Non-noble-metal Ni nanoparticles modified N-doped g-C3N4 for efficient photocatalytic hydrogen evolution

The use of non-noble-metal to replace precious metal as co-catalyst in solar-driven hydrogen evolution reaction (HER) is important for lowering hydrogen production cost. In this work, nickel metal nanoparticles loaded nitrogen-doped graphite carbon nitride (NiNCN3) was prepared, which significantly enhanced the HER activity of nitrogen-doped graphite carbon nitride. The hydrogen evolution rate of NiNCN3 can reach to 1507 μmol g⁻¹ h⁻¹, much higher than that of 3 wt % Pt/NCN (1055 μmol g⁻¹ h⁻¹). The distinguished photocatalytic performance is due to the accelerated electron transfer efficiency and inhibited photogenerated electron-hole recombination. Our study offers an alternative method to achieve the low-cost and effective noble-metal-free photocatalyst for HER.     

Synthesis of Ni2P/Ni5P4 on porous N decorated rGO foam for efficiently electrocatalytic hydrogen evolution reaction

As a new generation of non-precious metal catalysts, nickel phosphide is regarded as an ideal substitute for precious metal platinum in electrochemical hydrogen evolution. Here, a hydrogen evolution reaction (HER) electrocatalyst is developed by in situ growth of Ni₂P/Ni₅P₄ heterostructures on porous N decorated rGO foam (named Ni₂P/Ni₅P₄/N-rGO). The porous rGO foam structure provides a larger surface area and abundant active sites. The Ni₂P/Ni₅P₄ nanoparticles with heterostructures are uniformly distributed on the rGO sheet, which enhance the charge transfer ability. The decorating of N element also correspondingly improves the HER performance. The as-prepared Ni₂P/Ni₅P₄/N-rGO exhibits excellent HER performance in alkaline medium. When the current density is 10 mA cm^?2, the overpotential is only 22 mV. No obvious loss of HER activity after 2000 cyclic voltammetry indicates that the composite has excellent stability. This work presents a valuable route for fabricating inexpensive and high-performance catalysts for electrocatalysis.     

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.     

Construction of Cu7.2S4/g-C3N4 photocatalyst for efficient NIR photocatalysis of H2 production

Graphite-like carbon nitride (g-C₃N₄) has been regarded as a promising photocatalyst for solar-to-chemical conversion. Nevertheless, the narrow absorption of light extremely limited its photocatalytic performance under near-infrared (NIR) irradiation. Herein, the Cu₇.₂S₄ with outstanding NIR absorption was successfully introduced to g-C₃N₄ nanosheets through a simple in-situ growth procedure. As expected, the constructed Cu_7.2S₄/g-C₃N₄ (CSCN) photocatalysts exhibit superior H₂ production activity of 82 μmol g⁻¹ h⁻¹ under NIR light irradiation (λ > 800 nm), which outperforms currently reported g–C₃N₄–based NIR-driven H₂ production systems. Especially, the optimal sample CSCN-5 displays a robust activity of 66 μmol g⁻¹ h⁻¹ at λ = 850 nm monochromatic light irradiation. The excellent photocatalytic performance is linked to the extended optical absorption as well as the efficient separation efficiency of photoinduced carriers, which are evidenced by the UV-visible absorption spectroscopy and photoelectrochemical test. This work provides an effective approach for constructing a Cu_7.2S₄/g-C₃N₄ photocatalytic system for the transformation of NIR solar energy into hydrogen.     

Ag2CO3-derived Ag/g-C3N4 composite with enhanced visible-light photocatalytic activity for hydrogen production from water splitting

In this paper, Ag-based g-C₃N₄ composites have been successfully fabricated through two deferent synthetic methods: (i) a facile and efficient precipitation-calcination strategy (denoted as D–CN–xAg, x represents the dosage of Ag₂CO₃, the same below), (ii) a calcination method (denoted as Z–CN–xAg). All Ag-based g-C₃N₄ composites exhibit the enhanced photocatalytic activities under visible-light irradiation. Moreover, the optimal dosage of Ag₂CO₃ in the D–CN–xAg composite is found to be 5%, the corresponding hydrogen production capacity is 153.33 μmol g⁻¹ h⁻¹, which is 4.6 times higher than that of Z–CN–5%Ag composite. This might be attributed to appropriate content of metallic Ag and more active sites exposed on the surface of D–CN–5%Ag composite. Meanwhile, combining with photoelectrochemical results, it could be inferred that LSPR effect and the intimate interfacial between metallic Ag and g-C₃N₄ in the system play also important role for the improvement of photocatalytic activity. These results demonstrate that the appropriate loading of metallic Ag originated from Ag₂CO₃ into g-C₃N₄ could accelerate the separation and transfer of photogenerated electron-hole pairs, leading to the improvement of photocatalytic activity for hydrogen production from water splitting. Finally, a possible photocatalytic mechanism is proposed.     

