Adsorption Characteristics of 1, 3-Propanedithiol on Gold: Surface

The adsorption behavior of 1,3-propanedithiol (1,3-PDT) on a colloidal gold surface has been investigated by means of surface-enhanced Raman scatterin...
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J. Phys. Chem. B 2000, 104, 6218-6224

Adsorption Characteristics of 1,3-Propanedithiol on Gold: Surface-Enhanced Raman Scattering and Ellipsometry Study Sang Woo Joo, Sang Woo Han, and Kwan Kim* Laboratory of Intelligent Interface, School of Chemistry and Center for Molecular Catalysis, Seoul National UniVersity, Seoul 151-742, Korea ReceiVed: January 13, 2000; In Final Form: March 24, 2000

The adsorption behavior of 1,3-propanedithiol (1,3-PDT) on a colloidal gold surface has been investigated by means of surface-enhanced Raman scattering (SERS). 1,3-PDT chemisorbed dissociatively on gold as a monothiolate. Moreover, we could clearly identify the S-S stretching band that reveals the formation of multilayers of 1,3-PDT on the colloidal gold surface. As for the case of 1,2-ethanedithiol, the intermolecular disulfide bond appeared to form even at a submonolayer coverage limit, implying that the adsorption of R,ω-alkanedithiol on gold does not take place homogeneously from the early stages of self-assembly. On the other hand, referring to the ab initio calculated vibrational frequencies of AuSCH2CH2CH2Cl, we could also analyze the concentration-dependent SER spectral features of 1,3-PDT in terms of the conformational isomerism around the two C-S bonds. Upon the increase of the bulk concentration of 1,3-PDT, a transition from a GG to GT conformation appeared to occur on the sol surface near the monolayer coverage limit; herein the first letter designates the conformation with respect to the C-C bond near the gold surface. On the basis of the concentration-dependent SER peak intensities, the 1,3-PDT-derivatized Au particles appeared to consist in nearly contact configurations. A separate ellipsometry measurement performed with vacuum-evaporated gold substrate revealed that at best one and one-half layered film could be assembled on gold in polar and protic solvents while up to three layers were assembled on gold in n-hexane solution of 1,3-PDT.

Introduction Adsorption and self-assembly of organic molecules on metal surfaces have been extensively studied during the past decade not only from the point of view of basic research but also from the aspect of practical applicability.1-3 Understanding the processes of self-assembly is advantageous for manipulating the physicochemical properties of interfaces for a variety of heterogeneous phenomena such as catalysis, corrosion inhibition, and lubrication.4-7 Recent interest has focused on the application prospects of self-assembled monolayers (SAMs) for efficient electronic and optical devices,8,9 chemical sensors,10 artificial membranes,11 and electron-transfer barriers.12 The most widely studied and well-characterized systems include alkanethiols,13 dialkyl sulfides,14 and dialkyl disulfides15 on gold and silver. Aliphatic as well as aromatic dithiols are known to adsorb on gold as monothiolates by forming a single Au-S covalent bond.16 The other thiol is pendent with respect to the gold surface. Kohli et al.17 further reported from ellipsometry studies that R,ω-dithiols could organize up to eight covalently attached layers on gold in n-hexane and ethanol. The linking chemistry between layers was claimed to be the oxidative formation of a sulfur-sulfur bond. However, in an investigation of the structure and photooxidation of R,ω-aliphatic dithiols on gold, Rieley et al.18 were able to interpret their X-ray photoelectron spectra by presuming that the thiol molecules were to form only monolayers on gold in ethanol. A similar presumption was made by Nakanishi et al.19 during their fabrication of CdS nanoparticulate layers on a SAM of R,ω-aliphatic dithiols on gold. * To whom all correspondence should be addressed. Tel: +82-28806651. Fax: +82-2-8743704. E-mail: [email protected].

Recently, we have provided the spectroscopic evidence that aliphatic as well as aromatic dithiols can constitute multilayers on gold by forming intermolecular S-S bonds.20,21 By means of surface-enhanced Raman scattering (SERS), the S-S stretching bands were clearly identified at ∼505 and ∼509 cm-1, respectively, for 1,2-ethanedithiol20 and p-xylene-R,R′-dithiol21 in aqueous gold sols. In addition, using reflection-absorption infrared spectroscopy as well as ellipsometry, we confirmed that R,ω-aliphatic dithiols could assemble multilayers up to nine covalently attached layers when the gold substrates were immersed in dithiol solutions in n-hexane.22 Furthermore, although dithiol molecules have previously been known to adsorb on silver as dithiolates by forming two Ag-S bonds,23-25 R,ω-aliphatic dithiols also appeared to assemble multilayered films on silver in an n-hexane medium. In this article, we present the adsorption characteristics of 1,3-propanedithiol (1,3-PDT) on gold revealed by SERS in aqueous gold sol. For a more reliable analysis of SER spectra in gold sol, the feasibility of the formation of multilayers on the vacuum-evaporated gold was also examined by means of ellipsometry. 1,3-PDT can be present as a mixture of various conformational isomers around the two C-C bonds and the two C-S bonds. On this basis, the primary aims of the present work were not only to confirm that 1,3-PDT assembles multilayers by forming intermolecular S-S bonds but also to elucidate whether there are conformational preponderances in forming multilayers on gold. To achieve the latter aim, we also performed ab initio vibrational frequency calculations for 1,3-PDT and its related analogues. The calculated frequencies in various conformational isomerisms were successfully applied to the analysis of the SER spectral features of 1,3-PDT in gold sol.

