Photocatalytic H2 Evolution Reaction Research Articles

Homogeneous distribution of highly dispersed Pt species over O and P co-doped g-C3N4 and its superior photocatalytic H2 evolution activity

In this study, we synthesized O and P co-doped graphitic C3N4 (OPCN) materials by a one-step calcination method and prepared Pt/OPCN photocatalysts for the photocatalytic H2 evolution reaction. Among the Pt/OPCN catalysts with various O and P contents, Pt/OPCN2 exhibited the highest H2 evolution rate of 2198 μmol·g−1 (AQE = 7.33%), which was 1.65 times higher than that of Pt/CN (1330 μmol·g−1). Moreover, Pt/OPCN2 demonstrated excellent photocatalytic properties, such as suitable band gap (2.50eV) for visible light and efficient charge separation of photoexcited electron–hole pairs, which was confirmed by ultraviolet-visible spectroscopy, electrochemical impedance spectroscopy, photoluminescence spectroscopy, and time-resolved photoluminescence measurements. Fundamentally, the Pt species effectively interacted with neighboring P=O and C=O functional groups that optimally located defective OPCN2, resulting in a homogeneous formation of highly dispersed PtO evenly distributed over Pt/OPCN2, as confirmed by X-ray photoelectron spectroscopy, high-angle annular dark-field scanning transmission electron microscopy, and density functional theory calculations. However, excessive O and P contents of OPCN3 damaged the morphological structure of g-C3N4, inducing a poor interaction between the Pt species and OPCN3 and forming isolated Pt nanoparticles, resulting in the low photocatalytic activity of Pt/OPCN3.

MoB2 modified g-C3N4: A Schottky junction with enhanced interfacial redox activity and charge separation for efficient photocatalytic H2 evolution

The g-C3N4 material, which exhibits a sensitivity to visible light and has a band position that is appropriate for the photoreduction of H2, has garnered significant interest. However, the ability of g-C3N4 to use light for photocatalysis is restricted due to its insufficient capacity to absorb light and its poor reaction kinetics on the surface. The researchers opted for the metallic MoB2 to create a Schottky junction coupled with g-C3N4. The incorporation of MoB2 nanoparticles onto g-C3N4 enhances its light absorption capacity and promotes the efficient separation and transfer of photogenerated charge. In addition, MoB2 nanoparticles served as active sites to accelerate H2 evolution. Compared to g-C3N4, the MoB2@g-C3N4 composite exhibited a nearly 51-fold increase in H2 production rate, indicating a significant improvement in photocatalytic activity. The present investigation demonstrated the potential of metallic MoB2 as a modified material during a photocatalytic H2 evolution reaction.

14.8% Quantum Efficient Gallium Phosphide Photocatalyst for Hydrogen Evolution.

Gallium phosphide is an established photoelectrode material for H2 or O2 evolution from water, but particle-based GaP photocatalysts for H2 evolution are very rare. To understand the reasons, we investigated the photocatalytic H2 evolution reaction (HER) of suspended n-type GaP particles with iodide, sulfite, ferricyanide, ferrous ion, and hydrosulfide as sacrificial electron donors, and using Pt, RhyCr2-yO3, and Ni2P HER cocatalysts. A record apparent quantum efficiency of 14.8% at 525 nm was achieved after removing gallium and oxide charge trapping states from the GaP surface, adding a Ni2P cocatalyst to reduce the proton reduction overpotential, lowering the Schottky-barrier at the GaP-cocatalyst interface, adjusting the polarity of the depletion layer at the GaP-liquid interface, and optimizing the electrochemical potential of the electron donor. The work not only showcases the main factors that control charge separation in suspended photocatalysts, but it also explains why most known HER photocatalysts in the literature are based on n-type and not p-type semiconductors.

