MoO3/g-C3N4 Z-scheme (S-scheme) system derived from MoS2/melamine dual precursors for enhanced photocatalytic H2 evolution driven by visible light

A dual-precursors co-pyrolysis strategy was adopted to prepare photocatalysts of MoO₃/g-C₃N₄ composite from MoS₂/melamine through a facile one-pot method. By following this strategy, MoS₂ was in-situ transformed to MoO₃ accompanying with the pyrolysis and polymerization of melamine to g-C₃N₄ nanosheets. A set of samples about MoO₃/g-C₃N₄ composite containing different MoO₃ contents were prepared and the visible-light driven photocatalytic performance of samples were investigated. The photocatalytic activity was notably enhanced and the highest evolution rate for H₂ reached 13.9 times and 2.5 times that of the pristine g-C₃N₄ and the MoO₃/g-C₃N₄ derived from MoO₃/melamine, respectively. On the one hand, layered MoS₂ used as the precursor of MoO₃ contributed to the intimate contact with g-C₃N₄ nanosheets and the high dispersity of derived MoO₃, and on the other hand, the morphology and electronic structure of g-C₃N₄ were actually changed by the strong interaction between the MoO₃ and g-C₃N₄, thus inducing an efficient Z-scheme (S-scheme) system in the composite. The above-mentioned co-operation effect in the composite resulted in the significant improvement on the activity of photocatalytic H₂ evolution. This work presented a valuable reference, by taking MoO₃/g-C₃N₄ as an example, for the construction of composite-photocatalyst system in visible-light-driven H₂ evolution.     

Nitrogen vacancy enhanced photocatalytic selective oxidation of benzyl alcohol in g-C3N4

Efficient photocatalytic selective conversion of organic under mild condition remains challengeable for green chemistry. In this work, we determined that nitrogen vacancies (NVs) in graphitic carbon nitride (g-C₃N₄) could enhance photocatalytic selective oxidation of benzyl alcohol to benzaldehyde. The as-prepared N-deficient g-C₃N₄ samples (CN_x) exhibited superior conversion efficiency and excellent product selectivity than pristine g-C₃N₄ upon visible light irradiation. By combining experimental characterizations and density functional theory calculations, the NVs in g-C₃N₄ could significantly boost carrier separation and act as specific sites to enhance the adsorption and activation of reactants. Thus, our work could provide insight into the pathways of reaction between defects and reactants, and point out a feasible strategy for green fine chemical production.     

1T-phase MoS2/holey ultrathin g-C3N4 nanosheets based 2D/2D heterostructure for enhanced photocatalytic hydrogen production

Although graphitic carbon nitride (g-C₃N₄) is widely used for photocatalytic hydrogen production, its practical application is restricted by the high recombination rate of photoinduced electron-hole pairs and limited active sites. In this work, holey ultrathin g-C₃N₄ nanosheets (HCN NSs) with rich active sites are prepared, followed by the growth of 1T-MoS₂ NSs on their surfaces to construct 2D/2D 1T-MoS₂/HCN heterostructure. Due to the high surface area and abundant hydrogen active sites of the hybrid, large and intimate 2D nanointerface between MoS₂ and HCN, hydrogen ion adsorption and charge separation/transport ability are greatly enhanced. As a result, 1T-MoS₂/HCN-4 with the optimal 1T-MoS₂ content of 8 wt% displays the highest H₂ production rate of 2724.2 μmol^?1 h^?1 g^?1 under simulated solar light illumination with apparent quantum efficiency of 8.1% (λ = 370 nm). Moreover, the 1T-MoS₂/HCN-4 hybrid manifests improved stability after a long-time test. This study opens the door to design highly-efficient g-C₃N₄ based 2D/2D heterostructures for photocatalytic H₂ production.     

