- Email: [email protected]
An infrared study of the few-layer graphene | ionic liquid interface: Reintroduction of in situ electroreflectance spectroscopy
An infrared study of the few-layer graphene — ionic liquid interface: Reintroduction of in situ electroreflectance spectroscopy Ove Oll, Tavo Romann, Enn Lust PII: DOI: Reference:
S1388-2481(14)00155-6 doi: 10.1016/j.elecom.2014.05.023 ELECOM 5168
To appear in:
Electrochemistry Communications
Received date: Revised date: Accepted date:
21 April 2014 19 May 2014 20 May 2014
Please cite this article as: Ove Oll, Tavo Romann, Enn Lust, An infrared study of the few-layer graphene — ionic liquid interface: Reintroduction of in situ electroreflectance spectroscopy, Electrochemistry Communications (2014), doi: 10.1016/j.elecom.2014.05.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT An infrared study of the few-layer graphene | ionic liquid interface: reintroduction of in situ electroreflectance spectroscopy Ove Oll, Tavo Romann and Enn Lust*
PT
Institute of Chemistry, University of Tartu, Ravila 14A, 50144 Tartu, Estonia
RI
* Corresponding author. Tel.: +372 737 5165; fax: +372 737 5264.
AC CE P
TE
D
MA
NU
SC
E-mail address: [email protected] (E. Lust)
1
ACCEPTED MANUSCRIPT Abstract A simple method of producing high quality few-layer graphene (FLG) electrodes (<5 nm thick)
PT
has been described and the in situ infrared measurements of the FLG| 1-ethyl-3-
RI
methylimidazolium tetrafluoroborate system have been reported. Ideal polarizability of the system has been established and three different spectral modes have been discussed in order to
SC
provide a varied understanding of both the electronic and structural effects at the interface. The
NU
method of in situ electroreflectance spectroscopy has been extended to the study of FLG | ionic liquid interface, providing new information about the method and possibilities for future studies
MA
of specific adsorption and electronic structure of the interface. Plasmonic enhancement of the
double layer and infrared sensors.
D
spectra has been demonstrated, providing excellent opportunities for the study of the electric
AC CE P
spectroscopy
TE
Keywords: Few-layer graphene; EMImBF4; In situ infrared spectroscopy; Electroreflectance
2
ACCEPTED MANUSCRIPT 1. Introduction Electroreflectance (ER) spectroscopy is a widely used technique for the study of semiconductor
PT
electronic structure [1]. In the mid 60’s it was discovered that the technique could also be used to
RI
probe metallic interfaces in (the) in situ electrochemical conditions [2] using the external reflection technique with near infrared to ultraviolet irradiation. The thorough research that
SC
continued found that, differently from semiconductors, ER could be used to selectively probe the
NU
surface states of a metallic interface, thus providing an extremely powerful tool for the analysis of interfacial phenomena from an electronic standpoint. The readers are referred to an excellent
MA
review analyzing the fundamentals and capabilities of ER [3]. Although ER provided some fascinating insight into the electronic effects of metallic interfaces, many of the discovered
D
experimental phenomena are still not adequately explained. Due to some technical difficulties
TE
and the fact that very few groups around the world had the knowledge and capability for in situ
AC CE P
ER, the method was abandoned in the early 90’s. Our communication seeks to reintroduce ER by extending the scope of it to the semimetallic few-layer graphene (FLG) interface within the midinfrared spectral region.
In our preceding study [4], clear differences in spectral behavior were shown for different types of carbon electrodes (thin film amorphous and carbide-derived carbons). However, largely due to technical and practical limitations, clear assessments of the interfacial structure were not able to be established. The aim of this study was to show the applicability of FLG electrodes for in situ infrared studies and to provide a background for future more complex studies. 2. Experimental Atomic force microscopy (AFM) data were obtained by an Agilent TechnologiesTM Series 5500 system. Spectroscopic measurements were performed using a PerkinElmer Spectrum GX FTIR equipped with a liquid nitrogen-cooled mid-range MCT detector and the electrochemical 3
ACCEPTED MANUSCRIPT measurements using an Autolab PGSTAT 30 potentiostat in a three-electrode glass cell with an Ag|AgCl wire in the same ionic liquid (IL) as a pseudo-reference electrode [4]. Impedance
PT
spectra were measured within ac frequency range from 10−2 to 105 Hz with 5 mV ac modulation.
