Nanostructure Engineering and Doping of Conjugated Carbon Nitride Semiconductors for Hydrogen Photosynthesis

Construction of the 1D Covalent Organic Framework/2D g-C3N4 Heterojunction with High Apparent Quantum Efficiency at 500 nm

The reasonable construction of heterojunction photocatalysts with clear nanostructures and a good interface contact especially the one-dimensional/two-dimensional (1D/2D) composite heterojunction with unique morphology is considered one of the most effective strategies for designing highly efficient photocatalysts. Herein, a series of the 1D β-keto-enamine-based covalent organic framework (COF)/2D g-C₃N₄ composite materials COF-CN (1:x; where 1:x represents the mass ratio of COF and g-C₃N₄, x = 2.5, 5, 10, 15, 20) is prepared through the in situ reaction of 2,4,6-triformylphloroglucinol (Tp) and benzidine (BD) in stripped g-C₃N₄ suspension. A series of characterizations, such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), have verified their 1D/2D heterojunction structure. With the introduction of 1D COF nanobelts, the absorption of the composite is largely extended to 560 nm. Photocatalytic experiments reveal that the composite COF/CN shows evidently superior photocatalytic performance than individual COF and g-C₃N₄. The optimized COF-CN (1:10) exhibits a H₂ production rate of 12.8 mmol g^-1·h^-1 under visible-light (λ ≥ 420 nm) irradiation, which is about 62 and 284 times higher than those of COF and g-C₃N₄, respectively. The apparent quantum efficiency (AQE) of COF-CN (1:10) is about 15.09% under 500 nm light irradiation, which is one of the highest among previous COF- or g-C₃N₄-based materials. This work provides important strategies for designing and constructing high-efficiency heterojunction photocatalysts with multidimensional features.

Cost-Effective Electrochemical Activation of Graphitic Carbon Nitride on the Glassy Carbon Electrode Surface for Selective Determination of Serotonin

A simple one-step electrochemical deposition/activation of graphitic carbon nitride (g-C₃N₄) is highly desired for sensor configurations and remains a great challenge. Herein, we attempt an electrochemical route to exfoliate the g-C₃N₄ nanosheets in an aqueous solution of pH 7.0 for constructing a sensor, which is highly sensitive for the detection of serotonin (5-HT). The significance of our design is to exfoliate the g-C₃N₄ nanosheets, a strong electrocatalyst for 5-HT detection. Investigations regarding the effect of neutral pH (pH 7.0) on the bulk g-C₃N₄ and g-C₃N₄ nanosheets, physical characterization, and electrochemical studies were extensively carried out. We demonstrate that the g-C₃N₄ nanosheets have a significant electrocatalytic effect for the 5-HT detection in a dynamic linear range from 500 pM to 1000 nM (R² = 0.999). The limit of detection and sensitivity of the designed 5-HT sensor was calculated to be 150 pM and 1.03 µA µM⁻¹ cm⁻², respectively. The proposed sensor has great advantages such as high sensitivity, good selectivity, reproducibility, and stability. The constructed g-C₃N₄ nanosheets-based sensor platform opens new feasibilities for the determination of 5-HT even at the picomolar/nanomolar concentration range.

Advancing Applications of Black Phosphorus and BP-Analog Materials in Photo/Electrocatalysis through Structure Engineering and Surface Modulation

Black phosphorus (BP), an emerging 2D material semiconductor material, exhibits unique properties and promising application prospects for photo/electrocatalysis. However, the applications of BP in photo/electrocatalysis are hampered by the instability as well as low catalysis efficiency. Recently, tremendous efforts have been dedicated toward modulating its intrinsic structure, electronic property, and charge separation for enhanced photo/electrocatalytic performance through structure engineering. Simultaneously, the search for new substitute materials that are BP-analogous is ongoing. Herein, the latest theoretical and experimental progress made in the structural/surface engineering strategies and advanced applications of BP and BP-analog materials in relation to photo/electrocatalysis are extensively explored, and a presentation of the future opportunities and challenges of the materials is included at the end.

Unraveling the mechanisms of S-doped carbon nitride for photocatalytic oxygen reduction to H2O2

An in-depth understanding of the microscopic reaction mechanism on a nonmetal-doped catalytic system at the atomic level is one of the critical approaches to developing new efficient catalysts. Herein, the effects of S-doping on melon-based carbon nitride (CN) for the photocatalytic selective oxygen reduction reaction (ORR) have been comprehensively investigated by first-principles calculations. The configurations, electronic properties, optical properties, and the reaction performance of the S-doped melon-based CN have been studied and discussed. The results demonstrate that the decoration with S atoms exhibited substantial effects, involving the redistribution of the charge density and tuning of the bandgap, which promote the photocatalytic selective ORR activity. Accordingly, O2 is activated on the S-doped system with about 0.4 e of charge obtained from catalytic surfaces, leading to the thermodynamically feasible H2O2 and H2O formation, which is in good agreement with the experimental results. Our results provide theoretical insights into the design and development of polymeric carbon nitride (PCN) as well as other metal-free photocatalysts for the selective ORR.

