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

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.

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.

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.

Self-assembled ultrathin closely bonded 2D/2D heterojunction for enhanced visible-light-induced photocatalytic oxidation and reaction mechanism insights

Two-dimensional (2D) layered heterojunctions with a staggered band structure and unique interface properties exhibit promising application prospects in photocatalytic pollutant removal, water splitting, and CO₂ reduction. Ultrathin 2D/2D heterojunctions with a large specific surface area and a short migration path of the photogenerated charge always illustrate a better photocatalytic performance than non-ultrathin 2D heterojunction photocatalysts. In this study, a novel ultrathin 2D/2D heterojunction of the Bi₂O₂(OH)(NO₃)/BiOBr nanosheet composite (ultrathin BION/BiOBr) was in situ self-assembled though a cetyltrimethylammonium bromide assisted one-step hydrothermal method. Benefiting from the advantage of the unique ultrathin heterojunction structure, the ultrathin 2D/2D BION/BiOBr heterojunctions exhibit a greatly improved photocatalytic removal effect for multiple pollutants compared to the nanocrystal BION/BiOBr, pure BION. As a representative, the ultrathin 2D/2D Br-modified BION/BiOBr heterojunction shows an enhanced tetracycline degradation rate of 76%, which corresponded to a higher photodegradation rate constant of 0.01116 min^-1 when compared to pure BION (17%, 0.00161 min^-1) and nanocrystal BION/BiOBr (24%, 0.00223 min^-1) under visible-light irradiation for 2 h. A series of characterization and density functional theory calculations demonstrate the enhanced separation and migration efficiency of the photogenerated electrons and holes over the ultrathin heterojunction, facilitating the formation of oxidizing groups for the organic pollutant removal. The possible mechanism of the TC photodegradation and the possible photodegradation pathway are also investigated in detail. This work provides a feasible method for constructing ultrathin 2D/2D heterojunction materials for environmental purification.

Construction of Chemically Bonded Interface of Organic/Inorganic g-C3N4/LDH Heterojunction for Z-Schematic Photocatalytic H2 Generation

The design and synthesis of a Z-schematic photocatalytic heterostructure with an intimate interface is of great significance for the migration and separation of photogenerated charge carriers, but still remains a challenge. Here, we developed an efficient Z-scheme organic/inorganic g-C3N4/LDH heterojunction by in situ growing of inorganic CoAl-LDH firmly on organic g-C3N4 nanosheet (NS). Benefiting from the two-dimensional (2D) morphology and the surface exposed pyridine-like nitrogen atoms, the g-C3N4 NS offers efficient trap sits to capture transition metal ions. As such, CoAl-LDH NS can be tightly attached onto the g-C3N4 NS, forming a strong interaction between CoAl-LDH and g-C3N4 via nitrogen-metal bonds. Moreover, the 2D/2D interface provides a high-speed channel for the interfacial charge transfer. As a result, the prepared heterojunction composite exhibits a greatly improved photocatalytic H2 evolution activity, as well as considerable stability. Under visible light irradiation of 4 h, the optimal H2 evolution rate reaches 1952.9 μmol g-1, which is 8.4 times of the bare g-C3N4 NS. The in situ construction of organic/inorganic heterojunction with a chemical-bonded interface may provide guidance for the designing of high-performance heterostructure photocatalysts.

Spontaneous DNA translocation through a van der Waals heterostructure nanopore for single-molecule detection

Solid-state nanopore detection and sequencing of a single molecule offers a new paradigm because of its several well-recognized features such as long reads, high throughput, high precision and direct analyses. However, several key technical challenges are yet to be addressed, especially the abilities to control the speed and direct the translocation of the target molecules. In this work, using molecular dynamics (MD) simulations, we found a spontaneous translocation of single-stranded DNA (ssDNA) through a van der Waals (vdW) heterostructure nanopore formed by stacking two graphenic materials, namely those of BC₃ and C₃N. Our results showed that, without using an external stimulus, ssDNA can be spontaneously transported through such a vdW nanopore from its BC₃ side to its C₃N side, with the C₃N surface demonstrating a stronger capability than the BC₃ surface to attract DNA bases. Thus, the distinct binding strengths of BC₃ and C₃N were concluded to drive the ssDNA translocation. The results indicated the vdW forces playing a leading role during the translocation process. Our simulations also showed, at the edges of the nanopore, a clear energy barrier for nucleotides, resulting in a translocation speed slowed to a value of 0.2 μs per base, i.e., twice as slow as that indicated for the latest published methods. The present findings provide a new architecture for biomolecule detection and sequencing, which may be considered some of the most important functions of nanomaterials in biological and chemical analyses.

