Anal. Chem. 2009, 81, 2017–2021
Rotating Cell for in Situ Raman Spectroelectrochemical Studies of Photosensitive Redox Systems Ladislav Kavan,*,†,‡ Pavel Janda,*,†,‡ Matthias Krause,†,§ Frank Ziegs,† and Lothar Dunsch*,† Group of Electrochemistry and Conducting Polymers, Leibniz-Institute of Solid State and Materials Research, Helmholtzstrasse 20, D-01069 Dresden, Germany, J. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Dolejsˇkova 3, CZ-182 23 Prague 8, Czech Republic, and Forschungszentrum Dresden-Rossendorf, Institute for Ion Beam Physics and Materials Research, D-01314 Dresden, Germany A recently developed rotating spectroelectrochemical cell for in situ Raman spectroscopic studies of photoreactive compounds without marked decomposition of the sample is presented. Photochemically and thermally sensitive redox systems are difficult to be studied under stationary conditions by in situ spectroelectrochemistry using laser excitation as in Raman spectroscopy. A rotating spectroelectrochemical cell can circumvent these difficulties. It can be used for any type of a planar electrode and for all electrode materials in contact with aqueous or nonaqueous solutions as well as with ionic liquids. The innovative technical solution consists of the precession movement of the spectroelectrochemical cell using an eccentric drive. This precession movement allows a fixed electrical connection to be applied for interfacing the electrochemical cell to a potentiostat. Hence, any electrical imperfections and noise, which would be produced by sliding contacts, are removed. A further advantage of the rotating cell is a dramatic decrease of the thermal load of the electrochemical system. The size of the spectroelectrochemical cell is variable and dependent on the thickness of the cuvettes used ranging up to ∼10 mm. The larger measuring area causes a higher sensitivity in the spectroscopic studies. The as constructed spectroelectrochemical cell is easy to handle. The performance of the cell is demonstrated for ordered fullerene C60 layers and the spectroelectrochemical behavior of nanostructured fullerenes. Here the charge transfer at highly ordered fullerene C60 films was studied by in situ Raman spectroelectrochemistry under appropriate laser power and accumulation time without marked photodecomposition of the sample. Raman spectroscopy is an important spectroscopic method for the characterization of a large variety of redox species at electrodes.1-4 However, in Raman measurements the photostability of the system under study has to be considered for getting defined insight into the vibrational pattern of molecules and/or * To whom correspondence should be addressed. † Leibniz-Institute of Solid State and Materials Research. ‡ J. Heyrovsky´ Institute of Physical Chemistry. § Institute for Ion Beam Physics and Materials Research. (1) Kavan, L.; Dunsch, L. ChemPhysChem 2007, 8, 974. 10.1021/ac802336y CCC: $40.75 2009 American Chemical Society Published on Web 02/04/2009
solids. The risk of photodecomposition of studied systems under laser light asks for low intensity of illumination and short accumulation time, but this reduces the sensitivity of the method and the reliability of the measured data. Hence an experimental set up is required, allowing efficient photon flux and heat dissipation on the sample surface during laser illumination. There have been number of electrochemical cells used in Raman spectroelectrochemistry which fit the electrochemical requirements and are successfully used to study the spectroscopic behavior of either the electrode material or the redox system in solution.5 Furthermore, the general principle of a rotating sample is very often used in spectroscopy and was already applied in Raman spectroscopy some decades before.6,7 While for ex situ measurements the rotational movement of the sample suits well, for in situ measurements the electrochemical cell with the sample has to be electrically connected to the potentiostat/galvanostat. This is necessary for