One-Step Nickel Foam Assisted Synthesis of Holey G-Carbon Nitride Nanosheets for Efficient Visible-Light Photocatalytic H2 Evolution

Highly Sensitive Diethylamine Detection at Room Temperature Using g-C3N4 Nanosheets Decorated with CuO Hollow Polyhedral Structures

Chemiresistive-based metal oxide semiconductor (MOS) gas sensors are widely used in gas sensing due to their advantageous properties. Graphitic carbon nitride (g-C3N4) and metal oxide heterostructure materials can improve charge transport properties, selectivity, and sensitivity in MOS gas sensor materials. Herein, for the first time, CuO hollow polyhedral structures (HPSs) were synthesized via a hydrothermal technique and annealed at different temperatures, with the 400 °C annealed (CuO-400 HPSs) demonstrating remarkable sensing capabilities for diethylamine (DEA) gas at room temperature (RT). The x-g-C3N4 nanosheets were decorated with CuO HPSs in varying amounts (x = 0.8, 1.8, 2.1, and 3.1 wt %) and then annealed at 400 °C for x-g-C3N4-CuO-400 hollow polyhedral heterostructures (HPHSs). Indeed, among the synthesized samples, the 1.8%-g-C3N4-CuO-400 HPHSs have a higher sensitivity to DEA (resistance change in gas (Rg) and air (Ra); Rg/Ra= 65 @ 20 ppm), a low detection limit (Rg/Ra= 6 @ 500 ppb), wide dynamic response (Rg/Ra= 190 @ 80 ppm), strong stability (30 days), and 21.6 times higher sensitivity than pure CuO at RT toward 20 ppm of DEA. The exceptional gas-sensing behavior can be attributed to various factors, including controlled annealing conditions that result in the formation of well-defined structures and greater porosity, efficient charge transfer properties resulting from an optimized ratio of g-C3N4 to CuO in HPHSs, an abundance of defects, unsaturated Cu sites, and synergistic effects. The study presents a universal strategy for generating sensitive and selective g-C3N4-based composite materials for low-temperature gas sensors.

Transition metals-doped g-C3N4 nanostructures as advanced photocatalysts for energy and environmental applications

Graphitic carbon nitride (g-C3N4)-based heterostructured photocatalysts have received significant attention for its potential applications in the treatment of wastewater and hydrogen evolution. The utilization of semiconductor materials in heterogeneous photocatalysis has recently received great attention due to their potential and eco-friendly properties. Doping with metal ions plays a crucial role in altering the photochemical characteristics of g-C3N4, effectively enhancing photoabsorption into the visible range and thus improving the photocatalytic performance of doped photocatalysts. As an emerging nanomaterial, nanostructured g-C3N4 represents a visible light-active semiconducting photocatalyst that has attracted significant interest in the photocatalysis field, particularly for its practical water treatment applications. To the best of our knowledge, investigations of functionalized photocatalytic (PC) materials on 3d transition metal-doped g-C3N4 remain unexplored in the existing literature. g-C3N4 based heterohybrid photocatalysts have demonstrated excellent reusability, making them highly promising for wastewater treatment applications. This paper describes the overview of numerous studies conducted on the heterostructured g-C3N4 photocatalysts with various 3d metals. Research studies have revealed that the introduction of element doping with various 3d transition metals (e.g., Ti, Mn, Fe, Co, Ni, Cu, Zn, etc.) into g-C3N4 is an efficient approach to enhance degradation efficacy and boost photocatalytic activity (PCA) of doped g-C3N4 catalysts. Moreover, the significance of g-C3N4 heterostructured nanohybrids is highlighted, particularly in the context of wastewater treatment applications. The study concludes by providing insights into future perspectives in this developing area of research, with a specific focus on the degradation of various organic contaminants.

