent work targeting high-performance 2D photocatalysts for efficient solar energy conversion, thus laying a firm base for future advancements in this rapidly expanding area of photocatalysis research.     

Recent Advances and Perspectives of Core-Shell Nanostructured Materials for Photocatalytic CO2 Reduction

Photocatalytic CO₂ conversion into solar fuels is a promising technology to alleviate CO₂ emissions and energy crises. The development of core-shell structured photocatalysts brings many benefits to the photocatalytic CO₂ reduction process, such as high conversion efficiency, sufficient product selectivity, and endurable catalyst stability. Core-shell nanostructured materials with excellent physicochemical features take an irreplaceable position in the field of photocatalytic CO₂ reduction. In this review, the recent development of core-shell materials applied for photocatalytic reduction of CO_{2 is introduced}. First, the basic principle of photocatalytic CO₂ reduction is introduced. In detail, the classification and synthesis techniques of core-shell catalysts are discussed. Furthermore, it is also emphasized that the excellent properties of the core-shell structure can greatly improve the activity, selectivity, and stability in the process of photocatalytic CO₂ reduction. Hopefully, this paper can provide a favorable reference for the preparation of efficient photocatalysts for CO₂ reduction.     

2D g-C3N4 for advancement of photo-generated carrier dynamics: Status and challenges

2D organic g-C₃N₄ photocatalysts are low cost materials with facile fabrication, suitable bandgap, tunable functionalization, excellent thermal/chemical-physical stability and exceptional photocatalytic behavior, raising considerable interest in photocatalytic and redox research areas. The photocatalytic performance of g-C₃N₄ mostly relies on the separation/transfer of photo-generated carriers. The mobility properties of the carrier largely determine the formation of reactive species, which have a high impact on surface reactions in the photocatalytic systems based on g-C₃N₄. This review paper outlines the works carried out so far on the optimization of the carrier mobility dynamics of 2D g-C₃N₄ materials via the internal and external modification strategies. The peculiar layered planar structure of g-C₃N₄ allows charge carrier mobility at the interface, in-plane and interlayer, and mechanisms of the charge separation/transfer will also be discussed. Comprehensive conclusions and perspectives on the modification of g-C₃N₄ leading to satisfactory carrier mobility will be given as well.     

Metal–Organic‐Framework‐Based Catalysts for Photoreduction of CO2

Photoreduction of CO₂ into reusable carbon forms is considered as a promising approach to address the crisis of energy from fossil fuels and reduce excessive CO₂ emission. Recently, metal–organic frameworks (MOFs) have attracted much attention as CO₂ photoreduction‐related catalysts, owing to their unique electronic band structures, excellent CO₂ adsorption capacities, and tailorable light‐absorption abilities. Recent advances on the design, synthesis, and CO₂ reduction applications of MOF‐based photocatalysts are discussed here, beginning with the introduction of the characteristics of high‐efficiency photocatalysts and structural advantages of MOFs. The roles of MOFs in CO₂ photoreduction systems as photocatalysts, photocatalytic hosts, and cocatalysts are analyzed. Detailed discussions focus on two constituents of pure MOFs (metal clusters such as Ti–O, Zr–O, and Fe–O clusters and functional organic linkers such as amino‐modified, photosensitizer‐functionalized, and electron‐rich conjugated linkers) and three types of MOF‐based composites (metal–MOF, semiconductor–MOF, and photosensitizer–MOF composites). The constituents, CO₂ adsorption capacities, absorption edges, and photocatalytic activities of these photocatalysts are highlighted to provide fundamental guidance to rational design of efficient MOF‐based photocatalyst materials for CO₂ reduction. A perspective of future research directions, critical challenges to be met, and potential solutions in this research field concludes the discussion.     