Elucidating the structure of the graphitic carbon nitride nanomaterials via X-ray photoelectron spectroscopy and X-ray powder diffraction techniques

By using the most popular method of thermal condensation of dicyandiamide in a semi-closed system, graphitic carbon nitrides (gCNs) were synthesized at 500, 550, and 600 °C. The resulting materials were comprehensively analyzed via X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (XRD)techniques. We show that the use of routine analytical methods provides an insight into the structure of the carbon nitride materials. The analysis of geometric linear structures and fully condensed structure of polymeric carbon nitrides was performed and the ranges within which the contents of different nitrogen species (pyridine, amine, imine and quaternary nitrogen) can change were determined. This analysis, in combination with quantitative XPS studies, permits to state that the carbon nitride structure prepared by the thermal condensation of dicyandiamide is closer to the structure in which poly(aminoimino)heptazine subunits are linked into chains rather than the structure involving fully-condensed polyheptazine network. The XRD analysis proved that the 3D crystal structure of carbon nitride is described more correctly by the orthorhombic cell and space group P21212 applied to condensed chains of poly(aminoimino)heptazine (melon) and not by the hexagonal cell with the space group P6m2.

Isolated FeN4 Sites for Efficient Electrocatalytic CO2 Reduction

The construction of isolated metal sites represents a promising approach for electrocatalyst design toward the efficient electrochemical conversion of carbon dioxide (CO2). Herein, Fe-doped graphitic carbon nitride is rationally prepared by a simple adsorption method and is used as template to construct isolated FeN4 sites through a confined pyrolysis strategy, which avoids the agglomeration of metal atoms to particles during the synthesis process and thus provides abundant active sites for the CO2 reduction reaction. The isolated FeN4 sites lower the energy barrier for the key intermediate in the CO2 reduction process, leading to the enhanced selectivity for CO production with a faradaic efficiency of up to 93%.

Achieving cadmium selenide-decorated zinc ferrite@titanium dioxide hollow core/shell nanospheres with improved light trapping and charge generation for photocatalytic hydrogen generation

We report the rational design and fabrication of magnetically separable zinc ferrite@titanium dioxide (ZnFe₂O₄@TiO₂) hollow core/shell nanospheres as photocatalysts for efficient H₂ evolution by loading the TiO₂ shell layer on the prepared ZnFe₂O₄ hollow nanospheres using the kinetics-controlled coating method. Meanwhile, the incident light absorption, photogenerated charge transfer and separation and photocatalytic hydrogen evolution activity were remarkably improved by well anchoring cadmium selenide (CdSe) quantum dots on the ZnFe₂O₄@TiO₂ hollow core/shell nanospheres. This unique design integrates the structural and functional merits of the ZnFe₂O₄, TiO₂, and CdSe quantum dots into porous hollow nanospheres with the double-shell heterostructure. This design significantly accelerates the separation and transport of photogenerated charge carriers, enhances the light absorption, and offers more active sites for the photocatalytic H₂ evolution reaction. Benefitting from the unique structural and component merits, the optimized magnetically separable ZnFe₂O₄@TiO₂/CdSe hollow core/shell nanospheres exhibit excellent photocatalytic hydrogen evolution performance with a high H₂ generation rate (266.0 μmol h^-1·g^-1) and high stability.

Facile Synthesis of Lacunary Keggin-Type Phosphotungstates-Decorated g-C3N4 Nanosheets for Enhancing Photocatalytic H2 Generation

In this work, the lacunary Keggin-type phosphotungstates of [PW9O34]9- (PW9) clusters were loaded onto the g-C3N4 nanosheets (NSs) to synthesize the phosphotungstate clusters-decorated 2D heterojunction photocatalysts by using the electrostatic-force driven self-assembly process. The surface charge polarity of g-C3N4 NSs was changed from a negative to a positive charge through the acidizing treatment. The positively-charged g-C3N4 NSs allowed the negatively-charged PW9 clusters to be adsorbed and deposited onto the g-C3N4 NSs, forming the PW9/g-C3N4 heterojunction NSs. The as-synthesized samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and UV-VIS absorption spectra, respectively. The photocatalytic activity tests indicated that, upon simulated sunlight irradiation, the photocatalytic H2-generation rate of PW9/g-C3N4 heterojunction NSs (~23.8 μmol h-1) was ~3.3 times higher than that of the pure g-C3N4 NSs (~7.3 μmol h-1). The enhanced photocatalytic activity of PW9 cluster-decorated g-C3N4 NSs could be attributed to the enhanced separation process of the photoinduced charge-carriers, due to the Z-scheme-mediate charge transfer behavior across their hetero-interface.

Polaron and bipolaron induced charge carrier transportation for enhanced photocatalytic H2 production

Photocatalysis is one of the facile approaches for efficient solar energy conversion and storage. However, rapid charge carrier recombination considerably decreases solar to energy conversion efficiency. Herein, polaron and bipolaron rich polypyrrole (PPy) has been utilized as a solid support for effective photogenerated charge carrier separation. Simple oxidative polymerization using a high concentration of ammonium persulfate (APS) induces radical cation/bipolaron formation in PPy due to the cleavage of π-bonds as confirmed by electron paramagnetic resonance spectroscopy (EPR). The formation of radical cations led to an increase of the dielectric constant which retards the charge carrier recombination and thereby enhances the conductivity. Moreover, the polarons and bipolarons induced charge carrier separation in photocatalytic H₂ production was studied with the well-known g-C₃N₄ photocatalyst. It is worth mentioning that compared to bare g-C₃N₄, the PPy supported system showed a drastically enhanced photocatalytic H₂ production rate. A maximum H₂ production rate of 1851 μmoles per g is achieved, which is ∼51 times higher than that of the bare g-C₃N₄ catalyst due to efficient charge carrier separation assisted by radical cations/bipolarons. Thus, utilizing this simple polaron and bipolaron rich PPy solid support could be an effective strategy and alternative for using noble metal cocatalysts to enhance charge carrier separation.

