Surface Modification of Carbon Nitride Polymers by Core-Shell Nickel/Nickel Oxide Cocatalysts for Hydrogen Evolution Photocatalysis

A new Ni-based cocatalyst nickel thiocarbonate enhancing graphitic carbon nitride photocatalytic hydrogen production by constructing a built-in electric field

Despite the excellent photocatalytic activity under visible light, graphitic carbon nitride (g-C3N4) exhibits a high overpotential for hydrogen evolution. To address this issue, cocatalysts have been utilized to modify g-C3N4. However, the use of high-performance cocatalysts typically involves noble metals such as platinum and palladium, which are cost-prohibitive for practical applications. Therefore, the development of efficient and cost-effective cocatalysts is crucial for advancing photocatalysis. In this study, we synthesized a new Ni-based cocatalyst, nickel thiocarbonate (NiCS3), to enhance the photocatalytic hydrogen evolution reaction (HER) on g-C3N4. The NiCS3/g-C3N4 composite demonstrated a significantly increased hydrogen evolution rate of 951 μmol·h-1·g-1 under visible light, representing more than a 105-fold improvement compared to pure g-C3N4. Theoretical calculations suggested that the enhanced performance in photocatalytic hydrogen production can be attributed to the generation of a built-in electric field within the composite, facilitating efficient charge carrier separation and migration. Additionally, the C site in NiCS3 provides a favorable Gibbs free energy of adsorbed H* (ΔGH∗). This work underscores the potential of NiCS3 as a viable alternative to precious metals in photocatalytic hydrogen production using g-C3N4.

ZnIn2S4-based multi-interface coupled photocatalyst for efficient photothermal synergistic catalytic hydrogen evolution

Photothermal synergistic catalysis is a novel technology that converts energy. In this study, ZnIn2S4 with S-vacancy (ZIS-Vs) is combined with Nickel, Nickle Oxide and Carbon Nanofiber aggregates (Ni-NiO@CNFs) to create a multi-interface coupled photocatalyst with double Schottky barrier, double channel and mixed photothermal conversion effect. Theoretical calculation confirms that the Gibbs free energy (ΔG*H) of the S-scheme heterojunction in the composite material is -0.07 eV, which is close to 0. This promotes the adsorption of H* and accelerates the formation of H2. Internal photothermal catalysis is achieved by visible-near infrared (Vis-NIR, RT) irradiation. The internal photothermal catalytic hydrogen production rate of the best sample (0.9Ni-NiO@CNFs/ZIS-Vs) is as high as 17.24 mmol·g-1·h-1, and its photothermal conversion efficiency (η) is as high as 61.42 %. Its hydrogen production efficiency is 20.52 times that of ZIS-Vs (0.84 mmol·g-1·h-1) under visible light (Vis, RT) conditions. When the Vis-NIR light source is combined with external heating (75 ℃), the hydrogen production efficiency is further improved, and the hydrogen production efficiency (29.16 mmol·g-1·h-1) is 26.75 times that of ZIS-Vs (1.09 mmol·g-1·h-1, Vis-NIR, RT). Further analysis shows that the increase in hydrogen production resulted from the apparent activation energy (Ea) of the catalyst decreasing from 16.7 kJ·mol-1 to 9.28 kJ·mol-1. This study provides a valuable prototype for the design of an efficient photothermal synergistic catalytic system.

Graphitic Carbon Nitride Structures on Carbon Cloth Containing Ultra- and Nano-Dispersed NiO for Photoactivated Oxygen Evolution

The development of low-cost and high-efficiency oxygen evolution reaction (OER) photoelectrocatalysts is a key requirement for H2 generation via solar-assisted water splitting. In this study, we report on an amenable fabrication route to carbon cloth-supported graphitic carbon nitride (gCN) nanoarchitectures, featuring a modular dispersion of NiO as co-catalyst. The synergistic interaction between gCN and NiO, along with the tailoring of their size and spatial distribution, yield very attractive OER performances and durability in freshwater splitting, of great significance for practical end-uses. The potential of gCN electrocatalysts containing ultra-dispersed, i. e. "quasi-atomic" NiO, exhibiting a higher activity than the ones containing nickel oxide nanoaggregates, is further highlighted by their activity even in real seawater. This work suggests that efficient OER catalysts can be designed through the construction of optimized interfaces between transition metal oxides and carbon nitride, yielding inexpensive and promising noble metal-free systems for real-world applications.

Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Development of Efficient and Recyclable ZnO-CuO/g-C3N4 Nanocomposite for Enhanced Adsorption of Arsenic from Wastewater

Arsenic (III) is a toxic contaminant in water bodies, especially in drinking water reservoirs, and it is a great challenge to remove it from wastewater. For the successful extraction of arsenic (III), a nanocomposite material (ZnO-CuO/g-C₃N₄) has been synthesized by using the solution method. The large surface area and plenty of hydroxyl groups on the nanocomposite surface offer an ideal platform for the adsorption of arsenic (III) from water. Specifically, the reduction process involves a transformation from arsenic (III) to arsenic (V), which is favorable for the attachment to the -OH group. The modified surface and purity of the nanocomposite were characterized by SEM, EDX, XRD, FT-IR, HRTEM, and BET models. Furthermore, the impact of various aspects (temperatures, pH of the medium, the concentration of adsorbing materials) on adsorption capacity has been studied. The prepared sample displays the maximum adsorption capacity of arsenic (III) to be 98% at pH ~ 3 of the medium. Notably, the adsorption mechanism of arsenic species on the surface of ZnO-CuO/g-C₃N₄ nanocomposite at different pH values was explained by surface complexation and structural variations. Moreover, the recycling experiment and reusability of the adsorbent indicate that a synthesized nanocomposite has much better adsorption efficiency than other adsorbents. It is concluded that the ZnO-CuO/g-C₃N₄ nanocomposite can be a potential candidate for the enhanced removal of arsenic from water reservoirs.

Nickel clusters accelerating hierarchical zinc indium sulfide nanoflowers for unprecedented visible-light hydrogen production

As a typical two-dimensional (2D) metal chalcogenides and visible-light responsive semiconductor, zinc indium sulfide (ZnIn₂S₄) has attracted much attention in photocatalysis. However, the high recombination rate of photogenerated electrons and holes seriously limits its performance for hydrogen production. In this work, we report in-situ photodeposition of Ni clusters in hierarchical ZnIn₂S₄ nanoflowers (Ni/ZnIn₂S₄) to achieve unprecedented photocatalytic hydrogen production. The Ni clusters not only provide plenty of active sites for reactions as evidenced by in-situ photoluminescence measurement, but also effectively accelerate the separation and migration of the photogenerated electrons and holes in ZnIn₂S₄. Consequently, the Ni/ZnIn₂S₄ composites exhibit good stability and reusability with highly enhanced visible-light hydrogen production. In particular, the best Ni/ZnIn₂S₄ photocatalyst exhibits an unprecedented hydrogen production rate of 22.2 mmol·h^-1·g^-1, 10.6 times that of the pure ZnIn₂S₄ (2.1 mmol·h^-1·g^-1). And its apparent quantum yield (AQY) is as high as 56.14% under 450 nm monochromatic light. Our work here suggests that depositing non-precious Ni clusters in ZnIn₂S₄ is quite promising for the potential practical photocatalysis in solar energy conversion.

Electronegativity Assisted Synthesis of Magnetically Recyclable Ni/NiO/g-C3N4 for Significant Boosting H2 Evolution

A magnetically recyclable Ni/NiO/g-C₃N₄ photocatalyst with significantly enhanced H2 evolution efficiency was successfully synthesized by a simple ethanol-solvothermal treatment. The presence of electronegative g-C₃N₄ is found to be the key factor for Ni⁰ formation in ternary Ni/NiO/g-C₃N₄, which provides anchoring sites for Ni²⁺ absorption and assembling sites for Ni⁰ nanoparticle formation. The metallic Ni⁰, on one side, could act as an electron acceptor enhancing carrier separation and transfer efficiency, and on the other side, it could act as active sites for H₂ evolution. The NiO forms a p–n heterojunction with g-C₃N₄, which also promotes carrier separation and transfer efficiency. The strong magnetic property of Ni/NiO/g-C₃N₄ allows a good recyclability of catalyst from aqueous solution. The optimal Ni/NiO/g-C₃N₄ showed a full-spectrum efficiency of 2310 μmol·h⁻¹·g⁻¹ for hydrogen evolution, which is 210 times higher than that of pure g-C₃N₄. This ethanol solvothermal strategy provides a facile and low-cost synthesis of metal/metal oxide/g-C₃N₄ for large-scale application.

