Z-scheme System Research Articles

G-C3N4/Carbon doped ammonium phosphotungstate heterojunction with pyromellitic diimide as organic electron mediator for efficient acetamiprid photocatalytic degradation

The high recombination of photogenerated electron-hole pairs and low redox ability in single photocatalysts are still challenges in improving photocatalytic degradation efficiency. In this research, carbon-doped ammonium phosphotungstate (CNPW) was successfully loaded on the surface of g-C3N4 with pyromellitic diimide (PMDI) as the new kind of organic electron mediator. The photocatalytic degradation rate constant k for acetamiprid of g-C3N4/PMDI/CNPW under visible light is 0.528 h−1 which is about 13.2 and 24 times higher compared with g-C3N4 and CNPW, respectively. We demonstrate the photogenerated charge carriers of g-C3N4/PMDI/CNPW obey the Z-scheme heterojunction mechanism rather than the type-Ⅱ heterojunction mechanism. PMDI can efficiently reduce the recombination of electron-hole pairs and enhance the photocatalytic degradation ability in the Z-scheme system. Furthermore, we have found that urea can act as a protective agent to protect g-C3N4 from decomposing in synthesizing g-C3N4/PMDI/CNPW. This research provides a new strategy for synthesizing high redox ability Z-scheme heterojunction. Introducing PMDI as an organic electron mediator enables a new pathway for enhancing photogenerated charge carriers transferring in a Z-scheme heterojunction system.

Internal electric field enhanced photoelectrochemical water splitting in direct Z-scheme GeC/HfS2 heterostructure: A first-principles study

Designing direct Z-scheme heterostructure photocatalysts has received enormous attention due to the efficient separation of photo-generated carriers in water splitting. Based on first-principles calculations, electronic properties and a photocatalytic mechanism of a GeC/HfS2 van der Waals (vdW) heterostructure are systematically explored. From the analysis of band arrangement and the built-in electric field, the heterostructure, with an indirect bandgap of 0.40 eV, is demonstrated to be a typical direct Z-scheme system. Remarkably, there is also a 0.40 eV interlayer work function difference in the heterostructure, which is helpful to further drive carrier separation and enhance the water splitting ability by partially bending the redox potential of water. The Gibbs calculation shows that the GeC/HfS2 vdW heterostructure can achieve overall photocatalytic water splitting spontaneously under neutral conditions. Moreover, excellent visible light absorption ability (∼5×105 cm−1) and giant carrier mobilities (5823 cm2 V−1 s−1) also make GeC/HfS2 heterostructure highly competitive in numerous photocatalytic materials and optoelectronic devices. The bandgap can be flexibly adjusted by biaxial strain, enabling a wider application of the heterostructure. All these significant properties not only demonstrate the great application potential of GeC/HfS2 heterostructure as photocatalysis but also provide ideas for designing novel electric field-enhanced heterostructures.

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.

InN/XS2 (X = Zr, Hf) vdW heterojunctions: promising Z-scheme systems with high hydrogen evolution activity for photocatalytic water splitting.

Z-scheme van der Waals heterojunctions are very attractive photocatalysts attributed to their excellent reduction and oxidation abilities. In this paper, we designed InN/XS2 (X = Zr, Hf) heterojunctions and explored their electronic structure properties, photocatalytic performance, and light absorption systematically using first-principles calculations. We found that the valence-band maximum (VBM) and conduction-band minimum (CBM) of the InN/XS2 (X = Zr, Hf) heterojunctions are contributed by InN and XS2, respectively. Photo-generated carriers transferring along the Z-path can accelerate the recombination of interlayer electron-hole pairs. Therefore, the photogenerated electrons in the CBM of the InN layer can be maintained making the hydrogen evolution reaction occur continuously, while photogenerated holes in the VBM of the Ti2CO2 layer make the oxygen evolution reaction occur continuously. The band edge positions of heterojunctions can straddle the required water redox potentials, while pristine InN and XS2 (X = Zr, Hf) can only be used for photocatalytic hydrogen evolution or oxygen evolution, respectively. Furthermore, the HER barriers can be tuned by transition metal doping. With Cr doping, the hydrogen evolution reaction (HER) barriers decrease to -0.12 for InN/ZrS2 and -0.05 eV for InN/HfS2, very close to the optimal value (0 eV). In addition, the optical absorption coefficient is as high as 105 cm-1 in the visible and ultraviolet regions. Therefore, the InN/XS2 (X = Zr, Hf) heterojunctions are expected to be excellent photocatalysts for water splitting.

