Metal-Free 2D/2D Phosphorene/g-C3 N4 Van der Waals Heterojunction for Highly Enhanced Visible-Light Photocatalytic H2 Production

2D/2D Inorganic/Organic Hybrid of Lead-Free Cs2AgBiBr6 Double Perovskite/Covalent Triazine Frameworks with Boosted Charge Separation and Efficient CO2 Photoreduction

Heterojunction construction, especially the inorganic/organic hybrids, is regarded as a universal and effective strategy to achieve high-performance photocatalysts. Herein, a 2D/2D inorganic/organic hybrid photocatalyst was constructed by the electrostatic self-assembly of the lead-free double-perovskite of Cs₂AgBiBr₆ nanosheets (NSs) and covalent triazine framework (CTF) NSs. The resultant Cs₂AgBiBr₆/CTF-1 (CABB/CTF-1) hybrid possessed a large surface-to-surface contact area, ensuring intimate interfacial interaction and efficient charge transfer/separation. Meanwhile, the periodical pore structure of CTF-1 endowed the CABB/CTF-1 hybrid with enhanced CO₂ adsorption/activation capacity. Consequently, the 2D/2D CABB/CTF-1 hybrid exhibited a remarkable photocatalytic performance toward CO₂ reduction. Based on the band structure analysis and various characterization techniques, for example, X-ray photoelectron spectra and electron spin resonance, an S-scheme charge transfer mechanism was proposed. This study presents a new protocol for designing 2D/2D inorganic/organic hybrid photocatalytic systems, which hold great potentials in solar fuel applications.

Phosphorus Tailors the d-Band Center of Copper Atomic Sites for Efficient CO2 Photoreduction under Visible-Light Irradiation

Photoreduction of CO₂ into solar fuels has received great interest, but suffers from low catalytic efficiency and poor selectivity. Herein, two single-Cu-atom catalysts with unique Cu configurations in phosphorus-doped carbon nitride (PCN), namely, Cu₁ N₃ @PCN and Cu₁ P₃ @PCN were fabricated via selective phosphidation, and tested in visible light-driven CO₂ reduction by H₂ O without sacrificial agents. Cu₁ N₃ @PCN was exclusively active for CO production with a rate of 49.8 μmol_CO  g_cat ^-1  h^-1 , outperforming most polymeric carbon nitride (C₃ N₄ ) based catalysts, while Cu₁ P₃ @PCN preferably yielded H₂ . Experimental and theoretical analysis suggested that doping P in C₃ N₄ by replacing a corner C atom upshifted the d-band center of Cu in Cu₁ N₃ @PCN close to the Fermi level, which boosted the adsorption and activation of CO₂ on Cu₁ N₃ , making Cu₁ N₃ @PCN efficiently convert CO₂ to CO. In contrast, Cu₁ P₃ @PCN with a much lower Cu 3d electron energy exhibited negligible CO₂ adsorption, thereby preferring H₂ formation via photocatalytic H₂ O splitting.

Constructing novel graphitic carbon nitride-based nanocomposites - From the perspective of material dimensions and interfacial characteristics

Two-dimensional (2D) graphitic carbon nitride (g-C₃N₄), a fascinating metal-free conjugated polymer, has garnered immense interest in the fields of solar power generation and environmental remediation. The construction of g-C₃N₄-based nanocomposites with materials of various dimensions can further improve their photocatalytic activities by surface area enlargement, bandgap tuning, heterojunction formation, etc. In this paper, we comprehensively reviewed the design, synthesis, and functionalities of g-C₃N₄-based nanocomposites based on their applications in hydrogen evolution, CO₂ reduction, and pollutants removal. We provided detailed analyses on the integration of 2D g-C₃N₄ with zero-, one-, two-, and three-dimensional materials with a focus on their interfacial characteristics and functional improvement. This review aims to stimulate fresh ideas on the interfacial engineering of g-C₃N₄-based nanocomposites to broaden their future applications.

Novel scheme towards interfacial charge transfer between ZnIn2S4 and BiOBr for efficient photocatalytic removal of organics and chromium (VI) from water

Construction of Z-scheme heterostructure is an effective strategy to enhance the charge carriers' separation. However, successfully achieving this on the defect heterojunction to improve the photocatalytic activity remains challenging. This work successfully obtained sulfur vacancy in the ZnIn₂S₄/BiOBr (SZIS/BOB) heterojunction composites with S-O covalent bonding using a hydrothermal method. As a result, they exhibited superior photocatalytic and stability performance. The optimized SZIS/BOB-10 exhibited excellent rhodamine B degradation (95.2%) and chromium (VI) reduction (97.8%) within 100 min under visible light. The enhanced composites with S-vacancies, S-O bond, and internal electric field induced the Z-scheme charge transfer mechanism. We had verified this mechanism based on the surface photovoltage spectra, electron spin response spectra, and density functional theory calculations. This work not only provides valuable insights into designing photocatalysts with a direct Z scheme heterostructure but also delineates a promising strategy for developing efficient photocatalysts to degrade organic pollutants.