Bridge engineering in photocatalysis and photoelectrocatalysis

Solar driven photocatalysis and photoelectrocatalysis have emerged as promising strategies for clean, low-cost, and environmental-friendly production of renewable energy and removal of pollutants. There are three crucial steps for the photocatalytic and photoelectrochemical (PEC) processes: light absorption, charge separation and transportation, and surface catalytic reactions. While significant achievement has been made in developing multiple-component photocatalysts to optimize the three steps for improved solar-to-chemical energy conversion efficiency, it remains challenging when weak interfacial contact between components/particles hinders charge transfer, restricts electron-hole separation and lowers the structural stability of catalysts. Moreover, owing to the mismatch of energy bands, an undesirable charge transfer direction leads to an adverse consequence. To tackle these challenges, bridges are implemented to smoothen the interfacial charge transfer, improve the stability of catalysts, mediate the charge transfer directions and improve the photocatalytic/PEC performance. In this review, we present the advances in bridge engineering in photocatalytic/PEC systems. Starting with the definition and classifications of bridges, we summarize the architectures of the reported bridged photocatalysts. Then we systematically discuss the insight into the roles and fundamental mechanisms of bridges in various photocatalytic/PEC systems and their contributions to activity enhancement in various reactions. Finally, the challenges and perspectives of bridged photocatalysts are featured.

Rationally Designed Copper-Modified Polymeric Carbon Nitride as a Photocathode for Solar Water Splitting

Polymeric carbon nitride has been considered to be an active photocathode for catalyzing the generation of H₂ through water splitting. However, the application of this material in photoelectrochemical cells remains a challenge owing to the intrinsically sluggish kinetics of charge separation. Herein, a facile salt-melt method is developed for fabricating Cu-modified polymeric carbon nitride as an effective photocathode material for solar water splitting. Various characterization data confirm that Cu-modified polymeric carbon nitride contains both free CuCl, derived from precursors, and coordinated Cu species incorporated into the polymeric carbon nitride, which can generate type-II heterojunctions. This special heterojunction energy structure contributes to a significantly enhanced photocurrent density for hydrogen evolution. The proposed strategy for synthesizing the Cu-modified polymeric carbon nitride can stimulate research for the development of highly efficient visible-light-active photocathodes.

Construction of functional nanonetwork-structured carbon nitride with Au nanoparticle yolks for highly efficient photocatalytic applications

We report a novel and versatile fabrication strategy for functional nanonetwork-structured carbon nitride with Au nanoparticle yolks (FNNS-C3N4-Au) based on hairy poly(acrylic acid)-grafted SiO2 nanospheres (Au@SiO2-g-PAA). Benefiting from the three-dimensional nanonetwork structure and well-distributed Au nanoparticles, the as-prepared nanocomposites demonstrated excellent photocatalysis performances (degradation rate constant: 1.8 × 10-2 min-1).

Surface engineering of graphitic carbon nitride polymers with cocatalysts for photocatalytic overall water splitting

Overall water splitting for the stoichiometric generation of H₂ and O₂ has been achieved by rational cocatalyst modification of g-C₃N₄ polymers to modulate the surface redox reaction kinetics.

Graphitic carbon nitride based polymers, being metal-free, accessible, environmentally benign and sustainable, have been widely investigated for artificial photosynthesis in recent years for the photocatalytic splitting of water to produce hydrogen fuel. However, the photocatalytic stoichiometric splitting of pure water into H₂ and O₂ with a molecular ratio of 2 : 1 is far from easy, and is usually hindered by the huge activation energy barrier and sluggish surface redox reaction kinetics. Herein, we provide a concise overview of cocatalyst modified graphitic carbon nitride based photocatalysts, with our main focus on the modulation of the water splitting redox reaction kinetics. We believe that a timely and concise review on this promising but challenging research topic will certainly be beneficial for general readers and researchers in order to better understand the property–activity relationship towards overall water splitting, which could also trigger the development of new organic architectures for photocatalytic overall water splitting through the rational control of surface chemistry.

