Microwave-Induced Structural Engineering and Pt Trapping in 6R-TaS2 for the Hydrogen Evolution Reaction

Rhombohedrally stacked layered transition metal dichalcogenides and their electrocatalytic applications

Layered transition metal dichalcogenides (TMDCs) are extensively investigated as catalyst materials for a wide range of electrochemical applications due to their high surface area and versatile electronic and chemical properties. Bulk TMDCs are van der Waals solids that possess strong in-plane bonding and weak inter-layer interactions. In the few-layer 2D TMDCs, several polymorphic structures have been reported as each individual layer can either retain octahedral or trigonal prismatic coordination. Among them, 1T (tetragonal), 2H (hexagonal) and 3R (rhombohedral) phases are very common. These polymorphs can display discrepancies in their catalytic activity as their electronic structure diverges due to different d orbital filling states. The broken inversion symmetry and large exposed edge sites of some of the 3R-phase TMDCS such as MoS2, NbS2 and TaS2 appear to be advantageous for electrocatalytic water reduction and battery applications. We describe recent studies in phase engineering of 2D TMDCs, particularly aiming at the 3R polytype and their electrocatalytic properties. Redox ability primarily depends on a distinct polymorphic phase in which TMDCs are isolated, and hence, with rich polymorphic structures being reported, numerous new catalytic applications can be realized. Phase conversion from 2H to 3R phase in some TMDCs enhances structural integrity and establishes robustness under harsh chemical conditions.

Advanced Microwave Strategies Facilitate Structural Engineering for Efficient Electrocatalysis

In the dynamic realm of energy conversion, the demand for efficient electrocatalysis has surged due to the urgent need to seamlessly integrate renewable energy. Traditional electrocatalyst preparation faces challenges like poor controllability, elevated costs, and stringent operational conditions. The introduction of microwave strategies represents a transformative shift, offering rapid response, high-temperature energy, and superior controllability. Notably, non-liquid-phase advanced microwave technology holds promise for introducing novel models and discoveries compared to traditional liquid-phase microwave methods. This review examines the nuanced applications of microwave technology in electrocatalyst structural engineering, emphasizing its pivotal role in the energy paradigm and addressing challenges in conventional methods. The ensuing discussion explores the profound impact of advanced microwave strategies on electrocatalyst structural engineering, highlighting discernible advantages in optimizing performance. Various applications of advanced microwave techniques in electrocatalysis are comprehensively discussed, providing a forward-looking perspective on their untapped potential to propel transformative strides in renewable energy research. It provides a forward-looking perspective, delving into the untapped potential of microwaves to propel transformative strides in renewable energy research.

Rapid and Universal Synthesis of 2D Transition Metal (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) Sulfides through Oxide Sulfurization in CS2 Vapor

Transition metal (TM) sulfides belong to the class of 2D materials with a wide application range. Various methods, including solvothermal, hydrothermal, chemical vapor deposition, and quartz ampoule-based approaches, have been employed for the synthesis of TM sulfides. Some of them face limitations due to the low stability of TM sulfides and their susceptibility to oxidation, and others require more sophisticated equipment or complex and rare precursors or are not scalable. In this work, we propose an alternative approach for the synthesis of 2D TM sulfides by sulfurization of corresponding metal oxides in the vapor of CS2 at elevated temperature. Subsequent treatment in liquid nitrogen allows exfoliation of created sulfides to a 2D structure. A proposed approach was successfully applied to nine transition metals: Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W. The resulting materials were extensively characterized using various analytical techniques with a focus on their crystalline structure and 2D nature. Our approach offers several advantages including the use of simple precursors (CS2 and metal oxides), universality (in all cases, the sulfides were obtained), equipment simplicity (tube furnace and quartz reactor), short preparation time (3 h), and the ability of morphology and phase tuning (in particular cases) of the created materials by adjusting the temperature. In addition, gram-scale bulk materials can be obtained in the entry-level laboratories using the proposed approach.