the precise control of the applied electrode potential at the working electrode and for simultaneous measurements of current-voltage characteristics. The sliding contacts are used often for such arrangements, in which the cell rotation is requested. However, the sliding contacts represent a source of signal noise, besides mechanical and electrical instability of the set up with the liquid-filled electrochemical cell. These requirements are to be addressed in the construction of a novel spectroelectrochemical cell. While the stability of carbon nanotubes8,9 and fullerene peapods10-12 is well established under Raman conditions, the (2) Chang, R. K. Raman Spectroscopic Techniques in Interfacial Electrochemistry, in: Spectroscopic and Diffraction Techniques in Interfacial Electrochemistry. In Spectroscopic and Diffraction Techniques in Interfacial Electrochemistry; Gutierez, C., Melendres, C., Eds.; Kluwer: Dordrecht, The Netherlands, 1990; pp 155-180. (3) Birke, R. L.; Lombardi, J. R. Surface Enhanced Raman Scattering; Plenum Press: New York, 1988. (4) McCreery, R. L. Raman Spectroscopy for Chemical Analysis; Wiley: New York, 2000. (5) Burba, C. M.; Frech, R. Appl. Spectrosc. 2006, 60, 490. (6) Kiefer, W.; Bernstein, H. J. Appl. Spectrosc. 1971, 25, 609. (7) Kiefer, W.; Bernstein, H. J. Appl. Spectrosc. 1971, 25, 500. (8) Kavan, L.; Rapta, P.; Dunsch, L. Chem. Phys. Lett. 2000, 328, 363. (9) Kavan, L.; Rapta, P.; Dunsch, L.; Bronikowski, M. J.; Willis, P.; Smalley, R. E. J. Phys. Chem. B 2001, 105, 10764. (10) Kavan, L.; Dunsch, L.; Kataura, H. Chem. Phys. Lett. 2002, 361, 79. (11) Kavan, L.; Dunsch, L.; Kataura, H.; Oshiyama, A.; Otani, M.; Okada, S. J. Phys. Chem. B 2003, 107, 7666. (12) Kavan, L.; Dunsch, L. ChemPhysChem 2003, 4, 944.
Analytical Chemistry, Vol. 81, No. 5, March 1, 2009
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situation is more difficult with fullerenes in thin films. Among our studies on the electrochemistry of fullerene layers,13-20 the stability of the fullerenes turned out to be a problem under Raman conditions and had to be solved by construction of a new cell. In general the photoreaction of the C60 fullerenes can be followed most easily by both the intensity drop and the softening of the pentagonal pinch, Ag(2) mode. An electron transfer into the C60 LUMO also causes an Ag(2) frequency downshift of about 6 cm-1 per electron per C60 molecule. Regarding the individual reduction steps, the frequency downshift is the largest for the first electron transfer (11 cm-1) and approaches ∼5.5 cm-1 for the steps from C603- (1448 cm-1) to C606- (1432 cm-1) (see ref 20 and citations therein). The separate study of these two effects, viz., photoreaction or charge transfer, requires the suppression of one of them. EXPERIMENTAL SECTION Raman Cell and the Rotation Device. The device developed in this study consists of an eccentric rotational arrangement for a three-electrode electrochemical cell, which is connected to a potentiostat by wires. As shown in Figure 1, the cell construction is dominated by a metallic frame onto which a 12 V electromotor and an anchor plate were mounted. The cell-holder was fixed to the anchor plate which rotates eccentrically with respect to the axis. This adds a translational movement to result in precession revolutions, directed by a telescopic setup on the upper part of the device. As a result, the laser illuminated sample area describes a defined circle around the rotation axis, while the perpendicular orientation of the electrochemical cell is retained. This geometry of the cell movement allows a fixed interfacing of three electrical connections between the cell and the potentiostat/galvanostat. Hence, any signal noise is eliminated due to the absence of sliding contacts, and this is the crucial innovation of this cell design. The electrochemical cell was made from a conventional 2 mm optical cell (Hellma, Germany). The cell was capped by an airtight silicone-rubber lid, through which three metallic wires were