LETTER pubs.acs.org/JPCL
Ion Adsorption at the Graphene/Electrolyte Interface Daniel J. Cole,‡ Priscilla K. Ang, and Kian Ping Loh* Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
bS Supporting Information ABSTRACT: The segregation of ions (Na+, OH , H3O+, and Cl ) at the graphene/water interface, as well as at the graphene oxide/water interface, is investigated by classical molecular dynamics simulations employing polarizable force fields. Hydronium and hydroxide ions show a strong affinity for the hydrophobic graphene/water interface. This behavior is rationalized by consideration of both the amphiphilic nature of the two ions and the favorable interactions between the surface-induced electrical potential gradient and the permanent and induced dipoles of the ions. Ionizable groups on the graphene oxide surface are able to interact strongly with sodium ions that are repelled from the pristine graphene surface. SECTION: Surfaces, Interfaces, Catalysis
he unique electrical1,2 and electrochemical3 properties of graphene position it as an attractive candidate for applications in carbon-based electronic devices, energy storage, and chemical biosensors. The technological challenge facing the development of graphene electrochemical electrode devices hinges upon its atomic structure, which influences sensitivity toward its local chemical environment. This is especially pertinent for the operation of graphene in an aqueous environment where analyte interactions with sp2 carbon atoms differ from interactions with oxygenated functional groups.4,5 Recently, graphene field effect transistors and electrochemical electrodes have shown promise in the electrical detection of inorganic6 and biochemical4,7 analytes. In the solution gate field effect transistor (SGFET) configuration, modulation of the channel conductance is achieved by applying a gate potential from a reference electrode placed on top of the channel, across an electrolyte, which acts as the dielectric. The gate potential induces the buildup of ions at the graphene/solution interface, which in turn induces charge carriers by capacitive charging of the ideally polarizable interface. In particular, it has been shown that, because of the ambipolar nature of graphene, adsorbed hydroxide (OH ) and hydronium (H3O+) ions are able to modulate the channel conductance by inducing holes and electrons, respectively.6 The traditional picture of an aqueous/low-dielectric interface, in which the media are treated as continuous dielectrics and the ions as point charges, predicts repulsion of ions from the interface by image charges of the same sign.8 However, ab initio simulations of the interface between hydrogenated graphene and water reveal that both H3O+ and OH may act as surfactants at hydrophobic interfaces with OH more localized in the surface layer.9 This segregation is attributed to the amphiphilic nature of the two ions; the O of H3O+ and the H of OH are relatively weak hydrogen bond acceptors and donors, and hence, there is a
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direct hydrophobic attraction between the surface and the ions. A classical simulation employing polarizable force fields has found OH buildup in the surface layer at a hydrophobic wall.10 Additionally, it has been shown that interaction between the permanent dipole of OH and surface water is the dominant adsorption mechanism for nonpolarizable hydroxide ions at rigid, hydrophobic interfaces, such as that between graphene and water,11,12 and is important even for large biomolecules.13 Appreciable buildup of H3O+ and OH at the graphene/ electrolyte interface via the mechanisms described above leads to modulation of the channel conductance by the induction of charge carriers within the graphene sheet. The dependence of the carrier density on the applied gate potential in a graphene SGFET has recently been modeled using an extended Poisson Boltzmann equation, which employs a spatially varying dielectric for water and a double layer capacitance via the potential of mean force (PMF) for each ion at the graphene/electrolyte interface.14,15 Although the agreement with experiment is reasonable, the PMFs for each ion are obtained from nonpolarizable molecular dynamics (MD) simulations of ions at hydrophobic selfassembled monolayers of alkane chains. Furthermore, it is unknown whether ion densities obtained from the Poisson Boltzmann equation at nonzero graphene surface charge are a good approximation to those obtained from atomistic simulations and whether ion segregation may be controlled by the addition of oxygenated functional groups. To address these issues, in this Letter, we calculate the densities of ions (Na+, Cl , OH , H3O+) at a model of the graphene/electrolyte interface, accounting for the polarizability of the ions and considering Received: June 7, 2011 Accepted: July 5, 2011 Published: July 05, 2011 1799
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Figure 1. Ion densities at the positive, neutral, and negative graphene/water interfaces. O and H (neutral surface only) of H3O+ and OH are represented by solid and dashed lines, respectively. (Inset) Typical snapshots of H3O+ and OH in the first water layer.
