Vacancy Engineering in 2D Transition Metal Chalcogenide Photocatalyst: Structure Modulation, Function and Synergy Application

Transition metal chalcogenides (TMCs) are widely used in photocatalytic fields such as hydrogen evolution, nitrogen fixation, and pollutant degradation due to their suitable bandgaps, tunable electronic and optical properties, and strong reducing ability. The unique 2D malleability structure provides a pre-designed platform for customizable structures. The introduction of vacancy engineering makes up for the shortcomings of photocorrosion and limited light response and provides the greatest support for TMCs in terms of kinetics and thermodynamics in photocatalysis. This work reviews the effect of vacancy engineering on photocatalytic performance based on 2D semiconductor TMCs. The characteristics of vacancy introduction strategies are summarized, and the development of photocatalysis of vacancy engineering TMCs materials in energy conversion, degradation, and biological applications is reviewed. The contribution of vacancies in the optical range and charge transfer kinetics is also discussed from the perspective of structure manipulation. Vacancy engineering not only controls and optimizes the structure of the TMCs, but also improves the optical properties, charge transfer, and surface properties. The synergies between TMCs vacancy engineering and atomic doping, other vacancies, and heterojunction composite techniques are discussed in detail, followed by a summary of current trends and potential for expansion.

Facile synthesis of Zn0.5Cd0.5S nanosheets with tunable S vacancies for highly efficient photocatalytic hydrogen evolution

In order to effectively improve the separation efficiency of photogenerated charge carriers and thus the photocatalytic activity, in this work, porous Zn0.5Cd0.5S nanosheets with a controlled amount of S vacancies were prepared by a multistep chemical transformation strategy using the inorganic-organic hybrid ZnS-ethylenediamine (denoted as ZnS(en)0.5) as a hard template. The amount of S vacancies and the morphology of the Zn0.5Cd0.5S nanostructures were tailored by adjusting the hydrolysis time. Furthermore, we report the observation of S vacancies in porous Zn0.5Cd0.5S nanosheets at the atomic level using spherical aberration-corrected (Cs-aberrated) transmission electron microscopy (Cs-corrected-TEM). The results revealed that Zn0.5Cd0.5S nanosheets with S vacancies absorb more visible light and generate more electron-hole carriers due to their porous nanosheet structure. At the same time, sulfur vacancies are introduced into the Zn0.5Cd0.5S nanosheets to capture the electrons generated by the light and further extend the lifetime of the carriers. As expected, the photocatalytic activity of Zn0.5Cd0.5S nanosheets prepared by 4 h hydrolysis is 20.5 times higher than that of Zn0.5Cd0.5S(en)x intermediates. Moreover, Zn0.5Cd0.5S-4h showed excellent cycling stability. This work provides a new strategy for the optimization of Zn0.5Cd0.5S photocatalysts to improve photocatalytic hydrogen evolution.

Sulfur vacancy-enhanced In2S3-x hollow microtubes for photocatalytic Cr (VI) and tetracycline removal

Morphological regulation and defect engineering are efficient methods for photocatalytic technology by improving photon absorption and electron dissociation. Herein, In2S3-x hollow microtubes with S-vacancies (MIS) were fabricated via a simple solvothermal reaction using In-based metal-organic frameworks (In-MOFs) as a precursor. Experimental results demonstrate that the hollow structure and optimal S-vacancies can jointly accelerate the photocatalytic reaction, attributed to a larger specific surface area, more active sites, and faster electron transfer efficiency. The champion MIS(2) displayed significantly better photocatalytic activity for Cr(VI) reduction and tetracycline (TC) degradation. The Cr(VI) reduction rate by MIS(2) is 3.67 and 2.82 times higher than those of optimal In2S3 template-free (HIS(2)) and MIS(1) with poor S-vacancies, respectively. The removal efficiency of TC by MIS(2) is 1.37 and 1.15 times higher than those of HIS(2) and MIS(1). Further integration of MIS(2) with aerogel simplifies the recovery process significantly.

