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Probing Electrocatalysis at Individual Au Nanorods via Correlated Optical and Electrochemical Measurements Partha Saha, Joshua W. Hill, Joshua D. Walmsley, and Caleb M. Hill Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03360 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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Analytical Chemistry

Probing Electrocatalysis at Individual Au Nanorods via Correlated Optical and Electrochemical Measurements Partha Saha, Joshua W. Hill, Joshua D. Walmsley, and Caleb M. Hill* Department of Chemistry, University of Wyoming, Laramie, WY 82071 ABSTRACT: A novel analytical methodology based on correlated optical and electroanalytical measurements was developed to probe electrocatalytic reactions at individual nanoparticles (NPs) with well-defined geometries. The developed methodology, Optically Targeted ElectroChemical Cell Microscopy (OTECCM), relies on a combination of optical hyperspectral imaging, to locate individual NPs and provide structural information, and Scanning ElectroChemical Cell Microscopy (SECCM), to provide direct information on the electrochemical behavior of the same NPs. This complementary strategy allows for SECCM measurements to be carried out in a “targeted” fashion, offering significant throughput advantages over conventional, scanning-based approaches. The developed methodology was applied to study the electrocatalytic oxidation of hydrazine at individual Au nanorods (NRs). Correlated electron microscopy investigations were carried out to conclusively demonstrate the ability of the proposed methodology to probe electrochemical reactions at individual NRs. A wide variety in behavior of the individual NRs was observed, with surface reactions at Au playing a prominent role in the observed response. In situ spectroscopic investigations at individual NRs suggest surface restructuring and/or residual ligand desorption leads to significant changes in catalytic activity over time. Results from the correlated electron microscopy investigations as well as the statistical analysis of data obtained for hundreds of individual nanostructures suggest that the gross geometry of a NR is a poor predictor of its electrocatalytic performance.

The intelligent design of improved nanomaterials for electrochemical applications such as electrocatalysis, sensing, energy storage, etc. depends critically on insights into the often-complicated relationships between a material’s structure and reactivity. Due to the inherently heterogeneous nature of nanostructured materials, such insights are often difficult to generate through traditional electrochemical studies on macroscopic amounts of material. A general strategy that has been increasingly explored in recent years has been to shift away from ensemble methods which probe macroscopic quantities of material to those which probe individual nanostructures. Such methods allow for the systematic evaluation of particular structures, possibly elucidating how fine variations in geometric parameters, composition, etc. affect reactivity. Measurements at this scale present an obvious analytical challenge, and ideal methodologies for carrying out such measurements must possess a combination of (a) direct electrochemical detection, to provide unambiguous information on reaction rates, (b) simultaneous structural information, to allow for the correlation of observed reaction rates with structure, and (c) high throughput to allow for hundreds of nanostructures to be examined, providing statistically meaningful results. Several experimental strategies for studying electrochemical reactions at individual nanostructures have been explored previously using electrochemical and optical detection schemes. Electrochemical schemes which have been explored include the direct attachment/electrodeposition of individual nanoparticles (NPs) onto ultramicroelectrodes (UMEs),1–8 the analysis of stochastic “collision” events between NPs and UMEs,9–16 and probe-based techniques such as Scanning ElectroChemical Microscopy (SECM)17–21 and Scanning ElectroChemical Cell Microscopy (SECCM).22–28 Optical schemes have also been explored for studying electrochemical reactions occurring at