Synergy between palladium single atoms and small nanoparticles co-anchored on carbon atom self-doped graphitic carbon nitride boosting photocatalytic H2 generation

Supramolecule self-assembly of dicyandiamide and uracil followed by thermal polymerization route is designed to prepare carbon atom self-doped g-C3N4 (CCNx), and then wet reduction is applied to fabricate Pd single atoms (Pd1) and nanoparticles (PdNPs) co-anchored CCNx heterojunctions (Pd1+NPs/CCNx). In Pd1+NPs/CCNx structure, interlayer Pd−N4 coordination is the most favorable for chemically stabilizing Pd1, while PdNPs accumulate on the in-plane of CCNx. Pd1+NPs/CCNx heterojunctions exhibit remarkably enhanced photocatalytic H2 evolution reaction (HER) activity, and HER rate and AQY value reach up to 24.1 mmol g−1 h−1 and 17.1% (400 nm) over the optimized Pd1+NPs/CCNx catalyst. Mechanism studies unveil that synergy of as-built interlayer N−Pd−N electron transfer channels at the atomic-scale and surface Mott–Schottky effect of small Pd nanoparticles notably accelerates migration of photogenerated electrons, which leads to plentiful electrons accumulation around Pd single atoms and small nanoparticles to decrease the energy barrier of H* activation and boost HER photodynamics significantly.

Influence of Pt Oxidation State on the Activity and Selectivity of g-C3N4-Based Photocatalysts in H2 Evolution Reaction

Graphitic carbon nitride g-C3N4 has been modified using platinum and platinum oxide (0.5–5 wt.%) and studied in photocatalytic H2 evolution reactions with ethanol aqueous solution under visible light irradiation (λ = 409 nm). An analysis of the by-products of the reaction (CO2, CH4, C2H6 etc.) was also carried out. The morphology, particle size distribution, and optical properties of the photocatalysts, and the chemical states of platinum cations were examined using various methods. The photocatalysts were investigated using a wide range of methods to clarify the morphology, particle size distribution, optical properties, and the chemical states of platinum cations. Factors affecting not only the activity, but also the selectivity of the photocatalyst in the target process of hydrogen production, have been established. The highest rate of H2 evolution achieved over 0.5 wt.% Pt/g-C3N4 photocatalyst is 0.6 mmol h−1 g−1 (selectivity 98.9%), which exceeds the activity of pristine g-C3N4 by 250 times. Increasing the Pt or PtO content up to 5 wt.% leads to an increase in the rate of formation of by-products (CH4, C2H6, and CO2) and a decrease in the selectivity of H2 evolution. The study also delves into the role of platinum and the mechanism of charge transfer in PtO/g-C3N4 and Pt/g-C3N4 photocatalysts due to light irradiation.

Efficient charge carrier separation over carbon-rich graphitic carbon nitride for remarkably improved photocatalytic performance in emerging organic micropollutant degradation and H2 production

Graphitic carbon nitride (g-C3N4) shows great potentials in visible-light-driven catalytic oxidation of organic micropollutants to harmless products and reduction of water to H2 but suffers from drawback of sluggish charge carrier separation and transfer dynamics. To overcome this drawback, here a novel point defect engineering strategy, by a nicotinic acid or barbituric acid-assisted supramolecule self-assembly of dicyandiamide followed by thermal polymerization, is designed to prepare pyridine unit-incorporated g-C3N4 (CPyr-CNx) and carbon atom self-doped g-C3N4 (C-CNx). The strategy leads to well-regulated chemical structures of CPyr-CNx and C-CNx and thus the precisely controlled electronic structures. The CPyr-CNx and C-CNx both exhibit remarkably improved visible-light photocatalytic activity in degradation of emerging organic micropollutants (methylparaben, acetaminophen and bisphenol A) and water-splitting to H2 production in comparison of bulk g-C3N4 and carbon-rich g-C3N4 prepared by a direct thermal copolymerization of nicotinic acid or barbituric acid with dicyandiamide, and their photocatalytic redox activity depends on carbon doping level. Experimental results combined with theoretical simulations reveal that the superior photocatalytic redox performance of CPyr-CNx and C-CNx is mainly dominated by the significantly boosted charge carrier separation and transfer dynamics driven by carbon doping induced-local electric field and -midgap states, which finally generates abundant reactive oxidation species including •O2− anion radicals, •OH radicals and 1O2 for the deep oxidation of target organic micropollutants to significantly reduce their ecotoxicity; additionally, such efficient charge carrier separation and transfer also favors high mobility for free electron-involved photocatalytic H2 evolution reaction.

