Role of Interfaces in Two-Dimensional Photocatalyst for Water Splitting

Engineering 2D Materials for Photocatalytic Water-Splitting from a Theoretical Perspective

Splitting of water with the help of photocatalysts has gained a strong interest in the scientific community for producing clean energy, thus requiring novel semiconductor materials to achieve high-yield hydrogen production. The emergence of 2D nanoscale materials with remarkable electronic and optical properties has received much attention in this field. Owing to the recent developments in high-end computation and advanced electronic structure theories, first principles studies offer powerful tools to screen photocatalytic systems reliably and efficiently. This review is organized to highlight the essential properties of 2D photocatalysts and the recent advances in the theoretical engineering of 2D materials for the improvement in photocatalytic overall water-splitting. The advancement in the strategies including (i) single-atom catalysts, (ii) defect engineering, (iii) strain engineering, (iv) Janus structures, (v) type-II heterostructures (vi) Z-scheme heterostructures (vii) multilayer configurations (viii) edge-modification in nanoribbons and (ix) the effect of pH in overall water-splitting are summarized to improve the existing problems for a photocatalytic catalytic reaction such as overcoming large overpotential to trigger the water-splitting reactions without using cocatalysts. This review could serve as a bridge between theoretical and experimental research on next-generation 2D photocatalysts.

Bridging electrocatalyst and cocatalyst studies for solar hydrogen production via water splitting

Solar-driven water-splitting has been considered as a promising technology for large-scale generation of sustainable energy for succeeding generations. Recent intensive efforts have led to the discovery of advanced multi-element-compound water-splitting electrocatalysts with very small overpotentials in anticipation of their application to solar cell-assisted water electrolysis. Although photocatalytic and photoelectrochemical water-splitting systems are more attractive approaches for scaling up without much technical complexity and high investment costs, improving their efficiencies remains a huge challenge. Hybridizing photocatalysts or photoelectrodes with cocatalysts has been an effective scheme to enhance their overall solar energy conversion efficiencies. However, direct integration of highly-active electrocatalysts as cocatalysts introduces critical factors that require careful consideration. These additional requirements limit the design principle for cocatalysts compared with electrocatalysts, decelerating development of cocatalyst materials. This perspective first summarizes the recent advances in electrocatalyst materials and the effective strategies to assemble cocatalyst/photoactive semiconductor composites, and further discusses the core principles and tools that hold the key in designing advanced cocatalysts and generating a deeper understanding on how to further push the limits of water-splitting efficiency.

Sn-doped BiOI assisted the excellent photocatalytic performance of multi-shelled ZnO microspheres under simulated sunlight

To deal with the environmental pollution and energy crises, it is indispensable to find green and efficient means to overcome these challenges. Herein, the Sn-doped BiOI modified multi-shelled ZnO heterojunction composite with a high performance are designed and prepared. The results prove the formation of heterojunction structure in the composites and the morphology is shown as the multi-shelled microsphere. The performances of the composites are evaluated by different kinds of antibiotic degradation and H₂-evolution under simulation sunlight irradiation. The measurements present that the Sn-BiOI/ZnO (SBZs) could completely remove ciprofloxacin (CIP) within 100 min, which is 4.18 times that of ZnO in kinetics. Typically, the degradation rate of CIP for SBZ6 is over 99.9%, which is more than 25% higher than that of pure ZnO microspheres. In addition, the rate of H₂ production could reach 3.08 mmol g^-1∙ h^-1, which is 1.79 times of the pure ZnO microspheres. The boosted performance of the composites may originate from the enhanced electronic transmission efficiency and improved separation and recombination efficiency of electrons/holes. The charge transfer mode in the SBZs heterojunction composites is proposed and verified as the Z-scheme by the active species experimental and the possible electron transfer path analysis. Therefore, Sn-doped BiOI is introduced into multi-shelled ZnO microsphere to form contact heterojunction interfacial, which greatly improves the photocatalytic performances of the SBZs. Furthermore, this work supplies a strategy for designing and preparing highly active ZnO-based heterojunction composites, which could effectively address the challenges of environmental remediation and clean energy production.

An ab initio study of two-dimensional anisotropic monolayers ScXY (X = S and Se; Y = Cl and Br) for photocatalytic water splitting applications with high carrier mobilities

Recently, metal oxyhalides have been broadly studied due to their hierarchical structures and promising functionalities. Herein, a thorough study of newly modeled monolayers ScXY (X = S and Se; Y = Cl and Br), a class of derivates of ScOBr monolayers, was conducted using first-principles calculations. We theoretically confirm that these ScXY monolayers are mechanically, dynamically, and thermally stable. Young's modulus and Poisson's ratio calculated for all these ScXY monolayers obviously exhibit anisotropic properties. All these monolayers are indirect-gap semiconductors with bandgaps in the range of 2.35-3.18 eV, and their conduction band minimum (CBM) and valence band maximum (VBM) can straddle the reduction and oxidation potential of water very well, respectively. Particularly, ScSeCl and ScSeBr monolayers have the most propitious bandgaps and band alignments to be used as promising photocatalysts, and the predicted carrier mobility is much larger than that of many other two-dimensional materials. Moreover, the predicted anisotropic carrier mobilities and indirect bandgaps will diminish the recombination and facilitate the migration of photo-generated electron and hole pairs. Moreover, biaxial strain (-5% to 5%) effects on the band alignments and bandgaps are discussed. Our findings highlight that ScSeCl and ScSeBr monolayers are envisioned to act as promising photocatalytic and photoelectronic materials with anisotropic ultrahigh carrier mobilities.