individual NPs, which usually rely on measuring changes in the localized surface plasmon resonance (LSPR) of individual metallic NPs29,30,39–44,31–38 or through the detection of reaction products with large optical cross-sections.45–53 These methods, though powerful, each carry significant drawbacks from an analytical perspective which prevent them from meeting the criteria outline above. These drawbacks include poor sample throughput (single NP attachment, probebased techniques), inability to provide correlated structural information (collisions), indirect data analysis (LSPR-based detection schemes), and limited applicable reaction systems (optical detection via fluorescence or Raman scattering). Improved strategies are therefore needed before studies at individual nanostructures can regularly generate insights useful in the design of improved functional materials. This report details the development of an improved methodology for studying electrochemical reactions at individual nanostructures based on correlated optical and electrochemical measurements and its application to study hydrazine oxidation at individual Au nanorods (NRs) with well-defined geometries. The described method, termed “Optically Targeted ElectroChemical Cell Microscopy” (OTECCM), utilizes a combination of optical hyperspectral imaging, to localize individual nanostructures and probe their structure, and SECCM carried out in a “targeted” fashion to efficiently probe their electrochemical properties. The results described here demonstrate the potential of this method to efficiently screen nanostructured materials for electrocatalytic applications through measurements at individual NPs.

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EXPERIMENTAL SECTION Hyperspectral Imaging. The instrumentation employed in the present studies is depicted schematically in Figure S8 in the SI. Optical measurements were carried using a homebuilt hyperspectral imaging system. Samples were mounted on an inverted optical microscope (Olympus IX-73) equipped with a long working distance objective (Olympus, 50x, NA=0.5). The position of the sample was controlled using a piezo system (ThorLabs NanoMax). The bottom of the sample was illuminated at a high angle of incidence (~50°) with a white light source (Energetiq EQ-99, total intensity at the sample plane of ~50 W/cm2). Scattered light collected by the objective was directed onto the slit of a spectrometer equipped with an EMCCD camera (Andor Newton DU970-P/Shamrock SR-303i). Hyperspectral images were constructed by collecting EMCCD images while moving the sample in a linear fashion using the piezo system. All data acquisition was carried out using custom LABVIEW programs. SECCM Measurements. Electrochemical measurements at individual NRs were made in a 2-electrode configuration (ITO working electrode, Ag/AgCl wire counter electrode) using a patch-clamp style electrometer (Dagan CHEM-CLAMP). SECCM probes were manufactured by preparing patch type pipettes from borosilicate capillaries (1 mm OD, 0.5 mm ID, Sutter) using a pipette puller (Sutter P-2000, HEAT = 350, FIL = 4, VEL = 30, DEL = 200, PULL = 0), filling with the electrolyte solution of interest, and inserting a Ag/AgCl wire into the pipette. Pipettes were characterized through a combination of optical microscopy and conductivity measurements in 100 mM KCl (see SI) and were found to possess an average pore radius of 0.3 µm and a half angle of 7°. The position of the SECCM probes was controlled using a piezo system (Physik Instrumente P-611.3S). All data acquisition was carried out using a National Instruments DAQ interface and custom LABVIEW software. All electrochemical measurements were carried out inside of a homebuilt Cu Faraday cage. Unless otherwise noted, all potentials are reported vs. the Ag/AgCl counter/reference electrodes employed. Methodology. The general process employed to characterize individual NPs in the present studies is outlined schematically in Figure 1. The NPs of interest are first dispersed across an inert, non-catalytic substrate electrode (ITO) at a low coverage (< 0.1 µm-2) via drop coating. Dispersing the NPs at these low densities is critical for two reasons: (1) it enables individual NPs to be probed optically, which requires individual NPs to be isolated from other optical features by a distance greater than the diffraction limit of the optical system (~1 µm in the present system) and (2) it enables individual NPs to be probed electrochemically with relatively large probes with dimensions on the order of the inter-NP separation in the sample. Washing the NPs several times via centrifugation before deposition and thoroughly rinsing the NPs after deposition were found to be necessary to observe significant electrochemical responses from individual NPs (see SI for details)

RESULTS AND DISCUSSION In a typical experiment, the optical properties of a 20 µm x 30 µm region of the sample were first characterized using a hyperspectral imaging protocol, yielding spectral information at each point across the interrogated area with diffraction-limited spatial resolution (~1 µm). Features on the sample with physical dimensions

