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In-situ Nanoscale Redox Mapping Using Tip-enhanced Raman Spectroscopy Gyeongwon Kang, Muwen Yang, Michael Mattei, George C. Schatz, and Richard P. Van Duyne Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00313 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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Nano Letters

In-situ Nanoscale Redox Mapping Using Tipenhanced Raman Spectroscopy

Gyeongwon Kang, Muwen Yang, Michael Mattei, George C. Schatz,* and Richard P. Van Duyne* Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States

KEYWORDS: Tip-enhanced Raman Spectroscopy (TERS), nanoscale electrochemical imaging, site-dependent electrochemistry, Nernst equation

1 ACS Paragon Plus Environment

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ABSTRACT

Electrochemical atomic force microscopy tip-enhanced Raman spectroscopy (EC-AFM-TERS) was used for the first time to spatially resolve local heterogeneity in redox behavior on an electrode surface in-situ and at the nanoscale. A structurally well-defined Au(111) nanoplate located on a polycrystalline ITO substrate was studied to examine nanoscale redox contrast across the two electrode materials. By monitoring the TERS intensity of adsorbed Nile Blue (NB) molecules on the electrode surface, TERS maps were acquired with different applied potentials. The EC-TERS maps showed a spatial contrast in TERS intensity between Au and ITO. TERS line scans near the edge of a 20-nm-thick Au nanoplate demonstrated a spatial resolution of 81 nm under an applied potential of -0.1 V vs Ag/AgCl. The intensities from the TERS maps at various applied potentials followed Nernstian behavior, and a formal potential (E0’) map was constructed by fitting the TERS intensity at each pixel to the Nernst equation. Clear nanoscale spatial contrast between the Au and ITO regions was observed in the E0’ map. In addition, statistical analysis of the E0’ map identified a statistically significant 4 mV difference in E0’ on Au vs ITO. Electrochemical heterogeneity was also evident in the E0’ distribution, as a bimodal distribution was observed in E0’ on polycrystalline ITO, but not on gold. A direct comparison between an AFM friction image and the E0’ map resolved the electrochemical behavior of individual ITO grains with a spatial resolution of ~40 nm. The variation in E0’ was attributed to different local surface charges on the ITO grains. Such site-specific electrochemical information with nanoscale spatial and few mV voltage resolutions is not available using ensemble spectroelectrochemical methods. We expect that in-situ redox mapping at the nanoscale using EC-AFM-TERS will have a crucial impact on understanding the role of nanoscale surface features in applications such as electrocatalysis.


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Understanding nanoscale electrochemistry on heterogeneous electrode surfaces has become an important area of research due to recent developments in the nanofabrication of electrode materials1-4 for applications including sensors2 and electrocatalysis.4 In particular, nanoscale features on the electrode surface play an important role in determining the mechanism, kinetics, and thermodynamics of interfacial redox reactions,5-8 however the relationship between local redox behavior and electrode features at the nanoscale is poorly defined in most work. The most common techniques to map this spatially varying electrochemistry are scanning electrochemical microscopy (SECM)9,

10

and scanning ion-conductance microscopy (SICM).11 SECM measures electrode

reactivity at the nanoscale via the current produced by a redox mediator cycling between a nanoelectrode tip and a substrate electrode.9,

12, 13

SICM detects local variations in surface

topography and reactivity by measuring the ionic current through a nanopipette tip. The spatial resolution of these methods has been mainly limited by the tip geometry and diffusion. Recent efforts to fabricate structurally well-defined nanoelectrodes2, 3, 14-16 have enabled nanoscale spatial resolution imaging of local electrochemical currents.6,

17-22

However, a detailed chemical

understanding of adsorbate-surface interactions cannot be readily obtained from the electrochemical current alone. Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) based on atomic force microscopy (AFM)23, 24 and scanning tunneling microscopy (STM)25-28 has recently been developed as a powerful tool for selectively probing different electrode surface locations. TERS is known to provide detailed chemical information with sub-nanometer lateral resolution under ultra-high vacuum conditions29-31 and a few nanometers under ambient32, 33 or liquid conditions.27 In ECTERS, subdiffraction-limited spatial resolution can be achieved using the nanoscale resolution of STM and AFM,34-36 and this is combined with the single-molecule sensitivity and rich chemical


