Supported data and manuscript "A density functional theory study of magnetic transition in MnO2 adsorbed vanadium carbide (V2C) MXene"

A density functional theory study of magnetic transition in MnO 2 adsorbed vanadium carbide (V 2 C) MXene Mahjabeen Fatima a,b , Saleem Ayaz Khan c,* , Syed Rizwan a,* a Physics Characterization and Simulations Lab (PCSL), Department of Physics & Astronomy, School of Natural Sciences (SNS), National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan b Department of Physics and Astronomy, School of Natural Sciences, The University of Manchester, United Kingdom c New Technologies Research Centre, University of West Bohemia, Univerzitni 2732, 306 14 Pilsen, Czech Republic ARTICLE INFO Keywords: V 2 C MXene Density Functional Theory (DFT) Magnetism ABSTRACT The work reports nonmagnetic behavior (0.04 µB) in two-dimensional (2D) V 2 C-OF MXene and ferromagnetism in MnO 2 adsorbed V 2 C-OF MXene. The density functional theory (DFT) calculations were carried out to study the magnetic moments of V 2 C-OF and MnO 2 @V 2 C-OF MXene. The MXene, which is derived from the exfoliation of its parent V 2 AlC MAX phase, shows a good potential to be a ferromagnetic when MnO 2 is adsorbed on it. The V 2 C MXene and MnO 2 adsorbed V 2 C MXene were successfully synthesized, as characterized using X-ray diffraction, showing an increased c-lattice parameter from 22.6 Å to 27.2 Å after MnO 2 adsorption. The DFT study confirmed that MnO 2 adsorbed V 2 C MXene changed from nonmagnetic (in V 2 C MXene) to a strong ferromagnetic with a magnetic moment of 4.48 μ B for Mn adsorbed V 2 C-OF MXene. The current work is a step-forward towards understanding of magnetism in two-dimensional materials for future 2D spintronics. 1. Introduction The tuning of electronic and magnetic properties of a material through adsorption of one element or compound over varying class of MXenes is an effective strategy for the enhancement in energy storage systems and spintronic devices [1–3]. With the growing requirement of novel and smart materials, several compounds have been engineered for the development of nanotechnology industry. The first 2D material, graphene [4], showed intriguing properties for diverse applications after which, many other 2D materials like borophene, hexagonal boronnitride, phosphorene, transition metal dichalcogenide and phosphorene bismuthene were discovered and are used in the applications of biosensors, hydrogen evolution reactions (HER), photonics, energy storage systems, etc. [5–10]. The 2D MXenes are a result of exfoliated MAX phase which is a bulk, three-dimensional material that represents a huge (60+members) family of transition metal carbides, nitrides, and carbonitrides. These possess lamellar hexagonally symmetric structures (space group P63/ mmc) with the generalized formula of M n+1 AX n (n =1,2,3), whereas ‘M’ represents a primary transition metal (Ti, Nb, Ta, Mo, V and many more), ‘A’ shows element from groups III-A and IV-A of the periodic table, and ‘X’ symbolizes carbon or nitrogen [11–15]. In order to get the MXene, the chemical etching of MAX phase is carried out using a suitable chemical etchant to remove the A-layer from MAX [16,17]. The examples of MXene include V 2 C, Nb 2 C, Cr 2 C, Ti 2 C, Ti 3 C 2 , Nb 4 C 3 , Hf 2 C, Mo 2 C, etc [18,19]. MXenes involve (n +1) M layers that enfolds ‘n’ layers of X in an [MX] n M sequence. Moreover, chemically-etched pristine MXenes are hard to exist because of its highly reactive surface which results in absorbance of moisture from the atmosphere and may form a bonding with oxygen, hydroxide, fluorine (from chemical etchant), or an oxyfluoride (–OH, −O,-F, −OF) and are typically named as surface terminations (T x ) as they tend to lower the reactivity of the MXenes [20,21]. Owing to the presence of transition metals which possess valence electrons in d-orbitals and tendency to form the bonds, MXenes become very important material candidate for their magnetic properties. Nonmagnetism, ferromagnetism and anti-ferromagnetism of various kinds have been studied and predicted in MXene families which changes with presence of M element [22]. Gao et al. discussed monolayer Ti 2 C and Ti 2 N, through first-principles calculations, that exhibited half-metallic ferromagnetism. They also discussed that V 2 C and V 2 N exhibit a nonmagnetic nature [23]. Khazaei et al. calculated the magnetic moment (M) of Cr 2 C and Cr 2 N which showed a narrow band gap of semiconducting nature revealing its ferromagnetism [24]. Shien et al. * Corresponding authors. E-mail addresses: [email protected] (S.A. Khan), [email protected] (S. Rizwan). Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm https://doi.org/10.1016/j.jmmm.2024.172749 Received 16 April 2024; Received in revised form 14 November 2024; Accepted 20 December 2024 Journal of Magnetism and Magnetic Materials 614 (2025) 172749 Available online 21 December 2024 0304-8853/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). elaborated the structure of Ti n+1 AlC n and Ti n+1 C n MXenes along with the formation energy required for Ti 2 C and Ti 3 C 2 [25]. Their results indicated increased stability with a higher Ti–C bond count, with Alcontaining nano-blocks proving more stable and exhibiting possible magnetic ordering in the Ti layers. Enyashin et al. studied the effect of functional groups on magnetism of different MXenes using LDA +U, GGA +U and PBEsol functional and reported that the presence of functional groups modifies the spin–orbit coupling of 5d-orbital transition metal that results in the presence of ferrimagnetism and anti-ferromagnetism in Ta 3 C 2 , anti-FM in Ta 3 C 2 , FM and antiFM in Ta 2 C [26,27]. Zhao et al. discussed the magnetism in different compounds of M 2 C MXene under various mechanical strains showing the Meissner effect [28]. Recently, the Nb-doped as well as lanthanides-doped Ti 3 C 2 MXene showed variable ferromagnetism and anti-ferromagnetism [29–31]. Herein, we used first-principles calculation for computational analysis of magnetic properties of well-prepared V 2 C and MnO 2 adsorbed V 2 C MXene using density functional theory (DFT) and discussed the effect on their magnetic properties that changes from non-magnetic (in V 2 C) to ferromagnetic (in MnO 2 -adsorbed V 2 C MXene). 2. Experimental details In Fig. 1, the schematic of selective etching of Al layer from V 2 AlC to obtain V 2 CT x is shown. The synthesis of V 2 CT x MXene was initiated by taking 1 g of V 2 AlC MAX (300 mesh) and treating it with the chemical etchant (49 % concentrated hydrofluoric acid, ACS grade, BDH) for optimized 116 h at room-temperature. Magnetic stirring was constantly carried out by a Teflon-coated magnetic stirrer at 300 rpm. Etched sample was washed 4 to 5 times by using DI water and ethanol after centrifugation at 4500 rpm till the supernatant obtained a pH of 5. MXene was then filtered out with the help of vacuum filtration process in which, the solution was rinsed through DI water along with absolute ethanol using a celgard porous membrane having a pore size of 0.22 μ m. Powder sample of V 2 C MXene was obtained after drying in a vacuum oven for 24 h. MnO 2 -V 2 C nanocomposite was synthesized by liquid-phase precipitation method at high temperature. At first, 200 mg of V 2 C powder was dispersed in 100 mL, 1 mM aqueous solution of MnO 2 with constant magnetic stirring at 40 ◦C for 6 h. Afterwards, the 100 mL, 1 mM KMnO 4 aqueous solution was gradually poured in formerly stirred solution and was mixed under magnetic stirrer for further 30 min. A precipitate was collected at the end by centrifugation and rinsing consecutively with ethanol and DI water separately for 3 times with help of vacuum filtration. The powder obtained was then dried out in the vacuum oven (<0.09 MPa) at 55 ◦C for 24 h. The X-Ray diffraction (XRD) of the samples was done using the Bruker D8 Advance system. For obtaining elemental description, energy dispersive X-ray spectroscopy (EDX) was carried out using TESCON VEGA 3. 