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Discerning the Redox-Dependent Electronic and Interfacial Structures in Electroactive Self-Assembled Monolayers Raymond A Wong, Yasuyuki Yokota, Mitsuru Wakisaka, Junji Inukai, and Yousoo Kim J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05885 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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Journal of the American Chemical Society

Discerning the Redox-Dependent Electronic and Interfacial Structures in Electroactive Self-Assembled Monolayers Raymond A. Wong†, Yasuyuki Yokota†,*, Mitsuru Wakisaka‡, Junji Inukai§ and Yousoo Kim†,* †

Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

‡

Graduate School of Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama 939-0398, Japan

§

Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu, Yamanashi 400-8510, Japan

ABSTRACT: We explore the redox-dependent electronic and structural changes of ferrocene-terminated self-assembled monolayers (Fc SAMs) immersed in aqueous solution. By exploiting X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS), we can electrochemically control the Fc SAMs and spectroscopically probe the induced changes with the Fe oxidation state related to the ferrocene/ferrocenium (Fc/Fc+) redox center, formation of 1:1 Fc+-ClO4- ion pairs, molecular orientation and monolayer thickness. We further find the insignificant involvement of interfacial water in the Fc SAMs irrespective of redox state. Electrolyte dependencies could also be identified with 0.1 M NaClO4 and HClO4 when probing partially oxidized Fc/Fc+ SAMs. Corroborating the occurrence of electrochemically induced oxidation, EC-UPS shows that oxidation to Fc+ is accompanied by a shift of the highest occupied molecular orbital toward higher binding energy. The oxidation to Fc+ is also met with an increase in work function ascribed to the induced negative interfacial dipole caused by the presence of Fc+-ClO4- ion pairs along with a contribution from the reorientation of the Fc+ SAMs. The reversibility of our observations is confirmed upon conversion from Fc+ back to the neutral Fc. The approach shown here is beneficial for a broad range of redox-responsive systems to aid in the elucidation of structure-function relationships.

INTRODUCTION One of the key challenges in understanding redox processes is determining the precise changes with the electronic structure and physical environment of the electroactive species. The detailed knowledge of such redox-dependent changes is critical to the rational design and performance characteristics of bio(chemical) sensors,1 molecular electronics,2-3 actuators4 and energy storage/conversion devices.5-7 To address this, a vital prerequisite is the thorough elucidation of interfacial charge transfer with model systems. Ferrocene-terminated alkanethiol self-assembled monolayers (Fc SAMs) on Au are an ideal model system owing to their practical significance and versatility with structural modifications.8-14 The corresponding ferrocene/ferrocenium (Fc/Fc+) redox couple consists of single electron transfer and ion pairing reactions with anions (X-) as follows: (Fc)SAM + (X-)sol ⇆ (Fc+X-)SAM + e-

(eq. 1)

Although characterizing the electrochemical behavior with classical electrochemical methods such as cyclic voltammetry (CV) can allow for ion pairing events to be inferred,15-16 CV is largely limited to probing the overall current response as a function of potential.17 As a result, complementary methods are necessary to better understand the so-called “structure-function” relationships. More importantly, an alteration of the redox state invariably changes the interfacial structure with respect to the double layer, interactions with the electrolyte and work func-

tion.18-20 For instance, the use of surface-enhanced Raman spectroscopy (SERS)21 and electrochemical quartz crystal microbalance (EQCM) have indicated the occurrence of ion pairing.22 In particular, EQCM can provide the mass-to-charge ratio for quantitative insights into the behavior of ion pair formation. Infrared (IR) spectroscopy and surface plasmon resonance (SPR) studies have shown the interplay between Fc/Fc+ redox, SAM orientation and changes in monolayer thickness.23-26 However, to obtain a more complete picture including the interfacial electronic structures (i.e. changes in the redox center and work function) alternative methods are necessary. In contrast, X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) utilizes the photoelectric effect to provide quantitative and semi-quantitative insights into the chemical state and electronic structure. For instance, Yokota et al. utilized XPS/UPS to correlate Fc SAMs containing different electron donating groups and found a linear relationship between the observed electrochemical oxidation potential and the highest occupied molecular orbital (HOMO) energy from UPS with a slope of 0.7.11, 27 Similarly, Nijhuis and coworkers have extensively used XPS/UPS to elucidate the molecular electronic and supramolecular structures on the performance of molecular diodes.10, 28 As a whole, these studies have been restricted in probing the neutral state of the electroactive termini. Namely, there are a lack of studies on the analysis of all the redox states that can be accessible via electrochemical control. Improved knowledge of the resulting changes associated with neutral and