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.

Epitaxial Growth of Flower-Like MoS2 on One-Dimensional Nickel Titanate Nanofibers: A “Sweet Spot” for Efficient Photoreduction of Carbon Dioxide

Herein, a full spectrum-induced hybrid structure consisting of one-dimensional nickel titanate (NiTiO₃) nanofibers (NFs) decorated by petal-like molybdenum disulfide (MoS₂) particles was designed through a facile hydrothermal method. The key parameters for tailoring the morphology and chemical, surface, and interfacial properties of the heterostructure were identified for efficient and selective conversion of CO₂ into valuable chemicals. Introducing MoS₂ layers onto NiTiO₃ NFs provided superior CO₂ conversion with significantly higher yields. The optimized hybrid structure produced CO and CH₄ yields of 130 and 55 μmol g⁻¹ h⁻¹, respectively, which are 3.8- and 3.6-times higher than those from pristine NiTiO₃ nanofibers (34 and 15 μmol g⁻¹ h⁻¹, respectively) and 3.6- and 5.5-times higher than those from pristine MoS₂ (37 and 10 μmol g⁻¹ h⁻¹, respectively). This improved performance was attributed to efficient absorption of a wider spectrum of light and efficient transfer of electrons across the heterojunction. Effective charge separation and reduced charge carrier recombination were confirmed by photoluminescence and impedance measurements. The performance may also be partly due to enhanced hydrophobicity of the hierarchical surfaces due to MoS₂ growth. This strategy contributes to the rational design of perovskite-based photocatalysts for CO₂ reduction.

Efficient Dye Contaminant Elimination and Simultaneously Electricity Production via a Bi-Doped TiO2 Photocatalytic Fuel Cell

TiO₂ develops a higher efficiency when doping Bi into it by increasing the visible light absorption and inhibiting the recombination of photogenerated charges. Herein, a highly efficient Bi doped TiO₂ photoanode was fabricated via a one-step modified sol-gel method and a screen-printing technique for the anode of photocatalytic fuel cell (PFC). A maximum degradation rate of 91.2% of Rhodamine B (RhB) and of 89% after being repeated 5 times with only 2% lost reflected an enhanced PFC performance and demonstrated an excellent stability under visible-light irradiation. The excellent degradation performance was attributed to the enhanced visible-light response and decreased electron-hole recombination rate. Meanwhile, an excellent linear correlation was observed between the efficient photocurrent of PFC and the chemical oxygen demand of solution when RhB is sufficient.

Sheet-on-sheet TiO2/Bi2MoO6 heterostructure for enhanced photocatalytic amoxicillin degradation

Developing sheet-on-sheet (2D/2D) heterostructure with built-in electric field (BIEF) is effective in boosting the performance of photocatalysts for emerging contaminants degradation. Herein, the 2D/2D microtopography and (-)TiO₂/(+)Bi₂MoO₆ BIEF were precisely integrated into hierarchical nanosheets, which can provide the basis and driving force for charge transfer both in in-plane and interface of heterojunction. The prepared photocatalyst (TiO₂/Bi₂MoO₆) showed high-efficiency and stable performance for photocatalytic amoxicillin (AMX) degradation, which was 18.2 and 5.7 times higher than TiO₂ and Bi₂MoO₆, respectively. More importantly, TiO₂/Bi₂MoO₆ showed more efficient photocatalytic activity and photogenerated charge separation than TiO₂@Bi₂MoO₆ (different morphology). Besides, four possible pathways of AMX degradation were proposed depending on Gaussian calculations and intermediates analysis by GC-MS and HPLC-TOFMS. This work sheds light on the design and construction of unique 2D/2D heterostructure photocatalysts for AMX degradation.

MOF nanosheet-derived carbon-layer-coated CoP/g-C3N4 photocatalysts with enhance charge transfer for efficient photocatalytic H2 generation

Catalysts with low cost, high efficiency, and no need for precious metals cocatalysts, are the key to realizing the maximum utilization of solar energy-efficient hydrogen (H2) production. In this study,...

Direct Z-scheme WTe2/InSe van der Waals heterostructure for overall water splitting

WTe2/InSe is a direct Z-scheme vdW heterostructure for water splitting. The Te-vacancy can effectively lower the energy of the HER, and the overall water splitting can proceed spontaneously on the surface of the WTe2/InSe heterostructure when pH > 7.

