Significantly Raised Visible-Light Photocatalytic H2 Evolution on a 2D/2D ReS2 /In2 ZnS4 van der Waals Heterostructure

Defect Emission and Its Dipole Orientation in Layered Ternary Znln2 S4 Semiconductor

Defect engineering is promising to tailor the physical properties of 2D semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors are not clarified. Here, the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2 S4 is thoroughly studied via versatile spectroscopic tools in combination with theoretical calculations. It is demonstrated that the Zn-In antistructural defects induce the formation of a series of donor and acceptor energy levels and sulfur vacancies induce donor energy levels, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair in Znln2 S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef ). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2 S4 is directly revealed by back focal plane imagining, where it presents obviously in-plane dipole orientation within a dozen-layer thickness of Znln2 S4 . These unique features of defects in Znln2 S4 including extrinsic absorption, rich recombination paths, gate tunability, and in-plane dipole orientation are definitely a benefit to the advanced orientation-functional optoelectronic applications.

Accelerate charge separation in Cu2O/MoO2 photocathode for photoelectrocatalytic hydrogen evolution

Photoelectrocatalyzing water reduction is a potential approach to building a green and sustainable society. As a benchmark photocathode, Cu2O receives much attention but faces serious charge recombination and photocorrosion. This work prepared an excellent Cu2O/MoO2 photocathode via in situ electrodeposition. A systematical study of theory and experiment demonstrates that MoO2 not only effectively passivates the surface state of Cu2O as well as accelerates reaction kinetics as a cocatalyst, but also promotes the directional migration and separation of photogenerated charge. As expected, the constructed photocathode exhibits a highly enhanced photocurrent density and an appealing energy transformation efficacy. Importantly, MoO2 can inhibit the reduction of Cu+ in Cu2O via a formed internal electric field and shows excellent photoelectrochemical stability. These findings pave the way to designing a high-activity photocathode with high stability.

A π-Conjugated Van der Waals Heterostructure Between Single-Atom Ni-Anchored Salphen-Based Covalent Organic Framework and Polymeric Carbon Nitride for High-Efficiency Interfacial Charge Separation

Semiconductor-based heterostructures have exhibited great promise as a photocatalyst to convert solar energy into sustainable chemical fuels, however, their solar-to-fuel efficiency is largely restricted by insufficient interfacial charge separation and limited catalytically active sites. Here the integration of high-efficiency interfacial charge separation and sufficient single-atom metal active sites in a 2D van der Waals (vdW) heterostructure between ultrathin polymeric carbon nitride (p-CN) and Ni-containing Salphen-based covalent organic framework (Ni-COF) nanosheets is illustrated. The results reveal a NiN2 O2 chemical bonding in NiCOF nanosheets, leading to a highly separated single-atom Ni sites, which will function as the catalytically active sites to boost solar fuel production, as confirmed by X-ray absorption spectra and density functional theory calculations. Using ultrafast femtosecond transient adsorption (fs-TA) spectra, it shows that the vdW p-CN/Ni-COF heterostructure exhibits a faster decay lifetime of the exciton annihilation (τ = 18.3 ps) compared to that of neat p-CN (32.6 ps), illustrating an efficiently accelerated electron transfer across the vdW heterointerface from p-CN to Ni-COF, which thus allows more active electrons available to participate in the subsequent reduction reactions. The photocatalytic results offer a chemical fuel generation rate of 2.29 mmol g-1 h-1 for H2 and 6.2 µmol g-1 h-1 for CO, ≈127 and three times higher than that of neat p-CN, respectively. This work provides new insights into the construction of a π-conjugated vdW heterostructure on promoting interfacial charge separation for high-efficiency photocatalysis.

Methylthio-functionalized UiO-66 to promote the electron-hole separation of ZnIn2S4 for boosting hydrogen evolution under visible light illumination

Solar-driven water splitting offers a leading-edge approach to storing abundant and intermittent solar energy and producing hydrogen as a clean and sustainable energy carrier. More importantly, constructing well-designed photocatalysts is a promising approach to develop clean hydrogen energy. In this paper, flower spherical UiO-66-(SCH₃)₂/ZnIn₂S₄ (UiOSC/ZIS) photocatalysts are successfully synthesized by a simple two-step hydrothermal method, and they exhibit high hydrogen production activity in light-driven water splitting. The optimized 30-UiOSC/ZIS (the content of UiOSC was 30 mg) composite exhibits optimal hydrogen production activity with a hydrogen production of 3433 μmol g^-1 h^-1, which is 5 and 235 times higher than that of pure ZIS and UiOSC, respectively. In addition, a long-cycling stability test has shown that the UiOSC/ZIS composite has good stability and recyclability. Experimental and characterization results show the formation of a type-II heterojunction between UiOSC and ZIS. This effectively suppresses the recombination of electrons-holes and promotes the carrier transfer, thus significantly improving the hydrogen production performance. This research further promotes the application of UiO-66-(SCH₃)₂ in the field of photocatalytic hydrogen production and provides a reference for the rational design of UiO-66-based composite photocatalysts.

