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Gap mode induced photocatalytic oxidation of p-alkyl thiophenol molecules on silver films
Chemical Physics Letters 675 (2017) 63–68
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Gap mode induced photocatalytic oxidation of p-alkyl thiophenol molecules on silver films Keitaro Akai, Masayuki Futamata ⇑ Saitama University, Saitama 338-8570, Japan
a r t i c l e
i n f o
Article history: Received 7 January 2017 In final form 27 February 2017 Available online 28 February 2017 Keywords: Plasmon induced phtocatalytic reaction p-Methyl thiophenol Gap mode Surface enhanced Raman scattering
a b s t r a c t We found that a gap mode plasmon induced photocatlaytic oxidation of p-alkyl thiophenol (TP) molecules such as p-methyl TP, p-ethyl TP, p-isopropyl TP and p-tertiary butyl TP to p-carboxyl TP, in which 532 nm laser (<1 lW/lm2) impinged on a sample of silver (Ag) nanoparticles/p-alkyl TP/Ag films/BK-7 prism under attenuated total reflection (ATR) and external geometry. In contrast to the parasubstituted TP, o-methyl TP and m-methyl TP did not show the oxidation. The present observation on the oxidation of alkyl TP that is not thermally activated provides insights into the mechanism of plasmon mediated photocatalytic reactions. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Considerable interest has been placed on salient role of surface plasmons of metal nanostructures to exploit in various applications such as nano-optics, biosensing, photothermal therapy and phtocatalytic reactions. Photocatalytic reactions can be induced by a hot electron-hole pair (a hot carrier) that is excited by surface plasmons on metal nanoparticles, but are not always efficiently due to extremely short lifetime of 10 fs [1,2]. Prolonged lifetime of hot carriers by particular nanostructures, such as closely adjacent metal nanoparticles yielding 1–2 ps [3,4], therefore, facilitates hot carrier transfer to 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 [5]. Although the role of lifetime of hot carriers is still a matter for debate, innovative plasmon induced photocatalytic reactions have been reported for various reactants [2,6]. Gap-mode enhanced Raman scattering (GERS), which relies on huge enhancement of electric field at metal nanostructures in particular at a nanogap between metal nanoparticles or between metal nanoparticles and metal substrates, is inherently suitable for in-situ elucidation of relevant photocatalytic reactions. For instance, Tian reported that p-aminothiophenol (PATP) adsorbed on silver and gold surfaces dimerizes to p, p0 mercaptoazobenzene (DMAB) under laser illumination from SERS spectral changes in which new peaks appeared at 1142, 1388, and 1432 cm1 [7,8]. Alternatively, Osawa [9,10] and Kim [11] ⇑ Corresponding author. E-mail address: [email protected] (M. Futamata). http://dx.doi.org/10.1016/j.cplett.2017.02.081 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
accounted for such spectral changes by charge transfer effect, by which b2 modes of PATP, inactive in normal Raman spectroscopy, can be detected. The origin of the observed spectral changes of PATP has been investigated under various conditions, such as at different pH and electrode potentials [7,12,14], but still under debate. Beside those on PATP, only a few reports can be seen in literatures on photochemical reaction of adsorbates on metal surfaces, such as reduction of p-nitrothiophenol (PNTP) on silver under illumination of laser at 632.8 nm [9,10], or on superstructure of silver nanoparticles (AgNPs) [13]. In addition, photocatalytic decarboxylation is observed for p-mercaptobenzoic acid (PMBA) on gold electrodes [15,16], which is also discussed in the present contribution in relation to the oxidation of p-alkyl TP. It is prerequisite to enhance electric field for adsorbates on metal surfaces or between metal nanostructures for efficient transfer of hot carriers to molecules to drive photocatalytic reactions. The gap mode plasmon employed here generates markedly enhanced electric field by a factor of 104–105 at a nanogap between metal nanoparticles, e.g. AgNPs and AuNPs on various transition metal substrates [11,17,18]. Indeed, Al, Ni, Zn as well as Ag, Au and Cu substrates provide promising SERS enhancement factors of 107–109 for adsorbed TP molecules using AuNPs [19,20]. We noted that adsorbed species on metal substrates are slightly decomposed or evaporated due to huge electric field at a nanogap. For instance, various alkane thiol molecules adsorbed on Ag substrates with an SAAg covalent bond are prone to be evaporated or decomposed to form amorphous carbon (a-carbon) even under quite weak laser power density of 1 lW/lm2. Such instability of adsorbates disturbs Raman spectral measurements under gap mode resonance. The photochemical decomposition of adsorbates
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is also serious in tip-enhanced Raman spectroscopy (TERS), in which the gap mode is critical to detecting extremely weak Raman signal from tiny amount of adsorbates in the close vicinity of a probe. Pettinger investigated suppressing such carbonation of dye molecules using ultrahigh vacuum conditions [21,22]. Hayazawa also reported that a-carbon seriously contaminates the Si tip during or after the evaporation of Ag based on TERS spectra [23], which vary even during measurements. Hence, the probes or substrates for TERS are mostly contaminated by decomposition of adsorbates during metal coating and/or spectral measurements. Thus, elucidation of each photochemical reaction or decomposition of various adsorbates is essential in overcoming the issue of instability of adsorbates to detect their inherent Raman spectra, and to exploit related photocatalytic reactions. Concerning plasmon induced instability and reactions, we have observed invariable Raman spectra from rhodamine dye, DNA bases and various thiols between AgNPs and Ag films under a gap mode resonance [19,20]. 2. Experimental Preparation of samples. Silver nanoparticles (AgNPs) were synthesized by a citrate reduction method [19,20] in which citrate molecules reduce Ag+ ions, and also stabilize AgNPs in suspensions. Silver films (thickness, t = 45 nm) were deposited on Ge (t = 1 nm) films pre-deposited on a hemicylindrical prism (BK-7) to diminish roughness of Ag films [24]. Self-assembled-monolayers of substituted TP molecules were formed on fresh Ag films by immersing
the prism/Ge/Ag films in ethanol solutions of substituted TP molecules (1 mM) for 1 h, and rinsed with pure methanol to remove excess and uncombined thiol molecules. The substituted TP-SAM coated Ag/Ge/BK-7 prism was immersed in an AgNP suspension for 1 h to immobilize AgNPs on Ag films through van der Waals interaction [25]. Surface coverage of AgNPs (hAgNP) 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. Here hAgNP was evaluated for each sample by SEM observations (Hitachi S4100). Spectral measurements. A PSP coupled gap mode was excited for a sample of AgNPs immobilized on TP-SAM/Ag films/prism in air under ATR geometry with a solid laser (k = 532.0 nm, typically with a power of 10 mW corresponding to a power density of 1 lW/lm2 using a conventional objective of f = 100 mm) at the incident angles of 42.5°, which is a resonance angle for PSP coupled gap mode at hAgNP of 5% in air. External geometry was also used to excite a gap mode by illuminating from air side of the same sample. Reflectivity and Raman spectra of TP were measured using a photodiode and a Raman spectrometer consisting of conventional optics, Chromex 250 is spectrometer, and Andor CCD detector (DU420). Photocatalytic reaction was studied by observing Raman spectral changes of substituted TP-SAM on Ag films under ATR and external geometry in air, if otherwise noted. Different conditions such as different laser power between 1 mW and 20 mW (532 nm) and measurements in vacuum (101–102 Torr) as well as in the air were employed to examine the effect of laser power, and oxygen on the photocatalytic reaction.
