Transition Metal Atoms Research Articles

Universal electronic descriptors for optimizing hydrogen evolution in transition metal-doped MXenes

The integration of transition metal (TM) atoms into two-dimensional (2D) transition metal carbides and nitrides (MXenes) has been identified as a promising strategy for enhancing hydrogen evolution reaction (HER) performance. However, the vast combinatorial space presents challenges for rapid catalyst screening. In response, high-throughput calculations were conducted on Ti2CO2 and Zr2CO2 doped with a wide array of TM single atoms. The local structural and corresponding electronic structural changes were analyzed, with an emphasis on their implications for HER performance. Furthermore, universal electronic descriptors were developed using the Sure Independence Screening and Sparsifying Operator (SISSO) method, harmonizing the Gibbs free energy of hydrogen adsorption (ΔGH*) trends across different MXenes substrates doped with TM atoms. These descriptors enabled the prediction of Cr, Fe, Ru, Pd, Os, and Ir as effective dopants for optimizing the ΔGH* of Hf2CO2, resulting in a reduction of ΔGH* from 1.082 eV to within ±0.2 eV. Our work not only highlights the potential of these TM dopants in significantly enhancing the catalytic activity of MXenes but also underscores the value of universal electronic structure descriptors in rapidly identifying and developing high-performance HER catalysts.

DFT Study on the Spin States of Polyaniline-3d Transition-Metal (Sc-Zn) Composites and Their Sensing Application to Detect Chemical Warfare Agents.

Organic-inorganic composite materials, combining polymers with transition metal (TM) atoms based on PAni and 3d TMs, have been designed and investigated in various spin states by performing density functional calculations. These designed composites were analyzed for their stability in different spin states as well as for their calculated electronic properties, including binding energies, frontier molecular orbitals, and dipole moments. Additionally, 3D isosurfaces and 2D scattered plots of reduced density gradient as a function of (sign λ2)ρ provide insights into the noncovalent interactions between the composite units. The most stable Mn@PAni composite has been assessed as a sensing material for chemical warfare blood agents (HCN, NCCl, NCBr, NCCN, and AsH3) using density functional-based calculations. The reduced band gap and significant red/blue shift in the UV-vis spectra obtained through TDDFT calculations underline the selectivity and efficiency of the Mn@PAni composite toward different analytes.

Evaluation of the structural, electronic and magnetic properties of modified 8-16-4 graphyne by 3d transition metals: A DFT-D2 study

A new two-dimensional carbon allotrope called 8-16-4 graphyne (GY) has been recently introduced (in 2021). This allotrope consists of sp. and sp2 hybridized carbon atoms. To investigate the adsorption behavior of single atom of 3d transition metals (TM) on 8-16-4 GY, corrected density functional theory (DFT-D2) was employed. The results reveal that single TM atoms prefer to bind tightly in the center of the 16-carbon ring of GY, and the adsorption energy of TMs decreases from Sc to Zn with a reducible trend from −9.282 to −1.158 eV, respectively. These high adsorption energies show that diffusion of TM atoms and clustering of them are prevented on the surface of the carbon frame. Pristine and 3d TM decorated GY have some Dirac points, zero band gap, and conductivity properties. Also, pristine and all of TM decorated GY except Cr, Mn, and Fe-decorated GY haven't magnetic properties. These findings suggest that 8-16-4 GY is a suitable carbonic substrate to prevent the clustering of TM atoms that is essential for some applications such as effective single-atom catalysts, hydrogen storage, CO2 capture, etcetera.

The germanides ScTGe2 (T = Fe, Co, Ru, Rh) – crystal chemistry, 45Sc solid-state NMR and 57Fe Mössbauer spectroscopy