Visible light responsive CuS/ protonated g-C3N4 heterostructure for rapid sterilization

As the environment deterioration is becoming more serious, bacterial pollution is threatening the health of human beings. Hence, it is vital to develop rapid and safe sterilization strategy. Herein, CuS/protonated g-C₃N₄(CuS/PCN) composites were synthesized by simple hydrothermal method and electrostatic adsorption. This heterostructured system exhibited enhanced photocatalytic properties under visible light compared with CuS or g-C₃N₄ alone, ascribing to the rapid separation of photogenerated electron-hole pairs. Meanwhile, the obvious photothermal effects of CuS/PCN were achieved and the temperature increased with the increased amount of CuS in the composites due to the more light absorption. However, when the CuS content is more than 10 %, photocurrent density is decreased with increasing the amount of CuS, indicating the increased recombination of photogenerated electron-hole pairs. When the CuS content is 20 %, the composite can perform the optimized synergistic effects of both photothermal action and photocatalysis under light irradiation for 20 min. The corresponding bacteria-killing efficiency against Staphylococcus aureus and Escherichia coli is 98.23 % and 99.16 %, respectively. The underlying mechanism is that the bacterial membrane can be weakened by reactive oxygen species and bacterial activities are inhibited by hyperthermia. This CuS/PCN heterojunction is promising for environmental disinfection including water and public facilities sterilization.

Defects Engineering Leads to Enhanced Photocatalytic H2 Evolution on Graphitic Carbon Nitride-Covalent Organic Framework Nanosheet Composite

Graphitic carbon nitride nanosheet (CNS) represents an attractive candidate for solar fuel production. However, the abundant defects in CNS lead to serious charge recombination and limit the photocatalytic performance. Herein, the synthesis of a CNS-covalent organic framework (CNS-COF) nanosheet composite is presented for the first time. CNS with significantly reduced defects is first obtained by rationally tuning the thermal exfoliation conditions of bulk carbon nitride. Subsequent modification of the CNS with trace COF nanosheet through chemical imine bonding can not only passivate the surface termination of carbon nitride in the boundary region, but also establish strong electronic coupling between these two components. As a consequence, enhanced charge separation and photocatalytic activity are realized on the resulting CNS-COF nanosheet composite. Under optimum conditions, hydrogen is evolved at a rate of 46.4 mmol g^-1 h^-1 . This corresponds to an apparent quantum efficiency of 31.8% at 425 nm, which is among the best values ever reported for carbon nitride-based materials.

Efficient Photocatalytic Hydrogen Evolution and CO2 Reduction: Enhanced Light Absorption, Charge Separation, and Hydrophilicity by Tailoring Terminal and Linker Units in g-C3N4

Although graphitic carbon nitride (g-C₃N₄) has been identified as a promising photocatalyst, pristine g-C₃N₄ has a limited light absorption, insolubility, small specific surface area, and rapid electron-hole pair recombination. In this study, hydroxyl-grafted oxygen-linked tri-s-triazine-based polymer (HGONTP) is achieved through the polycondensation of hydrothermally pretreated dicyandiamide (DCDA). The content of C-O-C linkers and terminal OH groups in HGONTP can be regulated by the cyclization and hydrolysis degrees of DCDA through the replacement of the pendant NH₂ groups with OH groups. The HGONTP photocatalyst exhibits an outstanding light absorption from UV to near-IR, possessing a narrow band gap of 2.18 eV, a hydrophilic surface, a large specific surface area of 96.1 m² g^-1, and reduced charge recombination. As a result, HGONTP exhibits a hydrogen evolution rate 27.7-fold higher than that for pristine g-C₃N₄ (6.54 vs 0.236 mmol g^-1 h^-1). The apparent quantum yield reaches 12.6% at 420 nm and 4.1% at 500 nm. In addition, the photocatalytic conversion efficiency of CO₂ to CO reaches as high as 3.3 μmol g^-1 h^-1 without cocatalysts and sacrificial agents. The selectivity of CO₂ to CO achieves 88.4%. The proposed strategy paves a new avenue to design high-performance polymeric photocatalysts used in water.

