Article pubs.acs.org/JPCC
Interactions of Organic Solvents at Graphene/α-Al2O3 and Graphene Oxide/α-Al2O3 Interfaces Studied by Sum Frequency Generation Jennifer L. Achtyl,† Ivan V. Vlassiouk,‡ Sheng Dai,§,∥ and Franz Geiger*,† †
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Measurement Science & System Engineering Division and §Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37931, United States ∥ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States ‡
S Supporting Information *
ABSTRACT: The adsorption of 1-hexanol from cyclohexane-d12 at single-layer graphene/α-Al2O3 interfaces was probed at mole percent values as low as 0.05 in the C−H stretching region using vibrational sum frequency generation (SFG). The SFG spectra are indiscernible from those obtained in the absence of graphene, and from those obtained in the presence of graphene oxide films prepared via oxygen plasma treatment of pristine single-layer graphene. A Langmuir adsorption model yields observed free adsorption energies of −19.9(5) to −20.9(3) kJ/mol for the three interfaces. The results indicate that the molecular structure of the hexanol alkyl chain is subject to the same orientation distribution when graphene, oxidized or not, is present or absent at the α-Al2O3/cyclohexane-d12 interface. Moreover, it appears that the adsorption of 1-hexanol in this binary mixture is driven by hexanol interactions with the underlying oxide support, and that a single layer of graphene does not influence the extent of this interaction, even when defects are introduced to it. Finally, our structural and quantitative thermodynamic data provide important benchmarks for theoretical calculations and atomistic simulations of liquid/graphene interfaces. We hypothesize that defects emerging in graphene during operation of any device application that relies on layered solvent/graphene/oxide interfaces have little impact on the interfacial structure or thermodynamics, at least for the binary mixture and over the range of defect densities probed in our studies. competition and interaction of solvents at the interface.22,23 Moreover, there is a need to understand how defects in graphene films and how the underlying substrate impact the interfacial properties of interest.4,21,30−35 Due to the lack of experimental techniques that are appropriate for accessing graphene/liquid interfaces with surface specificity and without damage to the surface, many studies have been restricted to computational approaches such as molecular dynamics simulations and density functional theory calculations to examine the structure and adsorption of ionic liquids,13,29,36,37 hydrocarbons,18,26,38 small molecules,26 and water26,27,39 at graphene interfaces. However, as recently shown, the nonlinear optical techniques of sum frequency generation (SFG) and second harmonic generation (SHG) can be utilized as experimental methods to examine structure, orientation, and adsorption selectively at the graphene/liquid interface in situ, under relevant experimental conditions, and without causing damage to the sample or a need for vacuum conditions.12,40,41 For instance, SFG has been used to probe the molecular orientation of the ionic liquids, 1-butyl-3-methylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium
I. INTRODUCTION Graphene is recognized as a material with enormous potential. Due to its beneficial and tunable electronic, chemical, mechanical, thermal, and magnetic properties, graphene and graphene derivatives are being utilized for a large variety of applications spanning fields of energy storage and conversion, separation, sensing, biomedicine, catalysis, and material science.1−11 Many of these technologies rely on processes occurring at a fluid−graphene interface.13,15−21 For example, graphene applications such as supercapacitors, rechargeable batteries, biosensors, chemiresistors, dye-sensitized solar cells, separation membranes, and graphene preparation and modification methods involve a liquid/graphene interface that may include ionic liquids,12−14 organic solutions,15−19 aqueous solutions, 16,17,20 and often heterogeneous solvent mixtures.21−25 Despite the promise of a wide applicability of graphene, there remains a general lack in the molecular level understanding of the graphene/liquid interface. This situation has motivated calls for fundamental experimental and computational studies to examine the chemical and physical properties of the graphene interface from a molecular perspective.12,26−28 Important properties to examine include the liquid structure at the interface,13,21−23,27,29 the strength of the interfacial interaction energies,21,23,24,26,27,29 and with the case of mixed solvents, the © 2014 American Chemical Society
Received: May 14, 2014 Revised: July 8, 2014 Published: July 8, 2014 17745
dx.doi.org/10.1021/jp5047547 | J. Phys. Chem. C 2014, 118, 17745−17755
The Journal of Physical Chemistry C
Article
delocalized electrons within the substrate. With single-layer graphene, we have previously reported that we do not observe a strong nonresonant signal, and a weak SFG response of the bare graphene surface is only achieved with a 20-min-long signal integration time.40 The resonant portion of χ(2) depends on the number and overall orientation of adsorbates at the interface, which is defined by
dicyanamide, over graphene surfaces, and the stretching vibration of chemically bound hydrogen on monolayer graphene.12,35,42−44 In our previous publications, we utilized SHG to examine the electrical double-layer structure and ion adsorption energies in aqueous environments over single-layer defected and pristine graphene interfaces; we found that the underlying supporting substrate greatly influences graphene interfacial properties.40,41 These results are similar to those reported by Baldelli et al., who reported that the underlying CaF2 and BaF2 substrates may dictate the adsorption and orientation of ionic liquids at the graphene surface.12,44 Here, we extend our studies of single-layer graphene in aqueous systems40,41 to binary mixtures of organic molecules with the goal of determining if the hydrophobic graphene surface alters the orientation and adsorption of polar and nonpolar organic solvents. As many applications could involve mixtures of solutions, it is necessary to understand which solvents, if any, are present at the surface, and if certain solvents may be excluded from the interface at certain mixing ratios. In our previous work,45 we probed the structure and thermodynamics of solute/solvent interactions of 1-hexanol and cyclohexane at the α-Al2O3 interface and found that the solute becomes the solvent at a 10% mole fraction at the interface. Here, we work with this well-characterized model system to determine if the presence of a hydrophobic single layer of graphene over an α-Al2O3 substrate perturbs the adsorption of a polar organic solute (1-hexanol) in a nonpolar solvent (cyclohexane). Additionally, we introduce defects into the graphene film and examine how defects affect the interaction of 1-hexanol and cyclohexane. The studies presented in this report will serve as a benchmark for future experimental and computational studies of the liquid/graphene interface.
