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.     

High-efficient C3N4-viologen charge transfer systems for promoting photocatalytic H2 evolution through band engineering

Accelerating the charge transfer (CT) capability of photocatalysts is an efficient way to improve the overall photocatalytic performance, yet the precise regulation of CT in photocatalyst systems is still lacking. In this paper, a series of hybrid photocatalysts composed of graphitic carbon nitride (CN) and various viologens (V) were prepared for the photocatalytic hydrogen evolution (PHE) from water splitting under visible-light irradiation. Considering the fixed energy structure of CN, the different electron-withdrawing substituents were introduced to engineer the band structure of V delicately and modulate the CT process between CN and V. It was shown that all the hybrid photocatalysts CN-x%V_y exhibited higher photocatalytic performance, of which CN–1%V₃, possessing the strongest electron withdrawing group (-NO₂), demonstrated the best PHE performance (3572.3 μmol g⁻¹ h⁻¹), exceeding 29 times over the unmodified CN. It was proposed that the introduction of V can optimize the interfacial photogenerated electron transfer (CN→V→Pt) of the whole photocatalytic system effectively. We highlighted the V as an efficient chemical segment to modify semiconductors toward enhanced activity due to the following unique characteristics: (i) the unique redox ability, (ii) the easy synthetic methods for controlling the band structures precisely, and (iii) the inherent positively charged feature. This work provides a deep understanding of CT for the rational design of high-performance photocatalysts through band engineering.     

Active-center-enriched Ni0.85Se/g-C3N4 S-scheme heterojunction for efficient photocatalytic H2 generation

Photocatalysts with abundant active sites are essential for photocatalytic H₂ evolution from water. Herein, Ni_0.85Se-deposited g-C₃N₄ was obtained by a physical solvent evaporation method. The investigation shows that Ni_0.85Se with unsaturated active Se atoms can significantly improve the photocatalytic activity of g-C₃N₄, and the H₂ production rate of Ni_0.85Se/g-C₃N₄ can reach 8780.3 μmol g^?1 h^?1, which is 3.5 and 92.9 times higher than that of Ni_0.85+xSe/g-C₃N₄ (2497.9 μmol g^?1 h^?1) and pure g-C₃N₄ (94.5 μmol g^?1 h^?1), respectively. This improvement can be attributed to the quick charge transfer between Ni_0.85Se and g-C₃N₄ with S-scheme heterojunction feature based on a series of trapping experiments and photoelectrochemical analysis. Moreover, abundant unsaturated Se atoms could provide more H₂ evolution active sites. This work sheds light on the construction of heterojunctions with abundant active sites for H₂ production.     

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.     

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.     

Enhanced charge transfer for efficient photocatalytic H2 evolution over UiO-66-NH2 with annealed Ti3C2Tx MXenes

The annealed Ti₃C₂T_x MXenes retained original layered morphology and gave rise to the formation of TiO₂ is anticipated to achieve improved photocatalytic hydrogen evolution performance as a noble-metal-free co-catalyst. In this work, a novel Ti₃C₂/TiO₂/UiO-66-NH₂ hybrid was rationally designed for the first time by simply introducing annealed Ti₃C₂T_x MXenes over water-stable Zr-MOFs (UiO-66-NH₂) precursors via a facile hydrothermal process. As expected, the rationally designed Ti₃C₂/TiO₂/UiO-66-NH₂ displayed significantly improvement in photocatalytic H₂ performance (1980 μmol·h⁻¹·g⁻¹) than pristine UiO-66-NH₂ under simulated sunlight irradiation. The excellent photocatalytic HER activity can be attributed to the formation of multi-interfaces in Ti₃C₂/TiO₂/UiO-66-NH₂, including Ti₃C₂/TiO₂/UiO-66-NH₂, Ti₃C₂/TiO₂ and Ti₃C₂/UiO-66-NH₂ interfaces, which constructed multiple pathways at the interfaces with Schottky junctions to accelerate the separation and transfer of charge carriers and endowed the accumulation of photo-generated electrons on the surface of Ti₃C₂. This work expanded the possibility of porous MOFs for the development of efficient photocatalytic water splitting using annealed MXenes.     

