Graphene Meets Ionic Liquids: Fermi Level Engineering via

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Graphene Meets Ionic Liquids: Fermi Level Engineering via Electrostatic Forces

Gangamallaiah Velpula,†,¶ Roald Phillipson,†,¶ Jian Xiang Lian,‡ David Cornil,‡ Peter Walke,† Ken Verguts,§,⊥ Steven Brems,⊥ Hiroshi Uji-i,†,∥ Stefan De Gendt,§,⊥ David Beljonne,‡ Roberto Lazzaroni,‡ Kunal S. Mali,*,† and Steven De Feyter*,† Downloaded via UNIV OF TEXAS AT DALLAS on March 12, 2019 at 19:19:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Division of Molecular Imaging and Photonics, Department of Chemistry, KU Leuven, Celestijnenlaan, 200F, B-3001 Leuven, Belgium ‡ Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, 7000 Mons, Belgium § Molecular Design and Synthesis, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium ⊥ imec vzw, Kapeldreef 75, B-3001 Leuven, Belgium ∥ RIES, Hokkaido University, N20 W10, Kita-Ward, Sapporo 001-0020, Japan S Supporting Information *

ABSTRACT: Graphene-based two-dimensional (2D) materials are promising candidates for a number of different energy applications. A particularly interesting one is in next generation supercapacitors, where graphene is being explored as an electrode material in combination with room temperature ionic liquids (ILs) as electrolytes. Because the amount of energy that can be stored in such supercapacitors critically depends on the electrode− electrolyte interface, there is considerable interest in understanding the structure and properties of the graphene/IL interface. Here, we report the changes in the properties of graphene upon adsorption of a homologous series of alkyl imidazolium tetrafluoroborate ILs using a combination of experimental and theoretical tools. Raman spectroscopy reveals that these ILs cause n-type doping of graphene, and the magnitude of doping increases with increasing cation chain length despite the expected decrease in the density of surface-adsorbed ions. Molecular modeling simulations show that doping originates from the changes in the electrostatic potential at the graphene/IL interface. The findings described here represent an important step in developing a comprehensive understanding of the graphene/IL interface. KEYWORDS: graphene, ionic liquids, Raman spectroscopy, doping, Fermi level engineering

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Enhancing the energy density by maximizing the electrode surface area and/or increasing the operating voltage has thus become the holy grail of contemporary supercapacitor research.1 Because the amount of energy that can be stored in a supercapacitor critically depends on the specific surface area and the maximum cell voltage, the past few years have witnessed significant activity in both the field of electrode materials and the chemistry of electrolytes. Graphene, a single atom thick sheet of sp2-hybridized carbon,2 showcases several key properties suitable for electrode materials such as high specific surface area (2630 m2/g), good electrical conductivity,

lectric double-layer (EDL) capacitors, also known as supercapacitors or ultracapacitors, are emerging devices for electrical energy storage that consist of two conductive electrodes ionically connected to each other via an electrolyte. Supercapacitors store electrical energy via reversible adsorption of ions on the surface of these conductive electrodes. The energy density (ED) of EDL capacitors is a function of the operating voltage (V0) and the specific capacitance C of the electrode−electrolyte system and is given by ED = ξCV02, where ξ is a constant. The specific capacitance in turn depends on the specific surface area of the electrode accessible to the electrolyte, its interfacial doublelayer capacitance, and the density of the electrode material. The operating voltage of the supercapacitor is generally limited by the electrochemical window of the electrolyte material, wherein it remains stable without being oxidized or reduced. © XXXX American Chemical Society

Received: December 28, 2018 Accepted: March 4, 2019

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DOI: 10.1021/acsnano.8b09768 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. (a) Molecular structures of the ILs employed in this study. (b) Scheme showing the influence of the adsorption of ILs with increasing chain length on the Fermi level of graphene. An increase in the chain length leads to a decrease in the density of surface-adsorbed ions. However, the amount of n-type doping increases.