Examination of photocatalytic Z-scheme system for overall water splitting with its electronic structure.

Although the solar-to-hydrogen (STH) conversion efficiency of a photocatalytic Z-scheme system for overall water-splitting with a solid-state electron mediator composed of a hydrogen evolution cocatalyst (HEC) nanoparticles/hydrogen evolution photocatalyst (HEP) particle layer with an Rh,La-codoped SrTiO3/conductor with an Au/oxygen evolution photocatalyst (OEP) particle layer with Mo-doped BiVO4/oxygen evolution cocatalyst (OEC) nanoparticles reached the highest value (1.1%) in 2016, it was still insufficient for practical application, resulting in a proposal in a previous paper to develop HEP and OEP particles with longer wavelength absorption edges. While progress has been rather slow since then, the Z-scheme system has been analyzed in this paper from a new point of view, i.e., the electronic structure of the system on the basis of solid-state physics, in order to seek for new ideas to enhance its STH conversion efficiency. In addition to the proposal in the previous paper, new ideas in this paper include the formation of a built-in potential to enhance electron (positive hole) transfer from the HEP (OEP) to the HEC (OEC) by putting positive (negative) charges on the HEC (OEC) nanoparticles, enhancement of the reduction (oxidation) of water by an electron (a positive hole) transferred from the HEP (OEP) to the HEC (OEC) by using the quantum-size effect of HEC and OEC nanoparticles, enhancement of the transfer of a photo-created positive hole (electron) from the HEP (OEP) to the conductor by controlling the Schottky barrier between them, and enhancement of the movement of electronic charge carriers together with depression of their recombination in highly doped HEP and OEP particles by the use of ionic relaxation processes in the particles.

Piezoelectric-effect-enhanced photocatalytic performance in Cr/Nb modified Bi4Ti3O12/g-C3N4 Z-scheme system

Polarizations within ferroelectric or pizeoelectric materials are conducive to improve photocatalytic activity by promoting interior photogenerated carriers separation. However, the photocatalytic activity of pristine ferroelectric or pizeoelectric semiconductor is generally limited by its insufficient polarization and spectral absorption. Herein, Bi4Ti3-2nCrnNbnO12/x-g-C3N4 (BTCNO-n/x-CN) heterojunctions was prepared by loading ultrathin g-C3N4 on ferroelectric BTCNO synthesized by co-doping Cr3+-Nb5+ pairs into Bi4Ti3O12 (BTO). The Cr3+-Nb5+ pairs co-doping not only endows the heterojunctions stronger pizeoelectric performance but also further promotes the spectral absorption, which were confirmed through the hysteresis loop, piezoelectric potential, density functional theory calculations and UV–vis diffuse reflectance spectra (DRS). The piezo-photocatalytic performance of BTCNO-n/x-CN was investigated in detail by RhB degradation and H2 evolution under light and ultrasound. According to these results, the optimal loading content of g-C3N4 and doping concentration of Cr3+-Nb5+ pairs were determined as n = 0.1 and x = 5 %, and BTCNO-0.1/5%-CN showed an optimal photocatalytic activity for 98.7 % RhB removal ratio after 45 min, and 696 µmol·g−1·h−1 of H2 production rates. The possible Z-scheme charge transfer mechanism in BTCNO-n/x-CN heterojunction was proposed based on the results of capture experiments, which improves photocatalytic performance through enhancing the separation of photogenerated carriers and promotes the generation of O2− and OH. This work utilizes the piezoelectric effect and heterojunctions to enhance interior and surface carriers separation to improve the catalytic activity, which offers a new scheme to design excellent photocatalysts.