3. Results and discussion The crystallographic information was obtained from XRD as shown in Fig. 2a. The XRD pattern of V 2 AlC, V 2 C etched with 49 % HF solution and 10 % MnO 2 -V 2 C nanocomposite reveals a sharp peak at 2θ =13.28◦ and 41.09◦of MAX precursor which shows its high crystallinity. The shifted peak is the consequence of an increased c-lattice parameter when MAX phase was etched and transformed into 2D MXene sheets. Moreover, the smaller peaks of MAX phase in MXene correspond to the presence of small unetched MXene [32–34]. XRD patterns of nanocomposite shows MnO 2 presence along with V 2 CT x . The broadened as well as shifted pattern of (002) diffraction peak to a lower angle suggested an increase in the interlayer spacing of the composite. The additional peaks in the MnO 2 -V 2 CT x nanocomposite at 2θ of 35.5◦and 39.6◦are attributed to (112) and (101) polycrystalline planes of orthorhombic MnO 2 (JCPD 00-0300820) [35,36]. Additionally, peak broadening has been observed which is due to the reduced crystallinity and presence of MnO 2 over V 2 CT x sheets. Moreover, Fig. 2c and 2d shows the micrographs of V 2 C lamellar structure and MnO 2 @V 2 C MXene. The lamellar structure attained in Fig. 2c has not been destroyed (Fig. 2d) and persists even after the adsorption of MnO 2 on V 2 C MXene. However, the interlayer spacing of V 2 C sheets has evidently increased. Additionally, from the previous study, it is shown that pristine V 2 C is generally metallic [24], characterized by a high density of states at the Fermi level and the metallic behavior is due to the delocalized electron states from the vanadium atoms, which contributes to high electrical conductivity. Furthermore, V 2 C with surface terminated oxygen and Fluorine (V 2 C-OF) can significantly alter its electronic structure. Oxygen atoms on the surface can introduce localized states that may reduce the metallic character, potentially creating a small bandgap or narrowing the density of states at the Fermi level. This change depends on the extent of functionalization. When Mn is adsorbed on the surface of V 2 COF, it forms localized states associated with the Mn atoms and the oxygen functional groups. This adsorption modifies the DOS near the Fermi level, depending on the strength of interaction between Mn and V 2 C-OF. While Mn adsorption does not fully integrate into the lattice, it Fig. 1. Schematic illustration of MnO 2 -V 2 C nanocomposite. M. Fatima et al. Journal of Magnetism and Magnetic Materials 614 (2025) 172749 2 affects surface conductivity. Moreover, the magnetic properties of Pristine V 2 C typically does not exhibit any intrinsic magnetism [23], as it lacks unpaired electrons that could generate a magnetic moment. The material is usually nonmagnetic. However, V 2 C-OF might exhibit slight magnetic effects, as oxygen functionalization can induce localized spin polarization in the V atoms. However, this magnetism tends to be weak or negligible, and V 2 C-OF is often treated as either weakly magnetic or non-magnetic. Moreover, Mn adsorption induces localized magnetic moments on the surface, which leads to magnetic responses. Furthermore, the localized spin of Mn atoms results in ferromagnetic behaviour depending on Mn concentration and distribution. This is justified via detailed computational analysis described below. Fig. 2. A) XRD of V 2 AlC, prepared V 2 CTx and MnO 2 -V 2 C nanocomposite b) EDS of MnO 2 -V 2 C nanocomposite, c) Micrograph of pristine V 2 C d) Micrograph of MnO 2 - V 2 C nanocomposite. Fig. 3. A) Structure of V 2 C-OF b) Structure of Mn doped V 2 C-OF in 2x2x1 supercell, c) Structure of Mn adsorbed V 2 C-OF in 2x2x1 supercell d) Structure of Mn adsorbed V 2 C-OF in 4x4x1 supercell. M. Fatima et al. Journal of Magnetism and Magnetic Materials 614 (2025) 172749 3 3.1. Crystal structure and computational details The crystal structure of V 2 C-OF and Mn adsorbed V 2 C-OF is modeled by a supercell of slabs. For slab construction the bulk V 2 C structure was optimized to obtain the optimized lattice constants. This optimized structure was then used