2D MOFs enriched g-C3N4 nanosheets for highly efficient charge separation and photocatalytic hydrogen evolution from water

Here we report a 2D-2D heterostructure of g-C₃N₄/UMOFNs photocatalysts via mechanical grinding two kinds of two-dimensional nanosheets of g-C₃N₄ nanosheets and UMOFNs, which exhibits enhanced H₂ evolution from water with simulated solar irradiation. g-C₃N₄ nanosheets are in close contact with UMOFNs, and there is a certain interaction between them, showing the effect of superimposition on the two-dimensional layer. The 2D-2D heterostructure offers a maximal photocatalytic hydrogen production activity of 1909.02 μmol g⁻¹ h⁻¹ with 3 wt% of UMOFNs, which is 3-fold higher than that of g-C₃N₄ nanosheets (628.76 μmol g⁻¹ h⁻¹) and 15-flod higher than that of bulk g-C₃N₄ (124.30 μmol g⁻¹ h⁻¹). The significant increasement of photocatalysis is due to 2D-2D heterostructure possessing a short charge transfer distance and large contact area between g-C₃N₄ and UMOFNs. The highly dispersed NiO, CoO and π-π bonds in UMOFNs of 2D-2D structure also promote charge transfer and enhance the photocatalytic activity.     

Hierarchical TiO2 spheroids decorated g-C3N4 nanocomposite for solar driven hydrogen production and water depollution

A novel hierarchical TiO₂ spheroids embellished with g-C₃N₄ nanosheets has been successfully developed via thermal condensation process for efficient solar-driven hydrogen evolution and water depollution photocatalyst. The photocatalytic behaviour of the as-prepared nanocomposite is experimented in water splitting and organic pollutant degradation under solar light irradiation. The optimal ratio of TiO₂ spheroids with g-C₃N₄ in the nanocomposite was found to be 1:10 and the resulting composite exhibits excellent photocatalytic hydrogen production of about 286 μmol h^?1g^?1, which is a factor of 3.4 and 2.3 times higher than that of pure TiO₂ and g-C₃N₄, respectively. The outstanding photocatalytic performance in this composite could be ascribed as an efficient electron-hole pair's separation and interfacial contact between TiO₂ spheroids with g-C₃N₄ nanosheets in the formed TiO₂/g-C₃N₄ nanocomposite. This work provide new insight for constructing an efficient Z-scheme TiO₂/g-C₃N₄ nanocomposites for solar light photocatlyst towards solar energy conversion, solar fuels and other environmental applications.     

Hybridization of g-C3N4 quantum dots with 1D branched TiO2 fiber for efficient visible light-driven photocatalytic hydrogen generation

The hybrid 1D branched TiO₂ loaded with g-C₃N₄ QDs was successfully fabricated that plays a significant role in photocatalysis. The 1D branched TiO₂ prepared by electrospinning followed by alkali-hydrothermal process, and g-C₃N₄ QDs were grafted over it by a chemical vapor deposition method. The composite display enhancement in photocatalytic hydrogen evolution is about 10.57 mmol. g⁻¹.h⁻¹ in comparison to the g-C₃N₄ sample that only produces 0.32 mmol. g⁻¹.h⁻¹ while the HBTiO₂ sample evolved a negligible amount of hydrogen under visible light. The composite sample shows quantum efficiency for HER at 420 nm light is 18.6% that is much higher than the other two samples. The specific surface area of the composite sample is 92.39 m²g^-1 that is about 13 times more than bulk g-C₃N_4. The bandgap of HBTiO₂/g-C₃N₄ QDs, g-C₃N₄, and HBTiO₂ samples calculated as 2.71 eV, 2.67eV, and 3.24eV, respectively. The TRPL spectra imply that the duration of the lifetime of composite becomes longer which effectually overwhelm the electron-hole recombination. The 1D branched TiO₂ fiber reduces the charge recombination by fast transfer of electron while g-C₃N₄ QDs facilitate the visible light absorption by improving the optical properties. The formation of the type II heterostructure system remarkably promotes the separation and transfer of electron holes and facilitates the photo-reduction reaction.     

