Optimized Ag Nanovoid Structures for Probing Electrocatalytic Carbon

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Optimized Ag nanovoid structures for probing electrocatalytic carbon dioxide reduction using operando surface enhanced Raman spectroscopy Denis Öhl, Yasin Ugur Ugur Kayran, Joao R. C. Junqueira, Vera Essmann, Tim Bobrowski, and Wolfgang Schuhmann Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02501 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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Optimized Ag nanovoid structures for probing electrocatalytic carbon dioxide reduction using operando surface enhanced Raman spectroscopy Denis Öhl, Yasin U. Kayran, João R. C. Junqueira, Vera Eßmann, Tim Bobrowski, Wolfgang Schuhmann* Analytical Chemistry – Center for Electrochemical Sciences (CES); Faculty of Chemistry and Biochemistry; Ruhr University Bochum; Universitätsstrasse 150, D-44780 Bochum (Germany)

E-mail: [email protected]

carbon dioxide reduction, Raman spectroscopy, Ag nanovoids, SERS, bipolar electrochemistry

ACS Paragon Plus Environment

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ABSTRACT

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical tool and a strongly surface structure dependent process. Importantly, it can be coupled with electrochemistry to simultaneously record vibrational spectroscopic information during electrocatalytic reactions. Highest Raman enhancements are obtained using precisely tuned nanostructures. The fabrication and evaluation of a high number of different nanostructures with slightly different properties is time consuming. We present a strategy to systematically determine optimal nanostructure properties of electrochemically generated Ag voids structures in order to find the void size providing the high signal enhancement for Raman spectroscopy. Ag-coated Si wafers were decorated with a monolayer of differently sized polymer nanospheres using a Langmuir-Blodgett (LB) approach. Subsequently, bipolar electrochemistry was used to electrodeposit a gradient of differently sized void structures. The gradient structures were locally evaluated using Raman spectroscopy of a surface adsorbed Raman probe and the surface regions exhibiting the highest Raman enhancement were characterized by means of scanning electron microscopy (SEM). High throughput scanning droplet cell (SDC) experiments were utilized to determine suitable conditions for the electrodeposition of a highly-active structure in a three-electrode electrochemical cell. This structure was subsequently employed as working electrode in operando surface enhanced Raman measurements to verify its viability as signal amplifier and to spectroscopically rationalize the complex electrochemical reduction of carbon dioxide.


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Introduction Commodity chemicals and fuels consisting of hydrocarbons may be produced by means of electrocatalytic reduction of the greenhouse gas CO2. The CO2 reduction reaction (CO2RR) can proceed via different reaction pathways involving different numbers of transferred electrons and thus yield different products depending on the material and applied overpotential.1 The synthesis of higher hydrocarbons by CO2RR is usually achieved using Cu-based catalysts2 whereas noble metals such as Ag and Au yield predominatly CO.3–5 CO is of particular interest because of its potential use in the Fischer-Tropsch process where it can be further converted to hydrocarbons.6 The production of CO at the necessary overpotentials is accompanied by the hydrogen evolution reaction (HER). Herein, Ag is employed as catalyst material which was previously reported to predominatly yield CO and H2 as main products.7 It has been claimed that the formation of CO proceeds via formate as intermediate product yielding small quantities of ethanol and methanol.1,8 Mechanistic insight into the reaction mechanism of the CO2RR at Ag has been obtained using vibrational spectroscopy techniques such as ATR-FT-IR8 and Raman measurements on roughened Ag surfaces.9 It was proposed that CO2 adsorbs and is protonated to form a formate intermediate either in a proton coupled electron transfer reaction (Eq. 1) or in two separate steps (Eq. 2 and 3).2 + ∗ + + → ∗

(1)

+ ∗ + → ∗

(2)

∗ + → ∗

(3)

CO is formed by reaction with another proton and subsequent desorption (Eq. 4 and 5).1,8


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∗ + + → ∗ +

(4)

∗ → ( ) + ∗

(5)

As stated in earlier reports8 the first reaction step is rate limiting. Hence, it is important to distinguish if the CO2RR proceeds via equation 1 or 2 in order to ultimately design superior catalyst materials. Probing of intermediate reaction products during electrochemical reactions can be achieved by investigating the working electrode surface by means of vibrational spectroscopy. Specifically, the ability to fabricate plasmonic metallic nanostructured is a prerequisite for the advancement of surface-enhanced Raman spectroscopy (SERS). State-of-the-art nanostructures can amplify the Raman signal intensity of analyte molecules by several orders of magnitude.10 Substrate nanoengineering has led to a variety of different materials boosting the sensitivity up to single molecule resolution.11 Both, bimetallic and metallic substrates are utilized for SERS applications whereas the most commonly used metals are Ag and Au.12 Substrates can consist of nanoparticle (NP) dimers13, colloidal NP14,15, or of arrays of for example voids16–18, domes19,20 or wires21. Substrate fabrication can be achieved by different approaches such as lithography, electroless methods22 or electrodeposition.23 SERS is highly sensitive to the surface morphology and the variation of structure and morphology of the substrate can tremendously impact on the Raman enhancement.24 Since substrate production is often time consuming, the reproducible fabrication of a large number of different nanostructured surfaces to finally find the high surface enhancement is tideous. High Raman enhancement may be governed by e.g. spacing between particles25 or void size24. Hence, it is desirable to have access to a method providing a high variability of different nanostructures which can then be evaluated with respect to Raman signal enhancement.16 A large number of different Ag void structures were fabricated using bipolar electrochemistry, and the


