Hot-electron-assisted S-scheme heterojunction of tungsten oxide/graphitic carbon nitride for broad-spectrum photocatalytic H2 generation

Advances and challenges in the modification of photoelectrode materials for photoelectrocatalytic water splitting

With the increasing consumption of fossil fuels, the development of clean and renewable alternative fuels has become a top priority. Hydrogen (H2) is an ideal primary clean energy source for its extremely high gravimetric energy density, carbon-free combustion, and abundant natural resources. Photoelectrocatalytic (PEC) water splitting is among the most promising approaches for converting sunlight and water into H2. However, the cost-effectiveness and the overall solar to hydrogen conversion efficiency of PEC water splitting are still big challenges. In the past few decades, several studies have been devoted to this technology, and it is essential to develop economical photoelectrocatalysts with high conversion efficiency. Therefore, there is an urgent need for a comprehensive and updated review of recent advances in the design, manufacture, and modification of efficient PEC water splitting systems. This review first starts with the basic mechanism of photoelectrochemical water splitting. Then the problems in PEC water splitting are discussed, and the methods of photoelectrode modulation such as nanostructure fabrication, doping engineering, surface modification, and heterojunction construction are introduced. Finally, the critical challenges and future trends/perspectives in the PEC water splitting are discussed.

Sugars induced exfoliation of porous graphitic carbon nitride for efficient hydrogen evolution in photocatalytic water-splitting reaction

Photocatalytic hydrogen evolution holds great promise for addressing critical energy and environmental challenges, making it an important area in scientific research. One of the most popular photocatalysts is graphitic carbon nitride (gCN), which has emerged as a noteworthy candidate for hydrogen generation through water splitting. However, ongoing research aims to enhance its properties for practical applications. Herein, we introduce a green approach for the fabrication of porous few-layered gCN with surface modifications (such as oxygen doping, carbon deposition, nitrogen defects) with promoted performance in the hydrogen evolution reaction. The fabrication process involves a one-step solvothermal treatment of bulk graphitic carbon nitride (bulk-gCN) in the presence of different sugars (glucose, sucrose, and fructose). Interestingly, the conducted time-dependent process revealed that porous gCN exfoliated in the presence of fructose at 180 °C for 6 h (fructose_6h) exhibits a remarkable 13-fold promotion of photocatalytic hydrogen evolution compared to bulk-gCN. The studied materials were extensively characterized by microscopic and spectroscopic techniques, allowing us to propose a reaction mechanism for hydrogen evolution during water-splitting over fructose_6h. Furthermore, the study highlights the potential of employing a facile and environmentally friendly fructose-assisted solvothermal process to improve the efficiency and stability of catalysts based on graphitic carbon nitride.

V-doped Ni2P nanoparticle grafted g-C3N4 nanosheets for enhanced photocatalytic hydrogen evolution performance under visible light

Exploring low-cost and highly active photocatalysts with noble metal-free cocatalysts is of great significance for photocatalytic hydrogen evolution under simulated sunlight irradiation. In this work, a novel V-doped Ni₂P nanoparticle loaded g-C₃N₄ nanosheet is reported as a highly efficient photocatalyst for H₂ evolution under visible light irradiation. The results demonstrate that the optimized 7.8 wt% V-Ni₂P/g-C₃N₄ photocatalyst exhibits a high hydrogen evolution rate of 271.5 μmol g^-1 h^-1, which is comparable to that of the 1 wt% Pt/g-C₃N₄ photocatalyst (279 μmol g^-1 h^-1), and shows favorable hydrogen evolution stability for five successive runs within 20 h. The remarkable photocatalytic hydrogen evolution performance of V-Ni₂P/g-C₃N₄ is mainly due to the enhanced visible light absorption ability, the facilitated separation of photo-generated electron-hole pairs, the prolonged lifetime of photo-generated carriers and the fast transmission ability of electrons.