Composite of g-C3N4 and poly(3-hexylthiophene) prepared by polymerizing thiophene-3-acetic acid treated g-C3N4 and 3-hexylthiophene for enhanced photocatalytic hydrogen production

Composite of g-C₃N₄ and poly(3-hexylthiophene) (P3HT) with enhanced photocatalytic H₂ production activity was prepared by polymerizing 3-hexylthiophene and g-C₃N₄, which was treated with thiophene-3-acetic acid (T3A). The morphology, chemical structure, and light absorption properties of samples were characterized by SEM, TEM, BET, XRD, FT-IR, XPS, UV–visible diffuse reflectance spectra (UV–vis). The migration and separation efficiency of charge carriers were characterized by photoluminescence (PL) emission spectra, Time resolved photoluminescence spectra, transient photocurrent responses, and electrochemical impedance spectroscopy (EIS). The photocatalytic activity of the catalysts were tested as the H₂ evolution rate from water under visible light irradiation in the presence of triethanolamine as sacrifice agent. The results indicated that g-C₃N₄-P3HT composite shows significant enhanced migration and separation efficiency of charge carriers, and photocatalytic H₂ production activity from water. The intrinsic nature causing the significance enhanced photocatalytic performance was discussed. Our findings here may provide a new strategy to design composite photocatalyst with high photocatalytic activity.     

Electron-donating tris(p-fluorophenyl)phosphine-modified g-C3N4 for photocatalytic hydrogen evolution and p-chlorophenol degradation

Incorporating aromatics into g-C₃N₄ is an effective strategy to extend electron delocalization. A novel intramolecular donor-acceptor conjugated g-C₃N₄ was synthesized via thermal copolymerization of urea and tris(p-fluorophenyl)phosphine (TPP). FTIR and XPS spectra showed that the incorporation of TPP did not destroy the framework of g-C₃N₄. DFT calculation displayed that the HOMO of TPP-modified g-C₃N₄ (TPP-CN) came mainly from p_z orbital of phosphorus. The change of the electronic property led to a narrowed bandgap, extended delocalization of π-electrons through benzene rings, and accelerated migration of photoexcited electrons via intramolecular charge transfer. The optimal Pt-loaded TPP-CN showed the highest rate of H₂ generation of 12.45 mmol h^?1 g^?1, 5 times of that of pure g-C₃N₄, and the apparent quantum efficiency of 24.9% at 420 nm. The degradation of p-chlorophenol over the optimal TPP-CN was 4 times of that of pure g-C₃N₄. The mechanism of photocatalytic p-chlorophenol degradation was proposed based on mass spectrometry analysis.     

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.     

Highly efficient visible-light-assisted photocatalytic hydrogen generation from water splitting catalyzed by Zn0.5Cd0.5S/Ni2P heterostructures

Development of heterostructured photocatalysts which can facilitate spatial separation of photo-generated charge carriers is crucial for achieving improved photocatalytic H₂ production. Consequently, herein, we report the synthesis of Zn_0.5Cd_0.5S/Ni₂P heterojunction photocatalysts with varying amount of Ni₂P, 0.5 (S1), 1 (S2), 3 (S3), 5 (S4) and 10wt% (S5) for the efficient visible-light-assisted H₂ generation by water splitting. The heterostructures were characterized thoroughly by PXRD, FE-SEM, EDS, HR-TEM and XPS studies. FE-SEM and HR-TEM analyses of the samples unveiled the presence of Zn_0.5Cd_0.5S microspheres composed of smaller nanocrystals with the surface of the microspheres covered with Ni₂P nanosheets and the intimate contact between the Zn_0.5Cd_0.5S and the Ni₂P. Further, visible-light-assisted photocatalytic investigation of the samples showed excellent water splitting activity of the heterostructure, Zn_0.5Cd_0.5S/1wt%Ni₂P (S2) with very high H₂ generation rate of 21.19 mmol h^?1g^?1 and the AQY of 21.16% at 450 nm with turnover number (TON) and turnover frequency (TOF) of 251,516 and 62,879 h^?1 respectively. Interestingly, H₂ generation activity of S2 was found to be about four times higher than that of pure Zn_0.5Cd_0.5S (5.0 mmol h^?1g^?1) and about 240 times higher than that of CdS/1wt%Ni₂P. The enhanced H₂ generation activity of S2 has been attributed to efficient spatial separation of photogenerated charge carriers and the presence of highly reactive Ni₂P sites on the surface of Zn_0.5Cd_0.5S microspheres. A possible mechanism for the enhanced photocatalytic H₂ generation activity of Zn_0.5Cd_0.5S/1wt%Ni₂P (S2) has been proposed and is further supported by photoluminescence and photocurrent measurements. Furthermore, the catalyst, S2 can be recycled for several cycles without significant loss of catalytic activity and photostability. Remarkably, the H₂ generation activity of S2 was found to be even higher than the reported examples of Zn_xCd_1-xS doped with noble metal cocatalysts. Hence, the present study highlights the importance of Zn_0.5Cd_0.5S/Ni₂P heterostructures based on non-noble metal co-catalyst for efficient visible-light-driven H₂ production from water splitting.     