Atomic-Level Regulated 2D ReSe2 : A Universal Platform Boostin Photocatalysis

Solar hydrogen (H₂ ) generation via photocatalytic water splitting is practically promising, environmentally benign, and sustainably carbon neutral. It is important therefore to understand how to controllably engineer photocatalysts at the atomic level. In this work, atomic-level engineering of defected ReSe₂ nanosheets (NSs) is reported to significantly boost photocatalytic H₂ evolution on various semiconductor photocatalysts including TiO₂ , CdS, ZnIn₂ S₄ , and C₃ N₄ . Advanced characterizations, such as atomic-resolution aberration-corrected scanning transmission electron microscopy (AC-STEM), synchrotron-based X-ray absorption near edge structure (XANES), in situ X-ray photoelectron spectroscopy (XPS), transient-state surface photovoltage (SPV) spectroscopy, and transient-state photoluminescence (PL) spectroscopy, together with theoretical computations confirm that the strongly coupled ReSe₂ /TiO₂ interface and substantial atomic-level active sites of defected ReSe₂ NSs result in the significantly raised activity of ReSe₂ /TiO₂ . This work not only for the first time realizes the atomic-level engineering of ReSe₂ NSs as a versatile platform to significantly raise the activities on different photocatalysts, but, more importantly, underscores the immense importance of atomic-level synthesis and exploration on 2D materials for energy conversion and storage.

ReS2 with unique trion behavior as a co-catalyst for enhanced sunlight hydrogen production

The interfacial catalytic reaction plays a crucial role in determining hydrogen production efficiency of a photocatalyst. In this work, hollow spherical nano-shell composite (g-C₃N₄/CdS/ReS₂) formed by graphitic carbon nitride (g-C₃N₄), cadmium sulfide (CdS), and rhenium disulfide (ReS₂) was prepared for photocatalytic hydrogen production, with ReS₂ introduced as a relatively inexpensive co-catalyst with excellent performance. It was found that two-electron catalytic reaction took place in this photocatalytic system due to the unique trion behavior of ReS₂ co-catalyst, which greatly enhances the rate of photocatalytic hydrogen production. The tightly bound excitons in the ReS₂ co-catalyst could easily capture the photogenerated electrons in the photocatalytic system to form trions, while g-C₃N₄ in the inner shell and CdS in the middle shell provided sufficient electrons for the formation of trions. The active edge sites of ReS₂ also facilitated the generation and desorption of hydrogen, which creates conditions favoring two-electron catalytic reaction. In addition, oxidation and reduction reactions occurred inside and outside of the hollow spherical nano-shell, respectively, which effectively inhibits the recombination of photogenerated carriers. The unique trion behavior of ReS₂ alters the interfacial catalytic reaction compared to the widely used platinum (Pt) co-catalyst in photocatalytic hydrogen production, which provides a new approach for enhancing the activity of photocatalytic systems.

Photocatalytic Applications of ReS2-Based Heterostructures

ReS2-based heterostructures, which involve the coupling of a narrow band-gap semiconductor ReS2 with other wide band-gap semiconductors, have shown promising performance in energy conversion and environmental pollution protection in recent years. This review focuses on the preparation methods, encompassing hydrothermal, chemical vapor deposition, and exfoliation techniques, as well as achievements in correlated applications of ReS2-based heterostructures, including type-I, type-II heterostructures, and Z-scheme heterostructures for hydrogen evolution, reduction of CO2, and degradation of pollutants. We believe that this review provides an overview of the most recent advances to guide further research and development of ReS2-based heterostructures for photocatalysis.

