Atomic-Scale Understanding Of The Na And Cl Remotion From Seawater Promoted By Mo1.33C(OH)2-Mxene

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Guerrero-Sanchez, Dalia M. Muñoz-Pizza, Ma Guadalupe Moreno-Armenta, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-1383073/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 8 You are reading this latest preprint version Abstract Drinking water scarcity in arid and semi-arid regions is a reality that may turn into a global healthcare problem in the next few years. The scientific community is always looking for new materials to achieve effective sea and brackish water desalination to reduce water scarcity. Commonly, theoretical and experimental methods make a synergy to better understand and explain the chemical and physical processes in water desalination electrodes. In this way, experimental evidence pointed to Mo 1.33 CT z MXene as an efficient ion intercalation material, in which both Na cations and Cl anions are removed. However, the atomic-scale understanding of the physico-chemical processes due to the cation and anion interaction with the MXene is still unknown. We report the Na cation and Cl anion interaction with an OH functionalized Mo 1.33 C monolayer through a comprehensive first-principles density functional theory assessment. Results demonstrate that Na atoms attach to Oxygen, whereas Cl atoms bond through hydrogen bonds to the functional groups in the Mxene, these bonds have two energy contributions: electrostatic and charge transfer, which increases its adsorption energy. Electrostatic potential isosurface calculations evidenced reactive sites. Oxygen atoms have an affinity for the electropositive Na atoms, whereas hydrogen atoms -of the hydroxyl groups- interact with the electronegative Cl atoms. Bader charge analysis and non-covalent interaction isosurfaces help clarifying the way cations and anions attach to the MXene layer. Our findings explain why OH-functionalized Mo 1.33 C can efficiently remove both anions and cations based on their affinities with the functional groups present in the MXene. ion and cation trapping electronegativity charge transfer water desalination hydroxyl groups. Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Freshwater scarcity is a critical global problem. Global warming and the increased demand for agricultural, domestic, and industrial use are drivers of the lower availability of this resource. Additionally, limitations in the development of circular systems for better use, and pollution ( 1 ) generate freshwater scarcity area also problems, as well. Between the alternatives to obtain clean water, the desalination process is widely considered. Despite the growing number of desalination plants, currently 17000 ( 2 ) , there are important challenges due to their high costs of operation ( 3 ) , mainly in energy consumption. The desalination technologies include membrane distillation, electrodialysis, reverse osmosis, ion exchange, thermal based processes, and nanofiltration ( 4 , 5 ) . Nanofiltration is applied to improve the performance in the desalination technologies, allowing operation at low pressures, energy efficiency, and increase in the permeability and selectivity ( 6 ) . Application to improve salt rejection, ion exchange and hydrophobic characteristics have been proved using carbon nanotubes ( 7 , 8 ) , TiO 2 nanofilms ( 9 ) , and graphene oxide nanotubes ( 10 ) . Recently the use of the two-dimensional MXenes family has been recognized, due to the high capacity to rejection, permeability, and selectivity with potential applications on seawater desalination ( 11 , 12 ) . 2D MXenes were discovered in 2011 by Michael Naguib et al. They reported a new 2D material -Ti 3 C 2 T x - with a potential wide range of applications ( 13 ) . This paper established the birth of a new family of versatile 2D materials, MXenes, with applications that span from energy, optical, electronic, environmental, sensors, catalysis, and several other ( 14 ) . Conventional MXenes could be achieved upon exfoliating them from their 3D counterpart, the MAX phases. Until today, just some MXenes have been fabricated in the lab. However, it is expected to have a large quantity of these materials since there exist > 70 MAX phases ( 15 ) . Conventional MXenes can have seven, five, and three layers, which can be carbides or nitrides. A transition metal layer is always sandwiched by either two nitrogen or carbon monolayers. The versatility of these materials also relies on their facility to be engineered into multiple transition metals MXenes, in which, more than one transition metal can be accommodated in the same monolayer, called o-MXenes ( 14 ) . As an example, a Ti x Ta 4−x C 3 monolayer have been already evaluated as an anode for lithium storage, showing a larger efficiency than its conventional Ti 4 C 3 counterpart ( 16 ) . Such efficiency is related to the enhanced Li-ion transport generated by the mixed TiTa layer ( 17 ) . Although the MXene family is already large enough, other members of the MXene family needs to be mentioned, these are the i-MXenes, that can be also synthesized from their i-MAX phases. There exist 32 i-MAX phases ( 18 ) which will generate a variety of monolayers with random and ordered vacancies. The first i-MAX phase ((Mo 2/3 Sc 1/2 ) 2 AlC) was synthesized in 2017 ( 19 ) and it was the precursor of the Mo1.33C monolayer which is obtained through selective etching. Such monolayer possesses ordered Sc vacancies. Another example is the Nb1.33C i-MXene in which the vacancies are randomly distributed, and it is obtained from the precursor i-MAX phase (Nb 2/3 Sc 1/3 ) 2 AlC ( 20 ) . The randomly distributed vacancies in this monolayer are generated because in the MAX phase, the Sc and Nb atoms are forming a solid solution. In this way, it can be seen that the precursor i-MAX phase is key for the atomic arrangement of the resultant i-MXene phase ( 19 , 20 ) . The i-MXenes may be also as versatile as the conventional or the o-MXenes. They could be also engineered to form randomly or ordered o-like MXenes, if advantage is taken of the intrinsic vacancies in the monolayers. i-MXenes can be used in electronic and optical devices ( 14 ) as well as in energy, catalysis, and environmental applications ( 18 , 21 ) . Due to its high versatility, high surface area, and easy way of modification by adding different functional groups, MXenes point to be key in environmental remediation applications ( 22 , 23 ) . From these, desalination of seawater and brackish water is key. However, MXene membrane science is still in an early stage and research is needed to understand and explain the desalination processes, the device performance, and industrial scaling ( 24 ) . In this way, several research papers have appeared in which MXenes enhance the cation trapping to get efficient desalination membranes. Ti 3 C 2 T x have demonstrated high selectivity to metal cations with different charges as well as effectivity larger than the one shown by graphene oxide and some other carbon-based nano materials ( 25 ) . Ti 3 C 2 T x membranes were also engineered with Al 3+ ions to prevent swelling while retaining a high salt rejection and fast water fluxes, promising scalability ( 26 ) . Ti 3 C 2 T x has also been used as an intercalation material in membranes to enhance the desalination via capacitive deionization ( 27 , 28 ) . Also, fabricating surface charged MXene membranes generates a performance during nanofiltration and/or forward osmosis processes ( 29 ) . Research in desalination processes has been focused to Ti 3 C 2 T x MXene. From an atomic perspective, ion sieving has been analyzed where ions of different charges feel different energy barriers for its diffusion through the membrane, where the MXene interlayer distance contracts or expands upon interaction with such ions ( 30 ) . In a recent work, desalination via intercalation was analyzed for the Mo 1.33 CT z i-MXene/Carbon nanotube electrodes, where cation and anion are efficiently removed in the treatment of seawater and brackish water. Such process was achieved without the need of an exchange membrane, so it is hypothesized that the MXene layer is the one carrying the nanofiltration process by trapping Cl and Na at the same time. The carbon nanotubes in the electrode serve to avoid MXene re-stacking ( 31 ) . Although Mo 1.33 C MXene demonstrates a high efficiency for the trapping of Cl and Na in the experimental setup, no atomic scale understanding of such process exists in the literature. Considering