Formation of BNC Coordination to Stabilize the Exposed Active Nitrogen Atoms in g-C3 N4 for Dramatically Enhanced Photocatalytic Ammonia Synthesis Performance

It is an important issue that exposed active nitrogen atoms (e.g., edge or amino N atoms) in graphitic carbon nitride (g-C₃ N₄ ) could participate in ammonia (NH₃ ) synthesis during the photocatalytic nitrogen reduction reaction (NRR). Herein, the experimental results in this work demonstrate that the exposed active N atoms in g-C₃ N₄ nanosheets can indeed be hydrogenated and contribute to NH₃ synthesis during the visible-light photocatalytic NRR. However, these exposed N atoms can be firmly stabilized through forming BNC coordination by means of B-doping in g-C₃ N₄ nanosheets (BCN) with a B-doping content of 13.8 wt%. Moreover, the formed BNC coordination in g-C₃ N₄ not only effectively enhances the visible-light harvesting and suppresses the recombination of photogenerated carriers in g-C₃ N₄ , but also acts as the catalytic active site for N₂ adsorption, activation, and hydrogenation. Consequently, the as-synthesized BCN exhibits high visible-light-driven photocatalytic NRR activity, affording an NH₃ yield rate of 313.9 µmol g^-1 h^-1 , nearly 10 times of that for pristine g-C₃ N₄ . This work would be helpful for designing and developing high-efficiency metal-free NRR catalysts for visible-light-driven photocatalytic NH₃ synthesis.

Engineering Two-dimensional Nanomaterials to Enable Structure-Activity Relationship Studies in Nanosafety Research

Emerging, two-dimensional engineered nanomaterials (2DNMs) possess unique and diverse physical and chemical properties, such as extreme aspect ratios, adjustable electronic properties as well as functional lattice defects and surface chemistry which underpin their interactions with biological systems. This perspective highlights the need for structure activity relationship (SAR) studies for key properties of emerging grapheme-related and inorganic 2DNMs upon prioritization based on their potential impact and trajectory for large-scale production and applications. Further, it is discussed how a synthesis platform of microbiologically sterile, size-sorted, "model" 2DNMs with precise structure would enable SAR toxicological studies and allow for the sustainable and safe translation of 2D nanotechnology to real-world applications.

Molecular Junctions on Polymeric Carbon Nitrides with Enhanced Photocatalytic Performance

Molecular catalysts (MC), namely homogeneous catalysts, have demonstrated great promise for efficient solar-to-chemical energy conversion in the hybrid system. However, the poor interfacial interaction between MC and photosensitizers (PS) impedes the efficient and fast interfacial electron transfer. To promote interfacial communication between PS and MC, a proof-of-concept method was developed for the combination of polymeric carbon nitride (PCN) PS with bipyridine cobalt [Co(bpy)₃ ²⁺ ] MC by covalent bonds, creating molecular junctions to promote interfacial electron transfer as confirmed by transient photoluminescence lifetime and electrochemical measurements. As a result, the binary photocatalyst [Co(bpy)₃ ²⁺ /BINA₂ -CN] showed extensively enhanced photocatalytic activity such as H₂ and CO₂ reduction in comparison with the physical mixture of Co(bpy)₃ ²⁺ and PCN. This observation highlights the importance of construction of surface molecular junctions between PS and MC to accelerate the interfacial charge-carrier mobility and, consequently, improve the photocatalytic activity.

Hierarchical Self-assembly of Well-Defined Louver-Like P-Doped Carbon Nitride Nanowire Arrays with Highly Efficient Hydrogen Evolution

Self-assembled nanostructure arrays integrating the advantages of the intrinsic characters of nanostructure as well as the array stability are appealing in advanced materials. However, the precise bottom-up synthesis of nanostructure arrays without templates or substrates is quite challenging because of the general occurrence of homogeneous nucleation and the difficult manipulation of noncovalent interactions. Herein, we first report the precisely manipulated synthesis of well-defined louver-like P-doped carbon nitride nanowire arrays (L-PCN) via a supramolecular self-assembly method by regulating the noncovalent interactions through hydrogen bond. With this strategy, CN nanowires align in the outer frame with the separation and spatial location achieving ultrastability and outstanding photoelectricity properties. Significantly, this self-assembly L-PCN exhibits a superior visible light-driven hydrogen evolution activity of 1872.9 μmol h-1 g-1, rendering a ~ 25.6-fold enhancement compared to bulk CN, and high photostability. Moreover, an apparent quantum efficiency of 6.93% is achieved for hydrogen evolution at 420 ± 15 nm. The experimental results and first-principles calculations demonstrate that the remarkable enhancement of photocatalytic activity of L-PCN can be attributed to the synergetic effect of structural topology and dopant. These findings suggest that we are able to design particular hierarchical nanostructures with desirable performance using hydrogen-bond engineering.

Novel carbon and defects co-modified g-C3N4 for highly efficient photocatalytic degradation of bisphenol A under visible light

Graphite carbon nitride (g-C₃N₄, CN) is considered as a promising semiconductor for environmental catalysis. However, pure CN can not meet the requirements for actual applications due to its high recombination rate of photogenerated electron-hole pairs and a relatively large band gap preventing full utilization of solar energy. In this work, we report synthesis of a novel carbon and defects co-modified g-C₃N₄ (C_xCN) by calcination of melamine activated by oxalic. This new catalyst C_xCN has porous structure with much higher surface areas compared with pristine CN. UV-vis analysis and DFT calculations show that C_xCN has a lower bandgap for enhancing visible light adsorption compared with CN. Photoluminescence (PL) and photoelectrochemical analyses show that C_xCN has a low recombination rate of photogenerated electron-hole pairs, which improves the utilization of solar energy. As a result, C_xCN samples show high efficiency for the degradation of bisphenol A (BPA) under visible light irradiation, where the best catalyst of C_xCN (C_1.0CN) samples shows about 22 times higher photocatalytic degradation rate than that of CN. Moreover, C_1.0CN shows high mineralization rate and can degrade BPA into CO₂ and H₂O by the generated active species, like superoxide radicals (O₂^-) and holes (h⁺).

