Metal Monolayers Research Articles

Bi-metallic electrochemical deposition on 3D pyrolytic carbon architectures for potential application in hydrogen evolution reaction

ABSTRACT 3D printing has emerged as a highly efficient process for fabricating electrodes in hydrogen evolution through water splitting, whereas metals are the most popular choice of materials in hydrogen evolution reactions (HER) due to their catalytic activity. However, current 3D printing solutions face challenges, including high cost, low surface area, and sub-optimal performance. In this work, we introduce metal-deposited 3D printed pyrolytic carbon (PyC) as a facile and cost-effective HER electrode. We adopt an integrated approach of resin 3D printing, pyrolysis, and electrochemical metal deposition. 3D printing of a resin and its subsequent pyrolysis led to 3D complex architectures of the conductive substrate, facilitating the electrochemical metal deposition and leading to layered 3D metal architecture. Both monolayers of metals (such as copper and nickel) and bi-metallic 3D PyC structures are demonstrated. Each metal layer thickness ranges from 6 to10 µm. The metal coatings, particularly the bi-metallic configurations, result in achieving significantly higher mechanical properties under compressive loading and improved electrical properties due to the synergistic contributions from each metal counterpart. The metalized PyC structures are further demonstrated for HER catalysts, contributing to the development of highly efficient and durable catalyst systems for hydrogen production. Among the materials studied here, Ni@Cu bimetallic 3D PyC electrodes are particularly well-suited, demonstrating a low HER overpotential value of 264 mV (100 mA/cm2, KOH (1 M)) with corresponding Tafel slopes of 107 mV/dec, with exceptional stability during a 10 h operation at a high applied current of −50 mA/cm2.

Green synthesis of nanoscale zero-valent iron impregnated walnut shell biochar as efficient adsorbent for metal(loid)s purification: Performance and mechanism insight

The escalation of metal(loid) contamination in global water poses severe risks to ecological environment safety. Meeting the ‘pollution control with waste’ strategy, we utilized eco-friendly waste tea leaf extract and walnut shell to first fabricate a greenly synthesized nanoscale zero-valent iron impregnated walnut shell biochar (G-nZVI/WSB) for the simultaneous sequestration of cadmium (Cd), lead (Pb), and arsenic (As) from aqueous systems. Since the interactions among these metal(loid)s during adsorption on G-nZVI/WSB and their corresponding mechanisms remain unclear, we conducted a series of batch experiments coupled with characterization techniques. The optimal pyrolysis temperature and Fe/C ratio for G-nZVI/WSB were determined to be 650 °C and 20 %, respectively. Under optimal conditions (contact time = 24 h, pH = 6), G-nZVI/WSB exhibited theoretical maximum adsorption capacities for Cd(II), Pb(II), and As(III) of 52.96, 135.1, and 8.414 mg g−1, respectively, which were significantly higher than those of WSB carrier. In single and combined systems, the monolayer and chemical adsorption of metal(loid)s were predominant, with their competitive affinity for active sites following the order of Pb(II) > Cd(II) > As(III). Characterization analyses using FTIR, SEM, XRD, and XPS confirmed the specific adsorption mechanisms of G-nZVI/WSB for these metal(loid)s, including cation-π interactions, coprecipitation, oxidation, and coordinated complexation. Notably, G-nZVI/WSB exhibited excellent adsorption stability over five cycles of adsorption-desorption, which achieved the efficient purification of multi-contaminated actual wastewaters to meet the standards for wastewater discharge or drinking water. These findings demonstrate the significant potential of G-nZVI/WSB as an eco-friendly and promising nanocomposite for treating metal(loid)-contaminated wastewater.

First principles theoretical of designing Sandwich-like Metallic BP4 Monolayer as Anode for Alkali Metal-ion Batteries.