Figure 2. (a) The densities of oxygen and hydrogen atoms of water molecules at the uncharged graphene surface. (b) Separation of the atomic densities leads to charge density oscillations that vary with the surface charge.
both neutral and charged surface models, as well as oxidized and hydroxylated graphene. We show that ion segregation at the graphene/electrolyte interface involves a balance between the loss of the ions’ solvation shells and favorable interactions between the surface and ions and also between the surface-induced water structure and the permanent and induced dipoles of the ions. The density profiles of ions (H3O+ and Cl in acidic solution and OH and Na+ ions in basic solution) at the uncharged pristine graphene surface are shown in Figure 1 (black lines). Na+ and Cl segregation follows the trend observed in a previous simulation employing polarizable classical force fields to describe ion adsorption at hydrophobic surfaces.16 Na+ loses ions from its solvation shell of 5.4 water molecules as it approaches the hydrophobic surface. Its polarizability is negligible, and therefore, there are no compensating dipole dipole interactions, and the ion is repelled from the interface. In contrast, Cl shows relatively strong adsorption at the uncharged graphene/water interface. The number of nearest-neighbor water molecules decreases as the ion approaches the surface, from 6.7 in the bulk to 5.9, and therefore, there must be some compensating driving force to adsorption. Any surface in contact with bulk water will have an effect on the intrinsic ordering of water molecules in its
proximity. In particular, rigid hydrophobic surfaces, such as graphene, will disrupt water water hydrogen bonds in the liquid, resulting in a layered structure of water molecules close to the surface, as observed in a number of computational studies.10 13 The density distributions of water H atoms do not overlap with those of the O atoms in the vicinity of the surface (Figure 2a). This anisotropy in the orientations of the water molecules, along with their permanent electric dipoles, results in an oscillating charge distribution (Figure 2b) at the surface/water interface that penetrates around 8.5 Å into the solvent and, in this case, acts to stabilize the highly polarizable Cl ion via dipole dipole interactions (Figure 3) in a similar manner to that proposed by Zangi and Engberts for OH adsorption at a hydrophobic surface.12 In full agreement with Car Parrinello MD simulations of OH at the hydrogen-terminated graphene surface,9 we find here that the ion is found in the first water layer, with its H atom pointing toward the interface. A smaller, second peak can be seen at 5.5 Å from the interface, which matches that in simulations employing nonpolarizable force fields.12 These adsorption sites are stabilized by interactions between the ion’s permanent and induced dipoles and the surface-induced solvent charge density oscillations. In the surface layer, the H atom points toward the 1800
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The Journal of Physical Chemistry Letters surface, forming a hydrophobic interaction with the graphene surface, which counters the loss of hydrogen-bonding interactions with water (on average, it has 5.2 neighbors in the first water layer, compared with 5.7 in the second). H3O+ is strongly peaked at a separation of 3.1 Å from the graphene sheet, within the first water layer. Its H atoms point toward the bulk liquid, enabling the formation of three hydrogen bonds with three first layer water molecules. Indeed, at the interface, H3O+ has, on average, 3.0 nearest neighbors, identical to the number in the bulk, and therefore, adsorption is favored by direct hydrophobic interaction with the surface, with no associated loss of the ion’s solvation shell, as has previously been proposed following classical MD simulations of H3O+ at the air/ water interface.17 Interestingly, we have also observed a second stable adsorption site for H3O+, at a separation of 6.2 Å from the graphene sheet, which has not been observed in simulations of low-concentration electrolytes.10 At this site, there are significant interactions between H3O+ and Cl ions adsorbed in the inner water layers. We propose that these ion ion pairs are stabilized by favorable interactions with the solvent charge density gradient induced by the graphene surface. As shown in Figure 3, the induced dipole on the H3O+ ion at 6.2 Å interacts favorably with the positive solvent charge density gradient at that point, while the oppositely polarized Cl ion at 5.5 Å is stabilized by the negative charge density gradient. As discussed in the Supporting Information, these interactions are not strong enough to cause significant ion segregation at low electrolyte concentrations, indicating that both ion solvent and ion ion interactions are required to stabilize adsorption at sites farther from the graphene surface.
