In Situ SERS Study of Azobenzene Derivative ... - ACS Publications

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In Situ SERS Study of Azobenzene Derivative Formation from 4‑Aminobenzenethiol on Gold, Silver, and Copper Nanostructured Surfaces: What Is the Role of Applied Potential and Used Metal? Marcela Dendisová,† Lukás ̌ Havránek,† Milan Ončaḱ ,‡ and Pavel Matějka*,‡ †

Department of Analytical Chemistry, Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic Department of Physical Chemistry, Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic

‡

S Supporting Information *

ABSTRACT: The aromatic mercapto derivative 4-aminobenzenethiol (4-ABT) is a substance that can be easily adsorbed on Au, Ag, and Cu surfaces, but in some studies, formation of 4,4′-dimercaptoazobenzene (4,4′-DMAB) on Ag and Au is described. We have studied 4-ABT on all three SERS-active metals in a spectroelectrochemical cell aiming at the role of the metal and electrode potential on formation of 4,4′-DMAB at 785-nm excitation. In the case of Au, intense bands of 4,4′-DMAB are observed in a potential range from +0.2 to −0.8 V. Only at very negative potentials do these bands almost disappear and only spectral features of 4-ABT are observed. In the case of Ag, a similar spectral behavior is observed, but relative bands intensities are weaker than on Au. In the case of Cu, there is no spectral evidence of 4,4′-DMAB at any potential value. Only characteristic bands of 4ABT are observed in the whole potential range; the highest signals are obtained at potentials around −0.6 V. Experimental results are supported by DFT calculations. We can conclude that the crucial aspect of surface photocatalytic formation of 4,4′-DMAB from 4ABT is the metal. The reaction is very effective on Au, and it is inhibited on Cu.

1. INTRODUCTION 4-Aminobenzenethiol (4-ABT) is a typical probe molecule able to form a self-assembled monolayer via its thiol group on appropriate metal surfaces. Surface-enhanced Raman scattering (SERS) spectroscopy has become an appropriate technique for studies of analyte layers on solution/metal surface interfaces.1,2 Raman spectral signals of analytes adsorbed on appropriate metal surfaces can be enhanced significantly by factor 105 or higher.3−6 The SERS effect can be used to study small amounts of substances adsorbed on noble metal nanofeatures but only under certain conditions: The introduced molecules or products of surface reaction have to be adsorbed on the surface or located in very close vicinity of the metal surface.7 Roughness of metal surface has to be in nano- and/or microscale level (depending on the excitation wavelength) in order that so-called SERS activity can be gained. The suitable method for the approach of the nanostructured surface is electrochemical roughening using a metal coating of a suitable substrate8 or oxidation−reduction cycles (ORCs)9 when surface morphology and thus the enhancement factor is varied and optimal conditions are sought. The magnitude of the enhancement factor depends on several factors, e.g., applied potential, excitation wavelength, surface coverage, and the above-mentioned morphology.10 The enhancement effects of surface Raman signals of adsorbed molecules arise from two main aspects: physical and chemical enhancement mechanisms. The so-called electromagnetic (EM) mechanism is considered as the main contribution to the enhancement. Chemical © 2013 American Chemical Society

enhancement can be caused, e.g., by a photoinduced charge transfer mechanism,11 tautomerization of surface species, or the charge tunnelling enhancement mechanism.12 Raman bands corresponding to the vibrational modes perpendicular to the surface are significantly enhanced. The thiol group of 4-ABT interacts very strongly with the metal surface and a very strong covalent bond is formed between surface atoms and the sulfur atom.12 The newly formed bond between the sulfur and metal is very strong, but the bond strength differs for the metals used; Roguska et al.13 demonstrated that it is comparable for silver and gold and a rapidly weaker bond was observed on a copper surface. In a past study, performed in electrochemical arrangement, where spectral features were studied in relation to applied potential, the new bands at about 1142, 1392, and 1439 cm−1 of 4-ABT on silver surfaces were assigned to b2 modes from 4-ABT.14 Wang et al.15 observed on silver−gold bimetallic nanostructures similar bands (1141, 1392, and 1435 cm−1) at 514.5-nm excitation as strong spectral features while they were very weak at 1064-nm excitation. The calculated enhancement factors at 514.5-nm excitation were higher for these bands (assigned to b2 modes based on a previous study14) than for a1 vibration modes of 4-ABT. Wu et al.16 calculated that there were no b2 modes in spectra of 4-ABT adsorbed on a silver surface, because the results of DFT calculations showed Received: April 25, 2013 Revised: September 13, 2013 Published: September 16, 2013 21245