Enhanced charge separation in dual Z-scheme Au decorated LaFeO3-g-C3N4-BiFeO3 system for efficient H2 production

The artificial Z-scheme system that mimics natural photosynthesis is encompassed of two semiconductors which generates two sets of charge carriers at different locations under solar irradiation. Researchers are trying to improve the efficacy of these systems by engineering highly stable redox couples, enhancing their light absorption capability, and discovering new cocatalysts. Herein, we report the fabrication of Au decorated LaFeO3-g-C3N4-BiFeO3 (Au/LFO-CN-BFO) catalyst as dual Z-scheme system for photocatalytic water splitting reactions. The photocatalytic performances of Au/LFO-CN-BFO, LFO-CN-BFO, LaFeO3, g-C3N4 and BiFeO3 were compared and the mechanism on the symmetric double Z-scheme Au/LFO-CN-BFO photocatalytic system was proposed. The results display that Au/LFO-CN-BFO has more superior photocatalytic performance compared to its counter parts (LaFeO3, g-C3N4 and BiFeO3), which is due to the symmetric double Z-scheme nature of photocatalyst. The as-fabricated Au/LFO-CN-BFO photocatalyst reveals high performance for photocatalytic water reduction to evolve H2 (i.e., 698.4 µmol h-1g−1) under visible light irradiation (λ ≥ 420 nm), which is far greater compared to that of its counterparts. As confirmed from various observations, the improved H2 evolution performance of the dual Z-scheme system is accredited to the improved light absorption and significantly promoted charge carrier’s separation. The design of this highly efficient dual Z-scheme composite photocatalyst would trigger the development of new artificial Z-scheme scheme systems that will solve the energy and associated environmental issues.

Constructing a Visible-Active CoFe2O4@Bi2O3/NiO Nanoheterojunction as Magnetically Recoverable Photocatalyst with Boosted Ofloxacin Degradation Efficiency.

Constructing visible-light-active Z-scheme heterojunctions has proven fruitful in enhancing the photocatalytic activity of photocatalysts for superior water clean-up. Herein, we report the fabrication of a CoFe2O4@Bi2O3/NiO (CBN) Z-scheme nanoheterojunction. The obtained CBN heterojunction was used for visible-light-assisted degradation of ofloxacin (OFL) in water. The OFL degradation efficiency achieved by the CBN heterojunction was 95.2% in 90 min with a rate constant of kapp = 0.03316 min-1, which was about eight times that of NiO and thirty times that of CoFe2O4. The photocatalytic activity of a Bi2O3/NiO Z-scheme heterojunction was greatly enhanced by the visible activity and redox mediator effect of the cobalt ferrite co-catalyst. Higher charge-carrier separation, more visible-light capture, and the Z-scheme mechanism in the Z-scheme system were the important reasons for the high performance of CBN. The scavenging experiments suggested ●O2- as an active species for superior OFL degradation. The possible OFL degradation pathway was predicted based on LC-MS findings of degradation intermediate products. The magnetic nature of the CBN helped in the recovery of the catalyst after reuse for six cycles. This work provides new insights into designing oxide-based heterojunctions with high visible-light activity, magnetic character, and high redox capabilities for potential practical applications in environmental treatment.

Molecular Z-Scheme for Solar Fuel Production via Dual Photocatalytic Cycles.

Natural photosynthesis uses an array of molecular structures in a multiphoton Z-scheme for the conversion of light energy into chemical bonds (i.e., solar fuels). Here, we show that upon excitation of both a molecular photocatalyst (PC) and a substituted naphthol (ROH) in the presence of a sacrificial electron donor and proton source, we achieve photocatalytic synthesis of H2. Data support a multiphoton mechanism that is catalytic with respect to both PC and ROH. The use of a naphthol molecule as both a light absorber and H2 producing catalyst is a unique motif for Z-scheme systems. This molecular Z-scheme can drive a reaction that is uphill by 511 kJ mol-1 and circumvents the high-energy constraints associated with the reduction of weak acids in their ground state, thus offering a new paradigm for the production of solar fuels.

Enwrapping g-C3N4 on In2O3 hollow hexagonal tubular for photocatalytic CO2 conversion: Construction, characterization, and Z-scheme mechanism insight