the previous discussion and the proved importance of the Mo 1.33 C MXene in the Brackish and Seawater water desalination process via cation and anion intercalation ( 31 ) , in this work we have performed a comprehensive atomic scale density functional theory study to understand the Cl and Na interaction process with the Mo 1.33 CT z monolayer, T z =OH groups. We explain the viability of intercalation in terms of the atomic Cl and Na electronegativities. Cl atoms prefer to interact with the hydrogen atoms of the hydroxyl groups whereas the Na atoms form bonds with the oxygen atoms of the functional groups. Our study suggests that functionalizing MXenes with hydroxyl groups will make them viable for Brackish and Seawater water desalination in which both Cl and Na ions are removed. Method The atomic scale understanding of the brackish and sea desalination water process was treated though density functional theory calculations, as implemented in the Vienna Ab-initio Package ( 32 – 35 ) code. The one-electron wavefunctions were treated with the projector augmented wave method as derived by Kresse and Joubert ( 36 ) . The electronic states were expanded in a plane wave basis set with an optimized cutoff energy of 400 eV. Non-classical electron-electron interactions (exchange-correlation) were approximated with the generalized gradient approximation using the Perdew-Burke-Ernzerhof functional form ( 37 ) . i-MXenes are experimentally obtained with several functional groups, when hydroxyl groups are involved, long-range interactions may appear.Therefore, we considered the description of dispersion-corrected van der Waals interactions through the Grimme-D3 functional correction ( 38 ) . Na and Cl were adsorbed on different i-MXene high symmetry sites. To reach convergence in a system, the following criteria was used: the energy must be lower than 1×10 − 4 eV for two consecutive electronic steps, and the norm of all atomic forces must be smaller than 0.01 eV/Å. To evaluate the electronic states at the reciprocal lattice, the Brillouin zone was sampled using an equally distributed ( 39 ) k-points mesh with a volume of 12×12×1. To obtain the electrostatic potential isosurface and the Bader charges, the most stable adsorption models were converged with a denser k-points mesh of 24×24×1. The Bader charge analysis code ( 40 ) was used to obtain the valence charge transfer in the stable models. Results And Discussion In this section, we describe the Mo 1.33 CT z monolayer and explain -at the atomic scale- the desalination process of both Cl and Na from saline and brackish water. The Mo 1.33 C structure is typically functionalized with OH, O, and F atoms. It has been found that the Mo 1.33 CT 2 , with T = O, is unstable ( 41 ) . However, T = OH and T = F generate stable structures which are consistent with the experimental findings. It was also shown that when there is combination of F and O as functional groups (1O:2F, 2O:1F), the system also is also stable ( 41 ) . The species observed experimentally by XPS ( 41 ) are 50% F, 25% O and 25% OH suggesting that the O/F ratio is 50/50, but with half of the O atoms in OH form. In this work, we used the stable OH functionalized structure in which the ordered vacancies are occupied by OH functional groups (See Fig. 1 a). A similar structure has been reported in previous studies by ( 21 , 41 ) . Once the atomic structure for the Mo 1.33 C(OH) 2 was defined (structural parameters reported in the supplementary section), we proceeded to calculate its electrostatic potential isosurfaces (electron accumulation is depicted in red, whereas electron depletion is depicted in blue). As expected, since the O atoms are the most electronegative, they have electron accumulation, whereas the H and Mo atoms have electron depletion (see Fig. 1 b). Considering their electronegativities, O atoms would be available to bond with electropositive atoms like Na and electropositive atoms in the structure, like H, would bond to electronegative atoms like Cl. In the work by Srimuk et al. ( 31 ) , it was demonstrated experimentally that Mo 1.33 CT z was able to eliminate Na and Cl atoms from brackish and seawater with high salt concentration. However, in their work the role of the functional groups was not clarified. It was hypothesized that O or F atoms may be the ones involved in the trapping. However, considering their electronegativities -very similar to Cl-, they would bond to Na but not to Cl atoms ( 42 ) . Also, Srimuk et al. ( 31 ) , did not mention the OH groups, which are 25% of the coverage in the Mo 1.33 CT z MXene. OH groups are more promising for Cl and Na trapping since they possess both electronegative O and H atoms. Using the information coming from the electrostatic potential isosurface, we tried several adsorption sites for both Cl and Na atoms. Adsorption sites were considered on top of the surface layer and bonded to Mo atoms. To avoid getting trapped in a local minimum, for each site, we started at different perpendicular distances, see Figure S1. For example, site S 1 is the same as S 1a but the last one is closer to the Mo layer. Upon structural optimization of all models, we obtained the adsorption energy for all stable sites, see Table 1 . As a general trend, after atomic relaxation of the adatoms at different positions, we observed that all S xa models transform into the S x ones. Upon adsorbing Cl atoms on all defined sites (see Figure S1), S 1 , S 5 , and S 8 sites end up as S 7 structures in which the Cl atom is forming three hydrogen bonds with the surface H atoms. In contrast, S 6 stabilizes on a bridge-like site formed by two OH groups. S 4 stabilizes as a distorted S 1 site and is not further considered. Finally, the S 3 site turns into the S 2 site, which is the most stable structure. Three main adsorption sites were identified as S 7 , S 6 , and S 2 , the last being more stable than S 7 and S 6 by 0.09 eV and 0.26 eV, respectively. S 7 and S 2 are similar. In both sites, Cl forms three hydrogen bonds with the OH groups, the only difference is that S 2 is located at the Mo vacancy site. The S 6 site is structurally different because the Cl atom is located at a bridge-like position interacting with a couple of Hydrogen atoms. Since S 6 and S 7 are less stable than the S 2 site, they are not further considered for the analysis. However, the main interaction in all observed models for Cl is through hydrogen bonds. The most stable S 2 site is depicted in Fig. 2a. S 7 and S 6 adsorption sites are shown in Figure S2. Table 1 Adsorption energies (in eV) of the stable adsorption models for Cl and Na atoms on the Mo 1.33 C(OH) 2 MXene. Adsorption Site (Cl) Adsorption energy Adsorption Site (Na) Adsorption energy S 2 -2.04 S 2 -0.99 S 6 -1.78 S 3 -1.36 S 7 -1.95 S 4 -1.22 S 5 -0.82 S 6 -1.21 Figure 2. (a) The most stable adsorption sites (a) and (b) for Cl and Na on the Mo 1.33 CT z i-MXene, where T z are OH functional groups. (c) and (d) electrostatic potential isosurfaces showing the electron accumulation (red) and depletion (blue) for both Cl and Na atoms interacting with the MXene. Unit cell in (a) and (b) is highlighted as a dashed blue rectangle. Figure 2c shows the electrostatic potential isosurface for Cl in the S 2 site, which makes evident the depletion of charge on the hydrogen atoms and the accumulation of charge on the adsorbed Cl atoms and the O functional groups. Notice that there is a local charge redistribution due to the Cl atoms. With respect to Na adsorption (see Table 1 ), the same sites were considered (see Figure S1). As expected, different results emerge due to the chemistry of the Na atoms. Since Na atoms are electropositive, the interaction with the monolayer is through the oxygen atoms. The most stable site is S 3 . In this case, the Na atom forms three bonds with the oxygen atoms, whereas the neighbor Hydrogen atoms reorient to preclude interaction with Na, an expected behavior since H and Na are both electropositive (see Fig. 2b). Site S 2 is less stable than site S 3 by 0.37 eV and experiences a slight shifting towards S 5 (see Figure S3). S 4 site stabilizes in a threefold coordination with O atoms, however it is 0.14 eV less stable than the S 3 site. The main difference is that S 4 sits on top of a molybdenum atom (See Figure S3) and the S 3 site is on top of a hollow site. Upon adsorbing the Na atom on S 5 site (Figure S3), we noticed that it forms a double bonded configuration with the O atoms of the OH functional groups. This adsorption site is the less stable, with