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.     

In situ fabrication of CDs/g-C3N4 hybrids with enhanced interface connection via calcination of the precursors for photocatalytic H2 evolution

Novel carbon dots (CDs)/graphitic carbon nitride (g-C₃N₄) hybrids were fabricated via an in situ thermal polymerization of the precursors, urea and glucose. This heterojunction catalyst exhibited enhanced photocatalytic H₂ evolution activity under visible-light (λ > 420). A sample of CDs/g-C₃N₄ hybrids, CN/G0.5, which was prepared from 0.5 mg of glucose in 6.0 g of urea (8.3 × 10^?3 wt% glucose), exhibited the best photocatalytic performance for hydrogen production from water under visible light irradiation, which is about 4.55 times of that of the bulk g-C₃N₄ (BCN). The improvement of photocatalytic activity was mainly attributed to the construction of built-in electric field at the interface of CDs and g-C₃N₄, which could improve the separation of photogenerated electron-hole pair. Moreover, the tight connection of CDs with g-C₃N₄ would serve as a well electron transport channel, which could promote the photocatalytic H₂ evolution ability.     

Crystallinity and phase controlling of g-C3N4 /CdS hetrostructures towards high efficient photocatalytic H2 generation

The heterostructures of graphitic carbon nitride (g-C₃N₄) and CdS were synthesized by controlling the crystalline degree of g-C₃N₄ and the phase composition of CdS. A thermal polycondensation process of N precursors was adjusted to get amorphous and crystalline g-C₃N₄. A multistep adsorption method was used to deposit CdS nanoparticles on g-C₃N₄. An annealed process was used to adjust the phase composition of CdS from cubic to hexagonal. The morphology of CdS was changed to rod. Amorphous g-C₃N₄/CdS heterostructures revealed enhanced photocatalytic activity because the amorphous g-C₃N₄ has a lower crystallinity, it is easier to form a heterojunction with the CdS. Further, an annealing process resulted in the phase transfer and morphology change of CdS. The high stability and rod morphology of CdS make the heterostructures with high H₂ generation rate to 5440 μmol h-1 g-1 which is ~5 times high compared with g-C₃N₄.     

Greatly enhanced photocatalytic hydrogen production on CdS/(Pt/g-C3N4) via dual functions of Pt on semiconductors interface

CdS and g-C₃N₄ are famous semiconductors in photocatalytic hydrogen evolution, however, their low efficiencies limit their further application. Here, a highly efficient ternary catalyst CdS/(Pt/g-C₃N₄) was reported and its photocatalytic hydrogen production activity reached up to 1465.9 μmol/h/g, which is 5.3 times of Pt/CdS and 4.0 times of Pt/g-C₃N₄, respectively. TEM and HRTEM images demonstrate the Pt nanoparticles exists on the interface of between CdS and g-C₃N₄ acting as a cocatalyst for hydrogen evolution. SPV spectra and electrochemical tests demonstrate that Pt as bridge between CdS and g-C₃N₄ also accelerates the electrons transforming which benefits for the inhibition of the recombination of photoexcited electrons and holes. This study demonstrated the dual roles of interface Pt and provides a new method to design a highly efficient photocatalyst.     

Construction of ternary hybrid layered reduced graphene oxide supported g-C3N4-TiO2 nanocomposite and its photocatalytic hydrogen production activity