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information available from surface-enhanced Raman spectroscopy (SERS). Recently, Mattei et al. reported the observation of single- and few-molecule electrochemistry of Nile Blue (NB) adsorbed on an indium tin oxide (ITO) electrode24 based on the AFM-TERS platform (EC-AFMTERS).23, 24 In that work, TERS intensity-potential curves, so-called TERS voltammograms, were obtained by acquiring TER spectra concurrently with surface cyclic voltammetry (CV). The acquired TERS voltammograms showed single or double steps, and the step-like results were quantitatively analyzed using the Laviron model. Surface site heterogeneity across the ITO surface was reflected in spatial variations in the formal potential (E0’) extracted from the quantitative analysis. Although this work provided a description of how the local electrochemistry is affected by molecule-surface interactions, the correlation of local variations in E0’ with electrode structure was lacking. Herein, we present potential-dependent and nanoscale TERS maps of NB, acquired in tandem with AFM imaging, on a heterogeneous electrode surface. The TERS intensity at each pixel in a series of images at different applied potentials was fit to the Nernst equation. The resulting site-dependent E0’ map represents the first TERS study of nanoscale electrochemical heterogeneity correlated with local electrode structure. All EC-AFM-TERS experiments were performed on a home-built TERS setup described previously.23, 24 In order to produce a well-defined electrode surface for correlated EC-TERS and AFM imaging, single crystal Au nanoplates (typically either hexagons or equilateral triangles, and some of the latter having truncated tips) were synthesized by reducing AuCl4- with lemon grass extract following a previously reported procedure.37, 38 50 g of lemongrass was finely chopped and boiled in 250 mL of water for 5 min. 5 mL of room temperature extract was added to 45 mL of 1 mM HAuCl4 (Sigma-Aldrich) and left to react for ~24 hours without stirring. The solutioncontaining plates were purified by centrifuging 25 mL aliquots at 5000 rpm for 10 minutes, pouring


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off the supernatant, and redispersing in 25 mL of Mili-Q water. The cleaning process was repeated once more. ITO coverslips (40 mm × 22 mm, 8-12 Ω resistivity, SPI Supplies) were cut into 20 mm × 22 mm pieces, sonicated in ethanol (Thermo Fischer Scientific) for 20 minutes, and dried under nitrogen. 100 µL of cleaned Au nanoplate solution was drop-cast onto an ITO coverslip and was dried under ambient conditions overnight. To remove weakly bound Au nanoplates and nanospheres (side products), the sample was sonicated in ethanol and water for 3 minutes each. The sample was then incubated in 10 µM Nile Blue perchlorate (Sigma-Aldrich) solution in ethanol overnight. The excess Nile Blue solution was removed by touching the side of ITO to a piece of Kimwipe and incubating the sample in 20 mL of ethanol for 20 minutes. After drying the sample, copper tape (Ted Pella) was attached to the edge of the ITO to make an electrical contact. The sample (working electrode) was then sealed in a custom-made electrochemical cell24 with a silicone O-ring. Pt and Ag/AgCl (Pt and Ag wires from Alfa Aesar) wires were used as counter and quasi-reference electrodes, respectively, and fixed into the cell using Hysol epoxy (Loctite). A mixture of 50 mM Tris buffer at pH = 7.1 and 50 mM NaCl was used as a supporting electrolyte. Au coated contact mode silicon AFM tips with a cantilever frequency of 15 kHz and a spring constant of 0.2 N/m were used (NaugaNeedles). A closed-loop scanner with active XY feedback was used and the instrument is in a thermally insulated chamber to minimize drift. A drift rate of ~3.4 nm/min was measured. (Figure S1) A 633 nm continuous wave He-Ne laser (Spectra Physics) was coupled into a single mode fiber (Thorlabs) and directed into an isolation chamber housing the AFM-TERS system. TER signals collected from the AFM-TERS setup were then passed through a custom multimode fiber bundle (50 µm core diameter, FiberTech Optical), which was directed to a spectrograph (Isoplane SCT-320, Princeton Instruments) equipped with a CCD camera (Pixis 400, Princeton Instruments).


