Nanoscale Chemical Reaction Imaging at the Solid-Liquid Interface

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Nanoscale Chemical Reaction Imaging at the Solid-Liquid Interface via TERS Ashish Bhattarai, and Patrick Z. El-Khoury J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00935 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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The Journal of Physical Chemistry Letters

Nanoscale Chemical Reaction Imaging at the SolidLiquid Interface via TERS Ashish Bhattarai* and Patrick Z. El-Khoury*

Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA

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[email protected]; [email protected]

ABSTRACT Not all regions of optical field nano-localization and enhancement are suitable sites for chemical transformations on plasmonic metals. We illustrate the concept using chemically functionalized monocrystalline gold platelets in aqueous solution imaged using an Au-coated tip-enhanced Raman scattering (TERS) probe. For our proof-of-principle study, we select a model plasmon driven chemical process, namely, the dimerization of p-nitrothiophenol (NTP) to dimercaptoazobenzene. Consistent with recent observations from our group, we find that TERS maps at vibrational resonances corresponding to NTP trace the optical fields that are maximally enhanced towards the edges of the platelets. Conversely, simultaneously recorded product maps reveal that the dimerization process only occurs at specific sites on our substrate. Given the uniformity of the structures and local optical fields at the edges of the gold platelets, our results suggest that molecular crowding and steric effects play a key role in our case of plasmon driven NTP dimerization at the gold-water interface. 1 ACS Paragon Plus Environment

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The powerful combination of scanning probe microscopy and Raman spectroscopy in the so-called tip-enhanced Raman scattering (TERS1-4) scheme has revolutionized chemical imaging. Sub-nm spatial resolution has been demonstrated using scanning tunneling microscopy-based TERS in ultra-high vacuum and at cryogenic temperatures.5-8 Under ambient laboratory conditions, atomic force microscopy (AFM)-based TERS with single/few nanometer precision has also been reported,9-11 which paves the way for broader applicability of this powerful technique. While chemical fingerprinting with high sensitivity and nanometer spatial resolution has been one of the drives behind advancements in TERS, chemical reaction imaging remains the ultimate goal. This is particularly the case in solution where most chemical transformations of interest to the biological and energy sciences take place. Several pioneering works illustrated that surface plasmon-enhanced Raman scattering can be used to monitor chemical transformations. Recently, the plasmon-driven polymerization of thiols was visualized through TERS.12 Other interesting demonstrations include tracking tautomerization of porphycene at plasmonic hotspots,13 plasmon-enhanced N-demethylation of methylene blue,14 photo-induced and plasmon-driven cis-trans isomerization,15,16 and photocatalytic reduction of CO2 following visible light irradiation.17 The oxidative and reductive dimerization of p-nitrothiophenol (NTP) and p-aminothiophenol (ATP) to dimercaptoazobenzene (DMAB) on plasmonic substrates is of particular relevance to this work and has been extensively scrutinized by several groups.18-21 Although controversial in its early days,22-25 subsequent analyses of these dimerization processes convincingly established plasmon-enhanced DMAB formation. Overall, the DMAB product is routinely observed throughout the course of plasmon-enhanced Raman scattering measurements targeting both ATP and NTP.18-21,24,26 Several mechanistic investigations have subsequently been performed, with some consensus that hot-electrons drive



azo bond formation to yield DMAB.21,27-30 Note that all of the aforementioned works investigated DMAB formation through (nominally diffraction-limited) surface-enhanced Raman scattering measurements that lack the nanoscale spatial resolution that is afforded through TERS. The first attempt at monitoring the NTP → DMAB reaction via TERS nanoscopy was reported by Lantman et al.31 The prior investigation employed a combination of a 633 nm laser source to probe the parent (NTP) through TERS and a 532 nm light source to induce the dimerization process. TERS maps of catalytic hot-spots were also recently recorded with a lateral spatial resolution of ~20 nm.32 Nonetheless, similar measurements in solution/at the solid-liquid interface continue to be scarce, particularly in the TERS scheme.33-35 Very recently, Kumar et al. reported TERS maps of ATP → DMAB chemical transformations at the solid-liquid interface using unique TERS probes that offer both strong TERS signal enhancement and stable operation in liquids over extended periods of time.36 The choice of heterogenous substrate in the prior work36 nonetheless poses challenges in the quest to understand SERS vs TERS vs chemical reaction hotspots in the context of plasmon-assisted chemical conversion. This is the subject of our present work, whereby we use in situ TERS nanoscopy37 to both locate hotspots at which the NTP → DMAB reaction occurs and to distinguish between regions of optical field localization and enhancement and chemical reaction hotspots in a single measurement. Figure 1 highlights our experimental platform. Figure 1A shows an AFM image of gold platelets supported by a glass coverslip in H2O. The gold structures are imaged using a ~100 nm Au-coated AFM probe. The inset of Figure 1A shows a topographic cross-section taken towards the left edge of the stacked plates. The cut reveals stacked and displaced microplates, with ~30 nm apparent step heights. Using an inverted optical microscopy scheme (bottom excitationcollection) with an in-plane polarized 633-nm laser source,37 we record hyperspectral TERS