Defect and Donor Manipulated Highly Efficient Electron-Hole Separation in a 3D Nanoporous Schottky Heterojunction

Given the rapid recombination of photogenerated charge carriers and photocorrosion, transition metal sulfide photocatalysts usually suffer from modest photocatalytic performance. Herein, S-vacancy-rich ZnIn2S4 (VS-ZIS) nanosheets are integrated on 3D bicontinuous nitrogen-doped nanoporous graphene (N-npG), forming 3D heterostructures with well-fitted geometric configuration (VS-ZIS/N-npG) for highly efficient photocatalytic hydrogen production. The VS-ZIS/N-npG presents ultrafast interfacial photogenerated electrons captured by the S vacancies in VS-ZIS and holes neutralization behaviors by the extra free electrons in N-npG during photocatalysis, which are demonstrated by in situ XPS, femtosecond transient absorption (fs-TA) spectroscopy, and transient-state surface photovoltage (TS-SPV) spectra. The simulated interfacial charge rearrangement behaviors from DFT calculations also verify the separation tendency of photogenerated charge carriers. Thus, the optimized VS-ZIS/N-npG 3D hierarchical heterojunction with 1.0 wt % N-npG exhibits a comparably high hydrogen generation rate of 4222.4 μmol g-1 h-1, which is 5.6-fold higher than the bare VS-ZIS and 12.7-fold higher than the ZIS without S vacancies. This work sheds light on the rational design of photogenerated carrier transfer paths to facilitate charge separation and provides further hints for the design of hierarchical heterostructure photocatalysts.

Syntheses, crystal structure, and photoelectric properties of two Zn-based chalcogenidoantimonates Zn-Sb-Q (Q = S, Se)

Metal chalcogenides are a special class of semiconducting materials characterized by their rich structures and compositions, making them a promising option for a broad range of applications in the field of inorganic chemistry. However, the path forward is not without its challenges, notably in the realms of interface management and enhancing carrier concentration. To address these issues, we solvothermally synthesized two novel chalcogenidoantimonates [Zn(tren)]2Sb2Se5 (1) [tren = tris (2-aminoethyl) amine] and [Zn(tepa)H]2Sb2S6 (2) (tepa = tetraethylenepentamine) utilizing transition metal Zn by band gap optimization strategy in the visible region. Both compounds exhibited distinct zero-dimensional cluster structures, with transition metal complex cations acting as structure-directing agents. A comprehensive analysis of the electronic structure, band gap, and photocurrent response of these crystals was undertaken, revealing significantly enhanced photocatalytic properties compared to preceding studies. This research underscores the potential of antimony chalcogenides in the realm of photoelectric properties and promotes the applications of chalcogenides.

Highly efficient flower-like ZnIn2S4/CoFe2O4 photocatalyst with p-n type heterojunction for enhanced hydrogen evolution under visible light irradiation

The construction of a p-n heterojunction structure is considered to be an effective method to improve the separation of electron-hole pairs in photocatalysts. A series of ZnIn₂S₄/CoFe₂O₄ (ZIS/CFO) photocatalysts with p-n heterojunctions were prepared via a method involving ultrasonication and calcination. The synthesized photocatalysts were tested and analyzed via various testing techniques, and their hydrogen evolution rates were evaluated. Compared with pure ZIS, ZIS/CFO with different mass ratios of CFO to ZIS showed improved photocatalytic hydrogen production performance, and the optimal photoactivity showed a nearly 12-fold increase, which can be attributed to the formation of p-n junctions and the formed internal electric field, accelerating the separation of electron-hole pairs and effectively improving the photocatalytic hydrogen evolution rate. The excellent stability of the ZIS/CFO composite was proven by three cycle experiments. In addition, the ZIS/CFO composite also possessed excellent magnetic properties to realize facial magnetic recoverability. This work paves the way for the design and preparation of magnetically recoverable p-n heterojunction photocatalysts.