Phosphorene has been widely used as anode material for batteries. However, the huge volume change during charging and discharging process, the semiconductor properties, and the high open circuit voltage limit its application. Based on this, by introducing the electron-deficient boron atoms into blue phosphorene, we proposed a P-rich sandwich-like BP4 monolayer by density functional theory calculation and particle swarm optimization. The BP4 monolayer shows good thermodynamic and dynamic stability, as well as chemical stability in O2 atmosphere, mainly due to the strengthened P-P bond of the outer layer by the middle boron atoms adopting sp3 hybridization. According to the band structure, the BP4 monolayer shows metallic property, which is beneficial to electron conductivity. Furthermore, compared with blue phosphorene and black phosphorene, the P-rich BP4 monolayer shows higher theoretical capacity for Li, Na, and K of 1193.90, 1119.28, and 397.97 mA·h·g-1, respectively. The lattice constant of BP4 monolayer increases only 3.73 (Li), 2.52 (Na) after Li/Na fully adsorbed on the anode. More importantly, the wettability of BP4 monolayer in the electrolyte is comparable to that of graphene. These findings show that the stable sandwich-like BP4 monolayer has potential as a lightweight anode material.

Effects of vacancy defects and atomic doping on the electronic and magnetic properties of puckered penta-like PdPSe monolayer: an Ab initio study

The experimental knowledge of two-dimensional penta-like PdPSe monolayer is largely based on a recent publication (Liet al2021Adv. Mater. 2102541). Therefore, the aim of our research is consequently to explore the effect of vacancy defects and substitutional doping on the electronic properties of the novel penta-PdPSe monolayer by using first-principles calculations. Penta-like PdPSe is a semiconductor with an indirect bandgap of 1.40 eV. We show that Pd and Se vacancy defected structures are semiconductors with band gaps of 1.10 eV and 0.95 eV respectively. While P single vacancy and double vacancy defected structures are metals. The doping with Ag (at Pd site) and Si (at P site) convert the PdPSe to nonmagnetic metallic monolayer while the doping with Rh (at Pd site), Se (at P site) and As (at site Se) convert it to diluted magnetic semiconductors with the magnetic moment of 1µB. The doping with Pt (at the Pd site), As (at the P site), S and Te (at Se site) are indirect semiconductors with a bandgap of ∼1.2 eV. We undertook this theoretical study to inspire many experimentalists to focus on penta-like PdPSe monolayer growth incorporating different impurities and by defect engineering to tune the novel two dimensional materials (PdPSe) properties for the advanced nanoelectronic application.

Two-dimensional metallic Be2Al monolayer as a potential anode material for lithium-ion/sodium-ion batteries

Developing anode materials that combine excellent robustness, high-volume capacity, and superior electrical conductivity is essential for advancing high-performance ion batteries. Through comprehensive first-principles calculations, this paper illustrates the possibility of a two-dimensional Be2Al monolayer as an anode candidate for lithium/sodium-ion batteries. The Be2Al monolayer exhibits a high theoretical energy storage capacity (2382/2382 mAh/g for Li/Na) and an ultra-low diffusion barrier (124/77 meV for Li/Na). Its average open-circuit voltage (0.35/0.52 V for Li/Na) falls within the desired ideal range. Additionally, the Be2Al monolayer demonstrates excellent electrical conductivity, facilitating efficient electron transport. These promising outcomes indicate that the Be2Al monolayer can be an effective anode for next-generation lithium-ion and sodium-ion batteries.

Theoretical investigation into potential thermoelectric material Monolayer InI with direct bandgap and ultralow lattice thermal conductivity

Monolayer metal iodides are garnering widespread attention due to their ultralow lattice thermal conductivity and their potential applications in the field of thermoelectrics. Here, we utilized first-principles calculations and Boltzmann transport theory to thoroughly investigate the thermoelectric behavior of monolayer InI. Results indicate that this material functions as a direct bandgap semiconductor and features an exceptionally low lattice thermal conductivity of 0.27 W m−1K−1 at 300 K, a result of its low group velocities and short phonon lifetimes. Furthermore, transport coefficients were calculated with enhanced precision through the Wannier interpolation method, incorporating the HSE06 hybrid functional and spin-orbit coupling (SOC) effects. Remarkably, the maximum thermoelectric figure of merit (ZT) values for monolayer InI demonstrate substantial growth with increased temperatures, surging from 0.31 at 300 K to 1.20 at 450 K for p-type, and escalating from 0.26 at 300 K to 0.72 at 450 K for n-type. These promising low-temperature thermoelectric properties exhibited by monolayer InI suggest its suitability for integration into the production of thermoelectric devices.