The reduction of CO2 achieved by photocatalysis can simultaneously alleviate the energy crisis and solve environmental issues. Nevertheless, it remains challenging for the rational design of photocatalysts with high-efficiency carrier migration ability. Herein, the Z-scheme g-C3N4/In2O3 (CN/INO) heterostructure was fabricated via metal–organic frameworks (MOFs) assisted thermal deposition which could form a fully encapsulated hollow tubular structure. The unique structure was based on the MOFs-derived hollow hexagonal In2O3 tubular integrated with ultrathin g-C3N4. The Z-scheme CN/INO heterojunction exhibited a larger specific surface area and excellent charge separation efficiency. Benefiting from the above features, the Z-scheme CN/INO heterojunction demonstrated superior performance on photocatalytic CO2 reduction. The formation of CO and CH3OH over the optimized CN/INO-2 catalyst was 7.94 and 1.44 µmol⋅g−1⋅h−1, respectively. Moreover, the density functional theory (DFT) calculations and Kelvin probe force microscopy (KPFM) was carried out to further investigate the situation of charge transfer on the interface of CN/INO. The in-situ Fourier transform infrared spectroscopy (FTIR) was measured to confirm the immediate products and the possible mechanism of photocatalytic CO2 reduction was proposed. This work provided a MOFs-assisted strategy to construct a Z-scheme system for photocatalytic CO2 reduction.

Defect-engineered plasmonic Z-scheme heterostructures for superior photoelectrochemical water oxidation

The Z-scheme photoelectrochemical (PEC) catalytic system that mimics natural photosynthesis is considered an encouraging method to improve the catalytic activities of the catalysts and finally address the global energy crisis. Herein, a plasmon-enhanced Z-Scheme heterostructure is synthesized via interface-structure-designing and defect-engineering. The optimized TiO2/Au/ZnIn2S4OS exhibits an excellent PEC catalytic activity with a photocurrent density of 3.80 mA/cm2 at 1.23 V (vs RHE), which is 2.1 times higher than that of TiO2 nanorod arrays. The density functional theory (DFT) calculation results demonstrate that the synergy of O, S-defects can tailor the charge density distribution and electronic structure of ZnIn2S4, which effectively improves the electrical conductivity of the photocatalyst. The kinetic and thermodynamic behavior investigation, as well as theoretical simulations, indicate that the successful construction of the Z-scheme system can significantly improve the redox capacity and photo-excited electron-hole separation efficiency of the catalysts, thereby promoting the water oxidation activity. Furthermore, the Au nanodots sandwiched between TiO2 and ZnIn2S4OS are found to play multifunctional roles in boosting the PEC catalytic performance. This work provides a valuable approach to constructing defects-dominated Z-scheme systems at the atomic scale and sheds light on new insight into the LSPR effect for improving the catalytic performance in Z-scheme systems.

Construction of self-activating Z-scheme g-C3N4/AgCl heterojunctions for enhanced photocatalytic property

The Z-scheme g-C3N4/AgCl heterojunctions were prepared by precipitation method. Different loading amounts of AgCl on the surface of g-C3N4 in the g-C3N4/AgCl heterojunction photocatalysts have an important influence on the photocatalytic performance. The high anchoring amount of AgCl (From 5 to 35 wt%) on g-C3N4 leads to large interface contact areas, excellent charge transfer efficiency and many surface active points, which can promote the photocatalytic activity of g-C3N4/AgCl heterojunctions. The photocatalytic efficiency of g-C3N4/35 wt% AgCl (denoted as S4) is obviously the highest among g-C3N4/AgCl heterojunctions, pristine g-C3N4 and AgCl. It indicates that S4 exhibits enhanced degradation efficiencies, which are 96.1% of Rhodamine B (RhB) and 77.8% of Tetracycline (TC) for 120 min, respectively. The orders of removal efficiency for RhB and TC are all g-C3N4/35 wt% AgCl (S4) > g-C3N4/25 wt% AgCl (S3) > g-C3N4/15 wt% AgCl (S2) > g-C3N4/5 wt% AgCl (S1) > AgCl > C3N4 under illumination of ultraviolet–visible (UV–Vis) light or simulated sunlight. The best degradation rate constants (k) of S4 for RhB and TC are 7.46–1.71 times higher than those of pure g-C3N4 or AgCl and S1–S3, respectively. With prolonging photocatalytic time, the k of S4 becomes more and more quickly, and the photocatalytic activity can be improved due to its self-activating effect under the UV–Vis light illumination. The electron spin resonance (ESR) analysis and radical trapping experiment show that the electrons at conduction band (CB) of AgCl and the holes at valence band (VB) of g-C3N4 have no redox abilities, while the electrons at the CB of C3N4 and the holes at the VB of AgCl exhibit strong redox capability, respectively. The charge transfer of the heterojunctions follows Z-scheme mode. This Z-scheme system can change from direct Z-scheme photocatalytic system to all-solid-state Z-scheme mode due to the reduction from AgCl to Ag under light illumination, which also improves the photocatalytic performance. Additionally, the photodegradation mechanism was proposed.