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structures were subsequently locally investigated by menas of SERS to determine void morphologies offering high surface enhancement. As reported earlier, appropriate nanofabrication can convert Ag to a signal enhancing surface for SERS measurements.17 Coupling electrochemistry with SERS at catalytically active surfaces enables the elucidation of reaction pathways by operando detection of especially intermediate and surface adsorbed products. Operando Raman spectroscopy was already used for a variety of important electrochemical reactions such as the CO2RR,26 the oxygen evolution reaction (OER),27 or the oxygen reduction reaction (ORR).28 We aim on the reproducible fabrication of Ag nanovoid structures for SERS applications using three differently sized polymer nanosphere templates (200, 300 and 500 nm). Templated layer thickness gradients were fabricated by means of bipolar electrodeposition. The void structure gradient was evaluated for high Raman enhancement. Subsequently, the reproduction parameters of the optimal void structures in a three electrode electrochemical cell were elucidated by means of high throughput scanning droplet cell (SDC) electrodeposition using the results from the bipolar electrochemistry deposition as starting point for the optimization. The thus obtained nanovoid structures with high SERS activity were used as electrode surfaces for operando Raman measurements during electrocatalytic CO2RR. Experimental Section Langmuir-Blodgett (LB) based modification of Ag surfaces with polymer nanobeads. Agcoated Si wafers (200 nm thickness) were provided by the chair of Physical Chemistry I (RuhrUniversität Bochum). The wafers were cut to the desired dimensions that are around 5x1 cm2 rectangles for the bipolar electrodeposition, 2x2 cm2 squares for scanning droplet cell (SDC) experiments and 3x4 cm2 rectangle for operando electrochemistry/Raman measurements. Prior to polymer beads deposition, the Ag surface and the LB trough (KSV Instruments, FIN) were


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rinsed thoroughly with EtOH and H2O. The polystyrene nanosphere solutions (5% w/w, Ø = 200, 300, or 500 nm, Thermo Scientific, USA) were diluted with EtOH (1:2, 150 µL nanospheres and 300 µL EtOH, respectively) and spread on the water surface. The target surface pressure was kept at around 50 mN m-1 using a 2x1 cm2 Pt Wilhelmy plate as reference during vertical retraction of the Ag-coated Si wafer (2 mm min-1). For each polymer nanosphere deposition, the success was controlled using SEM imaging (EM Quanta 3D ESEM, FEI, USA). For further experiments only samples with a homogeneously covered polymer bead surface were used. Bipolar electrodeposition for generation of a void gradient. Bipolar Ag electrodeposition was carried out in an in-house built cell.16,29 An electric field gradient was applied to the templated Ag-wafer by applying a predefined voltage between carbon rods as feeder electrodes (Ø = 6 mm, 9 cm length, 16 cm distance, SGL Carbon, GER) for 240 s using an Autolab PGSTAT302N potentiostat (Metrohm-Autolab, NED). That configuration does not require a connection of the bipolar electrode, namely the Ag and naobeads coated Si wafer, to the feeder electrodes or to a power supply. The voltage applied to the bipolar electrode (BE) depends on its length and was varied to generate the same potential gradient for each BE (9.5 V for Ø = 200 nm, 9.9 V for Ø = 300 nm, 9.1 V for Ø = 500 nm). The carbon rods were cleaned after each Ag deposition by ultrasonication in EtOH for 15 min in an ultrasonic bath (Sonorex Digitec DT 103H, Bandelin, GER). The Ag plating solution (MetSil 500 CNF, 31 g L-1 Ag, Metalor Technologies, CH) was used as received. Before Ag deposition, the polymer-coated samples were incubated for 10 min with the plating solution to assure a sufficiently wetted Ag surface. Moreover, the solution was refreshed for each electrodeposition to maintain the same Ag concentration. Afterwards, the bipolar electrodes were rinsed with EtOH and H2O and incubated for 60 min in dichloromethane (DCM) to dissolve the polymer beads.