Reduced graphene oxide (rGO) supported g-C₃N₄-TiO₂ ternary hybrid layered photocatalyst was prepared via ultrasound assisted simple wet impregnation method with different mass ratios of g-C₃N₄ to TiO₂. The synthesized composite was investigated by various characterization techniques, such as XRD, FTIR, Raman Spectra, FE-SEM, HR-TEM, UV vis DRS Spectra, XPS Spectra and PL Spectra. The optical band gap of g-C₃N₄-TiO₂/rGO nanocomposite was found to be red shifted to 2.56 eV from 2.70 eV for bare g-C₃N₄. It was found that g-C₃N₄ and TiO₂ in a mass ratio of 70:30 in the g-C₃N₄-TiO₂/rGO nanocomposite, exhibits the highest hydrogen production activity of 23,143 μmol g^?1h^?1 through photocatalytic water splitting. The observed hydrogen production rate from glycerol-water mixture using g-C₃N₄-TiO₂/rGO was found to be 78 and 2.5 times higher than g-C₃N₄ (296 μmol g^?1 h^?1) and TiO₂ (11,954 μmol g^?1 h^?1), respectively. A direct contact between TiO₂ and rGO in the g-C₃N₄-TiO₂/rGO nanocomposite produces an additional 10,500 μmol g^?1h^?1 of hydrogen in 4 h of photocatalytic reaction than the direct contact between g-C₃N₄ and rGO. The enhanced photocatalytic hydrogen production activity of the resultant nanocomposite can be ascribed to the increased visible light absorption and an effective separation of photogenerated electron-hole pairs at the interface of g-C₃N₄-TiO₂/rGO nanocomposite. The effective separation and transportation of photogenerated charge carriers in the presence of rGO sheet was further confirmed by a significant quenching of photoluminescence intensity of the g-C₃N₄-TiO₂/rGO nanocomposite. The photocatalytic hydrogen production rate reported in this work is significantly higher than the previously reported work on g-C₃N₄ and TiO₂ based photocatalysts.     

Bimetallic PtNi/g-C3N4 nanotubes with enhanced photocatalytic activity for H2 evolution under visible light irradiation

Bimetallic PtNi-decorated graphitic carbon nitride (g-C₃N₄) nanotubes were prepared through calcining the mixture of urea and thiourea in the presence of Pluronic F127, followed by deposition of bimetallic PtNi nanoparticles (NPs) via chemical reduction. It is found that the photocatalytic activity of PtNi/g-C₃N₄ nanotubes is strongly dependent on the molar ratio of Pt/Ni and the highest activity is observed for Pt₁Ni₁/g-C₃N₄. Under visible light (λ > 420 nm) irradiation, the H₂ generation rate over Pt₁Ni₁/g-C₃N₄ nanotubes is 104.7 μmol h^?1 from a triethanolamine (10 vol%) aqueous solution, which is higher than that of Pt/g-C₃N₄ nanotubes (98.6 μmol h^?1) and is about 47.6 times higher than that of pure g-C₃N₄ nanotubes. The cyclic photocatalytic reaction indicates that our Pt₁Ni₁/g-C₃N₄ nanotubes function as a stable photocatalyst for visible light-driven H₂ production. The effect of bimetallic PtNi NPs in the transfer and separation of photogenerated charge carriers occurring in the excited g-C₃N₄ nanotubes was investigated by performing photo-electrochemical and photoluminescence measurements. Our results reveal that bimetallic PtNi could replace Pt as a promising cocatalyst for photocatalytic H₂ evolution with better performance and lower cost.     

ZnCr LDH nanosheets modified graphitic carbon nitride for enhanced photocatalytic hydrogen production

ZnCr layered double hydroxides (ZnCr LDH) nanosheets modified graphitic carbon nitride (g-C₃N₄) nanohybrids were fabricated via a self-assembly procedure through electrostatic interaction between these two components. Such 2D-2D inorganic-organic hybrid material was employed for photocatalytic hydrogen production under visible light for the first time. The physical and photophysical properties of the hybrid nanocomposites were investigated to reveal the effect of ZnCr LDH nanosheets on the photocatalytic activities of g-C₃N₄. It was found that 1 wt% ZnCr LDH nanosheets modified g-C₃N₄ was optimal for the formation of intimate interfacial contact. The visible light photocatalytic H₂ production activity over g-C₃N₄ was enhanced about 2.8 times after ZnCr LDH nanosheets modification. The significant enhancement in photocatalytic performance for ZnCr LDH/g-C₃N₄ heterojunction should be attributed to the promoted charge transfer and separation efficiency, resulting from the intimate interfacial contact and Type II band alignment between ZnCr LDH and g-C₃N₄.     