Figure 1. Generalized procedure for probing individual NPs via OTECCM. Samples are prepared by dispersed the NPs of interest onto a non-catalytic substrate electrode at low coverages (< 0.1 µm-2) via dropcoating (a). The resulting samples are first characterized optically using a hyperspectral imaging protocol (b). This data is used to locate individual NPs and compare their optical signatures to that expected for individual NPs. Targeted electrochemical measurements are then made via SECCM on identified NPs (c). The overall process is extremely efficient, typically taking just a few minutes to characterize 5-10 NPs. smaller than the diffraction limit, such as NPs, appear as roughly Gaussian spots of diffraction-limited size in the resulting images. For NPs, this optical data provides both spectral information, which can be used to infer information about the composition, size, and/or geometry of the NP in question, and, most critical to the present studies, the spatial location of the NP from the spatial intensity profile. The appropriate fitting of these spatial intensity profiles, which forms the basis of several super-resolution optical techniques now commonly employed, can be used to localize individual optical emitters with rather extreme accuracies (within a few 10’s of nm), though such accuracies were not necessary for the present studies.

After optical imaging, the positions of any NPs of interest are noted and used as a “roadmap” for subsequent electrochemical measurements. A probe consisting of a pipette filled with an electrolyte solution of interest and a counter electrode (Ag/AgCl wire) was brought into contact with the sample over each NP, forming a miniaturized electrochemical cell where the NP is “trapped” inside. The probe approach procedure followed was similar to that previously described by Unwin et al., wherein cell currents are monitored during approach and


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Figure 2 Correlated electrochemical (bottom), optical (top-left), and scanning electron microscopy (SEM, top-right) measurements at three individual Au nanorods (a-c). Electrochemical measurements were taken in a solution of 2 mM N2H4, 25 mM citric acid, and 25 mM trisodium citrate with a sweep rate of 1000 mV/s. Different sweep segments in the obtained CVs are denoted by color, with the given label indicating the cycle number and polarity (c = cathodic, a = anodic) of each segment. Normalized experimental (black) and simulated (red) scattering spectra are both given for each nanorod. Simulations were carried out for hemispherically capped nanorods at the interface of an ITO substrate ( 2) and air ( 1), using nanorod dimensions from the SEM images. Scale bars in the SEM images represent 100 nm. Dimensions of each nanorod are given below the respective SEM image.

contact between the probe and sample is detected as a sudden current spike (see SI for details on the approach procedure).22 The desired voltammetric measurements are then carried out immediately and the probe is retracted. The probes employed in these experiments were large with respect the NPs being probed (~500 nm vs. ~50 nm), making it feasible to trap entire NPs within the solution volume during measurements. In typical experiments, an electrochemical response was observed from 80% of the locations investigated, a number which, due to the high accuracy of the piezo system employed (< 10 nm), primarily reflects variations in activity among the deposited NPs. Finite element simulations demonstrate that for pipettes of this size, the pipette radius and cone angle have significant impacts on the currents measured in this geometry, both of which can be characterized straightforwardly for pipettes of this size. The targeting accuracy and meniscus height were found to have rather weak effects. Details on these simulations are given in the SI. The combined optical and electrochemical approach employed here has several advantages over existing approaches based on purely optical or electrochemical detection schemes. Unlike optical detection schemes, the present method provides direct, unambiguous electrochemical detection which is simple to interpret and does not depend on a narrow selection of probe reactions. Compared to other probe-based electrochemical schemes, the proposed method offers a significant improvement in sample throughput. While scanning-based methods often require tens of minutes to characterize individual NPs, the present strategy is orders of magnitude faster, requiring only ~60 s to optically map a substrate and ~10 s to electrochemically probe each localized nanostructure. These faster throughputs are critical to the useful application of single NP electrochemical measurements as a screening technique to generate chemically relevant information (e.g., the identity of exceptionally reactive structures). Additionally, since reactions are not being driven across the entire sample during an experiment, complications arising from the deactivation of NPs over time (i.e., decreased currents in successive cycles)