Emerging beyond-graphene elemental 2D materials for energy and catalysis applications

Elemental two-dimensional (2D) materials have emerged as promising candidates for energy and catalysis applications due to their unique physical, chemical, and electronic properties. These materials are advantageous in offering massive surface-to-volume ratios, favorable transport properties, intriguing physicochemical properties, and confinement effects resulting from the 2D ultrathin structure. In this review, we focus on the recent advances in emerging energy and catalysis applications based on beyond-graphene elemental 2D materials. First, we briefly introduce the general classification, structure, and properties of elemental 2D materials and the new advances in material preparation. We then discuss various applications in energy harvesting and storage, including solar cells, piezoelectric and triboelectric nanogenerators, thermoelectric devices, batteries, and supercapacitors. We further discuss the explorations of beyond-graphene elemental 2D materials for electrocatalysis, photocatalysis, and heterogeneous catalysis. Finally, the challenges and perspectives for the future development of elemental 2D materials in energy and catalysis are discussed.

Photodepositing CdS on the Active Cyano Groups Decorated g-C3 N4 in Z-Scheme Manner Promotes Visible-Light-Driven Hydrogen Evolution

g-C₃ N₄ /CdS heterojunctions are potential photocatalysts for hydrogen production but their traditional type-II configuration generally leads to weak oxidative and reductive activity. How to construct the novel Z-scheme g-C₃ N₄ /CdS counterparts to address this issue remains a great challenge in this field. In this work, a new direct Z-scheme heterojunction of defective g-C₃ N₄ /CdS is designed by introducing cyano groups (NC-) as the active bridge sites. Experimental observations in combination with density functional theory (DFT) calculations reveal that the unique electron-withdrawing feature of cyano groups in the defective g-C₃ N₄ /CdS heterostructure can endow this photocatalyst with numerous advantageous properties including high light absorption ability, strong redox performance, satisfactory charge separation efficiency, and long lifetime of charge carriers. Consequently, the resultant photocatalytic system exhibits more active performance than CdS and g-C₃ N₄ under visible light and reaches an excellent hydrogen evolution rate of 1809.07 µmol h^-1 g^-1 , which is 6.09 times higher than pristine g-C₃ N₄ . Moreover, the defective g-C₃ N₄ /CdS photocatalyst maintains good stability after 40 h continuous test. This work provides new insights into design and construction of Z-scheme heterojunctions for regulating the visible-light-induced photocatalytic activity for H₂ evolution.

Emerging Cocatalysts on g-C3 N4 for Photocatalytic Hydrogen Evolution

Over the past few decades, graphitic carbon nitride (g-C₃ N₄ ) has arisen much attention as a promising candidate for photocatalytic hydrogen evolution reaction (HER) owing to its low cost and visible light response ability. However, the unsatisfied HER performance originated from the strong charge recombination of g-C₃ N₄ severely inhibits the further large-scale application of g-C₃ N₄ . In this case, the utilization of cocatalysts is a novel frontline in the g-C₃ N₄ -based photocatalytic systems due to the positive effects of cocatalysts on supressing charge carrier recombination, reducing the HER overpotential, and improving photocatalytic activity. This review summarizes some recent advances about the high-performance cocatalysts based on g-C₃ N₄ toward HER. Specifically, the functions, design principle, classification, modification strategies of cocatalysts, as well as their intrinsic mechanism for the enhanced photocatalytic HER activity are discussed here. Finally, the pivotal challenges and future developments of cocatalysts in the field of HER are further proposed.

Visible-light-responsive Z-scheme system for photocatalytic lignocellulose-to-H2 conversion

A Z-scheme system was successfully constructed for visible-light-driven photocatalytic H₂ production from lignocelluloses, the highest H₂ evolution rate of this Z-scheme system is 5.3 and 1.6 μmol h^-1 in α-cellulose and poplar wood chip aqueous solutions, respectively, under visible light irradiation.

Iodide-Induced Fragmentation of Polymerized Hydrophilic Carbon Nitride for High-Performance Quasi-Homogeneous Photocatalytic H2 O2 Production

Polymeric carbon nitride (PCN) as a class of two-electron oxygen reduction reaction (2 e^- ORR) photocatalyst has attracted much attention for H₂ O₂ production. However, the low activity and inferior selectivity of 2 e^- ORR greatly restrict the H₂ O₂ production efficiency. Herein, we develop a new strategy to synthesize hydrophilic, fragmented PCN photocatalyst by the terminating polymerization (TP-PCN) effect of iodide ions. The obtained TP-PCN with abundant edge active sites (AEASs), which can form quasi-homogeneous photocatalytic system, exhibits superior H₂ O₂ generation rate (3265.4 μM h^-1 ), far surpassing PCN and other PCN-based photocatalysts. DFT calculations further indicate that TP-PCN is more favorable for electron transiting from β spin-orbital to the π* orbitals of O₂ , which optimizes O₂ activation and reduces the energy barrier of H₂ O₂ formation. This work provides a new concept for designing functional photocatalysts and understanding the mechanism of O₂ activation in ORR for H₂ O₂ production.