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images of the rectangular area that is highlighted in Figure 1A. TERS results are shown in Figures 1B and 1C. The two panels show simultaneously recorded TERS image slices at frequency shifts that have been previously assigned to the parent NTP (1330 cm-1, Figure 1B) and product DMAB species (1425 cm-1, Figure 1C). As a result of the inverted excitation-collection geometry, only the edges and their immediate vicinity are visible in our TERS maps.37 With that in mind, the overall TERS image of the reactants (Figure 1B) traces the local optical fields that are sustained towards the edges, as elaborated in a prior work from our group.37 This observation is also generally consistent with several prior reports from our group that employed molecular TERS to image local optical fields.38,39 Herein, the variations in signal levels towards the edges of the platelets are attributed to inhomogenous molecular coverage and/or the comparatively coarse (10 nm) lateral step size in our current work.37 Overall, our measurements report on topography (Figure 1A), local optical fields (Figure 1B),37 and chemical reaction hotspots (Figure 1C). The simultaneously recorded TERS image at 1425 cm-1 (Figure 1C) shows that product formation is favored at very specific sites on the substrate. For instance, the product signatures are relatively dim towards the outer edge of the construct (see Figures 1A and 1C), which is in contrast with the parent map that traces the enhanced local fields along the same edge (see Figure 1B), barring heterogeneity in molecular coverage. In other words, the product map does not necessarily follow the overall spatial profile and magnitude of the local optical fields, as gauged through the recorded TERS maps at the NTP resonances.37,38,39 Further spatio-spectral analysis in Figure 1D shows three consecutively recorded TERS spectra that further illustrate the concept. These spectra were extracted from three adjacent pixels (10 nm lateral step size) and clearly show pixel-to-pixel variation and different relative contributions of the product to the overall spectra in each case.



These observations thus reveal that the dimerization process is site-specific, occurring at some (not all) sites where electric fields are optimally enhanced. Moreover, it appears that our images track the dimerization process at the solid-liquid interface with sub-10 nm spatial resolution (pixel limited). Further evidence in support of this statement is given in the supporting information section, see Figure S2. 120

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Figure 1: A) A representative AFM image of gold micro-plates in H2O. The inset shows a topographic cross-sectional cut. The highlighted rectangular area in A) is visualized through TERS. Panels B) and C) show simultaneously recorded TERS images at 1330 (NTP resonance) and 1425 cm-1 (DMAB resonance). Note that the substrate is consistently towards the left in A-C. Panel (D) shows three spectra taken at adjacent pixels (marked by 1, 2, and 3 in C). The black asterisks (*) highlight the major Raman modes for DMAB. The TERS images were recorded with a lateral step size of 10 nm, and the spectra were integrated for 0.5 s at each pixel.

After a reaction cite is located (e.g., site-2 in Figure 1C), the tip is positioned at the identified position and brought into contact. Subsequently, sequential TERS spectra are recorded to track sluggish (limited by the integration time used) product formation kinetics. Figure 2A shows a waterfall plot of the Raman trajectory at site-2 (see Figure 1C), henceforth termed junction 1. In this case, the first 8 spectra/frames show a persistent presence of the parent (NTP), and no observable product (DMAB) peaks. Conversely, frames 9 through 14 show a dominant presence of DMAB bands for a time period of 2.5 seconds. After a brief disappearance of the product peak (frame 15), the DMAB signature is recovered at frame 16 and is sustained for a time period of 5 6 ACS Paragon Plus Environment

