Leaflike Structured Multilayer Assembly of Dimercaptothiadiazole on

Inset: Magnified spectra in the region of 2550−2400 and 1100−980 cm−1. .... (62, 65-69) The −OH group of the water molecule shows a binding en...
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J. Phys. Chem. C 2009, 113, 10176–10184

Leaflike Structured Multilayer Assembly of Dimercaptothiadiazole on Gold Surface Palraj Kalimuthu, Palanisamy Kalimuthu, and S. Abraham John* Department of Chemistry, Gandhigram Rural UniVersity, Gandhigram-624 302, Dindigul, India ReceiVed: NoVember 25, 2008; ReVised Manuscript ReceiVed: April 25, 2009

Adsorption of a heteroaromatic dithiol, 2,5-dimercapto-1,3,4-thiadiazole (DMT), on Au surface from a completely deaerated aqueous solution leads to the formation of a multilayer assembly via hydrogen bonding with water molecules, whereas adsorption from acetonitrile, ethanol, dimethyl sulfoxide, and chloroform solutions leads to the formation of a monolayer. Cyclic voltammetry (CV), attenuated total reflectance (ATR) infrared spectroscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were used to characterize the monolayer and multilayer assemblies of DMT on Au surface. Since DMT contains two S-H groups, it chemisorbs on Au surface through one of its two S-H groups, while the other S-H group is pointing away from the surface. The presence of free S-H groups on the Au surface was confirmed by CV, ATR-FT-IR, and XPS. It is presumed that the free S-H groups of DMT on the Au surface form a hydrogen bond with the water molecules. Subsequently, DMT molecules in solution form a hydrogen bond with the water molecules attached with DMT on Au surface, and this type of hydrogen bonding network goes on increasing when the soaking time of the Au surface in an aqueous solution of DMT increases. The involvement of water molecules in the multilayer formation was confirmed from the appearance of a broad stretching band around 3300 cm-1 corresponding to the hydrogen bonded O-H in the ATR-FT-IR spectrum, in addition to a binding energy peak at 533.4 eV due to hydrogendonating water molecules in the O 1s region of XPS. The absence of a stretching band characteristic for S-S at 537 cm-1 in the Raman spectrum confirmed that the multilayer assembly was not formed via S-S linkage. However, exposure of DMT multilayer to air shows a stretching band characteristic of S-S, indicating that aerial oxidation leads to the formation of S-S bond. SEM images show that DMT forms a leaflike structured multilayer assembly on Au surface. Introduction

CHART 1: Structure of Thiadiazole Compounds

The heteroaromatic dithiol, 2,5-dimercapto-1,3,4-thiadiazole (DMT) and its derivatives (Chart 1) have been extensively studied as chelating agents and cathode-active material for lithium secondary batteries.1-5 In comparison with many other thiol compounds, a distinguishing feature of DMT and its derivatives is the formation of tautomeric structures having either the thioamide [-NHC(dS)-] or the thiol [-NC(SH)-] form (Scheme 1).6,7 The tautomerization influences the reactivity of the thiadiazoles, which has been demonstrated for polymerization process8 and substitution reactions.9 In the solid state, DMT exists as a thione-thiol tautomer,6 while in solution a solventdependent equilibrium exists between all forms (Scheme 1). DMT has acid dissociation constants of pKa1 ) -1.36 and pKa2 ) 7.5, making its neutral form a very strong acid and its anionic form a weak acid.5,10 Oyama and Buttry have extensively studied how the acid-base chemistry of DMT affects the spectral and redox properties of DMT in different solvents containing weak bases and acids.8,11-15 On the basis of the spectral and electrochemical studies, they concluded that DMT exists as both undissociated (dithiol) and dissociated (thiolate) forms in ethanol and DMSO, since these solvents are capable of dissociating DMT into thiolate species while it exists exclusively in dithiol form in acetonitrile (AN), which is not a sufficiently strong proton acceptor for the deprotonation of DMT.8,11-15 Although the spectral and electrochemical properties of DMT were not studied in an aqueous * To whom correspondence should be addressed. Fax: + 91 451 245 3071. E-mail: [email protected].