Abstract The TiMnSi2-type (space group Pbam) germanides ScTGe2 (T = Fe, Co, Ru, Rh) were synthesized from the elements by arc-melting. Single crystals were grown by annealing sequences of the arc-melted buttons in an induction furnace. The structures of ScFeGe2, ScRuGe2 and ScRhGe2 were refined from single-crystal X-ray diffraction data. In ScRuGe2, the ruthenium atoms have distorted octahedral germanium coordination (242–268 pm Ru–Ge). Three trans-face-sharing octahedra form a sub-unit which is condensed via common edges in c direction and connected via common corners with four adjacent blocks, forming a three-dimensional [RuGe2 type] substructure. The two crystallographically independent scandium sites have coordination numbers 15 (Sc1@Ge8Ru4Sc3) and 17 (Sc2@Ge7Ru6Sc4). Electronic band structure calculations for ScCoGe2 and ScRuGe2 show a net charge transfer from the scandium to the transition metal and germanium atoms, leading to a description with polyanionic networks Sc δ+[TGe2]δ−. The two crystallographically independent Sc sites are easily distinguishable by 45Sc magic-angle spinning (MAS)-NMR spectroscopy. Isotropic chemical shift values and nuclear electric quadrupolar interaction parameters were deduced from an analysis of the triple-quantum (TQ)-MAS NMR spectra. The electric field gradient parameters deduced from these experiments are in good agreement with quantum-chemical calculations using the Wien2k code. Likewise, the two crystallographically independent iron sites in ScFeGe2 could be discriminated in the 57Fe Mößbauer spectra through their isomer shifts and quadrupole splitting parameters: δ = 0.369(1) mm s−1 and ∆E Q = 0.232(2) mm s−1 for Fe1 and δ = 0.375(2) mm s−1 and ∆E Q = 0.435(4) mm s−1 for Fe2 (data at T = 78 K).

Direct Exchange in Ultra‐Thin Ferromagnetic Janus MXenes

The development of spintronic devices urgently requires ultra‐thin two‐dimensional (2D) ferromagnetic materials with high Curie temperature (Tc), TC however, there are few natural intrinsic ferromagnetic 2D materials. The successful synthesis of Janus monolayer MoSSe in experiments provides a new approach for designing new 2D materials. By replacing transition metal carbides with two different transition metal atoms, we have designed over 70 Janus MXence materials and determined that 30+ materials have ferromagnetic ground states, of which three have robust ferromagnetism through density functional theory analysis. The ferromagnetic coupling in such materials mainly originates from the direct exchange of d‐orbitals between transition metal atoms in different layers. Further, using K‐nearest neighbours (KNN) machine learning (ML) method, six out of the remaining 360 Janus MXence systems were screened for ferromagnetism, with one system exhibiting strong ferromagnetism. This work provides an alternative and convenient method for developing ultra‐thin 2D magnetic materials for next generation spintronic device applications.

Adsorption behaviors of organic molecules on Nb/Zr–modified bismuthene: A first–principles study

In this work, the geometric and electronic properties of the adsorption of volatile organic molecules (VOCs) such as CH2O, C2H4O, C2H6O on bismuthene monolayer with transition metal (TM = Nb, Zr) atoms adsorbed are investigated by using first–principles methods, aiming to explore the potential applications of TM–modified bismuthene monolayer in the field of organic molecular gas sensors. In contrast to the adsorption of organic molecules on Zr–bismuthene, Nb–bismuthene appears the chemisorption with multiple organic molecules, and it can be found that Nb–bismuthene could stably adsorb target molecules and exhibit outstanding charge transfer. The longer desorption time of organic molecules on Nb–bismuthene and the higher organic molecular adsorption capacity of the substrate at most temperatures, which suggests that Nb–bismuthene is suitable as a disposable sorbent for organic molecules, providing a viable direction for the application of Nb–bismuthene monolayer as a sensitive material for the removal of organic molecules.

Single rhodium atom embedded two dimensional MoSi2N4: A promising electrocatalyst for oxygen reduction reaction

It is essential to develop electrocatalysts that exhibit excellent performance in the oxygen reduction reaction (ORR) for electrochemical energy applications. Recently, MoSi2N4 monolayer has been synthesized and attracted widespread interest. Herein, we constructed a series of structures by embedding transition metal (TM = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt and Au) atom into MoSi2N4 with N vacancy (VN-MSN), denoted as TM@VN-MSN, and investigated their ORR activity systematically by first-principles calculations. It is found that TM@VN-MSN systems are stable and there is charge transfer between TM atom and MoSi2N4. After screening, Rh@VN-MSN has the best ORR catalytic activity with an overpotential of 0.35 V, even lower than that of classical Pt (111). Furthermore, an implicit solvation model is used to evaluate the solvent effect. The volcano plots and scaling relations are established based on free energies of intermediates, and verified ΔG*OH can use as a descriptor to explore the ORR activity on TM@VN-MSN. Moreover, d-band center theory and crystal orbital Hamilton populations (COHP) are used to reveal the ORR activity origin. This work not only provide a promising method for designing a new type of MoSi2N4-based ORR catalysts, but also expands its application in the field of electrocatalysis.