can be avoided, an advantage shared with any SECCM-based approach. Hydrazine Oxidation at Individual Au NRs. The methodology described above was employed to study the oxidation of hydrazine (N H → N 5H 4e ) at individual Au nanorods (NRs). The Au NRs employed in these studies were synthesized via wet chemical methods, following the wellstablished seed-mediated growth procedure based on the reduction of HAuCl4 in the presence of AgNO3 and cetyltrimethylammonium bromide (CTAB).54 These NRs presented an ideal test case for the proposed methodology, as they display strong, narrow optical resonances which are easily detectable at the single NR level via dark field scattering. In order to conclusively demonstrate the ability of the developed methodology to probe electrocatalysis at individual nanostructures, correlated optical, electrochemical, and scanning electron microscopy (SEM) investigations were carried out, the results of which are summarized in Figure 2. Samples were first characterized optically and electrochemically according to the procedure outlined above, rinsed with H2O to remove residual electrolyte, and imaged via SEM in the same locations where optical and electrochemical measurements were made. The correlated SEM imaging was made possible by the optical “roadmap” obtained during the hyperspectral imaging process. The optical spectra obtained for individual Au NRs were roughly Lorentzian in shape, with full widths at half max (FWHM, ), of < 40 nm. These experimental spectra were compared to finite element simulations of scattering from NRs on an ITO substrate in air, a situation which is approximated by a hemispherically-capped rod with a wavelengthdependent refractive index corresponding to Au55 at the planar interface between two dielectrics, ITO ( 2) and air ( 1). Simulations were carried out for NRs with dimensions taken from the SEM images (see SI), and the resulting agreement with the experimentally obtained spectra is fair. Several factors may be responsible for the observed disagreement, including residual ligands on the NR surface changing the effective medium refractive index or deviations from the



ideal, planar interface assumed in the calculations. The latter is especially likely, as the grain size of the ITO employed is rather large (~20-30 nm), and NRs often appear to adsorb in the vicinity of large grains (see SEM images in Figure 2). These findings suggest that with the ITO substrates employed, the interpretation of these optical measurements should be limited to locating individual nanostructures on a substrate and using the line widths to predict whether the located structure is an individual particle, not for conclusively evaluating the geometry of the particle in question. Though, improved substrates with a more consistent geometry at the nanoscale could enable the use of optical data for structural determinations. CVs obtained at these same NRs in an electrolyte solution containing hydrazine are also given in Figure 2, which constitute a small, but representative cross-section of the types of behavior observed in the >200 individual experiments carried out in these studies. Two features are generally present in these CVs: an anodic wave between 0.7 V and 1.2 V vs. Ag/AgCl on anodic sweeps (denoted “a” in the figure) and an anodic peak at ~0.6 V on the cathodic sweeps (denoted “c” in the figure), both attributable to hydrazine oxidation. The strikingly variable behavior observed at these individual NRs, which are each quite comparable in size (24 nm x 50 nm, 21 nm x 44 nm, and 19 nm x 45 nm), leads to several important conclusions. First, the large variability in catalytic activity observed (peak hydrazine oxidation currents of ~25 pA, ~50 pA, and ~12 pA) indicates that heterogeneities in nm-scale structural features and/or residual ligand coverage dominate the observed behavior, not gross geometry. Second, the evolution of catalytic activity over time is variable, with some NRs exhibiting increases in activity with cycling (panel b) while others lose activity (panel a). This indicates that the chemical nature of the NR surface is evolving over time, likely involving multiple processes which are more prevalent in some NRs than others (e.g., surface restructuring, ligand desorption, or dissolution). Influence of Surface Reactions on the Electrochemical Response of Individual Au NRs. The general behavior observed in the electrocatalytic oxidation of hydrazine at individual Au NRs presented above, anodic current peaks on both the anodic and cathodic scans, is indicative of an electrocatalytic system in which the electrode surface is undergoing dynamic, reversible chemical changes which alter the kinetics of the reaction. This can be attributed to the formation and subsequent reduction of anodic gold oxide ( ) films on the NR surface,56 which is clearly illustrated in the bulk voltammetry of Au in the same electrolyte depicted in Figure 3. Here, Au displays an anodic peak at ~1.2 V vs. Ag/AgCl ( → ) and a cathodic peak at ~0.7 V vs. Ag/AgCl ( → ). Interestingly, these surface processes do not appear to significantly affect hydrazine oxidation at bulk, polycrystalline Au electrodes. The response of such an electrode under similar conditions yields a response which is a simple superposition of the ⇌ peaks and an irreversible anodic peak due to hydrazine oxidation. This difference is due to the significantly higher mass transfer rates of electroactive species to individual nanoparticles as compared to planar electrodes, which amplifies the impact of kinetic effects. It is important to note that the currents measured at individual NRs are due to hydrazine oxidation alone, not surface reactions involving / . The currents associated with these reactions will scale directly with the surface area

