Hydrocarbon on Carbon: Coherent Vibrational Spectroscopy of

Letter pubs.acs.org/JPCL

Hydrocarbon on Carbon: Coherent Vibrational Spectroscopy of Toluene on Graphite Jennifer L. Achtyl, Avram M. Buchbinder, and Franz M. Geiger* Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States ABSTRACT: The ability to study the interactions of hydrocarbons on carbon surfaces is an integral step toward gaining a molecular level understanding of the chemical reactions and physical properties occurring on them. Here, we apply vibrational sum frequency generation (SFG) to determine the tilt angle of toluene, a common organic solvent, on millimeter-thick highly oriented pyrolytic graphite (HOPG). The combination of a time-delay technique, which results in the successful suppression of the nonresonant SFG response, and a null angle method is shown to overcome the “strong optical absorber” problem posed by macroscopically thick carbon samples and yields a molecular tilt angle of toluene in the range of 37° to 42° from the surface normal. The implications of this approach for determining the orientation of organic species adsorbed on carbon interfaces, which are important for energy-relevant processes, are discussed. SECTION: Kinetics, Spectroscopy

T

Figure 1 shows the SFG response in the C−H stretching region of HOPG (SPI Supplies, grade 2, 1 cm × 1 cm) for three

he surfaces of carbon-based materials enable physical and chemical process that are important in applications relevant for energy storage,1,2 materials science,3−5 and catalysis.6,7 In addition, they are key players in heterogeneous processes that are now well-known to impact the climate system8 and public health.9 Yet, the relevant chemical reactions and physical processes occurring on the surfaces of these materials remain poorly understood on the molecular level, mainly because surface-selective and molecularly specific studies applied to the surfaces of carbon materials are sparse, and if they exist, they have been applied to samples having a thickness of just a few nanometers or less.10−13 So far, the surface-selective molecular-level spectroscopic interrogation of carbon materials having dimensions relevant for the energy-, catalysis-, and environment-related processes mentioned above has been curtailed by the optical properties of these strong absorbers. Here, we show how one can overcome this classical “strong absorber problem” of carbon materials by using femtosecond pulses from a kHz amplifier laser system,14 which should be short enough to avoid thermal damage15,16 while enabling the interrogation of the basal plane surfaces of millimeter-thick samples of highly ordered pyrolytic graphite (HOPG) using vibrational sum frequency generation (SFG)17 spectroscopy. We find that (a) the material generates well-defined nonresonant SFG responses from the delocalized electrons within HOPG for upconverter input energies below 800 nJ; (b) the nonresonant SFG signals are polarization dependent but rotationally invariant; (c) well-established time-delay methods18 readily suppress the nonresonant SFG response, yielding spectra that are void of C−H stretching modes for bare HOPG; and (d) the molecular orientation of toluene is easily determined at the HOPG surface through its C−H stretches. © 2012 American Chemical Society

Figure 1. Vibrational sum frequency generation response from the basal plane of a millimeter-thick sample of HOPG for the ppp, ssp, and sps polarization combinations. The shift in center frequencies is attributed to the anisotropy of the optical materials properties of HOPG. Inset: Rotational invariance of the ppp-polarized SFG response at 2900 cm−1 from four trials (gray traces) and average (black trace). Please see the text for details.

commonly used polarization combinations, each probing different elements of the second order nonlinear susceptibility tensor χ(2).19 In general, the ssp and sps polarization combinations can be utilized to probe for vibrational transition components oriented mainly perpendicular and parallel to the surface, respectively, while the ppp polarization combination probes the χzzz tensor element. For the bare HOPG, C−H Received: December 22, 2011 Accepted: January 6, 2012 Published: January 6, 2012 280

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stretches are not observed in any of the spectra, indicating a lack of vibrational transitions that can be detected within the detection limit of the experiment. Instead, we find that, as has been pointed out by Kim et al.,12 the SFG response from HOPG is similar to that obtained from neat gold surfaces,20 which yield instantaneous electronic responses void of molecular vibrations. As shown in Figure 1, we find here that the nonresonant SFG response from the HOPG surface is invariant with rotation around the surface normal, irrespective of the polarization combination. This finding suggests that unlike in single graphene sheets,10 the crystalline domains on the HOPG basal surfaces studied here are smaller than the 30 μm spot size of our laser beams, consistent with AFM images such as those provided in the TOC graphic. We proceeded to determine the molecular orientation of a common organic species at the carbon interface by drop-casting HOPG with toluene,21,22 which has a well-known and strong SFG contribution in the C−H stretching region. For each experiment, a fresh HOPG surface is prepared by cleaving a few layers of graphene from the surface using Scotch Tape. Figures 2B and 3B show that the methyl symmetric C−H stretch of

