Folding of Graphene Nanostructures Driven by Ionic Liquids

Aug 20, 2014 - Vitaly V. Chaban , Eudes Eterno Fileti , and Oleg V. Prezhdo ... Taimoory , Volodymyr Semeniuchenko , Ganesh Kamath , John F. Trant...
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Folding of Graphene Nanostructures Driven by Ionic Liquids Nanodroplets Mert Atilhan† and Santiago Aparicio*,‡ †

Department of Chemical Engineering, Qatar University, Doha, Qatar Department of Chemistry, University of Burgos, 09001 Burgos, Spain

‡

ABSTRACT: The folding of graphene nanostructures driven by ionic liquid nanodroplets is studied by molecular dynamics simulations. Nanodroplets formed by nine different ionic liquids comprising alkylimidazolium, cholinium, and N-methylpiperazinium cations, paired with BF4, Tf2N, lactate, salicylate, and benzoate anions were studied. The interaction between these fluids and graphene nanoribbons was considered by the analysis of the effect of nanoribbons width and also complex geometries such as flower-like and planar graphene flakes. Likewise, the possibility of folding of graphene nanostructures supported on SiO2 was also analyzed. The folding mechanism driven by nine ionic liquids is analyzed showing the evolution of the different interaction energies. The reported results show suitable application of ionic liquid nanodroplets for controlling the folding of graphene nanoribbons, although those supported on SiO2 surfaces could not be folded by the studied systems.

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INTRODUCTION The attention attracted by graphene-based materials in the last years has led the development of graphene-based structures including complex geometries,1,2 which require new nanoengineering methods for a precise control of fabrication methods.3 Graphene nanoribbons (GNRs) are one-dimensional strips of graphene whose properties are different to those of large-area graphene sheets. GNRs can be prepared using several methods4 such as nanoscale peeling5 or lithographic patterning of graphene,6 cutting of carbon nanotubes using plasma etching7 or chemical attack,8 or bottom-up synthetic strategies using chemical vapor deposition9 and chemical synthesis.10 The properties of GNRs can be tuned considering their width and edge structure. Moreover, GNRs are a suitable material for preparing complex carbon nanostructures in bottom-up fabrication strategies11 improving the control of synthetic procedures. Likewise, GNRs show very interesting mechanisms of interaction with other nanostructures such as carbon nanotubes.12 Folded GNRs show exotic properties such as metal to half- metal transitions, edge states and zero-energy flat bands, which can be controlled through the stacking produced in the folding mechanism.13 GNRs can be used as nanofillers to improve the mechanical properties of polymeric nanocomposites and epoxy composites. Moreover, they are known to increase the mechanical properties of biodegradable polymeric nanocomposites of poly(propylene fumarate) at low weight percentage loading of oxidized GNRs. The mechanisms of GNRs bending, twisting, stretching, and folding have been analyzed both using experimental and computational approaches.14−21 Patra et al.22 proposed the use of water nanodroplets as a method for activating and guiding the folding of graphene nanoribbons or even of more complex structures such as planar or flower-like flakes. Their results showed that complex nanostructures such as knots or © 2014 American Chemical Society

nanocapsules can be created using appropriated nanodroplets, and controlled through the selection of suitable sizes of nanodroplets in comparison with GNRs width. Bellido and Seminario23 analyzed the GNRs folding mechanisms driven by water nanodroplets for free and supported GNRs, considering different substrates and GNRs geometries, and showed that folding can be produced and controlled through a suitable selection of GNRs geometry, substrate and droplet sizing. Catheline et al.24 analyzed folding driven by tetrahydrofuran (THF) using both transmission electronic microscopy and molecular dynamics simulations, confirming that it is activated and driven by THF solvent nanodroplets. Therefore, although it is technically challenging to control the placing of solvent nanodroplets on the different positions of GNRs and graphene flakes, the use of solvent nanodroplets is a very attractive alternative for developing functional nanodevices with novel properties, through the development of new carbon-based three-dimensional nanostructures.14 The use of low volatile solvents for the development of composites with individualized graphene flakes was remarked by Catheline et al.24 Ionic liquids (ILs) are considered in this work as fluids for nanodroplet driven folding of GNRs and graphene flakes. ILs are fluids with almost null volatility25 whose effect on GNRs folding was not previously studied, and thus, the advantages of nonevaporating nanodroplets need to be studied in detail. Likewise, the possibility of controlling ILs properties through the judicious combination of anions and cations among the huge quantity of available ILs26 would expand the properties of the used nanodroplets with regard to GNRs, in comparison with those based on common solvents. Received: March 6, 2014 Revised: July 30, 2014 Published: August 20, 2014 21081

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(e.g., imidazolium-based cations or BF4 anions). Important parameters such as the effect of nanodroplets size, GNR width, and shape with regard to the folding mechanism were studied.

The main objective of this work is to analyze the folding mechanisms of GNRs and graphene flakes driven by IL nanodroplets. The modeling and analysis of nanoscale phenomena in relationship with graphene related materials using molecular dynamics (MD) simulations have proven to be a powerful and useful approach, providing valuable information, which is unreachable using purely experimental approaches.27−31 Previously reported relevant molecular dynamics studies on the behavior of ILs on graphene sheets, and related systems such as carbon nanotubes, may be useful to analyze the ILs role as GNR folding agents. Zhou et al.32 carried out MD studies for 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([BMIM][Tf2N]) on uncharged graphene surface showing a highly dense first adsorbed layer, with anion−cation mixed layering, showing a strong interaction of ions with the substrate at least in the first adsorbed layer. Fedorov et al.33 analyzed the mechanism of interfacial behavior for 1,3-dimethylimidazolium chloride on neutral graphite showing a preferential adsorption of cations on the surface, which led to a highly dense adsorbed layer. The behavior of 1butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) on graphene was analyzed by Wang et al.34 showing again parallel arrangement of imidazolium cation on top of graphene surface and the appearance of up to four solid-like adsorbed layers. Zhao et al.35 proved through quantum chemistry and MD calculations the driving role of collective van der Waals interaction between graphene and alkylimidazolium-based ILs for graphene dispersion. Although most of the available studies were reported for imidazolium-based ILs, densification and layering in the vicinity of the graphene surface was also inferred for piperazinium-31 and cholinium-based36 ILs leading to solid-like layers for these adsorbed ions. Therefore, MD simulations were carried out in this work to analyze the behavior of ILs nanodroplets, considering nine different ILs. The selected ILs for the study were: cholinium lactate ([CH][LA]), cholinium benzoate ([CH][BE]), cholinium salicylate ([CH][SA]), N-methylpiperazinium lactate ([MP][LA]), methylpiperazinium benzoate ([MP][BE]), methylpiperazinium salicylate ([MP][SA]), 1-ethyl-3-methylimidazolium lactate ([EMIM][LA]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]), and 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([EMIM][Tf 2 N]), Figure 1. ILs were selected due to their environmentally friendly, low toxicity, and biocompatible ions nature (e.g., cholium or lactate) together with classical ions