Fig. 1. Raman spectral changes with duration of time for p-methyl TP molecules under a gap mode resonance#: (a) between 1750 and 550 cm1, (b) at 1600 cm1 region, (c) temporal variation of the Raman peak intensity at 586 cm1 from p-methyl TP and at 1184 cm1 from PMBA. #The samples of AgNP/p-substituted TP/Ag film (45 nm)/BK-7 prism sample were illuminated by laser at 532 nm with 0.44 lW/lm2 under ATR geometry (hi = 43°) in air, if otherwise noted in Figs. 1–4. The symbols of a, b, c, d, e, f, and g in (a) denote duration of laser irradiation 0, 6, 15, 24, 33, 42, 51 and 60 min. The symbol of * in Figs. 1, 2 and 4 shows Raman peaks from reactant molecules.
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3. Results and discussion We here report photocatalytic oxidation of p-alkyl TP to PMBA in air, which was not observed for o-methyl TP or m-methyl TP. Also, the oxidation of p-methyl TP was perturbed in vacuum. Laser illumination at 532 nm (1 lW/lm2) yielded changes in Raman spectra of p-alkyl TP-SAM formed on Ag films with underlaid Ge thin layer on a hemicylindrical prism (BK-7). Indeed, pronounced peaks of p-alkyl TP at 1594, 1383, 1186, 1075, 1010, 777 and 586 cm1 for p-methyl TP were temporally replaced with those at 1584, 1378, 1184, 1073, 1010, 997, 824 and 687 cm1 during laser illumination in the gap mode geometry (Figs. 1a and b and S1a–S1c). The latter group of Raman peaks are consistent with those of PMBA (Fig. S2a and S2c) at a gap between Ag films and AgNPs, indicating p-alkyl TP is oxidized to PMBA in 1 h (Fig. 1c). Slight differences appeared in Raman spectra of the reaction product from different p-alkyl TP molecules such as a broad peak at 1380 cm1 (msymCOO) for p-methyl TP (Fig. 1a), and a rather sharp peak at 1665 cm1 (mC@O) for p-ethyl TP (Fig. S1a), whereas other peaks such as at 1582, 1184, 1073, 1011 and 660 cm1 were commonly observed for different p-alkyl TP molecules (Figs. 1a and b, 2a–c, S1a–S1c). The broad peak at 1380 cm1 observed for p-methyl TP, and the sharp peak at 1665 cm1 for p-ethyl TP are assigned to a symmetric stretching mode of carboxylate anion (msymCOO) of deprotonated PMBA, and a C@O stretching mode (mC@O) of protonated PMBA [20], respectively. Namely, the changes in observed spectra are attributed to PMBA
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owing to distinct difference in surface pH for different reactants; which is lower for Ag surface coated by p-tertiary butyl TP and higher for that by p-methyl TP than pKa of PMBA (5.9) [20]. Thus, PMBA molecules were formed by the photocatalytic oxidation of palkyl TP under a gap mode resonance, which did not give any evidence for further instability or photocatalytic reactions. In contrast, we observed that PMBA molecules adsorbed on Ag films from ethanol solutions were decarboxylated to form TP by a gap mode irradiation at 532 nm (1 lW/lm2) as evidenced by Raman spectral changes by a laser irradiation (Figs. S2a–S2d) as reported earlier [15,16]. As both of these PMBA molecules commonly adsorb on Ag films via an SAAg bond, the distinct photocatalytic stability is not due to differences in orientation but possibly due to those in electronic state of Ag surfaces as described in SI-(2). With respect to the oxidation of an alkyl group into a carboxyl group, noble metal catalysts such as gold nanoparticles (AuNPs) have been developed in addition to traditional catalysts like KMnO4 and K2Cr2O7. Indeed, various alcohol molecules are catalytically oxidized by quite small AuNPs (with a diameter of 2–6 nm), which are immobilized on metal oxide like TiO2 to have well large surface area and specific electronic state, in particular at peripheral interface between AuNPs and TiO2 substrate [26,27]. Not only alcohol but various organic molecules such as different alkenes and amines are catalytically transformed to industrially valuable materials [26,27]. In contrast to small gold nanoparticles, we first negatively anticipated that our AgNPs with exceedingly larger sizes (30–40 nm) offers no such catalytic properties due to their bulk
Fig. 2. Raman spectral changes at 1600 cm1 region with duration of time for p-alkyl TP molecules under a gap mode resonance#: (a) p-ethyl TP, (b) p-isopropyl TP, (c) ptertiary butyl TP, and (d) schematic figures for the oxidation of p-alkyl TP. See also Fig. S1a–S1c in more detail.