RI
1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) from Solvionic (99.5 %, H2O ~100 ppm) was additionally dried in ultra-vacuum at 100 °C for 48 hours, until reaching a pressure of
SC
10−9 torr and water content below the detection limit of Karl Fischer method (< 5 ppm).
NU
Our constructed system [4-5] uses an exfoliated highly oriented pyrolytic graphite (HOPG) (Veeco) layer on the flat side of a 10 mm diameter infrared transparent ZnSe (n = 2.4)
MA
hemisphere as the working electrode. A thin (~5 µm) HOPG layer is glued onto the ZnSe hemisphere with a thin layer (~300 nm) of dielectric epoxy (EPO) glue and exfoliated with scotch
D
tape. Usually only one exfoliation is required to produce a see-trough layer of FLG on the
TE
hemisphere. Although the produced surface is somewhat uneven, the hemisphere setup requires
AC CE P
only the middle, infrared active part of the hemisphere to be uniformly covered. Optical transmission and Raman spectroscopy measurements suggest the thinner parts of the electrode to compose of <10 layers of graphene. Figure 1 shows the experimental setup of the measurements and a contact mode AFM image of the working electrode. It can be seen from the AFM image how the thin FLG layer conforms to the slightly wavy surface of the EPO glue. All the surface terraces observed are of single layer height. The average width of the terraces is over 4 µm. Spectra in the potential region of −1.8 V < U < 1.8 V were measured relative to the fixed reference potential −0.2 V (the potential of zero charge (pzc) [4]). Details about the measurement technique and electrochemical setup have been previously reported [5]. 3. Results and discussion
4
ACCEPTED MANUSCRIPT Figure 2a shows the impedance phase angle diagram for the FLG|EMImBF4 system measured at the pzc. It is seen that the system is ideally polarizable in a wide range of ac frequencies (Phase
PT
angle ≥ −89.8°) which is a prerequisite for the unambiguous interpretation of ER spectra. The
RI
potential-capacitance (CU) curve is shown in figure 2b. The characteristic V-shape is seen,
[6], explained by the semimetallic nature of FLG.
SC
similar to the CU curves demonstrated for graphene and FLG electrodes in electrolyte solution
NU
Figure 3 shows the potential dependent in situ infrared spectra for the FLG|EMImBF4 interface. Three distinct areas of different spectral information have been outlined in the figure. The most
MA
striking and perhaps most interesting are the extremely wide (approximately 1000 cm−1 wide) Gaussian shaped peaks with very strong potential-position dependence. Such features could not
D
be characterized by conventional theory of vibrational spectroscopy and are instead interpreted to
TE
result from the potential induced change of the reflective properties of the FLG electrode; as in
AC CE P
situ ER spectroscopy data. From a fundamental standpoint, the peaks signify the excitation of electrons to the empty π-orbitals of FLG at the K point [7] by infrared irradiation, which shift due to the applied electric potential at the interface. Due to the limited spectral range and other spectral features, the peaks are either not seen or easily detected within the range from 0.4 to −0.6 V. Figure 4a shows the potential dependence of the peak dips (maxima). It can be seen that there is an almost perfectly linear dependence between the peak dips and electrode potential, with exactly the same absolute slope of 0.233 eV V−1 at both the positive and negative potential side, in excellent agreement with the surface electronic structure of FLG [7]. Similar graphs of Edip vs. U have been shown for low index crystal faces of Au and Ag [3, 8]. However, the exact mechanism generating this effect is still under discussion. While it has been proposed that this dependency represents the applied effective field strength on the respective material surface states, such assumptions could not be confirmed by theory [3]. On the contrary, large differences 5
ACCEPTED MANUSCRIPT exist between different crystal faces, metals and cathodic and anodic regimes for which many unconfirmed explanations have been given. More interestingly, the Edip vs. U plot slopes for
PT
metal interfaces are always positive [3], contrary to what is seen in case of our measurements.