Fabrication of Ag-doped MoO3 and its nanohybrid with a two-dimensional carbonaceous material to enhance photocatalytic activity

Recently, two-dimensional (2D) carbon-based materials and their nanocomposites have gained considerable fascination as a photocatalysts due to their remarkable contribution towards photocatalytic water splitting and remediation. Herein, a novel 2D reduced graphene oxide (rGO) based silver doped molybdenum trioxide (Ag/MoO₃) photocatalyst was synthesized successfully via hydrothermal and ultra-sonication methods. The surface structure, morphology, functional group characterization, and bandgap of the synthesized photocatalysts were analyzed using advanced physicochemical techniques. The photocatalytic performance of the prepared materials was scrutinized for Methylene blue (MB) dye degradation under solar light illumination. Because of its lower charge transfer resistance (19.54 Ω) and higher electrical conductivity (12.74 × 10² Sm^?1) the rGO/Ag/MoO₃ photocatalyst demonstrated significantly higher photocatalytic activity for dye removal than pure MoO₃ and Ag/MoO₃ photocatalysts. In particular, the rGO/Ag/MoO₃ photocatalyst illustrated about 98% dye degradation at a rate constant (0.0571 min^?1) greater than MoO₃ (0.0097 min^?1) and Ag/MoO₃ (0.0184 min^?1). Ag doping and the addition of rGO sheets led to enhanced optical absorbance and effectual separation of photo-induced electron-hole pairs, causing major progress in the photocatalytic behavior of MoO₃. Transient photocurrent results revealed longstanding photo-excited charge carriers in the graphene-based material.     

Mesoporous Polymeric Cyanamide‐Triazole‐Heptazine Photocatalysts for Highly‐Efficient Water Splitting

Conjugated polymers are promising light harvesters for water reduction/oxidation due to their simple synthesis and adjustable bandgap. Herein, both cyanamide and triazole functional groups are first incorporated into a heptazine‐based carbon nitride (CN) polymer, resulting in a mesoporous conjugated cyanamide‐triazole‐heptazine polymer (CTHP) with different compositions by increasing the quantity of cyanamide/triazole units in the CN backbone. Varying the compositions of CTHP modulates its electronic structures, mesoporous morphologies, and redox energies, resulting in a significantly improved photocatalytic performance for both H₂ and O₂ evolution under visible light irradiation. A remarkable H₂ evolution rate of 12723 µmol h^?1 g^?1 is observed, resulting in a high apparent quantum yield of 11.97% at 400 nm. In parallel, the optimized photocatalyst also exhibits an O₂ evolution rate of 221 µmol h^?1 g^?1, 9.6 times higher than the CN counterpart, with the value being the highest among the reported CN‐based bifunctional photocatalysts. This work provides an efficient molecular engineering approach for the rational design of functional polymeric photocatalysts.     

Carbon–Graphitic Carbon Nitride Hybrids for Heterogeneous Photocatalysis

Polymeric graphitic carbon nitride (g-C₃N₄) and various carbon materials have experienced a renaissance as viable alternates in photocatalysis due to their captivating metal-free features, favorable photoelectric properties, and economic adaptabilities. Although numerous efforts have focused on the integration of both materials with optimized photocatalytic performance in recent years, the direct parameters for this emerging enhancement are not fully summarized yet. Fully understanding the synergistic effects between g-C₃N₄ and carbon materials on photocatalytic action is vital to further development of metal-free semiconductors in future studies. Here, recent advances of carbon/g-C₃N₄ hybrids on various photocatalytic applications are reviewed. The dominant governing factors by inducing carbon into g-C₃N₄ photocatalysts with involving photocatalytic mechanism are highlighted. Five typical carbon-induced enhancement effects are mainly discussed here, i.e., local electric modification, band structure tailoring, multiple charge carrier activation, chemical group functionalization, and abundant surface-modified engineering. Photocatalytic performance of carbon-induced g-C₃N₄ photocatalysts for addressing directly both the renewable energy storage and environmental remediation is also summarized. Finally, perspectives and ongoing challenges encountered in the development of metal-free carbon-induced g-C₃N₄ photocatalysts are presented.     