A novel nonmetal intercalated high crystalline g-C3N4 photocatalyst for efficiency enhanced H2 evolution

Introducing nickel foam as assistant, a novel nonmetal intercalated high crystalline graphitic carbon nitride (g-C₃N₄) catalyst was successfully fabricated by β-cyclodextrin (β-CD) pretreated melamine with one step thermal polymerization process. The final results show that 0.3-NCCN presented a remarkably visible-light (λ > 400 nm) photocatalytic H₂ evolution reaching 9297  μmol g^?1 h^?1, and the reaction process follows the zero-order kinetic model. The increase in crystallization of 0.3-NCCN implies a higher-ordered arrangement of tris-s-triazine. The nonmetal interlayer formed by oxygen-contained graphitized carbon can extend the π-conjugated system. Both above significantly benefit the rapid migration and separation of charge-carrier. Moreover, the narrowed band gap provides a stronger thermodynamic driving force for improving the photocatalytic water splitting efficiency. Our work paved a new method to construct high performance photocatalyst for water splitting.     

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.     

Exfoliated and plicated g-C3N4 nanosheets for efficient photocatalytic organic degradation and hydrogen evolution

Exfoliated and plicated g-C₃N₄ nanosheets (CNsF) were prepared through a thermal-chemical exfoliation in which the bulk g-C₃N₄ was obtained first under thermal exfoliation, and then was exposed to an acidic etching using hydrofluoric acid under hydrothermal condition. The acidic etching not only exfoliated g-C₃N₄ nanosheets by disrupting weak van der Waals forces between layers, which led to formation of a monolayer or a few layers of g-C₃N₄ nanosheets, but also made disordered defects on its surface and created plicated g-C₃N₄ nanosheets. Under visible-light illumination, the optimized sample (CNsF-6%) showed a hydrogen evolution rate of 54.13 μmol h^?1g^?1 (without co-catalyst) and a specific surface area of 121.4 m² g^?1, which were about 4.7 and 2.5 times, respectively, higher than pristine g-C₃N₄. It also showed remarkably enhanced photocatalytic performance in removing various organic pollutants. This remarkable improvement probably arises from the porous nanosheets and an increased number of active sites resulting from the CNsF, which subsequently enhanced the charge separation efficiency. This work provided an alternative way to obtain highly active g-C₃N₄ photocatalysts.     

Construction of heterostructured g-C3N4@TiATA/Pt composites for efficacious photocatalytic hydrogen evolution

Construction of heterostructured photocatalysts is a feasible method for improving hydrogen production from water splitting because of its good charge transport efficiency. Herein, we coupled the Ti-MOFs (TiATA) with metal-free graphitic carbon nitride (g-C₃N₄) to synthesize composites, g-C₃N₄@TiATA, in which a heterostructure was formed between g-C₃N₄ and TiATA. The establishment of heterojunctions not only broadens the light absorption range of g-C₃N₄@TiATA (490 nm) by contrast with g-C₃N₄ (456 nm), but also greatly accelerates charge migration. Photocatalytic studies present that the construction of heterostructure steering the charges flow from g-C₃N₄ to TiATA and then delivery to the cocatalyst of Pt nanoparticles, exhibiting an impressively photocatalytic hydrogen production rate (265.8 μmol·h⁻¹) in assistance of 300 W Xenon lamp, which is about 3.4 times as much as g-C₃N₄/Pt.     

Fabrication of noble-metal-free Cd0.5Zn0.5S/NiS hybrid photocatalyst for efficient solar hydrogen evolution

Up to now, most of the semiconductor photocatalysts can only achieve their high photocatalytic activity for hydrogen production with the loading of noble metals, such as Pt or Ru, as cocatalysts, which drastically increases the total cost of the designed photocatalyst. Herein, we report the design and fabrication of a highly efficient Cd_0.5Zn_0.5S photocatalyst decorated with nanosized NiS surface heterojunctions. The hydrogen evolution rate over this photocatalyst reached 1.4 mmol/h, with a remarkable quantum yield of 33.9%. This efficiency is even much higher than many noble metal loaded photocatalysts. In this hybrid photocatalyst, the nanosized NiS on the surface can serve as electron trapping sites, by which, photogenerated electrons were extracted from Cd_0.5Zn_0.5S substrate, leading to spatially separated photoreduction and oxidation reactions. More interestingly, it was found that NiS played a similar role as noble metal, providing active sites for proton reduction, and hence efficiently enhancing the overall hydrogen production rate. Our work demonstrates the possibility of substitution of noble metal cocatalyst by a properly engineered surface hetero-junction to achieve efficient and low cost photocatalytic hydrogen production.     

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.     