Dispersed Cu (Ni, Co) in MN3 moiety on graphene as active site via electrolytic water towards electro-epoxidation of ethylene

Ethylene oxide (EO) is an important petrochemical product, widely used in printing and dyeing, automotive, medicine, and other fields. Direct epoxidation of ethylene by electrocatalysis is a simple and sustainable route to produce EO, which can avoid the disadvantage of traditional industrial production, such as high temperature and CO2 emissions. An efficient electrocatalyst could generate active oxygen intermediate during the electrolytic water process, which escape from the tremendous activation barrier of breaking molecular oxygen. In addition, ethylene and active oxygen are both required to be adsorbed or produce interaction on the same active site, which is more effective to directly transfer oxygen. Motivated by this strategy, we used first-principle to screen the possibility of 27 single transition metal atoms in MN3 moiety as active site starting from the Gibbs free energy change (ΔG) of *O + C2H4 → *+ C2H4O. Dispersed Cu, Ni, Co can absorb both active oxygen and ethylene, and exhibits the excellent catalytic performance through stability, selectivity, thermodynamics and kinetics evaluation. Electronic structure analysis indicates that the catalytic activity roots from the high electron transfer from the absorbed ethylene to the empty M–O anti-bonding, which weaken the M–O bonding and promote the oxygen transfer. Notably, for CuN3@graphene, the applied potential to produce active oxygen intermediate is 1.53 V vs RHE, and the kinetic energy barrier for subsequent oxygen transfer to form EO is 1.00 eV. This study verified the feasibility of single atomic electrocatalysts for EO production and offers valuable clues to design/discover active center to explore experimentally.

The Guidelines for the Design and Synthesis of Transition Metal Atom Doped Carbon Dots.

Atoms doping is a practical approach to modulate the physicochemical properties of carbon dots (CDs) and thus has garnered increasing attention in recent years. Compared to non-metal atoms, transition metal atoms (TMAs) possess more unoccupied orbitals and larger atomic radii. TMAs doping can significantly alter the electronic structure of CDs and bestow them with new intrinsic characteristics. TMAs-doped CDs have exhibited widespread application potential as a new class of single-atom-based nanomaterials. However, challenges remain for the successful preparation and precise design of TMAs-doped CDs. The key to successfully preparing TMA-doped CDs lies in anchoring TMAs to the carbon precursors before the reaction. Herein, taking the formation mechanism of TMAs-doped CDs as a starting point, we systematically summarized the ligands employed for synthesizing TMAs-doped CDs and proposed the synthetic strategy involving multiple ligands. Additionally, we summarize the functional properties imparted to CDs by different TMA dopants to guide the design of TMA-doped CDs with different functional characteristics. Finally, we describe the bottlenecks TMAs-doped CDs face and provide an outlook on their future development.

Exploring the adsorption characteristics of toxic CO gas on pristine, defective, and transition metal-doped I-AsP monolayer

Utilizing first-principles calculations based on density functional theory, the adsorption characteristics of toxic carbon monoxide (CO) gas adsorbed on pristine, defective, and transition metal (TM) atoms (Sc, Mn, Fe, Ni, Cu, and Zn)-doped blue arsenic-phosphorene (I-AsP) are investigated to design potential sensing platforms. The pristine and defective monolayers exhibited weak CO gas sensing performance due to weak adsorption and unaltered electronic properties. In contrast, TM-doped monolayers exhibited strong interactions and enhanced the adsorption sensitivity due to their chemisorption behavior towards CO molecule. Furthermore, electronic properties obtained through the spin density of states analysis show significant changes in the adsorption of CO molecules on the doped I-AsP monolayer, influencing electrical conductivity. The significant change in the work function of system monolayers after CO adsorption confirmed that the TM-doped I-AsP monolayer is sensitive to CO gas. Therefore, the TM-doped I-AsP monolayer possesses strong sensitivity to CO gas, among which the Sc and Cu-doped monolayers emerge as promising candidates for toxic CO gas detection.

