Facile fabrication of nickel/porous g-C3N4 by using carbon dot as template for enhanced photocatalytic hydrogen production

Ni/porous g-C₃N₄ was prepared by high temperature thermal polymerization process using carbon dots as soft template and photodeposition. With nickel nanoparticles supported as co-catalyst, the hydrogen evolution reaction (HER) activity of the photocatalyst has been significantly enhanced under visible light, which is up to 1273.58 μmol g⁻¹ h⁻¹, superior to pristine g-C₃N₄ (4.12 μmol g⁻¹ h⁻¹). This is attributed to the inhibited recombination of photogenerated electron-hole pairs and the much better electron transport efficiency. The formed porous structure of carbon nitride could facilitate light utilization and together with nickel nanoparticles, better charge separation can be realized which are proved by the photoluminescence, time-resolved photoluminescence spectra, transient photocurrent measurements and electrochemical impendence spectroscopy. This work provides a useful route to obtain less expensive and efficient photocatalyst containing no noble metals for hydrogen production.     

Bimetallic PtNi alloy modified 2D g-C3N4 nanosheets as an efficient cocatalyst for enhancing photocatalytic hydrogen evolution

Platinum-based alloy materials as effective cocatalysts in improving the performance of photocatalytic H₂ production have raised great interest. Herein, a facile strategy of chemical reduction is established to synthesize bimetallic PtNi nanoparticles on 2D g-C₃N₄ nanosheets with excellent photocatalytic activity. The addition of PtNi nanoparticles can provide new H⁺ reduction sites and increase more active sites of the material. The synergistic effect between PtNi alloy nanoparticles and 2D g-C₃N₄ nanosheets can regulate electronic structure, narrow the band, accelerate charge transfer efficiency and inhabit the recombination of photo-induced electron (e⁻) and hole pairs (h⁺), contributing to the improvement of hydrogen evolution activity. The optimal hydrogen evolution rate of Pt_0.6Ni_0.4/CN shows higher hydrogen evolution rate (9528 μmol·g⁻¹·h⁻¹), which is 13.1 times than that of pure g-C₃N₄ nanosheets. Besides, a possible mechanism of photocatalytic hydrogen generation has been brought up according to a series of physical and chemical characterization. This work also provides a potential idea of developing cocatalysts integrating metal alloys with 2D g-C₃N₄ nanosheets for promoting photocatalytic hydrogen evolution.     

Heterostructured thin LaFeO3/g-C3N4 films for efficient photoelectrochemical hydrogen evolution

The deposition of LaFeO₃ at the surface of a graphitic carbon nitride (g-C₃N₄) film via magnetron sputtering followed by oxidation for photoelectrochemical (PEC) water splitting is reported. The LaFeO₃/g-C₃N₄ film was investigated by various characterization techniques including SEM, XRD, Raman spectroscopy, XPS and photo-electrochemical measurements. Our results show that the hydrogen production rate of a g-C₃N₄ film covered by a LaFeO₃ film, exhibiting both a thickness of ca. 50 nm, is of 10.8 μmol h⁻¹ cm⁻² under visible light irradiation. This value is ca. 70% higher than that measured for pure LaFeO₃ and g-C₃N₄ films and confirms the effective separation of electron-hole pairs at the interface of LaFeO₃/g-C₃N₄ films. Moreover, the LaFeO₃/g-C₃N₄ films were demonstrated to be stable and retained a high activity (ca. 70%) after the third reuse.     