an energy 0.54 eV higher than S 3 . S 1 and S 7 are not stable, after structural relaxation evolve to S 5 . This happens since these sites try to preclude the direct H-Na interaction generated with their neighboring OH group. Finally, the S 6 site (Figure S3) stabilizes with the same relative energy than S 4 (they are degenerated in energy). The main difference between S 6 and S 4 is that S 6 sits nearby the Mo vacancy site -occupied by an OH group- whereas S 4 is far from the vacancy site, see Figure S3 for structural details. The electrostatic potential isosurface of the most stable S 3 site can be seen in Fig. 2d. Notice that in the vicinity of the Na atom, O atoms accumulate less charge due to the interaction with Na. On the other hand, the Na atom shows a strong charge depletion. The electrostatic potential isosurface of the atoms far from the adsorbates is similar to the one depicted in the surface without adsorbates, evidencing a local effect of the adsorbed atoms. To gain a deeper understanding into the charge transfer between the surface and the adsorbates, we determine the Bader charges through the atoms-in-molecules topological analysis ( 40 ) . Figure 3 a shows the charge transfer between the Cl atom and the monolayer. A charge density transfer of 0.62e from the monolayer to the Cl atom is seen, despite their weak interaction. It is well known that hydrogen bonds are result of several energy contributions. These are: electrostatic interaction, charge transfer interactions, \(\pi\) -resonance assistance, cooperative effects, Pauli repulsion, dispersion interactions, and secondary electrostatic interactions ( 43 ) . In this case, one part of the hydrogen bond is the charge transfer. However, it seems that there are other contributions of electrostatic character. To prove this, we plotted the non-covalent interaction index which is based on the density and its derivatives (the so-called reduced density gradient). In principle, from density -key in DFT- one can obtain all properties of the physical systems. Then, the reduced density gradient allows to identify non-covalent interactions at low-density (low gradient) regions. They appear as spikes, each related to a different non-covalent interaction ( 44 ) . These interactions have different character, they may be attractive (hydrogen bonding) or repulsive (steric hindrance). To distinguish between them, the second eigenvalue of the electron-density matrix Hessian is used ( 44 ) . This eigenvalue takes negative values for bonding interactions, like hydrogen bond. In contrast, it takes positive values when interactions are antibonding ( 44 ) . Then, the second eigenvalue of the Hessian matrix multiplied by the density (which gives us details of the interaction strength) is a way visualize and understand weak interactions. In Fig. 4 , we plotted the non-covalent interaction index isosurfaces considering s = 0.5 in a range of \(sign\left({\lambda }_{2}\right)\rho\) from − 0.05 to 0.05. A red-green-blue color code is used to depict the interaction zones. Red depicts repulsive interactions, green stands for weak van der Waals interactions that could be either repulsive or attractive depending on the sign they appear on, and blue shows the strong bonding interactions. Here we can see that the Cl atom has a strong attractive interaction through hydrogen bonds (blue ellipsoidal isosurfaces in Fig. 4 ) with three hydrogen atoms from the substrate. This characteristic dispersion interaction together with the charge transfer (0.62e from the monolayer to the Cl atom) and the electrostatic character of the interaction, help explain the large adsorption energy obtained for Cl adsorption (see Table 1 ). Green isosurfaces evidence O-O weak interactions between the functional groups. On the other hand, for the Na case, the charge density transfer is now from the adsorbate to the monolayer. The Na atom transfers 0.87e to the monolayer (see Fig. 3 b). Although Na atom experiences a chemical interaction with the monolayer, the adsorption energy is lower than the one obtained for Cl adsorption as previously explained in the above paragraphs. The piece of evidence here presented helps to understand the interaction mechanism that explains the way both Cl and Na are efficiently captured by the i-MXene monolayer. Also, we provide an atomic scale understanding of the Cl and Na elimination from brackish and seawater with high saline content, as previously reported in the literature ( 31 ) . Conclusions Experimental evidence has demonstrated that Mo 1.33 CT z MXene is an efficient electrode to capture Cl and Na atoms from sea and brackish water [Srimuk et al. ACS Sustainable Chem. Eng. 2018, 6, 3, 3739–3747]. Understanding, at an atomic scale, the chemical and physical processes that happen in these electrodes is key to tune and improve the water desalination processes. In this paper, we report on the Na cation and Cl anion trapping on the Mo 1.33 C(OH) 2 monolayer by first-principles density functional theory calculations. Results show that Na atoms bond to Oxygen atoms, whereas Cl atoms bond through hydrogen bonds to the functional groups in the i-MXene, these bonds have two energy contributions: electrostatic and charge transfer. This fact explains its large adsorption energy observed upon adsorbing it on the i-MXene layer. Electrostatic potential isosurfaces show that Oxygen atoms have an affinity for the electropositive Na atoms, whereas hydrogen atoms -of the hydroxyl groups- interact with the electronegative Cl atoms. Bader charge analysis and non-covalent interaction isosurfaces clarify how cations and anions attach to the MXene layer. Na atoms donate charge to the i-MXene, whereas for Cl atoms, the charge density goes from the substrate to the adsorbate. Our findings explain at the atomic scale how anions and cations are removed from sea and brackish water based on their affinities with the functional groups present in the Mo 1.33 C(OH) 2 i-MXene. Declarations Acknowledgements We thank DGAPA-UNAM projects IN105722, IN110820, and IA100822, and CONACyT grant A1-S-9070, for partial financial support. Calculations were performed in the DGCTIC-UNAM Supercomputing Center, projects LANCAD-UNAM-DGTIC-368, LANCAD-UNAM-DGTIC-051, and LANCAD-UNAM-DGTIC-150. J.G.S. acknowledges LNS-BUAP and THUBAT KAAL IPICYT supercomputing center, projects 202201042N and TKII-JGSA001, for computational resources. We thank E. Murillo and Aldo Rodriguez-Guerrero for his technical support. Data availability Authors declare that the main data supporting the findings of this study are contained within the paper. The row data is available from the corresponding author upon reasonable request. References (1) Zapata-Sierra, A.; Cascajares, M.; Alcayde, A.; Manzano-Agugliaro, F. Worldwide research trends on desalination, Desalination , 2021 , 519, 115305. 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All-MXene-Based Integrated Membrane Electrode Constructed using Ti3C2Tx as an Intercalating Agent for High-Performance Desalination, Environ. Sci. Technol ., 2020 , 54, 7, 4554–4563. https://doi.org/10.1021/acs.est.9b05759. (28) Agartan, L.; Hantanasirisakul, K.; Buczek, S.; Akuzum, B.; A. Mahmoud, K.; Anasori, B.; Gogotsi, Y.; Caglan Kumbur, E. Influence of operating conditions on the desalination performance of a symmetric pre-conditioned Ti3C2Tx-MXene membrane capacitive deionization system, Desalination , 2020, 477, 114267. https://doi.org/10.1016/j.desal.2019.114267. (29) Meng, B.; Liu, G.; Mao, Y.; Liang, F.; Liu, G.; Jin, W. Fabrication of surface-charged MXene membrane and its application for water desalination, Journal of Membrane Science , 2021 , 623, 119076, https://doi.org/10.1016/j.memsci.2021.119076. (30) R. Berdiyorova, G.; E. Madjet, M.; A. Mahmoud, K. Ionic sieving through Ti3C2(OH)2 MXene: First-principles calculations, Appl. Phys. Lett. , 2016 , 108, 113110, https://doi.org/10.1063/1.4944393. (31) Srimuk, P.; Halim, J.; Lee, J.; Tao, Q.; Rosen, J.; Presser, V. Two-Dimensional Molybdenum Carbide (MXene) with Divacancy Ordering for Brackish and Seawater Desalination via Cation and Anion Intercalation, ACS Sustainable Chem. Eng., 2018 , 6, 3, 3739–3747. https://doi.org/10.1021/acssuschemeng.7b04095. (32) Kresse, G., Hafner, J., Ab initio molecular dynamics for liquid metals. Phys. Rev. B, 1993 47, 558. https://doi.org/10.1103/PhysRevB.47.558. (33) Kresse, G., Hafner, J. Ab initio molecular-dynamics simulation of the liquid-metal–amorphous-semiconductor transition in germanium. Phys. Rev. B, 1994, 49, 