10.1021/jp0001790 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/10/2000

Adsorption of 1,3-Propanedithiol on Gold

J. Phys. Chem. B, Vol. 104, No. 26, 2000 6219

Experimental Section The method of preparation of the aqueous gold sol was reported previously.26 Initially, 133.5 mg of KAuCl4 (Aldrich) was dissolved in 250 mL of water, and the solution was brought to boiling. A solution of 1% sodium citrate (25 mL) was then added to the KAuCl4 solution under vigorous stirring, and boiling was continued for ca. 20 min. The resulting Au sol solution was stable for several weeks. To 1 mL of Au sol solution, 10-1-10-5 M aqueous 1,3-PDT solution was added dropwise to a final concentration of 3 × 10-3-5 × 10-7 M using a micropipet. The purple gold sol became bluish-green by the addition of 1,3-PDT. The chemical 1,3-PDT was purchased from Lancaster and used as received. Other chemicals unless specified were reagent grade, and triply distilled water of resistivity greater than 18.0 MΩ‚cm was used in making aqueous solutions. Raman spectra were obtained using a Renishaw Raman system model 2000 spectrometer equipped with an integral microscope (Olympus BH2-UMA). The 632.8 nm line from a 17 mW air-cooled He/Ne laser (Spectra Physics model 127) was used as the excitation light source; an interference filter (CVI Laser Corp.) was put in front of the He/Ne laser source to reject various unwanted plasma lines. Raman scattering was detected with 180° geometry using a Peltier-cooled (-70 °C) CCD camera (400 × 600 pixels). A glass capillary (Kimax-51) with an outer diameter of 1.3-1.8 mm was used as a sampling device. With an objective lens, the laser beam was focused onto a spot of approximately 2 µm diameter. Data acquisition times were usually 90 s. The holographic grating (1800 grooves/mm) and the slit allowed the spectral resolution to be 1 cm-1. The Raman band of a silicon wafer at 520 cm-1 was used to calibrate the spectrometer, and the accuracy of the spectral measurement was estimated to be better than 1 cm-1. The Raman spectrometer was interfaced with an IBM-compatible PC, and the spectral data were analyzed using Renishaw WiRE software v. 1.2 based on the GRAMS/32C suite program (Galatics). Ellipsometric measurements were made for 1,3-PDT layers self-assembled on gold substrates. Initially, gold substrates were prepared by resistive evaporation of titanium (Aldrich, >99.99%) and gold (Aldrich, >99.99%) at ∼10-6 Torr on batches of glass slides, cleaned previously by sequentially sonicating in ethanol, hot piranha solution (1:3 H2O2 (30%)/H2SO4), and distilled deionized water. Titanium was deposited prior to gold to enhance adhesion of the gold to the substrate. After the deposition of approximately 200 nm of gold, the evaporator was back-filled with nitrogen. The gold substrates were immersed subsequently into a solution of 1,3-PDT in n-hexane or methanol for a predetermined period of time. After the substrates were removed from the solution, they were rinsed with excess solvent and then dried in an N2 gas stream. The ellipsometric thickness of such self-assembled 1,3-PDT films was estimated using a Rudolph Auto EL II optical ellipsometer. The measurement was performed using the 632.8 nm line of a He/Ne laser incident upon the sample at 70°. The ellipsometric parameters, ∆ and Ψ, were determined for both the bare clean substrate and the self-assembled film. The so-called DafIBM program supplied by the Rudolph Technologies was employed to determine the thickness values, assuming that the refractive index of the organic film was 1.45.27 At least four different sampling points were considered in order to obtain averaged thickness values. Computational Methods Ab initio self-consistent field vibrational frequencies were calculated for 1,3-PDT, 1,3-dichloropropane (1,3-DCP), and

Figure 1. (a) OR spectrum of neat 1,3-PDT. (b) SER spectrum of 5 × 10-5 M 1,3-PDT in aqueous gold sol. (c) OR spectrum of neat dipropyl disulfide. The spectral region between 2400 and 1700 cm-1 was omitted due to the absence of any information.