to construct the V 2 C −OF surface slab. In Fig. 3a, the carbon atom is sandwiched between vanadium layers. The O and F atomic layers were inserted to the system as surface terminations. A supercell of 2 ×2 ×1 was initially generated introducing vacuum of 11 Å to investigate the stability of Manganese (Mn) in slab using different positions of Mn as shown in Fig. 3b-c. The internal geometry was further optimized with different Mn configuration. In internal geometry optimization the positions of the atoms were allowed to move in the direction of the force until the equilibrium has been attained. The doping and adsorption of Mn in V 2 C-OF were observed to calculate formation energies and justify the Mn stability in the reported compound. Furthermore, the stable Mn-adsorbed V 2 C-OF system was then studied in 4 ×4 ×1 supercell as shown in Fig. 3d. The aim of the extended supercells in Fig. 3(a-c) was to ensure that all cases were consistent with Fig. 3(d). The computational analysis was performed via ab-initio all-electron FLAPW method, as executed in the WIEN2k code [37]. The calculations were initiated using Pedrew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) exchange–correlation functional for structure relaxation [38–40]. The single consistent field (SCF) calculation are converged using GGA +U with U =3.0 eV for Mn-d state. In the interstitial regions, wave function was expanded in plane waves, and the plane wave cut-off chosen was R MT Kmax =7.0 where R MT represents the smallest radius of the atomic sphere and Kmax as the largest wave-vector magnitude. The R MT were taken as 1.86 a.u. for V-atoms, 1.55 a.u. for Oatoms, 1.68 a.u. for F-atoms, 1.63 a.u. for C-atoms and 1.80 a.u. for Mnatom. For structure relaxation 2 k-points were used in irreducible brillouin zone with k-grid of 2 ×2 ×1. For energy convergence 54 k-points in IBZ with k grid of 6 ×6 ×3. Moreover, the forces relaxation criteria were kept at 10 -4 Ryd and energy convergence criteria was fixed at 10 -5 Ryd. V 2 C-OF system is modelled by a supercell of slabs. 3.2. Structure stability In DFT, formation energy is a crucial parameter that significantly contributes to our understanding of the relative stability of various atomic substitutions within crystal structures and their implications in chemical reactions. The formation energy that can be calculated using the equations. HV2−xMnxC−OF f=E(V2−xMnxC−OF) + E(V2C−OF) − E(Mn)(1) HMnadd−V2C−OF f=E(Mnadd −V2C−OF) − E(V2C−OF) − E(Mn)(2) The formation energy of Mn doped V 2 C-OF structure, calculated by Equation (1), is 1.9014 eV/unit cell. For Mn-adsorbed V 2 C-OF, it was calculated by Equation (2) and is −0.2388 eV/unit cell which clearly describes that Mn adsorbed V 2 C-OF structure shows better stability than Mn doped V 2 C-OF structure. Mn-adsorbed V 2 C-OF system was then studied in 4 ×4 ×1 supercell. The schematic diagram of V 2 C-OF with different configuration of Mn is shown in Fig. 3(a-d). The V 2 C-OF exhibits non-magnetic properties with the net magnetic moment (M) of about 0.14 µB per formula unit which is very small, and is consistent with the previous work [23,41] whereas, M for Mn adsorbed V 2 C-OF is found to be 4.48 μ B . The increased M obtained in our calculations justifies the presence of strong ferromagnetism in Mn adsorbed V 2 C-OF as compared to non-magnetic V 2 C-OF. Mn itself exhibits a ferromagnetic nature and readily forms a bond with oxygen. Thus, when Mn is adsorbed on the surface of V 2 C-OF, manganese atom forms a bond with oxygen and fluorine. In Fig. 3d, we discuss the attachment of functional groups with Mn atom. There is 1O and 2F’s surrounding Mn-atoms. Due to the bond formation with the oxygen atom, the magnetic moment of Vanadium atom becomes small because the number of free electrons is reduced due to charge sharing. The interaction is however complex when Mn is adsorbed on its surface, but clearly, the huge difference between the magnetic moments is due to the shielding effect of the functional groups attached (oxygen and fluorine). As