MoSSe/Hf(Zr)S2 heterostructures used for efficient Z-scheme photocatalytic water-splitting

HfS2/SMoSe, HfS2/SeMoS, ZrS2/SMoSe, and ZrS2/SeMoS heterostructures are promising overall water-splitting photocatalysts.

Atomically Thin 2D Photocatalysts for Boosted H2 Production from the perspective of Transient Absorption Spectroscopy

Excited state photophysical processes play the most important role in deciding the efficiency of any photonic applications like solar light driven H2 evolution, which is considered to be the next...

Unveiling the mechanism of high-performance hydrogen evolution reaction on noble-metal-free (113)-faceted Ni3C: ab initio calculations

To examine the reactivity of noble-metal-free Ni3C towards hydrogen evolution reaction (HER), we report a comprehensive first-principles density functional theory (DFT) study on the stability, geometric structure, electronic characteristics, and catalytic activity for HER on the Ni3C crystal (113) surfaces with different surface terminations, namely the C-rich and Ni-rich terminated surface of Ni3C (113). The results indicate that C-rich and some stoichiometric surfaces are thermodynamically stable. The bridge-site of C-rich Ni3C (113) is indispensable for HER because it not only displays improved electrocatalytic activity, but also possesses appropriate hydrogen adsorption energy, overpotential and robust stability. The ΔG H (0.02 eV) and overpotential obtained by C-rich Ni3C outperformed that obtained by Pt determined by computation (ΔG H = -0.07 eV). Thus, the bridge-sites of C-rich Ni3C (113) function as both excellent and stable active sites and adsorption/desorption sites. Increasing the density of active sites through doping or enlarging the surface area renders a prospective strategy to ameliorate the HER activity further. Overall, this study elucidates new insights into the surface properties of Ni3C for HER from water splitting and opens up a fascinating avenue to optimize the performance of solar energy conversion devices by synthesizing preferentially exposed catalyst facets.

Effect of the cross-linker length of thiophene units on photocatalytic hydrogen production of triazine-based conjugated microporous polymers

Conjugated microporous polymers (CMPs) have been investigated in the field of photocatalytic hydrogen production because of their extended π-conjugation, tunable chemical structure and excellent thermal stability. Herein, we construct three CMPs based on thiophenes and triazine, and prove the effect of cross-linker length on photocatalytic activity of CMPs. BTPT-CMP1 exhibits blue-shifted optical absorption compared to BTPT-CMP2 and BTPT-CMP3 with long cross-linkers, however, possesses higher photocurrent because of the large specific surface area and small interface charge transfer resistance of BTPT-CMP1. It was found that BTPT-CMP1 (5561.87 μmol g-1 h-1) with short cross-linkers exhibits better photocatalytic performance compared to BTPT-CMP2 (1840.86 μmol g-1 h-1) and BTPT-CMP3 (1600.48 μmol g-1 h-1). Also, BTPT-CMP1 possesses a higher hydrogen evolution rate than most reported 1,3,5-triazine based conjugated polymers. These results demonstrate that the cross-linker length has great influence on the photocatalytic properties of conjugated microporous polymers, which offers theoretical direction for designing high-performance CMPs.

Triple cascade nanocatalyst with laser-activatable O2 supply and photothermal enhancement for effective catalytic therapy against hypoxic tumor

Nanozymes have been combined with glucose oxidase (GOx) for dual-enzyme cascade catalytic therapy. However, their catalysis efficiency is restricted because of the hypoxia tumor microenvironment (TME). Although many methods are developed for O₂ supply, the O₂ leakage and consumption of H₂O₂ compromised their practical application. Herein, a biocompatible carbon nitride (C₃N₄)/nanozyme/GOx triple cascade nanocatalyst was designed with laser-activatable O₂ self-supply via water splitting to relieve tumor hypoxia and thus improve the catalysis efficiency. To this end, polydopamine (PDA) nanosphere was prepared and attached with C₃N₄ nanosheet to improve water splitting efficiency and realize photothermal-enhanced catalysis, simultaneously. The PDA@C₃N₄ composite was then coated with MIL-100 (Fe), where GOx was loaded, to form C₃N₄/MIL-100/GOx triple cascade nanocatalyst. The triple cascade catalysis was realized with laser-activatable O₂ supply from PDA@C₃N₄, H₂O₂ generation with GOx, and •OH production from peroxidase-like MIL-100 (Fe) for tumor therapy. Upon 808 nm irradiation, PDA, as a photothermal agent, realized photothermal therapy and enhanced the catalytic therapy. Thus, the synergy of laser-activatable O₂ supply and photothermal enhancement in our triple cascade nanocatalyst improved the performance of catalytic therapy without drug resistance and toxicity to normal tissues.