Figure 3. CVs (100 mV s-1) of a 2 mm diameter Au inlaid disk electrode in a 25 mM citric acid, 25 mM trisodium citrate buffer with and without 2 mM N2H4 (a) and simulated CVs for a bulk Au electrode (b) and an individual 25 nm x 50 nm Au NR for varying values of the stoichiometric coefficient in reaction 1 (c). CVs were simulated for hydrazine oxidation at a 25 nm x 50 nm Au NR on a non-catalytic, planar substrate electrode.

of the Au structures in question, and the observed current densities of ~0.5 mA cm-2 at bulk Au would be expected to result in signals on the order of ~10 fA for the Au NRs employed here, well below the pA-level signals observed in these experiments. Thus, the absence of such features in the single NR data is to be expected. However, there are strong correlations between (a) the → oxidation peak at bulk Au and the loss in activity at ~1.2 V vs. Ag/AgCl observed at individual NRs and (b) the → reduction peak and the anodic peak centered at ~0.6 V vs. Ag/AgCl on the cathodic sweeps at individual NRs. Taken together, these observations suggest the surface / reactions can effectively “turn on” and “turn off” hydrazine oxidation at the Au NRs. This behavior can be summarized through the following generic mechanism: ⇌ ! ∗ #$1% ! ∗ → 5 4' #$2% 2 3 ⇌ 6 6' #$3% where hydrazine first attaches to the NR surface to form an adsorbed intermediate, ∗ , and subsequently undergoes a series of subsequent oxidation events, one of which is the slow, rate determining step. Due to the involvement of the Au surface in the adsorption step (1), the formation and subsequent reduction of films on the NR surface (3) can be


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Figure 4. Plasmon resonance wavelength shifts (Δ+, ) for individual NRs during electrochemical cycling at 100 mV s-1 in an aqueous solution of 2 mM N2H4, 25 mM citric acid, and 25 mM trisodium citrate with different anodic limits. Data is given for two NRs with an anodic limit of 1.0 V (a) and two NRs with an anodic limit of 1.5 V (b). The solid red lines in panel (a) depict fits of a triangular waveform to the data. The dashed red lines in panel (b) are a guide to the eye.

expected to effectively “turn off” and “turn on” hydrazine oxidation. Assuming the rate limiting step is a single electron transfer which follows Butler-Volmer kinetics, the following general rate expression could be derived (see SI): 7

2 -. /0 -11 ' $34%1566 8 #$4%

where 9 :/;$?@%

>,

45?A ! ?A, 8

>,

4B

?C ?C ! D + +,

#$5%

where > is the scattering intensity, >, is the peak intensity, ?A is the photon energy, ?A, is the peak photon energy, +, is the peak wavelength, and is the full width at half max. Representative results from these experiments, presented as peak wavelength shifts (Δ+, ), are given in Figure 4. Similar results were obtained without the inclusion of N2H4 in the electrolyte. When the NRs were cycled with an anodic limit of 1.0 V vs. Ag/AgCl (Figure 4a), their spectra displayed repeatable, stable shifts which mirrored the applied potential waveform. These stable, reversible shifts can be attributed to capacitive charging, which alters the electron density within the NR and thus the bulk plasmon frequency.32,42,57 Both the polarity (red



