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Structure and Dynamics of Ionic Liquids Confined in Amorphous Porous Chalcogenides Guido Ori,†,‡,§ Carlo Massobrio,§ Annie Pradel,† Michel Ribes,† and Benoit Coasne*,†,‡,⊥ †

Institut Charles Gerhardt Montpellier, CNRS-UMR 5253, ENSCM, Université Montpellier, Place Eugène Bataillon 34095 Montpellier Cedex 5, France ‡ Multiscale Materials Science for Energy and Environment 2, UMI 3466 CNRS−MIT, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § Institut de Physique et de Chimie des Matériaux de Strasbourg, Université de Strasbourg and CNRS UMR 7504, 23 rue du Loess, F-67034 Strasbourg Cedex 2, France ⊥ Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 United States S Supporting Information *

ABSTRACT: Besides the abundant literature on ionic liquids in porous silica and carbon, the confinement of such intriguing liquids in porous chalcogenides has received very little attention. Here, molecular simulation is employed to study the structural and dynamical properties of a typical ionic liquid confined in a realistic molecular model of amorphous chalcogenide with various pore sizes and surface chemistries. Using molecular dynamics in the isobaric−isothermal (NPT) ensemble, we consider confinement conditions relevant to real samples. Both the structure and self-dynamics of the confined phase are found to depend on the surface-to-volume ratio of the host confining material. Consequently, most properties of the confined ionic liquid can be written as a linear combination of surface and bulk-like contributions, arising from the ions in contact with the surface and the ions in the pore center, respectively. On the other hand, collective dynamical properties such as the ionic conductivity remain close to their bulk counterpart and almost insensitive to pore size and surface chemistry. These results, which are in fair agreement with available experimental data, provide a basis for the development of novel applications using hybrid organic−inorganic solids consisting of ionic liquids confined in porous chalcogenides.

1. INTRODUCTION Amorphous porous chalcogenides (chalcogels) are considered promising materials for applications in catalysis, gas separation, electrochemistry, and heavy metal remediation.1−5 Nevertheless, the synthesis of porous chalcogenides still remains a challenge, with current strategies relying on high temperature treatments, corrosive gases, and long processing times.6−8 Another difficulty in the synthesis of porous chalcogenides concerns their stability in water solutions with nonconclusive experiments reported in the literature.7,9 As an alternative to water, ionic liquids (IL) are receiving increasing attention as solvents for the synthesis of porous chalcogenides.10−12 In addition to being versatile solvents with a large range of chemical compositions and physicochemical properties, they allow synthesizing hybrid solids made up of chalcogels filled with IL which combine the unique properties of the IL and chalcogenide elements. Such hybrid materials are of particular interest as high-voltage supercapacitors that combine the electrochemical properties of the IL and those of a host solid matrix.13,14 © 2015 American Chemical Society

A large number of experimental and theoretical works on confined IL have considered silica and carbon matrices.15−18 As in the case of molecular liquids, these works have shown that the physics of the confined IL departs from the bulk with drastically modified structural and dynamical properties and shifted phase transitions. Besides silica and carbon materials, very little is known about the physics and chemistry of hybrid materials made up of IL confined in porous chalcogels, and the specific interactions between chalcogenide surfaces and IL remain largely unexplored.10−12,19,20 Despite a few experimental papers on the synthesis of nanoporous chalcogenides, there is no theoretical/molecular simulation study on the specific physical interactions between chalcogenide and IL. Consequently, considering the promising properties of chalcogenide systems and IL, it is of considerable interest to identify key variables such as the surface/volume ratio, etc. Identifying Received: March 17, 2015 Revised: May 30, 2015 Published: June 1, 2015 6742