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COMPUTATIONAL METHODS Force field parametrization for all the studied ILs, and graphene materials were reported in previous works.36−40 Classical molecular dynamics simulations were carried out using the MDynaMix v.5.0 molecular modeling package.41 Spherical nanodroplets, with diameters, D, equal to 2.5, 3.0, 3.5, 4.0, and 4.5 nm, were built, with the number of molecules for each nanodroplet fixed considering the experimental densities of each compound at 303 K and atmospheric pressure. The temperature selected for this study was slightly above the ambient one to improve ionic diffusion considering the large viscosity of some of the studied ILs, and thus, allowing good property sampling in the studied simulation time. NVT simulations at 303 K and 5 ns long were carried out for each isolated nanodroplet for equilibration purposes, which was ensured through constant values of total potential energy. Hydrogen-terminated GNRs with dimensions w × 15.0 nm2, with w = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 nm were built, Figure 2a. The selection of nanodroplet diameters and GNR widths was guided to analyze in a systematic way the relationships D− w on the GNRs folding mechanism.

Figure 2. Starting structures using for molecular dynamics simulations of ionic liquids on top of (a) GNRs (15.0 nm long and w wide), (b) rectangular flakes, and (c) flower-like flakes. [CH][LA] droplets shown in this figure, with [CH] cations in blue and [LA] anions in green.

Equilibrated nanodroplets were placed on top of each GNR (∼0.3 nm separation) at one end, whereas the other GNR end was fixed, to simulate GNR fixation to some kind of substrate.22 Two more complex geometries were considered: (i) structure F1, a flake composed of two 5.0 × 3.3 nm2 rectangular pieces joined by a 0.7 × 2.2 nm2 stripe (Figure 2b); and (ii) structure F2, a flower-like structure composed of four petals joined to a central flake totalling a 9.2 × 9.2 nm2 structure (Figure 2c). The structure and sizing of these nanostructures were selected for comparison purposes with previous literature studies on GNRs folding using water and organic solvents.22,23 In the case of F1

Figure 1. Molecular structures of ions involved in ionic liquids studied in this work. Atom color code: (gray) C, (white) H, (red) O, (blue) N, (cyan) F, (pink) B, and (yellow) S. 21082

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ns), 1.0 (at t = 0.89 ns), and 1.5 (at t = 0.92 ns) nm, for which GNRs fold, whereas for w = 2.0 nm, this effect is not obtained and, thus, GNR does not fold, Figure 3b. The behavior reported in Figure 3 for w = 0.5 nm is very different to those for w = 1.0 and 1.5 nm (which are almost exactly the same). ΔEGNR‑GNR is remarkably lower for w = 0.5 nm than for w = 1.0 and 1.5 nm, Figure 3a, which could be justified considering the structures after folding reported in Figure 3b. In the case of w = 0.5 nm nanodroplet, most of the ions remains confined inside of the folded GNR whereas this is not the case for w = 1.0 and 1.5 nm. Therefore, the strength of GNR-GNR interactions is lower in the case of w = 0.5 nm because the number of π−π GNR interactions is lower (a lower number of π-stacked aromatic rings). At the same time, the time required for folding is larger for w = 0.5 nm, because in the case of w = 1.0 and 1.5 nm ions are expelled from the confined droplet improving the folding mechanism. A closer view of the time evolution of ΔEGNR‑GNR allows analyzing the kinetics of the folding mechanism, Figure 4. This kinetic mechanism is analogous for all the cases obtained in this work in which folding is obtained, although the exact times for which each kinetic step appears depend on the droplet radius and GNR width. ΔEGNR‑GNR increases (in absolute value) in the first stages when GNR starts to wrap the IL nanodroplet (point (2) in Figure 4); after that, ΔEGNR‑GNR remains almost

and F2 structures, liquid nanodroplets were placed on top of the central stripe of F1 and on top of the central flake of F2, both at ∼0.3 nm separation. One of the ends of F1 was fixed along the simulations, whereas F2 was allowed to freely move. Folding for supported graphene was analyzed using a SiO2 substrate (8.2 × 11.2 × 1.0 nm3), F1 flake was placed on top of SiO2 at 0.3 nm of separation, and then nanodroplets were placed on top of F1. SiO2 substrate was fixed along the simulations, and the SiO 2 charges and Lennard-Jones parameters were obtained from Bellido and Seminario.23 Simulations for nanodroplets on top of GNRs, F1 (unsupported and SiO2 supported) and F2 nanostructures were carried out in the NVT ensemble for 5 ns, using 25.0 × 25.0 × 25.0 nm3 periodic cells for all the studied systems.

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RESULTS AND DISCUSSION Nanodroplets on Rectangular GNRs. The effect of GNRs width and ILs nanodroplets diameter on the mechanism of GNRs folding was analyzed in a first stage. For this purpose, simulations with [CH][LA] were carried out. For a [CH][LA] nanodroplet with D = 3.0 nm, the possibility of GNRs folding was analyzed using GNRs with w = 0.5, 1.0, 1.5, and 2.0 nm. The results showed GNRs folding for all the studied w with the exception of w = 2.0 nm. GNRs simulations carried out in absence of ILs nanodroplets do not lead to folding for the studied time frame (5 ns) for any of the studied GNRs. The folding of the studied ribbons should lead to a lower energy structure in comparison with unfolded ribbons, because of the π−π interactions, and thus, we have calculated the time evolution of the GNR interaction energy, ΔEGNR‑GNR, defined according to eq 1: ΔEGNR‐GNR = EGNR,withNANO(COUL + LJ) − EGNR,withoutNANO(COUL + LJ)

(1)

where EGNR,withNANO (COUL + LJ) and EGNR,withoutNANO (COUL + LJ) stand for the sum of Coulombic and van der Waals (Lennard-Jones) energy of GNR with and without IL nanodroplets, respectively. Therefore, ΔEGNR‑GNR may be used as a direct measurement of the folding of GNRs upon time evolution. Figure 3 shows ΔEGNR‑GNR as a function of w for [CH][LA] nanodroplet with D = 3.0 nm. The reported results show a sudden change in ΔEGNR‑GNR for w = 0.5 (at t = 1.15

Figure 3. (a) GNR interaction energy, ΔEGNR‑GNR, defined according to eq 1, as a function of simulation time, t, in the system [CH][LA] nanodroplet (D = 3.0 nm) + GNR, as a function of GNR width (w × 15.0 nm2); (b) structures for the same systems for t = 5 ns. All values obtained at 303 K. (b) [CH] cations in blue and [LA] anions in green.

