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Specific photocatalytic reaction of p-methyl thiophenol and related molecules under the gap mode resonance
Chemical Physics Letters 730 (2019) 568–574
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Research paper
Specific photocatalytic reaction of p-methyl thiophenol and related molecules under the gap mode resonance Kanae Tabei, Keitaro Akai, Masayuki Futamata
T
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Saitama University, Saitama 338-8570, Japan
H I GH L IG H T S
information was obtained for the plasmon induced photocatalytic reaction. • Vital thiophenol is site-selectively oxidized to p-MBA in a gap mode resonance. • P-alkyl oxidation is not due to thermal heating but due to hot carrier transfer. • The oxidation is accelerated by oxygen, while partly suppressed by nitrogen gas. • The • In contrast, o-, m-, p-mercaptobenzyl alcohol are oxidized to o-, m- and p-MBA.
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasmon induced photocatalytic reaction P-methyl thiophenol Gap mode Surface enhanced Raman scattering
Para-alkyl thiophenol (p-AlTP) molecules with normal and branched alkyl groups are selectively oxidized into pmercaptobenzoic acid under the gap mode resonance, whereas o- and m-methyl TP (o-, m-MeTP) do not show such reactions [1]. We investigated here detailed properties of the photocatalytic reaction specific to p-AlTP in terms of the following contrasting observations. (1) The oxidation of p-MeTP was not induced by thermal heating of the samples up to 373 K. Subtle temperature increase during the oxidation was also corroborated by the measurements of the Stokes and anti-Stokes scattering intensity. (2) The oxidation of p-MeTPs was accelerated by oxygen atmosphere at room temperature, whereas nitrogen atmosphere yielded an intermediate species. (3) Ortho-, m-, and p-mercaptobenzyl alcohol molecules were oxidized to corresponding o-, m- and pmercaptobenzoic acid (p-MBA) with no definite site selectivity.
1. Introduction Metal nanoparticles (MNPs) are utilized solely or after immobilization on metal oxides like SiO2 or TiO2 for catalyzing chemical transformation of organic molecules at the temperatures lower than ∼370 K. For instance, silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) catalyze the oxidation of alcohol to aldehyde (the Wolf rearrangement) [2] and the Diels-Alder reaction [3]. Photocatalytic reactions can also be implemented by MNPs, which are driven by hot carriers, electrons and holes excited during dephasing process of surface plasmons (SP). The efficiency of the photocatalytic reactions supported by SP is inherently restricted due to extremely short lifetime of SP ≤ 10 fs [4–7]. Prolonged lifetime of hot carriers by chemical interface dumping (CID) [6,7], chemical interface scattering (CIS), or particular nanostructures such as closely adjacent metal nanoparticles yielding 1–2 ps [8,9], therefore, facilitates hot carrier transfer to
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adsorbates to cause chemical reaction, which is also known as the origin of chemical enhancement in surface enhanced Raman scattering (SERS) on metal surfaces with atomic scale roughness [10]. Although the role of the lifetime of hot carriers is still a matter for debate, crucial reactions have been reported for plasmon induced oxidation of ethylene [11] or dissociation of hydrogen [12]. Inherently, a localized surface plasmon (LSP), which generates high density of hot carriers and enormously enhanced electric field available for highly-sensitive spectroscopy, is quite suitable for in-situ elucidation of photocatalytic reactions on MNPs. For instance, p-aminothiophenol (PATP) adsorbed on silver and gold surfaces is dimerized into p, p′mercaptoazobenzene (DMAB) under laser irradiation, in which reaction process was elucidated using new SERS peaks appeared at 1142, 1388, and 1432 cm−1 [13–15]. Recent density functional theory (DFT) calculations, combined with SERS and other analytical methods, for metal clusters adsorbed by PATP provide detailed information on hybridized
Corresponding author. E-mail address: [email protected] (M. Futamata).
https://doi.org/10.1016/j.cplett.2019.06.052 Received 29 April 2019; Received in revised form 17 June 2019; Accepted 18 June 2019 Available online 19 June 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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MeTP, o-, m- and p-mercaptobenzyl alcohols (o-, m- and p-MBAl) was formed on fresh Ag films by immersing the Ag(t = 45)/Ge(t = 1)/Si substrates in ethanol solutions of corresponding thiol molecules (1 mM) for 1 h, and rinsed thoroughly with neat ethanol to remove excess and unbound thiophenol molecules. Subsequently, the thiophenol-SAM coated Ag/Ge/Si substrates were immersed in an AgNP suspension for 1 h to immobilize AgNPs on Ag films through van der Waals interaction [22]. Surface coverage of AgNPs on Ag films given by the ratio of occupied area by AgNPs to the entire surface area of Ag films was adjusted, typically ∼ 5%, by the concentration of AgNP suspensions [1].
electronic state and hot carrier transfer between metals and adsorbed molecules during the dimerization of PATP [14,15]. Among various SERS probes such as roughened metal surfaces, MNPs and arrays of metal nanostructures, the gap mode plasmon generating markedly enhanced electric field by a factor of 104–105 at a nanogap between MNPs on various transition metal substrates [16–18] plays a salient role for this purpose. Indeed, Al, Ni, Zn as well as Ag, Au and Cu substrates provide promising SERS enhancement factors of 107–109 for adsorbed thiophenol (TP) molecules using AuNPs [19,20]. Under the gap mode geometry, we recently found that p-alkyl thiophenol (p-AlTP) molecules, such as methyl, ethyl, iso-propyl, or tertiary butyl groups in AgNP/p-AlTP/Ag film (t = 45 nm)/Ge film (t = 1 nm)/Si substrates are oxidized into p-mercaptobenzoic acid (p-MBA) by irradiating laser light at 532 nm (< ∼1 μW/μm2). In contrast, o- and m-methyl TP (o-, mMeTP) molecules do not show the oxidation [1]. AgNPs were crucial to cause the oxidation of p-MeTP on Ag films [1].
2.2. Spectral measurements A gap mode for AgNPs/p-MeTP-SAM (or o-, m-, p-MBAl)/Ag/Ge/Si substrates was excited in the air with a solid laser (λ = 532.0 nm, typically with a power (Ip) of 10 mW corresponding to a power density of ∼0.8 μW/μm2 using a conventional objective of f = 50 mm) under external geometry at an incident angle of ∼50° with p-polarized light. SERS spectra of p-MeTP were measured using a micro Raman spectrometer consisting of conventional transfer optics, an upright microscope (Olympus), a single polychromator (Bunko-Keiki MK-300), and a charge coupled device (CCD) detector (Andor, iVAc). Photocatalytic reaction was monitored by observing Raman spectral changes of pMeTP molecules in AgNPs/p-MeTP (or o-, m-, p-MBAl)/Ag(45)/Ge(1)/ Si substrates under laser illumination with a constant power (1–50 mW) for 60 min in the air, if otherwise noted.
p-AlTP (p-alkyl thiophenol), p-MeTP (p-methyl thiophenol) for Al=CH3
2.3. Thermal heating Microscope cryostat, MicrostatN (Oxford Instrum.), was employed to heat the sample substrates (AgNP/p-MeTP/Ag/Ge/Si) from room temperature (∼297 K (24 °C)) up to 373 K (100 °C), of which temperature was adjusted with a conventional resistivity heater and a Proportional-Integral-Differential (PID) controller. SERS spectra of the p-MeTP sample were measured through a quartz window (t = 1 mm). The cryostat was also used to investigate the effect of nitrogen and oxygen atmosphere on the oxidation of p-MeTP.
