How Does a Plasmon-Induced Hot Charge Carrier Break a CâC Bond

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24715−24724

How Does a Plasmon-Induced Hot Charge Carrier Break a C−C Bond? Hyun Huh, Hoa Duc Trinh, Dokyung Lee, and Sangwoon Yoon* Department of Chemistry, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 06974, Korea

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S Supporting Information *

ABSTRACT: Hot-electron chemistry at gold nanoparticle (AuNP) surfaces has received much attention recently because its understanding provides a basis for plasmonic photocatalysis and photovoltaics. Nonradiative decay of excited surface plasmons produces energetic hot charge carriers that transfer to adsorbate molecules and induce chemical reactions. Such plasmon-driven reactions, however, have been limited to a few systems, notably the dimerization of 4-aminobenzenethiol to 4,4′-dimercaptoazobenzene. In this work, we explore a new class of plasmon-driven reactions associated with a unimolecular bond cleavage process. We unveil the mechanism of the decarboxylation reaction of 4-mercaptobenzoic acid and extend the mechanism to account for the β-cleavage reaction of 4-mercaptobenzyl alcohol. Combining the construction of wellcontrolled nanogap systems and sensitive Raman spectroscopy with methodical changes of experimental conditions (laser wavelengths, interface materials, pH, ambient gases, etc.), we track the hot charge carriers from the formation to the transfer to reactants, which provides insights into how plasmon excitation eventually leads to the C−C bond cleavage of the molecules in the nanogap. KEYWORDS: plasmon-driven reaction, hot charge carrier, plasmonic catalysis, decarboxylation, nanoparticle-on-mirror, surface-enhanced Raman spectroscopy

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INTRODUCTION Gold nanoparticles (AuNPs) are widely used in many fields as sensors, biological imaging agents, surface-enhanced Raman scattering (SERS) substrates, catalysts, photothermal therapeutic agents, and photovoltaic materials.1−9 These applications are based on the unique optical properties of AuNPs that arise from surface plasmons, i.e., the collective oscillation of conduction electrons in response to incident electromagnetic fields.10 Understanding the mechanism of the rise and decay of surface plasmons is essential for the further development of AuNP applications. The resonantly excited surface plasmons decay through either radiative or nonradiative pathways.11−14 Radiative relaxation leads to strong scattering of light or localized electric fields around the nanoparticles, which enables many surface-enhanced spectroscopies, such as SERS and surfaceenhanced fluorescence.15−17 Nonradiative decay occurs through electron−electron, electron−phonon, and/or electron−interface scattering. Nonthermal hot charge carriers (electrons and holes) are generated during this process, subsequently transfer to adsorbate molecules, and induce chemical reactions at the surface. Alternatively, the hot charge carriers are thermalized and dissipated as heat that promotes thermal reactions. Recently, plasmon-driven chemical reactions that rely on the nonradiative decay of surface plasmons have drawn much © 2019 American Chemical Society

attention because of their potential photocatalytic and photovoltaic applications.18−20 However, the library of such reactions is currently limited to a few systems; the dimerization of 4-aminobenzenethiol (ABT) to 4,4′-dimercaptoazobezene (DMAB) is the most extensively studied example.21−24 High yields, particularly on Ag nanoparticles, and clear Raman spectroscopic evidence that distinguishes between the reactant and the product has transformed this reaction into the poster child of plasmon-driven reactions. Other reaction systems, including the reduction of CO2, conversions of aldehydes into esters, and water splitting, are mostly followed by gas chromatography, which is a less straightforward method for studying the fundamental principles of plasmon-driven reactions.25−29 In this paper, we extend the list of plasmon-driven reactions to include bond cleavage reactions: particularly, the decarboxylation of 4-mercaptobenzoic acid (MBA) to benzenethiol (BT). This unimolecular reaction is much simpler than the transformation of ABT into DMAB, which requires ABT molecules to be located very close to each other for dimerization to occur. Consequently, this decarboxylation reaction is free from constraints on the relative positions or Received: March 28, 2019 Accepted: June 13, 2019 Published: June 13, 2019 24715

DOI: 10.1021/acsami.9b05509 ACS Appl. Mater. Interfaces 2019, 11, 24715−24724

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Figure 1. Scheme for studying the plasmon-driven decarboxylation reaction of MBA to BT. The NPoM system is formed by adsorbing AuNPs (61 nm) onto MBA SAMs on Au substrates. The reaction laser (λ = 785, 633, or 532 nm) is focused on the NPoM system through a 50× objective that excites AuNP surface plasmons. The reaction products are detected by SERS (785 nm excitation). Raman spectral analysis reveals the identity of the product and the dependence of the reactivity on the experimental parameters.