Figure 4. GNR interaction energy, ΔEGNR‑GNR, defined according to eq 1, and the corresponding snapshots of the folding of a GNR with w = 1.5 nm activated by a [CH][LA] nanodroplet with D = 3.0 nm. Simulations at 303 K. [CH] cations in blue and [LA] anions in green. 21083

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For w = 2.0 nm, the [CH][LA] nanodroplet does not fold the GNR, but ions tend to spread on the GNR, covering almost the whole GNR length, and thus, anion−cation interaction is weakened because the system evolves from an almost spherical droplet to a film on top of the GNR. Therefore, GNRs folding leads to a weakening of anion−cation interactions, which should be balanced by ion−GNR new interactions to justify the ILs ability to fold the nanoribbons. The strength of ion−GNR interactions, and thus, ion−GNR interaction energies, split into Coulombic and Lennard-Jones (van der Waals) contributions, are reported for a [CH][LA] nanodroplet (D = 3.0 nm) on top of GNRs as a function of GNR width are reported in Figure 6. The interaction of [LA] anion with GNR for those cases in which folding is produced, w = 0.5, 1.0, and 1.5 nm, is characterized by an increase of anion-GNR Coulombic interaction energy, EA‑GNR, for the initial stages of the folding (simulation time < 0.9 ns), although in the case of w = 0.5 nm the changes are poorly defined because of the noisy character of the energy plots in that time frame. Then, sudden decreases are produced at roughly 0.9 ns (also poorly defined for w = 0.5 nm), when the extreme of the GNR touches the other side of the GNR for the first time, Figure 6a. These decreases lead to well-defined minima after which energy increases to an asymptotic value. In the case of cation−GNR Coulombic interaction energy, Figure 6b, energy increases (in absolute value) upon nanodroplet wrapping by the GNR, and once the GNR reaches the other side of the ribbon, it evolves toward a maxima, after which it decreases toward an asymptotic negative value. The case of ion-GNR Lennard-Jones interaction energies follow very similar trends both for anion and cation, although for a selected w, cation-GNR show larger (in absolute value) than A-GNR values, Figures 6c,d. Likewise, ion-GNR LennardJones contributions are characterized by a peak for those cases in which folding is produced, whereas in the case of ions spreading on GNRs (w = 2.0), Lennard-Jones decrease toward an asymptotic value once the GNR is covered by the ions layer. Therefore, in the case of folding, these results are in agreement with increasing ion-GNR interactions upon GNR wrapping, sudden changes when both layers of GNRs reach contact, and then, almost constant values of the properties once ions are expelled from the nanodroplet but reach new positions on the GNR surface, Figure 3. Therefore, the sudden changes of EA‑GNR and EC‑GNR, in the 1−2 ns time frame could be explained considering that ions are expelled from the confined droplet moving to the outer regions of the GNR, and being placed on top of the GNRs, and thus, leading to the increase (in absolute value) for all the ion−GNR interaction energies reported in Figure 6 for w = 1.0 and 1.5 nm. These changes are less remarkable for w = 0.5 because most of the ions remains confined inside the GNRs. In the case of w = 2.0 nm, as ions are spread on top of GNRs, without folding, ion−GNR interaction energies increase (in absolute value) for all the considered contribution, and thus, in parallel anion−cation interaction energy decrease (in absolute value) when ions spread on top of the nanoribbon (Figure 5). Figures 5 and 6 show that the balance between anion−cation interactions and ion−GNR interactions determine the ability of a nanodroplet, with a selected size, to fold a GNR or to spread on it. In the case of [CH][LA] with D = 3.0 nm, ion−GNR interactions increase with GNR width, Figure 6, whereas anion−cation decreases with increasing w, Figure 5, and thus, from a critical value of w, the weakening of anion−cation

constant, while the nanodroplet is being fully wrapped by the GNR (point (2) to point (3) in Figure 4). At point (3), the GNR extreme touches the other side of the GNR (folding), and ΔEGNR‑GNR increases (in absolute value) very quickly, while the GNR extreme moves toward the other extreme (point (3) to point (4)). In the region between points (4) and (5), the folded GNR compresses the IL nanodroplet increasing the number of stacked rings, and thus increasing (in absolute value) ΔEGNR‑GNR. Molecules from the IL nanodroplet start to leave the folded ribbons for the region between points (5) and (6), thus, allowing a better interaction between top and bottom layers of folded GNR, which leads to an increase of ΔEGNR‑GNR although less remarkable than for the region between points (4) and (5). Some of the IL molecules once leaving the region inside the folded ribbons are placed on top of the GNR, but this behavior does not affect the GNR−GNR interaction because it remains constant from point (6) to 5 ns, and thus, an almost constant number of ions remains confined between the top and bottom layers of the GNRs without allowing a further increase of GNR folding. Figures 3 and 4 show the driving role of ILs nanodroplet for GNRs folding. Therefore, it is useful to analyze the changes of ILs nanodroplets properties along the GNRs folding. Anion− cation interaction energy, EA‑C, for the [CH][LA] nanodroplet with D = 3.0 nm on GNRs as a function of GNR width is reported in Figure 5. In the case of very narrow GNRs, w = 0.5

Figure 5. Anion−cation interaction energy, EA‑C (summing Coulombic and Lennard-Jones contributions), in the system formed by a [CH][LA] nanodroplet with D = 3.0 nm on a GNR with dimensions w × 15.0 nm2. Simulations at 303 K.

nm, which are easily folded by the IL nanodroplet, the structure of the IL does not change remarkably upon GNR folding, and thus, anion−cation interaction is almost equal for the nanodroplet with folded GNR than for the nanodroplet in absence of the GNR (t = 0 in Figure 5). For w = 1.0 and 1.5 nm GNRs, which are also folded by the nanodroplet, the time evolution of EA‑C is very different to that for w = 0.5, especially for the first ns of the simulations, corresponding to the different folding mechanism reported in Figure 3, EA‑C first decreases (in absolute value) up to roughly 0.5 ns, and then, increases (in absolute value) up to 1 ns. Once the extreme of the GNRs touches the GNR, the EA‑C increases (in absolute value), very sharply reaching a minimum for the time when both GNR extremes are in contact (∼0.9 ns). After that, the folding mechanism that tends to increase the contact between the two layers of GNRs hinders anion−cation interactions, even many molecules are expelled, and thus, EA‑C decreases remarkably. 21084

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Figure 6. Anion−GNR interaction energy, EA‑GNR, and cation−GNR interaction energy, EC‑GNR, split in Coulombic and Lennard-Jones (LJ) contributions, in the system formed by a [CH][LA] nanodroplet with D = 3.0 nm on a GNR with dimensions w × 15.0 nm2. Simulations at 303 K. Panels (a) and (c) are for anion-GNR interactions, panels (b) and (d) are for cation-GNR.

