Enhanced Photocatalytic Activity of WS2 Film by Laser Drilling to Produce Porous WS2/WO3 Heterostructure

How can heteroatoms boost the performance of photoactive nanomaterials for wastewater purification?

Photocatalysis, as an alternative for treating persistent water pollutants, holds immense promise. However, limitations hinder sustained treatment and recycling under varying light conditions. This comprehensive review delves into the novel paradigm of metal and non-metal doping to overcome these challenges. It begins by discussing the fundamental principles of photocatalysis and its inherent limitations. Understanding these constraints is crucial for developing effective strategies. Band gap narrowing by metal and non-metal doping modifies the band gap, enabling visible-light absorption. Impurity energy levels and oxygen vacancies influenced the doping energy levels and surface defects. Interfacial electron transfer and charge carrier recombination are the most important factors that impact overall efficiency. The comparative analysis of nanomaterials are reviewed on various, including nanometal oxides, nanocarbon materials, and advanced two-dimensional structures. The synthesis process are narratively presented, emphasizing production yields, selectivity, and efficiency. The review has potential applications in the environment for efficient pollutant removal and water purification, economic cost-effective and scalable production and technological advancement catalyst design, in spite of its challenges in material stability, synthesis methods and optimizing band gaps. The novelty of the review paper is on the proposal of a new paradigm of heterojunctions of doped metal and non-metal photocatalysts to promise highly efficient water treatment. This review bridges the gap between fundamental research and practical applications, offering insights into tailored nano photocatalysts.

GW Quasiparticle Energies and Bandgaps of Two-Dimensional Materials Immersed in Water

Computational simulations have become of major interest to screen potential photocatalysts for optimal band edge positions which straddle the redox potentials. Unfortunately, these methods suffer from a difficulty in resolving the dynamic solvent response on the band edge positions. We have developed a computational method based on the GW approximation coupled with an implicit solvation model that solves a generalized Poisson equation (GPE), that is, GW-GPE. Using GW-GPE, we have investigated the band edge locations of (quasi) 2D materials immersed in water and found a good agreement with experimental data. We identify two contributions of the solvent effect, termed a "polarization-field effect" and an "environmental screening effect", which are found to be highly sensitive to the atomic and charge distribution of the 2D materials. We believe that the GW-GPE scheme can pave the way to predict band edge positions in solvents, enabling design of 2D material-based photocatalysts and energy systems.

Improving Photoelectrochemical Activity of Magnetron-Sputtered Double-Layer Tungsten Trioxide Photoanodes by Irradiation with Intense Pulsed Ion Beams

The photoelectrochemical (PEC) activity of metal oxide photoelectrodes for water-splitting applications can be boosted in several different ways. In this study, we showed that PEC activity can be significantly improved with a double-layer (crystalline-amorphous) configuration of WO₃ thin films irradiated with intense pulsed ion beams (IPIB) of a nanosecond duration. It was found that IPIB irradiation promotes the formation of crystalline and sponge-like WO₃ structures on the surface. Due to an increase in the active surface and light scattering in irradiated samples, photocurrent generation increased by ~80% at 1.23 reversible hydrogen electrodes (RHE).

Lateral layered semiconductor multijunctions for novel electronic devices

Layered semiconductors, represented by transition metal dichalcogenides, have attached extensive attention due to their unique and tunable electrical and optical properties. In particular, lateral layered semiconductor multijunctions, including homojunctions, heterojunctions, hybrid junctions and superlattices, present a totally new degree of freedom in research on electronic devices beyond traditional materials and their structures, providing unique opportunities for the development of new structures and operation principle-based high performance devices. However, the advances in this field are limited by the precise synthesis of high-quality junctions and greatly hampered by ambiguous device performance limits. Herein, we review the recent key breakthroughs in the design, synthesis, electronic structure and property modulation of lateral semiconductor multijunctions and focus on their application-specific devices. Specifically, the synthesis methods based on different principles, such as chemical and external source-induced methods, are introduced stepwise for the controllable fabrication of semiconductor multijunctions as the basics of device application. Subsequently, their structure and property modulation are discussed, including control of their electronic structure, exciton dynamics and optical properties before the fabrication of lateral layered semiconductor multijunction devices. Precise property control will potentially result in outstanding device performances, including high-quality diodes and FETs, scalable logic and analog circuits, highly efficient optoelectronic devices, and unique electrochemical devices. Lastly, we focus on several of the most essential but unresolved debates in this field, such as the true advantages of few-layer vs. monolayer multijunctions, how sharp the interface should be for specific functional devices, and the superiority of lateral multijunctions over vertical multijunctions, highlighting the next-phase strategy to enhance the performance potential of lateral multijunction devices.

