Cobalt Phosphorous Trisulfide as a High-Performance Electrocatalyst for the Oxygen Evolution Reaction

Van der Waals heterostructures via spontaneous self-restacked assembling for enhanced water oxidation

The pursuit of sustainable energy solutions has identified water oxidation as a crucial reaction, with the oxygen evolution reaction (OER) serving as a decisive efficiency determinant in water technologies. This study presents a novel van der Waals (vdW) heterostructure catalyst, synthesized through a spontaneous self-restacking of nickel-iron-based phosphorus-sulfur compounds (NiPS3 and FePS3). Density Functional Theory (DFT) calculations underpinned the thermodynamic spontaneity of the restacking process, uncovering an electronic transition that significantly amplifies electrocatalytic functionality. The catalyst demonstrates a remarkable OER performance, achieving a low overpotential of 257 mV at 20 mA cm-2 and a Tafel slope of 49 mV dec-1 and demonstrates remarkable durability sustaining 500 mA cm-2 for 140 hours. In addition to its high performance, the material's rapid reconstruction facilitated by surface electron enrichment and the release of phosphate and sulfate during the OER underscores a dual enhancement in both activity and stability. The universality of the synthesis method is further demonstrated by extending the approach to other MPS3 materials (M = Mn, Co, Zn), establishing a generalized platform for developing high-performance OER catalysts. This work represents a significant advancement in the application of restacked vdW heterostructures as a foundation for advanced electrocatalytic materials.

High-Speed and Low-Energy Resistive Switching with Two-Dimensional Cobalt Phosphorus Trisulfide for Efficient Neuromorphic Computing

Two-dimensional (2D) materials hold significant potential for the development of neuromorphic computing architectures owing to their exceptional electrical tunability, mechanical flexibility, and compatibility with heterointegration. However, the practical implementation of 2D memristors in neuromorphic computing is often hindered by the challenges of simultaneously achieving low latency and low energy consumption. Here, we demonstrate memristors based on 2D cobalt phosphorus trisulfide (CoPS3), which achieve impressive performance metrics including high switching speed (20 ns), low switching energy (1.15 pJ), high switching ratio (>400), and low switching voltages (1.05 V for set and -0.89 V for reset). The creation of sulfur vacancies in CoPS3 through an electroforming process facilitates the formation of conductive filaments, leading to uniform fast switching with minimal energy requirements. The CoPS3 memristors also show linear conductance modulation and long-term memory retention, enabling high-accuracy modeling of artificial neural networks for handwritten digit recognition and convolutional neural networks for image processing. Furthermore, robust memristive switching is achieved in solution-processed large-scale CoPS3 films, underscoring their potential for wafer-scale, low-temperature integration. The combination of rapid switching, low energy consumption, extended memory retention, high switching ratio, linear conductance update, and scalability manifests the potential of 2D CoPS3 materials for energy-efficient neuromorphic computing circuits.

XPS Analysis of FexNiyPS3 vdW Materials Used in the Hydrogen Evolution Processes

In response to the global demand for sustainable energy solutions, the quest for stable and cost-effective hydrogen production has garnered significant attention in recent decades. Here, the emergence of layered metal phosphorus trichalcogenides (MPX3, M: transition metal, X: chalcogen) materials and their two-dimensional counterparts with customizable composition and electronic structure holds great promise for such purposes. In the present study, we successfully synthesized large-scale and high-quality FePS3, NiPS3, and an alloyed counterpart, Fe0.5Ni0.5PS3. Subsequent systematic investigations were conducted to probe their respective electronic structures and assess their hydrogen evolution reaction (HER) properties. Remarkably, our results unveiled the successful modulation of the bandgap for FexNiyPS3, ultimately bestowing it with the most favorable HER performance for Fe0.5Ni0.5PS3 when compared to the other two samples. Furthermore, our exploration into the evolution of the X-ray photoelectron spectroscopy (XPS) spectra demonstrated that the charge conversions of metal cations play a pivotal role in the HER reactions. This critical insight further enriches our understanding of the fundamental mechanisms governing the performance of the prepared layered MPX3-based electrocatalysts, thus facilitating a comprehensive and detailed analysis of the pre- and post-HER reactions. This work not only sheds light on the intricate interplay between composition, electronic structure, and catalytic performance in the realm of novel electrocatalysts, but also contributes to the broader scientific community's pursuit of sustainable and efficient hydrogen production.