A BiOBr/Bi4MoO9 edge-on heterostructure with fast electron transport for efficient photocatalytic activity

This study demonstrates the rational design and construction of a BiOBr/Bi₄MoO₉ edge-on heterostructure by growing fish scale-like BiOBr nanosheets on the surface of Bi₄MoO₉. Such structural and compositional merits expedite electron transport and offer a large interfacial contact area and abundant reactive sites. Optimized BiOBr/Bi₄MoO₉ exhibited outstanding TC degradation activity.

Structure modulation of g-C3N4 in TiO2{001}/g-C3N4 hetero-structures for boosting photocatalytic hydrogen evolution

Structure design of photocatalysts is highly desirable for taking full advantage of their abilities for H2 evolution. Herein, the highly-efficient TiO2{001}/g-C3N4 (TCN) heterostructures have been fabricated successfully via an in situ ethanol-thermal method. And the structure of g-C3N4 in the TCN heterostructures could be exfoliated from bulk g-C3N4 to nanosheets, nanocrystals and quantum dots with the increase of the synthetic temperature. Through detailed characterization, the structural evolution of g-C3N4 could be attributed to the enhanced temperature of the ethanol-thermal treatment with the shear effects of HF acid. As expected, the optimal TCN-2 heterostructure shows excellent photocatalytic H2 evolution efficiency (1.78 mmol h-1 g-1) under visible-light irradiation. Except for the formed built-in electric field, the significantly enhanced photocatalytic activity of TCN-2 could be ascribed to the enhanced crystallinity of TiO2{001} nanosheets and the formed g-C3N4 nanocrystals with large surface area, which could extend the visible light absorption, and expedite the transfer of photo-generated charge carriers further. Our work could provide guidance on designing TCN heterostructures with the desired structure for highly-efficient photocatalytic water splitting.

2D/2D Heterojunction systems for the removal of organic pollutants: A review

Photocatalysis is considered to be an effective way to remove organic pollutants, but the key to photocatalysis is finding a high-efficiency and stable photocatalyst. 2D materials-based heterojunction has aroused widespread concerns in photocatalysis because of its merits in more active sites, adjustable band gaps and shorter charge transfer distance. Among various 2D heterojunction systems, 2D/2D heterojunction with a face-to-face contact interface is regarded as a highly promising photocatalyst. Due to the strong coupling interface in 2D/2D heterojunction, the separation and migration of photoexcited electron-hole pairs are facilitated, which enhances the photocatalytic performance. Thus, the design of 2D/2D heterojunction can become a potential model for expanding the application of photocatalysis in the removal of organic pollutants. Herein, in this review, we first summarize the fundamental principles, classification, and strategies for elevating photocatalytic performance. Then, the synthesis and application of the 2D/2D heterojunction system for the removal of organic pollutants are discussed. Finally, the challenges and perspectives in 2D/2D heterojunction photocatalysts and their application for removing organic pollutants are presented.

Synergetic Advantages of Atomically Coupled 2D Inorganic and Graphene Nanosheets as Versatile Building Blocks for Diverse Functional Nanohybrids

2D nanostructured materials, including inorganic and graphene nanosheets, have evoked plenty of scientific research activity due to their intriguing properties and excellent functionalities. The complementary advantages and common 2D crystal shapes of inorganic and graphene nanosheets render their homogenous mixtures powerful building blocks for novel high-performance functional hybrid materials. The nanometer-level thickness of 2D inorganic/graphene nanosheets allows the achievement of unusually strong electronic couplings between sheets, leading to a remarkable improvement in preexisting functionalities and the creation of unexpected properties. The synergetic merits of atomically coupled 2D inorganic-graphene nanosheets are presented here in the exploration of novel heterogeneous functional materials, with an emphasis on their critical roles as hybridization building blocks, interstratified sheets, additives, substrates, and deposited monolayers. The great flexibility and controllability of the elemental compositions, defect structures, and surface natures of inorganic-graphene nanosheets provide valuable opportunities for exploring high-performance nanohybrids applicable as electrodes for supercapacitors and rechargeable batteries, electrocatalysts, photocatalysts, and water purification agents, to give some examples. An outlook on future research perspectives for the exploitation of emerging 2D nanosheet-based hybrid materials is also presented along with novel synthetic strategies to maximize the synergetic advantage of atomically mixed 2D inorganic-graphene nanosheets.

Ultrathin indium vanadate/cadmium selenide-amine step-scheme heterojunction with interfacial chemical bonding for promotion of visible-light-driven carbon dioxide reduction

The formation of interfacial chemical bonding in heterostructures plays an important role in the transport of carriers. Herein, we firstly prepared ultrathin InVO₄ nanosheet (Ns) with a thickness of 1.5 nm. Diethylenetriamine-modified CdSe (CdSe-DETA) nanobelts are in-situ deposited on the surface of ultrathin InVO₄ Ns to build a InVO₄/CdSe-DETA step-scheme (S-scheme) heterojunction photocatalysts. The protonated DETA acts as an amine-bridge to promote the formation of a tight chemical bond at the interface of InVO₄/CdSe-DETA, thereby promoting the transfer of carriers at the interface. For photocatalytic CO₂ reduction, the rationally designed InVO₄/CdSe-DETA S-scheme photocatalyst exhibits a remarkable CO generation rate of 27.9 µmol h^-1 g^-1 at 420 nm, which is 3.35 and 3.39 times higher than that of CdSe-DETA and InVO₄ Ns, respectively. The new method by using interfacial chemical bonding to facilitate interfacial charge transportation provide a promising strategy for improve photocatalysis.