Interfacial engineering of Ti3C2 MXene/CdIn2S4 Schottky heterojunctions for boosting visible-light H2 evolution and Cr(VI) reduction

Developing efficient heterojunction photocatalysts that have a high charge carrier separation rate and improved light-harvesting capacity is a crucial step in solving energy crisis and reducing environmental pollution. Herein, we synthesized few-layered Ti₃C₂ MXene sheets (MXs) by a manual shaking process, and combined with CdIn₂S₄ (CIS) to construct novel Ti₃C₂ MXene/CdIn₂S₄ (MXCIS) Schottky heterojunction through a solvothermal method. The strong interface between two-dimensional (2D) Ti₃C₂ MXene and 2D CIS nanoplates led to enhanced light-harvesting capacity and promoted charge separation rate. Additionally, the presence of S vacancies on the MXCIS surface helped to trap free electrons. The optimal sample, 5-MXCIS (with 5 wt% MXs loading), exhibited outstanding performance for photocatalytic hydrogen (H₂) evolution and Cr(VI) reduction under visible light due to the synergistic effect of enhanced light-harvesting capacity and charge separation rate. The charge transfer kinetics was thoroughly studied using multiple techniques. The reactive species of ^•O₂^-, ^•OH and h⁺ were generated in 5-MXCIS system, and e^- and ^•O₂^- radicals were found to be the main contributors to Cr(VI) photoreduction. Based on the characterization results, a possible photocatalytic mechanism for H₂ evolution and Cr(VI) reduction was proposed. On the whole, this work provides new insights into the design of 2D/2D MXene-based Schottky heterojunction photocatalysts for boosting photocatalytic efficiency.

Oxygen-Defect-Mediated ZnCr2O4/ZnIn2S4 Z-Scheme Heterojunction as Photocatalyst for Hydrogen Production and Wastewater Remediation

Photocatalysis has long been considered a promising technology in green energy and environmental remediation. Since the poor performance of single components greatly limits the practical applications, the construction of heterostructures has become one of the most important technical means to improve the photocatalytic activity. In this work, based on the synthesis of oxygen-vacancy-rich ZnCr₂O₄ nanocrystals, ZnCr₂O₄/ZnIn₂S₄ composites are prepared via a low-temperature in situ growth, and the oxygen-vacancy-induced Z-scheme heterojunction is successfully constructed. The unique core-shell structure offers a tight interfacial contact, increases the specific surface area, and promotes the rapid charge transfer. Meanwhile, the oxygen-vacancy defect level not only enables wide-bandgap ZnCr₂O₄ to be excited by visible light enhancing the light absorption, but also provides necessary conditions for the construction of Z-scheme heterojunctions promoting charge separation and migration and allowing more reactive charges. The reaction rates of visible-light-driven photocatalytic hydrogen production (3.421 mmol g^-1 h^-1), hexavalent chromium reduction (0.124 min^-1), and methyl orange degradation (0.067 min^-1) of the composite reach 3.6, 6.5, and 8.4 times those of pure ZnIn₂S₄, and 15.8, 41.3, and 67.0 times those of pure ZnCr₂O₄, respectively. This work presents a novel option for constructing high-performance photocatalysts.

Sulfur Vacancy and Ti3 C2 Tx Cocatalyst Synergistically Boosting Interfacial Charge Transfer in 2D/2D Ti3 C2 Tx /ZnIn2 S4 Heterostructure for Enhanced Photocatalytic Hydrogen Evolution

Constructing an efficient photoelectron transfer channel to promote the charge carrier separation is a great challenge for enhancing photocatalytic hydrogen evolution from water. In this work, an ultrathin 2D/2D Ti3 C2 Tx /ZnIn2 S4 heterostructure is rationally designed by coupling the ultrathin ZnIn2 S4 with few-layered Ti3 C2 Tx via the electrostatic self-assembly strategy. The 2D/2D Ti3 C2 Tx /ZnIn2 S4 heterostructure possesses larger contact area and strong electronic interaction to promote the charge carrier transfer at the interface, and the sulfur vacancy on the ZnIn2 S4 acting as the electron trap further enhances the separation of the photoinduced electrons and holes. As a consequence, the optimal 2D/2D Ti3 C2 Tx /ZnIn2 S4 composite exhibits a high photocatalytic hydrogen evolution rate of 148.4 µmol h-1 , which is 3.6 times and 9.2 times higher than that of ZnIn2 S4 nanosheet and flower-like ZnIn2 S4 , respectively. Moreover, the stability of the ZnIn2 S4 is significantly improved after coupling with the few-layered Ti3 C2 Tx . The characterizations and density functional theory calculation demonstrate that the synergistic effect of the sulfur vacancy and Ti3 C2 Tx cocatalyst can greatly promote the electrons transfer from ZnIn2 S4 to Ti3 C2 Tx and the separation of photogenerated charge carriers, thus enhancing the photocatalytic hydrogen evolution from water.

