A new family of copper-based MXenes

In this work, we demonstrate, through first-principles calculations, the existence of a new family of copper-based MXenes. These add up new structures to the previously reported universe and span the interest of such 2D materials for applications in heterogeneous catalysis, ion-based batteries, sensors, biomedical applications, and so on. First, we propose the MXene-like structures: Cu2N, Cu2C, and Cu2O. Phonon spectra calculations confirmed their dynamical stability by showing just positive frequencies all through the 2D Brillouin zone. The new MXenes family displays metallic characteristics, mainly induced by the Cu-3d orbitals. Bader charge analysis and charge density differences depict bonds with ionic character in which Cu is positively charged, and the non-metal atom gets an anionic character. Also, we investigate the functionalization of the proposed structures with Cl, F, O, and OH groups. Results show that the H3 site is the most favorable for functionalization. In all cases, the non-magnetic nature and metallic properties of the pristine MXenes remain. Our results lay the foundations for the experimental realization of a new MXenes family.

Atomic understanding of 2D systems is a critical factor to help in the nanoscale revolution. The hunt for 2D systems with engineered properties has been a constant since the graphene discovery in 2004 1 . To mention, silicene 2 , germanene 3 , phosphorene 4 , borophene 5 , and very recently, the plumbene monolayer 6 . All these systems with a great variety of properties and applications. Nevertheless, that is not it. Other binary and ternary 2D materials have emerged as well [7][8][9] , between them, 2D Janus 10 and MXenes 11 . The later with several applications in different fields 12 . It is expected that the MXenes family could be one of the largest in the 2D realm. Since the outbreak work by Michael Naguib et al. 11 on Ti 3 C 2 , several experimental and theoretical investigations have appeared on different MXenes. For example, antiferromagnetic Cr 2 N undergoes a half-metal transition just by adsorbing O on it, being applicable in spintronics 13 . Also, MXenes find applications in next-generation shielding 14 , nano-optoelectronics 15 , as key to efficient catalytic processes 16 , and in energy conversion and storage 17 . So, the spectrum of applications that pristine MXenes bring is still under construction and could be complemented with the addition of new MXene structures. In this way, several research groups are looking for new and novel MXenes with engineered properties through computational and experimental methods [18][19][20][21][22][23][24][25][26] . MXenes modification through surface functionalization is a possibility. For example, carboxyl functionalized Ti 3 C 2 T x MXene shows a high capability to trap and remove U and Eu 20 . Also, sulfur-modified MXenes achieve high storage in Li-ion-based batteries 21 .
Mixing two transition metals into the same structure through the formation of either solid solutions or ordered double-transition metal MXenes is another way to engineer the MXenes properties. Ordered doubletransition MXene sheets present an optically controlled ferrimagnetic to ferromagnetic transition 23 . Mixed functionalized double MXenes show an antiferromagnetic behavior useful to construct 2D-based spin field-effect transistors 26 . Also, double non-magnetic transition metal MXenes have been used as Li-storage anodes, in which surface storage is enhanced due to the ordered double-transition metal nature of the MXene layer 27 .
MXenes are commonly obtained from the precursor MAX phase through selective etching of the A layer, employing aqueous hydrofluoric acid solutions 28 . However, the selective atomic substitution method has been proposed as another way to obtain MXenes. Urbankowski et al. 29 reported the synthesis of Mo 2 NT x and V 2 NT x using as precursors the Mo 2 CT x and V 2 CT x by ammoniation. Sun et al. 30 and Cao et al. 31 synthesized the twodimensional Mo 5 N 6 employing MoS 2 layers as a precursor in a furnace with NH 3 flux, and they demonstrated the versatility of this approach synthesizing W 5 N 6 . Besides, theoretical investigations have demonstrated that the substitution of M atoms in the MXene phase by other metallic atoms is possible 32,33 ; for example, Ti 3 C 2 has been modified by substituting Ti atoms with 3d, 4d, and 5d transition metals 32 . The MXenes MCO 2 (M = Ti, V, Zr, Hf, and Ta) were modified by the replacement of the M atom by Cu, Zn, Mo, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au and Hg 33 . Taking into account that MXenes not only come from the MAX phases -they can also be synthesized-opens the door to include more members to the MXenes family. www.nature.com/scientificreports/ Metallic copper has a wide range of applications; due to its high electromigration, it is widely used to construct integrated circuits. Also, Cu is an efficient current collector due to its high electric conduction 34 . Large area and conformal thin Cu films can be achieved on solid surfaces via atomic layer deposition, in which a Cu-based organometallic molecule interacts with the solid surface to generate the growth. The process happens through ligand-exchange reactions 35 . Also, copper surfaces can be used as a substrate to grow graphene, being a natural support to achieve graphene-based electrodes 34 .
That is not it; Cu is also widely used in single atom alloy catalysts 36 , in which single Pt atoms are incorporated into the Cu surface. Cu generates a high selectivity in hydrogenation processes due to its favored Cu-O interaction with the aldehyde molecules, whereas Pt is the hydrogen source.
We have great expectations of the potential advantages of bringing a 2D material with two exposed Cu surface layers and a large surface area. For example, due to its affinity to graphene, it is natural to think in new vdW heterostructures, which will be highly interesting from the scientific point of view as well as for their potential applications. On the other hand, the feasibility of mixing MXenes with other transition metals assures the formation of single-atom alloy catalysts, in which Pt, Pd, or other relatives can be incorporated. Several other applications related to copper should be expected as well.
Considering the previous discussion, we propose a new copper-based MXenes family: Cu 2 C, Cu 2 N, and Cu 2 O, which are isostructural to the well-known and already studied Cr 2 C, Cr 2 N, Ti 2 C, Ti 2 N structures 13,32,37,38 . Our results demonstrate that the new MXenes family presents positive frequencies and metallic behavior, even after functionalizing them with Cl, F, O, and OH, a key characteristic that remains from pure copper, pointing out the importance of such 2D structures experimental realization.

