Electrochemical STM Tip-Enhanced Raman Spectroscopy Study of

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C: Physical Processes in Nanomaterials and Nanostructures

Electrochemical STM Tip-Enhanced Raman Spectroscopy Study of Electron Transfer Reactions of Covalently Tethered Chromophores on Au(111) Xu Chen, Guillaume Goubert, Song Jiang, and Richard P. Van Duyne J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03163 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 8, 2018

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The Journal of Physical Chemistry

Electrochemical STM Tip-Enhanced Raman Spectroscopy Study of Electron Transfer Reactions of Covalently Tethered Chromophores on Au(111) Xu Chen,†, ‡ Guillaume Goubert,‡1 Song Jiang,‡ and Richard P. Van Duyne†, ‡, * †

Applied Physics Graduate Program, and ‡Department of Chemistry, Northwestern University,

Evanston, Illinois 60208, United States Corresponding Author *E-mail: [email protected]. Tel: 847-491-3516. Fax: 847-491-7713. Author Contributions: Xu Chen and Guillaume Goubert contributed equally to this work.

1

Guillaume Goubert present address: Laboratory of Organic Chemistry, D-CHAB, ETH Zurich, Vladimir-Prelog-

Weg 3, 8093 Zürich, Switzerland

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The ability to study electron transfer reactions at the solid-liquid interface with nanometer resolution has the potential to critically improve our understanding of electrocatalytic processes. However, few techniques are capable of studying electrode surfaces in situ at the nanoscale. We study the redox reactions of Nile Blue (NB) covalently tethered to a Au(111) electrode using in situ tip-enhanced Raman spectroscopy (TERS) and show that TERS amplitude decreases reversibly as NB is reduced. The potential dependent TERS intensity allows us to associate an electrochemical wave with the loss of electronic resonance of NB and another with the peak of fluorescence of tethered NB, which we tentatively attribute to the disassembly of on-surface NB aggregates. The study of the electrochemical activity of immobile adsorbates at the solid-liquid interface with TERS is an essential step towards the realization of in situ spectroscopic mapping at the nanoscale.

INTRODUCTION The study of electrochemical processes at the solid-liquid interface is of paramount interest for the development of renewable chemical fuels using water splitting or CO2 reduction.1 The design of better materials and catalysts requires detailed knowledge of active sites on the surface, which requires a fundamental understanding of the relationship between the surface structure and the reactivity at the nanoscale.2 Scanning probe microscopy (SPM) techniques such as scanning tunneling microscopy (STM) or atomic force microscopy (AFM) provide nanoscale topographic information both in situ and ex situ. However, they only yield a limited amount of chemical information, hindering the measurement of nanoscale chemical activity. In contrast, vibrational spectroscopic techniques give us rich chemical information. Single-molecule sensitivity has been demonstrated with surface-enhanced Raman spectroscopy (SERS), a technique in which the


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concentration of the electromagnetic field in a nanoscale volume enhances the Raman signal by up to 6 to 8 orders of magnitude.3 However, the diffraction of light limits the spatial resolution of these optical techniques. A growing number of surface analytical techniques have emerged that combine an SPM technique with a vibrational spectroscopic tool to provide rich chemical information at the nanoscale.4–9 TERS is at the forefront of these developments. In TERS, an STM tip or an AFM cantilever made of plasmonic material such as Au or Ag is precisely positioned to create a single localized hotspot at its apex. Several groups have reported TERS at 1 nm or sub-nm resolution in ambient or in vacuum.9–11 Recently Van Duyne, Ren, and Domke have also demonstrated in situ TERS under electrochemical conditions.12–15 However, these reports either did not study electron transfer reactions,14,15 or did not employ atomically welldefined surface.12,13 Here, we use electrochemical TERS (EC-TERS) to study the electron transfer reactions of NB covalently tethered to a Au(111) single crystal electrode in situ, which is a crucial step towards unraveling the structure-activity relationships at the nanoscale. NB has been identified as a model probe for single-molecule Raman spectro-electrochemistry.13,16–19 The redox reaction of NB has been described as a two-electron two-proton transfer process in acidic conditions (pH < 6).16 As the electrode potential is swept in the negative direction, the Raman signal drops significantly near the formal potential of NB due to the loss of the resonance Raman contribution at 633 nm.12,19 This potential dependent spectroscopic response has been used in electrochemical SERS studies to discriminate between different waves observed in cyclic voltammograms (CV).17,20