Photocatalytic C–H activation for C–C/CN/C–S bond formation over CdS: effect of morphological regulation and S vacancies

CdS catalytic materials were utilized to fabricate C–C, CN and C–S bonds for drug intermediates or other value-added products through the high bond energy, low polarity and strong inertia C–H bonds activation.

Regulating the surface state of ZnIn2S4 by gamma-ray irradiation for enhanced photocatalytic hydrogen evolution

Surface state of ZnIn2S4 is regulated with γ-ray radiation, achieving over 10 times enhanced photocatalytic H2-evolution performance, which provides reasonable inspiration to regulate the surface vacancies of photocatalysts for enhanced activity.

Robust visible-light photocatalytic H2 evolution on 2D RGO/Cd0.15Zn0.85In2S4–Ni2P hierarchitectures

Unique 2D ternary hierarchitectures constructed from reduced graphene oxide nanosheets grown with ultrathin Cd0.15Zn0.85In2S4 nanosheets and Ni2P nanoparticles exhibited an outstanding capability for visible-light photocatalytic H2 production.

ZnIn2 S4 -Based Photocatalysts for Energy and Environmental Applications

As a fascinating visible-light-responsive photocatalyst, zinc indium sulfide (ZnIn₂ S₄ ) has attracted extensive interdisciplinary interest and is expected to become a new research hotspot in the near future, due to its nontoxicity, suitable band gap, high physicochemical stability and durability, ease of synthesis, and appealing catalytic activity. This review provides an overview on the recent advances in ZnIn₂ S₄ -based photocatalysts. First, the crystal structures and band structures of ZnIn₂ S₄ are briefly introduced. Then, various modulation strategies of ZnIn₂ S₄ are outlined for better photocatalytic performance, which includes morphology and structure engineering, vacancy engineering, doping engineering, hydrogenation engineering, and the construction of ZnIn₂ S₄ -based composites. Thereafter, the potential applications in the energy and environmental area of ZnIn₂ S₄ -based photocatalysts are summarized. Finally, some personal perspectives about the promises and prospects of this emerging material are provided.

Molybdenum sulfide-modified metal-free graphitic carbon nitride/black phosphorus photocatalyst synthesized via high-energy ball-milling for efficient hydrogen evolution and hexavalent chromium reduction

Improving the photocatalytic property of metal-free photocatalyst is still a challenging work. Herein, a novel high-efficiency molybdenum sulfide (MoS₂)-modified metal-free graphitic carbon nitride (g-C₃N₄)/black phosphorus (BP) photocatalyst (MCN/BP/MS) was synthesized on a large scale via high-energy ball milling process. The optimized MCN/BP/MS exhibits the excellent hydrogen evolution rate of 2146.8 µmol·g^-1·h^-1, and hexavalent chromium (Cr(Ⅵ)) reduction activity with an apparent rate constant of 0.1464 min^-1 and a degradation rate of 100% in 25 min. Detailed characterizations and mechanism research verified that the significantly improved photocatalytic activity of MCN/BP/MS mainly profited from the matched band structure, enhanced light absorption, intense interface contact, as well as the type-Ⅰ/Z hybrid charge transfer mechanism, which gave rise to a consecutive multistep charge migration, thus the photocarriers transfer and separation can be greatly promoted, and the photogenerated electrons with high reducing capacity can be preserved. This work not only provides a high-efficiency g-C₃N₄-based noble-metal-free photocatalyst, but also affords a beneficial inspiration for improving the photocatalytic property of the metal free photocatalyst.