Recent advances in Co-based co-catalysts for efficient photocatalytic hydrogen generation

Recent progress in photocatalytic hydrogen generation reaction highlights the critical role of co-catalysts in enhancing the solar-to-fuel conversion efficiency of diverse band-matched semiconductors. Because of the compositional flexibility, adjustable microstructure, tunable crystal phase and facet, cobalt-based co-catalysts have stimulated tremendous attention as they have high potential to promote hydrogen evolution reaction. However, a comprehensive review that specifically focuses on these promising materials has not been reported so far. Therefore, this present review emphasizes the recent progress in the pursuing of highly efficient Co-based co-catalysts for water splitting, and the advances in such materials are summarized through the analysis of structure-activity relationships. The fundamental principles of photocatalytic hydrogen production are profoundly outlined, followed by an elaborate discussion on the crucial parameters influencingthe reaction kinetics. Then, the co-catalytic reactivities of various Co-based materials involving Co, Co oxides, Co hydroxides, Co sulfides, Co phosphides and Co molecular complexes, etc, are thoroughly discussed when they are coupled with host semiconductors, with an insight towards the ultimateobjective of achieving a rationally designed photocatalyst for enhancing water splitting reaction dynamics. Finally, the current challenge and future perspective of Co-based co-catalysts as the promising noble-metal alternative materials for solar hydrogen generation are proposed and discussed.

Vertical growth of SnS2 nanobelt arrays on CuSbS2 nanosheets for enhanced photocatalytic reduction of CO2

Two-dimensional SnS₂ nanobelt arrays vertically grown on two-dimensional CuSbS₂ nanosheet (2D SnS₂⊥2D CuSbS₂) heterostructures were synthesized via a facile solution-phase growth route. The resultant SnS₂⊥CuSbS₂ heterostructures showed enhanced photocatalytic activity for CO₂ reduction because of unique structural advantages and the p-n heterojunction with matched energy band alignment.

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.

3D Tungsten Disulfide/Carbon Nanotube Networks as Separator Coatings and Cathode Additives for Stable and Fast Lithium-Sulfur Batteries

Commercial application of Li-S batteries is greatly restricted by their unsatisfactory cycle retention and poor cycling life originating from the lithium polysulfide (LiPS) shuttling effect and sluggish sulfur redox kinetics. Various strategies have been proposed to boost the performances of Li-S batteries, including nanostructured sulfur composites, functional separators/interlayers, electrode/electrolyte additives, and so on. However, how to combine two or more strategies to efficiently settle these challenging issues confronted by Li-S batteries is in desperate need. Here, we demonstrate a powerful combined strategy of introducing novel 3D WS₂/carbon nanotube (CNT) networks built by hybridization of 1D CNTs with 2D WS₂ into Li-S batteries, simultaneously serving as a functional cathode additive and separator coating. Such 3D WS2/CNTs networks with abundant edge sites, a large active surface, and a fast electron pathway twice perform functions from the cathode side and separator surface: (1) to suppress polysulfide diffusion through a physical barrier and chemical interactions; (2) to accelerate LiPS conversion reactions; and (3) to enhance conductivity for better sulfur reactivation and high utilization. As a result, the as-built WS2/CNTs-incorporated battery configuration achieves a commendable combination of capacity, rate, and cycle stability (1491 mA h g^-1 at 0.2 C, 754 mA h g^-1 at 5 C, and initial capacity of 1069 mA h g^-1 with an ultralow decay rate of 0.040% per cycle over 1000 cycles at 1 C) along with remarkably mitigated anode corrosion and low self-discharge.

One-Pot Synthesis of Chlorophyll-Assisted Exfoliated MoS2/WS2 Heterostructures via Liquid-Phase Exfoliation Method for Photocatalytic Hydrogen Production

Developing strategies for producing hydrogen economically and in greener ways is still an unaccomplished goal. Photoelectrochemical (PEC) water splitting using photoelectrodes under neutral electrolyte conditions provides possibly one of the greenest routes to produce hydrogen. Here, we demonstrate that chlorophyll extracts can be used as an efficient exfoliant to exfoliate bulk MoS2 and WS2 to form a thin layer of a MoS2/WS2 heterostructure. Thin films of solution-processed MoS2 and WS2 nanosheets display photocurrent densities of -1 and -5 mA/cm2, respectively, and hydrogen evolution under simulated solar irradiation. The exfoliated WS2 is significantly more efficient than the exfoliated MoS2; however, the MoS2/WS2 heterostructure results in a 2500% increase in photocurrent densities compared to the individual constituents and over 12 h of PEC durability under a neutral electrolyte. Surprisingly, in real seawater, the MoS2/WS2 heterostructure exhibits stable hydrogen production after solar illumination for 12 h. The synthesis method showed, for the first time, how the MoS2/WS2 heterostructure can be used to produce hydrogen effectively. Our findings highlight the prospects for this heterostructure, which could be coupled with various processes towards improving PEC efficiency and applications.