RI
Thus, it is suggested that, under the condition of ideal polarizablity, the Edip vs. U plot slope is specific only to the electrode surface electronic states, therefore carrying no information about
SC
the electrolyte side of the electric double layer. That said, these peaks have been confirmed [3] to
NU
offer extremely interesting information for systems with partial charge-transfer or specific adsorption and therefore ER allows conducting the electron transfer studies of FLG interfaces.
MA
While this is the first article in which in situ spectral features in the mid-infrared region have been looked at from the perspective of ER spectroscopy, these features probably exist for a large
D
number of interfaces not yet discussed completely, but are usually disregarded as infrared
TE
background effects. This was the case in one of our previous studies analyzing the semimetallic
AC CE P
thin-layer Bi electrode behavior [5] where similar albeit much wider (approximately 3000 cm−1 wide) bands appeared as the infrared background. It is now clearer that these bands characterize the influence of electric potential on the electronic properties of the semimetallic surface layer characteristics.
Another feature in the potential dependence spectra is the G band, characteristic of sp2 carbon materials at approximately 1570 cm−1. While the ER spectra represent the shift in empty surface states, the G band is specific to the bonding between carbon atoms and thus to the filled electron orbitals. Figure 4b shows the potential dependence of the maxima of the G band relative to the applied electrode potential. A familiar V-shape dependence of the G band wavenumber on U can be seen, characteristic of the shift in electronic structure due to the applied electrode potential. Exactly the same effect has been seen in in situ Raman measurements of the graphene interface [9] for which the potential dependence is very similar. 6
ACCEPTED MANUSCRIPT Finally, the spectrum also shows the vibrational energy levels of the electrolyte side of the interface- the infrared absorption (IRA) spectra. The observed peaks are interpreted to be
PT
characteristic mainly of the diffuse part of the electrical double layer. These features will be
RI
discussed in more detail in a future publication; however, three important points have to be made. Firstly, the peak area- potential dependence with a minimum confirms the earlier assumption by
SC
our group [4] and others [10] that the in situ infrared spectra for porous carbon electrodes are
NU
unable to give information about the changes at the carbon interface and instead result from the actuation of the electrode. Secondly, the IRA spectra are very intensive, up to 50 times (200 times
MA
compared to HOPG measured in an infrared reflection-absorption setup) more so than those measured for the amorphous carbon interface [4]. Although the IRA spectra also represent
D
different interfacial properties and structures as well as different electrode thicknesses (20 nm
TE
thick amorphous carbon compared to <5 nm thick FLG), such increase of signal (usually known
AC CE P
as an enhancement effect) is rare even for rough metallic surfaces. Theoretical models [11] and experiments with graphene micro-ribbons [12] have predicted that the plasmonic resonance of graphene is applicable for spectroscopy in the terahertz frequency range. A recent article [13] has also shown that graphene nano-ribbon arrays exhibit plasmonic enhancement of adsorbed structures. Thus, it is concluded that the plasmonic resonance of graphene [14] is applicable for the investigation of the electrical double layer structure and biosensors, however, the exact mechanism of this enhancement will be the focus of our future studies. At last, the shape of the CE curve and potential dependence of the IRA spectra do not confirm the assumptions made in a recent theoretical article about the interface between graphite and an ionic liquid [15]. On the contrary, the experimental results suggest the theoretical treatments to be oversimplified and that more comprehensive models are required for a fundamental understanding of the complex processes at the electrode | ionic liquid interface. 7
ACCEPTED MANUSCRIPT 4. Conclusion The applicability of mechanically exfoliated few-layer graphene (FLG) electrodes (<5 nm thick)
PT
for in situ infrared studies has been shown. The infrared spectra are seen to carry very different
RI
spectral information characterizing both the electronic structure of the electrode and chemical structure of the electrical double layer. New information about the fundamental aspects of in situ
SC
ER spectroscopy has been established that could help in providing a better understanding of this
NU
method. Contrary to popular wisdom [16], it has been shown that a pure carbon electrode possesses plasmonic properties that provide some spectral enhancement of the interfacial
MA
structure. These results introduce some new possibilities for the studies of the interface of graphene and FLG in electrolyte solutions.