which excites surface plasmons in the AuNPs. The decays of these excited surface plasmons drive chemical reactions that yield products. In addition, the narrow nanogaps permit the detection of molecules by SERS. The reaction product is probed by SERS, which provides information on the identities and structures of molecules with great sensitivity. NPoM systems also allow us to readily modify the experimental conditions; we are able to change the wavelength of the reaction laser, the type of the adsorbed nanoparticle, the reactant molecule to ones with slightly modified functional groups, and the environment (pH and ambient gases), which leads to a better understanding of the reaction mechanism.

geometries of the reactants. Furthermore, the Raman spectra of the reactant and product are distinctively different, which facilitates the detailed investigation of the dynamics and mechanism of the reaction in detail. Among carboxylic acids, 1,3-diacids readily decarboxylate over several hours upon heating to 120−180 °C.30 For aromatic carboxylic acids, however, much harsher conditions using Cu or Pd catalysts are usually required.30−32 In this work, we demonstrate that the difficulties associated with the cleavage of the C−C bond are readily overcome through the use of surface plasmons. To the best of our knowledge, the plasmon-driven decarboxylation reaction of MBA has been reported for the first time by Yao and co-workers.33 Zhao and co-workers also observed Raman peaks that correspond to products from the decarboxylation of MBA; however, they wrongly assigned these peaks to the charge transfer SERS of MBA.34 Yao and co-workers prepared AuNP (30 nm) monolayer films on Si wafers and adsorbed MBA molecules onto the AuNPs. Although they correctly identified the new Raman peaks as corresponding to the decarboxylation product, BT, following irradiation, the dependence of the reactivity on experimental parameters is rather ambiguous. Their study also lacks an in-depth investigation into how the reaction occurs and what the key reaction parameter is. This limitation partly arises from their nanostructure system. The nanogaps (locations, gap distances, etc.) in their AuNP monolayer films are poorly defined and very inhomogeneous. As a result, it is not clear where the SERS signal arises from, how the MBA molecules in the probe volume are oriented, and how they interact with the AuNPs. In this work, we prepare more controlled nanogaps and probe plasmon-driven reactions in these nanogaps in situ (Figure 1). To achieve this, we use nanoparticle-on-mirror (NPoM) systems. The reactant molecules form highly ordered self-assembled monolayers (SAMs) on Au substrates. AuNP adsorption onto these SAMs creates nanogaps between the AuNP surfaces and Au substrates. Because SAM formation and AuNP adsorption are carried out sequentially, the MBA molecules are oriented in the nanogaps such that their carboxylic acid groups closely face the AuNPs. These welldefined nanogap structures are irradiated by the reaction laser,

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RESULTS AND DISCUSSION Observation of Decarboxylation Reaction. We constructed NPoM systems to explore the plasmon-driven decarboxylation reaction of MBA in well-controlled nanogaps. Highly ordered MBA SAMs are prepared on Au substrates (150 nm film thickness); subsequently, AuNPs (61 nm) are adsorbed onto the SAMs. As a result, nanogaps are formed between the AuNPs and the Au substrate. A SAM of the reactant MBA molecules exists inside the nanogaps. Such small nanogaps not only promote plasmon-driven reactions using excited plasmons but also enable the SERS detection of the reaction product. We first acquired SERS spectra of MBA in the nanogaps prior to irradiation to gain information on the initial MBA molecular state (Figure 2a, bottom panel). The Raman spectra are dominated by two large peaks at 1076 and 1588 cm−1 that are assigned to a CCC in-plane bending and C−S stretching combination band (δCCC + νCS) and the CC stretching mode of the benzene ring (νCC), respectively.35,36 The absence of vibrational peaks associated with S−H bonds (δSH at 917 cm−1 and νSH at 2567 cm−1) indicates that the MBA is chemisorbed on the Au substrate through Au−S bonding to form a stable SAM.37 The very small peak at 718 cm−1 (CCC out-of-plane bend, γCCC) also suggests that MBA adsorbs onto the Au surface in an upright structure rather than a horizontal one, which facilitates intermolecular π−π interactions leading to a well-packed SAM structure. The presence of CO stretching (νCO, 1716 cm−1), and COO− stretching (νCOO−, 1397 24716