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seconds. Throughout the remainder of this time trajectory, the product signal disappears, exposing the Raman signature of NTP. Figure 2B shows time-averaged spectra of NTP (first 8 frames) and DMAB (frames 9-14), which can be clearly delineated through their distinct Raman lines. The time evolving signals shown in Figure 2 suggest that the spectra of both parents and product arise from small ensembles. A more in-depth analysis of the DMAB spectra reveals that that the relative intensities of the 1130, 1370, and 1425 cm-1 bands remain constant throughout the time trajectory, which supports our inference of an ensemble averaged optical response, see Figure S3. A cross-correlation slice at 1130 cm-1 (a DMAB resonance) taken from a 2D correlation of the recorded time trajectory (see supporting Figure S4) is shown in Figure 2C. The trace shows that the product peaks are non-correlated to the parent peaks, e.g., the predominant 1330 cm-1 NTP band. It therefore appears that a small fraction of NTP molecules in our probing volume have undergone dimerization into DMAB. On the basis of the results shown in Figure 2, the fate of the product species can be understood in two ways. First, the conversion of NTP to DMAB may be reversible. Second, the parent and product species can diffuse out of the effective probing area over the timescale of our measurement. The absence of correlation between the product and parent peaks, albeit suggestive of diffusion, cannot be used to rule out the decomposition of DMAB into two NTP molecules at the junction. While some prior evidence supports DMAB photo-reduction into ATP at plasmonic junctions,40,41 and to NTP in basic media,42 we (i) do not observe the optical signatures of ATP, and (ii) perform our measurements in Millipore H2O (Millipore, pH = 7). Overall, our first trajectory cannot be used to rigorously preclude any of the mechanisms for product disappearance.



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Figure 2: Panel (A) shows the time evolution of point TERS spectra at a hot-spot. Panel (B) shows two time-averaged TERS spectra of NTP (black) and DMAB (red). Panel (C) shows a crosscorrelation slice at 1130 cm-1 of the data presented in (A). Black asterisks denote the Raman modes that are associated with the product, DMAB, whereas red asterisks mark the 1330 cm-1 resonance of NTP. Each spectrum was integrated for a time period of 0.5 s. Raman trajectories at other hotspots (e.g., junction 2) paint a different picture, see Figure 3. Unlike junction 1, sporadic DMAB formation events are now observed. Three consecutive spectra shown in Figure 3B taken at 90th, 91st, and 92nd frames (0.25 s/frame in this case) illustrate the concept. On the basis of this data, it appears that all NTP molecules in our effective probing volume undergo dimerization. Note that the observation of similar patterns in a previous SERS study21 was associated with single/few molecular events at plasmonic hotspots. Unlike the case at junction 1, the relative intensities of the Raman lines of the DMAB product exhibit pronounced intensity fluctuations (see Figure S5), which was also previously associated with Raman scattering in single/few molecule regime.43 In the absence of more concrete evidence, we refrain from associating the observed signatures to those of single/a few molecules. Rather, a 2D correlation analysis of our Raman time trajectory aids in the interpretation of temporally varying parent and product signatures, see Figures 3C and 3D. A cross-correlation slice at 1130 cm-1 (a DMAB resonance) shows that the product peaks at 1130, 1370, and 1425 cm-1 are strongly correlated to each other. This is expected, as the lines arise from the same species. Interestingly, the same slice shows an inverse correlation between the product and parent peaks (e.g., the predominant 1330


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cm-1 NTP resonance). These results paint a different picture than the one for junction 1, see above. Namely, DMAB formation at junction 2 depletes the parent NTP species therein, and vice versa. In other words, the reaction is reversible.

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Figure 3: Panel A shows a TERS spectral trajectory at a hot-spot (junction 2). Panel B shows sequentially recorded TERS spectra (black) at 90th, 91st, and 92nd time series. The red spectra in this panel correspond to PBE/def2-TZVP Raman spectra of the parent and product species. A 2D correlation analysis of the time trajectory is shown in panel C and a cross-correlation cut at 1130 cm-1 is shown in panel D. Black and red asterisks highlight the peaks associated with the product and parent, respectively. Each spectrum was integrated for a time period of 0.25 s. Throughout the course of our TERS measurements, we additionally occasionally observed a unique set of vibrational lines, e.g., at 1465 cm-1 and 1500 cm-1 (Figure 4C, red spectrum) that 9 ACS Paragon Plus Environment