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Figure 1. (A) Schematic of EC-AFM-TERS imaging experiment. Redox reaction mechanism of NB involving 2 e- and 1 H+ at pH = 7 is shown in the top left corner. (B) Representative TER spectrum of NB (10 µM incubation concentration) acquired on a Au nanoplate using 500 µW of 633 nm excitation. 110 TER spectra obtained with a 1 s acquisition time were averaged. The asterisk at 520 cm-1 indicates the Si signal from the AFM tip underneath the Au film. (C) AFM image of a Au nanoplate on ITO. (3 µm × 3 µm) (D) Two AFM line profiles corresponding to the blue and red lines in (C). Figure 1A shows a schematic of the EC-AFM-TERS imaging experiment. At pH = 7, the electronic resonance of NB at ~634 nm results in an extra enhancement of the TERS intensity with 633 nm laser excitation.23 At reducing potentials, NB undergoes a two-electron, one-proton transfer reaction. Because this process breaks the conjugation of the phenoxazine moiety, the intensity of the TER spectrum decreases upon the reduction of the molecule. In order to investigate the redox behavior on a well-defined nanoscale electrode feature, Au nanoplates with a thickness 6 ACS Paragon Plus Environment

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of 15 ~ 25 nm were placed on an ITO electrode substrate. As-synthesized, the Au nanoplates are known to be single-crystalline with the (111) face predominantly exposed,37 in contrast to the polycrystalline ITO surface. The ultrathin nanoplates allow enough transmission of both excitation and TERS photons through the electrode surface for sufficient TERS signal to be collected. Some absorption is still expected due to the finite thickness of the nanoplates, as will be discussed later in the paper. TER spectra were acquired while raster scanning the Au-coated AFM tip over the border between the Au nanoplate and the ITO substrate. The average of 110 TER spectra of NB with the tip located on the nanoplate is shown in Figure 1B. The TER spectra of NB on ITO have been published previously23, 24 and no significant difference in Raman shift or relative intensity was observed on Au compared to ITO. (Figure S2) An AFM image of the electrode surface (Figure 1C) reveals that the polycrystalline ITO surface has a significant number of grains with lateral size of 50 ~ 100 nm. In contrast, the Au nanoplate has a well-defined triangular shaped surface with a ~1.6 µm edge length. Two AFM line scans selected in Figure 1C are shown in Figure 1D. Both line scans show the well-defined topography of a ~20 nm thick Au nanoplate, with a few protrusions due to the roughness of ITO grains underneath the plate. A SEM image of a similar nanoplate supported on ITO also shows clear morphological differences in the surface of the Au nanoplate and ITO. (Figure S3)


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Figure 2. (A) AFM image of the corner of a Au nanoplate on ITO. (320 nm × 320 nm) (B) – (H) TERS intensity maps showing the variations of 591 cm-1 peak area of NB TERS acquired from the electrode surface shown in (A) with a 1 s acquisition time per pixel. Potentials were held at -0.1, 0.3, -0.4, -0.45, -0.5, -0.6, and -0.8 V vs Ag/AgCl, respectively. White dotted lines represent the border line of the Au nanoplate. Each TERS pixel size is 20 nm × 20 nm. (I) Selected zoomed-in TER spectra around the 591 cm-1 NB TERS peak with a 1 s acquisition time obtained on Au (black) and ITO (red) pixels under each potential. Tip-retracted spectrum after the imaging experiment is shown as a blue spectrum. Figure 2A shows an AFM image of the corner of a Au nanoplate on ITO. We chose this welldefined electrode feature for TERS mapping experiments. TERS maps were constructed by acquiring TER spectra pixel by pixel while raster scanning the AFM tip over the surface, followed by integrating the NB 591 cm-1 mode. In our previous EC-AFM-TERS work regarding single- or few-molecule electrochemistry, the redox behavior of NB was extracted from the integrated TERS intensity while concurrently sweeping the potential.24 Conversely, in this work, the electrode was held at a constant potential for the duration of each image. It is known that the kinetics of the NB 8 ACS Paragon Plus Environment