eration,14,15 and scanning probe microscopy, namely, atomic force microscopy (AFM)16−18 and scanning tunneling microscopy (STM),19−21 have been employed to directly probe the structure of graphene/IL interfaces. In comparison, those interfaces have been studied rather extensively using theoretical tools, mostly density functional theory (DFT) calculations22,23 and molecular dynamics (MD) simulations.24−27 The general consensus that has emerged from these studies reveals that ILs display pronounced layering at the graphene (graphite) interface, and the layering extends up to more than a nanometer from the solid surface and then diminishes into the bulk IL. The density of ions is significantly higher in the interfacial region. The cations typically adsorb closer to the surface than the anions, and the cationic rings with their alkyl side chains lie flat on the graphene surface. As a result of the proximity of the cationic layer to the graphene surface, several additional layers are formed, and such layering appears to be a generic trait for ILs adsorbed on solid surfaces. Thus, the EDL at the graphene/IL interface is thicker than a single compact layer. Despite the numerous reports devoted to the structural elucidation of the graphene/IL interface, very little is known about the influence of IL adsorption on the properties of graphene. Graphene, being a “surface only” material, is known to be extremely sensitive to adsorbates. The charge carrier concentration (Fermi level) of graphene is readily affected upon adsorption of a variety of materials.28 Understanding the influence of ILs on the properties of graphene is especially relevant in the context of further optimization of graphene as an electrode material. Although Fermi level engineering of graphene has been explored in the recent past in the context of graphene functionalization,28,29 there exists no information on how IL adsorption influences the electronic properties of graphene. In this contribution, we provide a detailed account of the influence of IL adsorption on graphene using a combination of Raman spectroscopy, tight-binding DFT (DFTB), and MD simulations. Graphene grown using chemical vapor deposition (CVD) and transferred to SiO2 substrates was used as the model substrate. We study adsorption of a homologous series of imidazolium ionic liquids with tetrafluoroborate counterions (Figure 1a) on graphene. Raman spectroscopy reveals that adsorption of neat CnMIM-BF4 ILs on graphene induces ntype doping of graphene. Furthermore, the magnitude of ntype doping increases with increasing chain length, contrary to the anticipated reduction in the density of ions at the graphene/IL interface (Figure 1b). Doping originates from

and exceptional chemical stability. Graphene, reduced graphene oxide, and graphene−carbon nanotube composites are currently being explored as high surface area electrode materials.3,4 The pursuit of higher operating voltages, on the other hand, has fueled the exploration of room temperature ionic liquids (RTILs, hereafter referred to as ILs) as electrolytes.5 ILs are pure salts composed of organic or inorganic anions and organic cations that are liquid under ambient conditions and display wide electrochemical windows. Typical electrochemical windows vary between 4.5 V to 5 V; however, in some cases, windows of up to 6 V have been reported.6,7 ILs are composed of large, nonsymmetric, polyatomic cations (ammonium, pyrrolidinium, imidazolium, etc.) substituted with alkyl chains. The anions are also mostly polyatomic. Besides their wide electrochemical windows, ILs exhibit several other favorable properties such as low vapor pressure and high thermal stability, and they are known to dissolve a wide array of materials. Furthermore, their properties can be easily tuned via choice of cations and anions.8 Given the exceptional properties of the two materials, the graphene/IL combination has emerged as a promising alternative for conventional electrode/electrolyte combinations employed in EDL supercapacitors. This promise has prompted a flurry of activity in the development of graphene-based EDL supercapacitors that employ ILs as electrolytes.9−12 The energy storage in these devices relies on the charge separation that occurs upon charging where the ions of the IL form an EDL on the surface of graphene. The device performance thus critically depends on the graphene/IL interface, especially the EDL. Given that graphene/IL-based double-layer supercapacitors still lag well behind lithium ion batteries in terms of specific energy, there is significant room for improving the performance of such devices. This calls for a thorough understanding of the graphene/IL interface.13 The graphene/IL interface has been intensively studied theoretically and somewhat less so experimentally. A majority of these efforts were directed toward elucidating the structure of the interface. Experimental probing of the graphene/IL interface is further complicated by the rich array of complex interactions that exist between the constituent ions: Coulombic, dipolar, π−π, van der Waals, and hydrogen bonding interactions are all known to coexist, leading to formation of nanostructured domains with segregation of charged groups and apolar alkyl chains in the bulk phase as well as at solid interfaces. Despite its complex nature, secondorder nonlinear spectroscopy, namely, sum-frequency genB


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Figure 2. Raman spectra of graphene before and after functionalization with CnMIM-BF4 ionic liquids. (a) Typical Raman spectra before and after functionalization with C2MIM-BF4. (b) Typical Raman spectra before and after functionalization with C10MIM-BF4. For the complete series of ILs, see Figure S3 in the Supporting Information.