2.4. Laser heating p-MBA (p-mercaptobenzoic acid)
First, intensity ratio of the Stokes and anti-Stokes scattering (IS/IaS) of 384 cm−1 from p-MeTP in AgNP/p-MeTP/Ag/Ge/Si samples, immobilized in the cryostat as aforementioned, was measured at different temperatures between 297 and 373 K using a weak laser power (1 m W, which is much weaker than threshold power of 10 mW for the oxidation) and a notch filter (Iridian Narrow band NF-532 nm, centered at 532 nm with a notch width of 12 nm). The observed intensity ratio IS/ IaS was theoretically evaluated based on the Boltzmann distribution and the asymmetry factor of cross section and enhancement in SERS for the Stokes and anti-Stokes regions [23]. Subsequently, SERS intensity ratio (IS/IaS) was measured during the oxidation excited at 532 nm (1–50 mW) without temperature control. The value IS/IaS was analyzed to evaluate gap mode driven heating of the p-MeTP samples.
Further information such as the influence of substrate temperature, gap mode induced temperature increase, nitrogen or oxygen atmosphere, and different functional group on the oxidation of p-MeTP are prerequisite for comprehensive understanding and utilization of photocatalytic reactions. Here we reports decisive observations that p-alkyl TPs are oxidized not via thermal process but via hot electron transfer from metal to adsorbates, while observing an intermediate species under nitrogen atmosphere and resembled oxidation of mercaptobenzyl alcohol molecules. 2. Experimental 2.1. Preparation of samples AgNPs were synthesized by a citrate reduction method [19,20], in which citrate molecules reduce Ag+ ions, and also stabilize AgNPs in suspensions. The average size of our AgNPs was found to be around 20–30 nm in radii based on scanning electron microscope (SEM) observations. Silver films with a thickness (t) of 45 nm were evaporated in vacuum on Ge (t = 1 nm) pre-deposited on a Si wafer to diminish roughness of Ag films [21], while monitoring the thickness with quartz crystal sensor, Q-pod (Inficon). Self-assembled-monolayer (SAM) of p-
2.5. SERS under different atmosphere Nitrogen (99.99%) or oxygen gas (99.99%) was flowed into the sample compartment of MicrostatN, in which AgNP/p-MeTP/Ag/Ge/Si substrates are immobilized, at a rate of 300 or 1000 mL/min. The gas inside the sample compartment is exchanged 10–20 times per minute, as its volume is ∼ 20 mL.
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Fig. 1. SERS spectra of p-MeT in AgNP/p-MeT/Ag film/Ge/Si substrates under gap mode resonance, at which the excitation laser of 532 nm (1–50 mW) was irradiated under external geometry: (a) at 1 mW, (b) at 10 mW, (c) 20 mW, (d) the peak shift of the marker band at ∼ 1600–1590 cm−1 vs. duration of time for the laser irradiation, showing the oxidation efficiency (see the main text in more detail). Please see also Figs. S1a–g.
3. Results and discussion
negligibly small in our experimental conditions as dwelled in the next section.
3.1. The effect of laser power 3.2. The effect of substrate temperature The stronger laser power density inherently provides the higher efficiency for the oxidation of p-MeTP due to increased density of hot carriers at the nanogap between AgNP and Ag films [24]. We measured SERS spectra of p-MeTP in AgNP/p-MeTP/Ag film (45 nm)/Ge(1 nm)/ Si substrates at various laser power of 1–50 mW (λ0 = 532 nm) in the air (Figs. 1a–c, and S1a–g). Laser power less than 5 mW (∼0.4 μW/ μm2) gave faint spectral changes, namely SERS bands observed at 1600, 1492, 1388, 1221, 1189, 1079, 1016, 795, 620 and 381 cm−1 did not show significant variations by the laser illumination for ∼ 60 min. In contrast, laser power higher than 10 mW (∼0.8 μW/μm2) provided pronounced spectral changes, such as marked downshift of the peak at 1599 cm−1 to 1590 cm−1 in addition to new peaks appeared at ∼ 1370, 1140, 836, 709, 688, 513, and ∼ 354 cm−1, all of which are attributed to p-MBA (Fig. S1h1). Thus, apparent threshold power for the oxidation of p-MeTP is around 10 mW in the air. The SERS peak at 1599 cm−1 of p-MeTP, assigned to a CeC stretching mode coupled with a CeH inplane bending mode of a phenyl ring based on DFT calculations, showed a drastic downshift by about 10 cm−1 upon the oxidation of pMeTP to p-MBA. We adopted this peak as a marker band for this oxidation (Figs. S1a–g). Higher laser power from 1 mW up to 50 mW increased the efficiency of the oxidation of p-MeTP (Fig. 1d). For instance, the oxidation was almost completed in 60 min at 20, 30, 40 and 50 mW yielding down shift of 7–9 cm−1 for the marker band, whereas faint spectral changes less than 1 cm−1 were observed at 1, 2, 5 mW in 60 min. These results are probably caused by the higher hot carrier density generated at the nanogap between AgNPs and Ag films at the higher laser power. Simultaneously, the laser irradiation on the samples under a gap mode resonance may raise the temperature of p-MeTP via dephasing surface plasmons and dissipation of its energy, which is
SERS spectra of p-MeTP in AgNP/p-MeTP/Ag film (45 nm)/Ge (1 nm)/Si substrate were measured at different temperatures between 297 and 373 K. Modest laser power of 1 mW, much lower than the threshold value of 10 mW, was used to derive the intrinsic influence of sample temperature on the oxidation. SERS spectra of p-MeTP observed at 1600, 1492, 1388, 1221, 1189, 1079, 1016, 795, 620 and 381 cm−1 did not show any significant changes in peak intensity and positions at 297 K for more than 30 min (Fig. 1a). Essentially the same results were observed at higher temperature up to 373 K as those at 297 K (Fig. 2), indicating that the oxidation of p-MeTP is not thermally caused up to 373 K. On the other hand, plasmon induced thermal effect, which is given by dephasing of surface plasmons via electron-electron and electronlattice scattering, recently attracts much attention to utilize it for reshaping of nanostructures, thermal microscopy or photo thermal therapy [25]. It may increase temperature of p-MeTP during the oxidation under a gap mode resonance. We therefore evaluated an increase in temperature of the sample based on absorption of light and subsequent thermal diffusion into surrounding media. A continuous wave (CW) laser as well as a pulse laser yields temperature of nanoparticles as exemplified for outside of MNPs by the following equation [25,26].