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Raman scanning along the void gradient. Raman scanning experiments at different positions across the Ag-void modified electrode were carried out using a Jubin-Yvon iHR 550 spectrometer (HORIBA Jobin-Yvon, GER) equipped with a 20x objective (Zeiss, GER). The nanovoid surfaces were incubated in 5 mM 4-nitrothiophenol (4-NTP, Sigma Aldrich, GER) for 3 h under exclusion of light. The Raman scan was started at the former cathodic pole of the bipolar electrodeposition for a length of 2.25 cm with an increment of 100 µm and 10 µm, respectively. The laser was operated at a power of 2 mW at a wavelength of 532 nm. Raman spectra were recorded from 900 to 1700 cm-1 with an exposure time of 10 s averaged over 3 spectra. The Raman scans were repeated 3 times per sample (3 lines with a vertical distance of 2 mm). Afterwards, the position with the highest SERS intensity was characterized by means of SEM. Image analysis was carried out with ImageJ V1.51.30 Finding suitable electrodeposition parameters by means of SDC. To determine the conditions under which the preferred void opening size was electrodeposited in the bipolar experiment, SDC experiments were carried out using an in-house built setup31. It consists of a measuring cell fixed to x-y-z step motors (OWIS, GER) with an integrated counter electrode (CE, platinum wire) and reference electrode (RE, Ag/AgCl/3 M KCl). The electrochemical droplet cell is connected to a PGU-100 bi-potentiostat (IPS Jaissle, GER) and the current signal is read out using an AD/DA converter. The experiments were performed using an in-house programmed software operated under Microsoft Visual Basic 6. Multiple experiments were conducted in order to determine optimal Ag deposition conditions. Prior to each electrodeposition (300 µl plating solution per spot), a waiting time of 10 min was programmed to ensure electrolyte diffusion into the void volume in between the nanospheres. With each experiment, the conditions were narrowed down until the electrodeposition reliably yielded a Ag surface structure yielding high Raman


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enhancement. The variation of the experimental conditions is summarized in Table 1. The obtained Ag surface structure was investigated by means of tapping-mode atomic force microscopy (AFM) utilizing a Nanowizard III (JPK Instruments, GER). Table 1. Parameters of SDC experiments for Ag deposition optimization Applied potential (vs. Ag/AgCl/3 M KCl) / mV

Time / s

Increment of variable parameter

Number of deposited spots

-900

20-540

20 s

81

-900, -850, -800

55-130

5s

48

-850, -800, -750

100-290

10 s

60

240

5 mV

63

-700 – (-800)

Operando Raman spectroscopy characterization of the CO2RR at Ag nanovoid surfaces. Operando Raman measurements were carried out with the aforementioned Raman spectrometer equipped with an immersible 60x objective (Zeiss, GER). The objective was immersed in the electrolyte in an in-house developed electrochemical cell consisting of a PTFE cell body mounted on the catalytically active nanostructured Ag substrate. The cell was sealed using a tightly pressed O-ring to prevent electrolyte leakage. The measurements were performed in 0.1 M KHCO3 with a Pt mesh as CE and a Ag/AgCl/3 M KCl RE. The nanovoid-modified Ag surface was connected as WE to an Autolab PGSTAT302N potentiostat (Metrohm-Autolab, NED) and each potential was applied for 120 s. 1 min after potential application Raman spectra were recorded using a grating of 1200 grooves mm-1, an exposure time of 3 s and 3 repetitions under illumination with a 532 nm laser with a power of 2.5 mW. Prior to the measurements, the 0.1 M KHCO3 solution was saturated with CO2 for 30 min and during the meaurements the solu-


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tion was bubbled with CO2 and the electrodes were cycled 4 times in that solution up to -1.2 V to ensure a fully reduced metallic Ag surface. Results and Discussion The concept of the performed experiments is summarized in Scheme 1 aiming at the elucidation of reaction pathways of CO2RR at nanovoid modified Ag-electrode surfaces. The strategy involves the optimization of nanovoid-modfied Ag sufaces with respect to optimized Raman enhancement using a deposition gradient invoked by means of bipolar electrochemistry, highthroughput refinement of the Ag electrodeposition parameters using a SDC followed by the reproducible fabrication of these surfaces for the investigation of the CO2RR simultaneously with SERS detection of reaction intermediates and products.

Scheme 1. Experimental steps for the fabrication and optimization of SERS substrates for operando Raman measurements. (1) Polystyrene nanospheres are deposited onto flat Ag surfaces using a Langmuir trough. (2) A nanovoid gradient using the polymer nanospheres as template is fabricated using a commercial Ag plating solution and variable local deposition potentials are imposed by bipolar electrochemistry. (3) To monitor the SERS characteristics, the 4-NTP-


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modified BE is locally evaluated using Raman spectroscopy from the cathodic towards the anodic edge. The point of highest Raman enhancement is then characterized by SEM. (4) The parameters for Ag electrodeposition are determined using high-throughput SDC using the Ag deposition parameters from the bipolar electrochemistry deposition as the starting values for further refinement in a three-electrode electrochemical cell. (5) The obtained Ag nanovoid surfaces are employed as WE for operando surface-enhanced Raman investigation of the electrochemical reduction of CO2. Bipolar electrodeposition of Ag on nanosphere coated Ag substrate. Polymer nanosphere patterning of flat Ag surfaces was achieved by means of Langmuir-Blodgett deposition which has proven to be a suitable technique for the deposition of ordered layers of nanoscaled materials such as metallic nanoparticles32,33 or organic molecules34 (see SI Fig. S1 for detailed information). Subsequently, bipolar electrochemistry was exploited to electrochemically generate a void gradient along the nanosphere-templated Ag-coated Si wafer. By application of a potential difference between the feeder electrodes (Vfeeder), an electric field in solution (dashed line Fig. 1 a) is generated. The electric field drops between the BE poles (∆BE) depending on its length (lBE) and on the distance between the feeder electrodes (dfeeder) according to Eq. (6)16,35. ∆BE was kept constant for each bipolar electrodeposition experiment to obtain a similar Ag gradient for differently sized nanospheres. ∆BE was kept at 2.8 V for 240 s to generate a potential drop sufficient to induce the electrodeposition gradually across the electrode.