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redox reaction (k0 = 10 s-1) are ~10 times faster than the acquisition time in our TERS measurements (1 spectrum/s).24 Thus, the kinetics of the reaction are obscured, and only the thermodynamics (E0’) will be discussed. TERS intensity maps at applied potentials of -0.1, -0.3, 0.4, -0.45, -0.5, and -0.6 V vs Ag/AgCl, respectively, are shown in Figures 2B - H. The TERS intensity is generally stronger on Au than on ITO at potentials positive of E0’ (Figures 2B - D) but is similar after the reduction. (Figures 2E - H) No edge effect was observed in the TERS intensity maps, unlike the work by El-Khoury et al. in which a stronger TERS signal of 4mercaptobenzonitrile at the Au(111) step edge under ambient conditions was reported.39 The possibility of a lower molecular coverage on the nanoplate edge can contribute to this result, but a major factor is likely the coarse spatial resolution of the present measurements compared to ElKhoury. In addition, resonant NB is less sensitive to the local electric field intensity and polarization than 4-mercaptobenzonitrile which is non-resonant and forms a well-defined selfassembled monolayer. As expected from the surface CV measurements (details in S1 and S2, and Figure S4) and the potential-sweep TERS voltammogram curves24, the overall TERS intensities in the TERS maps steeply decrease around the E0’ (~ -0.4 V vs Ag/AgCl) of NB. Also, the calculated surface coverages of NB on ITO and Au are 0.005 and 0.035 monolayer, respectively, which corresponds to 2 and 14 molecules in a 20 nm × 20 nm pixel area. (Details in S3) Since there is likely at least one molecule residing in each pixel, non-zero TERS intensity is observed everywhere in TERS maps. Multiple cycles of surface CVs confirmed that the coverage of NB on both Au(111) single crystal and ITO does not change on the timescale of our imaging experiments. (Figure S5) We observe no significant change in the average TERS intensity during the course of each imaging experiment, verifying the chemical stability of our measurements. (Figure S6) A tipretracted spectrum after the imaging experiment was acquired to confirm if the tip apex is not


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contaminated (Figure 2I). There seems to be a small NB peak originating from few molecules adsorbed on the upper part of the tip that is electrochemically inaccessible. This background peak is not critical in our experiment due to its low signal-to-noise-ratio. (Table S1) Further, in the TERS images discussed below, we still observe electrochemical contrast in spite of this small amount of tip contamination, confirming that it has no significant impact on our results. Representative TER spectra at the marked pixels in the intensity maps shown in Figure 2I indicate that TERS intensities on both Au and ITO decrease as potential becomes more negative due to the reduction of NB.

Figure 3. (A) AFM image shown in Figure 2A with an AFM line scan (red horizontal line) and TERS line scan pixels marked. (red vertical lines) (B) AFM topography line scan along the horizontal line in (A), across the Au nanoplate and ITO. (Nanoplate thickness = 20 nm) (C) TERS


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line scans (dots) acquired at pixels marked in (A) and fitting curves at potentials of -0.1 (red) and -0.2 (blue) V vs Ag/AgCl. The lateral resolutions are 81 and 90 nm, respectively. In (B) and (C), red shaded region indicates the border range dividing Au (left) and ITO (right) in the AFM topography. Calculated (D) penetration depth profile and (E) Raman collection efficiency through a 20 nm Au nanoplate using dielectric constants of Au. Red vertical line indicates the excitation laser wavelength (633 nm) used in the experiment. (F) Simulated gap-mode enhancement factor profile, (EAU/EITO)4, along the axis normal to the surface and the tip apex with a tip-surface distance of 5 nm at 633 nm excitation wavelength. The spatial resolution of our TERS images was determined by plotting TERS line scans across the border of the Au nanoplate. Figure 3A is the same AFM image shown in Figure 2A, but with a red horizontal line indicating the trace on which an AFM topography profile was obtained. Figure 3B shows that the thickness of the imaged Au nanoplate is 20 nm, with a 70 nm wide red shaded region indicating the edge of the nanoplate. The AFM topography at the nanoplate edge slowly decreases rather than showing a vertical drop. However, the edge of the nanoplate shows a vertical drop in the SEM image. (Figure S3) The gradual drop in the AFM topography is an artifact due to the steep drop at the edge of the nanoplate and the radius of the AFM tip.40, 41 At this length scale, we have reached the resolution limit of our system. In Figure 3C, TERS line scans at -0.1 and -0.2 V vs Ag/AgCl are shown to obtain the spatial resolution of our measurements. At each acquisition point indicated in Figure 3A, a 2 × 2 array of TERS image pixels was averaged to obtain a smoothed TERS line scan (40 nm × 40 nm acquisition area for each point; raw TERS line scans are shown in Figure S7). The averaging was performed due to signal fluctuations commonly observed in few molecule TERS measurements.42, 43 The pixels in each line scan are divided into two categories by the nanoplate edge (red shaded) region, where the left four pixels and the right 11 ACS Paragon Plus Environment

