Removal of heavy metals from wastewater using 2D MXenes: A theoretical study

Removal of heavy metals from wastewater using 2D MXenes: A theoretical study | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Removal of heavy metals from wastewater using 2D MXenes: A theoretical study Xin Wu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3960842/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Water is an indispensable material for human life. Unfortunately, the developments of industrial activities have reduced the quality of water resources in the world. Meantime, heavy metals are an important factor in water pollution due to their toxicity. This study highlights the method for the capture of heavy metal ions from wastewater using the procedure of adsorption. The adsorption of toxic heavy metal ions (Pb 2+ , Hg 2+ , and Cd 2+ ) on Ca 2 C and Cr 2 C MXene monolayers is investigated using the density functional theory. We have optimized the considered MXenes by nine DFT functionals: PBE, TPSS, BP86, B3LYP, TPSSh, PBE0, CAM-B3LYP, M11, and LC-WPBE. Our results have shown a good agreement with previously measured electronic properties of the Ca2C and Cr 2 C MXene layers and PBE DFT method. The calculated cohesive energy for the Ca2C and Cr 2 C MXene monolayers are − 4.12 eV and − 4.20 eV, respectively, which are in agreement with the previous studies. The results reveal that the adsorbed heavy metal ions have a substantial effect on the electronic properties of the considered MXene monolayers. Besides, our calculations show that the metal/MXene structures with higher electron transport rates display higher binding energy and charge transfers between the metal and Ca 2 C and Cr 2 C layers. Time-dependent density functional analysis also displayed “ligand to metal charge transfer” excitations for the metal/MXene systems. The larger Ebin for the Pb@Ca 2 C as well as Pb@Cr 2 C are according to larger redshifts which are expected (∆λ = 45 nm and 71 nm, respectively). Our results might be helpful for future research toward the application of MXene-based materials for removing wastewater pollutants. MXene Heavy metals Wastewater treatment Ca2C Density functional theory Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Recently, two-dimensional (2D) materials have been representing wide applications such as sensing devices, catalysis, optoelectronic applications, and medicine. 1–4 Besides, there have been many studies on a new class of 2D materials, carbide-nitride (MXenes), with interesting properties. 5–8 MXenes are also successfully synthesized by the elements of group IIIA or IVA and a carbon or nitrogen. Similar to the 2D transition metal dichalcogenides, MXenes can be semiconducting or metallic. 9–15 Furthermore, these novel 2D materials show much higher conductivity than the graphene layer. 16, 17 High performance of the 2D MXene layers have been verified in divers’ fields such as bio-sensors 18 and hydrogen storage applications. 19–24 For instance, Liu et al. 25, 26 investigated the application of 2D Ti 3 C 2 in the hydrogen sorption behavior of MgH 2 . Motivated by the successful synthesis 27, 28 of the 2D MXene monolayers with calcium and chromium carbide compositions (Ca 2 C and Cr 2 C), in this work we investigate the application of the Ca 2 C and Cr 2 C monolayers as adsorbent for use in next generation removal heavy metals (HMs). While the interesting properties of the considered Ca 2 C and Cr 2 C MXenes attract high attentions, research on the applications of the considered 2D materials to remove toxic HM ions are still scarce. Capture of HMs from wastewater is a vital process in the industrial procedures. 29–36 Previously, divers techniques for removing of HMs from wastewater have been proposed. Among these techniques, adsorption is known as a low-cost approach for eliminating the HMs from wastewater. 37–39 As an fascinating experimental research, various isotherms of HMs adsorption on the nanomaterials were recently investigated. 40 While diverse materials have been explored to eliminate of HMs from wastewater, 38–42 it has not been performed a comprehensive study to remove the HMs using Ca 2 C and Cr 2 C MXene monolayers. Therefore, previous fascinating studies have inspired us for examining the application of synthesized Ca 2 C and Cr 2 C 27, 28 MXene monolayers as an adsorbent to capture toxic HMs. In another word, this research is conducted to explore the performance of Ca 2 C and Cr 2 C MXene layers to capture Cd 2+ , Hg 2+ , and Pb 2+ from wastewater. 2. Computational details All calculations of the Ca 2 C and Cr 2 C MXene layers were performed by GAMESS package. 43 The solvent effect (water, ε = 78.4) was considered using the polarized continuum technique. 44 We have carried out the optimization of the considered MXenes by nine DFT functionals: 45 PBE, 46 TPSS, 47 BP86, 48, 49 B3LYP, 50, 51 TPSSh, 52 PBE0, 53 CAM-B3LYP, 54 M11, 55 and LC-WPBE. 56 Our results have showed a good agreement with previously measured electronic properties 27, 28 of the Ca 2 C and Cr 2 C MXene layers and PBE DFT method. Accordingly, the PBE functional is chosen in our study to calculate the properties of the MXene and HM@MXene complexes. The van der Waals (vdW) bindings are also computed using the DFT-D3 technique. 57 The stability of the HM@MXene complexes was also examined by frequency calculations. Besides, cohesive energy (E coh ) of the MXene layers was computed using by: $${E}_{coh}=\left({E}_{tot}-\sum _{i}{n}_{i}{E}_{i}\right)/j$$ 1 The binding energy (E bin ) can be also calculated by: E bin =E complex - E HM - E MXene ( 2 ) Also, the E bin at PBE mrthod is corrected for basis set superposition error (BSSE): The rate constant (k) for the charge hopping 58–60 between two fragments can be calculated by the Marcus theory. 61–64 What we are mostly interested: $$k={t}^{2}\sqrt{\frac{\pi }{{\hslash }^{2}{k}_{B}T\lambda }}\text{exp}\left[-\frac{\lambda }{4{k}_{B}T}\right]$$ 4 $${\lambda }^{\pm }=\left[{E}_{o}^{\pm }-{E}_{\pm }^{\pm }\right]+[{E}_{\pm }-{E}_{o}]$$ 5 $$t=\frac{{E}_{L+1 }- {E}_{L}}{2}$$ 6 Here, \(t\) is the transfer integral, λ is the reorganization energy, \(\hslash\) is the Planck constant, \({k}_{B}\) is the Boltzmann constant and T is the temperature (298 K, in the present study). The λ for the self-exchange hole and electron transfer processes can be computed using Nelsen’s four-point method, 63 where \({\lambda }^{+}\) and \({\lambda }^{-}\) are the hole and electron reorganization energy, respectively. \({E}_{o}^{\pm }\) is the energy of the cation or anion computed with the optimized structure of the neutral molecule, \({E}_{\pm }^{\pm }\) is the energy of the cation or anion calculated with the optimized cation or anion structure, \({E}_{\pm }\) is the energy of the neutral molecule calculated at the cationic or anionic state, and \({E}_{o}\) is the energy of the neutral molecule at the ground state. In the present work, the electron reorganization energy has been calculated for the considered HM@monolayer systems. To study the excitations of HM@MXene systems against UV-Vis light, time-dependent DFT (TDDFT) analysis 65 was also performed. Finally, charge transfer between HMs and MXene sheets was explored using the natural bond orbital (NBO) approach. 