New carbon nitride close to C6N7 with superior visible light absorption for highly efficient photocatalysis

The rational design and construction of novel two-dimensional (2D) carbon nitrides (CNs) beyond g-C₃N₄ is a hot topic in the fields of chemistry and materials. Inspired by the polymerisation of urea, we have prepared a series of novel C-C bridged heptazine CNs UO_x (where x is the ratio of urea to oxamide, x = 1, 1.5, 2, 2.5, and 3), which are similar to (C₆N₇)_n, upon the introduction of oxamide. As predicted using density functional theory (DFT) calculations, the conjugated structure of UO_x was effectively extended from an individual heptazine to the entire material. Consequently, its bandgap was reduced to 2.05 eV, and its absorption band edge was significantly extended to 600 nm. Furthermore, its carrier transfer and separation were significantly enhanced, establishing its superior photocatalytic activity. The optimised UO₂ exhibits a superior photocatalytic hydrogen production rate about 108.59 μmol h^-1 (using 10 mg of catalyst) with an apparent quantum efficiency (AQE) of 36.12% and 0.33% at 420 and 600 nm, respectively, which is one of the most active novel CNs reported to date. Moreover, UO₂ exhibits excellent photocatalytic activity toward the oxidation of diphenylhydrazine to azobenzene with conversion and selectivity reaching ~100%, which represents a promising highly efficient 2D CN material. Regarding phenols degradation, UO₂ also displayed significantly higher activity and durability during the degradation of phenol when compared to traditional g-C₃N₄, highlighting its significant potential for application in energy, environment and photocatalytic organic reactions.

Water-splitting photoelectrodes consisting of heterojunctions of carbon nitride with ap-type low bandgap double perovskite oxide

Quinary and senary non-stoichiometric double perovskites such as Ba₂Ca_0.66Nb_1.34-xFe_xO_6-δ(BCNF) have been utilized for gas sensing, solid oxide fuel cells and thermochemical CO₂reduction. Herein, we examined their potential as narrow bandgap semiconductors for use in solar energy harvesting. A cobalt co-doped BCNF, Ba₂Ca_0.66Nb_0.68Fe_0.33Co_0.33O_6-δ(BCNFCo), exhibited an optical absorption edge at ∼800 nm,p-type conduction and a distinct photoresponse up to 640 nm while demonstrating high thermochemical stability. A nanocomposite of BCNFCo and g-C₃N₄(CN) was prepared via a facile solvent-assisted exfoliation/blending approach using dichlorobenzene and glycerol at a moderate temperature. The exfoliation of g-C₃N₄followed by wrapping on perovskite established an effective heterojunction between the materials for charge separation. The conjugated 2D sheets of CN enabled better charge migration resulting in increased photoelectrochemical performance. A blend composed of 40 wt% perovskites and CN performed optimally, whilst achieving a photocurrent density as high as 1.5 mA cm^-2for sunlight-driven water-splitting with a Faradaic efficiency as high as ∼88%.

Superior tunable photocatalytic properties for water splitting in two dimensional GeC/SiC van der Waals heterobilayers

The photocatalytic characteristics of two-dimensional (2D) GeC-based van der Waals heterobilayers (vdW-HBL) are systematically investigated to determine the amount of hydrogen (H2) fuel generated by water splitting. We propose several vdW-HBL structures consisting of 2D-GeC and 2D-SiC with exceptional and tunable optoelectronic properties. The structures exhibit a negative interlayer binding energy and non-negative phonon frequencies, showing that the structures are dynamically stable. The electronic properties of the HBLs depend on the stacking configuration, where the HBLs exhibit direct bandgap values of 1.978 eV, 2.278 eV, and 2.686 eV. The measured absorption coefficients for the HBLs are over ~ 105 cm-1, surpassing the prevalent conversion efficiency of optoelectronic materials. In the absence of external strain, the absorption coefficient for the HBLs reaches around 1 × 106 cm-1. With applied strain, absorption peaks are increased to ~ 3.5 times greater in value than the unstrained HBLs. Furthermore, the HBLs exhibit dynamically controllable bandgaps via the application of biaxial strain. A decrease in the bandgap occurs for both the HBLs when applied biaxial strain changes from the compressive to tensile strain. For + 4% tensile strain, the structure I become unsuitable for photocatalytic water splitting. However, in the biaxial strain range of - 6% to + 6%, both structure II and structure III have a sufficiently higher kinetic potential for demonstrating photocatalytic water-splitting activity in the region of UV to the visible in the light spectrum. These promising properties obtained for the GeC/SiC vdW heterobilayers suggest an application of the structures could boost H2 fuel production via water splitting.

Engineering of g-C3N4-based photocatalysts to enhance hydrogen evolution

The technology of photocatalytic hydrogen production that converts abundant yet intermittent solar energy into an environmentally friendly alternative energy source is an attractive strategy to mitigate the energy crisis and environmental pollution. Graphitic carbon nitride (g-C₃N₄), as a promising photocatalyst, has gradually received focus in the field of artificial photosynthesis due to its appealing optical property, high chemical stability and easy synthesis. However, the limited light absorption and massive recombination of photoinduced carriers have hindered the photocatalytic activity of bare g-C₃N₄. Therefore, from the perspective of theoretical calculations and experiments, many valid approaches have been applied to rationally design the photocatalyst and ameliorate the hydrogen production performance, such as element doping, defect engineering, morphology tuning, and semiconductor coupling. This review summarized the latest progress of g-C₃N₄-based photocatalysts from two perspectives, modification of pristine g-C₃N₄ and interfacial engineering design. It is expected to offer feasible suggestions for the fabrication of low-cost and high-efficiency photocatalysts and the photocatalytic mechanism analyses assisted by calculation in the near future. Finally, the prospects and challenges of this exciting research field are discussed.

