Comparative Studies of Photoelectron Spectroscopy and Voltammetry

Article pubs.acs.org/JPCC

Comparative Studies of Photoelectron Spectroscopy and Voltammetry of Ferrocene-Terminated Self-Assembled Monolayers Possessing Different Electron-Donating Abilities Yasuyuki Yokota,*,† Yoshitada Mino,† Yuta Kanai,† Toru Utsunomiya,† Akihito Imanishi,† Matthaü s A. Wolak,‡ Rudy Schlaf,‡ and Ken-ichi Fukui*,† †

Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan ‡ Department of Electrical Engineering, University of South Florida, Tampa, Florida 33620, United States ABSTRACT: The relationship between the oxidation potential determined by electrochemical measurements and the ionization energies measured by gas-phase ultraviolet photoelectron spectroscopy (UPS) has long been a focus of research of various groups. The focus of this study is to reveal such a correlation for redox molecules, which are chemically attached to metal electrodes. X-ray photoelectron spectroscopy, UPS, and cyclic voltammetry were performed for three types of ferrocene-terminated self-assembled monolayers possessing different electron-donating abilities. The results of these experiments indicate a linear relation with a slope of ∼0.7 between the UPS-derived energy of the highest occupied molecular orbital (HOMO) and the electrochemical oxidation potential. This indicates that the HOMO energy can be used to determine the oxidation potential of chemically modified electrodes. electrochemically gated molecular circuits,8 requires knowledge about the electronic structure of redox molecules that are chemically attached to the electrodes. The electronic states of chemically modified electrodes have long been studied by various methods,9 especially by ultraviolet photoelectron spectroscopy (UPS) in ultrahigh vacuum (UHV).10−14 These UPS studies revealed that the ionization energy or HOMO energy is a complicated function of the surrounding microscopic environment of the molecules. This is in contrast to the gas phase experiments, where the molecules are isolated.15,16 For example, it is well established that screening effects of ionic states by surrounding molecules and/or electrodes are responsible for a significant part of the energy shifts with respect to the gas phase spectra.15 Hence, with regard to the use of the gas-phase ionization energy or HOMO energy, when available, care should be taken for the redox molecules chemically attached to the electrode. The electrochemical properties of chemically modified electrodes have also been a significant focus of research in recent decades.17 Self-assembled monolayers (SAMs) of thiols on gold electrodes are a mainstay for surface modifications with molecular layers.18,19 Since the pioneering work of Chidsey,20 a large number of studies has been reported in the literature concerning ferrocene-terminated SAMs.18 They include kinetic

1. INTRODUCTION The assessment of the energy cost to remove an electron from molecules is essential for many applications, including chemical and electrochemical processes. The relationship between ionization energies measured by gas-phase ultraviolet photoelectron spectroscopy (UPS) and the corresponding oxidation potential determined by electrochemical measurements has been studied by various groups for half a century.1−6 As early as 1961, Loveland and Dimeler pointed out that half-wave potentials E1/2 of seven aromatic hydrocarbons have a linear relation to their gas-phase ionization potentials Ip.1 In 1963, Pysh and Yang have reported the linear relation for various organic compounds with the following empirical equation:2 E1/2 = 0.68Ip + C

(1)

where C is a constant determined by the particular reference electrode. As a possible reason for the deviation from the simple expectation of a unity slope, they described that both the Ip and the solvation energy in the electrolyte solutions are functions of the molecular structure of the compounds. While relatively large values of 0.83 and 0.89 were reported in refs 3 and 4, recent systematic studies have yielded slope values of ∼0.7 for ferrocene derivatives5 as well as aromatic hydrocarbons.6 Using eq 1, the oxidation potential can be deduced from the ionization energy or the highest occupied molecular orbital (HOMO) of the compounds. Recent progress in the development of sophisticated electrochemical devices, such as dye-sensitized solar cells7 and © 2014 American Chemical Society

