Design of nanohoneycomb structured Ta3N5/WO3 Z-scheme for enhanced visible light photocatalytic hydrogen evolution

Ta₃N₅ has suitable band positions for visible light photocatalytic hydrogen production from water splitting. However, fast self-recombination of electrons and holes is a drawback for its low efficiency. A Ta₃N₅/WO₃ Z-scheme was therefore considered to solve this problem. Furthermore, a nanohoneycomb (nHC) structure was designed and fabricated based on solution-based nanosphere lithography to offer higher surface area for reaction. The cell size of the nHC was controlled by using 400-, 200-, and 100-nm polystyrene nanospheres as the mask. Under visible light irradiation, the hydrogen generation rates of Ta₃N₅@WO₃ film and Ta₃N₅@100-nm WO₃ nHC were measured to be 4.6 and 8.1 μmol/g·h, respectively, whereas that of pure Ta₃N₅ nHC was negligible. With deposition of Pt cocatalyst, the hydrogen generation rates for Ta₃N₅@WO₃ film and Ta₃N₅@100-nm WO₃ nHC were further raised to 9.9 and 16.6 μmol/g·h, confirming the effectiveness of the structure design.     

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 CoS_x 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/CoS_x 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/CoS_x 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.     

Indirect Z-scheme hydrogen production photocatalyst based on two-dimensional GeC/MoSi2N4 van der Waals heterostructures

The stacked two-dimensional materials with suitable band gap are crucial for photocatalytic hydrogen production. Here, using first-principles calculations, the GeC/MoSi₂N₄ heterojunction with a band gap of 1.80 eV is calculated thoroughly. The indirect band alignment of Z-scheme and high carrier mobility boost the separation of electron-hole pairs, allowing more electrons and holes participating in the reactions. Additionally, the band-edge potential perfectly satisfies the requirements for redox potential of water splitting. Furthermore, the Gibbs free energy (−0.552 eV) close to zero indicates the heterojunction can conduct HER exceedingly well, providing a guarantee for photocatalytic hydrogen production. Remarkably, the light absorption coefficient peak is about 1.39 × 10⁵ cm⁻¹ within the visible light range enables the heterojunction to absorb more visible light from the spectrum. In short, results demonstrate the GeC/MoSi₂N₄ heterojunction is a promising photocatalyst for visible light water splitting, which will pave the way for the development of water splitting hydrogen production.     

Two-dimensional C3N/WS2 vdW heterojunction for direct Z-scheme photocatalytic overall water splitting

The development and design of efficient photoelectric catalysts is of great significance for environmental friendliness. This paper is devoted to finding a new two-dimensional van der Waals heterojunction to realize hydrogen production from water splitting. Based on first-principles calculations, a direct type-Z C₃N/WS₂ heterojunction was successfully designed by combining highly active two-dimensional transition metal dichalcogenides (TMDs) with C₃N similar in structure to graphene. The staggered band structure of the direct Z-scheme and high carrier mobility facilitates the efficient separation of photogenerated electron and hole pairs. More importantly, the C₃N/WS₂ direct Z-type heterojunction can perfectly realize total water splitting from pH = 0 to pH = 7. What's more, the Gibbs free energy and overpotential demonstrate the excellent hydrogen evolution capability and the oxygen evolution capacity of the material. In summary, these studies provide new ideas for designing high-performance photoelectric catalysts for visible-light water splitting.     

Special Z-scheme CdS@WO3 hetero-junction modified with CoP for efficient hydrogen evolution

In order to explore new materials capable of producing the new energy hydrogen, co-catalyst CoP was successfully modified Z-scheme hetero-junction CdS@WO₃ to achieve efficient splitting of water under visible light. With the lactic acid solution as sacrificial agent, the H₂ production was 736.89 μmol corresponding the apparent quantum yield of 1.72%, which was 20.2 and 24.5 times than pure CdS and WO₃, respectively. The results of XRD, TEM and FESEM characterization showed that the catalyst has obvious micro-morphology and high crystallinity. The valence distribution and composition of elements in the catalyst were measured by XPS. UV-vis, PL and electrochemical detection showed that the catalyst has excellent optical and electrical properties such as rapidly photo-generated charge transfer efficiency. Not only the energy band structure of the catalyst was calculated and analyzed, simultaneously the charge transfer mechanism and HER mechanism were explored and proposed.     