Figure 7. Radial distribution functions, g(r), of GNR carbon atoms around the center-of-mass of (a) [LA] and (b) [CH], calculated for the reported time frames, in the system formed by a [CH][LA] nanodroplet with D = 3.0 nm on a GNR with dimensions 1.5 × 15.0 nm2. Simulations at 303 K.

interactions can be compensated only through the spreading of ions on the GNR surface, leading to a nonfolding behavior. The changes in [CH][LA] nanodroplets upon GNR wrapping and folding are also analyzed in Figure 7 using the distribution functions of GNR carbon atoms around the centerof-mass of the corresponding ions. Results reported in Figure 7 show stronger peaks for [CH]-GNR than for [LA]-GNR, which is in agreement with the stronger interaction energies for [CH] reported in Figure 6. Likewise, the time evolution of the distribution functions shows an increase in the 0.00−0.92 ns time frame, which rises from the wrapping of the nanodroplet by the GNR; after that, the intensity of the peaks decrease because of the folding that leads to larger GNR−GNR interactions and, thus, to weaker ion-GNR ones. Once some ions begin to leave the droplet trapped by the folded GNR and move to the top of the GNR (t > 1.58 ns), the ion−GNR interactions start to increase and so does the distribution functions reported in Figure 7. The ability of IL nanodroplets to drive GNR’s folding should rise from the balance of intermolecular interaction energies between all the involved compounds in folded and unfolded scenarios. Therefore, total nanodroplet−GNR interaction energy, ΔE, is defined according to eq 2, which could be used to quantify the stabilization energy upon GNR folding:

ΔE = E intertotal,withNANO − (E intertotalIL,withoutGNR + EGNR,withoutNANO)

(2)

where Eintertotal,withNANO stands for the total intermolecular interaction energy (ion−ion, ion−GNR, GNR−GNR), summing up Coulombic and Lennard-Jones contributions; EintertotalIL,withoutGNR stands for the total intermolecular interaction energy (Coulombic + Lennard-Jones) calculated for the droplet in absence of the GNR; and EintertotalIL,withoutNANO is defined as in eq 2. The values calculated upon GNR (w × 15.0 nm2) folding (averaging 2.0 to 5.0 ns time frame) for [CH][LA] with D = 3.0 nm are reported in Figure 8, which show the large stabilization rising upon the interaction between the IL nanodroplet and the GNR, increasing with increasing GNR width but reaching a minimum separating folding from nonfolding regimes. The effect of nanodroplet sizing on GNRs folding was also analyzed, and thus, the properties of [CH][LA] nanodroplets with several diameters on top of a 2.0 × 15.0 nm2 GNR were studied. Increasing nanodroplet size leads to changes from nonfolding situations, D = 3.0 nm, to folding scenarios, D = 3.5 and 4.0 nm, Figure 9a. Nevertheless, although increasing nanodroplet size should favor GNRs folding, it also increases 21085

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nonfolding from folding with sliding behavior regions for D < 5 nm (although extending D values to larger nanodroplets, the behavior reported by Patra et al.22 was cubic), the calculated slope of this linear trend was 1.91 nm−1. In the case of [CH][LA] ionic liquid nanodroplets, results reported in Figure 10 also show a linear behavior for the studied nanodroplet

Figure 8. Stabilization energy, ΔE, defined according to eq 2, in the system formed by a [CH][LA] nanodroplet with D = 3.0 nm on a GNR with dimensions w × 15.0 nm2. Simulations at 303 K.

the time required for this folding because of the longer time required to wrap the whole nanodroplet for the bigger systems. Therefore, ongoing from D = 3.5 nm to D = 4.0 nm, the time required to completely wrap the nanodroplet and touch the other side of the GNR is 1.16 and 2.52 ns, respectively. Likewise, the use of bigger nanodroplets hinders the contact between GNRs folded layers, results from Figure 9a show that the use of D = 3.5 nm nanodroplets leads to a situation roughly 400 kJ mol−1 larger (in absolute value) than for the case of D = 4.0 nm nanodroplets. Likewise, bigger nanodroplets do not change remarkably the characteristics of anion−cation interactions, which remains almost constant during the nanodroplet wrapping and GNR folding, Figure 9b. Moreover, the interaction of ions with GNRs is obviously improved with the increase of nanodroplets size, but this effect is almost negligible for the [CH] cation, Figure 9d, and not very strong for the [LA] anion, Figure 9c. Patra et al.22 summarized the characteristics of GNRs folding driven by water nanodroplets using a so-called phase diagram, in which boundary lines separating folding from nonfolding regions where plotted in a w versus D diagram, showing the regions for which sliding or rolling after folding is produced. Therefore, this diagram was also built in this work for [CH][LA] nanodroplets and plotted in comparison with water results from Patra et al.22 for discussion purposes. Water nanodroplets show a linear boundary separating

Figure 10. Boundary lines separating folding and nonfolding regions for water and [CH][LA] nanodroplets with diameter D on GNRs with width w. Values for water obtained from Patra et al.22 Simulations at 303 K.

diameters, but the slope is 1.01 nm−1, which is remarkably lower than for water nanodroplets. Likewise, water and [CH][LA] boundaries crosses at D = 3.9 nm and w = 2.65 nm, that is to say, for GNRs with w < 2.65 nm smaller nanodroplets are required to fold GNRs when using [CH][LA] in comparison with water ones, whereas for w > 2.65 nm, the behavior is opposite. Moreover, the rolling folding mechanism obtained for water nanodroplets is also obtained for [CH][LA], but the boundary separating sliding from rolling regions is shifted for [CH][LA] in comparison with water, and thus, smaller [CH][LA] nanodroplets are required to roll GNRs with low w values than using water droplets. The discussion in the previous sections is centered in [CH][LA] ionic liquid nanodroplets, but one of the main characteristics of ILs is the possibility of tuning properties by the suitable selection of anions and cation, and thus, the GNRs folding using additional ILs will be analyzed in the next sections. The comparisons for the nine selected ILs were done