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cm−1) and bending (δCOO−, 847 cm−1) bands indicates that MBA probably exists as a mixture of protonated and deprotonated forms.35,38 The SERS spectrum of MBA in the nanogaps changes upon irradiation of the NPoM system with the 785 nm (15 mW) laser. Figure 2a displays the evolution of the SERS spectrum of MBA in the NPoM system as the irradiation time is increased from 0 to 10 min (middle panel); the emergence and rise of new peaks at 419, 998, 1022, and 1573 cm−1, indicated by asterisks, are the most pronounced changes. Concomitantly, a few peaks that correspond to MBA (521, 1397, and 1716 cm−1) are observed to decrease in intensity (an enlarged spectrum is shown in Figure S1). By comparing these spectra with the SERS spectra of a few possible reaction products including benzenethiol (BT), 4-mercaptophenol (MP), and biphenyl-4-thiol (BPT), we find that the SERS spectrum obtained after irradiation for 10 min contains the spectral features of BT (top panel in Figure 2a; see also Figure S2 for the spectra of other species). The peaks at 419, 998, 1022, and 1573 cm−1 are assigned to the νCS + δCCC, δCCC, δCH, and νCC modes of BT, respectively.37 The observed spectral changes strongly suggest that MBA inside the nanogap of the NPoM system transforms into BT upon irradiation at 785 nm. From the relative peak intensity of the 998 cm−1 band against the 1076 cm−1 band, which is common to both MBA and BT, we estimate that 34 ± 2% of the MBA is converted into BT by irradiation at 785 nm. We determine how much irradiation time is required to produce BT from MBA. Figure 2b shows the relationship between the intensity of the peak at 998 cm−1 (normalized against the 1076 cm−1 peak at t = 0) and irradiation time. The rise is best fitted to an exponential function with a rate constant of 0.59 ± 0.14 min−1. This result suggests that the irradiation-induced transformation of MBA into BT in the NPoM nanogap follows pseudo-first-order reaction kinetics with excess photon flux. The intensity of the peak at 998 cm−1 monotonically increases, and the reaction becomes faster, with increasing laser irradiation power (Figure S3).

Figure 2. (a) SERS spectrum of MBA reactants in NPoM nanogaps and peak assignments (lower panel). The SERS spectrum evolves as the NPoM is irradiated at 785 nm for the time shown on the right (middle panel). In particular, new peaks emerge, indicated by asterisks. The top panel shows the SERS spectrum of BT for comparison with the reaction product from the irradiation of MBA in the NPoM. (b) Intensity of the Raman peak at 998 cm−1 as a function of irradiation time at 785 nm. This plot reflects the rate at which the BT product is formed by irradiation. The rise is best fitted to an exponential function with a rate constant of 0.59 ± 0.14 min−1.

Figure 3. (a) Evolution of the Raman spectrum of MBA in the NPoM system upon irradiation at 532 nm over 10 min (green spectra), followed by irradiation at 785 nm at the same spot for a further 10 min (red spectra). The right panel displays the evolution of the intensity of the Raman peak at 998 cm−1 upon irradiation. The Raman acquisition conditions are identical for all spectra (λexc = 785 nm, exposure time = 3 s). (b) Comparison of decarboxylation reactivity between different irradiation wavelengths. The intensities of the product Raman peak at 998 cm−1 are plotted against the irradiation time at three different wavelengths, 785, 633, and 532 nm. Exponential fitting yields rate constants of 0.60 ± 0.22 min−1 (785 nm), 0.35 ± 0.04 min−1 (633 nm), and 0.17 ± 0.04 min−1 (532 nm). 24717


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Figure 4. (a) Transmission electron microscopy (TEM) images of silica-encapsulated AuNPs (Au@SiO2); the shells are 2.8 ± 0.6 nm thick. (b) Raman spectra of MBA molecules between Au substrates and Au@SiO2 before and after irradiation at 785 nm. (c) Control experiment from the same SAMs but with AuNPs adsorbed, instead of Au@SiO2. In the case of the Au NPoM system shown in (c), irradiation yields the BT product whose peaks are indicated by the arrows in the Raman spectrum, whereas no reaction is observed for the Au@SiO2 NPoM system, as shown in panel (b).

Wavelength Dependence. Our observations evidently indicate that irradiation at 785 nm drives the decarboxylation of MBA to BT in the nanogaps between the AuNPs and the Au substrate. Because this reaction occurs at the nanogap junction, we believe that surface plasmons excited at 785 nm play key roles in driving this decarboxylation reaction. We confirm this hypothesis by changing the reaction laser wavelength and correlating the wavelength-dependent reaction yield with the resonance conditions of the prepared NPoM structure. We find that decarboxylation is very inefficient upon irradiation at 532 nm, in marked contrast to irradiation at 785 nm. The NPoM system with the MBA molecules in the nanogap is irradiated at 532 nm (15 mW) for 10 min; the changes in the MBA molecules are monitored by Raman spectroscopy at an excitation wavelength of 785 nm. Figure 3a (green lines) shows that the SERS spectrum remains largely unchanged when the NPoM is irradiated at 532 nm, indicating that irradiation at 532 nm does not induce the decarboxylation of MBA to BT at the nanogap. The laser wavelength is then switched from 532 to 785 nm, while identical conditions are maintained (position, focus, laser power, Raman acquisition conditions, etc.). We find that the peaks at 998 and 1022 cm−1 grow with increasing irradiation time. Plotting the peak intensity at 998 cm−1 from the commencement of laser irradiation clearly shows the onset of the decarboxylation reaction as the wavelength is switched from 532 to 785 nm (Figure 3a right panel). We also conducted experiments using a 633 nm laser. Figure 3b shows the intensities of the peak at 998 cm−1, when irradiated at 785, 633, and 532 nm, as functions of time. Clearly, irradiation at 785 nm is most effective for driving the decarboxylation reaction. Fitting these plots to exponential functions yields rate constants of 0.60 ± 0.22 min−1 (785 nm), 0.35 ± 0.04 min−1 (633 nm), and 0.17 ± 0.04 min−1 (532 nm). The reaction is very slow and ineffective when irradiated at 532 nm.

