Solid-state Electron Mediator Research Articles

Carbon Nanotubes as a Solid-State Electron Mediator for Visible-Light-Driven Z-Scheme Overall Water Splitting.

So-called Z-scheme systems, which typically comprise an H2 evolution photocatalyst (HEP), an O2 evolution photocatalyst (OEP), and an electron mediator, represent a promising approach to solar hydrogen production via photocatalytic overall water splitting (OWS). The electron mediator transferring photogenerated charges between the HEP and OEP governs the performance of such systems. However, existing electron mediators suffer from low stability, corrosiveness to the photocatalysts, and parasitic light absorption. In the present work, carbon nanotubes (CNTs) were shown to function as an effective solid-state electron mediator in a Z-scheme OWS system. Based on the high stability and good charge transfer characteristics of CNTs, this system exhibited superior OWS performance compared with other systems using more common electron mediators. The as-constructed system evolved stoichiometric amounts of H2 and O2 at near-ambient pressure with a solar-to-hydrogen energy conversion efficiency of 0.15%. The OWS reaction was also promoted in the case that this CNT-based Z-scheme system was immobilized on a substrate. Hence, CNTs are a viable electron mediator material for large-scale Z-scheme OWS systems.

Poly Meta-Aminophenol (PmAP) as a Solid-State Electron Mediator in the Z-Scheme, Ag3PO4/CoFe2O4 Heterojunction: Mineralization of Highly Concentrated Bisphenol-A and Reactive Dyes Water Pollutants.

This study demonstrated the effectiveness of poly meta-aminophenol (PmAP) as a solid electron mediator in the Z-scheme photocatalytic system for organic pollutants (viz. bisphenol-A and reactive dyes) mineralization and also illustrated how PmAP transported the photogenerated electrons from an O2-emitting photocatalyst (Ag3PO4) to a H2-emitting photocatalyst (CoFe2O4) enabling enhanced photocatalytic activity under visible light irradiation. The PmAP/Ag3PO4-CoFe2O4 (PAC-10), was prepared by a two-step process and characterized by various analytical methods to assess the impact of PmAP on optical, photocatalytic, and electrochemical characteristics of the CoFe2O4 (CFO)/Ag3PO4 composite. The morphological investigation revealed that the PmAP sheet was nicely decorated with evenly distributed Ag3PO4 and CoFe2O4 particles. The M-S plot and impedance analyses were used to assess the electrochemical capabilities of the catalyst. Z-scheme charge transfer pathways were well supported by the radical trapping experiment and HRTEM analysis of Pt photodeposited PAC-10 photocatalysts during the photoreaction. Because of its magnetic nature and ease of synthesis, the PAC-10 offers an easily recyclable Z-scheme photocatalytic system that has the potential for purifying wastewater with high concentrations (up to 100 mg/L) of organic pollutants within 30 min of visible light exposition.

Stabilizing the S-scheme ZnO/ZnCr2O4/Bi6Cr2O15 heterojunction through the application of carbon xerogel as both a solid-state mediator and reducing agent

This study aims to evaluate the application of the carbonaceous material known as carbon xerogel as a solid-state mediator in the ZnO/ZnCr2O4/Bi6Cr2O15 heterojunction, aiming to stabilize the charge transfer pathway to an S-scheme configuration. Furthermore, the carbon xerogel was also chosen as a reducing agent for the formation of the proposed heterojunction under high temperatures, due to the reduction of the Bi6Cr2O15 into Bi0 and Cr2O3, with posterior formation of the ZnCr2O4 phase through solid-state reaction with the ZnO. The photocatalytic performance of the materials was evaluated through sulfamerazine degradation under both simulated sunlight and visible light. The X-ray diffractometry and Raman spectrometry show the formation of the ZnO, ZnCr2O4, Bi6Cr2O15, and Bi0 phases on the composite material, as expected. Furthermore, morphological modifications were observed for the zinc oxide (ZnO) after the addition of both the carbon xerogel and bismuth-based phases, resulting in a significant enhancement of specific surface area and pore volume of the materials. Photocatalysis-wise, the composite material developed obtained the highest efficiency among the materials evaluated, as the stabilization of the S-scheme heterojunction between ZnO, ZnCr2O4, and Bi6Cr2O15 by the carbon xerogel as a solid-state electron mediator and the plasmonic resonance effect provided by the metallic bismuth particles functioned towards an increase in the photocurrent generation capacity and charge carrier lifetime during photonic excitation. The hydroxyl radical was defined as the major species involved in the degradation of the sulfamerazine, indicating the successful stabilization of the S-scheme charge transport pathway between the proposed components of the photocatalyst.