Recent Advances in WS2 and Its Based Heterostructures for Water-Splitting Applications

The energy from fossil fuels has been recognized as a main factor of global warming and environmental pollution. Therefore, there is an urgent need to replace fossil fuels with clean, cost-effective, long-lasting, and environmentally friendly fuel to solve the future energy crisis of the world. Therefore, the development of clean, sustainable, and renewable energy sources is a prime concern. In this regard, solar energy-driven hydrogen production is considered as an overriding opening for renewable and green energy by virtue of its high energy efficiency, high energy density, and non-toxicity along with zero emissions. Water splitting is a promising technology for producing hydrogen, which represents a potentially and environmentally clean fuel. Water splitting is a widely known process for hydrogen production using different techniques and materials. Among different techniques of water splitting, electrocatalytic and photocatalytic water splitting using semiconductor materials have been considered as the most scalable and cost-effective approaches for the commercial production of sustainable hydrogen. In order to achieve a high yield of hydrogen from these processes, obtaining a suitable, efficient, and stable catalyst is a significant factor. Among the different types of semiconductor catalysts, tungsten disulfide (WS2) has been widely utilized as a catalytic active material for the water-splitting process, owing to its layered 2D structure and its interesting chemical, physical, and structural properties. However, WS2 suffers from some disadvantages that limit its performance in catalytic water splitting. Among the various techniques and strategies that have been constructed to overcome the limitations of WS2 is heterostructure construction. In this process, WS2 is coupled with another semiconducting material in order to facilitate the charge transfer and prevent the charge recombination, which will enhance the catalytic performance. This review aims to summarize the recent studies and findings on WS2 and its heterostructures as a catalyst in the electrocatalytic and photocatalytic water-splitting processes.

Laser printed two-dimensional transition metal dichalcogenides

Laser processing is a highly versatile technique for the post-synthesis treatment and modification of transition metal dichalcogenides (TMDCs). However, to date, TMDCs synthesis typically relies on large area CVD growth and lithographic post-processing for nanodevice fabrication, thus relying heavily on complex, capital intensive, vacuum-based processing environments and fabrication tools. This inflexibility necessarily restricts the development of facile, fast, very low-cost synthesis protocols. Here we show that direct, spatially selective synthesis of 2D-TMDCs devices that exhibit excellent electrical, Raman and photoluminescence properties can be realized using laser printing under ambient conditions with minimal lithographic or thermal overheads. Our simple, elegant process can be scaled via conventional laser printing approaches including spatial light modulation and digital light engines to enable mass production protocols such as roll-to-roll processing.

Ultrasonically prepared photocatalyst of W/WO3 nanoplates with WS2 nanosheets as 2D material for improving photoelectrochemical water splitting

A sonochemical treatment has been an emerged technique as an interesting method for fabricating different photocatalysts with unique photoelectrochemical (PEC) properties. This study investigated the PEC performance of WO3 with WS2 nanosheets as a 2D material before calcination (WO3/WS2-90) and after calcination (WO3/WS2-450) prepared with sonochemical treatment. The WS2 nanosheets were prepared from a liquid exfoliation phase with few-layer nanosheets, approximately 6.5 nm in thickness. The nanosheets were confirmed by UV-Vis spectroscopy and atomic force microscopy. Further, XPS, RAMAN, and SEM-EDAX analyses indicated that, following calcination of the WO3/WS2 electrode, the WS2 nanosheets initially transformed to 2D-WO3. After depositing the WS2 nanosheets on the WO3, the photocurrent density increased substantially. The WO3/WS2-450 films after calcination showed a photocurrent density of 5.6 mA.cm-2 at 1.23 V vs. Ag/AgCl, which was 3.1 and 7.2 times higher, respectively than those of the WO3/WS2-90 before calcination and pure WO3. Mott-Schottky and electrochemical impedance spectroscopy analyses confirmed the fabrication of the WO3/WS2 photoanode after calcination. The deposition of WS2 nanosheets onto pure WO3 increased the donor concentration (24-fold), reduced the space charge layer (4.6-fold), and decreased the flat band potential (1.6-fold), which could all help improve the photoelectrochemical efficiency. Moreover, the incorporation of WO3 with WS2 nanosheets as a 2D material (WO3/WS2-450) enhanced the incident photon current efficiency (IPCE) by 55%. In addition, the applied-bias photon-to-current conversion efficiency of the WO3/WS2-450 films was approximately 2.26% at 0.75 V (vs. Ag/AgCl), which is 5.6 and 9 times higher, respectively than those of WO3/WS2-90 and pure WO3.

Photoluminescence of PdS2 and PdSe2 quantum dots

PdS₂ and PdSe₂ QDs are fabricated via liquid exfoliation using NMP solvent. The PL behaviors of these QD solutions are studied. The obtained results suggest promising optoelectronic applications with group-10 TMD QDs in the future.