Novel Z-scheme AgI/Sb2WO6 heterostructure for efficient photocatalytic degradation of organic pollutants under visible light: Interfacial electron transfer pathway, DFT calculation and mechanism unveiling

Developing highly efficient heterostructured photocatalysts with robust redox ability is of great significance to wastewater purification. Herein, a novel Z-scheme AgI/Sb2WO6 heterojunction was successfully constructed via a chemical-precipitation method. The Z-scheme system can serve as a highly efficient photocatalyst for degradation of organic pollutants in water. Under visible light illumination, the degradation efficiency of rhodamine B and tetracycline over the optimal Z-scheme heterojunction can achieve 95% in 12 min and 80% in 8 min, which is 10.8 and 11.4 times higher than that over single Sb2WO6, respectively. Interestingly, low amounts of Ag0 can be generated and attached on the surface of Sb2WO6 during the photocatalytic process, further enhancing the photocatalytic activity of the Z-scheme heterojunction. Based on theoretical calculations, the interfacial internal electric field (IEF) can facilitate the photoexcited electrons at the conduction band (CB) of AgI to consume the photoexcited holes at the valence band (VB) of Sb2WO6, which greatly promotes the Z-scheme charge transfer path. Quenching experiments and electron spin resonance analyses demonstrate superoxide radicals play a major role in the photocatalytic reactions. The concept of constructing a Z-scheme heterojunction photocatalyst with efficient interfacial charge transfer shall provide a design guide for wastewater purification.

Constructing Z-scheme g-C3N4/TiO2 heterostructure for promoting degradation of the hazardous dye pollutants

The use of dyes and segments has increased widely in recent years, but it poses a serious health risk to ecosystems. In this work, TiO2 and two-dimensional g-C3N4 nanosheets (g-CN) were fabricated through co-precipitation and thermal polymerization technique, respectively. The g–CN–TiO2 photocatalyst (1: 3, 2: 2, 3: 1) in various weight percentages was prepared using a simple impregnation process. The photocatalytic behaviour of the g-CN, TiO2 NPs, and different weight percentages of g–CN–TiO2 photocatalyst was evaluated against methylene blue (MB) dye under UV–visible light illumination. Compared to pristine and other weight percentages of the g–CN–TiO2 nanocomposite, the optimized g–CN–TiO2 nanocomposite (3:1) showed promoted performance against MB dye. The enriched catalytic efficiency can be accredited to the low amount of TiO2 nanoparticles deposited on gCN nanosheets, possibly due to the boosted transport properties of the electron-hole pairs. The enriched photocatalytic behaviour can be attributed to the development of the Z-scheme system between TiO2 and g-CN. The current study is an outstanding demonstration of the development of maximum catalytic efficiency for destroying hazardous chemical dyes.

Efficient hollow cubic Co9S8@defective ZnS/g-C3N4 for multi-pollutants removal via cascade Z-scheme heterojunction

Multi-pollutants environment coexisted organic and inorganic pollutants has become a common phenomenon, seriously affecting water biodiversity and human water security. Currently, the removal of organic and inorganic pollutants by photocatalysis are mostly in own single-pollutant environment, not in multi-pollutants system, where the common photocatalysts cannot achieve efficiently charge separate and transfer with low reactive oxygen species (ROS) efficiency. Herein, we constructed an efficient cascaded Z-scheme heterojunction Co9S8 @defective ZnS/g-C3N4 (C@DZ/N) composed of defective engineering in hollow nanocubes, which achieved excellent photodegradation activity (98.93 % for ciprofloxacin (CIP) and 97.18 % for Cr(VI) at 90 min) in multi-pollutants environment, significantly higher than the hollow cubic with Z-scheme heterojunction in single-pollutant environment (98.07 % for CIP and 98.72 % for Cr(VI) at 140 min). Defective ZnS acts as the core for direct charge transfer and separation to form a unique cascaded Z-scheme heterojunction system that blocks the type II charge transfer pathway to achieve ultra-efficient charge transfer. Furthermore, the structural characterization and ROS experimental results based on multi-pollutants models show that the cascade Z-scheme system greatly facilitates the ROS release kinetics and further leads to more electron-deficient cubic surfaces while increase the directional conversion to singlet oxygen (1O2) in multi-pollutants environment. Further tests of intermediates and reusability indicated its potential for the high-efficient treatment of multiple pollution systems.