χR(2) ∝ Ns⟨βv ⟩ ∝ Ns
(3)
3. EXPERIMENTAL SECTION 3.1. Laser System. Details of our laser setup and sample cells for the SFG experiments have been described in detail previously.49,50,72,73 Briefly, a regeneratively amplified kHz Ti:sapphire laser system (Spectra Physics Spitfire Pro, 800 nm, 2.5 mJ/pulse) produces 800 nm laser light with 120 fs pulse duration. This beam is directed through a 50/50 beamsplitter, half of which pumps an optical parametric amplifier (OPA-800CF, Spectra Physics) to produce broadband IR laser light (∼140 cm−1 fwhm) specifically tuned to the CH stretching region (3.0−4.0 μm). The second half of the beam is used as the 800 nm visible upconverter and is directed though two variable density filters (Edmund Optics) to attenuate the power to 1 μJ per pulse, which is well below the damage threshold of the graphene films. The attenuated visible light then passes through a home-built time delay stage, which is used to optimize temporal overlap of the two beams, and then through a narrow bandpass filter (F1.1-8000.0-UNBLK-1.00, CVI Melles Griot) providing SFG spectral resolution of 10 cm−1. The IR and visible beams are then overlapped spatially and temporally at the sample interface at incident angles of 60° and 45° from surface normal, respectively. The resulting SFG signal then passes through the appropriate filters (long-pass filter 600 nm cutoff, and Notch Plus Filter, Kaiser Optical Systems Inc.) to remove any reflected 800 nm light and to separate contributions from other nonlinear processes before it is dispersed and detected using a 0.5 m spectrograph (Acton Research) coupled to a liquid nitrogen cooled, back-thinned, charged couple device (CCD) camera (Roper Scientific, 1340 × 100 pixels). By selecting the polarization of the incident visible and IR beams, and the resulting SFG beam, we selectively probe for components of the vibrational modes aligned at specific orientations from the surface. For the work presented here, all SFG spectra were collected with the SSP-polarization
(1)
Here, |χ | is the square modulus of the second-order susceptibility tensor of the interface, and Ivis and IIR are the intensities of the input visible and IR light fields, respectively. The χ(2) term is composed of both resonant, χ(2) R , and nonresonant, χ(2) , contributions, NR (2) 2
(2) −iϕ χ (2) = χR(2) + χNR e
ωv − ωIR − i Γv
Here, Ns is the number of adsorbates at the surface, ⟨βv⟩ is the orientational average of the molecular hyperpolarizability, AK is the IR transition moment, MIJ is the Raman transition probability, ωv is the frequency of the resonant mode, ωIR is the frequency of the incident IR beam, and Γv is related to the natural line width of the transition. SFG signal resonance enhancement is achieved when the IR frequency is onresonance with a vibrational mode at the interface. Specifically, for the analyses used here, our IR field was tuned to be onresonance with the C−H stretching vibrational modes. Thus, using the intensity of the SFG signal from the various C−H stretching vibrational modes, we can obtain valuable thermodynamic and structural information relating to the surface activity and relative orientation/order of species in the interfacial region.
2. SUM FREQUENCY GENERATION (SFG) Vibrational sum frequency generation is a coherent nonlinear optical spectroscopy that is highly sensitive to molecular structure, orientation, and adsorption at interfaces.45−57 Details on SFG theory have been previously reported;58−66 here we provide just a brief summary. SFG arises when two electric fields, typically produced from incident infrared and visible laser beams, combine at an interface spatially and temporally to produce a third field, which is the sum of the two incident frequencies. Under the electric dipole approximation, the SFG signal is forbidden in centrosymmetric media.66,67 As bulk species exhibit inversion symmetry and because symmetry is inherently broken at the interface, SFG is surface specific. The intensity of the sum frequency signal, ISFG is given by ISFG ∝ |χ (2) |2 I visIIR
AK MIJ
(2)
where the resonant and nonresonant terms are related by a phase, ϕ. The nonresonant term is minimal for the oxide surfaces we have studied previously23,34,43 but can be large for metal surfaces35,68−71 or for carbon materials such as highly ordered pyrolytic graphite (HOPG)54 due to electronic transition states and the instantaneous response of the 17746
dx.doi.org/10.1021/jp5047547 | J. Phys. Chem. C 2014, 118, 17745−17755
The Journal of Physical Chemistry C
Article
Solutions of binary mixtures were prepared by mass and serial dilution using 1-hexanol (>99%, Sigma-Aldrich) and cyclohexane-d12 (99.6 atom % D, CDN Isotopes). The liquid solutions were introduced and removed from the Teflon cell with a syringe, and the cell was capped during the duration of the experiment to prevent evaporation. Following injection in the sample cell, the solution was allowed to equilibrate for a minimum of 15 min before data acquisition. Changes in the SFG intensities were not observed over the time scale of our experiment (36 min). SFG spectra of binary 1-hexanol and cyclohexane solutions were collected of ascending mole fraction of 1-hexanol. 3.3. Raman, X-ray Photoelectron Spectroscopy (XPS), and Contact Angle Measurements. Raman spectra of the graphene films were recorded with an Acton TriVista CRS Confocal Raman system. The spectra were collected using a 514.5 nm excitation wavelength, attenuated to a power density