The study on influence factors of contact properties of metal-MoS2 interfaces

The metal and two-dimension (2D) semiconductor contact interfaces have a more considerable contact resistance hindering carrier injection, which makes the performance of 2D semiconductor devices less than the theory. The contact properties of Ni, Au, and Mo with MoS2 are simulated by the first-principles method. The interface dipole caused by the interface charge redistribution changes the work function difference at the metal-MoS2 interface, so the interface charge redistribution is one of the important factors for correctly evaluating the contact properties. Due to the metal-induced gap states (MIGS) at metal-monolayer (ML) MoS2 interfaces, the Fermi level is strongly pinned to fixed energy, and the Schottky barrier height (SBH) cannot be regulated efficiently by the metal work function. Although the work function of Au is bigger than Ni, the Fermi level of Au is pinned at a higher position. In the meantime, the bandgap of MoS2 narrows and metallization occurs due to the larger MIGS. In the Mo-MoS2 interface, the Fermi level is pinned near the conduction band minimum of MoS2. The contact resistances (Rc) of the three structures are tested by the Circular Transfer Length Method (CTLM), which is consistent with the prediction of the simulation. The Mo-MoS2 has the smallest Rc. The results indicate that contact resistance of 2D semiconductors cannot be simply predicted by soled work functions or Fermi level pinning, but is determined by several factors.

Theoretical Study of Dissolved Gas Molecules in Transformer Oil Adsorbed on Intrinsic and TM (Ta, V)-Doped MoTe2 Monolayer.

In this paper, CH4, C2H2, H2, and CO adsorbed on intrinsic MoTe2 monolayer and transition metal atom (Ta, V)-doped MoTe2 monolayer have been investigated with density functional theory based on first-principles study. The adsorption energy, geometries, band structures, and density of states of four gases (CH4, C2H2, H2, and CO) adsorbed on the MoTe2 and doped MoTe2 surfaces were analyzed. The results shown that the gas adsorption performance of transition metal atom (Ta, V)-doped MoTe2 monolayers is more superior than that of intrinsic MoTe2, and the adsorption energy and charge transfer of the adsorbed gases on the TM-MoTe2 monolayer are significantly increased in comparison with both sides. Among them, Ta-MoTe2 has the largest Eads value in the adsorbed CO system with a very small adsorption distance, as well as a more suitable recovery time of CO at room temperature, so Ta-MoTe2 can be a candidate material for CO detection. New atoms were introduced during the doping process, which increased the carrier density and carrier mobility of the material, thus improving the charge transfer at the surface of the material. which provides a direction for the gas-sensitive properties of metal Ta-modified MoTe2 materials.

Systematic combination of palladium facets and monolayer metal hydroxide nanosheets for promotion of ethanol oxidation reaction

The design of catalyst surfaces that exhibit high activities for ethanol oxidation reaction (EOR) is essential to achieving practical application of direct ethanol fuel cells. Composite catalysts of Pd nanoparticles and metal hydroxide are promising candidates for EOR; however, a guide of catalyst design, including appropriate valence states and interfacial structures between component materials, has not been identified. Herein, we synthesize composite electrocatalysts of cubic and octahedral Pd nanocrystals with well-defined facets and monolayer metal hydroxide nanosheets with an identical structure and different composition containing Ni, Fe and Mn, and systematically elucidate highly active interfacial sites between Pd and metal hydroxides for EOR. Promotion of OH− adsorption by the hydroxide nanosheets enhances the catalytic activities of the Pd nanocrystals and the nanosheet having higher affinity to OH− result in higher EOR performances of the composite catalysts. The improved activities are greater for (111) facet than for (100) facet of the Pd whereas the (100) facet is superior for EOR to the (111) facet for the bare Pd nanocrystals. Our study suggests that combination of the Pd (111) facet and metal hydroxides is favorable for developing highly active electrocatalysts for EOR.

Kagomerization of transition metal monolayers induced by two-dimensional hexagonal boron nitride

The kagome lattice is an exciting solid state physics platform for the emergence of nontrivial quantum states driven by electronic correlations: topological effects, unconventional superconductivity, charge and spin density waves, and unusual magnetic states such as quantum spin liquids. While kagome lattices have been realized in complex multi-atomic bulk compounds, here we demonstrate from first-principles a process that we dub kagomerization, in which we fabricate a two-dimensional kagome lattice in monolayers of transition metals utilizing an hexagonal boron nitride (h-BN) overlayer. Surprisingly, h-BN induces a large rearrangement of the transition metal atoms supported on a fcc(111) heavy-metal surface. This reconstruction is found to be rather generic for this type of heterostructures and has a profound impact on the underlying magnetic properties, ultimately stabilizing various topological magnetic solitons such as skyrmions and bimerons. Our findings call for a reconsideration of h-BN as merely a passive capping layer, showing its potential for not only reconstructing the atomic structure of the underlying material, e.g. through the kagomerization of magnetic films, but also enabling electronic and magnetic phases that are highly sought for the next generation of device technologies.