Figure 3. (A) ppp-Polarized response at 2880 cm−1 from the basal plane of a millimeter-thick sample of HOPG as a function of energy of the visible upconverter. The straight line is a linear least-squares fit to the data. Inset: Response up to 8 microJ showing departure from the expected SFG response (straight line) above 1 μJ. (B) Time-delayed ppp-polarized SFG spectra of toluene on the basal plane of a millimeter-thick HOPG sample before (black spectrum, offset for clarity) and after (gray spectrum) preparing a freshly cleaved surface using Scotch Tape.

applied the null angle method21,25,26 to determine the molecular orientation of toluene at the carbon interface. Figure 2C shows the SFG intensity from both the bare HOPG surface, and toluene on it can be diminished, albeit not fully extinguished, when the polarizer analyzer is set to 63 ± 5° and 78 ± 5°, respectively, when probing with p-polarized infrared and visible light plane polarized 45° away from the surface normal. The null angles of both surfaces are determined by collecting the SFG intensity at 2910 cm−1, coinciding with methyl symmetric C−H stretch. The difference in these null angles, which amounts to 15 ± 5° as determined from four separate measurements carried out on four different days and samples, results in a tilt angle of the methyl C3v axis that ranges from about 37° to 42° when assuming monomodal tilt angle distribution functions with widths that range from 1° to 40°, optical properties of toluene we applied in prior work,21 and a refractive index for the basal HOPG substrate of 3.04.27 Given that the nonresonant SFG response obtained from these thick HOPG samples is rotationally invariant around the azimuth (inset in Figure 1), our tilt angle calculations assume that the molecular orientation of toluene on HOPG is also invariant with rotation around the azimuth. Control studies show linear increases in the SFG response with incident visible pulse energy, which is expected for surface SFG, up to around 800 nJ (Figure 3A).19 Departure from linearity, which indicates that processes other than SFG contribute to the signal, is observed at higher input energies. As a result, the measurements were performed using a visible pulse energy of approximately 400 nJ, while the infrared pulse energy was kept around 1 μJ. Likewise, applying Scotch Tape to the HOPG substrate after toluene adsorption and removing the top material resulted in SFG spectra without vibrational C−H stretching modes, indicating that toluene absorption into the HOPG bulk is negligible within the detection limit of the experiment (Figure 3B). The SFG spectra shown in Figures 2B and 3B differ by the spectral feature at 2850 cm−1. The former was obtained with incident broadband IR radiation having a slightly higher center frequency (bandwidth of 140 cm−1) such that the third mode was not accessible, and only two modes were observed. In conclusion, we have obtained coherent spectroscopic responses of the surfaces of HOPG substrates in the presence

Figure 2. ppp-Polarized SFG spectra of toluene on the basal plane of a millimeter-thick HOPG sample using zero (A) and 500 fs (B) upconverter time delays, null angle measurements, run in quadruplet, of HOPG in the absence (●) and presence (○) of toluene (C) with molecular orientation analysis (D).

toluene on HOPG is clearly detectable at 2910 cm−1, as indicated by the circular mark, when the visible upconverter is time-delayed by 500 fs from the infrared broadband pulse. The time needed to acquire these spectra ranged from 4 to 5 min. While Figures 2B and 3B still show some nonresonant background across the spectral range of interest, its intensity is minor compared to that of the methyl C−H stretching modes. As reported in the literature, the vibrational modes apparent in Figures 2B and 3B can be assigned as a methyl overtone at 2850 cm−1, a symmetric stretch at 2910 cm−1, and either a methyl asymmetric or methyl Fermi resonance at 2970 cm−1.22−24 Having isolated the vibrational SFG response of a molecular species adsorbed to millimeter-thick HOPG samples, we 281