14251. https://doi.org/10.1103/PhysRevB.49.14251. (34) Kresse, G. & Furthmüller, J. Efciency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mat. Sci., 1996, 6, 15–50. https://doi.org/10.1016/0927-0256(96)00008-0. (35) Kresse, G. & Furthmüller, J. Efcient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B, 1996, 54, 11169. https://doi.org/10.1103/PhysRevB.54.11169. (36) G. Kresse and D. Joubert, 1999, From ultrasoft pseudopotentials to the projector augmented-wave method, Physical Review B, 59, 1758. https://doi.org/10.1103/PhysRevB.59.1758. (37) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77 (18), 3865–3868. https://doi.org/10.1103/PhysRevLett.77.3865. (38) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132 (15). https://doi.org/10.1063/1.3382344. (39) Hendrik J. Monkhorst and James D. Pack, 1976, Special points for Brillouin-zone integrations, Phys. Rev. B 13, 5188. https://doi.org/10.1103/PhysRevB.13.5188. (40) W. Tang, E. Sanville, G. Henkelman, 2009, A grid-based bader analysis algorithm without lattice bias , J. Phys.: Condens. Matter, 21, 084204. https://doi.org/10.1088/0953-8984/21/8/084204. (41) H. Lind, J. Halim, S. I. Simak, and J. Rosen, 2017, Investigation of vacancy-ordered Mo1.33C MXene from first principles and x-ray photoelectron spectroscopy, Phys. Rev. Mat., 1, 044002. DOI: 10.1103/PhysRevMaterials.1.044002. (42) The investigation on the adsorption of Cl and Na atoms on the O-F functionalized Mo1.33CTz (i-MXene) is an ongoing project. (43) Stephanie C. C. van der Lubbe, Célia Fonseca Guerra, 2019, The Nature of Hydrogen Bonds: A Delineation of the Role of Different Energy Components on Hydrogen Bond Strengths and Lengths, Chemistry An Asian Journal, 14, 2760-2769. https://doi.org/10.1002/asia.201900717. (44) Erin R. Johnson, Shahar Keinan, Paula Mori-Sánchez, Julia Contreras-García, Aron J. Cohen, Weitao Yang, 2010, Revealing Noncovalent Interactions, J. Am. Chem. Soc., 132, 18, 6498–6506. https://doi.org/10.1021/ja100936w. Additional Declarations No competing interests reported. Supplementary Files Supplementaryinformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major revision 11 Apr, 2022 Reviews received at journal 22 Mar, 2022 Reviewers agreed at journal 13 Mar, 2022 Reviewers invited by journal 07 Mar, 2022 Editor assigned by journal 28 Feb, 2022 Editor invited by journal 28 Feb, 2022 Submission checks completed at journal 28 Feb, 2022 First submitted to journal 21 Feb, 2022 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-1383073","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":87088878,"identity":"bee3f81e-bb76-4277-8d55-daa2f3e62cb5","order_by":0,"name":"J. Guerrero-Sanchez","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA/0lEQVRIiWNgGAWjYDADftK1SDaQrMXgALEq5SOSj32uqLgnZ3y89/AHhpo7idsZeMwe/MxhiMZliOGNtOSZZ84UG5udOZcmwXDsWeLOBh5zw95tDLkzcTjVcEaOMWNjW0Liths5ZgyMDYcTNxzgMZPgBWrpx+Ewwxn5nxkb/yUkbp7/xvgDTIvkX6CWNlx+kchhZmxsSEjcIMFjIAHTIo3PFgOeZ8aMDccSjCXO5JhJJBw7bLzhMFuZtOw2CZx+kW9PfszYUJMgx99+xvjDh5rDshuON2+TfLvNJncDjhBDDckEBgbHBmYwUwKHs4C2oNtuj1PpKBgFo2AUjFgAAHEBW2EbNjRWAAAAAElFTkSuQmCC","orcid":"","institution":"Universidad Nacional Autónoma de México","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"J.","middleName":"","lastName":"Guerrero-Sanchez","suffix":""},{"id":87088879,"identity":"0a7a63fb-fb97-424b-9830-b4ae7bdb65dc","order_by":1,"name":"Dalia M. Muñoz-Pizza","email":"","orcid":"","institution":"Colegio de la Frontera Norte","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Dalia","middleName":"M.","lastName":"Muñoz-Pizza","suffix":""},{"id":87088883,"identity":"e5b19513-3343-4ab1-9326-d269a5d56856","order_by":2,"name":"Ma Guadalupe Moreno-Armenta","email":"","orcid":"","institution":"Universidad Nacional Autónoma de México","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Ma","middleName":"Guadalupe","lastName":"Moreno-Armenta","suffix":""},{"id":87088885,"identity":"4439b572-26e3-4151-b087-13791af71892","order_by":3,"name":"Noboru Takeuchi","email":"","orcid":"","institution":"Universidad Nacional Autónoma de México","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Noboru","middleName":"","lastName":"Takeuchi","suffix":""}],"badges":[],"createdAt":"2022-02-22 02:29:09","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-1383073/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-1383073/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":18783215,"identity":"626f4622-fc53-4353-8e7a-60cd8646780e","added_by":"auto","created_at":"2022-03-02 15:28:26","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":869240,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) \u003c/strong\u003eModel system of the Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e i-MXene, where T\u003csub\u003ez\u003c/sub\u003e are OH functional groups. \u003cstrong\u003e(b) \u003c/strong\u003eelectrostatic potential isosurface showing the electron accumulation (red) and depletion (blue). The unit cell is highlighted as a dashed blue square.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-1383073/v1/d8a301f7ca3be7ad4e97cbe2.png"},{"id":18782724,"identity":"9105998d-ba41-4d12-b614-01dcc572a706","added_by":"auto","created_at":"2022-03-02 15:25:26","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":988330,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) \u003c/strong\u003eThe most stable adsorption sites \u003cstrong\u003e(a)\u003c/strong\u003e and \u003cstrong\u003e(b)\u003c/strong\u003e for Cl and Na on the Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e i-MXene, where T\u003csub\u003ez\u003c/sub\u003e are OH functional groups. \u003cstrong\u003e(c)\u003c/strong\u003e and \u003cstrong\u003e(d) \u003c/strong\u003eelectrostatic potential isosurfaces showing the electron accumulation (red) and depletion (blue) for both Cl and Na atoms interacting with the MXene. Unit cell in (a) and (b) is highlighted as a dashed blue rectangle.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-1383073/v1/fdd58cf90401fc426c94ef2f.png"},{"id":18782720,"identity":"29729d54-3c10-4280-85f8-c269db5b8c36","added_by":"auto","created_at":"2022-03-02 15:25:26","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":537828,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) \u003c/strong\u003eSchematic of the charge transfer from the substrate to the Cl atom, and \u003cstrong\u003e(b) \u003c/strong\u003eatomic model for the charge transfer from the Na atom to the substrate. The charge transfer was obtained through a Bader charge analysis. Violet arrows denote the charge transfer direction.\u003c/p\u003e\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-1383073/v1/fa7576507aeb72f80e8ad3fb.png"},{"id":18782721,"identity":"1bc550e6-aa94-47f2-9661-ff2ad926208a","added_by":"auto","created_at":"2022-03-02 15:25:26","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":213794,"visible":true,"origin":"","legend":"\u003cp\u003e See image above for figure legend.\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-1383073/v1/ece5a1edff3ed11473e98a75.png"},{"id":18783216,"identity":"917fe313-113f-44d6-8417-bc7f9b2ceea5","added_by":"auto","created_at":"2022-03-02 15:28:29","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":364648,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-1383073/v1/8775bd32-bac3-4b06-ba01-98cc28d45446.pdf"},{"id":18782723,"identity":"74178675-400f-4911-bc82-5570cc3186f1","added_by":"auto","created_at":"2022-03-02 15:25:26","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1827966,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-1383073/v1/8c7e1f4efb11a1e28baf4718.