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like electronic state. However, the large AgNPs on BK-7 glass prism demonstrated catalytic behavior under illumination of light, indicating the observed oxidation is photocatalytic reaction. Intriguingly, solely PSP without using AgNPs did not initiate the oxidation of p-alkyl TP in ATR configurations, probably due to insufficiently high density of photon flux at a nanogap under such conditions. The reaction took place explicitly at above a power density of 0.03 lW/lm2 at 532 nm (Fig. S3a–S3c), and gradually proceeded in 40–60 m at a laser power of 0.8 lW/lm2 under a gap mode resonance. The reaction rate increased almost linearly with the laser power (Fig. S3d), which was accelerated by a factor of 7 by increasing the laser power 1–0.88 lW/lm2. The linear increase in the rate with increasing power is indication of photon-induced oxidations, such as photocatalytic and photothermal reactions, of p-alkyl TP, which is in contrast to Arrhenius type non-linear increase due to local heating of the nanogap [6]. This possibility was supported by a modest increase in temperature of p-alkyl TP under a gap mode resonance [28]. Indeed, the intensity ratio of Stokes and anti-Stokes scattering for the same Raman peak of TP showed only slight temperature increase less than 1 K (see the next paragraph), which allows us to neglect the effect of local temperature increase at adsorbates. As discussed in detail in SI, other possibilities than photocatalytic oxidation causing the observed reactions of p-alkyl TP to PMBA, such as oxidation of palkyl TP by O2 molecules with/without laser illumination, and by direct photochemical reaction, are safely neglected. Consequently, the oxidation of p-alkyl TP is induced by the gap mode plasmon in the air, ruling out other possibilities like oxidation solely by O2 molecules, local heating, and direct electronic excitation of palkyl TP We found that the oxidation of alkyl-TP molecules has site selectivity. Indeed, p-alkyl TP molecules such as p-methyl TP, pethyl TP, p-isopropyl TP and p-tertiary butyl TP molecules, irrespective of normal or branched alkyl groups, were transformed
to p-carboxyl TP (p-mercapto benzoic acid, PMBA) as confirmed by the Raman spectral changes from 1594, 1395, 1080, and 575 cm1 for p-alkyl TP to 1584, 1380, 1076, and 570 cm1 for PMBA (Figs. 1a, and S1a–S1c). On the other hand, o-methyl TP, m-methyl TP, 2,6 (o, o0 )-dimethyl TP, 3,5 (m, m0 )-dimethyl TP did not exhibit such spectral changes, but yielded slight decrease in peak intensity of their Raman bands indicating its faint evaporation under a gap mode resonance (Fig. 3a–d). Note that p-tertiary butyl (t-butyl) TP was oxidized under a gap mode resonance in contrast to the case for chemical oxidation using KMnO4 or K2Cr2O7 in acidic conditions. In the latter case, adsorption of rather large ions like MnO 4 on a t-butyl group and further oxidation is sterically hindered, while benzyl groups (ACHRAPh, where R denotes H or alkyl groups) are oxidized to form benzoic acid. Hence, steric hindrance at a para-alkyl group is not a limitation in the present reaction, possibly because activated oxygen molecules, much smaller than bulky MnO 4 ions, work for the oxidation. We next consider the origin of the site selectivity in the observed photocatalytic reaction, which is specific for substituted aromatic thiol molecules bound to Ag surfaces with an ASAAg bond. Such site-selective reaction of organic molecules is basically attributed to distinct electronic or steric effect of a substituent group at different positions. For example, Hammett’s rule is known to account for slightly different reactivity at m- and p-site substituent in aromatic rings on the basis of electronic and resonance effect [29]. Dissociation constant of p-carboxyl aniline is significantly larger than that of m-carboxy aniline, while the value for p-carboxy toluene is smaller than that for m-carboxy toluene. This type of electronic interaction does not provide the observed drastic site-selectivity that only p-alkyl TP is oxidized on Ag substrates. Instead of such intramolecular effect as in Hammett’s rule, we should take into account the role of AgNPs in gap mode geometry. Because alkyl TP molecules are oriented vertically or in a slightly tilted configuration to Ag substrates [30], hot carriers can be pref-
Fig. 3. Raman spectral changes with duration of time for various o, m-substituted TP molecules under a gap mode resonance#: (a) o-methyl TP, (b) m-methyl TP, (c) 2, 6dimethyl TP, (d) 3, 5-dimethyl TP.
K. Akai, M. Futamata / Chemical Physics Letters 675 (2017) 63–68
Fig. 4. Raman spectral changes with time for p-methyl TP at different oxygen circumstances under a gap mode resonance#: (a) in vacuum, (b) in air after the measurement of (a). The symbols of A, B, C, D, E, F and G in (a) denote 0, 1, 3, 6, 9, 12 and 15 m, respectively. Also, those of a0 , b0 , c0 , d0 , e0 , f0 , g0 , h0 and i0 in (b) denote 0, 30, 60, 90, 120, 150, 180, 210 and 220 m, respectively.
erentially transferred to adsorbed molecules through the paraposition. Indeed, the crucial role of AgNPs in the oxidation of palkyl groups was confirmed by the measurements of the same pmethyl TP molecules on roughened Ag films. For this purpose, Ag films (t=10–15 nm) were evaporated on a BK-7 prism without underlaid Ge thin layers, which have sufficiently large roughness to exploit localized surface plasmon (LSP) for SERS measurements. Note that the oxidation rate of p-methyl TP on this roughened Ag films (without AgNPs) was significantly reduced to 1/4 of that under a gap mode resonance (the data not shown). Incomplete suppression of the oxidation is likely due to discontinuous nature of Ag island films containing nano-crevice, where p-alkyl TP molecules are trapped and bonded with their thiol and alkyl groups. Thus, SERS activity to give pronounced intensity using an LSP is necessary but not sufficient for photocatalytic oxidation of palkyl TP. Consequently, AgNPs are essential in the excitation of a gap mode plasmon, and in the transfer of a multitude of hot carrier from Ag surfaces to p-alkyl TP to initiate their oxidation. In addition, we should note that oxygen molecules, most likely activated to O 2 by a hot electron [6,31,32], are imperative to cause the oxidation of p-alkyl TP (see the next section). Details on hybridized electronic states consisting of oxygen molecules, p-alkyl TP and Ag surfaces are critically important to elucidate the entire oxidation mechanism, which is in our future scope. Nevertheless, specific reaction observed for p-alkyl groups clearly indicates that hot carrier transfer from plasmonic nanostructures, consisting AgNP and Ag substrates, to adsorbed p-alkyl TP initiates the reaction, and then oxygen molecules transform it to PMBA.