SiO2 is often heavily p-type doped due to adsorption of water and oxygen mediated by a strong interaction with the SiO2 substrate.36 We note that the initial Pos(G) of unfunctionalized graphene does not influence the shifts observed after functionalization. Similar experiments carried out with graphene deposited on hexagonal boron nitride (h-BN), where the graphene sample is close to charge neutral, yielded similar results (vide infra). Measurements of the ID/IG peak ratio indicate that the pristine graphene samples are of good quality; that is, the defect density is low. Upon deposition of neat ILs, additional peaks located between 1300 cm−1 and 1575 cm−1 appear in the Raman spectrum (Figure S1, Supporting Information). These additional peaks are associated with the imidazolium ring and aliphatic chain vibrations of the CnMIM+ cation.37 These assignments were validated by performing Raman measurements on the bulk ionic liquid sample for C4MIM-BF4 (Figure S2, Supporting Information) and indicate that there is no/limited convolution with the native bands of graphene. The most interesting feature of the Raman spectra obtained after deposition of ILs, however, is the shift of both Pos(G) and Pos(2D) to lower wavenumbers relative to the pristine (unfunctionalized) graphene. The shifts observed for C10MIM-BF4 are higher than those for C2MIMBF4. Furthermore, the intensity ratio of the G and 2D band (I2D/IG) increases for C10MIM-BF4 (Figure 2b). These spectral changes clearly indicate a reduction in p-type doping, and therefore, it can be concluded that CnMIM-BF4 ILs induce ntype doping of graphene. Examination of graphene/IL interfaces involving ILs with intermediate chain lengths (C4-, C6-, and C8-MIM-BF4) clearly revealed a trend where both Pos(G) and Pos(2D) gradually shift to lower wavenumbers with increasing chain length (Figure S3, Supporting Information). To quantify the extent of doping by CnMIM-BF4 ILs as a function of alkyl chain length, statistically significant results were acquired by recording Raman maps (10 × 10 μm2 in size containing 10 × 10 pts2) at three positions, separated at least by several hundreds of micrometers, for each sample before and after functionalization. As stated above, the parameters in the Raman spectrum of graphene that depend on the charge carrier concentration are the positions of the G and 2D band, their relative intensities, and the full width at half-maximum of the G band [FWHM(G)]. Observing these changes, however, is often complicated by secondary factors such as the presence of defects, bilayers, and substrate-induced mechanical strain. In order to discount the influence of defects and bilayer areas on

either (often partial) charge transfer driven by the difference in chemical potentials between the IL and the substrate or a surface-dipole-induced electrostatic potential that acts as a gate and modifies the vacuum level, or the combination of both. DFTB and MD simulations indicate that the gating effect associated with the layered organization at the graphene interface is the main source of doping. The present report is an important step in developing a comprehensive fundamental understanding of graphene/IL interfaces, which will be potentially useful for improving the capacitance of graphenebased 2D materials.

RESULTS AND DISCUSSION Figure 1a displays the molecular structures of the ILs used in this study. Imidazolium-based ILs are among the most commonly used ILs, and CnMIM-BF4 ILs have also been explored recently as electrolytes for EDL supercapacitors.30−32 The length of the alkyl side chain was varied from ethyl to decyl (C12MIM-BF4 is a solid at room temperature). Such variation in the length of alkyl side chains provides a handle on the predominant intermolecular interaction, which changes from mostly long-range electrostatic to mostly short-range van der Waals with increasing chain length. Raman Characterization of the Graphene/IL Interface. The influence of IL adsorption on graphene was studied by monitoring the changes in the Raman spectrum of graphene before and after drop-casting neat IL onto the surface. Raman spectroscopy is an indispensable tool for characterization of graphene and has been used intensively to monitor properties of graphene such as electron density, number of layers, density of defects, stress, and strain.33−35 The primary Raman spectrum of graphene consists of three main bands: the G band (∼1580 cm−1), the D band (∼1350 cm−1), and the 2D band (∼2650 cm−1) found at the positions indicated under the excitation source used in this study (633 nm). By examining the relative shifts in intensity and position of these bands, changes in the properties of graphene can be unraveled. Figure 2 shows representative Raman spectra before and after deposition of neat (a) C2MIM-BF4 and (b) C10MIM-BF4 on the surface of CVD-grown graphene transferred to SiO2. For both samples, Pos(G) is located around 1600 cm−1 before functionalization, which is shifted to higher wavenumbers compared to that of charge-neutral graphene (1581 cm−1) and indicates that the graphene samples used in this study are ptype doped. It is well-known that CVD graphene transferred to C


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Figure 3. Plots of Pos(2D) versus Pos(G) for pristine and IL-functionalized graphene: (a) C2MIM-BF4, (b) C4MIM-BF4, (c) C6MIM-BF4, (d) C8MIM-BF4, (e) C10MIM-BF4 (also see Figure S5 in the Supporting Information).