ZIF-L-derived porous C-doped ZnO/CdS graded nanorods with Z-scheme heterojunctions for enhanced photocatalytic hydrogen evolution

This study presents a novel approach for synthesizing C–ZnO/CdS graded nanorods derived from metal–organic frameworks (MOFs) that can be applied as a catalyst for photocatalytic hydrogen evolution from pure water. Porous C-doped ZnO was prepared by a self-template method using imidazole-like metal–organic backbone (ZIF-L) as a precursor through a two-step calcination method. CdS nanoparticles were deposited on ZIF-L surface by chemical deposition. The two-step calcination method introduced elemental C, and the unique architecture of ZIF-L played an essential role in forming the hierarchical structure of the porous ZnO nanorods. Compared with other ZnO/CdS catalysts, the C-doped ZnO/CdS graded nanorods exhibited remarkable photocatalytic activity for hydrogen production. The highest hydrogen production rate of 20.25 mmol g^?1 h^?1 with an apparent quantum yield (AQY) of 24.7% at 365 nm obtained over C–ZnO/CdS with Pt as co-catalyst, which was 24.4 and 65.3 times higher than that over CdS (0.83 mmol g^?1 h^?1) and ZnO (0.31 mmol g^?1 h^?1), respectively. This outcome was attributed to (i) the formation of Z-scheme heterojunction that significantly promoted the separation and migration of photogenerated electron–hole pairs; (ii) C doping that reduced the bandgap of ZnO and broadened its spectral response range; and (iii) the ordered arrangement of porous nanorods that effectively reduced the recombination rate of the electron–hole pairs.     

A novel direct Z-scheme heterojunction of CoTiO3/ZnIn2S4 for enhanced photocatalytic H2 evolution activity

The shortage of fossil energy has become a growing global concern. It is particularly important to make full use of the infinite solar energy resources, and transform them into sustainable and clean energy. The development of hydrogen energy has become a feasible solution to solve the energy shortage problem. The preparation of photocatalysts featuring efficient charge transfer channels and high hydrogen production activity provides a pathway for the development of hydrogen energy. In this paper, we report for the first time the direct assembly of 2D ZnIn₂S₄ (ZIS) nanosheets on the surface of CoTiO₃ (CTO). The synthesized CoTiO₃/ZnIn₂S₄ (CTO/ZIS) photocatalyst features a direct Z-scheme charge transfer channel, which enhances the separation rate of photogenerated carriers, and accelerates the photocatalytic H₂ evolution (PHE) rate. Without the assistance of any co-catalyst, the PHE rate of prepared CoTiO₃/ZnIn₂S₄ was as high as 5.21 mmol g^?1 h^?1. Moreover, the H₂ evolution rate of CoTiO₃/ZnIn₂S₄ almost did not decrease significantly after four consecutive 4 h cycles. This investigation provides a valuable approach for the exploitation of novel and efficient Z-scheme photocatalysts in the application of solar energy to hydrogen energy conversion.     

In-situ construction of direct Z-scheme sea-urchin-like ZnS/SnO2 heterojunctions for boosted photocatalytic hydrogen production

Constructing direct Z-scheme heterostructure is an effective way to promote the separation of photogenerated carriers and optimize the redox ability of the photocatalytic system. This work reports the in-situ synthesis of sea-urchin-like ZnS/SnO₂ Z-scheme heterojunctions via a one-step hydrothermal method. Both experimental results and density functional theory (DFT) calculations indicate that the tight interfaces derived from in-situ precursor dissociation can ensure a fast transfer for photogenerated carriers, meanwhile, the Z-scheme type of heterojunctions can increase the carrier separation efficiency and maintain the high reduction ability of photogenerated electrons. As expected, the photocatalytic hydrogen evolution rate of the as-optimized ZnS/SnO₂ sample can reach 2.17 mmol g^?1 h^?1, which is 15.5 times higher than that of the commercial ZnS. This work can offer a novel strategy for designing Z-scheme heterojunction as well as controlling the contact interface for boosted photocatalytic activity.     

Constructing direct Z-scheme CuO/PI heterojunction for photocatalytic hydrogen evolution from water under solar driven

In this article, direct Z-scheme CuO/PI heterojunction photocatalyst is constructed by stripping CuO nanoparticles on the surface of polyimide (PI) through solvothermal method. TEM images display the CuO nanoparticles with {002} active exposure facet highly dispersed on the surface of PI. UV–vis, PL and TRPL spectrums confirm that these photocatalysts possess extend absorption towards visible light region. In-situ XPS technique and photodeposition characterization evidence the direct Z-scheme mechanism. The highest activity of H₂ production rate achieved with EY-sensitized 20%CuO/PI photocatalyst reaches to 418.4 μmol g^?1 under visible light irradiation, which is 32 times higher than that of pristine PI. The enhanced photocatalytic performance can be attributed to the direct Z-scheme charge transfer in the intimate interfacial between CuO and PI. Yet, no apparent photocatalytic activity decline is detected after four runs. Our work highlights the role of conjugated polymer in constructing efficient low-cost CuO/polymer heterojunction photocatalyst.     