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The oxidation of p-alkyl group in TP requires both of the excitation of a gap mode plasmon and oxygen molecules. We convinced that the oxidation of p-alkyl TP efficiently proceeds in the air, but is significantly perturbed in a vacuum. Indeed, new peaks appeared at 1594, 1564, 1381, 1171, 989 and 826 cm1 for p-methyl TP in a vacuum (Fig. 4a), which are significantly different from those of original p-methyl TP and those of PMBA observed in the air (Fig. 1a). The distinct Raman spectra observed in a vacuum are possibly attributed to partial oxidation of p-methyl TP to p-mercapto benzyl alcohol or p-mercapto benzaldehyde molecules, which are not observed in the air even at quite weak laser power (0.03 lW/lm2). Incomplete suppression of the oxidation of palkyl TP in a vacuum is most likely due to insufficiently low pressure (101–102 Torr) in our vacuum cell employed with a conventional rotary pump. Further oxidation to PMBA was observed after switching off the vacuum pump to expose our sample to the air (Fig. 4b). Further study is now being continued to examine the effect of oxygen pressure on the oxidation of p-alkyl TP under a gap mode. These results are consistent with slower rate (<1/2) of the oxidation of p-alkyl TP observed in pure water, in which the same oxidation of p-alkyl TP to PMBA was accomplished as evidenced by the same Raman spectra observed as those in the air (data not shown). Thus, oxygen molecules are required to oxidize p-alkyl TP to PMBA under a gap mode. Recently, Linic reported the activation of O2 molecules on Ag(100) surfaces under a plasmon resonance, in which hot e is transferred from Ag surfaces to form O 2 anions that work for the oxidation of ethylene [6,31,32]. Hence, they suggested the activation of O2 molecules initiates the reaction. However, since the para-alkyl groups were preferentially oxidized (as shown above), hot carrier transfer to p-alkyl TP is the first step in the present reaction. After then, oxygen molecules work for the oxidation of p-alkyl groups to a carboxy group. Although O2 or activated O2 species have never been detected in SERS measurements [8], crucial role of oxygen molecules was convinced in the gap mode induced oxidation of p-alkyl TP. Detailed mechanism of this photocatalytic reaction will be elucidated in the near future. Consequently, we found that alkyl groups at para-position in TP molecules are selectively oxidized to a carboxylic group to form PMBA under a gap mode resonance in air. Ortho- or meta-methyl TP did not show the oxidation. The oxygen molecules are essential to driving the oxidation, while the reaction rate is accelerated by increasing laser power especially at shorter wavelengths. From the p-site selectivity, the oxidation is probably initiated by a nanogap mode plasmon via hot carrier transfer to p-alkyl TP which reacts with activated oxygen molecules. Acknowledgement This work was partly supported by KAKENHI (25286014) by JSPS, by the Salt Science Research Foundation (No. 1618 for 2016) and also by the Foundation of Iron and Steel Institute of Japan (ISIJ, 2016-2017). One (MF) of the authors appreciates Prof. Mitsuru Ishikawa (Josai University) for useful discussions. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2017.02. 081. References [1] M.L. Brongersma, N. Halas, P. Nordlander, Nat. Nanotech. 10 (2015) 25–34. [2] M. Kale, T. Avanesian, P. Christopher, ACS Catal. 4 (2013) 116–128.
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[3] K. Watanabe, D. Menzel, N. Nillius, H.-J. Freund, Chem. Rev. 106 (2006) 4301– 4320. [4] A. Manjavacas, J.G. Liu, V. Kulkarni, P. Nordlander, ACS Nano 8 (2014) 7630– 7638. [5] A. Otto, I. Mrozek, H. Grabhorn, W. Akemann, J. Phys.: Condens. Matter 4 (1992) 1143–1212. [6] P. Christopher, H. Xin, S. Linic, Nat. Chem. 3 (2011) 467–472. [7] Y.-F. Huang, H.-P. Zhu, G.-K. Liu, D.-Y. Wu, Bin Ren, Z.-Q. Tian, J. Am. Chem. Soc. 132 (2010) 9244–9246. [8] L.-B. Zhao, M. Zhang, Bin Ren, Z.-Q. Tian, D.-Y. Wu, J. Phys. Chem. C 118 (2014) 27113–27122. [9] M. Osawa, N. Matsuda, K. Yoshii, I. Uchida, J. Phys. Chem. 98 (1994) 12702– 12707. [10] N. Matsuda, K. Yoshii, K. Ataka, M. Osawa, T. Matsue, I. Uchida, Chem. Lett. 21 (1992) 1385–1388. [11] K. Kim, K.L. Kim, D. Shin, J.-Y. Choi, J. Phys. Chem. C 116 (2012) 4774–4779. [12] M.R. Lopez-Ramirez, D.A. Ruiz, F.J.A. Ferrer, S.P. Centeno, J.F. Arenas, J.C. Otero, J. Soto, J. Phys. Chem. C 120 (2016) 19322–19328. [13] W. Xie, S. Schlucker, S. Nat. Commun. 6 (2015) 7570. [14] P. Xu, L. Kang, N.H. Mack, K.S. Schanze, X. Han, H.-L. Wang, Sci. Rep. 3 (2013) 2997. [15] A. Michota, J. Bukowska, J. Raman Spectrosc. 34 (2003) 21–25.