Synthesis of a Gold-Inserted Iron Disilicide and Rutile Titanium Dioxide Heterojunction Photocatalyst via the Vapor-Liquid-Solid Method and Its Water-Splitting Reaction.

A solid-state Z-scheme system is constructed whereby rutile titania (TiO2) and beta-iron disilicide (β-FeSi2) were combined to act as oxygen (O2)- and hydrogen (H2)-evolution photocatalysts, respectively, connected by gold (Au). β-FeSi2 island grains with diameters in the 0.5-2 μm range were formed on the surface of Au-coated TiO2 powder by the co-sputtering method. On the surface of TiO2 powder, the Au-Si liquidus phase was obtained via a Au-Si eutectic reaction, which contributed to the selective deposition and crystallization of β-FeSi2 island grains onto Au. After the loading of the H2-evolution cocatalysts platinum and chromium oxide onto β-FeSi2, the system obtained catalyzed the evolution of H2 and O2 in a stoichiometric ratio from pure water under ultraviolet light irradiation. The transfer of photoexcited electrons in the conduction band (CB) of β-FeSi2 to Pt causes the reduction of protons to H2, and the photogeneration of holes in the valence band (VB) of TiO2 causes the oxidation of water to O2. In addition, the photogenerated holes in the VB of β-FeSi2 and the photoexcited electrons in the CB of TiO2 combined with each other in the Au layer, affording the completion of the overall photocatalytic water-splitting.

Synthesis of WO3/AgI photocatalysts applying for degradation of antibiotics in water

In this paper, AgI was successfully synthesized in the presence of WO3 to form AgI/WO3 Z scheme hetero-junction by solid-phase heating method and by varying the WO3 mole ratio (1:0.5, 1:1, 1:2 and 1:3) with respect to the AgI. The PL spectra indicate that the introduction of WO3 to AgI can efficiently suppress the recombination of photo-generated charge carrier. The photocatalytic activity of WO3/AgI was investigated under visible light by using the Amoxicillin (AMX) antibiotic as an organic target in aqueous solution. The WO3/AgI photoactivity for AXM was greatly enhanced when both materials were coupled to form a Z-scheme system. The highest degradation percentage was reached using the WO3/AgI material ratio mole of 1/1. As compared with to the pure WO3 and AgI, the WO3/AgI hybrid material show remarkably improved visible-induced photocatalytic activities in degrading AMX for the enhanced transport ability of electrons and holes.

Halide and Oxide Double Perovskites As Promising Semiconductor Photocatalysts Candidates for Artificial Photosynthesis of Solar Fuels

