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Heavy-Metal Adsorption Behavior of Two-Dimensional AlkalizationIntercalated MXene by First-Principles Calculations Jianxin Guo,†,§ Qiuming Peng,*,† Hui Fu,† Guodong Zou,† and Qingrui Zhang*,‡ †

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China Hebei Provincial Key Lab of Optoelectronic Information Materials, College of Physics Science and Technology, Hebei University, Baoding 071002, PR China ‡ Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 5, 2015 | http://pubs.acs.org Publication Date (Web): August 31, 2015 | doi: 10.1021/acs.jpcc.5b05426

§

S Supporting Information *

ABSTRACT: The two-dimensional (2D) layered MXene (Ti3C2(OH)xF2−x) material can be alkalization intercalated to achieve heavy-metal ion adsorption. Herein the adsorption kinetics of heavy-metal ions and the effect of intercalated sites on adsorption have been interpreted by first-principles with density functional theory. When the coverage of the heavy-metal ion is larger than 1/9 monolayer, the two-dimensional alkalization-intercalated MXene (alk-MXene: Ti3C2(OH) 2) exhibits strong heavy-metal ion absorbability. The hydrogen atoms around the adsorbed heavymetal atom are prone to form a hydrogen potential trap, maintaining charge equilibrium. In addition, the ion adsorption efficiency of alk-MXene decreases due to the occupation of the F atom but accelerates by the intercalation of Li, Na, and K atoms. More importantly, the hydroxyl site vertical to the titanium atom shows a stronger trend of removing the metal ion than other positions.

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A large family of 2D materials, labeled MXenes,16 are attracting much research enthusiasm because they combine hydrophilic surfaces, good structural and chemical stabilities, excellent electrical conductivities, and environment-friendly characteristics. MXenes are a group of layered 2D materials,17−21 produced by removing the Al layer, Mn+1AXn (n = 1, 2, 3) or MAX phase, where M represents an early transition metal; A corresponds to III A or IV A group elements; and X is C or N powders in HF solutions at low temperatures. This facile one-step method is desirable for industrial applications. More recently, we confirmed the selective metal ion adsorption behavior of MXene in addition to its use in energy storage devices such as electronic device, supercapacitors, Li batteries, and hydrogen storage.22−29 The exposed surface of MXene is initially terminated by the OH or F group after exfoliation. However, after alkalization interaction treatment, the functional groups on the surface of alk-MXene are mainly composed of −OH, −F, and intercalation ion (Na+, Li+, K+ etc.) groups. Consequently, comprehensive knowledge of the effect of different sites on metal ion adsorption plays an important role on tailoring the adsorption process of alk-MXene.

INTRODUCTION

Heavy-metal ions in drinking water are toxic, and they are difficult to decompose or metabolize in organisms. Thus, they continuously concentrate to exceed permissible limits, which seriously threatens human’s health.1−4 Additionally, in contrast to other high level light-metal ions such as Ca2+, Mg2+, Li+, K+, and Na+, the concentration of heavy-metal ions is far lower. Therefore, it becomes a main challenge to trap heavy-metal ions without varying light-metal ions at a low cost with high efficiency in drinking water treatment. Various techniques such as membrane processes,5 electrochemical method,6 chemical coagulation,7 and adsorption,8−10 have been developed to remove heavy-metal ions from water. Among these approaches, the adsorption treatment has attracted much attention in the past decades due to its suitable cost and simplicity of operation. Compared with other sorption counterparts, two-dimensional (2D) materials have two obvious virtues, large surface areas and abundant active sites, and become ideal adsorbents for environment remediation. For example, graphene oxide and its derivatives with a typical 2D structure11 have potential applications in the extraction of heavy-metal ions,12 arsenate,13 and organic dyes.14,15 However, their intrinsic properties, such as only single element and the simple surface functionality this may cause because of the lack of the kinds of surface groups, and the expensive cost limit their practical applications. © XXXX American Chemical Society

Received: June 7, 2015 Revised: August 7, 2015

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DOI: 10.1021/acs.jpcc.5b05426 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C Recently, we report lead uptake of alk-MXene.30 The simple adsorption mechanism was explained. However, some deep and detailed issues need to be further clarified for extending its applications: (i) other heavy metals adsorption ability of alkMXene, (ii) the effect of different hydroxyl sites and different functional groups on adsorption behavior of heavy-metal ion, and (iii) the effect of aqueous solution. Herein, theoretical elucidation on heavy-metal ion adsorption behavior was conducted based on the first-principles calculations.