Fabrication of direct Z-scheme Cu2O@V-CN (octa) heterojunction with exposed (111) lattice planes and nitrogen-rich vacancies for rapid sterilization

The Z-scheme heterojunction has demonstrated significant potential for promoting photogenerated carrier separation. However, the rational design of all-solid Z-scheme heterojunctions catalysts and the controversies about carrier transfer path of direct Z-scheme heterojunctions catalysts face various challenges. Herein, a novel heterojunction, Cu₂O@V-CN (octa), was fabricated using V-CN (carbon nitride with nitrogen-rich vacancies) in-situ electrostatic self-wrapping Cu₂O octahedra. Density functional theory (DFT) calculations revealed that the separation of carriers across the Cu₂O@V-CN (octa) heterointerface was directly mapped to the Z-scheme mechanism compared to Cu₂O/V-CN (sphere). This is because the Cu₂O octahedra expose more highly active (111) lattice planes with more terminal Cu atoms and V-CN with abundant nitrogen vacancies to form delocalized electronic structures like electronic reservoirs. This facilitates the wrapping of Cu₂O octahedra by V-CN and protects their stability via tighter interfacial contact, thus enhancing the tunneling of carriers for rapid photocatalytic sterilization. These findings provide novel approaches for designing high-efficiency Cu₂O-based photocatalytic antifoulants for practical applications.

More Accurate Method for Evaluating the Activity of Photocatalytic Hydrogen Evolution and Its Reaction Kinetics Equation

Photocatalytic water splitting to hydrogen is a sustainable energy conversion method. However, there is a lack of sufficiently accurate measurement methods for an apparent quantum yield (AQY) and a relative hydrogen production rate (r_H2) at the moment. Thus, a more scientific and reliable evaluation method is highly required to allow the quantitative comparison of photocatalytic activity. Herein, a simplified kinetic model of photocatalytic hydrogen evolution was established, the corresponding photocatalytic kinetic equation was deduced, and a more accurate calculation method is proposed for the AQY and the maximum hydrogen production rate v_H2,max. At the same time, new physical quantities, absorption coefficient k_L and specific activity SA, were proposed to sensitively characterize the catalytic activity. The scientificity and practicality of the proposed model and the physical quantities were systematically verified from the theoretical and experimental levels.

One-step thermal polymerization synthesis of nitrogen-rich g-C3N4 nanosheets enhances photocatalytic redox activity

Graphitic carbon nitride (g-C₃N₄) has attracted enormous attention as a visible-light-responsive carbon-based semiconductor photocatalyst. However, fast charge recombination seriously limits its application. Therefore, it is urgent to modify the electronic structure of g-C₃N₄ to obtain excellent photocatalytic activity. Herein, we reported a one-step thermal polymerization synthesis of nitrogen-rich g-C₃N₄ nanosheets. Benefiting from the N self-doping and the ultrathin structure, the optimal CN-70 exhibits its excellent performance. A 6.7 times increased degradation rate of rhodamine B (K = 0.06274 min^-1), furthermore, the hydrogen evolution efficiency also reached 2326.24 μmol h^-1 g^-1 (λ > 420 nm). Based on a series of characterizations and DFT calculations, we demonstrated that the N self-doping g-C₃N₄ can significantly introduce midgap states between the valence band and conduction band, which is more conducive to the efficient separation of photogenerated carriers. Our work provides a facile and efficient method for self-atom doping into g-C₃N₄, providing a new pathway for efficient photocatalysts.

Pyridazine doped g-C3N4 with nitrogen defects and spongy structure for efficient tetracycline photodegradation and photocatalytic H2 evolution