AuSCH2CH2CH2Cl. They were obtained using the Gaussian 94 program28 following geometry optimization with the LANL2DZ basis set. For the optimized geometries, no imaginary frequencies were obtained. The scaling factors were determined by matching the calculated frequencies of the antisymmetric stretching band of the methylene group to the observed ones at ∼2920 cm-1. Results and Discussion Vibrational Assignment of 1,3-PDT. In Figure 1a is shown the ordinary Raman (OR) spectrum of neat 1,3-PDT. The assignments of the OR peaks are somewhat complicated because 1,3-PDT can exist as a mixture of various rotational isomers related to the conformations around the two C-C bonds and the two C-S bonds. Som and Mukherjee29 have shown, however, that it is necessary to consider the conformation around the two C-C bonds only for the assignment of the vibrational spectra of the molecule in both the solid and neat liquid states. Then, depending on the trans (T) or gauche (G) conformation around each C-C bond, three overall conformations are possible, namely TT, TG, and GG. Som and Mukherjee assigned the bands at 663, 701, and 765 cm-1 in Figure 1a to the C-S stretching vibrations of the GG, GG, and TT conformers, respectively. When an SH group is treated as a point mass, the molecular geometry of an aliphatic mercaptan should be similar to the corresponding aliphatic chloride. Since both the effective masses and the force constants are much the same for these two groups, the vibrational spectrum of an aliphatic mercaptan is known to be very similar to that of the corresponding halide. In this respect, it is informative that Thorbjørnsrud et al.30 and Grindheim and Stφlevik31 observed six bands in the C-Cl stretching region of the infrared and Raman spectra of 1,3dichloropropane (1,3-DCP). Two bands at 641 and 678 cm-1 were assigned to the GG conformer, two bands at 657 and 727 cm-1 to the TG conformer, and two bands at 697 and 784 cm-1 to the TT conformer. In the OR spectrum of 1,3-PDT (Figure 1a), six bands are observed in the C-S stretching region, namely the bands at 656, 663, 701, 720, 765, and 784 cm-1. Previously, we have correlated these bands with those at 641, 657, 678, 697, 727, and 784 cm-1, respectively, appearing in the vibrational spectra of 1,3-DCP.24 Such a correlation was partly due to the similarity of the intensity patterns in the same spectral regions of the OR spectra of the two molecules. Then, the six

6220 J. Phys. Chem. B, Vol. 104, No. 26, 2000 TABLE 1: Observed and Calculated Vibrational Frequencies of 1,3-PDT and 1,3-DCPa 1,3-PDT

1,3-DCP

ORb

calcdc

ORb,d

calcdc

assgnte

656 663 701 720 (sh) 765 784 2561 2923 2929

658 670 680 698 737 797 2535 2923 2929

641 657 678 697 727 784

632 648 665 671 730 782

2919 2930

2919 2930

CS/CCl stretch (GG) CS/CCl stretch (GT/TG) CS/CCl stretch (GG) CS/CCl stretch (TT) CS/CCl stretch (GT/TG) CS/CCl stretch (TT) ν(SH) νas(CH2) (GG) νas(CH2) (TG,TT)

0.9084 0.9085 0.9036

0.9038 0.9045 0.8964

scaling factorf GG GT/TG TT

All data given in cm-1. b OR spectral data in the neat state. Calculated frequencies at the RHF/LANL2DZ level. For the scaling factors, see footnote e below. d Taken from ref 30. e G and T refer to the gauche and trans conformers, respectively, with respect to the C-C bond of 1,3-PDT or 1,3-DCP. f Determined by matching the calculated frequencies of νas(CH2) to those of the observed ones. a

c

bands in this region were assigned to the C-S stretching vibrations of 1,3-PDT with GG, TG, GG, TT, TG, and TT conformations, respectively. These assignments disagree with those by Som and Mukherijee.29 In particular, the band at 784 cm-1 in Figure 1a which was assigned to the CH2 rocking mode by Som and Mukherijee is too strong for such an assignment. However, it is possible that this band contains some contribution from a CH2 rocking vibration. Such a view is in complete agreement with the assignment of the band at the same position in the vibrational spectra of 1,3-DCP made by Thorbjφrnsrud et al.30 The appropriateness of the above vibrational assignments is separately examined in this work by performing an ab initio quantum mechanical calculation on 1,3-PDT and 1,3-DCP. In fact, as collectively summarized in Table 1, the above vibrational assignments were surprisingly consistent with the theoretical results. On these grounds, we could also assign the vibrational peaks of 1,3-propanedithiolate. For instance, in the OR spectrum of the aqueous 1,3-propanedithiolate dianion,24 three bands appear in the C-S stretching region, namely at 656, 716, and 773 cm-1. These bands can be correlated with the bands at 663, 720, and 784 cm-1 in Figure 1a. This implies that only the bands due to TG and TT conformers appear in the Raman spectrum of aqueous dianions. The 1,3-propanedithiolate dianion seems not to exist as a GG conformer probably because the Coulombic repulsion between the negative charges destabilizes such a conformation. Also, the information from the dianion spectrum provides partial support for the assignments of the C-S stretching vibrations on 1,3-PDT. SERS of 1,3-PDT in Au Sol: General Spectral Features. Before discussing the SER spectral features of 1,3-PDT in Au sol, it will be worthwhile to recall the SER spectral features observed in Ag sol.24 In aqueous silver sol, the S-H stretching band which appears at 2558 cm-1 in the OR spectrum of 1,3PDT is completely missing. This implies that 1,3-PDT adsorbs dissociatively on silver after losing two thiol protons; two Ag-S bonds are formed. In the C-S stretching region, four bands were identified in the SER spectrum in silver sol, namely at 598, 655, 698, and 748 cm-1. Invoking the fact that when a mercaptan adsorbs on a silver surface its C-S stretching frequency red-shifts by 20-50 cm-1 from the value for neat liquid, the above four bands can be correlated with the bands