Fabrication of NiO/Ta2O5 composite photocatalyst for hydrogen production under visible light

A series of novel composite photocatalysts, NiO/Ta₂O₅, were synthesized by the solid‐state reaction and successfully characterized by X‐ray diffraction, Transmission electron microscopy, diffused reflectance ultraviolet and visible (DRUV‐vis) spectroscopy, Photoluminescence and X‐ray photoelectron spectroscopy. Powder X‐ray diffraction (PXRD) pattern indicated the formation of composite material. The red shift in the absorption edges of the newly prepared composite photocatalysts were well observed from the DRUV‐vis spectra. The composite photocatalyst prepared at metal ratio (1:3) showed highest result toward hydrogen production under ultraviolet and visible light irradiation in the presence of methanol as a sacrificial agent. Copyright © 2011 John Wiley & Sons, Ltd.     

CdS-dots decorated WO3 nanorods photocatalyst with highly ordered interfacial structure for enhanced photocatalytic activities via Z-scheme mechanism

Constructing heterostructure is regarded as one of the most promising strategies for the enhancement of photocatalytic activities, because it can make charge carriers separated more efficiently at the interface. Herein, CdS-WO₃ heterostructure photocatalysts with highly ordered and intimate interfacial structure between the two constituent phases have been successfully prepared via a heterogeneous nucleation and growth of CdS nanoparticles on the surface of WO₃ nanorods, which were fulfilled through a controlled release of S^2? in the aqueous solution containing Cd²⁺ by the reaction of NH₃ with thioacetamide. The as-prepared photocatalysts were carefully studied in morphology and interfacial structure by FESEM and HRTEM, along with the other characterizations by XRD, XPS, and UV–visible absorption spectra. Under the irradiation of mercury lamp, the photocatalyst with 6 wt% CdS could afford a degradation rate of methyl orange (MO) of 94.6% in 70 min, 5.84 and 2.51 times as high as WO₃ and CdS, through a photocatalytic degradation process mainly controlled by·O₂^? as active species. In view of the distinctive alignment of energy bands of CdS-WO₃, the enhanced photocatalytic activities should be attributed to the more efficient Z-scheme mechanism that allows the photogenerated holes in WO₃ and electrons in CdS to function more efficiently thanks to the efficient interfacial recombination of the electrons in WO₃ and holes in CdS.     

Facile approach for Z-scheme type Pt/g-C3N4/SrTiO3 heterojunction semiconductor synthesis via low-temperature process for simultaneous dyes degradation and hydrogen production

The platinum/graphite-like carbon nitride/strontium titanate (Pt/g-C₃N₄/SrTiO₃) heterojunction semiconductor was synthesized using a facile approach for simultaneous photocatalytic dye degradation and hydrolysis of hydrogen production from simulated dyeing wastewater. Using SrTiO₃, trace Pt, and the addition of an appropriate amount of electron donors, it can effectively absorb sunlight and achieve 93% dye degradation and 471 μmol h⁻¹ g⁻¹ hydrogen yield. The analysis result indicates that the semiconductor is a Z-scheme type composite. It was also showed that the addition of electron donors effectively promoted the degradation rate, whereas the addition of Pt changed the photocatalytic reaction pathway, which resulted in a reduced degradation rate and a significant improvement of hydrogen evolution. A reaction mechanism for this phenomenon is also proposed.     

Photocatalytic overall water splitting by Z-scheme g-C3N4/BiFeO3 heterojunction

Solar water splitting by photocatalysis in the absence of sacrificial agent has been identified as a promising approach to produce green hydrogen. Increasing photocatalytic efficiency is the core issue in this process. Forming heterojunctions is one potential solution to improve photoactivity. Herein, we successfully synthesized several g-C₃N₄/BiFeO₃ composites containing different mass ratio of g-C₃N₄ by an uncomplicated and cost-effective method. The composite samples exhibited remarkably enhanced photocatalytic performance compared with the bare BiFeO₃ and g-C₃N₄. The highest hydrogen production rate obtained is ~ 160.75 μmol h⁻¹.g-¹ under UV irradiation and ~23.31 μmol h⁻¹.g-¹ under visible light irradiation. This enhanced photocatalytic activity is attributed to the synergistic effect of the junction and Z-scheme charge transfer mechanism between BiFeO₃ and g-C₃N₄, which can effectively accelerate the separation of photogenerated electron-hole pairs and also capability of carrying out redox reaction.     