Nanoscale boron carbonitride semiconductors for photoredox catalysis

The conversion of solar energy to chemical energy achieved by photocatalysts comprising homogeneous transition-metal based systems, organic dyes, or semiconductors has received significant attention in recent years. Among these photocatalysts, boron carbon nitride (BCN) materials, as an emerging class of metal-free heterogeneous semiconductors, have extended the scope of photocatalysts due to their good performance and Earth abundance. The combination of boron (B), carbon (C), and nitrogen (N) constitutes a ternary system with large surface area and abundant activity sites, which together contribute to the good performance for reduction reactions, oxidation reactions and orchestrated both reduction and oxidation reactions. This Minireview reports the methods for the synthesis of nanoscale hexagonal boron carbonitride (h-BCN) and describes the latest advances in the application of h-BCN materials as semiconductor photocatalysts for sustainable photosynthesis, such as water splitting, reduction of CO₂, acceptorless dehydrogenation, oxidation of sp³ C-H bonds, and sp² C-H functionalization. h-BCN materials may have potential for applications in other organic transformations and industrial manufacture in the future.

Nitrogen-doped Carbon Nanospheres-Modified Graphitic Carbon Nitride with Outstanding Photocatalytic Activity

Metals and metal oxides are widely used as photo/electro-catalysts for environmental remediation. However, there are many issues related to these metal-based catalysts for practical applications, such as high cost and detrimental environmental impact due to metal leaching. Carbon-based catalysts have the potential to overcome these limitations. In this study, monodisperse nitrogen-doped carbon nanospheres (NCs) were synthesized and loaded onto graphitic carbon nitride (g-C₃N₄, GCN) via a facile hydrothermal method for photocatalytic removal of sulfachloropyridazine (SCP). The prepared metal-free GCN-NC exhibited remarkably enhanced efficiency in SCP degradation. The nitrogen content in NC critically influences the physicochemical properties and performances of the resultant hybrids. The optimum nitrogen doping concentration was identified at 6.0 wt%. The SCP removal rates can be improved by a factor of 4.7 and 3.2, under UV and visible lights, by the GCN-NC composite due to the enhanced charge mobility and visible light harvesting. The mechanism of the improved photocatalytic performance and band structure alternation were further investigated by density functional theory (DFT) calculations. The DFT results confirm the high capability of the GCN-NC hybrids to activate the electron-hole pairs by reducing the band gap energy and efficiently separating electron/hole pairs. Superoxide and hydroxyl radicals are subsequently produced, leading to the efficient SCP removal.

In situ addition of Ni salt onto a skeletal Cu7S4 integrated CdS nanorod photocatalyst for efficient production of H2 under solar light irradiation

Development of earth-abundant, low cost, skeletal-type copper sulfide superstructures and in situ addition of Ni salts plays a prominent role to enhance the activity of CdS semiconductor nanostructures for photocatalytic H₂ production.

Enhanced and selective photocatalytic reduction of CO2 by H2O over strategically doped Fe and Cr into porous boron carbon nitride

Strategic doping of metals and non-metals into a photocatalyst can help in tuning the band gap and alignment of band structure.

Phosphorus-doped inverse opal g-C3N4 for efficient and selective CO generation from photocatalytic reduction of CO2

In this work, inverse opal (IO) structure construction and phosphorus doping were combined to modify carbon nitride (g-C₃N₄) for the photocatalytic reduction of CO₂.

Nitrogen deficient carbon nitride for efficient visible light driven tetracycline degradation: a combination of experimental and DFT studies

The narrow visible-light absorption range and a high recombination rate of photo-excited electrons and holes are the main reasons for the confined photocatalytic performance of graphitic carbon nitride (g-C₃N₄).

Z-scheme 2D-m-BiVO4 networks decorated by a g-CN nanosheet heterostructured photocatalyst with an excellent response to visible light

For economical water splitting and degradation of toxic organic dyes, the development of inexpensive, efficient, and stable photocatalysts capable of harvesting visible light is essential. In this study, we designed a model system by grafting graphitic carbon nitride (g-C₃N₄) (g-CN) nanosheets on the surface of 2D monoclinic bismuth vanadate (m-BiVO₄) nanoplates by a simple hydrothermal method. This as-synthesized photocatalyst has well-dispersed g-CN nanosheets on the surface of the nanoplates of m-BiVO₄, thus forming a heterojunction with a high specific surface area. The degradation rate for bromophenol blue (BPB) shown by BiVO₄/g-CN is 96% and that for methylene blue (MB) is 98% within 1 h and 25 min, respectively. The 2D BiVO₄/g-CN heterostructure system also shows outstanding durability and retains up to ∼95% degradation efficiency for the MB dye even after eight consecutive cycles; the degradation efficiency for BPB does not change too much after eight consecutive cycles as well. The enhanced photocatalytic activities of BiVO₄/g-CN are attributed to the larger surface area, larger number of surface active sites, fast charge transfer and improved separation of photogenerated charge carriers. We proposed a mechanism for the improved photocatalytic performance of the Z-scheme photocatalytic system. The present work gives a good example for the development of a novel Z-scheme heterojunction with good stability and high photocatalytic activity for toxic organic dye degradation and water splitting applications.