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three pixels represent TERS intensities from NB on Au and ITO, respectively. At both potentials, the averaged TERS intensity on Au is 21 % higher on Au than on ITO. TERS line scans were fit to the normal cumulative distribution function with the nonlinear curve-fitting function (lsqcurvefit function) implemented in Matlab to determine the spatial resolution of each line scan. (Details in S4) The fits yield spatial resolution values of 81 and 90 nm at -0.1 and -0.2 V vs Ag/AgCl, respectively. A recent study reported that the tip-broadening effect results in a lower lateral resolution in AFM-TERS measurements due to the interaction between the tip shaft and the side of an imaged object.44 Therefore, the origin of the measured spatial resolution is likely limited by the AFM resolution which is mainly determined by the radius of curvature and the geometry of the tip. The origin of the observed intensity contrast needs to be discussed since the contrast plays an important role in determining the spatial resolution of our measurements. Considering the ~8 times higher coverage of NB on Au compared to ITO obtained from surface CVs (Figure S4), the observed intensity contrast is smaller than expected. One of the factors contributing to this unexpected weaker intensity contrast is due to the absorption of light by the Au nanoplate. DeckertGaudig et al.45 reported Au nanoplates as ideal substrates for TERS due to their optical transparency. However, a significant power loss is still expected through the nanoplate due to the finite thickness of Au. The intensity of an electromagnetic field transmitted through a material with a thickness of z is described by the Beer-Lambert law: 𝐼 𝑧 = 𝐼% 𝑒 '()

(1)

where I0 and I(z) are the intensities before and after transmission, respectively, and a is the absorption coefficient of the material. A penetration depth (dp) is defined to be the inverse of a


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which can be described with respect to the complex index of refraction, 𝐼𝑚(𝑛 l ), where l is the wavelength of the laser: 𝛼 =

. /0

=

12 3

𝐼𝑚(𝑛 l )

(2)

Then dp can be described with dielectric constants and excitation wavelengths as described in the following equation: 𝛿6 =

3 12 78(9 l )

(3)

Figure 3D shows a plot of dp with varying laser wavelengths calculated using the above equation with the experimentally measured dielectric function of Au.46 At 633 nm, the penetration depth of Au is calculated to be 15 nm. Following Equation (1), the intensity after the 633 nm laser is transmitted through a 20-nm-thick Au plate is 26 % relative to the initial intensity. In a bottom illumination geometry, the intensity of the scattered light is also attenuated by absorption by the nanoplate. Assuming equal attenuation of the incident and scattered light, the collected Raman signal is proportional to the square of the transmission efficiency. In Figure 3E, the attenuation of the collected Raman signal for a bottom illumination optical setup through a 20-nm-thick Au plate with varying laser wavelengths is shown. This plot indicates the Raman collection efficiency through the plate at 633 nm is 7 % of the collection efficiency on ITO. After taking the coverage difference of NB on Au and ITO in addition to the attenuation effect into account, the Raman collection efficiency through the Au nanoplate is still expected to be 54 % of the efficiency on ITO. This value is ~2.3 times smaller than the observed TERS intensity contrast. The extra enhancement on Au is attributed to the gap-mode enhancement from the coupling between the plasmonic tip and the substrate. To confirm the coupling between the tip and the substrate, a boundary element method (BEM) based on the MNPBEM Matlab toolkit47 was carried out.