Recent advances in g-C3N4-based photocatalysts for hydrogen evolution reactions

Graphitic carbon nitride (g-C₃N₄), having unique properties, like suitable electronic band structure, ease of functionalization, easy synthesis, and high stability is a polymeric semiconductor. These properties make it suitable to act as a photocatalyst and have attracted researchers to use it for hydrogen evolution reactions (HER). This review provides the recent advances (2019 onwards) in the development of g-C₃N_4-based photocatalysts to be employed for HER, starting with the fundamentals of g-C₃N₄, designing and engineering g–C₃N₄–based photocatalysts categorized as doped-g-C₃N_4, composites of g-C₃N₄, and engineered-g-C₃N₄ are discussed. Analysis of characteristics and advantages of g-C₃N_4-based heterojunctions is also provided, followed by current challenges and future perspectives, leading to the conclusion. It is expected to offer valuable information for rational design of novel and efficient g-C₃N_4-based photocatalysts for visible-light-driven HER.     

Guanidine carbonate assisted supramolecular self-assembly for synthesizing porous g-C3N4 for enhanced photocatalytic hydrogen evolution

Graphitic carbon nitride (g-C₃N₄) is taken as one of the most promising polymer semiconductor photocatalysts for energy conversion. However, the photocatalytic activity of g-C₃N₄ is usually impeded by the low light absorption and fast recombination of photogenerated carriers. Herein, three-dimensional porous g-C₃N₄ with controllable morphology are synthesized by thermal polycondensation of supramolecular preorganization assembly of melamine, cyanuric acid and guanidine carbonate (1:1:x, x means the ratio of guanidine carbonate). By adjusting the amount of guanidine carbonate in the assembly, the precursors’ morphology can be changed from microrods to polyhedrons, which affects the g-C₃N₄ structure accordingly. The optimized hollow porous polyhedral g-C₃N₄ shows the enhanced light absorption and improved photogenerated carriers separation efficiency, thus exhibiting a 7.7-fold hydrogen evolution activity and 9-fold apparent quantum efficiency (AQE) higher than microtube without addition of guanidine carbonate. This work paves a complementary way towards synthesizing highly efficient photocatalysts through the guanidine carbonate-assisted supramolecular assembly.     

Decoration of nicale phosphide nanoparticles on CdS nanorods for enhanced photocatalytic hydrogen evolution

Manipulation of the co-catalyst plays an important role in charge separation and reactant activation to enhance the activity of CdS based photocatalysts. Transition-metal phosphides have aroused widespread interest in catalysis owing to their special structure and catalytic behavior. Herein, Ni₂P as a cocatalyst coupled with CdS for efficient photocatalytic hydrogen evolution with a rate of 483.25 mmol g^?1.h^?1, which was nearly 525 and 1.92 times higher than that of CdS (0.92 mmol g^?1.h^?1) and 1 wt% noble metal Pt modified CdS (251.29 mmol g^?1.h^?1), respectively. Its apparent quantum yield reaches 70% at 420 nm. Based on data analysis, Schottky heterostructure was constructed by combining Ni₂P with CdS. The Schottky junction provides a convenient way for photoinduced electrons to transfer and promotes the effective separation of photoinduced carriers.     

In situ loading amorphous CoSx on N-doped g-C3N4 through light assisted synthesis for enhanced photocatalytic hydrogen generation

Loading co-catalysts are an effective strategy to break the confinement of bulk carbon nitride in photocatalysis. Employing this strategy, N-doped g-C₃N₄ decorated with CoS_x was successfully prepared through a photochemical synthesis route. The optimum hydrogen evolution performance of N-CN-CoS_x-4 was 1757 μmol g⁻¹ h⁻¹ under visible light irradiation. Superior interfacial carrier transfer properties and improved light absorption of N-CN-CoS_x-4 could elucidate its better photocatalytic activity. This research offers a reference for the construction of high-efficiency, stable and low-priced photocatalysts.     