For economical water splitting and degradation of toxic organic dyes, the development of inexpensive, efficient, and stable photocatalysts capable of harvesting visible light is essential.

Graphitic carbon nitride-based materials for photocatalytic reduction of U(vi)

This work reports the photocatalytic reduction of U(vi) using g-C₃N₄-based materials and discusses the factors affecting the photocatalytic reduction of U(vi).

Modified Graphitic Carbon Nitride Nanosheets for Efficient Photocatalytic Hydrogen Evolution

Considerable research efforts have been devoted to develop noble-metal-free cocatalysts coupled with semiconductors for highly efficient photocatalytic H₂ evolution as part of the challenge toward solar-to-fuel conversion. Herein, a new cocatalyst with excellent activity in the electrocatalytic H₂ evolution reaction (HER) that is based on Co sheathed in N-doped graphitic carbon nanosheets (Co@NC) was fabricated by a surfactant-assisted pyrolysis approach and then coupled with g-C₃ N₄ nanosheets to construct a 2 D-2 D g-C₃ N₄ /Co@NC composite photocatalyst by a simple grinding method. As a result of advantages in effective electrocatalytic HER activity, suitable electronic band structure, and rapid interfacial charge transfer brought about by the 2 D-2 D spatial configuration, the g-C₃ N₄ /Co@NC photocatalyst that contained 4 wt % Co@NC presented a high photocatalytic H₂ generation rate of 15.67 μmol h^-1 under visible-light irradiation (λ≥400 nm), which was 104.5 times higher than that of pristine g-C₃ N₄ . The optimum g-C₃ N₄ /Co@NC photocatalyst showed a high apparent quantum efficiency of 10.82 % at λ=400 nm.

Tailoring of crystalline structure of carbon nitride for superior photocatalytic hydrogen evolution

Light absorption and carrier transfer, are two sequential and complementary steps related to photocatalysis performance, whereas the collective integration of these two aspects into graphitic carbon nitride (g-C₃N₄) photocatalyst through polycondensation optimization have seldom been achieved. Herein, we report on tailoring the crystalline structure of g-C₃N₄ by avoiding the formation of incompletely reacted N-rich intermediates and selective breaking the hydrogen bonds between the layers of g-C₃N₄ simultaneously. The obtained layer plane ordered porous carbon nitride (LOP-CN) material shows efficient photocatalytic H₂ generation performance. The highest H₂ evolution rate achieved is 53.8 μmol under λ ≥ 400 nm light irradiation, which is 7.4 times higher than that of g-C₃N₄ prepared by convention thermal polycondensation. The substantially boosted photocatalytic activity is mainly ascribed to the efficient charge separation on long-range atomic order layer plane and the extended visible light harvesting ability. This work highlights the importance of crystalline structure tailoring in improving charge separation and light absorption of g-C₃N₄ photocatalyst for boosting its photocatalytic H₂ evolution activity.

Dual-defect-modified graphitic carbon nitride with boosted photocatalytic activity under visible light

The development of photocatalysts that efficiently degrade organic pollutants is an important environmental-remediation objective. To that end, we report a strategy for the ready fabrication of oxygen-doped graphitic carbon nitride (CN) with engendered nitrogen deficiencies. The addition of KOH and oxalic acid during the thermal condensation of urea led to a material that exhibits a significantly higher pseudo-first-order rate constant for the degradation of bisphenol A (BPA) (0.0225 min-1) compared to that of CN (0.00222 min-1). The enhanced photocatalytic activity for the degradation of BPA exhibited by the dual-defect-modified CN (Bt-OA-CN) is ascribable to a considerable red-shift in its light absorption compared to that of CN, as well as its modulated energy band structure and more-efficient charge separation. Furthermore, we confirmed that the in-situ-formed cyano groups in the Bt-OA-CN photocatalyst act as strong electron-withdrawing groups that efficiently separate and transfer photo-generated charge carriers to the surface of the photocatalyst. This study provides novel insight into the in-situ dual-defect strategy for g-C3N4, which is extendable to the modification of other photocatalysts; it also introduces Bt-OA-CN as a potential highly efficient visible-light-responsive photocatalyst for use in environmental-remediation applications.

Carbon Nitride Co-catalyst Activation Using N-Doped Carbon with Enhanced Photocatalytic H2 Evolution

Photocatalytic water splitting holds huge potential to meet the current challenges of energy and environments. Among them, polymeric carbon nitride (CN) has drawn much attention as a promising metal-free photocatalyst. As it is known, a number of promising co-catalysts have been developed to improve catalytic reactions, Pt nanoparticles is still among the best co-catalysts for CN in photocatalytic H₂ evolution, due to the suitable Fermi level to transfer excited electrons and the low overpotential for H₂ reduction. Herein, we report the interface engineering of urea-derived bulk CN and Pt co-catalyst by using a small portion of N-doped carbon (N-C) as a transition layer with a boosted photocatalytic activity up to 7 times. It was revealed that the activation energy of the Pt co-catalyst for water reduction was lowered in the presence of N-C, and the intimate interaction between CN and N-C, ascribing to the similar elemental composition and crystal structure, promoted the efficient separation and migration of charge carriers. This study may open a new avenue to develop CN-based photocatalysts for solar fuel conversion with even higher activity by photocatalyst/co-catalyst interface engineering.