Designing SnS/MoS2 van der Waals heterojunction for direct Z-scheme photocatalytic overall water-splitting by DFT investigation

Construction of direct Z-scheme photocatalytic heterojunctions with an internal electric field has been proposed as an outstanding method to achieve efficient utilization of solar energy for photocatalytic overall water-splitting. In this work, the properties of van der Waals (vdW) heterojunctions formed by group-IV mono-chalcogenides (MXs) (M = Ge, Sn; X = S, Se, Te) and MoS₂ are systematically studied by first-principles calculations, including the vdW binding energy, the direction of an internal electric field and the electronic structure. The results predict that GeS/MoS₂, GeSe/MoS₂ and SnS/MoS₂ vdW heterojunctions are potential direct Z-scheme water-splitting photocatalysts with appropriate band alignments, a wide light absorption range and low effective charge-carrier mass. Furthermore, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) activities of the heterojunctions as photocatalysts are predicted. The results indicate that SnS/MoS₂ with the Sn vacancy has a low Gibbs free energy of the HER (0.06 eV), and MoS₂ with the S edge can offer OER active sites. This study provides a theoretical basis for the further design and preparation of a new two-dimensional overall water-splitting photocatalyst, which is conducive to the development of efficient two-dimensional photocatalysts in the field of clean energy.

CdS Nanocubes Adorned by Graphitic C3N4 Nanoparticles for Hydrogenating Nitroaromatics: A Route of Visible-Light-Induced Heterogeneous Hollow Structural Photocatalysis

Modulating the transport route of photogenerated carriers on hollow cadmium sulfide without changing its intrinsic structure remains fascinating and challenging. In this work, a series of well-defined heterogeneous hollow structural materials consisting of CdS hollow nanocubes (CdS NCs) and graphitic C3N4 nanoparticles (CN NPs) were strategically designed and fabricated according to an electrostatic interaction approach. It was found that such CN NPs/CdS NCs still retained the hollow structure after CN NP adorning and demonstrated versatile and remarkably boosted photoreduction performance. Specifically, under visible light irradiation (λ ≥ 420 nm), the hydrogenation ratio over 2CN NPs/CdS NCs (the mass ratio of CN NPs to CdS NCs is controlled to be 2%) toward nitrobenzene, p-nitroaniline, p-nitrotoluene, p-nitrophenol, and p-nitrochlorobenzene can be increased to 100%, 99.9%, 83.2%, 93.6%, and 98.2%, respectively. In addition, based on the results of photoelectrochemical performances, the 2CN NPs/CdS NCs reach a 0.46% applied bias photo-to-current efficiency, indicating that the combination with CN NPs can indeed improve the migration and motion behavior of photogenerated carriers, besides ameliorating the photocorrosion and prolonging the lifetime of CdS NCs.

Constructing Heterogeneous Photocatalysts Based on Carbon Nitride Nanosheets and Graphene Quantum Dots for Highly Efficient Photocatalytic Hydrogen Generation

Although graphitic carbon nitride nanosheets (CNs) with atomic thickness are considered as promising materials for hydrogen production, the wide band gap (3.06 eV) and rapid recombination of the photogenerated electron-hole pairs impede their applications. To address the above challenges, we synergized atomically thin CNs and graphene quantum dots (GQDs), which were fabricated as 2D/0D Van der Waals heterojunctions, for H₂ generation in this study. The experimental characterizations indicated that the addition of GQDs to the π-conjugated system of CNs can expand the visible light absorption band. Additionally, the surface photovoltage spectroscopy (SPV) confirmed that introducing GQDs into CNs can facilitate the transport of photoinduced carriers in the melon chain, thus suppressing the recombination of charge carriers in body. As a result, the H₂ production activity of the Van der Waals heterojunctions was 9.62 times higher than CNs. This study provides an effective strategy for designing metal-free Van der Waals hetero-structured photocatalysts with high photocatalytic activity.

N-hexane-assisted synthesis of plasmonic Au-mediated polymeric carbon nitride photocatalyst for remarkable H2 evolution under visible-light irradiation

Plasmonic Au-mediated polymeric carbon nitride (PCN) has been recognized as one of the promising materials for photocatalytic applications due to its excellent properties in wide visible light spectrum, however it is still hindered by low catalytic efficiency. In this work, it was established that strong metal-support interactions (MSI) at the interface between plasmonic Au nanoparticles (NPs) and PCN nanosheets (PCNS) improves its photocatalytic efficiency. The resulting Au/PCNS_2.5 exhibits excellent photocatalytic activity with H₂ evolution rate up to 4.84 mmol g^-1h^-1 under visible light, 12.4 times higher than that of bulk PCN. Such strong MSI significantly strengthens the localized surface plasmon resonance (LSPR) effect of Au NPs and the charge "pump" role of Schottky junctions at Au-PCNS interface, resulting in broad light absorption range as well as effective separation and transfer of charge carrier. This work provides a new way to design the plasmonic photocatalysts for splitting water as well as other plasmon-driven chemical reactions.

A Transparent, High-Performance, and Stable Sb2 S3 Photoanode Enabled by Heterojunction Engineering with Conjugated Polycarbazole Frameworks for Unbiased Photoelectrochemical Overall Water Splitting Devices

Developing low-cost, high-performance, and durable photoanodes is essential in solar-driven photoelectrochemical (PEC) energy conversion. Sb₂ S₃ is a low-bandgap (≈1.7 eV) n-type semiconductor with a maximum theoretical solar conversion efficiency of ≈28% for PEC water splitting. However, bulk Sb₂ S₃ exhibits opaque characteristics and suffers from severe photocorrosion, and thus the use of Sb₂ S₃ as a photoanode material remains underexploited. This study describes the design and fabrication of a transparent Sb₂ S₃ -based photoanode by conformably depositing a thin layer of conjugated polycarbazole frameworks (CPF-TCzB) onto the Sb₂ S₃ film. This structural design creates a type-II heterojunction between the CPF-TCzB and the Sb₂ S₃ with a suitable band-edge energy offset, thereby, greatly enhancing the charge separation efficiency. The CPF-TCzB/Sb₂ S₃ hybrid photoanode exhibits a remarkable photocurrent density of 10.1 mA cm^-2 at 1.23 V vs reversible hydrogen electrode. Moreover, the thin CPF-TCzB overlayer effectively inhibits photocorrosion of the Sb₂ S₃ and enables long-term operation for at least 100 h with ≈10% loss in photocurrent density. Furthermore, a standalone unbiased PEC tandem device comprising a CPF-TCzB/Sb₂ S₃ photoanode and a back-illuminated Si photocathode can achieve a record solar-to-hydrogen conversion efficiency of 5.21%, representing the most efficient PEC water splitting device of its kind.