=

(6)

The equation assumes that the potential drop at the interface feeder electrodes/ electrolyte is negligible in comparison to the ohmic drop across the electrolyte and thus can be neglected when calculating the eletric field. The total potential drop hence is the sum of the potential drop at the feeder electrode/electrolyte interface and the potential drop induced by the electrolyte resistance. The metal deposition rate correlates with the applied feeder electrode voltage. Hence, an altered


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potential induces a variation of the metal deposition rate. The electric field drops linearly along the electrolyte/BE interface and thus the maximum polarization potential of the BE is reached at the respective extremities. Therefore, positions closer to the cathodic pole of a BE experience a faster metal electrodepositon rate. The poles of the BE are oriented in opposite polarity as the feeder electrodes as schematically depicted in Fig. 1 a. Since Ag can be electrochemically dissolved at oxidizing conditions, we found that the Ag dissolves up to about the middle of the BE. This was confirmed by energy dispersive X-ray (EDX) mapping (Fig. 1 b).

Figure 1. a) Schematic representation of a BPE setup. An electrical field is generated between the two feeder electrodes resulting in a linearly changing potential across the electrolyte and the BE. b) The red cross marks the position on the BE shown in Fig. 1a. Anodic dissolution of Ag until the middle of the BE is shown via EDX mapping of Ag and Si. The left side indicates the position close to the cathodic pole which is still covered with Ag. On the opposite side towards the anodic pole the Ag is completely dissolved and small Ag EDX signals and high Si signals are measured. c) SEM characterization of the void gradient across the BE from the cathodic pole (upper left) towards the middle of the BE (end of layer thickness gradient, lower right).


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Ag on the cathodic side is separated (left, cathodic BE pole) from the side facing the anodic pole, where only Si remained. At the very end of the cathodic edge the voids are overgrown with an amorphous Ag structure due to the high cathodic potential leading to fast Ag electrodeposition. With increasing distance from the edge of the cathodic pole, Ag-voids with different opening sizes occur corresponding to different growth rates at the respective positions on the BE. The images show a gradual increase of the void opening until the equator of the bowls from which the opening size starts to decrease again (Fig. 1c). Raman scanning across the Ag nanovoid gradient. After the preparation of differently templated void gradients caused by different sizes of the assembled polymer beads, each sample was laterally investigated by recording 4-NTP SERS spectra along the Ag nanovoid-modified sample to obtain a rough estimation of the structure-dependent Raman enhancement. To screen a large variety of void compositions, 3 different Ag-nanovoid surfaces were prepared using polymer nanobead sizes of 200, 300, and 500 nm diameter, followed by creating a Ag gradient using bipolar electrochemistry. After adsorption of the probe molecule 4-NTP SERS spectra were obtained along the samples with lateral displacement increments of 100 µm (Fig. 2 a-c). The nanosphere template diameter scales with the position of the area of maximum intensity, which is shifted towards the cathodic edge for larger polymer bead sizes. The maximum intensity for the 200 nm nanosphere voids was reached at 3.8 mm distance from the cathodic edge while for the 300 nm and the 500 nm nanosphere voids the maximum Raman enhancement was reached at distances of 3.5 mm and 0.6 mm from the cathodic edge, respectively. Furthermore, the highest enhancement is reached when 300 nm nanospheres were used as templates accounting for a threefold increase as compared with the other samples. An overall length of 22.5 mm was measured for each sample and roughly 5-10 % of the surface morphologies are suitable for SERS. The


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intensity of the highest Raman peak of 4-NTP at 1343 cm-1 corresponding to the NO2 symmetric stretch vibration36 was plotted as function of the position on the bipolar electrode (Fig. 2d and e). The 300 nm-based Ag-void structure exhibits a sharp enhancement maximum at around 3.5 mm distance from the cathodic edge. The 100 µm increment line scans were performed using a 532 nm laser (λ) equipped with a 20x objective (numerical aperture (NA) = 0.3), resulting in a theoretical spot size of 2.16 µm according to Eq. (7).