a reduction in p-type doping and again confirms that CnMIMBF4 ILs cause n-type doping of graphene. Comparison of the shifts of Pos(G) for the different ILs clearly indicates that the degree of n-type doping gradually increases with increasing alkyl chain length of the imidazolium cation, with the full results summarized in Table S1 in the Supporting Information. Figure 4 shows a plot of shifts in

the Raman data, the data sets were corrected by removing nonrelevant data points based on the intensity of the D band and outlying I2D/IG values, respectively.38 For graphene on SiO2, substrate-mediated mechanical strain typically arises from surface corrugation of graphene caused by roughness of the underlying substrate.39 Strain can influence the Raman spectrum of graphene in a variety of different ways depending on factors such as its magnitude, directionality relative to the axes of the carbon lattice, and whether it is compressive or tensile in nature.33 Importantly, as the 2D band is a second-order process involving two phonons, Pos(2D) shows a much stronger sensitivity to strain than Pos(G). Thus, to disentangle the influence of strain from doping, the Raman data were analyzed based on a reported method40 that relies on the correlation of Pos(2D) with Pos(G). This method uses the different fractional variation of Pos(2D) and Pos(G): whereas the variation in strain causes data points to fall on a single line with a slope of 2.2 ± 0.2, the variation in p-type doping leads to the points shifting along a line with a slope of 0.7 ± 0.1. This difference implies that every point in Pos(G)−Pos(2D) space represents a single value for the amount of doping and strain. Figure 3 displays plots of Pos(2D) versus Pos(G) before and after deposition of CnMIM-BF4 ILs with varying alkyl chain lengths on the surface of graphene. Before IL deposition, most data points have a Pos(G) located around 1600 cm−1 and are aligned along a line with a slope of ≈2.0. This slope is in close agreement with the reported 2.2 ± 0.2, despite the different laser excitation wavelength used in this study (Figure S4, Supporting Information). These results indicate a uniform amount of p-type doping and large variations in strain along the surface. After deposition of CnMIM-BF4 ILs, the collection of data points shifts to lower values of Pos(G) for all IL derivatives, while maintaining a similar variation in strain. This down shift of Pos(G) indicates

Figure 4. Shift in Pos(G) versus the number of carbon atoms present in the alkyl chain on the imidazolium cation. The plot clearly reveals that the shift in Pos(G), which is indicative of the extent of doping, increases with increasing chain length.

Pos(G) versus the number of carbon atoms in the alkyl chains attached to the imidazolium cations. It is clear that the magnitude of the shift increases with increasing chain length. As such, the amount of doping increases with increasing alkyl chain length. Estimated values of the amount of doping and Fermi level shift are presented in Table 1. Note that for C8MIM-BF4, the Fermi level shift is larger compared to that of C6MIM-BF4 and C10MIM-BF4, despite having a lower change in charge carrier concentration. This difference is caused by the lower amount of p-type doping [lower Pos(G)] before functionalization for C8MIM-BF4 compared to that of the D


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of piperidine, where the extent of n-type doping was found to be dependent on its surface coverage.43 Thus, the interpretation of the trend observed here for the n-type doping by CnMIM-BF4 ILs is not straightforward and thus merits detailed scrutiny. Molecular Modeling of the Graphene/IL Interface. To understand the origin of the doping and its dependence on alkyl chain length, we resort here to a computational study combining quantum ab initio and classical approaches. The organization of the ions on graphene was first investigated using molecular dynamics simulations considering two models that implement periodic boundary conditions but differ in size and complexity: (i) Model 1 includes a small (∼3 × 3 × 4 nm3) IL-filled box sandwich between two graphene flakes; (ii) Model 2 comprises a larger (∼6 × 6 × 9 nm3) slab of the IL capped on both sides by a few-layer graphene contact. Although we anticipate that Model 2 is more representative of the actual experiments, Model 1 is amenable to a validation of the classical electrostatic calculations against more reliable quantum-chemical results, here obtained using the tightbinding DFTB approach (single-point calculation on a representative snapshot along the MD run).44 It is important to stress that, whereas the classical force field simulations only account for purely electrostatic effects inside the ILs, the DFTB calculations also include (dynamic) electronic polarization effects within the IL together with charge reorganization and/or transfer at the graphene/IL interface. Tight Binding DFT Calculations. Benchmarking studies were performed for CnMIM-BF4, with n = 2 and 8. Figure S9b in the Supporting Information compares the plane-averaged electrostatic potentials calculated at the MD and DFTB levels of theory. Good agreement is found between the two methods that both predict a large downshift in potential when going from n = 2 to n = 8. As discussed in detail below, the origin of this effect is primarily electrostatic in origin and associated with the layering of the cations and anions, which is well captured using a purely classical theory. However, we note that DFTB yields a larger average electrostatic potential in the IL region compared to MD, irrespective of the length of the alkyl side chains. This is due to a small partial charge transfer taking place from graphene to the neighboring IL molecules (by ∼+0.01 |e|/nm2 for C2MIM-BF4 and +0.02 |e|/nm2 for C8MIM-BF4) and included/ignored at the DFTB/MD level. However, we stress that (i) the electrostatic layering effect dominates, especially at large n; (ii) the partial charge transfer is expected to be reduced for the slightly p-doped graphene samples at hand here. Overall, we thus feel confident that the MD simulations provide a robust description of the main effects responsible for the doping of graphene as well as the overall trend with alkyl chain lengths. MD Simulations. We thus next moved on to running extensive MD simulations on the larger Model 2 (Figure S10 in the Supporting Information) and considering the whole series of molecules (C2, C4, C6, and C8). Figure 5 displays the calculated number density profiles of center of mass (COM) for CnMIM-BF4 ILs with varying chain lengths. In accordance with previous reports, oscillations in the number density profiles extending up to ∼15 Å from the surface to the bulk indicate that the ILs are organized in a layered fashion.24,25 Interestingly, the first density peaks (at 4 Å and 8 Å from the graphene surface) of the BF4− anions are superimposed to that corresponding to the CnMIM+ cation, which indicates that the CnMIM+ cations and the BF4− anions are coadsorbed, as