MoS2-Based Nanocomposites for Photocatalytic Hydrogen Evolution and Carbon Dioxide Reduction.

Photocatalysis is a facile and sustainable approach for energy conversion and environmental remediation by generating solar fuels from water splitting. Due to their two-dimensional (2D) layered structure and excellent physicochemical properties, molybdenum disulfide (MoS2) has been effectively utilized in photocatalytic H2 evolution reaction (HER) and CO2 reduction. The photocatalytic efficiency of MoS2 greatly depends on the active edge sites present in their layered structure. Modifications like reducing the layer numbers, creating defective structures, and adopting different morphologies produce more unsaturated S atoms as active edge sites. Hence, MoS2 acts as a cocatalyst in nanocomposites/heterojunctions to facilitate the photogenerated electron transfer. This review highlights the role of MoS2 as a cocatalyst for nanocomposites in H2 evolution reaction and CO2 reduction. The H2 evolution activity has been described comprehensively as binary (with metal oxide, carbonaceous materials, metal sulfides, and metal-organic frameworks) and ternary composites of MoS2. Photocatalytic CO2 reduction is a more complex and challenging process that demands an efficient light-responsive semiconductor catalyst to tackle the thermodynamic and kinetic factors. Photocatalytic reduction of CO2 using MoS2 is an emerging topic and would be a cost-effective substitute for noble catalysts. Herein, we also exclusively envisioned the possibility of layered MoS2 and its composites in this area. This review is expected to furnish an understanding of the diverse roles of MoS2 in solar fuel generation, thus endorsing an interest in utilizing this unique layered structure to create nanostructures for future energy applications.

Ag-Doped WO3 Nanoplates as Heterogenous Multifunctional Catalyst for Glycerol Acetylation, Electrocatalytic and Enhanced Photocatalytic Hydrogen Production.

Herein, we report a hydrothermal method to synthesize pristine and Ag-doped WO3 nanoplates and study their multifunctional competence in the accomplishment of enhanced catalytic organic conversion and highly efficient photocatalytic and electrocatalytic H2 evolution reactions. The as-synthesized nanoplates were characterized by using various techniques including X-ray diffraction, field emission scanning electron microscopy-energy-dispersive X-ray analysis, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and BET surface area studies. The significant catalytic performance was shown by 1% Ag-doped WO3 nanoplates with 100% glycerol conversion and 90% triacetin selectivity. The photocatalytic activity was also examined toward water splitting H2 evolution reaction which demonstrates the highest H2 evolution of 12.06 mmol g-1 catalyst for 1% Ag-doped WO3 nanoplates in a time span of 8 h. Moreover, the electrocatalytic hydrogen evolution reaction was also monitored in acidic media (0.1 M H2SO4) which demonstrates good results for 1% Ag-doped WO3 nanoplates with a low overpotential of 0.53 V and a low Tafel slope of 40 mV dec-1.

Strongly coupled NH2NH-modified high crystallinity Graphene quantum dots/Carbon Nitride for efficient photocatalytic hydrogen evolution

For the application of photocatalysis, the development of noble metal-free cocatalysts is crucial. Here, a simple molecular fusion method was employed to create high crystallinity GQDs with the –NHNH2 modification, which were then used as an effective metal-free cocatalyst in the photocatalytic H2 evolution reaction. The modified –NHNH2 Lewis base active sites, the quick charge separation channels, and the improved electric conductivity allowed the optimized NHNH2-GQDs/PUCN sample to exhibit an H2 evolution rate of 204.2 μmol g−1 h−1, which is an 80-fold greater activity than that of pure PUCN. This work will provide insights into the design of metal-free cocatalysts for photocatalytic H2 evolution applications.