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Figure 1. X-ray and ultraviolet photoelectron spectroscopy combined with an electrochemical cell (EC-XPS/UPS) used to probe ferrocene-terminated alkanethiol self-assembled monolayers (Fc SAMs). (a) Schematic showing the EC and XPS/UPS analysis chambers separated by a gate valve, which permits electrochemical characterization and transfer without exposure to air. (b) Schematic of EC cell with ‘hanging meniscus’ with working (WE), counter (CE) and reference (RE) electrodes. (c) Molecular structure of Fc SAMs (11-ferrocenyl-1-undecanethiol) used in study. (d) Conceptual representation of the interfacial structural changes between ferrocene (Fc) and ferrocenium (Fc+). Sulphur is denoted by the black circles while the electrolyte anions are denoted by the green circles. (e) Steady state cyclic voltammograms (CV) of Fc SAMs on Au(111) using 0.1 M NaClO4 with scan rates of 10, 30 and 50 mV s-1. The vertical dashed lines indicate the potentials at which the electrode was polarized for the subsequent XPS and UPS measurements.

oxidized states has crucial implications in understanding the durability and degradation mechanisms of molecular scale devices.5, 29 The aforementioned shortcomings have motivated us to adopt XPS/UPS to enable the investigation of electrochemically accessible redox states. Herein, we report on our progress in discerning the interfacial environment of Fc SAMs with respect to the redox-dependent electronic and structural changes. We accomplish this by coupling electrochemistry with XPS/UPS. As XPS/UPS is an ultrahigh vacuum (UHV) technique, electrochemical measurements are performed in a separate EC chamber under Ar atmosphere, which permits evacuation and sample transfer to the XPS/UPS analysis chamber without exposure to air. This setup is crucial as oxidized moieties including Fc+ cations are known to decompose in the presence of oxygen in the air.27, 30 Subsequently, we can spectroscopically probe changes to the Fc/Fc+ redox center, the formation of 1:1 Fc+-ClO4- ion pairs, monolayer thickness and SAM orientation. UPS shows that following oxidation to Fc+, the HOMO energy shifts toward higher binding energy and is accompanied by an increase in work function ascribed the reorientation of the molecular dipole. Namely, the induced negative dipole from the formation of Fc+-ClO4- ion pairs and Fc+ SAM reorientation. Taken together, these findings allow us to formulate a more complete picture of the electrode-SAMelectrolyte double layer region. Figures 1a-b and S1 show the experimental setup (see experimental section for details) containing the electrochemical chamber connected via gate valve to the XPS/UPS analysis

chamber. This quasi in situ EC-XPS/UPS technique31-34 with immersed electrodes was introduced by Kolb and Hansen with the intent of unraveling the electrochemical double layer (EDL) regarding the surface concentration of ions, point of zero charge determination and potential-dependent work function changes.17, 34-35 To the extent of our knowledge, our work represents the first report on the utilization of this methodology on a reversible redox-active system (surface bound Fc SAMs, Figure 1c-d). This methodology provides us the unique opportunity to correlate the electrochemistry with the redox-induced electronic and structural changes of Fc SAMs. We further note that the use of surface-bound Fc probes also shows how UHVEC methods32, 36 can be highly effective in discerning the electrode-SAM-electrolyte double layer region. RESULTS AND DISCUSSION Cyclic voltammograms (CVs) of Fc SAMs on Au(111) using 0.1 M NaClO4 as the electrolyte (Figure 1e) show reversible peaks at 0.25 and 0.37 V that scale linearly with scan rate, thus confirms that the Fc termini are directly tethered to the Au electrode. The widely observed non-ideal behavior as seen by peak broadening and multiple CV peaks is rationalized by the heterogeneity in intermolecular interactions resulting in different electrochemical microenvironments (see supporting information (SI) on non-ideal behavior).26, 37-39 The Fc SAM coverage (Γ) determined by the integral charge transferred is 4.6 × 10-10 mol cm-2 and is in line with the theoretical value of 4.5 ×