NH4Cl-assisted synthesis of TaON nanoparticle applied to photocatalytic hydrogen and oxygen evolution from water

Oxynitride semiconductors are promising photocatalyst materials for visible light-driven water splitting, while the synthesis of well crystalized oxynitride still remains challenge. In present work, narrow-bandgap TaON nanoparticles are synthesized via heating a vacuum-sealed mixture of KTaO3, Ta and NH4Cl. This method possesses multiple advantages in terms of lower calcination parameter, higher N conversion efficiency and superior photocatalytic activity in comparison with the traditional thermal ammonolysis using NH3 gas as a nitrogen source. Through the analysis of intermediates produced upon the elevation of heating temperature, a gas–solid-phase reaction between TaCl5 and Ta2O5 is demonstrated as the final step, which is conducive to decreasing thermal energy barrier and accelerating nitridation process. Precise control of preparation conditions, including calcination temperature and duration, allows for the regulation of surface O/N ratio of TaON particles to unity, resulting in optimized photocatalytic activity. Photoelectrochemical assessment and intensity modulated photocurrent spectroscopy provide convincing evidence for improved charge transfer efficiency of photoexcited holes at TaON surface. A Z-scheme overall water splitting is accomplished by employing the TaON as an effective oxygen evolution photocatalyst, SrTiO3:Rh as a hydrogen evolution photocatalyst, and reduced graphene oxide (rGO) as a solid-state electron mediator. This work presents a promising strategy for the synthesis of high-quality oxynitride materials in application to photocatalytic water splitting.

Efficient and stable visible-light-driven Z-scheme overall water splitting using an oxysulfide H2 evolution photocatalyst

So-called Z-scheme systems permit overall water splitting using narrow-bandgap photocatalysts. To boost the performance of such systems, it is necessary to enhance the intrinsic activities of the hydrogen evolution photocatalyst and oxygen evolution photocatalyst, promote electron transfer from the oxygen evolution photocatalyst to the hydrogen evolution photocatalyst, and suppress back reactions. The present work develop a high-performance oxysulfide photocatalyst, Sm2Ti2O5S2, as an hydrogen evolution photocatalyst for use in a Z-scheme overall water splitting system in combination with BiVO4 as the oxygen evolution photocatalyst and reduced graphene oxide as the solid-state electron mediator. After surface modifications of the photocatalysts to promote charge separation and redox reactions, this system is able to split water into hydrogen and oxygen for more than 100 hours with a solar-to-hydrogen energy conversion efficiency of 0.22%. In contrast to many existing photocatalytic systems, the water splitting activity of the present system is only minimally reduced by increasing the background pressure to 90 kPa. These results suggest characteristics suitable for applications under practical operating conditions.

Augmented photocatalysis induced by 1T-MoS2 bridged 2D/2D MgIn2S4@1T/2H-MoS2 Z-scheme heterojunction: mechanistic insights into H2O2 and H2 evolution.

In the realm of composite photocatalysts, the fusion of the co-catalyst effect with interfacial engineering is recognized as a potent strategy for facilitating the segregation and migration of photo-induced charge carriers. Herein, an innovative mediator-based Z-scheme hybrid, i.e. MIS@1T/2H-MoS2, has been well designed by pairing MIS with 1T/2H-MoS2via a facile hydrothermal strategy as a competent photocatalyst for H2O2 and H2 generation. The co-catalyst, i.e. metallic 1T-phase bridging between semiconducting 2H-MoS2 and MIS, serves as a solid state electron mediator in the heterostructure. Morphological findings revealed the growth of 1T/2H-MoS2 nanoflowers over MIS microflowers, verifying the close interaction between MIS and 1T/2H-MoS2. By virtue of accelerated e-/h+ pair separation and migration efficiency along with a proliferated density of active sites, the MMoS2-30 photocatalyst yields an optimum H2O2 of 35 μmol h-1 and H2 of 370 μmol h-1 (ACE of 5.9%), which is 3 and 2.7 fold higher than pristine MIS. This obvious enhancement can be attributed to photoluminescence and electrochemical aspects that substantiate the diminished charge transfer resistance along with improved charge carrier separation, representing a good example of a noble metal-free photocatalyst. The proposed Z-scheme charge transfer mechanism is aided by time-resolved photoluminescence (TRPL), XPS, radical trapping experiments, and EPR analysis. Overall, this endeavour provides advanced insights into the architecture of noble metal-free Z-scheme heterostructures, offering promising prospects in photocatalytic applications.