Novelty All-Inorganic Titanium-Based Halide Perovskite for Highly Efficient Photocatalytic CO2 Conversion

Lead halide perovskite materials have great potential for photocatalytic reaction due to their low fabrication cost, unique optical absorption coefficient, and suitable band structures. However, the main problems are the toxicity and instability of the lead halide perovskite materials. Therefore, a facile synthetic method is used to prepare lead-free environmentally friendly Cs₂TiX₆(X = Cl, Cl_0.5Br_0.5, Br) perovskite materials. Their structural and optical characteristics are systematically investigated. The band gaps of the produced samples are illustrated to be from 1.87 to 2.73 eV. Moreover, these materials can keep high stability in harsh environments such as illumination and heating, and the Cs₂Ti(Cl_0.5Br_0.5)₆ microcrystals demonstrate the yields of 176 µmol g⁻¹ for CO and 78.9 µmol g⁻¹ for CH₄ after light irradiation for 3 h, which is of the first report of Ti-based perovskite photocatalysts. This finding demonstrates that the Ti-based perovskites will create opportunities for photocatalytic applications, which may offer a new idea to construct low-cost, eco-friendly, and bio-friendly photocatalysts.     

A Review of Phosphorus Structures as CO2 Reduction Photocatalysts

Effective photocatalytic carbon dioxide (CO₂) reduction into high-value-added chemicals is promising to mitigate current energy crisis and global warming issues. Finding effective photocatalysts is crucial for photocatalytic CO₂ reduction. Currently, metal-based semiconductors for photocatalytic CO₂ reduction have been well reviewed, while review of nonmetal-based semiconductors is almost limited to carbon nitrides. Phosphorus is a promising nonmetal photocatalysts with various allotropes and tunable band gaps, which has been demonstrated to be promising non-metallic photocatalysts. However, no systematic review about phosphorus structures for photocatalytic CO₂ reduction reactions has been reported. Herein, the progresses of phosphorus structures as photocatalysts for CO₂ reduction are reviewed. The fundamentals of photocatalytic CO₂ reduction, corresponding properties of phosphorus allotropes, photocatalysts with phosphorus doping or phosphorus-containing ligands, research progress of phosphorus allotropes as photocatalysts for CO₂ reduction have been reviewed in this paper. The future research and perspective of phosphorus structures for photocatalytic CO₂ reduction are also presented.     

Enhanced Photocatalytic Performances of CeO2/TiO2 Nanobelt Heterostructures

CeO₂/TiO₂ nanobelt heterostructures are synthesized via a cost‐effective hydrothermal method. The as‐prepared nanocomposites consist of CeO₂ nanoparticles assembled on the rough surface of TiO₂ nanobelts. In comparison with P25 TiO₂ colloids, surface‐coarsened TiO₂ nanobelts, and CeO₂ nanoparticles, the CeO₂/TiO₂ nanobelt heterostructures exhibit a markedly enhanced photocatalytic activity in the degradation of organic pollutants such as methyl orange (MO) under either UV or visible light irradiation. The enhanced photocatalytic performance is attributed to a novel capture–photodegradation–release mechanism. During the photocatalytic process, MO molecules are captured by CeO₂ nanoparticles, degraded by photogenerated free radicals, and then released to the solution. With its high degradation efficiency, broad active light wavelength, and good stability, the CeO₂/TiO₂ nanobelt heterostructures represent a new effective photocatalyst that is low‐cost, recyclable, and will have wide application in photodegradation of various organic pollutants. The new capture–photodegradation–release mechanism for improved photocatalysis properties is of importance in the rational design and synthesis of new photocatalysts.     

Composite hydroxyapatite/TiO2 materials for photocatalytic oxidation of NOx

Hydroxyapatite/TiO₂ composite photocatalysts were obtained from sol–gel prepared TiO₂ and commercial hydroxyapatite (HA) powders. Composites with different HA/TiO₂ ratio were studied to assess the influence of HA on the morphology and the photocatalytic behavior of the materials. Morphological SEM analysis revealed that the presence of HA diminishes the aggregation of TiO₂ particles and leads to their higher dispersion in the composites that was confirmed by the N₂ adsorption–desorption isotherms and Barret–Joyner–Halenda analysis. The photocatalytic activity of the prepared catalysts was examined by monitoring photocatalytic oxidation of NO_x model gases over catalysts under UV illumination. The NO_x oxidation over the composite catalysts was improved in comparison with pure TiO₂ powder. Moreover, the decrease of the TiO₂ content, which is the photocatalytically active component in the composites, resulted in enhanced NO_x removal. Maximum activity was recorded for composites with HA/TiO₂ ratios 1 and 2 that was related to improved TiO₂ dispersion and NO₂ trapping by the composite materials.     