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Figure 2. XPS of the pristine Fc SAMs and after polarization at 0.5 and 0.1 V in 0.1 M NaClO4, corresponding to oxidized (Fc+) and neutral (Fc) states, respectively. (a) Fe 2p, (b) Cl 2p, (c) O 1s, (d) Au 4f, (e) C 1s, and (f) S 2p spectra.

10-10 mol cm-2 (assuming hexagonal packing and Fc as spheres with diameter of 0.66 nm),10, 24, 26 thus, indicates the successful preparation of densely packed Fc SAMs (theoretical coverage of unsubstituted n-alkanethiol SAM is 7.6 × 10-10 mol cm-2).4041

The subsequent step is to correlate the electrochemistry with the corresponding electronic and structural changes. XPS was performed on the pristine sample and after polarization at 0.5 and 0.1 V (Figure 1e), which corresponds to the oxidized (Fc+) and neutral (Fc) states, respectively. The pristine XPS spectra (Figure 2), as a whole, is consistent with previous reports showing the presence of only Fe, C, and S.11, 27, 42-43 Specifically, the pristine Fe 2p spectrum in Figure 2a shows the distinct sharp symmetry of the Fe 2p3/2 and 2p1/2 peaks at 707.9 and 720.6 eV respectively, associated with diamagnetic FeII (containing no unpaired electrons) in the Fc termini.44-45 This indicates that pristine sample is exclusively comprised of the neutral state Fc. The reversibility as shown from CV is corroborated by the similarity in all the pristine and Fc XPS spectra (Figure 2 and S2). The integrated Fe 2p3/2 peak areas of Fc and Fc+ relative to the pristine sample are close to unity (Table 1 and Figure 2a), while the S 2p spectra (Figure 2f) exhibits no notable change irrespective of redox state. These results indicate that the SAM coverage is consistent following electrochemistry and sample transfer. Substantial changes in the spectra are revealed upon oxidation to Fc+. The oxidation event (eq. 1) is met with several structural changes as shown schematically in Figure 1d, which include (1) ion pair formation with counter anions due to charge compensation of the positive Fc+,15-16, 19 and (2) orientation changes caused by repulsive electrostatic interactions of the positive Fc+ and steric effects from the incorporation of ion pairs.23-24 Firstly,

the occurrence of oxidation is corroborated by the increase in the Fe oxidation state (to FeIII) as seen by the clear shift toward higher binding energy with the Fe 2p3/2 and 2p1/2 peaks (Figure 2a) at 709.4 and 722.6 eV, respectively. The noteworthy asymmetry and peak broadening and shoulder at 711.4 eV is ascribed to an assortment of final state effects including multiplet splitting44 and is indicative of the unpaired electron present with the paramagnetic FeIII.45 This also leads to the noticeable change in the spin-orbit splitting induced separation46 of the Fe 2p3/2 and 2p1/2 peaks, where pristine and Fc exhibits 12.8 eV in separation, while this is larger at 13.2 eV in the case of Fc+. In the characterization of Fc and Fc+ containing imidazolium-based ionic liquids, Taylor and Licence found a similar trend in the separation of the Fe 2p peaks.45 Turning our attention to the Cl 2p and O 1s spectra (Figure 2b-c), which clearly indicates that the Fc+ termini are interacting to some degree with ClO4- anions. The corresponding Fe to Table 1. Summary of XPS results and effective thickness Fe 2p3/2 (area ratio)

Fe : Cl atomic ratio

O : Cl atomic ratio

Effective thickness (Å)a

Pristine

1.00

1:0

0:0

17.5 ± 0.4

Fc+

0.97

0.9 : 1

4.1 : 1

18.6 ± 0.2

Fc

0.95

1:0

0:0

17.2 ± 0.7

The values are obtained from Figure 2. aEstimation of thickness determined by equation 2 where the standard deviation is from 3 independent measurements.