Figure 3. Interaction between the induced ionic polarization and solvent charge density oscillations (black line, right axis) as a function of distance from the graphene/water interface. Negative polarization denotes negative charge pointing toward the surface. Interactions between induced ionic dipoles and the solvent charge density gradient favor ion segregation to the surface (permanent polarization shows the same trend).
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The above results indicate that ions do adsorb asymmetrically at the graphene/water interface, which induces mirror charges in the graphene sheet. H3O+ adsorbs closer to the surface and at higher concentration than Cl and hence, at low pH, will induce the accumulation of electrons. Conversely, at high pH, there is a depletion of electrons and accumulation of holes due to the preferential surface adsorption of OH ions over small, positively charged ions. Application of a gate voltage across a SGFET alters both the direct surface ion interactions (via the Coulomb interaction) and, significantly, the solvent structure at the electrode interface. In particular, Figure 2b reveals that the magnitude of the charge density oscillations in the solvent increases in the first water layer (within 4 Å of the surface) at the negative graphene electrode but decreases at the positive electrode. In contrast, charge density oscillations are enhanced in the 4 8 Å region at the positive electrode over the neutral and negatively charged surfaces. Figure 1 shows the ion density distributions at surfaces carrying charges of (4.8 μC/cm2, and Figure 4 reveals that, in general, the net ionic charge distribution shows the expected response to changes in surface charge, indicating that the segregation of OH , Na+, and Cl is dominated by surface ion Coulomb interactions and, as such, should be well-described by a mean field Poisson Boltzmann model. Importantly, however, we do not observe the expected decrease in the concentration of H3O+ in the second peak (at 6.2 Å) at the positively charged surface. That is, the ion ion and enhanced dipole dipole interactions in the second water layer are able to overcome the surface ion Coulomb repulsion. As such, the density distribution of ions at charged interfaces should account for both the molecular nature of the ion ion and ion solvent interactions as well as the applied field within the Poisson Boltzmann approach. For nonpolarizable ions, such as Na+, ionic interactions with the hydroxylated or oxidized graphene surface are one way to
Figure 5. Na+ density distributions at the bare graphene surface and the C OH and C O terminated surfaces. (Inset) Na+ penetrates into the first water layer.