Baumberg and co-workers determined the plasmon coupling of NPoMs with AuNPs of different sizes using dark-field scattering spectroscopy.39 The NPoM with 61 nm AuNPs, as used in our experiments, have a strong dipolar coupling plasmon mode at about 730 nm. In our case, the resonance wavelength is more likely to be shifted to an even longer wavelength because our gap is smaller than those of the Baumberg NPoM systems (MBA vs BPT).40 Consequently, excitation at 785 nm is very close to the resonance of the coupled plasmon mode of our NPoM, which leads to the best reactivity. Excitation at 633 nm is removed from this resonance, and plasmons are effectively not excited at 532 nm because 532 nm is completely off-resonance compared to the plasmon energy of the NPoM. This result conclusively reveals that the decarboxylation of MBA in NPoM is driven by plasmon excitation. Furthermore, the coupled plasmon mode, rather than the surface plasmons of individual nanoparticles, requires excitation for this plasmon-driven reaction. Hot Charge Carrier Transfer Reaction or Thermal Reaction? From the results presented in Figure 3, we learned that surface plasmons induced by resonant excitation of NPoM drives the decarboxylation reaction of MBA inside the nanogaps. As we described in the Introduction, nonradiative decay of surface plasmons generates either hot charge carriers or localized heat.41 We use silica-coated AuNPs (Au@SiO2) to determine which decay pathway is responsible for the decarboxylation reaction. Silica shells are thermally conductive but electrically insulating;42−44 hence, plasmon excitation of Au@SiO2 facilitates thermal reactions at the surface through localized heating, whereas it hinders hot charge carrier-induced reactions because hot charge carrier transport to the surface is blocked.45 A comparison of the reactivities of the Au- and Au@ SiO2-NPoMs should reveal which decay pathway is responsible for the observed plasmon-driven decarboxylation reaction. The AuNPs (61 nm) are encapsulated with silica shells that are 2.8 ± 0.6 nm thick (Figure 4a); these Au@SiO2 particles are adsorbed onto the MBA SAMs. For comparison, we also 24718


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ACS Applied Materials & Interfaces prepared bare-AuNP-adsorbed MBA SAMs. The intensity of the Au@SiO2 NPoM Raman spectrum acquired before irradiation is about one order of magnitude less than that of the Au NPoM because the gap between the Au substrate and the AuNP is larger in the silica-shell containing system, which results in weaker SERS enhancement (bottom panels in Figure 4b,c). No Raman peaks attributable to the BT product are observed when Au@SiO2 NPoM is irradiated at 785 nm (25 mW) for 10 min (Figure 4b, upper panel), whereas peaks at 998 and 1022 cm−1 that correspond to the BT product are clearly observed in the spectrum when the bare Au NPoM is used (indicated by arrows in Figure 4c, upper panel). The increased nanogap due to the silica shell shifts the coupled plasmon mode of the Au@SiO2 NPoM to the shorter wavelength region, compared to that of the bare Au NPoM. The consequent off-resonance with 785 nm excitation could be the reason for the lack of reactivity of the Au@SiO2 NPoM. Thus, we explored shorter excitation wavelengths, such as 633 and 532 nm (Supporting Information). We found that the decarboxylation reaction does not occur even when the Au@ SiO2 NPoM is irradiated at 633 or 532 nm (Figure S4). There are many factors to consider for the role of the silica shell in this inhibition of the reaction, such as the porous structure, tunneling of electrons, and binding of the carboxylic acid group to the silica surface (Supporting Information). However, the most likely scenario is that the silica shell prevents the transfer of hot charge carriers from the AuNPs to the surface, which significantly hampers the decarboxylation reaction, as reported by Takeyasu et al.45 Therefore, we conclude that hot charge carrier transfer, rather than plasmonic heating, is responsible for the decarboxylation reaction observed in the Au NPoM. Hot Charge Carrier Transfer at the Interface. We further investigated how hot charge carriers are transferred to the reactant molecules, MBA, in the nanogap of the NPoM system and thereafter how the decarboxylation reaction proceeds. We note that bond cleavage associated with the decarboxylation reaction often requires the formation of radicals. Hence, it stands to reason that the hot charge carriers from the AuNPs are involved in the formation of radicals. We hypothesize that the radical is more readily formed when the carboxyl group is in the form of the carboxylate anion (−COO−) rather than the neutral carboxylic acid (−COOH). If this is the case, increasing (decreasing) the population of carboxylate in the SAM sandwiched between the AuNPs and the Au substrate will promote (hinder) the decarboxylation reaction. We tested this hypothesis by exploring the pH dependency of reactivity, and by replacing the carboxylic acid (−COOH) with the corresponding methyl ester (−COOCH3). We prepared NPoM systems containing MBA SAMs under basic (pH 12), neutral (pH 7), and acidic (pH 3) conditions. The SERS spectra acquired prior to irradiation clearly reveal changes in the protonation state of the carboxylic acid moiety of the MBA in NPoM according to pH (Figure 5, gray spectra in each pair). The SERS spectra of MBA under basic and neutral conditions exhibit distinctive high-intensity peaks at 1419 cm−1 (νCOO− mode) and no peak in the νCO region, indicating that MBA is predominantly in the carboxylate form (−COO−). In contrast, under acidic conditions, the νCOO− peak is significantly less intense and a νCO peak is observed, which is characteristically associated with the presence of the carboxylic acid group (−COOH). The deprotonated MBA