Cl atomic ratio is close to unity at 0.9:1 (Table 1), indicating that the Fc termini has overwhelmingly converted to Fc+. The Fc+ termini in the presence of weakly solvated ClO4- anions results in the facile formation 1:1 Fc+-ClO4- contact ion pairs.15-16 This result provides quantitative spectroscopic evidence of ion pair formation and shows the viability of the EC-XPS/UPS method in discerning such ion pairing events. Additionally, the oxidation to Fc+ leads to macroscopic changes resulting in an increase in electrode wettability, owing to the change in surface free energy with the polar Fc+.47 Regarding the electronic structure, the increase in wettability is related to a change in molecular dipole with the Fc+ SAMs (vide infra, UPS results).48-49 Further, we can ascertain whether interfacial water is incorporated within the Fc+ SAMs. The corresponding O:Cl atomic ratio of 4.1:1 (Table 1) is indicative of the ClO4- anion, which together with the near 1:1 Fe:Cl ratio indicates that interfacial water is not a predominant process. This is attributed to the densely packed Fc SAMs with the negligible presence of pinhole defects (i.e. exposed Au surfaces) that are accessible to water50 as well as the highly favorable formation of Fc+-ClO4- contact ion pairs. This result complements previous EQCM studies in showing that the formation of 1:1 ion pairs is not met with the incorporation of interfacial water.22

Figure 3. Effective film thickness from consecutive switching between Fc+ and Fc. Data obtained from attenuation of Au 4f7/2 photoemission intensity with eq 2. Each data point is based on at least 3 independent measurements.

Further insights into the monolayer thickness and orientation can be obtained from the Au 4f and C 1s spectra (Figure 2d-e). Bain and Whitesides,51 showed that the effective thickness (dSAM) of alkanethiol SAMs on Au is related to the attenuation of the Au photoemission intensity caused by inelastic scattering as shown in the following relation: /

(eq. 2)

where λSAM denotes the attenuation wavelength, while ISAM and IAu denotes the intensity of the SAM covered and bare Au surfaces, respectively. It is noteworthy to mention that this is a general equation for the attenuation of photoelectrons from a substrate by an overlayer.46 By using this approach, Yokota et al27 estimated the effective thicknesses of Fc SAMs, which was found to be in good agreement with reported values using complementary methods.43, 52 In our case, by using the Au 4f7/2 intensities and λSAM of 42 Å (see SI for determination of λSAM),27, 51 we observe that the effective thickness of the pristine and Fc+ SAMs are 17.5 and 18.6 Å, respectively (Table 1), which sub-

sequently decreases to 17.2 Å upon reduction back to the neutral Fc. This reproducible switching behavior is confirmed by consecutively cycling between the Fc and Fc+ states (Figure 3). The 1.5 Å difference between Fc and Fc+ is comparable to existing reports25-26 and when taken together with the C 1s spectra (vide infra), can be interpreted as a combination of reorientation events that have been reported to occur consisting of (1) rotation of the Fc+ termini where the cyclopentadienyl (Cp) ligands are more perpendicular to the surface24 and (2) tilting of the alkyl chain toward a more vertical position.23 It is noteworthy that as the calculated maximum thickness change from Fc+ rotation is 0.585 Å, which implies that there is a contribution from the tilting of the alkyl chain.25 This is related to the C 1s spectra (Figure 2e), which consists of a superposition of the Cp ligands and alkyl chains. The pristine and Fc C 1s spectra are notably narrower (FWHM ~1.1) than the asymmetric Fc+ spectrum (FWHM ~1.6). In examining mixed decanethiol/Fc SAMs, Watcharinyanon et al43 concluded that the sharpness of the Fc SAM C 1s spectrum indicates that the alkanethiol and Fc components strongly overlap. Additionally, the higher binding energy components of Fc+ based compounds has been ascribed to the loss of electron density from the Cp ligands,45 which should be similar in our case. Since there is the absence of residual unoxidized Fc termini according to the Fe 2p spectrum, the lower binding energy constituents of the Fc+ C 1s spectrum (Figure 2e) can be interpreted as the reorientation of the alkyl chain.

