Visible-light Photocatalytic H2 Evolution Research Articles

Graphitic carbon nitride nanosheets prepared by gaseous molecules assembling for enhanced photocatalytic performance

Graphitic carbon nitride (g-C3N4), a new two-dimensional (2D) polymeric semiconductor, is considered as one of the most promising candidates for visible-light photocatalytic H2 evolution. The synthesis of thin-layered g-C3N4 is a facile approach to enhancing its photocatalytic properties. Here, a novel, cost-efficient, and time-saving strategy is reported to obtain 2D g-C3N4. In this equipment, bulk g-C3N4 and thin-layered g-C3N4 (denoted as G-CN) assembled from gaseous molecules were obtained simultaneously. The thin-layered g-C3N4 possesses more negative conduction-band minimum (~ 0.18 eV) relative to the bulk counterparty, leading to stronger redox ability. What is more, the carrier mobility and separation efficiency are both improved. As a result, the water splitting performance and photodegradation efficiency for methylene blue on thin-layered g-C3N4 are dramatically improved. The H2 evolution rate and half-time of photodegradation obtained by kinetic fitting reached 48.83 μmol h−1 and about 1.5 h, which is much more superior to that of bulk g-C3N4. Generally, the present work may bring out new thinking to synthesize thin-layered 2D materials.

One-pot synthesis of metakaolin/g-C3N4 composite for improved visible-light photocatalytic H2 evolution

The conversion of solar energy to chemical energy via photocatalysis is a potential pathway for future energy supplying, and the development of graphitic carbon nitride (g-C3N4) for visible-light photocatalytic hydrogen evolution reaction (HER) has attracted increasing attention. In this work, raw kaolinite was used to enhance the photocatalytic performance of g-C3N4 by constructing the metakaolin/g-C3N4 composites. A facile one-pot method, heat-treating their mixed precursors, was employed. The as-prepared MK/CN-70.4% exhibits a favorable hydrogen evolution rate of 288 μmol g−1 h−1, which is 1.5 times that of bulk g-C3N4. The improved HER benefits from both the electronegativity of metakaolin and the nanosizing of g-C3N4. The proposed strategy has the potential to inspire the utilization of other clay minerals to improve the photocatalytic activity of g-C3N4.

Fabrication of Hierarchical Two-Dimensional MoS2 Nanoflowers Decorated upon Cubic CaIn2S4 Microflowers: Facile Approach To Construct Novel Metal-Free p-n Heterojunction Semiconductors with Superior Charge Separation Efficiency.

Due to the enormous demand for effective conversion of solar energy and large-scale hydrogen production, cost-effective and long-lasting photocatalysts are believed to be necessary for global production of sustainable and clean hydrogen fuel. Robust and highly efficient p-n heterojunction photocatalysts have a striking ability to enhance light-harvesting capacity and retard the recombination of photoexcitons. A series of p-MoS2/n-CaIn2S4 heterojunction composites with different MoS2 contents have been synthesized via a facile two-step hydrothermal technique in which rose-like p-MoS2 nanoflowers are decorated upon n-type cubic CIS microflowers. In the synthesis protocol highly dispersed MoS2 nanoflowers provided more active edge sites for the growth of c-CIS nuclei, leading to a hierarchical architecture with intimate interfacial contact. The formation of a hierarchical flower-like morphology of the photocatalyst has been established by an HRTEM and FESEM study. Electrochemical characterization, especially the slope of the curve from Mott-Schottky analysis and nature of the current from LSV, reveals the p-n heterojunction nature of the composite photocatalyst. The fabricated heterojunction photocatalysts were further examined for visible light photocatalytic H2 evolution. Far exceeding those for the neat c-CIS and MoS2, it is seen that the p-MoS2/n-CIS heterojunction photocatalyst with an optimum content of MoS2 exhibited enhanced H2 evolution using a 0.025 M Na2S/Na2SO3 solution as hole quenching agent under visible light illumination. The 0.5 wt % p-MoS2/n-CIS photocatalyst presents a higher H2 production rate of 602.35 μmol h-1 with 0.743 mA cm-2 photocurrent density, 19 times and 8 times higher than those of neat c-CIS, respectively. This superior photocatalyic activity is due to the efficient separation of electron-hole charge carriers at the interface, as supported by a photoluminescence study and EIS measurements.