Figure 4. Net ionic charge density at graphene surfaces of varying applied electric fields. Interactions of H3O+ with both inner-layer negative ions and the surface-induced water layers leads to significant adsorption, even at the positively charged surface. 1801
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The Journal of Physical Chemistry Letters overcome loss of the ion’s solvation shell upon adsorption. The concentrations of H3O+, OH , and Cl ions are reduced at the functionalized surface relative to the pristine, uncharged graphene surface because the surface groups form few hydrogen bonds directly with the three ions. In contrast, Na+ concentration is enhanced at the hydroylated surface and further enhanced at the oxidized surface, and two new density peaks are observed (Figure 5). This change in adsorption behavior is a result of Na+ retaining its solvation shell of approximately five oxygen atoms by adsorbing at the surface, surrounded by four water molecules and one oxide group. Under basic conditions and at positive gate voltages, we would expect both the applied field and direct surface ion interactions to favor sodium ion segregation close to an oxidized graphene electrode. The adsorption of these ions near the surface in response to the ionizable groups on the graphene oxide surface ( OH and COOH) contributes to the pH sensitivity of the oxygenated interface. To correctly describe the transport properties of H3O+ and OH in a hydrogen-bonded water network requires expensive quantum chemical calculations, and quantitative measures of ion concentration at a surface/electrolyte interface depend strongly on factors such as force field parametrization and surface rigidity.11 However, we have shown here that classical, polarizable force fields are able to capture the important qualitative features of ion segregation at an uncharged hydrophobic interface.9 We have considered the technologically interesting case of graphene, though the results are expected to be representative of any rigid, hydrophobic surface. Segregation of ions involves a balance between ion surface and ion water interactions, which may be affected by an external electric field or the presence of oxygenated defects. The asymmetric adsorption of ions suggests that electrical response on the pristine graphene surface arising from a shift in the flat band potential or conductivity will be more sluggish because the ion segregation is heavily influenced by dipole dipole interactions in the structured water layer. In contrast, hydroxylated or oxidized graphene surfaces can interact directly with nonpolarizable ions such as Na+ and may show stronger pH response.
’ COMPUTATIONAL METHODS Classical simulations were performed using the AMBER10 MD package,18 using an integration time step of 1 fs and the SHAKE algorithm to constrain H-containing bonds.19 Longranged Coulomb interactions were treated using the particle mesh Ewald sum,20 with a real space cutoff of 10 Å. The vdW interaction cutoff length was set to 10 Å. The temperature was controlled by the weak-coupling algorithm with a 2 ps time constant, and during equilibration, the systems were heated at a rate of 50 K per 20 ps. The graphene surface model consisted of a single, rigid sheet of 448 C atoms, which were fixed to their initial positions. The surface area of the sheet was 11.8 nm2, with a vacuum gap of approximately 40 Å separating periodic images in the z-direction. To investigate water layering at the graphene surface, the vacuum layer was filled with 1564 POL3 polarizable water molecules,21 and the height of the supercell was adjusted in the z-direction to obtain a water density of approximately 1 g/cm3 between the graphene sheets. Interactions between the surface and water molecules were represented by a Lennard-Jones potential, using the standard Lorentz Berthelot rules to combine the parameters of the POL3 water model with the parameters of an sp2 carbon atom