Oxygen has the valence shell configuration of 2p 4 while Flourine has 2p 5 , so when there is an arrangement in which 2F-atoms and 1O-atom forms a bond with Vanadium atom and the adsorption of Mn-atom occurs at the surface, the density of states increases drastically as compared to other reasonable configurations [29]. Consequently, the overall magnetic moment of Mn-adsorbed V 2 C-OF becomes higher than that of V 2 C-OF. This increased magnetic moment is discussed in detail by the help of Fig. 4. Fig. 4a reveals the electronic band gap of Mn-adsorbed V 2 C-OF system. The zero electronic band gap shows that the electronic density of states is much higher and significant number of electrons are present in the conduction band. The density of states is obtained by Kohn-Sham eigen values calculation on a fine k-grid in the irreducible Brillioun Zone [42]. In Fig. 4b, a peak is observed around −12 eV to −11 eV for both V 2 C-OF and Mn adsorbed structures which usually occurs due to hybridization of s, p and d orbitals. Whereas in the region from −8eV to −2eV, the density of spin-up electrons is higher compared to the spindown. At fermi-level, there is a little difference between spin up and spin down peaks which is then onwards persisting in the conduction band, i.e. for 0 to 1.6 eV for V 2 C-OF system and 0 to 2 eV for Mnadsorbed V 2 C-OF. The total density of states (TDOS) versus energy plot is as shown in Fig. 4c. At low energy around −12 eV to −11 eV the peak is mainly formed by C-s along with small contribution of V-d state. The DOS peaks around −8 eV are mainly originated from induced magnetized F-p and O-p states that are attached as functional group with Mn atom. From −7 eV to −3 eV region bands are formed by nonmagnetic Fp and O-p state that are attached functional group to the Vanadium. Below −2 eV the Vd state is hybridized with O-p state. Also, from −2 eV to 8 eV the valance and conduction bands are formed by predominant V-d state. For a comprehensive analysis of the partial density of states, please refer to Figure I in the Supplementary File. 3.3. Magnetic analysis Magnetism in solids generally comes from localized electrons or delocalized electrons [42]. The interplay between these localized and delocalized electrons of in crystal structure defines the overall magnetic behavior of the solid. In this system, magnetism is induced by Mn-atoms in other elements specifically Fluorine atoms bonded with Mn. For more details of induce magnetization see Table 1 in Supplementary File. The F-p and O-p orbital are hybridized with Mn-d state results induce magnetization in F and O atoms. Also, the magnetic properties of Mn adsorbed V 2 C originates from the d-orbitals of V-atoms. The V d-orbital electrons and localization of the d electrons by surface terminations (F, OH, H, or Cl) are also inducing magnetism in the compound [29]. In any case, magnetism comes particularly from exchange splitting subsequently resulting in a partial occupation of states, which differ between the spin-up (M↑) and spin-down (M↓) electrons. The corresponding magnetic moment μ B is the difference between these occupation numbers (M =M↑ −M↓). In ferromagnetic metals, the order is collinear. Apart from this system, many elements for example, in Fe, Co, Ni shows collinear ordering and ferromagnetic nature [42]. In this work the total magnetic moment of the Mn ad V 2 C-OF is found to be 4.48 μ B . Also, nonmagnetic-to-magnetic transition state relies upon applying a relatively small amount of strain. The 2D half-metallic Ti 2 C that is a ferromagnetic material and changes into a half-metal, a spin-gapless semiconductor, and then a metal under continuously applied biaxial strain. However, 2D Ti 2 N does not show any changes when a similar biaxial strain is applied to it [23]. To make the magnetic moment of V, O, F and C distinct, we show M. Fatima et al. Journal of Magnetism and Magnetic Materials 614 (2025) 172749 4 graph in Fig. 5 up to 0.2 μ B . We can observe the positioning of Vanadium atoms from 1 to 32 which shows