Pt-Modified High Entropy Rare Earth Oxide for Efficient Hydrogen Evolution in pH-Universal Environments

The development of efficient and stable catalysts for hydrogen production from electrolytic water in a wide pH range is of great significance in alleviating the energy crisis. Herein, Pt nanoparticles (NPs) anchored on the vacancy of high entropy rare earth oxides (HEREOs) were prepared for the first time for highly efficient hydrogen production by water electrolysis. The prepared Pt-(LaCeSmYErGdYb)O showed excellent electrochemical performances, which require only 12, 57, and 77 mV to achieve a current density of 100 mA cm-2 in 0.5 M H2SO4, 1.0 M KOH, and 1.0 M PBS environments, respectively. In addition, Pt-(LaCeSmYErGdYb)O has successfully worked at 400 mA cm-2 @ 60 °C for 100 h in 0.5 M H2SO4, presenting the high mass activity of 37.7 A mg-1Pt and turnover frequency (TOF) value of 38.2 s-1 @ 12 mV, which is far superior to the recently reported hydrogen evolution reaction (HER) catalysts. Density functional theory (DFT) calculations have revealed that the interactions between Pt and HEREO have optimized the electronic structures for electron transfer and the binding strength of intermediates. This further leads to optimized proton binding and water dissociation, supporting the highly efficient and robust HER performances in different environments. This work provides a new idea for the design of efficient RE-based electrocatalysts.

Beyond the Platinum Era─Scalable Preparation and Electrochemical Activation of TaS2 Flakes

Among 2D materials, transition-metal dichalcogenides (TMDCs) of group 5 metals recently have attracted substantial interest due to their superior electrocatalytic activity toward hydrogen evolution reaction (HER). However, a straightforward and efficient synthesis of the TMDCs which can be easily scaled up is missing. Herein, we report an innovative, simple, and scalable method for tantalum disulfide (TaS₂) synthesis, involving CS₂ as a sulfurizing agent and Ta₂O₅ as a metal precursor. The structure of the created TaS₂ flakes was analyzed by Raman, XRD, XPS, SEM, and HRTEM techniques. It was demonstrated that a tuning between 1T (metallic) and 3R (semiconductor) TaS₂ phases can be accomplished by varying the reaction conditions. The created materials were tested for HER, and the electrocatalytic activity of both phases was significantly enhanced by electrochemical self-activation, up to that comparable with the Pt one. The final values of the Tafel slopes of activated TaS₂ were found to be 35 and 43 mV/dec for 3R-TaS₂ and 1T-TaS₂, respectively, with the corresponding overpotentials of 63 and 109 mV required to reach a current density of 10 mA/cm². We also investigated the mechanism of flake activation, which can be attributed to the changes in the flake morphology and surface chemistry. Our work provides a scalable and simple synthesis method to produce transition-metal sulfides which could replace the platinum catalyst in water splitting technology.

Recent progress in microwave-assisted preparations of 2D materials and catalysis applications

On the urgency of metal-free catalysts, two-dimensional materials (2DMs) have caused extensive researches because of distinctive optical and electronic properties. In the last decade, microwave methods have emerged in rapid and effective preparations of 2DMs for catalysis. Microwave heating offers several advantages namely direct, fast, selective heating and uniform reaction temperature compared to conventional heating methods, thus bringing about high-yield and high-purity products in minutes or even seconds. This review summarizes recent advances in microwave-assisted preparations of 2DMs-based catalysts and their state-of-the-art catalytic performances. Microwave heating mechanisms are briefly introduced mainly focusing on microwave-matter interactions, which can guide the choice of precursors, liquid media, substrates, auxiliaries and experiment parameters during microwave radiation. We especially provide a detailed insight into various microwave-assisted procedures, classified as exfoliation, synthesis, doping, modification and construction towards different 2DMs nanomaterials. We also discuss how microwave affects the synthetic composition and microstructure of 2DMs-based catalysts, thereby deeply influencing their optical and electronic properties and the catalytic performances. Finally, advantages, challenges and prospects of microwave-assisted approaches for 2DMs nanomaterials are summarized to inspire the effective and large-scale fabrication of novel 2DMs-based catalysts.

Sulfonated NbS2-based proton-exchange membranes for vanadium redox flow batteries

In this work, novel proton-exchange membranes (PEMs) based on sulfonated poly(ether ether ketone) (SPEEK) and two-dimensional (2D) sulfonated niobium disulphide (S-NbS₂) nanoflakes are synthesized by a solution-casting method and used in vanadium redox flow batteries (VRFBs). The NbS₂ nanoflakes are produced by liquid-phase exfoliation of their bulk counterpart and chemically functionalized with terminal sulfonate groups to improve dimensional and chemical stabilities, proton conductivity (σ) and fuel barrier properties of the as-produced membranes. The addition of S-NbS₂ nanoflakes to SPEEK decreases the vanadium ion permeability from 5.42 × 10^-7 to 2.34 × 10^-7 cm² min^-1. Meanwhile, it increases the membrane σ and selectivity up to 94.35 mS cm^-2 and 40.32 × 10⁴ S min cm^-3, respectively. The cell assembled with the optimized membrane incorporating 2.5 wt% of S-NbS₂ nanoflakes (SPEEK:2.5% S-NbS₂) exhibits high efficiency metrics, i.e., coulombic efficiency between 98.7 and 99.0%, voltage efficiency between 90.2 and 73.2% and energy efficiency between 89.3 and 72.8% within the current density range of 100-300 mA cm^-2, delivering a maximum power density of 0.83 W cm^-2 at a current density of 870 mA cm^-2. The SPEEK:2.5% S-NbS₂ membrane-based VRFBs show a stable behavior over 200 cycles at 200 mA cm^-2. This study opens up an effective avenue for the production of advanced SPEEK-based membranes for VRFBs.