3D/2D Heterojunction of CeO2/Ultrathin MXene Nanosheets for Photocatalytic Hydrogen Production

Two-dimensional (2D) nanomaterials benefit from the high specific surface area, unique surface properties, and quantum size effects, which have attracted widespread scientific attention. MXenes add many members to the 2D material family, mainly metal conductors, most of which are dielectrics, semiconductors, or semimetals. With excellent electron mobility, beneficial to electron-hole separation, and large functional groups that can be tightly coupled with other materials, MXenes have broad application prospects in photocatalysis. Meanwhile, the application of CeO₂-based materials in organic catalysis, photocatalytic water splitting, and photodegradation of organic pollutants has been extensively explored, and studies have found that CeO₂-based materials show good photocatalytic performance. In view of this, we synthesized regular octahedral CeO₂ with a homojunction in one step by a hydrothermal method and compounded it with ultrathin 2D material MXene, which exhibited fast carrier migration efficiency and a good interfacial effect, making the material show excellent photocatalytic activity. The results showed that the photocatalytic H₂ evolution performance of the MXene/CeO₂ heterojunction was significantly improved. In this study, a low-cost catalyst with high photocatalytic activity was prepared, presenting a new research idea for achieving a combined 3D/2D photocatalytic system.

2D/2D CsPbBr3/BiOCl Heterojunction with an S-Scheme Charge Transfer for Boosting the Photocatalytic Conversion of CO2

The rational design of a two-dimensional (2D)/2D "face-to-face" heterojunction photocatalyst is crucial for the mediation of interfacial charge transfer/separation. Herein, a unique 2D/2D step-scheme (S-scheme) photocatalyst of CsPbBr₃/BiOCl is constructed by the self-assembly of CsPbBr₃ and BiOCl nanosheets (NSs). Profiting from the effective interface contact and appropriate band structures between CsPbBr₃ and BiOCl NSs, a valid S-scheme heterojunction of CsPbBr₃/BiOCl is established. Density functional theory (DFT) calculations and a series of characterization techniques including X-ray photoelectron spectra (XPS), photoassisted Kelvin probe force microscopy (KPFM), and electron spin resonance (ESR) systematically corroborate the S-scheme charge-transfer mechanism between CsPbBr₃ and BiOCl. The formation of an S-scheme heterojunction endows the photocatalyst with boosted charge separation and retainment of the highest redox ability. As a result, the obtained 2D/2D CsPbBr₃/BiOCl S-scheme photocatalyst shows much superior CO₂-reduction performance to single CsPbBr₃ and BiOCl. This investigation provides new insights into the construction of novel S-scheme heterojunctions based on 2D/2D photocatalytic systems.

Recent advances and challenges in 2D/2D heterojunction photocatalysts for solar fuels applications

Although photocatalytic technology has emerged as an effective means of alleviating the projected future fuel crisis by converting sunlight directly into chemical energy, no visible-light-driven, low-cost, and highly stable photocatalyst has been developed to date. Due to considerably higher interfacial contact with numerous reactive sites, effective charge transmission and separation ability, and strong redox potentials, the focus has now shifted to 2D/2D heterojunction systems, which have exhibited effective photocatalytic performance. The fundamentals of 2D/2D photocatalysis for different applications and the classification of 2D/2D materials are first explained in this paper, followed by strategies to improve the photocatalytic performance of various 2D/2D heterojunction systems. Following that, current breakthroughs in 2D/2D metal-based and metal-free heterojunction photocatalysts, as well as their applications for H₂ evolution via water splitting, CO₂ reduction, and N₂ fixation, are discussed. Finally, a brief overview of current constraints and predicted results for 2D/2D heterojunction systems is also presented. This paper lays out a strategy for developing efficient 2D/2D heterojunction photocatalysts and sophisticated technology for solar fuel applications in order to address the energy issue.

Photocatalytic hydrogen evolution based on carbon nitride and organic semiconductors

Photocatalytic hydrogen evolution (PHE) presents a promising way to solve the global energy crisis. Metal-free carbon nitride (CN) and organic semiconductors photocatalysts have drawn intense interests due to their fascinating properties such as tunable molecular structure, electronic states, strong visible-light absorption, low-cost etc. In this paper, the recent progresses of photocatalytic hydrogen production based on organic photocatalysts, including CN, linear polymers, conjugated porous polymers and small molecules, are reviewed, with emphasis on the various strategies to improve PHE efficiency. Finally, the possible future research trends in the organic photocatalysts are prospected.