dx.doi.org/10.1021/jp4040985 | J. Phys. Chem. C 2013, 117, 21245−21253

The Journal of Physical Chemistry C

Article

electrolyte in spectroelectrochemical cell consisted a 0.1 M aqueous solution of potassium sulfate. 4-Aminobenzenethiol (Figure 1) was purchased from Sigma-Aldrich (97%) and diluted in methanol (p.a.).

that surface photocatalytic coupling reactions yield a new surface species of 4,4′-dimercaptoazobenzene. Furthermore, Fang et al.17 suggested that the so-called b2-modes of 4-ABT belong to −NN− vibrational modes of 4,4′-dimercaptoazobenzene (4,4′-DMAB) produced from 4-ABT by a selective catalytic coupling reaction on silver nanoparticles. Cao et al.18 observed bands of 4,4′-DMAB on Au(core)/Cu(shell) nanoparticles. They compared unmodified Au nanoparticles with Au/Cu systems. It should be noted that the continuous coverage of Au(core) with the Cu layer was not evidenced and the effect of interaction of 4-ABT with Au(core) cannot be excluded. Furthermore, the 633-nm visible excitation was used which represents higher photon energy than 785-nm radiation used in this work. Based on several published studies16,19−22 a new azobenzene compound (4,4′-DMAB) can be formed from 4-ABT by a photocatalytic coupling reaction on both gold and silver surfaces. The marker bands of 4,4′-DMAB are intense maxima at ca. 1142, 1392, and 1439 cm−1. Formation of 4,4′-DMAB and its SERS bands intensities are dependent on many factors such as laser power, SERS-active substrate, excitation wavelength of laser,23 applied potential,23 polarization angle,24 and measurement temperature.25 Intensities and band positions of surface-enhanced Raman signals of 4-ABT and 4,4′-DMAB may depend on temperature as was published by Canpean et al.26 Recently, we have used 4-ABT to evaluate effectiveness of surface-enhanced Raman scattering (SERS) on different Cu substrates at 1064-nm excitation.4 No formation of 4,4′dimercaptoazobenzene (4,4′-DMAB) indicated by the absence of bands around 1430, 1395, and 1140 cm−1 was observed. The observation has been in contrast to the above-mentioned results published for Ag and Au substrates at excitation in the visible range showing that near-infrared excitation on a copper surface reduces the risk of photocatalytic processes. The unsolved issues are (1) the role of copper as a substrate (reducing the risk of surface reaction) and/or (2) the effect of excitation wavelength (as the features of 4,4′-DMAB were extremely weak at this excitation on bimetallic substrate15). The nature and reorientation of the molecule adsorbed on SERS-active nanostructured surfaces (gold, silver, or copper) can be readily determined by electrochemical surface-enhanced Raman scattering (EC SERS) spectroscopy.13,27 The combination of SERS spectroscopy and electrochemistry allows much more comprehensive studying of adsorption processes, enabling us to tune the conditions of analyte sorption onto the metal surface by changing the experimental electrochemical parameters. In this study the special electrochemical cell (previously used for study of antioxidants)28 is used for EC SERS study of 4-ABT adsorbed on gold, silver, and copper substrates to elucidate the role of the applied potential and the metal used. Our results demonstrate that (1) adsorption of 4-ABT is influenced significantly by the applied electrode potential and (2) the formation of the azobenzene derivative depends substantially on the kind of metal used. The importance of the interaction of different metals with the amino group is suggested.

Figure 1. Chemical structures of 4-ABT (a) and 4,4′-DMAB (b).