Joo et al. at 656, 701, 720, and 784 cm-1 in the OR spectrum of 1,3PDT (Figure 1a), respectively. Namely, the first two bands at 598 and 655 cm-1 in the SER spectrum can be assigned to the GG conformer while the remaining two at 698 and 748 cm-1 are attributable to the TT conformer. Although the SER spectral pattern is barely dependent on the bulk concentration of 1,3PDT, the GG bands are much stronger than the TT bands, implying that the GG conformation of the adsorbate is dominant on the silver surface. The TG conformation, which is dominant in neat liquid, is hardly present on the silver surface. This indicates that the steric effect plays a major role in determining the conformation of adsorbed 1,3-PDT as is the case for 1,2ethanedithiol.14,20 The SER spectrum of 1,3-PDT obtained at a bulk concentration of 5 × 10-5 M in gold sol is shown in Figure 1b. According to the TEM measurement, the average diameter of gold particles was 17 nm. Assuming that the adsorbate is oriented perpendicularly with respect to the gold surface (vide infra), the concentration of 1,3-PDT required for monolayer coverage is estimated to be 3.0 × 10-6 M.21 This implies that the SER spectrum shown in Figure 1b corresponds to 1,3-PDT on gold above the monolayer coverage. One can notice a strong band at 260 cm-1 in Figure 1b which can be attributed to the ν(AuS) band. This indicates that 1,3-PDT has adsorbed on gold after the rupture of its thiol proton. On the other hand, in contrast to the case in Ag sol,24 the S-H stretching band is identified, albeit weak, at 2512 cm-1 in Figure 1b. This suggests that 1,3-PDT should bind to gold forming only one Au-S bond. Another noticeable feature in the Au-sol SER spectrum is the appearance of a very distinct peak at 502 cm-1. Its counterpart is not identified at all in the infrared and OR spectra of 1,3-PDT in neat and anion states, nor in the Ag-sol SER spectrum. Consulting the vibrational spectral data reported for dialkyl disulfides, the peak has to be attributed to the S-S stretching vibration; in the OR spectrum of dipropyl disulfide, the S-S stretching band is identified at 509 cm-1 (see Figure 1c). This suggests that multilayers can be assembled for 1,3-PDT on the gold surface. The prominent appearance of the νas(CH2) and δ(CH2) bands at 2899 and 1413 cm-1, respectively, in Figure 1b would also be associated with the formation of multilayered films. We have to mention that no disulfide species was detected by 1H NMR spectroscopy for 1 M 1,3-PDT in C6D6 left for 20 h in an ambient condition. In addition, the S-S stretching band was not detected at all in the OR spectra of 1,3-PDT in methanol and ethanol. Hence, the present SERS observations dictate that a brushlike polysulfide forms by adsorbing on the gold sol particles (vide infra). Although identified at 2512 cm-1, the SH stretching peak is not intense in Figure 1b. This may arise from a rather parallel orientation of the S-H bond with respect to the gold sol surface. The low intensity of the SH stretching band may also be associated with the long distance of the -SH moiety from the sol surface, caused by the formation of a multilayered film. On the other hand, the fact that the SH stretching frequency in Figure 1b is lower by ∼49 cm-1 than that in Figure 1a may indicate that the pendent SH group is interacting with water or other SH groups by forming hydrogen bonds. On these grounds, we have attempted to see whether the adsorption characteristics of 1,3-PDT on gold are dependent on the surface coverage. SERS of 1,3-PDT in Au Sol: Concentration-Dependent Spectral Features. Figure 2a,b shows the SER spectra in the wavenumber regions of 3000-2400 and 1500-200 cm-1, respectively, taken in gold sol with various concentrations of 1,3-PDT from 5 × 10-7 to 3 × 10-3 M. Unlikely silver sol, the