Modification of an oxyhalide solid-solution photocatalyst with an efficient O2-evolving cocatalyst and electron mediator for two-step photoexcitation overall water splitting.

Two-step photoexcitation overall water splitting based on particulate photocatalysts represents a promising approach for low-cost solar hydrogen production. The performance of an O2-evolution photocatalyst and electron mediator between two photocatalysts crucially influences the construction of an efficient two-step excitation water-splitting system. Bismuth-tantalum oxyhalides are emerging photocatalysts for O2 evolution reactions and can be applied in two-step water-splitting systems. In this study, a highly crystalline Bi4TaO8Cl0.9Br0.1 solid solution with microplatelet morphology was synthesized by the dual flux method. The light absorption intensity and charge transfer efficiency of the Bi4TaO8Cl0.9Br0.1 solid solution were higher than those of Bi4TaO8Cl and Bi4TaO8Br; thus, the sacrificial O2 evolution activity of Bi4TaO8Cl0.9Br0.1 photocatalyst was obviously enhanced. The two-step excitation water splitting with a solid-state electron mediator was successfully constructed using Bi4TaO8Cl0.9Br0.1 as the O2-evolution photocatalyst and Ru/SrTiO3:Rh as the H2-evolution photocatalyst. The CoOx cocatalyst and reduced graphene oxide decorations on the surface of Bi4TaO8Cl0.9Br0.1 promoted the catalytic O2 generation process on Bi4TaO8Cl0.9Br0.1 and electron transfer between CoOx/Bi4TaO8Cl0.9Br0.1 and Ru/SrTiO3:Rh photocatalysts, respectively. As a result, the apparent quantum yield for this overall water-splitting system was 1.26% at 420 nm, which surpassed the present performance of the two-step excitation water-splitting systems consisting of metal oxyhalide photocatalysts. This study demonstrates the validity of high-quality solid-solution photocatalysts with suitable surface modification for efficient solar hydrogen production from water splitting.

Solar-based photocatalytic ozonation employing novel S-scheme ZnO/Cu2O/CuO/carbon xerogel photocatalyst: effect of pH, salinity, turbidity, and temperature on salicylic acid degradation.

This paper proposes the study of a solar-based photocatalytic ozonation process for the degradation of salicylic acid (SA) using a novel S-scheme ZnO/Cu2O/CuO/carbon xerogel photocatalyst. The incorporation of CuO and Cu2O aims to enhance charge mobility through the formation of p-n heterojunctions with ZnO, whereas the carbon xerogel (XC) was selected due to its eco-friendly nature, capacity to stabilize S-scheme heterojunctions as a solid-state electron mediator, and ability to function as a reducing agent under high temperatures. The characterization of the composites demonstrates that the presence of the XC during the calcination step led to the reduction of a fraction of the CuO into Cu2O, forming a ternary semiconductor heterojunction system. In terms of photocatalysis, the XC/ZnO-CuxO 5% composite achieved the best efficiency for salicylic acid degradation, mainly due to the stabilization of the S-scheme charge transfer pathway between the ZnO/CuO/Cu2O semiconductors by the XC. The total organic carbon (TOC) removal during heterogeneous photocatalysis was 80% for the solar-based process and 68% for the visible light process, after 300 min. The solar-based photocatalytic ozonation process was highly successful regarding the degradation of SA, achieving a 75% increase in the apparent reaction rate constant when compared to heterogeneous photocatalysis. Furthermore, a 78% TOC removal was achieved after 150 min, which is half the time required by the heterogeneous photocatalysis to obtain the same result. Temperature, salinity, and turbidity had major effects on the efficiency of the photocatalytic ozonation process; the system's pH did not cause any major performance variation, which holds relevance for industrial applications.