Noble-metal-free heterostructure for efficient hydrogen evolution in visible region: Molybdenum nitride/ultrathin graphitic carbon nitride

Molybdenum nitride (MoN) has been attracting increasing attention in recent years for the application of electrocatalytic hydrogen evolution reaction. However, the application of MoN for photocatalytic H2 evolution has seldom been reported. Here, we report that the sample with the heterostructure between Mo2N and 2D ultrathin g-C3N4 nanosheet (CN) is fabricated via boil bathing method for the use of the visible light photocatalytic H2 evolution. Without using any noble metal cocatalyst, the H2 evolution rate of Mo2N/CN under visible light irradiation reaches 0.89 μmolg−1 h−1, which is ≈148 times higher than that of pure CN. The photoelectron-chemical experiments suggest that the carriers in Mo2N/CN can separate and transfer easier than those in CN due to the effect of Mo2N as the modification of CN. The combination of the Mo2N and CN represents not only a simple but also an economical and powerful method for the highly effective photocatalytic H2 generation in the visible region.

Facile preparation of hollow-nanosphere based mesoporous g-C3N4 for highly enhanced visible-light-driven photocatalytic hydrogen evolution

Mesoporous g-C3N4 consisting of hollow nanospheres (MCNHN) was prepared via a facile vapor-deposition method. It demonstrated an excellent visible-light photocatalytic H2 evolution activity of 22.3 times greater than bulk g-C3N4. The special hollow nanosphere structure, increased specific surface area and modified crystalline enabled MCNHN to realize stronger light harvest and better photoinduced electron-hole separation, which leaded to highly improved photocatalytic performance. The proposed method and setup are easy to be established, and have the potential for preparing nanoscale g-C3N4 with other tailored nanostructures.

Facile synthesis of ternary CdIn2S4/In(OH)3/Zn2GeO4 nanocomposite with enhanced visible-light photocatalytic H2 evolution

A novel in situ hydrolysis reaction method has been developed for the controllable synthesis of ternary CdIn2S4/In(OH)3/Zn2GeO4 nanocomposite. The CdIn2S4 and In(OH)3 content in the ternary system can be conveniently tuned by varying the pH value of the reaction system. The ternary CdIn2S4/In(OH)3/Zn2GeO4 (pH = 6.5) nanocomposite sample exhibited the highest photocatalytic H2 production rate of 1426.90 μmol h−1 g−1, without the assistance of a noble metal. A possible mechanism of H2-production activity was proposed. The photo-generated electrons in the CB of CdIn2S4 can transfer to the CB of In(OH)3 and Zn2GeO4, resulting in a rapid separation of photo-generated electrons and holes. This work can not only provide an excellent candidate for visible-light H2 evolution, but also provide an efficient strategy for designing of new system with high activity for solar hydrogen generation.

Enhanced activity of visible-light photocatalytic H2 evolution of sulfur-doped g-C3N4 photocatalyst via nanoparticle metal Ni as cocatalyst

Sulfur doped graphitic carbon nitride g-S-C3N4 (denoted as SCN) was successfully prepared, and the SCN was further combined with metal Ni to prepare the photocatalyst with high visible-light photocatalytic activity. The Ni/g-S-C3N4 (denoted as NiSCN) composite photocatalyst was obtained by photodeposition method. Metal Ni species with the particle size about 5 nm were effectively supported on the surface of SCN. The NiSCN photocatalyst with the optimal Ni loading amounts exhibited an excellent H2-production rate of 2021.3 μmol g−1 h−1 under the visible light (λ > 420 nm). In addition, some Ni atoms might be embedded in the lattice structure of SCN. According to the experimental and computational results, the good utilization of visible light was attributed to the S and Ni doping, which upshifted the VB potential and narrowed the band gap of the photocatalyst. The metal Ni loaded on SCN surface improved the electron transfer rate and charge separation efficiency of the photocatalyst. Then, the H+ in water obtained the electrons on metal Ni to form H2 via the H radical intermediate.