66, 67 3. Result and discussions 3.1. Ca 2 C and Cr 2 C MXene monolayers Ca 2 C and Cr 2 C MXene monolayers have the hexagonal symmetry, in which the C atom is located between two Ca (Cr) atoms, as shown in Fig. 1 . Figure. 1 represents the optimized geometry of the MXenes with the Ca-C (Cr-C) 2.63 Å (1.95 Å) and Ca-Ca (Cr-Cr) 3.40 Å (2.47 Å) bond lengths, and Ca-Ca-C (49°), Cr-Cr-C (49°), Ca-C-Ca (81), Cr-C-Cr (81°), C-Ca-C (99°), and C-Cr-C (97°) angles which are in agreement with the previous studies. 68, 69 No imaginary frequencies were obtained, representing that the MXene layers are dynamically stable and belongs to minima in the potential energy surface. The calculated cohesive energy for the Ca 2 C and Cr 2 C MXene monolayers are − 4.12 eV and − 4.20 eV, respectively, that are in agreements with the previous studies. 70 Note that, all the cohesive energies are between the values of already synthesized 2D systems such as graphene (-7.90 eV) 71 and silicene (-4.01 eV). 72 Our calculations (Figure. 2) also reveal that the HOMO-LUMO energy gap (E gap ) of the Ca 2 C and Cr 2 C are 0.21 eV and 0.28 eV, respectively, offering the MXene monolayers should present metallic nature, that are in agreements with the previous studies. 68, 69 In contrast with the Ca 2 C, HOMO and LUMO orbitals of the Cr 2 C layer are distributed on the MXene surface (Figure. 2). To verify the electronic structure of MXene monolayers, the corresponding density of states (DOSs) are analyzed and presented in Figure. 2. As shown in this Figure, the metallic nature of considered layers has been verified with DOS analysis. 3. 2. HM@MXene complexes We investigate the adsorption of a HM atom on the surface of MXene layers. Structurally, 5 adsorption positions are found on the Ca 2 C and Cr 2 C layers, such as top of a trigon (T3), top of a carbon atom (TC), top of a Ca atom (TCa), top of a Cr atom (TCr), top of a Ca-C bond (TCCa), top of a Cr-C bond (TCCr), top of a Ca-Ca bond (TCaCa), and top of a Cr-Cr bond (TCrCr). The calculated E bin values of a Cd 2+ cation on the surface of Ca 2 C and Cr 2 C layers are shown in Figure. 3. Our results for the Cd@Ca 2 C and Cd@Cr 2 C systems present that the Cd 2+ adsorbed on the top of carbon atom have the most negative E bin values (-1.98 eV and − 2.10 eV, respectively). Hence, for the Hg 2+ and Pb 2+ cations, the top of C atom position of the MXene layers are explored and the most stable structures are presented in Figure. 4. In contrast with the HM@Ca 2 C, our results reveal that the HOMO and LUMO of the HM@Cr 2 C systems are more localized on the cations, particularly for the Pb@Cr 2 C complex. The E gap of the HM@MXene systems are also computed and exhibited in Figure. 4. We found that the E gap of the MXene monolayers could change significantly with the HM adsorbing. Thus, upon the cation adsorption occurs, E gap of the Ca 2 C and Cr 2 C are meaningly increased and decreased, respectively. Our calculations indicated that the E gap of Cr 2 C monolayers decreased after HM adsorption about 79%, 89%, and 97%, for the Cd@Cr 2 C, Hg@Cr 2 C, and Pb@Cr 2 C complexes, respectively. However, the enhanced in the E gap of the Ca 2 C after HM cations adsorptions are about 48%, 52%, and 71% for the Cd@Ca 2 C, Hg@Ca 2 C, and Pb@Ca 2 C systems, respectively (Figure. 4). The lowest E gap is for the Pb@Cr 2 C with a value of 0.009 eV (∆E gap =97%). To verify the effect of cations on the E gap of the MXene layers, the corresponding DOSs are analyzed and presented in Figure. 5. The DOS plots present that in the HM@Ca 2 C complexes, LUMOs shift to higher energies, which results in a prominent increase in the E gap as compared to the pure Ca 2 C MXene monolayer. However, in the HM@Cr 2 C complexes, LUMOs and HOMOs shift to lower and higher energies, respectively, which results in a prominent decrease in the E gap as compared to the pure Cr 2 C MXene monolayer. The DOS analysis also shows that the HM adsorbing is an effective method of generating MXene-based materials with tunable E gap than before. The quantitative results of various properties of HM@MXene systems such as interaction distance between HM ion and MXene layers ( r ), charge difference between the isolated ions (q = + 2e) and HM adsorbed on the MXene (∆ Q ), and binding energy (E bin ) are calculated and presented in Figure. 6. The r values between HMs and MXene layers of each HM@MXene systems are presented in Figure. 6 (a). However, the Pb@MXnene complexes show the lowest r values (2.18 Å and 2.02 Å for the Pb@Ca 2 C and Pb@Cr 2 C, respectively) than other complexes. Our results show that the Pb 2+ cation links strongly on the Cr 2 C MXene layer with a lower r than other HMs. The reason for the difference in the nature of the interaction of Pb ion with respect to Hg and Cd ions can be related to the difference in their electron configurations (Hg 2+ ([Xe] 4 f 14 5 d 10 ), Cd 2+ ([Kr] 4 d 10 ), and Pb 2+ ([Xe] 4 f 14 5 d 10 6 s 2 )). 73–76 According to the electron configurations, Pb 2+ ion has vacant p orbitals and can interact with the MXene surface through electrostatic interactions. In comparison with Pb 2+ ion, Hg 2+ and Cd 2+ ions do not have vacant orbitals. Thus, they have to interact with the surfaces only through vdW interactions. The NBO analysis also shows a high charge transfer from the MXene to the HMs; thus, the HM@MXene bindings are mainly ionic. As shown in Figure. 6 (b), among of charge transfers to the HMs are in the order of Pb@MXene > Hg@MXene > Cd@MXene for both Ca 2 C and Cr 2 C layers. Also, the large charge transfers between HMs and Mxnene layers are maybe responsible for the stabilization of cations on the MXnene sheets. Our calculations also reveal that the charge transfers and binding strength of the HM@MXene complexes vary in a similar fashion (Figure. 6 (c)), which increase along the following series: Pb@Cr 2 C (-3.16 eV) > Hg@Cr 2 C (-2.66 eV) > Cd@Cr 2 C (-2.60 eV) > Pb@Ca 2 C (-2.52 eV) > Hg@Ca 2 C (-2.35 eV) > Cd@Ca 2 C (-2.21 eV). Therefore, the HM@MXene systems with the higher charge transfers show larger E bin . Also, as results show, the computed values of corrected adsorption energy for BSSE vary from − 2.15 eV (for Cd@Ca 2 C) to -3.05 eV (for Pb@Cr 2 C). The percentage of the BSSE to the raw adsorption energy is in a range of 2–4% for the considered complexes indicating the negligible BSSE in these systems. 3. 