ΔTCW (r) =
I0 σabs for r > RAgNP 4πκ m r
(1)
Here, ΔT , I0, σabs, κm, r and RAgNP denote temperature increase (K), laser power density (W/m2), absorption cross section (m2), thermal conductivity of media (J/s K m), distance from the center of MNPs, and radius of MNPs, respectively. For instance, experimental conditions CW
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equation, laser irradiation (1 μW/μm2) to AgNP under a gap mode resonance raises temperature at Ag surface (ΔTCW (RAgNP)) by about 1.59 K (in the air) or 0.0656 K (in water), whereas much higher temperature increase of ∼ 160 K is evaluated for tightly focused laser of 1 mW/μm2 (10 mW) used in conventional laser trapping experiments [27]. Thus, the temperature increase of AgNPs in our experiments is estimated to be around 2 K, which is too small to give rise to thermal oxidation of p-MeTP as aforementioned. We experimentally ascertained this prediction for temperature increase in our samples during the oxidation of p-MeTP using intensity ratio of the Stokes and anti-Stokes scattering IS/IaS. Prior to this evaluation, relation between the intensity ratio IS/Ia-S for the Raman band of p-MeTP at 384 cm−1 and temperature of the substrates between 297 K and 373 K was measured using sufficiently weak laser power of 1 mW (532 nm). The intensity ratio IS/Ia-S at 384 cm−1, which is attributed to a skeletal stretching and bending mode of p-MeTP, notably altered with increasing the temperature (Figs. 3a and b, and S2a–d). The observed intensity ratio ρ = IS/Ia-S was evaluated based on the theoretical equation [23].
ρ= Fig. 2. SERS (the Stokes shift) spectra of p-MeT in AgNP/p-MeT/Ag film/Ge/Si substrates under gap mode resonance (532 nm, 1 mW) at various temperature of substrates between 297 and 373 K.
IS hν = A asymexp ⎛ m ⎞ IaS ⎝ kT ⎠
(2)
The pumping term in the original equation [23], which contributes at much higher power densities, typically 102–103 μW/μm2, compared to the present study (∼1 μW/μm2 or less), is neglected. Here the asymmetry parameter Aasym = (ηS/ηaS)×(σS,m/σaS,m)×|Gs/GaS| is dependent on wavelength via sensitivity of detector ηS/ηaS, scattering cross section of the Stokes (S) and anti-Stokes (aS) shift signal (σS,m/ σaS,m) = {(νL − νm)/(νL + νm)}3 for frequency of an excitation laser νL and a vibrational mode νm, and enhancement factor |GS/GaS| in SERS. It turned out that the observed relation of the intensity ratio IS/IaS to temperature is governed by the Boltzmann factor exp(hνm/kT) (Fig. 3c).
such as I0 = 1 μW/μm2 = 106 W/m2, σabs = 2.58 × 10−14 m2 for r = 20 nm (AgNP) under the gap mode, κm = 0.0241 J/s K m (air) or 0.582 J/s K m (water) are used in the present study. Note that the absorption cross section σabs is inherently determined by metal species, and hence quite similar for isolated particles (1.44 × 10−14 m2) and gap mode geometry (2.58 × 10−14 m2). According to the above
Fig. 3. Stokes and anti-Stokes SERS spectra of p-MeTP in AgNP/p-MeTP/Ag film/Ge/Si substrates under gap mode resonance (532 nm, 1 mW in (a)–(c) and 1–50 mW in (d)) at various temperatures of substrates between 297 and 373 K (See also Figs. S2a–d): (a) at 297 K, (b) at 373 K, (c) Intensity ratio of the Stokes and anti-Stokes shift at 384 cm−1 vs. temperature of the sample (filled red circle), and (d) Intensity ratio of the Stokes and anti-Stokes shift at ∼ 384 cm−1 vs. laser power. The intensity ratio (IS/Ias) at different laser power (green triangle) was also plotted in (c) in addition to theoretical Boltzmann factor with (dotted line) and without asymmetry factor (broken-dotted line) in scattering intensity. The numbers in (c) attached to the symbol (green triangles) denote the laser power. 571
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Fig. 4. SERS spectra of p-MeT in AgNP/p-MeT/Ag film/Ge/Si substrates under gap mode resonance (λex = 532 nm, Ip = 10 mW) under N2 flow (300 mL/min.) for 60 min. followed by being exposed to the air for 30 min: (a) SERS spectral changes during the entire time course, (b) a spectrum at 0 min (from p-MeTP, black line), that at 60 min. under N2 flow (from an intermediate species, red line), and that at 30 min in the air after exposed in N2 flow for 60 min. (from p-MBA, blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. SERS spectra of p-MeT in AgNP/p-MeT/Ag film/Ge/Si substrates under gap mode resonance (λex = 532 nm, Ip) under O2 flow (1000 mL/min): (a) at Ip = 20 mW, (b) wavenumber shift for the Raman band at 1590–1600 cm−1 from p-MeTP and p-MBA vs. time, showing the oxidation efficiency of p-MeTP molecules (see also Figs. S4a–d for the data observed at Ip = 1–20 mW under O2 flow of 0.3 L/min).
IS/IaS for the SERS peak at ∼ 384 cm−1 during the oxidation of p-MeTP based on the above relation (Fig. 3c). Note that laser illumination at 1–50 mW gave temperature increase of the samples only less than 10 K in the air (Fig. 3d), which is primarily given by fluctuation of the SERS intensity in each measurement. Subtle increase in sample temperature less than 10 K corresponds well with the predicted value of 1.6 K based on Eq. (1) [25,26]. Clearly, such minute increase in temperature of p-MeTP samples does not induce the oxidation (Fig. 2). Thus the oxidation of p-MeTP is not caused by thermal heating but photocatalytic reaction via a gap mode plasmon, which is accelerated at stronger laser power as stated in the Section 1.
