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. Our results lay the foundations for the experimental realization of a new MXenes family.
Research Article
A New Family of Copper-based MXenes
https://doi.org/10.21203/rs.3.rs-333539/v1
This work is licensed under a CC BY 4.0 License
You are reading this latest preprint version
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 [7-9], between them, the recently discovered Janus [10] and MXenes [11]. The latest 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 Ti3C2, several experimental and theoretical investigations have appeared on different MXenes. For example, antiferromagnetic Cr2N 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 [17], and in energy conversion and storage [17]. So, the spectrum of applications that 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 through computational and experimental methods [18-22]. Since some MAX phases have Cu as the intermediated layer A atom, it is possible that these atoms may occupy M sites to form a new family of Cu-based MXenes. To the best of our knowledge, no MXene structures have been reported using Cu.
Considering the previous discussion, we propose a new copper-based MXenes family: Cu2C, Cu2N, and Cu2O, which are isostructural to the well-known and already characterized Cr2C, Cr2N, Ti2C, Ti2N structures. Our results demonstrate that the new MXenes family presents positive frequencies and metallic characteristics, pointing out the importance of such 2D structures’ experimental realization.
Calculations on this report were performed using the Vienna Ab initio Simulation Package (VASP) [23-26]. Exchange-correlation energies are treated according to the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) parameterization [27]. The electronic states are expanded using the projector-augmented wave basis (PAW) [28, 29] with an energy cutoff of 500 eV. The MXene-type structures Cu2X (X= O, N, C) are simulated with the supercell method. A monolayer of Cu2X 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 1x10-4 eV. Integration of electronic states in the Brillouin zone was performed using a Monkhorst-Pack mesh of 15×15×1 k-points [30]. Phonon dispersions were obtained using the finite-differences method [31] combined with phonopy code [32].
The atomic representation of the new Cu-based MXenes is depicted in Figure 1. Top and side views of the Cu2O, Cu2N, and Cu2C are shown in Figures1a, 1b, and 1c, respectively. After full structural optimization without any constraints, the Cu2O, Cu2N, and Cu2C 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 Cu2O, Cu2N, and Cu2C, respectively.
We calculated the electron density difference (Δ⍴) defined as 
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 (X= O, N, C) respectively. The isosurfaces for each Cu-based MXene are depicted in Figure 1, blue color suggests depletion, and the red color corresponds to charge density accumulation. The three different Cu-based MXenes denote charge depletion around the Cu atoms. Also, we observed the accumulation of charge close to the Oxygen atoms. This behavior suggests bonds with ionic behavior. Bader charge density analysis for the Cu2O MXene-type structure reveals that each Cu atom transfers 0.50e to the three nearest O atoms (0.166e/O); also, each O atom accepts 1.00e from the six Cu nearest neighbors. In the Cu2N MXene-type structure, Cu atoms donate 0.60e (0.20e/O atom) to the nearest N atoms, which accept 1.20e from the six Cu nearest neighbors. Similar behavior is observed for the Cu2C MX-type structure, where Cu atoms lose 0.48e (0.16e/O atom) and the O atoms accept 0.96e from the six nearest Cu atoms.
The electronic properties of the new Cu-based MXene-type structures are investigated. The results are summarized in Figure 2. The density of states (DOS) and projected DOS (pDOS) for the Cu2O, Cu2N, and Cu2C are shown in the upper, middle, and lower panels of Figure 2. Fermi level is set as the reference energy in all cases. Positive and negative values along the DOS axis are for spin-up and -down, respectively. In all cases, spin-up and spin-down show no asymmetry as a clear indication of the materials’ non-magnetic nature. Also, all the Cu-based MXenes present metallic characteristics. Cu-3d orbitals dominate DOS in all cases, from -3 eV to 1 eV, with small contributions coming from the O-2p, N-2p, and C-2p orbitals.
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 (Figure 3). Dispersions for Cu2O, Cu2N, and Cu2C are shown in Figures3a, 3b, and 3c, respectively. In all cases, the absence of negative frequencies indicates dynamical stability of the three different Cu-based MXenes.
In conclusion, by using spin-polarized first-principles total-energy calculations, we predicted the structural and electronic properties of the new Cu-based MXenes Cu2O, Cu2N, and Cu2C. Calculations demonstrate the non-magnetic nature of the MXenes, with metallic characteristics. Also, phonon dispersion with only positive frequencies indicates the dynamical stability of the Cu-based MXenes. Therefore, this new MXenes family could be obtained experimentally, opening an avenue to include the large number of applications that Cu can bring to the realm of 2D materials.
Acknowledgments
We thank DGAPA-UNAM projects IA100920 and IN110820 for partial financial support. Calculations were performed in the DGCTIC-UNAM Supercomputing Center, projects No. LANCAD-UNAM-DGTIC-368 and LANCAD-UNAM-DGTIC-150. We thank Aldo Rodriguez-Guerrero for technical support. R.P.P. and S.J.G.O. acknowledges DGAPA-UNAM for the postdoctoral position.
[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science, 306 (2004) 666–669.
[2] P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M.C. Asensio, A. Resta, B. Ealet, Guy Le Lay, Phys. Rev. Lett., 108 (2012) 155501.