Adsorption of 1,3-Propanedithiol on Gold SER spectral pattern was dependent on the bulk concentration of 1,3-PDT in gold sol. It can be noticed that the SH and SS stretching bands become intensified upon increasing the bulk concentration of 1,3-PDT. As will be discussed later, the relative peak intensities of the CS stretching bands are also dependent on the bulk concentration of 1,3-PDT. On the other hand, even at a submonolayer coverage limit, the νas(CH2) and δ(CH2) bands appearing respectively at ∼2900 and ∼1420 cm-1 are the most dominant in the SER spectra along with the Au-S stretching band at 260 cm-1. Together with the appearance of the SS stretching band even at 5 × 10-7 M, this seems to indicate that multilayered films are assembled from a very early stage of self-assembly of 1,3-PDT on gold. In the OR spectrum of 1,3-PDT, the νas(CH2) band appears at 2923 cm-1. Its counterpart appears at 2912 cm-1 in the Agsol SER spectrum.24 However, in the Au-sol SER spectra, the νas(CH2) band appears at 2893 cm-1 at 5 × 10-7 M concentration but at 2900 cm-1 at 3 × 10-3 M. The lower frequency in Au sol than in Ag sol may reflect the different adsorbate structures of 1,3-PDT on gold and silver. The concentrationdependent frequency change in Au sol is presumed, on the other hand, to arise from the different extent of the multilayers. Regarding the SH stretching band in Au sol, its peak position was barely dependent on the bulk concentration of 1,3-PDT, even though the peak intensity became gradually more distinct upon increasing the bulk concentration. This may suggest that the local environment of the terminal SH group is little affected by the change in the surface coverage. As mentioned previously, the CS stretching peaks of neat 1,3-PDT could be classified in terms of the conformational isomers. The CS stretching peaks appearing in the Ag-sol SER spectrum were also successfully assigned to appropriate isomers. Since the adsorption behavior of 1,3-PDT on gold is different from that on silver, the assignments made to interpret the Agsol SER spectrum may not be applied to the Au-sol SER spectra, however. To help analyze the CS stretching peaks in the Ausol SER spectra, we have thus performed ab initio quantum mechanical calculations for a hypothetical compound AuSCH2CH2CH2-Cl in four different conformations, GG, GT, TG, and TT; herein, G and T refer to the gauche and trans conformers, respectively, with respect to the C-C bond, and the first letter designates the conformation near to the gold atom. As collectively summarized in Table 2, the calculated frequencies can be correlated surprisingly well with those in the Ausol SER spectra. The two SER bands at 608 and 659 cm-1 can be assigned to the GG conformer whereas the other two SER bands at 626 and 744 cm-1 are assigned to the TG conformer; actually, the SER peak at 608 cm-1 was blended with the stronger peak at 626 cm-1 at low bulk concentration but the two peaks became separated as the bulk concentration was increased. An SER spectral feature at 649 cm-1 is thought to be a mixture of the GT and TT bands. The other pair of GT and TT conformers are assigned to the feature at 702 cm-1 and the shoulder peak at 762 cm-1, respectively. In the first column of Table 2, the OR peak frequencies of 1,3-PDT are listed in parentheses. It can be noticed that one of the two paired CS stretching modes has considerably red-shifted for all conformers upon adsorbing on the gold surface. If one recalls that the electron donation from sulfur to gold should result in the weakening of the C-S bond,23,24 these highly shifted bands could be associated with the stretching motion of the C-S bonds directly anchored to the gold surface. According to the ab initio quantum mechanical calculations on AuS-CH2CH2CH2-Cl, the GG conformer appeared ener-

J. Phys. Chem. B, Vol. 104, No. 26, 2000 6221 TABLE 2: Comparison of SER Peaks of 1,3-PDT with Theoretical Peaks of AuS-CH2CH2CH2-Cla SERSb,c 1,3-PDT 260 502 608 (656) 626 (663) 649 (663) (720) 659 (701) 702 (765) 744 (765) 762 (sh) (784) 2512 2899

calcdd AuS-CH2CH2CH2-Cl

603 618 652 652 662 692 731 761 2899 0.9032 0.8966 0.9006 0.8904

assgnte ν(AuS) ν(SS) CS stretch (GG) CS stretch (TG) CS stretch (GT) CS stretch (TT) CS stretch (GG) CS stretch (GT) CS stretch (TG) CS stretch (TT) ν(SH) νas(CH2) scaling factorf GG GT TG TT

a All data given in cm-1. b SER spectral data in Au sol. c Values in parentheses represent the OR peaks of 1,3-PDT. d Calculated at the RHF/LANL2DZ level. For the scaling factors, see footnote e. e G and T refer to the gauche and trans conformers, respectively, with respect to the C-C bond of 1,3-PDT or AuS-CH2CH2CH2-Cl. The first letter designates the conformation near the gold atom. f Determined by matching the calculated frequencies of νas(CH2) to those of the observed ones.