Figure 5. SERS spectra of MBA in NPoM prepared in basic (pH 12), neutral (pH 7), and acidic (pH 3) media. The gray spectrum in each pair is that prior to irradiation, while the black spectrum is that following irradiation at 785 nm (15 mW) for 20 min. The dotted vertical lines mark the vibrational frequencies that correspond to νCOO− and νCO modes, while the arrows highlight the vibrational peaks that correspond to the BT product.

(basic and neutral conditions) converts into BT far more efficiently than the protonated form when irradiated at 785 nm. After irradiation, the Raman peaks at 998 and 1022 cm−1, which are attributed to the BT product, are much more intense in the spectrum of MBA in alkaline and neutral media than that of MBA under acidic conditions. These results clearly indicate that the carboxylate form is the key species involved in the plasmon-driven transformation of MBA into BT. Replacement of the carboxylic acid group (−COOH) in MBA with the corresponding methyl ester (−COOCH3) significantly hinders the formation of the carboxylate anion, irrespective of the environment. Hence, decarboxylation is unlikely if the carboxylate is required for the reaction to proceed. We tested this hypothesis with an NPoM constructed with AuNPs on SAMs of methyl 4-mercaptobenzoate (MBAMe, SH−C6H4−COOCH3). Figure 6 displays the Raman spectral evolution of the NPoM upon irradiation at 785 nm (15 mW) for 10 min. We find that the νCO peak persists throughout the irradiation process and that no peaks around the νCOO− region are detected, indicating that MBA-Me is retained in the ester form. Consequently, decarboxylation does not occur, and no peaks that are assignable to the decarboxylation product (998 and 1022 cm−1) are observed in Figure 6. Therefore, we conclude that the carboxylate anion is the key species involved in the plasmon-driven decarboxylation of MBA in nanogaps. Direct Transfer vs Oxygen-Mediated Reaction. So far, we have discovered that hot charge carriers from plasmon excitation and carboxylate anions play crucial roles in the decarboxylation reaction of MBA in NPoM. It is natural to assume that hot charge carriers that are transferred to the carboxylate anion in the next step lead to decarboxylation. However, it is prudent to consider another possible hot charge carrier acceptor, namely molecular oxygen in air. Electron transfer from AuNPs to ambient oxygen is reportedly a key intermediate step in many plasmon-driven reactions, particularly those associated with oxidation.24,46,47 We examine the possibility that the observed decarboxylation reaction is mediated by superoxide radical anions produced by the 24719


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completely purged with Ar (Figure 7b). This result appears to indicate that ambient oxygen is necessary for decarboxylation. However, we note that the νCO peak is distinctly more intense when air is replaced by Ar. In other words, MBA remains in its carboxylic acid form (−COOH) in Ar, not in the carboxylate anion form. This observation implies that the acid form, rather than the lack of oxygen, is possibly the reason for inefficient decarboxylation in Ar. We tested this hypothesis by adding water vapor to the Ar gas by bubbling it through water; peaks corresponding to the BT product are observed in the presence of water vapor, even in the absence of oxygen (Figure 7c). MBA is deprotonated readily in the presence of water, and the resulting carboxylate accelerates the subsequent decarboxylation process. Consequently, aerobic oxygen is not a factor that contributes to driving the decarboxylation reaction in nanogaps; rather, the formation of the carboxylate anion is more important. These results suggest that the hot charge carriers (probably hot holes in this case) induced by plasmon excitation of AuNPs are transferred directly to carboxylate anions (not to molecular oxygen), that ultimately undergo C− C bond cleavage to yield CO2 and BT. Mechanism of the Plasmon-Driven Decarboxylation Reaction. By considering our combined experimental observations, we propose a mechanism for the decarboxylation of MBA in nanogaps as follows. First, the deprotonated form of MBA, i.e., the carboxylate, is the key species required for this reaction to proceed (Figure 8a). The high reaction yields observed under alkaline or neutral conditions (Figure 5), and the lack of reaction when the carboxylic acid group is replaced with the ester group (Figure 6) strongly support the importance of the carboxylate. Resonant excitation of coupled plasmon modes generates hot charge carriers in the AuNPs. These hot charge carriers are transported to the AuNP surfaces and subsequently transferred to the molecules near the surfaces (Figure 8b). This plasmondriven process is unequivocally corroborated by the dependence of the reaction efficiency on the excitation wavelength (Figure 3) and the lack of reaction when hot charge carrier transfer from the AuNPs is blocked by silica shells (Figure 4). The direct transfer of hot holes to carboxylate anions produces carboxyl radicals (Figure 8c). The depletion of these hot holes in the AuNPs is compensated for by hot-electron transfer to protons. Direct charge transfer to the carboxylate