Received: March 9, 2014 Revised: April 30, 2014 Published: May 6, 2014 10936

dx.doi.org/10.1021/jp5023899 | J. Phys. Chem. C 2014, 118, 10936−10943

The Journal of Physical Chemistry C

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studies of the electron transfer,21 the properties of the electrical double layer,22 and the influence of the redox microenvironment on the electron transfer.23 Among the most important features of chemically modified electrodes is the shift of the oxidation potentials as a function of intermolecular interactions,24 counteranion species,25 and electrolyte concentrations,26 which are insignificant in the case of the redox molecules dissolved in solutions.27 Comprehensive studies of both electronic states and electrochemical properties for chemically modified electrodes, however, are very limited due to the difficulty of preparing electrodes with systematically changing properties.28 In this study, we performed photoelectron spectroscopy and electrochemical measurements for three types of ferroceneterminated SAMs to reveal the correlation between the oxidation potential and the HOMO energy. The molecular structures used in this study are shown in Chart 1, where FcCO

triphenylphosphonium bromide (2) were prepared according to the literature.29,30 2.1.1. (CH3)8FcC11H20OH (3). Compound 2 (3.51 g, 7.0 mmol) was stirred in THF (80 mL) at 0 °C under Ar. A 1.6 M hexane solution of n-butyllithium (4.40 mL, 7.0 mmol) was added, and the solution was stirred for 30 min. A THF solution (20 mL) of compound 1 (2.28 g, 7.0 mmol) was added dropwise over 10 min, followed by stirring for 2 h at room temperature. After the reaction was quenched by the addition of a saturated ammonium chloride aqueous solution (20 mL), the reaction mixture was extracted with ether. The solution was concentrated and then purified by alumina column chromatography using 3:1 hexane/ether as an eluent to yield 0.79 g (24%) of 3 as an orange solid. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.10−1.90 (m, 40H), 3.26 (s, 1H), 3.61 (q, 2H, J = 6.4 Hz), 5.55−5.75 (m, 1H), 6.00 (d, 1H, J = 16.0 Hz). 2.1.2. (CH3)8FcC11H20SH (OctFc). Diisopropyl azodicarboxylate (DIAD) (0.63 mL, 3.2 mmol) was added to a stirred solution of triphenylphosphine (PPh3) (0.85 g, 3.2 mmol) in THF (20 mL) at 0 °C under Ar. The mixture was stirred at 0 °C for 30 min. A THF solution (10 mL) of compound 3 (0.75 g, 1.6 mmol) with thioacetic acid (0.23 mL, 3.2 mmol) was added dropwise over 10 min, and the solution was stirred for 1 h. The solution was concentrated and then purified by alumina column chromatography using hexane as an eluent to give 0.11 g (14%) of OctFc as an orange oil. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.10−1.90 (m, 40H), 2.50 (q, 2H, J = 7.2 Hz)), 3.23 (s, 1H), 5.55−5.65 (m, 1H), 6.11 (d, 1H, J = 11.6 Hz). HRMS (FAB): Calcd for C29H46SFe: 482.2670; Found: 482.2678. 2.2. Gold Substrates and Monolayer Preparation. A gold film on mica with a (111)-oriented surface was prepared by thermal deposition of gold metal (Furuya Metal) in a vacuum chamber at ∼1 × 10−5 Pa (SVC700TM, Sanyu Electron). The Au(111) substrates were annealed in a butane flame and immersed in a 0.1 mM ferrocene derivative acetone solution for at least 12 h.33,34 The substrates, fully covered by SAMs, were rinsed with pure acetone and dried with N2 gas. 2.3. Photoelectron Spectroscopy. XPS measurements were performed on a combined XPS-UPS Kratos Axis Ultra with an average base pressure of 10−9 Torr.35 XPS data were collected with monochromatic Al Kα radiation (1486.7 eV) operating at 150 W at a pass energy of 40 eV. UP-spectra were obtained using 21.2 eV He (I) excitation (Omicron VUV Lamp, HIS 13) at a pass energy of 5 eV. The photoelectrons were detected normal to the surface. Binding energies for XPS are referenced to the Au 4f7/2 peak at 84.0 eV, and those for UPS to the Fermi edge of bare gold surfaces (0 eV). 2.4. Electrochemical Measurements. A 0.1 M HClO4 aqueous solution was prepared as the electrolyte solution. The electrode potential was controlled by a potentiostat (PGSTAT128N, Metrohm Autolab) or a bipotentiostat (AFCBP1, Pine Instrument Company). Au/AuOx was used as the reference electrodes and presented with respect to Ag/