Namely the intensity ratio ρobs = 8(IS/IaS)obs = 6.42 (297 K) and 4.35 (373 K) corresponds well to the Boltzmann factor of 6.48 (297 K) and 4.41 (373 K). It suggests that the asymmetry factor does not contribute to the intensity ratio IS/IaS. In other words, the ratio of scattering cross section in the Stokes shift to the anti-Stokes shift, (σS,m/σaS,m) = 8{(νS)/ (νaS)}3 = 0.88 (< 1), is probably compensated by that of enhancement factor |GS/GaS| in SERS. This is supported by the observations that the gap mode of AgNP/gap/Ag substrates is resonantly excited at ∼ 440 nm and also at ∼ 600 nm as evidenced by transmission spectra measured for AgNP/p-MeTP/Ag/Ge/cover glass substrates (Fig. S3a), and by enhanced electric field at the nanogap using a finite difference time domain (FDTD) method (Fig. S3b). Larger enhancement factor |GS/GaS| is expected at the Stokes shift side of the excitation wavelength (532 nm), depending on the diameter of AgNPs and the gap sizes. For instance, the gap mode in AgNP (r nm in radii)/gap (dgap, nm)/Ag substrates under external geometry (angle of incidence (AOI) = 45°) provides larger enhancement at the Stokes shift side for r ≥ 30 nm (dgap = 1 nm, Fig. S3b) or r = 20 nm (dgap = 0.5 nm, the data not shown), which accounts for the experimental observations (see also Supporting Data). Next, we evaluated the sample temperature using the intensity ratio
3.3. The effect of nitrogen and oxygen atmosphere We previously reported that p-MeTP is photocatalytically oxidized in the air and in aqueous solutions, while partially suppressed under low pressure of the air (low vacuum) [1]. Here, we investigated the effect of nitrogen and oxygen atmosphere on the photo-oxidation of pMeTP in AgNP/p-MeTP/Ag/Ge/Si substrates. First, the sample substrate was irradiated by laser at 532 nm (10 mW) under nitrogen gas flow (300 mL/min) (Fig. 4a and b). SERS spectra of p-MeTP gradually 572
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Next, we investigated the effect of oxygen atmosphere on the oxidation of p-MeTP, in which oxygen was introduced to the same substrate at a flow rate of 300 or 1000 mL/min while varying the laser power between 1 and 20 mW (Figs. 5a, and S4a–d). Peak position of the marker band at 1600 cm−1 (1598 cm−1) gradually down-shifted to 1598 cm−1 (1595 cm−1) even at 1 mW (5 mW, Figs. S4a and 5b), while those at 1599 (at 5 mW, Figs. S4b and 5b) and 1598 cm−1 (10 mW, Figs. S4c and 5b) down-shifted to 1595 and 1592 cm−1. These observations demonstrate that the oxidation proceeded even with such weak laser power. Accordingly, there is no apparent threshold power for the oxidation of p-MeTP under oxygen flow, which is in contrast to that in the air (Fig. 1d). Correspondingly, the oxidation efficiency of pMeTP was accelerated by oxygen, such as 100% at 45 min under O2 flow(1 L/min) higher than that ∼ 80% at 60 min in the air at the laser power of 20 mW (Figs. S4d, 5a and b). 3.4. Oxidation of o-, m- and p-mercaptobenzyl alcohol Gap mode driven oxidation should have a site selectivity due to appreciable differences in hot carrier density for a reactant group in SAM molecules located between AgNP and Ag films. Indeed, only pMeTP is oxidized unlike o- and m-MeTP molecules [1]. Here, the oxidation of o-, m- and p-MBAl relevant to MeTP molecules was investigated in terms of their site selectivity and intermediate species for the oxidation of p-MeTP. We found that all of o-, m- and p-MBAl are oxidized to corresponding o-, m- and p-mercaptobenzoic acid (o-, m-, and p-MBA) under a gap mode resonance (Fig. 6a and b, and S5). For instance, SERS spectra of o-MBAl observed at 1676, 1651, 1585, 1564, 1461, 1439, 1264, 1203, 1164, 1124, ∼1050, 1033, 857, 831, 810, 786, 687, 641 cm−1 varied their intensity and peak positions, and finally gave the spectra at 1583, 1560, 1463, 1428, 1376, 1282, 1259, 1152, 1117, 1027, 995, 824, ∼670, and 615 cm−1 (Fig. 6a) after ∼ 60 min at 20 mW (532 nm). In particular, the SERS band at ∼ 1203 cm−1, which is assigned to a coupled vibration of a stretching mode of νC-CH2OH and a deformation mode of δC-OH relating to a benzyl alcohol group, disappeared while new peaks at 1360–1380 cm−1 assigned to a symmetric stretching mode of a COO− group. It clearly proves an oxidation of a benzyl alcohol group (–CH2OH) into a carboxyl group (–COOH). Indeed, SERS spectra for omercaptobenzyl alcohol (o-MBAl) irradiated for 60 min almost accord with those of o-mercaptobenzoic acid (o-MBA) in AgNP/o-MBA/Ag (45)/Ge(1)/Si substrates. Essentially the same results as those for oMBAl were observed for m-MBAl (Fig. S5) and p-MBAl (Fig. 6b), indicating MBAl are oxidized to MBA under a gap mode without showing site selectivity of the benzyl alcohol group. In addition, the observed SERS spectrum from p-MBAl before the laser irradiation (at 0 min, Fig. 6b) resembles those of the intermediate species of p-MeTP detected in nitrogen atmosphere (Fig. 4a and b). However, the Raman bands at 1570, 998, 460 cm−1 were observed only for the intermediate species of p-MeTP, which are compelling evidence that the intermediate species is not p-mercaptobenzyl alcohol. The intermediate species is presumably attributed to p-mercaptobenzyl radical based on DFT calculations (Fig. S6a–d), which is crucial to elucidate the oxidation mechanism of p-MeTP to p-MBA. Further details are under investigation using different molecules to identify the intermediate species and also using DFT calculations for the unified electronic state including AgNPs, adsorbed species and Ag films under a gap mode resonance.
Fig. 6. SERS spectra of (a) o-MBAl, and (b) p-MBAl in AgNP/o- or m-MBAl/Ag film/Ge/Si substrates under gap mode resonance (λex = 532 nm, Ip = 10 mW, see also Figs. S5 for the spectral changes of m-MBAl) at different duration of laser irradiation as given in parentheses from 0, 30 and 60 min. SERS spectra of o-MBA and p-MBA, shown on top of each figure (in red line) accord with those observed from photocatalytic oxidation of o- and p-MBAl as well as that from m-MBAl (Fig. S5). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
varied to new peaks at 1570, 1423, 1360, 1298, 1226, 1176, 1075, 998, 889, 808, 658, 508, and 460 cm−1 (Fig. 4a) after about 1 hr. Interestingly, these new peaks again altered to those of p-MBA at 1591, 1388, 1370, 1186, 1082, 1017, 841, 801, 718, 626, 548 and 518 cm−1, when the nitrogen flow was switched off to expose the samples to the air (Fig. 4b). Thus, the photo-oxidation of p-MeTP is partially suppressed by nitrogen atmosphere giving a metastable species, whereas the oxidation again resumed in the air. The intermediate species may be assigned to p-mercaptobenzaldehyde or p-mercaptobenzyl alcohol, which are partially oxidized molecules of p-MeTP at a methyl group in conventional reactions using oxidants like MnO4− but probably not the case in the current study (as will be seen in the next section). Complete suppression of the transfer of p-MeTP to the intermediate species can be realized if tightly sealed vacuum cells are employed instead of using a nitrogen flow system in the present study.
p-MBAl (p-mercaptobenzyl alcohol) 573
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doi.org/10.1016/j.cplett.2019.06.052. References [1] [2] [3] [4] [5]
p-MBR (mercaptobenzyl radical)
[6] [7] [8]
In conclusion, we found that the oxidation of p-MeTP under a gap mode resonance is not caused by thermal reaction but by photocatalytic reaction. The oxidation was accelerated by the larger power density and oxygen atmosphere. An intermediate species was detected under nitrogen atmosphere, which is further oxidized by introducing the air. All of o-, m- and p-mercaptobenzylalcohol molecules, possible candidates for the intermediate species for the oxidation of p-MeTP, are oxidized to o-, m- and p-mercaptobenzoic acid with no specific site selectivity. Thus, vital information was obtained to get insight into the mechanism of the plasmon driven photocatalytic reactions of p-alkyl TP and related molecules.
[9] [10] [11] [12] [13] [14] [15]
Acknowledgement
[16] [17] [18]
This work was supported by KAKENHI (17H02722) by JSPS – Japan, and Salt Science Foundation (No. 1815 for 2018). One of the authors (MF) appreciates Prof. Mitsuru Ishikawa (Josai University) and Prof. De-Yin Wu (Xiamen University) for useful discussion. This paper is devoted to the memory of Prof. em. Andreas Otto (Heinrich-Henie Univ. Düsseldorf, Germany), at which laboratory MF initiated to learn and investigate surface enhanced Raman scattering under ATR-(Otto) geometry.