Figure 6. Raman spectra of the NPoM system with the carboxylic acid of MBA (−COOH) replaced by the methyl ester (−COOCH3, the methyl group being highlighted as the red bar in the illustration on the top) when irradiated at 785 nm for the times indicated on the right. The high intensity of the νCO peak and the lack of a νCOO− peak indicate that MBA-Me remains in its ester form. The lack of peaks at 998 and 1022 cm−1 that correspond to the BT product clearly shows that the decarboxylation reaction requires the formation of the carboxylate.

transfer of hot electrons from the AuNPs to aerobic oxygen. To distinguish between the hot charge carrier transfer pathways involving carboxylate anions and molecular oxygen, we performed experiments in an oxygen-free atmosphere. The NPoM sample is placed in a homebuilt gas flow cell (Figure S8) and air is purged with Ar. The SERS spectrum is monitored during irradiation of the sample at 785 nm. When the MBA NPoM system is irradiated in air (prior to purging), peaks (998, 1022 cm−1) that correspond to the BT product are observed to grow in the SERS spectrum (Figure 7a). On the other hand, no BT-related peaks are observed when the cell is

Figure 7. Evolution of the Raman spectrum of MBA in NPoM upon irradiation at 785 nm for the listed times on the right side of each panel. The NPoM sample in a gas flow cell is exposed to (a) air, after which the cell is replenished with (b) Ar, and then (c) Ar bubbled through water. The arrows indicate the positions of the peak corresponding to the BT product. In (b), the dotted line highlights the pronounced νCO peak, consistent with the carboxylic acid (−COOH) form of MBA under these conditions. 24720


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Figure 8. Proposed plasmon-driven decarboxylation reaction process. (a) AuNP is positioned close to the carboxylic acid moiety of MBA in the NPoM structure by adsorption onto the MBA SAM on the Au substrate. The deprotonation of MBA in the nanogap to the carboxylate anion and a proton is a prerequisite for the reaction. (b, c) Resonant excitation of the coupled plasmon mode of NPoM creates hot charge carriers. Hot holes are transferred to carboxylate anions, while hot electrons are transferred to protons, leading to the formation of carboxyl radicals and hydrogen atoms. (d) Homolytic cleavage of the C−C bond yields (e) CO2 and the 4-mercaptophenyl radical. (f) 4-Mercaptophenyl radical then combines with a hydrogen atom to form the BT product.

shows that irradiation at 785 nm does induce the β-cleavage reaction for MBnOH, leading to the formation of BT under alkaline conditions although the product yield is much smaller than the decarboxylation of MBA. The lower yield is presumably due to the higher pKa value of MBnOH (pKa = 15.4),48 compared to that of MBA (pKa = 4.79).49 The reaction does not occur under acidic conditions, supporting our proposed mechanism where the hydroxide anion is the key intermediate to accept the hot holes from the AuNPs. More indepth investigations are currently underway to reveal the detailed energetics of the β-cleavage reaction of MBnOH in comparison with the decarboxylation reaction of MBA; the results will be published separately. Nevertheless, our observation of the β-cleavage reaction product from MBnOH in nanogaps validates the proposed mechanism in Figure 8. Lingering Issues. Although the hot-carrier transfer mechanism explains the bond cleavage reactions reasonably well as discussed above, there are still remaining issues to be resolved. First, we cannot exclude the effect of strong electromagnetic fields on the reactions. We were able to discriminate the two major nonradiative plasmonic decay pathways (i.e., hot charge carriers and heat) that may lead to the reaction using the silica-coated AuNPs. However, radiative decay is another important decay pathway that should not be overlooked. Recently, Kim and co-workers demonstrated that the plasmon-induced dissociation of disulfide occurs via direct intramolecular excitation of the molecule by the plasmonic field.50 Radiative decay of the excited plasmon generates intense localized electromagnetic fields around the nanoparticles. When the frequency of the plasmonic field matches with the transition energy of the adsorbed molecules to the excited state that leads to products, the reaction is accelerated. However, distinguishing between the hot-carrier-transfer mechanism and the direct intramolecular excitation mechanism is challenging. Electronic structure calculations of the adsorbed molecules and more extensive wavelength-dependent experiments are required. Second, a fundamental understanding of the formation and transfer dynamics of the hot charge carriers is highly desired. For example, it is not clear yet how hot charge carrier transfer occurs overcoming the fast electron−hole recombination rate. In this regard, it is interesting to note that recent studies revealed that it is important to make both reduction (hotelectron transfer) and oxidation (hot-hole transfer) pathways available.47,51 In these studies, halide anions play an important role in reacting with hot holes. The depletion of hot holes