When n is 2 and 4, the products of H+ are H5O2+ (Zundel cation) and H9O4+ (Eigen cation) (see Supporting Information). The corresponding formation energy (ΔH) is expressed as the following

COMPUTATIONAL DETAILS All calculations were carried out by the Vienna ab initio Simulation Package (VASP) code31 based on the density functional theory (DFT). The generalized gradient approximation with the Perdew−Burke−Ernzerhof (PBE)32 was used to describe the exchange-correlation functional, and the ion− electron interaction was treated by the projector augmented wave (PAW) method.33 The cutoff energy of the plane-wave basis was set at 520 eV. The conjugated gradient method was adopted during geometry optimization, and the criterion of energy convergence was 10−6 eV/cell. The Brillouin zone was represented by Monkhorst−Pack special k-points mesh34 of 6 × 6 × 1, 4 × 4 × 1, 3 × 3 × 1, and 2 × 2 × 1 for 2 × 2, 3 × 3, 4 × 4, and 5 × 5 supercells of Ti3C2(OH)2, respectively, corresponding to 1/4, 1/9, 1/16, and 1/25 ML (monolayer) of metal coverage on the surface. The smearing width of the partial occupancies was 0.1 eV, which were determined using the Methfessel−Paxton smearing scheme.35 For all simulations, the van der Waals (vdW) interaction is included using a dispersion correction term with the DFT-D3 method.36,37 A large vacuum space of 20 Å was used to avoid any interaction between an MXene sheet. The electronic structure including density of states (DOS) and electron localization function (ELF)38−41 computations of 2 × 2, 3 × 3, and 4 × 4 supercells including pristine and metal occupation MXene sheets were calculated using 24 × 24 × 1, 16 × 16 × 1, and 12 × 12 × 1 kpoints, respectively. The chemical reaction of heavy metals Y (Y = Pb, Cu, Zn, Pd, Cd) uptake of alk-MXene is shown as the following

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ΔHTi3C2(O2H2−2mPbm) = E Ti3C2(O2H2−2mPbm) + 2mE(H2O)n H+ − E Ti3C2(OH)2 − mE(H2O)2n Pb+

According to this definition, a negative ΔH indicates that it is energetically favorable for the reagents to form more stable products and vice versa.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 5, 2015 | http://pubs.acs.org Publication Date (Web): August 31, 2015 | doi: 10.1021/acs.jpcc.5b05426

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RESULTS AND DISCUSSION Structural Evolution of Intercalation. A Ti 3 AlC 2 compound can be described as an intergrowth structure with alternative stacking of hexagonal Ti3C2 layers and close-packed Al atom layers along the c-axis (Figure 1A). The calculated

Figure 1. Schematic illustration of the delaminating process from Ti3AlC2 to Ti3C2(OH)2. (A) Ti3AlC2; (B) Ti3C2; (C) I-Ti3C2(OH)2; (D) II-Ti3C2(OH)2; (E) III-Ti3C2(OH)2.