Chart 1. Ferrocene Derivatives Used in This Study

and OctFc molecules contain electron withdrawing and donating substitution groups, respectively, to shift the energy level and oxidation potential. X-ray photoelectron spectroscopy (XPS) and UPS were used to reveal the core and valence electronic states, and the oxidation potential was determined by cyclic voltammetry. The results confirm the linear relation between the oxidation potential and HOMO energy with a slope of ∼0.7.

2. EXPERIMENTAL SECTION 2.1. Materials. The electrolyte solutions were prepared using ultrapure grade HClO4 (Cica-Merck) and Milli-Q water (Nihon Millipore). The ferrocene derivatives, 11-mercaptoundecanoylferrocene (FcCO) and 11-ferrocenyl-1-undecanethiol (Fc), were purchased from Asemblon Inc. or Prochimia Surface and DOJINDO Laboratories, respectively. All other commercial chemicals were of reagent grade or better, and used without further purification. The ferrocene derivatives, OctFc, were synthesized according to the modification of reported procedures.29−32 The synthetic route is shown in Scheme 1. Octamethylformylferrocene (1) and (10-hydroxydecyl)Scheme 1. Synthetic Route of the OctFc Molecule

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molecules in the SAM.19 Although the Fc SAM has a four times larger coverage than the OctFc SAM, the observed S 2p (Figure 1(b)) and Au 4f (Figure 1(c)) intensities of these SAMs were comparable. Large intensities for the OctFc SAM indicate that the photoelectrons ejected from the S 2p and Au 4f core levels can easily escape from the sample surface due to the thin effective thickness.47 However, the FcCO SAM gave the smallest S 2p and Au 4f intensities. According to the literature, the inclusion of a carbonyl group like in the FcCO molecules tends to change the thiol adsorption scheme from monolayer to multilayer formation with a strong dependence on the thiol concentration.48,49 This suggests that the FcCO SAM is covered by a partial multilayer formed from unbound molecules, which reduces the photoelectron escape probability from the sulfur/ gold interface. This hypothesis is supported by the accompanying electrochemical measurements discussed below, as well as a peak position analysis of the S 2p signal. The corresponding XPS C 1s spectra of the FcCO, Fc, and OctFc SAMs are shown in Figure 1(d). The C 1s peak of the Fc SAM was observed at 284.5 eV, which is within the range of the peak positions of ferrocene-terminated SAMs in a previous report.13,40,43 Watcharinyanon et al. reported that carbon atoms in alkyl chains yield a C 1s peak at 284.8 eV, while C 1s related to carbon atoms in the ferrocene rings has a binding energy of 284.2 eV.40 These measurements were performed with synchrotron radiation. This suggests that the 284.5 eV peak seen here contains both components. The full width at halfmaximum (fwhm) of the peak is approximately 0.8 eV and in excellent agreement with the value obtained by Watcharinyanon et al. for the Fc SAM (0.80−0.85 eV).40 In contrast, the FcCO and OctFc SAMs yielded a broad peak at higher binding energy due to the structural inhomogeneity and/or the presence of carbon atoms in several functional groups other than the ferrocene and alkyl chains.28,50 3.2. UV Photoelectron Spectroscopy. UPS was used to study the valence electronic structures of the ferroceneterminated SAMs. The electronic structure directly affects the oxidation potential determined by electrochemical measurements. Figure 2(a) depicts the valence region spectra of the FcCO, Fc, and OctFc SAMs, where the binding energy scale is referenced to the Fermi level. The spectral shapes measured on the Fc and OctFc SAMs are similar to those of ferrocene and decamethylferrocene solids, respectively,51 indicating that the