Accelerated Photoreduction of CO2 to CO over a Stable Heterostructure with a Seamless Interface

Photocatalytic CO₂ reduction is a means of alleviating energy crisis and environmental deterioration. In this work, a rising two-dimensional (2D) material rarely reported in the field of photocatalytic CO₂ reduction, black phosphorus (BP) nanosheets, is synthesized, on which Co₂P is in situ grown by solvothermal treatment using BP itself as a P source. Co₂P on the BP nanosheets (BPs) surface can prevent the destruction of BPs in ambient air and, in the meantime, favor charge separation and CO₂ adsorption and activation during the catalytic process. Upon light irradiation, Co₂P can extract the photogenerated electrons effectively across the intimate interface and lower the CO₂ activation energy barrier, supported by both experimental characterizations and theoretical calculations. Benefitting from integrated advantages of BPs and Co₂P, the optimal Co₂P/BPs exhibit photocatalytic reduction of CO₂ to CO at a rate of 25.5 μmol g^-1 h^-1 with a selectivity of 91.4%, both of which are higher than those of pristine BPs. This work presents ideas for stabilizing BPs and improving their CO₂ reduction performance simultaneously.

Photocatalytic hydrogen evolution over a nickel complex anchoring to thiophene embedded g-C3N4

Evolution of hydrogen from water by utilizing solar energy and photocatalysts is one of the most promising ways to solve energy crisis. However, designing a cost-effective and stable photocatalyst without any noble metals is of vital importance for this process. Herein, an extremely active molecular complex cocatalyst NiL₂(Cl)₂ is successfully designed. After being covalently linked to thiophene-embedded polymeric carbon nitride (TPCN), the hybrid catalyst NiL₂(Cl)₂/TPCN exhibits extraordinary H₂ production activity of 95.8 μmol h^-1 without Pt (λ ≥ 420 nm), together with a remarkable apparent quantum yield of 6.68% at 450 nm. In such a composite catalyst, the embedded π-electron-rich thiophene-ring not only extends the π-conjugated system to enhance visible light absorption, but also promotes the charge separation through electron-withdrawing effect. It turns out that the CN covalent bonds formed between NiL₂(Cl)₂ and TPCN skeleton accelerate the transfer of electrons to the Ni active sites. Our finding reveals that the strategy of embedding π-electron-rich compounds to graphitic carbon nitride provides potentials to develop excellent photocatalysts. The strong covalent combination of molecular complexes cocatalyst onto organic semiconductors represents an important step towards designing noble-metal-free photocatalysts with superior activity and high stability for visible light driven hydrogen evolution.

Photocatalytic overall water splitting without noble-metal: Decorating CoP on Al-doped SrTiO3

CoP, a noble-metal-free cocatalyst, was first introduced onto the surface of Al-doped SrTiO₃ (Al:STO) via an in situ photodeposition-phosphorization method for photocatalytic overall water splitting (POWS) into stoichiometric H₂ and O₂. Compared with pure Al:STO, the POWS activity was enhanced by a factor of ~ 421 over 1.0%CoP/Al:STO, with the highest evolution rates of 2106 and 1002 μmol h^-1 g^-1 for H₂ and O₂, respectively. The mechanism for the remarkably boosted POWS activity was systematically analyzed based on the comprehensive characterization. On the one hand, benefiting from the in situ photodeposition process, CoP with metallic character were intimately decorated onto the surface of Al:STO and accelerated the separation and migration of photoinduced charge carriers. On the other hand, CoP, serving as reactive sites for H₂ evolution reaction, lowered the overpotential and facilitated the surface reduction reaction, thereby enhancing the POWS activity. Furthermore, Cr₂O₃ was photodeposited on the surface of 1.0%CoP/Al:STO composite to suppress the undesired reverse reaction and the POWS activity was further enhanced up to 3558 and 1722 μmol h^-1 g^-1 for H₂ and O₂, respectively, with apparent quantum yield of 7.1% at 350 ± 10 nm. This work presents a new avenue for designing POWS system without noble-metal cocatalyst.

Band offset in semiconductor heterojunctions

Semiconductor heterojunctions are widely applied in solid-state device applications, including semiconductor lasers, solar cells, and transistors. In photocatalysis they are of interest due to their capability to hinder charge carriers' recombination. A key role in the performance of heterojunctions is that of the alignment of the band edges of the two units composing the junction. In this work, we compare the performances of three widely applied approaches for the simulation of semiconductors heterostructures, based on density functional theory calculations with hybrid functionals. We benchmark the band offsets of ten semiconductors heterostructures for which experimental values are available: AlP/GaP, AlP/Si, AlAs/GaAs, AlAs/Ge, GaAs/Ge, GaP/Si, ZnSe/Ge, ZnSe/AlAs, ZnSe/GaAs, and TiO₂/SrTiO₃. The methods considered are (i) the alternating slabs junction (ASJ), (ii) the surface terminated junction (STJ), and (iii) the independent units (IU) approach. Moreover, two different ways to determine a common reference have been considered, (i) the plane averaged electrostatic potential, and (ii) the energy of the core levels. Advantages, drawbacks and overall performances of each method are discussed. The results suggest that the accuracy in the estimation of the band offsets is ∼0.2 eV when the ASJ method is applied. The STJ approach provides a similar accuracy, while the neglection of any interface effect, as in the IU method, provides only a qualitative estimate of the band offset and can result in significant deviations from the experiment.