getically the most stable among the four isomers; the relative stability was in the order of GG > TG > GT > TT. For the species adsorbed on metal surfaces, however, one also has to consider the intermolecular interaction as well as the surface effect. As mentioned previously, one can understand on this basis why the TG conformer is scarcely identifiable in the Agsol SER spectra of 1,3-PDT, although the TG conformer is a dominant species in neat liquid.24 The TG conformation is inappropriate for 1,3-PDT to form two Ag-S bonds when adsorbing on the silver surface. In the Au-sol SER spectra, the CS stretching peak of TG conformers was observed to be dominant at low as well as high surface coverage limits. The CS stretching peak of the TT conformer was negligibly weak at all bulk concentrations. On the other hand, the CS stretching peak of the GT conformer grew as the bulk concentration of 1,3-PDT was increased above the monolayer coverage limit. This can be deduced from Figure 3a, in which the GT-to-GG intensity ratio in the SER spectra is plotted as a function of the bulk concentration of 1,3-PDT in Au sol. The concentration dependence is a sigmoid shape whose inflection point occurs at ∼10-5 M close to a monolayer coverage limit. Although the SER intensity itself may not correctly reflect the relative abundance of specific conformational isomers, the present observation is consonant with the presumed adsorbate structure of 1,3-PDT on gold. For the growth of multilayered film, the GG as well as TT conformations will be energetically unfavorable owing to the closeness of the terminal SH group to gold. Since the latter kind of steric effect could be negligible for the GT and TG conformations, multilayers would form readily from these isomers by forming intermolecular S-S bonds. The fact that the SER peak of ν(CS) of the GG conformer is nonetheless identified even at high bulk concentration may be attributed to its intrinsic, high Raman scattering cross section. In Figure 3b,c are shown the SER peak intensities of the ν(SH) and ν(SS) bands drawn with respect to that of the νas(CH2) band as a function of the bulk concentration of 1,3-PDT in Au sol. For both peaks, a sigmoid shape is identified. Although more S-S bonds might form at high bulk concentra-

6222 J. Phys. Chem. B, Vol. 104, No. 26, 2000

Joo et al.

Figure 2. SER spectra of 1,3-PDT at 5 × 10-7-3 × 10-3 M in gold sol, in the wavenumber regions of (a) 3000-2400 cm-1 and (b) 1500200 cm-1. The peak intensities are normalized with respect to those of νas(CH2) bands in each SER spectrum.

Figure 4. Solvent-dependent ellipsometric measurements of 1,3-PDT on gold substrates in 1 mM n-hexane and 5 mM methanol solutions.

Figure 3. (a) GT-to-GG intensity ratio of the CS stretching modes at 702 and 659 cm-1, respectively, in the SER spectra versus the bulk concentration of 1,3-PDT and SER peak intensities of (b) ν(SH) and (c) ν(SS) bands in Figure 2a,b, respectively, drawn as a function of the concentration of 1,3-PDT in gold sol.

tion, the ν(SS) band intensity attained a plateau at 5 × 10-4 M. At the moment, it is difficult to assess the number of disulfide bonds formed on the gold surface, but the latter observation suggests that the length of polydisulfide is not extensive. Recalling that the νas(CH2) band was upshifted in the Au-sol SER spectra upon increasing the bulk concentration of 1,3-PDT, the multilayers would assume more disordered structures upon the increase in the number of disulfide linkages. We have also to mention that the SER band at 502 cm-1 in Figure 1b, attributed to the SS stretching vibration, is at least five times

weaker than the ν(SS) band at 509 cm-1 in the OR spectrum of dipropyl disulfide (Figure 1c) when their intensities are normalized with respect to those of the νas(CH2) bands. As assumed for the low intensity of the SH stretching band, the low intensity of the SS stretching band may be attributed, on one hand, to the long distance of the -SS- moiety from the sol surface and/ or the parallel alignment of the S-S bonds with respect to the sol surface. In fact, in the region of the S-S stretching vibration, only broad features were observed in the Au-sol SER spectra of larger alkanedithiols such as 1,4-butanedithiol, 1,5-pentanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol, and 1,9nonanedithiol. On the other hand, we have also to mention that the low intensity of the SS stretching band may simply reflect the formation of quite imperfect multilayers specifically on the aqueous, colloidal gold surface. The plausible structure of 1,3PDT on gold deducible from the aforementioned SER spectral features will be described later in a separate section. Ellipsometric Evidence of Multilayer Formation of 1,3PDT. The feasibility of the formation of multilayers of 1,3PDT on the gold surface was separately examined by measuring the ellipsometric thickness of 1,3-PDT layers on vacuumevaporated gold substrates. In Figure 4 are shown the ellipsometric thicknesses of 1,3-PDT layers self-assembled on vacuumevaporated gold substrates in 1 mM n-hexane and 5 mM methanol solution. Even after taking account of the surface roughness factor for the vacuum-evaporated gold film (∼1.3),32 1 mM solution is thought to be concentrated enough for the formation of multilayers. It is seen that the thickness of the selfassembled layer is dependent on the kind of solvent. After a prolonged immersion (for 22 h) in 1 mM n-hexane solution, the thickness of the 1,3-PDT layer attained 1.9 nm, while in 5 mM methanol (for 17 h) the value was 0.7 nm. Assuming that the monothiolate, 1,3-PDT-, is tilted by 30° from the surface normal with a trans-zigzag conformation as is the case for aliphatic monothiolate on Au(111), the thickness of a fullcovered 1,3-PDT monolayer is estimated to be 0.57 nm using the known bond lengths, bond angles, and van der Waals atomic