[19] [20] [21] [22] [23] [24] [25] [26] [27]
Appendix A. Supplementary material Supplementary data to this article can be found online at https://
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Research paper
Specific photocatalytic reaction of p-methyl thiophenol and related molecules under the gap mode resonance Kanae Tabei, Keitaro Akai, Masayuki Futamata
T
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Saitama University, Saitama 338-8570, Japan
H I GH L IG H T S
information was obtained for the plasmon induced photocatalytic reaction. • Vital thiophenol is site-selectively oxidized to p-MBA in a gap mode resonance. • P-alkyl oxidation is not due to thermal heating but due to hot carrier transfer. • The oxidation is accelerated by oxygen, while partly suppressed by nitrogen gas. • The • In contrast, o-, m-, p-mercaptobenzyl alcohol are oxidized to o-, m- and p-MBA.
A R T I C LE I N FO
A B S T R A C T
Keywords: Plasmon induced photocatalytic reaction P-methyl thiophenol Gap mode Surface enhanced Raman scattering
Para-alkyl thiophenol (p-AlTP) molecules with normal and branched alkyl groups are selectively oxidized into pmercaptobenzoic acid under the gap mode resonance, whereas o- and m-methyl TP (o-, m-MeTP) do not show such reactions [1]. We investigated here detailed properties of the photocatalytic reaction specific to p-AlTP in terms of the following contrasting observations. (1) The oxidation of p-MeTP was not induced by thermal heating of the samples up to 373 K. Subtle temperature increase during the oxidation was also corroborated by the measurements of the Stokes and anti-Stokes scattering intensity. (2) The oxidation of p-MeTPs was accelerated by oxygen atmosphere at room temperature, whereas nitrogen atmosphere yielded an intermediate species. (3) Ortho-, m-, and p-mercaptobenzyl alcohol molecules were oxidized to corresponding o-, m- and pmercaptobenzoic acid (p-MBA) with no definite site selectivity.
1. Introduction Metal nanoparticles (MNPs) are utilized solely or after immobilization on metal oxides like SiO2 or TiO2 for catalyzing chemical transformation of organic molecules at the temperatures lower than ∼370 K. For instance, silver nanoparticles (AgNPs) and gold nanoparticles (AuNPs) catalyze the oxidation of alcohol to aldehyde (the Wolf rearrangement) [2] and the Diels-Alder reaction [3]. Photocatalytic reactions can also be implemented by MNPs, which are driven by hot carriers, electrons and holes excited during dephasing process of surface plasmons (SP). The efficiency of the photocatalytic reactions supported by SP is inherently restricted due to extremely short lifetime of SP ≤ 10 fs [4–7]. Prolonged lifetime of hot carriers by chemical interface dumping (CID) [6,7], chemical interface scattering (CIS), or particular nanostructures such as closely adjacent metal nanoparticles yielding 1–2 ps [8,9], therefore, facilitates hot carrier transfer to
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adsorbates to cause chemical reaction, which is also known as the origin of chemical enhancement in surface enhanced Raman scattering (SERS) on metal surfaces with atomic scale roughness [10]. Although the role of the lifetime of hot carriers is still a matter for debate, crucial reactions have been reported for plasmon induced oxidation of ethylene [11] or dissociation of hydrogen [12]. Inherently, a localized surface plasmon (LSP), which generates high density of hot carriers and enormously enhanced electric field available for highly-sensitive spectroscopy, is quite suitable for in-situ elucidation of photocatalytic reactions on MNPs. For instance, p-aminothiophenol (PATP) adsorbed on silver and gold surfaces is dimerized into p, p′mercaptoazobenzene (DMAB) under laser irradiation, in which reaction process was elucidated using new SERS peaks appeared at 1142, 1388, and 1432 cm−1 [13–15]. Recent density functional theory (DFT) calculations, combined with SERS and other analytical methods, for metal clusters adsorbed by PATP provide detailed information on hybridized
Corresponding author. E-mail address: [email protected] (M. Futamata).
https://doi.org/10.1016/j.cplett.2019.06.052 Received 29 April 2019; Received in revised form 17 June 2019; Accepted 18 June 2019 Available online 19 June 2019 0009-2614/ © 2019 Elsevier B.V. All rights reserved.
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MeTP, o-, m- and p-mercaptobenzyl alcohols (o-, m- and p-MBAl) was formed on fresh Ag films by immersing the Ag(t = 45)/Ge(t = 1)/Si substrates in ethanol solutions of corresponding thiol molecules (1 mM) for 1 h, and rinsed thoroughly with neat ethanol to remove excess and unbound thiophenol molecules. Subsequently, the thiophenol-SAM coated Ag/Ge/Si substrates were immersed in an AgNP suspension for 1 h to immobilize AgNPs on Ag films through van der Waals interaction [22]. Surface coverage of AgNPs on Ag films given by the ratio of occupied area by AgNPs to the entire surface area of Ag films was adjusted, typically ∼ 5%, by the concentration of AgNP suspensions [1].
electronic state and hot carrier transfer between metals and adsorbed molecules during the dimerization of PATP [14,15]. Among various SERS probes such as roughened metal surfaces, MNPs and arrays of metal nanostructures, the gap mode plasmon generating markedly enhanced electric field by a factor of 104–105 at a nanogap between MNPs on various transition metal substrates [16–18] plays a salient role for this purpose. Indeed, Al, Ni, Zn as well as Ag, Au and Cu substrates provide promising SERS enhancement factors of 107–109 for adsorbed thiophenol (TP) molecules using AuNPs [19,20]. Under the gap mode geometry, we recently found that p-alkyl thiophenol (p-AlTP) molecules, such as methyl, ethyl, iso-propyl, or tertiary butyl groups in AgNP/p-AlTP/Ag film (t = 45 nm)/Ge film (t = 1 nm)/Si substrates are oxidized into p-mercaptobenzoic acid (p-MBA) by irradiating laser light at 532 nm (< ∼1 μW/μm2). In contrast, o- and m-methyl TP (o-, mMeTP) molecules do not show the oxidation [1]. AgNPs were crucial to cause the oxidation of p-MeTP on Ag films [1].
2.2. Spectral measurements A gap mode for AgNPs/p-MeTP-SAM (or o-, m-, p-MBAl)/Ag/Ge/Si substrates was excited in the air with a solid laser (λ = 532.0 nm, typically with a power (Ip) of 10 mW corresponding to a power density of ∼0.8 μW/μm2 using a conventional objective of f = 50 mm) under external geometry at an incident angle of ∼50° with p-polarized light. SERS spectra of p-MeTP were measured using a micro Raman spectrometer consisting of conventional transfer optics, an upright microscope (Olympus), a single polychromator (Bunko-Keiki MK-300), and a charge coupled device (CCD) detector (Andor, iVAc). Photocatalytic reaction was monitored by observing Raman spectral changes of pMeTP molecules in AgNPs/p-MeTP (or o-, m-, p-MBAl)/Ag(45)/Ge(1)/ Si substrates under laser illumination with a constant power (1–50 mW) for 60 min in the air, if otherwise noted.
p-AlTP (p-alkyl thiophenol), p-MeTP (p-methyl thiophenol) for Al=CH3
2.3. Thermal heating Microscope cryostat, MicrostatN (Oxford Instrum.), was employed to heat the sample substrates (AgNP/p-MeTP/Ag/Ge/Si) from room temperature (∼297 K (24 °C)) up to 373 K (100 °C), of which temperature was adjusted with a conventional resistivity heater and a Proportional-Integral-Differential (PID) controller. SERS spectra of the p-MeTP sample were measured through a quartz window (t = 1 mm). The cryostat was also used to investigate the effect of nitrogen and oxygen atmosphere on the oxidation of p-MeTP.