Microbial coupled photocatalytic fuel cell with a double Z-scheme g-C3N4/ZnO/Bi4O5Br2 cathode for the degradation of different organic pollutants

A novel heterostructure of g-C₃N₄/ZnO/Bi₄O₅Br₂ (ZB-3) was designed, and used in the microbial coupled photocatalytic fuel cell (MPFC). It can effectively improve electron utilization efficiency and pollutant degradation using this double Z-scheme heterojunction structure. The current–time (I–t) curves demonstrated that the current density of ZB-3 was higher than that of ZnO, ZnO/Bi₄O₅Br₂ (ZB-1), g-C₃N₄/ZnO (ZB-2). Electrochemical impedance spectroscopy (EIS) indicated ZB-3 possessed the minimum charge-transfer resistance. This MPFC for degrading rhodamine B (RhB) and tetracycline (TC) under different conditions were developed using these materials. Even in the dark condition, MPFC with g-C₃N₄/ZnO/Bi₄O₅Br₂ demonstrated 93% and 82% degradation efficiency for RhB and TC, respectively. Furthermore, the electron transport mechanism of the MPFC and ZB-3 were proposed. It paves the approach for more efficient pollutant degradation via MFC photocatalysis.     

Boosted photogenerated carriers separation in Z-scheme Cu3P/ZnIn2S4 heterojunction photocatalyst for highly efficient H2 evolution under visible light

Developing low-cost, highly efficient and robust photocatalystic hydrogen evolution system is a promising solution to environmental and energy crisis. Herein, a Z-scheme Cu₃P/ZnIn₂S₄ heterojunction photocatalyst was successfully constructed for the first time via a facile solution-phase hybridization method. The optimized Cu₃P/ZIS composite exhibited the highest H₂ production rate of 2561.1 μmol g⁻¹ h⁻¹ under visible light irradiation (>420 nm), which was 5.2 times greater than that of bare ZnIn₂S₄ and even exceeded the photocatalytic performance of Pt/ZIS composite. The apparent quantum yield of 10 wt% Cu₃P/ZnIn₂S₄ can reach 22.3% at 420 nm. The huge boost of photocatalytic hydrogen evolution activity is ascribed to the formation of heterojunction with the built in electric field within Cu₃P/ZnIn₂S₄ and Z-scheme charge carriers transfer pathway, which result in efficient separation and migration of charge carriers. In addition, both experimental and theoretical calculation confirmed that the charge-carriers transfer pathway of Cu₃P/ZnIn₂S₄ photocatalyst follows the Z-scheme mechanism instead of conventional type-Ⅱ heterojunction mechanism. This work is considered helpful for getting a great deal of insight into constructing high-activity and cost-effective transition metal phosphides (TMPs) based photcatalytic hydrogen production system and rationally designing Z-scheme heterojunction photocatalyst.     

Electron transfer via a carbon channel for efficient Z-scheme solar hydrogen production

Z-scheme photocatalysis provides a promising solution to photocatalytic solar water splitting, yet restricted by inferior interfacial charge transfer. Here, we demonstrate a Z-scheme composite photocatalyst made of Fe₂O₃, a carbon layer, and g-C₃N₄ that can achieve efficient hydrogen generation from solar water decomposition. The success relies on in-situ preparation of core-shell Fe₂O₃@C structure at the surface of g-C₃N₄. Carbon as an intermediate layer thus acts as a bridge that significantly accelerates the migration of photogenerated electrons from Fe₂O₃ conduction band to g-C₃N₄ valence band. As a result, the highest rate of H₂ generation reaches 5.26 mmol h⁻¹g⁻¹. This activity is approximately 33-time greater than that achieved over pristine g-C₃N₄ and about 4-time larger than that obtained over a Fe₂O₃/g-C₃N₄ heterojunction without internal carbon layer. This work explicates the potential insight of the composite and paves a promising way to engineer the charge transfer behavior.     

Deposition of heterojunction of ZnO on hydrogenated TiO2 nanotube arrays by atomic layer deposition for enhanced photoelectrochemical water splitting

The heterojunction of ZnO was deposited on hydrogenated TiO₂ nanotube arrays (H–TiO₂) by atomic layer deposition (ALD) with various cycles. The ZnO was uniformly wrapped with the H–TiO₂ samples and the thickness could be accurately controlled by the cycle numbers of ALD. The higher growth rate ~2.7 Å/cycle was obtained due to the surface amorphous layer, compared with the air-treated samples (A-TiO₂), ~2.3 Å/cycle. When the cycle numbers increased to 200, nanowire arrays appeared. Interestingly, the absorption in the visible light region improved more significantly when ALD ZnO was employed for the H–TiO₂ rather than the A-TiO₂ samples. The H–TiO₂ samples with 42 nm of ALD ZnO exhibited enhanced photoelectrochemical water splitting performances, compared with the A-TiO₂ with 42 nm of ALD ZnO. This was related to the higher degree of the electronic band bending and improved photo-response in the UV and visible light region, resulting from the oxygen vacancies.     