Application of density functional theory to study the electronic structure and magnetic behavior of clusters MnPS3 (M = Fe, Co, Ni; n = 0 ~ 3)

CONTEXT

The article explores and compares the electronic structure and magnetic properties of transition metal phosphate materials, namely FePS₃, CoPS₃, and NiPS₃.

RESEARCH FINDINGS

Analysis of the optimized configuration reveals significant insights into the electronic properties of M_nPS₃ clusters. Electrons within the cluster exhibit a flow from the metal atom M and the non-metal atom P to the non-metal atom S. The S atom serves as the primary site for electrophilic reactions within the cluster, while the metal atom hosts the main site for nucleophilic reactions. Configurations 2a⁽²⁾, 2b⁽²⁾, 3a⁽⁴⁾, 3b⁽³⁾, and 3c⁽²⁾ exhibit enhanced electron mobility and optimal electronic properties. Moreover, the analysis of the magnetic properties of the optimized configurations demonstrates that the magnetic behavior of M_nPS₃ clusters is influenced by the spin motion of α electrons in the p orbital. Metal atoms make a relatively significant contribution to the magnetic properties of M_nPS₃ clusters. Configurations 1b⁽³⁾, 2c⁽⁴⁾, and 3a⁽⁴⁾ exhibit comparatively higher magnetic properties compared to other configurations of the same size. This study identifies the optimal configuration for the magnetic and electronic properties of transition metal phosphorothioate materials. It also elucidates the trends in magnetic and electronic properties as the number of metal atoms varies, thereby providing valuable theoretical support for the application of these materials in the fields of magnetic materials and electronic devices.

METHODS

In this study, the Fe-based transition elements, namely Fe, Co, and Ni, are selected as the metal atoms M. The cluster MPS₃ is used to simulate the local structure of the material, allowing for an investigation into the influence of the metal atoms on its electronic and magnetic properties. By increasing the number of metal atoms and expanding the cluster size, the variations in these properties are explored. Density functional theory (DFT) calculations are performed using the B3LYP functional within the Gaussian09 software package. The M_nPS₃ cluster is subjected to optimal calculations and vibrational analysis at the def2-tzvp quantization level, resulting in optimized configurations with different spin multiplet degrees. Quantum chemistry software GaussView, wave function analysis software Multiwfn, and plotting software Origin are utilized for data characterization and graphical representation of the magnetic and electronic properties of the optimized configurations. Through the employment of these computational tools, valuable insights into the magnetic and electronic properties of the M_nPS₃ cluster and its dependency on different metal atoms are obtained.

Recent Progress in Metal Phosphorous Chalcogenides: Potential High-Performance Electrocatalysts

Since the discovery of graphene, research on the family of 2D materials has been a thriving field. Metal phosphorous chalcogenides (MPX₃ ) have attracted renewed attention due to their distinctive physical and chemical properties. The advantages of MPX₃ , such as tunable layered structures, unique electronic properties, thermodynamically appropriate band alignments and abundant catalytic active sites on the surface, make MPX₃ material great potential in electrocatalysis. In this review, the applications of MPX₃ electrocatalysts in recent years, including hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction, are summarized. Structural regulation, chemical doping and multi-material composite that are often effective and practical research methods to further optimize the catalytic properties of these materials, are introduced. Finally, the challenges and opportunities for electrocatalytic applications of MPX₃ materials are discussed. This report aims to advance future efforts to develop MPX₃ and related materials for electrocatalysis.