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taken from the AMBER parm99SB force field. To simulate the effect of an applied electric field, the entire system was doubled in the direction perpendicular to the surface, resulting in two distinguishable graphene sheets separated from each other by 43.5 Å of water in a simulation cell of total height 87.0 Å. A positive charge of 0.008 e was assigned to each carbon atom of one surface such that the total surface charge density was +4.8 μC/cm2. An equal and opposite surface charge was applied to the second surface, resulting in a net electric field across the water layer. Following equilibration of each of the above systems at 300 K, 10 approximately evenly spaced water molecules were replaced by 5 hydronium (or hydroxide) and 5 chloride (or sodium) ions within each region between the graphene sheets, to represent 0.14 M acidic (or basic) solutions. H3O+ and OH structures and interaction parameters were taken from refs 8 and 10, which have been parametrized against ab initio structural and energetic properties of water clusters.8 The parameter set for the OH ion has been shown to reproduce experimental coordination numbers of ions in bulk water and to be stable to small changes in the parametrization.10 Further, the relative concentrations of H3O+ and OH ions at the hydrophobic water/air interface have been shown to be consistent with vibrational sum frequency generation spectroscopy22 and infrared spectroscopic observation of ice nanocrystal surfaces.23 Eigen and Zundel forms of the hydronium ion have been shown to have similar binding affinities for the water/air interface,23 and therefore, we considered only the more abundant Eigen form here. Parameters for Na+ and Cl ions were taken from refs 16 and 24 and have been shown to reproduce experimental energetic and structural properties of the solvated ions.24 Following ref 10, the polarizabilities of hydroxide oxygen atoms and chloride ions were reduced from 2.10 to 1.70 Å3 and from 3.69 to 3.50 Å3, respectively, to avoid possible “polarization catastrophes” in long dynamical simulations. At the uncharged surface, the solutions were equilibrated for 1 ns, and production runs were continued for 10 ns, at which point the average number of ions adsorbed within 8.5 Å of the graphene surface was converged. At the charged surfaces, longer equilibration periods of 7 ns were required as ions traversed between the surfaces, followed by 6 ns production runs. All simulations were performed in the NVT ensemble, and particle coordinates and induced dipole moments were saved every 0.1 ps during production runs. To model the functionalized graphene surfaces, we have constructed a hydroxylated graphene surface, where 20 OH groups were bonded to C atoms on each side of the graphene sheet at a surface coverage of 4%. O and H atoms of the hydroxyl group were assigned partial charges of 0.5 and +0.3 e , respectively, and to maintain charge neutrality, C atoms bonded to a hydroxyl group were assigned charges of +0.2 e . C O bonds and C C O angles remained rigid, whereas C O H angles and Lennard-Jones parameters for oxygen and hydrogen of the hydroxyl groups were taken from the AMBER parm99SB force field. The structure of this simple model is in reasonable agreement with ab initio simulations of a hydroxylated graphene sheet.25 A graphene oxide sheet with a net charge of 12 e was created by removing the H atom from each hydroxyl group. Twelve aqueous Na+ counterions were used to ensure charge neutrality. As before, both surfaces were equilibrated in 0.14 M acidic and basic solutions for 1 ns before ion density distributions were calculated over 7.5 ns production runs. 1802
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’ ASSOCIATED CONTENT
bS
Supporting Information. Discussion of ion ion correlations. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Current Addresses ‡
TCM Group, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom.