Polytypic Phase Transition of Nb1-xVxSe2 via Colloidal Synthesis and Their Catalytic Activity toward Hydrogen Evolution Reaction

Polytypes of two-dimensional transition metal dichalcogenide can extend the architecture and application of nanostructures. Herein, Nb_1-xV_xSe₂ alloy nanosheets in the full composition range (x) were synthesized by a colloidal reaction. At x = 0.1-0.3, a phase transition occurred from various hexagonal (three 2H and one 4H types) phase NbSe₂ to an atomically homogeneous 1T phase VSe₂. Density functional theory calculations also revealed a polytypic phase transition at x = 0.3, which was shifted close to 0 in the presence of Se vacancies. Furthermore, the calculations validate favorable formation of Se vacancies at the phase transition. The sample at x = 0.3 exhibited enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄. The Gibbs free energy indicates that the catalytic HER performance is correlated with the active Se vacancy sites of polytypic structures.

Topochemical Transformation of Two-Dimensional VSe2 into Metallic Nonlayered VO2 for Water Splitting Reactions in Acidic and Alkaline Media

The engineering of the structural and morphological properties of nanomaterials is a fundamental aspect to attain desired performance in energy storage/conversion systems and multifunctional composites. We report the synthesis of room temperature-stable metallic rutile VO₂ (VO₂ (R)) nanosheets by topochemically transforming liquid-phase exfoliated VSe₂ in a reductive Ar-H₂ atmosphere. The as-produced VO₂ (R) represents an example of two-dimensional (2D) nonlayered materials, whose bulk counterparts do not have a layered structure composed by layers held together by van der Waals force or electrostatic forces between charged layers and counterbalancing ions amid them. By pretreating the VSe₂ nanosheets by O₂ plasma, the resulting 2D VO₂ (R) nanosheets exhibit a porous morphology that increases the material specific surface area while introducing defective sites. The as-synthesized porous (holey)-VO₂ (R) nanosheets are investigated as metallic catalysts for the water splitting reactions in both acidic and alkaline media, reaching a maximum mass activity of 972.3 A g^-1 at -0.300 V vs RHE for the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ (faradaic efficiency = 100%, overpotential for the HER at 10 mA cm^-2 = 0.184 V) and a mass activity (calculated for a non 100% faradaic efficiency) of 745.9 A g^-1 at +1.580 V vs RHE for the oxygen evolution reaction (OER) in 1 M KOH (overpotential for the OER at 10 mA cm^-2 = 0.209 V). By demonstrating proof-of-concept electrolyzers, our results show the possibility to synthesize special material phases through topochemical conversion of 2D materials for advanced energy-related applications.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

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.

Inverted perovskite solar cells with enhanced lifetime and thermal stability enabled by a metallic tantalum disulfide buffer layer

Perovskite solar cells (PSCs) have proved their potential for delivering high power conversion efficiencies (PCE) alongside low fabrication cost and high versatility. The stability and the PCE of PSCs can readily be improved by implementing engineering approaches that entail the incorporation of two-dimensional (2D) materials across the device's layered configuration. In this work, two-dimensional (2D) 6R-TaS2 flakes were exfoliated and incorporated as a buffer layer in inverted PSCs, enhancing the device's PCE, lifetime and thermal stability. A thin buffer layer of 6R-TaS2 flakes was formed on top of the electron transport layer to facilitate electron extraction, thus improving the overall device performance. The optimized devices reach a PCE of 18.45%, representing a 12% improvement compared to the reference cell. The lifetime stability measurements of the devices under ISOS-L2, ISOS-D1, ISOS-D1I and ISOS-D2I protocols revealed that the TaS2 buffer layer retards the intrinsic, thermally activated degradation processes of the PSCs. Notably, the devices retain more than the 80% of their initial PCE over 330 h under continuous 1 Sun illumination at 65 °C.