D =

!. #

%$(7)

Figure 2. Raman scanning across the nanostructured samples obtained from 200 nm, 300 nm, and 500 nm polymer bead templates with a 100 µm distance increment. The Raman scans were started at the very edge of the former cathodic pole of the BE (at 0 mm in the graphs). a) 200 nm, b) 300 nm, c) 500 nm polymer bead templates. The samples were incubated with 4-NTP for 3 h. d) Raman spectrum of 4-NTP at 3.5 mm distance from the cathodic edge at the sample prepared


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using 300 nm polymer nanospheres. e) The variation of the intensity of the N-O symmetric stretching band at 1343 cm-1 across the nanovoid-modified Ag gradients. Hence, the spatial resolution may not be sufficient to detect the highest active areas owing to averaging effects and the large distance increment during the lateral Raman scan. Consequently, the Raman line scan was repeated with a higher spatial resolution using 10 µm increments and a 60x objective (according to Eq. (7): theoretical spot size of 650 nm, NA = 1.00) for the 300 nm substrate (Fig. 3 a,b).

Figure 3. Raman line scan using a 60x objective and a 10 µm lateral displacement along the 300 nm nanovoid structure for a total length of 8 mm. b) The variation of the intensity of the NO symmetric stretching band at 1343 cm-1 as a function of Raman spectrometer position is shown in a). c) SEM picture of the nanovoid structures. d) Void diameter distribution (N = 4000, group size 5 nm) of the position of the substrate determined in b) and c) reveals a maximum at around 190 nm. The nanovoid structure offering the highest Raman intensity is in good agreement with the previous measurements (3.51 mm). Moreover, it is a narrow area (around 50 µm in length), which exhibits an extraordinary signal enhancement. The signal intensity is increased by roughly one order of magnitude as compared with Fig. 2 probably due to a second incubation with 4-NTP which may further increase the number of chemisorbed molecules and the use of an objective


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with increased NA, which may account for a substantially higher recorded intensity.37 SEM imaging (Fig. 3 c) at that particular position revealed an average void opening diameter of 190 nm (Fig. 3 d). Since the templates are spherical, each diameter (different from the largest) is observed twice on the BE: once above and once below the equator. AFM (Fig. S3) suggests that the voids displayed in Fig. 3 c are electrodeposited above the equator. The determined depth of these voids is around 290 nm. Refinement of Ag electrodeposition by means of SDC. A refinement of the Ag electrodeposition parameters was performed using high-throughput SDC measurements to ultimately obtain parameters for the reproducible formation of Ag-nanovoid structures with highest Raman enhancement. In contrast to previous strategies,16 we determine appropriate conditions for the electrodeposition in a three-electrode cell by means of an SDC instead of laterally positioning a reference electrode during the bipolar Ag electrodepsoition. Briefly, different sets of experiments were automatically performed using the SDC in order to determine parameters (potential and time) suitable for synthesizing the previously determined structure for maximum Raman enhancement (see SI Fig. S2 for detailed information). Chronoamperometry at different Ag deposition overpotentials for the same time (240 s) shows different growth behavior (Fig. 4 a). If the different potentials are translated to the corresponding void opening sizes it can be deduced that a potential of at least -780 mV is necessary to induce significant morphological changes during Ag electrodeposition (Fig. 4 b). Furthermore, the dashed line in Fig.4 b indicates the void opening size of 190 nm and the corresponding potential of -790 mV to be most suitable to deposit the anticipated Ag structure. From the electrochemical data, the growth rate until the equator of the nanospheres can be determined from the peak shaped curve recorded at suitable Ag deposition potentials (e.g. -790 and -800 mV) (Fig. 4 a). That particular shape can be explained by different


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diffusional properties depending on the height of the deposited structures and its electrochemically accessible surface area (ECSA). Initially, the ECSA exposed by the Ag coating is comparably high, thus high cathodic currents are recorded (Fig. 4 a, area 1) which start to decrease upon Ag growth between the templating polymer nanospheres until a minimal current is recorded when the Ag growth is reaching the bead equator (area Fig. 4 a, area 2). Due to the increased layer thickness the ECSA decreases to a minimum where neighboring nanospheres are closest to each other. Upon further growth of the Ag deposit, a higher ECSA is exposed leading again to increasing reductive currents (Fig. 4 a, area 3). From the time necessary to reach the lowest current and hence the bead equator a growth rate for Ag deposition can be determined (Fig. 4 c). The average growth rate fluctuates in a potential window from -700 mV to -790 mV from around 1-2 nm s-1 and rises to around 6 nm s-1 for a potential of -900 mV.

Figure 4. Electrodeposition of Ag SDC spots. a) Chronoamperometric curves for Ag electrodeposition at varying potentials for 240 s. At higher overpotentials, a different curve shape is recorded. Numbers mark areas with different characteristic properties. b) Relation of void opening size induced different electrodeposition potentials. Until an applied overpotential of -780 mV no significant void size change is detected via SEM. At higher overpotentials smaller size openings are


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detected due to a faster electrodeposition rate. The dashed line represents a diameter of 190 nm. c) Average growth rate until the equator of the voids extracted from chronoamperometric data. Operando Raman spectroscopy during CO2RR on Ag nanovoids. Potential dependent operando SERS measurements of the CO2RR were carried out in a potential window between -0.1 V and -1.3 V vs. Ag/AgCl/3 M KCl. The potential was stepped in 0.1 V and 0.05 V steps, respectively, and at each potential Raman spectra were recorded. The i-V curve (Fig. 5 a) shows a steep cathodic current increase at potentials beyond -0.9 V vs. Ag/AgCl/3 M KCl which suggests CO2RR and HER taking place simultaneously.