without the involvement of ambient gas species is evidenced by experiments under Ar and in the presence of water vapor (Figure 7). The carboxyl radical undergoes the homolytic cleavage of the C−C bond (Figure 8d) to form CO2 and the 4mercaptophenyl radical (Figure 8e), which then combines with a hydrogen atom, to give the BT product (Figure 8f). Applications of the Proposed Mechanism to Other Bond Cleavage Reactions. An understanding of the mechanism allows us to explore other reactions that may occur through a similar process. The proposed mechanism for the decarboxylation reaction (Figure 8) can be applied to the β-cleavage reaction of 4-mercaptobenzyl alcohol (MBnOH) in nanogaps, as illustrated in Figure S5. To examine this possibility, we constructed the NPoM with MBnOH and adjusted the protonation state of the hydroxyl group. Figure 9

Figure 9. Application of the proposed mechanism in Figure 8 to the plasmon-driven β-cleavage reaction of MBnOH in nanogaps. The fainter spectra represent the SERS spectra of MBnOH prepared in NPoM under alkaline (pH = 12.4) and acidic (pH = 3.0) conditions. The bolder spectra are the ones acquired after irradiation on each sample at 785 nm for 8 min. The vertical dotted lines indicate the Raman peak position of the BT product from the β-cleavage reaction of MBnOH. The plasmon-driven β-cleavage reaction does occur to MBnOH under alkaline conditions although the product yield is lower than the decarboxylation of MBA. 24721


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Silica-coated AuNPs (Au@SiO2) are prepared by the method reported by Tian and co-workers.54 AuNPs are first coated with (3aminopropyl)trimethoxysilane, after which the addition of sodium silicate solution leads to condensation and the formation of silica shells around the AuNPs that are 2.8 ± 0.6 nm thick. NPoM systems are constructed by adsorbing AuNPs onto MBA SAMs formed on Au substrates. The Au substrates (150 nm thick Au film on 4″ Si wafers) were purchased from the National Nanofab Center (Daejeon, Korea) and cut into 1 cm × 1 cm pieces. After cleaning, each Au substrate is immersed in a solution of MBA in ethanol (2 mL, 1 mM) for 16 h to form SAMs of the MBA molecules. Drop casting AuNPs (3 μL) onto the SAMs completes the creation of nanogaps that are defined by the AuNPs and the Au substrates, spaced by MBA SAMs. Au@SiO2 NPoMs are constructed in a similar manner. After drop casting, the NPoM system is vacuum-dried for 1 h. On average, ∼150 AuNPs are located in the probe region (Figure S7). Alkaline (pH 12), neutral (pH 7), and acidic (pH 3) solutions are prepared for pH-dependent experiments using NaOH, deionized water, and HCl, respectively. An aliquot of each solution is dropped onto pre-formed NPoM samples, allowed to react for 10 min, after which the samples are dried under mild vacuum conditions. The prepared NPoM system is placed under a microscope objective for laser-induced reactions and Raman spectroscopy. Three lasers, operating at wavelengths of 785 nm (Invictus, Kaiser Optical Systems), 633 nm (633-MLD, Cobolt), and 532 nm (Excelsior 532, Spectra-Physics), are used for these reactions. The laser beam is guided into the backport of the microscope (Leica DMLP) illuminator and focused to a diameter of ∼5 μm onto the NPoM through an objective (Leica NPLAN, 50×, N.A. 0.75). The power at the sample is adjusted to 15 mW for all three wavelengths, which corresponds to 2800 W/cm2. Raman spectra are acquired by exciting the samples at 785 nm (6.0 mW) through a 50× objective. Raman scattering is collected by the same objective and transmitted to a spectrometer with a spectral resolution of 5 cm−1 (Raman Rxn 1, Kaiser Optical Systems). The total exposure time is typically 3 s unless noted otherwise. Due to inhomogeneity in the AuNP distribution and inherent fluctuations in SERS intensity, we acquired Raman spectra from at least three different spots to ensure that the Raman spectral features are consistent. The ambient conditions of the NPoM system are adjusted using a homebuilt gas flow cell (Figure S8). After the NPoM sample is placed inside the cell, the cell is purged with Ar gas at a rate of 300 cm3/min for 30 min. The purged gas exits through silicone vacuum oil to prevent the backflow of air or moisture. To introduce water vapor into the cell, a water bubbler is inserted in the Ar gas line. The cell is designed to be mountable on a microscope stage. Because of the distance between the quartz window of the cell and the sample, we use a 10× objective lens (Leica NPLAN, N.A. 0.25, w.d. 17.6 mm) for proper sample focusing. Accordingly, the laser power is raised to 50 mW to induce reactions. Raman spectra are acquired using the same objective lens at a laser power of 6.0 mW and a total acquisition time of 3 s. TEM (JEM-2100, JEOL) is used to characterize the sizes and shapes of the AuNPs and Au@SiO2. We determined the distributions of AuNPs on the SAMs by scanning electron microscopy (SEM, Sigma, Zeiss). UV−vis spectroscopy (Lambda 25, PerkinElmer) is used to determine the surface plasmon resonance properties of nanoparticles at the ensemble level.