Adsorption of 1,3-Propanedithiol on Gold radii and the approximate distance between the sulfur atom and the gold surface (0.15 nm).27 Although the present estimate is somewhat rough, it suggests that a trilayered film is formed on gold in 1 mM n-hexane solution while only a one and one-half layered film is formed in methanol solution. We also examined the multilayers of 1,3-PDT in tetrahydrofuran, acetone, and ethanol. In polar and protic solvents, extensive multilayers did not form, as was the case in methanol. A disulfide linkage seemed to form more favorably in a nonpolar medium. We have to mention that we did in fact add a methanol solution of 1,3PDT to the Au sol solution to obtain the SER spectra due to the low solubility of 1,3-PDT in water. Considering that at least a one and one-half layered film can be assembled on gold in a methanol medium, it is not unreasonable to observe the S-S stretching band in the Au-sol SER spectra (see Figure 2). We mentioned previously that no disulfide species was detected by 1H NMR spectroscopy for 1 M 1,3-PDT in C6D6 left for 20 h in an ambient condition. In addition, the S-S stretching band was not detected at all in the OR spectra of 1,3-PDT in methanol and ethanol. Considering that the formation of disulfides from thiols is known to be the first step on an oxidative cascade for S-S species, the apparent absence of the disulfide species in the aqueous as well as the organic medium is intriguing. Although the formation of disulfides does not proceed rapidly in the dissolved state, they may form readily once the dithiol molecules are assembled on a solid substrate; presumably, the activation barrier to form disulfide bonds is lowered following the adsorption of 1,3-PDT on gold. It may also be instructive to recall recent studies that organothiol monolayers on gold oxidize to sulfates and sulfonates under the presence of excess oxidizers.33 If these functional groups are readily formed even in the ambient conditions, multilayers will not be assembled for 1,3-PDT on the gold surface. In the present work, however, we could not identify any evidence for oxidized sulfur species in all SER spectra. This suggests that, for 1,3-PDT on gold, the multilayer formation must be more favorable than the oxidation of thiol groups at least under the absence of excess oxidizers. Plausible Structure of 1,3-PDT on Colloidal Au. Through the C-S stretching peak assignment, 1,3-PDT was confirmed to adsorb on gold by forming a single Au-S bond, assuming the GT and TG conformations with respect to the C-C bonds; the first letter designates the conformation near the gold surface. On the other hand, even at a submonolayer coverage limit, a peak attributable to the S-S stretching vibration was clearly identified in the SER spectrum, illustrating that multilayers can be assembled for 1,3-PDT on the gold surface. In fact, a separate ellipsometry measurement revealed that up to three layers could be assembled on gold in an n-hexane solution of 1,3-PDT. In polar and protic solvents, extensive multilayers seemed not to form on the gold surface, however; a one and one-half layered film was assembled, for instance, in a methanol medium. These ellipsometry measurements suggest that only a limited multilayers will form by 1,3-PDT on the aqueous Au sol surface. The concentration-dependent SER spectral features can be understood on these presumptions. As can be seen in Figure 2, the SER intensity ratio of the ν(CH2) and ν(AuS) bands is nearly independent of the surface concentration of 1,3-PDT on gold. If an extensive multilayer is formed on the gold sol surface, the ν(CH2) band will grow more rapidly than the ν(AuS) band upon increase in the bulk concentration of 1,3-PDT. On the other hand, it can be noticed from Figure 3b,c that the SER intensity of the ν(SH) band changes similarly as that of the ν(SS) band as a function of the

J. Phys. Chem. B, Vol. 104, No. 26, 2000 6223

Figure 5. Plausible adsorbate structures of 1,3-PDT on (a) gold and (b) silver.