2.2. Preparation Procedures. SERS-active large-scale substrates were prepared by electrochemical (cathodic) metal coating of a massive platinum target in two-electrode arrangement from an electrochemical bath containing the corresponding ions. Current sequences (based on previously published procedures8,9,29−31) consisted of two to six individual steps (see Table 1). The surface morphology of the gold substrate was modified using ORCs in the potential window −280 to +1220 mV with scan rate 50 mV/s, and 40 cycles were carried out. Table 1. Current Sequences Used for Preparation of LargeScale Substrates bath

Au

Ag

Cu

[Au(CN)2]−

[Ag(NH3)2]+

[Cu(NH3)4]2+

step

I [mA]

t [min]

I [mA]

t [min]

I [mA]

t [min]

1 2 3 4 5 6

5 10 15

15 10 5

7 10

5 15

10 20 30 40 50 60

10 10 10 10 10 10

2.3. Morphological Characterization of SERS-Active Surfaces. Scanning electron microscopy (SEM) was used for characterization of all studied substrates. The microscope JCM5700 CarryScope (JEOL, Japan) with a resolution of 5 nm and a magnification range of 8−30 000× was used for recording the images which were saved as TIFF files. After surface modification using ORCs, nanofeatures were apparent from the SEM images shown in Figure 2. Small cigar nanofeatures, layered polygons, and so-called cauliflower structures were present on gold, silver, and copper surfaces, respectively. The ability of the used surfaces to enhance the scattered signal was defined by enhancement factors. The approximate factors were calculated according to a previous publication.4 The surface morphologies were very diverse when comparing individual metals. Thus, we calculated enhancement factors (EF) for each of the surfaces using different geometrical approximations. Nanofeatures on gold and silver surfaces were approximated by cylinders, and “cauliflowers” on the copper surface were approximated by spheres. Band intensities were compared for stretching CC vibration mode of aromatic ring at ca. 1590 cm−1; the spectra measured at a potential ca. 0 mV were compared mutually. Approximate calculated values of the EFs were 2.06 × 105, 1.15 × 106, and 2.12 × 105 for gold, silver, and copper surface, respectively. The values for Au and Cu are

2. EXPERIMENTAL SECTION 2.1. Chemicals. For preparation of SERS active substrates, citric acid, sodium citrate, potassium dicyanoaurate, sodium hydroxide, silver nitrate, ammoniac solution, and cupric chloride were used. Oxidation−reduction cycles were performed in an aqueous solution of potassium chloride. The 21246



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Figure 2. SEM images of Au, Ag, and Cu surfaces.

Table 2. Experimental Conditions of in Situ Spectroelectrochemical Measurements Au Ag Cu

direction

start potential [mV]

final potential [mV]

forward backward forward backward forward backward

200 −1200 200 −1200 100 −1100

−1100 200 −1100 200 −1100 100

focus distance [mm]

laser power [mW]

integration time [s]

number of accumulations

9.68

100

4

5

9.84

50

2

10

10.70

100

5

5

comparable; silver exhibit 1 order of magnitude higher values than Au and Cu. The order of magnitude for Cu is comparable with our previous results on Cu.4 2.4. Raman Spectroscopy. All in situ SERS spectra were collected using a Raman spectrometer Dimension P2 (Lambda Systems, USA). The spectrometer was equipped with a diode laser emitting radiation at 785 nm and thermoelectrically cooled CCD detector. The laser beam irradiating the surface of the target through a glass window was focused using a micrometer positioning device.28 Measurement conditions as laser power and integration time were optimized for each of the spectral sets as described in Table 2. We should notice that we did not compare the absolute spectral intensities, but we focused on relative intensity ratios of 4-ABT and 4,4′-DMAB features observed on individual metals. The number of repeated (and averaged) accumulations was 5 or 10 considering the time of individual accumulation. Spectra were recorded with varying resolution 6−8 cm−1 with respect to the wavelength of scattered radiation. 2.5. In Situ Spectroelectrochemistry. All measurements were performed in a special electrode cell28 designed as a threeelectrode arrangement and equipped with a salt bridge suitable for various reference electrodes, platinum plate used as auxiliary electrode and contact for a changeable SERS-active working electrode (massive metal target with diameter up to 10 mm coated with various SERS-active metal layers). The electrolyte contained 4-ABT (ca. 5 × 10−5 mol L−1) in 0.1 M K2SO4. Cyclic series of potentials were applied to the working electrode in the direction from positive to negative values (see Table 2) and backward with a step of 100 mV (Table 2); SERS spectra were recorded at each (stabilized during data accumulation) applied potential. The Raman probe head was connected to the spectrometer Dimension P2 via fiber optics as it is illustrated in Scheme 1. (The spectral background of all spectra was corrected uniformly using a previously published procedure based on finite impulse (FIR) filtration.32) 2.6. Theoretical Calculations. Theoretical calculations were performed using Gaussian 2009 quantum chemical