3. UV-Vis analysis The results of TDDFT analysis are reported in Table 1 . The Ca 2 C and Cr 2 C have λ max at 452 nm and 521 nm, respectively. By adsorbing the HMs on the MXene layers, redshifts for λ max are obtained which show strong bindings between the layers and HMs. This analysis presents a charge transfer transition between MXene layers and HMs. The nature of charge transfer between HMs and MXene sheets can be explored by the NBO analysis (Table 1 ). As shown in Table 1 , for the Cd@Ca 2 C, Cd@Cr 2 C, Hg@Ca 2 C, and Hg@Cr 2 C the main contribution in λ max is an electronic excitation from HOMO to LUMO + 1 MOs. Moreover, the NBO results of the Cd@Ca 2 C and Cd@Cr 2 C complexes show − 0.05220 a. u. and − 0.05334 a. u. for the energy of 2p x orbital of carbon atom. The occupation number of 2p x orbitals are calculated 0.63227 and 0.68207, respectively. However, in the Cd@Ca 2 C and Cd@Cr 2 C, the energy of 4d x2y2 orbitals for Cd 2+ is calculated − 0.52451 a. u. as a 1.27102 occupation number and − 0.58287 a. u. as a 1.28145 occupation number, respectively. Therefore, it can be concluded that these excitations are ligand to metal (MXene→cation) charge transfer. As shown in Table 1 , Hg@Ca 2 C and Hg@Cr 2 C systems also present MXene to cation charge transfer in their λ max vertical excitation. The redshift for Hg @Ca 2 C and Hg@Cr 2 C systems are a quite large values (∆λ = 40 nm and 42 nm, respectively). Lastly, the results in Table 1 are in excellent agreement with strong binding strength as well as large charge transfers between Pb 2+ and the MXene layers. The larger E bin for the Pb@Ca 2 C as well as Pb@Cr 2 C are according to larger redshifts which are expected (∆λ = 45 nm and 71 nm, respectively). Table 1 Calculated maximum wavelength (λ max ), oscillator strength ( f ), transition energy (ΔE), and charge transfer excitations for the HM@MXene systems. System λ max (nm) f ΔE (a. u.) crucial transition Ca 2 C 452 0.0677 0.100 HOMO→LUMO (80%) Cr 2 C 521 0.0860 0.087 HOMO→LUMO (80%) Cd@ Ca 2 C 486 0.0856 0.093 HOMO→LUMO + 1 (60%) Hg@ Ca 2 C 492 0.0966 0.092 HOMO→LUMO + 1 (70%) Pb@ Ca 2 C 497 0.1030 0.091 HOMO→LUMO + 2 (70%) Cd@ Cr 2 C 554 0.1066 0.082 HOMO→LUMO + 1 (60%) Hg@ Cr 2 C 563 0.1177 0.080 HOMO→LUMO + 1 (75%) Pb@ Cr 2 C 592 0.1330 0.077 HOMO→LUMO + 2 (75%) System donor orbital/energy(occupancy)/location acceptor orbital/energy(occupancy)/location Cd@ Ca 2 C 2p x / -0.05220 (0.63227)/C 4d x2y2 / -0.52451 (1.27102)/Cd Hg@ Ca 2 C 2p x / -0.06531 (0.65301)/C 5d xz / -0.55875 (1.36351)/Hg Pb@ Ca 2 C 2p x / -0.07316(0.83356)/C 6s/ -0.76815 (1.69685)/Pb Cd@ Cr 2 C 2p x /-0.05334(0.68207)/C 4d x2y2 / -0.58287 (1.28145)/Cd Hg@ Cr 2 C 2p x / -0.05416(0.66356)/C 5d xz / -0.56875 (1.46351)/Hg Pb@ Cr 2 C 2p x /-0.05334(0.73127)/C 6s/ -0.86815 (1.88685)/Pb 3. 4. Charge transport properties To examine how the linking of the HMs affect the electronic properties of the MXene sheets, the charge transport properties can be considered. Using the MXene as the source of the electrons and the HMs as the electron-hole, we can form donor-acceptor system due to the charge transfer between the MXenes and the HMs. The obtained electron transport parameters for the HM@MXene systems are gathered in Table 2 . We found that among the considered complexes, the Pb@MXene have the largest electron transfer rates. In the Cd@Ca 2 C, Cd@Cr 2 C, Hg@Ca 2 C, and Hg@Cr 2 C the λ are slightly enhanced but the reduction of t keeps the charge transfer rates in a similar value. However, an obvious change in the λ and t for the Pb@Ca 2 C and Pb@Cr 2 C leads to a considerable increase in electron transfers. Besides, the t of the Pb@Cr 2 C (0.60 eV), revealing that Pb@Cr 2 C is the best electron transport material among all the studied HM@MXene systems. As reported in Table 1 , Table 2 , and Figure. 6, the Pb@Cr 2 C complex with higher k , presents the higher binding energy, higher charge transfers, and larger redshift. Table 2 Charge transport properties for HM@CNC systems. System t (eV) λ (eV) k (s − 1 ) Cd@ Ca 2 C 0.21 0.62 1.21×10 12 Hg@ Ca 2 C 0.23 0.68 2.72×10 12 Pb@ Ca 2 C 0.31 0.32 1.26×10 14 Cd@ Cr 2 C 0.25 0.58 4.90×10 12 Hg@ Cr 2 C 0.28 0.55 8.45×10 12 Pb@ Cr 2 C 0.60 0.21 1.72×10 15 4. Conclusion In summary, we examined the binding properties of the Cd 2+ , Hg 2+ , and Pb 2+ with Ca 2 C and Cr 2 C MXene monolayers. We found that the linking HMs on the MXene layers give the complexes, wherein the HM@MXene systems present the donor-acceptor frameworks. In addition, the efficiency of HM adsorption on the MXene obeys a Pb@MXene > Hg@MXene > Cd@MXene trend. We found that among the considered complexes, the Pb@MXene have the largest electron transfer rates (1.26×10 14 and 1.72×10 15 s − 1 for the Pb@Ca 2 C and Pb@Cr 2 C, respectively). TDDFT analysis was also considered to the HM@MXene systems to explore excitations of systems. The most probable excitations for the HM@MXene complexes showed ligand to metal transitions. Nevertheless, this theoretical study might be helpful for the researchers toward designing adsorbents based on MXene for removing toxic pollutants from wastewater. Declarations Conflicts of interest There are no conflicts to declare Author Contribution Xin Wu: Supervision, Resources, Project administration, Writing-review & editing. Acknowledgments The authors are grateful to Zhejiang University for computational resources and financial supports. References A. Omidvar, Assessment of boroxine covalent organic framework as Li-ion battery anodes, Journal of Molecular Liquids 339 (2021) 116822. A. Mohajeri, A. 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Kamath, Adsorption mechanism of toxic heavy metal ions on oxygen-passivated nanopores in graphene nanoflakes, Journal of Materials Science 55 (2020) 15826–15844. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted 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-3960842","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":273190666,"identity":"75376380-aa1e-4ae6-bec8-8d128fe43767","order_by":0,"name":"Xin Wu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAu0lEQVRIiWNgGAWjYDACZijNDyISCkjRItkA0mJAim0GB8AkMSqP8x6T5mE4HG18fnXihwcGDPL8Ygfwa5Fs5ksDacndduPtZgmgwwxnzk7Ar4WfmcfsNkTL2Q0gLQkGtwloYYNp2Tzj7OYfRGmB27KBv3cbcbZINvOY/5xjkJ474wbvNosEAwnCfjE4f8bY4E2FdW5//9nNN39U2MjzSxPQAtUIxBJglRLEKIcB/gOkqB4Fo2AUjIKRBACwsD6AT9Tp5gAAAABJRU5ErkJggg==","orcid":"","institution":"Zhejiang University","correspondingAuthor":true,"prefix":"","firstName":"Xin","middleName":"","lastName":"Wu","suffix":""}],"badges":[],"createdAt":"2024-02-16 09:34:22","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3960842/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3960842/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":51294007,"identity":"1454174d-d095-4c0c-89dd-4d9e1f2c70b0","added_by":"auto","created_at":"2024-02-19 03:06:35","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":317868,"visible":true,"origin":"","legend":"\u003cp\u003eTop and side views of optimized Ca\u003csub\u003e2\u003c/sub\u003eC (a and b) and Cr\u003csub\u003e2\u003c/sub\u003eC (c and d) MXene monolayers. The Ca, Cr, and C atoms are shown in green, blue, and gray colors, respectively.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-3960842/v1/a945db027d351edcd0455571.png"},{"id":51293822,"identity":"82ed20d6-e4fb-4043-8087-99a59af5f48e","added_by":"auto","created_at":"2024-02-19 02:58:33","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":281719,"visible":true,"origin":"","legend":"\u003cp\u003eFrontier molecular orbital analysis (a and c) and the density of state diagrams (b and d) of the Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayers, respectively. The numerical values of the bandgap energies are also shown.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-3960842/v1/9f5df42406a57eca72ea4b2b.png"},{"id":51293820,"identity":"2d3e77ae-a424-465b-80cf-d9fdd23109c7","added_by":"auto","created_at":"2024-02-19 02:58:33","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":21915,"visible":true,"origin":"","legend":"\u003cp\u003eCalculated binding energies of a Cd\u003csup\u003e2+\u003c/sup\u003e cation on the various positions of the Ca\u003csub\u003e2\u003c/sub\u003eC (green) and Cr\u003csub\u003e2\u003c/sub\u003eC (blue) MXene monolayers.