Construction of Electrostatic Self-Assembled 2D/2D CdIn2S4/g-C3N4 Heterojunctions for Efficient Visible-Light-Responsive Molecular Oxygen Activation

Molecular oxygen activated by visible light to generate radicals with high oxidation ability exhibits great potential in environmental remediation The efficacy of molecular oxygen activation mainly depends on the separation and migration efficiency of the photoinduced charge carriers. In this work, 2D/2D CdIn₂S₄/g-C₃N₄ heterojunctions with different weight ratios were successfully fabricated by a simple electrostatic self-assembled route. The optimized sample with a weight ratio of 5:2 between CdIn₂S₄ and g-C₃N₄ showed the highest photocatalytic activity for tetracycline hydrochloride (TCH) degradation, which also displayed good photostability. The enhancement of the photocatalytic performance could be ascribed to the 2D/2D heterostructure; this unique 2D/2D structure could promote the separation and migration of the photoinduced charge carriers, which was beneficial for molecular oxygen activation, leading to an enhancement in photocatalytic activity. This work may possibly provide a scalable way for molecular oxygen activation in photocatalysis.

P Doping Promotes the Spontaneous Visible-Light-Driven Photocatalytic Water Splitting in Isomorphic Type II GaSe/InS Heterostructure

The development and design of clean and efficient water splitting photocatalysts is important for the current situation of energy shortage and environmental pollution. A new type of isomorphic GaSe/InS heterostructure is constructed, and the optoelectronic properties were studied through first-principles calculations. The results show that GaSe/InS vdW heterostructure is a type II semiconductor with a band gap of 2.09 eV. However, through the analysis of the energy band edge position and Gibbs free energy change of water splitting, it is found that the GaSe/InS heterostructure is difficult to undergo overall water splitting. Therefore, nonmetallic element P doping is considered, the established P-doped GaSe/InS (P-GaSe/InS) heterostructure could maintain the type II band arrangement, and under acidic conditions, P-GaSe/InS heterostructure could spontaneously undergo overall water splitting thermodynamically. Furthermore, the low exciton binding energy of P-GaSe/InS heterostructure highlights better light absorption performance. Therefore, these findings indicate that P-GaSe/InS heterostructure is a promising photocatalyst in overall water splitting.

Differences and Similarities of Photocatalysis and Electrocatalysis in Two-Dimensional Nanomaterials: Strategies, Traps, Applications and Challenges

Photocatalysis and electrocatalysis have been essential parts of electrochemical processes for over half a century. Recent progress in the controllable synthesis of 2D nanomaterials has exhibited enhanced catalytic performance compared to bulk materials. This has led to significant interest in the exploitation of 2D nanomaterials for catalysis. There have been a variety of excellent reviews on 2D nanomaterials for catalysis, but related issues of differences and similarities between photocatalysis and electrocatalysis in 2D nanomaterials are still vacant. Here, we provide a comprehensive overview on the differences and similarities of photocatalysis and electrocatalysis in the latest 2D nanomaterials. Strategies and traps for performance enhancement of 2D nanocatalysts are highlighted, which point out the differences and similarities of series issues for photocatalysis and electrocatalysis. In addition, 2D nanocatalysts and their catalytic applications are discussed. Finally, opportunities, challenges and development directions for 2D nanocatalysts are described. The intention of this review is to inspire and direct interest in this research realm for the creation of future 2D nanomaterials for photocatalysis and electrocatalysis.

In Situ Growth of ZnIn2S4 on MOF-Derived Ni-Fe LDH to Construct Ternary-Shelled Nanotubes for Efficient Photocatalytic Hydrogen Evolution

A rational design of a novel ternary-shelled nanotube is attractive in photocatalytic water splitting. Herein, ZnIn₂S₄ nanosheets were in situ grown on the surface of MIL-88A-derived Ni-Fe layered double hydroxide (LDH) to fabricate ternary-shelled nanotubes (ZIS@Ni-Fe LDH) via a self-assembly strategy. Characterization indicates that the ZIS@Ni-Fe LDH heterostructure exhibits a high surface area and a well-defined ternary-shelled hollow structure. The optimal heterostructure presents a remarkably improved photocatalytic hydrogen production rate (2035.81 μmol g^-1 h^-1) compared with bare ZnIn₂S₄ and MIL-88A-derived Ni-Fe LDH under visible light illumination. The effect of ZnIn₂S₄ loading on the photocatalytic performance and stability of ZIS@Ni-Fe LDH is systematically studied. The ZIS@Ni-Fe LDH heterostructure can make better use of the inner space, provide abundant reactive sites, improve light harvesting, accelerate interfacial electron transfer, and further promote photocatalytic hydrogen evolution. Based on the electrocatalytic performance, the probable photocatalytic mechanism and the electron transfer pathway can be proposed. Our work provides a facile and efficient strategy to construct ternary-shelled heterojunction photocatalysts.