Goldene: An Anisotropic Metallic Monolayer with Remarkable Stability and Rigidity and Low Lattice Thermal Conductivity.

In a recent breakthrough in the field of two-dimensional (2D) nanomaterials, the first synthesis of a single-atom-thick gold lattice of goldene has been reported through an innovative wet chemical removal of Ti3C2 from the layered Ti3AuC2. Inspired by this advancement, in this communication and for the first time, a comprehensive first-principles investigation using a combination of density functional theory (DFT) and machine learning interatomic potential (MLIP) calculations has been conducted to delve into the stability, electronic, mechanical and thermal properties of the single-layer and free-standing goldene. The presented results confirm thermal stability at 700 K as well as remarkable dynamical stability of the stress-free and strained goldene monolayer. At the ground state, the elastic modulus and tensile strength of the goldene monolayer are predicted to be over 226 and 12 GPa, respectively. Through validated MLIP-based molecular dynamics calculations, it is found that at room temperature, the goldene nanosheet can exhibit anisotropic tensile strength over 9 GPa and a low lattice thermal conductivity around 10 ± 2 W/(m.K), respectively. We finally show that the native metallic nature of the goldene monolayer stays intact under large tensile strains. The combined insights from DFT and MLIP-based results provide a comprehensive understanding of the stability, mechanical, thermal and electronic properties of goldene nanosheets.

First-principles study of CH2O, CH4, Cl2, HCN, and CNCl gas adsorption behavior by h-BN boron site modification

The potential application of metallic boron bit-doped hexagonal boron nitride monolayers (h-BN@M, M = Fe, Sn, Cu, Zn) as materials for the detection of hazardous gases (CH2O, CH4, Cl2, HCN, and CNCl) has been investigated based on the first nature principle. h-BN@Fe exhibits chemisorptive properties for the adsorption of HCN and CNCl and h-BN@Sn exhibits chemisorption and appropriate humidity resistance. The recovery time of HCN on the h-BN@Fe surface and CNCl on the h-BN@Sn surface was significantly shortened under the 392 K ultraviolet light attempt frequency. In the presence of an applied electric field, HCN and CNCl on the h-BN@Fe surface and CNCl on the h-BN@Sn surface showed adjustable recovery times. This suggests that it is possible to design gas sensing devices with good adsorption characteristics on metallic boron bit-doped hexagonal boron nitride monolayers. In the future, the performance of these materials for the detection of hazardous gases will be improved by changing their structure and composition.

Ultrathick MA2N4(M'N) Intercalated Monolayers with Sublayer‐Protected Fermi Surface Conduction States: Interconnect and Metal Contact Applications

AbstractRecent discovery of ultrathick MoSi2N4(MoN)n monolayers open up an exciting platform to engineer two‐dimensional (2D) material properties via intercalation architecture. In this study, a series of ultrathick MA2N4(M'N) monolayers (M, M' = Mo, W; A = Si, Ge) is computationally investigated under both homolayer and heterolayer intercalation architectures, in which the same and different species of transition metal nitride inner core sublayer are intercalated by outer passivating nitride sublayers, respectively. The MA2N4(M'N) are stable metallic monolayers with excellent mechanical strength. Intriguingly, the metallic states around Fermi level are localized within the inner core sublayer. Carrier conduction mediated by electronic states around the Fermi level is thus spatially insulated from the external environment by the native outer nitride sublayers, suggesting the potential of MA2N4(M'N) in back‐end‐of‐line metal interconnect applications. N and Si (or Ge) vacancy defects at the outer sublayers create ‘punch through’ states around the Fermi level that bridges the carrier conduction in the inner core sublayer and the outer environment, forming an electrical contact akin to the ‘via' structures of metal interconnects. It is further shown that MoSi2N4(MoN) can serve as a quasi‐Ohmic contact to 2D WSe2. These findings reveal the potential of ultrathick MA2N4(MN) monolayers in interconnect and metal contact applications.

