Activation of the Basal Plane in Two Dimensional ... - ACS Publications

Article Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Activation of the Basal Plane in Two Dimensional Transition Metal Chalcogenide Nanostructures Jae Hyo Han,†,‡,§,∥ Hong Ki Kim,⊥,#,∥ Bongkwan Baek,†,‡,§ Jeonghee Han,†,‡,§ Hyun S. Ahn,§ Mu-Hyun Baik,*,#,⊥ and Jinwoo Cheon*,†,‡,§ †

Center for NanoMedicine, Institute for Basic Science (IBS), Seoul 03722, Republic of Korea Yonsei-IBS Institute, Yonsei University, Seoul 03722, Republic of Korea § Department of Chemistry, Yonsei University, Seoul 03722, Republic of Korea ⊥ Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea # Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 34141, Republic of Korea

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S Supporting Information *

ABSTRACT: Achieving a molecular level understanding of chemical reactions on the surface of solid-state nanomaterials is important, but challenging. For example, the fully saturated basal plane is believed to be practically inert and its surface chemistry has been poorly explored, while two-dimensional (2D) layered transition-metal chalcogenides (TMCs) display unique reactivities due to their unusual anisotropic nature, where the edges consisting of unsaturated metals and chalcogens are sites for key chemical reactions. Herein, we report the use of Lewis acids/bases to elucidate the chemical reactivity of the basal plane in 2D layered TMCs. Electrophilic addition by Lewis acids (i.e., AlCl3) selectively onto sulfides in the basal plane followed by transmetalation and subsequent etching affords nanopores where such chemical activations are initiated and propagated from the localized positions of the basal plane. This new method of surface modification is generally applicable not only to various chemical compositions of TMCs, but also in crystal geometries such as 1T and 2H. Nanoporous NbS2 obtained by this method was found to have an enhanced electrochemical energy storage capacity, offering this chemical strategy to obtain functional 2D layered nanostructures.

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INTRODUCTION Two-dimensional (2D) nanostructures such as graphene, hexagonal boron nitride (h-BN), and transition-metal chalcogenide (TMC) have attracted much attention due to their unique optical, electrical and catalytic properties.1,2 Despite being of central importance for various applications, the chemistry of 2D nanomaterials remains poorly understood. In particular, the basal plane of these materials that represents the majority of the surface of 2D nanostructures is thought to be chemically inert, whereas the edges are proven to be chemically reactive.3−5 Hence, previous approaches of activating the basal plane utilized nonselective and high energy processes involving radical species, plasma or electron beam irradiation,5−11 which are hard to understand in plausible chemical terms and challenging to control. We questioned if a simple and much milder chemical approach can be employed as an alternative. Conceptually, the sulfides in the basal plane surface are Lewis bases that may be vulnerable to an electrophilic attack from Lewis acids such as AlCl3, GaCl3, SnCl4, and TiCl4, which may allow creating new and interesting nanostructures with a higher degree of control than previously possible. To evaluate both the energetic and mechanistic scenarios of such reactions, we © XXXX American Chemical Society

combined our experimental explorations with computer simulations based on density functional theory (DFT) calculations. This approach allowed for exploring the reactivities and the corresponding potential energy surfaces for the putative reactions of the sulfide ions in the basal and edge facets toward Lewis acids. We considered both octahedral (1T) hexagon- and trigonal prismatic (2H) triangle-shapes in classical electrophilic substitution (SE2) reactions between the Lewis-acids and the Lewis-basic sulfide ions in TMC (Figure 1a and 1b).

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RESULTS AND DISCUSSION Colloidal Synthesis of NbS2 Nanosheets for Reactivity Study. We chose a group-V 2D layered TMC, 2H-NbS2, as a representative material for the study. Its intrinsically metallic electronic structure derived from a half-filled 4dz2 orbital12 affords excellent electrical conductivity, transparency, and flexibility that are useful for transparent electrodes and energy storage devices.13,14 We successfully synthesized ultrathin triangular 2H-NbS2 nanosheets with an edge length of 500 Received: May 24, 2018

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DOI: 10.1021/jacs.8b05477 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