In this study, with thiourea and 3-aminopyridazine as precursors, the graphite-phase carbon nitride (ACN-x) with nitrogen defects and sponge structure is prepared via the introduction of the benzene-like ring structure of pyridazine replacing a "melem" group through hydrothermal procedure combined with calcination. It is made possible by the attraction of three hydrogen bond receptors for 3-aminopyrazine to lone pair electrons on the "melem" molecule. The remarkable extensively photocatalytic activity can be attributed to three effects of the introduction of 3-aminopyridazine: (i)formation of nitrogen defects between adjacent tri-s-triazine groups; (ii)formation of effective charge transfer channels within the tri-s-triazine group; (iii)the spongy structure exposed abundant amino groups(-NH₃) at edge sites, combining with the internal amino group and as hole stabilizer to prolong the excited state life of photocatalyst. The photogenerated carrier migration and separation efficiency improved effectively through the tuning synergy. As a result, ACN-x exhibits excellent photocatalytic activity, with hydrogen production efficiency of up to 11331.74 μmol g^-1 h^-1, which is approximately 94.5 times that of the pristine g-C₃N₄ (119.88 μmol g^-1 h^-1). The degradation constants of TC and RhB are 0.0498min^-1 and 0.129min^-1, which are 3.32 and 6.35 times of the pristine g-C₃N₄, respectively. The TC degradation in different initial concentrations, pH, dissolved organic matter concentrations, and water sources is conducted to prove the environmental adaptability of the ACN-x system. The mechanism of the system indicates that ·O₂^- plays an important role, and the ·OH and h⁺ play a minor role in the TC photocatalytic degradation. Finally, the TC degradation possible pathway is proposed.

Recent advances in visible-light graphitic carbon nitride (g-C3N4) photocatalysts for chemical transformations

Graphitic carbon nitride (g-C3N4) has emerged as a new research hotspot, attracting broad interdisciplinary attention in the form of metal-free and visible-light-responsive photocatalysts in the field of solar energy conversion and environmental remediation. These photocatalysts have evolved as attractive candidates due to their non-toxicity, chemical stability, efficient light absorption capacity in the visible and near-infrared regions, and adaptability as a platform for the fabrication of hybrid materials. This review mainly describes the latest advances in g-C3N4 photocatalysts for chemical transformations. In addition, the typical applications of g-C3N4-based photocatalysts involving organic transformation reactions are discussed (synthesis of heterocycles, hydrosulfonylation, hydration, oxygenation, arylation, coupling reactions, etc.).

Facile fabrication of surface vulcanized Co-Fe spinel oxide nanoparticles toward efficient 4-nitrophenol destruction

Developing efficient modulation strategies to boost the degradation efficiencies of non-noble metal catalysts for toxic phenolic compounds involving peroxymonosulfate (PMS)-based oxidation processes is essential but remains an arduous challenge. This study reports the one-pot construction of in-situ surface vulcanized CoFe₂O₄ @carbon (S_x-CF@C) to boost the PMS activation for 4-nitrophenol (4-NP) destruction. The direct pyrolysis of an aerogel precursor consisted of cobalt nitrate, ferric nitrate, melamine, and thiourea enables the as-formed S_x-CF@C with hierarchical structure, rich oxygen vacancies, and electron/mass transfer, thereby considerably promoting PMS activation performance of S_x-CF@C toward 4-NP degradation. Specifically, the optimal S_0.2-CF@C can achieve a removal efficiency of 99% for 4-NP destruction (20 mg/L) through PMS activation. Meanwhile, the catalyst also has generality to degrade a variety of antibiotic and dye organic pollutants. The radical quenching and electron paramagnetic resonance tests reveal the radical and non-radical activation mechanism in the S_0.2-CF@C/PMS system. The degradation pathway for 4-NP destruction over the S_0.2-CF@C/PMS system is proposed. This study provides an efficient approach to modulate the PMS activation performance of ferrite spinel materials toward the degradation of acute phenolic compounds.

Oxygen-containing groups and P doped porous carbon nitride nanosheets towards enhanced photocatalytic activity

Metal-free polymer graphite carbon nitride (CN) is a promising photocatalyst that has garnered significant research attention. However, unmodified CN possesses several shortcomings such as low specific surface area, poor dispersibility in water, and rapid photogenerated electron-hole recombination, which have severely impacted its mass adoption. Here, this study proposed a two-step heat treatment method to incorporate P dopant and the containing-oxygen groups successively into CN. The final product, denoted as PO-CN, possessed a porous ultrathin nanosheet-like morphology. The introduction of P dopant altered the intrinsic electronic structure of CN. Meanwhile, the presence of oxygen-containing groups improved the dispersibility of PO-CN in water. Also, it led to the formation of a porous ultrathin structure that could provide more active sites. Through the synergistic effects of these two methods, PO-CN demonstrated superior photocatalytic performance compared to the unmodified counterpart. Based on the collective results obtained experimentally and theoretically, PO-CN possessed a porous ultrathin structure, low resistance, and low carrier recombination. The results show an optimal hydrogen evolution rate of PO-CN (997.7 mol h^-1 g^-1), which was 11.2 times and 3.22 times that of the CN (88.89 mol h^-1 g^-1) and PCN (310.3 mol h^-1 g^-1). Moreover, PO-CN was then used in the degradation of Rhodamine B, and a degradation kinetic constant (k) of 0.15009 was calculated, which was 18.42 times and 8.22 times higher as compared to those of CN (0.00815) and PCN (0.01826). Hence, this work provides a new strategy for the alteration of the morphology and electronic structure of CN.