AgCl (3.0 M NaCl). Pt wire was used as the counter electrode. The geometric area of the working electrode was ∼0.35 cm2. The roughness factor of the gold surface was determined from the ratio between the charge required for the oxidation of the Fc SAM and the corresponding literature value (43.4 μC/ cm2).36

3. RESULTS AND DISCUSSION 3.1. X-ray Photoelectron Spectroscopy. SAMs of FcCO, Fc, and OctFc were characterized by XPS to quantify the adsorption of the thiols onto the Au(111) substrates. Figure 1(a) shows Fe 2p spectra for each SAM. The Fe 2p3/2 and 2p1/2

Figure 1. XPS (a) Fe 2p, (b) S 2p, (c) Au 4f, and (d) C 1s spectra of FcCO, Fc, and OctFc SAMs.

signals for the Fc SAM were observed at 707.8 and 720.6 eV, respectively, in accordance with previous reports.13,37−43 The lack of the signal at around 710 eV indicates that the ferrocene moiety is not oxidized42 and possesses electrochemical activity on the surface.43 Although the Fe 2p peaks for FcCO and OctFc SAMs were shifted to higher and lower binding energy, respectively, the ferrocene moiety of these SAMs was also not oxidized during SAM formation. The direction of the peak shift is correlated to the inductive effect of the substitution groups of the ferrocene moiety. The details of the peak positions of Fe 2p3/2 are being further discussed below. Figure 1(b) shows the XPS S 2p spectra of FcCO, Fc, and OctFc SAMs. The S 2p3/2 and 2p1/2 signals for Fc and OctFc SAMs were observed at 162.0 and 163.2 eV, respectively, with a 2:1 intensity ratio, in accordance with the well-established values for n-alkanethiol SAMs.13,40,43−46 This observation indicates that these SAMs do not contain unbound thiol chains.44,46 In contrast, the S 2p3/2 and 2p1/2 signals for the FcCO SAM show a deviation from the 2:1 ratio due to the additional component at 163−165 eV, indicating a presence of unbound thiol in the case of FcCO SAM.44,46 The signal intensity of the Fe 2p spectra correlates with the coverage of molecules at the surface.38−41 The ratio of the coverage of FcCO, Fc, and OctFc SAMs estimated via the Fe 2p3/2 signal was 0.87:1.00:0.28. Since the OctFc molecule has a bulky substitution group compared to the Fc and FcCO molecules, a smaller coverage can be expected for the OctFc SAM, considering the close-packing of the constituent

Figure 2. (a) UPS valence band spectra of FcCO, Fc, and OctFc SAMs. (b) Magnified spectra of the HOMO emission range. The dotted lines represent the simulated DOS curves obtained by the convolution of the molecular orbital energies by a Gaussian function (σ = 0.27 eV). All simulated DOS curves were shifted by 2.5 eV on the binding energy scale to match the experimental HOMO region peak of the Fc SAM. Note that the contribution of the localized −SH orbital is not included in the simulated DOS curves. 10938