Fabrication of MnO2/g-C3N4 hetero-junction for hydrogen production and rhodamine B/methylene blue dye degradation

Over the past few decades, significant progress has been made in the area of clean and renewable energy resources to cope with the rapid diminish of fossil fuels. To suffice this purpose, photocataytic hydrogen production using heterojunctions is drawing a lot of attention. For this purpose a MnO2/g-C3N4 heterojunctions has been fabricated using hydrothermal method. The fabricated sample has been characterized by PXRD, SEM and EDX analysis. The XPS analysis reveals about the presence of Mn, C, N and O elements in the sample. The bandgap for MnO2/g-C3N4 narrowed to 1.75 eV, favouring the harvesting of visible-light. In the photocatalytic H2 evolution reaction (HER), MnO2/g-C3N4 heterojuctions exhibited promising results with 3855 μmol g−1 H2 production via Z-scheme mechanism, where e− produced in MnO2 are accumulated in the valance band of g-C3N4, increasing the lifetime of generated e− and h+. Fabricated MnO2/g-C3N4 heterojunctions have also been used for degradation of industrial dye viz. Rhodamine B (RhB) and Methyl orange (MO).

In-situ synthesis of structurally oriented hierarchical UiO-66(–NH2)/CdIn2S4/CaIn2S4 heterostructure with dual S-scheme engineering for photocatalytic renewable H2 production and asulam degradation

The in-situ fabrication of dual S-scheme ternary heterostructure is a promising approach to isolate photo-induced electron-hole and boost the efficiency of a semiconductor photocatalyst. However, the dual S-scheme heterostructure based on MOFs are rarely narrated. Herein, a S-vacancy rich hierarchical UiO-66(–NH2)/CdIn2S4/CaIn2S4 ternary hybrid material is successfully fabricated by in-situ preferential growth of ultrathin CaIn2S4 nanosheets and CdIn2S4 nanorods in presence of UiO-66(–NH2) (UN) spherical nanoparticles by using a simple reflux route. The physiochemical characteristics and optoelectronic features of the developed materials are illustrated in detail. The efficacy of UiO-66(–NH2)/CdIn2S4/CaIn2S4 ternary hybrid material is explored for photocatalytic H2 evolution reaction and decontamination of asulam (ASM) herbicide. The development of S-vacancy facilitates interfacial charge carrier migration and reduces the recombination rate. The enhanced photocatalytic H2 production and ASM degradation could be ascribed to staggered band alignment between UiO-66(–NH2) (UN), CdIn2S4 (CDS) and CaIn2S4 (CAS), which support S-scheme charge channelization in the heterojunction. Due to its physio-chemical advantages, the optimal 15UN/CDS/CAS30 hybrid photocatalyst exhibits highest photocatalytic H2 evolution rate of 4931 μmol g−1h−1 with apparent conversion efficiency of 31.5% and ASM degradation > 93% (k = 0.02 min−1) under irradiation of visible light. This work furnishes a new perceptive into the in-situ construction of dual S-scheme photocatalytic systems for renewable energy production and decontamination of organic pollutants. In addition, this study for the first time illustrates mineralization of Asulam using a heterogeneous photocatalytic route.