Journal of the American Chemical Society

Figure 1. Surface activation, colloidal synthesis, and characterization of 2D layered TMCs. (a) Surface activation of the basal plane in both octahedral (1T) and trigonal prismatic (2H) colloidal 2D layered TMCs with Lewis acids preferentially dissolute the localized region of the basal plane forming nanopores. (b) Electrophilic substitution (SE2) between Lewis acid and Lewis-basic sulfide ions of NbS2 on the initial stage. (c) 2HNbS2 nanosheets produced by colloidal synthesis of NbCl5 and 1-DDT in oleylamine. (d) Illustration of triangle shaped five-layered 2H-NbS2. (e) Low-magnification TEM image of as-synthesized NbS2 nanosheets. Inset: SAED pattern of corresponding TEM image. (f) HRTEM image showing lattice fringes with spacing of 1.6 and 2.8 Å corresponding to the (110) and (100) planes, respectively. Inset: FFT pattern of corresponding TEM image. (g) AFM image of deposited NbS2 nanosheets on a silicon wafer. Scale bars, 200 nm (e), 1 nm (f) and 200 nm (g).

Surface Activation of NbS2 Nanosheets with Lewis Acidic AlCl3. For the chemical reactivity study, the fivelayered triangular NbS2 nanosheet was tested with one of the classical Lewis acids, AlCl3 (0.12 mmol) at 120 °C under Ar atmosphere, and the time dependent structural response was observed (Figure 2a). The effect of the AlCl3 concentration on the final structure was additionally tested (Figure S2). Figure 2b shows the TEM images where changes from clean nanosheets ((i), 0 h) to multiple nanopores with average pore diameter (d) of 2.3 (±0.7) nm embedded within nanosheets ((ii), 1 h) are observed. As the reaction proceeds, the nanopores become larger in size and the morphological transformation into sponge-like structure is observed ((iii), 2 h). Within 3 h, the nanosheet is significantly etched and eroded, as shown in Figure 2b (iv) and Figure S3. We also observed that some floccules emerged after 2 h, which appear to be due to the adsorption of excessively etched fragments on the surfaces (vide infra). Figure 2c shows the low-magnification TEM image and the corresponding electron diffraction pattern (inset of Figure 2c) at 1 h, which indicate that the surface activated NbS2 remains to be single-crystalline, whereas the diffraction spots are slightly broadened, possibly arising from the relaxation of atoms near the nanopores that stabilize the initial framework.19 Crystallographic analysis by HRTEM shows that the lattice

nm and a thickness of 3.9 nm utilizing a one-pot heat-up method based on colloidal synthesis from the molecular precursors of niobium pentachloride (NbCl5) and 1-dodecanethiol (1-DDT) in oleylamine (Figure 1c, Figure 1d, and Figure 1e).15 The triangle shape is a representative of the 2Hgeometry in TMCs.16 Each layer of the 2H-NbS2 nanosheet is held together by van der Waals (vdWs) forces with an interlayer spacing of 6.5 Å, and the layered nanostructures comprise of S−Nb−S triatomic layers, as illustrated in Figure 1d. The high-resolution transmission electron microscope (HRTEM) image shows an atomic arrangement of the lattice fringes with spacings of 1.6 and 2.8 Å, which corresponds to the (110) and (100) planes, respectively (Figure 1f). Selected area electron diffraction (SAED) patterns show its singlecrystallinity (inset of Figure 1e) and the thickness of nanosheets is characterized by atomic force microscopy (AFM) to be 3.9 nm, which corresponds to roughly five layers with the monolayer of NbS2 being ∼0.7 nm thick (Figure 1g).1,17 The X-ray diffraction (XRD) patterns of NbS2 nanosheets that we prepared match well with literature precedence (Figure S1). When compared to bulk NbS2, the significant reduction in peak intensity of (002), (006) and (103) reflections of c-axis and the strong appearance of (100), (110) and (200) of a- and b-axis provide good evidence for the ultrathin nature18 of our sample. B


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Figure 2. Morphological transformation of NbS2 nanosheets driven by a Lewis acid (AlCl3). (a) Schematic illustration of morphological transformation of NbS2 nanosheets by AlCl3. (b) High-magnification TEM images at each stage of NbS2 nanosheets transforming into porous structures by adding AlCl3 for (i) 0 h, (ii) 1 h, (iii) 2 h, and (iv) 3 h, respectively. (c) Low-magnification TEM image of the NbS2 nanosheets with multiple nanopores at stage (ii). Inset: a SAED pattern of the corresponding TEM image. (d) HRTEM image of the lattice fringes around the nanopores with spacings of 1.6 and 2.8 Å corresponding to the (110) and (100) planes, respectively. (e) AFM image of nanopore-NbS2 nanosheets. (f) XRD patterns of NbS2 and transformed nanosheets of (i) 0 h, (ii) 1 h, (iii) 2 h and (iv) 3 h, respectively. Scale bars, 50 nm (b), 100 nm (c), 1 nm (d) and 100 nm (e).