In- situ solid-phase fabrication of Ag/AgX (X=Cl,Br, I)/g-C3N4 composites for enhanced visible-light hydrogen evolution

In this work, a series of Ag/AgX (X = Cl, Br, I)/g-C₃N₄ (Ag/AgX/CN) composites were successfully fabricated by an in-situ solid phase method. The morphology and structure, photoluminescence and photoelectrochemical properties of composites were investigated in detail. The as-prepared Ag/AgX/CN composites were used as H₂ evolution photocatalysts under visible-light irradiation with a sacrificial agent. The experimental results revealed that Ag/AgI/CN-4 composite possesses highest-H₂ evolution rate (up to 59.22 μmol g⁻¹ h⁻¹) which are approximately 31 times higher than that of pure g-C₃N₄ (1.94 μmol g⁻¹ h⁻¹). In addition, Ag/AgCl/CN-4 and Ag/AgBr/CN-4 composites also present high photocatalytic activities yielding, 26.39 and 18.05 μmol_H2 g⁻¹ h⁻¹_, respectively. The enhanced photocatalytic activities of Ag/AgI/CN-4 composite might be attributed to the synergistic effect between Ag/AgI nanoparticles and g-C₃N₄ and the localized surface plasmon resonance effect of metallic Ag. Moreover, Ag/AgI/CN-4 composite showed excellent recyclability and stability after five cycling photocatalytic tests (about 25 h). Furthermore, the possible photocatalytic mechanism of Ag/AgI/CN composites is proposed.     

Construction of 1D/0D/2D Zn0.5Cd0.5S/PdAg/g-C3N4 ternary heterojunction composites for efficient photocatalytic hydrogen evolution

A high-efficiency and easy-available approach was developed to obtain a ternary heterojunction composites with advanced hydrogen evolution reaction (HER) performance under visible light by water split. PdAg bimetallic nanoparticles make a close contact interface between g-C₃N₄(CN) and Zn_0.5Cd_0.5S(ZCS). Under visible light irradiation, CN and ZCS are both excited to generate electron-hole pairs, PdAg bimetallic nanoparticles act as a bridge between CN and ZCS. Not only can the photogenerated electrons from CN be captured, but they can also be quickly transferred to the surface of ZCS and participate in the photocatalytic reaction to release H₂, and the recombination of charge carriers between the contact interface of ZCS and CN can be significantly inhibited. In addition, the thin CN layer reduces the photocorrosion of the ZCS and enhances the specific surface area of the composite material. After testing, the composite material with 30 wt% ZCS and 4 wt% PdAg demonstrates hydrogen evolution performance, up to 6250.7 μmol g^?1h^?1, which is 753 times the hydrogen evolution rate of single-component CN and 12.6 times of ZCS/CN. Compared with single-component and two-component photocatalysts, the ternary ZCS/PdAg/CN photocatalyst achieves significantly enhanced photocatalytic activity.     

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.     

g-C3N4/B doped g-C3N4 quantum dots heterojunction photocatalysts for hydrogen evolution under visible light

Developing high activity and eco-friendly photocatalysts for water splitting is still a challenge in solar energy conversion. In this paper, B doped g-C₃N₄ quantum dots (BCNQDs) were prepared via a facile molten salt method using melamine and boron oxide as precursors. By introducing BCNQDs onto the surface of g-C₃N₄, g-C₃N₄/BCNQDs heterojunction was constructed via hydrothermal treatment. The resulting g-C₃N₄/BCNQDs heterojunction exhibits enhanced hydrogen evolution performance for water splitting under visible light irradiation. The mechanism underlying the improved photocatalytic activity was explored and discussed based on the formation of heterojunction between g-C₃N₄ and BCNQDs with well-matched band structure.     

New insights in the performance and reuse of rGO/TiO2 composites for the photocatalytic hydrogen production

The viability of the photocatalytic hydrogen production is closely related to the performance and long term stability of the photocatalyst. In this work rGO/TiO₂ composites have been synthetized with graphene oxide (GO) ratios from 1% to 10% and experimentally assessed towards hydrogen generation from methanol solutions. The performance of the composite with 2% of rGO (2 GT) has been compared to bare TiO₂ working with 20% volume methanol solution. The hydrogen production initial rate showed similar values with both photocatalysts decreasing after about 24 h. Further analysis of the photocatalytic process at longer times showed the negative influence of hydrogen accumulation in the reaction system. Thus, an experimental procedure with argon purge was developed and the behavior of TiO₂ and 2 GT photocatalysts was compared. It is concluded that TiO₂ keeps its activity after 8 operation cycles while 2 GT performance reduces progressively. This can be attributed to the further reduction of GO and the increase of defects in its structure.     

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.