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SAMs gave a reproducible response from the first potential scan, the FcCO SAM showed a gradual decrease of the redox wave (∼10%) and required several scans to reach a steady-state. The analysis of the XPS S 2p emission suggests that the gradual decrease of the current response is caused by a desorption of unbound FcCO molecules during the potential scans. After settling into the steady state the peak current was found to be proportional to the scan rate for all SAMs. Thus, it can be concluded that the redox peaks observed in the present study are due to the redox reaction of ferrocene moieties immobilized on the gold electrode.18 The surface coverage calculated from the integration of the peak area is discussed in the next section. As shown in Figure 3, the Fc SAM cyclic voltammogram exhibits the characteristic double peaks at 0.19 and 0.32 V, whose positions are within the range of the formal potentials of Fc SAMs and similar SAMs, as reported previously.18,24,26,33,60−68 According to Lennox et al., the lower and higher potential peaks are derived from the contribution of “isolated” and “clustered” ferrocene moieties in the SAMs, respectively.24 Just recently, Rudnev et al. performed more systematic studies using a number of different single crystal electrodes. These experiments showed that even subtle differences of the electrode structure changes the shape of the redox waves, which led them to propose a three-peak model.68 The redox wave of the OctFc SAM showed three peaks at −0.08, 0.10, and 0.15 V, presumably due to the same origins as compared to the Fc SAM. It should be noted that the OctFc SAM can easily oxidize as expected from the HOMO peak position in the UP-spectra. Compared to the Fc and OctFc SAMs, a single peak with a large fwhm (∼0.2 V) was observed for the steady-state response of the FcCO SAM. The observation of this single peak has been reported previously by various groups and the peak position is within the ranges of the formal potentials of FcCO SAMs and similar SAMs.18,63,65,66,69 The fwhm in an ideal case, where an ideal Nernstian reaction under Langmuir isothermal conditions occurs at 25 °C, is about 0.09 V, and a deviation from this fwhm in ideal CV diagnostics can reveal details about the redox center.18 The large fwhm value suggests the presence of a distribution of formal potentials (due to a distribution of local environments around the redox centers), although the contribution of other factors needs to be acknowledged (i.e., repulsive interaction18 and double-layer effects22). The origin of multiple or single peak behaviors of ferrocene-terminated SAMs could stem from differences between the solvent−SAM interactions depending on the presence of hydrophilic functional groups close to the ferrocene moiety, such as carbonyl and carboxyl groups.36 3.4. Comparison between Photoelectron Spectroscopy and Electrochemical Measurements. In this section, XPS, UPS, and CV measurements, as well as DFT calculations related to the electronic states and the electrochemical properties of ferrocene moieties are compared. First, we discuss the coverage of the FcCO, Fc, and OctFc SAMs estimated by the peak areas of relevant features in the XPS Fe 2p, UPS HOMO, and CV measurements. Subsequently, the relative peak shift of the XPS Fe 2p and UPS HOMO-derived peaks are compared, followed by an analysis of the relative peak positions of the CV measurements. Finally, the correlation between the formal potential and the HOMO energy of the SAMs is examined. Table 1 shows the relative coverage of the SAMs estimated from the integration of the Fe 2p3/2 XPS core level, the HOMO