Figure 5. a) Potential step experiment on Ag nanovoid surfaces during Raman investigation. Each potential was held for 120 s. b) Raman spectra (counts per mW and s) recorded 60 s after application of a new potential show reversible changes in the peak pattern. All reference spectra are recorded under OCP conditions (one spectrum recorded before (OCP: 83 mV, averaged over 120 s) and two spectra recorded after the whole set of experiments (OCP of 32 mV)).


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For the electrocatalytic CO2RR on Ag nanovoid surfaces the commonly detected products at high overpotentials are CO with a substantial formation of H2 as by-product.7 Expectedly, no major changes in the Raman spectra occur for potentials above -0.9 V vs. Ag/AgCl/3 M KCl since neither CO2RR nor HER are proceeding. However, at potentials below -0.9 V, significant changes are detected. First of all, peak intensities decrease at 1300-1400 cm-1 corresponding to CO2 because surface adsorbed CO2 is consumed during the electrochemical reaction.26,38,39. In particular, the bands at 1307 cm-1 and 1404 cm-1 are assigned to CO2 vibrational modes.39–42 The CO2 peak intensity plotted as a function of the applied potential clearly displays its consumption at high turnover rates and its recovery when the electrolyte is exchanged by freshly CO2 saturated solution (Fig. S5). During the consumption of CO2 different products may be formed with CO and H2 reportedly being the main product for Ag-based catalysts.43 The stretching modes of adsorbed CO on different metals can, depending on its orientation on the surface, occur at around 450 cm-1 corresponding to the metal-carbon stretch44 and at 2100 cm-1 9,45–49 for linear, on top bound CO. Furthermore, it was observed that at 1950 cm-1 a band for bridge-bound CO occurs.47,50 We found that predominantly on top bound linear CO is formed, accompanied by small quantities of bridge bound CO (Fig. 6 a).


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Figure 6. a) Potential dependent peak intensities of CO associated peaks during the CO2RR. b) Stark tuning of the CO peak during a cathodic potential sweep from -1 V to -1.1 V vs. Ag/AgCl/3 M KCl. Additionally, the most prominent spectral feature at 2097 cm-1 at -1 V broadens and red-shifts to 2074 cm-1 at -1.1 V (Fig. 6 b). It was reported, that such a frequency shift, namely the vibrational Stark tuning, can be evoked by compression/dissipation of the CO adlayer48,51 and especially by the applied electrical field.52 Dissipation of the CO adlayer may be introduced by the production of other products such as CH-containing species. At potentials of at least -1 V vs. Ag/AgCl/3 M KCl sharp peaks in the CH stretch vibration region from 2700 cm-1 until 3000 cm-1 emerge in parallel to the formation of CO. These distinct peaks at 2827 cm-1, 2860 cm-1 and 2929 cm-1 correspond to CH vibrations in alkanes whereas signals at 2715 cm-1 may account for CH stretch vibrations in aldehydes.26,53 Previously, these peaks were assigned altogether to a growth of long methylene chains on the surface, from which a base signal (peaks at 2860 and 2929 cm-1) is still present after the first cycling up to CO2-RR overpotentials. The retained intensities (2860 and 2929 cm-1) may be assigned to methylene containing species adsorbed on the surface. The emergence of the peaks at 2724 and 2827 cm-1 can be induced by reorientation of molecules on the surface until they finally desorb. 53–56 Additionally, as indicated in Fig. 5, bands in the region of around 1000 cm-1 start to increase corresponding to saturated hydrocarbons.26,53 In particular, the band at 1001 cm-1 indicates the formation of C-C bonds (Fig. 7).57,58 Recorded reference spectra of methanol and ethanol which are reported products of the CO2RR on Ag59 (Fig. S6) lead to the assumption that the emerging peaks correspond to other species than those. Furthermore, the observed peak pattern does not account for ethylene.60 Additionally, we could not find evidence accounting for the formation of formic acid during the CO2RR, which would exhibit prominent spectral features at 1800 cm-1.61,62 That may be explained by a


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very short lifetime and a direct follow-up reaction of the intermediately formed formate to higher products, such as C2 species. That the evolution of CO and CH-derived peaks is specific to the CO2-RR and the presence of CO2 was rationalized by similar Raman measurements in Ar saturated KHCO3 solution (Fig. S7), where the above reported peaks do not grow or emerge in the same manner as in a solution containing CO2. It should be noted that obtaining spectral information at potentials beyond -1.2 V vs. Ag/AgCl/3M KCl is impaired by visible bubbles formation at the surface which block the optical path.63, whereas mixing of the solution and subsequent bubble removal reverses the effect.