Architecture and Kinetic Studies of Photocatalytic H2O2 Generation and H2 Evolution through Regulation of Spatial Charge Transfer via Z-Scheme Path over a (001) Facet Engineered TiO2@MXene/B-g-C3N4 Ternary Hybrid.

Spatial charge separation and migration are the critical shortcomings dominating the core energy conversion corridors of photocatalytic systems. Here, a biomimetic multi-interfacial architecture providing strong coupled interaction and rapid charge transmission for photostable and competent photocatalytic H2O2 production and H2 evolution is proposed. The triple-hybrid all-solid-state Z-scheme system was formed with the (001) facet exposed TiO2 nanosheets derived from MXene layers and B-g-C3N4 nanosheets (M/(001)TiO2@BCN) through an electrostatic self-assembly strategy with intimate electronic interaction due to Ti orbital modulation and proper stacking among the hybrids. The metallic and highly conductive MXene layers act as solid state electron mediators in the Z-scheme heterojunction that promote electron-hole separation and migration efficiency. Specifically, the MTBCN-12.5 composite provides optimum yield of H2O2 up to 1480.1 μmol h-1 g-1 and a H2 evolution rate of 408.4 μmol h-1 (with ACE 6.7%), which are 4 and 20 fold greater than the pristine BCN, respectively. The enhanced photocatalytic performance is systematically identified by the increased surface area, higher cathodic and anodic current densities of -1.01 and 2.27 mA cm-2, delayed charge recombination as supported by PL and EIS measurement, and excellent photostability. The Z-scheme charge transfer mechanism is validated by time-resolved photoluminescence (TRPL) analysis, cyclic voltametric analysis, and the radical trapping experiment as detected by PL analysis. This research marks a substantial advancement and establishes the foundation for future design ideas in accelerating charge transfer.

A Cu medium designed Z-scheme ZnO-Cu-CdS heterojunction photocatalyst for stable and excellent H2 evolution, methylene blue degradation, and CO2 reduction.

Solar photocatalysis has emerged as a pollution-free and inexhaustible technique that has been extensively researched in the domains of environmental remediation and energy production. Herein, we have integrated ZnO and CdS nanoparticles through Cu as a solid-state electron mediator to design a ZnO-Cu-CdS Z-scheme heterosystem via a sol-gel route and further tested this as a photocatalyst for dye degradation, H2 evolution, and CO2 reduction. Within 60 min of visible light exposure, about 97% of methylene blue (MB) is degraded with a degradation rate constant of 0.042 min-1 for the ZnO0.45Cu0.1CdS0.45 catalyst. The MB degradation with this catalyst is 84, 21, 4.8, and 2 times as high as those of ZnO, CdS, ZnO0.5CdS0.5, and Cu0.1ZnO0.9 catalysts. The ZnO-Cu-CdS catalyst manifests an H2 evolution efficiency of 5579 μmol h-1 g-1, which is 169, 41, 3.9, and 3.5 times as high as those of ZnO, CdS, ZnO0.5CdS0.5, and Cu0.1ZnO0.9 catalysts. Using H2 as a reducing agent, the CO production rate over the ZnO0.45Cu0.1CdS0.45 catalyst reaches 770 μmol h-1 g-1, which is 3 and 1.8 times higher than those of ZnO0.5CdS0.5 and Cu0.1ZnO0.9 catalysts. Besides, the optimal CH4 production rate over ZnO0.45Cu0.1CdS0.45 reaches 890 μmol h-1 g-1. The improved photocatalytic response of the ZnO-Cu-CdS catalyst is assigned to the delayed recombination of photoexcited charge carriers through a Z-scheme charge transport mode, maintaining the photocarriers with strong redox potentials and the dual role of Cu to serve as a conductive bridge to accelerate the charge transfer rate and enhance the light absorption due to its SPR phenomenon. This research offers a promising strategy for developing binary/ternary Z-scheme heterojunction photocatalytic systems for different photocatalytic applications.