Interface Engineering in 2D/2D Heterogeneous Photocatalysts

Assembling different 2D nanomaterials into heterostructures with strong interfacial interactions presents a promising approach for novel artificial photocatalytic materials. Chemically implementing the 2D nanomaterials' construction/stacking modes to regulate different interfaces can extend their functionalities and achieve good performance. Herein, based on different fundamental principles and photochemical processes, multiple construction modes (e.g., face-to-face, edge-to-face, interface-to-face, edge-to-edge) are overviewed systematically with emphasis on the relationships between their interfacial characteristics (e.g., point, linear, planar), synthetic strategies (e.g., in situ growth, ex situ assembly), and enhanced applications to achieve precise regulation. Meanwhile, recent efforts for enhancing photocatalytic performances of 2D/2D heterostructures are summarized from the critical factors of enhancing visible light absorption, accelerating charge transfer/separation, and introducing novel active sites. Notably, the crucial roles of surface defects, cocatalysts, and surface modification for photocatalytic performance optimization of 2D/2D heterostructures are also discussed based on the synergistic effect of optimization engineering and heterogeneous interfaces. Finally, perspectives and challenges are proposed to emphasize future opportunities for expanding 2D/2D heterostructures for photocatalysis.

In situself-assembly fabrication of ultrathin sheet-like CuS modified g-C3N4heterojunction and its enhanced visible-light photocatalytic performance

Photocatalysts with heterojunction structure have been widely used for organic degradation. In this study, CuS/g-C₃N₄heterojunction was formed byin situself-assembly via a simply hydrothermal method. A series of characterizations were applied to analyzing the morphology, structure, optical properties and photo-induced electron transfer of the samples. The effect of CuS mass ratio in the CuS/g-C₃N₄composite on methyl blue (10 mg l^-1) degradation under visible-light illumination was discussed. When CuS mass ratio was 60%, CuS/g-C₃N₄behaved the highest photocatalytic efficiency which is 17 times higher than that of pure g-C₃N₄, and the optimal heterojunction exhibited promising photocatalytic stability as well. The synthesized CuS/g-C₃N₄with intimate contact and promising photocatalytic performance provides important implications on analogous researches on g-C₃N₄-based heterojunctions for photocatalytic applications.

Junction of ZnmIn2S3+m and bismuth vanadate as Z-scheme photocatalyst for enhanced hydrogen evolution activity: The role of interfacial interactions

A series of Zn_mIn₂S_3+m photocatalysts were synthesized to show tunable band gap energy with the variation of Zn/S atomic ratio. The junction of Zn_mIn₂S_3+m and BiVO₄ led to intimate interfacial contacts. Both experimental and theoretical results implied that electrons flowed from Zn_mIn₂S_3+m to BiVO₄ at the Zn_mIn₂S_3+m/BiVO₄ interface to form built-in electric field due to the variation of Fermi level, which promised Z scheme charge transfer feature for improving separation of charge carriers for enhanced photocatalytic performance. A higher degree of charge transfer process occurred for Zn₂In₂S₅/BiVO₄ heterostructure promised stronger built-in electric field, higher charge separation efficiency and improved photocatalytic activity in comparison to ZnIn₂S₄/BiVO₄ and Zn₃In₂S₆/BiVO₄ heterojunctions. The optimal hydrogen production rate of Zn₂In₂S₅/BiVO₄ photocatalyst is 8.42 mmol•g^-1•h^-1 with apparent quantum yield of 22.32 % at 435 nm, which is about 2.2 and 1.5 times higher than that of ZnIn₂S₄/BiVO₄ and Zn₃In₂S₆/BiVO₄, respectively.

Promoted photocarrier separation by dipole engineering in two-dimensional perovskite/C2N van der Waals heterostructures

Due to the aggravation of environmental pollution and the energy crisis, it is urgent to develop and design environment-friendly and efficient photocatalysts for water splitting. van der Waals heterostructures composed of different two-dimensional materials offer an easily accessible way to combine properties of individual materials for applications. Herein, a novel Cs₃Bi₂I₉/C₂N heterostructure is proposed through first-principles calculations. The structural, electronic, and optical properties, as well as the charge transfer mechanism at the interface of Cs₃Bi₂I₉/C₂N are systematically investigated. Due to the difference between the work functions of Cs₃Bi₂I₉ and C₂N monolayers, when they are constructed into heterostructures, redistribution of charge occurs in the whole structure, and some of the charge transfer occurs at the interface due to the formation of an internal electric field. The band structure of Cs₃Bi₂I₉/C₂N has type-II band alignment, and the band edge position as well as the band-gap value of the heterostructure are suitable for visible light water splitting. The in-plane biaxial strain, interfacial spacing, and external electric field can effectively modulate the electronic structure and photocatalytic performance of the heterostructure. Under certain conditions, the heterostructure can be changed from type-II to type-I band alignment, accompanied by the transition from an indirect band-gap semiconductor to a direct band-gap semiconductor. Moreover, the intrinsic anion defect (I vacancy) at different positions, as donor defects, can introduce defect levels near the conduction band edge, which affects the transition of photogenerated carriers in these systems. Our findings provide a theoretical design for strategies to improve the performance of two-dimensional perovskites/C₂N in photocatalytic and optoelectronic applications.

