Direct Z-scheme Heterojunction Research Articles

Rational designing 0D/1D Z-scheme heterojunction on CdS nanorods for efficient visible-light-driven photocatalytic H2 evolution

Rational design and construction of unique transmission channel in photocatalyst system is of great significance for promoting the efficient spatial separation of photo-induced charge carriers. This work reports a novel 0D/1D direct Z-scheme heterojunction photocatalyst synthesized by in-situ anchoring W18O49 QDs onto the surface of CdS NRs. Due to this unique structural design, both bulk and surface photogenerated carriers are separated efficiently, achieving a dramatically improved photocatalytic H2 evolution performance. The optimal of H2 production rate of the W18O49/CdS heterostructure is as high as 55.24 mmol·h−1·g−1, with lactic acid as sacrificial reagents under visible light irradiation, which is approximately 110.5 and 3.7 times as that of pure CdS NRs and Pt/CdS, respectively. Even in the absence of hole scavengers, the optimal WCS300 composite exhibits an obvious photocatalytic H2 evolution activity accompanied by the generation of considerable H2O2. This work provides a new protocol for the development of efficient photocatalyst systems.

Fabrication of 1D/2D CdS/CoSx direct Z-scheme photocatalyst with enhanced photocatalytic hydrogen evolution performance

In this work, we fabricate a 1D/2D heterojunction photocatalyst composed of n-type CdS nanorods and p-type CoSx nanoflake. This photocatalyst achieves a hydrogen evolution rate of 9.47 mmol g−1 h−1, which is 13.7 times higher than that of pure CdS nanorods. Scanning Kelvin Probe, Mott-Schottky plots, UV–Vis absorption spectra and surface photocarrier orienting reaction results indicate that the enhanced photocatalytic performance of CdS/CoSx is owing to the fabrication of direct Z-Scheme heterojunction system which greatly improves the utilization, migration and separation rate of photo-generated carriers. To the best of our knowledge, this work is the first time to describe a CdS/CoSx direct Z-scheme system with 1D/2D nanostructure, which can expedite the transfer process of photogenerated carriers with strong redox energy to participate in photocatalytic reactions.

Highly dispersed Zn0.5Cd0.5S nanoparticles anchored on NiCo2O4 nanosheets as the direct Z-scheme heterojunction for enhanced visible light photocatalysis

Highly dispersed Zn0.5Cd0.5S nanoparticles anchored on NiCo2O4 nanosheets as the direct Z-scheme heterojunction for enhanced visible light photocatalysis

Photocatalytic degradation of diclofenac using TiO2-CdS heterojunction catalysts under visible light irradiation.

The present study reports the photocatalytic degradation of analgesic drug diclofenac using the hydrothermally prepared TiO2-CdS heterojunction catalyst. The results suggest that the prepared catalysts exhibited excellent photocatalytic activity under visible light irradiation. The photodegradation kinetics were well fitted to the pseudo-first-order reaction. The apparent reaction rate constant for TC5 catalyst in the diclofenac degradation was 0.02316min-1. Mineralisation of diclofenac using TC5 photocatalyst was around 86% within 4h of irradiation time. The operating parameters such as optimal catalyst dosage, apparent solution pH and the effect of initial diclofenac concentration were also studied using the TC5 catalyst. The role of active species in the degradation mechanism was elucidated and it was found that the hydroxyl radical is the main active species in the diclofenac degradation mechanism. The charge transfer between heterojunction catalysts is facilitated by direct Z-scheme heterojunction structure. The coupled photocatalysts also showed good photochemical stability and reusability over five successive reaction cycles. The tentative degradation pathway has been devised based on LC-MS peaks, and it is found that only m/z 224, m/z 178 and m/z 124 were persisted at the end of the reaction.

Construction of novel AgIO4/ZnO/graphene direct Z-scheme heterojunctions for exceptional photocatalytic hydrogen gas production

Construction of novel AgIO4/ZnO/graphene direct Z-scheme heterojunctions for exceptional photocatalytic hydrogen gas production

Two-dimensional graphitic carbon nitride/N-doped carbon with a direct Z-scheme heterojunction for photocatalytic generation of hydrogen.