Defect engineering of 1T′ MX 2 (M = Mo, W and X = S, Se) transition metal dichalcogenide-based electrocatalyst for alkaline hydrogen evolution reaction

The alkaline electrolyzer (AEL) is a promising device for green hydrogen production. However, their energy conversion efficiency is currently limited by the low performance of the electrocatalysts for the hydrogen evolution reaction (HER). As such, the electrocatalyst design for the high-performance HER becomes essential for the advancement of AELs. In this work, we used both hydrogen (H) and hydroxyl (OH) adsorption Gibbs free energy changes as the descriptors to investigate the catalytic HER performance of 1T′ transition metal dichalcogenides (TMDs) in an alkaline solution. Our results reveal that the pristine sulfides showed better alkaline HER performance than their selenide counterparts. However, the activities of all pristine 1T′ TMDs are too low to dissociate water. To improve the performance of these materials, defect engineering techniques were used to design TMD-based electrocatalysts for effective HER activity. Our density functional theory results demonstrate that introducing single S/Se vacancy defects can improve the reactivities of TMD materials. Yet, the desorption of OH becomes the rate-determining step. Doping defective MoS2 with late 3d transition metal (TM) atoms, especially Cu, Ni, and Co, can regulate the reactivity of active sites for optimal OH desorption. As a result, the TM-doped defective 1T′ MoS2 can significantly enhance the alkaline HER performance. These findings highlight the potential of defect engineering technologies for the design of TMD-based alkaline HER electrocatalysts.

Hydroxide Modified Synthesis of Atomically‐Doped Photoluminescent WS2 Monolayers

AbstractAtomically doping in thin film poses a significant challenge due to the potential for dopant precipitation caused by self‐purification effects. This challenge is particularly pronounced in the case of doping monolayers of photoluminescent transition metal dichalcogenides, where high energy barriers are also required to break the in‐plane bonds between the transition metal (TM) and chalcogen atoms. To address this issue, this study introduces hydroxide ions to adsorb onto the surface of WS2 monolayers. This results in a significant reduction in the formation energy of the TM─S bonds, enabling them to substitute for W sites and overcoming the self‐purification effect of WS2 monolayers. The in‐plane doping of TMs including Cr, Mn, Fe, Co, and Ni atoms is confirmed through precise atomic‐scale chemical imaging using scanning transmission electron microscopy with electron energy loss spectroscopy mapping. Photoluminescence measurements reveal that the band structure of WS2 monolayers can be systematically modulated by different doping metals, owing to their distinct atomic sizes. In addition, the atomically‐doped WS2 monolayers exhibit room‐temperature ferromagnetism, which has never been seen in pristine WS2 monolayers. The modulation of the band structure and the emergence of magnetism in TMs‐doped WS2 monolayers hold significant promise for optoelectronic and magnetoelectric applications.

The rational co-doping strategy of transition metal and non-metal atoms on g-CN for highly efficient hydrogen evolution by DFT and machine learning

As a clean energy source, hydrogen has attracted high interest in developing efficient hydrogen evolution reaction (HER) catalysts due to its sustainable and renewable characteristics. In this work, we have systematically investigated the HER activity of the g-CN two-dimensional materials. The catalytic activity in HER is enhanced by doping non-metallic atoms (C, N, B, Si) and transition metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) in the vacancies of g-CN. Based on the first principle calculation, we screened 40 structures and found that the ΔGH* of Sc@C3–CN, V@C3–CN, Mn@C3–CN, Sc@N3–CN, and Ti@Si3–CN was close to zero. Among them, Ti@Si3–CN has the lowest Gibbs free energy change (−0.01 eV) and has excellent HER performance. In addition, we explored HER activity's origin by using machine learning (ML) algorithms. The results show that the atomic relative distance in the TM@X3-CN structure significantly affects the catalytic activity.

Electronic and magnetic properties of transition-metal-doped monolayer B2S2 within GGA + U framework.