Superior sponge-like carbon self-doping graphitic carbon nitride nanosheets derived from supramolecular pre-assembly of a melamine-cyanuric acid complex for photocatalytic H2 evolution

The photocatalytic evolution of hydrogen (H₂) from water splitting is considered a promising route to overcome the energy crisis, and the key lies in the preparation of efficient photocatalysts. Herein, superior ordered sponge-like carbon self-doped graphitic carbon nitride (g-C₃N₄) nanosheets (SCCNS) were prepared via a combined strategy of melamine-cyanuric acid complex supramolecular pre-assembly and solvothermal pre-treatment using ethylene glycol (EG) aqueous solutions (EG:water = 50:50 vol.%) as a solvent and carbon doping source. The following pyrolysis converts the naturally arranged melamine-EG-cyanuric acid supramolecular intermediates to highly crystalline SCCNS with large specific surface areas. The optimal SCCNS-180 exhibits superior photocatalytic H₂ evolution activities (∼4393 and 11 320 μmol h^-1 g^-1) when irradiated with visible light and simulated sunlight; these values are up to ∼17- and ∼18-fold higher than that of bulk g-C₃N₄. The quantum efficiency of SCCNS-180 at λ = 420 nm can reach 6.0%. The excellent photocatalytic performance of SCCNS-180 derives from its distinct ordered sponge-like nanosheet structure with highly crystallinity and the carbon doping, leading to its improved optical absorption, accelerated photoinduced electron-hole pair transfer and separation rate and enlarged specific surface area (134.4 m² g^-1).

Structural Insight on Defect-Rich Tin Oxide for Smart Band Alignment Engineering and Tunable Visible-Light-Driven Hydrogen Evolution

Herein, a series of defect-rich tin oxides, Sn_xO_y, were synthesized with tunable Sn²⁺/Sn⁴⁺ composition ratio and defect chemistry, aiming to explore the impact of local structural modulation, non-stoichiometric chemistry, and defective center on the modulation of band gap values, band edge potential positions, and photocatalytic hydrogen evolution performance. The phase structure, morphology, surface component, and photoelectric properties were analyzed by multiple testing methods. The modulation of the Sn²⁺/Sn⁴⁺ molar ratio was analyzed by X-ray photoelectron spectroscopy and the spectra of Mossbauer and electron spin resonance, which indicated the existence of interstitial tin and oxygen vacancy, predicting a highly disordered local structure. In addition, the photocatalytic activity was evaluated by water splitting for hydrogen production under visible light. The optimal photocatalytic activity toward H₂ production rate reached 58.6 μmol·g^-1·h^-1 under visible light illumination. However, the photocatalytic activity gradually decreased with an increase of synthetic temperature. Much higher Sn²⁺/Sn⁴⁺ molar ratio in the present defective tin oxide gave rise to more negative band edge potentials for hydrogen production. Meanwhile, the driving force was decreased with the diminished Sn²⁺. Large amounts of hydroxyl groups, Sn²⁺, and relatively negative potential of conduction band in non-stoichiometric Sn_xO_y play critical roles in visible light harvesting and photocatalytic water splitting. Furthermore, the relationships among crystal structure, electronic properties, and photocatalytic activities were clarified by theoretical calculation. This work provides a novel strategy for the development of highly efficient photocatalytst by regulating the internal electronic structure and surface defects.