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(Details in S5) On the ITO surface, higher electric field intensity is observed in proximity to the tip apex which is in line with the plasmonic nature of the Au nanostructure. On the Au surface, the plasmon mode of the tip-nanoplate assembly is dominated by that of the Au nanoplate. (Figure S8) The calculated gap-mode enhancement factor ((Eau/EITO)4) within the tip-surface junction ranges from 2.6 to 15.7 which is in good agreement with our experimental result. (Figure 3F) Thus, we claim the gap-mode enhancement is the major factor for the observed spatial contrast in our TERS intensity maps.


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Figure 4. Formal potential (E0’) maps with (A) 20 nm × 20 nm and (B) 40 nm × 40 nm pixel sizes. (C), (D) Histograms of the E0’ distribution from Au (purple) and ITO (cyan) pixels from (A) with (C) unimodal and (D) bimodal Gaussian fits. The histograms were constructed from the E0’ map with 20 nm × 20 nm pixels. The unimodal Gaussian fits are shown as solid curves in (C). Two bimodal Gaussian curves are shown as dashed curves and the sums of the two curves are plotted as solid curves in (D). (Grey and black curves: Au, red and orange curves: ITO. Inset in (D) is a zoomed-in plot around -0.41 V vs Ag/AgCl to show a Gaussian curve that is not dominant in the Au distribution.) In order to quantitatively investigate the site dependence of NB electrochemical behavior, we determined E0’ values at each pixel in our potential-dependent TERS images. In the equilibrium regime the TERS intensity at a single image pixel will vary with potential according to the Nernst equation: ;

E = E% − ;

E = E % −

=> ?@ => ?@

ln

CDEFG

ln

CDHI J ' CDHI CDHI

CDHI

(4) (5)

where [NBox] and [NBred] refer to the coverages of oxidized and reduced forms of NB, respectively, and [NBox]0 is the initial coverage of oxidized NB. The number of electrons transferred through the process is assumed to be 2 at pH = 7 (n = 2). Assuming that the TERS intensity is proportional to the coverage of the oxidized form of NB ([NBox]), the coverage expression in Equation (5) can be converted into an expression that depends on the TERS intensity: ;

E = E % −

=> ?@

ln

KLMNO,QRI 'KLMNO KLMNO

(6)

where ITERS is the TERS intensity at the potential, E, and ITERS,max is the maximum TERS intensity over the potential window. ITERS can be now written in terms of potential E: 15 ACS Paragon Plus Environment

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I>T=U =

KLMNO,QRI .VWXY ['

[\ NL

;

T'TJ ]

+ I>T=U,_`?

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(7)

Here, ITERS is shifted by ITERS,min to compensate for the weak background signal detected regardless of the potential. The TERS intensity averaged over the whole surface, and separately over the Au and ITO regions at each potential was fit to Equation (7) and shown to follow Nernstian behavior (Figure S9). It is important to note that in our TERS imaging experiments, E0’ represents an average value for the forward and reverse reactions. In our previous potential sweep experiments, E0’ values for the forward and reverse reactions were considered separately due to unusual quasi-reversibility, and would more accurately be called half-wave potentials.24 To further investigate spatial variations in the redox behavior of NB, a distribution of E0’ over the imaging window was obtained by fitting the TERS intensities in Figure 2 over the potentials at each TERS pixel to the Nernst equation. Figure 4A shows the resulting E0’ map, and it is evident that there is a clear contrast between Au (left) and ITO (right) where E0’ values are generally more negative on Au than on ITO. This comparison is in good agreement with the ensemble E0’ values obtained from the surface CVs on Au(111) single crystal and ITO, where E0’ on Au is 16 mV more negative than on ITO (Figure S4). Figure 4B is the reconstructed E0’ map comprised of 40 nm × 40 nm size pixels by averaging the intensities of 4 neighboring pixels (a 2 × 2 grid of 20 nm × 20 nm pixels) in TERS maps followed by fitting TERS intensities at each pixel to the Nernst equation. This pixel conversion was performed since the aforementioned spatial resolution was based on a 40 nm × 40 nm pixel size. Even with a coarser binning of the map, the spatial contrast in the E0’ map is still observed. To statistically analyze the E0’ values and study their site dependence, the pixels in Figure 4A were categorized using the white lines that determine the Au nanoplate border. Figure 4C shows a histogram of the distribution of E0’ values on the two different substrates overlaid with unimodal 16 ACS Paragon Plus Environment