In situ loading of Ni2P on Cd0.5Zn0.5S with red phosphorus for enhanced visible light photocatalytic H2 evolution

Using non-toxic red phosphorus as the phosphorus source, Ni2P was synthesized and in situ loaded on Cd0.5Zn0.5S photocatalyst by a simple solvothermal method for photocatalytic H2 evolution under visible light (λ ≥ 420 nm) irradiation. The as-prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), ultraviolet–visible diffuse reflectance spectra (UV–Vis DRS), Brunauer–Emmett–Teller (BET) surface area measurements and electrochemistry techniques. The photocatalytic hydrogen evolution activity over Cd0.5Zn0.5S is significantly increased by in situ-loaded Ni2P due to the intimate interface contact and efficient charge transfer from Cd0.5Zn0.5S to Ni2P. The 4 wt%Ni2P-loaded Cd0.5Zn0.5S photocatalyst shows five times higher H2 evolution rate than that of the pristine Cd0.5Zn0.5S. The highest H2 evolution rate of 65.6 μmol/h and the apparent quantum yield (AQY) of 29% at 420 nm is achieved under the optimized conditions. The noble metal free Ni2P is stable during 20 h photocatalytic reaction.

Nanoporous g-C3N4 nanosheets: Facile synthesis and excellent visible-light photocatalytic H2 evolution performance

A facile melamine-involved vapor deposition method was proposed and demonstrated to grow graphitic carbon nitride (g-C3N4) on natural kaolin clay. Nanoporous g-C3N4 nanosheets of 5 nm in thickness containing nanopores of 5–100 nm in diameter were obtained after removing the kaolin. They showed 14 times higher visible-light photocatalytic H2 evolution efficiency than traditional bulk g-C3N4, benefitting from highly increased specific surface area and improved electron transport ability. This vapor deposition strategy can inspire the synthesis of nanoscale g-C3N4 with various tailored structures using diverse natural minerals or other cheap templates.

One-step synthesis of nanostructured g-C3N4/TiO2 composite for highly enhanced visible-light photocatalytic H2 evolution

Improving the photocatalytic property of g-C3N4 by combined strategies has attracted increasing attention recently. In this work, we realized the structure nanosizing of g-C3N4 and its synchronous compounding with TiO2 nanoparticles in one step, using a facile melamine-involved vapor deposition method coupled with a simple and easy setup. Nanostructured g-C3N4/TiO2 heterojunction was well-established and the resultant nanocomposite demonstrated an excellent visible-light photocatalytic H2 evolution performance 10.8 times higher than that of bulk g-C3N4. The structure nanosizing coupled with the heterojunction construction contributed together to the improvement of photoinduced electron-hole separation and final photocatalytic efficiency. The proposed simple method and setup have the potential to be used for preparing other g-C3N4-based nanocomposites with advanced photocatalytic properties.

Zirconium-Porphyrin-Based Metal-Organic Framework Hollow Nanotubes for Immobilization of Noble-Metal Single Atoms.

Single atoms immobilized on metal-organic frameworks (MOFs) with unique nanostructures have drawn tremendous attention in the application of catalysis but remain a great challenge. Various single noble-metal atoms have now been successfully anchored on the well-defined anchoring sites of the zirconium porphyrin MOF hollow nanotubes, which are probed by aberration-corrected scanning transmission electron microscopy and synchrotron-radiation-based X-ray absorption fine-structure spectroscopy. Owing to the hollow structure and excellent photoelectrochemical performance, the HNTM-Ir/Pt exhibits outstanding catalytic activity in the visible-light photocatalytic H2 evolution via water splitting. The single atom immobilized on MOFs with hollow structures are expected to pave the way to expand the potential applications of MOFs.

Defect-rich O-incorporated 1T-MoS2 nanosheets for remarkably enhanced visible-light photocatalytic H2 evolution over CdS: The impact of enriched defects

Structure and phase engineering in low-cost and earth-abundant MoS2 materials have been widely explored in catalytic domains. Designing MoS2 nanostructures associated with the number of catalytically active edge sites is of particular importance for obtaining an effective HER photocatalytic system. We herein report hydrothermal-synthesized defect-rich O-incorporated 1T-MoS2 nanosheets (denoted as DRM), and their implementation as cocatalysts to form photocatalytic heterostructure with CdS nanorods (denoted as DRM-C). For comparison, defect-free O-incorporated 1T-MoS2 nanosheets (denoted as DFM) and corresponding heterostructure i.e. DFM-C were simultaneously synthesized. A carefully comparative study on DRM and DFM demonstrates that the DRM cocatalyst exhibits more abundant defects in the basal planes and edges, and thus leading to the higher density of active sites beneficial to the photocatalytic HER. As a result, the optimized 20 wt% DRM-C exhibits an extraordinary visible-light (λ > 420 nm) photocatalytic H2 production rate of 132.4 mmol h−1 g−1, obviously higher than that of 20 wt% DFM-C (102.1 mmol h−1 g−1). Interestingly, such an impressive activity of 20 wt% DRM-C is demonstrated to be superior to that of pure CdS (20.0 mmol h−1 g−1) and conventional Pt/CdS (89.0 mmol h−1 g−1). To the best of our knowledge, this DRM-C photocatalyst shows the advanced visible-light-driven HER performance among reported MoS2/CdS composites.