Enhanced plasmonic photocatalytic performance of C3N4/Cu by the introduction of a reduced graphene oxide interlayer

Cu nanoparticles (NPs) are low-cost surface plasmonic resonance (SPR) metal nanostructures, and their SPR properties can be used to enhance the photocatalytic hydrogen evolution performance of carbon nitride (C₃N₄). But their actual performance is usually limited, and one key factor is their poor interfacial quality. In this work, a highly conductive reduced graphene oxide (RGO) interlayer is introduced between protonated C₃N₄ (PCN) nanosheets and Cu NPs, which can act as an efficient sink for photogenerated electrons from C₃N₄ and hot electrons from Cu NPs, and simultaneously serve as reaction sites for the hydrogen evolution reaction, and accelerate the charge transport by the formed C-O-C and C-O-Cu bonds. The optimal hydrogen evolution rate of the optimized PCN/RGO/Cu is 1.30 mmol g^-1 h^-1, which is 6.76, 2.47 and 2.41 times that of PCN, PCN/RGO and PCN/Cu, respectively, and it can further reach up to 13.22 mmol g^-1 h^-1 by loading moderate Pt NPs. Meanwhile, the introduced RGO can effectively anchor Cu NPs to enhance the stability of the photocatalyst. In addition, due to the broad SPR response of Cu NPs, a near-infrared photocatalytic performance is realized for PCN/RGO/Cu with an apparent quantum efficiency of 0.46% at 765 nm.

Surface anchoring of nickel sulfide clusters as active sites and cocatalysts for photocatalytic antibiotic degradation and bacterial inactivation

Achieving photocatalytic antibiotic degradation and bacterial inactivation with high efficiency remains a challenging mission to originate a clean environment. In this work, ultra-small NiS clusters were in-situ grew on photoactive ZnIn₂S₄ nanoflower supports to form a NiS/ZnIn₂S₄ heterojunction, in which a strong and surface-limited binding was formed between the NiS clusters and ZnIn₂S₄ supports. The in-situ formed NiS clusters not only appeased interfacial charge transfer resistance of the heterojunction but also eventuated a strong built-in electric field, resulting a fast electron migration from ZnIn₂S₄ to NiS clusters functioned as cocatalyst and active sites to boost the separation efficiency of photogenerated carriers. As a result, the optimal 2NiS/ZnIn₂S₄ heterojunction expressed a higher photocatalytic Escherichia inactivation activity (99.23 % for 3 h) and a raised antibiotic degradation performance, including tetracycline (60 % for 20 min), ofloxacin (62 % for 20 min), oxytetracycline (63 % for 20 min) compared to that of pure ZnIn₂S₄ (39.14 % for Escherichia inactivation and 44 % for tetracycline degradation). This work furnishes a great promise to develop inorganic clusters coupled photocatalysts for light-driven environmental application.

Engineering S-scheme CuO/ZnO heterojunctions sonochemically for eradicating RhB dye from wastewater under solar radiation

In this research, S-scheme heterojunctions composed of different concentrations of CuO and ZnO nanoparticles are fabricated for eradicating rhodamine B dye under solar radiation. ZnO nanoparticles are designed through a facile sol-gel route employing Triton X-100. Spherical CuO nanoparticles of 15.2 nm and 1.5 eV band gap energy are deposited on ZnO nanoparticles in an ultrasonic bath of 300 W intensity. The physicochemical performance of the photocatalyst is explored by HRTEM, SAED, BET, XRD, DRS and PL. The in situ homogeneous growth of spherical CuO nanoparticles on ZnO active centers shifts the photocatalytic response to the deep visible region and enhances the efficiency of charge carrier separation and transportation. Among all heterojunctions, ZnCu10 containing 10 wt% CuO displays the best photocatalytic rate for expelling 93% of RhB dye within 240 min, which is twenty-fold higher than that of pristine ZnO and CuO. Reactive oxygen species are the predominant species in degrading the dye pollutant on the heterojunction surface, as shown from scrubber trapping experiments and PL spectrum of terephthalic acid. Coupling ZnO as an oxidative photocatalyst and CuO as a reductive photocatalyst generates an efficient S-scheme heterojunction with strong redox power in destructing various organic pollutants.

Effect of Functional Group Modifications on the Photocatalytic Performance of g-C3 N4

In recent years, photocatalysis has received increasing attention in alleviating energy scarcity and environmental treatment, and graphite carbon nitride (g-C₃ N₄ ) is used as an ideal photocatalyst. However, it still remains numerous challenges to obtain the desirable photocatalytic performance of intrinsic g-C₃ N₄ . Functional group functionalization, formed by introducing functional groups into the bulk structure, is one of the common modification techniques to modulate the carrier dynamics and increases the number of active sites, offering new opportunities to break the limits for structure-to-performance relationship of g-C₃ N₄ . Nevertheless, the general overview of the advance of functional group modification of g-C₃ N₄ is less reported yet. In order to better understand the structure-to-performance relationship at the molecular level, a review of the latest development of functional group modification is urgently needed. In this review, the functional group modification of g-C₃ N₄ in terms of structures, properties, and photocatalytic activity is mainly focused, as well as their mechanism of reaction from the molecular level insights is explained. Second, the recent progress of the application of introducing functional groups in g-C₃ N₄ is introduced and examples are given. Finally, the difficulties and challenges are presented, and based on this, an outlook on the future research development direction is shown.