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eAtomic-Scale Understanding Of The Na And Cl Remotion From Seawater Promoted By Mo\u003csub\u003e1.33\u003c/sub\u003eC(OH)\u003csub\u003e2\u003c/sub\u003e-Mxene\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eFreshwater scarcity is a critical global problem. Global warming and the increased demand for agricultural, domestic, and industrial use are drivers of the lower availability of this resource. Additionally, limitations in the development of circular systems for better use, and pollution \u003csup\u003e(\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e)\u003c/sup\u003e generate freshwater scarcity area also problems, as well. Between the alternatives to obtain clean water, the desalination process is widely considered. Despite the growing number of desalination plants, currently 17000 \u003csup\u003e(\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e)\u003c/sup\u003e, there are important challenges due to their high costs of operation \u003csup\u003e(\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e)\u003c/sup\u003e, mainly in energy consumption. The desalination technologies include membrane distillation, electrodialysis, reverse osmosis, ion exchange, thermal based processes, and nanofiltration \u003csup\u003e(\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e)\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eNanofiltration is applied to improve the performance in the desalination technologies, allowing operation at low pressures, energy efficiency, and increase in the permeability and selectivity \u003csup\u003e(\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e)\u003c/sup\u003e. Application to improve salt rejection, ion exchange and hydrophobic characteristics have been proved using carbon nanotubes \u003csup\u003e(\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e)\u003c/sup\u003e, TiO\u003csub\u003e2\u003c/sub\u003e nanofilms \u003csup\u003e(\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e)\u003c/sup\u003e, and graphene oxide nanotubes \u003csup\u003e(\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e)\u003c/sup\u003e. Recently the use of the two-dimensional MXenes family has been recognized, due to the high capacity to rejection, permeability, and selectivity with potential applications on seawater desalination \u003csup\u003e(\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e)\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e2D MXenes were discovered in 2011 by Michael Naguib et al. They reported a new 2D material -Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003ex\u003c/sub\u003e- with a potential wide range of applications \u003csup\u003e(\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e)\u003c/sup\u003e. This paper established the birth of a new family of versatile 2D materials, MXenes, with applications that span from energy, optical, electronic, environmental, sensors, catalysis, and several other \u003csup\u003e(\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e)\u003c/sup\u003e. Conventional MXenes could be achieved upon exfoliating them from their 3D counterpart, the MAX phases. Until today, just some MXenes have been fabricated in the lab. However, it is expected to have a large quantity of these materials since there exist\u0026thinsp;\u0026gt;\u0026thinsp;70 MAX phases \u003csup\u003e(\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e)\u003c/sup\u003e. Conventional MXenes can have seven, five, and three layers, which can be carbides or nitrides. A transition metal layer is always sandwiched by either two nitrogen or carbon monolayers. The versatility of these materials also relies on their facility to be engineered into multiple transition metals MXenes, in which, more than one transition metal can be accommodated in the same monolayer, called o-MXenes \u003csup\u003e(\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e)\u003c/sup\u003e. As an example, a Ti\u003csub\u003ex\u003c/sub\u003eTa\u003csub\u003e4\u0026minus;x\u003c/sub\u003eC\u003csub\u003e3\u003c/sub\u003e monolayer have been already evaluated as an anode for lithium storage, showing a larger efficiency than its conventional Ti\u003csub\u003e4\u003c/sub\u003eC\u003csub\u003e3\u003c/sub\u003e counterpart \u003csup\u003e(\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e)\u003c/sup\u003e. Such efficiency is related to the enhanced Li-ion transport generated by the mixed TiTa layer \u003csup\u003e(\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e)\u003c/sup\u003e. Although the MXene family is already large enough, other members of the MXene family needs to be mentioned, these are the i-MXenes, that can be also synthesized from their i-MAX phases. There exist 32 i-MAX phases \u003csup\u003e(\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e)\u003c/sup\u003e which will generate a variety of monolayers with random and ordered vacancies. The first i-MAX phase ((Mo\u003csub\u003e2/3\u003c/sub\u003eSc\u003csub\u003e1/2\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eAlC) was synthesized in 2017 \u003csup\u003e(\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e)\u003c/sup\u003e and it was the precursor of the Mo1.33C monolayer which is obtained through selective etching. Such monolayer possesses ordered Sc vacancies. Another example is the Nb1.33C i-MXene in which the vacancies are randomly distributed, and it is obtained from the precursor i-MAX phase (Nb\u003csub\u003e2/3\u003c/sub\u003eSc\u003csub\u003e1/3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eAlC \u003csup\u003e(\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e)\u003c/sup\u003e. The randomly distributed vacancies in this monolayer are generated because in the MAX phase, the Sc and Nb atoms are forming a solid solution. In this way, it can be seen that the precursor i-MAX phase is key for the atomic arrangement of the resultant i-MXene phase \u003csup\u003e(\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e)\u003c/sup\u003e. The i-MXenes may be also as versatile as the conventional or the o-MXenes. They could be also engineered to form randomly or ordered o-like MXenes, if advantage is taken of the intrinsic vacancies in the monolayers. i-MXenes can be used in electronic and optical devices \u003csup\u003e(\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e)\u003c/sup\u003e as well as in energy, catalysis, and environmental applications \u003csup\u003e(\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e)\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eDue to its high versatility, high surface area, and easy way of modification by adding different functional groups, MXenes point to be key in environmental remediation applications \u003csup\u003e(\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e)\u003c/sup\u003e. From these, desalination of seawater and brackish water is key. However, MXene membrane science is still in an early stage and research is needed to understand and explain the desalination processes, the device performance, and industrial scaling \u003csup\u003e(\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e)\u003c/sup\u003e. In this way, several research papers have appeared in which MXenes enhance the cation trapping to get efficient desalination membranes. Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003ex\u003c/sub\u003e have demonstrated high selectivity to metal cations with different charges as well as effectivity larger than the one shown by graphene oxide and some other carbon-based nano materials \u003csup\u003e(\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e)\u003c/sup\u003e. Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003ex\u003c/sub\u003e membranes were also engineered with Al\u003csup\u003e3+\u003c/sup\u003e ions to prevent swelling while retaining a high salt rejection and fast water fluxes, promising scalability \u003csup\u003e(\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e)\u003c/sup\u003e. Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003ex\u003c/sub\u003e has also been used as an intercalation material in membranes to enhance the desalination via capacitive deionization \u003csup\u003e(\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e)\u003c/sup\u003e. Also, fabricating surface charged MXene membranes generates a performance during nanofiltration and/or forward osmosis processes \u003csup\u003e(\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e)\u003c/sup\u003e. Research in desalination processes has been focused to Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eT\u003csub\u003ex\u003c/sub\u003e MXene. From an atomic perspective, ion sieving has been analyzed where ions of different charges feel different energy barriers for its diffusion through the membrane, where the MXene interlayer distance contracts or expands upon interaction with such ions \u003csup\u003e(\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e)\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn a recent work, desalination via intercalation was analyzed for the Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e i-MXene/Carbon nanotube electrodes, where cation and anion are efficiently removed in the treatment of seawater and brackish water. Such process was achieved without the need of an exchange membrane, so it is hypothesized that the MXene layer is the one carrying the nanofiltration process by trapping Cl and Na at the same time. The carbon nanotubes in the electrode serve to avoid MXene re-stacking \u003csup\u003e(\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e)\u003c/sup\u003e. Although Mo\u003csub\u003e1.33\u003c/sub\u003eC MXene demonstrates a high efficiency for the trapping of Cl and Na in the experimental setup, no atomic scale understanding of such process exists in the literature.