D
Acknowledgements
TE
This work was supported by the Estonian Ministry of Education and Research (Project IUT20-
AC CE P
13), Estonian Science Foundation grant No. 8357, and Estonian Centres of Excellence in Research Project TK117T “High-technology Materials for Sustainable Development”. References
[1] M. Cardona, K. L. Shaklee, F. H. Pollak, Phys. Rev. 154 (1967) 696. [2] J. Feinleib, Phys. Rev. Lett. 16 (1966) 1200. [3] D. M. Kolb, In: Spectroelectrochemistry. Springer US, 1988, pp. 87-188. [4] T. Romann, O. Oll, P. Pikma, H. Tamme, E. Lust, Electrochim. Acta. 125 (2014) 183. [5] T. Romann, O. Oll, P. Pikma, E. Lust, Electrochem. Commun. 23 (2012) 118. [6] H. Ji, et al. Nat. Commun. 5 (2014) 3317. [7] B. Partoens, F. M. Peeters, Phys. Rev. B. 74 (2006) 075404. [8] D. M. Kolb, C. Franke, Appl. Phys. A 49 (1989) 379. [9] J. H. Zhong, et al. Electrochim. Acta 110 (2013) 754. 8
ACCEPTED MANUSCRIPT [10] F. W. Richey, B. Dyatkin, Y. Gogotsi, Y. A. Elabd, J. Am. Chem. Soc. 135 (2013) 12818. [11] F. Rana, IEEE Trans. Nanotehnol. 7 (2008) 91.
PT
[12] L. Ju, et al. Nat. Nanotechnol. 6.10 (2011) 630.
RI
[13] Y. Li, et al. NanoLett. 14 (2014) 1573.
[14] A. N. Grigorenko, M. Polini, K. S. Novoselov, Nat. Photonics 6 (2012) 749.
SC
[15] A. A. Kornyshev, N. B. Luque, W. Schmickler. J.Solid State Electrochem. (2013) 1.
NU
[16] P.A. Christensen, in: A.J. Bard, M. Stratmann (Eds.), Encyclopedia of Electrochemistry, vol. 3, Wiley, New York, 2003, 530.
MA
Figures
Fig. 1. Spectral configuration for the measurement of FLG electrodes (a) (not in scale) and
D
contact mode AFM image of the electrode topography.
TE
Fig. 2. Electrochemical impedance spectroscopy phase angle diagram (a) for the FLG| EMImBF4
AC CE P
system and the same data represented as the differential capacitance- frequency dependence. Potential- capacitance curve of the FLG|EMImBF4 system (b) measured at 200 Hz. Fig. 3. P-polarized in situ infrared spectra of the FLG|EMImBF4 system. The spectra are shifted in the vertical direction for clarity. Areas where different spectral information has been extracted have been outlined.
Fig. 4. The dependences of Edip (a) and G band position (b) on electrode potential from the data seen in Figure 3.
9
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC CE P
TE
D
Figure 1
10
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NU
SC
RI
PT
ACCEPTED MANUSCRIPT
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D
Figure 2
11
AC CE P
Figure 3
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
12
AC CE P
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D
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NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Figure 4
13
ACCEPTED MANUSCRIPT
PT
Highlights
Fabrication of high quality few-layer graphene electrodes is described.