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.     

Carbon intercalated MoS2 cocatalyst on g-C3N4 photo-absorber for enhanced photocatalytic H2 evolution under the simulated solar light

Despite MoS₂ being a promising non-precious-metal cocatalyst, poor electronic conductivity and low activity for hydrogen evolution caused by serious agglomeration have been identified as critical roadblocks to further developing MoS₂ cocatalyst for photocatalytic water splitting using solar energy. In this work, the density functional theory calculations reveal that carbon intercalated MoS₂ (C-MoS₂) has excellent electronic transport properties and could effectively improve catalytic activity. The experiment results show that the prepared tremella-like C-MoS₂ nanoparticles have large interlayer spacing along the c-axis direction and high dispersion because of intercalation of the carbon between adjacent MoS₂ layers. Furthermore, the heterostructure photocatalyst of C-MoS₂@g-C₃N₄ formed by loading the cocatalyst of C-MoS₂ onto g-C₃N₄ nanosheets exhibits the H₂ evolution rate of 157.14 μmolg⁻¹h⁻¹ when containing 5 wt% C-MoS₂. The high photocatalytic H₂ production activity of the 5 wt% C-MoS₂@g-C₃N₄ can be attributed to the intercalated conductive carbon layers in MoS₂, which leads to efficient charge separation and transfer as well as increased activities of the edge S atoms for H₂ evolution. We believe that the C-MoS₂ will offer great potential as a photocatalytic H₂ evolution reaction cocatalyst with high efficiency and low cost.     

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.     

AQ-coupled few-layered g-C3N4 nanoplates obtained by one-step mechanochemical treatment for efficient visible-light photocatalytic H2O2 production

Solar-driven photocatalytic H₂O₂ production is a sustainable and clean technique with respect to the traditional route. Here, the efficient H₂O₂ generation was accomplished by π?π coupling of AQ onto the few-layered graphitic carbon nitride (g-C₃N₄) nanoplates through one-step mechanochemical treatment. A H₂O₂ generation rate of 231 μM h^?1 was obtained using AQ-coupled g-C₃N₄ nanoplates under visible light illumination, exceeding that of the g-C₃N₄ nanoplates and bulk g-C₃N₄ by 7-time and 14-time, respectively. Experimental results showed that the high oxygen reduction efficiency could be ascribed to the enhanced surface area, more exposed active sites and the distinct AQ roles of the electrons storage and restraining the charge recombination. This work inspired future work in synthesizing H₂O₂ through a sustainable and green route.     

Novel ReS2/g-C3N4 heterojunction photocatalyst formed by electrostatic self-assembly with increased H2 production

Binary heterostructures (named as CN@Re) composed of ReS₂ nanospheres and g-C₃N₄ nanosheets are constructed by electrostatic self-assembly method. The ReS₂ nanospheres were prepared by hydrothermal method and the g-C₃N₄ nanosheets were treated with surface charge modification. Hydrogen production efficiency of modified CN and CN@Re nanostructures was evaluated in a simulated solar environment. To our surprise, CN5@Re5% exhibits the highest H₂ production up to 1823 μmol g^?1h^?1 of CN5@Rey, which is 3.2 times as high as CN. The improvement of the photocatalytic hydrogen production efficiency of modified CN is attributed to its interaction with the hole sacrificing agent lactic acid, while the improvement of the photocatalytic activity of CN@Re nanostructure is attributed to the efficient electron transfer efficiency between CN and ReS₂ and the enhanced light absorption capacity brought by ReS₂. In addition, the photocatalytic stability of CN5@Re5% has been studied, which can maintain a stable rate of hydrogen production over four cycles. The apparent quantum efficiency is as high as 4.10% at 365 nm and 2.82% at 420 nm.     