parameters (a = 3.081 Å, c = 18.659 Å) of Ti3AlC2 are in agreement with experimental values42,43 and previous theoretical results.44,45 Ti3C2 retains the atomic geometry of the parent MAX phase after Al removal. Compared with those of Ti3AlC2, the lattice parameter (a = 3.102 Å) of Ti3C2 increases, but the thickness of block [Ti3C2] (D = 4.649 Å, defined by the distances between two opposite Ti faces of [Ti3C2] blocks) decreases (Figure 1B). The values of a and D change to 3.090 and 4.718 Å, respectively, due to the intercalation of the OH group. The bonding energy (2.124 eV) between the [Ti3C2] layer and Al layer becomes weaker in comparison with the bonding energy (8.418 eV) of [Ti3C2] layers, demonstrating Ti3AlC2 can be potentially etched by hydrofluoric in theoretical aspect.16,26 During the alkalization-intercalated process, the Ti3C2 coverts into three Ti3C2(OH)2 structures with different sites, which are defined as I-Ti3C2(OH)2, II-Ti3C2(OH)2, and IIITi3C2(OH)2 (Figure 1C−E). For I-Ti3C2(OH)2, the OH group locates on the top site of the inner Ti atom; for II-Ti3C2(OH)2, the OH group lies above the top site of the C atom; the IIITi3C2(OH)2 structure can be described as a combination of ITi3C2(OH)2 and II-Ti3C2(OH)2, in which one side is similar to I-Ti3C2(OH)2 and the other side is similar to II-Ti3C2(OH)2.

Ti3C2(OH)2 + m Y(NO3)2 → Ti3C2(O2 H 2 − 2mYm) + 2mHNO3

(1)

The formation energy (ΔH) of the chemical reaction to describe the stability of heavy metals Y (Y = Pb, Cu, Zn, Pd, Cd) uptake of alk-MXene is defined as the following ΔH(Ti3C2)T2−2m(O2Y)m = E(Ti3C2)T2−2m(O2Y)m + 2mE HNO3−E Ti3C2T2 − mE Y(NO3)2 (2)

where E is the total energy of the corresponding substance. T represents the different terminated surfaces of Ti3C2 such as OH, OX (X = Li, Na, K), F, and their mixture. In addition, all heavy metals will exist in the form of hydrate in aqueous solution. The chemical reaction of Ti3C2(OH)2 and Pb(NO3)2 (as the typical example in this work) in the water solvent was described by Ti3C2(OH)2 + m(H 2O)2n Pb2 + → Ti3C2(O2 H 2 − 2mPbm) + 2m(H 2O)n H+

(4)

(3) B


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The Journal of Physical Chemistry C


The energy results indicate that the order of structural stability follows I-Ti3C2(OH)2 > III-Ti3C2(OH)2 > II-Ti3C2(OH)2, which is consistent with the previous result.25 Adsorption Mechanism. Generally, the toxic heavy metals contain Pb, Cu, Zn, Pd, Cd, etc. To judge the adsorption role of alk-MXene (Ti3C2(OH)2), the formation energies are calculated for 1/9 ML. Figure 2 shows that the heavy metals can be

Figure 2. Formation energies of different heavy metals.

Figure 3. Schematic illustration of the formation of the Ti3C2(O2H2−2mPbm) structure. (A) Ti3C2(OH)2; (B) Ti3C2(O2H2−2mPbm); (C) top view of Ti3C2(O2H2−2mPbm) structure when Pb coverage is 1/9 ML. S1−S6 represent the different atom sites of the hydroxyl.

well removed by alk-MXene in terms of the negative values of formation energies. It indicates that alk-MXene is an ideal material for removing toxic heavy metals. The different formation energies of heavy metals originate from the different interaction roles of outside layered electrons between heavy metals and MXene or NO3− according to the chemical reaction. This work is continuously followed by our previous result,30 and thus Pb is selected as an example, which can not only interpret its adsorption behavior in detail but also achieve the most optimal condition for the following large-scale applications. The structure and electronic properties of transition metal adsorption atoms (Cu, Zn, Pd, and Cd) are similar to the Pb atom as a typical example in this work by the check of the GGA+U method (Figures S1 and S2). In addition, since the other heavy metals are transition metals, the Cd, as a typical example of transition metals, is selected to compare the similarity and difference between Pb and Cd (transition metal). The schematic illustration where Pb2+ replaces H+ of Ti3C2(OH)2 to form the Ti3C2(O2H2−2mPbm) structure (Figure 3A and B) reveals that Pb2+ occupies the center of a hydroxyl potential trap which consists of eight hydroxyl groups (blue circles) and two oxygen atoms (the hydroxyls losing H atoms) (black circles). The formation energy of Ti3C2(O2H2−2mPbm) increases with decreasing Pb coverage. When the Pb adsorbs on the surface of Ti3C2(OH)2, the lattice parameter a and the thickness D of the [Ti3C2] block varies (Table 1). With increasing the Pb coverage, a decreases and D increases, suggesting the [Ti3C2] block compresses along the a-axis and expands along the c-axis. Concurrently, both dPb−O (the distance of Pb and O at the S1 or S2 site in Figure 3C) and dPb−H (the distance of Pb and H at the S3 or S4 site in Figure 3C) become short, and d(O−H)′ increases with increasing Pb coverage (the distance of O and H at the S3 or S4 site in Figure 3C, Table 1). Comparatively, the distances of other hydroxyls far from the Pb atom, for instance S5 or S6 sites in Figure 3C, maintain invariable. In comparison, Ti3C2(O2H2−2mCdm) has the same structure and the changing rule as Ti3C2(O2H2−2mPbm).