EXPERIMENTAL METHOD



EC-TERS results were acquired using a home-built instrument based on an STM platform with side-illumination (backscattering) geometry, compatible with opaque electrodes like Au(111). A detailed description of the apparatus can be found in the supporting information. To suppress molecular desorption and diffusion on the surface during EC-TERS measurements, we covalently tethered NB to the electrode, which has been reported in the literature via different chemical routes.21,22 Willets and coworkers have shown that the electrochemical response of tethered NB, using amide coupling to a thiol, presents two well-separated waves in CV and differential pulse voltammetry.17,20 We performed a similar EDC-coupling procedure, as is detailed in the supporting information. We chose a short thiol linker, 3-methylpropionic acid, to keep NB molecules near the Au(111) surface and maintain an efficient fluorescence quenching.

RESULTS AND DISCUSSION


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Figure 1. (A) CV of covalently tethered NB in 0.1 M HClO4 on Au(111) at various scan rates (B) CV of physisorbed NB on Au(111) in the same electrolyte. The insets show the mechanisms for the redox reactions at pH 1 for physisorbed and tethered NB.16,20

We performed CV for EDC-coupled NB (EDC-NB) on Au(111) in 0.1 M HClO4, as shown in Figure 1 (A). The structure of EDC-NB is drawn as an inset in the top-left corner. Both waves in the CV of EDC-NB exhibit a linear dependence of peak amplitude on the scan rate (Figure S2 in supporting information), indicating surface processes. The two waves are found at E01 = +20 mV and E02 = −96 mV vs. Ag/AgCl (3 M NaCl). For comparison, we performed CV for physisorbed NB on Au(111) in the same electrolyte and found a single electrochemical wave at E0NB = −74 mV vs. Ag/AgCl (3 M NaCl), as shown in Figure 1 (B). The structure of physisorbed NB is drawn as and inset in the lower-left corner. The discrepancy between E02 and E0NB can be explained either by fluctuations of the pH values of electrolytes, or by the covalent attachment of NB to the surface. By integrating the background-subtracted CV of EDC-NB in Figure 1, we obtained the amount of charge passed during both the forward and backward sweeps, which are directly proportional to the number of molecules reduced and oxidized respectively, corresponding to a surface excess of 4.2×1012 molecules/cm2 on Au(111). Electrochemical STM images of Au(111) in 0.1 M HClO4 (Figure S3 in the supporting information) demonstrate that the sample consists of large atomically smooth terraces with a small number of step edges. We take the active TERS area to



be a disc with a 10 nm radius, half the expected radius of curvature of the tip apex,23 and estimate the upper limit for the number of molecules probed by the tip to be 13 on average. Recent high resolution TERS experiments have shown that this may be an overestimation of the TERS-active area.9,10

Figure 2. TER spectra in 0.1 M HClO4 on Au(111), using an insulated Au tip. The potential of the sample is presented on the right. The bias between sample and tip was maintained at 0.1 V. Each spectrum was accumulated 120 s at a constant potential. The excitation wavelength was 633 nm, using a power of 0.94 mW. The potential dependent TER spectra are plotted from bottom to top in the same order as they were acquired. The spectra are background subtracted using the procedure described in Figure S4 in the supporting information.