Table 1. Number of Injected Electrons and Shift in Fermi Level Function after Functionalization with CnMIM-BF4 ILsa number of injected electrons (×1012 cm−2) shift in Fermi level (eV)

C2

C4

C6

C8

C10

3.0

3.8

10.0

8.3

13.2

−0.07

−0.10

−0.17

−0.23

−0.21

a

Values are extracted based on the shift of Pos(G) and reference 42.

other ILs. Therefore, the Fermi level shift increases with increasing chain length and saturates after C8MIM-BF4. As noted above, the unfunctionalized graphene samples are p-type doped, and such doping is a generic trait for CVD graphene samples transferred to SiO2. Previous reports have attributed the doping to the interaction of graphene with the SiO2 surface and also to adsorption of moisture and oxygen.36 In this scenario, the influence of this initial p-type doping of the graphene samples must be disentangled from the observed Raman shifts. In order to address this issue, we studied the influence of IL adsorption (C2- and C8MIM-BF4) on graphene supported by h-BN flakes deposited onto SiO2. The results of these control experiments reproduced the trends discussed above (see Figure S6 and discussion provided in the Supporting Information). We note that CnMIM-BF4 ILs are hydrophilic in nature and thus tend to adsorb water upon storage under ambient conditions. In fact, almost all ILs are known to be hygroscopic in nature.41,42 Therefore, the influence of the amount of water present in the IL samples on the observed Raman shifts needs to be scrutinized carefully. To address this issue, the water content in C4MIM-BF4 and C8MIM-BF4 was estimated using Karl Fischer titration. Two measurements were carried out separated by almost a year. As expected from the hygroscopic nature of CnMIM-BF4 ILs, the water content increased significantly during storage. Corresponding Raman measurements carried out on these two “grades” of ILs, however, yielded comparable values for the shifts in Pos(G), irrespective of the increase in the water content (Figure S7 in the Supporting Information). Furthermore, the graphene/IL interfaces remained stable under ambient conditions, and the observed shifts in the Raman spectra did not change significantly when measurements were carried out on samples several days apart (Figure S8 in the Supporting Information). These control experiments demonstrate that the observed changes in the charge carrier concentration are not caused by the water present in these systems and that the CnMIM-BF4/ graphene interface is stable. The observed trend in n-type doping of graphene by CnMIM-BF4 ILs is in contrast to what has been observed for typical (alkylated) organic compounds. It is readily expected that the density of ions adsorbed on the graphene surface decreases with an increase in the alkyl chain length, given that the alkyl chains are known to adsorb parallel to the graphene surface.25 However, despite the decrease in the density, the amount of n-type doping is found here to increase with increasing chain length. On the contrary, n-type doping of graphene caused by adsorption of alkyl substituted primary amines was found to exhibit the opposite trend. As the density of amine functional groups (and thus the dopant species) adsorbed on the surface decreased as a result of an increasing chain length, the extent of n-type doping was also found to decrease.38 Similar observations were recorded for adsorption E