Screening ideal MXene electrodes for monolayer MoSe2: A first-principles study

Exploring proper electrodes with low-resistance contacts is vital for promoting the performance of nanoelectronics. We have screened 35 kinds of metallic monolayer (ML) MXenes for possible low-resistance contact electrodes with ML MoSe2 through first-principles calculations. The lattice mismatch and work function (WF) are used as two indicators to screen for possible electrodes. Then, we validate the interface contact characteristics of ten selected van der Waals (vdW) heterostructures. Notably, n-type Ohmic contacts are observed for the MoSe2/Mo2C(OH)2, MoSe2/Zr2C(OH)2, MoSe2/Sc2N(OH)2, MoSe2/Nb2C(OH)2, MoSe2/Zr2N(OH)2, and MoSe2/Zr2NF2 interfaces, while p-type (quasi) Ohmic contacts are observed at the MoSe2/Mo2NO2 and MoSe2/Sc2CO2 interfaces. All the above interfaces are stable, and the bands from each component layer are well preserved. The screening strategy is also suitable for identifying other low-resistance vdW contacts.

Quantum spin liquid signatures in monolayer 1T-NbSe2

Quantum spin liquids (QSLs) are in a quantum disordered state that is highly entangled and has fractional excitations. As a highly sought-after state of matter, QSLs were predicted to host spinon excitations and to arise in frustrated spin systems with large quantum fluctuations. Here we report on the experimental observation and theoretical modeling of QSL signatures in monolayer 1T-NbSe2, which is a newly emerging two-dimensional material that exhibits both charge-density-wave (CDW) and correlated insulating behaviors. By using scanning tunneling microscopy and spectroscopy (STM/STS), we confirm the presence of spin fluctuations in monolayer 1T-NbSe2 by observing the Kondo resonance as monolayer 1T-NbSe2 interacts with metallic monolayer 1H-NbSe2. Subsequent STM/STS imaging of monolayer 1T-NbSe2 at the Hubbard band energy further reveals a long-wavelength charge modulation, in agreement with the spinon modulation expected for QSLs. By depositing manganese-phthalocyanine (MnPc) molecules with spin S = 3/2 onto monolayer 1T-NbSe2, new STS resonance peaks emerge at the Hubbard band edges of monolayer 1T-NbSe2. This observation is consistent with the spinon Kondo effect induced by a S = 3/2 magnetic impurity embedded in a QSL. Taken together, these experimental observations indicate that monolayer 1T-NbSe2 is a new promising QSL material.

Synthesis and Raman Detection of 5-Amino-2-mercaptobenzimidazole Self-Assembled Monolayers in Nanoparticle-on-a-Mirror Plasmonic Cavity Driven by Dielectric Waveguides.

Functionalization of metallic surfaces by molecular monolayers is a key process in fields such as nanophotonics or biotechnology. To strongly enhance light-matter interaction in such monolayers, nanoparticle-on-a-mirror (NPoM) cavities can be formed by placing metal nanoparticles on such chemically functionalized metallic monolayers. In this work, we present a novel functionalization process of gold surfaces using 5-amino-2-mercaptobenzimidazole (5-A-2MBI) molecules, which can be used for upconversion from THz to visible frequencies. The synthesized surfaces and NPoM cavities are characterized by Raman spectroscopy, atomic force microscopy (AFM), and advancing-receding contact angle measurements. Moreover, we show that NPoM cavities can be efficiently integrated on a silicon-based photonic chip performing pump injection and Raman-signal extraction via silicon nitride waveguides. Our results open the way for the use of 5-A-2MBI monolayers in different applications, showing that NPoM cavities can be effectively integrated with photonic waveguides, enabling on-chip enhanced Raman spectroscopy or detection of infrared and THz radiation.