DOI: 10.1021/acs.langmuir.5b00982 Langmuir 2015, 31, 6742−6751

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Langmuir

The initial GeS2 surface consists of a slab of 480 atoms and has a size of 2.358 × 2.358 × 2.358 nm3. We replicated this slab 2 × 2 times along the xy plane, while z is the direction normal to the surface. By inserting the slab having a thickness Δz in a simulation box of a size hz, one defines a slit pore of a width H = hz −Δz thanks to the use of periodic boundary conditions. More precisely, we took the pore width H as the distance between the mean positions of the S and Ge atoms from opposite surfaces. We prepared systems with H = 2.4, 3.6, and 4.8 nm in order to obtain GeS2 slit-like nanopores with different widths. For each pore width, two surfaces chemistries were considered: bare and hydrogenated GeS2 surfaces. To build a hydrogenated GeS2 surface, the undercoordinated S atoms (S atoms with less than two neighbors) located at a distance 3 nm).51 As suggested in ref 51, the choice made for the force field also affects the orientational ordering of the confined IL. The above picture suggests that many structural properties of the confined IL, such as the conformation of the anion, can be described as a linear combination of two populations. The amount of trans conformers in a pore of a size H can be estimated as Q (H ) = Q bulk +

2δ (Q surf − Q bulk ) H

Figure 5. Percentage of the IL anion trans conformer in GeS2 pores as a function of pore width H: (circles) bare and (triangles) hydrogenated. The solid black line shows the percentage of the IL anion trans conformer obtained by simulation for bulk IL. The solid red and dashed blue lines are the predictions from the two population model described in the text for the bare and hydrogenated pores, respectively.

MSD) were calculated for both the IL cation and anion along the xy plane parallel to the chalcogenide surfaces. Figure 6

(4)

where δ is the thickness of the vicinal IL layer with properties modified by the presence of the GeS2 surface. Figure 5 displays a fit of eq 4 against the amount of trans conformers versus the pore size H. We obtain Qsurf = 39 ± 8% of trans conformers in the vicinity of the bare GeS2 surface (Qsurf = 35 ± 7% for the hydrogenated GeS2 surface) and Qbulk = 93 ± 9% as the percentage of trans conformation of the IL anion in the bulk and δ ∼ 1.1 nm. Equation 4 is in very good agreement with the molecular simulation data. This shows that the structure of the IL confined in GeS2 pores can be fitted against a simple model in which structural properties are a function of the surface-tovolume ratio of the host confining material (surface and volume hereafter refer to the surface and volume of the porosity, i.e., the specific surface area and porous volume). 3.2. Dynamical Properties. The dynamics of the confined IL was investigated by means of mean square displacements and ionic conductivity. 2D-mean square displacements (2D-

Figure 6. 2D-mean square displacements of the center of mass for IL cation (top) and anion (bottom) for different pore widths H = 2.4 nm, red; 3.6 nm, blue; 4.8 nm, black. We show the data for the bare and hydrogenated GeS2 surfaces (solid and dashed lines, respectively) as well as the data for the bulk IL (yellow lines). The gray lines represent the power law expected for the Fickian diffusion (MSD ∼ t).

displays 2D-MSD for the IL as a function of the surface chemistry for the three pore sizes H. The 2D-MSD rapidly increases within the first few ps (ballistic regime) and then increases less rapidly at larger times as the dynamics of the IL cation and anion becomes close to the Fickian regime (where MSD ∼ t). The 2D-MSD for the confined IL is smaller than for the bulk due to confinement and surface forces. This is corroborated by the fact that a slowing down of the selfdynamics is observed for both the cation and anion upon decreasing the pore size. For all pore H, the IL anion is slower 6748

DOI: 10.1021/acs.langmuir.5b00982 Langmuir 2015, 31, 6742−6751

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Figure 7. 2D-mean square displacements (2D-MSD) of the centers of mass for the IL cation (circles) and anion (triangles) in the xy plane parallel to the chalcogenide surface. We show the 2D-MSD data calculated over a time step of 3.5 ns for both the bare (left) and hydrogenated surfaces (right). The solid lines are the two population model (see text). The dashed line is the 2D-MSD for the bulk IL.