A ratiometric multicolor fluorescence biosensor for visual detection of alkaline phosphatase activity via a smartphone

Herein we designed a selective and smartphone-based strategy for visual detection of alkaline phosphatase (ALP) by utilizing the property of amino-functionalized copper (II)-based metal-organic frameworks (NH₂-Cu-MOFs) with oxidase mimic property and fluorescence property. Surprisingly, the oxidase mimic property of NH₂-Cu-MOFs can work well at a high pH value 8.0. Thus, a cascade reaction between ALP and NH₂-Cu-MOFs was realized for the construction of a ratiometric multicolor sensing platform through the controllable catalytic activity of NH₂-Cu-MOFs by pyrophosphate (PPi) and ALP. The catalytic activity of NH₂-Cu-MOFs was greatly inhibited because of the binding ability of Cu²⁺ with PPi. When the ALP was added, the catalytic activity of NH₂-Cu-MOFs was restored and then further catalyzed the o-phenylenediamine to form the 2, 3-diaminophenazine due to the hydrolysis function of ALP towards PPi into orthophosphates. RGB analysis of the fluorescent sample images was adopted for ALP quantitative analysis. Besides, a hydrogel test kit and mobile app for ALP detection were designed as conceptual products for point-of-care. The LODs of the fluorescence sensing platform was 0.078 mU mL^-1 and 0.35 mU mL^-1 by solution analysis and hydrogel test kit analysis, respectively. This fluorescent visual method was applied to ALP detection in serum samples with satisfying results, which opened a promising horizon for the diagnosis of other biomarkers in clinical serum samples based on ALP-mediated enzyme-linked immunosorbent assay for the development of biomedicine and clinical diagnosis.

Artificial Photosynthesis with Polymeric Carbon Nitride: When Meeting Metal Nanoparticles, Single Atoms, and Molecular Complexes

Artificial photosynthesis for solar water splitting and CO₂ reduction to produce hydrogen and hydrocarbon fuels has been considered as one of the most promising ways to solve increasingly serious energy and environmental problems. As a well-documented metal-free semiconductor, polymeric carbon nitride (PCN) has been widely used and intensively investigated for photocatalytic water splitting and CO₂ reduction, owing to its physicochemical stability, visible-light response, and facile synthesis. However, PCN as a photocatalyst still suffers from the fast recombination of electron-hole pairs and poor water redox reaction kinetics, greatly restricting its activity for artificial photosynthesis. Among the various modification approaches developed so far, decorating PCN with metals in different existences of nanoparticles, single atoms and molecular complexes, has been evidently very effective to overcome these limitations to improve photocatalytic performances. In this Review article, a systematic introduction to the state-of-the-art metal/PCN photocatalyst systems is given, with metals in versatility of nanoparticles, single atoms, and molecular complexes. Then, the recent processes of the metal/PCN photocatalyst systems in the applications of artificial photosynthesis, e.g., water splitting and CO₂ reduction, are reviewed. Finally, the remaining challenges and opportunities for the development of high efficiency metal/PCN photocatalyst systems are presented and prospected.

Enhancing Visible-Light Hydrogen Evolution Performance of Crystalline Carbon Nitride by Defect Engineering

Crystalline carbon nitride (CCN)-based semiconductors have recently attracted widespread attention in solar energy conversion. However, further modifying the photocatalytic ability of CCN always results in a trade-off between high crystallinity and good photocatalytic performance. Herein, a facile defect engineering strategy was demonstrated to modify the CCN photocatalysts. Results confirmed that the obtained D-CCN maintained the high crystallinity; additionally, the hydrogen production rate of D-CCN was approximately 8 times higher than that of CCN. Particularly, it could produce H₂ even if the incident light wavelength extended to 610 nm. The significantly improved photocatalytic activity could be ascribed to the introduction of defects into the CCN polymer network to form the midgap states, which significantly broadened the visible-light absorption range and accelerated the charge separation for photoredox catalysis.

Modulation of Polymeric Carbon Nitrides through Supramolecular Preorganization for Efficient Photocatalytic Hydrogen Generation

Supramolecular pre-assembly by design is an effective strategy to adapt the physicochemical properties of polymeric carbon nitrides (PCNs) to improve their solar conversion performance. A new supramolecular preorganization protocol, which employs H₂ O as the self-assembly medium and sodium persulfate as a modifier, is proposed to modulate the textural and photoelectronic features of PCNs for efficient visible-light H₂ evolution. Sodium persulfate is revealed to precisely tailor the final carbon nitride polymers with unusual porous layered structures and promoted charge separation and migration kinetics. As a result, the modulated PCNs with optimized structures show a greatly enriched activity for photocatalytic H₂ generation compared to the analogous materials derived from melamine without the modifier.