Montmorillonite-hybridized g-C3N4 composite modified by NiCoP cocatalyst for efficient visible-light-driven photocatalytic hydrogen evolution by dye-sensitization

The photocatalytic hydrogen evolution performance of g-C₃N₄ was enhanced via the hybridization with montmorillonite (MMT) and using NiCoP as cocatalyst. The highest hydrogen-evolution rate from water splitting under visible-light irradiation observed over MMT/g-C₃N₄/15%NiCoP was 12.50 mmol g⁻¹ h⁻¹ under 1.0 mmol L⁻¹ of Eosin Y-sensitization at pH of 11, which was ∼26.0 and 1.6 times higher than that of MMT/g-C₃N₄ (0.48 mmol g⁻¹ h⁻¹) and g-C₃N₄/15%NiCoP (7.69 mmol g⁻¹ h⁻¹). The apparent quantum yield at 420 nm reached 40.3%. The remarkably improved photocatalytic activity can be ascribed to the increased dispersion of g-C₃N₄ layers, staggered conduction band potentials between g-C₃N₄ and NiCoP, as well as the electrostatic repulsion originated from negatively charged MMT. This work demonstrates that MMT can be an outstanding support for the deposition of catalytically active components for photocatalytic hydrogen production.     

Direct Z-scheme ZnIn2S4/LaNiO3 nanohybrid with enhanced photocatalytic performance for H2 evolution

The direct Z-scheme ZnIn₂S₄/LaNiO₃ nanohybrid based on ZnIn₂S₄ nanosheets and LaNiO₃ cubes was synthesized by a facile hydrothermal method. The ZnIn₂S₄/LaNiO₃ nanohybrid showed improved photocatalytic H₂ evolution and stability. The photocatalytic H₂ evolution activity of ZnIn₂S₄/LaNiO₃ nanohybrid is 3-fold enhanced than that of bare ZnIn₂S₄. The enhanced performance of ZnIn₂S₄/LaNiO₃ nanohybrid is mainly ascribed to the formation of heterojunction between LaNiO₃ and ZnIn₂S₄. The heterojunction can facilitate charge transport on the interface between LaNiO₃ and ZnIn₂S₄ and suppress the recombination of photo-generated charge carriers over ZnIn₂S₄/LaNiO₃ nanohybrid, which were well demonstrated by photoelectrochemical tests. Moreover, the direct Z-scheme photocatalytic reaction mechanism was proposed to elucidate the improved performance of ZnIn₂S₄/LaNiO₃ nanohybrid photocatalyst. This study may provide some guidance on the construction of direct Z-scheme photocatalytic system for photocatalytic H₂ evolution.     

Increasing electron transfer and light absorption in graphited nanocarbon-conjugated g-C3N4 nanosheet heterostructure for photocatalytic hydrogen evolution

Accelerating the charge separation and transfer as well as increasing the visible light absorption is of great importance for photocatalysts to realize efficient photocatalytic hydrogen evolution via water splitting. Herein, for the first time, we fabricated in-plane graphited nanocarbon-conjugated polymeric carbon nitride (GNC-C₃N₄) nanosheet heterostructure photocatalyst from melamine and hexaketocyclohexane octahydrate mixture via an amino-carbonyl reaction. The incorporation of GNC into conjugate network of C₃N₄ can not only dramatically enhance the light harvesting but also significantly promote the charge separation and transfer by the built-in electric field and intimate interface in the coplanar GNC-C₃N₄ heterostructure. Accordingly, the optimal GNC-C₃N₄ photocatalyst demonstrates a more than 15-fold enhancement for photocatalytic hydrogen evolution from water under visible light irradiation, compared to C₃N₄.     

A novel pathway toward efficient and stable C3N4-based photocatalyst for light driven H2 evolution: The synergistic effect between Pt and CoWO4

A photocatalytic integrated system containing Pt nanoclusters, CoWO₄ nanoclusters and C₃N₄ nanosheets was achieved through a hydrothermal process followed by photodeposition. Meanwhile, the photocatalytic activity of the as-prepared system was explored for light driven H₂ evolution. Finally, the photocatalytic mechanism was explored roughly. The results show that there exists strong synergistic effect between Pt and CoWO₄. The photocatalytic activity of C₃N₄ can be significantly enhanced utilizing the aforementioned synergistic effect. When the as-prepared photocatalytic system is used, the fastest evolution rate of H₂ can be up to 14.2 μmol h⁻¹, which is 2.1 times as high as that over the Pt modified C₃N₄ nanosheets (6.7 μmol h⁻¹). And the quantum yield of the as-prepared photocatalytic system at 400 nm (0.018%) is also much higher than that of the Pt modified C₃N₄ nanosheets (0.004%). Here, this remarkable photocatalytic activity ought to be attributed to superior separation of the electro-hole pair caused by efficient charge transfer in the photocatalytic system which follows a Z-scheme-like mechanism. Therein, Pt nanoclusters may serve as an electron transfer pathway between CoWO₄ and C₃N₄ as well as active sites while CoWO₄ nanoclusters may play a water oxidation cocatalyst.     