Structuring phase junction between tri-s-triazine and triazine crystalline C3N4 for efficient photocatalytic hydrogen evolution

Herein a novel carbon nitride composite with triazine-based crystalline carbon nitride (tri-C3N4) vertically aligned on tri-s-traizine-based crystalline carbon nitride (tri-s-tri-C3N4) is first proposed for efficient visible light driven (λ > 420 nm) photocatalytic H2 evolution. The well matched lattice fringes between (002) plane of tri-s-tri-C3N4 and (102) plane of tri-C3N4 characterized by TEM unambiguously demonstrate the successful construction of tight crystalline junction between tri-s-tri-C3N4 and tri-C3N4. As a result, the crystalline carbon nitride phase junction (tri-/tri-s-tri-C3N4) shows a high visible light photocatalytic H2 evolution activity of 144 μmol/h, which is 30 times higher than that of pristine g-C3N4. This outstanding photocatalytic H2 evolution performance could be attributed to the fact that the construction of crystalline tight junction can greatly enhance the transfer and separation efficiency of photoinduced carriers. This study may provide a new way for rational design of carbon nitride heterojunction for gaining high photocatalytic activity.

Scalable one-step production of porous oxygen-doped g-C3N4 nanorods with effective electron separation for excellent visible-light photocatalytic activity

Photoinduced electron transfer and separation from its home atom to form spatial isolated electron/hole pairs is the most crucial factor in artificial photocatalysis field. The scalable production of nanoscale g-C3N4 with remarkable electron separation efficiency in one-step, green, economic and “bottom-up” approach is of great challenge. Herein, one-dimensional porous architectural g-C3N4 nanorods have been facilely prepared by direct calcination of hydrous melamine nanofibers precipitated from aqueous solution of melamine. Porous morphologies with increased interfacial area enhance light capture capacity and accelerate catalysis reaction. It is noted that oxygen atoms were simultaneously doped into g-C3N4 matrix, which broke the symmetry of pristine g-C3N4, resulting in more effective separation of electron/hole pairs. Thus, the oxygen-doped g-C3N4 nanorods loaded with Pt presented excellent visible-light photocatalytic hydrogen evolution in the triethanolamine solution (732μmolg−1h−1) and in overall water splitting (29.6μmolg−1h−1), and 2,4-dinitrophenol degradation for O-doped g-C3N4 nanorod (removal efficiency of 100% within 75min). Subsequently, the visible-light photocatalytic H2 evolution (96μmolg−1h−1) with simultaneous 2,4-dinitrophenol degradation were achieved for Pt@g-C3N4 nanorod. The proposed synthesis strategy overcomes long-standing, stubborn and serious stacking/agglomeration for g-C3N4 synthesis, and paves the pathway for industrial scale-up production of high-performance g-C3N4 and application in practical energy and environment area.

Significantly enhanced photocatalytic hydrogen generation over graphitic carbon nitride with carefully modified intralayer structures

Graphitic carbon nitride has been widely investigated as promising photocatalyst for solar-driven photocatalytic hydrogen production. In this study, the intralayer properties of graphitic carbon nitride was modified by a facile hydrothermal treatment with appropriate amount of ammonium hydroxide to promote its photocatalytic H2 production performance. Transition from intralayer modification to bulk structural transformation occurred once the concentration of NH3·H2O is over certain concentration. The destruction of heptazine units resulted in the partial oxidation of C atoms in g-C3N4 during the hydrothermal treatment process and the introduction of OH and CO can effectively enhance the interaction between g-C3N4 with reactants. Band structure measurement revealed that the thus obtained g-C3N4 possessed elevated conduction band bottom indicative of improved reduction capacities. The significantly enhanced redox ability on the surface reaction sites overcame the deceased light absorption and higher charge recombination efficiency of charge carriers, and finally induced a significantly improved photocatalytic H2 evolution (707.58μmol·h−1·g−1). The highest visible light photocatalytic H2 evolution rate was 7.5 times higher than that of pristine graphitic carbon nitride (94.46μmol·h−1·g−1), with an apparent quantum yield of 5.18% at 420nm. This study demonstrates a facile and effective strategy to tune the electronic and photocatalytic properties of carbon nitride for improved solar energy conversion efficiency which are expected to be extended to the modifications of other photocatalytic materials.