Phosphorus and sulphur co-doping of g-C3N4 nanotubes with tunable architectures for superior photocatalytic H2 evolution

Non-metal doping not only optimizes the energy band structure of g-C₃N₄ to improve the absorption of visible light, but also exacerbates the distortion of lowest and highest unoccupied molecular orbital plane, causing polarization, thereby improving photocatalytic activity. For the first time, S and P are co-introduced into g-C₃N₄ network to enhance photocatalytic performance and create various tubular morphologies. The ratio of S to P is crucial to control the tubular morphology and property. In the photocatalytic process, the separation of electrons and holes causes by the polarization of the S and P elements and the synergy of the tubular morphology results in new migration paths for photogenerated electrons and holes. Using optimized preparation conditions, g-C₃N₄ tubes co-doped with S and P (CNSP) reveal very high H₂ generation efficiency (163.27 μmol/h), which is two orders of magnitude higher compared to that of pure g-C₃N₄ and apparent quantum yield is 18.93% at 420 nm. Fast degradation of Rhodamine B by using CNSP occurs within 5 min under visible light irradiation. Because of the reproducible process, the synthetic strategy provides a novel method for controlling the morphology of g-C₃N₄-based materials with super activity.     

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.     

Hydrogen generation by photocatalytic reforming of glucose with heterostructured CdS/MoS2 composites under visible light irradiation

A binary heterostructured CdS/MoS₂ flowerlike composite photocatalysts was synthesized via a simple one-pot hydrothermal method. This photocatalyst demonstrated higher photocatalytic hydrogen production activity than pure MoS₂. The heterojunction formed between MoS₂ and CdS seems to promote interfacial charge transfer (IFCT), suppress the recombination of photogenerated electron–hole pairs, and enhance the hydrogen generation. Based on the good match between the conduction band (CB) edge of CdS and that of MoS₂, electrons in the CB of CdS can be transferred to MoS₂ easily through the heterojunction between them, which prevents the accumulation of electrons in the CB of CdS, inhibiting photocorrosion itself and greatly enhancing stability of catalyst. Hydrogen evolution reaction (HER) using Na₂S/Na₂SO₃ or glucose as sacrificial agents in aqueous solution was investigated. The ratio between CdS and MoS₂ plays an important role in the photocatalytic hydrogen generation. When the ratio between CdS and MoS₂ reaches 40 wt%, the photocatalyst showed a superior H₂ evolution rate of 55.0 mmol g⁻¹ h⁻¹ with glucose as sacrificial agent under visible light, which is 1.2 times higher than using Na₂S/Na₂SO₃ as sacrificial agent. Our experimental results demonstrate that MoS₂-based binary heterostructured composites are promising for photocorrosion inhibition and highly efficient H₂ generation.     

CoCO3 hierarchical structure embedded on g-C3N4 nanosheets to assemble 3D/2D Z-scheme heterojunction towards efficiently and stably photocatalytic hydrogen production

Structure and interface control of heterojunction is usually a challenging issue to improve the photocatalytic performance. Herein, a new 3D/2D CoCO₃/g-C₃N₄ heterojunction is assembled by embedding hexahedral CoCO₃ on g-C₃N₄ nanosheets. The unique step-like hierarchical structure of CoCO₃, the formed built-in electric field and Z-scheme charge transfer behavior at the interface jointly drive the high-efficient separation of photogenerated carriers to boost the photocatalytic H₂ production. It achieves the optimal H₂ production rate that is almost 2.6 times than g-C₃N₄, apparent quantum efficiency (AQE) of 10.1% at 400 nm and continuous running of 60 h over the 3D/2D CoCO₃/g-C₃N₄ heterojunction. This work endows a fresh structural control strategy for the fabrication of 3D/2D Z-scheme heterojunction to improve the photocatalytic H₂ production performance.     

Fabrication of 0D/1D/2D ZnS–CuS nanodots/GNRs/g-C3N4 heterojunction photocatalyst for efficient photocatalytic overall water splitting

Photocatalytic water splitting has become a significant challenge in modern chemistry. In this process, the rate-determining step is the hydrogen evolution reaction (HER). In the present work, a surface modification approach for graphitic carbon nitride (g-C₃N₄) was applied to improve its photocatalytic HER. 0D ZnS–CuS nanodots were synthesized with the hydrothermal method as a co-catalyst to enhance the capability and stability of water splitting in the presence of visible light irradiation. Also, graphene nanoribbons were synthesized from CNTs unzipping to reduce the energy barrier of HER, improve the HE kinetic, and enhance the catalytic performance of the g-C₃N₄. By using ZnS–CuS/GNRs₍₂₎/g-C₃N₄ photocatalyst, a low onset potential of 130 mV, slight Tafel slope of 41 mV dec^?1, as well as excellent stability of 2000 s was obtained in acidic media. This efficient performance is attributed to the increased visible light absorption level in the proposed photocatalyst and the high stability in electron-hole pairs.     

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.     

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.     

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.