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-3960842/v1/60ea2c32d20e235206dc5e77.png"},{"id":51294008,"identity":"877e9bcc-018a-40c9-97be-331dbb18c29d","added_by":"auto","created_at":"2024-02-19 03:06:37","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":743522,"visible":true,"origin":"","legend":"\u003cp\u003eThe most stable geometries, MOs, and E\u003csub\u003egap\u003c/sub\u003e values of the HM@MXene systems.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-3960842/v1/5e67d3c2b15f727ceb02fe24.png"},{"id":51293825,"identity":"cfe14393-ba80-4504-917d-7a409f2f0e9a","added_by":"auto","created_at":"2024-02-19 02:58:34","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":208404,"visible":true,"origin":"","legend":"\u003cp\u003eThe DOSs for the (a) Cd@Ca\u003csub\u003e2\u003c/sub\u003eC, (b) Hg@Ca\u003csub\u003e2\u003c/sub\u003eC, (c) Pb@Ca\u003csub\u003e2\u003c/sub\u003eC, (d) Cd@Cr\u003csub\u003e2\u003c/sub\u003eC, (e) Hg@Cr\u003csub\u003e2\u003c/sub\u003eC, and (f) Pb@Cr\u003csub\u003e2\u003c/sub\u003eC.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-3960842/v1/838ff48ca603afac7c7b7a49.png"},{"id":51293824,"identity":"7540bdd6-4d30-49f4-aae8-3291381f7759","added_by":"auto","created_at":"2024-02-19 02:58:33","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":29480,"visible":true,"origin":"","legend":"\u003cp\u003eCalculated (a) interaction distance (\u003cem\u003er\u003c/em\u003e), (b) charge transfers (\u003cem\u003e∆Q\u003c/em\u003e), and (c) binding energy (E\u003csub\u003ebin\u003c/sub\u003e) for the HM@Ca\u003csub\u003e2\u003c/sub\u003eC (green bars) and HM@Cr\u003csub\u003e2\u003c/sub\u003eC (blue bars) complexes.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-3960842/v1/4f943f4ba21e2880dfb8a6ab.png"},{"id":51312944,"identity":"137b1dc9-7043-4f0a-aaf1-a796ce2bbe0b","added_by":"auto","created_at":"2024-02-19 11:42:02","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1724460,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3960842/v1/81f04be9-0f95-4dd6-80a2-572277ae5710.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Removal of heavy metals from wastewater using 2D MXenes: A theoretical study","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eRecently, two-dimensional (2D) materials have been representing wide applications such as sensing devices, catalysis, optoelectronic applications, and medicine. \u003csup\u003e1\u0026ndash;4\u003c/sup\u003e Besides, there have been many studies on a new class of 2D materials, carbide-nitride (MXenes), with interesting properties.\u003csup\u003e5\u0026ndash;8\u003c/sup\u003e MXenes are also successfully synthesized by the elements of group IIIA or IVA and a carbon or nitrogen. Similar to the 2D transition metal dichalcogenides, MXenes can be semiconducting or metallic.\u003csup\u003e9\u0026ndash;15\u003c/sup\u003e Furthermore, these novel 2D materials show much higher conductivity than the graphene layer.\u003csup\u003e16, 17\u003c/sup\u003e High performance of the 2D MXene layers have been verified in divers\u0026rsquo; fields such as bio-sensors\u003csup\u003e18\u003c/sup\u003e and hydrogen storage applications.\u003csup\u003e19\u0026ndash;24\u003c/sup\u003e For instance, Liu et al.\u003csup\u003e25, 26\u003c/sup\u003e investigated the application of 2D Ti\u003csub\u003e3\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003e in the hydrogen sorption behavior of MgH\u003csub\u003e2\u003c/sub\u003e. Motivated by the successful synthesis\u003csup\u003e27, 28\u003c/sup\u003e of the 2D MXene monolayers with calcium and chromium carbide compositions (Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC), in this work we investigate the application of the Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC monolayers as adsorbent for use in next generation removal heavy metals (HMs). While the interesting properties of the considered Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXenes attract high attentions, research on the applications of the considered 2D materials to remove toxic HM ions are still scarce.\u003c/p\u003e \u003cp\u003eCapture of HMs from wastewater is a vital process in the industrial procedures.\u003csup\u003e29\u0026ndash;36\u003c/sup\u003e Previously, divers techniques for removing of HMs from wastewater have been proposed. Among these techniques, adsorption is known as a low-cost approach for eliminating the HMs from wastewater. \u003csup\u003e37\u0026ndash;39\u003c/sup\u003e As an fascinating experimental research, various isotherms of HMs adsorption on the nanomaterials were recently investigated.\u003csup\u003e40\u003c/sup\u003e While diverse materials have been explored to eliminate of HMs from wastewater,\u003csup\u003e38\u0026ndash;42\u003c/sup\u003e it has not been performed a comprehensive study to remove the HMs using Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayers. Therefore, previous fascinating studies have inspired us for examining the application of synthesized Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC\u003csup\u003e27, 28\u003c/sup\u003e MXene monolayers as an adsorbent to capture toxic HMs. In another word, this research is conducted to explore the performance of Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene layers to capture Cd\u003csup\u003e2+\u003c/sup\u003e, Hg\u003csup\u003e2+\u003c/sup\u003e, and Pb\u003csup\u003e2+\u003c/sup\u003e from wastewater.\u003c/p\u003e"},{"header":"2. Computational details","content":"\u003cp\u003eAll calculations of the Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene layers were performed by GAMESS package.\u003csup\u003e43\u003c/sup\u003e The solvent effect (water, \u0026epsilon;\u0026thinsp;=\u0026thinsp;78.4) was considered using the polarized continuum technique.\u003csup\u003e44\u003c/sup\u003e We have carried out the optimization of the considered MXenes by nine DFT functionals:\u003csup\u003e45\u003c/sup\u003e PBE,\u003csup\u003e46\u003c/sup\u003e TPSS,\u003csup\u003e47\u003c/sup\u003e BP86,\u003csup\u003e48, 49\u003c/sup\u003e B3LYP,\u003csup\u003e50, 51\u003c/sup\u003e TPSSh,\u003csup\u003e52\u003c/sup\u003e PBE0,\u003csup\u003e53\u003c/sup\u003e CAM-B3LYP,\u003csup\u003e54\u003c/sup\u003e M11,\u003csup\u003e55\u003c/sup\u003e and LC-WPBE.\u003csup\u003e56\u003c/sup\u003e Our results have showed a good agreement with previously measured electronic properties\u003csup\u003e27, 28\u003c/sup\u003e of the Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene layers and PBE DFT method. Accordingly, the PBE functional is chosen in our study to calculate the properties of the MXene and HM@MXene complexes. The van der Waals (vdW) bindings are also computed using the DFT-D3 technique.