Stability of single-atom iron complexes on graphene double vacancy
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2023-12-10 |
| Journal | PoverhnostŹ¹ |
| Authors | O.S. Karpenko, Š. Š. ŠŠ¾Š±Š°Š½Š¾Š², M. T. Šartel |
| Institutions | Chuiko Institute of Surface Chemistry |
| Analysis | Full AI Review Included |
Executive Summary
Section titled āExecutive SummaryāThis study utilizes Density Functional Theory (DFT) to investigate the structural stability and electronic properties of single-atom iron (Fe) complexes anchored on nitrogen-doped graphene double vacancies (N4-V2 sites).
- High Stability Confirmed: The formation of the single-atom complex, [C90N4H24Fe]0, is highly favorable, exhibiting a large exothermic binding energy of -7.37 eV, confirming the stability of the Fe atom at the defect site.
- Coordination Environment: The Fe atom is coordinated by four nitrogen atoms in a square planar geometry, corresponding to D4h local symmetry, analogous to metal-porphyrin complexes.
- Bonding Mechanism: The binding between the Fe atom and the N4 ligand is exclusively due to sigma (Ļ)-bonding interactions involving the Fe d-orbitals and the ligand molecular orbitals (MOs). Pi (Ļ)-bonding is excluded due to symmetry conditions.
- Electronic State: The Ground Electronic State (GES) of the complex is a triplet (M=3), indicating the presence of unpaired electrons, which is critical for magnetic and catalytic applications.
- Theoretical Framework: The binding mechanism and d-orbital splitting are successfully rationalized using concepts from classical coordination chemistry and Ligand Field Theory, providing a robust model for designing single-atom catalysts on graphene.
Technical Specifications
Section titled āTechnical SpecificationsāThe following parameters were determined via DFT (B3LYP/6-31G** with Grimme corrections) using the C96H24 PAH model.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Fe Complex Formation Energy | -7.37 | eV | [C90N4H24Fe]0 (Exothermic binding) |
| Double Vacancy (V2) Formation Energy | +16.73 | eV | C96H24 ā C94H24 (Endothermic) |
| N-Doping Energy (4C ā 4N) | +7.77 | eV | C94H24 ā C90N4H24 (Endothermic) |
| Fe-N Coordination Bond Order | 0.474 | N/A | Calculated bond strength |
| Nā¦N Diagonal Distance (Ligand) | 3.840 | Angstrom | C90N4H24 (before Fe binding) |
| Nā¦N Diagonal Distance (Complex) | 3.808 | Angstrom | [C90N4H24Fe]0 (after Fe binding) |
| Local Coordination Symmetry | D4h | N/A | Square planar field |
| Ground Electronic State (GES) | Triplet (M=3) | N/A | Spin multiplicity of the complex |
| Fe Atomic Orbital Configuration | 3d64s2 | N/A | Neutral iron atom (Fe0) |
Key Methodologies
Section titled āKey MethodologiesāThe study relied entirely on computational modeling using established quantum chemistry techniques to simulate the defect formation and metal binding process.
- Model Selection: A polycyclic aromatic hydrocarbon (PAH), C96H24, was chosen as a finite, representative model for the infinite graphene plane.
- Computational Framework: Density Functional Theory (DFT) was employed using the B3LYP functional.
- Basis Set and Correction: The 6-31G** basis set was used, supplemented by Grimme corrections to accurately incorporate long-range dispersion interactions, which are crucial for surface adsorption studies.
- Defect Generation (V2): A double vacancy (V2) was created by computationally removing a diatomic C2 molecule from the C96H24 model, resulting in C94H24.
- Doping and Ligand Formation: Four carbon atoms adjacent to the V2 defect were substituted with four nitrogen atoms, forming the C90N4H24 ligand with a local N4 coordination center.
- Complexation: A neutral iron atom (Fe0) was introduced into the N4 coordination center to form the [C90N4H24Fe]0 complex.
- Orbital Analysis: Molecular Orbital (MO) theory and Ligand Field Theory were applied to analyze the symmetry matching between the Fe d-orbitals and the ligand MOs (specifically HOMO-4 and HOMO-5), confirming the nature of the Ļ-bonding interaction.
Commercial Applications
Section titled āCommercial ApplicationsāThe creation of stable, single-atom transition metal sites on graphene is foundational for advanced materials engineering, particularly in electrochemistry and spintronics.
- Single-Atom Catalysis (SACs): The Fe-N4 site is a highly efficient and stable active center for various catalytic reactions, including the Oxygen Reduction Reaction (ORR) in proton exchange membrane (PEM) fuel cells, offering a cost-effective alternative to platinum-group metals.
- Spintronic Devices: The triplet ground state (M=3) of the complex introduces localized magnetic moments, enabling the use of this material in developing next-generation spintronic devices, such as spin filters and magnetic sensors, leveraging the unique electronic properties of graphene.
- Energy Storage and Conversion: Fe-N-doped graphene materials enhance the performance and durability of electrodes in lithium-ion batteries and supercapacitors by improving conductivity and providing stable anchor points for charge carriers.
- Chemical Sensing: Controlled defect engineering and metal doping allow for tuning the electronic bandgap of graphene, creating materials with high selectivity and sensitivity for detecting specific gas molecules or chemical species.
- Micro/Nanoelectronics: The ability to introduce a controlled bandgap via doping and defect creation overcomes the zero bandgap limitation of pristine graphene, facilitating its integration into semiconductor-based microelectronic circuits.
View Original Abstract
The equilibrium and spatial structure of the polycyclic aromatic hydrocarbon C96H24, chosen as a model of the graphene plane, as well as the systems obtained from it by removing the diatomic molecule C2 (C94H24) and then replacing four carbon atoms with four nitrogen atoms (C90N4H24) have been studied by the DFT method (B3LYP) in the 6-31G** basis using Grimme corrections to account for dispersion interactions. In the same approximation, the energetics of the formation of a complex of an iron atom in zero oxidation degree (Fe0) with C90N4H24 ([C90N4H24Fe]0) in the square planar field of the ligand has been studied. The types of molecular orbitals of the ligand, which correspond to the symmetry of the atomic d-orbitals of the Fe atom, have been determined. Interaction diagrams of the d-orbitals of the Fe atom with some molecular orbitals of the ligand C90N4H24 of the corresponding symmetry are constructed. It is concluded that the binding of the transition metal atom on the double vacancy of the graphene plane can be rationally described based on the local symmetry of the coordination center and molecular orbitals of the ligand and the formed complex.