’ ACKNOWLEDGMENT L.K.P. thanks the NRF-CRP grant “Graphene Related Materials and Devices” R-143-000-360-281. Computational resources were provided by the Cambridge HPC Service. ’ REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666–669. (2) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652–655. (3) Pumera, M. Graphene-Based Nanomaterials and their Electrochemistry. Chem. Soc. Rev. 2010, 39, 4146–4157. (4) Pumera, M.; Scipioni, R.; Iwai, H.; Ohno, T.; Miyahara, Y.; Boero, M. A Mechanism of Adsorption of β-Nicotinamide Adenine Dinucleotide on Graphene Sheets: Experiment and Theory. Chem.— Eur. J. 2009, 15, 10851–10856. (5) Lim, C. X.; Hoh, H. Y.; Ang, P. K.; Loh, K. P. Direct Voltammetric Detection of DNA and pH Sensing on Epitaxial Graphene: An Insight into the Role of Oxygenated Defects. Anal. Chem. 2010, 82, 7387–7393. (6) Ang, P. K.; Chen, W.; Wee, A. T. S.; Loh, K. P. Solution-Gated Epitaxial Graphene as pH Sensor. J. Am. Chem. Soc. 2008, 130, 14392–14393. (7) Ang, P. K.; Jaiswal, M.; Lim, C. H. Y. X.; Wang, Y.; Sankaran, J.; Li, A.; Lim, C. T.; Wohland, T.; Barbaros, O.; Loh, K. P. A Bioelectronic Platform Using a Graphene Lipid Bilayer Interface. ACS Nano 2010, 4, 7387–7394. (8) Vacha, R.; Buch, V.; Milet, A.; Devlin, J. P.; Jungwirth, P. Autoionization at the Surface of Neat Water: Is the Top Layer pH Neutral, Basic, or Acidic? Phys Chem. Chem. Phys. 2007, 9, 4736–4747. (9) Kudin, K. N.; Car, R. Why Are Water Hydrophobic Interfaces Charged? J Am. Chem. Soc. 2008, 130, 3915–3919. (10) Vacha, R.; Horinek, D.; Berkowitz, M. L.; Jungwirth, P. Hydronium and Hydroxide at the Interface between Water and Hydrophobic Media. Phys. Chem. Chem. Phys. 2008, 10, 4975–4980. (11) Vacha, R.; Zangi, R.; Engberts, J. B. F. N.; Jungwirth, P. Water Structuring and Hydroxide Ion Binding at the Interface between Water and Hydrophobic Walls of Varying Rigidity and van der Waals Interactions. J. Phys. Chem. C 2008, 112, 7689–7692. (12) Zangi, R.; Engberts, J. B. F. N. Physisorption of Hydroxide Ions from Aqueous Solution to a Hydrophobic Surface. J. Am. Chem. Soc. 2005, 127, 2272–2276. (13) Cole, D. J.; Payne, M. C.; Colombi Ciacchi, L. Water Structuring and Collagen Adsorption at Hydrophilic and Hydrophobic Silicon Surfaces. Phys. Chem. Chem. Phys. 2009, 11, 11395–11399. (14) Dankerl, M.; Hauf, M. V.; Lippert, A.; Hess, L. H.; Birner, S.; Sharp, I. D.; Mahmood, A.; Mallet, P.; Veuillen, J.-Y.; Stutzmann, M.;
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Garrido, J. A. Graphene Solution-Gated Field-Effect Transistor Array for Sensing Applications. Adv. Funct. Mater. 2010, 20, 3117–3124. (15) Schwierz, N.; Horinek, D.; Netz, R. R. Reversed Anionic Hofmeister Series: The Interplay of Surface Charge and Surface Polarity. Langmuir 2010, 26, 7370–7379. (16) Horinek, D.; Netz, R. R. Specific Ion Adsorption at Hydrophobic Solid Surfaces. Phys. Rev. Lett. 2007, 99, 226104. (17) Petersen, M. K.; Iyengar, S. S.; Day, T. J. F.; Voth, G. A. The Hydrated Proton at the Water Liquid/Vapor Interface. J. Phys. Chem B 2004, 108, 14804–14806. (18) Case, D. A.; Darden, T. A.; Cheatham, T. E., III; Simmerling, C. L.; Wang, J.; Duke, R. E; Luo, R.; Crowley, M.; Walker, R. C.; Zhang, W.; et al. Amber10; University of California: San Francisco, CA, 2008. (19) Ryckaert, J.-P.; Ciccotti, G.; Berendsen, H. J. C. Numerical Integration of the Cartesian Equations of Motion of a System with Constraints: Molecular Dynamics of n-Alkanes. J. Comput. Phys. 1977, 23, 327–341. (20) Essmann, U.; Perera, L.; Berkowitz, M. L.; Darden, T.; Lee, H.; Pedersen, L. G. A Smooth Particle Mesh Ewald Method. J. Chem. Phys. 1995, 103, 8577–8593. (21) Caldwell, J. W.; Kollman, P. A. Structure and Properties of Neat Liquids Using Nonadditive Molecular Dynamics: Water, Methanol, and N-Methylacetamide. J. Phys. Chem. 1995, 99, 6208–6219. (22) Mucha, M.; Frigato, T.; Levering, L. M.; Allen, H. C.; Tobias, D. J.; Dang, L. X.; Jungwirth, P. Unified Molecular Picture of the Surfaces of Aqueous Acid, Base, and Salt Solutions. J. Phys. Chem. B 2005, 109, 7617–7623. (23) Buch, V.; Milet, A.; Vacha, R.; Jungwirth, P.; Devlin, J. P. Water Surface is Acidic. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 7342–7347. (24) Dang, L. X. Computational Study of Ion Binding to the Liquid Interface of Water. J. Phys. Chem. B 2002, 106, 10388–10394. (25) Schniepp, H. C.; Li, J. -L.; McAllister, M. J.; Sai, H.; HerreraAlonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide. J. Phys. Chem. B 2006, 110, 8535–8539.
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