Au nanoparticle-controlled formation of metallic and oxidized Pt nanoparticles on graphitic carbon nitride nanosheets for H2 evolution

Surface decoration of noble-metal cocatalysts on graphitic phase carbon nitride (g-C3N4) with high efficiency and trace content for water splitting is exciting but difficult to achieve. Herein, we report the anchoring of Au and metallic/oxidized Pt nanoparticles (NPs) on g-C3N4 as cocatalysts via a photoreduction process for enhancing photocatalytic H2 production. Au NPs are preferentially decorated on g-C3N4, which can control the formation of metallic/oxidized Pt complex species. The well dispersed Au and metallic/oxidized Pt NPs improved the light-harvesting and the photo-generated carrier separation of g-C3N4. G-C3N4 sequentially decorated with Au (0.3 wt%) and metallic/oxidized Pt (0.3 wt%) cocatalysts, exhibited the highest and stable H2 evolution rates of 2560 and 139 μmol h-1 g-1 under simulated sunlight and visible light (λ ≥ 420 nm) irradiation, respectively, compared to the samples that are simultaneously and sequentially decorated with the same content of Pt and Au on g-C3N4. The enhanced photocatalytic activity is attributed to the synergistic effect of Au and metallic/oxidized Pt cocatalysts, i.e., the effective localized surface plasma resonance coupling between Pt and Au NPs, as well as electron-sink function of metallic Pt, which promote the generation and transfer of more carriers from g-C3N4 to the Pt species, in addition to the superior hydrogen evolution capacity of metallic and oxidized Pt. This work maximizes the performance of noble-metal cocatalysts with minimized content and provides the possibility of realizing efficient solar-to-fuel conversion.

Molybdenum sulfide-modified metal-free graphitic carbon nitride/black phosphorus photocatalyst synthesized via high-energy ball-milling for efficient hydrogen evolution and hexavalent chromium reduction

Improving the photocatalytic property of metal-free photocatalyst is still a challenging work. Herein, a novel high-efficiency molybdenum sulfide (MoS₂)-modified metal-free graphitic carbon nitride (g-C₃N₄)/black phosphorus (BP) photocatalyst (MCN/BP/MS) was synthesized on a large scale via high-energy ball milling process. The optimized MCN/BP/MS exhibits the excellent hydrogen evolution rate of 2146.8 µmol·g^-1·h^-1, and hexavalent chromium (Cr(Ⅵ)) reduction activity with an apparent rate constant of 0.1464 min^-1 and a degradation rate of 100% in 25 min. Detailed characterizations and mechanism research verified that the significantly improved photocatalytic activity of MCN/BP/MS mainly profited from the matched band structure, enhanced light absorption, intense interface contact, as well as the type-Ⅰ/Z hybrid charge transfer mechanism, which gave rise to a consecutive multistep charge migration, thus the photocarriers transfer and separation can be greatly promoted, and the photogenerated electrons with high reducing capacity can be preserved. This work not only provides a high-efficiency g-C₃N₄-based noble-metal-free photocatalyst, but also affords a beneficial inspiration for improving the photocatalytic property of the metal free photocatalyst.

The fabrication of two-dimensional g-C3N4/NaBiO3·2H2O heterojunction for improved photocatalytic CO2 reduction: DFT study and mechanism unveiling

Photocatalytic CO₂ reduction is typically limited by the separation efficiency of photogenerated carriers for a single semiconductor. Thus, fabricating a two-dimensional/two-dimensional (2D/2D) heterojunction photocatalyst with high separation efficiency of photogenerated carriers has become a research priority. Here, a 2D/2D g-C₃N₄/NaBiO₃·2H₂O (CN/NBO) heterojunction photocatalyst was successfully synthesized for CO₂ photoreduction. With the assistance of the nature of CN, the 10CN/NBO composite showed the best performance with the production yield rates of 110.2 and 43.8 µmol g^-1 for CO and CH₄, respectively. Our experiments showed that the introduction of CN in CN/NBO composites, which is under the step-scheme (S-step) transfer direction of photogenerated carriers, could greatly inhibit the recombination of photogenerated e^--h⁺ pairs to prolong the carriers' lifetime, which was further confirmed by analysis of photoluminescence and photochemical characterization. As we expected, the CN/NBO composites show improved photocatalytic CO₂ reduction activity. The in situ infrared spectroscopy was also performed to study the intermediate products of the photocatalytic CO₂ reduction process. This study provides a way to design CN-based heterojunction photocatalysts for CO₂ photoreduction.