Interfacial chemical bond and internal electric field modulated Z-scheme Sv-ZnIn2S4/MoSe2 photocatalyst for efficient hydrogen evolution

Construction of Z-scheme heterostructure is of great significance for realizing efficient photocatalytic water splitting. However, the conscious modulation of Z-scheme charge transfer is still a great challenge. Herein, interfacial Mo-S bond and internal electric field modulated Z-scheme heterostructure composed by sulfur vacancies-rich ZnIn₂S₄ and MoSe₂ was rationally fabricated for efficient photocatalytic hydrogen evolution. Systematic investigations reveal that Mo-S bond and internal electric field induce the Z-scheme charge transfer mechanism as confirmed by the surface photovoltage spectra, DMPO spin-trapping electron paramagnetic resonance spectra and density functional theory calculations. Under the intense synergy among the Mo-S bond, internal electric field and S-vacancies, the optimized photocatalyst exhibits high hydrogen evolution rate of 63.21 mmol∙g^-1·h^-1 with an apparent quantum yield of 76.48% at 420 nm monochromatic light, which is about 18.8-fold of the pristine ZIS. This work affords a useful inspiration on consciously modulating Z-scheme charge transfer by atomic-level interface control and internal electric field to signally promote the photocatalytic performance.

Constructing defect-related subband in silver indium sulfide QDs via pH-dependent oriented aggregation for boosting photocatalytic hydrogen evolution

Surface engineering of quantum dots (QDs) plays critical roles in tailoring carriers' dynamics of I-III-VI QDs via the interplay of QDs in aggregates or assembly, thus influencing their photocatalytic activities. In this work, an aqueous synthesis and the followed pH tuned oriented assembly method are developed to prepare network-like aggregates, dispersion, or sheet-like assembly of GSH-capped Silver Indium Sulfide (AIS). FTIR, DLS, and HRTEM investigation revealed that surface protonation or deprotonation of QDs occurred at pH < 6 or pH > 12 favors the formation of network-like aggregates with various defects or sheet-like assembly with perfect crystal lattice, respectively, via the surface charge induced interaction among AIS QDs. Further UV-vis, steady and transient PL investigation confirm the narrowed band gaps and the prolonged PL lifetime of the acidic network-like aggregates. As a result, the optimized network-like aggregates (3.0-AIS) exhibits superior photocatalytic H₂ evolution (PHE) rates (5.2 mmol·g^-1·h^-1), about 113 times that of alkaline sheet-like assembly (13.0-AIS) or 2.7 times higher than that of dispersed AIS QDs (AIS-8.0). The formation of defects and their roles in PHE mechanisms are discussed. This work is expected to give some new insight for designing efficient non-cadmium/non-novel metal I-III-VI photocatalysts for boosting PHE.

Sulfur vacancies and morphology dependent sodium storage properties of MoS2-x and its sodiation/desodiation mechanism

Creating rich vacancies and designing distinct micro-morphology are considered as effective strategies for boosting the electrochemical performances of sodium ion battery (SIB) electrode materials. In this paper, a variety of MoS₂ nanostructures with different sulfur vacancies concentration and morphologies are successfully constructed by a hydrothermal method combined with various-temperature calcination treatment in a Ar/H₂ mixed atmosphere. Employed as a free-standing anode for SIBs, the flower-like MoS_2-x microspheres assembled by the intertwined nanosheet arrays (MoS_2-x-800) delivers highest specific capacity of 525.3 mAh g^-1 and rate capability, as well as extraordinarily stable cycle life with almost no loss of capacity after 420 cycles. The favorable sodium storage properties are mainly ascribed to the cooperated effects of superior intrinsic conductivity and richer active sites generated by sulfur vacancies, and numerous interspace achieved by the intersection of neighbouring nanosheets. Meanwhile, through ex situ analyses, the reversible charge/discharge mechanism of the obtained MoS_2-x-800 is revealed reasonably. This work not only brings new insights into the design of high-performance electrode materials for SIBs, but also makes a great step forward in the practical applications of transition metal sulfides in energy storage systems.

Spinel-type FeNi2S4 with rich sulfur vacancies grown on reduced graphene oxide toward enhanced supercapacitive performance

Due to the coexistence of rich sulfur vacancies and rGO, the r-FeNi₂S₄-rGO electrode shows a superior specific capacitance.