An efficient and multifunctional S-scheme heterojunction photocatalyst constructed by tungsten oxide and graphitic carbon nitride: Design and mechanism study

The design of multifunctional photocatalyst with strong redox performance is the key to achieve sustainable utilization of solar energy. In this study, an elegant S-scheme heterojunction photocatalyst was constructed between metal-free graphitic carbon nitride (g-C₃N₄) and noble-metal-free tungsten oxide (W₁₈O₄₉). As-established S-scheme heterojunction photocatalyst enabled multifunctional photocatalysis behavior, including hydrogen production, degradation (Rhodamine B) and bactericidal (Escherichia coli) properties, which represented extraordinary sustainability. Finite-difference time-domain (FDTD) simulations manifested that the integration of double-layer hollow g-C₃N₄ nanotubes with W₁₈O₄₉ nanowires could expand the light harvesting ability. Demonstrated by density functional theory (DFT) calculations and electron spin resonance (ESR) measurements, the S-scheme heterojunction not only promoted the separation of carriers, but also improved the redox ability of the catalyst. This work provides a theoretical basis for enhancing the photocatalytic performances and broadening the application field of photocatalysis.

Ternary nanocomposites of CdS/WO3/g-C3N4 for hydrogen production

The sustainable rise in global warming and the consumption of fossil fuels considerably contribute to energy and environmental issues. To address these issues, semiconductor heterostructures can be used to generate clean energy sources as alternative energy sources and to reduce environmental impacts. Herein, we report the synthesis of a ternary semiconductor of the CdS/WO₃/g-C₃N₄ (i.e. C-CNW) nanostructured composite for hydrogen production and dye degradation under visible-light irradiation. The structural properties of the prepared materials were studied using a series of investigational analyses. The 3C-CNW nanostructure photocatalyst exhibited faster malachite green (MG) dye photodegradation within 105 min and the highest hydrogen production rate is 868.23 μmol g^-1 h^-1 under visible light illumination. Moreover, the photocatalytic hydrogen production of the 3C-CNW nanostructure photocatalyst with various scavengers was analyzed. Its higher photocatalytic activity is ascribed to the Z-scheme mechanism, which induces rapid diffusion of photoinduced charges within the ternary photocatalyst with its optical bandgap. This proposed strategy is useful to improve photocatalysts that play a role in mitigating energy and environmental issues.

Room-Temperature Optoelectronic Gas Sensor Based on Core-Shell g-C3N4@WO3 Heterocomposites for Efficient Ammonia Detection

The ever-growing modern industry promotes the evolution of gas sensors for environmental monitoring and safety inspection. However, traditional chemiresistive gas sensors still suffer from drawbacks of high power consumption and detection limit, mainly due to the insufficient charge-transfer ability of gas-sensing materials. Here, an optoelectronic gas sensor that can detect ppb-level ammonia at room temperature is constructed based on core-shell g-C3N4@WO3 heterocomposites. The growth of WO3 nanosheets on graphitic g-C3N4 nanosheets was precisely controlled, achieving well-defined g-C3N4@WO3 core-shell architectures. Based on the synergism between light activation and the amplification effect of in situ-formed heterojunctions, the g-C3N4@WO3 sensor exhibits improved sensing characteristics for reliable ammonia detection. As compared with the pristine g-C3N4 sensor, the sensor response toward ammonia is enhanced 21 times and the detection limit is reduced from 308 to 108 ppb. This work provides a successful approach for the in situ formation of core-shell g-C3N4@WO3 interfacial composites and offers an easy solution for the rational design of advanced optoelectronic gas sensors.