Figure 4. XPS of Fc SAMs immersed in 0.1 M NaClO4 and 0.1 M HClO4 without electrochemical control which induced partially oxidized Fc/Fc+ states. (a) Fe 2p, and (b) Cl 2p spectra. The immersion time was 5 minutes.

The ability to investigate partially oxidized Fc/Fc+ states could also be attained. Following anodic polarization at 0.37 V, corresponding to the second oxidation peak from the CV, the Fe 2p spectrum (Figure S3) shows the presence of both FeII and FeIII. The corresponding Fe:Cl ratio is 1.2:1, which is in agreement with the FeIII:FeII area ratio (Figure S3). The comparison of the XPS spectra with the integral charge transferred from CV are in reasonable agreement showing that ~70% of the Fc have oxidized to Fc+. Furthermore, partially oxidized Fc/Fc+ could be induced without electrochemical control depending on the composition of the electrolyte. When solely immersing in 0.1 M HClO4 (for 5 mins without electrochemical control), the Fe and Cl 2p spectra (Figure 4) shows the presence of FeII and FeIII and Cl, whereas


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Figure 5. UPS of the pristine Fc SAM and after polarization at 0.5 and 0.1 V in 0.1 M NaClO4, corresponding to oxidized Fc+ and neutral Fc states. (Left panel) Secondary electron cut-off region showing the changes in work function. The intercepts of the dashed lines are used to define the secondary electron cut-off. (Middle panel) HOMO emission region, and (Right panel) Enlarged HOMO emission region near the Fermi level.

in the case of 0.1 M NaClO4, the FeII oxidation state is conserved. From the Fe:Cl ratio and FeIII:FeII area ratio, we can estimate that there is ~80% yield in the conversion to Fc+. Attempts to polarize the electrode under cathodic conditions also yielded partially oxidized Fc/Fc+ (Figure S4). It should be noted that the electrochemical response and formal potentials in both HClO4 and NaClO4 are known to be virtually identical (Figure S5),53 suggesting similar mechanisms and the absence of proton-coupled electron transfer.50, 54 Further, all XPS measurements following electrochemistry in 0.1 M NaClO4 indicates the absence of any involvement with Na species (Figure S6). Typically, oxidation can be induced by the adsorption of electroactive species that are capable of acting as oxidizing or reducing agents.55 Nevertheless, it is possible that the partial oxidation and charge compensation at the interface may be mediated by H3O+ to yield H2. It is noteworthy to mention that the inability for the Fc state to be conserved in HClO4 has implications relating to other immersion type analyses (i.e. other UHVEC methods)56 and in applications such as charge storage.57 The specifics of pH-dependence may provide further insights into the HClO4 induced oxidation along with alternative spectroscopic techniques with sufficient spatial or time resolution. UPS provides complementary insights into the valance structure and work function (Φ) (Figure 5 and S7). At first glance, the reversibility of the Fc/Fc+ redox couple is clear from the peak at ~1.6 eV which is only present with the pristine and Fc

SAMs. This peak is predominantly ascribed to the six electrons in the 3d orbitals of Fe, which form the HOMO of Fc, giving (a1g)2(e2g)4 as the electronic configuration.12, 58 Following oxidation to Fc+, the HOMO is expected to shift to towards higher binding energy with Fc+ possessing the electronic configuration (a1g)2(e2g)3.45 It is noteworthy that Yokota et al. observed nearly matching shifts in the Fe 2p3/2 peak from XPS and the HOMO peak from UPS of Fc SAMs with different electron donating groups ascribed to the Fc HOMO being localized with the Fe atom.11 In our case, comparison of the HOMO emission region (Figure 5 middle and right panels) and the binding energy difference in the XPS Fe 2p3/2 spectra of Fc and Fc+, suggests that the HOMO related features of Fc+ SAMs are located at 2-5 eV with a pronounced feature centered at 4.4 eV. The work function as determined by the difference in the source energy (21.2 eV) and the width of the UPS spectrum (Figure 5, left panel), shows that the pristine and Fc is ~4.6 eV, while Fc+ is found to be consistently larger (~5.2 eV). The lower work function of Fc relative to bare Au (typically 5-5.3 eV) is due to the net positive dipole moment contributed by the Authiolate bond, alkyl chain and Fc termini.42, 49 Consequently, the increase in work function with Fc+ originates from the reorientation of the molecular dipole (see SI for magnitude of dipole moment change).43, 48 With Fc+, the compensation of charge by the formation of Fc+-ClO4- ion pairs19 along with SAM reorientation, consequently induces a negative interfacial dipole and