Ultrasound-assisted synthesis of BiVO4/C-dots/g-C3N4Z-scheme heterojunction photocatalysts for degradation of minocycline hydrochloride and Rhodamine B: optimization and mechanism investigation

BiVO₄/C-Dots/g-C₃N₄Z-scheme photocatalyst with carbon dots as the electron mediators efficiently degrades minocycline hydrochloride and dye wastewater.

Enhanced photocatalytic activity of Ag/Ag2Ta4O11/g-C3N4 under wide-spectrum-light irradiation: H2 evolution from water reduction without co-catalyst

Designing a superior and stable catalyst toward H₂ evolution under solar light to solve the energy crisis has attracted wide concern. Herein, we have constructed a novel heterojunction photocatalyst Ag/Ag₂Ta₄O₁₁/g-C₃N₄ by in situ assembly, which can efficiently split water to generate H₂ by utilizing wide-spectrum-light irradiation. Optimal H₂ production reaches highly to 253.03 μmol g^-1 h^-1 under the simulated solar light. Moreover, the catalyst presented well stability by the retained 98% photocatalytic activity and invariable textural structure after five recycling tests. The mechanism of H₂ generation over the prepared material was carefully investigated through scanning electron microscope (SEM), transmission electron microscopy (TEM), UV-Vis absorption spectra (UV-Vis), photoluminescence analysis (PL), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectra (EPR), and several electrochemical measurements. It is proposed that the carriers are efficiently separated through Ag-mediated Z-scheme route in space, retaining their strong redox ability. Ag particles produced by in situ reduction from the component Ag₂Ta₄O₁₁ could devote to the quick electron migration as the bridge center, effective solar light harvesting due to their surface plasmon resonance, and excellent stability by inhibiting their agglomeration and elution. This research offers a new idea for constructing full solid Z-scheme photocatalysts under wide-spectrum-light irradiation.

Polymeric structure optimization of g-C3N4 by using confined argon-assisted highly-ionized ammonia plasma for improved photocatalytic activity

The optimization of the polymeric structure and the modulation of surface amino groups in graphitic carbon nitride (g-CN) are critical but challenging in improving the photoelectric and photocatalytic performances of this polymer semiconductor. Ammonia plasma treatment may provide a fast and useful approach to optimize g-CN materials yet is seriously restricted by the low ionization ability of ammonia. Herein, a confined fast and environmental-friendly ammonia plasma method based on argon-assisted high ionization of NH₃ was developed for efficient modification of raw g-CN. Compared with the weakly-ionized pure ammonia plasma which can only introduce amino group onto the surface g-CN, the argon-assisted highly-ionized ammonia plasma treatment obviously contributes to the comprehensively polymeric structure optimization of g-CN, and thus plays a key role in enhancing its light-harvesting and decelerating the recombination of the photogenerated charge carriers. As a result, the argon-assisted highly-ionized ammonia plasma-treated g-CN-Ar+NH₃ outperformed the raw g-CN by a 2.5-fold higher photocatalytic reduction of hexavalent chromium and a remarkable 3.8-fold higher photocatalytic H₂ evolution activity (up to 957.8 μmol·h^-1·g^-1) under visible light irradiation. Our findings suggest the great prospects of this novel highly-ionized ammonia plasma treatment method in the controllable modification of semiconductors and polymers.

Amorphous Bimetallic Cobalt Nickel Sulfide Cocatalysts for Significantly Boosting Photocatalytic Hydrogen Evolution Performance of Graphitic Carbon Nitride: Efficient Interfacial Charge Transfer