Artificial photosynthesis of solar fuels is deemed as one of the Holy grails of renewable energy technology for simultaneously solving energy and environmental issues. At the present, it is one of the most involved programs in the international Mission Innovation Challenge for Accelerating the Clean Energy Revolution. Solar-driven water splitting and CO2 conversion are the main research application of artificial photosynthesis. However, these reactions are extremely challenging due to energetically uphill (G>0) and non-spontaneous multi-electron transfer processes, which are difficult to be understood by traditional knowledge of catalysis. An efficient solar energy conversion system must simultaneously deal well with light absorption, charge separation and transfer, surface redox reactions. Particularly, efficient charge separation and transfer by retarding back electron transfer, are often regarded as the key determining steps for overall solar energy conversion. To solve the high recombination rates of photogenerated electron-hole pairs and their low reduction and oxidation abilities in a single photocatalyst, heterojunction manipulation is urgently required. Two mainstream heterojunctions—type-II and Z-scheme heterojunctions have been widely acknowledged [1].Recently, lead halide perovskites (LHPs) with the chemical formula of ABX3, where A is an organic or inorganic cation (A= Cs+, MA+, FA+), B is Pb2+, and X is a halogen anion (Br-, Cl-, I-), such as CH3NH3PbI3,FAPbI3, and CsPbBr3 have been widely investigated as auspicious semiconductor photocatalysts for photocatalytic H2 production and photocatalytic CO2 reduction owing to their impressive photoelectrochemical properties, facile to synthesize, high carrier mobility, low exciton binding energy, and long carrier lifetime [2]. However, the high toxicity and notorious instability upon exposure to light, moisture, and high temperature are the major obstacles to their practical use.Therefore, developing alternative lead-free semiconductor photocatalysts with similar optoelectronic properties to the LHPs is highly needed.Inorganic halide double-perovskites and analogous oxide double-perovskites with the chemical formula of A2B'B"X6 and A2B'B"O6, respectively, are layered 3D materials, which have been considered as a novel ecofriendly visible light responsive semiconductor photocatalysts to replace the toxic lead-halide perovskite. The main feature of the halide and oxide double-perovskites is that their structures can be accommodated with different transition metal combinations on B' and B" site cations to tune their intrinsic properties such as light absorption, carrier mobilities, chemical diversity, and so on. Theoretically, an auspicious photocatalytic activity can be realized from them owing to their impressive photophysical properties. However, poor charge separation and severe charge recombination have restricted their practical photocatalytic application. Recently, several halides and oxides double perovskites have been demonstrated as visible light-responsive photocatalysts for photocatalytic CO2 reduction and photocatalytic half-reaction (oxygen and hydrogen evolution reactions) such as Cs2AgBrBr6, Cs2AgSbBr6, Sr2CoTaO6, Sr2CoWO6, etc. [3-5]. However, for oxide double perovskites even though they have shown bifunctional photocatalytic oxygen and hydrogen two half-reactions with visible light but their potential as photocatalytic CO2 reduction and one-step overall water splitting have not been achieved so far. Therefore, further improvement of the material design and synthesis by assembling heterostructure based on two eco-friendly halide and oxide double perovskites may play a key role in achieving high efficient photocatalytic performance under visible-light-irradiation. Thought is a challenging task, but holds great potential in advancing science and technology in photocatalysis. References Liao, C. Li, S.-Y. Liu, B. Fang, H. Yang, Emerging frontiers of Z-scheme photocatalytic systems, Trends in Chemistry. 4 (2022) 111–127.-C. Wang, N. Li, A.M. Idris, J. Wang, X. Du, Z. Pan, Z. Li, Surface Defect Engineering of CsPbBr3 Nanocrystals for High Efficient Photocatalytic CO2 Reduction, Solar RRL. 5 (2021) 2100154.Idris, A. M.; Liu, T.; Shah, J. H.; Zhang, X.; Ma, C.; Malik, A. S.; Jin, A. Solar RRL 2020, 4 (3), 1900456.Idris, A. M.; Liu, T.; Hussain Shah, J.; Han, H.; Li, C. ACS Sustainable Chemistry&Engineering 2020, 8 (37), 14190-14197.Wang, H. Huang, Z. Zhang, C. Wang, Y. Yang, Q. Li, D. Xu, Lead-free perovskite Cs2AgBiBr6@g-C3N4 Z-scheme system for improving CH4 production in photocatalytic CO2 reduction, Applied Catalysis B: Environmental. 282 (2021).

A Van Der Waals Heterojunction Based on Monolayers of MoS2 and WSe2 for Overall Solar Water Splitting