Figure 9. (a) GNR interaction energy, ΔEGNR‑GNR, defined according to eq 1, (b) anion−cation interaction energy, EA‑C (summing Coulombic and Lennard-Jones contributions), (c) anion−GNR interaction energy, EA‑GNR (summing Coulombic and Lennard-Jones contributions), and (d) cation− GNR interaction energy, EC‑GNR (summing Coulombic and Lennard-Jones contributions), in the system formed by [CH][LA] nanodroplets with D diameter on a GNR with dimensions 2.0 × 15.0 nm2. Simulations at 303 K. 21086

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for ILs nanodroplet with D = 3.0 nm and GNRs with w = 0.5, 1.0, and 2.0 nm. The time required to completely wrap the nanodroplet by the GNRs is reported in Table 1 for the studied

[CH] cation, or even do not wrap for [Tf2N] anion. In the case of anions, the ILs containing [BE] anion show very fast GNRs wrapping, and thus, the combinations [CH][BE] or [MP][BE] seems the more suitable for folding purposes. A comparison for the ion−GNR interaction energies at the initial stages of the nanodroplet wrapping by the GNR is reported in Figure 11 for the nine studied ILs and GNR with w = 1.0. Cation−GNR interaction energies are remarkably larger than anion−GNR ones for all the studied ILs, and thus, this fact seems to control nanodroplet wrappings by the GNR considering that for GNRs with w = 1.0 nm [CH]-based ILs are able to wrap and fold the GNR more effectively than [MP]- or [EMIM]-based ones. Results in Figure 11a, shows cation−GNR interaction energy for ILs containing a common anion ([LA]), [MP] cation has a remarkably larger affinity for GNR than [CH] or [EMIM], which would justify the trend of [MP][LA] to spread on the GNR, thus, leading to a nonfolding situation, Table 1. The effect of anion on cation−GNR interactions for a common cation ([CH]) is reported in Figure 11b, with the reporting results showing small differences and, thus, similar wrapping times, Table 1. The analysis of anion−GNR interactions for ILs containing a common anion, Figure 11c, or a common cation, Figure 11d, show almost negligible differences with the exception of [MP][LA], which is the only nonfolding IL of those included in Figure 11. Nanodroplets on Flakes. The ability of ionic liquid nanodroplets to fold more complex geometries than rectangular GNRs reported in previous sections was also studied using nanodroplets with D = 3.0 for rectangular and flower like flakes reported in Figure 2. First, it should be remarked that simulations carried out for the rectangular flake in absence of the IL nanodroplet did not led to folding for the studied time

Table 1. Time Required to Completely Wrap, for GNRs, or Encapsulate, for Rectangular and Flower-Like Blades (Figure 2), t, in the Systems Formed by Ionic Liquid Nanodroplets with D = 3.0 nma flakes

GNRs w (nm) ionic liquid

0.5

1.0

2.0 t (ns)

rectangular

flower-like

[CH][LA] [CH][BE] [CH][SA] [MP][LA] [MP][BE] [MP][SA] [EMIM][LA] [EMIM][BF4] [EMIM][Tf2N]

1.15 0.56 1.04 0.92 0.45 0.99 1.23 1.34 0.84

0.89 0.53 0.90 N 0.36 N 1.30 2.39 N

N N N N N N N N N

0.35 0.21 0.45 N 0.12 N 0.41 0.33 0.54

0.26 0.33 0.29 0.09 0.29 0.09 0.44 0.78 0.25

GNRs with dimensions w × 15.0 nm2. N shows those cases for which no folding is produced for the studied timeframe (5 ns). 3% uncertainty for the reported time values. a

ILs, showing that none of the considered ILs is able to fold the w = 2.0 nm GNR in the studied time frame, all of them spread very quickly over the GNR surface avoiding the bending of the GNR to start the folding process. Values for narrower GNRs reported in Table 1 show that ILs containing [MP] cation, with the exception of those paired with [BE] anion, are not able to wrap the w = 1.0 nm GNR, and also those containing [EMIM] cation also show or larger wrapping times than those containing

Figure 11. Cation−GNR (C-GNR, panels a and b) and anion−GNR (A-GNR, panels c and d) interaction energy, E, in the systems ionic liquid nanodroplets (D = 3.0 nm) with GNRs (w = 1.0 nm). Simulations at 303 K. 21087

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Figure 12. Snapshots of rectangular flake folding driven by a [CH][LA] nanodroplet with D = 3.0 nm. [CH] cations in blue and [LA] anions in green; the two rectangular flakes are plotted in red and yellow sticks for the sake of visibility. Simulations at 303 K.

frame (5 ns). Snapshots for the folding of rectangular flakes driven by [CH][LA] nanodroplets are reported in Figure 12. Simulations at the initial stages of folding shows that both rectangular flakes bind with the nanodroplets by the bending of the central strip leading to a sandwich-like structure, but for this structure, both flakes are not parallel; once the flakes are folded on one side of the top flake reaches the parallel side of the bottom flake (Figure 12, panel reporting results for 0.35 ns) and immediately the nanodroplet begins to be squeezed away. The time required to form this sandwiched structure by a [CH][LA] nanodroplet (0.35 ns) is larger than the time required by water nanodroplets, Patra et al.22 and Bellido and Seminario23 reported values of roughly 0.05 ns for the sandwiching of water droplets. Once the sandwiched nanostructure is formed with the [CH][LA] nanodroplet, the IL nanodroplet is completely squeezed away very rapidly, in roughly 0.07 ns the nanodroplet is out of the sandwiched structure and the two rectangular flakes are placed parallel one on top of the other forming a two-layered structure (with AB stacking and 0.347 nm interlayer separation), then in an additional 0.26 ns the droplet moves toward the top graphene layer covering the whole available surface. The evolution of the main energy contribution is reported in Figure 13, the