Note that the metal ions generally exist in the form of hydrates in aqueous solution, and then the water medium has also been considered in the chemical reaction. Taking the Pb ion as a typical example, the possible sites and models were calculated. Basically, the formation energy (Supporting Information) indicates that the water medium would not change the reaction direction. Therefore, the models without water molecules were used for their simplicity and convenience. To get further insight into the variation from Ti3AlC2 to Ti3C2(O2H2−2mPbm) during the treatment of chemical exfoliation and alk-intercalation, the total and projected density of states (TDOS and PDOS) are calculated for Ti3AlC2, Ti3C2, Ti3C2(OH)2, and Ti3C2(O2H2−2mPbm). Figure 4A shows that the TDOS of Ti3AlC2 is related to the contribution of C 2(s,p)orbitals and Ti 3d-orbitals. However, Al 3(s,p,d)-orbitals and Ti 3d-orbitals merely form a weak orbital hybridization at −0.9 eV. The valence states move to a lower level, and the DOS configuration becomes narrow after Al removal (Figure 4B), which is consistent with the previous value.44 Compared with the DOS of Ti3C2, a new peak is observed at −9.0 eV when the hydroxyl groups are attached (Figure 4C), which originates mainly from the hybridization of Ti 3d-, C 2s-, O 2p-, and H 1sorbitals. Another peak at −6.5 eV corresponds to the hybridization of Ti 3d- and O 2p-orbitals. The DOS of Ti3C2(O2H2−2mPbm) (Figure 4D) shows that Pb 5d-orbitals and Pd 6s-orbitals hybridize with O 2s-orbitals at about −19.1 and −17.3 eV, with O 2p- and H 1s-orbitals between −9.3 and −8.3 eV, respectively. For Cd, the interactions of Cd and Ti3C2(O2H2−2m) are mainly from the hybridization of Cd 4dorbitals and O 2p-orbitals (Figure S1d). Figure 5 (Figure S3) shows the PDOS of Oa (the O atom locating at the S3 site in Figure 3C), Ob (the O atom locating at the S1 site in Figure 3C), and Oc (the O atom locating at the S5 site in Figure 3C). The difference in PDOS between three O atoms is related to the different roles in the interaction of the Pb atom. Compared C


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Table 1. dPb−O, dPb−H, d(O−H)′, d(O−H)″ (Others Far From Pb), a, and D of the Ti3C2(O2H2−2mPbm) System coverage (Θ)

a (Å)

D (Å)

dPb−O (Å)

dPb−H (Å)

d(O−H)′ (Å)

d(O−H)″ (Å)

1/25 1/16 1/9

3.083 3.082 3.074

4.738 4.740 4.811

2.414 2.413 2.407

2.606 2.606 2.594

1.003 1.003 1.005

0.972 0.972 0.973

Figure 4. Total and projected density of states for (A) Ti3AlC2, (B) Ti3C2, (C) Ti3C2(OH)2, (D) Ti3C2(O2H2−2mPbm).

Figure 6. Bader charge change of the different atoms for Ti3C2(O2H2−2mPbm) compared with the corresponding atoms of Ti3C2(OH)2. Ti1 and Ti2 represent the outer layer and inner layer Ti atoms, respectively. The number of the horizontal axis means the different positions of atoms including H, O, Ti, C, and Pb.