EDTA-dominated hollow tube-like porous graphitic carbon nitride towards enhanced photocatalytic hydrogen evolution

Graphite carbon nitride (g-C₃N₄) as metal-free photocatalyst has been widely studied recently in photocatalytic water reduction, which is considered as one of the promising routes to realizing the hydrogen energy-based society in the future. The generally used preparation process based on thermal polymerization of precursors easily brought the formation of aggregated nanosheets morphology, severely limiting its photocatalytic activity. Herein, the hollow tube-like morphology with porous surface was elaborately obtained by ethylene diamine tetraacetic acid (EDTA)-involved hydrothermal treatment of melamine precursor. The hollow and porous features shortened the migration distance of photo-generated carriers, trapped the incident lights, and provided more photocatalytic reactive sites, then realizing the enhanced photocatalytic H₂-evolution activity up to 7.1 times that of pristine g-C₃N₄. The presence of EDTA acted as the pivotal role to control the recrystallization process of melamine and its derivative, cyanuric acid, and thus to determine the framework formation of the hollow tube-like microstructure. Moreover, complete thermal decomposition of cyanuric acid during the thermal polymerization of precursors was responsible for the hollow and porous features. This work extends the morphology regulation cognition of g-C₃N₄ based on hydrothermal treatment of precursors, and is expected to bring deep understanding and feasible strategies to design morphology-dominated highly-efficient g-C₃N₄ photocatalysts.

Promoting Charge Separation in Hollow-Structured C/MoS2@ZnIn2S4/Co3O4 Photocatalysts via Double Heterojunctions for Enhanced Photocatalytic Hydrogen Evolution

A reasonable design of samples with efficient spatial separation for photoinduced electron-hole pairs toward the photocatalytic hydrogen evolution reaction (HER) has gained significant attention. Herein, a new C/MoS₂@ZnIn₂S₄/Co₃O₄ composite with a core-shell structure is designed toward photocatalytic hydrogen production on C/MoS₂ and Co₃O₄ dual electron collectors. Co₃O₄ nanoparticles as the co-catalyst would form a Schottky junction with ZnIn₂S₄ nanosheets while the C/MoS₂ hollow core would form the step-scheme (S-scheme) heterojunction with ZnIn₂S₄ sheets, which provides a dual photogenerated electron transfer pathway during the light irradiation process. In addition, the unique core-shell architecture offers large contact interfaces favoring the exposure of rich active sites, which facilitated the separation and the transfer of charges. Consequently, all these factors endowed the C/MoS₂@ZnIn₂S₄/Co₃O₄ composite with enhanced light absorption ability and an increased hydrogen evolution rate of 6.7 mmol·g^-1·h^-1 under 420 nm light irradiation, which is about 23.4- and 4.5-fold that of ZnIn₂S₄ and CMZ, respectively. This work offers a guideline for designing efficient composite photocatalysts toward the photocatalytic HER.

Preparation of metal-free BP/CN photocatalyst with enhanced ability for photocatalytic tetracycline degradation

The successful application of photocatalysis in practical water treatment opreations relies greatly on the development of highly efficient, stable and low-cost photocatalysts. The low-cost metal-free photocatalyst made up of black phosphorus (BP) and graphitic carbon nitride (CN) was successfully constructed and firstly used for the photocatalytic treatment of antibiotic contaminants in this work. Compared with bare CN, the BP/CN photocatalyst exhibited the enhanced photocatalytic performance for tetracycline hydrochloride (HTC) degradation, that 99% of HTC was removed by 6BP/CN (doping amount of BP was 6%) within 30 min under the simulated visible-light irradiation. The efficiency was even comparable to those of some high-efficiency photocatalysts recently-reported such as Fe⁰@POCN, CuInS₂/Bi₂MoO₆ and Cu₂O@HKUST-1. Under natural sunlight illumination, the determined apparent rate constant for degradation of HTC by BP/CN was 2.7 times as that by P25 TiO₂. The experimental results indicated that loading BP on CN could enhance the separation of charge carriers and promote the ability of light absorption for visible-light, thus leading to a greater catalytic activity. Meanwhile, the influences of different operating variables (pH, water, ion and HTC concentration) on HTC degradation were studied in detail. Furthermore, the degradation pathway of HTC was also proposed. In addition, the photocatalytic activity of the BP/CN for production of hydrogen peroxide (H₂O₂) was also studied, which could reach up to 501.04 μmol g^-1h^-1. It is anticipated that BP/CN photocatalyst could be used for practical water treatment.

Synthesis and application of CdS nanorods for LED-based photocatalytic degradation of tetracycline antibiotic

With the rapid development of pharmaceutical industrialization, increased consumption of drugs and discharged sewage contains antibiotics that lead to water contamination. For this purpose, removal of antibiotics from aquatic environment is emphasizing the need to produce clean water using easy separable catalysts through photocatalytic water remediation and thus the semiconductor photocatalysts have presently gained fascinating unprecedented research attention. Herein, we present the synthesis of semiconductor CdS nanorods by a facile hydrothermal procedure using ethylene diamine as a coordinating agent. Then, we subsequently studied the photocatalytic activity of CdS nanorods under blue and white LED light irradiation for the degradation of tetracycline antibiotic as a model compound. The light dependent photocatalytic activity of CdS nanorods demonstrated that CdS nanorods possess higher catalytic efficiency in presence of blue light compared to white light toward the photocatalytic degradation of tetracycline antibiotic. We have also studied the photocatalytic activity in presence of various light intensity. These CdS nanorods exhibited the highest tetracycline degradation efficacy of 95.6% within 60 min in presence of blue light (intensity: 200W/m²) without any supplementary oxygen sources during the degradation reaction. The photocatalytic mechanism of the tetracycline degradation has been elucidated by scavenging experiment. The experimental results indicate the formation of reactive oxygen species during photocatalytic degradation of tetracycline antibiotic. This work represents an alternative route to develop heterogeneous photocatalyst for antibiotics degradation due to the outstanding efficiency and stability of the CdS nanorods as well as easy separation through simple filtration method. It is anticipated that this work will shed light in the practical applications of CdS nanorods for environmental remediation through wastewater treatment.