Noble metals usually work as the cocatalyst for photocatalytic water splitting, but their rare and expensive properties narrowed their wide development. Transition-metal sulfides have appeared to be promising non-noble metal cocatalysts in the hydrogen evolution reaction (HER) to meet future energy demands. Meanwhile, many studies focus on the fabrication of bimetallic catalysts because of their remarkably superior catalytic activity compared with monometallic substances. Herein, amorphous bimetallic cobalt nickel sulfide (CoNiS_x) was fabricated to work as a cocatalyst in the photocatalytic H₂ evolution reaction, which can couple with pristine graphitic carbon nitride (g-C₃N₄). CoNiS_x-CN exhibits a larger specific surface area compared with g-C₃N₄, making it possess more reaction active sites. Moreover, the contacted interface in the CoNiS_x-CN composite photocatalyst contributes to higher separation efficiency of photogenerated carriers, which was proved by experimental and theoretical calculations. More importantly, the theoretical calculation also verified that CoNiS_x-CN has relatively closer Gibbs free energy to zero than pure g-C₃N₄ and corresponding monometallic cocatalyzed g-C₃N₄. Therefore, the prepared CoNiS_x-CN composite exhibited a dramatic photocatalytic HER performance of 2.366 μmol mg^-1 h^-1, which is about 76-fold higher in comparison with pristine g-C₃N₄ and comparable to g-C₃N₄ with Pt as a cocatalyst under 420 nm light irradiation. This study reveals a promising and efficient bimetallic cocatalyst for the photocatalytic H₂ evolution reaction.

Multiple Doped Carbon Nitrides with Accelerated Interfacial Charge/Mass Transportation for Boosting Photocatalytic Hydrogen Evolution

The interaction of water molecule with catalysts is crucial to photocatalysis, but the surface property manipulation still remains a great challenge. In this study, we report an in situ multiple heteroelement (sodium, oxygen, and iodide) doping strategy based on a molten salt-assisted route to prepare a green-colored carbon nitride (GCN). The as-prepared GCN yields 25.5 times higher H₂ evolution rate than that of bulk polymeric carbon nitride under visible light. Experimental characterization data demonstrate that the GCN delivers upshift of the conduction band and increased specific surface area and hydrophilicity. As confirmed by time-resolved PL spectra, DMPO spin-trapping EPR analysis, and so on, the excellent activity is dominantly ascribed to the greatly enhanced hydrophilicity and, subsequently, efficient interfacial charge transfer and hydrogen liberation. Moreover, through surface charge modification, the GCN also shows an increased degradation activity of rhodamine B. This work highlights the importance of surface modulation through multiple earth-abundant element incorporation for designing of advanced and practical photocatalysts.

Three-Dimensional Hierarchical g-C3N4 Architectures Assembled by Ultrathin Self-Doped Nanosheets: Extremely Facile Hexamethylenetetramine Activation and Superior Photocatalytic Hydrogen Evolution

Photocatalytic hydrogen evolution has broad prospects as a clean solution for the energy crisis. However, the rational design of catalyst complex, the H₂ evolution efficiency, and the yield are great challenge. Herein, three-dimensional hierarchical g-C₃N₄ architectures assembled by ultrathin carbon-rich nanosheets (3D CCNS) were prepared via an extremely facile hexamethylenetetramine activation approach at the bulk scale, indicating the validation of scale-up production process. The two-dimensional ultrathin carbon-rich nanosheets were several hundred nanometers in width but only 5-6 nm in thickness and gave rise to a unique 3D interconnected network. The unique composition and structure of the nanosheets endow them with a remarkable light absorption spectrum with the tunable band gap, high electrical conductivity, fast charge separation, and large surface areas with abundant reaction active sites, and thus significantly improved H₂ production performance. As high as ∼7.8%, quantum efficiency can be achieved by irradiating 3D CCNS at 420 nm with a H₂ evolution rate >2.7 × 10⁴ μmol/g/h, which is ∼31.3 times higher than that of the pristine g-C₃N₄. Our work introduces an extremely facile route for mass production of doping modified 3D g-C₃N₄-based photocatalyst with excellent H₂ evolution performances.

Facile fabrication of oxygen and carbon co-doped carbon nitride nanosheets for efficient visible light photocatalytic H2 evolution and CO2 reduction

C₃N₄ nanosheets with oxygen and carbon co-doping were successfully designed for H₂ evolution and CO₂ reduction. A mechanistic study was also performed.

Nanopore enriched hollow carbon nitride nanospheres with extremely high visible-light photocatalytic activity in the degradation of aqueous contaminants of emerging concern

Nanopore enriched hollow carbon nitride nanospheres exhibit extremely high visible-light photocatalytic activity in the degradation of aqueous contaminants of emerging concern.