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(10) Dean, J. J.; van Driel, H. M. Graphene and Few-Layer Graphite Probed by Second-Harmonic Generation: Theory and Experiment. Phys. Rev. B 2010, 82, 125411. (11) Dean, J. J.; van Driel, H. M. Second Harmonic Generation from Graphene and Graphitic Films. Appl. Phys. Lett. 2009, 95, 261910-3. (12) Kim, H.; Balgar, T.; Hasselbrink, E. The Stretching Vibration of Hydrogen Adsorbed on Epitaxial Graphene Studied by SumFrequency Generation Spectroscopy. Chem. Phys. Lett. 2011, 508, 1−5. (13) Andrews, A. B.; McClelland, A.; Korkeila, O.; Demidov, A.; Krummel, A.; Mullins, O. C.; Chen, Z. Molecular Orientation of Asphaltenes and PAH Model Compounds in Langmuir-Blodgett Films Using Sum Frequency Generation Spectroscopy. Langmuir 2011, 27, 6049−6058. (14) Buchbinder, A. M.; Weitz, E.; Geiger, F. M. When the Solute Becomes the Solvent: Orientation, Ordering, and Structure of Binary Mixtures of 1-Hexanol and Cyclohexane over the (0001) α−Al2O3 Surface. J. Am. Chem. Soc. 2010, 132, 14661−14668. (15) Hicks, J. M.; Urbach, L. E.; Plummer, E. W.; Dai, H.-L. Can Pulsed Laser Excitation of Surfaces Be Described by a Thermal Model? Phys. Rev. Lett. 1988, 61, 2588−2591. (16) Balooch, M.; Schildbach, M.; Tench, R.; Allen, M.; Siekhaus, W. J. Surface Site Specificity on the Basal Plane of Graphite: 1.06 Mm Laser Damage Threshold and Reactivity with Oxygen between 350 and 2300 K. J. Vac. Sci. Technol. B 1991, 9, 1088−1091. (17) Zhu, X. D.; Suhr, H.; Shen, Y. R. Surface Vibrational Spectroscopy by Infrared-Visible Sum-Frequency Generation. Phys. Rev. B 1987, 35, 3047−3050. (18) Lagutchev, A.; Hambir, S. A.; Dlott, D. D. Nonresonant Background Suppression in Broadband Vibrational Sum-Frequency Generation Spectroscopy. J. Phys. Chem. C 2007, 111, 13645−13647. (19) Heinz, T. F. In Nonlinear Surface Electromagnetic Phenomena; Ponath, H.-E., Stegeman, G. I., Eds.; Elsevier: Amsterdam, The Netherlands, 1991; p 353. (20) Richter, L. J.; Petralli-Mallow, T. P.; Stephenson, J. C. Vibrationally Resolved Sum-Frequency Generation with BroadBandwidth Infrared Pulses. Opt. Lett. 1998, 23, 1594−1596. (21) Stokes, G. Y.; Buchbinder, A. M.; Gibbs-Davis, J. M.; Scheidt, K. A.; Geiger, F. Chemically Diverse Environmental Interfaces and Their Reactions with Ozone Studied by Sum Frequency Generation. Vib. Spectrosc. 2009, 50, 86−98. (22) Hommel, E. L.; Allen, H. C. The Air-Liquid Interface of Benzene, Toluene, M-Xylene, and Mesitylene: A Sum Frequency, Raman, and Infrared Spectroscopic Study. Analyst 2003, 128, 750− 755. (23) Yang, Z.; Li, Q.; Hua, R.; Gray, M. R.; Chou, K. C. Competitive Adsorption of Toluene and N-Alkanes at Binary Solution/Silica Interfaces. J. Phys. Chem. C 2009, 113, 20355−20359. (24) Opdahl, A.; Somorjai, G. A. Solvent Vapor Induced Ordering and Disordering of Phenyl Side Branches at the Air/Polystyrene Interface Studied by SFG. Langmuir 2002, 18, 9409−9412. (25) Hicks, J. M.; Kemnitz, K.; Eisenthal, K. B.; Heinz, T. F. Studies of Liquid Surfaces by Second Harmonic Generation. J. Phys. Chem. 1986, 90, 560−562. (26) Simpson, G. J.; Rowlen, K. L. Orientation-Insensitive Methodology for Second Harmonic Generation. 2. Application to Adsorption Isotherm and Kinetics Measurements. Anal. Chem. 2000, 72, 3407− 3411. (27) Lee, S.; Virtanen, J. A.; Virtanen, S. A.; Penner, R. M. Assembly of Fatty Acid Bilayers on Hydrophobic Substrates Using a Horizontal Deposition Procedure. Langmuir 1992, 8, 1243−1246.

and absence of toluene, a common organic solvent. The use of femtosecond laser pulses has enabled substantial reductions in the nonresonant SFG response from millimeter-thick carbon samples that has been plaguing the spectroscopic investigation of surfaces of carbon samples that are even just a few nanometers thick (Figure 2A,B).10−12 The approach is surface selective and allows us to triangulate the molecular tilt angle of toluene (Figure 2C,D) to between 37° and 42°, with varying widths. Finally, the use of femtosecond pulses combined with a time delay between them avoids thermal damage, as indicated in the control studies (Figure 3A). This new capability to apply surface-selective coherent vibrational spectroscopy to organic species on macroscopically thick carbon materials has the potential to enable molecularlevel investigations of interfacial processes relevant for the chemistry and physics of energy- and environmentally related materials, catalysis, and the many other technological areas in which the various forms of elemental carbon matter. Coherent vibrational spectroscopies will also enable the tracking of interfacial processes in real time and space under operating conditions. Finally, our orientational and spectroscopic data will serve as useful benchmarks for scanning probe studies and theoretical calculations of physical and chemical processes occurring at the surfaces of these fascinating materials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

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