only a relatively mild reaction temperature of 50 °C and significant morphological deformation could be seen after 1 h (Figure 3c and 3d). To rationalize this reactivity trend, the frontier orbitals of the reactants were analyzed,20 focusing on the lowest unoccupied molecular orbital (LUMO) of the Lewis acids and the highest occupied molecular orbital (HOMO) of the NbS2 substrate. The electronic structure of NbS2 was discussed in detail elsewhere.12,21 In short, NbS2 is believed to be isoelectric with a work function of approximately −6.0 eV. The LUMO energies of Lewis acids are −4.03 (TiCl4), −3.56 (SnCl4), −2.28 (GaCl3) and −1.71 eV (AlCl3), as illustrated in Figure 4a. The Lewis acid/base interaction will naturally be strong if the energy difference between the HOMO of the Lewis base and the LUMO of the Lewis acid are small, which provides a plausible explanation for why the Ti and Sn based Lewis acids react much more readily than the main-group acids based on Ga and Al. An alternative way of analyzing these results more quantitatively is deriving the local hardness/ softness of these reactive centers22,23 within Pearson’s principles of hard-soft-principles of hard-soft-acids-bases (HSAB).24 To qualitatively evaluate the strength of Lewis acids to NbS2 within the framework of the HSAB theory, the maximum hardness principle (MHP) is employed.25−28 During the formation of a chemical bond in a Lewis acid/base interaction, two driving forces are important. One is the molecular chemical hardness that serves as primary determinant for bringing acids and bases together that have well-matched hardness kernels within the HSAB principle, as described in eq

spacings are 1.6 and 2.8 Å corresponding to the (110) and (100) planes, respectively (Figure 2d). The thickness of NbS2 nanosheets at 1 h measured by AFM suggests a thickness of 3.9 nm, equivalent to the pristine nanosheet at 0 h (Figure 2e). The XRD analyses indicate that the crystallinity of the activated NbS2 sheet remains persistent after 1 h, as the pattern is identical to the pristine nanosheets at 0 h (Figure 2f (i−ii)). However, the fading diffraction patterns from 2 to 3 h indicate that crystallographic corrosion occurs during this time period (Figure 2f (iii−iv)). Time dependent TEM monitoring combined with XRD pattern analyses suggests an etching process as follows. (i) Lewis acid activation of NbS2 results in multiple spots of nanopores in central region of the nanosheet that is five layers thick. (ii) As the reaction proceeds, surface dissolutions at the center, vertex and edge of top and bottom layers generate fragmentations in the first and fifth layers. (iii) Meanwhile, the inner second to fourth layers proceed from stage (ii), and the remaining layers determine the overall triangular shape. Elucidation of Sulfide Reactivity of NbS2 Nanosheets Using Various Lewis Acids. Inspired by the successful preparation of these 2D nanostructures, we focused on the electronic structure of NbS2 to better understand its reactivity toward various Lewis acids, such as TiCl4, SnCl4, GaCl3, and AlCl3. Interestingly, AlCl3 and GaCl3 required reaction temperatures above 100 °C and the extended reaction time of 2 h for the sequential morphological transformation into nanopore and porous structures, respectively (Figure 3a and 3b). SnCl4 and TiCl4 were much more effective and needed C


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Figure 3. A series of Lewis acids in different acidity examined for the basal plane activation of NbS2. Time dependent TEM images of activation promoted by (a) AlCl3, (b) GaCl3, (c) SnCl4 and (d) TiCl4 for (i) 0 h and (ii) 1 h at 50 °C. For (a) AlCl3 and (b) GaCl3, relatively high reaction temperature of 120 °C is required for activation to proceed in (iii) 1 h and (iv) 2 h. Scale bars, 100 nm.

Figure 4. Reactivity of NbS2 nanosheets toward various Lewis acids. (a) Orbital energy diagram of NbS2 and classical Lewis acids employed in this study from computed ionization potentials (LUMO, −IP) and electron affinities (HOMO, −EA), and the work function of NbS2 (W = −6.0 eV).12 (b) Correlation between the values of the molecular chemical hardness (η) and the maximum hardness index (Yh) between various Lewis acids and NbS2. (c) Ball-and-stick model of Nb-zigzag terminated NbS2 cluster model with coordination numbers (CN) of metal sites on the basal and edge plane. (d) Electrostatic potential map of the 9-line NbS2 cluster.