•
Ideal polarizability of an ionic liquid | electrode interface is established.
•
In situ electroreflectance spectroscopy is extended to the mid-infrared region.
•
Potential dependent reflective properties of graphene are discussed.
•
Plasmonic spectral enhancement is shown for a pure carbon electrode.
AC CE P
TE
D
MA
NU
SC
RI
•
14
S1388-2481(14)00155-6 doi: 10.1016/j.elecom.2014.05.023 ELECOM 5168
To appear in:
Electrochemistry Communications
Received date: Revised date: Accepted date:
21 April 2014 19 May 2014 20 May 2014
Please cite this article as: Ove Oll, Tavo Romann, Enn Lust, An infrared study of the few-layer graphene — ionic liquid interface: Reintroduction of in situ electroreflectance spectroscopy, Electrochemistry Communications (2014), doi: 10.1016/j.elecom.2014.05.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT An infrared study of the few-layer graphene | ionic liquid interface: reintroduction of in situ electroreflectance spectroscopy Ove Oll, Tavo Romann and Enn Lust*
PT
Institute of Chemistry, University of Tartu, Ravila 14A, 50144 Tartu, Estonia
RI
* Corresponding author. Tel.: +372 737 5165; fax: +372 737 5264.
AC CE P
TE
D
MA
NU
SC
E-mail address: [email protected] (E. Lust)
1
ACCEPTED MANUSCRIPT Abstract A simple method of producing high quality few-layer graphene (FLG) electrodes (<5 nm thick)
PT
has been described and the in situ infrared measurements of the FLG| 1-ethyl-3-
RI
methylimidazolium tetrafluoroborate system have been reported. Ideal polarizability of the system has been established and three different spectral modes have been discussed in order to
SC
provide a varied understanding of both the electronic and structural effects at the interface. The
NU
method of in situ electroreflectance spectroscopy has been extended to the study of FLG | ionic liquid interface, providing new information about the method and possibilities for future studies
MA
of specific adsorption and electronic structure of the interface. Plasmonic enhancement of the
double layer and infrared sensors.
D
spectra has been demonstrated, providing excellent opportunities for the study of the electric
AC CE P
spectroscopy
TE
Keywords: Few-layer graphene; EMImBF4; In situ infrared spectroscopy; Electroreflectance
2
ACCEPTED MANUSCRIPT 1. Introduction Electroreflectance (ER) spectroscopy is a widely used technique for the study of semiconductor
PT
electronic structure [1]. In the mid 60’s it was discovered that the technique could also be used to
RI
probe metallic interfaces in (the) in situ electrochemical conditions [2] using the external reflection technique with near infrared to ultraviolet irradiation. The thorough research that
SC
continued found that, differently from semiconductors, ER could be used to selectively probe the
NU
surface states of a metallic interface, thus providing an extremely powerful tool for the analysis of interfacial phenomena from an electronic standpoint. The readers are referred to an excellent
MA
review analyzing the fundamentals and capabilities of ER [3]. Although ER provided some fascinating insight into the electronic effects of metallic interfaces, many of the discovered
D
experimental phenomena are still not adequately explained. Due to some technical difficulties
TE
and the fact that very few groups around the world had the knowledge and capability for in situ
AC CE P
ER, the method was abandoned in the early 90’s. Our communication seeks to reintroduce ER by extending the scope of it to the semimetallic few-layer graphene (FLG) interface within the midinfrared spectral region.
In our preceding study [4], clear differences in spectral behavior were shown for different types of carbon electrodes (thin film amorphous and carbide-derived carbons). However, largely due to technical and practical limitations, clear assessments of the interfacial structure were not able to be established. The aim of this study was to show the applicability of FLG electrodes for in situ infrared studies and to provide a background for future more complex studies. 2. Experimental Atomic force microscopy (AFM) data were obtained by an Agilent TechnologiesTM Series 5500 system. Spectroscopic measurements were performed using a PerkinElmer Spectrum GX FTIR equipped with a liquid nitrogen-cooled mid-range MCT detector and the electrochemical 3
ACCEPTED MANUSCRIPT measurements using an Autolab PGSTAT 30 potentiostat in a three-electrode glass cell with an Ag|AgCl wire in the same ionic liquid (IL) as a pseudo-reference electrode [4]. Impedance
PT
spectra were measured within ac frequency range from 10−2 to 105 Hz with 5 mV ac modulation.