sandwiched structure is formed and the nanodroplet is squeezed away and it spreads on the top graphene layer the anion−cation energy contribution reaches again the same values than those previous to folding. The ion-GNR interaction energies suffer the most remarkable changes in the initial folding stages in which they increase (in absolute value) very remarkably, with sudden changes when droplets are squeezed away. The folding of rectangular flakes was also analyzed for the other eight studied ILs, with D = 3.0 nm for all the nanodroplets. For the studied ILs, only [MP][LA] and [MP][SA] were not able to fold the studied rectangular flake, Table 1. All the IL nanodroplets which folded the flake led to nanodroplet expelling out of the initial sandwiched configuration, leading to a parallel arrangement of both flake’s layer. Nevertheless, some nanodroplets spread only on the top layer ([CH][LA], [CH][BE] and [EMIM][BF4]), whereas for other nanodroplets the ILs spread both on the top and the bottom flake layers ([CH][SA], [MP][BE], and [EMIM][Tf2N]). Results reported in Figure 13 show a comparison between ion− flake and flake−flake interaction energies for [CH][LA] and [CH][SA] showing very similar patterns for both nanodroplets, the main difference rising in the ion-flake interaction energies, which are stronger for those ILs spreading on both layers of the folded flake (e.g., [CH][SA]) than for those which spread only on the top layer (e.g., [CH][LA]). All the studied ILs are able to fold the flower-like flake, even those which did not fold the rectangular flake or the w = 1.0 nm GNR, with the folding time being very short for all of them, Table 1, and without expelling the nanodroplet for times lower than 5 ns. The folding mechanism of the flower-like flake by ILs with the following encapsulation of the nanodroplet is reported in Figure 14 for [CH][LA]; likewise, the evolution of ΔEGNR‑GNR with simulation time is shown in Figure 15. Once the four petals are closed and the nanodroplet is encapsulated, t = 0.26 ns for [CH][LA] nanodroplet, the size of the nanodroplet allows the movement of one petal to be placed on top of the neighbor petal giving rise to the first step in energy reported in Figure 15. Then, the nanodroplet is compressed and for t = 1.10 ns, the other two petals adopt a parallel arrangement, and thus, another energy step is produced in Figure 15. This encapsulation of the IL nanodroplet, with two petals on top and other two on the bottom, both in AB stacking, is maintained in the studied time frame without expelling the nanodroplet. Results reported in Figure 15 for the flower-like flake in absence of the nanodroplet show that no folding is produced. Patra et al.22 and Bellido and Seminario,23 also reported the folding mechanism of water driven flower-like flakes, and thus, simulations for water nanodroplets with the same diameter as those used for ILs studies were also carried out in this work, Figure 15. Results reported in this work for water nanodroplets also follow a three-step folding mechanism for water: (i) bending of the four petals leading to a closed structure (t = 0.03 ns), (ii) the first two petals are placed one

Figure 13. GNR interaction energy, ΔEGNR‑GNR, defined according to eq 1, anion−cation interaction energy; EA‑C; anion−GNR interaction energy, EA‑GNR; and cation−GNR interaction energy, EC‑GNR, all of them summing Coulombic and Lennard-Jones contributions, in the system formed by a [CH][LA] (continuous lines) or [CH][SA] (dashed lines) nanodroplets, both with D = 3.0 nm on the rectangular blade reported in Figure 12. Simulations at 303 K.

ΔEGNR‑GNR shows a sudden change at 0.35 ns rising from the beginning of the sandwiching process, followed by a very fast increase (in absolute value from 0.35 to 0.44 ns) corresponding to the squeezing away of the nanodroplet and the formation of the two-layer structure, after which the energy remains constant. The anion−cation structure only suffers remarkable changes in the initial stages of the folding, in which the interaction with the graphene rectangular flakes decreases this energy in a 15% for [CH][LA] nanodroplet, but once the 21088

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Figure 14. Snapshots of flower-like flake folding driven by a [CH][LA] nanodroplet with D = 3.0 nm. [CH] cations in blue and [LA] anions in green; the four petals are plotted in different colors for the sake of visibility. Simulations at 303 K.

Figure 15. GNR interaction energy, ΔEGNR‑GNR, defined according to eq 1, in the systems formed by a nanodroplets of [CH][LA] or water, both with D = 3.0 nm diameter, on a flower-like flake. Results for flower-like flake without nanodroplets are also reported. Simulations at 303 K.

Figure 16. Interaction of a [CH][LA] nanodroplet (D = 3.0 nm) with a rectangular flake supported on SiO2 after 5 ns simulations. [CH] cations in blue and [LA] anions in green. Simulations at 303 K.

on top of the other (t = 0.07 ns), and (iii) the remaining two petals are also placed in a parallel arrangement (t = 0.33 ns). These three steps are followed for water (but not for the ILs) by a fourth step rising from the expelling of some water molecules, which leave the encapsulated nanodroplet placed outside of the folded structure and, thus, allowing that the four blades are partially placed one on top of the other (the last step in Figure 15 for water nanodroplet) developing a four-layered structure with a small water nanodroplet encapsulated in the vicinity of the central flake connecting the four blades. Simulations by Patra et al.22 showed analogous results to those reported in this work (in spite of the slight differences between the studied flower-like flakes), but their simulations were limited to 1 ns time frame, and thus comparison for expelling molecules obtained in this work and longer times could not be done. Results by and Bellido and Seminario23 were obtained for simulations up to 0.20 ns, and they did report only blades folding around the water nanodroplet but layering of blades were not reported possibly because of the characteristics of their flakes and droplets, which are different to those in this work. Nanodroplets on Supported Flakes. Simulation of rectangular flakes supported on SiO2 substrates were also carried out following a similar approach to that by Bellido and Seminario23 for water nanodroplets, which were also studied in this for comparative purposes. A snapshot of the [CH][LA] nanodroplet on top of the supported rectangular flake is reported in Figure 16; after 5 ns, the nanodroplet is not able to fold the flake, which remains on top of the SiO2 at an average distance of 0.32 nm. The ions spread over one of the rectangular flakes, whereas the other one remains empty; likewise, some of the ions also spread over the SiO2 surface. Results for water obtained in this work show that the studied

nanodroplet is also not able to fold the supported flakes, in agreement with Bellido and Seminario.23 Any of the nine studied ILs was able to fold the supported rectangular flakes for t < 5 ns. The different energy contributions in the supported flake systems were analyzed to infer the reasons hindering the flakes folding, Figure 17, showing that ion−flake interactions are weaker than flake-SiO2 ones, and thus, [CH][LA] nanodroplets

Figure 17. Energy terms for the system formed by a [CH][LA] nanodroplet (D = 3.0 nm) with a rectangular flake supported on SiO2. Simulations at 303 K. 21089

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are not able to fold the studied graphene flakes when supported on SiO2 surfaces. These results are in agreement with experimental measurements of contact angles of ILs on polar and nonpolar surfaces. From a macroscopic viewpoint, two main factors control the behavior of ionic liquids on solid surfaces: (i) increasing surface tension tends to increase contact angles, and (ii) increasing polarity of the substrate tends to decrease contact angle. Sedev42 analyzed the behavior of several ionic liquids with regard to different substrates in relationship with ionic liquid surface tension and substrate polarity. Restolho et al.43 showed that, for classic imidazolium-based ionic liquids contact angles on glass surfaces are roughly 20°, whereas on apolar surfaces such as PTFE angles in the 80 to 100° range were measured. These results on silica surfaces were confirmed experimental and computationally by Bovio et al.,44 who showed strong interaction between imidazolium ILs and silica polar substrates. Therefore, ionic liquids spreading on polar surfaces have been clearly reported in the literature, in agreement with behavior reported in Figure 16, whereas moderate contact angles on nonpolar surfaces have been shown experimentally. Likewise, Baldelli et al.45 measured the contact angle of 1-butyl-3-methylimidazolium methylsulfate on graphene (supported on CaF2) reporting 58 ± 2°, which is remarkably lower than the 90° value reported for water on the same surface, thus, showing the ability of the ionic liquid to wet even the nonpolar graphene surface.