ferrocene moieties are located at the outermost surface in these SAMs. Note that He (I) UPS has a smaller escape depth than XPS due to the lower kinetic energy of the photoelectrons; the mean free path for 15 eV electrons is less than 1 nm.52,53 The Fc SAM spectra contain five distinct features at 1.5, 3.3, 4.0, 6.9, and 8.2 eV, in good agreement with spectra shown in the literature.14 When the FcCO spectrum is shifted by −0.4 eV, the spectral shapes of FcCO and Fc SAMs are virtually the same. The density of states (DOS) of gas-phase FcCO, Fc, and OctFc molecules were calculated using Gaussian 09 to facilitate the assignment of the UPS features.54 The equilibrium geometry and the molecular orbitals of the three molecules were computed by density functional theory (DFT) with PBE functional55 and the 6-31G* bases set. All DOS curves were obtained by convoluting the molecular orbital energies with a Gaussian function (σ = 0.27 eV) and shifting by 2.5 eV on the binding energy scale to match the experimental HOMO region peak of the Fc SAM. The dotted lines in Figure 2(a) show the simulated DOS curves. It is apparent that they match the lower binding energy region of the UP-spectra fairly well for all three SAMs. The pronounced peaks at 5.2 eV in the simulated DOS curves (see asterisks) are related to the alkyl-chain states. The lack of these peaks in the UP-spectra supports that the photoelectrons ejected from alkyl chains provide only a minor contribution to the whole spectrum. Figure 2(b) shows the UP-spectra of the HOMO region for three SAMs with simulated DOS curves. It is well-accepted from experimental51,56,57 and theoretical58,59 investigations that the HOMO peak of ferrocene is composed of three ironlocalized orbitals, namely e2g and a1g. The DFT calculations also suggest that the HOMO peak includes these orbitals. As in the case of the XPS Fe 2p peaks, the calculated DOS features, as well as the UP-spectra measured on the FcCO and OctFc SAMs were shifted to higher and lower binding energy, respectively, compared to the plain Fc SAM. This is in accordance with the chemical intuition. The details of the position and the intensity of HOMO peaks will be discussed below. It is interesting to note that the UPS HOMO peak of the OctFc SAM has a shoulder at around 1.4 eV, whose origin is not known at this stage. It may be that this is caused by the existence of ferrocene moieties with different environments due to high steric hindrance15,16 or (less likely) by a contribution from the S−Au bonding state.52,53 3.3. Cyclic Voltammetry. Figure 3 shows steady-state cyclic voltammograms (CVs) of three SAMs in a 0.1 M HClO4 solution. A redox wave due to the one-electron oxidation processes appeared for all SAMs. While the Fc and OctFc

Figure 3. Cyclic voltammograms of FcCO, Fc, and OctFc SAMs in 0.1 M HClO4. 10939



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discussion. The coverages obtained by UPS and CV were consistent due to the cancellation of two effects: decrease of escaped photoelectrons due to the multilayer formation for the former and desorption of molecules during the potential cycles for the latter. Thus, the coverage of the FcCO SAM as determined by CV is about half of the Fc SAM, although the molecules have almost the same estimated length as determined by molecular modeling. In the following the peak shifts of the FcCO, Fc, and OctFc SAMs as obtained via XPS, UPS, and DFT calculations will be discussed. Figure 4(a)−(c) shows the magnified Fe 2p3/2 XPspectra, UP-spectra, and DOS curves, whose reference is shifted to be the corresponding OctFc peaks. The shoulder peak at 0.38 eV for the UP-spectrum of the OctFc SAM is ignored for the following discussion because the peak assignment is not entirely clear, as discussed in the previous section. Remarkably, the peak shifts of the XP- and UP-spectra and DOS curves are almost consistent. It is known that the electron density of the molecule is decreased (increased) with the presence of electron withdrawing (donating) substitution groups. Hence, the fact that the binding energy of the FcCO SAM is increased, while the binding energy of the OctFc SAM is decreased relative to the value of the Fc SAM is consistent with chemical intuition. However, the energy shift of the XP- and UP-spectra is expected to be different since the core levels of molecules are generally more localized than their valence levels. The HOMO, HOMO-1, and HOMO-2 levels of ferrocene derivatives that form the HOMO peak of the UP-spectra are composed of localized Fe 3d orbitals with a slight hybridization with the cyclopentadienyl ligands. This localization of the HOMO level could be the reason for the same energy shift of the XP- and UP-spectra. Figure 4(d) shows linear sweep voltammograms for the oxidation of the FcCO, Fc, and OctFc SAMs. Note that the reference potentials were shifted to the major peak of the OctFc SAM. As mentioned in the previous section, the oxidation peak of the Fc and OctFc SAMs were split into two and three peaks, respectively. It is preferable to select the “isolated” peaks (vertical solid lines) rather than the “clustered” peaks (vertical dotted lines) for these SAMs since the “clustered” peaks of the OctFc SAM only have a minor contribution to the whole oxidation peak. The difference between the oxidation potentials of the FcCO and OctFc SAMs (0.59 V) is close to the previously reported value of the similar SAMs measured in an organic electrolyte solution (0.6 V).70 Note that the single peak of the FcCO SAM can include both components.