1.29 A second factor, known as the principle of maximization of hardness (MH)27 is the tendency of the adduct to minimize the energetic fluctuations of the bonds at equilibrium

distances. Because these two forces are linked to each other, the maximum hardness index (Yh) was recently proposed to unify the HSAB and MH principles in respect to each other.28 D


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Figure 5. A proposed mechanism for chemical activation of the basal plane with Lewis acidic AlCl3 in 2H-NbS2. Calculated potential energy surface diagram in periodic boundary conditions shows that the reaction pathway involves electrophilic addition followed by transmetalation, and dimerization steps lead to a nanopore.

| o o o o Acid } o o o o hard − soft(soft − hard)o ~

At first, chemical hardness (η) between two chemical species can be estimated by eq 2 where εLUMO is the LUMO energy of Lewis acid and εHOMO is the HOMO energy of Lewis base.30−32

hard − hard soft − soft

l Yh ≥ 0.5, η≥1 o o o o o 0 ≤ Yh < 0.5, 1/ 2 ≤ η < 1 − Baseo m o o o o o o 0 < η < 1/ 2 n Yh < 0,

A hard − Bsoft + A soft − B hard ↔ A hard − B hard + A soft − Bsoft

η=

εLUMO − εHOMO 2

(1)

Adopting the eq 2 and 5, the maximum hardness in terms of molecular chemical hardness is found against η = 0.98, 1.21, 1.86, and 2.14 to be Yh = 0.48, 0.66, 0.86 and 0.89, respectively (see Figure 4b). Thus, TiCl4 and SnCl4 can be categorized as soft and borderline Lewis acid in reference to NbS2 whereas GaCl3 and AlCl3 are classified as hard acids. Correlating the experimental results and theoretical approximations, we found that the reactivity between NbS2 and classical Lewis acids properly conform to the typical hard/soft Lewis acid/base interactions and the degree of reactivity relies on the frontier molecular orbital (FMO) interaction between the HOMO of Lewis basic TMCs and the LUMO of Lewis acids. One open question is why the sulfides in the basal plane are attacked first by the Lewis acids, whereas the edges that are generally believed to be more reactive remain mostly unreactive at the beginning of the reaction.33 The quantum chemical slab model consists of metal-terminated Nb-zigzag (Nb-zz) clusters exhibiting straight edges without kink sites as in S-zz triangles,34 giving rise to two distinctly different metal sites on the edges with a coordination number (CN) of 4 and in the basal plane with a CN of 6, as illustrated in Figure 4c. Our model calculations suggest that the electrostatic potential is most negative at the center of a triangular slab (Figure 4d and Figure S4). The negative electrostatic potential indicates that Lewis acids are generally attracted to the localized center of a nanosheet over the edge and vertex sites, whereas external

(2)

The Yh is initially derived from quantifying the bond stability from the differences between the hard−hard (η/η) and soft− hard (S/η) ratio, as shown in eq 3.28 Yh =

η−S η S = − η η η

(3)

S refers the chemical softness index that is the inverse of the chemical hardness (eq 4). S=

1 2η

(4)

With that, the Yh can be simplified as Yh = 1 −

1 2η2

(5)

The physicochemical meaning of maximum hardness index can be understood as prescribing the quantum elucidation of the nature of bonding between the hard and soft in the acid/base chemical reactions against the domains of chemical hardness (η). Hard acids can be classified as Yh ≥ 0.5 and η ≥ 1 and soft acids can be defined to be characterized by 0 ≤ Yh < 0.5 and 1/ √2 ≤ η < 1, as follows: E


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Figure 6. General applicability of the localized activation on the basal plane of 2D layered TMCs. (a) Illustration of colloidal 2H-MoS2 nanosheets and its ball-and-stick model. (b) High-magnification TEM images of 2H-MoS2 nanosheets transforming into the porous structure by AlCl3 in (i) 0 h and (ii) 1 h. (c) Illustration of colloidal 1T-TiS2 nanosheets and its ball-and-stick model. (d) High-magnification TEM images of 1T-TiS2 nanosheets transforming into the porous structure by AlCl3 in (i) 0 and (ii) 1 h. Scale bars, 50 nm (b), 20 nm (d).