Designs of composite photocatalysts for their enhanced performance based on photoinduced charge transfer

The author and coworkers focused on the fabrication of composite photocatalysts and charge transfer between composite constituents for increased activity and sensitivity to visible light, aiming at developing materials for environmental preservation through the oxidative decomposition of organic pollutants and clean energy production through water splitting for hydrogen generation. Cu2+ ion-grafted titanium dioxide (TiO2) was designed on the basis of visible-light-induced interfacial charge transfer from the valence band (VB) of TiO2 to Cu2+, generating high oxidative decomposition activity owing to the utilization of photogenerated holes in the VB of TiO2. Cu+ produced by electron injection was converted back to Cu2+ by oxygen (O2) reduction through multi-electron O2 reduction reaction. As for water splitting, zinc rhodium oxide (ZnRh2O4) and bismuth vanadate (Bi4V2O11) as H2 and O2 evolution photocatalysts, respectively, were connected with silver (Ag), acting as a solid-state electron mediator, to prepare a composite photocatalyst that is sensitive to red light. The key function of the heterojunction photocatalyst is the transfer of photoexcited electrons from the conduction band (CB) of Bi4V2O11 to the VB of ZnRh2O4 via Ag. Thus, the photoexcited electrons in the CB of ZnRh2O4 and the holes in the VB of Bi4V2O11 effectively reduced and oxidized water, respectively, thereby splitting water and liberating H2 and O2 at a stoichiometric ratio.

Carbon dots as solid-state electron mediator and electron acceptor in S-scheme heterojunction for boosted photocatalytic hydrogen evolution

Nowadays, it is still an attracting work to photocatalytic water splitting into hydrogen production using solar energy owing to its ecological and sustainable development significance. Herein, a ternary ZnIn2S4/carbon dots/g-C3N4 (ZIS/CDs/CN) S-scheme heterojunction photocatalyst with solid-state electron mediator and electron acceptor dual roles of CDs (∼5 nm) as a co-catalyst are designed via a calcination method follow-up water bath process for boosted photocatalytic H2 evolution. Compared to individual ZIS and pure CN, the photocatalytic efficiency of H2 evolution over ZIS/CDs/CN is enhanced by about 2 and 10 times, respectively. Combined with density functional theory (DFT) calculations, the heterojunction type between ZIS and CN is determined to be S-scheme heterojunction. In order to corroborate the role of CDs in S-scheme heterojunction, ZnIn2S4/g-C3N4 (ZIS/CN) was synthesized for comparative experiments and characterization, and its promotion of S-scheme heterojunction was confirmed. Furthermore, ZIS/CDs/CN S-scheme heterojunction photocatalyst still express excellent practicality in seawater environment and structural stability after four successive cycling reactions for photocatalytic H2 evolution. In addition, this bi-functional CDs is also applicable to S-scheme heterojunction constructed from other two different semiconductors, which enrich the construction method of later S-scheme heterojunctions and advance the development of S-scheme heterojunctions.

Construction of carbon dots-deposited TiO2 Photocatalysts with visible-light-induced photocatalytic activity for the elimination of pollutants

Element decomposition is a promising modification technique for enhancing the photocatalytic activity of TiO2. Firstly, (i) It may accelerate photogenerated electron–hole separation as electron traps, (ii) broaden the sunlight absorption region into the visible range and promote surface electron excitation by plasmon resonances excited under visible light and (iii) modify the surface performance of semiconductors. Herein, we describe the introduction of carbon dots (CDs) as solid-state electron mediator on TiO2 sheets (TNs) via a highly effective hydrothermal process. The as-prepared CDs/TNs nanocomposites was measured by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, UV–visible diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy analysis. The results proved that CDs were firmly scattered in the TNs interface. Furthermore, the resultant CDs/TNs nanocomposites exhibited remarkable photocatalytic activity for the degradation of different model pollutants under visible light, which is considerably superior to that of bare TNs. The enhanced photocatalytic activity highlights that CDs can promote the separation and migration efficiency of photoexcited electron–hole pairs. The striking stability of CDs/TNs was investigated by performing five successive cycles of degradation of Congo red. Moreover, the degradation intermediates of three pollutants were monitored by liquid chromatography-tandem mass spectrometry, and potential removal pathways are proposed. We firmly believe that this work may inspire new insights and approaches for the design of highly efficient photocatalysts.