Considering the significant role of magnetism induction in two-dimensional (2D) semiconductor materials, we systematically investigate the effects of various dopants from the 3d and 4d transition metal (TM) series, including Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag and Cd, on the electronic and magnetic properties of monolayer B2S2 through first-principles calculations. The calculated formation energies indicate that substitutional doping at the B site with various TM atoms could be achieved under S-rich growth conditions. What matters is that with the exception of systems doped with Cu, Tc, and Ag elements, which exhibit non-magnetic semiconductor properties, all other doped systems demonstrate magnetism. Specifically, the Cr-, Ni- and Pd-doped monolayers are magnetic half-metals, while the rest are magnetic semiconductors. We have also performed calculations of magnetic couplings between two TM atoms with an impurity concentration of 3.12%, revealing the prevalence of weak magnetic coupling in the majority of the magnetic systems examined. Moreover, the monolayers doped with Cr, Zr and Pd atoms exhibit ferromagnetic ground states. These findings strongly support the high potential for inducing magnetism in the B2S2 monolayer through B-site doping.

Theoretical study of highly efficient VS2-based single-atom catalysts for lithium-sulfur batteries.

Lithium-sulfur (Li-S) batteries have become a research hotspot due to their high energy density. However, they also have certain disadvantages and limitations. To enhance the performance of Li-S batteries, this study focuses on the utilization of transition metal (TM)-embedded vanadium disulfide (VS2) materials as cathode catalysts. Using density functional theory (DFT), comprehensive calculations and atomic-level screening of ten TM atoms were conducted to understand the underlying mechanisms and explore the potential of TM@VS2 catalysts for enhancing battery performance. The computational results indicate that five selected catalysts possess sufficient bonding strength towards high-order lithium polysulfide intermediates by the formation of a significant covalent bond between S atoms in Li2Sn and TM atoms, thereby effectively suppressing the shuttle effect. The Ni@VS2 catalyst can effectively decrease the decomposition energy barrier of Li2S in the charge reaction and can have an optimal Gibbs free energy at the rate-determining step among TM@VS2 catalysts for the discharge reaction. This study elucidates the mechanism of VS2-based transition-metal single-atom catalysts and provides an effective reference for the anchoring of TM atoms on other materials.

Modulation of the electronic and magnetic properties of an MnCrNO2 ferromagnetic semiconductor MXene.

MXenes are members of the rapidly expanding family of two-dimensional materials known for their electronic and magnetic properties and hold significant promise for advancements in electronics and spintronics technologies. In this study, we identified a stable MnCrNO2 MXene characterized by a band gap of 2.68 eV, a magnetic moment of 6μB, and a magnetic anisotropy energy of 78.6 μeV per transition metal atom. These properties were computed using density functional theory with an on-site Coulomb potential and HSE06 hybrid functional calculations. Manipulating the band gap and magnetic properties offers considerable advantages for tailoring MXenes for specific applications. Our investigation extended to exploring the property behaviors under biaxial strain, as well as the adsorption of Group-I and Group-II ions onto the newly discovered MXene. Our findings underscore a highly linear relationship between strain and band gap, supported by an impressive R2 score of 0.997 for the best-fit straight line. Moreover, we demonstrated the linear tunability of the material's magnetic anisotropy energy under biaxial strain, achieving an R2 score of 0.982. Adsorption of 2.2% Group-I and Group-II ions onto the MnCrNO2 MXene reveals the potential for a semiconductor-to-half-metal phase transition with K, Rb, Be, Mg, and Ca ions. These results provide pathways for leveraging MXenes for the development of next-generation electronic and spintronic devices.

Unveiling the electronic and magnetic landscape of 3d transition metal doped hydrogenated borophenes: a first-principles study.