[16] Y. Zong, G. Guo, M. Xu, Y. Yuan, R. Gu, J. Yao, RSC Adv. 4 (2014) 31810–31816. [17] P.K. Aravind, H. Metiu, Surf. Sci. 124 (1984) 506–528. [18] K. Ikeda, J. Sato, N. Fujimoto, N. Hayazawa, S. Kawata, K. Uosaki, J. Phys. Chem. C 113 (2009) 11816–11821. [19] K. Akai, C. Iida, M. Futamata, J. Opt. 11 (2015) 114008. [20] M. Futamata, M. Ishikura, C. Iida, S. Handa, Faraday Discuss. 178 (2015) 203– 220. [21] B. Pettinger, Topics Appl. Phys. 103 (2006) 217–240. [22] J. Steidtner, B. Pettinger, Phys. Rev. Lett. 100 (2008) 236101. [23] N. Hayazawa, T. Yano, S. Kawata, J. Raman Spectrosc. 43 (2012) 1177–1182. [24] H. Liu, B. Wang, E.S.P. Leong, P. Yamg, Y. Zong, G. Si, J. Teng, S.A. Maier, ACS Nano 4 (2010) 3139. [25] W. Sun, Phys. Chem. Chem. Phys. 16 (2014) 5846–5854. [26] M. Haruta, Faraday Discuss. 152 (2011) 11–32. [27] T. Takei, T. Akita, I. Nakamura, T. Fujitani, M. Okumura, K. Okazaki, J. Huang, T. Ishida, M. Haruta, Adv. Catal. 55 (2012) 1–124. [28] A. Lehmuskero, P. Johansson, H. Rubinsztein-Dunlop, M. Käll, ACS Nano 9 (2015) 3453–3469. [29] C. Hansch, A. Leo, R.W. Taft, Chem. Rev. 91 (1991) 165–195. [30] J. Nara, S. Higashi, Y. Morikawa, T. Ohno, J. Chem. Phys. 120 (2004) 6705–6711. [31] S. Linic, U. Aslam, C. Boerigter, M. Morabito, Nat. Mater. 14 (2015) 567–576. [32] C. Boerigter, R. Campana, M. Morabito, S. Linic, Nat. Commun. 7 (2016) 10545.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Gap mode induced photocatalytic oxidation of p-alkyl thiophenol molecules on silver films Keitaro Akai, Masayuki Futamata ⇑ Saitama University, Saitama 338-8570, Japan
a r t i c l e
i n f o
Article history: Received 7 January 2017 In final form 27 February 2017 Available online 28 February 2017 Keywords: Plasmon induced phtocatalytic reaction p-Methyl thiophenol Gap mode Surface enhanced Raman scattering
a b s t r a c t We found that a gap mode plasmon induced photocatlaytic oxidation of p-alkyl thiophenol (TP) molecules such as p-methyl TP, p-ethyl TP, p-isopropyl TP and p-tertiary butyl TP to p-carboxyl TP, in which 532 nm laser (<1 lW/lm2) impinged on a sample of silver (Ag) nanoparticles/p-alkyl TP/Ag films/BK-7 prism under attenuated total reflection (ATR) and external geometry. In contrast to the parasubstituted TP, o-methyl TP and m-methyl TP did not show the oxidation. The present observation on the oxidation of alkyl TP that is not thermally activated provides insights into the mechanism of plasmon mediated photocatalytic reactions. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Considerable interest has been placed on salient role of surface plasmons of metal nanostructures to exploit in various applications such as nano-optics, biosensing, photothermal therapy and phtocatalytic reactions. Photocatalytic reactions can be induced by a hot electron-hole pair (a hot carrier) that is excited by surface plasmons on metal nanoparticles, but are not always efficiently due to extremely short lifetime of 10 fs [1,2]. Prolonged lifetime of hot carriers by particular nanostructures, such as closely adjacent metal nanoparticles yielding 1–2 ps [3,4], therefore, facilitates hot carrier transfer to 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 [5]. Although the role of lifetime of hot carriers is still a matter for debate, innovative plasmon induced photocatalytic reactions have been reported for various reactants [2,6]. Gap-mode enhanced Raman scattering (GERS), which relies on huge enhancement of electric field at metal nanostructures in particular at a nanogap between metal nanoparticles or between metal nanoparticles and metal substrates, is inherently suitable for in-situ elucidation of relevant photocatalytic reactions. For instance, Tian reported that p-aminothiophenol (PATP) adsorbed on silver and gold surfaces dimerizes to p, p0 mercaptoazobenzene (DMAB) under laser illumination from SERS spectral changes in which new peaks appeared at 1142, 1388, and 1432 cm1 [7,8]. Alternatively, Osawa [9,10] and Kim [11] ⇑ Corresponding author. E-mail address: [email protected] (M. Futamata). http://dx.doi.org/10.1016/j.cplett.2017.02.081 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
accounted for such spectral changes by charge transfer effect, by which b2 modes of PATP, inactive in normal Raman spectroscopy, can be detected. The origin of the observed spectral changes of PATP has been investigated under various conditions, such as at different pH and electrode potentials [7,12,14], but still under debate. Beside those on PATP, only a few reports can be seen in literatures on photochemical reaction of adsorbates on metal surfaces, such as reduction of p-nitrothiophenol (PNTP) on silver under illumination of laser at 632.8 nm [9,10], or on superstructure of silver nanoparticles (AgNPs) [13]. In addition, photocatalytic decarboxylation is observed for p-mercaptobenzoic acid (PMBA) on gold electrodes [15,16], which is also discussed in the present contribution in relation