RI
1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) from Solvionic (99.5 %, H2O ~100 ppm) was additionally dried in ultra-vacuum at 100 °C for 48 hours, until reaching a pressure of
SC
10−9 torr and water content below the detection limit of Karl Fischer method (< 5 ppm).
NU
Our constructed system [4-5] uses an exfoliated highly oriented pyrolytic graphite (HOPG) (Veeco) layer on the flat side of a 10 mm diameter infrared transparent ZnSe (n = 2.4)
MA
hemisphere as the working electrode. A thin (~5 µm) HOPG layer is glued onto the ZnSe hemisphere with a thin layer (~300 nm) of dielectric epoxy (EPO) glue and exfoliated with scotch
D
tape. Usually only one exfoliation is required to produce a see-trough layer of FLG on the
TE
hemisphere. Although the produced surface is somewhat uneven, the hemisphere setup requires
AC CE P
only the middle, infrared active part of the hemisphere to be uniformly covered. Optical transmission and Raman spectroscopy measurements suggest the thinner parts of the electrode to compose of <10 layers of graphene. Figure 1 shows the experimental setup of the measurements and a contact mode AFM image of the working electrode. It can be seen from the AFM image how the thin FLG layer conforms to the slightly wavy surface of the EPO glue. All the surface terraces observed are of single layer height. The average width of the terraces is over 4 µm. Spectra in the potential region of −1.8 V < U < 1.8 V were measured relative to the fixed reference potential −0.2 V (the potential of zero charge (pzc) [4]). Details about the measurement technique and electrochemical setup have been previously reported [5]. 3. Results and discussion
4
ACCEPTED MANUSCRIPT Figure 2a shows the impedance phase angle diagram for the FLG|EMImBF4 system measured at the pzc. It is seen that the system is ideally polarizable in a wide range of ac frequencies (Phase
PT
angle ≥ −89.8°) which is a prerequisite for the unambiguous interpretation of ER spectra. The
RI
potential-capacitance (CU) curve is shown in figure 2b. The characteristic V-shape is seen,
[6], explained by the semimetallic nature of FLG.
SC
similar to the CU curves demonstrated for graphene and FLG electrodes in electrolyte solution
NU
Figure 3 shows the potential dependent in situ infrared spectra for the FLG|EMImBF4 interface. Three distinct areas of different spectral information have been outlined in the figure. The most
MA
striking and perhaps most interesting are the extremely wide (approximately 1000 cm−1 wide) Gaussian shaped peaks with very strong potential-position dependence. Such features could not
D
be characterized by conventional theory of vibrational spectroscopy and are instead interpreted to
TE
result from the potential induced change of the reflective properties of the FLG electrode; as in
AC CE P
situ ER spectroscopy data. From a fundamental standpoint, the peaks signify the excitation of electrons to the empty π-orbitals of FLG at the K point [7] by infrared irradiation, which shift due to the applied electric potential at the interface. Due to the limited spectral range and other spectral features, the peaks are either not seen or easily detected within the range from 0.4 to −0.6 V. Figure 4a shows the potential dependence of the peak dips (maxima). It can be seen that there is an almost perfectly linear dependence between the peak dips and electrode potential, with exactly the same absolute slope of 0.233 eV V−1 at both the positive and negative potential side, in excellent agreement with the surface electronic structure of FLG [7]. Similar graphs of Edip vs. U have been shown for low index crystal faces of Au and Ag [3, 8]. However, the exact mechanism generating this effect is still under discussion. While it has been proposed that this dependency represents the applied effective field strength on the respective material surface states, such assumptions could not be confirmed by theory [3]. On the contrary, large differences 5
ACCEPTED MANUSCRIPT exist between different crystal faces, metals and cathodic and anodic regimes for which many unconfirmed explanations have been given. More interestingly, the Edip vs. U plot slopes for
PT
metal interfaces are always positive [3], contrary to what is seen in case of our measurements.