minimizes the rate of the electron−hole recombination and thus increases the number of hot electrons, which consequently accelerates the reduction of H+ or O2, leading to the production of ABT or 4-nitrobenzenethiol. The presence of both channels seemingly keeps the electrons and the holes separated. Notably, the mechanism we propose also includes both transfer pathways (Figure 8b). Other issues regarding the hot-carrier transfer mechanism include, but are not limited to, the structural and morphological parameters of nanostructures that determine the spatial and the energy distribution of hot carriers, differences between the hotelectron and hot-hole transfer, and so on.52

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CONCLUSIONS In this study, we investigate the decarboxylation reaction of MBA in NPoM structures by irradiation at 785 nm and reveal the complete decarboxylation mechanism through systematic studies. We prepare NPoM systems by adsorbing AuNPs onto MBA SAMs on Au substrates. The sample is laser irradiated at 785, 633, or 532 nm, and probed at the same spot by SERS, excited at 785 nm. Irradiation of MBA in NPoM at 785 nm produces new Raman peaks, notably at 998 and 1022 cm−1, that are identified as corresponding to BT by comparison with an authentic sample. These results clearly indicate that irradiation at 785 nm induces the decarboxylation of MBA in the nanogaps of the NPoM system. The best reaction yield is observed when the sample is irradiated at the wavelength that corresponds to the resonance energy of the plasmon coupling mode of the NPoM system. The reaction yield decreases as the irradiation wavelength moves away from the energy of the plasmon coupling mode. These results suggest that the observed reaction is driven by the excitation of the surface plasmons of NPoM. Excited surface plasmon decay generates hot charge carriers, and their transfer to MBA induces decarboxylation. This transfer is blocked and no BT products are observed when the AuNPs are coated with silica shells. We also find that the charge-transfer acceptor is the carboxylate anion of MBA through pH adjustment, esterification, and ambient gas-exchange experiments. Hot holes and hot electrons from the AuNPs are transferred to carboxylate anions and protons to produce carboxyl radicals and hydrogen atoms, respectively. Homolytic C−C bond cleavage of the carboxyl radical yields CO2 and the BT product. We find that the proposed mechanism can be applied to other bond cleavage reactions. The MBnOH in nanogaps undergoes the βcleavage reaction to yield the BT upon excitation of surface plasmons under alkaline conditions. A fundamental understanding of the mechanism for the plasmon-driven decarboxylation reaction of MBA in nanogaps, as a model system for studying the relationship between nanostructures and hot charge carrier dynamics, will lead to a wide range of applications and eventually contribute to the development of highly efficient plasmon-based photovoltaic or photocatalytic materials and devices.

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ASSOCIATED CONTENT

S Supporting Information *

MATERIALS AND METHODS

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05509.

Detailed experimental methods are provided in the Supporting Information. Briefly, AuNPs are synthesized using the seeded growth method.53 The citrate-reduction of HAuCl4 produces AuNP seed particles. Repeated addition of citrate and HAuCl4 to the seed solution at 90 °C yields AuNPs (500 mL, 50 pM) with diameters of 61 ± 8 nm (Figure S6).

Expanded SERS spectra; SERS spectra of possible products; power-dependence data; effects of the laser wavelength on the reactivity of MBA in Au@SiO2 24722


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NPoM; mechanism of the plasmon-driven β-cleavage process for MBnOH; detailed experimental methods (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sangwoon Yoon: 0000-0001-5705-4569 Author Contributions

This manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support from the National Research Foundation (NRF) of Korea (Grant No. 2016R1A2B2007259) for this work. This research was also supported by Chung-Ang University Research Grants (2017). We thank Prof. Seung Wook Ham for helpful discussions.

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REFERENCES

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