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Ivanovskii, A. Graphene-Based and Graphene-Like Materials. Russ. Chem. Rev. 2012, 81, 571−605. (2) Zhang, T.; Xue, Q.; Zhang, S.; Dong, M. Theoretical Approaches to Graphene and Graphene-Based Materials. Nano Today 2012, 7, 180−200. (3) Luo, J.; Kim, J.; Huang, J. Materials Processing of Chemically Modified Graphene: Some Challenges and Solutions. Acc. Chem. Res. 2013, 46, 225−2234. (4) Ma, L.; Wang, J.; Ding, F. Recent Progress and Challenges in Graphene Nanoribbon Synthesis. ChemPhysChem 2013, 14, 47−54. (5) Sasaki, N.; Okamoto, H.; Masuda, S.; Miura, K.; Itamura, N. Simulated Nanoscale Peeling Process of Monolayer Graphene Sheet: Effect of Edge Structure and Lifting Position. J. Nanomater. 2010, 1− 12. (6) Han, M. Y.; Ozyilmaz, B.; Zhang, Y.; Kim, P. Energy Band-Gap Engineering of Graphene Nanoribbons. Phys. Rev. Lett. 2007, 98, 206805. (7) Liao, L.; Zhang, L.; Wang, X.; Diankov, G.; Dai, H. Narrow Graphene Nanoribbons from Carbon Nanotubes. Nature 2009, 458, 877−880. (8) Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K.; Tour, J. M. Longitudinal Unzipping of Carbon Nanotubes to Form Graphene Nanoribbons. Nature 2009, 458, 872− 876. (9) Sprinkle, M.; Ruan, M.; Hu, Y.; Hankinson, J.; Rubio-Roy, M.; Zhang, B.; Wu, X.; Berger, C.; de Heer, W. A. Scalable Templated Growth of Graphene Nanoribbons on SiC. Nat. Nanotechnol. 2010, 5, 727−731. (10) Cai, J.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X.; Mullen, K.; Fasel, R. Atomically Precise Bottom-Up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470−473. (11) Chen, L.; Hernandez, Y.; Feng, X.; Müllen, K. From Nanographene and Graphene Nanoribbons to Graphene Sheets: Chemical Synthesis. Angew. Chem., Int. Ed. 2012, 51, 7640−7654. (12) Chamberlain, T. W.; Biskupek, J.; Rance, G. A.; Chuvilin, A.; Alexander, T. J.; Bichoutskaia, U.; Khlobystov, A. N. Size, Structure, and Helical Twist of Graphene Nanoribbons Controlled by Confinement in Carbon Nanotubes. ACS Nano 2012, 6, 3943−3953. (13) Yin, W. J.; Xie, Y. E.; Liu, L. M.; Chen, Y. P.; Wang, R. Z.; Wei, X. L.; Zhong, J. X.; Lua, L. Atomic Structure and Electronic Properties of Folded Graphene Nanoribbons: A First-Principles Study. J. Appl. Phys. 2013, 113, 173506. (14) Shenoy, V. B.; Gracias, D. H. Self-Folding Thin-Flm Materials: From Nanopolyhedra to Graphene Origami. MRS Bull. 2012, 37, 847−854. (15) Koskinen, P. Graphene Nanoribbons Subject to Gentle Bends. Phys. Rev. 2012, 85, 205429. (16) Kagimura, R.; Mazzoni, M. S. C.; Chacham, H. Knots in a Graphene Nanoribbon. Phys. Rev. B 2012, 85, 125415. (17) Li, X.; Wang, X.; Zhang, L.; Lee, S.; Dai, H. Chemically Derived, Ultrasmooth Graphene Nanoribbon Semiconductors. Science 2008, 319, 1229−1232. (18) Chuvilin, A.; Bichoutskaia, E.; Gimenez-Lopez, M. C.; Chamberlain, T. W.; Rance, G. A.; Kuganathan, N.; Biskupek, J.; Kaiser, U.; Khlobystov, A. N. Self-Assembly of a Sulphur-Terminated Graphene Nanoribbon within a Single-Walled Carbon Nanotube. Nat. Mater. 2011, 10, 687−692. (19) Kim, K.; Lee, Z.; Malone, B. D.; Chan, K. T.; Aleman, B.; Regan, W.; Gannett, W.; Crommie, M. F.; Cohen, M. L.; Zettl, A. Multiply Folded Graphene. Phys. Rev. B 2011, 83, 245433.

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CONCLUSIONS Results on the folding of graphene-based nanostructures using ionic liquids nanodroplets using classical molecular dynamics simulations are reported in this work. The analysis of the folding mechanisms for graphene nanoribbons was done analyzing the changes in the main terms of interaction energies. The study of the effects of GNRs width and nanodroplets diameters allowed building the phase diagram separating folding and nonfolding regions, which compared with folding using water nanodroplets showed narrow GNRs are folded more efficiently (using smaller droplets) by ionic liquids nanodroplets. The folding of rectangular flakes by ILs nanodroplets led to a fast process leading to a two-layered structure with AB stacking with nanodroplets placed only on top of one sheet or on top of both sheets depending on the used IL. In the case of the studied flower-like flakes, the four petals adopt a structure upon folding assisted by ILs nanodroplets characterized by two AB stacked petals encapsulating the nanodroplet. Likewise, the study of flakes supported on SiO2 surfaces showed that the studied nine ILs are not able to fold the flake, the strong flake−SiO2 interactions are not balanced by the ion-flake ones, and thus, new ILs leading to stronger interactions with graphene flakes or a new mechanism to allow the folding of supported flakes using IL nanodroplets should be developed. The ability of tailoring ILs to be suitable for considered technologies leads to new methods for controlling GNR folding. Results reported in this work show the prevailing role developed by the cation on the folding mechanism, for very different types of cations, in contrast with the marginal role of the studied anions. Therefore, ILs containing ions with moderate affinity toward the graphene, neither too high for spreading on the sheet nor too low to hinder folding, should be designed. Cholinium-based cations are remarkable candidates for which new anions, which improve graphene affinities, will be considered in future works. 21090