Co3O4/CeO2 p-n heterojunction construction and application for efficient photocatalytic hydrogen evolution

The construction of p-n type heterojunction is an effective way to enhance the efficiency of photocatalytic hydrogen evolution. In this work, Co₃O₄/CeO₂ p-n heterojunction was construct by a simple hydrothermal method. This heterojunction mainly uses the internal electric field formed and accelerate the separation of electrons and holes in the opposite direction. In addition, according to SEM and TEM characterization, it was found that the granular cobalt oxide nanoparticles prepared by in-situ hydrothermal method were firmly and uniformly dispersed in cerium oxide, which effectively increased the active sites of hydrogen evolution. And combined with the BET results, it shows that the growth of cobalt oxide effectively increases the specific surface area and increases the active sites for hydrogen evolution. By exploring the hydrogen evolution capacity of different ratios of the complex, the test results showed that in all different ratios of the catalyst, CC-0.16 showed the best performance, and the hydrogen production efficiency reached 2298.52 μmol g⁻¹h⁻¹, which was 71 times that of nanobelt CeO₂ and 2.72 times that of Co₃O₄. According to the characterization results, the photocatalytic water splitting mechanism of the p-n heterojunction was proposed, and the charge transfer mechanism in the process was discussed in depth.     

Constructing g-C3N4 quantum dots modified g-C3N4/GO nanosheet aerogel for UV-Vis-NIR driven highly efficient photocatalytic H2 production

The development of ultraviolet to near-infrared (UV-Vis-NIR) responsive photocatalysts offers a unique opportunity for the full use of solar energy to solve the energy and the environmental problems. Here, successful preparation of a three-dimensional (3D) porous photocatalyst of graphitic carbon nitride quantum dot (CNQDs) modified g-C₃N₄/graphene oxide composite aerogel (CNGO/CNQDs) via hydrothermal and vacuum injection method was reported. In this unique ternary 3D photocatalyst, graphene oxide could improve the separation of photogenerated electrons and holes and promote the charge separation, while the aerogel's 3D network structure provided a rich active site. Simultaneously, due to the appropriate up-conversion performance of the nitrogen carbide quantum dots, CNGO/CNQDs achieved a light response from ultraviolet (UV) to near-infrared (NIR). These properties endow it with a good photocatalytic performance. The hydrogen production efficiency of CNGO/CNQDs reached 1231 μmol h⁻¹, which was 16 times more than that of matrix material. In addition, the apparent quantum yields (AQY) of CNGO/CNQDs at wavelengths of 420 nm and 700 nm were 13% and 0.116%, respectively.     

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.     

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.     

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.     

A direct one-step synthesis of ultrathin g-C3N4 nanosheets from thiourea for boosting solar photocatalytic H2 evolution

Two-dimensional (2D) graphitic carbon nitride (g-C₃N₄) nanosheets, as the promising photocatalyst with fascinating properties, have become a “rising star” in the field of photocatalysis. Although g-C₃N₄ nanosheets exfoliated from the bulk g-C₃N₄ powders are extensively emerged, developing a simple synthetic approach is still full of challenge. To this end, here we report a direct polymerization strategy to fabricate the ultrathin g-C₃N₄ nanosheets, that is only heating treatment of thiourea in air without addition of any template. The photocatalytic activities of as-prepared samples were evaluated by photoreduction of water to hydrogen (H₂) using triethanolamine as sacrificial agent and Pt as co-catalyst under visible-light irradiation (λ > 420 nm). As a result, our few-layered g-C₃N₄ nanosheets with an average thickness of 3.5 nm exhibit a superior visible-light photocatalytic H₂ evolution rate (HER) of 1391 μmol g⁻¹ h⁻¹ and a remarkable apparent quantum efficiency of 6.6% at 420 nm. Eventually, the HER of as-fabricated ultrathin g-C₃N₄ nanosheets is not only much higher than the dicyandiamide-derived g-C₃N₄ or melamine-derived g-C₃N₄, but also greater than the thermal-oxidation etched g-C₃N₄ nanosheets under the same condition.     