ZnIn2S4 nanosheets decorating WO3 nanorods core-shell hybrids for boosting visible-light photocatalysis hydrogen generation

We here report the fabrication of a core-shell WO₃@ZnIn₂S₄ heterostructure by an interfacial seeding growth strategy, which is implemented by direct growth of ZnIn₂S₄ nanosheets on the surface of WO₃ nanorods with forming a strong electronic interaction between two semiconductors that are beneficial for promoting the interfacial charge transfer. Systematic studies demonstrate that the WO₃@ZnIn₂S₄ nanohybrids hold superior performance for photocatalytic hydrogen generation under visible light irradiation with a production rate of 3900 μmol g⁻¹ h⁻¹. This work provides an effective approach to construct the direct Z-scheme photocatalytic systems for efficient photocatalytic hydrogen evolution, which would be significant for the design of more direct Z-scheme system for various photocatalytic applications.     

Application of a photostable silver-assisted Z-scheme NiTiO3 nanorod/g-C3N4 nanocomposite for efficient hydrogen generation

The performance and reaction mechanism of a silver (Ag)-assisted one-dimensional NiTiO₃ nanorod/CN heterostructure nanocomposite (NTACN) photocatalyst for hydrogen (H₂) production were explored with simulated sunlight. The physicochemical properties of the synthesized catalysts were examined using various spectrophotometers. The newly developed NTACN samples displayed an enhanced photocatalytic activity in producing hydrogen. Specifically, the H₂ production rate of NTACN-5 (with a NT-to-ACN weight ratio of 5) was 3351 μmol/g-h, which was 1.42 times higher than that of ACN-4 with a Ag-to-CN ratio of 4 (2325 μmol/g-h). The effects of the Ag-to-CN and NiTiO₃-to-ACN ratios on the photocatalytic activity of NTACN photocatalysts were determined. The NTACN photocatalysts exhibited a high long-term photostability under simulated sunlight irradiation. The increased photocatalyst performance and photostability were primarily ascribed to an improved charge separation efficiency due to a Z-scheme reaction mechanism as well as the assistance provided by Ag as a charge transfer shuttle and in the surface plasmon resonance effect. A photocatalytic mechanism for hydrogen generation over the NTACN photocatalysts under simulated sunlight irradiation is suggested.     

Largely elevated photocatalytic hydrogen generation over Eu doped g-C3N4 photocatalyst

Graphitic carbon nitrides (g-C₃N₄) have come into researchists’ horizons for their diversified merits, such as the especial graphite-phase 2D laminar framework, competitive price, innoxious, eligible bandgap (∼2.7 eV) and acceptable consistency. Whereas limited by the disadvantages of inferior specific surface area and fast recombination of photo-generated charge pairs, the pragmatic applicability of g-C₃N₄ turns out to be still lacking. In our work, g-C₃N₄ (GCN) and Eu-doped g-C₃N₄ (Eu/CN) with different Eu/g-C₃N₄ molar ratios (1%, 2%, 3%, 4%) were synthesized by an impregnating method and characterized through a series of measurements. Photocatalytic activities of Eu/CN catalysts manifest preeminent H₂ generation capacity and stability excited by solar light. The highest H₂ generation rate without any co-catalyst is 128.8 μmol g⁻¹ h⁻¹ over the 3% Eu/CN, achieving 117.1-fold as high as that of GCN (1.1 μmol g⁻¹ h⁻¹). Eu doping is proven to slightly widen the bandgap of the samples, resulting in the conduction band of samples more negative and the reduction reaction more effortlessly. Simultaneously, Eu doping changes the molecular structure of g-C₃N₄ and forms more nitrogen defects. Photo-excited electrons can be captured by the defective sites derived from the defect levels, and the recombination rate of photoinduced carriers will be significantly inhibited, accordingly facilitating the high-efficiency separation of photo-induced carriers and improving photocatalytic efficiency. This study provides an advantageous instruction for the implementation of rare earth metals application in improving the separation and transfer rate of photo-induced electrons (e⁻) and holes (h⁺) over g-C₃N₄.