Green synthesis of Fe2O3 deposited g-C3N4: Addition of rGO promoted Z-scheme ternary heterojunction for efficient photocatalytic degradation and H2 evolution reaction

Construction of heterojunction photocatalysts with robust charge separation, efficiency light utilization and better charge transportation are hotspot in photocatalytic energy and environmental remediation applications. Herein, we have prepared the Z-scheme ternary heterojunction (rGO/g-C3N4/Fe2O3) photocatalysts using green tea extract assisted hydrothermal method for visible-light active photocatalytic degradation and H2 production. The heterojunction formation provides the more active sites for photocatalytic redox reaction in comparison with pure photocatalysts, which was confirmed by BET analysis. Moreover, elemental trapping experiment, photocurrent analysis, and band alignment derived from valance band XPS, clearly revealed that, the prepared photocatalyst shows Z-scheme charge separation. Photocatalytic activity demonstrated that, the prepared Z-scheme heterojunction exhibits 97 and 94% of degradation efficiency against methylene blue (MB) and ciprofloxacin (CIP) model organic pollutants. Moreover, the ternary heterojunction photocatalyst exhibits almost 100% of recycling efficiency towards H2 production even at 5th cycle with 20,786 μmol/g. Hence, we hope that this green synthesis approach would provide the new way to prepare the heterojunction photocatalysts for energy and environmental remediation applications.

One-pot synthesis of carbon coated Cu-doped ZnIn2S4 core–shell structure for boosted photocatalytic H2-evolution

Carbon-coated Cu doped ZnIn2S4 nanostructure (Cu-ZIS@C) was synthesized through one-pot hydrothermal method. The obtained core–shell Cu-ZIS@C exhibit outstanding enhanced performance toward photocatalytic H2-evolution reaction. The optimized doping amount and coating could displayed a H2 generation rate of 1455.98 μmol·g−1·h−1. The doped Cu sites could facilitate the separation of photo-generated charge carriers. Besides, the carbon layer would contribute to the improved conductivity of whole photocatalytic system and adjust the mobility of charge transfer in bulk structure. We hope this paper could offer a reference for the fabrication of core–shell nanostructure for efficient H2-evolution reaction.

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.

Intermetallic molybdenum disilicide: a new, active, and stable cocatalyst for efficient solar hydrogen production

Intermetallic molybdenum disilicide (MoSi2) has been identified as a new, active, and stable cocatalyst for photocatalytic H2evolution reactions in both dye-sensitized and semiconductor-based systems.

Upcycling endogenous Fe from coal gasification slag waste into a cocatalyst for the photocatalytic H2 evolution reaction

Endogenous Fe from coal gasification slag waste can be upcycled into an active, durable, and versatile supported Fe2P cocatalyst for photocatalytic H2 evolution reactions in dye-sensitized systems and on semiconductor photocatalysts.

Ordered and carbon-doped porous polymeric graphitic carbon nitride nanosheets toward enhanced visible light absorption and efficient photocatalytic H2 evolution.

An effective and rational pathway to tune the electronic bandstructure and visible light absorption properties of low-cost organic graphitic carbon nitride (g-C3N4, GCN) photocatalysts is still very challenging. Here, an efficient strategy is validated to tailor the bandstructure of g-C3N4 and C-doping can be regulated by polymerizing melamine with malonic acid, which can greatly extend the photoresponse range to 900 nm. The optimized GCN exhibits an improved photocatalytic hydrogen production rate of 663.6 μmol g-1 h-1 under visible light irradiation and an apparent quantum yield of 11% at 420 nm, which is three times higher than that of traditional bulk g-C3N4. This superior performance is derived from the unique ordered and porous structure of GCN, which effectively improves its light absorption and provides a larger specific surface area. In addition, the introduction of malonic acid into melamine and the subsequent thermal polymerization reaction further optimize the band structure of GCN, extend its light absorption via C-doping, and improve the photoinduced charge separation, resulting in high photocatalytic performance. This strategy provides a novel platform to design highly efficient GCN-based photocatalysts with precisely tunable operation windows and enhanced charge separation.