Facile fabrication of mesoporous In2O3/LaNaTaO3 nanocomposites for photocatalytic H2 evolution

The incorporation of In₂O₃ nanoparticles on mesoporous La_0.02Na_0.98TaO₃ photocatalysts is very interesting for promoting the H₂ production under UV illumination in the presence of [10%] glycerol as a hole scavenger. It is demonstrated that an outstanding mesoporous In₂O₃/La_0.02Na_0.98TaO₃ photocatalyst can be constructed by incorporating In₂O₃ nanoparticles (0-2 wt%) and mesoporous La_0.02Na_0.98TaO₃ nanocomposites for highly promoting photocatalytic H₂ evolution. The maximum yield of H₂ ~ 2350 μmol g⁻¹ was obtained over mesoporous 1%In₂O₃/La_0.02Na_0.98TaO₃ nanocomposite. The mesoporous 1%In₂O₃/La_0.02Na_0.98TaO₃ nanocomposite exhibited further enhancement H₂ production, in which the rate of H₂ evolution can be as high as 235 μmol g⁻¹ h⁻¹, 435 times higher than those of mesoporous La_0.02Na_0.98TaO₃. The results showed that the 1%In₂O₃/La_0.02Na_0.98TaO₃ photocatalyst possesses high stability and durability for H₂ evolution by implying almost no photoactivity reduce after five cycles for 45 h continuous illumination. The measurement of photoluminescence spectroscopy, transient photocurrent spectra and UV- diffuse reflectance spectra for all synthesized samples exhibited that the promoted H₂ production is mainly explained by its effective electron-hole separation and broaden photoresponse region due to its compositions and structures of the obtained heterostructures.     

SnO2 promoted carrier separation in superior thin g-C3N4 nanosheets for enhanced photocatalytic degradation and H2 generation

The construction of heterostructures is an efficient approach to improve the photocatalystic performance of semiconductors. In this paper, SnO₂-g-C₃N₄ (SnO₂–CN) nanocomposites were created via thermal polymerization using SnO₂ nanoparticles and layered g-C₃N₄ nanosheets. A mechano-chemical pre-reaction and the second thermal polymerization of bulk g-C₃N₄ play important roles for the formation of SnO₂/g-C₃N₄ heterostructures with improved interface nature. The heterostructures with an optimized SnO₂ weight ratio of 10% was obtained by adjusting parameters for enhanced photocatalytic reactions in visible light region. Hydrogen generation and the degradation of rhodamine B (Rh B) were tested to characterize the photocatalytic performance of the SnO₂–CN nanocomposites. The degradation of a 20 mg/L Rh B solution was finished within 15 min, in which the degradation rate was about twice compared with superior thin g-C₃N₄ nanosheets prepared by a two-step polymerization procedure. The SnO₂–CN nanocomposite with 10% SnO₂ revealed a H₂ generation rate of 2569.5 μmol g⁻¹L⁻¹. The enhanced photocatalytic performance is ascribed to a type II heterostructure formed and improved interface properties between g-C₃N₄ and SnO₂. In addition, the improved conductivity of SnO₂ promoted the photogenerated carrier separation and transfer. The result provided a new idea for the construction of g-C₃N₄ heterostructures with improved interface characterization and the improvement of photocatalytic properties.     

Excellent visible light photocatalytic efficiency of Na and S co-doped g-C3N4 nanotubes for H2 production and organic pollutant degradation

Na and S co-doped g-C₃N₄ nanotubes (Na_xSCNNTs) were synthesized via thermal polymerization using NaHCO₃ and thiourea as Na and S source, respectively. The co-doping of Na and S in g-C₃N₄ nanotubes was verified by FTIR, SEM elemental mapping and XPS measurements. After loading Pt, the optimal Na_0.1SCNNT produced H₂ at a rate of 173.7 μmol h⁻¹, which is 1.76 times and 14 times of that of Na₀SCNNT and bulk g-C₃N₄, respectively. Moreover, the performance of Na_0.1SCNNT was increased by 50% after replacing Pt with PtCo. The apparent quantum efficiency of Na_0.1SCNNT/Pt and Na_0.1SCNNT/PtCo were 6.7% and 10.2% at λ = 420 nm, respectively. Na_0.1SCNNT also displayed the best photocatalytic activity for both p-chlorophenol and rhodamine B degradation, which are 3.1 and 3.4 times of that of bulk g-C₃N₄, respectively. Cyclic photocatalytic experiments demonstrated the high stability of Na_0.1SCNNT. The enhanced photocatalytic activity of Na_0.1SCNNT is resulted from the large specific surface area, narrowed bandgap, enhanced visible light absorption, and down-shifted valance band, which are supported by steady-state PL spectra and time-resolved transient PL decay, as well as photoelectrochemical analysis. Finally, the possible photocatalytic mechanisms for H₂ production, and degradation of rhodamine B and p-chlorophenol are proposed.