A review on vertical and lateral heterostructures of semiconducting 2D-MoS2 with other 2D materials: a feasible perspective for energy conversion

Fossil fuels as a double-edged sword are essential to daily life. However, the depletion of fossil fuel reservoirs has increased the search for alternative renewable energy sources to procure a more sustainable society. Accordingly, energy production through water splitting, CO₂ reduction and N₂ reduction via photocatalytic and electrocatalytic pathways is being contemplated as a greener methodology with zero environmental pollution. Owing to their atomic-level thickness, two-dimensional (2D) semiconductor catalysts have triggered the reawakening of interest in the field of energy and environmental applications. Among them, following the unconventional properties of graphene, 2D MoS₂ has been widely investigated due to its outstanding optical and electronic properties. However, the photo/electrocatalytic performance of 2D-MoS₂ is still unsatisfactory due to its low charge carrier density. Recently, the development of 2D/2D heterojunctions has evoked interdisciplinary research fascination in the scientific community, which can mitigate the shortcomings associated with 2D-MoS₂. Following the recent research trends, the present review covers the recent findings and key aspects on the synthetic methods, fundamental properties and practical applications of semiconducting 2D-MoS₂ and its heterostructures with other 2D materials such as g-C₃N₄, graphene, CdS, TiO₂, MXene, black phosphorous, and boron nitride. Besides, this review details the viable application of these materials in the area of hydrogen energy production via the H₂O splitting reaction, N₂ fixation to NH₃ formation and CO₂ reduction to different value-added hydrocarbons and alcohol products through both photocatalysis and electrocatalysis. The crucial role of the interface together with the charge separation principle between two individual 2D structures towards achieving satisfactory activity for various applications is presented. Overall, the current studies provide a snapshot of the recent breakthroughs in the development of various 2D/2D-based catalysts in the field of energy production, delivering opportunities for future research.

Two-Dimensional Gallium Sulfide Nanoflakes for UV-Selective Photoelectrochemical-type Photodetectors

Two-dimensional (2D) transition-metal monochalcogenides have been recently predicted to be potential photo(electro)catalysts for water splitting and photoelectrochemical (PEC) reactions. Differently from the most established InSe, GaSe, GeSe, and many other monochalcogenides, bulk GaS has a large band gap of ∼2.5 eV, which increases up to more than 3.0 eV with decreasing its thickness due to quantum confinement effects. Therefore, 2D GaS fills the void between 2D small-band-gap semiconductors and insulators, resulting of interest for the realization of van der Waals type-I heterojunctions in photocatalysis, as well as the development of UV light-emitting diodes, quantum wells, and other optoelectronic devices. Based on theoretical calculations of the electronic structure of GaS as a function of layer number reported in the literature, we experimentally demonstrate, for the first time, the PEC properties of liquid-phase exfoliated GaS nanoflakes. Our results indicate that solution-processed 2D GaS-based PEC-type photodetectors outperform the corresponding solid-state photodetectors. In fact, the 2D morphology of the GaS flakes intrinsically minimizes the distance between the photogenerated charges and the surface area at which the redox reactions occur, limiting electron-hole recombination losses. The latter are instead deleterious for standard solid-state configurations. Consequently, PEC-type 2D GaS photodetectors display a relevant UV-selective photoresponse. In particular, they attain responsivities of 1.8 mA W^-1 in 1 M H₂SO₄ [at 0.8 V vs reversible hydrogen electrode (RHE)], 4.6 mA W^-1 in 1 M Na₂SO₄ (at 0.9 V vs RHE), and 6.8 mA W^-1 in 1 M KOH (at 1.1. V vs RHE) under 275 nm illumination wavelength with an intensity of 1.3 mW cm^-2. Beyond the photodetector application, 2D GaS-based PEC-type devices may find application in tandem solar PEC cells in combination with other visible-sensitive low-band-gap materials, including transition-metal monochalcogenides recently established for PEC solar energy conversion applications.