cannot be assigned to NTP or DMAB. Note that up to this stage, all of our discussion and assignments were concerned with trans-DMAB. While it is not uncommon for azobenzenes to undergo cis-trans isomerization,16,44 the majority of the reported plasmon-enhanced NTP → DMAB conversion processes in literature implicate trans-isomer of the product, besides a few theoretical demonstrations.45,46,47 Figure 4A and B show simultaneously recorded TERS maps at 1330 cm-1 and 1500 cm-1 around the edge of a gold microplate. Much like in Figure 1, we show two simultaneously recorded maps at 1330 and 1500 cm-1. Figure 4C shows 4 spectra representing experimental NTP (position 1 in Figure 4A and B) and product (position 2 in Figure 4A and B) spectra, along with their theoretical analogues. Although the relative intensities of the predicted peaks for the cis-DMAB species are not in agreement with their theoretical analogues, the measured (1465 cm-1 and 1500 cm-1) and predicted vibrational resonances (1455 cm-1 and 1488 cm-1) are reasonably well-aligned. We tentatively assign the product spectrum to cis-DMAB on this basis. Difference in the relative intensities of the observable states can otherwise be rationalized on the basis of modified TERS selection rules38 and/or the removal of orientational averaging in our case of Raman scattering from a nanoscopic probing volume.48

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denoted in (A) and (B), along with the PBE/def2-TZVP Raman spectra of the two species (red). The highlighted area in panel (C) isolated the major peaks of cis- DMAB. Each spectrum was integrated for a time period of 1 s. Several conclusions seem inescapable on the basis of our observations. First and foremost, plasmon-assisted chemical transformations do not take place at all regions of high local optical field enhancement. This is evidenced through simultaneous local field (Figure 1B) and chemical reaction (Figure 1C) imaging. The effect has thus far been unclear through prior works that suffer from limited spatial resolution and that are otherwise further complicated by heterogeneities in the structures, and hence properties, of typical plasmonic substrates used to study plasmon-assisted chemical conversion. Second, our correlation analysis of TERS spectral sequences reveals that the NTP → DMAB process is reversible, and that both the trans- and cis- forms of the product are formed at the solid-liquid interface. To the best of our knowledge, this is the first observation of cis-DMAB throughout the course of the plasmon-assisted dimerization of NTP. Finally, given the uniformity of our monocrystalline Au platelets and the local field around their edges, as established in a prior work from our group,37 it is reasonable to deduce that molecular crowding and steric effects play an important role in plasmon-assisted chemical transformations at the solid-liquid interface. This observation motivates more fundamental research ultimately aimed at understanding realistic plasmon-enhanced photocatalytic systems, whereby heterogeneities in both molecular coverage and plasmonic nanostructures come into full force.

Methods Gold nanoplates (AP1-10/1000-CTAB-DIH-1-5, NanoPartzTM) stock is deposited and allowed to dry onto a glass bottom solution cell (Cellvis), followed by sonicating the cell in ethanol for ~30 seconds and rigorously washing the substrate using the same solvent. Subsequently, 100



uL of 3 mM ethanolic 4-nitrothiophpenol (NTP, Tokyo Chemicals Industries) solution was added and allowed to react for an hour. The mixture is then sonicated for ~30 seconds and washed with ethanol. The alcohol is allowed to dry prior to adding 150 l of Millipore water for the liquid TERS measurements that are described below. Our protocol for TERS nanoscopy in solution is described in a recent work.37 For the purpose of this work, initial topographic AFM measurements were performed in tapping mode feedback using a silicon tip (Opus, 160AC-NN) coated with 100 nm of Au by arc-discharge physical vapor deposition (target: Ted Pella Inc., 99.99% purity). A 633 nm laser (100-200 μW) is incident onto the apex of the TERS probe using a 100X air objective (Nikon, NA=0.85) using the bottom (inverted) excitation channel. The polarization of the laser is controlled with a half waveplate and is orthogonal to the long axis of the AFM probe (in-plane). The scattered radiation is collected through the same objective and filtered through a series of filters. The resulting light is detected by a CCD camera (Andor, Newton EMCCD) coupled to a spectrometer (Andor, Shamrock 500). A dedicated TERS imaging mode (SpecTop, patent pending from AIST-NT) was employed for simultaneous AFM-TERS mapping in H2O. Using this mode, TERS signals are collected when the tip is in direct contact with the surface with a typical force in the 10−25 nN range. A semicontact mode is then used to move the sample relative to the tip (pixel to pixel) to preserve the sharpness and optical properties of the tip and to minimize the lateral forces that otherwise perturb the substrate.


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AUTHOR INFORMATION Corresponding Author *[email protected], [email protected] The authors declare no competing financial interest.