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Figure 6b shows a snapshot of C8MIM-BF4 at the interface within 6 Å above the graphite surface. The alkyl chains of the cations are mostly in extended conformation and flat on the graphene surface. Compared to C2MIM-BF4 (Figure 6a), the density of BF4− anions in close contact with the surface is smaller in the case of C8MIM-BF4 (Figure 6b), as the longer alkyl chains of the cations form a protecting coating over graphite.25 The corresponding charge density profiles for the ILs are displayed in Figure 7. Similar to the number density profile shown in Figure 5, we observe an oscillation of the charge density, corroborating a layered structure at the graphite/IL interface, as well as an enhancement of the charge density, compared to the bulk region. In the case of ILs with shorter chain lengths (i.e., C2MIM-BF4), distinct peaks can be seen at ∼5 Å and ∼8 Å from graphite (peaks 2 and 4, respectively, in Figure 7a). As the size of the alkyl side chain increases (C8MIM-BF4), the charge density peak at 8 Å (peak 4) is strongly reduced compared to that of C2MIM-BF4. Thus, with increasing chain length, the cations and anions beyond the first two charge density peaks (i.e., beyond ∼6 Å − 7 Å) from graphite become more and more randomly distributed along the z-axis. Considering a planar interface, the surface electrostatic potential φ(z) along the surface normal direction is calculated from the charge density by integrating Poisson’s equation. Because the bottom and top graphite layers are equivalent, the calculated electrostatic potential reported in Figure 7b is an average of the surface potential in both directions (“upward” and “downward”). As one moves away from the graphite surface, there is an accumulation of positive charge density (peak 1 in Figure 7a) associated with the cations, followed by a large peak at negative density due to the corresponding anions (peak 6 in Figure 7a). Because of the protective effect of the alkyl chains (see above), the anions are continuously pulled away from the surface into the bulk as the alkyl chain length increases; see, in particular, the negative charge density (peak 6) in the inset of Figure 7a. For all alkyl chain lengths, this results in a net drop of the electrostatic potential at the interface (Figure 7b). A potential drop leads to a decrease in

Figure 5. Number density profiles of the COM of CnMIM-BF4 layers sandwiched between two graphene planes (only the bottom half is shown for clarity). The cations and the anions are shown in blue and red, respectively. The graphite surface is located at z = 0.

previously reported for C4MIM-PF6.45 The position of the peaks in the number density profile of CnMIM-BF4 does not depend on the size of the alkyl side chain of the cation, and the layering of the ILs at the graphite surface is still preserved in all cases. The intensity of the peaks, however, decreases with increasing chain length as the bigger cations occupy more space at the graphite surface. At the graphene/IL interface, corresponding to the first density peak, the density of IL is increased, which stems from (i) the CH−π intermolecular interactions between the alkyl side chain and the graphite surface, leading to a favorable adsorption energy (whereas the unfavorable entropy loss at the interface is expected to have a minor effect in this case);46 and (ii) the π-stacking interactions between the imidazolium ring of the cation and the graphite surface.25

Figure 6. Snapshots of C2MIM-BF4 and C8MIM-BF4 within 6 Å above the graphite surface (in light gray). The CnMIM+ cations are in cyan, and the BF4− anions are in red. F


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Figure 7. Charge density and surface electrostatic potential profiles along the z-axis of CnMIM-BF4. (a) Charge density profiles; the arrows highlight the charge density peaks in the interfacial region, which extends up to 14 Å. Inset: Close-up view of the first two peaks is shown. (b) Calculated potential profile is an average over both directions. The point of zero charge (PZC) values corresponding to each IL are also displayed.

engineering aspects of this research appear to have significantly outpaced the complementary fundamental studies. Nevertheless, given the dependence of energy density on the nature of the graphene/IL interface, there is rising appreciation of fundamental studies aimed at elucidating the microscopic picture of this functional interface. In the results described above, we have investigated how an increase in the alkyl chain length on the cation of an ionic liquid changes the electronic properties of graphene. The experimental results clearly demonstrate that the charge carrier concentration in graphene increases with increasing chain length of the IL. Combined DFT and classical MD simulations show that the n-type doping of graphene in contact with the IL arises from electrostatic layering effects, with direct charge transfer contributing a small and opposite effect. Although this study shines light on the fundamental aspects of graphene/IL interactions, a number of facets still remain unexplored. It is well-known that the physicochemical properties of ILs depend on the particular combination of the cation and the anion. Thus, such studies need to be extended to a wider range of ILs exploring different cations and anions. Furthermore, to be directly useful for applications related to energy storage devices, the spectroscopic measurements need to be carried under the influence of an electric field to simulate the environment in an EDL supercapacitor. Last but not least, the basic information obtained from such studies, changes in charge carrier concentration, for example, need to be considered in the context of other important parameters such as electrochemical windows, viscosity, and wettability of ILs, which are known to be crucial for device efficiency. Some of the outstanding aspects mentioned above are currently the subject of an ongoing investigation.