Recent Advancement of the Current Aspects of g-C3 N4 for its Photocatalytic Applications in Sustainable Energy System

Being one of the foremost enticing and intriguing innovations, heterogeneous photocatalysis has also been used to effectively gather, transform, and conserve sustainable sun's radiation for the production of efficient and clean fossil energy as well as a wide range of ecological implications. The generation of solar fuel-based water splitting and CO₂ photoreduction is excellent for generating alternative resources and reducing global warming. Developing an inexpensive photocatalyst can effectively split water into hydrogen (H₂ ), oxygen (O₂ ) sources, and carbon dioxide (CO₂ ) into fuel sources, which is a crucial problem in photocatalysis. The metal-free g-C₃ N₄ photocatalyst has a high solar fuel generation potential. This review covers the most recent advancements in g-C₃ N₄ preparation, including innovative design concepts and new synthesis methods, and novel ideas for expanding the light absorption of pure g-C₃ N₄ for photocatalytic application. Similarly, the main issue concerning research and prospects in photocatalysts based g-C₃ N₄ was also discussed. The current dissertation provides an overview of comprehensive understanding of the exploitation of the extraordinary systemic and characteristics, as well as the fabrication processes and uses of g-C₃ N₄ .

A nanocubicle-like 3D adsorbent fabricated by in situ growth of 2D heterostructures for removal of aromatic contaminants in water

Focusing on the emergence of organic pollutants in aqueous environments, attempts to assemble two-dimensional (2D) materials into three-dimensional (3D) structures are expected to improve their pollution control performance. However, most 3D heterostructural nanomaterials are constructed by mechanical mixing methods, which result in structures that are randomly arranged and prone to collapse. Two typical 2D carbon materials, reduced graphene oxide (rGO) and covalent triazine frameworks (CTFs), have exhibited excellent effects in the fields of contaminant adsorption and photocatalysis, respectively. However, their regular packing structure could not provide an interconnected pore network suitable for the diffusion or adsorption of pollutants. In this study, a series of heterostructures named rGCs were fabricated by direct growth of 2D CTFs with different ratios on the surface of rGO layers. The rGCs were designed to remove trace concentrations of naphthalene (NAP) and benzophenone (BP) from water, which can be regenerated under sunlight. rGC-20, in which nanocubicle-like 3D heterostructures were successfully constructed, not only adsorbed NAP and BP with superb normalized adsorption capacities (5000-5300 μmol/g) but also could be regenerated with an exceptional percentage recovery of 90-95% in the 4th cycle. The microenvironment created in nanocubicle-like 3D heterostructures enhances the adsorption of pollutants, the excitation of electrons and utilization of radicals, which further promotes the adsorption and photocatalysis of rGCs. This work provides a promising adsorbent with outstanding adsorption-regeneration ability for aromatic contaminant removal from water. DATA AVAILABILITY: The main data that support the findings of this study are available from the article and its Supplementary Information. Extra data are available from the corresponding author on request.

Emerging 2D Materials for Electrocatalytic Applications: Synthesis, Multifaceted Nanostructures, and Catalytic Center Design

Currently, the development of advanced 2D nanomaterials has become an interdisciplinary subject with extensive studies due to their extraordinary physicochemical performances. Beyond graphene, the emerging 2D-material-derived electrocatalysts (2D-ECs) have aroused great attention as one of the best candidates for heterogeneous electrocatalysis. The tunable physicochemical compositions and characteristics of 2D-ECs enable rational structural engineering at the molecular/atomic levels to meet the requirements of different catalytic applications. Due to the lack of instructive and comprehensive reviews, here, the most recent advances in the nanostructure and catalytic center design and the corresponding structure-function relationships of emerging 2D-ECs are systematically summarized. First, the synthetic pathways and state-of-the-art strategies in the multifaceted structural engineering and catalytic center design of 2D-ECs to promote their electrocatalytic activities, such as size and thickness, phase and strain engineering, heterojunctions, heteroatom doping, and defect engineering, are emphasized. Then, the representative applications of 2D-ECs in electrocatalytic fields are depicted and summarized in detail. Finally, the current breakthroughs and primary challenges are highlighted and future directions to guide the perspectives for developing 2D-ECs as highly efficient electrocatalytic nanoplatforms are clarified. This review provides a comprehensive understanding to engineer 2D-ECs and may inspire many novel attempts and new catalytic applications across broad fields.

Metal-free BN quantum dots/graphitic C3N4 heterostructure for nitrogen reduction reaction

Exploring high-efficiency metal-free electrocatalysts towards N₂ reduction reaction (NRR) is of great interest for the development of electrocatalytic N₂ fixation technology. Herein, we combined boron nitride quantum dots (BNQDs) and graphitic carbon nitride (C₃N₄) to design a metal-free BNQDs/C₃N₄ heterostructure as an effective and durable NRR catalyst. The electronically coupled BNQDs/C₃N₄ presented an NH₃ yield as high as 72.3 μg h^-1 mg^-1 (-0.3 V) and a Faradaic efficiency of 19.5% (-0.2 V), far superior to isolated BNQDs and C₃N₄, and outperforming nearly all previously reported metal-free catalysts. Theoretical computations unveiled that the N₂ activation could be drastically enhanced at the BNQDs-C₃N₄ interface where interfacial BNQDs and C₃N₄ cooperatively adsorb N₂ and stabilize *N₂H intermediate, leading to the significantly promoted NRR process with an ultra-low overpotential of 0.23 V.