In-situ fabrication of CuS/g-C3N4 nanocomposites with enhanced photocatalytic H2-production activity via photoinduced interfacial charge transfer

In this work, novel CuS/g-C₃N₄ composite photocatalysts were successfully prepared via a simple in-situ growth method. CuS nanoparticles, with an average diameter of ca.10 nm, were well dispersed on the surface of g-C₃N₄, revealing that g-C₃N₄ nanosheets were promising support for in-situ growth of nanosize materials. The CuS/g-C₃N₄ composites exhibited highly enhanced visible light photocatalytic H₂ evolution from water-splitting compared to pure g-C₃N₄. The optimum photocatalytic activity of 2 wt% CuS/g-C₃N₄ composite photocatalytic H₂ evolution was about 13.76 times higher than pure g-C₃N₄. The enhanced photocatalytic activity is attributed to the interfacial charge transfer (IFCT). In this system, electrons in the valence band (VB) of g-C₃N₄ can transfer directly to CuS clusters, causing the reduction of partial CuS to Cu₂S, which can act as an electron sink and co-catalyst to promote the separation and transfer of photo-generated electrons. The accumulated photoinduced electrons in CuS/Cu₂S clusters could effectively reduce H⁺ to produce H₂. This work provides a possibility for constructing low-cost CuS as a substitute for noble metals in the photocatalytic production of H₂ via a facile method based on g-C₃N₄.     

In-situ growth of CdS quantum dots on g-C3N4 nanosheets for highly efficient photocatalytic hydrogen generation under visible light irradiation

Well dispersed CdS quantum dots were successfully grown in-situ on g-C₃N₄ nanosheets through a solvothermal method involving dimethyl sulfoxide. The resultant CdS–C₃N₄ nanocomposites exhibit remarkably higher efficiency for photocatalytic hydrogen evolution under visible light irradiation as compared to pure g-C₃N₄. The optimal composite with 12 wt% CdS showed a hydrogen evolution rate of 4.494 mmol h⁻¹ g⁻¹, which is more than 115 times higher than that of pure g-C₃N₄. The enhanced photocatalytic activity induced by the in-situ grown CdS quantum dots is attributed to the interfacial transfer of photogenerated electrons and holes between g-C₃N₄ and CdS, which leads to effective charge separation on both parts.     

Rapid and energy-efficient preparation of boron doped g-C3N4 with excellent performance in photocatalytic H2-evolution

Microwaving heating method, which shows significantly higher efficiency in synthesis time and energy utilization than conventional heating method, was firstly applied to synthesize boron doped g-C₃N₄ (B-g-C₃N₄). Results demonstrated that B-g-C₃N₄ could be prepared in 35 min via the method. XRD and XPS analyses verified the successfully doping of B element into the structure of g-C₃N₄. The doped B improved the photoabsorption performance of g-C₃N₄. Meanwhile, it greatly hindered the annihilation of charge carriers, prolonging the lifetime of photogenerated electrons, and thereby resulting in the excellent photocatalytic activity. The optimal B-g-C₃N₄ exhibited a H₂ production rate of 1439 and 400 μmol g^?1 h^?1 under simulated sunlight and visible light irradiation, respectively. This values are about 2.4 times higher than that of pure g-C₃N₄. This study offered an energy-efficient and rapid way for the synthesis of external atom doped g-C₃N₄ with high catalytic efficiency.     