Type-II heavy Fermi liquids and the magnetic memory of 4Hb−TaS2

The interplay of quantum spin liquids with itinerant conduction electrons is of crucial interest for understanding layered structures composed of frustrated magnet and metal monolayers. Using parton-mean-field theory, we demonstrate here that a type-II heavy Fermi liquid, which is characterized by a vortex lattice in the slave boson condensate, can occur in the vicinity of the quantum phase transition separating fractionalized and heavy Fermi liquid phases. The magnetic flux threading each such vortex is about vf/137c times smaller than the magnetic flux threading vortices in type-II superconductors, where vf is the speed of magnetic excitations and c the speed of light. This makes a magnetic observation of this effect challenging. We propose scanning tunneling spectroscopy instead and investigate its signatures. If a type-II heavy Fermi liquid is cooled into a type-II superconductor, vortices in the slave boson condensate and in the superconducting condensate mutually attract. We argue that the type-II heavy Fermi liquid thereby provides a compelling explanation for the magnetic memory observed recently [Persky , ] in thermal cycles of 4Hb−TaS2. Published by the American Physical Society 2024

Two-dimensional Strontium Tantalate: 2D-Sr[formula omitted]TaO[formula omitted

We propose and characterize a novel 2D material with a perovskite structure, Sr2TaO4 monolayer. Using a first principles approach, we study the stability, elastic and electronic properties of the 2D-Sr2TaO4 system. In this metallic monolayer system, the tantalum atom is surrounded by six oxygen atoms in an octahedral configuration. The rotation of the octahedra structures yields antiferrodistortive distortions favoring a phase transition. We have demonstrated the stability of this perovskite-like monolayer using phonons, molecular dynamics, and elasticity criteria.

2D Metal/Graphene and 2D Metal/Graphene/Metal Systems for Electrocatalytic Conversion of CO2 to Formic Acid

AbstractEfficiently transforming CO2 into renewable energy sources is crucial for decarbonization efforts. Formic acid (HCOOH) holds great promise as a hydrogen storage compound due to its high hydrogen density, non‐toxicity, and stability under ambient conditions. However, the electrochemical reduction of CO2 (CO2RR) on conventional carbon black‐supported metal catalysts faces challenges such as low stability through dissolution and agglomeration, as well as suffering from high overpotentials and the necessity to overcome the competitive hydrogen evolution reaction (HER). In this study, we modify the physical/chemical properties of metal surfaces by depositing metal monolayers on graphene (M/G) to create highly active and stable electrocatalysts. Strong covalent bonding between graphene and metal is induced by the hybridization of sp and d orbitals, especially the sharp , , and orbitals of metals near the Fermi level, playing a decisive role. Moreover, charge polarization on graphene in M/G enables the deposition of another thin metallic film, forming metal/graphene/metal (M/G/M) structures. Finally, evaluating overpotentials required for CO2 reduction to HCOOH, CO, and HER, we find that Pd/G, Pt/G/Ag, and Pt/G/Au exhibit excellent activity and selectivity toward HCOOH production. Our novel 2D hybrid catalyst design methodology may offer insights into enhanced electrochemical reactions through the electronic mixing of metal and other p‐block elements.

2D Metal/Graphene and 2D Metal/Graphene/Metal Systems for Electrocatalytic Conversion of CO2 to Formic Acid.

Efficiently transforming CO2 into renewable energy sources is crucial for decarbonization efforts. Formic acid (HCOOH) holds great promise as a hydrogen storage compound due to its high hydrogen density, non-toxicity, and stability under ambient conditions. However, the electrochemical reduction of CO2 (CO2 RR) on conventional carbon black-supported metal catalysts faces challenges such as low stability through dissolution and agglomeration, as well as suffering from high overpotentials and the necessity to overcome the competitive hydrogen evolution reaction (HER). In this study, we modify the physical/chemical properties of metal surfaces by depositing metal monolayers on graphene (M/G) to create highly active and stable electrocatalysts. Strong covalent bonding between graphene and metal is induced by the hybridization of sp and d orbitals, especially the sharp , , and orbitals of metals near the Fermi level, playing a decisive role. Moreover, charge polarization on graphene in M/G enables the deposition of another thin metallic film, forming metal/graphene/metal (M/G/M) structures. Finally, evaluating overpotentials required for CO2 reduction to HCOOH, CO, and HER, we find that Pd/G, Pt/G/Ag, and Pt/G/Au exhibit excellent activity and selectivity toward HCOOH production. Our novel 2D hybrid catalyst design methodology may offer insights into enhanced electrochemical reactions through the electronic mixing of metal and other p-block elements.