Figure 5. DOS of three different situations of O atoms of Ti3C2(O2H2−2mPbm): (a) Oa, (b) Ob, (c)Oc.

with far Oc, both Oa and Ob contribute to the adsorption of Pb. Nevertheless, the interactive orbitals are different (−9.3 and −8.3 eV for Oa; −19.1 and −17.3 eV for Ob, Figure 5). The discrepancy between Oa and Oc results in the presence of a double peak (Figure 5 and Figure S3). Effect of Different Hydroxyl Sites. To determine the charge variation during the adsorption process, the Bader decomposition of electron density46 has been carried out by virtue of the charge analysis method and plotted in Figure 6. Compared with those of Ti3C2(OH)2, the H and O atoms (H_2, O_2, H_4, and O_4) of the two hydroxyl groups close to the Pb atom at the S3 and S4 sites in Figure 3C, respectively,

change significantly. They pick up electrons rather than losing electrons like others. The C_4 and C_6 nearest neighbor to the Pb atom change remarkably, suggesting that those C atoms play an important role in the adsorption of Pb. The charges of all atoms of Ti3C2(O2H2−2mPbm) have altered after the Pb insertion, compared with the pristine Ti3C2(OH)2. Furthermore, the Pb trapped 1.6765 |e| from Ti3C2(OH)2, while the obtained electrons of two H atoms (which are substituted by the Pb atom) are 0.8558 |e|. It demonstrates that the trapped electrons not only derive from O_8 and O_9 (at S1, S2 sites in Figure 3C) but also the hydroxyls around the Pb atom. D


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Figure 7. ELFs of the different Pb coverages for Ti3C2(O2H2−2mPbm). (A) 1/4 ML; (B) 1/16 ML. 1 and 2 represent (110) and (−110) planes, respectively.

Similarly, the Bader charge of Ti3C2(O2H2−2mCdm) was calculated (Figure S4). The variation of Bader charge is stronger than Ti3C2(O2H2−2mPbm), resulting in its lower formation energy. ELF is derived from this measure of Pauli repulsion and is confined to the [1, 0] interval.47 ELF = 1 corresponds to the perfect localization (covalent bonding), and ELF = 0.5 corresponds to electron−gas-like pair probability (metallic bonding).48 ELF = 0 represents ionic bonding or a precursor to metallic bonding. The ELF has been calculated to obtain the bonding information on Ti3C2(O2H2−2mPbm) (Ti3C2(O2H2−2mCdm)) for different Pb (Cd) coverages (Figure 7, Figure S2d, and Figure S5). The adsorption of the Pb (Cd) atom is associated with the hydroxyl, the same as the oxygen atom below Pb (Cd). The uniform surface electrons of the pristine Ti3C2(OH)2 are concentrated in the vicinity of the Pb30 (Cd) atom. The number of opposite-spin electron pairs around the Pb (Cd) atom increases with decreasing Pb (Cd) coverage. On the basis of the variation of formation energy, it is confirmed that enough opposite-spin electron pairs are necessary to form the Pb (Cd) atom in the adsorption process. Namely, the formation of the hydroxyl potential trap plays an important role in the continuous adsorption of Pb (Cd). For the hydroxyl groups which locate at other sites such as IIor III-Ti3C2(OH)2, the distribution of opposite-spin electron pairs of Ti3C2(OH)2 rapidly decreases (Figure 8). It reveals that the Pb2+ uptake capacity of II- or III-Ti3C2(OH)2 reduces due to a lack of bonding between Pb and the hydroxyl group. According to the formation energies of I-Ti3C2(O2H2−2mPbm) (−1.091 eV), II-Ti3C2(O2H2−2mPbm) (0.183 eV), and IIITi3C2(O2H2−2mPbm) (−0.539 eV), it can be seen that ITi3C2(OH)2 shows a stronger trend of removing Pb2+ than IIand III-Ti3C2(OH)2. For Cd, the ELF distinction between different hydroxyl located sites is not obvious (Figure S6). The formation energies of I-, II-, and III-Ti3C2(O2H2−2mCdm) are −3.157, −1.194, and −2.246 eV, respectively. All types of the Ti3C2(O2H2−2mCdm) display stronger adsorption ability than Ti3C2(O2H2−2mPbm). However, I-Ti3C2(O2H2−2mCdm) is still the most stable adsorption structure. In addition, II- and IIITi3C2(OH)2 can transfer to I-Ti3C2(OH)2 by the relaxation of atoms according to the minimum energy principle, indicating that the Ti3C2(OH)2 structures hardly affect heavy metal uptake capacity. Effect of Different Functional Groups. The substitution of the F atom affects the Pb adsorption on three possible OH