the basal plane are generally operative for 2D layered TMCs regardless of its chemical composition and crystal geometry. Different chemical compositions of 2H-MoS239 exhibited similar trends when multiple pores are generated in the basal plane with an average diameter of 1.3 nm upon treatment of AlCl3 at 280 °C for 1 h (Figure 6a and Figure 6b). In addition, such process is also applicable for group-IV 1T-TMCs where hexagon-shape of 1T-TiS2 with hexagonal shape with edge length and thickness of 30 and 6.9 nm is tested (Figure 6c).40,41 At 280 °C in 3 h with AlCl3 as a reaction promoter, nanopores with an average diameter of 3.1 (±0.3) nm are formed in the center of basal plane of TiS2 nanosheets (Figure 6d). The nanopore-MoS2 and TiS2 conserved its original 2H and 1T crystal geometry confirmed by Raman spectroscopy (Figure S7). Electrochemical Energy Storage of NbS2 Nanosheets. Miniaturization into nanoscale size and porosity generated by chemical activations are routes to invoke high surface area and facile electrolyte diffusion pathways, which are known strategies for enhancing the electrochemical energy storage property of 2D layered materials (Figure 7a).9,42 Cyclic voltammogram (CV) of as-synthesized and porous NbS2 nanosheetssolid-NbS2 (0 h), nanopore-NbS2 (1 h) and fully corroded-NbS2 (2 h)were recorded at scan rates ranging from 10 to 500 mV/s, yielding areal capacitance of 175, 302, and 115 mF/cm2 at 10 mV/s, respectively. A linear dependence of the discharge current on the scan rate was observed up to 200 mV/s (Figure 7b). Brunauer−Emmett− Teller (BET) method was employed to determine the surface area of the solid-, nanopore- and corroded-NbS2 nanosheets, yielding 50.3, 86.6, and 89.1 m2/g, respectively (Figure S8). Despite the larger surface area of corroded-NbS2 nanosheets, the decrease in the capacitance value is possibly attributed to the collapsed charge storage areas of excessively etched regions and poor crystallinity of nanosheets. Figure 7c shows galvanostatic charge/discharge measurement performed at 1 A/g, and the obtained curves were close to the ideal triangular capacitive cycles. The specific capacitance values can be derived from the charge/discharge curves (Figure 7d). The

Lewis bases will prefer the edges, where the electrostatic potential is positive. To further clarify the preferential activation of sulfides on the localized region of the basal plane rather than the edge by AlCl3 in the first activation stage, the bond formation energies of AlCl3 at various sulfides in NbS2 cluster model for the center, edge and vertex location were calculated as −46.1, −33.7 and −37.7 kcal/mol, respectively (Figure S5). Calculated Mechanism. DFT calculations indicate that the most plausible mechanism for the reactivity in the basal plane involves the initial electrophilic addition of the AlCl3 to the sulfide, followed by transmetalation,35,36 as highlighted in Figure 1b. This attack forms an Al−S bond and disrupts the bonding in the [Nb−S−Nb] subunit, injecting a partial positive charge into the activated crystal lattice, labeled as intermediates 2 and 3 in Figure 5. The dianionic sulfide ion is extracted and replaced formally by a chloride anion in the process. Our calculations indicate that this step is assisted by a dithiolate formation in intermediate 3. Energetically, the initial adduct formation is 25.4 kcal/mol downhill and the chloride injection accompanied by dithiolate formation is found at a relative energy of only 3.5 kcal/mol. Next, the bond between chloride and the aluminum metal cleaves as another sulfide attacks the Al center to afford intermediate 4. Among the many possible pathways that we considered, the lowest involves the addition of another equivalent of AlCl3 to the dithiolate to form intermediate 5, which is calculated to be at a relative energy of −32.4 kcal/mol. The two aluminum centers work in concert to cleave the dithiolate bond and extract the sulfide anion, as illustrated in 6. The anionic chloride ion that was injected into the lattice in exchange for the dianionic sulfide ion is unable to support the structural integrity, of course, and acts as a defect site that can invite further reactions,37,38 ultimately leading to a nanopore, as illustrated in Figure 5. We were able to observe the byproducts anticipated from this mechanism such as the NbCl5 and Al2SCl5− by ESI-MS (Figure S6). Chemical Activation of other 2D TMC Nanosheets. Lewis acids promoted pore formations in the localized areas of F