interconnected. They contacted the Ag pseudoreference electrode, the Pt counter electrode, and the working electrode. The working electrode body was from highly oriented pyrolytic graphite (HOPG) and was prepared by peeling-off the topmost HOPG leaf with deposited C60 film (see below), contacted by Cu wire, and sealed by lamination in polyethylene foil, which had a hole of (13) Dunsch, L.; Janda, P.; Krieg, T. In Recent Advances in the Physics and Chemistry of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society: Pennington, NJ, 1998; pp 1335-1343. (14) Krieg, T.; Janda, P.; Dunsch, L. Elektrochemisches Verhalten und Struktur von C60 Schichten auf Elektroden. In Elektrochemische Reaktionstechnik und Synthese, GDCh Monographie Bd 14; Russow, J., Sandstede, G., Staab, R., Eds.; GDCh: Frankfurt am Main, Germany, 1999; pp 188-195. (15) Janda, P.; Krieg, T.; Dunsch, L. Adv. Mater. 1998, 17, 1434. (16) Dunsch, L.; Janda, P.; Krieg, T.; Georgi, P. ; Gembicka, M.; Oswald, S.; Mattern, N. Elektrochemische Strukturierung von Kohlenstoff-Schichten. In Elektrochemische Verfahren fu ¨ r Neue Technologien, GDCh-Monographie Bd. 21; Kolb, D. M., Mund, K., Russow, J., Eds.; GDCh, Frankurt an Main, Germany, 2000; pp 71-79. (17) Touzik, A.; Hermann, H.; Janda, P.; Dunsch, L.; Wetzig, K. Europhys. Lett. 2002, 60, 411. (18) Deutsch, D.; Tarabek, J.; Krause, M.; Janda, P.; Dunsch, L. Carbon 2004, 42, 1137. (19) Krause, M.; Deutsch, D.; Janda, P.; Kavan, L.; Dunsch, L. Fullerenes, Nanotubes, Carbon Nanostruct. 2005, 13, 159. (20) Krause, M.; Deutsch, D.; Janda, P.; Kavan, L.; Dunsch, L. Phys. Chem. Chem. Phys. 2005, 7, 3179.
2018
Analytical Chemistry, Vol. 81, No. 5, March 1, 2009
Figure 1. The device for in situ Raman spectroelectrochemistry with an eccentric rotation of the electrochemical cell. The working electrode is positioned out of the motor axis (dashed-dotted line) using the eccentric holder.
varying diameter providing the contact between the working electrode and the electrolyte solution. The assembled cell was positioned at the eccentrical rotator and adjusted by a screw. The diameter of the trace of the laser spot on the working electrode was in the range of 1-8 mm. The cell rotated with a speed of 50-300 rpm during data acquisition. Materials and Equipment. The highly ordered fullerene (C60) film was prepared by heteroepitaxial deposition in vacuum (8 × 10-6 mbar, deposition rate 0.2 nm/s) on the freshly cleaved basal plane of highly oriented pyrolytic graphite (HOPG, ZYH grade, Advanced Ceramics) at 240 °C.15,18-20 The electrochemical instrumentation consisted of HEKA IEEE-488 or EG&G PAR 273A potentiostats/galvanostats. The electrolyte solution was aqueous 4 M KF or RbF with 0.1 M KOH or RbOH, respectively, and deoxygenated by nitrogen. Spectrolectrochemical Measurements. Raman spectra were excited by an Ar+ laser, λ ) 514.5 nm, (Innova 305, Coherent). The spectra were recorded on a T-64000 spectrometer (Instruments, SA) with a spectral resolution of 2 cm-1. The spectroelectrochemical cell was mounted in front of the macroentrance objective of the spectrometer. The spectrometer was calibrated before each series of measurements by using the F2g mode of Si at 520.2 cm-1. The nanostructure and surface arrangement of the deposited fullerene films were examined ex situ using atomic force microscope Nanoscope IIIa (Veeco) prior to the spectroelectrochemical experiment.19,20 This film was used for subsequent Raman spectroscopic and spectroelectrochemical studies. RESULTS AND DISCUSSION The Raman spectra of the C60/HOPG working electrode in the electrochemical cell (Figure 1) were acquired for varying times of laser illumination without and with the sample rotation.
Figure 2. Raman intensity of the Ag(2) mode region of a 500 nm thick C60 film on HOPG in contact with an aqueous 4 mol/L RbF + 0.10 mol/L RbOH solution as a function of time without sample rotation. The laser wavelength was 514.5 nm, the laser power 1.25 mW, and the accumulation time 240 s in each spectral run.