a time t, and N = Ncat + Nan the total number of ions. The total ionic conductivity S of the confined IL is shown in Table 2 for

than the cation as previously observed for the same IL confined in silica pores.24,35 This result also agrees with experimental data for the same bulk IL, showing that the cation is faster than the anion. The faster cation dynamics is due to the homogeneous electrostatic field around this ion,24 which leads to weaker electrostatic interactions with its surrounding and its structural flexibility due to the alkyl chain and planar imidazolium ring. Comparison between data for the three pore sizes shows that the hydrogenation of the surface induces a slowing down of the IL in comparison with the bare chalcogenide surface. This effect is of the same order of magnitude for the IL cation and anion. As discussed above, the presence of H atoms at the surface of GeS2 promotes a stronger interaction with the IL anion. This affects the dynamics of both the IL cation and anion. For the pore with H = 4.8 nm, the IL reaches a regime close to the Fickian regime after ∼2 ns. In contrast, for the smallest pores, 4 ns were not enough to reach the Fickian regime. Because of the large viscosity of the IL at room temperature, extended simulations of hundreds of ns are typically necessary to reach the Fickian regime.60 In particular, because of the absence of Fickian regime on the time scale of our simulations, we decided to probe the self-dynamics of the IL upon confinement by estimating the 2D-MSD at 3.5 ns, i.e. ⟨Δrt=3.5ns⟩. Figure 7 shows ⟨Δrt=3.5ns⟩ for the IL cation and anion versus the pore size H. In the spirit of the treatment of the structural data, we compare the data in Figure 7 with the two population models described above; we assume that ⟨Δrt=3.5ns⟩ is the weighted distributions of ⟨Δrt=3.5ns⟩ in the bulk and surface regions. Again, we define the surface region of the confined IL as the zone at a distance smaller than δ from the surfaces. This simple model quantitatively describes the average dynamical quantities obtained by means of MD simulations (compare the blue and red lines in Figure 7). We also computed the ionic conductivity of the confined IL in the framework of the Green−Kubo formalism: S=

1 3kBTV

∫0

∞

Table 2. Ionic Conductivity S for Bulk IL and IL Confined in Bare and Hydrogenated GeS2 Pores of a Width Ha bare GeS2

a

H, nm

2.4

3.6

4.8

2.4

3.6

4.8

bulk

S (mS/cm)

21

25

17

23

16

22

21

The maximum error is ±8 mS/cm.

different pore sizes H and surface chemistries (we also show the bulk ionic conductivity). S for the confined IL is found to be close to its bulk counterpart. Although the self-dynamics of the IL becomes slower upon confinement, this last result shows that the ionic conductivity at 300 K is not very sensitive to confinement and surface interactions. This result can be ascribed to the fact that ionic conductivity is a collective motion effect (which is less sensitive to confinement than the self-dynamics). Finally, this result agrees with experimental data showing that the conductivity at temperature below or close to room temperature of an ionogel (50% of IL loading) is close to the bulk ionic conductivity.16

4. CONCLUSION Molecular dynamics of a typical IL trapped in amorphous chalcogenide pores are performed in order to shed light on this new class of hybrid materials. Both the structural and dynamical properties of the confined IL are found to be modified because of confinement, surface chemistry, and reduced dimension of the system. Despite the significant screening of the Coulombic interactions due to the small Debye length in IL, the structural properties of the confined IL are mostly governed by electrostatic interactions. However, owing to their different roughness and charge surface properties with respect to silica, GeS2 surfaces affect the properties of the confined IL to a lesser extent. We also show in this work that the hydrogenation of the GeS2 surface affects both the structural and dynamical properties of the confined IL. This effect is due to the stronger interaction of the IL anion with the hydrogen atoms of chalcogenide surfaces. On the other hand, owing to its collective nature, the ionic conductivity of the IL confined at room temperature is similar to the bulk ionic conductivity for both the bare and hydrogenated surfaces.