Scheme 1. Scheme of Electrochemical Cell with Potentiostat for in Situ SERS Spectroelectrochemistry Equipped with Platinum Plate (AE), Salt Bridge for Reference Electrode (RE), Contact for Working Electrode (WE), Raman Probe (λ) for Laser Beam (Solid Line), and Back Scattered Signal (Dashed Line)

package.33 Standard set of convergence criteria was used, as implemented in the Gaussian program package. No scaling factor was applied for theoretical calculations. The Raman intensities are given in A∧4/AMU. Note that normal Raman spectra, i.e. not the surface enhanced Raman spectra are presented. Different calculation methods and basis were tested for the reproduction of the Raman spectra of the 4-ABT molecule, namely B3LYP functional and MP2 method also with basis sets LANL2DZ, 6-31+G*, and 6-31++G**. Calculated spectra were from the qualitative point of view comparable but bands intensities ratios were slightly differed (as it is shown in Figure S1). While the spectra provided with the less time21247



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Vibration modes of single molecules (4-ABT and 4,4′DMAB) were assigned using more accurate calculations described above (Table S1). Characteristic vibration modes correspond to theoretical and experimental data shown in Table 3 with expected small differences in Raman shift that are caused by basic principles of normal Raman and surfaceenhanced Raman scattering spectroscopies. 3.1. Gold Surface. In situ SERS spectra recorded on gold substrate from +200 to −1200 mV exhibited overall maximal spectral intensity at potential values of about −600 mV. The bands of azobenzene product are evident at ca. 1437, 1390, and 1140 cm−1 characteristic for (i) in-plane bending symmetrical vibration of C−H bonds in aromatic rings, (ii) coupled vibration mode of stretching N−N vibration with stretching C−C vibration and bending in-plane asymmetrical C−H vibration mode, and (iii) dominantly bending in-plane asymmetrical vibration of the C−H group in ortho position (ν (C−N) is referred in some studies20,23). An apparent shift of the band at ca. 1430 cm−1 (from 1438 to 1421 cm−1) was observed changing the potential from positive to negative potential values indicating probably some rearrangement of adsorbed molecules (Figure 4A). Furthermore, the band intensities of 4,4′-DMAB (Figure 5A) depended on applied potential rather differently than the bands of 4-ABT (Figure 5B). The features of 4,4′-DMAB disappeared practically at negative potentials around −1000 mV and only the bands of 4ABT were observed. Intensities of characteristic vibration modes assigned to 4-ABT (e.g., at ca. 1176 cm−1 assigned to δip (C−H) vibration mode) slightly increased to negative potential values, and at reverse potential change the intensities increased up to ca. −800 (−600) mV and then slowly decreased or were almost unchanged. Below 1600 cm−1 we observed at potential around −1000 mV only one band of aromatic ring vibrations assigned to quadrant stretching vibration of 4-ABT at ca. 1590 cm−1 while at more positive potentials the spectra exhibited two overlapped bands of quadrant stretching modes attributed to both 4-ABT and 4,4′-DMAB at ca. 1590 and 1574 cm−1, respectively (Table 3 and Figure 4A). The potential dependence of the 1574 cm−1 band indicates that this band is another characteristic feature of 4,4′-DMAB. We suppose that the band shift (from ca. 1590 to 1574 cm−1) of the same type of vibration mode is associated with the effect of the azo NN bond on the aromatic ring in the case of 4,4′-DMAB. The described observations mean that both 4-ABT and 4,4′-DMAB can be present simultaneously on the surface, and their ratio depends on the applied potential. Only at very negative potentials (around −1000 mV) the presence of azoderivative was not evident; only bands of 4-ABT were apparent. Huang et al.19 described in detail the reaction mechanism on the silver surface and they assume that the compound 4,4′DMAB is reduced to 4-ABT at such negative potential values. From this comparison we can conclude that the formation of 4,4′-DMAB is a reversible process depending on the applied potential. 3.2. Silver Surface. In spectra measured on the silver substrate from +200 to −1200 mV quite similar spectral behavior as in the case of the gold substrate was observed. Vibration modes characteristic for the azo-derivative at 1438 and 1391 cm−1 (assigned to the same vibrations as in the case of gold surface) were observed almost in the whole potential range (Figure 6A) excluding very negative values of potential. Furthermore, the band at 1140 cm−1 (Figure 6B and Table 3) assigned to asymmetrical in-plane bending vibration of C−H