RI
Thus, it is suggested that, under the condition of ideal polarizablity, the Edip vs. U plot slope is specific only to the electrode surface electronic states, therefore carrying no information about
SC
the electrolyte side of the electric double layer. That said, these peaks have been confirmed [3] to
NU
offer extremely interesting information for systems with partial charge-transfer or specific adsorption and therefore ER allows conducting the electron transfer studies of FLG interfaces.
MA
While this is the first article in which in situ spectral features in the mid-infrared region have been looked at from the perspective of ER spectroscopy, these features probably exist for a large
D
number of interfaces not yet discussed completely, but are usually disregarded as infrared
TE
background effects. This was the case in one of our previous studies analyzing the semimetallic
AC CE P
thin-layer Bi electrode behavior [5] where similar albeit much wider (approximately 3000 cm−1 wide) bands appeared as the infrared background. It is now clearer that these bands characterize the influence of electric potential on the electronic properties of the semimetallic surface layer characteristics.
Another feature in the potential dependence spectra is the G band, characteristic of sp2 carbon materials at approximately 1570 cm−1. While the ER spectra represent the shift in empty surface states, the G band is specific to the bonding between carbon atoms and thus to the filled electron orbitals. Figure 4b shows the potential dependence of the maxima of the G band relative to the applied electrode potential. A familiar V-shape dependence of the G band wavenumber on U can be seen, characteristic of the shift in electronic structure due to the applied electrode potential. Exactly the same effect has been seen in in situ Raman measurements of the graphene interface [9] for which the potential dependence is very similar. 6
ACCEPTED MANUSCRIPT Finally, the spectrum also shows the vibrational energy levels of the electrolyte side of the interface- the infrared absorption (IRA) spectra. The observed peaks are interpreted to be
PT
characteristic mainly of the diffuse part of the electrical double layer. These features will be
RI
discussed in more detail in a future publication; however, three important points have to be made. Firstly, the peak area- potential dependence with a minimum confirms the earlier assumption by
SC
our group [4] and others [10] that the in situ infrared spectra for porous carbon electrodes are
NU
unable to give information about the changes at the carbon interface and instead result from the actuation of the electrode. Secondly, the IRA spectra are very intensive, up to 50 times (200 times
MA
compared to HOPG measured in an infrared reflection-absorption setup) more so than those measured for the amorphous carbon interface [4]. Although the IRA spectra also represent
D
different interfacial properties and structures as well as different electrode thicknesses (20 nm
TE
thick amorphous carbon compared to <5 nm thick FLG), such increase of signal (usually known
AC CE P
as an enhancement effect) is rare even for rough metallic surfaces. Theoretical models [11] and experiments with graphene micro-ribbons [12] have predicted that the plasmonic resonance of graphene is applicable for spectroscopy in the terahertz frequency range. A recent article [13] has also shown that graphene nano-ribbon arrays exhibit plasmonic enhancement of adsorbed structures. Thus, it is concluded that the plasmonic resonance of graphene [14] is applicable for the investigation of the electrical double layer structure and biosensors, however, the exact mechanism of this enhancement will be the focus of our future studies. At last, the shape of the CE curve and potential dependence of the IRA spectra do not confirm the assumptions made in a recent theoretical article about the interface between graphite and an ionic liquid [15]. On the contrary, the experimental results suggest the theoretical treatments to be oversimplified and that more comprehensive models are required for a fundamental understanding of the complex processes at the electrode | ionic liquid interface. 7
ACCEPTED MANUSCRIPT 4. Conclusion The applicability of mechanically exfoliated few-layer graphene (FLG) electrodes (<5 nm thick)
PT
for in situ infrared studies has been shown. The infrared spectra are seen to carry very different
RI
spectral information characterizing both the electronic structure of the electrode and chemical structure of the electrical double layer. New information about the fundamental aspects of in situ
SC
ER spectroscopy has been established that could help in providing a better understanding of this
NU
method. Contrary to popular wisdom [16], it has been shown that a pure carbon electrode possesses plasmonic properties that provide some spectral enhancement of the interfacial
MA
structure. These results introduce some new possibilities for the studies of the interface of graphene and FLG in electrolyte solutions.