Figure 6. Schematic representation of the electrode-SAM-electrolyte double layer region. (a) Fc and (b) Fc+ states. Dipole moments are qualitatively denoted by the arrowhead symbol pointing from the positive to negative direction.48 Solvated ions are denoted by the encased circles. The electrolyte does not permeate into the SAMs. Note that the precise position of the counter anion is not conclusively known.19



increases the work function. As the component of dipole moment with respect to the normal of the Au electrode determines the work function,43 at the present stage, it is difficult to disentangle the relative contribution of the dipole from ion pair formation and the reorientation towards the increase in work function.

the first application of EC-XPS/UPS to analyze a reversible redox-active system. We envision that the approach shown here, will be beneficial in the elucidation of structure-function relationships to aid in the engineering of sophisticated redox-responsive molecular-scale devices.

Taken together, we can formulate a picture of the electrodeSAM-electrolyte double layer region as schematically shown in Figure 6. The electrified SAM-electrolyte (and metal-electrolyte) interface is typically described as the Stern layer (inner Helmholtz plane) comprised of a compact layer of counter ions and/or water molecules directly at the electrode surface followed by the Gouy-Chapman or diffuse layer comprised of a distribution of solvated ions stretching into the bulk of the electrolyte.59-60 The use of surface-bound electroactive moieties allows for the composition of the Stern layer to be determined. Namely, the Stern layer of the oxidized Fc+ state is predominately composed of 1:1 Fc+-ClO4- ion pairs along with minor degrees of water and solvated ClO4-. This situation is analogous to the compensation of the surface charge with specifically adsorbed ions at the charged metal-solution or SAM-solution interfaces.59, 61-62 In contrast, in the neutral Fc state, the Stern layer consists of weakly interacting solvent molecules and solvated ions.34 Concomitantly, this results in differences in the net dipole moment of the SAMs and therefore, the work function (Figure 6). In all, the differences in the interfacial structures are also evident in the CV (Figure 1e), with the apparent double layer capacitances of Fc (at 0.1 V) clearly increasing following oxidation to Fc+ (at 0.5 V).

EXPERIMENTAL SECTION

Lastly, the work shown here with EC-XPS/UPS provides a stepping stone to explore an assortment of other redox-responsive systems18, 63-64 including electrochemical induced doping65 and multi-electron transfer systems,3 which are often significantly more readily accessible in an electrochemical environment. Moreover, the EC chamber methodology can be extended to other UHV methods such as scanning probe microscopy, which opens the door in enabling real-space observations to reveal the local environment of electroactive species prior to and after redox reactions. CONCLUSION We have probed the redox-dependent electronic and interfacial structures of Fc SAMs by using an integrated EC cell and XPS/UPS (EC-XPS/UPS) setup. The Fc SAMs can be electrochemically controlled with the redox-induced changes analyzed. We successfully probed changes to the Fc/Fc+ redox center, determination of 1:1 Fc+-ClO4- ion pair formation, orientation, effective thickness, and degree of interfacial water could be determined. UPS confirms the occurrence of oxidation by showing that the HOMO energy shifts to higher binding energy following oxidation to Fc+. The increase in work function following oxidation to Fc+ is ascribed to the change in the molecular dipole formation of Fc+-ClO4- ion pairs along with a contribution from SAM reorientation which induces a negative dipole normal to the electrode. These Fc SAMs exhibit the useful ability to tune the interfacial structure regarding molecular orientation and work function and macroscopic properties including wettability. To the extent of our knowledge, our work represents