2.4. Laser heating p-MBA (p-mercaptobenzoic acid)
First, intensity ratio of the Stokes and anti-Stokes scattering (IS/IaS) of 384 cm−1 from p-MeTP in AgNP/p-MeTP/Ag/Ge/Si samples, immobilized in the cryostat as aforementioned, was measured at different temperatures between 297 and 373 K using a weak laser power (1 m W, which is much weaker than threshold power of 10 mW for the oxidation) and a notch filter (Iridian Narrow band NF-532 nm, centered at 532 nm with a notch width of 12 nm). The observed intensity ratio IS/ IaS was theoretically evaluated based on the Boltzmann distribution and the asymmetry factor of cross section and enhancement in SERS for the Stokes and anti-Stokes regions [23]. Subsequently, SERS intensity ratio (IS/IaS) was measured during the oxidation excited at 532 nm (1–50 mW) without temperature control. The value IS/IaS was analyzed to evaluate gap mode driven heating of the p-MeTP samples.
Further information such as the influence of substrate temperature, gap mode induced temperature increase, nitrogen or oxygen atmosphere, and different functional group on the oxidation of p-MeTP are prerequisite for comprehensive understanding and utilization of photocatalytic reactions. Here we reports decisive observations that p-alkyl TPs are oxidized not via thermal process but via hot electron transfer from metal to adsorbates, while observing an intermediate species under nitrogen atmosphere and resembled oxidation of mercaptobenzyl alcohol molecules. 2. Experimental 2.1. Preparation of samples AgNPs were synthesized by a citrate reduction method [19,20], in which citrate molecules reduce Ag+ ions, and also stabilize AgNPs in suspensions. The average size of our AgNPs was found to be around 20–30 nm in radii based on scanning electron microscope (SEM) observations. Silver films with a thickness (t) of 45 nm were evaporated in vacuum on Ge (t = 1 nm) pre-deposited on a Si wafer to diminish roughness of Ag films [21], while monitoring the thickness with quartz crystal sensor, Q-pod (Inficon). Self-assembled-monolayer (SAM) of p-
2.5. SERS under different atmosphere Nitrogen (99.99%) or oxygen gas (99.99%) was flowed into the sample compartment of MicrostatN, in which AgNP/p-MeTP/Ag/Ge/Si substrates are immobilized, at a rate of 300 or 1000 mL/min. The gas inside the sample compartment is exchanged 10–20 times per minute, as its volume is ∼ 20 mL.
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Fig. 1. SERS spectra of p-MeT in AgNP/p-MeT/Ag film/Ge/Si substrates under gap mode resonance, at which the excitation laser of 532 nm (1–50 mW) was irradiated under external geometry: (a) at 1 mW, (b) at 10 mW, (c) 20 mW, (d) the peak shift of the marker band at ∼ 1600–1590 cm−1 vs. duration of time for the laser irradiation, showing the oxidation efficiency (see the main text in more detail). Please see also Figs. S1a–g.
3. Results and discussion
negligibly small in our experimental conditions as dwelled in the next section.
3.1. The effect of laser power 3.2. The effect of substrate temperature The stronger laser power density inherently provides the higher efficiency for the oxidation of p-MeTP due to increased density of hot carriers at the nanogap between AgNP and Ag films [24]. We measured SERS spectra of p-MeTP in AgNP/p-MeTP/Ag film (45 nm)/Ge(1 nm)/ Si substrates at various laser power of 1–50 mW (λ0 = 532 nm) in the air (Figs. 1a–c, and S1a–g). Laser power less than 5 mW (∼0.4 μW/ μm2) gave faint spectral changes, namely SERS bands observed at 1600, 1492, 1388, 1221, 1189, 1079, 1016, 795, 620 and 381 cm−1 did not show significant variations by the laser illumination for ∼ 60 min. In contrast, laser power higher than 10 mW (∼0.8 μW/μm2) provided pronounced spectral changes, such as marked downshift of the peak at 1599 cm−1 to 1590 cm−1 in addition to new peaks appeared at ∼ 1370, 1140, 836, 709, 688, 513, and ∼ 354 cm−1, all of which are attributed to p-MBA (Fig. S1h1). Thus, apparent threshold power for the oxidation of p-MeTP is around 10 mW in the air. The SERS peak at 1599 cm−1 of p-MeTP, assigned to a CeC stretching mode coupled with a CeH inplane bending mode of a phenyl ring based on DFT calculations, showed a drastic downshift by about 10 cm−1 upon the oxidation of pMeTP to p-MBA. We adopted this peak as a marker band for this oxidation (Figs. S1a–g). Higher laser power from 1 mW up to 50 mW increased the efficiency of the oxidation of p-MeTP (Fig. 1d). For instance, the oxidation was almost completed in 60 min at 20, 30, 40 and 50 mW yielding down shift of 7–9 cm−1 for the marker band, whereas faint spectral changes less than 1 cm−1 were observed at 1, 2, 5 mW in 60 min. These results are probably caused by the higher hot carrier density generated at the nanogap between AgNPs and Ag films at the higher laser power. Simultaneously, the laser irradiation on the samples under a gap mode resonance may raise the temperature of p-MeTP via dephasing surface plasmons and dissipation of its energy, which is
SERS spectra of p-MeTP in AgNP/p-MeTP/Ag film (45 nm)/Ge (1 nm)/Si substrate were measured at different temperatures between 297 and 373 K. Modest laser power of 1 mW, much lower than the threshold value of 10 mW, was used to derive the intrinsic influence of sample temperature on the oxidation. SERS spectra of p-MeTP observed at 1600, 1492, 1388, 1221, 1189, 1079, 1016, 795, 620 and 381 cm−1 did not show any significant changes in peak intensity and positions at 297 K for more than 30 min (Fig. 1a). Essentially the same results were observed at higher temperature up to 373 K as those at 297 K (Fig. 2), indicating that the oxidation of p-MeTP is not thermally caused up to 373 K. On the other hand, plasmon induced thermal effect, which is given by dephasing of surface plasmons via electron-electron and electronlattice scattering, recently attracts much attention to utilize it for reshaping of nanostructures, thermal microscopy or photo thermal therapy [25]. It may increase temperature of p-MeTP during the oxidation under a gap mode resonance. We therefore evaluated an increase in temperature of the sample based on absorption of light and subsequent thermal diffusion into surrounding media. A continuous wave (CW) laser as well as a pulse laser yields temperature of nanoparticles as exemplified for outside of MNPs by the following equation [25,26].