Table 1. Relative Coverage of SAMs as Estimated from XPS, UPS, and CV Measurements, as well as the Coverage and Size of the Ferrocene Backbone as Calculated from CV Curves SAMs

XPS Fe 2p3/2a

UPS HOMOa

CVa

CV (mol/cm2)b

size of ferrocene backbone (Å)c

FcCO Fc OctFc

0.87 1.00 0.28

0.50 1.00 0.25

0.51 1.00 0.33

2.3 × 10−10 4.5 × 10−10 1.5 × 10−10

9.2 6.6 11.5

a The Fe 2p3/2 peak of the XPS, the HOMO region in UPS, and the oxidation peak from CV were integrated with the linear background subtraction. bThe coverage of the Fc SAM was referenced to the literature value to normalize the contribution of the roughness factor of the Au substrates.36 cThe size of the ferrocene backbone was estimated from the CV coverage by determining the diameter of closepacked spheres.

region of the corresponding UP-spectrum, and the oxidation peak from the CV curve including a linear background subtraction. The coverages of the Fc and OctFc SAMs are consistent for the three separate measurements, as expected. In order to investigate the coverages in more detail, the surface coverages obtained by the CV measurements and the size of the ferrocene moieties as calculated from the diameter of closepacked spheres are shown in the last two columns in Table 1. The diameter for the OctFc molecule is 11.5 Å, larger than the expected one (10.6 Å: the length between the furthest hydrogen atoms in the ferrocene moiety with twice the van der Waals radius of the hydrogen atom (8.2 + 1.2 × 2 Å). However, the diameter for the Fc molecule (6.6 Å) is known to be smaller than the length estimated by molecular modeling (5.5 + 1.2 × 2 = 7.9 Å).36 It is reasonable to assume that the above difference originates from the large steric hindrance of the OctFc molecule. In contrast, the coverage of the FcCO SAM obtained from the XPS Fe 2p peak is larger than that obtained by UPS and CV. This may be a result of the unbound molecules forming a partial multilayer, which causes an overestimation of the coverage, since XPS measurements count all photoelectrons emitted from the Fe 2p level irrespective of the presence of chemical bonds between sulfur and the gold electrode. A possible origin of the multilayer formation is the presence of a carbonyl group next to the ferrocene moiety, which alters the solute−solvent interaction during SAM formation.36 It is interesting to note that the total coverage of the FcCO SAM was found to increase somewhat with the concentration of the thiol solution. However, this small change of the coverage (∼10%) does not affect the conclusions from the following

Figure 4. Comparison between (a) XPS Fe 2p3/2 spectra, (b) UP-spectra of the HOMO region, (c) simulated DOS curves, and (d) CVs of the three different SAMs. The origins of the abscissae are shifted to the main peak of the OctFc SAM. The lateral axes for the XP- and UP-spectra and the simulated DOS curves are reversed with respect to Figures 1 and 2 for easy comparison to the CVs. Vertical solid lines designate the peak positions as discussed in the main text, while the peaks indicated by the vertical dotted lines are not considered. 10940



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of the solvation energy is a prerequisite for a comprehensive understanding of the linear relationship. Recently, several groups, including the authors’, have succeeded in fabricating thin ionic liquid films on various substrates suitable for photoelectron spectroscopy in UHV.35,71−73 Since ionic liquids with a negligible vapor pressure are used as the electrolyte solution,39 the HOMO energy of redox molecules solvated within a matrix similar to an electrochemical environment can be determined by conventional UPS measurements. The investigation of ionic liquid/ferrocene-terminated SAMs is currently in progress in the authors’ group.