that the magnetic moment has been induced in Vanadium atoms by the adsorption of Mn-atom. Consequently, the behavior of Mn-adsorbed V 2 C-OF system overall is ferromagnetic. Also, the Oxygen atoms are labeled from 33 to 48 while Fluorine atoms are from 49 to 64. By the positioning of Oxygen and Fluorine atoms with Mn-atom, we can conclude that the magnetic moment of 2 Fluorine atoms and 1 Oxygen atom bonded with Mn-atom has induced in Fluorine and Oxygen atoms. The induced magnetization in Fluorine and Oxygen atoms is also shown in DOS figures (see Fig. 4 and Supplementary File). The peaks in Fig. 5 at atoms 41, 49 and 61 represent 1 Oxygen and 2 Fluorine bonded Mn atom. The µ B of Mn-atom is about 3.99551µ B however, the magnetic moments of other elements are comparatively very low. Therefore, to make the magnetic moment of other element visible we show Fig. 5 on low scale. For clear description, Table 1 is given in the Supplementary File which directly shows that the magnetic moment of pristine V 2 C-OF system is about 0.04034µ B . However, it is 4.48546µ B for Mn adsorbed V 2 C-OF system. 4. Conclusion The two-dimensional V 2 CT x was synthesized from its parent compound MAX. This article reports the theoretical results on magnetic properties of pristine V 2 C and MnO 2 adsorbed V 2 C nanocomposite. XRD results showed that c-lattice parameter is increased from 13.01 Å to 22.6 Å for V 2 AlC, V 2 C and 27.2 Å for MnO 2 -V 2 C nanocomposite respectively. Clearly, signifying the adsorption-dominant properties. SEM and EDX signified the adsorption of MnO 2 in V 2 C. In addition to it, the computational analysis revealed the strong ferromagnetic nature of MnO 2 adsorbed V 2 C while bare or V 2 C-OF shows a non-magnetic nature. The current work will lead to the understanding of two-dimensional materials and investigates potential of MXenes for diverse applications in the field of 2D spintronics. CRediT authorship contribution statement Mahjabeen Fatima: Writing – original draft, Investigation, Formal analysis. Saleem Ayaz Khan: Writing – review & editing, Formal analysis, Conceptualization. Syed Rizwan: Writing – review & editing, Supervision, Funding acquisition, Conceptualization. Funding 1. HEC: 20-14784/NRPU/R&D/HEC/2021. 2. QM4ST CZ.02.01.01/00/22_008/0004572. Fig. 4. a) Electronic bandgap of Mn-adsorbed V 2 C-OF b) Spin-polarized DOS Vs. Energy (eV) of V 2 C-OF and Mn-adsorbed V 2 C-OF system c) Total DOS Vs. energy (eV) of V 2 C-OF and Mn-adsorbed V 2 C-OF. Fig. 5. Computationally obtained Magnetic moments of V 2 C-OF and Mn (ad) V 2 C-OF systems. M. Fatima et al. Journal of Magnetism and Magnetic Materials 614 (2025) 172749 5 Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment Authors are thankful to the Higher Education Commission (HEC) of Pakistan for provision of funding of research under the Project No.: 2014784/NRPU/R&D/HEC/2021. Saleem Ayaz Khan acknowledges the support by the QM4ST project financed by the Ministry of Education of the Czech Republic grant no. CZ.02.01.01/00/22_008/0004572, cofunded by the European Regional Development Fund. Authors Contribution Mahjabeen Fatima analyzed computational data and wrote the manuscript, Saleem Ayaz Khan has extensively assisted in computational calculation and analysis, and Syed Rizwan conceived the research concept, assisted in manuscript writing and supervised the complete project. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jmmm.2024.172749. Data availability Data will be made available on request. References [1] M. Fatima, J. Fatheema, N.B. Monir, A.H. Siddique, B. Khan, A. Islam, D. Akinwande, S. Rizwan, Nb-doped MXene with enhanced energy storage capacity and stability, Front. Chem. 8 (2020) 168. [2] J. Hu, B. Xu, C. Ouyang, Y. Zhang, S.A. Yang, Investigations on Nb 2 C monolayer as promising anode material for Li or non-Li ion batteries from first-principles calculations, RSC Adv. 6 (33) (2016) 27467–27474. [3] M.A. Iqbal, S.I. Ali, F. Amin, A. Tariq, M.Z. Iqbal, S. 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