consuming BLYP/LANL2DZ method are quantitatively different from the results of more reliable methods of quantum chemistry, qualitative agreement of the peak positions is satisfactory and we use it therefore for calculation of the larger systems. In this study, we focus on systems of adsorbed molecules (4ABT and 4,4′-DMAB) with metal clusters (Au5−9, Ag5−9, and Cu9). As model systems we used 4-ABT molecule adsorbed on metal clusters via sulfur atom (via nitrogen atom only in the case of Cu) and 4,4′-DMAB adsorbed on two metal clusters via two sulfur atoms. Nano/microstructured surfaces were simulated by small metal clusters containing 9 copper, silver, or gold atoms for 4-ABT model adsorption and 5 atoms for each cluster for 4,4′-DMAB adsorption. Illustrations of adsorption of 4-ABT and 4,4′-DMAB to one or two metal clusters are shown as Supporting Information (Figure S2). Structures of several adsorbed species (i.e., 4-ABT adsorbed via sulfur, 4,4′-DMAB adsorbed via both sulfur atoms and 4,4′DMAB adsorbed via only one sulfur atom) with metal clusters which were crucial for comparison with experimental data are shown in Figure 3. Comparison of theoretical and experimental spectra for all metals is shown as Supporting Information (Figure S3−5).

Figure 3. Structures of used molecules with metal clusters: (a) 4-ABT adsorbed via sulfur atom to copper cluster, (b) 4,4′-DMAB adsorbed via sulfur atoms to two silver clusters, and (c) 4,4′-DMAB adsorbed via sulfur atom to one gold cluster.

3. RESULTS AND DISCUSSION The main purpose of this work was investigation of the role of proposed photoreaction causing formation of 4,4′-DMAB on gold, silver, and copper nanostructured surfaces under comparable conditions. Considering published studies12,16,17,19−23 it follows that the reaction carries out on gold and silver surface, but no published work has described the photoreaction on a pure copper surface at near-infrared excitation. We studied the possible reaction on different metal nanostructured substrates under electrochemical conditions applying various potential values to the metal surface. Experimental data of vibration modes in observed in situ SERS spectra were compared with theoretical calculated values in close vicinity of metal clusters. The bands positions with their relative intensities and their assignments for gold, silver, and copper surfaces are summarized in Table 3. 21248


21249

m-w m m-w vw w m-w

m-w w m m-s vs m-w w s vs m w m-w vs m

357 446 504 602 621 717

980 994 1047 1098 1120 1183 1187 1288 1306 1395 1445 1471 1560 1589

m w w w m-w m-w w m-w m-w vs s m m m-w m m vw m-s m-s

389 438 514 600 638 716 819 981 1006 1083 1140 1177 1189 1264 1386 1437 1484 1578 1592

rel. int

DFT

m-w m m vs m-w m-w s vs m m-w m-w vs m-s

1048 1095 1122 1181 1182 1283 1299 1395 1443 1465 1560 1590

m-w m m-w w w m-w

rel. int

983

355 451 502 601 622 716

Raman [cm−1]

Ag EXP

w

714

m-s vs m m m-s s w-m m

1074 1141 1192 1303 1390 1434 1472 1576

vw w

w vw

486 542

920 1005

w

rel. int

390

Raman [cm−1]