ΔTCW (r) =
I0 σabs for r > RAgNP 4πκ m r
(1)
Here, ΔT , I0, σabs, κm, r and RAgNP denote temperature increase (K), laser power density (W/m2), absorption cross section (m2), thermal conductivity of media (J/s K m), distance from the center of MNPs, and radius of MNPs, respectively. For instance, experimental conditions CW
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equation, laser irradiation (1 μW/μm2) to AgNP under a gap mode resonance raises temperature at Ag surface (ΔTCW (RAgNP)) by about 1.59 K (in the air) or 0.0656 K (in water), whereas much higher temperature increase of ∼ 160 K is evaluated for tightly focused laser of 1 mW/μm2 (10 mW) used in conventional laser trapping experiments [27]. Thus, the temperature increase of AgNPs in our experiments is estimated to be around 2 K, which is too small to give rise to thermal oxidation of p-MeTP as aforementioned. We experimentally ascertained this prediction for temperature increase in our samples during the oxidation of p-MeTP using intensity ratio of the Stokes and anti-Stokes scattering IS/IaS. Prior to this evaluation, relation between the intensity ratio IS/Ia-S for the Raman band of p-MeTP at 384 cm−1 and temperature of the substrates between 297 K and 373 K was measured using sufficiently weak laser power of 1 mW (532 nm). The intensity ratio IS/Ia-S at 384 cm−1, which is attributed to a skeletal stretching and bending mode of p-MeTP, notably altered with increasing the temperature (Figs. 3a and b, and S2a–d). The observed intensity ratio ρ = IS/Ia-S was evaluated based on the theoretical equation [23].
ρ= Fig. 2. SERS (the Stokes shift) spectra of p-MeT in AgNP/p-MeT/Ag film/Ge/Si substrates under gap mode resonance (532 nm, 1 mW) at various temperature of substrates between 297 and 373 K.
IS hν = A asymexp ⎛ m ⎞ IaS ⎝ kT ⎠
(2)
The pumping term in the original equation [23], which contributes at much higher power densities, typically 102–103 μW/μm2, compared to the present study (∼1 μW/μm2 or less), is neglected. Here the asymmetry parameter Aasym = (ηS/ηaS)×(σS,m/σaS,m)×|Gs/GaS| is dependent on wavelength via sensitivity of detector ηS/ηaS, scattering cross section of the Stokes (S) and anti-Stokes (aS) shift signal (σS,m/ σaS,m) = {(νL − νm)/(νL + νm)}3 for frequency of an excitation laser νL and a vibrational mode νm, and enhancement factor |GS/GaS| in SERS. It turned out that the observed relation of the intensity ratio IS/IaS to temperature is governed by the Boltzmann factor exp(hνm/kT) (Fig. 3c).
such as I0 = 1 μW/μm2 = 106 W/m2, σabs = 2.58 × 10−14 m2 for r = 20 nm (AgNP) under the gap mode, κm = 0.0241 J/s K m (air) or 0.582 J/s K m (water) are used in the present study. Note that the absorption cross section σabs is inherently determined by metal species, and hence quite similar for isolated particles (1.44 × 10−14 m2) and gap mode geometry (2.58 × 10−14 m2). According to the above
Fig. 3. Stokes and anti-Stokes SERS spectra of p-MeTP in AgNP/p-MeTP/Ag film/Ge/Si substrates under gap mode resonance (532 nm, 1 mW in (a)–(c) and 1–50 mW in (d)) at various temperatures of substrates between 297 and 373 K (See also Figs. S2a–d): (a) at 297 K, (b) at 373 K, (c) Intensity ratio of the Stokes and anti-Stokes shift at 384 cm−1 vs. temperature of the sample (filled red circle), and (d) Intensity ratio of the Stokes and anti-Stokes shift at ∼ 384 cm−1 vs. laser power. The intensity ratio (IS/Ias) at different laser power (green triangle) was also plotted in (c) in addition to theoretical Boltzmann factor with (dotted line) and without asymmetry factor (broken-dotted line) in scattering intensity. The numbers in (c) attached to the symbol (green triangles) denote the laser power. 571
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Fig. 4. SERS spectra of p-MeT in AgNP/p-MeT/Ag film/Ge/Si substrates under gap mode resonance (λex = 532 nm, Ip = 10 mW) under N2 flow (300 mL/min.) for 60 min. followed by being exposed to the air for 30 min: (a) SERS spectral changes during the entire time course, (b) a spectrum at 0 min (from p-MeTP, black line), that at 60 min. under N2 flow (from an intermediate species, red line), and that at 30 min in the air after exposed in N2 flow for 60 min. (from p-MBA, blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. SERS spectra of p-MeT in AgNP/p-MeT/Ag film/Ge/Si substrates under gap mode resonance (λex = 532 nm, Ip) under O2 flow (1000 mL/min): (a) at Ip = 20 mW, (b) wavenumber shift for the Raman band at 1590–1600 cm−1 from p-MeTP and p-MBA vs. time, showing the oxidation efficiency of p-MeTP molecules (see also Figs. S4a–d for the data observed at Ip = 1–20 mW under O2 flow of 0.3 L/min).
IS/IaS for the SERS peak at ∼ 384 cm−1 during the oxidation of p-MeTP based on the above relation (Fig. 3c). Note that laser illumination at 1–50 mW gave temperature increase of the samples only less than 10 K in the air (Fig. 3d), which is primarily given by fluctuation of the SERS intensity in each measurement. Subtle increase in sample temperature less than 10 K corresponds well with the predicted value of 1.6 K based on Eq. (1) [25,26]. Clearly, such minute increase in temperature of p-MeTP samples does not induce the oxidation (Fig. 2). Thus the oxidation of p-MeTP is not caused by thermal heating but photocatalytic reaction via a gap mode plasmon, which is accelerated at stronger laser power as stated in the Section 1.