Facile strategy to fabricate Ni2P/g-C3N4 heterojunction with excellent photocatalytic hydrogen evolution activity

Transition metal phosphides are considered as the most prospective replacements for noble metal cocatalysts used for H₂ evolution during photocatalytic water splitting. In this work, Ni₂P/g-C₃N₄ composite photocatalyst was synthesized using a simple in-situ hydrothermal method by one step. Benefiting from the excellent light trapping, efficient transfer of charge carriers and strong stability of Ni₂P nanoparticles, as well as the stable interface contact between Ni₂P and g-C₃N₄, the Ni₂P/g-C₃N₄ exhibit greatly enhanced H₂ evolution performance during photocatalytic water splitting. The optimized H₂ evolution rate can reach 3344 μmol h^?1 g^?1 over 17.5 wt% Ni₂P/g-C₃N₄, which is 68.2 times greater than that of pure g-C₃N₄ and even much greater than that of 15 wt% Pt/g-C₃N₄. The apparent quantum efficiency (QE) is about 9.1% under 420 nm monochromatic. The enhancement mechanism was demonstrated in detail by transient photocurrent responses, photoluminescence spectra and electrochemical impedance spectroscopy. This work develops a facile strategy to fabricate transition metal phosphide/semiconductor heterojunction systems with potential application for photocatalytic H₂ evolution.     

Meso-g-C3N4/g-C3N4 nanosheets laminated homojunctions as efficient visible-light-driven photocatalysts

Mesoporous g-C₃N₄/g-C₃N₄ (Meso-g-C₃N₄/g-C₃N₄) nanosheets laminated homojunctions have been fabricated via template-calcination strategy using melamine and amino cyanamide as co-precursors. The prepared Meso-g-C₃N₄/g-C₃N₄ nanosheets laminated homojunctions possess relative high surface area of 34 m² g^?1, large pore size of 15.0 nm and narrow band gap of 2.75 eV. The visible-light-driven photocatalytic reaction rate constant of methyl orange and hydrogen production rate (～115.6 μmol h^?1 g^?1) for Meso-g-C₃N₄/g-C₃N₄ nanosheets laminated homojunctions is about 12.5 and 6.5 times higher than that of the pristine g-C₃N₄, respectively. This may be attributed to the synergetic effect of the close-contact laminated structure contributing to the separation of photogenerated charge carriers and mesoporous structure facilitating the diffusion of reactants and products, and offering more surface active sites. This novel laminated homojunction may open up a new avenue for designing other high-efficient photocatalysts.     

Improved photocatalytic H2 production assisted by aqueous glucose biomass by oxidized g-C3N4

Chemically modified g-C₃N₄ for the photocatalytic H₂ evolution from water was explored. Bulk g-C₃N₄ was treated in hot HNO₃ aqueous solution to obtain the oxidized material (o-g-C₃N₄), tested in water containing glucose as model water-soluble sacrificial biomass, using Pt as co-catalyst, under simulated solar light. The behaviour of o-g-C₃N₄ was studied in relation with catalyst amount, Pt loading, glucose concentration. Results showed that H₂ production is favoured by increasing glucose concentration up to 0.1 M and Pt loading up to 3 wt%, and it resulted strongly enhanced using small amount of o-g-C₃N₄ (0.25 g L^?1). o-g-C₃N₄ possesses superior photocatalytic activity (～26-fold higher) compared to pristine g-C₃N₄, with H₂ evolution further improved by ultrasound-assisted exfoliation and evolution rates up to ca. 1370 μmol h^?1 per gram of catalyst, with excellent reproducibility (RSD < 6%, n = 3). Significant production was observed also in river water and seawater, with results far better (up to ca. 2500 μmol g^?1 h^?1) compared to commercial AEROXIDE^® P25 TiO₂ under natural solar light.     

In-situ growth of mesoporous Nb2O5 microspheres on g-C3N4 nanosheets for enhanced photocatalytic H2 evolution under visible light irradiation

Large-surface-area mesoporous Nb₂O₅ microspheres were successfully grown in-situ on the surface of g-C₃N₄ nanosheets via a facile solvothermal process with the aid of Pluronic P123 as a structure-directing agent. The resultant g-C₃N₄/Nb₂O₅ nanocomposites exhibited enhanced photocatalytic activity for H₂ evolution from water splitting under visible light irradiation as compared to pure g-C₃N₄. The optimal composite with 38.1 wt% Nb₂O₅ showed a hydrogen evolution rate of 1710.04 μmol h^?1 g^?1, which is 4.7 times higher than that of pure g-C₃N₄. The enhanced photocatalytic activity could be attributed to the sufficient contact interface in the heterostructure and large specific surface area, which leads to effective charge separation between g-C₃N₄ and Nb₂O₅.     