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Figure 7. Electrochemical characterization of solid-, nanopore- and corroded-NbS2 nanosheets. (a) Scheme that describes the miniaturization and pore generation in 2D nanostructures to afford enlarged surface areas and allow multiple electrolyte ion diffusion pathways which accounts for the significant enhancement in electrochemical energy storage capability. (b) Cyclic voltammetry curves (black: solid-, red: nanopore-, blue: corrodedNbS2 nanosheets) in aqueous 1 M KCl electrolytes. (c) Discharge currents over a range of scan rates from 10 to 500 mV/s at −0.2 V. (d) Galvanostatic charge/discharge curves of NbS2 nanosheets at a current density of 1 A/g. (e) Specific capacitance calculated at various current densities from 0.5 to 10 A/g.

A capacitor electrode fabricated from nanopore-NbS2 was durable in continued operations, enduring greater than 2000 charge/discharge cycles at a current rate of 2 A/g with greater than 96% capacity retention (Figure S10). This result shows that NbS2 possesses a high intrinsic potential for usage in nextgeneration charge storage electrodes and further combined optimizations on wide potential range electrolytes, electrode preparation and sophisticated pore architecture engineering will pave a new direction in the future.

nanopore-NbS2 exhibited high gravimetric capacitance of 553 F/g at a current density of 0.5 A/g and maintained a high value of 313 F/g in fast cycles at 10 A/g. In comparison, the gravimetric capacitances of solid-NbS2 and corroded-NbS2 were 388 and 200 F/g at 0.5 A/g and 128 and 66 F/g at 10 A/g, respectively (Figure 7e). The ion transport properties of solid-, nanopore- and corroded-NbS2 nanosheets were also measured by using electrochemical impedance spectroscopy (EIS) (Figure S9). The vacancies surrounding the nanopores can serve as thermodynamically favorable adsorption sites for ionic species and generate additional gap states for extra charge storages.4 Although these effects may be relevant, more detailed studies are required to confirm their role in group V TMC.

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CONCLUSION In summary, we observed that Lewis acids can show selective reactivity in surface activation of the basal plane of freestanding 2D TMCs. On the basis of the experimental G


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The suspension (5 μL) was then drop coated onto a glassy carbon disk and dried thoroughly at room temperature for 2 h in Ar atmosphere. This procedure was repeated several times to increase the loading amount. Then, 5 μL of 1% Nafion was cast on the electrode surface to maintain the stability of the electrode. 1.0 M KCl aqueous solution was used as the electrolyte at room temperature. The potential range for cyclic voltammetry (CV) and the galvanostatic charge−discharge test is from −0.5 to 0.1 V. The potential scan rate for CV was kept at 10, 20, 50, 100, 200, and 500 mV/s, respectively. Computational Details. All calculations for the mechanism of selective activation were performed with the Vienna ab initio simulation package (VASP).45,46 The generalized gradient approximation with Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional (GGA-PBE),47 and the electron−ion interaction with projector augmented wave (PAW) pseudopotentials48 were used. We employed 500 eV as an energy cutoff for the plane wave part of the wave function, a gamma k-point grid of (3 × 3 × 1), and the DFT-D3 method of Grimme49 to consider the van der Waals (vdWs) interaction in structural modeling for the mechanism of selective activation. In geometry optimizations, the total energy was converged to 10−4 eV and the forces on each relaxed atom were less than 0.001 eV/Å. The vacuum space was set to be at least 10 Å to prevent the interactive effect between copies of the replica within the periodic boundaries. The calculations for AlCl3, GaCl3, SnCl4, and TiCl4 were performed using density functional theory50 (DFT) as implemented in the Jaguar 9.1 suite51 of ab initio quantum chemistry programs. The geometry optimizations were performed with the B3LYP52−55 hybrid exchange and correlation functional. The 6-31G** basis set was used for atoms,56 except for the heavy atoms, which were represented by the Los Alamos LACVP basis set including relativistic effective core potential.57−59 The HOMO and LUMO energies of the optimized structures were reevaluated by single-point calculations using Dunning’s correlation consistent triple-ζ basis set cc-pVTZ(-f)60 that includes a double set of polarization functions. The electrostatic potential (ESP) calculations for 7-, 8-, and 9-line NbS2 slab models that geometrically optimized by VASP were performed by single-point calculations with the PBE/6-31G** level of theory47,61 with niobium represented by the Los Alamos LACVP basis set including relativistic effective core potential57−59 in Jaguar 9.1.51 ESP maps for the several NbS2 slab models were obtained using the Jmol62 software.