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The TER spectra of EDC-NB on Au(111) at different potentials are shown in Figure 2. All spectra are plotted after background subtraction following the procedure described in Figure S4 in the supporting information. The first (bottom) spectrum in Figure 2 was acquired at +0.109 V where NB was present only in the oxidized form. We then incrementally stepped the potential negative to −0.241 V and then positive back to +0.109 V, holding the potential constant during each of the 2 min TERS acquisitions. The TERS amplitude decreased around E02 as the potential was swept towards reducing potentials and completely disappeared at −0.241 V. During the reverse sweep towards oxidizing potentials, the TERS amplitude recovered. The normalized TERS intensity in Figure 2 is about 10 times lower than that in Reference 12, which can be attributed to three major differences between the STM-based and AFM-based setups. First, TERS tips in the AFM case are fabricated by thermal evaporation of Au onto commercial cantilevers; whereas TERS tips in the STM case are electrochemically etched from bulk metal wires, which causes variation of the enhancement factors between tips. Second, in the AFM case, neither the incident laser beam nor the signal being collected need to travel through the electrolyte solution; while in our STM case, both the laser beam and the signal suffer from the mismatch of refractive indices at the air/glass/solution interfaces, which also results in lower TERS intensity than Reference 12. We compared our EC-TER spectra of EDC-NB with those of physisorbed NB in Reference 12. The peak positions of EDC-NB match those of physisorbed NB, except for a few bands that are difficult to resolve due to lower signal-to-noise ratio (SNR) in our case compared to Reference 12. For the same reason, EC-TERS from the reduced EDC-NB at −0.241 V is below our detection limit. On the other hand, STM-based EC-TERS setup possesses advantages over AFMbased one. For instance, AFM tips perturb the electrical double-layer near the electrode, thus



locally alter the effective potential experienced by molecules,12 and often get contaminated by surface adsorbates due to direct contact with the molecules, which is less of a concern when using an STM.

Figure 3. Amplitude of fluorescence acquired in situ when tip was retracted, in blue. The sample potential vs. Ag/AgCl (3 M NaCl) is plotted in black. The fluorescence reached its maximum at around 0 V during the reducing sweep but did not come back during the oxidizing sweep.

Additionally, we observed a potential dependent fluorescence that peaks around 0 V in the reducing direction (Figure 3), a potential slightly more negative than E01 of EDC-NB. Since NB is known to form non-fluorescent H-aggregates in aqueous solution,24 it is likely that a proportion of the tethered NB molecules on Au(111) surface aggregate with untethered NB molecules during the EDC-coupling procedure, or that some NB molecules form aggregates in solution before being tethered on Au(111) surface. As the sample potential is stepped in the


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negative direction, we expect the dissociation of NB H-aggregates and the formation of NB monomers before the reduction of NB, which has been revealed by Ren et al using transient ECSERS.18 Since Au(111) surface should quench the fluorescence from physisorbed NB,25,26 we hypothesize that the fluorescence maximum around 0 V is caused by the release of some untethered NB monomers from Au(111) surface, which are newly formed as a result of the dissociation of H-aggregates. This hypothesis could be further tested by dilution experiments since the formation of aggregates in solution is hindered at very low concentrations. We did not observe the return of fluorescence after the potential cycle is finished. This indicates that the desorbed NB monomers diffused away from the optical path. It is also noteworthy that the fluorescence level never reaches zero throughout the potential cycle, which can be attributed to the contribution from tethered NB. We hypothesize that the distance between EDC-NB and Au(111) surface created by the thiol linker lowers the quenching efficiency, since change in proximity as small as a few angstroms can strongly modify the intensity of fluorescence.27–29 To analyze the potential dependence of the TERS amplitude, we assume that the sample attained equilibrium before each acquisition. We fit the amplitude of the 593 cm-1 band of NB as a function of potential to the Nernst model described by Equation 1. This 593 cm-1 mode is the most resonantly enhanced at a 633 nm excitation30 and is assigned to the in-plane skeletal motion of the phenoxazine ring.12 We assume that the TERS signal is proportional to the local coverage of oxidized EDC-NB and that the contribution of reduced EDC-NB to the TERS signal is negligible.

= 1+

+ Equation 1 Equation 1



In Equation 1, S is the TERS amplitude and n the number of electrons passed (n=2 for NB). E is the applied potential at the electrode, E0 the formal potential of EDC-NB, α the charge transfer coefficient, and b an offset. R, F and T are the gas constant, the Faraday constant and the absolute temperature, respectively.