BiVO4基纳米异质结光催化材料的研究进展

BiVO_4纳米异质结光催化剂因稳定性能好、无毒、禁带宽度小、在水溶液中具有高析氧活性等,成为当前研究的热点之一。本文综述了现阶段BiVO_4纳米异质结材料的研究进展,着重介绍了BiVO_4纳米异质结材料的类型、结构特点,并根据其结构阐述了电子-空穴的移动机理。同时,阐述了BiVO_4纳米异质结光催化材料在应用领域取得的成果。最后,对BiVO_4纳米异质结材料的研究方向进行了展望,期望能为研发具有优异光催化性能的BiVO_4异质结提供参考。     

Metallic MXene Saturable Absorber for Femtosecond Mode‐Locked Lasers

2D transition metal carbides, nitrides, and carbonitides called MXenes have attracted much attention due to their outstanding properties. However, MXene's potential in laser technology is not explored. It is demonstrated here that Ti₃CN, one of MXene compounds, can serve as an excellent mode‐locker that can produce femtosecond laser pulses from fiber cavities. Stable laser pulses with a duration as short as 660 fs are readily obtained at a repetition rate of 15.4 MHz and a wavelength of 1557 nm. Density functional theory calculations show that Ti₃CN is metallic, in contrast to other 2D saturable absorber materials reported so far to be operative for mode‐locking. 2D structural and electronic characteristics are well conserved in their stacked form, possibly due to the unique interlayer coupling formed by MXene surface termination groups. Noticeably, the calculations suggest a promise of MXenes in broadband saturable absorber applications due to metallic characteristics, which agrees well with the experiments of passively Q‐switched lasers using Ti₃CN at wavelengths of 1558 and 1875 nm. This study provides a valuable strategy and intuition for the development of nanomaterial‐based saturable absorbers opening new avenues toward advanced photonic devices based on MXenes.     

Ligand-Assistant Iced Photocatalytic Reduction to Synthesize Atomically Dispersed Cu Implanted Metal-Organic Frameworks for Photo-Enhanced Uranium Extraction from Seawater

Uranium extraction from natural seawater is one of the most promising routes to address the shortage of uranium resources. By combination of ligand complexation and photocatalytic reduction, porous framework-based photocatalysts have been widely applied to uranium enrichment. However, their practical applicability is limited by poor photocatalytic activity and low adsorption capacity. Herein, atomically dispersed Cu implanted UiO-66-NH₂ (Cu SA@UiO-66-NH₂) photocatalysts are prepared via ligand-assistant iced photocatalytic reduction route. N—Cu–N moiety acts as an effective electron acceptor to potentially facilitate charge transfer kinetics. By contrast, there exist Cu sub-nanometer clusters by the typical liquid phase photoreduction, resulting in a relatively low photocatalytic activity. Cu SA@UiO-66-NH₂ adsorbents exhibit superior antibacterial ability and improved photoreduction conversion of the adsorbed U(VI) to insoluble U(IV), leading to a high uranium sorption capacity of 9.16 mg-U/g-Ads from natural seawater. This study provides new insight for enhancing uranium uptake by designing SA-mediated MOF photocatalysts.     

Protonation‐Assisted Exfoliation of N‐Containing 2D Conjugated Polymers

Ultrathin 2D conjugated polymer nanosheets are an emerging class of photocatalysts for solar‐to‐chemical energy conversion. Until now, the majority of ultrathin 2D polymer photocatalysts are produced through exfoliation of layered polymers. Unfortunately, it still remains a great challenge to exfoliate layered polymers into ultrathin nanosheets with high yields. In this work, a liquid‐phase protonation‐assisted exfoliation is demonstrated to enable remarkably improved exfoliation yields of various 2D N‐containing conjugated polymers such as g‐C₃N₄, C₂N, and aza‐CMP. The exfoliation yields are only 2–15% in pure water whereas they can be substantially improved to 41–56% in 12 m HCl. The exfoliated ultrathin nanosheets possess average thicknesses less than 5 nm and can be easily dispersed in aqueous solutions. More importantly, the exfoliated nanosheets exhibit significantly enhanced photocatalytic activity toward photocatalytic water splitting compared to their bulk counterparts. Further characterizations and computational calculations reveal that protonation of the heterocyclic nitrogen sites in the conjugated polymer frameworks can lead to strong hydrogen bonding between the polymer surfaces and water molecules, resulting in facilitated exfoliation of polymers into the liquid phase. This study unveils an important protocol toward producing ultrathin 2D N‐containing conjugated polymer nanosheets for future solar energy conversion.     