Figure 7. a) Potential dependent development of Raman peak intensities in the CH-stretch region at around 2800 cm-1. Peaks corresponding to CH formation increase at cathodic potentials below -0.95 V vs. Ag/AgCl/3M KCl. b) Magnification of the relevant Raman peaks. Intensities of peaks at 2724 cm-1, 2860 cm-1 and 2929 cm-1 increase and a new peak emerges at 2827 cm-1 with decreasing potential. Conclusion. Ag-void structures for highest Raman signal enhancement were determined through synthesis of a void gradient by means of bipolar electrochemistry, followed by local Raman measurements using 4-NTP as surface adsorbed Raman probe. The void size yielding the highest


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Raman enhancement has an opening of around 190 nm and a depth of around 290 nm obtained from 300 nm polystyrene nanospheres as templates. High-throughput scanning-droplet cell experiments were performed to elucidate optimal parameters for the electrodeposition of Ag through the nanospere template in a three-electrode electrochemical cell aiming at a highly reproducible procedure for the fabrication of Ag nanovoid structures with uniform Raman enhancement. Since Ag is a common CO2RR electrocatalyst, mainly producing CO and H2,59 the Ag nanovoid structures were applied for operando Raman measurements. Changes in the Raman spectra occur upon application of sufficiently cathodic potentials corresponding to on top bound linear CO. Moreover, the formation of C2 species becomes evident, however, without the presence of formyl species, which are commonly assumed to be an intermediate product during the CO2RR suggesting a very short lifetime of intermediately formed formic acid. Supporting Information. Langmuir-Blodgett nanosphere deposition; SDC Ag-electrodeposition; Nanovoid AFM; Preparation of bigger electrodes; CO2 Raman peak potential dependency; Methanol, ethanol and formic acid reference Raman spectra; CO2 Electrolysis in Ar-saturated solution Corresponding Author *Prof. Dr. Wolfgang Schuhmann, [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT


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The authors are grateful to the Deutsche Forschungsgemeinschaft in the framework of the Forschergruppe (FOR 2397-1; SCHU 929/15-1) “Multi-scale analysis of complex three-phase systems: oxygen reduction on gas-diffusion electrodes in aqueous electrolyte” and in the framework of the Cluster of Excellence “Resolv” (EXC1069). Martin Trautmann is gratefully acknowledged for performing the AFM measurements. References (1) Kuhl, K. P., Cave, E. R., Abram, D. N., Jaramillo, T. F. New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 2012, 5, 7050. (2) Kortlever, R., Shen, J., Schouten, K. J. P., Calle-Vallejo, F., Koper, M. T. M. Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6, 4073–4082. (3) Kim, C., Jeon, H. S., Eom, T., Jee, M. S., Kim, H., Friend, C. M., Min, B. K., Hwang, Y. J. Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles. J. Am. Chem. Soc. 2015, 137, 13844–13850. (4) Lu, Q., Rosen, J., Zhou, Y., Hutchings, G. S., Kimmel, Y. C., Chen, J. G., Jiao, F. A selective and efficient electrocatalyst for carbon dioxide reduction. Nat. Commun. 2014, 5, 3242. (5) Mistry, H., Choi, Y.-W., Bagger, A., Scholten, F., Bonifacio, C. S., Sinev, I., Divins, N. J., Zegkinoglou, I., Jeon, H. S., Kisslinger, K., Stach, E. A., Yang, J. C., Rossmeisl, J., Roldan Cuenya, B. Enhanced Carbon Dioxide Electroreduction to Carbon Monoxide over DefectRich Plasma-Activated Silver Catalysts. Angew. Chem. 2017, 129, 11552–11556. (6) Hernández, S., Amin Farkhondehfal, M., Sastre, F., Makkee, M., Saracco, G., Russo, N. Syngas production from electrochemical reduction of CO2: Current status and prospective implementation. Green Chem. 2017, 19, 2326–2346. (7) Clark, E. L., Bell, A. T. Direct Observation of the Local Reaction Environment during the Electrochemical Reduction of CO2. J. Am. Chem. Soc. 2018, 140, 7012–7020. (8) Firet, N. J., Smith, W. A. Probing the Reaction Mechanism of CO2 Electroreduction over Ag Films via Operando Infrared Spectroscopy. ACS Catal. 2016, 7, 606–612. (9) Ichinohe, Y., Wadayama, T., Hatta, A. Electrochemical reduction of CO2 on silver as probed by surface-enhanced Raman scattering. J. Raman Spectrosc. 1995, 26, 335–340. (10) Cardinal, M. F., Vander Ende, E., Hackler, R. A., McAnally, M. O., Stair, P. C., Schatz, G. C., van Duyne, R. P. Expanding applications of SERS through versatile nanomaterials engineering. Chem. Soc. Rev. 2017, 46, 3886–3903. (11) Nie, S., Emory, R. Probing Single Molecules and Single Nanoparticles by SurfaceEnhanced Raman Scattering. Science 1997, 275, 1102–1106. (12) Schlücker, S. Surface-enhanced Raman spectroscopy: Concepts and chemical applications. Angew. Chem. Int. Ed. 2014, 53, 4756–4795. (13) Wustholz, K. L., Henry, A.-I., McMahon, J. M., Freeman, R. G., Valley, N., Piotti, M. E., Natan, M. J., Schatz, G. C., van Duyne, R. P. Structure-activity relationships in gold nanoparticle dimers and trimers for surface-enhanced Raman spectroscopy. J. Am. Chem. Soc. 2010, 132, 10903–10910.