Preparation of porous C doped g-C3N4 nanosheets controlled by acetamide for photocatalytic H2 evolution

Photocatalytic production H₂ of splitting water is an important branch of the application of photocatalytic materials. In this paper, porous C doped carbon nitride was successfully synthesized by calcining urea and acetamide at high temperature, and then analyzed by various characterization methods. XRD and FT-IR spectra show that the crystal structure of carbon nitride modified by acetamide are similar to bulk carbon nitride. SEM and TEM spectra show that the morphology and structure of carbon nitride and carbon nitride was modified by acetamide, which presents a porous nanosheets structure. XPS and EDS show that the content of carbon element in carbon nitride modified by acetamide increased significantly. The PL spectrum is obviously red shift, and the light absorption range is obviously enlarged, which is consistent with the absorption edge of UV–vis Spectrum. The BET of the optimized sample CN2-6 is 122.79 m²g^-1, which is 3.04 times higher than that of bulk carbon nitride CN1 (40.456 m²g^-1). In addition, the hydrogen production of CN2-6 is 13169.04 μmol/g/h under 300 W Xenon lamp, which is 14.95 times more than CN1 (880.82 μmol/g/h) hydrogen production rate. When the light source is a 10 W LED lamp, the apparent quantum efficiency (AQE) of sample CN2-6 reaches the maximum (2.91) under the wavelengths of 430 nm, which is 1.27 times more than CN1 under the same conditions. Meanwhile, prepared samples have excellent stability and durability under visible light.     

Simultaneously engineering K-doping and exfoliation into graphitic carbon nitride (g-C3N4) for enhanced photocatalytic hydrogen production

Doping and exfoliation are effective strategies to improve the photocatalytic activity of bulk graphitic carbon nitride (g-C₃N₄). Therefore, it can be inferred that engineering element-doping and exfoliation into g-C₃N₄ would further enhance the photocatalytic performance. Herein, we demonstrated a KOH-assisted hydrothermal-reformed melamine strategy for achieving the simultaneous K-doping and exfoliation of g-C₃N₄. The as-synthesized K-doped g-C₃N₄ ultrathin nanosheets displayed much enhanced photocatalytic hydrogen evolution rate (HER) of about 13.1 times higher than that of the bulk g-C₃N₄ under visible-light irradiation, achieving an apparent quantum efficiency of 6.98% at 420 nm. The improved photocatalytic HER can be attributed to the high surface area offering numerous photocatalytic active sites, enlarged conductive band edge optimizing photoreduction potential, and K-doping promoting charge generation and separation as well as the long life-time of photogenerated carriers. This work would provide a promising way to integrate co-doping and exfoliation into new gC₃N₄based materials.     

In-situ synthesis of ternary heterojunctions via g-C3N4 coupling with noble-metal-free NiS and CdS with efficient visible-light-induced photocatalytic H2 evolution and mechanism insight

The two-dimensional (2D) graphitic carbon nitride (g-C₃N₄) nanosheets based composites are prepared in the form of the NiS/g-C₃N₄, CdS/g-C₃N₄ and CdS/NiS/g-C₃N₄ using a facile and reliable method of chemical deposition. The TEM and HRTEM images demonstrated a spectacular representation of the 2D lamellar microstructure of the g-C₃N₄ with adequately attached CdS and NiS nanoparticles. The changes in crystallinity and the surface elemental valence states of composites with the incorporation of two metal sulphides are studied, which confirmed the formation of composites. The photocatalytic response of the composites was estimated by photodegradation of Rhodamine B (C₂₈H₃₁ClN₂O₃–RhB), and the ternary composite CdS/NiS/g-C₃N₄ samples exhibited the superior photocatalytic performance. Further, the free radical capture and electron paramagnetic resonance (EPR) spectroscopy experiments identified the main active species that contributed to the photocatalytic reaction. Besides, the samples’ photocatalytic performance was evaluated by photocatalytic hydrogen production. The stability of the performance-optimized composite was determined by employing cyclic experiments over five cycles. The CdS/NiS/g-C₃N₄ showed the highest efficiency of hydrogen production i.e. about 423.37 μmol.g^?1.h^?1, which is 2.89 times that of the pristine g-C₃N₄. Finally, two types of heterojunction structures were proposed to interpret the enhanced photocatalytic efficiency.     