Construction of a Z-scheme g-C3N4/NBGO/BiVO4 heterostructure with visible-light driven photocatalytic degradation of tetracycline: efficiency, reaction pathway and mechanism

In this study, an aminated graphene-like biochar (NGBO) was used as a solid-state electron mediator at a g-C3N4/BiVO4 interface to construct a hybrid Z-scheme system (g-C3N4/NBGO/BiVO4) for the photocatalytic degradation of tetracycline (TC) from aqueous solutions.

Highly active Z-scheme heterojunction photocatalyst of anatase TiO2 octahedra covered with C-MoS2 nanosheets for efficient degradation of organic pollutants under solar light

The construction of a Z-scheme photocatalyst by coupling semiconductors with conductors is an efficient way to achieve high pollutant degradation efficiency. In this study, a hydrothermal approach was used to fabricate a Z-scheme photocatalyst consisting of C-MoS2 sheets wrapped around octahedral anatase TiO2 nanocrystals. The catalyst showed excellent photocatalytic efficiency (99%) for methylene blue degradation with low catalyst loading (0.2 g L-1) under the simulated solar light within 60 min. High photocatalytic degradation efficiencies were also observed for Rhodamine B, methyl orange, and tetracycline under solar irradiation. The C-MoS2 acts as an electron mediator and serves as a carrier transmission bridge for the efficient electron-hole separation. The electron-rich (101)-faceted TiO2 benefits the Z-scheme recombination of electrons from the conduction band of TiO2 and holes at the valence band of MoS2. The semiconductor coupling of (101)-exposed octahedral TiO2 and 2H-MoS2 as well as the introduction of solid-state electron mediators, 1T-MoS2 and carbon, resulted in increased light absorption and accelerated charge transfer at the contact interface, which enhanced the photocatalytic activity of the photocatalyst significantly compared to those of P25, MoS2/TiO2, and C-MoS2. The efficient separation of electron-hole pairs prolongs their lifetime for oxidation and reduction reactions in the degradation process.

Controlled ultrasonic synthesis of TiO2@C3N4 nanocomposites with porphyrin as a solid-state electron mediator: A promising material for pollutant discoloration under visible light

This work describes a new ternary nanocomposite of TiO2 nanospheres decorated on the surface of graphitic carbon nitride (C3N4)-porphyrin (TCP) via the sonochemical method (WUC-D22H, Daihan Scientific, Korea). The ultrasonic frequency and power were 20 kHz and 100 W, respectively. The structure and morphology of the synthesized nanocomposites were systematically characterized by FT-IR, XRD, SEM, UV-DRS, and mapping analysis. Compared to its single and binary components, the ternary nanocomposite TiO2@C3N4-TCP clearly shows higher photocatalytic action against Rhodamine B (Rh–B) dye contaminants in the visible region. Ternary nanocomposite TiO2@C3N4-TCP catalysts have good stability and reusability for four successive photocatalytic discoloration tests. These materials consist of synergistic nanocomposites with enhanced visible light adsorption, effective charge separation, and low rate recombination dynamics. In addition, it has been clarified that hydroxyl and superoxide radical active species play some auxiliary roles, whereas the holes play the main role in the photodiscoloration of Rh–B. Moreover, as expected this work presents a novel approach towards advanced ternary nanocomposites in visible light adsorption aimed at the photodecomposition of pollutants via the photocatalytic process.

Integrated Z-Scheme Nanosystem Based on Metal Sulfide Nanorods for Efficient Photocatalytic Pure Water Splitting.