Constructing Multifunctional Metallic Ni Interface Layers in the g-C3N4 Nanosheets/Amorphous NiS Heterojunctions for Efficient Photocatalytic H2 Generation

The construction of exceptionally robust and high-quality semiconductor-cocatalyst heterojunctions remains a grand challenge toward highly efficient and durable solar-to-fuel conversion. Herein, novel graphitic carbon nitride (g-C₃N₄) nanosheets decorated with multifunctional metallic Ni interface layers and amorphous NiS cocatalysts were fabricated via a facile three-step process: the loading of Ni(OH)₂ nanosheets, high-temperature H₂ reduction, and further deposition of amorphous NiS nanosheets. The results demonstrated that both robust metallic Ni interface layers and amorphous NiS can be utilized as electron cocatalysts to markedly boost the visible-light H₂ evolution over g-C₃N₄ semiconductor. The optimized g-C₃N₄-based photocatalyst containing 0.5 wt % Ni and 1.0 wt % NiS presented the highest hydrogen evolution of 515 μmol g^-1 h^-1, which was about 2.8 and 4.6 times as much as those obtained on binary g-C₃N₄-1.0%NiS and g-C₃N₄-0.5%Ni, respectively. Apparently, the metallic Ni interface layers play multifunctional roles in enhancing the visible-light H₂ evolution, which could first collect the photogenerated electrons from g-C₃N₄, and then accelerate the surface H₂-evolution reaction kinetics over amorphous NiS cocatalysts. More interestingly, the synergetic effects of metallic Ni and amorphous NiS dual-layer electron cocatalysts could also improve the TEOA-oxidation capacity through upshifting the VB levels of g-C₃N₄. Comparatively speaking, the multifunctional metallic Ni layers are dominantly favorable for separating and transferring photoexcited charge carriers from g-C₃N₄ to amorphous NiS cocatalysts owing to the formation of Schottky junctions, whereas the amorphous NiS nanosheets are mainly advantageous for decreasing the thermodynamic overpotentials for surface H₂-evolution reactions. It is hoped that the implantation of multifunctional metallic interface layers can provide a versatile approach to enhance the photocatalytic H₂ generation over different semiconductor-cocatalyst heterojunctions.

Markedly enhanced visible-light photocatalytic H2 generation over g-C3N4 nanosheets decorated by robust nickel phosphide (Ni12P5) cocatalysts

Ni₁₂P₅ modified g-C₃N₄ photocatalysts exhibit significantly enhanced photocatalytic H₂-evolution activities under visible light irradiation.

Synthesis of 3D porous MoS2/g-C3N4 heterojunction as a high efficiency photocatalyst for boosting H2 evolution activity

A novel strategy was applied for the preparation of MoS₂/graphitic carbon nitride (g-C₃N₄) with porous morphology.

Novel composites of graphitic carbon nitride and NiO nanosheet arrays as effective photocathodes with enhanced photocurrent performances

Novel composites composed of graphitic carbon nitride (GCN) and p-type semiconductor of NiO nanosheet arrays were fabricated for the first time and demonstrated to be efficient photocathodes for enhancing charge separation and hole transfer.

Electron acceptor of Ni decorated porous carbon nitride applied in photocatalytic hydrogen production

Non-noble metal Ni as an electron acceptor can effectively improve photocatalytic hydrogen production.

Surface and interface design in cocatalysts for photocatalytic water splitting and CO2reduction

This review outlines the recent progress on designing the surface and interface of cocatalysts to create highly efficient photocatalysts for water splitting and CO₂reduction.

Nickel as a co-catalyst for photocatalytic hydrogen evolution on graphitic-carbon nitride (sg-CN): what is the nature of the active species?

Structural changes of a nickel co-catalyst on graphitic carbon nitride have been uncovered during photocatalytic proton reduction by using XPS and in situ EPR measurements.

An overview of the structural, textural and morphological modulations of g-C3N4 towards photocatalytic hydrogen production

This study highlights the recent trends in the structural, textural and morphological variations of g-C₃N₄ for visible-light-induced hydrogen evolution.

Efficient visible-light photocatalytic H2 evolution over metal-free g-C3N4 co-modified with robust acetylene black and Ni(OH)2 as dual co-catalysts

Enhanced photocatalytic visible-light H₂-evolution activity over metal-free g-C₃N₄ co-modified with acetylene black and Ni(OH)₂ co-catalysts is reported.

Recent progress in g-C3N4 based low cost photocatalytic system: activity enhancement and emerging applications

Noble metal free g-C₃N₄ based photocatalysts find promising applications in the fields of photocatalytic H₂ production, overall water splitting and CO₂ reduction. Their photocatalytic can be enhanced by depositing non-noble metal co-catalysts and exfoliation to nanosheets.