Defect engineering of photocatalysts for solar-driven conversion of CO2 into valuable fuels

Photoreduction of CO₂ into valuable fuels is a clean and sustainable way to mitigate the energy crisis and environmental problems. Factors limiting the efficiency of CO₂ photoreduction include narrow-band light absorption, poor charge carrier separation and transport, and sluggish activation/reaction of CO₂ on the surface of photocatalyst. In recent years, defect engineering of photocatalysts emerges as an effective method to improve their efficiency in the photocatalytic conversion of CO₂ into useful fuels. This review is focused on discussing how structural defects can be used to modulate the electronic structure of the photocatalysts and activate the inert CO₂ molecules. Special emphasis is placed on the important impact of defects on the charge carrier dynamics of the photocatalysts. Our discussions cover a variety of defective semiconductors, including metal oxides, metal sulfides, and two dimensional materials. In addition, the challenges and prospects of defect engineering in photoreduction of CO₂ are also analyzed. This review aims to provide useful information about the fundamental principles of photoreduction of CO₂ and guidance on the design and preparation of defective photocatalysts.     

Metal‐Free 2D/2D Phosphorene/g‐C3N4 Van der Waals Heterojunction for Highly Enhanced Visible‐Light Photocatalytic H2 Production

The generation of green hydrogen (H₂) energy using sunlight is of great significance to solve the worldwide energy and environmental issues. Particularly, photocatalytic H₂ production is a highly promising strategy for solar‐to‐H₂ conversion. Recently, various heterostructured photocatalysts with high efficiency and good stability have been fabricated. Among them, 2D/2D van der Waals (VDW) heterojunctions have received tremendous attention, since this architecture can promote the interfacial charge separation and transfer and provide massive reactive centers. On the other hand, currently, most photocatalysts are composed of metal elements with high cost, limited reserves, and hazardous environmental impact. Hence, the development of metal‐free photocatalysts is desirable. Here, a novel 2D/2D VDW heterostructure of metal‐free phosphorene/graphitic carbon nitride (g‐C₃N₄) is fabricated. The phosphorene/g‐C₃N₄ nanocomposite shows an enhanced visible‐light photocatalytic H₂ production activity of 571 µmol h^?1 g^?1 in 18 v% lactic acid aqueous solution. This improved performance arises from the intimate electronic coupling at the 2D/2D interface, corroborated by the advanced characterizations techniques, e.g., synchrotron‐based X‐ray absorption near‐edge structure, and theoretical calculations. This work not only reports a new metal‐free phosphorene/g‐C₃N₄ photocatalyst but also sheds lights on the design and fabrication of 2D/2D VDW heterojunction for applications in catalysis, electronics, and optoelectronics.     

Smart Utilization of Carbon Dots in Semiconductor Photocatalysis

Efficient capture of solar energy will be critical to meeting the energy needs of the future. Semiconductor photocatalysis is expected to make an important contribution in this regard, delivering both energy carriers (especially H₂) and valuable chemical feedstocks under direct sunlight. Over the past few years, carbon dots (CDs) have emerged as a promising new class of metal‐free photocatalyst, displaying semiconductor‐like photoelectric properties and showing excellent performance in a wide variety of photoelectrochemical and photocatalytic applications owing to their ease of synthesis, unique structure, adjustable composition, ease of surface functionalization, outstanding electron‐transfer efficiency and tunable light‐harvesting range (from deep UV to the near‐infrared). Here, recent advances in the rational design of CDs‐based photocatalysts are highlighted and their applications in photocatalytic environmental remediation, water splitting into hydrogen, CO₂ reduction, and organic synthesis are discussed.