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(20) Xie, Y. E.; Chen, Y. P.; Zhong, J. Electron Transport of Folded Graphene Nanoribbons. J. Appl. Phys. 2009, 106, 103714. (21) Feng, J.; Qi, L.; Huang, J. Y.; Li, J. Geometric and Electronic Structure of Graphene Bilayer Edges. Phys. Rev. B 2009, 80, 165407. (22) Patra, N.; Wang, B.; Krai, P. Nanodroplet Activated and Guided Folding of Graphene Nanostructures. Nano Lett. 2009, 9, 3766−3771. (23) Bellido, E. P.; Seminario, J. M. Molecular Dynamics Simulations of Folding of Supported Graphene. J. Phys. Chem. C 2010, 114, 22472−22477. (24) Catheline, A.; Ortolani, L.; Morandi, V.; Melle-Franco, M.; Drummond, C.; Zakri, C.; Penicaud, A. Solutions of Fully Exfoliated Individual Graphene Flakes in Low Boiling Point Solvents. Soft Matter 2012, 8, 7882−7887. (25) Ludwig, R.; Kragl, U. Do We Understand the Volatility of Ionic Liquids? Angew. Chem., Int. Ed. 2007, 46, 6582−6584. (26) Angell, C. A.; Ansari, Y.; Shzo, Z. Ionic Liquids: Past, Present and Future. Faraday Discuss. 2012, 154, 9−27. (27) Melle-Franco, M.; Zerbetto, F. Ejection Dynamics of a Simple Liquid from Individual Carbon Nanotube Nozzles. Nano Lett. 2006, 6, 969−972. (28) Jiang, Y.; Li, H.; Li, Y.; Yu, H.; Liew, K. M.; He, Y.; Liu, X. Helical Encapsulation of Graphene Nanoribbon into Carbon Nanotube. ACS Nano 2011, 5, 2126−2133. (29) Patra, N. D.; Song, Y.; Kral, P. Self-Assembly of Graphene Nanostructures on Nanotubes. ACS Nano 2011, 5, 1798−1804. (30) Jiang, Y.; Zhang, K.; Li, H.; He, Y.; Song, X. Spontaneous Encapsulation Behavior of Ionic Liquid into Carbon Nanotube. Nanoscale 2012, 4, 7063−7069. (31) Aparicio, S.; Atilhan, M. Molecular Dynamics Study of Carbon Nanostructures in N-Methylpiperazinium Lactate Ionic Liquid. J. Phys. Chem. C 2013, 117, 22046−22059. (32) Zhou, H.; Rouha, M.; Feng, G.; Lee, S. S.; Docherty, H.; Fenter, P.; Cummings, P. T.; Fulvio, P. F.; Dai, S.; McDonough, J.; Presser, V.; Gogotsi, Y. Nanoscale Perturbations of Room Temperature Ionic Liquid Structure at Charged and Uncharged Interfaces. ACS Nano 2012, 6, 9818−9827. (33) Fedorov, M. V.; Lynden-Bell, R. M. Probing the Neutral Graphene-Ionic Liquid Interface: Insights from Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2012, 14, 2552−2556. (34) Wang, Y. L.; Laaksonen, A.; Lu, Z. Y. Influence of Ionic Liquid Film Thickness on Ion Pair Distributions and Orientations at Graphene and Vacuum Interfaces. Phys. Chem. Chem. Phys. 2013, 15, 13559−13569. (35) Zhao, Y.; Hu, Z. Graphene in Ionic Liquids: Collective van der Waals Interaction and Hindrance of Self-Assembly Pathway. J. Phys. Chem. B 2013, 117, 10540−10547. (36) Aparicio, S.; Atilhan, M. Choline-Based Ionic Liquids on Graphite Surfaces and Carbon Nanotubes Solvation: A Molecular Dynamics Study. J. Phys. Chem. C 2012, 116, 12055−12065. (37) Karadas, F.; Köz, B.; Jacquemin, J.; Deniz, E.; Rooney, D.; Thompson, J.; Yavuz, C. T.; Khraisheh, M.; Aparicio, S.; Atilhan, M. High Pressure CO2 Absorption Studies on Imidazolium-Based Ionic Liquids: Experimental and Simulation Approaches. Fluid Phase Equilib. 2013, 351, 74−86. (38) Aparicio, S.; Atilhan, M. Nanoscopic Vision on Fuel Dearomatization Using Ionic Liquids: The Case of Piperazine-Based Fluids. Energy Fuel 2013, 27, 2515−2527. (39) Karadas, F.; Köz, B.; Jacquemin, J.; Deniz, E.; Rooney, D. W.; Thompson, J.; Yavuz, C. T.; Khraisheh, M.; Aparicio, S.; Atilhan, M. High Pressure CO2 Absorption Studies on Imidazolium-Based Ionic Liquids: Experimental and Simulation Approaches. Fluid Phase Equilib. 2013, 351, 74. (40) Atilhan, M.; Jacquemin, J.; Rooney, D. W.; Khraisheh, M.; Aparicio, S. On the Viscous Behavior of Imidazolium Based Ionic Liquids. Ind. Eng. Chem. Res. 2013, 52, 16774−16785. (41) Lyubartsev, A. P.; Laaksonen, A. MDynaMix - A Scalable Portable Parallel MD Simulation Package for Arbitrary Molecular Mixtures. Comput. Phys. Commun. 2000, 128, 565−589.

(42) Sedev, R. Surface Tension, Interfacial Tension and Contact Angles of Ionic Liquids. Curr. Opin. Colloid Interface Sci. 2011, 16, 310−316. (43) Restolho, J.; Mata, J. L.; Saramago, B. On the Interfacial Behavior of Ionic Liquids: Surface Tensions and Contact Angles. J. Colloid Interface Sci. 2009, 340, 82−86. (44) Bovio, S.; Podesta, A.; Milani, P.; Ballone, P.; del Popolo, M. G. Nanometric Ionic-Liquid Films on Silica: A Joint Experimental and Computational Study. J. Phys.: Condens. Matter 2009, 21, 424118. (45) Baldelli, S.; Bao, J.; Pei, S. S. Sum Frequency Generation Study on the Orientation of Room-Temperature Ionic Liquid at the Graphene−Ionic Liquid Interface. Chem. Phys. Lett. 2011, 516, 171− 173.

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