Photocatalysts with a direct Z-scheme heterojunction are promising by virtue of the effectively enhanced separation of charge carriers, high retention of redox ability and the absence of backward photocatalytic reactions. Their activity depends on band alignment and interfacial configurations between two semiconductors for charge carrier kinetics and the effective active sites for photochemical reactions. Herein, a two-dimensional (2D) graphitic carbon nitride/N-doped carbon (C3N4/NC) photocatalyst is synthesized by a gas template (NH4Cl)-assisted thermal condensation method. C3N4/NC has the synthetic merits of a direct Z-scheme heterojunction, 2D–2D interfacial contact, and enhanced specific surface area to improve charge separation kinetics and provide abundant active sites for photochemical reaction. It exhibits an over 46-fold increase of the photocatalytic hydrogen production rate compared to bulk C3N4 under visible light illumination. This work demonstrates the great potential of 2D Z-scheme heterojunctions for photocatalysis and will inspire more related work in the future.

Eosin Y-sensitized rose-like MoSx and CeVO4 construct a direct Z-scheme heterojunction for efficient photocatalytic hydrogen evolution

MoSx/CeVO4 was prepared by a simple hydrothermal method and a direct Z-scheme heterojunction was constructed. The existence of heterojunction provides a special transmission path for electron transfer between two materials.

Construction of a Z-scheme 1D/2D FeV3O8/g-C3N4 composite for ibuprofen degradation: mechanism insight, theoretical calculation and degradation pathway

A direct Z-scheme heterojunction of 1D FeV3O8 nanorods and 2D g-C3N4 nanosheets with enhanced charge separation efficiency was successfully fabricated.

Z-scheme heterojunction based on NiWO4/WO3 microspheres with enhanced photocatalytic performance under visible light.

The green treatment of dye wastewater has always been a research hotspot in the environmental field. The photocatalytic technology is considered to be a simple and effective strategy to remove dyes in wastewater. A new type of NiWO4/WO3 Z-scheme heterojunction microspheres were synthesized by a simple hydrothermal method and impregnation-calcination process. The crystal structure, microscopic morphology, optical and electrochemical properties of the samples were systematically characterized. The photocatalytic activity of methylene blue (MB) was studied by visible light irradiation. The results show that the direct Z-scheme heterojunction formed by NiWO4/WO3 effectively reduces the transfer resistance of photogenerated carriers and improves the separation efficiency of photogenerated carriers. The degradation rates of NiWO4/WO3-4 Z-scheme heterojunction microspheres to MB dye are 1.8 and 3.2 times higher than that of pure WO3·2H2O and WO3 microspheres, respectively. Combined with the Mott-Schottky curve and the active species capture experiments, a possible Z-scheme photogenerated carrier transfer mechanism is proposed. This study provides a method for the development and design of Z-scheme heterojunction photocatalysts in the field of wastewater purification.

Efficient solar photocatalytic hydrogen production using direct Z-scheme heterojunctions.

We report the preparation of a series of heterojunctions made of Ta3N5 and TiO2 nanoparticles that show good properties for photocatalytic hydrogen production. The composite photocatalyst with a light-response range up to 620 nm shows a hydrogen evolution rate of 250 μmol h-1. The apparent quantum efficiency at 330 nm can be as high as 46%. Particularly, normalized spectral studies indicate that the heterojunction is more active upon full-spectrum (without using optical filters) irradiation, and its activity is even superior to the total activity exhibited upon UV-light irradiation (λ ≤ 420 nm) and visible-infrared light irradiation (λ ≥ 420 nm). Moreover, in situ photodeposition of platinum nanoparticles on the surface of the photocatalyst as well as the band alignment analysis demonstrate the Z-scheme mechanism associated with the photocatalytic process. Specifically, photogenerated electrons from TiO2 will rapidly combine with the photogenerated holes from Ta3N5 through interfacial charge transfer, leaving the more active electrons and holes in Ta3N5 and TiO2, respectively, to facilitate redox reactions. Basically, TiO2 is only UV-light active, while Ta3N5 can be activated under visible-light irradiation. In this case, a synergy effect, upon simultaneous UV-light excitation and visible-light excitation, can be achieved by full-spectrum irradiation, leading to a much higher photocatalytic activity. This work thus provides a favorable and upward direction for the establishment of heterojunctions for high-efficiency hydrogen production and solar energy applications.