Substrate and monolayer preparation. The cylindrical-shaped Au (111) substrate with diameter of 6 mm (99.999%, MaTecK) was prepared by immersing in piranha solution containing 3:1 volume ratio of concentrated sulfuric acid (Kanto) and 30% hydrogen peroxide (Wako). Proper safety precautions must be taken when using piranha solution as it is a strong oxidizer and vigorously reacts with organics. The Au substrate was rinsed with Milli-Q water (18.2 MΩ, Millipore) and then dried with N2 gas before being annealed under a butane flame. The Au substrate was subsequently submerged in 0.1 mM 11-ferrocenyl-1undecanethiol (Dojindo Laboratories) in ethanol (Wako) for at least 12 hours to form the Fc SAMs, which was followed by rinsing in ethanol and drying with N2 gas. X-ray and ultraviolet photoelectron spectroscopy combined with electrochemical cell (EC-XPS/UPS). The EC-XPS/UPS setup is based on existing ultra-high vacuum (UHV-EC) systems.17, 33, 35 The X-ray/ultraviolet photoelectron spectroscopy (XPS/UPS) (Theta probe, Thermo Fisher Scientific) utilizes a monochromatic Al Kα X-ray source (1486.6 eV) with detection angle of 53o relative to the normal. The base pressure was ~107 Pa. XPS data collection was performed with the pass energy 100 eV. The binding energies with XPS are referenced to the Au 4f7/2 peak at 84.0 eV. XPS spectral fitting and relative atomic ratios were analyzed with sensitivity factors from the Avantage 5.52 software (Thermo Fisher Scientific) using Shirley background subtraction with Lorentzian and Gaussian ratio of 0.3. The UV source is comprised of He (I) excitation (21.2 eV). The UPS data collection was performed with pass energy of 2 eV with the UPS spectra referenced to the Fermi edge of Au at 0 eV. To resolve the secondary electron cut-off, a bias voltage of -12 V was applied. The EC chamber (Figure 1a-b and S1) contains a turbomolecular pump (Turbo V-551 Nagivator, Agilent) with base pressure of ~1×10-6 Pa. The retractable electrochemical cell is made of PTFE (polytetrafluorethylene) and utilizes a hanging meniscus setup and is connected via PFA (perfluoroalkoxy alkane) tubing to a syringe containing the electrolyte. Electrochemical measurements (HZ-7000 potentiostat, Hokuto Denko) were performed at ambient temperature (25oC) using 0.1 M NaClO4, (99.99% NaClO4·H2O, Merck) or 0.1 M HClO4 (Ultrapur, Kanto). Prior to use, the electrolyte was bubbled with Ar (>99.9999%, Tomoe Shokai) for at least 25 minutes. The counter electrode was a Pt wire, while the reference electrode consisted of an AuOx wire, with the data presented here converted with respect to Ag/AgCl (sat. KCl). The typical experimental procedure of EC-XPS/UPS is as follows: (1) Transfer of Fc SAM covered Au electrode to the EC chamber (2) Pressurization of EC chamber with Ar, which ensures that the chamber is effectively unexposed to air (3) im-


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Journal of the American Chemical Society mersing the electrode onto electrolyte followed by electrochemical measurements (4) Removal of immersed working electrode from electrolyte under electrochemical control (5) Evacuation of EC chamber and transfer to the analysis chamber for XPS or UPS. The evacuation and transfer steps takes ~15 minutes.

ASSOCIATED CONTENT Supporting Information. More on non-ideal CV behavior, attenuation length for effective thickness, magnitude of dipole moment change and Figures S1-S7. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected], [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported (in part) from the Japan Science and Technology Agency (JST) under the ACCEL project entitled, “Fundamentals and Applications of Diamond Electrodes” and also from the RIKEN FY2018 Incentive Research Projects. We thank Prof. Yasuaki Einaga (Keio University), Drs Hiroshi Imada and Emiko Kazuma (SISL, RIKEN) for fruitful discussions.

ABBREVIATIONS Cp cyclopentadienyl; CV cyclic voltammetry; EC electrochemical; Fc ferrocene; Fc+ ferrocenium ion; HOMO highest occupied molecular orbital; UHV ultra-high vacuum; UPS ultraviolet photoelectron spectroscopy; SAM self-assembled monolayer; XPS X-ray photoelectron spectroscopy

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