\u003csup\u003e57\u003c/sup\u003e The stability of the HM@MXene complexes was also examined by frequency calculations. Besides, cohesive energy (E\u003csub\u003ecoh\u003c/sub\u003e) of the MXene layers was computed using by:\u003c/p\u003e\n\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equ1\" class=\"mathdisplay\"\u003e$${E}_{coh}=\\left({E}_{tot}-\\sum _{i}{n}_{i}{E}_{i}\\right)/j$$\u003c/div\u003e\n\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eThe binding energy (E\u003csub\u003ebin\u003c/sub\u003e) can be also calculated by:\u003c/p\u003e\n\u003cp\u003eE\u003csub\u003ebin\u003c/sub\u003e=E\u003csub\u003ecomplex\u003c/sub\u003e- E\u003csub\u003eHM\u003c/sub\u003e- E\u003csub\u003eMXene\u003c/sub\u003e (\u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e)\u003c/p\u003e\n\u003cp\u003eAlso, the E\u003csub\u003ebin\u003c/sub\u003e at PBE mrthod is corrected for basis set superposition error (BSSE):\u003c/p\u003e\n\u003cp\u003e\u003cimg style=\"width: 316px;\" src=\"data:image/png;base64,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\" alt=\"\" /\u003e\u003c/p\u003e\n\u003cp\u003eThe rate constant (k) for the charge hopping\u003csup\u003e58\u0026ndash;60\u003c/sup\u003e between two fragments can be calculated by the Marcus theory.\u003csup\u003e61\u0026ndash;64\u003c/sup\u003e What we are mostly interested:\u003c/p\u003e\n\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equ2\" class=\"mathdisplay\"\u003e$$k={t}^{2}\\sqrt{\\frac{\\pi }{{\\hslash }^{2}{k}_{B}T\\lambda }}\\text{exp}\\left[-\\frac{\\lambda }{4{k}_{B}T}\\right]$$\u003c/div\u003e\n\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equ3\" class=\"mathdisplay\"\u003e$${\\lambda }^{\\pm }=\\left[{E}_{o}^{\\pm }-{E}_{\\pm }^{\\pm }\\right]+[{E}_{\\pm }-{E}_{o}]$$\u003c/div\u003e\n\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equ4\" class=\"mathdisplay\"\u003e$$t=\\frac{{E}_{L+1 }- {E}_{L}}{2}$$\u003c/div\u003e\n\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eHere, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(t\\)\u003c/span\u003e\u003c/span\u003e is the transfer integral, \u0026lambda; is the reorganization energy, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\hslash\\)\u003c/span\u003e\u003c/span\u003e is the Planck constant, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({k}_{B}\\)\u003c/span\u003e\u003c/span\u003e is the Boltzmann constant and T is the temperature (298 K, in the present study). The \u0026lambda; for the self-exchange hole and electron transfer processes can be computed using Nelsen\u0026rsquo;s four-point method,\u003csup\u003e63\u003c/sup\u003e where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\lambda }^{+}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\lambda }^{-}\\)\u003c/span\u003e\u003c/span\u003e are the hole and electron reorganization energy, respectively. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({E}_{o}^{\\pm }\\)\u003c/span\u003e\u003c/span\u003e is the energy of the cation or anion computed with the optimized structure of the neutral molecule,\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({E}_{\\pm }^{\\pm }\\)\u003c/span\u003e\u003c/span\u003e is the energy of the cation or anion calculated with the optimized cation or anion structure, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({E}_{\\pm }\\)\u003c/span\u003e\u003c/span\u003e is the energy of the neutral molecule calculated at the cationic or anionic state, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({E}_{o}\\)\u003c/span\u003e\u003c/span\u003e is the energy of the neutral molecule at the ground state. In the present work, the electron reorganization energy has been calculated for the considered HM@monolayer systems. To study the excitations of HM@MXene systems against UV-Vis light, time-dependent DFT (TDDFT) analysis\u003csup\u003e65\u003c/sup\u003e was also performed. Finally, charge transfer between HMs and MXene sheets was explored using the natural bond orbital (NBO) approach.\u003csup\u003e66, 67\u003c/sup\u003e\u003c/p\u003e"},{"header":"3. Result and discussions","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayers\u003c/h2\u003e \u003cp\u003eCa\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayers have the hexagonal symmetry, in which the C atom is located between two Ca (Cr) atoms, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Figure. 1 represents the optimized geometry of the MXenes with the Ca-C (Cr-C) 2.63 \u0026Aring; (1.95 \u0026Aring;) and Ca-Ca (Cr-Cr) 3.40 \u0026Aring; (2.47 \u0026Aring;) bond lengths, and Ca-Ca-C (49\u0026deg;), Cr-Cr-C (49\u0026deg;), Ca-C-Ca (81), Cr-C-Cr (81\u0026deg;), C-Ca-C (99\u0026deg;), and C-Cr-C (97\u0026deg;) angles which are in agreement with the previous studies.\u003csup\u003e68, 69\u003c/sup\u003e No imaginary frequencies were obtained, representing that the MXene layers are dynamically stable and belongs to minima in the potential energy surface. The calculated cohesive energy for the Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayers are \u0026minus;\u0026thinsp;4.12 eV and \u0026minus;\u0026thinsp;4.20 eV, respectively, that are in agreements with the previous studies.\u003csup\u003e70\u003c/sup\u003e Note that, all the cohesive energies are between the values of already synthesized 2D systems such as graphene (-7.90 eV)\u003csup\u003e71\u003c/sup\u003e and silicene (-4.01 eV).\u003csup\u003e72\u003c/sup\u003e Our calculations (Figure. 2) also reveal that the HOMO-LUMO energy gap (E\u003csub\u003egap\u003c/sub\u003e) of the Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC are 0.21 eV and 0.28 eV, respectively, offering the MXene monolayers should present metallic nature, that are in agreements with the previous studies.\u003csup\u003e68, 69\u003c/sup\u003e In contrast with the Ca\u003csub\u003e2\u003c/sub\u003eC, HOMO and LUMO orbitals of the Cr\u003csub\u003e2\u003c/sub\u003eC layer are distributed on the MXene surface (Figure. 2). To verify the electronic structure of MXene monolayers, the corresponding density of states (DOSs) are analyzed and presented in Figure. 2. As shown in this Figure, the metallic nature of considered layers has been verified with DOS analysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003e3. 2. HM@MXene complexes\u003c/h3\u003e\n\u003cp\u003eWe investigate the adsorption of a HM atom on the surface of MXene layers. Structurally, 5 adsorption positions are found on the Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC layers, such as top of a trigon (T3), top of a carbon atom (TC), top of a Ca atom (TCa), top of a Cr atom (TCr), top of a Ca-C bond (TCCa), top of a Cr-C bond (TCCr), top of a Ca-Ca bond (TCaCa), and top of a Cr-Cr bond (TCrCr). The calculated E\u003csub\u003ebin\u003c/sub\u003e values of a Cd\u003csup\u003e2+\u003c/sup\u003e cation on the surface of Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC layers are shown in Figure. 3. Our results for the Cd@Ca\u003csub\u003e2\u003c/sub\u003eC and Cd@Cr\u003csub\u003e2\u003c/sub\u003eC systems present that the Cd\u003csup\u003e2+\u003c/sup\u003e adsorbed on the top of carbon atom have the most negative E\u003csub\u003ebin\u003c/sub\u003e values (-1.98 eV and \u0026minus;\u0026thinsp;2.10 eV, respectively). Hence, for the Hg\u003csup\u003e2+\u003c/sup\u003e and Pb\u003csup\u003e2+\u003c/sup\u003e cations, the top of C atom position of the MXene layers are explored and the most stable structures are presented in Figure. 4. In contrast with the HM@Ca\u003csub\u003e2\u003c/sub\u003eC, our results reveal that the HOMO and LUMO of the HM@Cr\u003csub\u003e2\u003c/sub\u003eC systems are more localized on the cations, particularly for the Pb@Cr\u003csub\u003e2\u003c/sub\u003eC complex. The E\u003csub\u003egap\u003c/sub\u003e of the HM@MXene systems are also computed and exhibited in Figure. 