Band engineering design of g-C3N4-BiOClxBr1−x direct Z-scheme heterojunctions with remarkable enhancement of photocatalytic performance

A series of g-C3N4-BiOClxBr1−x direct Z-scheme heterojunctions were successfully constructed by direct pyrolysis and solvothermal routes. Direct Z-scheme system not only efficiently separated the charge carriers, but also retained their strong oxidizing and reducing capacity. In addition, the light absorption of BiOClxBr1−x solid solutions extended into the visible light region with the increase of bromine content. Benefiting from the joint utilization of heterojunction construction and solid solution fabrication strategies, g-C3N4-BiOCl0.5Br0.5 (g-C5B5) heterojunction presented notably improved photocatalytic activity with the apparent rate constant of 0.1698 min−1. The synergistic effects were mainly expounded as the enhanced visible-light response, promoted separation of electron-hole pairs and optimized redox potentials of charge carriers, which was further verified on the basis of photocatalytic experiment and a series of characterization. This work provides a novel insight to design high-efficiency photocatalysts with reasonable structure for removing environmental pollutants and converting solar energy.

A direct Z-scheme heterojunction with boosted transportation of photogenerated charge carriers for highly efficient photodegradation of PFOA: Reaction kinetics and mechanism

Z-scheme heterojunction photocatalyst is expected to boost the efficiency for the removal of refractory perfluorooctanoic (PFOA) because of its high carrier separation efficiency and strong redox ability. Here, a direct Z-scheme heterojunction of core-shell CeO2@NiAl-LDHs has been constructed and a high performance for PFOA photodegradation (90.2 % and 36.1 mg g–1 h–1) was observed. By exploring factors affecting the photodegradation performance, including pH, catalyst dosage, light intensity and reaction time, where the kinetics and thermodynamics of the degradation process were investigated in detail. In addition, the reaction order, activation energy, and the detailed form of the rate equation were obtained. In particular, according to the direction of transporation, quantity and rate of photogenerated electron transfer in the heterojunction, we proved the presence of a built-in electric field (BIEF) and confirmed its promoting on photogenerated carriers and the associated enhanced photocatalytic performance of CeO2@NiAl-LDHs. The boosted electron transfer process arises from the difference in the work function of the two phases that constitute the heterojunction. The efficient degradation of PFOA is a result of the synergistic effect between the reduction and defluorination of PFOA and its fluorine-containing intermediates by electrons and the oxidation of active free radicals (associated with the formation of hot electrons and holes).

Significantly enhanced photocatalytic hydrogen production performance of g-C3N4/CNTs/CdZnS with carbon nanotubes as the electron mediators

The electron mediator can effectively improve the performance of the direct Z-scheme heterojunction photocatalysts. However, it is still a great challenge to select cheap and efficient electron mediators and to design them into the Z-scheme photocatalytic system. In the present paper, the g-C3N4/CNTs/CdZnS Z-scheme photocatalyst was prepared using carbon nanotubes (CNTs) as the electron mediators, and its photocatalytic hydrogen production performance was studied. Compared with single-phase g-C3N4, CdZnS and biphasic g-C3N4/CdZnS photocatalysts, the photocatalytic hydrogen production performance of the prepared g-C3N4/CNTs/CdZnS has been significantly enhanced. Meanwhile, g-C3N4/CNTs/CdZnS possesses very good photocatalytic hydrogen production stability. The enhanced photocatalytic hydrogen production performance of g-C3N4/CNTs/CdZnS is attributed to the fact that CNTs, as an electron mediator, can accelerate the recombination of the photogenerated holes in the valence band of g-C3N4 and the photogenerated electrons in the conduction band of CdZnS, which makes the g-C3N4/CNTs/CdZnS Z-scheme photocatalyst be easier to escape the photogenerated electrons, increases the concentration of the photogenerated carriers and prolongs the lifetime of the photogenerated carriers. This work provides a theoretical basis for the further development and design of CNTs as the intermediate electron mediator of the Z-scheme heterojunction photocatalyst.