DFT

w m vs m-w vw m vs m m-w s vw

1560 1575

w w w w

621 716 798 988 1035 1094 1121 1180 1179 1285 1302 1394 1443

m w

rel. int

448 502

Raman [cm−1]

EXP

1594

1488

1176

817 980 1007 1079

636

396

Raman [cm−1]

Cu

s

m

m

w w-m w-m vs

w

m

rel. int

ν (C−S) + δip (C−C)Ar ν (C−S) γ (C−H)Ar δip (C−C)Ar + ν (C−S) + δip (C−H)Ar δip,as (C−C)Ar γ (C−C)Ar γ (C−C)Ar + γ (C−H) + ν (C−N) δip,as (C−C)Ar γas (C−H) ν (C−S) δip (C−H)Ar δip,as (C−H) (ortho)b δip,as (C−H)Ar δip (C−H) δip,s (C−H) ν (N−N) + δip,s (C−H) νas (C−C)Ar + δip,as (C−H) + ν (N−N) δip,s (C−H) + ν (N−N) δip (C−C)Ar + δip,s (C−H) + ν (C−N) νs (CC)Ar ν (CC)Ar

assignment

ν, stretching; δ, in plane bending ; γ, out of plane bending; s, symmetrical; as, asymmetrical; ip, in-plane; Ar, aromatic ring; intensity: vs, very strong; s, strong; m, medium; w, weak; vw, very weak. bOur assignment is based on DFT calculation and agrees with Cao et al.,18 while Canpean et al.20 and Huang et al.23 assigned the band to ν (C−N) and coupled ν (C−N) + δ (C−H), respectively.

a

4-ABT 4,4′-DMAB 4,4′-DMAB 4-ABT 4,4′-DMAB 4,4′-DMAB 4-ABT 4-ABT 4-ABT 4-ABT 4,4′-DMAB 4,4′-DMAB 4,4′-DMAB 4-ABT 4,4′-DMAB 4,4′-DMAB 4,4′-DMAB 4,4′-DMAB 4-ABT 4,4′-DMAB 4-ABT

EXP

Raman [cm−1]

rel. int

DFT

Raman[cm−1]

Au

Table 3. Comparison of Theoretical and Experimental Raman Shifts of 4-ABT and 4,4′-DMAB Vibration Modes and Their Assignmentsa

The Journal of Physical Chemistry C Article



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Figure 4. Details of in situ SERS spectra on Au surface recorded from +200 to −1200 mV.

Figure 5. (A) Intensities of 4,4′-DMAB characteristic bands recorded on the gold surface at decreasing potential (solid line) and reverse potential change (dashed line). (B) Intensities of 4-ABT characteristic bands recorded on the gold surface at decreasing potential (solid line) and reverse potential change (dashed line).

Figure 6. Details of in situ SERS spectra on the Ag surface recorded from +200 to −1200 mV.

−500 mV (Figure 7A). At negative potential values, the band intensities of DMAB were almost below the noise level, whereas bands intensities of 4-ABT represented for example by the 1592 cm−1 band were practically unchanged in the whole potential range. The intensity of the selected 4-ABT band is comparable and in certain ranges even weaker than the bands of 4,4′-DMAB in the case of the gold surface (Figure 7B) whereas in the case of the silver surface the selected 4-ABT band is always more intense than all bands of 4,4′-DMAB. The

group in the ortho position to carbon with nitrogen confirmed the presence of the azo-compound on silver surface. Similarly to the gold substrates a small shift of band at ca. 1433 cm−1 was related to potential changes. On the silver surface both compounds were observed together in a broad range of applied potential values; nevertheless, only features of 4-ABT were evident at negative potentials around −1000 mV. At decreasing direction of potential changes two maximal intensities of 4,4′-DMAB bands were observed at ca. −200 and 21250



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Figure 7. Comparison of silver (A) and gold (B). Intensities of characteristic bands of both 4-ABT (ca. 1592 cm−1) and 4,4′-DMAB (ca. 1433, 1389, and 1143 cm−1).