2D Bismuthene Metal Electron Mediator Engineering Super Interfacial Charge Transfer for Efficient Photocatalytic Reduction of Carbon Dioxide

The interfacial charge transfer still limits the photoactivity of artificial Z-scheme photocatalysts although they showed complementary light absorption and a strong photoredox ability. In this study, layered metallene is designed as an efficient electron mediator for constructing a C₃N₄/bismuthene/BiOCl 2D/2D/2D Z-scheme system. This bismuthene serves as a bridge processing superior charge conductibility, abundant metal-semiconductor contact sites, and the shortened charge diffusion distance, enhancing the photocatalytic CO₂ reduction reaction activity and stability. Density functional theory calculations show that the bismuthene creates a built-in electric field and congregates interfacial electrons, which is confirmed by the stable and consistent emission of the ultrafast transient absorption spectra. This work gives new insight into the interface design of Z-scheme photocatalysts by selecting a novel metallene electron mediator.

Highly active metal-free hetero-nanotube catalysts for the hydrogen evolution reaction

The development of low-cost, high-efficiency catalysts for the hydrogen evolution reaction is important for hydrogen production. In this study we investigate hydrogen adsorption at the interfaces of C/BN hetero-nanotubes using first-principles density functional theory calculations. Substantial charge redistributions associated with states near the Fermi level occur at the interfaces. More importantly, such electronic modification can enhance hydrogen adsorption at the interfacial atoms. As a result, the adsorption free energies ΔG_H*of hydrogen for the interfaces range from -0.26 to 0.30 eV, depending on hydrogen coverage. These values are much closer to zero than those for the basal plane, suggesting that the interfaces could be active sites for the hydrogen evolution reaction. The interfacial adsorption sites show a distinctive hybridization between the H s and C p orbitals, which accounts for the enhanced hydrogen adsorption at the interfaces. These findings have important implications for hydrogen energy applications.

Review of MXene-based nanocomposites for photocatalysis

Since multilayered MXenes (Ti₃C₂T_x, a new family of two-dimensional materials) were initially introduced by researchers at Drexel University in 2011, various MXene-based nanocomposites have received increased attention as photocatalysts owing to their exceptional properties (e.g., rich surface chemistry, adjustable bandgap structures, high electrical conductivity, hydrophilicity, thermal stability, and large specific surface area). Therefore, we present a comprehensive review of recent studies on fabrication methods for MXene-based photocatalysts and photocatalytic performance for contaminant degradation, CO₂ reduction, H₂ evolution, and N₂ fixation with various MXene-based nanocomposites. In addition, this review briefly discusses the stability of MXene-based nanophotocatalysts, current limitations, and future research needs, along with the various corresponding challenges, in an effort to reveal the unique properties of MXene-based nanocomposites.

Synthesis of ZnO nanoparticles using peels of Passiflora foetida and study of its activity as an efficient catalyst for the degradation of hazardous organic dye

Creating a sustainable and effective approach to handling organic contaminants from industrial waste is an ongoing problem. In the present study, ZnO nanoparticles (ZnO NPs) were synthesized under a controlled ultrasound cavitation technique using the extract of Passiflora foetida fruit peels, which act as a reducing (i.e., reduction of metal salt) and stabilizing agent. The formation of monodispersed and hexagonal morphology (average size approximately 58 nm with BET surface area 30.83m2/g). The synthesized ZnO NPs were characterized by a various technique such as UV–visible spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared (FTIR), Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Transmission electron microscopy (TEM), Thermogravimetric analysis (TGA) and Dynamic light scattering (DLS). Further, the XRD pattern confirmed the hexagonal wurtzite structure of synthesized ZnONPs. The ZnO NPs exhibit excellent degradation efficiency towards organic pollutant dyes, i.e., Methylene blue (MB) (93.25% removal) and Rhodamine B (91.06% removal) in 70 min, under natural sunlight with apparent rate constant 0.0337 min−1 (R2 = 0.9749) and 0.0347 min−1 (R2 = 0.9026) respectively.Zeta potential study shows the presence of a negative charge on the surface of ZnO NPs. The use of green synthesized ZnO NPs is a good choice for wastewater treatment, given their high reusability and photocatalytic efficiency, along with adaptability to green synthesis.

Nb2O5-Based Photocatalysts

Photocatalysis is one potential solution to the energy and environmental crisis and greatly relies on the development of the catalysts. Niobium pentoxide (Nb2O5), a typically nontoxic metal oxide, is eco-friendly and exhibits strong oxidation ability, and has attracted considerable attention from researchers. Furthermore, unique Lewis acid sites (LASs) and Brønsted acid sites (BASs) are observed on Nb2O5 prepared by different methods. Herein, the recent advances in the synthesis and application of Nb2O5-based photocatalysts, including the pure Nb2O5, doped Nb2O5, metal species supported on Nb2O5, and other composited Nb2O5 catalysts, are summarized. An overview is provided for the role of size and crystalline phase, unsaturated Nb sites and oxygen vacancies, LASs and BASs, dopants and surface metal species, and heterojunction structure on the Nb2O5-based catalysts in photocatalysis. Finally, the challenges are also presented, which are possibly overcome by integrating the synthetic methodology, developing novel photoelectric characterization techniques, and a profound understanding of the local structure of Nb2O5.