Topotactic Transformation of Solvated MgCr-LDH Nanosheets to Highly Efficient Porous MgO/MgCr2O4 Nanocomposite for Photocatalytic H2 Evolution

The hybrid structure of nanoparticles (NPs) with nanosheets has the advantage of both anisotropic properties of NPs and large specific surface areas of nanosheets, which is desirable for many technological applications. In this study, MgCr₂O₄ spinel NPs decorated on highly porous MgO nanosheets forming MgO/MgCr₂ O₄( x) nanocomposites were synthesized by a one pot coprecipitation method followed by a heat treatment process of the solvated wet gel of MgCr-LDH with polar solvent N, N-dimethylformamide (DMF) at 400 °C. This novel synthetic methodology generates materials consisting of porous metal oxides nanosheets adhered with spinel phase NPs due to the slow generation of gases such as H₂O, CO₂, and NH₃ under moderate temperature during the heat treatment process. The synergistic effect of much wider band gap MgO nanosheets and narrow band gap MgCr₂O₄ NPs added increased stability due to the stronger bonding coordination of MgCr₂O₄ NPs with MgO nanosheets. The obtained MgO/MgCr₂ O₄( x) nanocomposites possess large specific surface areas, highly porous structure, and excellent interface between MgCr₂O₄ NPs and MgO nanosheets, which proved from N₂ sorption isotherm, TEM, HR-TEM study. With metallic ratio of MgCr3:1, MgO/MgCr₂O₄(MgCr3:1) nanocomposites exhibit highest H₂ evolution rate of 840 μmolg^-12h^-1, which was 2 times higher than that of pure MgCr₂O₄(420 μmolg^-12h^-1). The LSV measurement study of MgO/MgCr₂O₄ (MgCr3:1) nanocomposite shows an enhancement of light current density of 0.22 μA/cm² at potential bias of -1.1 V. The Mott-Schottky analysis suggested the band edge positions of the n-type constituents and formation of n-n type heterojunctions in MgO/MgCr₂O₄ (MgCr3:1) nanocomposite, which facilitates the flow of charge carriers. The EIS and Bode phase plot of MgO/MgCr₂O₄ (MgCr3:1) nanocomposite signifies the lower interfacial charge transfer resistance and higher lifetime of electrons (2.7 ms) for enhanced H₂ production. Lastly, the enhanced photocatalytic H₂ production activity and long-term stability of MgO/MgCr₂O₄(MgCr3:1) could be attributed to maximum specific surface area, porous structure, close intimacy contact angle between two cubic phases of MgCr₂O₄ NPs and MgO nanosheets, abundant oxygen vacancies sites, reduced charge transfer resistance and suitable band edge potential to drive the thermodynamic energy for H₂ production. This work highlighted an effective strategy for the synthesis of cost-effective 2D porous heterojunctions nanocomposite photocatalyst for promising applications in the field of clean H₂ production utilizing abundant solar energy.

High-Performance All 2D-Layered Tin Disulfide: Graphene Photodetecting Transistors with Thickness-Controlled Interface Dynamics

Tin disulfide crystals with layered two-dimensional (2D) sheets are grown by chemical vapor deposition using a novel precursor approach and integrated into all 2D transistors with graphene (Gr) electrodes. The Gr:SnS₂:Gr transistors exhibit excellent photodetector response with high detectivity and photoresponsivity. We show that the response of the all 2D photodetectors depends upon charge trapping at the interface and the Schottky barrier modulation. The thickness-dependent SnS₂ measurements in devices reveal a transition from the interface-dominated response for thin crystals to bulklike response for the thicker SnS₂ crystals, showing the sensitivity of devices fabricated using layered materials on the number of layers. These results show that SnS₂ has photosensing performance when combined with Gr electrodes that is comparable to other 2D transition metal dichalcogenides of MoS₂ and WS₂.

Few-Layered Trigonal WS2 Nanosheet-Coated Graphite Foam as an Efficient Free-Standing Electrode for a Hydrogen Evolution Reaction

Few-layered tungsten disulfide (WS₂) with a controlled-phase ratio (the highest trigonal-phase ratio being 67%) was exfoliated via lithium insertion. The exfoliated WS₂ nanosheets were then anchored onto three-dimensional (3D) graphite foam (GF) to fabricate free-standing binder-free electrodes. The 3D GF can increase the interfacial contact between the WS₂ nanosheets and the electrolyte and facilitate ion transfer. Without the nonconductive binder, an intimate contact between the WS₂ and GF interface can be created, leading to the improvement of electrical conductivity. In comparison to the pure WS₂ nanosheets, the overpotential for a hydrogen evolution reaction is significantly decreased from 350 mV to 190 mV at 10 mA/cm², and no deactivation occurs after 1000 cycles. The density functional theory computations reveal that the efficient catalytic activity of the trigonal-phase WS₂/GF electrode is attributed to the lower Gibbs free energy for H* adsorption and higher electrical conductivity.