Fabrication of graphitic carbon nitride functionalized P-CoFe2O4 for the removal of tetracycline under visible light: Optimization, degradation pathways and mechanism evaluation

In this study, nano-sized CoFe₂O₄ composites were prepared through co-precipitation process. Then the phosphorus-doped strong magnetic graphitic carbon nitride hybrids composites (P-CoFe₂O₄@GCN) was stemmed from the CoFe₂O₄ composites via the thermal polymerization method. The TEM results show that the CoFe₂O₄ nanoparticles have been successfully embedded into the graphitic carbon nitride (GCN). The BET specific surface area of P-CoFe₂O₄@GCN-1 could reach 36.91 m²/g, which was 5.38 times higher than that of GCN. Thus, it provided sufficient reaction active sites to enhance the photocatalytic activity for tetracycline (TC) decomposition. The results from the photocatalytic experiments showed that the degradation efficiency of TC by P-CoFe₂O₄@GCN-1 could reach 96.2% within 60 min, which is 3.19 times higher than that of GCN. The h⁺, O₂•^- and •OH radicals detected by the electron spin resonance (ESR) were responsible for the TC decomposition in the photocatalytic reaction system. Persulfate (PS) can further activate the hybrid mixture system, and the fitting model predicted by the response surface methodology (RSM) indicated that the maximum tetracycline removal could reach 99.6% within 30 min. In addition, the degradation intermediates of TC were detected by HPLC-MS and the photodegradation mechanism was discussed.

Constructing Zn-P charge transfer bridge over ZnFe2O4-black phosphorus 3D microcavity structure: Efficient photocatalyst design in visible-near-infrared region

Black phosphorus (BP) is one of the most promising visible-near-infrared light-driven photocatalysts with favorite photoelectric properties and unique tunable direct band gap. Nevertheless, the further development of BP is hindered by the fast carrier recombination rate and high Gibbs free energy. Herein, an innovative strategy is developed for the controllable construction of Zn-P bonds induced zinc ferrite/black phosphorus (ZnFe₂O₄-BP) three dimensions (3D) microcavity structure. The Zn-P bonds serve as an efficient channel to optimize the carrier transport and Gibbs free energy of BP simultaneously. Besides, the unique 3D core-shell microcavity structure maintains the multiple reflections of sunlight inside the catalysts, which greatly improves the sunlight utilization upon photocatalysis. An optimized photocatalytic hydrogen production rate of 560 µmol h^-1g^-1 under near-infrared light (>820 nm) is achieved. A possible photocatalytic mechanism is proposed based on a series of experimental characterizations and theoretical calculations, this work provides a new sight to design high-quantity BP-based full-spectrum photocatalysts for solar energy conversion.

Target-switchable DNA hydrogels coupled with a Bi2Sn2O7/Bi2S3 heterojunction based on in situ anion exchange for the "signal-on" photoelectrochemical detection of DNA

In this paper, a novel photoelectrochemical (PEC) "signal-on" biosensor based on a Bi2Sn2O7/Bi2S3 heterojunctioncoupled with target-switchable DNA hydrogels is reported for the ultrasensitive detection of P53 gene DNA. For the first time, sulfide ions are discovered to display an excellent PEC sensitization effect on Bi2Sn2O7 material by forming the Bi2Sn2O7/Bi2S3 heterojunction. The sensitization amplitude increased by 63 times, and the photocurrent polarity switched from cathodic to anodic. When the target DNA-induced-cycling amplification process produced a mass of product chains (PD), PD was introduced into the target-switchable DNA hydrogels to quantitatively release sulfide ions, which were further introduced to the Bi2Sn2O7-modified PEC platform and resulted in an enormous enhancement of the PEC signal due to the significant sensitization effect of sulfide ions on Bi2Sn2O7via an anion-exchange reaction. The corresponding PEC signal change of the Bi2Sn2O7/Bi2S3 platform was used for the detection of target DNA. This biosensing strategy opens up a novel sulfide ion-sensitized PEC platform and exhibits excellent analytical performance with a wide linear range (100 fM-10 nM), which has broad application prospects in bioanalysis and clinical diagnosis.

Polymeric Carbon Nitride-Derived Photocatalysts for Water Splitting and Nitrogen Fixation

Photocatalysis is a promising energy conversion and environmental restoration technology. The main focus of photocatalysis is the development and manufacture of highly efficient photocatalysts. Semiconductor-based photocatalysis technology based on harnessing solar energy is considered as an attractive approach to solve the problems of global energy shortage and environmental pollution. Since 2009 pioneering work has been carried out on polymeric carbon nitride (PCN) for visible photocatalytic water splitting, thus PCN-based photocatalysis has become a hot research topic, demanding significant research attention. This article reviews the physical and chemical properties, synthesis methods, and the methods to control the morphology, heteroatom doping, and construction of heterojunctions to improve the performance of PCN-based photocatalysts in water splitting and nitrogen fixation. Through different design strategies, the photo-generated electron-hole pair separation efficiency of PCN materials can be effectively improved, thereby improving their photocatalytic performance. Finally, the challenges of PCN-based photocatalysts in water splitting and nitrogen fixation applications are discussed herein. It is strongly believed that through different design strategies, efficient PCN-based photocatalysts can be constructed for both water splitting and nitrogen reduction. These excellent modification strategies can be used as a guiding theory for photocatalytic reactions of other promising catalysts and further promote the development of photocatalysis.