Methodology
Calculations on this report were performed using the Vienna Ab initio Simulation Package (VASP) [39][40][41][42] . Exchange-correlation energies are treated according to the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) parameterization 43 . The electronic states are expanded using the projectoraugmented wave basis (PAW) 44,45 with an energy cutoff of 500 eV. The MXene-type structures Cu 2 X (X = O, N, C) are simulated with the supercell method. A monolayer of Cu 2 X composes each supercell in a 1 × 1 periodicity and a vacuum space of 15 Å to avoid spurious interactions. In geometry optimization, all force components must be lower than 0.01 eV/ Å and the energy differences less than 1 × 10 -4 eV. Integration of electronic states in the Brillouin zone was performed using a Monkhorst-Pack mesh of 15 × 15 × 1 k-points 46 . Phonon dispersions were obtained using the finite-differences method 47 combined with phonopy code 48 .

Results
The atomic representation of the new Cu-based MXenes is depicted in Fig. 1. Top and side views of the Cu 2 O, Cu 2 N, and Cu 2 C are shown in Fig. 1a-c, respectively. After full structural optimization without any constraints, the Cu 2 O, Cu 2 N, and Cu 2 C have cell parameters of the order of 2.80, 3.12, and 2.99 Å, respectively. The Cu-O, Cu-N, and Cu-C bonds have distances ~ 2.07, ~ 2.02, and ~ 2.00 Å. Each MXene-type structure comprises two monolayers (ML) of Cu as the most exposed and a central ML of X (X = O, N, C). The Cu-X interlayer distance is 1.30, 0.92, and 1.02 Å for the Cu 2 O, Cu 2 N, and Cu 2 C, respectively.
We calculated the electron density difference (Δ⍴) defined as �ρ = ρ MXene − ρ Cu − ρ X , where ⍴ MXene is the charge density of the MXene-type structure; ⍴ Cu and ⍴ X are the charge density of the isolated atoms of Cu and X The evidence that demonstrates the possible experimental realization of the Cu-based MXenes is the dynamic stability. In the phonon dispersion, positive frequencies stand for dynamically stable systems, whereas negative values suggest structural instabilities that may need to be corrected using strain or atomic displacements. Here we analyze the phonon dispersions of the new Cu-based MXenes along the path Γ-M-K-Γ (Fig. 3). Dispersions for Cu 2 O, Cu 2 N, and Cu 2 C are shown in Fig. 3a-c, respectively. In all cases, the absence of negative frequencies indicates dynamical stability of the three different Cu-based MXenes.
MXenes are generally obtained with some functional groups attached to them. The most common functional groups are Cl, F, O, and OH. We investigate the surface functionalization of the three different Cu-based MXenes with Cl, F, O, and OH to investigate the change in the electronic properties when functionalizing them.
To evaluate the most favorable site for the functionalization, we considered four high symmetry sites (see Fig. 1a). The Top site corresponds to the site on top of the Cu atom. The T4 site is on top of the central layer atom, H3 is the central point of the hexagon formed by the 1st and second layers, and the Bridge site is the middle point between two Cu atoms of the same layer. According to the calculations, the H3 site is the most favorable configuration in all cases. In general, the H3 site is the most stable functionalization site for all systems, followed   2 , Cu 2 N(OH) 2 , and Cu 2 C(OH) 2 MXenes, respectively. The atomistic models for the Cu 2 NT 2 (T = Cl, F, O, and OH) are displayed in Fig. 4.
To evaluate the change in electronic properties in the Cu 2 XT 2 family due to functionalization, we compute its electronic properties. The DOS and pDOS of the functionalized MXnes are summarized in Fig. 5. In all cases, the energy reference is the Fermi level, positive and negative values along the DOS axis refer to spin up and spin down, respectively. From left to right, the first column depicts the electronic properties of the Cu 2 OT 2 (T = Cl, F, O, and OH) functionalized MXenes. Note that in all cases, the non-magnetic nature of the pristine MXene remains. Also, the main contribution to the DOS around the Fermi level comes from the Cu-3d orbitals, followed by contributions of the O-2p orbitals and the T group p-orbitals. Note that the surface functionalization increases the electronic states close to the Fermi level, and therefore the MXenes remain metallic. Similar behavior is obtained for the Cu 2 NT 2 (T = Cl, F, O, and OH) functionalized MXenes (central column). Again, functionalized MXenes exhibit metallic characteristics with non-magnetic properties. DOS close to the Fermi is mainly composed of Cu-3d orbitals with a small N-2p and T-p orbitals contribution. The right column depicts the DOS and pDOS of the Cu 2 CT 2 (T = Cl, F, O, and OH) functionalized MXenes. Similar to the previous cases, the non-magnetic nature and the metallic behavior of the pristine Cu 2 C MXene remain. Around the Fermi level, the main contribution to the DOS comes from the Cu-3d orbitals. C-2p and T-p orbitals contribute to a lesser extent.