Figure 8. ELFs of the different hydroxyl sites of Ti3C2(OH)2 on the (110) plane. (A) II-Ti3C2(OH)2; (B) II-Ti3C2(O2H2−2mPbm); (C) IIITi3C2(OH)2; (D) III-Ti3C2(O2H2−2mPbm).

sites of Ti3C2(OH)2 (S4−S6 sites of Figure 3C). Table 2 lists the formation energies (ΔH) of Ti3C2(O2−xH2−x−2mFxPbm) with different F sites for 1/9 ML. For all F sites, the value of ΔH becomes smaller than those of Ti3C2(O2H2−2mPbm) without the F atom. On the basis of the ELF of the Ti3C2(O2H2−x−2mFxPbm) structure (Figure 9A and B), there is no opposite-spin electron pair between the Pb atom and F atom, indicating that the Pb uptake capacity of Ti3C2(OH)2 becomes weak due to the ion bonding between Pb and F. Therefore, it is confirmed that the F group inhibits the Pb2+ uptake of Ti3C2(OH)2. In the case of Cd, the F effect (Figure E


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Table 2. Formation Energies (ΔH) of Ti3C2(O2−xH2−x−2mFxPbm) and Ti3C2(O2H2−2yPby) after Replacing OX (X = Li, Na, K) Groups reaction equations

Pb coverage (Θ)

F or O sites

ΔH (eV)

Ti3C2(OH)2−xFx + mPb(NO3)2 → Ti3C2(O2−xH2−x−2mFxPbm) + 2mHNO3

1/9 1/9 1/9 1/9 1/9 1/9 1/4 1/9 1/16

1 2 3 1 2 3 -

−0.422 −0.806 −0.923 −0.191 −0.523 −0.582 (−1.043)Na (−1.818)Na (−1.827)Na

Ti3C2(OH)2−xOx + mPb(NO3)2 → Ti3C2(O2−xH2−x−2mOxPbm) + 2mHNO3

Ti3C2(O2H2−yXy) + yPb(NO3)2 → Ti3C2(O2H2−2yPby) + yHNO3 + yXNO3

(−1.520)Li (−2.248)Li (−2.324)Li

(−1.068)K (−2.040)K (−2.218)K


affect greatly the Pb adsorption for alk-MXene. The Li, Na, and K are replaced prior by Pb for different coverages as the X (X = Na, Li, K) and H coexist. Basically, the formation energies of Ti3C2(O2H2−yXy) (Table 2) show that the cation ion intercalation is beneficial. Namely, the existence of the OX group accelerates the Pb2+ uptake with respect to the lower formation energy. According to the above analysis, the OH groups replacing F groups and X ion intercalation are useful for the Pb ion removal. However, the surface of MXene is initially terminated by both OH and F groups after exfoliation.16 The alkalization process is of benefit to transfer the F group to the OH group and to generate the intercalation of X ions.26,30 The negative formation energies (Figure 10) indicate the F atom will be

Figure 10. Formation energies of the substitution of F atoms of Ti3C2F2 by the OH group (green) and H atom of Ti3C2(OH)2 by Li, Na, and K ions under different coverages.