\u003c/p\u003e \u003cp\u003eConsidering the previous discussion and the proved importance of the Mo\u003csub\u003e1.33\u003c/sub\u003eC MXene in the Brackish and Seawater water desalination process via cation and anion intercalation \u003csup\u003e(\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e)\u003c/sup\u003e, in this work we have performed a comprehensive atomic scale density functional theory study to understand the Cl and Na interaction process with the Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e monolayer, T\u003csub\u003ez\u003c/sub\u003e=OH groups. We explain the viability of intercalation in terms of the atomic Cl and Na electronegativities. Cl atoms prefer to interact with the hydrogen atoms of the hydroxyl groups whereas the Na atoms form bonds with the oxygen atoms of the functional groups. Our study suggests that functionalizing MXenes with hydroxyl groups will make them viable for Brackish and Seawater water desalination in which both Cl and Na ions are removed.\u003c/p\u003e"},{"header":"Method","content":"\u003cp\u003eThe atomic scale understanding of the brackish and sea desalination water process was treated though density functional theory calculations, as implemented in the Vienna Ab-initio Package \u003csup\u003e(\u003cspan additionalcitationids=\"CR33 CR34\" citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e)\u003c/sup\u003e code. The one-electron wavefunctions were treated with the projector augmented wave method as derived by Kresse and Joubert \u003csup\u003e(\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e)\u003c/sup\u003e. The electronic states were expanded in a plane wave basis set with an optimized cutoff energy of 400 eV. Non-classical electron-electron interactions (exchange-correlation) were approximated with the generalized gradient approximation using the Perdew-Burke-Ernzerhof functional form \u003csup\u003e(\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e)\u003c/sup\u003e. i-MXenes are experimentally obtained with several functional groups, when hydroxyl groups are involved, long-range interactions may appear.Therefore, we considered the description of dispersion-corrected van der Waals interactions through the Grimme-D3 functional correction \u003csup\u003e(\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e)\u003c/sup\u003e. Na and Cl were adsorbed on different i-MXene high symmetry sites. To reach convergence in a system, the following criteria was used: the energy must be lower than 1\u0026times;10\u0026thinsp;\u0026minus;\u0026thinsp;4 eV for two consecutive electronic steps, and the norm of all atomic forces must be smaller than 0.01 eV/\u0026Aring;. To evaluate the electronic states at the reciprocal lattice, the Brillouin zone was sampled using an equally distributed \u003csup\u003e(\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e)\u003c/sup\u003e k-points mesh with a volume of 12\u0026times;12\u0026times;1. To obtain the electrostatic potential isosurface and the Bader charges, the most stable adsorption models were converged with a denser k-points mesh of 24\u0026times;24\u0026times;1. The Bader charge analysis code \u003csup\u003e(\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e)\u003c/sup\u003e was used to obtain the valence charge transfer in the stable models.\u003c/p\u003e"},{"header":"Results And Discussion","content":"\u003cp\u003eIn this section, we describe the Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e monolayer and explain -at the atomic scale- the desalination process of both Cl and Na from saline and brackish water. The Mo\u003csub\u003e1.33\u003c/sub\u003eC structure is typically functionalized with OH, O, and F atoms. It has been found that the Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003e2\u003c/sub\u003e, with T\u0026thinsp;=\u0026thinsp;O, is unstable \u003csup\u003e(\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e)\u003c/sup\u003e. However, T\u0026thinsp;=\u0026thinsp;OH and T\u0026thinsp;=\u0026thinsp;F generate stable structures which are consistent with the experimental findings. It was also shown that when there is combination of F and O as functional groups (1O:2F, 2O:1F), the system also is also stable \u003csup\u003e(\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e)\u003c/sup\u003e. The species observed experimentally by XPS \u003csup\u003e(\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e)\u003c/sup\u003e are 50% F, 25% O and 25% OH suggesting that the O/F ratio is 50/50, but with half of the O atoms in OH form. In this work, we used the stable OH functionalized structure in which the ordered vacancies are occupied by OH functional groups (See Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). A similar structure has been reported in previous studies by \u003csup\u003e(\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e)\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOnce the atomic structure for the Mo\u003csub\u003e1.33\u003c/sub\u003eC(OH)\u003csub\u003e2\u003c/sub\u003e was defined (structural parameters reported in the supplementary section), we proceeded to calculate its electrostatic potential isosurfaces (electron accumulation is depicted in red, whereas electron depletion is depicted in blue). As expected, since the O atoms are the most electronegative, they have electron accumulation, whereas the H and Mo atoms have electron depletion (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Considering their electronegativities, O atoms would be available to bond with electropositive atoms like Na and electropositive atoms in the structure, like H, would bond to electronegative atoms like Cl. In the work by Srimuk et al. \u003csup\u003e(\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e)\u003c/sup\u003e, it was demonstrated experimentally that Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e was able to eliminate Na and Cl atoms from brackish and seawater with high salt concentration. However, in their work the role of the functional groups was not clarified. It was hypothesized that O or F atoms may be the ones involved in the trapping. However, considering their electronegativities -very similar to Cl-, they would bond to Na but not to Cl atoms \u003csup\u003e(\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e)\u003c/sup\u003e. Also, Srimuk et al. \u003csup\u003e(\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e)\u003c/sup\u003e, did not mention the OH groups, which are 25% of the coverage in the Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e MXene. OH groups are more promising for Cl and Na trapping since they possess both electronegative O and H atoms. Using the information coming from the electrostatic potential isosurface, we tried several adsorption sites for both Cl and Na atoms. Adsorption sites were considered on top of the surface layer and bonded to Mo atoms. To avoid getting trapped in a local minimum, for each site, we started at different perpendicular distances, see Figure S1. For example, site S\u003csub\u003e1\u003c/sub\u003e is the same as S\u003csub\u003e1a\u003c/sub\u003e but the last one is closer to the Mo layer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUpon structural optimization of all models, we obtained the adsorption energy for all stable sites, see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. As a general trend, after atomic relaxation of the adatoms at different positions, we observed that all S\u003csub\u003exa\u003c/sub\u003e models transform into the S\u003csub\u003ex\u003c/sub\u003e ones. Upon adsorbing Cl atoms on all defined sites (see Figure S1), S\u003csub\u003e1\u003c/sub\u003e, S\u003csub\u003e5\u003c/sub\u003e, and S\u003csub\u003e8\u003c/sub\u003e sites end up as S\u003csub\u003e7\u003c/sub\u003e structures in which the Cl atom is forming three hydrogen bonds with the surface H atoms. In contrast, S\u003csub\u003e6\u003c/sub\u003e stabilizes on a bridge-like site formed by two OH groups. S\u003csub\u003e4\u003c/sub\u003e stabilizes as a distorted S\u003csub\u003e1\u003c/sub\u003e site and is not further considered. Finally, the S\u003csub\u003e3\u003c/sub\u003e site turns into the S\u003csub\u003e2\u003c/sub\u003e site, which is the most stable structure. Three main adsorption sites were identified as S\u003csub\u003e7\u003c/sub\u003e, S\u003csub\u003e6\u003c/sub\u003e, and S\u003csub\u003e2\u003c/sub\u003e, the last being more stable than S\u003csub\u003e7\u003c/sub\u003e and S\u003csub\u003e6\u003c/sub\u003e by 0.09 eV and 0.26 eV, respectively. S\u003csub\u003e7\u003c/sub\u003e and S\u003csub\u003e2\u003c/sub\u003e are similar. In both sites, Cl forms three hydrogen bonds with the OH groups, the only difference is that S\u003csub\u003e2\u003c/sub\u003e is located at the Mo vacancy site. The S\u003csub\u003e6\u003c/sub\u003e site is structurally different because the Cl atom is located at a bridge-like position interacting with a couple of Hydrogen atoms. Since S\u003csub\u003e6\u003c/sub\u003e and S\u003csub\u003e7\u003c/sub\u003e are less stable than the S\u003csub\u003e2\u003c/sub\u003e site, they are not further considered for the analysis. However, the main interaction in all observed models for Cl is through hydrogen bonds. The most stable S\u003csub\u003e2\u003c/sub\u003e site is depicted in Fig.