Mesoporous g-C3N4 decorated by Ni2P nanoparticles and CdS nanorods together for enhancing photocatalytic hydrogen evolution

Ni₂P nanoparticles and CdS nanorods were grew together on a mesoporous g-C₃N₄ through a facile in-situ solvothermal approach. Under visible light (λ > 400 nm), the as-prepared ternary PCN–CdS-5% Ni₂P composite displays a high H₂ evolution rate with 2905.86 μmol g^?1 h^?1, which is about 14, 18 and 279 times that of PCN–CdS, PCN–Ni₂P and PCN, respectively. The enhanced photocatalytic activity is mainly attributed to the improved separation efficiency of the photocarriers by the type II PCN–CdS heterojunction and the effective extraction of photogenerated electrons by Ni₂P. Meanwhile, Ni₂P acts as co-catalyst to provide the photocatalytic active site for hydrogen reduction. In addition, PCN–CdS-5% Ni₂P composite exerts good stability in 12-h cycles.     

Increased photocatalytic hydrogen evolution and stability over nano-sheet g-C3N4 hybridized CdS core@shell structure

A novel visible-light-active CdS@g-C₃N₄ photocatalyst was synthesized via a chemisorption method. This core@shell structure catalyst exhibited enhanced photocatalytic H₂ production activity under visible-light (λ ≥ 420 nm) irradiation. The nano-sheet g-C₃N₄ was successfully coated on CdS nanoparticles with intimate contact. When the content of g-C₃N₄ in the hybridized composite is 3 wt. %, the hydrogen-production rate of the CdS@g-C₃N₄ is 2.5 and 2.2 times faster than pure CdS and bulk g-C₃N₄, respectively. Superior stability was also observed in the cyclic runs. The improvement in stability and activity result from the ability of the π-conjugated g-C₃N₄ material in transporting photo-induced holes. The core@shell structure promoted separation of the photo-generated electron-hole pair and accelerated the emigration speed of the hole from the valence band of CdS. This effect also results in a greatly improved amount of hydrogen production. The possible mechanism for the photocatalytic activity and stability of CdS@g-C₃N₄ are tentatively proposed.     

Direct Z-scheme CoS/g-C3N4 heterojunction with NiS co-catalyst for efficient photocatalytic hydrogen generation

Facilitating the separation of photoexcited electron-hole pairs and enhancing the migration of photogenerated carriers are essential in photocatalytic reaction. CoS/g-C₃N₄/NiS ternary photocatalyst was prepared by hydrothermal and physical stirring methods. The optimized ternary composite achieved a hydrogen yield of 1.93 mmol g^?1 h^?1, 12.8 times that of bare g-C₃N₄, with an AQE of 16.4% at 420 nm. The enhanced photocatalytic activity of CoS/g-C₃N₄/NiS was mainly ascribed to the synergistic interaction between the Z-scheme heterojunction constructed by CoS and g-C₃N₄ and the NiS co-catalyst featuring a large amount of hydrogen precipitation sites, which realized the efficient separation and migration of photogenerated carriers. In addition, the CoS/g-C₃N₄/NiS heterojunction-co-catalyst system exhibited excellent photocatalytic stability and recyclability.     

Fabrication of alveolate g-C3N4 with nitrogen vacancies via cobalt introduction for efficient photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution is a promising method for converting solar energy into chemical energy. Herein, on the basis of graphitic carbon nitride (g-C₃N₄) material with alveolate structure prepared via the hard template method, transition-metal cobalt oxide nanoparticles were reasonably introduced, and a highly efficient cobalt oxide composite alveolate g-C₃N₄ (ACN) photocatalyst was successfully prepared. A series of test methods were used to characterize the structural properties of the prepared samples systematically, and the photocatalytic activity of the catalysts in photocatalytic hydrogen evolution was explored. The composite materials have excellent photocatalytic performance mainly because the synergistic effect of the alveolate structure of ACN provides multiple scattering effects; nitrogen vacancies serves as the centers of photogenerated carrier separation; and cobalt oxides accelerates electron transfer. This study provides a new idea for the design of g–C₃N₄–based photocatalysts with wide light responses and simple structures.     