Functionalized MOF-Based Photocatalysts for CO2 Reduction

Metal-organic frameworks (MOFs) materials have become a research forefront in the field of photocatalytic CO₂ reduction attributed to their ultra-high specific surface area, adjustable structure, and abundant catalytic active sites. Particularly, MOFs can be facilely tuned to match CO₂ photoreduction by utilizing post-modification of metal nodes, functionalization of organic linkers, and combination with other active materials. Herein, the recent advances in the construction strategy of MOF-based photocatalysts materials for CO₂ reduction are highlighted. Some systematic modification strategies on MOF-based photocatalysts are also discussed, such as modification of metal sites and organic ligands, construction of heterojunction, introduction of single/dual-atom, and strain engineering. Finally, the future development directions of MOF-based photocatalysts in the field of CO₂ reduction are presented.

Robust S-scheme hierarchical Au-ZnIn2S4/NaTaO3: Facile synthesis, superior photocatalytic H2 production and its charge transfer mechanism

Designing Step-scheme (S-scheme) heterojunction with the directional charge transfer pathway has been considered as a promising strategy for realizing effective spatial separation of photo-generated carriers in a photocatalytic system by utilizing broadband solar energy. Herein, the novel and ternary S-scheme heterojunction photocatalysts were fabricated by embedding Au nanoparticles (NPs) on the surface of ZnIn₂S₄/NaTaO₃ composites through a facile two-step hydrothermal method for the first time. As expected, it showed an enhanced hydrogen evolution rate of 11404 μmol g^-1 h^-1, which was approximately 58 and 10 times higher than that of the pristine NaTaO₃ nanocubes (197 μmol g^-1 h^-1) and ZnIn₂S₄ microspheres (1180 μmol g^-1 h^-1) under simulated sunlight irradiation, respectively. An intimate heterojunction interface as well as Au nanoparticles as electron reservoir and reactive sites, which enhanced light absorption capacity and accelerated charge carrier separation, was answerable to the huge promotion in the photocatalytic performance. Most notably, XPS, EPR analysis and density functional theory (DFT) calculation results, revealed that the presence of strong interfacial electric fields promoted superior separation efficiency in the Au-ZnIn₂S₄/NaTaO₃ S-scheme heterojunction. This innovative work may shed light on a more appealing and meaningful approach to modify sodium tantalate for the promising application in photocatalytic hydrogen generation.

Emerging S-Scheme Photocatalyst

Photocatalysis is a green technology to use ubiquitous and intermittent sunlight. The emerging S-scheme heterojunction has demonstrated its superiority in photocatalysis. This article covers the state-of-the-art progress and provides new insights into its general designing criteria. It starts with the challenges confronted by single photocatalyst from the perspective of energy dissipation by borrowing the common behaviors in the dye molecule. Subsequently, other problems faced by single photocatalyst are summarized. Then a viable solution for these problems is the construction of heterojunctions. To overcome the problems and mistakes of type-II and Z-scheme heterojunctions, S-scheme heterojunction is proposed and the underlying reaction mechanism is summarized. Afterward, the design principles for S-scheme heterojunction are proposed and four types of S-scheme heterojunctions are suggested. Following this, direct characterization techniques for testifying the charge transfer in S-scheme heterojunction are presented. Finally, different photocatalytic applications of S-scheme heterojunctions are summarized. Specifically, this work endeavors to clarify the critical understanding on curved Fermi level in S-scheme heterojunction interface, which can help strengthen and advance the fundamental theories of photocatalysis. Moreover, the current challenges and prospects of the S-scheme heterojunction photocatalyst are critically discussed.

Dual plasmonic Au and TiN cocatalysts to boost photocatalytic hydrogen evolution

Employing a suitable cocatalyst is very important to improve photocatalytic H2 evolution activity. Herein, two plasmonic cocatalysts, Au nanoparticles and TiN nanoparticles were in-situ coupled over the g-C3N4 nanotube to form a ternary 0D/0D/1D Au/TiN/g-C3N4 composite via a successive thermal polycondensation and chemical reduction method. The g-C3N4 nanotube acted as a support for the growth of Au and TiN nanoparticles, leading to intimate contact between g-C3N4 nanotube with Au nanoparticles and TiN nanoparticles. As a result, multiple interfaces and dual-junctions of Au/g-C3N4 Schottky-junction and TiN/g-C3N4 ohmic-junction were constructed, which helped to promote the charged carriers' separation and enhanced the photocatalytic performance. Furthermore, loading plasmonic cocatalysts of Au nanoparticles and TiN nanoparticles can enhance the light absorption capacity. Consequently, the Au/TiN/g-C3N4 composite exhibited significantly enhanced photocatalytic H2 evolution activity (596 μmol g-1 h-1) compared to g-C3N4 or binary composites of Au/g-C3N4 and TiN/g-C3N4. This work highlights the significant role of cocatalysts in photocatalysis.