Figure 5. Observed relationships between the observed optical and electrochemical properties of individual Au nanostructures. The optical properties of 234 individual nanostructures were mapped and sorted into two classes based on the FWHM () of their spectra, those corresponding to individual NRs (“Single NRs”, E 40 F) and those corresponding to aggregates (“Aggregates”, G 40 F). Observed distributions of the observed peak scattering wavelengths (+,HI ) and peak currents (J,HI ) are given for each group in the top panels. The observed relationships between these parameters are given for each group in the bottom panels. The observed correlations are extremely weak, suggesting gross nanostructure geometry is not a strong predictor of electrocatalytic performance.

shifts at more positive potentials) and magnitude (~5 nm/V) of the observed shifts are consistent with previous reports. Distinctly different responses are obtained if cycling is carried out with more positive anodic limits, examples of which are given in Figure 4b. Similar results, redshifts proportional to the applied potential, are obtained up until a potential of ~1.2 V vs. Ag/AgCl, which coincides with the onset of formation. Upon reaching this potential, immediate blueshifts are observed, which vary in magnitude from NR to NR. Similar behavior is observed on subsequent cycles, with the magnitude of these effects getting progressively smaller. Gradualblueshifts over multiple cycles are also observed for some NRs (e.g., middle panel in Figure 4b). These results demonstrate that driving the ⇌23 oxidation/reduction reactions at individual NRs gradually lowers their spectral response due to capacitive charging. This dampening of capacitive effects suggests a decrease in the electrochemically active surface area through surface restructuring, eliminating high surface energy features and/or altering the faceting of the nanostructures over multiple cycles, or the formation of N2 bubbles, a product of hydrazine oxidation. The gradual blueshift observed for some NRs could be due to bubble formation or desorption of residual ligands, lowering the effective surrounding refractive index. All processes could be playing significant, likely opposing roles in the evolution of the catalytic behavior of these nanostructures over time, and further studies are needed to evaluate their relative importance. Relationships Between NR Geometry and Electrocatalytic Activity. A central motivating factor for carrying out investiga-

tions at individual nanoparticles is the establishment of detailed structure-function relationships. To this end, the data obtained in these investigations were analyzed to gain insights into the apparent relationships between NR geometry and the large variations in electrocatalytic behavior noted above. The data from correlated SEM investigations presented in Figure 2 suggest that there is little to no relationship between the observed electrocatalytic behavior and the gross NR geometry observable via SEM. Even though these NRs were of comparable dimensions (24 nm x 50 nm, 21 nm x 44 nm, and 19 nm x 45 nm), there was a large variation in both the magnitude of the observed currents (maxima of ~25, ~50, and ~5 pA) and in the behavior over successive scans, with some NRs gaining activity over time (middle panel) and some losing activity over time (left panel). These highly varying behaviors strongly suggest gross geometry is a poor predictor of catalytic behavior, with other factors, such as residual ligand coverage and sub nm-scale structural features, playing a dominant role in determining catalytic activity. While these correlated SEM investigations are convincing, the process involved (carrying out electrochemical measurements, indexing the sample for location in the SEM, carrying out high resolution imaging at each location of interest, etc.) is rather time consuming which prevents their use as an efficient, routine tool for analysis at individual nanostructures. A potentially attractive aspect of combined high-resolution optical and electrochemical measurements is the utilization of optical data to infer local structural information, supplementing or even eliminating the need for ex situ structural analyses via other methods such as SEM. In order to provide further insights into