Exfoliated 2D Layered and Nonlayered Metal Phosphorous Trichalcogenides Nanosheets as Promising Electrocatalysts for CO2 Reduction

Two-dimensional (2D) materials catalysts provide an atomic-scale view on a fascinating arena for understanding the mechanism of electrocatalytic carbon dioxide reduction (CO₂ ECR). Here, we successfully exfoliated both layered and nonlayered ultra-thin metal phosphorous trichalcogenides (MPCh₃ ) nanosheets via wet grinding exfoliation (WGE), and systematically investigated the mechanism of MPCh₃ as catalysts for CO₂ ECR. Unlike the layered CoPS₃ and NiPS₃ nanosheets, the active Sn atoms tend to be exposed on the surfaces of nonlayered SnPS₃ nanosheets. Correspondingly, the nonlayered SnPS₃ nanosheets exhibit clearly improved catalytic activity, showing formic acid selectivity up to 31.6 % with -7.51 mA cm^-2 at -0.65 V vs. RHE. The enhanced catalytic performance can be attributed to the formation of HCOO* via the first proton-electron pair addition on the SnPS₃ surface. These results provide a new avenue to understand the novel CO₂ ECR mechanism of Sn-based and MPCh₃ -based catalysts.

2D Layered Bimetallic Phosphorous Trisulfides MI MIII P2 S6 (MI = Cu, Ag; MIII = Sc, V, Cr, In) for Electrochemical Energy Conversion

Considerable improvements in the electrocatalytic activity of 2D metal phosphorous trichalcogenides (M₂ P₂ X₆ ) have been achieved for water electrolysis, mostly with M^II ₂ [P₂ X₆ ]^4- as catalysts for hydrogen evolution reaction (HER). Herein, M^I M^III P₂ S₆ (M^I  = Cu, Ag; M^III  = Sc, V, Cr, In) are synthesized and tested for the first time as electrocatalysts in alkaline media, towards oxygen reduction reaction (ORR) and HER. AgScP₂ S₆ follows a 4 e^- pathway for the ORR at 0.74 V versus reversible hydrogen electrode; CuScP₂ S₆ is active for HER, exhibiting an overpotential of 407 mV and a Tafel slope of 90 mV dec^-1 . Density functional theory models reveal that bulk AgScP₂ S₆ and CuScP₂ S₆ are both semiconductors with computed bandgaps of 2.42 and 2.23 eV, respectively and overall similar electronic properties. Besides composition, the largest difference in both materials is in their molecular structure, as Ag atoms sit at the midpoint of each layer alongside Sc atoms, while Cu atoms are raised to a similar height to S atoms, in the external segment of the 2D layers. This structural difference probably plays a fundamental role in the different catalytic performances of these materials. These findings show that M^I (Cu, Ag) together with Sc(M^III ) leads to promising achievements in M^I M^III P₂ S₆ materials as electrocatalysts.

Mixed Insulating State for van der Waals CoPS3

Large-scale high-quality van der Waals CoPS₃ single crystals are synthesized using a chemical vapor transport (CVT) method. The crystallographic structure and electronic properties of this layered material are systematically studied using different spectroscopic methods (XPS, NEXAFS, and resonant photoelectron spectroscopy) accompanied by density functional theory (DFT) calculations. All experimental and theoretical data allow assignment of this material to the class of mixed Mott-Hubbard/charge-transfer insulator with U_dd ≅ Δ. All obtained results can enrich the information on the new class of van der Waals materials, transition metal phosphorus trichalcogenides, and help to further effectively exploit their electronic, optical, and transport properties, which are important for adopting this kind of materials into different application areas, such as spintronics and catalysis.