\u0026nbsp;2a. S\u003csub\u003e7\u003c/sub\u003e and S\u003csub\u003e6\u003c/sub\u003e adsorption sites are shown in Figure S2.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAdsorption energies (in eV) of the stable adsorption models for Cl and Na atoms on the Mo\u003csub\u003e1.33\u003c/sub\u003eC(OH)\u003csub\u003e2\u003c/sub\u003e MXene.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAdsorption Site (Cl)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAdsorption energy\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAdsorption Site (Na)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAdsorption energy\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e-2.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eS\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-0.99\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS\u003csub\u003e6\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e-1.78\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eS\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-1.36\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS\u003csub\u003e7\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e-1.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eS\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-1.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eS\u003csub\u003e5\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-0.82\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eS\u003csub\u003e6\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e-1.21\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cb\u003eFigure 2. (a)\u003c/b\u003e The most stable adsorption sites \u003cb\u003e(a)\u003c/b\u003e and \u003cb\u003e(b)\u003c/b\u003e for Cl and Na on the Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e i-MXene, where T\u003csub\u003ez\u003c/sub\u003e are OH functional groups. \u003cb\u003e(c)\u003c/b\u003e and \u003cb\u003e(d)\u003c/b\u003e electrostatic potential isosurfaces showing the electron accumulation (red) and depletion (blue) for both Cl and Na atoms interacting with the MXene. Unit cell in (a) and (b) is highlighted as a dashed blue rectangle.\u003c/p\u003e \u003cp\u003eFigure 2c shows the electrostatic potential isosurface for Cl in the S\u003csub\u003e2\u003c/sub\u003e site, which makes evident the depletion of charge on the hydrogen atoms and the accumulation of charge on the adsorbed Cl atoms and the O functional groups. Notice that there is a local charge redistribution due to the Cl atoms.\u003c/p\u003e \u003cp\u003eWith respect to Na adsorption (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), the same sites were considered (see Figure S1). As expected, different results emerge due to the chemistry of the Na atoms. Since Na atoms are electropositive, the interaction with the monolayer is through the oxygen atoms. The most stable site is S\u003csub\u003e3\u003c/sub\u003e. In this case, the Na atom forms three bonds with the oxygen atoms, whereas the neighbor Hydrogen atoms reorient to preclude interaction with Na, an expected behavior since H and Na are both electropositive (see Fig.\u0026nbsp;2b). Site S\u003csub\u003e2\u003c/sub\u003e is less stable than site S\u003csub\u003e3\u003c/sub\u003e by 0.37 eV and experiences a slight shifting towards S\u003csub\u003e5\u003c/sub\u003e (see Figure S3). S\u003csub\u003e4\u003c/sub\u003e site stabilizes in a threefold coordination with O atoms, however it is 0.14 eV less stable than the S\u003csub\u003e3\u003c/sub\u003e site. The main difference is that S\u003csub\u003e4\u003c/sub\u003e sits on top of a molybdenum atom (See Figure S3) and the S\u003csub\u003e3\u003c/sub\u003e site is on top of a hollow site. Upon adsorbing the Na atom on S\u003csub\u003e5\u003c/sub\u003e site (Figure S3), we noticed that it forms a double bonded configuration with the O atoms of the OH functional groups. This adsorption site is the less stable, with an energy 0.54 eV higher than S\u003csub\u003e3\u003c/sub\u003e. S\u003csub\u003e1\u003c/sub\u003e and S\u003csub\u003e7\u003c/sub\u003e are not stable, after structural relaxation evolve to S\u003csub\u003e5\u003c/sub\u003e. This happens since these sites try to preclude the direct H-Na interaction generated with their neighboring OH group. Finally, the S\u003csub\u003e6\u003c/sub\u003e site (Figure S3) stabilizes with the same relative energy than S\u003csub\u003e4\u003c/sub\u003e (they are degenerated in energy). The main difference between S\u003csub\u003e6\u003c/sub\u003e and S\u003csub\u003e4\u003c/sub\u003e is that S\u003csub\u003e6\u003c/sub\u003e sits nearby the Mo vacancy site -occupied by an OH group- whereas S\u003csub\u003e4\u003c/sub\u003e is far from the vacancy site, see Figure S3 for structural details.\u003c/p\u003e \u003cp\u003eThe electrostatic potential isosurface of the most stable S\u003csub\u003e3\u003c/sub\u003e site can be seen in Fig.\u0026nbsp;2d. Notice that in the vicinity of the Na atom, O atoms accumulate less charge due to the interaction with Na. On the other hand, the Na atom shows a strong charge depletion. The electrostatic potential isosurface of the atoms far from the adsorbates is similar to the one depicted in the surface without adsorbates, evidencing a local effect of the adsorbed atoms.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo gain a deeper understanding into the charge transfer between the surface and the adsorbates, we determine the Bader charges through the atoms-in-molecules topological analysis \u003csup\u003e(\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e)\u003c/sup\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows the charge transfer between the Cl atom and the monolayer. A charge density transfer of 0.62e from the monolayer to the Cl atom is seen, despite their weak interaction. It is well known that hydrogen bonds are result of several energy contributions. These are: electrostatic interaction, charge transfer interactions, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\pi\\)\u003c/span\u003e\u003c/span\u003e-resonance assistance, cooperative effects, Pauli repulsion, dispersion interactions, and secondary electrostatic interactions \u003csup\u003e(\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e)\u003c/sup\u003e. In this case, one part of the hydrogen bond is the charge transfer. However, it seems that there are other contributions of electrostatic character. To prove this, we plotted the non-covalent interaction index which is based on the density and its derivatives (the so-called reduced density gradient). In principle, from density -key in DFT- one can obtain all properties of the physical systems. Then, the reduced density gradient allows to identify non-covalent interactions at low-density (low gradient) regions. They appear as spikes, each related to a different non-covalent interaction \u003csup\u003e(\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e)\u003c/sup\u003e. These interactions have different character, they may be attractive (hydrogen bonding) or repulsive (steric hindrance). To distinguish between them, the second eigenvalue of the electron-density matrix Hessian is used \u003csup\u003e(\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e)\u003c/sup\u003e. This eigenvalue takes negative values for bonding interactions, like hydrogen bond. In contrast, it takes positive values when interactions are antibonding \u003csup\u003e(\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e)\u003c/sup\u003e. Then, the