SCHEME 1: Proton Dissociation Equilibria of DMT

medium previously, it is expected that water would act as a slightly better proton acceptor than AN and be capable of deprotonating DMT. Recently, the adsorption behavior of n-alkanedithiols and aromatic dithiols on Au, Ag, and Pt surfaces has been reported.16-42 One report has shown that 1,6-hexanedithiol adsorbs flat on Au surface via both the sulfurs.26 However, other reports suggest that it adsorbs through one of the two SH groups and its molecular axis is oriented perpendicular to the metal surface.27-31 Several studies have shown that various structures

10.1021/jp810337s CCC: $40.75  2009 American Chemical Society Published on Web 05/12/2009

Multilayer Assembly of Dimercaptothiadiazole on Au and coverages can be formed by dithiols, and their adsorption geometry depends on the conditions of deposition.22,26,32-35 The adsorption behavior of DMT and its derivative, 5-methyl-2mercapto-1,3,4-thiadiazole, on Au and Cu surfaces has been studied recently.37-40 The objective of the present study is to examine the adsorption behavior of DMT on Au surface from different solvents. Since DMT mainly exists in dithiol form (Scheme 1) in AN, it is interesting to know whether it chemisorbs on Au electrode through both the S-H groups or one of the two S-H groups. If it adsorbs on Au surface via one of the two S-H groups, can the other S-H group engage in the formation of a multilayer through S-S linkage like n-hexanedithiol30 or aromatic dithiols?23,39 Interestingly, we found that adsorption of DMT on Au surface from an aqueous solution leads to the formation of a multilayer, while adsorption from other solvents leads to the formation of a monolayer. The formation of monolayer and multilayer assemblies of DMT was characterized by cyclic voltammetry (CV), attenuated total reflectance infrared spectroscopy (ATR-FT-IR), Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). A broad stretching band around 3300 cm-1, corresponding to a hydrogen bonded O-H in the ATR spectrum, and a binding energy at 533.4 eV in the O 1s region of XPS, due to hydrogen-donating water molecules, suggest that water molecules are responsible for the formation of DMT multilayer assemblies by hydrogen bonding with the free S-H groups of adsorbed DMT on Au surface. Experimental Section Chemicals. 2,5-Dimercapto-1,3,4-thiadiazole (DMT), 5-methyl2-mercapto-1,3,4-thiadiazole (MMT), and 5-amino-2-mercapto1,3,4-thiadiazole (AMT) (Chart 1) were purchased from Aldrich and were used as received. HPLC-grade ethanol (99.9%) purchased from Hayman and acetonitrile purchased from Merck were used. All other chemicals used in this work were of analytical grade. A 0.20 M phosphate buffer (PB) solution (pH ) 7.20) was prepared by using Na2HPO4 and NaH2PO4. The Au working electrode was polished with alumina powder (0.5 µm) and sonicated in water for 5-10 min. The polished Au electrodes were electrochemically cleaned by cycling the potential between -0.20 and +1.50 V in 0.05 M H2SO4 at a scan rate of 1.0 V s-1 for 5 min or until the CV characteristics for a clean Au electrode were obtained. Preparation of DMT Monolayer and Multilayer Assemblies on Au Surface. Monolayer and multilayer of DMT on Au substrates were prepared by varying the soaking time of Au electrodes (0.02 cm2) in 1 mM DMT dissolved in different solvents that were completely deaerated with ultrapure argon gas. During the immersion of electrode into a solution of DMT, an argon blanket was maintained over the solution. The DMT adsorbed Au electrode was removed from the solution and washed with respective solvents and water. Then the electrode was used for electrochemical and spectral measurements. Electrochemical reductive desorption measurements were carried out in completely deaerated 0.10 M KOH in the potential window of 0 to -1.20 V both directly and also after electrochemical oxidation in 0.20 M PB solution. Thin gold films produced by sputtering 99.99% pure gold on glass plates were used as substrates for ATR-FT-IR, Raman spectroscopy, XPS, and SEM measurements. Instrumentation. Electrochemical measurements were performed in a conventional two-compartment cell with a polished polycrystalline 1.6 mm Au as working electrode, Pt wire as

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Figure 1. Absorption spectra of DMT in (a) acetonitrile, (b) ethanol, and (c) water.