MOF nanosheet-derived carbon-layer-coated CoP/g-C3N4 photocatalysts with enhance charge transfer for efficient photocatalytic H2 generation

Catalysts with low cost, high efficiency, and no need for precious metals cocatalysts, are the key to realizing the maximum utilization of solar energy-efficient hydrogen (H2) production. In this study,...

Synergism between few-layer black phosphorus and graphitic carbon nitride enhances the photoredox C–H arylation under visible light irradiation

A volcano-shaped relation between the amount of FLBP in the FLBP/g-CN heterojunctions in the photoredox C–H arylation was exhibited. To understand the activity of 35 wt% FLBP/g-CN, band alignments of heterojunction was studied in detailed.

Highly crystalline sulfur and oxygen co-doped g-C3N4 nanosheets as an advanced photocatalyst for efficient hydrogen generation

Heteroatom doping has become a promising strategy to tailor the band structure and physicochemical properties of graphitic carbon nitride (g-C3N4). However, doping heteroatoms usually lead to decreased crystallinity, thereby an...

Crystallization kinetics of amorphous red phosphorus to black phosphorus by chemical vapor transport

The aRP–P4-HP–BP three-stage phase transition revealed the crystallization kinetics and nucleation mechanism of the high-quality BP crystal synthesized by the CVT reaction in the aRP–Sn–I system.

Atomic heterojunction-induced accelerated charge transfer for boosted photocatalytic hydrogen evolution over 1D CdS nanorod/2D ZnIn2S4 nanosheet composites

Design of highly efficient heterojunctions for photocatalytic hydrogen evolution is of significant importance to address the energy shortage and environmental crisis. Nevertheless, the smart design of semiconductor-based heterojunctions at the atomic scale still remains a significant challenge hitherto. Herein, we report novel atomic CdS/ZnIn₂S₄ heterojunctions by in-situ epitaxially growing 2D ZnIn₂S₄ nanosheets onto the surface of 1D defective CdS nanorods. The strong electronic coupling between defective CdS and ZnIn₂S₄ is confirmed by transient photocurrent response measurements, •O₂^- and •OH radicals experiments, and PL results, leading to accelerated interfacial charge separation and transfer. Additionally, the elevated charge transfer and electronic coupling are further confirmed by theoretical calculations. Consequently, CdS/ZnIn₂S₄ hybrids exhibit superior photocatalytic hydrogen generation activity to pristine CdS. Our findings offer a new paradigm for designing atomic 1D/2D heterojunctions for efficient solar-driven energy conversion.

Copper phosphide decorated g-C3N4 catalysts for highly efficient photocatalytic H2 evolution

Designing functional heterojunctions to enhance photocatalytic hydrogen evolution is still a key challenge in the field of efficient solar energy utilization. Copper phosphides become an ideal material to serve as the cocatalysts during photocatalytic hydrogen evolution by virtue of the lower prices. In this study, we synthesized graphitic carbon nitride (g-C₃N₄) based catalysts loaded with copper phosphide (Cu₃P, Cu₉₇P₃), which exhibit superior performance in photocatalytic H₂ evolution. Ultraviolet (UV)-visible spectroscopy illustrated that the absorption of light strengthened after the loading of copper phosphide, and the time-resolved transient photoluminescence (PL) spectra showed that the separation and transfer of the photoexcited carriers greatly improved. Moreover, both copper phosphide/g-C₃N₄ photocatalysts exhibited a relatively high H₂ evolution rate: Cu₃P/g-C₃N₄ (maximum 343 μmol h^-1 g^-1), Cu₉₇P₃/g-C₃N₄ (162.9 μmol h^-1 g^-1) while copper phosphide themself exhibit no photocatalytic activity. Thus, the copper phosphides (Cu₃P, Cu₉₇P₃) work as a cocatalyst during photocatalytic H₂ evolution. The cycling experiments illustrated that both copper phosphide/g-C₃N₄ photocatalysts perform excellent stability in the photocatalytic H₂ evolution. It is worth noting that while the NaH₂PO₂ was heated in the tube furnace for phosphorization to obtain Cu₃P, the excessive PH₃ could pass through the solution of CuSO₄ to obtain Cu₉₇P₃ at the same time, which significantly improved the utilization of PH₃ and reduced the risk of toxicity. This work could provide new strategies to design photocatalysts decorated with copper phosphide for highly efficient visible-light-driven hydrogen evolution.

Selective graphene-like metal-free 2D nanomaterials and their composites for photocatalysis

From the viewpoint of sustainability, graphene-like metal-free 2D nanomaterials (GMFs) hold great potential in different photocatalytic fields due to their distinct structures and properties. Although their lattice structures are highly similar, the properties of these nanomaterials are in vast diversity owing to the uniqueness of particular atomic arrangement, thus giving rise to their multi-faceted functionalities in photocatalytic process. In this review, we summarize the latest progress of GMFs and their hybrid composites in photocatalytic field, including graphene and its derivatives, hexagonal boron nitride (h-BN), graphitic carbon nitride (g-C₃N₄), black phosphorus (BP) and emerging 2D covalent organic frameworks (COFs). Their unique 2D structure and key photocatalytic properties are firstly briefly introduced. Then a critical discussion on their multiple roles in the activity enhancement of composite photocatalysts is emphasized, which in turn points out the direction of maximizing their functions and guides our efficient construction of hybrid photocatalysts based on above 2D nanomaterials. On this basis, a summary about the hybridization of above 2D metal-free materials is presented, and the merits of 2D/2D hybrid systems are elaborated. Last, we wrap up this review with some summative remarks, covering understanding their own unique strengths and weaknesses by comparison and proposing the major challenges and perspectives in this emerging field.