to the oxidation of p-alkyl TP. It is prerequisite to enhance electric field for adsorbates on metal surfaces or between metal nanostructures for efficient transfer of hot carriers to molecules to drive photocatalytic reactions. The gap mode plasmon employed here generates markedly enhanced electric field by a factor of 104–105 at a nanogap between metal nanoparticles, e.g. AgNPs and AuNPs on various transition metal substrates [11,17,18]. Indeed, Al, Ni, Zn as well as Ag, Au and Cu substrates provide promising SERS enhancement factors of 107–109 for adsorbed TP molecules using AuNPs [19,20]. We noted that adsorbed species on metal substrates are slightly decomposed or evaporated due to huge electric field at a nanogap. For instance, various alkane thiol molecules adsorbed on Ag substrates with an SAAg covalent bond are prone to be evaporated or decomposed to form amorphous carbon (a-carbon) even under quite weak laser power density of 1 lW/lm2. Such instability of adsorbates disturbs Raman spectral measurements under gap mode resonance. The photochemical decomposition of adsorbates
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is also serious in tip-enhanced Raman spectroscopy (TERS), in which the gap mode is critical to detecting extremely weak Raman signal from tiny amount of adsorbates in the close vicinity of a probe. Pettinger investigated suppressing such carbonation of dye molecules using ultrahigh vacuum conditions [21,22]. Hayazawa also reported that a-carbon seriously contaminates the Si tip during or after the evaporation of Ag based on TERS spectra [23], which vary even during measurements. Hence, the probes or substrates for TERS are mostly contaminated by decomposition of adsorbates during metal coating and/or spectral measurements. Thus, elucidation of each photochemical reaction or decomposition of various adsorbates is essential in overcoming the issue of instability of adsorbates to detect their inherent Raman spectra, and to exploit related photocatalytic reactions. Concerning plasmon induced instability and reactions, we have observed invariable Raman spectra from rhodamine dye, DNA bases and various thiols between AgNPs and Ag films under a gap mode resonance [19,20]. 2. Experimental Preparation of samples. Silver nanoparticles (AgNPs) were synthesized by a citrate reduction method [19,20] in which citrate molecules reduce Ag+ ions, and also stabilize AgNPs in suspensions. Silver films (thickness, t = 45 nm) were deposited on Ge (t = 1 nm) films pre-deposited on a hemicylindrical prism (BK-7) to diminish roughness of Ag films [24]. Self-assembled-monolayers of substituted TP molecules were formed on fresh Ag films by immersing
the prism/Ge/Ag films in ethanol solutions of substituted TP molecules (1 mM) for 1 h, and rinsed with pure methanol to remove excess and uncombined thiol molecules. The substituted TP-SAM coated Ag/Ge/BK-7 prism was immersed in an AgNP suspension for 1 h to immobilize AgNPs on Ag films through van der Waals interaction [25]. Surface coverage of AgNPs (hAgNP) 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. Here hAgNP was evaluated for each sample by SEM observations (Hitachi S4100). Spectral measurements. A PSP coupled gap mode was excited for a sample of AgNPs immobilized on TP-SAM/Ag films/prism in air under ATR geometry with a solid laser (k = 532.0 nm, typically with a power of 10 mW corresponding to a power density of 1 lW/lm2 using a conventional objective of f = 100 mm) at the incident angles of 42.5°, which is a resonance angle for PSP coupled gap mode at hAgNP of 5% in air. External geometry was also used to excite a gap mode by illuminating from air side of the same sample. Reflectivity and Raman spectra of TP were measured using a photodiode and a Raman spectrometer consisting of conventional optics, Chromex 250 is spectrometer, and Andor CCD detector (DU420). Photocatalytic reaction was studied by observing Raman spectral changes of substituted TP-SAM on Ag films under ATR and external geometry in air, if otherwise noted. Different conditions such as different laser power between 1 mW and 20 mW (532 nm) and measurements in vacuum (101–102 Torr) as well as in the air were employed to examine the effect of laser power, and oxygen on the photocatalytic reaction.
Fig. 1. Raman spectral changes with duration of time for p-methyl TP molecules under a gap mode resonance#: (a) between 1750 and 550 cm1, (b) at 1600 cm1 region, (c) temporal variation of the Raman peak intensity at 586 cm1 from p-methyl TP and at 1184 cm1 from PMBA. #The samples of AgNP/p-substituted TP/Ag film (45 nm)/BK-7 prism sample were illuminated by laser at 532 nm with 0.44 lW/lm2 under ATR geometry (hi = 43°) in air, if otherwise noted in Figs. 1–4. The symbols of a, b, c, d, e, f, and g in (a) denote duration of laser irradiation 0, 6, 15, 24, 33, 42, 51 and 60 min. The symbol of * in Figs. 1, 2 and 4 shows Raman peaks from reactant molecules.