Figure 4. The CV of the EDC-NB on Au(111) sample in 0.1 M HClO4 is plotted in black. The TERS intensity is calculated using the area under the 593 cm-1 band in TER spectra acquired for 2 min at each potential. The TERS acquired during the reducing sweep is shown in blue on top. The TERS collected during the oxidizing sweep is plotted in red at the bottom. The fitting results using Nernst model in Equation 1 are presented as dashed lines. Both CV branches are background subtracted to remove the contributions of double-layer capacitance and reduction of oxygen.


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The fitting results obtained using Equation 1, shown in Figure 4 are used to extract E0 from the TERS data. The TERS signal disappears over a range of potential near E02, while no systematic change is observed at potentials around E01. The midpoint potentials extracted from the Nernst fit are −94 mV and −101 mV in the reducing and oxidizing sweeps, respectively. We average these two values to obtain E0TERS = −97 mV as the formal potential of EDC-NB, which is very close to E02 =−96 mV extracted from the sample CV. During the reducing sweep, we observe a significant difference between the TERS intensity at the most positive potential and the following measurements, while in the oxidizing sweep, the signal slowly increases from +9 mV to +109 mV, where our simple model predicts a constant amplitude. This behavior could be explained by photobleaching of NB after spending a total of 34 min in the reduced form followed by a slow recovery as the potential is moved to the oxidative regime. Slow drift of the tip over the surface could further affect the TERS amplitude by changing the number of molecules being probed and bringing molecules that were not previously in the hotspot under the tip, potentially mitigating bleaching issues. In situ TERS mapping correlated with EC-STM imaging experiments are needed to probe the heterogeneity of surface coverage at the nanoscale and control for tip drifting. Our experiments show that the disappearance of the TERS signal around E0TERS due to the disruption of the phenoxazine ring resonance at 633 nm, coincides with the more negative set of CV peaks around E02. Our results agree with Willets’ EC-SERS studies, which showed that the SERS intensity of the 593 cm-1 mode of EDC-NB disappears at the more negative waves in CV and differential pulse voltammograms.17 However, they assigned the more positive electrochemical wave to a charge transfer to the amine, based on their study of different coupling



geometries.17,20 EC-TERS work using longer tethering thiol chains, as in Willets’ studies could help resolve this issue.

CONCLUSIONS In Summary, we used EC-TERS based on an STM instrument to study the quasi-reversible electron transfer reactions of NB covalently tethered to a Au(111) electrode via EDC-coupling. EDC-NB exhibits fluorescence that peaks at about 0 V vs. Ag/AgCl (3 M NaCl). We attribute the observed fluorescence maximum to the release of NB monomers on Au(111) surface due to the dissociation of non-fluorescence H-aggregates of tethered NB with untethered NB. CV of EDC-NB contains two electrochemical waves. The EC-TERS signal disappears over a range of potential around the more negative wave, proving that the more negative wave is associated with the loss of electronic resonance at 633 nm due to the disruption of the phenoxazine ring. Our ECTERS study of immobile adsorbate on an atomically well-defined substrate provides a steppingstone towards mapping site-specific electrochemical activities at the nanoscale and unravelling the heterogeneous electron transfer mechanism of EDC-NB, which are not accessible by ensemble methods like CV or EC-SERS. Recent UHV-TERS results have shown the feasibility to chemically address individual molecules in a high local coverage environment.31–33 This feat has not been achieved in electrochemical environments, thus represents the next frontier for ECTERS.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Experimental details and additional data (PDF)


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AUTHOR INFORMATION Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Air Force Office of Scientific Research MURI (FA9550-14-1-0003). The authors thanks Dr. Allen J. Bard, Dr. Henry White, Dr. Martin Edwards and Dr. Katherine A. Willets for helpful discussions as well as Michael Mattei for constructive suggestions regarding the manuscript preparation.

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