Nanostructure-based photoelectrochemical sensing platforms for biomedical applications

As a newly developed and powerful analytical method, the use of photoelectrochemical (PEC) biosensors opens up new opportunities to provide wide applications in the early diagnosis of diseases, environmental monitoring and food safety detection. The properties of diverse photoactive materials are one of the essential factors, which can greatly impact the PEC performance. The continuous development of nanotechnology has injected new vitality into the field of PEC biosensors. In many studies, much effort on PEC sensing with semiconductor materials is highlighted. Thus, we propose a systematic introduction to the recent progress in nanostructure-based PEC biosensors to exploit more promising materials and advanced PEC technologies. This review briefly evaluates the several advanced photoactive nanomaterials in the PEC field with an emphasis on the charge separation and transfer mechanism over the past few years. In addition, we introduce the application and research progress of PEC sensors from the perspective of basic principles, and give a brief overview of the main advances in the versatile sensing pattern of nanostructure-based PEC platforms. This last section covers the aspects of future prospects and challenges in the nanostructure-based PEC analysis field.

Bi2O3/BiVO4@graphene oxide van der Waals heterostructures with enhanced photocatalytic activity toward oxygen generation

The van der Waals (vdW) integration enables to create heterostructures with intimate contact and bring new opportunities. However, it is not confined to layered materials but can also be generally extended to 3D materials. Multidimensional Bi₂O₃/BiVO₄@graphene oxide (GO) van der Waals heterostructures are synthesized by one-pot wet chemistry method. Bi₂O₃/BiVO₄ composite nanoparticles are self-assembled with GO framework by vdW interaction to form vdW heterostructures, in which GO framework allows short electron transport distance and rapid charge transfer and provides massive reactive sites. Such self-assembled heterostructures show a superior high photoactivity towards oxygen evolution with an enhanced oxygen generation rate of 1828 µmol h^-1 g^-1, nearly 3 times than that of pure BiVO₄, attributed to the accelerated charge separation and transfer processes of Bi₂O₃/BiVO₄@GO vdW heterostructures. This study indicates that our strategy provides a new avenue towards fabricating multi-dimensional vdW heterostructures and inspiring more innovative insights in oxygen evolution field.

Bimetallic zeolite-imidazole framework-based heterostructure with enhanced photocatalytic hydrogen production activity

Bimetallic zeolite-imidazole frameworks with controllable flat band position, band gap and hydrogen evolution reaction characteristics were adopted as a photocatalytic hydrogen production catalyst. Furthermore, the g-C3N4-MoS2 2D-2D surface heterostructure was introduced to the ZnM-ZIF to facilitate the separation as well as utilization efficiency of the photo-exited charge carriers in the ZnM-ZIFs. On the other hand, the ZnM-ZIFs not only inhibited the aggregation of the g-C3N4-MoS2 heterostructure, but also improved the separation and transport efficiency of charge carriers in g-C3N4-MoS2. Consequently, the optimal g-C3N4-MoS2-ZnNi-ZIF exhibited an extraordinary photocatalytic hydrogen evolution activity 214.4, 37.5, and 3.7 times larger than that of the pristine g-C3N4, g-C3N4-ZnNi-ZIF and g-C3N4-MoS2, respectively, and exhibited a H2-evolution performance of 77.8 μmol h-1 g-1 under UV-Vis light irradiation coupled with oxidation of H2O into H2O2. This work will furnish a new MOF candidate for photocatalysis and provide insight into better utilization of porous MOF-based heterostructures for hydrogen production from pure water.

2D PtS nanorectangles/g-C3N4 nanosheets with a metal sulfide-support interaction effect for high-efficiency photocatalytic H2 evolution

Cocatalyst design is a key approach to acquire high solar-energy conversion efficiency for photocatalytic hydrogen evolution. Here a new in situ vapor-phase (ISVP) growth method is developed to construct the cocatalyst of 2D PtS nanorectangles (a length of ∼7 nm, a width of ∼5 nm) on the surface of g-C₃N₄ nanosheets. The 2D PtS nanorectangles/g-C₃N₄ nanosheets (PtS/CN) show an unusual metal sulfide-support interaction (MSSI), which is evidenced by atomic resolution HAADF-STEM, synchrotron-based GIXRD, XPS and DFT calculations. The effect of MSSI contributes to the optimization of geometrical structure and energy-band structure, acceleration of charge transfer, and reduction of hydrogen adsorption free energy of PtS/CN, thus yielding excellent stability and an ultrahigh photocatalytic H₂ evolution rate of 1072.6 μmol h^-1 (an apparent quantum efficiency of 45.7% at 420 nm), up to 13.3 and 1532.3 times by contrast with that of Pt nanoparticles/g-C₃N₄ nanosheets and g-C₃N₄ nanosheets, respectively. This work will provide a new platform for designing high-efficiency photocatalysts for sunlight-driven hydrogen generation.