Enhanced visible light photocatalytic H2 evolution over CeO2 loaded with Pt and CdS

CdS-Pt@CeO2 ternary composites are successfully prepared by an in situ redox precipitation followed by a subsequent precipitation. The prepared CdS-Pt@CeO2 sample exhibits high photocatalytic activity for hydrogen evolution from lactic acid aqueous solution under visible light irradiation, with a H2-evolation rate of 20.09 mmol h−1g−1 and apparent quantum yield (AQY) of 15.32%. With techniques such as XRD, TEM, DRS, XPS, N2 adsorption/desorption and electrochemistry measurements, the physicochemical properties of the CdS-Pt@CeO2 ternary composites are discussed, and a possible mechanism for the hydrogen evolution is proposed.

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.

Multi-node CdS hetero-nanowires grown with defect-rich oxygen-doped MoS2 ultrathin nanosheets for efficient visible-light photocatalytic H2 evolution

Developing low-cost and high-efficiency photocatalysts for hydrogen production from solar water splitting is intriguing but challenging. In this study, unique one-dimensional (1D) multi-node MoS2/CdS hetero-nanowires (NWs) for efficient visible-light photocatalytic H2 evolution are synthesized via a facile hydrothermal method. Flower-like sheaths are assembled from numerous defect-rich O-incorporated {0001} MoS2 ultrathin nanosheets (NSs), and {1120}-facet surrounded CdS NW stems are grown preferentially along the c-axis. Interestingly, the defects in the MoS2 NSs provide additional active S atoms on the exposed edge sites, and the incorporation of O reduces the energy barrier for H2 evolution and increases the electric conductivity of the MoS2 NSs. Moreover, the recombination of photoinduced charge carriers is significantly inhibited by the heterojunction formed between the MoS2 NSs and CdS NWs. Therefore, in the absence of noble metals as co-catalysts, the 1D MoS2 NS/CdS NW hybrids exhibit an excellent H2-generation rate of 10.85 mmol·g–1·h–1 and a quantum yield of 22.0% at λ = 475 nm, which is far better than those of Pt/CdS NWs, pure MoS2 NSs, and CdS NWs as well as their physical mixtures. Our results contribute to the rational construction of highly reactive nanostructures for various catalytic applications.

Black Phosphorus Revisited: A Missing Metal-Free Elemental Photocatalyst for Visible Light Hydrogen Evolution.

Metal-free elemental photocatalysts for hydrogen (H2 ) evolution are more advantageous than the traditional metal-based inorganic photocatalysts since the nonmetal elements are generally cheaper, more earth-abundant, and environmentally friendly. Black phosphorus (BP) has been attracting increasing attention in recent years based on its anisotropic 2D layered structure with tunable bandgap in the range of 0.3-2.0 eV; however, the application of BP for photocatalytic H2 evolution has been scarcely reported experimentally although being theoretically predicted. Herein, for the first time, the visible light photocatalytic H2 evolution of BP nanosheets prepared via a facile solid-state mechanochemical method by ball-milling bulk BP is reported. Without using any noble metal cocatalyst, the visible light photocatalytic hydrogen evolution rate of BP nanosheets reaches 512 µmol h-1 g-1 , which is ≈18 times higher than that of the bulk BP, and is comparable or even higher than that of graphitic carbon nitrides (g-C3 N4 ).

Earth-abundant WC nanoparticles as an active noble-metal-free co-catalyst for the highly boosted photocatalytic H2 production over g-C3N4 nanosheets under visible light

Enhanced visible-light photocatalytic H2 evolution over g-C3N4 nanosheets modified by earth-abundant WC nanoparticles as an active noble-metal-free co-catalyst.