N

⟨J ⃗ (t )J ⃗ (t0)⟩ dt with J ⃗ (t ) =

hydrogenated GeS2

∑ qivi⃗(t ) i=1

(5)

In this equation, kB is Boltzmann constant while T is the temperature and V the volume occupied by the IL. J(t) is the charge current while qcat = +1 and qan = −1 are the cation and anion charges, vi⃗ (t) the center-of-mass velocity of the ith ion at 6749

DOI: 10.1021/acs.langmuir.5b00982 Langmuir 2015, 31, 6742−6751

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(4) Bag, S.; Trikalitis, P. N.; Chupas, P. J.; Armatas, G. A.; Kanatzidis, M. G. Porous Semiconducting Gels and Aerogels from Chalcogenide Clusters. Science 2007, 317, 490−493. (5) Riley, B. J.; Chun, J.; Ryan, J. V.; Matyás,̌ J.; Li, X. S.; Matson, D. W.; Sundaram, S. K.; Strachan, D. M.; Vienna, J. D. Chalcogen-Based Aerogels as a Multifunctional Platform for Remediation of Radioactive Iodine. RSC Adv. 2011, 1, 1704−15172. (6) Xu, J.; Almeida, R. M. Preparation and Characterization of Germanium Sulfide Based Sol-Gel Planar Waveguides. J. Sol−Gel Sci. Technol. 2000, 19, 243−248. (7) Kalebaila, K. K.; Georgiev, D. G.; Brock, S. L. Synthesis and Characterization of Germanium Sulfide Aerogels. J. Non-Cryst. Solids 2006, 352, 232−240. (8) Vaughn, D. D.; Patel, R. J.; Hickner, M. A.; Schaak, R. E. SingleCrystal Colloidal Nanosheets of GeS and GeSe. J. Am. Chem. Soc. 2010, 132, 15170−15172. (9) Popescu, M. A. Non-Crystalline Chalcogenides; Kluwer Academic Publishers: New York, 2000; p 8. (10) Murugesan, S.; Kearns, P.; Stevenson, K. J. Electrochemical Deposition of Germanium Sulfide from Room-Temperature Ionic Liquids and Subsequent Ag Doping in an Aqueous Solution. Langmuir 2012, 28, 5513−5517. (11) Murugesan, S.; Akkineni, A.; Chou, B. P.; Glaz, M. S.; Vanden Bout, D. A.; Stevenson, K. J. Room Temperature Electrodeposition of Molybdenum Sulfide for Catalytic and Photoluminescence Applications. ACS Nano 2013, 7, 8199−8205. (12) Leyral, G.; Ribes, M.; Courthéoux, L.; Uzio, D.; Pradel, A. Synthesis and Structuring of NiMoS2 by Using an Ionic Liquid. Eur. J. Inorg. Chem. 2012, 2012, 4967−4971. (13) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nature Mater. 2008, 7, 845−854. (14) Merlet, C.; Rotenberg, B.; Madden, P. A.; Taberna, P.-L.; Simon, P.; Gogotsi, Y.; Salanne, M. On the Molecular Origin of Supercapacitance in Nanoporous Carbon Electrodes. Nature Mater. 2012, 11, 306−310. (15) Cerclier, C.; Zanotti, J.-M.; Embs, J.; le Bideau, J. Lithium Based Ionogel as Solid State Electrolyte: Dynamics of Confined Ionic Liquid, a Neutron Diffusion Study. ECS Trans. 2014, 64, 27−31. (16) Néouze, M.-A.; le Bideau, J.; Gaveau, P.; Bellayer, S.; Vioux, A. Ionogels, New Materials Arising from the Confinement of Ionic Liquids within Silica-Derived Networks. Chem. Mater. 2006, 17, 3931−3936. (17) Lynden-Bell, R. M.; del Pópolo, M. G.; Youngs, T. G. A.; Kohanoff, J.; Hanke, C. G.; Harper, J. B.; Pinilla, C. C. Simulations of Ionic Liquids, Solutions, and Surfaces. Acc. Chem. Res. 2007, 40, 1138− 1145. (18) Singh, R.; Monk, J.; Hung, F. R. A Computational Study of the Behavior of the Ionic Liquid [BMIM+][PF−6 ]Confined Inside Multiwalled Carbon Nanotubes. J. Phys. Chem. C 2010, 114, 15478−15485. (19) Zhang, Q.; Chung, I.; Jang, I. J.; Ketterson, J. B.; Kanatzidis, M. G. Chalcogenide Chemistry in Ionic Liquids: Nonlinear Optical WaveMixing Properties of the Double-Cubane Compound [Sb7S8Br2](AlCl4)3. J. Am. Chem. Soc. 2009, 131, 9896−9897. (20) Mathiaud, R.; Courthéoux, L.; Silly, G.; Albadri, H.; Levillain, J.; Gaumont, A.-C.; Ribes, M.; Pradel, A. Tuning the Morphology of GeS2 Hybrid Materials Using Ionic Liquids as Structuring Agents. Eur. J. Inorg. Chem. 2014, 36, 6232−6238. (21) Baldelli, S. Interfacial Structure of Room-Temperature Ionic Liquids at the Solid−Liquid Interface as Probed by Sum Frequency Generation Spectroscopy. J. Phys. Chem. Lett. 2013, 4, 244−252. (22) Feng, G.; Cummings, P. T. Supercapacitor Capacitance Exhibits Oscillatory Behavior as a Function of Nanopore Size. J. Phys. Chem. Lett. 2011, 2, 2859−2864. (23) Singh, R.; Monk, J.; Hung, F. R. Heterogeneity in the Dynamics of the Ionic Liquid [BMIM+][PF6−]Confined in a Slit Nanopore. J. Phys. Chem. C 2011, 115, 16544−16554. (24) Coasne, B.; Viau, L.; Vioux, A. Loading-Controlled Stiffening in Nanoconfined Ionic Liquids. J. Phys. Chem. Lett. 2011, 2, 1150−1154.