bulk concentration of 1,3-PDT in Au sol. Considering that an enough SERS enhancement can occur even for the second layer (vide infra), the comparable intensity variation of the ν(SH) and ν(SS) bands also suggests that the length of polydisulfide formed on the Au sol surface is not extensive. Along with the fact that the Au sol changes in color from purple to bluish-green by the addition of 1,3-PDT, these SER spectral features dictate that the 1,3-PDT-derivatized Au particles should consist in nearly contact configurations as depicted in Figure 5a. In such structures, the intensity ratio between the ν(CH2) and ν(AuS) bands will be nearly constant as seen in Figure 2. The increase of the ν(SH) band concomitant with the increase of the ν(SS) band in Figure 3 can also be explained by the decrease of the chance that the first monothiolate layer interacts with other gold particles due to the presence of the long disulfide. Although the ν(SH) and ν(SS) bands exhibited comparable intensity variations in the concentration-dependent SER spectra, their absolute intensities were not intense enough compared with those in the usual OR spectra; the SER ν(SS) band observed was, for instance, about five times weaker than its analogue in the OR spectrum of dipropyl disulfide when their intensities were normalized with respect to those of the ν(CH2) bands. In the earlier discussion, this was attributed to both parallel orientation and the long distance of the positions from the gold surface. In fact, recalling the electromagnetic (EM) model of SERS developed independently by Moskovits34 and Creighton,35 the enhancement factor of a vibrational dipole aligned parallel to the gold substrate should be ∼4 times smaller than that oriented perpendicular to the gold surface. On the other hand, the EM model dictates that the SERS enhancement factor on the gold sol surface should be proportional to [a/(a + d)],12 where a is the radius of the gold particle ()8.5 nm) and d is the distance from the surface.34,35 In this light, assuming that the thickness of a full-covered 1,3-PDT is 0.57 nm (vide supra), the intensity of the ν(CH2) band in the second layer should be at best 0.46 times weaker than that of the first layer. Therefore, as mentioned previously, unless an extensive multilayer is formed on the Au sol surface, the ν(CH2)-to-ν(AuS) SER intensity ratio will not change dramatically as a function of the surface coverage of 1,3-PDT on Au. Above EM-model based discussion can thus be viewed to support the appropriateness

6224 J. Phys. Chem. B, Vol. 104, No. 26, 2000 of the schematic sketch drawn in Figure 5a for the 1,3-PDTderivatized Au-sol particles. It is clear that 1,3-PDT adsorbs on gold as monothiolate by forming one single Au-S bond while the molecule adsorbs on silver as dithiolates by forming two Ag-S bonds (see Figure 5b). The origin of the different adsorption mechanism of 1,3PDT on gold and silver is not certain at the moment, however. Molecular mechanics calculations would be helpful for a better understanding of such a difference. Our previous molecular mechanics calculations have demonstrated that even the benzenethiolate and benzyl mercaptide can have different structures on Au(111).36 Since the theoretical work on monothiols such as HS and CH3S by Sellers et al.37 cannot be directly applied to the case of dithiols, it will also be worthwhile to perform a separate ab initio quantum mechanical calculation to clarify the different structures of dithiols on gold and silver. Our future work will be advanced toward such directions. Nonetheless, it will be informative to recall our earlier infrared spectroscopy study on the self-assembly of 1,2-benzenedithiol (1,2-BDT) on the vacuum-evaporated gold and silver.38 On both metals, 1,2BDT was chemisorbed by forming two metal-sulfur bonds after deprotonation. Furthermore, from the infrared peak intensities, the free energy of adsorption was estimated to be -27.3 kJ/ mol on gold and -39.6 kJ/mol on silver, suggesting that adsorption on silver should be energetically more favorable than that on gold. Summary and Conclusion We have demonstrated by SERS that 1,3-PDT is adsorbed on the colloidal gold surface as monothiolate by forming only one Au-S bond. The pendent SH group is shown on the other hand to form an intermolecular disulfide bond; the S-S stretching band is clearly identified at 502 cm-1. Surprisingly, as was the case for 1,2-ethanedithiol, the intermolecular disulfide bond appeared to form even at a submonolayer coverage limit, implying that the adsorption of R,ω-alkanedithiol on gold does not take place homogeneously from the early stage of selfassembly. We could also interpret the concentration-dependent SER spectral features of 1,3-PDT in terms of the predominance of the conformational isomers around the two C-S bonds on the gold surface. With reference to the ab initio vibrational frequency calculations on AuSCH2CH2CH2Cl, a transition from a GG to GT conformation appeared to occur on the sol surface near the monolayer coverage limit as the bulk concentration of 1,3-PDT was increased. The TG conformer seemed to be present on gold at low as well as high surface coverage limits while the TT conformer was hardly present at any bulk concentration. The unfavorable nature of the TT conformation could be attributed to the closeness of the terminal SH group to gold. On the basis of the concentration-dependent SER peak intensities, the 1,3-PDT-derivatized Au particles appeared to consist of nearly contact configurations as depicted in Figure 5a. A separate ellipsometry measurement revealed that at best one and one-half layered film could be assembled on gold in polar and protic solvents while up to three layers were assembled on gold in n-hexane solution of 1,3-PDT. Acknowledgment. This work was supported the by Korea Research Foundation Grant KRF-042-D00073. Authors also acknowledge the Korea Science and Engineering Foundation for providing an instrument purchasing fund through the Center for Molecular Catalysis in Seoul National University. S.W.J. and S.W.H. acknowledge the Korea Research Foundation for providing the BK21 fellowship.

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