[3] M.E. Dávila, L. Xian, S. Cahangirov, A. Rubio, G. Le Lay, New J. Phys. 16 (2014) 095002.
[4] Han Liu, Adam T. Neal, Zhen Zhu, Zhe Luo, Xianfan Xu, David Tománek, Peide D. Ye, ACS Nano, 8, 4, (2014) 4033–4041.
[5] Andrew J. Mannix, Xiang-Feng Zhou, Brian Kiraly, Joshua D. Wood, Diego Alducin, Benjamin D. Myers, Xiaolong Liu, Brandon L. Fisher, Ulises Santiago, Jeffrey R. Guest, Miguel Jose Yacaman, Arturo Ponce, Artem R. Oganov, Mark C. Hersam, Nathan P. Guisinger, Science, 350, 6267 (2015) 1513-1516.
[6] Junji Yuhara, Bangjie He, Noriaki Matsunami, Masashi Nakatake, Guy Le Lay, Adv. Mater. 31 (2019) 1901017.
[7] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, A. Kis, Nature Nanotechnology, 6 (2011) 147–150.
[8] Bevin Huang, Genevieve Clark, Efrén Navarro-Moratalla, Dahlia R. Klein, Ran Cheng, Kyle L. Seyler, Ding Zhong, Emma Schmidgall, Michael A. McGuire, David H. Cobden, Wang Yao, Di Xiao, Pablo Jarillo-Herrero, Xiaodong Xu, Nature, 546 (2017) 270–273.
[9] Cheng Gong, Lin Li, Zhenglu Li, Huiwen Ji, Alex Stern, Yang Xia, Ting Cao, Wei Bao, Chenzhe Wang, Yuan Wang, Z. Q. Qiu, R. J. Cava, Steven G. Louie, Jing Xia, Xiang Zhang, Nature, 546 (2017) 265–269.
[10] Jing Zhang, Shuai Jia, Iskandar Kholmanov, Liang Dong, Dequan Er, Weibing Chen, Hua Guo, Zehua Jin, Vivek B. Shenoy, Li Shi, Jun Lou, ACS Nano, 11, 8 (2017) 8192–8198.
[11] Michael Naguib, Murat Kurtoglu, Volker Presser, Jun Lu, Junjie Niu, Min Heon, Lars Hultman, Yury Gogotsi, Michel W. Barsoum, Adv. Mater., 23 (2011) 4248–4253.
[12] Yury Gogotsi, Babak Anasori, ACS Nano, 13 (2019) 8491−8494.
[13] G. Wang, J. Phys. Chem. C 120 (2016) 18850–18857
[14] A. Iqbal, P. Sambyal, C. Min Koo, Adv. Funct. Mater. 30 (2020) 2000883.
[15] X. Jiang, A. V. Kuklin, A. Baev, Y. Ge, H. Ågren, H. Zhang, P. N.Prasad, Physics Reports, 848 (2020) 1-58.
[16] Á. Morales-García, F. Calle-Vallejo, F. Illas, ACS Catal. 10, 22 (2020) 13487–13503.
[17] J. Pang, R. G. Mendes, A. Bachmatiuk, L. Zhao, H. Q. Ta, T. Gemming, H. Liu, Z. Liu, M. H. Rummeli, Chem. Soc. Rev., 48 (2019) 72-133.
[18] C. Zhang, Y. Ma, X. Zhang, S. Abdolhosseinzadeh,H. Sheng, W. Lan, A. Pakdel, J. Heier, F. Nuesch, Energy Environ. Mater. 3 (2020) 29–55.
[19] G. R. Berdiyorov, K. A. Mahmoud, Appl. Surf. Sci. 416 (2017) 725-730.
[20] P. Zhang, L. Wang, K. Du, S. Wang, Z. Huang, L. Yuan, Z. Li, H. Wang, L. Zheng, Z. Chai, W. Shi, J. Hazard. Mater. 396 (2020) 122731.
[21] X. Wang, Y. Cai, S. Wu, B. Li, Appl. Surf. Sci. 525 (2020) 146501
[22] J. J. Zhang, L. Lin, Y. Zhang, M. Wu, Boris I. Yakobson, and S. Dong, J. Am. Chem. Soc. 140, 30 (2018) 9768-9773.
[23] G. Kresse and J. Hafner, Phys. Rev. B, 47 (1993) 558.
[24] G. Kresse and J. Hafner, Phys. Rev. B, 49 (1994) 14251.
[25] G. Kresse and J. Furthmüller, Comput. Mat. Sci., 6 (1996) 15-50.
[26] G. Kresse and J. Furthmüller, Phys. Rev. B, 54 (1996) 11169.
[27] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.
[28] P. E. Blochl, Phys. Rev. B, 50 (1994) 17953.
[29] G. Kresse and D. Joubert, Phys. Rev. B, 59 (1999) 1758.
[30] H. Monkhorst, J. Pack, Phys. Rev. B, 13 (1976) 5188–5192.
[31] L. Chaput, A. Togo, I. Tanaka, G. Hug, Phys. Rev. B, 84 (2011) 094302.
[32] A. Togo, F. Oba, I. Tanaka, Phys. Rev. B, 78 (2008) 134106-1.
No competing interests reported.