CaH2-assisted structural engineering of porous defective graphitic carbon nitride (g-C3N4) for enhanced photocatalytic hydrogen evolution

The practical applications of graphitic carbon nitride (g-C₃N₄) for photocatalytic hydrogen evolution is strictly hindered by the low surface area, poor light harvesting capability and detrimental recombination of photoexcited charge carriers. Herein, using melamine as precursor and metal hydride (i.e., CaH₂) as active agent, we facilely incorporate various types of defects (i.e., nitrogen (N) vacancies (V_N), cyano groups (CN) and surface absorbed oxygen species(O_abs)) into g-C₃N₄ within a single step. The as-prepared material (denoted as MM-H) exhibits narrowed bandgap, promoted photoexcited electron-hole separation rate and facilitated charge transfer kinetics with enlarged BET surface area and massive porosity. As a result, a prominently enhanced photocatalytic H₂ productivity efficiency (1305.9 μmol h⁻¹g⁻¹) is shown on MM-H. This performance is better than that of g-C₃N₄ with CaH₂ post-treatment (617.3 μmol h⁻¹g⁻¹) and raw bulk-C₃N₄ (178.2 μmol h⁻¹g⁻¹). This work opens up a new dimension for designing high performance g–C₃N₄–based catalysts targeting various photocatalytic processes.     

CuS nanosheet-induced local hot spots on g-C3N4 boost photocatalytic hydrogen evolution

An ideal model composite made of CuS nanosheet (photothermal material) and g-C₃N₄ was constructed through in-situ assembly procedure, along with integration of dual photochemical effect and photothermal effect. Consequently, optimized CN/CuS composite approaches remarkable photocatalytic performance improved by 44.5 times with regarding to that of pristine CN. This superiority can be assignable to inherent characteristic of CuS as photochemical component, with improved charge separation and enriched surface active-sites. Additionally, the critical contribution of photothermal effect in boosting water photosplitting was also experimentally validated. CuS nanosheet as hot spots enables rapid temperature increment around photocatalysts under visible/near-infrared (NIR) light irradiation. The heat converted from solar is conducive to increase carrier density, accelerate carrier mobility, alleviate onset potential and facilitate surface redox kinetics, so as to promote photocatalytic activity. It is believed that synergetic incorporation of photothermal and photochemical conversion could be expanded to other photocatalytic systems towards effective solar energy conversion.     

N-doped carbon wrapped CoFe alloy nanoparticles with MoS2 nanosheets as electrocatalyst for hydrogen and oxygen evolution reactions

Developing efficient and cost-effective transition metal-based electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is crucial to generate clean and renewable hydrogen energy. The construction of hybrid catalysts with multiple active sites is an effective approach to promote catalytic performance. Herein, a molybdenum disulfide (MoS₂)-based hybrid with N-doped carbon wrapped CoFe alloy (MoS₂/CoFe@NC) was synthesized through a typical hydrothermal method. The MoS₂/CoFe@NC exhibits excellent electrocatalytic performance with overpotentials of 172 mV for HER and 337 mV for OER at 10 mA cm⁻², and long-term stability of 24-h electrolytic reaction in 1 M KOH solution. The chemical coupling between MoS₂ and CoFe@NC provides improved electronic structures and more accessible active sites. The CoFe@NC substrate accelerates the charge transfer to MoS₂ through a synergistic effect. This work demonstrates that the CoFe@NC is a promising substrate for depositing MoS₂ nanosheets (NSs) to achieve excellent catalytic performance for both HER and OER.     