Surface defect and rational design of TiO2−x nanobelts/ g-C3N4 nanosheets/ CdS quantum dots hierarchical structure for enhanced visible-light-driven photocatalysis

TiO_2-x/g-C₃N₄/CdS ternary heterojunctions are fabricated through thermal polymerization-chemical bath deposition combined with in-situ solid-state chemical reduction approach. The prepared materials are characterized by X-ray diffraction, Fourier transform infrared spectra, scanning electron microscopy, transmission electron microscopy, nitrogen adsorption-desorption, and X-ray photoelectron spectroscopy. The results show that the ternary heterojunctions are formed successfully and CdS quantum dots (QDs) and TiO₂ are anchored on surface of g-C₃N₄ nanosheets simultaneously. The visible-light-driven photocatalytic degradation ratio of Bisphenol A and hydrogen production rate are up to 95% and ∼254.8 μmol h⁻¹, respectively, which are several times higher than that of pristine TiO₂. The excellent visible-light-driven photocatalytic activity can be ascribed to the synergistic effect of TiO_2−x, g-C₃N₄ and CdS QDs which extend the photoresponse to visible light region and favor the spatial separation of photogenerated charge carriers.     

Facile synthesis of AuPd/g-C3N4 nanocomposite: An effective strategy to enhance photocatalytic hydrogen evolution activity

AuPd bimetallic nanoparticle (NP) modified ultra-thin graphitic carbon nitride nanosheet photocatalysts were synthesized via photochemical deposition-precipitation followed by hydrogen reduction. The crystal structure, chemical properties, and charge carrier behavior of these photocatalysts were characterized by X-ray diffraction (XRD), surface photovoltage spectroscopy (SPS), transient photovoltage spectroscopy (TPV), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and UV-Vis diffuse-reflectance spectroscopy (DRS). Photocatalytic H₂ evolution experiments indicate that the hydrogen treated AuPd nanoparticles can effectively promote the separation efficiency of electron-hole pairs photo-excited in the g-C₃N₄ photocatalyst, which consequently promotes photocatalytic H₂ evolution. The 1.0 wt% AuPd/g-C₃N₄ (H₂) composite photocatalyst showed the best performance with a corresponding photocatalytic H₂ evolution rate of 107 μmol h^?1. The photocatalyst can maintain most of its photocatalytic activity after four photocatalytic experiment cycles. These results demonstrate that the synergistic effect of light reduction and hydrogen reduction of AuPd and g-C₃N₄ help to greatly improve the photocatalytic activity of the composite photocatalyst.     

A latest overview on photocatalytic application of g-C3N4 based nanostructured materials for hydrogen production

Photocatalytic H₂ production is a hopeful technology to solving the environment problems and global energy. Consequently, it is essential to develop high efficient, nonprecious and stable photocatalysts. Graphitic carbon nitride (g-C₃N₄) was fascinated much concentration owed to this metal free n-type semiconductor possesses appropriate bandgap, unique two dimensional (2D) layered structures, low toxicity, high thermal and chemical stability, lowcost, facile preparation and visible light response. Moreover, the g-C₃N₄ composites are having huge promise on photocatalytic H₂ production but, the efficiency of pure g-C₃N₄ is at present limited by its poor visible light absorption and suffers from high recombination rate of g-C₃N₄ photogenerated electron/hole pairs resulting in low photocatalytic performance. Furthermore, the g-C₃N₄ has unique electronic structure, therefore renowned candidates have been coupled with different functional components to improve photocatalytic activity. In this contribution, we review the recent research progresses of transition metals, non metals, noble metals, semiconductor compounds, graphene, carbon nanotubes (CNTs), carbon dots and quantum dots, supported on g-C₃N₄ nanosheets, which were applied to photocatalytic H₂ production. In addition, different techniques used to synthesis the g-C₃N₄ based photocatalyst including with their corresponding examples have been described. We hope that this review will encourage the readers to extend the applications of g-C₃N₄ based heterostructure in the field of H₂ production in a green manner.     