D
Acknowledgements
TE
This work was supported by the Estonian Ministry of Education and Research (Project IUT20-
AC CE P
13), Estonian Science Foundation grant No. 8357, and Estonian Centres of Excellence in Research Project TK117T “High-technology Materials for Sustainable Development”. References
[1] M. Cardona, K. L. Shaklee, F. H. Pollak, Phys. Rev. 154 (1967) 696. [2] J. Feinleib, Phys. Rev. Lett. 16 (1966) 1200. [3] D. M. Kolb, In: Spectroelectrochemistry. Springer US, 1988, pp. 87-188. [4] T. Romann, O. Oll, P. Pikma, H. Tamme, E. Lust, Electrochim. Acta. 125 (2014) 183. [5] T. Romann, O. Oll, P. Pikma, E. Lust, Electrochem. Commun. 23 (2012) 118. [6] H. Ji, et al. Nat. Commun. 5 (2014) 3317. [7] B. Partoens, F. M. Peeters, Phys. Rev. B. 74 (2006) 075404. [8] D. M. Kolb, C. Franke, Appl. Phys. A 49 (1989) 379. [9] J. H. Zhong, et al. Electrochim. Acta 110 (2013) 754. 8
ACCEPTED MANUSCRIPT [10] F. W. Richey, B. Dyatkin, Y. Gogotsi, Y. A. Elabd, J. Am. Chem. Soc. 135 (2013) 12818. [11] F. Rana, IEEE Trans. Nanotehnol. 7 (2008) 91.
PT
[12] L. Ju, et al. Nat. Nanotechnol. 6.10 (2011) 630.
RI
[13] Y. Li, et al. NanoLett. 14 (2014) 1573.
[14] A. N. Grigorenko, M. Polini, K. S. Novoselov, Nat. Photonics 6 (2012) 749.
SC
[15] A. A. Kornyshev, N. B. Luque, W. Schmickler. J.Solid State Electrochem. (2013) 1.
NU
[16] P.A. Christensen, in: A.J. Bard, M. Stratmann (Eds.), Encyclopedia of Electrochemistry, vol. 3, Wiley, New York, 2003, 530.
MA
Figures
Fig. 1. Spectral configuration for the measurement of FLG electrodes (a) (not in scale) and
D
contact mode AFM image of the electrode topography.
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Fig. 2. Electrochemical impedance spectroscopy phase angle diagram (a) for the FLG| EMImBF4
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system and the same data represented as the differential capacitance- frequency dependence. Potential- capacitance curve of the FLG|EMImBF4 system (b) measured at 200 Hz. Fig. 3. P-polarized in situ infrared spectra of the FLG|EMImBF4 system. The spectra are shifted in the vertical direction for clarity. Areas where different spectral information has been extracted have been outlined.
Fig. 4. The dependences of Edip (a) and G band position (b) on electrode potential from the data seen in Figure 3.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Highlights
Fabrication of high quality few-layer graphene electrodes is described.
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Ideal polarizability of an ionic liquid | electrode interface is established.
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In situ electroreflectance spectroscopy is extended to the mid-infrared region.
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Potential dependent reflective properties of graphene are discussed.
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Plasmonic spectral enhancement is shown for a pure carbon electrode.
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