Solar water splitting (SWS) has been widely studied as a promising technology for generating carbon-free hydrogen. In overall water splitting, the cathode and the anode produce simultaneously H2 and O2, respectively. This process requires a bias of 1.23 V between the two electrodes. Considering the various intrinsic losses in the system (such as the catalytic overpotentials), the bias is reported as 1.6-2.4 V.1 In SWS, the active core of the system absorbs sunlight to generate non-equilibrium electron-hole pairs producing a bias. Considering an absorber made of a single material, this bias is inversely proportional to the amount of sunlight absorbed, leading to the Shockley–Queisser limit. To achieve high bias while maximising the absorption, one can use a type II heterojunction of two absorbers in a Z-scheme configuration.2 Moreover, to improve the overall water splitting process, the electron-hole pairs must be generated as close to the surface as possible. Here, we present a complete design of a direct Z-scheme system based on two-dimensional (2D) transition metal dichalcogenide (TMDC). We also implement a multi-physics model using density functional theory (DFT), the detailed balance method and the Butler-Volmer kinetics to compute the STH efficiency. TMDCs and their van der Waals heterojunctions (vdWH) are attractive for photovoltaic3 and catalytic4,5 applications and the production of 2D TMDC is easy and low-cost thanks to the exfoliation process.6 The active core of the presented system is a MoS2/WSe2 vdWH where MoS2 and WSe2 are the anode and the cathode, respectively.The scheme of the proposed cell is presented in Fig. 1(a) where two distinct regions form its active core: with and without a 2D hexagonal boron nitride (hBN) layer isolating the MoS2 and WSe2 layers. Fig. 1(b) represents the band diagram of region 1, where the two layers are isolated. In this region 1, electrons generated in WSe2 and holes generated in MoS2 produce the electrochemical reactions. As confirmed by our DFT calculations, the band edges positions are suitable with χO2 and χH2 equal to 0.66 eV and 0.50 eV respectively at pH 7 (see Fig 1(b)). The extra carriers generated in the first region (i.e., the electrons of MoS2 and the holes of WSe2) must recombine to avoid an accumulation of carriers which would stop the SWS process. They have to diffuse into region 2, without hBN. As depicted in Fig. 1(c), which presents the band diagram of this corresponding region, the recombination is ensured by the hybridisation of the electronic states in the valence band of the TMDCs. This hybridisation, which is confirmed by our DFT calculations, has also been experimentally observed7. Our ab initio calculations show very efficient recombination. Consequently, a small fraction of region 2 suffices to enable the recombination of all extra electron-hole pairs. This is advantageous for the design of the device since we can imagine stacking exfoliated flakes of MoS2, hBN and WSe2 randomly to produce the active core. The non-ideal stacking will inevitably create some regions 2 without hBN. Finally, we propose that the ultra-thin active core is protected and supported by mesoporous transparent oxide, enabling the free circulation of gases and water.Since some parameters of the considered materials are not well known, we calculate the STH efficiency using our multi-physics model considering 3 different cases: ideal, high-performance, and standard quality. In the latter case, with an optimized ratio of regions 1 and 2 and considering a high optical absorption, we find a 14 % efficiency Since the ultimate thinness of the active region limits the optical absorption, we propose that the mesoporous oxide serves as a support for absorption enhancement systems, such as photonic crystals, resonant cavities, nanoparticles.This work, therefore, proposes the design of a system that exceeds the critical 10% efficiency required to make SWS economically viable.8 Since our system is wireless and requires simple manufacturing processes (exfoliation), this result is remarkable.References M. G. Walter et al., Chem. Rev., 110, 6446–6473 (2010).Q. Xu et al., Materials Today, 21, 1042–1063 (2018).S. Das, D. Pandey, J. Thomas, and T. Roy, Adv. Mater., 31, 1802722 (2019).J. Wu et al., Adv. Mater. Interfaces, 3, 1500669 (2016).X. Yin et al., Chem. Soc. Rev., 50, 10087–10115 (2021).K. Si et al., Appl. Surf. Sci., 507, 145082 (2020).O. Karni et al., Phys. Rev. Lett., 123, 247402 (2019).Y. Fan, J. Wang, and M. Zhao, Nanoscale, 11, 14836–14843 (2019). Figure 1

Enhanced photoactivity and anti-photocorrosion of Z-scheme Zr2CO2/WSe2 heterostructure for overall water splitting

Enhancing the photocatalytic activity and suppressing the photocorrosion of two-dimensional transition metal dichalcogenides (TMDCs) remain great challenges. Herein, we established a novel heterostructured photocatalyst by stacking Zr2CO2 on a WSe2 monolayer with properties promising to promote the separation of photogenerated carriers and prevent photocorrosion existing in TMDCs. On the basis of first-principles calculations, the Zr2CO2/WSe2 system exhibits a direct Z-scheme band alignment with a suitable indirect bandgap of 1.35 eV. The Zr2CO2/WSe2 heterostructure has a low binding energy and high carrier mobilities, which highly benefits the separation and transportation of photogenerated carriers. Concomitantly, the synergistic effect of Zr2CO2 and WSe2 promotes the anti-photocorrosion ability of WSe2 monolayer and visible light optical responses of one separate material. Moreover, the Gibbs free energy shifts to negative values and continuously decreases during the process of water splitting into oxygen without additional overpotential under neural conditions, satisfying the thermodynamic requirements for water splitting. The solar-to-hydrogen (STH) efficiency of the Zr2CO2/WSe2 is calculated to be 15.57%. These findings shed new insight into the development of the visible-light response Z-scheme system for the high performance and stability of heterostructured photocatalysts.