Tunable charge transfer efficiency in HxMoO3@ZnIn2S4 hierarchical direct Z-scheme heterojunction toward efficient visible-light-driven hydrogen evolution

A Z-scheme system is ideal for photocatalysis hydrogen evolution due to spatially separating photogenerated electron–hole pairs and strong redox ability. However, the construction of such a system to achieve rapid charge transfer is still a big challenge. Hierarchical functional nanomaterials with controllable morphology, dimensionality, and electronic property can open more possibilities for designing active Z-scheme photocatalysts. Herein, we construct hierarchical direct Z-scheme composites of epitaxial ultrathin ZnIn2S4 nanosheets on HxMoO3 nanobelts, which exhibits 10.5 times higher H2-production activity (5.9 mmol·g−1·h−1) than pristine ZnIn2S4 with visible light. High conductivity and tunable charge transfer efficiency of HxMoO3 component resulting from hydrogen intercalation are favourable to the effective transport of charge carriers. Moreover, hierarchical architecture composed of 2D nanosheets increases intimate interface for effective separation of electron–hole pairs and exposes more reactive sites. The study provides a new sight in designing hierarchical direct Z-scheme photocatalyst for solar fuel generation.

Direct Z-scheme heterojunction of ZnO/MoS2 nanoarrays realized by flowing-induced piezoelectric field for enhanced sunlight photocatalytic performances

Direct Z-scheme heterojunction of ZnO/MoS2 nanoarray on Ni foam has been realized by flowing induced piezoelectric field for enhanced sunlight photocatalytic performances. The porous structure of Ni substrate can effectively induce applied deformation on nanorod arrays and generate built-in piezoelectric field by water flowing. Under sunlight irradiation for 50 min, the photocatalytic degradation efficiency of methyl orange is 50.6 %, and the degradation efficiency rapidly increases to 92.7 % under stirring-sunlight irradiation. The degradation rate increases with increasing stirring speed along an S-shaped relationship. In addition, the heterojunction photocatalyst also performs good repeatability in aqueous solution and even in alkaline solution. The role and amount of active radicals are investigated by radical scavenger experiments and electron spin resonance technique, indicating that only the amount of ∙OH radicals can be significantly increased by flowing induced piezoelectric field. This interesting phenomenon can be attributed to that piezoelectric field converts Ⅱ-type heterojunction into Z-scheme configuration.

In-situ constructing of one-dimensional SnIn4S8-CdS core-shell heterostructure as a direct Z-scheme photocatalyst with enhanced photocatalytic oxidation and reduction capabilities

Incorporating heterojunctions and constructing structures are reliable strategies to solve the problem of inefficient intrinsic charge generation in semiconductors. In this study, a direct Z-scheme heterojunction was formed ingeniously, via the intimate coaxial contact of SnIn4S8-CdS (SISCS) core-shell nanostructure. The prepared core-shell SISCS composites exhibited more superior photocatalytic oxidation and reduction capabilities in comparison with bare CdS and SnIn4S8. Significantly, the SISCS2 sample could reduce 98.9% of Cr (VI) (20 mg/L) and degrade almost all methyl orange (MO) (15 mg/L) within 24 min. The apparent rate constants of Cr (VI) and MO photodegradation over SISCS2 were 3.52 and 7.52 times than those of bare CdS nanorods, 4.57 and 5.59 folds as those of pure SnIn4S8, respectively. The intimate coaxial contact between CdS and SnIn4S8 efficiently enhanced the charge transfer, and the redox ability of the CdS and SnIn4S8 could be maintained crediting to the Z-scheme system heterojunction. Moreover, the enlarged specific surface area and the super-hydrophilic surface could both ensure excellent photocatalytic activity of SISCS composites. Furthermore, the Z-Scheme photocatalytic mechanism and the degradation pathways of MO were also discussed in depth.

A Mini Review on Bismuth-Based Z-Scheme Photocatalysts.

Recently, the bismuth-based (Bi-based) Z-scheme photocatalysts have been paid great attention due to their good solar energy utilization capacity, the high separation rate of their photogenerated hole-electron pairs, and strong redox ability. They are considerably more promising materials than single semiconductors for alleviating the energy crisis and environmental deterioration by efficiently utilizing sunlight to motivate various photocatalytic reactions for energy production and pollutant removal. In this review, the traits and recent research progress of Bi-based semiconductors and recent achievements in the synthesis methods of Bi-based direct Z-scheme heterojunction photocatalysts are explored. The recent photocatalytic applications development of Bi-based Z-scheme heterojunction photocatalysts in environmental pollutants removal and detection, water splitting, CO2 reduction, and air (NOx) purification are also described concisely. The challenges and future perspective in the studies of Bi-based Z-scheme heterojunction photocatalysts are discussed and summarized in the conclusion of this mini review.