weaker features of 4,4′-DMAB are less evident in spectra recorded on the silver surface compared to those of the gold surface (Figures 6 vs 4). 3.3. Copper Surface. In the case of copper substrate, where spectra of 4-ABT were recorded in situ in the range of applied potential from +100 to −1100 mV, no azo-benzene derivative bands were observed. The absence of bands at ca. 1430, 1390, and 1140 cm−1 confirmed that the reaction of 4-ABT to 4,4′DMAB was not observed at any potential values. Furthermore, no significant broadening, splitting, or shift of a band at ca. 1590 cm−1 (attributed to ring vibration of 4-ABT) was observed (Figure 8). In the spectra other bands characteristic

Figure 9. Intensities of characteristic bands recorded on the gold surface at decreasing potential (solid line) and reverse potential change (dashed line).

decreased. Different interaction of the 4-ABT on the copper surface can be caused by the different affinities of the metal used to the thiol and amino groups. The affinity of amino group to copper is much higher in comparison with other tested metals. The amino group is attracted to copper and the phenomenon defends formation of an azo-bond between two nitrogens of the bordering 4-ABT molecules. If we accept the hypothesis that the formation of 4,4′-DMAB is a photon-driven reaction, the excitation of adsorbed 4-ABT is necessary. It is hard to estimate correctly the required photon energy in the cases of individual metals, but we can tentatively propose that it is higher for 4-ABT adsorbed on the Cu surface than on Ag and Au. Hence, the photoreaction can be possible even on Cu, but only at shorter wavelengths using visible excitation (e.g., 488, 514, or 633 nm). We would like to note that bands characteristic for 4,4′-DMAB were observed as relatively weak bands on Cu nanosurfaces30,34,35 or on composite Cu/other material nanostructures34−38 at visible excitation wavelengths.

Figure 8. In situ SERS spectra on the Cu surface recorded from +100 to −1100 mV.

for 4-ABT were evident, e.g., a band at ca. 1176 cm−1 assigned to the in-plane bending vibration of C−H bonds of aromatic ring and a band about 1079 cm−1 assigned to the stretching C− S bond vibration mode. From the experimental data and absence of 4,4′-DMAB characteristic features, it is clear that photon-driven formation of 4,4′-DMAB does not occur on the copper surface, and only 4-ABT molecules are adsorbed on the copper substrate. On the copper surface only band intensities of 4-ABT vibration modes were evaluated (Figure 9) since any band confirming the presence of 4,4′-DMAB was not observed. Band intensities of the aromatic ring band increased generally with a decrease of potential, but they fluctuated concurrently. At backward potential change intensities of the bands were almost constant and then (at potentials above −400 mV) gradually

4. CONCLUSION In this work we studied a proposed photocatalytic reaction of 4aminobenzenethiol to 4,4′-dimercaptoazobenzene in the vicinity of metal surfaces of Au, Ag, and Cu. We confirmed the formation of an azo-derivative produced by a catalytic coupling reaction only in the cases of gold and silver surfaces. The formation of the azo-derivative was not observed on the 21251



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copper surface in this study even at shorter wavelengths (785 nm) than 1064 nm which was discussed in our previous study.4 Thus, we can conclude that (in a broader range of excitation energy) there is a crucial role of the metal on the photocatalytic formation of the azobenzene surface complex. We suggest that the reaction is affected by the affinity of heteroatoms (nitrogen and sulfur) to the used metals. Copper exhibited a much higher affinity to nitrogen than gold (and silver), and thus, it reduces the ability to form an azo-bond between two nitrogen atoms of neighboring 4-ABT molecules. Furthermore, the formation of 4,4′-DMAB on both gold and silver can be suppressed efficiently by applying relatively high negative potentials close to −1.0 V.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental and theoretically calculated spectra of 4-ABT and 4,4′-DMAB on different surfaces (Figures S1−S5). Theoretical Raman shifts of 4-ABT and 4,4′-DMAB vibration modes and their assignments (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: + 420 220 443 687. Fax: + 420 220 444 058. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Grant agency of Czech Republic (Project No. P206/11/0951) and from specific university research (MSMT No. 20/2013 − A2_FCHI_2013_042 and A1_FCHI_2013_003) are gratefully acknowledged.

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REFERENCES

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