Developing efficient metal sulfides for pure water splitting is a challenging topic in the field of photocatalysis. Herein, inspired by natural photosynthesis, an all-solid-state Z-scheme photocatalyst was constructed with Cd0.9 Zn0.1 S (CZS) for water reduction, red phosphorus (RP) for water oxidation, and metallic CoP as the electron mediator. RP@CoP core-shell nanostructures were uniformly attached on the CZS nanorods, which gave rise to multiple monodispersed nanojunctions. The integrated Z-scheme nanosystem exhibited an apparent quantum efficiency of 6.4 % at 420 nm for pure water splitting. Theoretical analysis and femtosecond transient absorption results revealed that the impressive performance was mainly due to efficient hole transfer of CZS, resulting from the intimate atomic contacts between CoP mediator and photocatalysts, together with favorable band alignment. Meanwhile, the multiple monodispersed Z-scheme nanojunctions could provide abundant reaction sites, which was also important for the boosted activity.

Recent progress on visible active nanostructured energy materials for water split generated hydrogen

Solar-driven water splitting is one amongst the fastest growing areas of research for hydrogen evolution. The technology employs application of nanostructured energy materials with featured charge-separation and light absorption characteristics. The high charge-carrier recombination and wide band-gap of energy materials are major material-related bottlenecks that restricted the large-scale implementation. Besides it, the co-catalyst, electron mediator and sacrificial electron donor (SED) were the other major components for further improving the H2 yield. A water splitting reactor with well-optimized photon and mass transfer properties is mandatory for studying hydrogen evolution at large scale. The present review compiles various strategies that were presented for reducing charge-carrier recombination and extending visible light absorption of materials, insights for loading earth-abundant co-catalysts on photocatalyst surface, recently developed reversible redox and non-noble metal-based solid-state electron mediators and advantage of applying organic contaminant and biomass-derived waste as SEDs. The discussions on several engineered reactor designs for the photocatalytic water splitting technology were also presented. A cost-effective way of designing a photocatalytic water splitting system with functionalized nanostructured energy materials, the earth-abundant co-catalysts, organic waste-based electron donors and efficient electron mediators for achieving improved hydrogen evolution rate is comprehensively discussed.

Silver as solid-state electron mediator in MoS2/Ag–AgVO3 Z-Scheme heterostructures for photocatalytic H2 generation

Heterostructured photocatalysts have attracted enoromous research interest in H2 generation through water splitting because of highly efficient interfacial charge-transfer characteristics of nanoarchitectures. Herein, Ag nanoparticles decorated AgVO3 nanorods and MoS2 nanosheets are combined to fabricate MoS2/Ag–AgVO3 heterostructures,in which Ag NPs act as a solid-state electron mediator. Owing to unique nanoarchitecture, MoS2/Ag–AgVO3 exhibited the best H2 generation (reaches 38.6 mmol g−1 in 2 h) whichis four times and twenty times higher than Ag–AgVO3 and bare MoS2 under visible light respectively. Transient photocurrent and Nyquist spectra reveal that the enhanced photocatalytic performance of MoS2/Ag–AgVO3 is mainly attributed to its effective carrier separation and transfer through the surfaces. Moreover, the band structures of the materials have been calculated from Mott-Schottky study, demonstrated a Z-scheme interfacial charge-transfer mechanism. This study describes a promising approach for harvesting solar energy to generate H2 from water by designing an efficient Ag mediated Z-scheme heterostuctures.

Highly efficient visible-light-driven photocatalytic hydrogen evolution by all-solid-state Z-scheme CdS/QDs/ZnIn2S4 architectures with MoS2 quantum dots as solid-state electron mediator

All-solid-state Z-scheme CdS/QDs/ZnIn2S4 architectures with MoS2 quantum dots as solid-state electron mediator were successfully designed and constructed by optimally combining one-dimensional CdS nanorods, zero-dimensional MoS2 quantum dots (QDs) and two-dimensional ZnIn2S4 nanosheets. The photocatalytic water splitting for hydrogen evolution demonstrated that such structural design can synergistically trigger remarkably improved visible-light-driven photocatalytic activity. Photocatalytic H2-evolution at 2107.5 μmol g−1 h−1 was achieved on this CdS/QDs/ZnIn2S4 architectures under visible light irradiation, exceeding those of bare CdS nanorods and pure ZnIn2S4 nanosheets by a factor of 26 and 62, respectively. This highly efficient photocatalytic activity arises from the effective charge separation and favourable electron mediator of MoS2 QDs. The Z-scheme charge separation mechanism was verified by the ESR, PL-TA and organic electron acceptor test.