the work function of graphene and, thus, promotes n-type doping. We note in passing that these changes are perfectly in line with the conclusions drawn from MD and DFTB calculations on the smaller Model 1 for Cn=2,8MIM-BF4 systems (Figure S9b). To further relate the theoretical calculations with the experimental results, the PZC is estimated based on the calculated electrostatic potential profile. The PZC is related to work function and thus the amount of doping.47 PZC can be defined as the potential drop (UEDL) within the EDL for uncharged electrodes,27 where UEDL is defined as the difference between the electrostatic potential on the electrode surface (φelectrode) and the potential in the bulk electrolyte (φbulk). In our work, φelectrode is set to zero, and φbulk can be estimated by averaging the potential in the central part of the box. From these calculations, UEDL is positive (see Figure 7b) and thus the PZC is also positive. In addition, the PZC increases with a magnitude that scales with the length of the alkyl side chains (see Figure 7b), which indicates an increasing amount of ntype doping with alkyl chain length. These results are in line with previously reported MD simulations, where the PZC for RMIM-BF4 ILs is positive and increases with alkyl chain length.24 The results of the calculations are fully consistent with the observed experimental trend observed here. In contrast, in the case of primary amines, the use of longer alkyl chains has been shown to yield less efficient n-type doping.42 This difference is related to the different role the alkyl chain plays in both situations. For amines, the alkyl chain acts as a spacer, reducing the density of n-type dopant moieties with increasing chain length. For ILs, however, the flat lying alkyl chains prevent anions from coming in close proximity to the surface (Figure 6), thereby increasing the net positive charge density near the surface with increasing chain length. The present results clearly indicate that, apart from the practical factors already identified,7 one also needs to take into account the microscopic picture of the graphene/IL interface in considering the material properties, for example, capacitance.

METHODS Raman Spectroscopy. All the ILs used in this study were purchased from IoLiTec-Ionic Liquids Technologies GmbH, and the stated purity of the ionic liquids is >99.5%. Water content of the ionic liquids was estimated by Karl Fischer titration with the aid of Metrohm 831 KF coulometer. All the ionic liquids were used as received. CVD-grown graphene samples (1 × 1 cm2) transferred to Si ++/SiO2 (300 nm) were obtained from Graphenea and were used as received. Raman experiments were performed at room temperature (21−23 °C) using an OmegaScopeTM 1000 (AIST-NT). Laser light from a He−Ne laser (632.8 nm) was reflected by using a dichroic mirror (Chroma, Z633RDC) and then focused onto the sample

CONCLUSIONS AND OUTLOOK Graphene/IL-based supercapacitors are promising energy storage devices. The surge of activity in this field is quite understandably catalyzed by the race to produce devices with high energy densities. As a consequence, the design and G