In this study, density functional theory (DFT) simulations have been utilized to probe the intricate electronic and magnetic properties of pristine and 3d transition metal doped hydrogenated borophenes. It has been investigated through electronic structure calculations that the hydrogenation of 2-Pmmn borophene leads to the emergence of an in-plane Dirac cone, elucidating its transformation into a potential Dirac material with fortified stability. By employing spin-polarized DFT calculations with the Hubbard U correction, we have estimated the electronic and magnetic states of transition metal doped hydrogenated borophenes. Our analysis reveals that the Cr doped hydrogenated borophene manifests the highest magnetic moment of 4.76μB, making it a promising magnetic 2D material. Furthermore, the exchange energy has been calculated by considering the interaction between two transition metal atoms, to assess its magnetic state (ferromagnetic/antiferromagnetic/non-magnetic). The mean field theory and Heisenberg model have been utilized for Néel and Curie temperature estimation, corresponding to anti-ferromagnetic and ferromagnetic states respectively. The present study contributes to the design and understanding of Dirac materials with tailored electronic and magnetic characteristics, highlighting the potential for novel applications in electronics and spintronics. The insights gained from this work may pave the way for future experimental investigations and the realization of functionalized 2D materials with tunable properties.

Structure searches and superconductor discovery in XB2 (X = Sc, Ti, V, Cr, and Tc).

With extensive structure searches for XB2 (X = Sc, Ti, V, Cr, and Tc) under pressures up to 100 GPa, we uncovered that the crystal structures of these compounds with the lowest enthalpy have the same space group (P6/mmm) as MgB2 at ambient pressure. Among them, ScB2, TiB2 and VB2 are dynamically stable at ambient pressure, but they do not superconduct. CrB2 becomes dynamically stable at 108 GPa and shows superconductivity with a transition temperature (Tc) of 26.0 K. TcB2 is not dynamically stable until 9 GPa. At 20 GPa, it has a Tc of 23.5 K. Further calculations indicate that CrB2 and TcB2 are also thermodynamically stable, suggesting that it is highly likely that they can be synthesized successfully in the laboratory. We found that transition metal atoms (Cr/Tc) dominate soft phonon vibrations and make significant contributions to the electron-phonon coupling (EPC) and superconductivity in CrB2/TcB2, which is in strong contrast to the case of MgB2, where high-frequency B vibrations dominate the EPC and superconductivity. Our work enriches the understanding of superconductivity in transition metal borides.

Conductance and assembly of quasi-1D coordination chain molecular junctions with triazole derivatives.

Incorporating transition metal atoms into metal-molecule-metal junctions presents opportunities for exploring the electronic properties of coordination complexes, organometallics and metal-organic materials on the single molecule level. Recent single molecule conductance studies have shown that in situ incorporation of electrode metal atoms into coordination chains formed in the junction can occur with deprotonated, negatively charged organic ligands, such as the imidazolate (Im-) anion. However, the mechanism and chemical principles, such as the role of the charge state of the ligand, for the construction of such coordination chains are still debated. Here, we probe the role of the ligand charge state and electronic structure in single-molecule conductance and formation of metal-molecule coordination chains. We perform break junction measurements with triazole isomers, which can bridge junctions both in their neutral and charged forms, and find that prior deprotonation of the ligands is not required for coordination complex assembly, but can affect the molecular conductance and junction formation probability. Our results indicate that coordination chains can form with neutral ligands, as long as the electron density in the frontier MOs is concentrated at the binding sites and along the direction of pulling, promoting ligand binding and incorporation of gold atoms into the junction during elongation. Our findings may provide insight into design principles for in situ assembled molecular wires with transition metal atoms and open the door to electronic and spintronic studies of such materials.

The oxygen evolution reaction on cobalt atom embedded nitrogen doped graphene electrocatalysts: a density functional theory study.

The oxygen evolution reaction (OER) is essential for the development of renewable energy conversion and storage technologies. Eight N-doped graphenes containing variable numbers of embedded cobalt atoms (Coxy-NG, x = 1-4, y = 1-3, where x represents the number of embedded Co atoms and y represents different configurations) were designed and their OER electrocatalytic activities were systematically studied through density functional theory calculations. The significant roles of the number of Co atoms and their configuration in their OER performance were discussed in detail. Co31-NG occupies the peak of the activity volcano plot with a low overpotential of 0.31 V, which is smaller than Co11-NG with only one Co atom and even superior to the widely used IrO2 (0.56 V). The electronic structure and electron density analysis reveal that the outstanding electrocatalytic performance is due to the orbital hybridization between Co and N atoms and the increased positive charge on in-plane Co due to the out-of-plane Co atoms/clusters. This work clarifies the important role of transition atoms and provides excellent examples for reducing the overpotential through embedding several transition metal atoms onto single-atom electrocatalysts.