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Gaussian fitting curves. Two E0’ distributions were confirmed to be statistically distinguishable by performing a two-sample t-test. (p-value: 6 × 10-11 and 5 % significance level) Therefore, the distributions show that the E0’ values on Au are more negative than on ITO. Parameters from the Gaussian fits using the unimodal Gaussian curves are given in Table 1. The mean value of the E0’ distribution is 4 mV more negative on Au, with comparable variance magnitudes. Although a single Gaussian curve fits the Au distribution well, the ITO E0’ distribution clearly deviates from a unimodal fit. The well-defined surface morphology of the Au nanoplate likely leads to the unimodal character of the E0’ distribution. In contrast, Figure 4D reveals that while a single Gaussian curve is sufficient to describe the Au distribution, a bimodal distribution appears to more accurately fit the ITO distribution. The parameters resulting from unimodal and bimodal fits for the Au and ITO distributions are reported in Table 2. It is noticeable that two Gaussians are evenly mixed to fit the ITO distribution with a ratio of 56:44 whereas one of the Gaussians dominates for the Au distribution. The goodness of fits for unimodal vs bimodal Gaussian curves were compared using a c2 test for the Au and ITO distributions. The results from the c2 test clearly indicate that a bimodal distribution fits the ITO E0’ distribution significantly better, whereas the Au E0’ distribution fits better to the unimodal Gaussian fit. (Table S2) In addition, one of the mean formal potentials for the ITO distribution (µ = -0.391 vs Ag/AgCl) becomes far more positive than the mean formal potential of the dominating Au distribution by 6 mV. On the other hand, the other mean formal potential for the ITO distribution (µ = -0.396 vs Ag/AgCl) is very close to the Au formal potential. This bimodal behavior for the ITO distribution is attributed to the polycrystalline nature of ITO, as observed in the AFM and SEM images in Figure 1C and Figure S3. Thus, we have demonstrated the power of EC-TERS imaging by detecting a small (~4 mV) but statistically significant difference in E0’ on Au and ITO.


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Table 1. Fit parameters from the unimodal Gaussian fit of E0’ distribution Mean potential, µ (V vs Ag/AgCl)

Standard deviation, s (mV)

Au

-0.397

4.95

ITO

-0.393

4.51

Table 2. Fit parameters from the bimodal Gaussian fit of E0’ distribution Mean potential, µ (V vs Ag/AgCl)

Standard deviation, s (mV)

Gaussian mixing proportion

Au (Gaussian 1)

-0.397

4.53

0.97

Au (Gaussian 2)

-0.408

6.34

0.03

ITO (Gaussian 1)

-0.391

4.13

0.56

ITO (Gaussian 2)

-0.396

3.02

0.44


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Figure 5. (A) Friction image of the same surface area shown in Figure 2A. (B) The E0’ map presented in Figure 4A is overlaid with the friction image with white crosses indicating the location of 13 ITO grains. The transparency of the two images was adjusted for a better comparison. (C) AFM friction line scan and (D) E0’ line scan along the white dashed line in (A). The locations of three ITO grains marked in (B) (grains 1, 2, and 3) are indicated as corresponding numbers in (C). The bimodal distribution in the ITO distribution reveals that the polycrystalline ITO surface affects the equilibrium nature of the NB redox reaction. In Figure 5, a direct comparison between the friction image and the E0’ map is shown to further investigate the origin of bimodal behavior in the ITO E0’ distribution. The friction image was acquired simultaneously with the topography image during the AFM imaging. (Figure 5A) The friction image depicts the lateral force