replaced by hydroxyl, and the X ions can replace all H ions at the alkaline environment. Note that the order of formation energy might be Li > Na > K or K > Na > Li according to the order of atom radius for Li, Na, and K. However, the actual order of formation energy is Li > K > Na (except 1 ML coverage). It reveals the main reasons are related to not only the different atom radius but also the distribution of valence electrons on the surface of MXene. On the basis of the ELFs (Figure 11) of Ti 3 C 2 O 2 H 2−y Li y , Ti 3 C 2 O 2 H 2−y Na y , and Ti3C2O2H2−yKy, we can find that the bonding of Na is stronger than those of Li and K with Ti3C2O2H2−y, revealing that the interaction of Na is more facile than K and Li. However, when all of the H ions are replaced by X ions (1 ML), the X ions prior locate at C top sites (Table S1). The order of formation energy changes to Li > Na > K. For Li and Na, the negative

Figure 9. ELFs of Ti3C2(OH)xF2−x (A) and Ti3C2(O2H2−x−2mFxPbm) (B). 1 and 2 represent (110) and (−110) planes, respectively.

S7) is consistent with that of Pb. Nevertheless, the different Flocated sites are similar according to the formation energies of F-site1 (−2.938 eV), F-site2 (−3.012 eV), and F-site3 (−3.028 eV). In addition, the O-terminated surface may be formed by thermal decomposition, and the effect is similar to the F ion, which weakens the adsorption ability of Ti3C2(OH)2 according to their formation energies (Table 2) and ELF (Figure S8). Due to a high concentration of X+ (X = Li, Na, K) ion sin solution after X+ ion intercalation treatment,22 those X+ ions F


Article

The Journal of Physical Chemistry C Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.


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ACKNOWLEDGMENTS We greatly acknowledge the NSFC (No. 51422105, 51578476), NSF of Hebei (No. E2015203404), Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0690), Heibei province scientific program (GCC2014058), and Heibei Province Technology Foundation for Selected Overseas Chinese, Outstanding Youth Scholar and Youth Top-notch Talent Program, and the High-Performance Computing Platform of Hebei University. We are grateful to Prof. Hai-Jun Wang in Hebei University for some computation supports with a “Molecule Aggregated Structure” workstation.

Figure 11. ELFs of the different metallic ions instead of H ion on the (110) plane. (A) Ti3C2(O2H2−yLiy); (B) Ti3C2(O2H2−yNay); (C) Ti3C2(O2H2−yKy).

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electron cloud (NES)49,50 forms above the metal layers (Figure S9A, B), which connects to the electrons of the O atom after Li or Na adsorption on Ti3C2O2 (Figure S9D). The effect of NES can be elucidated that the repulsion between the positive metal ions (adatom−adatom and Ti−adatom) by the electronic share forms metallic bonding. In the case of K, the electrons are more localized (Figure S9C), which may be the result of electron transfer from spherical s-orbitals to more localized nonspherical p- or d-orbitals.50 NES of K is isolated to cause the increased adsorption energy, compared with Li and Na. Using the 3 × 3 supercell model, the Pb adsorption was calculated for Ti3C2(OX)2, and the Ti3C2(OK)2 displays a better Pb uptake ability than Ti3C2(OLi)2 and Ti3C2(ONa)2 (Table S2). This interaction between K and Ti atoms is weaker than that for Li and Na. Thus, the substitution of Pb brings down the total energy of Ti3C2(OK)2.

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CONCLUSIONS The calculated results show that 2D alk-MXene is an effective adsorbent for removing heavy-metal ions in water. The formation of the hydrogen potential trap, which consists of hydrogen atoms of Ti3C2(OH)2 around the heavy-metal atom and O atoms below the heavy-metal atom, provides enough opposite-spin electron pairs to catch heavy-metal atoms. The ELF of Ti3C2(O2H2−2mPbm) indicates that the Pb atom and Ti3C2(O2H2−2m) form some stable chemical bonds. Furthermore, the residual of F and other hydrogen sites weakens the adsorption role of alk-MXene Ti3C2(OH)2. However, the intercalation of X+ (X = Li, Na, K) groups on the surface of Ti3C2(OH)2 strengthens the adsorption behavior.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b05426. The check of LDA+U method for transition metals, the effect of water solvent, and the different structures of hydrogen traps were involved (PDF)

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AUTHOR INFORMATION

Corresponding Authors

* (Q. M. Peng) E-mail: [email protected]. *(Q. R. Zhang) E-mail: [email protected]. G


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