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3. Results and discussion We here report photocatalytic oxidation of p-alkyl TP to PMBA in air, which was not observed for o-methyl TP or m-methyl TP. Also, the oxidation of p-methyl TP was perturbed in vacuum. Laser illumination at 532 nm (1 lW/lm2) yielded changes in Raman spectra of p-alkyl TP-SAM formed on Ag films with underlaid Ge thin layer on a hemicylindrical prism (BK-7). Indeed, pronounced peaks of p-alkyl TP at 1594, 1383, 1186, 1075, 1010, 777 and 586 cm1 for p-methyl TP were temporally replaced with those at 1584, 1378, 1184, 1073, 1010, 997, 824 and 687 cm1 during laser illumination in the gap mode geometry (Figs. 1a and b and S1a–S1c). The latter group of Raman peaks are consistent with those of PMBA (Fig. S2a and S2c) at a gap between Ag films and AgNPs, indicating p-alkyl TP is oxidized to PMBA in 1 h (Fig. 1c). Slight differences appeared in Raman spectra of the reaction product from different p-alkyl TP molecules such as a broad peak at 1380 cm1 (msymCOO) for p-methyl TP (Fig. 1a), and a rather sharp peak at 1665 cm1 (mC@O) for p-ethyl TP (Fig. S1a), whereas other peaks such as at 1582, 1184, 1073, 1011 and 660 cm1 were commonly observed for different p-alkyl TP molecules (Figs. 1a and b, 2a–c, S1a–S1c). The broad peak at 1380 cm1 observed for p-methyl TP, and the sharp peak at 1665 cm1 for p-ethyl TP are assigned to a symmetric stretching mode of carboxylate anion (msymCOO) of deprotonated PMBA, and a C@O stretching mode (mC@O) of protonated PMBA [20], respectively. Namely, the changes in observed spectra are attributed to PMBA
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owing to distinct difference in surface pH for different reactants; which is lower for Ag surface coated by p-tertiary butyl TP and higher for that by p-methyl TP than pKa of PMBA (5.9) [20]. Thus, PMBA molecules were formed by the photocatalytic oxidation of palkyl TP under a gap mode resonance, which did not give any evidence for further instability or photocatalytic reactions. In contrast, we observed that PMBA molecules adsorbed on Ag films from ethanol solutions were decarboxylated to form TP by a gap mode irradiation at 532 nm (1 lW/lm2) as evidenced by Raman spectral changes by a laser irradiation (Figs. S2a–S2d) as reported earlier [15,16]. As both of these PMBA molecules commonly adsorb on Ag films via an SAAg bond, the distinct photocatalytic stability is not due to differences in orientation but possibly due to those in electronic state of Ag surfaces as described in SI-(2). With respect to the oxidation of an alkyl group into a carboxyl group, noble metal catalysts such as gold nanoparticles (AuNPs) have been developed in addition to traditional catalysts like KMnO4 and K2Cr2O7. Indeed, various alcohol molecules are catalytically oxidized by quite small AuNPs (with a diameter of 2–6 nm), which are immobilized on metal oxide like TiO2 to have well large surface area and specific electronic state, in particular at peripheral interface between AuNPs and TiO2 substrate [26,27]. Not only alcohol but various organic molecules such as different alkenes and amines are catalytically transformed to industrially valuable materials [26,27]. In contrast to small gold nanoparticles, we first negatively anticipated that our AgNPs with exceedingly larger sizes (30–40 nm) offers no such catalytic properties due to their bulk
Fig. 2. Raman spectral changes at 1600 cm1 region with duration of time for p-alkyl TP molecules under a gap mode resonance#: (a) p-ethyl TP, (b) p-isopropyl TP, (c) ptertiary butyl TP, and (d) schematic figures for the oxidation of p-alkyl TP. See also Fig. S1a–S1c in more detail.
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like electronic state. However, the large AgNPs on BK-7 glass prism demonstrated catalytic behavior under illumination of light, indicating the observed oxidation is photocatalytic reaction. Intriguingly, solely PSP without using AgNPs did not initiate the oxidation of p-alkyl TP in ATR configurations, probably due to insufficiently high density of photon flux at a nanogap under such conditions. The reaction took place explicitly at above a power density of 0.03 lW/lm2 at 532 nm (Fig. S3a–S3c), and gradually proceeded in 40–60 m at a laser power of 0.8 lW/lm2 under a gap mode resonance. The reaction rate increased almost linearly with the laser power (Fig. S3d), which was accelerated by a factor of 7 by increasing the laser power 1–0.88 lW/lm2. The linear increase in the rate with increasing power is indication of photon-induced oxidations, such as photocatalytic and photothermal reactions, of p-alkyl TP, which is in contrast to Arrhenius type non-linear increase due to local heating of the nanogap [6]. This possibility was supported by a modest increase in temperature of p-alkyl TP under a gap mode resonance [28]. Indeed, the intensity ratio of Stokes and anti-Stokes scattering for the same Raman peak of TP showed only slight temperature increase less than 1 K (see the next paragraph), which allows us to neglect the effect of local temperature increase at adsorbates. As discussed in detail in SI, other possibilities than photocatalytic oxidation causing the observed reactions of p-alkyl TP to PMBA, such as oxidation of palkyl TP by O2 molecules with/without laser illumination, and by direct photochemical reaction, are safely neglected. Consequently, the oxidation of p-alkyl TP is induced by the gap mode plasmon in the air, ruling out other possibilities like oxidation solely by O2 molecules, local heating, and direct electronic excitation of palkyl TP We found that the oxidation of alkyl-TP molecules has site selectivity. Indeed, p-alkyl TP molecules such as p-methyl TP, pethyl TP, p-isopropyl TP and p-tertiary butyl TP molecules, irrespective of normal or branched alkyl groups, were transformed
to p-carboxyl TP (p-mercapto benzoic acid, PMBA) as confirmed by the Raman spectral changes from 1594, 1395, 1080, and 575 cm1 for p-alkyl TP to 1584, 1380, 1076, and 570 cm1 for PMBA (Figs. 1a, and S1a–S1c). On the other hand, o-methyl TP, m-methyl TP, 2,6 (o, o0 )-dimethyl TP, 3,5 (m, m0 )-dimethyl TP did not exhibit such spectral changes, but yielded slight decrease in peak intensity of their Raman bands indicating its faint evaporation under a gap mode resonance (Fig. 3a–d). Note that p-tertiary butyl (t-butyl) TP was oxidized under a gap mode resonance in contrast to the case for chemical oxidation using KMnO4 or K2Cr2O7 in acidic conditions. In the latter case, adsorption of rather large ions like MnO 4 on a t-butyl group and further oxidation is sterically hindered, while benzyl groups (ACHRAPh, where R denotes H or alkyl groups) are oxidized to form benzoic acid. Hence, steric hindrance at a para-alkyl group is not a limitation in the present reaction, possibly because activated oxygen molecules, much smaller than bulky MnO 4 ions, work for the oxidation. We next consider the origin of the site selectivity in the observed photocatalytic reaction, which is specific for substituted aromatic thiol molecules bound to Ag surfaces with an ASAAg bond. Such site-selective reaction of organic molecules is basically attributed to distinct electronic or steric effect of a substituent group at different positions. For example, Hammett’s rule is known to account for slightly different reactivity at m- and p-site substituent in aromatic rings on the basis of electronic and resonance effect [29]. Dissociation constant of p-carboxyl aniline is significantly larger than that of m-carboxy aniline, while the value for p-carboxy toluene is smaller than that for m-carboxy toluene. This type of electronic interaction does not provide the observed drastic site-selectivity that only p-alkyl TP is oxidized on Ag substrates. Instead of such intramolecular effect as in Hammett’s rule, we should take into account the role of AgNPs in gap mode geometry. Because alkyl TP molecules are oriented vertically or in a slightly tilted configuration to Ag substrates [30], hot carriers can be pref-
Fig. 3. Raman spectral changes with duration of time for various o, m-substituted TP molecules under a gap mode resonance#: (a) o-methyl TP, (b) m-methyl TP, (c) 2, 6dimethyl TP, (d) 3, 5-dimethyl TP.