Construction of S-scheme 1D/2D rod-like g-C3N4/V2O5 heterostructure with enhanced sonophotocatalytic degradation for Tetracycline antibiotics

Pharmaceutically active compounds are an emerging water contaminant that resists conventional wastewater treatments. Herein, the sonophotocatalytic degradation of Tetracycline (TC) antibiotics as a model contaminant was carried out over a rod-like g-C₃N₄/V₂O₅ (RCN-VO) nanocomposite. RCN-VO nanocomposite was synthesized via ultrasound-assisted thermal polycondensation method. The results showed that the RCN-VO nanocomposite could completely remove the TC in water within 60 min under simultaneous irradiation of visible light and ultrasound. Moreover, the sonophotocatalytic TC degradation (a synergy index of ∼1.5) was superior to the sum of individual sonocatalytic and photocatalytic degradation using RCN-VO nanocomposite. Besides, the enhanced sonophotocatalytic activity of RCN-VO can be attributed to the 1D/2D nanostructure and the S-scheme heterojunction formation between RCN and VO where the electrons migrated from RCN to VO across the RCN-VO interface. Under irradiation, the built-in electric field, band edge bending and Coulomb interaction can synergistically facilitate the unavailing electron-hole pair recombination. Thereby, the cumulative electron in RCN and holes in VO can actively take part in the redox reaction which generates free radicals and attack the TC molecules. This study provides insight into a novel S-Scheme heterojunction photocatalyst for the removal of various refractory contaminants via sonophotocatalytic degradation.

Construction of UiO-66/Bi4O5Br2 Type-II Heterojunction to Boost Charge Transfer for Promoting Photocatalytic CO2 Reduction Performance

One of the basic challenges of CO₂ photoreduction is to develop efficient photocatalysts, and the construction of heterostructure photocatalysts with intimate interfaces is an effective strategy to enhance interfacial charge transfer for realizing high photocatalytic activity. Herein, a novel UiO-66/Bi₄O₅Br₂ heterostructure photocatalyst was constructed by depositing UiO-66 nanoparticles with octahedral morphology over the Bi₄O₅Br₂ nanoflowers assembled from the Bi₄O₅Br₂ nanosheets via an electrostatic self-assembly method. A tight contact interface and a built-in electric field were formed between the UiO-66 and the Bi₄O₅Br₂, which was conducive to the photo-electrons transfer from the Bi₄O₅Br₂ to the UiO-66 and the formation of a type-II heterojunction with highly efficient charge separation. As a result, the UiO-66/Bi₄O₅Br₂ exhibited improved photocatalytic CO₂ reduction performance with a CO generation rate of 8.35 μmol h⁻¹ g⁻¹ without using any sacrificial agents or noble co-catalysts. This work illustrates an applicable tactic to develop potent photocatalysts for clean energy conversion.

A novel BiVO3/SnO2 step S-scheme nano-heterojunction for an enhanced visible light photocatalytic degradation of amaranth dye and hydrogen production

The destruction of toxic pollutants and production of hydrogen gas on the surface of semiconductors under light irradiation is the main significance of photocatalysis. Heterojunctions with matching in band gap energy are urgently required for enhancing the redox power of the charge carriers. A step S-scheme BiVO₃/SnO₂ nano-heterojunction was carefully synthesized for a successful photodegradation of amaranth dye and photocatalytic hydrogen evolution. Tetragonal SnO₂ nanoparticles of 80 m² g^-1 surface area and distinct mesoporous structure were fabricated by a sol-gel route in the presence of Tween-80 as the pore structure directing agent. BiVO₃ nanoparticles were deposited homogeneously on the SnO₂ surface in an ultrasonic bath of power intensity 300 W. The photocatalytic efficiency in the destruction of amaranth dye soar with increasing BiVO₃ contents of up to 10 wt%. The hydrogen evolution rate reached 8.2 mmol g^-1 h^-1, which is eight times stronger than that of pristine SnO₂. The sonicated nanocomposites were investigated by XRD, BET, FESEM, HRTEM, EDS, DRS and PL techniques. The step S-scheme heterojunction with superior oxidative and reductive power is the primary key for the exceptional photocatalytic process. The PL of terephthalic acid and the scavenger trapping experiments reveal the charge migration through the step S-scheme mechanism rather than the type (II) heterojunction mechanism.