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ACS Nano surface by using an objective (MITUTOYO, BD Plan Apo 100×, NA 0.7). The optical density at the sample surface was about 800 kW cm−2. Raman scattering was collected with the same objective and directed to a Raman spectrograph (Horiba JY, iHR-320) equipped with a cooled charge-coupled device camera operating at −100 °C (Andor, DU920P) through the dichroic mirror, a pinhole, and a longpass filter (Chroma, HQ645LP). Accumulation time for all spectra was 6 s. For each experiment, Raman maps were obtained for pristine as well as IL-functionalized graphene samples. A 10 μL drop of IL was placed onto a pristine graphene sample. The experiments were repeated in two or three sessions using different graphene samples for reproducibility and to avoid experimental artifacts, if any. Raman data were analyzed using MATLab software. Modeling. The following two models were built. Model 1. We used a 29.925 Å × 29.616 Å × 40.000 Å simulation box containing C2MIM-BF4 or C8MIM-BF4 molecules sandwiched between two graphene sheets. Model 2. The graphite (0001) substrate was built from an AB stacking of 10 and 9 graphene layers for the bottom and top graphite layers, respectively, with a separation of 86.79 Å. The simulation box dimension is 147.90 Å in the direction normal to the graphite surface. The graphene layers are separated by 3.395 Å and are 63.96 Å × 63.90 Å in size, corresponding to a 26 × 15 supercell of a rectangular unit cell with dimensions of 2.46 Å × 4.26 Å. In both models, the graphene layers were frozen during the molecular dynamics runs and simulated within periodic boundary conditions. Based on the volume mass density of the studied ILs, the space between the graphene layers was filled with a specific number of ions pairs. To describe the ILs at the classical level, we adopted the nonpolarizable force field as proposed by Canongia Lopes et al.48 The graphite layers were modeled as uncharged particles interacting with Lennard-Jones potential, corresponding to the sp2 carbon atoms in the AMBER force field.49 We used the Lorentz−Berthelot combination rule to handle the van der Waals interactions between ILs and the graphite layers. The nonbonded interactions were treated using the cutoff method with a cutoff distance of 12 Å, and the longrange electrostatic interactions were handled in the reciprocal space using the particle mesh Ewald approach,50 as implemented in the GROMACS software package.51 We used an integration time step of 2 fs and applied C−H bond constraints with the LINCS algorithm.52 Regarding the MD simulations, we performed a thermal annealing for 10 ns (NVT equilibration with the Berendsen thermostat), while decreasing the temperature from 1000 K to 600 K. Then, the analyses were performed on a MD trajectory of 20 ns at 450 K using the Berendsen thermostat in the NVT ensemble. The charge density distribution was calculated from the charge of the atoms and their average position (through the MD trajectory). The simulation box was divided along the z-direction into thin slices with dz = 0.1 Å, and the charge density was computed for each slice. The charge density distribution was then obtained as a function of z, by using the gmx_density program implemented in GROMACS. Assuming a planar capacitor, the electrostatic potential of the system can be calculated from the charge density distribution. The partial derivative of the potential with respect to the z-axis is the electric field intensity along the z-direction. According to Gauss’s law, the electric field intensity of a charged plate is equal to the surface charge density divided by the vacuum permittivity. Therefore, the surface charge density of each small box can be obtained from the previously calculated charge density of each box multiplied by dz and assuming a uniform charge distribution. In this way, the profile of the electric field intensity can be calculated as a function of z. Eventually, the distribution of the electrostatic potential can be obtained by integrating the electric field intensity in the z-direction. This can be done using the gmx_potential command in GROMACS. Tight-binding density functional calculations were performed using the 18.2 version of the DFTB+ package44 Single-point calculations have been done for C2MIM-BF4 and C8MIM-BF4 ILs on their geometry previously obtained by MD simulations. The calculation of the electrostatic potential was done for the gamma point only and using a (51 × 51 × 101) grid point covering all of the unit cell. The

surface potential along the normal of the graphene sheet (z-axis) was then extracted from this grid, and the electrostatic potential was calculated as an average of this surface potential in both directions.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b09768. Additional Raman data, Raman analysis of IL adsorption on graphene supported by h-BN, dependence of water content, supplementary DFT and MD simulation data (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Gangamallaiah Velpula: 0000-0002-0642-6892 David Cornil: 0000-0002-9553-1626 Peter Walke: 0000-0003-3586-0484 Hiroshi Uji-i: 0000-0002-0463-9659 David Beljonne: 0000-0002-2989-3557 Roberto Lazzaroni: 0000-0002-6334-4068 Kunal S. Mali: 0000-0002-9938-6446 Steven De Feyter: 0000-0002-0909-9292 Author Contributions ¶

G.V. and R.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS G.V. acknowledges the Marie Sklodowska-Curie individual fellowship award (No. 706314, GRAPHIL). This work is supported by the Fund of Scientific Research−Flanders (FWO), KU Leuven−Internal Funds, Belgian Federal Science Policy Office (IAP-7/05), and European Research Council under the European Union’s Seventh Framework Programme (FP7/2007−2013)/ERC Grant Agreement Nos. 340324 and 280064. The Mons−Leuven collaboration is supported by the Science Policy Office of the Belgian Federal Government (BELSPO; IAP project 7/5) and by the joint FWO-FNRS EOS project 30489208 (2Dto3D project). Research in Mons is also supported by the FNRS “Consortium des Equipements de Calcul Intensif−CECI” program Grant No. 2.5020.11. We also thank the group of Prof. Tianying Yan from Nankai University, China, for their help and very insightful discussions on molecular modeling. H.U. acknowledges KAKENHI (JP17H03003, JP17H05244, JP17H05458). D.B. is a FNRS Research Director. REFERENCES (1) Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845−854. (2) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (3) Raccichini, R.; Varzi, A.; Passerini, S.; Scrosati, B. The Role of Graphene for Electrochemical Energy Storage. Nat. Mater. 2015, 14, 271−279. (4) Bonaccorso, F.; Colombo, L.; Yu, G.; Stoller, M.; Tozzini, V.; Ferrari, A. C.; Ruoff, R. S.; Pellegrini, V. Graphene, Related TwoH


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