3.1. Ca2C and Cr2C MXene monolayers
Ca2C and Cr2C 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 Ca2C and Cr2C 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 (Egap) of the Ca2C and Cr2C 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 Ca2C, HOMO and LUMO orbitals of the Cr2C 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 Ca2C and Cr2C 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 Ebin values of a Cd2+ cation on the surface of Ca2C and Cr2C layers are shown in Figure. 3. Our results for the Cd@Ca2C and Cd@Cr2C systems present that the Cd2+ adsorbed on the top of carbon atom have the most negative Ebin values (-1.98 eV and − 2.10 eV, respectively). Hence, for the Hg2+ and Pb2+ 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@Ca2C, our results reveal that the HOMO and LUMO of the HM@Cr2C systems are more localized on the cations, particularly for the Pb@Cr2C complex. The Egap of the HM@MXene systems are also computed and exhibited in Figure. 4. We found that the Egap of the MXene monolayers could change significantly with the HM adsorbing. Thus, upon the cation adsorption occurs, Egap of the Ca2C and Cr2C are meaningly increased and decreased, respectively. Our calculations indicated that the Egap of Cr2C monolayers decreased after HM adsorption about 79%, 89%, and 97%, for the Cd@Cr2C, Hg@Cr2C, and Pb@Cr2C complexes, respectively. However, the enhanced in the Egap of the Ca2C after HM cations adsorptions are about 48%, 52%, and 71% for the Cd@Ca2C, Hg@Ca2C, and Pb@Ca2C systems, respectively (Figure. 4). The lowest Egap is for the Pb@Cr2C with a value of 0.009 eV (∆Egap=97%). To verify the effect of cations on the Egap of the MXene layers, the corresponding DOSs are analyzed and presented in Figure. 5. The DOS plots present that in the HM@Ca2C complexes, LUMOs shift to higher energies, which results in a prominent increase in the Egap as compared to the pure Ca2C MXene monolayer. However, in the HM@Cr2C complexes, LUMOs and HOMOs shift to lower and higher energies, respectively, which results in a prominent decrease in the Egap as compared to the pure Cr2C MXene monolayer. The DOS analysis also shows that the HM adsorbing is an effective method of generating MXene-based materials with tunable Egap 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 (Ebin) 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@Ca2C and Pb@Cr2C, respectively) than other complexes. Our results show that the Pb2+ cation links strongly on the Cr2C 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 (Hg2+ ([Xe] 4f14 5d10), Cd2+ ([Kr] 4d10), and Pb2+
([Xe] 4f14 5d10 6s2)).73–76 According to the electron configurations, Pb2+ ion has vacant p orbitals and can interact with the MXene surface through electrostatic interactions. In comparison with Pb2+ ion, Hg2+ and Cd2+ 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 Ca2C and Cr2C 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@Cr2C (-3.16 eV) > Hg@Cr2C (-2.66 eV) > Cd@Cr2C (-2.60 eV) > Pb@Ca2C (-2.52 eV) > Hg@Ca2C (-2.35 eV) > Cd@Ca2C (-2.21 eV). Therefore, the HM@MXene systems with the higher charge transfers show larger Ebin. Also, as results show, the computed values of corrected adsorption energy for BSSE vary from − 2.15 eV (for Cd@Ca2C) to -3.05 eV (for Pb@Cr2C). 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 Ca2C and Cr2C have λmax at 452 nm and 521 nm, respectively. By adsorbing the HMs on the MXene layers, redshifts for λmax 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@Ca2C, Cd@Cr2C, Hg@Ca2C, and Hg@Cr2C the main contribution in λmax is an electronic excitation from HOMO to LUMO + 1 MOs. Moreover, the NBO results of the Cd@Ca2C and Cd@Cr2C complexes show − 0.05220 a. u. and − 0.05334 a. u. for the energy of 2px orbital of carbon atom. The occupation number of 2px orbitals are calculated 0.63227 and 0.68207, respectively. However, in the Cd@Ca2C and Cd@Cr2C, the energy of 4dx2y2 orbitals for Cd2+ 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@Ca2C and Hg@Cr2C systems also present MXene to cation charge transfer in their λmax vertical excitation. The redshift for Hg@Ca2C and Hg@Cr2C 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 Pb2+ and the MXene layers. The larger Ebin for the Pb@Ca2C as well as Pb@Cr2C are according to larger redshifts which are expected (∆λ = 45 nm and 71 nm, respectively).
Table 1
Calculated maximum wavelength (λmax), oscillator strength (f), transition energy (ΔE), and charge transfer excitations for the HM@MXene systems.
System | λmax (nm) | f | ΔE (a. u.) | crucial transition |
Ca2C | 452 | 0.0677 | 0.100 | HOMO→LUMO (80%) |
Cr2C | 521 | 0.0860 | 0.087 | HOMO→LUMO (80%) |
Cd@ Ca2C | 486 | 0.0856 | 0.093 | HOMO→LUMO + 1 (60%) |
Hg@ Ca2C | 492 | 0.0966 | 0.092 | HOMO→LUMO + 1 (70%) |
Pb@ Ca2C | 497 | 0.1030 | 0.091 | HOMO→LUMO + 2 (70%) |
Cd@ Cr2C | 554 | 0.1066 | 0.082 | HOMO→LUMO + 1 (60%) |
Hg@ Cr2C | 563 | 0.1177 | 0.080 | HOMO→LUMO + 1 (75%) |
Pb@ Cr2C | 592 | 0.1330 | 0.077 | HOMO→LUMO + 2 (75%) |
System | donor orbital/energy(occupancy)/location | acceptor orbital/energy(occupancy)/location |
Cd@ Ca2C | 2px/ -0.05220 (0.63227)/C | 4dx2y2/ -0.52451 (1.27102)/Cd |
Hg@ Ca2C | 2px/ -0.06531 (0.65301)/C | 5dxz/ -0.55875 (1.36351)/Hg |
Pb@ Ca2C | 2px/ -0.07316(0.83356)/C | 6s/ -0.76815 (1.69685)/Pb |
Cd@ Cr2C | 2px/-0.05334(0.68207)/C | 4dx2y2/ -0.58287 (1.28145)/Cd |
Hg@ Cr2C | 2px/ -0.05416(0.66356)/C | 5dxz/ -0.56875 (1.46351)/Hg |
Pb@ Cr2C | 2px/-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@Ca2C, Cd@Cr2C, Hg@Ca2C, and Hg@Cr2C 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@Ca2C and Pb@Cr2C leads to a considerable increase in electron transfers. Besides, the t of the Pb@Cr2C (0.60 eV), revealing that Pb@Cr2C 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@Cr2C 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@ Ca2C | 0.21 | 0.62 | 1.21×1012 |
Hg@ Ca2C | 0.23 | 0.68 | 2.72×1012 |
Pb@ Ca2C | 0.31 | 0.32 | 1.26×1014 |
Cd@ Cr2C | 0.25 | 0.58 | 4.90×1012 |
Hg@ Cr2C | 0.28 | 0.55 | 8.45×1012 |
Pb@ Cr2C | 0.60 | 0.21 | 1.72×1015 |