In the following, the correlation between the oxidation potentials of the FcCO, Fc, and OctFc SAMs with the respective HOMO energy is discussed. Figure 5 shows the

4. CONCLUSIONS The relationship between the oxidation potential and the HOMO energies of ferrocene derivatives chemically attached to gold electrodes was systematically studied by various methods. In these studies, three ferrocene-terminated SAMs possessing different substitution groups were fabricated and their electronic states and electrochemical properties were characterized using XPS, UPS, and CV. XPS and UPS data revealed that the Fe 2p core levels and valence levels close to the HOMO orbital are shifted by a similar amount with the order of the binding energies being FcCO > Fc > OctFc SAMs. This result is in accordance with the well-known electron withdrawing and donating effects occurring in these systems. The oxidation potentials of these SAMs as measured in 0.1 M HClO4 solution were also shifted in the same order. A linear relation with a slope of ∼0.7 between the oxidation potential and the HOMO energy was found. This result indicates that the HOMO energy is a useful parameter to determine the oxidation potential of chemically modified electrodes.

Figure 5. Correlation of the oxidation potentials of the OctFc, Fc, and FcCO SAMs with the respective HOMO energy. The dotted line indicates a slope of 1 for comparison. The line through the data points has a slope of approximately 0.7. The error bars for the HOMO energy are ±0.1 eV, whereas those for the oxidation potentials are less than the size of the markers.

dependence of the oxidation potential on the HOMO energy as obtained from the data shown in Figure 4(b),(d). The dotted line shows the unit slope for comparison. The graph shows that the oxidation potentials Eox exhibit a linear correlation with the HOMO energy EHOMO with a slope factor of about 0.7: Eox = 0.7E HOMO

(2)

Matsumura-Inoue et al. developed the following equation for the relation between the peak potential of the oxidation of ferrocene derivatives measured in organic electrolyte solutions Ep,ox and the gas-phase ionization potentials Ip:

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AUTHOR INFORMATION

Corresponding Authors

ΔGs + C = 0.7Ip + C′ (3) F where ΔGs represents the difference of the solvation energy between the neutral and the oxidized ferrocene derivatives, F is the Faraday constant, and C and C′ are constants.5 Although the experimental conditions and the methods of data analysis are different, the slope values are the same as in eq 2. In order to explain the slope being less than unity, Matsumura-Inoue et al. proposed that the higher the ionization potential, the stronger the interaction between the solvent and the solute ferrocene derivatives. The microscopic origin of the ionization potential dependence of the solvation energy is not clear at this stage. Finally, we emphasize that the linear relationship obtained by the above comparison is adequate to reveal several open questions. First, whether the ferrocene moieties which yielded the “isolated” and “clustered” peaks in the CVs have the same electronic structure on the electrodes is not clear at this stage. The assessment of the relative importance between the HOMO energy and the solvation energy for the two species is important to attain microscopic understanding of the redox reaction for the chemically attached molecules. For example, ferrocene-terminated SAMs with only “isolated” ferrocene moieties, fabricated by the coadsorption with n-alkanethiol,13,23,24,34,36,38−40,60,67 are good candidates for systematic studies of these phenomena. Second, the origin of less than unity slope has long been an open question. As pointed out by Matsumura-Inoue et al. and other groups,2−5 the determination

*Tel/Fax: +81-6-6850-6238; e-mail: [email protected]. ac.jp. *Tel/Fax: +81-6-6850-6235; e-mail: [email protected]. ac.jp.

Ep,ox = Ip +

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Prof. Y. Morikawa and Mr. S. Akiyama for fruitful discussions. This work was partially supported by the Funding Program for Next Generation World-Leading Researchers (GR071) from the Japan Society for the Promotion of Science (JSPS), and by a Grant-in-Aid for Scientific Research No. 23750013 from the Ministry of Education, Culture, Sports, Science and Technology, Japan.

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REFERENCES

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