Figure 3. Raman intensity of the four Raman lines observed in the Ag (2) region of a C60 film on HOPG in an aqueous 4 mol/L RbF + 0.10 mol/L RbOH solution as a function of time without sample rotation. Raman parameters are the same as in Figure 2. The measurement at 2 min was not included in the exponential decay function, since for reference reasons the sample was rotated until the first accumulation was completed.
Table 1. Wave Numbers (ω), Relative Intensities (Irel), and Assignments of Lines Observed in the Region of the Ag (2) Mode of C60 Films on HOPGa C60 film, rotating sample
C60 film, fixed sample
Irel (%)
Irel (%)
ω (cm-1)
t)2 min
t ) 38 min
ω (cm-1)
t)2 min
t ) 38 min
assignment
1452 1459 1462 1468
0 0.4 0.8 100
0 0.7 1.2 95.4
1452 1459 1461 1468
0 0 0 100
2.7 7.7 3.7 14.8
(C60)n (C60)n (C60)2 C60
a The Raman measurements were carried out with the assembled photoelectrochemical cell, but the working electrode was kept at open circuit potential. Two different times (t) of laser illumination were tested, either in the fixed sample position or with rotation.
Figure 2 shows the spectra measured at open circuit potential without cell rotation. It is obvious that the electrochemical environment cannot prevent the C60 photoreaction under Raman conditions. The Raman intensity of the Ag (2) mode of C60 at 1468 cm-1 decreases, while new lines at 1462, 1459, and 1452 cm-1 appear (see also Table 1). Since the measurements were carried out at open circuit, these spectral changes cannot be ascribed to electrochemical reactions. Instead, and in agreement with previous studies of the C60 photoreactions in vacuum, these lines are assigned to the C60 dimer and polymer.21,22 The intensity/time profiles of these four lines are shown in Figure 3. The intensity of the C60 molecule line decays exponentially until an equilibrium state is reached, where ∼15% of the initial intensity is conserved (Table 1). The dimer (C60)2 is characterized by a line at 1462 cm-1. It is an intermediate product with an intensity maximum at 18 min of illumination at the used conditions (Figure 3). The lines assigned to polymer (C60)n are at 1459 and 1452 cm-1. Their intensities increase significantly during 40 min of data acquisition, but the intensity growth slows down for times longer than 18 min, when the dimer concentration has achieved its maximum. Obviously, the photodimer(21) Davydov, V. A.; Kashevarova, L. S.; Rakhmanina, A. V.; Senyavin, V. M.; Ceolin, R.; Szwarc, H.; Allouchi, H.; Agafonov, V. Phys. Rev. B 2000, 61, 11936. (22) Burger, B.; Winter, J.; Kuzmany, H. Z. Phys. B 1996, 101, 227.
Figure 4. Raman intensity of the Ag (2) mode region of a 500 nm thick C60 film on HOPG in contact with an aqueous 4 mol/L KF + 0.10 mol/L KOH solution as a function of time using a sample rotation frequency of 300 min-1. The laser wavelength was 514.5 nm, the laser power 2.5 mW, and the accumulation time 240 s in each spectral run.
ization/photopolymerization of C60 is undesirable in Raman spectroelectrochemical studies, because they interfere with the investigation of the spectral changes caused by an electrochemical charge transfer. The C60 photoreaction is almost completely suppressed by the sample rotation. As shown in Figure 4, the line assigned to C60 monomer (1468 cm-1) is stable during 40 min of laser illumination. The quantification of the time dependence by line shape analysis gives a conservation of >95% of the initial C60 Ag (2) line intensity after 40 min of irradiation (Figure 5). This is confirmed by data analysis, which gave a value of 95.3% for the intensity of the C60 monomer line based on an exponential decay after infinite time. Hence, even in the equilibrium, less than 5% of the C60 undergo a photoreaction, what is within the error of usual spectroscopic measurements. The fact that the exponential decay describes the