Phosphorylation of Polymeric Carbon Nitride Photoanodes with Increased Surface Valence Electrons for Solar Water Splitting

Overcoming the sluggish kinetics of the water oxidation is the key to a high performance for solar water splitting. Herein, phosphorylated polymeric carbon nitride (PCN) photoanodes were developed and showed enhanced photocurrent densities for solar water splitting. A photocatalytic efficiency of 120 μA cm^-2 was achieved in the basic solution (1.0 m NaOH) without sacrificial agents. In this system, phosphates were ionically anchored on the surface of PCN, and the modified films showed significantly increased density of valence electrons, and thus promoting photocatalytic efficiency.

Polymeric graphitic carbon nitride (g-C3N4)-based semiconducting nanostructured materials: Synthesis methods, properties and photocatalytic applications

In recent years, various facile and low-cost methods have been developed for the synthesis of advanced nanostructured photocatalytic materials. These catalysts are required to mitigate the energy crisis, environmental deterioration, including water and air pollution. Among the various semiconductors explored, recently novel classes of polymeric graphitic carbon nitride (g-C₃N₄)-based heterogeneous photocatalysts have established much greater importance because of their unique physiochemical properties, large surface area, low price, and long service life, ease of synthesis, product scalability, controllable band gap properties, low toxicity, and high photocatalytic activity. The present comprehensive review focuses on recent achievements in a number of facile chemical synthesis methods for semiconducting polymeric carbon nitrides and their heterogeneous nanohybrids with various dopants, nanostructured metals, metal oxides, and nanocarbons, as well as the parameters influencing their physiochemical properties and photocatalytic efficiency, which are discussed with reference to various catalytic applications such as air (NO_x) purification, wastewater treatment, hydrogen generation, CO₂ reduction, and chemical transformation. The mechanisms for the superior photocatalytic activity of polymeric g-C₃N₄-based heterogeneous photocatalysts are also discussed. Finally, the challenges, prospects, and future directions for photocatalytic polymeric g-C₃N₄-based semiconducting materials are described.

Doping-Induced Hydrogen-Bond Engineering in Polymeric Carbon Nitride To Significantly Boost the Photocatalytic H2 Evolution Performance

Unlike graphene, graphitic carbon nitride (CN) polymer contains a weak hydrogen bond and van der Waals (vdWs) interactions besides a strong covalent bond, which controls its final morphology and functionality. Herein, we propose a novel strategy, hydrogen-bond engineering, to tune hydrogen bonds in polymeric CN through nonmetal codoping. Incorporation of B and P dopants breaks partial hydrogen bonds within the layers and simultaneously weakens the vdWs interaction between neighboring layers, resulting in ultrathin codoped CN nanosheets. The two-dimensional structure of the ultrathin sheet, broken hydrogen bonds, and incorporated dopants endow them with efficient visible light harvesting, improved charge separation, and increased active edge sites that synergistically enhance the photocatalytic activity of doped CN. Specifically, the B/P-codoped CN exhibits an extremely high hydrogen-evolution rate of 10877.40 μmol h^-1 g^-1, much higher than most reported values of CN. This work demonstrates that hydrogen bond engineering is an effective strategy to modify the structure and properties of polymers for various applications.

LiCl as Phase-Transfer Catalysts to Synthesize Thin Co2 P Nanosheets for Oxygen Evolution Reaction

Inorganic salts have been widely studied as templates for the synthesis of 2D layer structures. However, these salts normally can only serve as templates without any catalytic activity. Here, we report that LiCl used for the synthesis of ultrathin nanosheets not only serves as template for the synthesis of ultrathin Co₂ P nanosheets with a thickness of 0.7 nm but also acts as a catalyst that induces the phase-transfer from CoP to Co₂ P. The Co₂ P nanosheets have a high electrochemical performance for oxygen evolution reaction.

Copper Phosphide-Enhanced Lower Charge Trapping Occurrence in Graphitic-C3N4 for Efficient Noble-Metal-Free Photocatalytic H2 Evolution

Graphitic carbon nitride (g-C₃N₄) fundamental photophysical processes exhibit a high frequency of charge trapping due to physicochemical defects. In this study, a copper phosphide (Cu₃P) and g-C₃N₄ hybrid was synthesized via a facile phosphorization method. Cu₃P, as an electron acceptor, efficiently captures the photogenerated electrons and drastically improved the charge separation rate to cause a significantly enhanced photocatalytic performance. Moreover, the robust and intimate chemical interactions between Cu₃P and g-C₃N₄ offers a rectified charge-transfer channel that can lead to a higher H₂ evolution rate (HRE, 277.2 μmol h^-1 g^-1) for this hybrid that is up to 370 times greater than that achieved from using bare g-C₃N₄ (HRE, 0.75 μmol h^-1 g^-1) with a quantum efficiency of 3.74% under visible light irradiation (λ = 420 nm). To better determine the photophysical characteristics of the Cu₃P-induced charge antitrapping behavior, ultrafast time-resolved spectroscopy measurements were used to investigate the charge carriers' dynamics from femtosecond to nanosecond time domains. The experimental results clearly revealed that Cu₃P can effectively enhance charge transfer and suppress photoelectron-hole recombination.