A visible-light-driven heterojunction for enhanced photocatalytic water splitting over Ta2O5 modified g-C3N4 photocatalyst

The photocatalytic water splitting for generation of clean hydrogen energy has received increasingly attention in the field of photocatalysis. In this study, the Ta₂O₅/g-C₃N₄ heterojunctions were successfully fabricated via a simple one-step heating strategy. The photocatalytic activity of as-prepared photocatalysts were evaluated by water splitting for hydrogen evolution under visible-light irradiation (λ > 420 nm). Compared to the pristine g-C₃N₄, the obtained heterojunctions exhibited remarkably improved hydrogen production performance. It was found that the 7.5%TO/CN heterojunction presented the best photocatalytic hydrogen evolution efficiency, which was about 4.2 times higher than that of pure g-C₃N₄. Moreover, the 7.5%TO/CN sample also displayed excellent photochemical stability even after 20 h photocatalytic test. By further experimental study, the enhanced photocatalytic activity is mainly attributed to the significantly improve the interfacial charge separation in the heterojunction between g-C₃N₄ and Ta₂O₅. This work provides a facile approach to design g-C₃N₄-based photocatalyst and develops an efficient visible-light-driven heterojunction for application in solar energy conversion.     

Accelerating photocatalytic hydrogen evolution of Ta2O5/g-C3N4 via nanostructure engineering and surface assembly

Despite that several strategies have been demonstrated to be effective for improving the catalytic hydrogen evolution activity of bulky g-C₃N₄, the large-scale hydrogen production over g–C₃N₄–based photocatalysts still confronts a big challenge. Here, a two-step calcination method is presented in constructing metal oxide/two-dimensional g-C₃N₄, i.e., Ta₂O₅/2D g-C₃N₄ photocatalyst. Thanks to the superiority of the synthetic method, nanostructure engineering forming 2D structure, and surface assembly with another semiconductor, can be realized simultaneously, in which ultrathin structure of 2D g-C₃N₄ and strong interfacial coupling between two components are two important characteristics. As a result, the structure engineered Ta₂O₅/2D g-C₃N₄ induces high photocatalytic hydrogen evolution half reaction rate of ~19,000 μmol g^?1 h^?1 under visible light irradiation (λ > 400 nm), and an external quantum efficiency (EQE) of 25.18% and 12.48% at 405 nm and 420 nm. The high photocatalytic performance strongly demonstrates the advance of the synchronous engineering of nanostructure and construction of heterostructure with tight interface, both of which are beneficial for the fast charge separation and transfer.     

Ball-milled Ni2P/g-C3N4 for improved photocatalytic hydrogen production

Transition metal phosphides (TMPs) are ideal candidates to replace precious cocatalysts for photocatalytic hydrogen evolution reaction (HER). Understanding the structure-activity relationships between TMPs and the host photocatalysts is an important criterion for the development of highly active TMPs for HER. In this work, the relations were explored with Ni₂P and g-C₃N₄ as prototypes. Ni₂P with a clear composition and structure was prepared first by a mild solvothermal method and was then loaded on g-C₃N₄ with an exact content through a ball milling process. The effects of the modification of Ni₂P on g-C₃N₄ structure, absorption, texture, HER activity, and photo-electrochemical properties were investigated. The results demonstrated that the crystal structure and the texture of g-C₃N₄ are less impacted by the decoration of Ni₂P, but the HER activity can be substantially improved. g-C₃N₄ modified with 3% Ni₂P showed the highest HER performance and it is ca. 9 times higher than the pristine g-C₃N₄ even Ni₂P was loaded just by a simple ball milling process. Ni₂P played a dual role as trapping sites to capture photoinduced electrons and the reactive centers to trigger the evolution of hydrogen for its high work function and the low HER overpotential. This work reveals some reliable structure-activity relationships between Ni₂P and g-C₃N₄ and offers a simple approach to synthesize highly efficient Ni₂P and its loading on host photocatalyst for enhanced HER.