This study highlights the behavior of IL in organic/inorganic (hybrid) materials by establishing that many structural and dynamical properties can be predicted from the surface-tovolume ratio of the host chalcogenide matrix. Important features such as the orientational ordering and conformation of the IL as well as its self-dynamics can be qualitatively estimated as a linear combination of surface and bulk-like contributions. While further work is required to evaluate the effect of other variables such as temperature, the present results contribute to a better understanding of the properties of IL confined in amorphous porous chalcogenides together with an improved assessment of the role of the surface-to-volume ratio of the host chalcogenide. The present work shows that IL confined in nanoporous chalcogenide materials share many features with those confined in more conventional matrices (silica, carbon, etc.). On the one hand, such similarities were expected as they are inherent to confinement and surface interaction effects. On the other hand, we believe that this paper highlights differences that can be expected for chalcogenide materials with respect to other materials. In fine, while more work is needed to better describe the behavior of confined IL, one should be able to build a simple, general model of IL confined in porous media as a function of a very limited set of parameters (surface energy, pore size or surface to volume ratio, temperature, etc.).

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ASSOCIATED CONTENT

S Supporting Information *

Details of the systems and force field parameters employed in this work. g(r) functions between the IL and the GeS2 surface and the calculated Ge(S)−S−H angle distributions of the GexSyHz clusters. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.langmuir.5b00982.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the French Research Agency ANR within the framework of the project Nanochalco (ANR-11BS08-015). This work was also supported by the ICoME2 Labex (ANR-11-LABX-0053) and the A*MIDEX projects (ANR-11-IDEX-0001-02) cofunded by the French program “Investissements d’Avenir” which is managed by the ANR. We acknowledge the Centre Informatique National de l’Enseignement Supérieur (CINES) for a generous allocation on the supercomputer YODA (projects: c2012096964 and c2013096964).

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DOI: 10.1021/acs.langmuir.5b00982 Langmuir 2015, 31, 6742−6751