ACKNOWLEDGMENTS AB is supported by the Department of Energy’s (DOE) Office of Biological and Environmental Research Bioimaging Technology project #69212. PZE is supported by the US DOE, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Part of the instrumentation that is required to perform TERS in solution was purchased through support from the PNNL LDRD program. This work was performed in the environmental and molecular sciences laboratory (EMSL), a DOE Office of Science User Facility sponsored by BER and located at PNNL. PNNL is operated by Battelle Memorial Institute for the DOE under contract number DE-AC05-76RL1830.

Supporting Information Available: (1) Comparison of tip-on vs. tip-off the edge spectra and cross-sectional profiles of the TERS map shown in Figure 1. (2) TERS image showing sub-10 nm resolution in our measurements. (3) DMAB spectra from the data presented in Figure 2. (4) 2D correlation map of the data shown in Figure 2(A). (5) DMAB spectra from the data presented in Figure 3. (6) Helium ion image of the Au-coated TERS probe that was used to record the spectral images shown in this work. (7) Helium ion image of stacked/displaced Au nanoplatelets coated with NTP.



References (1) Anderson, M. S. Locally Enhanced Raman Spectroscopy with an Atomic Force Microscope. Appl. Phys. Lett. 2000, 76, 3130-3132. (2) Stockle, R. M.; Suh, Y. D.; Deckert, V.; Zenobi, R. Nanoscale Chemical Analysis by TipEnhanced Raman Spectorscopy. Chem. Phys. Lett. 2000, 318, 131-136. (3) Pettinger, B.; Schambach, P.; Villagomez, C. J.; Scott N. Tip-Enhanced Raman Spectroscopy: Near-Fields Acting on a Few Molecules. Annu. Rev. Phys. Chem. 2012, 63, 379399. (4) Stadler, J.; Schmid, T.; Zenobi, R. Developments in and Practical Guidelines for TipEnhanced Raman Spectroscopy. Nanoscale 2012, 4, 1856-1870. (5) Zhang, R.; Zhang, Y.; Dong, Z. C.; Jiang, S.; Zhang, C.; Chen, L. G.; Zhang, L.; Liao, Y.; Aizpurua, J.; Luo, Y.; Yang, J. L.; Hour, J. G. Chemical mapping of a single molecule by plasmon-enhanced Raman scattering. Nature 2013, 498, 82–86. (6) Chiang, C.; Chen, X.; Goubert, G.; Chulhai, D. V.; Chen, X.; Pozzi, E. A.; Jiang, N.; Hersam, M. C.; Seidman, T.; Jenssen, L.; Van Duyne, R. P. Conformational Contrast of Surface-Mediated Molecular Switches Yields Ångstrom-Scale Spatial Resolution in Ultrahigh Vacuum TipEnhanced Raman Spectroscopy. Nano Lett. 2016, 16, 7774-7778. (7) Tallarida, N.; Lee, J.; Apkarian, V. A. Tip-Enhanced Raman Spectromicroscopy on the Angstrom Scale: Bare and CO-Terminated Ag Tips. ACS Nano 2017, 11, 11393-11401. (8) Zrimsek, A. N.; Chiang, N.; Mattei, M.; Zaleski, S.; McAnally, M. O.; Chapman, C. T.; Henry, A. –I.; Schatz, G. C.; Van Duyne, R. P. Single-Molecule Chemistry with Surface- and Tip-Enhanced Raman Spectroscopy. Chem. Rev. 2017, 117, 7583–7613. (9) He, Z.; Han, Z.; Kizer, M.; Linhardt, R. J.; Wang, X.; Sinyukov, A. M.; Wang, J.; Deckert, V.; Sokolov, A. V.; Hu, J.; Scully, M. O. Tip-Enhanced Raman Imaging of Single-Stranded DNA with Single Base Resolution. J. Am. Chem. Soc. 2019, 141, 753-757. (10) Bhattarai, A.; El-Khoury, P. Z. Imaging Localized Electric Fields with Nanometer Precision through Tip-Enhanced Raman Scattering. Chem. Commun. 2017, 53, 7310-7313. (11) Wang, X.; Huang, S.; Huang, T.; Su, H.; Zhong, J.; Zeng, Z.; Li, M.; Ren, B. Tip-enhanced Raman spectroscopy for surfaces and interfaces. Chem. Soc. Rev. 2017, 46, 4020-4041. (12) Szczerbinski, J.; Gyr, L.; Kaeslin, J.; Zenobi, R. Plasmon-Driven Photocatalysis Leads to Products Known from E-beam and X-ray-Induced Surface Chemistry. Nano Lett. 2018, 11, 6740-6749.


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