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Fig. 4. Raman spectral changes with time for p-methyl TP at different oxygen circumstances under a gap mode resonance#: (a) in vacuum, (b) in air after the measurement of (a). The symbols of A, B, C, D, E, F and G in (a) denote 0, 1, 3, 6, 9, 12 and 15 m, respectively. Also, those of a0 , b0 , c0 , d0 , e0 , f0 , g0 , h0 and i0 in (b) denote 0, 30, 60, 90, 120, 150, 180, 210 and 220 m, respectively.
erentially transferred to adsorbed molecules through the paraposition. Indeed, the crucial role of AgNPs in the oxidation of palkyl groups was confirmed by the measurements of the same pmethyl TP molecules on roughened Ag films. For this purpose, Ag films (t=10–15 nm) were evaporated on a BK-7 prism without underlaid Ge thin layers, which have sufficiently large roughness to exploit localized surface plasmon (LSP) for SERS measurements. Note that the oxidation rate of p-methyl TP on this roughened Ag films (without AgNPs) was significantly reduced to 1/4 of that under a gap mode resonance (the data not shown). Incomplete suppression of the oxidation is likely due to discontinuous nature of Ag island films containing nano-crevice, where p-alkyl TP molecules are trapped and bonded with their thiol and alkyl groups. Thus, SERS activity to give pronounced intensity using an LSP is necessary but not sufficient for photocatalytic oxidation of palkyl TP. Consequently, AgNPs are essential in the excitation of a gap mode plasmon, and in the transfer of a multitude of hot carrier from Ag surfaces to p-alkyl TP to initiate their oxidation. In addition, we should note that oxygen molecules, most likely activated to O 2 by a hot electron [6,31,32], are imperative to cause the oxidation of p-alkyl TP (see the next section). Details on hybridized electronic states consisting of oxygen molecules, p-alkyl TP and Ag surfaces are critically important to elucidate the entire oxidation mechanism, which is in our future scope. Nevertheless, specific reaction observed for p-alkyl groups clearly indicates that hot carrier transfer from plasmonic nanostructures, consisting AgNP and Ag substrates, to adsorbed p-alkyl TP initiates the reaction, and then oxygen molecules transform it to PMBA.
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The oxidation of p-alkyl group in TP requires both of the excitation of a gap mode plasmon and oxygen molecules. We convinced that the oxidation of p-alkyl TP efficiently proceeds in the air, but is significantly perturbed in a vacuum. Indeed, new peaks appeared at 1594, 1564, 1381, 1171, 989 and 826 cm1 for p-methyl TP in a vacuum (Fig. 4a), which are significantly different from those of original p-methyl TP and those of PMBA observed in the air (Fig. 1a). The distinct Raman spectra observed in a vacuum are possibly attributed to partial oxidation of p-methyl TP to p-mercapto benzyl alcohol or p-mercapto benzaldehyde molecules, which are not observed in the air even at quite weak laser power (0.03 lW/lm2). Incomplete suppression of the oxidation of palkyl TP in a vacuum is most likely due to insufficiently low pressure (101–102 Torr) in our vacuum cell employed with a conventional rotary pump. Further oxidation to PMBA was observed after switching off the vacuum pump to expose our sample to the air (Fig. 4b). Further study is now being continued to examine the effect of oxygen pressure on the oxidation of p-alkyl TP under a gap mode. These results are consistent with slower rate (<1/2) of the oxidation of p-alkyl TP observed in pure water, in which the same oxidation of p-alkyl TP to PMBA was accomplished as evidenced by the same Raman spectra observed as those in the air (data not shown). Thus, oxygen molecules are required to oxidize p-alkyl TP to PMBA under a gap mode. Recently, Linic reported the activation of O2 molecules on Ag(100) surfaces under a plasmon resonance, in which hot e is transferred from Ag surfaces to form O 2 anions that work for the oxidation of ethylene [6,31,32]. Hence, they suggested the activation of O2 molecules initiates the reaction. However, since the para-alkyl groups were preferentially oxidized (as shown above), hot carrier transfer to p-alkyl TP is the first step in the present reaction. After then, oxygen molecules work for the oxidation of p-alkyl groups to a carboxy group. Although O2 or activated O2 species have never been detected in SERS measurements [8], crucial role of oxygen molecules was convinced in the gap mode induced oxidation of p-alkyl TP. Detailed mechanism of this photocatalytic reaction will be elucidated in the near future. Consequently, we found that alkyl groups at para-position in TP molecules are selectively oxidized to a carboxylic group to form PMBA under a gap mode resonance in air. Ortho- or meta-methyl TP did not show the oxidation. The oxygen molecules are essential to driving the oxidation, while the reaction rate is accelerated by increasing laser power especially at shorter wavelengths. From the p-site selectivity, the oxidation is probably initiated by a nanogap mode plasmon via hot carrier transfer to p-alkyl TP which reacts with activated oxygen molecules. Acknowledgement This work was partly supported by KAKENHI (25286014) by JSPS, by the Salt Science Research Foundation (No. 1618 for 2016) and also by the Foundation of Iron and Steel Institute of Japan (ISIJ, 2016-2017). One (MF) of the authors appreciates Prof. Mitsuru Ishikawa (Josai University) for useful discussions. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2017.02. 081. References [1] M.L. Brongersma, N. Halas, P. Nordlander, Nat. Nanotech. 10 (2015) 25–34. [2] M. Kale, T. Avanesian, P. Christopher, ACS Catal. 4 (2013) 116–128.
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