A MOF-derived hierarchical CoP@ZnIn2S4 photocatalyst for visible light-driven hydrogen evolution

A hierarchical CoP@ZnIn₂S₄ photocatalyst was prepared via a MOF-templated strategy. Owing to the unique composition and morphology that can facilitate the separation of photoexcited carriers, enhance light absorption and provide high surface area, CoP@ZnIn₂S₄ exhibited a H₂ evolution rate of 0.103 mmol h^-1 and remained stable over 24 hours.

Hollow Multishell-Structured TiO2/MAPbI3 Composite Improves Charge Utilization for Visible-Light Photocatalytic Hydrogen Evolution

Interfacial photogenerated charge separation and transport have demonstrated great influence on photocatalytic performance. Herein, the composite photocatalysts of methylammonium lead iodide perovskite (MAPbI₃) in TiO₂ with a hollow multishell structure (HoMS) are designed and synthesized. The results indicate that the heterogeneous interface within the MAPbI₃/Pt/TiO₂-HoMS can help enhance the separation of photogenerated charges. HoMSs assembled with multiple shells can not only support large surfaces available for building a heterogeneous interface and photocatalytic reactions but also improve the light absorption capability of photocatalysts. Besides, the thin shell structure can also reduce the transmission distance of carriers so as to hinder charge recombination and improve charge utilization. As a result, samples of MAPbI₃/Pt/triple-shelled TiO₂ hollow structure displayed a H₂ yield of 6856.2 μmol h^-1 g^-1 under visible light, which is greatly better than that of bare MAPbI₃ (268.6 μmol h^-1 g^-1).

Highly Conjugated Graphitic Carbon Nitride Nanofoam for Photocatalytic Hydrogen Evolution

As a metal-free photocatalyst, graphitic carbon nitride (g-CN) shows great potential for photocatalytic water splitting, although its performance is significantly limited by structural defects due to incomplete polymerization. In the present work, we successfully synthesize highly conjugated g-CN nanofoam through an iodide substitution technique. The product possesses a high polymerization degree, low defect density, and large specific surface area; as a result, it achieves a hydrogen evolution rate of 9.06 mmol h^-1 g^-1 under visible light irradiation, with an apparent quantum efficiency (AQE) of 18.9% at 420 nm. Experimental analysis and theoretical calculations demonstrate that the recombination of photogenerated carriers at C-NH_x defects was effectively depressed in the nanofoam, giving rise to the high photocatalytic activity.

Optimizing Atomic Hydrogen Desorption of Sulfur-Rich NiS1+ x Cocatalyst for Boosting Photocatalytic H2 Evolution

Low-cost transition-metal chalcogenides (MS_x ) are demonstrated to be potential candidate cocatalyst for photocatalytic H₂ generation. However, their H₂ -generation performance is limited by insufficient quantities of exposed sulfur (S) sites and their strong bonding with adsorbed hydrogen atoms (SH_ads ). To address these issues, an efficient coupling strategy of active-site-enriched regulation and electronic structure modification of active S sites is developed by rational design of core-shell Au@NiS_{1+ x} nanostructured cocatalyst. In this case, the Au@NiS_{1+ x} cocatalyst can be skillfully fabricated to synthesize the Au@NiS_{1+ x} modified TiO₂ (denoted as TiO₂ /Au@NiS_{1+ x} ) by a two-step route. Photocatalytic experiments exhibit that the resulting TiO₂ /Au@NiS_{1+ x} (1.7:1.3) displays a boosted H₂ -generation rate of 9616 µmol h^-1 g^-1 with an apparent quantum efficiency of 46.0%  at 365 nm, which is 2.9 and 1.7 times the rate over TiO₂ /NiS_{1+ x} and TiO₂ /Au, respectively. In situ/ex situ XPS characterization and density functional theory calculations reveal that the free-electrons of Au can transfer to sulfur-enriched NiS_{1+ x} to induce the generation of electron-enriched S^{δ -} active centers, which boosts the desorption of H_ads for rapid hydrogen formation via weakening the strong SH_ads bonds. Hence, an electron-enriched S^{δ -} -mediated mechanism is proposed. This work delivers a universal strategy for simultaneously increasing the active site number and optimizing the binding strength between the active sites and hydrogen adsorbates.