In-situ growth of β-Bi2O3 nanosheets on g-C3N4 to construct direct Z-scheme heterojunction with enhanced photocatalytic activities

In order to construct tighter heterostructures, a novel Z-scheme β-Bi2O3/g-C3N4 composite (xBC) was synthesized by in-situ growth method. The crystal phase, morphology, chemical composition, optical and photoelectrochemical properties of the composites were investigated by XRD, FT-IR, SEM, TEM, XPS, UV–vis DRS, EIS, etc. The characterizations verified that β-Bi2O3 nanosheets grew uniformly on the surface of g-C3N4. Meanwhile, the photocatalytic performance of xBC composites were also evaluated by degrading Rhodamine B (RhB) and p-Nitrophenol (PNP) under visible-light irradiation. As a result, 0.5BC exhibited the most superior performance, the removal efficiencies reached 71.6% for PNP in 300min and 94.8% for RhB in 120 min, with a rate constant (0.0233min−1) of 7.1 and 5.8 times than that of g-C3N4 and β-Bi2O3, respectively. Evidences that Z-scheme carries transfer path were strongly confirmed by XPS, PL and reactive species analysis.

Oxygen vacancy-mediated sandwich-structural TiO2−x /ultrathin g-C3N4/TiO2−x direct Z-scheme heterojunction visible-light-driven photocatalyst for efficient removal of high toxic tetracycline antibiotics

A surface defect sandwich-structural TiO2−x/ultrathin g-C3N4/TiO2−x direct Z-scheme heterojunction photocatalyst is successfully constructed. The results manifest the existence of oxygen vacancies, sandwich structure and direct Z-scheme heterojunction. Noticeably, TiO2−x/ultrathin g-C3N4/TiO2−x efficiently eliminates high toxic tetracycline hydrochloride by means of·O2−, h+ and·OH, whose removal rate is 87.7% during 90 min and the pseudo-first-order rate constant reaches up to 31.7 min−1 × 10−3. The extraordinary performance can be attributed to the special 3D structure, Z-scheme heterojunction expediting charge transfer and promoting the generation of active species, meanwhile the oxygen vacancies enhancing the spatial separation of photo-induced carriers. Moreover, various environmental factors are systematically explored by statistics. SO42−, NH3-N and pH exhibit an obvious impact on removal rate. Meanwhile, TiO2−x/ultrathin g-C3N4/TiO2−x could also effectually remove tetracycline hydrochloride from complex actual-wastewater and exhibit high stability. Besides, the photocatalytic mechanism and degradation path of tetracycline hydrochloride are also elucidated.

Direct Z-scheme SnO2/Bi2Sn2O7 photocatalyst for antibiotics removal: Insight on the enhanced photocatalytic performance and promoted charge separation mechanism

A kind of direct Z-scheme SnO2/Bi2Sn2O7 heterojunction photocatalyst was fabricated aiming to explore the underlying photocatalyitc mechanism as well as the improved photocatalytic antibiotics degradation activity. Experimental results indicated that the existence of the built-in electric field between SnO2 and Bi2Sn2O7 led to a direct Z-scheme charge transfer. The efficient separation and fast transfer of photogenerated charge carriers contributed to boosting the photocatalytic performance of SnO2/Bi2Sn2O7. The optimal photocatalytic tetracycline degradation efficiency of SnO2/Bi2Sn2O7 was 88.4 %, which was 1.4-fold higher than that of pure Bi2Sn2O7 (62.9 %) and 12.5 times higher than pure SnO2 (7.1 %). Radical trapping experiments and electron paramagnetic results indicated that the photocatalytic degradation process in SnO2/Bi2Sn2O7 system was modulated by photogenerated electrons, holes and O2− active species. This work may offer a new insight for enhancing the photocatalytic activity of Bi2Sn2O7 by constructing a direct Z-scheme heterojunction structure.