Namely the intensity ratio ρobs = 8(IS/IaS)obs = 6.42 (297 K) and 4.35 (373 K) corresponds well to the Boltzmann factor of 6.48 (297 K) and 4.41 (373 K). It suggests that the asymmetry factor does not contribute to the intensity ratio IS/IaS. In other words, the ratio of scattering cross section in the Stokes shift to the anti-Stokes shift, (σS,m/σaS,m) = 8{(νS)/ (νaS)}3 = 0.88 (< 1), is probably compensated by that of enhancement factor |GS/GaS| in SERS. This is supported by the observations that the gap mode of AgNP/gap/Ag substrates is resonantly excited at ∼ 440 nm and also at ∼ 600 nm as evidenced by transmission spectra measured for AgNP/p-MeTP/Ag/Ge/cover glass substrates (Fig. S3a), and by enhanced electric field at the nanogap using a finite difference time domain (FDTD) method (Fig. S3b). Larger enhancement factor |GS/GaS| is expected at the Stokes shift side of the excitation wavelength (532 nm), depending on the diameter of AgNPs and the gap sizes. For instance, the gap mode in AgNP (r nm in radii)/gap (dgap, nm)/Ag substrates under external geometry (angle of incidence (AOI) = 45°) provides larger enhancement at the Stokes shift side for r ≥ 30 nm (dgap = 1 nm, Fig. S3b) or r = 20 nm (dgap = 0.5 nm, the data not shown), which accounts for the experimental observations (see also Supporting Data). Next, we evaluated the sample temperature using the intensity ratio
3.3. The effect of nitrogen and oxygen atmosphere We previously reported that p-MeTP is photocatalytically oxidized in the air and in aqueous solutions, while partially suppressed under low pressure of the air (low vacuum) [1]. Here, we investigated the effect of nitrogen and oxygen atmosphere on the photo-oxidation of pMeTP in AgNP/p-MeTP/Ag/Ge/Si substrates. First, the sample substrate was irradiated by laser at 532 nm (10 mW) under nitrogen gas flow (300 mL/min) (Fig. 4a and b). SERS spectra of p-MeTP gradually 572
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Next, we investigated the effect of oxygen atmosphere on the oxidation of p-MeTP, in which oxygen was introduced to the same substrate at a flow rate of 300 or 1000 mL/min while varying the laser power between 1 and 20 mW (Figs. 5a, and S4a–d). Peak position of the marker band at 1600 cm−1 (1598 cm−1) gradually down-shifted to 1598 cm−1 (1595 cm−1) even at 1 mW (5 mW, Figs. S4a and 5b), while those at 1599 (at 5 mW, Figs. S4b and 5b) and 1598 cm−1 (10 mW, Figs. S4c and 5b) down-shifted to 1595 and 1592 cm−1. These observations demonstrate that the oxidation proceeded even with such weak laser power. Accordingly, there is no apparent threshold power for the oxidation of p-MeTP under oxygen flow, which is in contrast to that in the air (Fig. 1d). Correspondingly, the oxidation efficiency of pMeTP was accelerated by oxygen, such as 100% at 45 min under O2 flow(1 L/min) higher than that ∼ 80% at 60 min in the air at the laser power of 20 mW (Figs. S4d, 5a and b). 3.4. Oxidation of o-, m- and p-mercaptobenzyl alcohol Gap mode driven oxidation should have a site selectivity due to appreciable differences in hot carrier density for a reactant group in SAM molecules located between AgNP and Ag films. Indeed, only pMeTP is oxidized unlike o- and m-MeTP molecules [1]. Here, the oxidation of o-, m- and p-MBAl relevant to MeTP molecules was investigated in terms of their site selectivity and intermediate species for the oxidation of p-MeTP. We found that all of o-, m- and p-MBAl are oxidized to corresponding o-, m- and p-mercaptobenzoic acid (o-, m-, and p-MBA) under a gap mode resonance (Fig. 6a and b, and S5). For instance, SERS spectra of o-MBAl observed at 1676, 1651, 1585, 1564, 1461, 1439, 1264, 1203, 1164, 1124, ∼1050, 1033, 857, 831, 810, 786, 687, 641 cm−1 varied their intensity and peak positions, and finally gave the spectra at 1583, 1560, 1463, 1428, 1376, 1282, 1259, 1152, 1117, 1027, 995, 824, ∼670, and 615 cm−1 (Fig. 6a) after ∼ 60 min at 20 mW (532 nm). In particular, the SERS band at ∼ 1203 cm−1, which is assigned to a coupled vibration of a stretching mode of νC-CH2OH and a deformation mode of δC-OH relating to a benzyl alcohol group, disappeared while new peaks at 1360–1380 cm−1 assigned to a symmetric stretching mode of a COO− group. It clearly proves an oxidation of a benzyl alcohol group (–CH2OH) into a carboxyl group (–COOH). Indeed, SERS spectra for omercaptobenzyl alcohol (o-MBAl) irradiated for 60 min almost accord with those of o-mercaptobenzoic acid (o-MBA) in AgNP/o-MBA/Ag (45)/Ge(1)/Si substrates. Essentially the same results as those for oMBAl were observed for m-MBAl (Fig. S5) and p-MBAl (Fig. 6b), indicating MBAl are oxidized to MBA under a gap mode without showing site selectivity of the benzyl alcohol group. In addition, the observed SERS spectrum from p-MBAl before the laser irradiation (at 0 min, Fig. 6b) resembles those of the intermediate species of p-MeTP detected in nitrogen atmosphere (Fig. 4a and b). However, the Raman bands at 1570, 998, 460 cm−1 were observed only for the intermediate species of p-MeTP, which are compelling evidence that the intermediate species is not p-mercaptobenzyl alcohol. The intermediate species is presumably attributed to p-mercaptobenzyl radical based on DFT calculations (Fig. S6a–d), which is crucial to elucidate the oxidation mechanism of p-MeTP to p-MBA. Further details are under investigation using different molecules to identify the intermediate species and also using DFT calculations for the unified electronic state including AgNPs, adsorbed species and Ag films under a gap mode resonance.
Fig. 6. SERS spectra of (a) o-MBAl, and (b) p-MBAl in AgNP/o- or m-MBAl/Ag film/Ge/Si substrates under gap mode resonance (λex = 532 nm, Ip = 10 mW, see also Figs. S5 for the spectral changes of m-MBAl) at different duration of laser irradiation as given in parentheses from 0, 30 and 60 min. SERS spectra of o-MBA and p-MBA, shown on top of each figure (in red line) accord with those observed from photocatalytic oxidation of o- and p-MBAl as well as that from m-MBAl (Fig. S5). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
varied to new peaks at 1570, 1423, 1360, 1298, 1226, 1176, 1075, 998, 889, 808, 658, 508, and 460 cm−1 (Fig. 4a) after about 1 hr. Interestingly, these new peaks again altered to those of p-MBA at 1591, 1388, 1370, 1186, 1082, 1017, 841, 801, 718, 626, 548 and 518 cm−1, when the nitrogen flow was switched off to expose the samples to the air (Fig. 4b). Thus, the photo-oxidation of p-MeTP is partially suppressed by nitrogen atmosphere giving a metastable species, whereas the oxidation again resumed in the air. The intermediate species may be assigned to p-mercaptobenzaldehyde or p-mercaptobenzyl alcohol, which are partially oxidized molecules of p-MeTP at a methyl group in conventional reactions using oxidants like MnO4− but probably not the case in the current study (as will be seen in the next section). Complete suppression of the transfer of p-MeTP to the intermediate species can be realized if tightly sealed vacuum cells are employed instead of using a nitrogen flow system in the present study.
p-MBAl (p-mercaptobenzyl alcohol) 573
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p-MBR (mercaptobenzyl radical)
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In conclusion, we found that the oxidation of p-MeTP under a gap mode resonance is not caused by thermal reaction but by photocatalytic reaction. The oxidation was accelerated by the larger power density and oxygen atmosphere. An intermediate species was detected under nitrogen atmosphere, which is further oxidized by introducing the air. All of o-, m- and p-mercaptobenzylalcohol molecules, possible candidates for the intermediate species for the oxidation of p-MeTP, are oxidized to o-, m- and p-mercaptobenzoic acid with no specific site selectivity. Thus, vital information was obtained to get insight into the mechanism of the plasmon driven photocatalytic reactions of p-alkyl TP and related molecules.
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Acknowledgement
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This work was supported by KAKENHI (17H02722) by JSPS – Japan, and Salt Science Foundation (No. 1815 for 2018). One of the authors (MF) appreciates Prof. Mitsuru Ishikawa (Josai University) and Prof. De-Yin Wu (Xiamen University) for useful discussion. This paper is devoted to the memory of Prof. em. Andreas Otto (Heinrich-Henie Univ. Düsseldorf, Germany), at which laboratory MF initiated to learn and investigate surface enhanced Raman scattering under ATR-(Otto) geometry.
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Appendix A. Supplementary material Supplementary data to this article can be found online at https://
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