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(32) Paul, S., Pearson, C., Molloy, A., Cousins, M. A., Green, M., Kolliopoulou, S., Dimitrakis, P., Normand, P., Tsoukalas, D., Petty, M. C. Langmuir−Blodgett Film Deposition of Metallic Nanoparticles and Their Application to Electronic Memory Structures. Nano Lett. 2003, 3, 533–536. (33) Tao, A., Kim, F., Hess, C., Goldberger, J., He, R., Sun, Y., Xia, Y., Yang, P. Langmuir−Blodgett Silver Nanowire Monolayers for Molecular Sensing Using SurfaceEnhanced Raman Spectroscopy. Nano Lett. 2003, 3, 1229–1233. (34) Simões Gamboa, A. L., Filipe, E. J. M., Brogueira, P. Nanoscale Pattern Formation in Langmuir−Blodgett Films of a Semifluorinated Alkane and a Polystyrene−Poly(Ethylene Oxide) Diblock Copolymer. Nano Lett. 2002, 2, 1083–1086. (35) Fosdick, S. E., Knust, K. N., Scida, K., Crooks, R. M. Bipolar electrochemistry. Angew. Chem. Int. Ed. 2013, 52, 10438–10456. (36) Liu, Y.-C., McCreery, R. L. Reactions of Organic Monolayers on Carbon Surfaces Observed with Unenhanced Raman Spectroscopy. J. Am. Chem. Soc. 1995, 117, 11254–11259. (37) Wei, A., Kim, B., Sadtler, B., Tripp, S. L. Tunable Surface-Enhanced Raman Scattering from Large Gold Nanoparticle Arrays. ChemPhysChem 2001, 2, 743–745. (38) Maynard, K. J., Moskovits, M. A surface enhanced Raman study of carbon dioxide coadsorption with oxygen and alkali metals on silver surfaces. J. Chem. Phys. 1989, 90, 6668– 6679. (39) Davis, A. R., Oliver, B. G. A vibrational-spectroscopic study of the species present in the CO2-H2O system. J. Solution Chem. 1972, 1, 329–339. (40) Berkesi, M., Hidas, K., Guzmics, T., Dubessy, J., Bodnar, R. J., Szabó, C., Vajna, B., Tsunogae, T. Detection of small amounts of H2O in CO2-rich fluid inclusions using Raman spectroscopy. J. Raman Spectrosc. 2009, 40, 1461–1463. (41) Falcke, H., Eberle, S. H. Raman spectroscopic identification of carbonic acid. Water Res. 1990, 24, 685–688. (42) Kneipp, K., Wang, Y., Berger, A. J., Dasari, R. R., Feld, M. S. Surface-enhanced Raman scattering of CO2 dissolved in aqueous colloidal solutions of silver and gold. J. Raman Spectrosc. 1995, 26, 959–962. (43) Choi, J., Kim, M. J., Ahn, S. H., Choi, I., Jang, J. H., Ham, Y. S., Kim, J. J., Kim, S.-K. Electrochemical CO2 reduction to CO on dendritic Ag–Cu electrocatalysts prepared by electrodeposition. Chem. Eng. J. 2016, 299, 37–44. (44) Zhang, Y., Weaver, M. J. Application of surface-enhanced Raman spectroscopy to organic electrocatalytic systems: Decomposition and electrooxidation of methanol and formic acid on gold and platinum-film electrodes. Langmuir 1993, 9, 1397–1403. (45) Beltramo, G. L., Shubina, T. E., Koper, M. T. M. Oxidation of formic acid and carbon monoxide on gold electrodes studied by surface-enhanced Raman spectroscopy and DFT. ChemPhysChem 2005, 6, 2597–2606. (46) Cooney, R. P., Fleischmann, M., Hendra, P. J. Raman spectrum of carbon monoxide on a platinum electrode surface. J. Chem. Soc., Chem. Commun. 1977, 235–237. (47) Mrozek, M. F., Xie, Y., Weaver, M. J. Surface-Enhanced Raman Scattering on Uniform Platinum-Group Overlayers: Preparation by Redox Replacement of UnderpotentialDeposited Metals on Gold. Anal. Chem. 2001, 73, 5953–5960. (48) Mrozek, I., Pettenkofer, C., Otto, A. Raman spectroscopy of carbon monoxide adsorbed on silver island films. Surf. Sci. 1990, 238, 192–198.


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Optimized Ag Nanovoid Structures for Probing Electrocatalytic Carbon

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