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experienced by the tip and contains contributions from the topography and the physical and chemical properties of the surface (e.g. stiffness or surface charge).48 This image provides a better spatial contrast in the surface structure of ITO. Figure 5B shows a comparison between the AFM friction image and the E0’ map in which 13 ITO grains are assigned based on the AFM friction image. This comparison reveals a strong correlation between specific grains which appear dark in the friction image and regions with more negative E0’. We propose that the negative shift in E0’ on these grains and the larger force evident in the friction image are due to a more negative local surface charge on these particular grains.49-51 A more negative surface charge would result in less stable binding of the neutral reduced form of NB compared to the cationic oxidized form, favoring a more negative E0’. Further, more negative surface charge would result in more repulsion of the tip, which is negatively charged under the negative applied potential, and therefore greater lateral force. The difference in E0’ of an adsorbed species occurs due to the differential binding of the two redox forms.52, 53 The surface charge is one of the dominant contributing factors to a change in differential binding free energy and thereby a change in E0’. A shift in E0’ of 5 – 10 mV has been previously attributed to variations in surface charge.53 The observed shift in E0’ in our work is within this range. Thus, variation in local surface charge can account for some of the observed bimodal behavior in the distribution of E0’ on ITO. A line scan containing three ITO grains in the AFM friction image (dashed line in Figure 5A) was selected (Figure 5C) and directly compared to the corresponding E0’ line scan (Figure 5D) to determine the spatial resolution of the E0’ map. This comparison clearly shows the E0’ value on each assigned grain is more negative than the neighboring pixels. We therefore determine the spatial resolution of the E0’ line scan to be a peak to peak distance (~40 nm) that is enough to resolve individual surface features and their


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corresponding E0’ values on ITO. Unlike the nanoplate edge, there is no steep topographic drop on ITO, thus the ITO grains are detectable without 3D effects of the tip.44 For a single crystal surface like the Au nanoplates, STM-TERS is likely a better method for distinguishing the chemistry on the terrace versus, for example, a the step edge. However, our EC-AFM-TERS system is better suited for resolving differences on the rough ITO surface. To conclude, our EC-TERS measurements have spatially resolved distinct regimes of electrochemical behavior on a polycrystalline ITO surface at the nanoscale and revealed the origin of bimodality in the E0’ distribution of ITO. In summary, we have successfully acquired TERS intensity maps representing the spatially dependent redox behavior of NB on a Au nanoplate on an ITO electrode using EC-AFM-TERS at the nanoscale for the first time. TERS intensity maps demonstrated a spatial resolution of 81 nm for distinguishing the border of the Au nanoplate and ITO electrode. A gap-mode enhancement with a factor of 2.3 difference between the gold nanoplate and the ITO was attributed to the origin of the observed spatial contrast. The site dependent E0’ of NB was obtained by fitting the TERS intensities to the Nernst equation and a 4 mV voltage difference in E0’ was resolved by statistical analysis of the E0’ distribution. The E0’ distribution also measures heterogeneity of the electrode surface since a bimodal distribution was observed for ITO while Au showed unimodal behavior. The observed bimodality was directly correlated to the surface heterogeneity of ITO with a spatial resolution of ~40 nm. Our EC-AFM-TERS imaging experiment with such spatial and voltage resolution has therefore provided new insight into our knowledge of the role of local electrode structure. Further technical improvements to achieve better spatial resolution will have a significant impact on the understanding of detailed chemical mechanisms at the molecular level, with profound implications for electrocatalysis research.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Details for sample preparation and procedure for surface CV measurements; details for surface excess calculation; procedure for determining the TERS line scan resolution; BEM simulation details; drift rate measurement of the instrument; TER spectra on Au nanoplate and ITO; SEM image of a synthesized Au nanoplate on an ITO substrate; surface CVs at different scan rates on ITO and Au(111) single crystal; coverage of NB on ITO and Au(111) single crystal; TERS intensity change in each TERS maps; raw TERS line scans; simulated electric field distribution at the tip-sample junction; Nernst fit of TERS intensity; peak current density plot from CVs; signalto-noise-ratio table; c2 test result table

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (G.C.S.) *Email: [email protected] (R.P.V.D.) ORCID Gyeongwon Kang: 0000-0002-8219-2717 Muwen Yang: 0000-0001-5472-765X Michael Mattei: 0000-0002-8276-5562


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George C. Schatz: 0000-0001-5837-4740 Richard P. Van Duyne: 0000-0001-8861-2228 Present Address M.M.: Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors acknowledge financial support from the Air Force Office of Scientific Research MURI (FA9550-14-1-0003) and the National Science Foundation Center for Chemical Innovation dedicated to Chemistry at the Space-Time Limit (CaSTL) (CHE-1414466). The authors thank Dr. Vitor Brasilience and Dr. Charles Cherqui for helpful discussions.

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