Rational design of the Z-scheme hollow-structure Co9S8/g-C3N4 as an efficient visible-light photocatalyst for tetracycline degradation

The development of photocatalysts with high catalytic activity that are capable of full utilization of solar energy is a challenge in the field of photocatalysis. Accordingly, in the present study, an efficient Z-scheme cage-structured Co₉S₈/g-C₃N₄ (c-CSCN) photocatalyst was constructed for the degradation of tetracycline antibiotics under visible-light irradiation. The Z-scheme charge-transfer mechanism accelerates the separation of photogenerated charge carriers and effectively improves photocatalytic activity. Moreover, c-CSCN has a hollow structure, allowing light to be reflected multiple times inside the cavity, thereby effectively improving the utilisation efficiency of solar energy. As a result, the photocatalytic activity of c-CSCN is 1.5-, 2.5-, and 5.8-times higher than those of sheet-type Co₉S₈/g-C₃N₄ (s-CSCN), c-Co₉S₈, and g-C₃N₄, respectively, for the degradation of tetracycline. c-CSCN maintains favourable photocatalytic activity over five consecutive degradation cycles, demonstrating its excellent stability. In addition, c-CSCN performs efficient tetracycline removal in different water substrates. Moreover, c-CSCN exhibits excellent ability to remove tetracycline under direct natural sunlight. This work fully demonstrates that c-CSCN has high catalytic activity and the potential for practical application as a wastewater treatment material.

Band Engineering and Morphology Control of Oxygen-Incorporated Graphitic Carbon Nitride Porous Nanosheets for Highly Efficient Photocatalytic Hydrogen Evolution

Graphitic carbon nitride (g-C₃N₄)-based photocatalysts have shown great potential in the splitting of water. However, the intrinsic drawbacks of g-C₃N₄, such as low surface area, poor diffusion, and charge separation efficiency, remain as the bottleneck to achieve highly efficient hydrogen evolution. Here, a hollow oxygen-incorporated g-C₃N₄ nanosheet (OCN) with an improved surface area of 148.5 m² g^-1 is fabricated by the multiple thermal treatments under the N₂/O₂ atmosphere, wherein the C-O bonds are formed through two ways of physical adsorption and doping. The physical characterization and theoretical calculation indicate that the O-adsorption can promote the generation of defects, leading to the formation of hollow morphology, while the O-doping results in reduced band gap of g-C₃N₄. The optimized OCN shows an excellent photocatalytic hydrogen evolution activity of 3519.6 μmol g^-1 h^-1 for ~ 20 h, which is over four times higher than that of g-C₃N₄ (850.1 μmol g^-1 h^-1) and outperforms most of the reported g-C₃N₄ catalysts.

Exploring edge functionalised blue phosphorene nanoribbons as novel photocatalysts for water splitting

1D phosphorene nanoribbon edges activating water molecules under sunlight.

Atomic-Level and Modulated Interfaces of Photocatalyst Heterostructure Constructed by External Defect-Induced Strategy: A Critical Review

Despite the existence of numerous photocatalyst heterostructures, their separation efficiency and charge flow precision remain low due to the poor study on interfacial properties. The photocatalysts with confined defects can effectively control the photogenerated carrier migration, but the metastability of such defects considerably decreases the photocatalyst stability. Meanwhile, the introduction of defective region can increase the coordinative unsaturation and delocalize local electrons to promote their interactions with the molecules/ions in that region. The selective growth of modulated heterogeneous interface by defect-induced strategy may not only increase the stability of defective structures, but also enhance the migration of interfacial charges. Using this method, photocatalytic heterostructures with low contact resistances and intimate interfaces are constructed to achieve the optimal charge migration in terms of efficiency and accuracy. In this work, the point, linear, and planar heterogeneous interfaces and related defect engineering techniques are discussed. Particularly, it is focused on the external, defect-induced interfacial heterogeneities with various spatial and dimensional configurations, which exhibit modulated and controllable interfacial properties. Furthermore, the main aspects of fabricating photocatalyst heterostructures by the defect-induced strategy, including the i) controllable generation of defects, ii) advanced characterization methods, and iii) elaborate construction of the minimal interface, are described.

Highly enhanced H2 evolution of MoO3/g-C3N4 hybrid composites based on a direct Z-scheme photocatalytic system

A direct Z-scheme MoO₃/g-C₃N₄ heterojunction with appropriate oxygen vacancies is successfully fabricated via an in situ method of a one-pot pyrolysis strategy.

Donor–acceptor covalent organic framework/g-C3N4 hybrids for efficient visible light photocatalytic H2 production

A donor–acceptor type covalent organic framework (COF) was used to construct TBTA/g-C₃N₄ hybrids for photocatalytic H₂ evolution. Under visible light irradiation, the hybrids exhibited an optimized H₂ evolution rate of 11.73 mmol g⁻¹ h⁻¹ without Pt.

Direct Z-scheme CdS–NiPc heterojunctions as noble metal-free photocatalysts for enhanced photocatalytic hydrogen evolution

A direct Z-scheme CdS-NiPc heterojunction was successfully synthesized by a one-step hydrothermal method. The optimal photocatalytic H2 production rate could reach 17.74 mmol h−1 g−1, which was 19.1 times higher than that of the pure CdS sample.