Facile construction of phosphate incorporated graphitic carbon nitride with mesoporous structure and superior performance for H2 production

Novel mesoporous phosphate incorporated g-C₃N₄ (CNM-Px) polymeric material was synthesized via a facile hydrothermal-calcination method, using melamine as precursor and phosphoric acid as dopant. The successful incorporation of phosphate into the framework of g-C₃N₄ nanosheets was verified by XRD, FT-IR and XPS characterizations and the possible formation mechanism was put forward. The as-fabricated CNM-Px samples were applied to photocatalytic hydrogen evolution reaction and exhibited remarkably improved photocatalytic performance both under simulated sunlight and visible light irradiation. The concentration of phosphoric acid was also well tuned and the optimal concentration was 2.5 mol L^?1. The hydrogen evolution rate of the optimized sample CNM-P2.5 (the concentration of treating phosphoric acid was 2.5 mol L^?1) reached 8163 μmol g^?1 h^?1 under simulated sunlight irradiation, which is 3.7 times higher than that of pristine g-C₃N₄ (CNM). It also showed dramatically improved hydrogen evolution rate under visible light irradiation, which was 2105 μmol g^?1 h^?1, about 6.7 times higher than that of CNM. The excellent photocatalytic activity of CNM-Px samples is due to the synergic advantages of larger surface area and reduced recombination of photo-generated electrons and holes. This study paves the way for tailoring design and synthesis of highly active metal-free carbon nitride materials for photocatalytic hydrogen evolution.     

The synergistic effect of potassium ions and nitrogen defects on carbon nitride for enhanced photocatalytic hydrogen evolution

Ion doping is an effective method to improve the photocatalytic activity of graphitic carbon nitride (g-C₃N₄) by providing a photocarriers transfer channel. But limited by the bonds in heptazine rings, photoelectrons are still trapped in the structure. Therefore, both potassium ions and nitrogen defects were successfully introduced into g-C₃N₄ by high temperature calcination to accelerate the charges transfer between both interlayers and intralayer of g-C₃N₄. The results showed that the hydrogen production rate of g-C₃N₄ modified simultaneously by nitrogen defects and potassium ions reaches 1722.4 μmol·g⁻¹·h⁻¹, which is 8 times that of pristine g-C₃N₄. Based on various characterization techniques and DFT calculations, we attributed the enhanced photocatalytic hydrogen evolution to the improved light adsorption, more delocalized HOMO-LUMO, and stronger interlayer interactions. This work will provide a promising way to enhance photocatalytic hydrogen evolution of g-C₃N₄ and a possible mechanism was proposed.     

Effectively extending visible light absorption with a broad spectrum sensitizer for improving the H2 evolution of in-situ Cu/g-C3N4 nanocomponents

Photocatalysts with broad spectrum absorption have been desired for a long time due to their ability to absorb more visible light. Herein, we developed an in-situ approach to specifically fabricate Cu nanoparticles onto the exterior surface of g-C₃N₄, followed by sensitization with Erythrosin B, to improve the photocatalytic H₂ evolution of g-C₃N₄ and extend the spectrum absorption. The photocatalytic H₂ evolution rate was significantly promoted, to more than 26 times that of pure g-C₃N₄, and the photocatalytic ability was maintained until reaching a wavelength of 700 nm. The origin of the improved activity was attributed to an in-situ Cu nanoparticle modification, which acts as an electron reservoir, and dye sensitization, which could extend the range of the visible light absorption, preventing charge recombination and enhancing the visible light utilization efficiency. In addition, the photocatalytic stability was investigated, and no significant attenuation was detected after six recycles.