4. We found that the E\u003csub\u003egap\u003c/sub\u003e of the MXene monolayers could change significantly with the HM adsorbing. Thus, upon the cation adsorption occurs, E\u003csub\u003egap\u003c/sub\u003e of the Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC are meaningly increased and decreased, respectively. Our calculations indicated that the E\u003csub\u003egap\u003c/sub\u003e of Cr\u003csub\u003e2\u003c/sub\u003eC monolayers decreased after HM adsorption about 79%, 89%, and 97%, for the Cd@Cr\u003csub\u003e2\u003c/sub\u003eC, Hg@Cr\u003csub\u003e2\u003c/sub\u003eC, and Pb@Cr\u003csub\u003e2\u003c/sub\u003eC complexes, respectively. However, the enhanced in the E\u003csub\u003egap\u003c/sub\u003e of the Ca\u003csub\u003e2\u003c/sub\u003eC after HM cations adsorptions are about 48%, 52%, and 71% for the Cd@Ca\u003csub\u003e2\u003c/sub\u003eC, Hg@Ca\u003csub\u003e2\u003c/sub\u003eC, and Pb@Ca\u003csub\u003e2\u003c/sub\u003eC systems, respectively (Figure. 4). The lowest E\u003csub\u003egap\u003c/sub\u003e is for the Pb@Cr\u003csub\u003e2\u003c/sub\u003eC with a value of 0.009 eV (∆E\u003csub\u003egap\u003c/sub\u003e=97%). To verify the effect of cations on the E\u003csub\u003egap\u003c/sub\u003e of the MXene layers, the corresponding DOSs are analyzed and presented in Figure. 5. The DOS plots present that in the HM@Ca\u003csub\u003e2\u003c/sub\u003eC complexes, LUMOs shift to higher energies, which results in a prominent increase in the E\u003csub\u003egap\u003c/sub\u003e as compared to the pure Ca\u003csub\u003e2\u003c/sub\u003eC MXene monolayer. However, in the HM@Cr\u003csub\u003e2\u003c/sub\u003eC complexes, LUMOs and HOMOs shift to lower and higher energies, respectively, which results in a prominent decrease in the E\u003csub\u003egap\u003c/sub\u003e as compared to the pure Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayer. The DOS analysis also shows that the HM adsorbing is an effective method of generating MXene-based materials with tunable E\u003csub\u003egap\u003c/sub\u003e than before. The quantitative results of various properties of HM@MXene systems such as interaction distance between HM ion and MXene layers (\u003cem\u003er\u003c/em\u003e), charge difference between the isolated ions (q\u0026thinsp;=\u0026thinsp;+\u0026thinsp;2e) and HM adsorbed on the MXene (∆\u003cem\u003eQ\u003c/em\u003e), and binding energy (E\u003csub\u003ebin\u003c/sub\u003e) are calculated and presented in Figure. 6. The \u003cem\u003er\u003c/em\u003e values between HMs and MXene layers of each HM@MXene systems are presented in Figure. 6 (a). However, the Pb@MXnene complexes show the lowest \u003cem\u003er\u003c/em\u003e values (2.18 \u0026Aring; and 2.02 \u0026Aring; for the Pb@Ca\u003csub\u003e2\u003c/sub\u003eC and Pb@Cr\u003csub\u003e2\u003c/sub\u003eC, respectively) than other complexes. Our results show that the Pb\u003csup\u003e2+\u003c/sup\u003e cation links strongly on the Cr\u003csub\u003e2\u003c/sub\u003eC MXene layer with a lower \u003cem\u003er\u003c/em\u003e than other HMs. The reason for the difference in the nature of the interaction of Pb ion with respect to Hg and Cd ions can be related to the difference in their electron configurations (Hg\u003csup\u003e2+\u003c/sup\u003e ([Xe] 4\u003cem\u003ef\u003c/em\u003e\u003csup\u003e14\u003c/sup\u003e 5\u003cem\u003ed\u003c/em\u003e\u003csup\u003e10\u003c/sup\u003e), Cd\u003csup\u003e2+\u003c/sup\u003e ([Kr] 4\u003cem\u003ed\u003c/em\u003e\u003csup\u003e10\u003c/sup\u003e), and Pb\u003csup\u003e2+\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e([Xe] 4\u003cem\u003ef\u003c/em\u003e\u003csup\u003e14\u003c/sup\u003e 5\u003cem\u003ed\u003c/em\u003e\u003csup\u003e10\u003c/sup\u003e 6\u003cem\u003es\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e)).\u003csup\u003e73\u0026ndash;76\u003c/sup\u003e According to the electron configurations, Pb\u003csup\u003e2+\u003c/sup\u003e ion has vacant \u003cem\u003ep\u003c/em\u003e orbitals and can interact with the MXene surface through electrostatic interactions. In comparison with Pb\u003csup\u003e2+\u003c/sup\u003e ion, Hg\u003csup\u003e2+\u003c/sup\u003e and Cd\u003csup\u003e2+\u003c/sup\u003e ions do not have vacant orbitals. Thus, they have to interact with the surfaces only through vdW interactions. The NBO analysis also shows a high charge transfer from the MXene to the HMs; thus, the HM@MXene bindings are mainly ionic. As shown in Figure. 6 (b), among of charge transfers to the HMs are in the order of Pb@MXene\u0026thinsp;\u0026gt;\u0026thinsp;Hg@MXene\u0026thinsp;\u0026gt;\u0026thinsp;Cd@MXene for both Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC layers. Also, the large charge transfers between HMs and Mxnene layers are maybe responsible for the stabilization of cations on the MXnene sheets. Our calculations also reveal that the charge transfers and binding strength of the HM@MXene complexes vary in a similar fashion (Figure. 6 (c)), which increase along the following series: Pb@Cr\u003csub\u003e2\u003c/sub\u003eC (-3.16 eV)\u0026thinsp;\u0026gt;\u0026thinsp;Hg@Cr\u003csub\u003e2\u003c/sub\u003eC (-2.66 eV)\u0026thinsp;\u0026gt;\u0026thinsp;Cd@Cr\u003csub\u003e2\u003c/sub\u003eC (-2.60 eV)\u0026thinsp;\u0026gt;\u0026thinsp;Pb@Ca\u003csub\u003e2\u003c/sub\u003eC (-2.52 eV)\u0026thinsp;\u0026gt;\u0026thinsp;Hg@Ca\u003csub\u003e2\u003c/sub\u003eC (-2.35 eV)\u0026thinsp;\u0026gt;\u0026thinsp;Cd@Ca\u003csub\u003e2\u003c/sub\u003eC (-2.21 eV). Therefore, the HM@MXene systems with the higher charge transfers show larger E\u003csub\u003ebin\u003c/sub\u003e. Also, as results show, the computed values of corrected adsorption energy for BSSE vary from \u0026minus;\u0026thinsp;2.15 eV (for Cd@Ca\u003csub\u003e2\u003c/sub\u003eC) to -3.05 eV (for Pb@Cr\u003csub\u003e2\u003c/sub\u003eC). The percentage of the BSSE to the raw adsorption energy is in a range of 2\u0026ndash;4% for the considered complexes indicating the negligible BSSE in these systems.\u003c/p\u003e\n\u003ch3\u003e3. 3. UV-Vis analysis\u003c/h3\u003e\n\u003cp\u003eThe results of TDDFT analysis are reported in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC have λ\u003csub\u003emax\u003c/sub\u003e at 452 nm and 521 nm, respectively. By adsorbing the HMs on the MXene layers, redshifts for λ\u003csub\u003emax\u003c/sub\u003e are obtained which show strong bindings between the layers and HMs. This analysis presents a charge transfer transition between MXene layers and HMs. The nature of charge transfer between HMs and MXene sheets can be explored by the NBO analysis (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). As shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, for the Cd@Ca\u003csub\u003e2\u003c/sub\u003eC, Cd@Cr\u003csub\u003e2\u003c/sub\u003eC, Hg@Ca\u003csub\u003e2\u003c/sub\u003eC, and Hg@Cr\u003csub\u003e2\u003c/sub\u003eC the main contribution in λ\u003csub\u003emax\u003c/sub\u003e is an electronic excitation from HOMO to LUMO\u0026thinsp;+\u0026thinsp;1 MOs. Moreover, the NBO results of the Cd@Ca\u003csub\u003e2\u003c/sub\u003eC and Cd@Cr\u003csub\u003e2\u003c/sub\u003eC complexes show \u0026minus;\u0026thinsp;0.05220 a. u. and \u0026minus;\u0026thinsp;0.05334 a. u. for the energy of 2p\u003csub\u003ex\u003c/sub\u003e orbital of carbon atom. The occupation number of 2p\u003csub\u003ex\u003c/sub\u003e orbitals are calculated 0.63227 and 0.68207, respectively. However, in the Cd@Ca\u003csub\u003e2\u003c/sub\u003eC and Cd@Cr\u003csub\u003e2\u003c/sub\u003eC, the energy of 4d\u003csub\u003ex2y2\u003c/sub\u003e orbitals for Cd\u003csup\u003e2+\u003c/sup\u003e is calculated \u0026minus;\u0026thinsp;0.52451 a. u. as a 1.27102 occupation number and \u0026minus;\u0026thinsp;0.58287 a. u. as a 1.28145 occupation number, respectively. Therefore, it can be concluded that these excitations are ligand to metal (MXene\u0026rarr;cation) charge transfer. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Hg@Ca\u003csub\u003e2\u003c/sub\u003eC and Hg@Cr\u003csub\u003e2\u003c/sub\u003eC systems also present MXene to cation charge transfer in their λ\u003csub\u003emax\u003c/sub\u003e vertical excitation. The redshift for \u003csup\u003eHg\u003c/sup\u003e@Ca\u003csub\u003e2\u003c/sub\u003eC and Hg@Cr\u003csub\u003e2\u003c/sub\u003eC systems are a quite large values (∆λ\u0026thinsp;=\u0026thinsp;40 nm and 42 nm, respectively). Lastly, the results in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e are in excellent agreement with strong binding strength as well as large charge transfers between Pb\u003csup\u003e2+\u003c/sup\u003e and the MXene layers. The larger E\u003csub\u003ebin\u003c/sub\u003e for the Pb@Ca\u003csub\u003e2\u003c/sub\u003eC as well as Pb@Cr\u003csub\u003e2\u003c/sub\u003eC are according to larger redshifts which are expected (∆λ\u0026thinsp;=\u0026thinsp;45 nm and 71 nm, respectively).\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\u003eCalculated maximum wavelength (λ\u003csub\u003emax\u003c/sub\u003e), oscillator strength (\u003cem\u003ef\u003c/em\u003e), transition energy (ΔE), and charge transfer excitations for the HM@MXene systems.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSystem\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eλ\u003csub\u003emax\u003c/sub\u003e (nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003ef\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eΔE (a. u.)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ecrucial transition\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e452\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.0677\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHOMO\u0026rarr;LUMO (80%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e521\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.0860\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.087\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHOMO\u0026rarr;LUMO (80%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCd@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e486\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.0856\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.093\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHOMO\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;1 (60%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHg@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e492\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.0966\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.092\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHOMO\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;1 (70%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePb@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e497\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.1030\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.091\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHOMO\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;2 (70%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCd@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e554\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.1066\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.082\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHOMO\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;1 (60%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHg@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e563\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.1177\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.080\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHOMO\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;1 (75%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePb@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e592\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.1330\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.077\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eHOMO\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;2 (75%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSystem\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003edonor orbital/energy(occupancy)/location\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003eacceptor orbital/energy(occupancy)/location\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCd@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e2p\u003csub\u003ex\u003c/sub\u003e/ -0.05220 (0.63227)/C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e4d\u003csub\u003ex2y2\u003c/sub\u003e/ -0.52451 (1.27102)/Cd\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHg@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e2p\u003csub\u003ex\u003c/sub\u003e/ -0.06531 (0.65301)/C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e5d\u003csub\u003exz\u003c/sub\u003e/ -0.55875 (1.36351)/Hg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePb@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e2p\u003csub\u003ex\u003c/sub\u003e/ -0.07316(0.83356)/C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e6s/ -0.76815 (1.69685)/Pb\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCd@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e2p\u003csub\u003ex\u003c/sub\u003e/-0.05334(0.68207)/C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e4d\u003csub\u003ex2y2\u003c/sub\u003e/ -0.58287 (1.28145)/Cd\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHg@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e2p\u003csub\u003ex\u003c/sub\u003e/ -0.05416(0.66356)/C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e5d\u003csub\u003exz\u003c/sub\u003e/ -0.56875 (1.46351)/Hg\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePb@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003e2p\u003csub\u003ex\u003c/sub\u003e/-0.05334(0.73127)/C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003e6s/ -0.86815 (1.88685)/Pb\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e\n\u003ch3\u003e3. 