Self-Supporting 3D Carbon Nitride with Tunable n → π* Electronic Transition for Enhanced Solar Hydrogen Production

Self-supporting 3D (SSD) carbon nitrides (UCN-X, X = 600-690; where X represents the pyrolytic temperature) consisting of curved layers, with plenty of wrinkles and enlarged size, are synthesized via a facile stepwise pyrolytic strategy. Such unique features of the SSD structure exhibiting dramatically improved charge mobility, extended π-conjugated aromatic system, and partial distortion of heptazine-based skeleton can not only keep the easier activation of the intrinsic π →  π* electronic transition but also awaken the n → π* electronic transition in carbon nitride. The n → π* electronic transition of UCN-X can be controllably tuned through changing the pyrolytic temperature, which can greatly extend the photoresponse range to 600 nm. More importantly, the change regularity of H₂ evolution rates is highly positive, correlated with the change tendency of n → π* electronic transition in UCN-X, suggesting the positive contribution of n → π* electronic transition to enhancing photocatalytic activity. The UCN-670, with optimal structural and optical properties, presents enhanced H₂ evolution rate up to 9230 µmol g^-1 h^-1 (Pt 1.1 wt%). This work realizes the synergistic optimization of optical absorption and exciton dissociation via fabricating an SSD structure. It offers a new strategy for the development of novel carbon nitride materials for efficient photocatalytic reactions.

Hierarchical CoS2/MoS2 flower-like heterostructured arrays derived from polyoxometalates for efficient electrocatalytic nitrogen reduction under ambient conditions

Electrochemical nitrogen reduction reaction (NRR) has been identified as a prospective alternative for sustainable ammonia production. Developing cost-effective and highly efficient electrocatalysts is critical for NRR under ambient conditions. Herein, the hierarchical cobalt-molybdenum bimetallic sulfide (CoS₂/MoS₂) flower-like heterostructure assembled from well-aligned nanosheets has been easily fabricated through a one-step strategy. The efficient synergy between different components and the formation of heterostructure in CoS₂/MoS₂ nanosheets with abundant active sites makes the non-noble metal catalyst CoS₂/MoS₂ highly effective in NRR, with a high NH₃ yield rate (38.61 μg h^-1 mg_cat.^-1), Faradaic efficiency (34.66%), high selectivity (no formation of hydrazine) and excellent long-term stability in 1.0 mol L^-1 K₂SO₄ electrolyte (pH = 3.5) at -0.25 V versus the reversible hydrogen electrode (vs. RHE) under ambient conditions, exceeding much recently reported cobalt- and molybdenum-based materials, even catch up with some noble-metal-based catalyst. Density functional theory (DFT) calculation indicates that the formation of N₂H* species on CoS₂(200)/MoS₂(002) is the rate-determining step via both the alternating and distal pathways with the maximum ΔG values (1.35 eV). These results open up opportunities for the development of efficient non-precious bimetal-based catalysts for NRR.

Engineering Nitrogen Vacancy in Polymeric Carbon Nitride for Nitrate Electroreduction to Ammonia

Electrocatalytic nitrate reduction to ammonia is of great interest in terms of energy conservation and environmental protection. However, the development of abundant metal-free electrocatalysts with high activity, selectivity, and stability is still a big challenge. Herein, polymeric graphitic carbon nitride (g-C₃N₄) with controllable numbers of nitrogen vacancies is reported to exhibit high Faradaic efficiency (89.96%), selectivity (69.78%), and stability toward nitrate-to-ammonia conversion. ¹⁵N isotope labeling experiments prove the produced ammonia originating from nitrate reduction. The combined results of ex situ and in situ characterizations unveil the reaction pathway based on the captured critical intermediates. Density functional theory calculations reveal that nitrogen vacancies could introduce a new electron state at the Fermi level and promote the adsorption, activation, and dissociation of nitrate. An appropriate content of nitrogen vacancies is beneficial for modulating the adsorption energies of reaction intermediates (*NO, *NOH, *NH₂, etc.), facilitating the enhancement in ammonia selectivity and Faradaic efficiency.

Coexistence of Photoelectric Conversion and Storage in van der Waals Heterojunctions

Van der Waals (vdW) heterojunctions, based on two-dimensional (2D) materials, have great potential for the development of ecofriendly and high-efficiency nanodevices, which shows valuable applications as photovoltaic cells, photodetectors, etc. However, the coexistence of photoelectric conversion and storage in a single device has not been achieved until now. Here, we demonstrate a simple strategy to construct a vdW p-n junction between a WSe_{2} layer and quasi-2D electron gas. After an optical illumination, the device stores the light-generated carriers for up to seven days, and then releases a very large photocurrent of 2.9 mA with bias voltage applied in darkness; this is referred to as chargeable photoconductivity (CPC), which completely differs from any previously observed photoelectric phenomenon. In normal photoconductivity, the recombination of electron-hole pairs occurs at the end of their lifetime; in contrast, infinite-lifetime photocarriers can be generated and stored in CPC devices without recombination. The photoelectric conversion and storage are completely self-excited during the charging process. The ratio between currents in full- and empty-photocarrier states below the critical temperature reaches as high as 10^{9}, with an external quantum efficiency of 93.8% during optical charging. A theoretical model developed to explain the mechanism of this effect is in good agreement with the experimental data. This work paves a path toward the high-efficiency devices for photoelectric conversion and storage.