observations and computational studies, we conclude that the sulfides of the basal plane in 2D layered TMCs act as soft Lewis bases. By judiciously choosing an external Lewis acid and the reaction time, it is feasible to control the structural dissolution as shown by the appearance of the nanopores during the initial reaction stage. Such chemically activated 2D layered TMCs showed a prominent electrochemical storage capability attracting future efforts. With this convenient, conceptually simple and general methodology in hand, several new directions of research and development can be envisioned immediately. For example, Lewis acid promoted reactions in 2D layered TMCs may enable uninterrupted expansion of 2D layered TMCs targeting more versatile, selective and mild surface activation techniques.43,44 The detailed molecular level understanding of surface reactions on 2D nanostructures that we disclosed here will open new avenues for designing orthogonal 2D materials chemistry in surface modifications, activations, chemical doping, and new structure formations, which are essential for better and undiscovered physicochemical properties.

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METHODS

Chemicals and Materials. All of the chemical reagents are obtained from commercial sources (Sigma-Aldrich) and were used without any further purifications. Synthetic Protocol for NbS2 Nanosheets. NbCl5 (0.023 g, 0.086 mmol), oleylamine (5.0 g, 18.7 mmol), and 1-dodecanethiol (0.14 g, 0.688 mmol) are added to a 25 mL three-neck round-bottom flask in a glovebox. The reaction mixture is first heated to 280 °C and maintained for additional 30 min. Then, the reaction is stopped and the mixture is cooled at room temperature. By addition of excess toluene, the resulting NbS2 nanosheets are precipitated and washed for several times by centrifugation. Synthetic Protocol for Group-IV TiS2 and VI MoS2 Nanosheets. TiS2: The colloidal TiS2 nanodiscs were synthesized using the previously reported solution based synthetic method.40 MoS2: Commercially available bulk MoS2 (Sigma-Aldrich) was sonicated in N-methyl-2-pyrrolidone (NMP) as previously reported by Coleman et al.39 The resulting solution went through the cascade centrifugations at 500 and 3000 rpm to isolate exfoliated MoS2 nanosheets. After centrifugations, the supernatant was purified with CHCl3 several times to remove NMP residues. Lewis Acids Treatment. As synthesized NbS2 nanosheets (0.010 g, 0.065 mmol), 1-octadecene (5.0 g, 19.8 mmol), and Lewis acids (0.12 mmol) are added to a 25 mL three-neck round-bottom flask under in a glovebox. The reaction mixture is first heated to the designated temperature of either 50 or 120 °C and maintained for extended period of time. Then, the reaction is stopped and the mixture is cooled at room temperature. By the addition of excess methanol and chloroform, the resulting NbS2 nanosheets are precipitated and washed several times by centrifugation. For TiS2 and MoS2 nanosheets, the identical reaction conditions are carried out except the reaction temperature at 280 °C for 3 h. Instruments. TEM and HRTEM analyses are performed using a JEM 2100 at 200 kV and ARM1300S at 1250 kV, respectively. Atomic force microscopy (AFM) images are obtained in a noncontact mode using a scanning probe microscope (Veeco). X-ray diffraction (XRD) studies are conducted using a Rigaku-G2005304 equipped with a Cu kα radiation source (30 kV, 15 mA). ESI-MS spectra are obtained with 1290 LC (Agilent) in a standard procedure for MS detection. The electrochemical performance of the samples was evaluated with Princeton Versastat 3. Electrochemical Characterization. To evaluate the electrochemical properties of the samples, working electrodes in a threeelectrode configuration were fabricated using the following procedure: a sample suspension with a concentration of 2 mg/mL was prepared by sonication to disperse the as-prepared powder samples in toluene.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b05477. Additional experimental results, Cartesian coordinates of DFT-optimized structures, and computational details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Jae Hyo Han: 0000-0003-1905-6971 Hong Ki Kim: 0000-0002-6115-384X Hyun S. Ahn: 0000-0002-6014-0916 Mu-Hyun Baik: 0000-0002-8832-8187 Jinwoo Cheon: 0000-0001-8948-5929 Author Contributions ∥

J.H.H. and H.K.K. contributed equally to this work.

Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS-R026-D1), (IBS-R10-D1) and S. J. Yoo for TEM analyses [KBSI-HVEM (JEM-ARM1300S)] in Korea.

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