4. Charge transport properties\u003c/h3\u003e\n\u003cp\u003eTo examine how the linking of the HMs affect the electronic properties of the MXene sheets, the charge transport properties can be considered. Using the MXene as the source of the electrons and the HMs as the electron-hole, we can form donor-acceptor system due to the charge transfer between the MXenes and the HMs. The obtained electron transport parameters for the HM@MXene systems are gathered in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. We found that among the considered complexes, the Pb@MXene have the largest electron transfer rates. In the Cd@Ca\u003csub\u003e2\u003c/sub\u003eC, Cd@Cr\u003csub\u003e2\u003c/sub\u003eC, Hg@Ca\u003csub\u003e2\u003c/sub\u003eC, and Hg@Cr\u003csub\u003e2\u003c/sub\u003eC the \u003cem\u003eλ\u003c/em\u003e are slightly enhanced but the reduction of \u003cem\u003et\u003c/em\u003e keeps the charge transfer rates in a similar value. However, an obvious change in the \u003cem\u003eλ\u003c/em\u003e and \u003cem\u003et\u003c/em\u003e for the Pb@Ca\u003csub\u003e2\u003c/sub\u003eC and Pb@Cr\u003csub\u003e2\u003c/sub\u003eC leads to a considerable increase in electron transfers. Besides, the \u003cem\u003et\u003c/em\u003e of the Pb@Cr\u003csub\u003e2\u003c/sub\u003eC (0.60 eV), revealing that Pb@Cr\u003csub\u003e2\u003c/sub\u003eC is the best electron transport material among all the studied HM@MXene systems. As reported in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, and Figure. 6, the Pb@Cr\u003csub\u003e2\u003c/sub\u003eC complex with higher \u003cem\u003ek\u003c/em\u003e, presents the higher binding energy, higher charge transfers, and larger redshift.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCharge transport properties for HM@CNC systems.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSystem\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003et (eV)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eλ (eV)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ek (s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCd@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.21\u0026times;10\u003csup\u003e12\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHg@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.72\u0026times;10\u003csup\u003e12\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePb@ Ca\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.26\u0026times;10\u003csup\u003e14\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCd@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.90\u0026times;10\u003csup\u003e12\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHg@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e8.45\u0026times;10\u003csup\u003e12\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePb@ Cr\u003csub\u003e2\u003c/sub\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.21\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.72\u0026times;10\u003csup\u003e15\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn summary, we examined the binding properties of the Cd\u003csup\u003e2+\u003c/sup\u003e, Hg\u003csup\u003e2+\u003c/sup\u003e, and Pb\u003csup\u003e2+\u003c/sup\u003e with Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayers. We found that the linking HMs on the MXene layers give the complexes, wherein the HM@MXene systems present the donor-acceptor frameworks. In addition, the efficiency of HM adsorption on the MXene obeys a Pb@MXene\u0026thinsp;\u0026gt;\u0026thinsp;Hg@MXene\u0026thinsp;\u0026gt;\u0026thinsp;Cd@MXene trend. We found that among the considered complexes, the Pb@MXene have the largest electron transfer rates (1.26\u0026times;10\u003csup\u003e14\u003c/sup\u003e and 1.72\u0026times;10\u003csup\u003e15\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for the Pb@Ca\u003csub\u003e2\u003c/sub\u003eC and Pb@Cr\u003csub\u003e2\u003c/sub\u003eC, respectively). TDDFT analysis was also considered to the HM@MXene systems to explore excitations of systems. The most probable excitations for the HM@MXene complexes showed ligand to metal transitions. Nevertheless, this theoretical study might be helpful for the researchers toward designing adsorbents based on MXene for removing toxic pollutants from wastewater.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflicts of interest\u003c/h2\u003e \u003cp\u003eThere are no conflicts to declare\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eXin Wu: Supervision, Resources, Project administration, Writing-review \u0026amp; editing.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThe authors are grateful to Zhejiang University for computational resources and financial supports.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eA. 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Kamath, Adsorption mechanism of toxic heavy metal ions on oxygen-passivated nanopores in graphene nanoflakes, Journal of Materials Science 55 (2020) 15826\u0026ndash;15844.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"MXene, Heavy metals, Wastewater treatment, Ca2C, Density functional theory","lastPublishedDoi":"10.21203/rs.3.rs-3960842/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3960842/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWater is an indispensable material for human life. Unfortunately, the developments of industrial activities have reduced the quality of water resources in the world. Meantime, heavy metals are an important factor in water pollution due to their toxicity. This study highlights the method for the capture of heavy metal ions from wastewater using the procedure of adsorption. The adsorption of toxic heavy metal ions (Pb\u003csup\u003e2+\u003c/sup\u003e, Hg\u003csup\u003e2+\u003c/sup\u003e, and Cd\u003csup\u003e2+\u003c/sup\u003e) on Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayers is investigated using the density functional theory. We have optimized the considered MXenes by nine DFT functionals: PBE, TPSS, BP86, B3LYP, TPSSh, PBE0, CAM-B3LYP, M11, and LC-WPBE. Our results have shown a good agreement with previously measured electronic properties of the Ca2C and Cr\u003csub\u003e2\u003c/sub\u003eC MXene layers and PBE DFT method. The calculated cohesive energy for the Ca2C and Cr\u003csub\u003e2\u003c/sub\u003eC MXene monolayers are \u0026minus;\u0026thinsp;4.12 eV and \u0026minus;\u0026thinsp;4.20 eV, respectively, which are in agreement with the previous studies. The results reveal that the adsorbed heavy metal ions have a substantial effect on the electronic properties of the considered MXene monolayers. Besides, our calculations show that the metal/MXene structures with higher electron transport rates display higher binding energy and charge transfers between the metal and Ca\u003csub\u003e2\u003c/sub\u003eC and Cr\u003csub\u003e2\u003c/sub\u003eC layers. Time-dependent density functional analysis also displayed \u0026ldquo;ligand to metal charge transfer\u0026rdquo; excitations for the metal/MXene systems. The larger Ebin for the Pb@Ca\u003csub\u003e2\u003c/sub\u003eC as well as Pb@Cr\u003csub\u003e2\u003c/sub\u003eC are according to larger redshifts which are expected (∆λ\u0026thinsp;=\u0026thinsp;45 nm and 71 nm, respectively). Our results might be helpful for future research toward the application of MXene-based materials for removing wastewater pollutants.\u003c/p\u003e","manuscriptTitle":"Removal of heavy metals from wastewater using 2D MXenes: A theoretical study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-02-19 02:58:29","doi":"10.21203/rs.3.rs-3960842/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"14269798-5bed-4def-aeb3-9c5478e3ba06","owner":[],"postedDate":"February 19th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-02-19T11:33:53+00:00","versionOfRecord":[],"versionCreatedAt":"2024-02-19 02:58:29","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3960842","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3960842","identity":"rs-3960842","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}