Activating Carbon Nitride by BP@Ni for the Enhanced Photocatalytic Hydrogen Evolution and Selective Benzyl Alcohol Oxidation

Two-dimensional (2D) semiconductors are promising photocatalysts; in order to overcome the relatively low efficiency of single-component 2D photocatalysts, heterostructures are fabricated for effective charge separation. Herein, a 2D heterostructure is synthesized by anchoring nickel nanoparticle-decorated black phosphorus (BP) nanosheets to graphitic carbon nitride (CN) nanosheets (CN/BP@Ni). The CN/BP@Ni heterostructure exhibits an enhanced charge separation due to the tight interfacial interaction and the cascaded electron-transfer channel from CN to BP and then to Ni nanoparticles. Possessing abundant active sites of Ni and P-N coordinate bonds, CN/BP@Ni shows a high visible-light-driven H₂ evolution rate of 8.59 mmol·h^-1·g^-1 with the sacrificial agent EtOH, about 10-fold to that of CN/BP. When applying benzyl alcohol to consume photogenerated holes, CN/BP@Ni enables the selective production of benzaldehyde; therefore, two value-added products are obtained in a single closed redox cycle. This work provides new insights into the development of photocatalysts without non-noble metals.

2D/2D Heterojunction systems for the removal of organic pollutants: A review

Photocatalysis is considered to be an effective way to remove organic pollutants, but the key to photocatalysis is finding a high-efficiency and stable photocatalyst. 2D materials-based heterojunction has aroused widespread concerns in photocatalysis because of its merits in more active sites, adjustable band gaps and shorter charge transfer distance. Among various 2D heterojunction systems, 2D/2D heterojunction with a face-to-face contact interface is regarded as a highly promising photocatalyst. Due to the strong coupling interface in 2D/2D heterojunction, the separation and migration of photoexcited electron-hole pairs are facilitated, which enhances the photocatalytic performance. Thus, the design of 2D/2D heterojunction can become a potential model for expanding the application of photocatalysis in the removal of organic pollutants. Herein, in this review, we first summarize the fundamental principles, classification, and strategies for elevating photocatalytic performance. Then, the synthesis and application of the 2D/2D heterojunction system for the removal of organic pollutants are discussed. Finally, the challenges and perspectives in 2D/2D heterojunction photocatalysts and their application for removing organic pollutants are presented.

Carbon-based metal-free electrocatalysts: from oxygen reduction to multifunctional electrocatalysis

Since the discovery of N-doped carbon nanotubes as the first carbon-based metal-free electrocatalyst (C-MFEC) for oxygen reduction reaction (ORR) in 2009, C-MFECs have shown multifunctional electrocatalytic activities for many reactions beyond ORR, such as oxygen evolution reaction (OER), hydrogen evolution reaction (HER), carbon dioxide reduction reaction (CO₂RR), nitrogen reduction reaction (NRR), and hydrogen peroxide production reaction (H₂O₂PR). Consequently, C-MFECs have attracted a great deal of interest for various applications, including metal-air batteries, water splitting devices, regenerative fuel cells, solar cells, fuel and chemical production, water purification, to mention a few. By altering the electronic configuration and/or modulating their spin angular momentum, both heteroatom(s) doping and structural defects (e.g., atomic vacancy, edge) have been demonstrated to create catalytic active sites in the skeleton of graphitic carbon materials. Although certain C-MFECs have been made to be comparable to or even better than their counterparts based on noble metals, transition metals and/or their hybrids, further research and development are necessary in order to translate C-MFECs for practical applications. In this article, we present a timely and comprehensive, but critical, review on recent advancements in the field of C-MFECs within the past five years or so by discussing various types of electrocatalytic reactions catalyzed by C-MFECs. An emphasis is given to potential applications of C-MFECs for energy conversion and storage. The structure-property relationship for and mechanistic understanding of C-MFECs will also be discussed, along with the current challenges and future perspectives.

Fe-Doped Graphitic Carbon Nitride for Methylene Blue Degradation with Visible-Light

In the present work, degradation of methylene blue (MB) dye in aqueous solution through H₂O ₂and iron doped g-C₃N₄ (Fe-g-C₃N₄) was studied. The hybrid was fabricated by thermal polymerization with iron (III) nitrate nonahydrate and melamine, and it was characterized by X-ray diffraction, Fourier transform infrared, UV-Vis diffuse reflectance spectrum, X-ray photoelectron spectroscopy, transmission electron microscope and Brunner-Emmet-Teller. The various experimental conditions such as doping amount, a dose of the sample, solution pH, the addition of H₂O₂, and concentration of MB on the degradation of MB dye were optimized. The maximum extent of degradation of methylene blue was obtained at pH 5, doping amount of 2.7 wt% and dose of 0.07 g. The molar ratio of Fe:H₂O₂ is 1:1000 showed 99% of MB (30 mg/L) decolorization over 60 min. The hybrid showed good stability and recyclability after three cycles of use. Photo-Fenton reaction exhibited a higher synergetic effect than the combination of Fenton and photocatalytic process.