Facile Solvent-Free Synthesis of Metal Thiophosphates and Their Examination as Hydrogen Evolution Electrocatalysts

The facile solvent-free synthesis of several known metal thiophosphates was accomplished by a chemical exchange reaction between anhydrous metal chlorides and elemental phosphorus with sulfur, or combinations of phosphorus with molecular P2S5 at moderate 500 °C temperatures. The crystalline products obtained from this synthetic approach include MPS3 (M = Fe, Co, Ni) and Cu3PS4. The successful reactions benefit from thermochemically favorable PCl3 elimination. This solvent-free route performed at moderate temperatures leads to mixed anion products with complex heteroatomic anions, such as P2S64−. The MPS3 phases are thermally metastable relative to the thermodynamically preferred separate MPx/ MSy and more metal-rich MPxSy phases. The micrometer-sized M-P-S products exhibit room-temperature optical and magnetic properties consistent with isolated metal ion structural arrangements and semiconducting band gaps. The MPS3 materials were examined as electrocatalysts in hydrogen evolution reactions (HER) under acidic conditions. In terms of HER activity at lower applied potentials, the MPS3 materials show the trend of Co > Ni >> Fe. Extended time constant potential HER experiments show reasonable HER stability of ionic and semiconducting MPS3 (M = Co, Ni) structures under acidic reducing conditions.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Mapping the electrocatalytic water splitting activity of VO2 across its insulator-to-metal phase transition

The electrocatalytic water splitting activity of V-based oxides has been rarely investigated, even though several polymorphs in VO₂ are expected to exhibit different electrocatalytic activities depending on their crystal and electronic structures. The rutile structure of VO₂(R), showing metallic character, is a good candidate for a new electrocatalyst since it undergoes insulator-to-metal transition (IMT) from the insulating VO₂(M1) at a low temperature of 68 °C, and involves a substantially increased electrical conductivity by three orders of magnitude. The extensive improvements in the electrocatalytic activity for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) are confirmed when the IMT is induced where the overpotential (η₁₀) is reduced from 1056 mV to 598 mV in the OER and 411 mV to 136 mV in the HER, respectively. This improvement is attributed to the increased electrochemically active surface area (ECSA), reduced charge transfer resistance, and increased electron density, driven by the IMT to the metallic VO₂(R) phase.

Synthesis of Magnesium Phosphorous Trichalcogenides and Applications in Photoelectrochemical Water Splitting

Promising applications of metal phosphorous trichalcogenides (M₂ P₂ X₆ or MPX₃ ) have been predicted in optoelectronics, photoelectrocatalysis, and water-splitting reactions, mainly due to its wide bandgap. Transition metals are widely used in the synthesis of MPX₃ , however, divalent cations of alkaline earth metals can also be constituents in MPX₃ 2D layered structures. Herein, MgPX₃ (X = S, Se) are synthesized and their photoelectrochemical (PEC) activity is tested in the hydrogen evolution and oxygen evolution reaction (OER) regions under a wide range of wavelengths. MgPSe₃ photoelectrode shows the best PEC performance with a response of 1.6 ± 0.1 mA cm^-2 under 420 nm. In the light-assisted OER, a 200 mV improvement is obtained in the overpotential at 10 mA cm^-2 for MgPSe₃ . The better performance of MgPSe₃ is consistent with its lower optical bandgap (E_g  = 3.15 eV), as a result of the variation of electronegativity between selenide and sulfide.

New Polymeric Composites Based on Two-Dimensional Nanomaterials for Biomedical Applications

The constant evolution and advancement of the biomedical field requires robust and innovative research. Two-dimensional nanomaterials are an emerging class of materials that have risen the attention of the scientific community. Their unique properties, such as high surface-to-volume ratio, easy functionalization, photothermal conversion, among others, make them highly versatile for a plethora of applications ranging from energy storage, optoelectronics, to biomedical applications. Recent works have proven the efficiency of 2D nanomaterials for cancer photothermal therapy (PTT), drug delivery, tissue engineering, and biosensing. Combining these materials with hydrogels and scaffolds can enhance their biocompatibility and improve treatment for a variety of diseases/injuries. However, given that the use of two-dimensional nanomaterials-based polymeric composites for biomedical applications is a very recent subject, there is a lot of scattered information. Hence, this review gathers the most recent works employing these polymeric composites for biomedical applications, providing the reader with a general overview of their potential.