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Analytical Chemistry structure function relationships in this system and provide statistically sound support for the conclusions drawn from the correlated SEM studies, the optical and electrochemical responses of >200 individual nanostructures were analyzed using the developed methodology. Structural information was inferred from the spectral responses observed at individual NRs, which were fit to Lorentzian functions (Eq. 5). The obtained spectra were first utilized to classify structures into two groups: those which displayed sharp, Lorentzian line shapes ( < 40 nm) corresponding to single, isolated NRs, and those which displayed broader Lorentzian or non-Lorentzian spectra, corresponding to aggregate structures (example spectra given in SI). Relationships between their optical and electrochemical responses were then analyzed using the peak scattering wavelength for each structure (+,HI ) and the peak current observed during voltammetry (J,HI ) as metrics, the results of which are summarized in Figure 5. It has been well established that the wavelength of the dominant localized surface plasmon resonance of a NR is largely determined by its aspect ratio (AR, length divided by diameter), with the resonance red-shifting as the AR increases.58,59 The structural characterization of the NRs employed in the present study via TEM (see Figure S1 in the SI) demonstrate that the AR is strongly correlated with the diameter of the NRs, but not their length. Thus, the optical resonance of a NR should be strongly correlated with NR diameter, with a shorter resonance wavelength corresponding to a larger, thicker NR. Based on this reasoning, the simplistic prediction is that +,HI and J,HI should be negatively correlated. As is clear in the presented data, extremely weak correlations are observed between the optical and electrochemical properties at both individual NRs and aggregates. This is in agreement with the correlated SEM investigations, and further supports that NR geometry is not a strong predictor of electrocatalytic activity. Further, this data demonstrates that other factors must dominate the response to such a degree where even weak correlations due to mass transfer (i.e., currents which on average scale with the radii of the structures involved) are not observable when hundreds of individual structures are analyzed, a conclusion made possible by the high throughput of the present method. As discussed above, possible explanations for this behavior are particle-to-particle variations in residual ligand coverage and/or the presence of sub-nm structural features on the NRs.

probe electrocatalytic reactions at individual, well-defined nanostructures previously synthesized via wet chemical methods. The electrocatalytic behavior of individual Au nanorods towards hydrazine oxidation was dominated by surface reactions, attributable to the formation and reduction of anodic gold oxide films. The observed behavior is consistent with an effective rate expression for hydrazine oxidation at the Au NRs that is sensitive to the surface activity of Au to second order. In situ spectroscopic investigations suggest irreversible structural changes (surface restructuring, ligand desorption, and/or N2 bubble formation) are responsible for changes in the catalytic activity of the NRs over time. Interestingly, both the correlated electron microscopy studies and statistical analyses of observed electrochemical and optical responses demonstrate that gross particle geometry is not an effective predictor of electrocatalytic performance at the individual NP level. This apparent discrepancy suggests that sub-nm level structural features and/or variations in ligand coverage play a dominant role in behavior at this regime.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Characterization of NRs, example scattering spectra for NRs and NR aggregates, details on pipette characterization and geometry effects, derivation of kinetic expressions, and details on finite element simulations.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT Financial support for this work from the University of Wyoming, the Wyoming NASA Space Grant Consortium (NASA Grant #NNX15AI08H), and the NIH Wyoming INBRE program (2P20GM103432) is gratefully acknowledged.

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CONCLUSIONS The work presented here demonstrates a valuable new approach to the electrochemical analysis of individual nanostructures based on correlated optical and electrochemical measurements. The demonstrated methodology, Optically Targeted ElectroChemical Cell Microscopy (OTECCM), utilizes highresolution optical hyperspectral imaging to locate individual nanostructures on samples and SECCM to probe the electrochemical properties of the located structures in a targeted fashion, eliminating the need for the slow, scanning-based approaches employed in SECM/SECCM while retaining true electrochemical information not obtainable in optical characterization methods. The utility of this approach was demonstrated through studies of electrocatalytic hydrazine oxidation at individual Au nanorods. Correlated electron microscopy investigations conclusively confirmed the ability of the proposed method to

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