second eigenvalue of the Hessian matrix multiplied by the density (which gives us details of the interaction strength) is a way visualize and understand weak interactions. In Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e, we plotted the non-covalent interaction index isosurfaces considering s\u0026thinsp;=\u0026thinsp;0.5 in a range of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(sign\\left({\\lambda }_{2}\\right)\\rho\\)\u003c/span\u003e\u003c/span\u003e from \u0026minus;\u0026thinsp;0.05 to 0.05. A red-green-blue color code is used to depict the interaction zones. Red depicts repulsive interactions, green stands for weak van der Waals interactions that could be either repulsive or attractive depending on the sign they appear on, and blue shows the strong bonding interactions. Here we can see that the Cl atom has a strong attractive interaction through hydrogen bonds (blue ellipsoidal isosurfaces in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e) with three hydrogen atoms from the substrate. This characteristic dispersion interaction together with the charge transfer (0.62e from the monolayer to the Cl atom) and the electrostatic character of the interaction, help explain the large adsorption energy obtained for Cl adsorption (see Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Green isosurfaces evidence O-O weak interactions between the functional groups.\u003c/p\u003e \u003cp\u003eOn the other hand, for the Na case, the charge density transfer is now from the adsorbate to the monolayer. The Na atom transfers 0.87e to the monolayer (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). Although Na atom experiences a chemical interaction with the monolayer, the adsorption energy is lower than the one obtained for Cl adsorption as previously explained in the above paragraphs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe piece of evidence here presented helps to understand the interaction mechanism that explains the way both Cl and Na are efficiently captured by the i-MXene monolayer. Also, we provide an atomic scale understanding of the Cl and Na elimination from brackish and seawater with high saline content, as previously reported in the literature \u003csup\u003e(\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e)\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eExperimental evidence has demonstrated that Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e MXene is an efficient electrode to capture Cl and Na atoms from sea and brackish water [Srimuk et al. \u003cem\u003eACS Sustainable Chem. Eng.\u003c/em\u003e 2018, 6, 3, 3739\u0026ndash;3747]. Understanding, at an atomic scale, the chemical and physical processes that happen in these electrodes is key to tune and improve the water desalination processes. In this paper, we report on the Na cation and Cl anion trapping on the Mo\u003csub\u003e1.33\u003c/sub\u003eC(OH)\u003csub\u003e2\u003c/sub\u003e monolayer by first-principles density functional theory calculations. Results show that Na atoms bond to Oxygen atoms, whereas Cl atoms bond through hydrogen bonds to the functional groups in the i-MXene, these bonds have two energy contributions: electrostatic and charge transfer. This fact explains its large adsorption energy observed upon adsorbing it on the i-MXene layer. Electrostatic potential isosurfaces show that Oxygen atoms have an affinity for the electropositive Na atoms, whereas hydrogen atoms -of the hydroxyl groups- interact with the electronegative Cl atoms. Bader charge analysis and non-covalent interaction isosurfaces clarify how cations and anions attach to the MXene layer. Na atoms donate charge to the i-MXene, whereas for Cl atoms, the charge density goes from the substrate to the adsorbate. Our findings explain at the atomic scale how anions and cations are removed from sea and brackish water based on their affinities with the functional groups present in the Mo\u003csub\u003e1.33\u003c/sub\u003eC(OH)\u003csub\u003e2\u003c/sub\u003e i-MXene.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank DGAPA-UNAM projects IN105722, IN110820, and IA100822, and CONACyT grant A1-S-9070, for partial financial support. Calculations were performed in the DGCTIC-UNAM Supercomputing Center, projects LANCAD-UNAM-DGTIC-368, LANCAD-UNAM-DGTIC-051, and LANCAD-UNAM-DGTIC-150. J.G.S. acknowledges LNS-BUAP and THUBAT KAAL IPICYT supercomputing center, projects 202201042N and TKII-JGSA001, for computational resources. We thank E. Murillo and Aldo Rodriguez-Guerrero for his technical support.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAuthors declare that the main data supporting the findings of this study are contained within the paper. The row data is available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003cp\u003e(1) Zapata-Sierra, A.; Cascajares, M.; Alcayde, A.; Manzano-Agugliaro, F. 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Soc., 132, 18, 6498\u0026ndash;6506.\u0026nbsp;https://doi.org/10.1021/ja100936w.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"ion and cation trapping, electronegativity, charge transfer, water desalination, hydroxyl groups.","lastPublishedDoi":"10.21203/rs.3.rs-1383073/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-1383073/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDrinking water scarcity in arid and semi-arid regions is a reality that may turn into a global healthcare problem in the next few years. The scientific community is always looking for new materials to achieve effective sea and brackish water desalination to reduce water scarcity. Commonly, theoretical and experimental methods make a synergy to better understand and explain the chemical and physical processes in water desalination electrodes. In this way, experimental evidence pointed to Mo\u003csub\u003e1.33\u003c/sub\u003eCT\u003csub\u003ez\u003c/sub\u003e MXene as an efficient ion intercalation material, in which both Na cations and Cl anions are removed. However, the atomic-scale understanding of the physico-chemical processes due to the cation and anion interaction with the MXene is still unknown. We report the Na cation and Cl anion interaction with an OH functionalized Mo\u003csub\u003e1.33\u003c/sub\u003eC monolayer through a comprehensive first-principles density functional theory assessment. Results demonstrate that Na atoms attach to Oxygen, whereas Cl atoms bond through hydrogen bonds to the functional groups in the Mxene, these bonds have two energy contributions: electrostatic and charge transfer, which increases its adsorption energy. Electrostatic potential isosurface calculations evidenced reactive sites. Oxygen atoms have an affinity for the electropositive Na atoms, whereas hydrogen atoms -of the hydroxyl groups- interact with the electronegative Cl atoms. Bader charge analysis and non-covalent interaction isosurfaces help clarifying the way cations and anions attach to the MXene layer. Our findings explain why OH-functionalized Mo\u003csub\u003e1.33\u003c/sub\u003eC can efficiently remove both anions and cations based on their affinities with the functional groups present in the MXene.\u003c/p\u003e","manuscriptTitle":"Atomic-Scale Understanding Of The Na And Cl Remotion From Seawater Promoted By Mo1.33C(OH)2-Mxene","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-03-02 15:25:24","doi":"10.21203/rs.3.rs-1383073/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2022-04-11T07:12:24+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2022-03-22T11:21:59+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"57fae105-42b5-4c7c-afaf-4f023167fde0","date":"2022-03-13T18:49:03+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-03-07T17:21:59+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-02-28T16:16:34+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2022-02-28T12:10:33+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-02-28T12:07:28+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2022-02-22T02:26:15+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"c99d45b3-5c6f-44e2-a899-aab2bc47da63","owner":[],"postedDate":"March 2nd, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2022-05-03T06:59:26+00:00","versionOfRecord":[],"versionCreatedAt":"2022-03-02 15:25:24","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-1383073","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-1383073","identity":"rs-1383073","version":["v1"]},"buildId":"FbvkV6FR0MCFSLy54lSbu","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}