Single LiCl-driven synthesis of heptazine-/triazine-based crystalline carbon nitride homojunction for enhanced solar hydrogen evolution

LiCl-based molten salts have been widely used to induce the polymerization of melamine-derived monomer or intermediate into triazine-based or heptazine-based crystalline carbon nitride. Nevertheless, the role of LiCl beyond lowering melting point has not been fully revealed, and pure LiCl was exclusively reported to induce the formation of triazine-based crystalline counterpart. Herein, we firstly found that single LiCl can promote the deep deamination of melon-based pristine carbon nitride (PCN) to heptazine-based crystalline counterpart. As a result, a heptazine-/triazine-based crystalline carbon nitride homojunction was in situ formed by single LiCl-induced deamination and phase transition of PCN. It exhibits a favorable photocatalytic H2 evolution reaction rate 18.3 times higher than PCN, which benefits from the promoted charges separation between heptazine and triazine units in phase homojunction. This work emphasizes that single LiCl can realize diverse modulation of carbon nitride framework with advanced photocatalytic activity.

Non-Metal-Doped Porous Carbon Nitride Nanostructures for Photocatalytic Green Hydrogen Production

Photocatalytic green hydrogen (H2) production through water electrolysis is deemed as green, efficient, and renewable fuel or energy carrier due to its great energy density and zero greenhouse emissions. However, developing efficient and low-cost noble-metal-free photocatalysts remains one of the daunting challenges in low-cost H2 production. Porous graphitic carbon nitride (gCN) nanostructures have drawn broad multidisciplinary attention as metal-free photocatalysts in the arena of H2 production and other environmental remediation. This is due to their impressive catalytic/photocatalytic properties (i.e., high surface area, narrow bandgap, and visible light absorption), unique physicochemical durability, tunable electronic properties, and feasibility to synthesize in high yield from inexpensive and earth-abundant resources. The physicochemical and photocatalytic properties of porous gCNs can be easily optimized via the integration of earth-abundant heteroatoms. Although there are various reviews on porous gCN-based photocatalysts for various applications, to the best of our knowledge, there are no reviews on heteroatom-doped porous gCN nanostructures for the photocatalytic H2 evolution reaction (HER). It is essential to provide timely updates in this research area to highlight the research related to fabrication of novel gCNs for large-scale applications and address the current barriers in this field. This review emphasizes a panorama of recent advances in the rational design of heteroatom (i.e., P, O, S, N, and B)-doped porous gCN nanostructures including mono, binary, and ternary dopants for photocatalytic HERs and their optimized parameters. This is in addition to H2 energy storage, non-metal configuration, HER fundamental, mechanism, and calculations. This review is expected to inspire a new research entryway to the fabrication of porous gCN-based photocatalysts with ameliorated activity and durability for practical H2 production.

Efficient water reduction by ruthenium-picolinate dye-sensitized photocatalyst under red light illumination

To utilize a wider region of the solar spectrum for solar-fuel production, we newly synthesized a heteroleptic Ru(II)-picolinate complex, [Ru(H4dpbpy)2(pic)]+ (Rupic; H4dpbpy = H4dpbpy = 2,2′-bipyridine-4,4′-diphosphonic acid, Hpic = picolinic acid) as a red light-absorbing molecular photosensitizer. Rupic exhibited metal-to-ligand charge-transfer (MLCT) absorption and emission (λabs and λem = 491 and 711 nm, respectively) over a longer wavelength region than that observed with the bipyridine derivative [Ru(H4dpbpy)2(bpy)]2+ (Rubpy, bpy = 2,2′-bipyridine). The Ru(III)/Ru(II) redox potential of Rupic (Eox = +1.07 vs NHE) was approximately 0.27 V more negative than that of Rubpy, owing to the weaker ligand field strength of pic than that of bpy. Moreover, as expected, owing to the red-light absorption ability of Rupic, the photocatalytic H2 evolution reaction activity of the Rupic-sensitized Pt-TiO2 nanoparticles (apparent quantum yield, AQY ∼ 3 %) under red light illumination (λ = 625 ± 35 nm) was significantly higher than that of its Rubpy-sensitized analog (AQY ∼ 0.5 %). These results indicate the superior photosensitizing performance of the Ru(II)-picolinate complex for H2 production under sunlight illumination.