counter electrode, and KCl saturated Ag/AgCl as reference electrode. All the electrochemical measurements were carried out with CHI model 650B (Austin, TX) electrochemical workstation. UV-vis spectra were recorded with a Perkin-Elmer Lamda 35 spectrophotometer. SEM measurements were carried out with a Hitachi S-300H and JEOL JSM-5610LV. A JASCO FT-IR 460 Plus model equipped with an ATR attachment of horizontal ZnSe crystal (Pike Technologies) was used for FTIR measurements. The resolution of the spectra was 4 cm-1, and the scans were repeated 100 times. Raman spectra were recorded with a WITEC focus innovation confocal Raman (WITEC Wissenschafid). XPS measurements were carried out by using Shimadzu Axis 165 high-performance multitechnique analysis using an Al KR source with pass energy of 80 eV, where the pressure in the analysis chamber was lower than 1 × 10-8 Torr and the dwell time was 458 ms. The binding energies for identical samples were reproducible within (0.10 eV. The charge under the reductive desorption waves was used to calculate the surface coverage (Γ) of DMT without subtracting the surface roughness factor of Au using the formula Γ ) Q/nFA; where Q is the charge consumed for the reductive desorption waves, n is 1, F is 96 485 C, and A is 0.02 cm2. The charge was calculated using CH Instruments (Austin, TX) software. Results and Discussion UV-Visible Spectra of DMT in Different Solvents. The existence of DMT in dithiol or thiolate form can easily be identified from its characteristic absorption peak in the UV-visible absorption spectrum. Figure 1 shows the absorption spectra for DMT in ethanol, AN, and water. It shows a predominant peak at 318 nm in AN, characteristic of the dithiol form (curve a).14,43 In ethanol, DMT shows a predominant peak at 335 nm due to the dithiol form and a shoulder wave at 261 nm due to thiolate form, indicating that DMT exists as both dithiol and thiolate forms (curve b).14,43 In water, a predominant peak at 329 nm with a shoulder band at 257 nm was observed (curve c), and these bands were assigned to the dithiol and thiolate forms of DMT, respectively. To confirm that the two bands observed are due to dithiol and thiolate, we have added different concentrations of HCl to an aqueous solution of DMT. While increasing the concentration of HCl, the intensity of the band due to thiolate decreased, whereas the intensity of the band due to dithiol increased (see Supporting Information, Figure S1). After the addition of 6 N HCl, the band due to thiolate vanished. These results clearly confirmed that DMT predominantly exists as dithiol form in water. The observed differences in the position

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Figure 2. (A). LSVs obtained for (a) bare Au and DMT adsorbed on Au electrodes from 1 mM DMT in AN for (b) 30 min and (c) 40 h in 0.2 M PB solution at a scan rate of 0.05 V s-1. (B). LSVs obtained for the reductive desorption of DMT adsorbed on Au electrodes for 40 h from AN before (a) and after (b) electrochemical oxidation in PB solution at a scan rate of 0.10 V s-1 in 0.1 M KOH.

of absorption peaks of DMT in three different solvents can be easily explained by the proton acceptor properties of the solvents. Since ethanol is a sufficiently strong base to serve as a proton acceptor, deprotonation of DMT occurs and it exists in a dissociated state (CH3CH2OH2+ and DMT-).14,43 Thus, DMT exists as both dithiol and thiolate forms in ethanol (curve b). AN is not a strong base like ethanol for the deprotonation of DMT, and thus the spectrum corresponds to the dithiol form of DMT (curve a).14 As water is a slightly better proton acceptor than AN, DMT exists in both thiolate and dithiol forms, as expected. Electrochemical Oxidation and Reductive Desorption of DMT Adsorbed on Au Electrodes from Acetonitrile. Figure 2A shows the first run of linear sweep voltammograms (LSVs) obtained for DMT adsorbed for 30 min and 40 h in a completely deaerated 1 mM DMT in acetonitrile (AN) on Au electrodes in 0.20 M PB solution at a scan rate of 50 mV s-1. It is evident from Figure 2A that bare Au electrode does not show any electrochemical response in 0.20 M PB solution (curve a). However, DMT adsorbed from AN for 30 min shows two oxidation waves at 0.15 and 0.38 V (curve b). This is due to the fact that, as evidenced from absorption spectrum (Figure 1a) in AN, DMT exists in dithiol form and it gets adsorbed on Au electrode via one of its -SH groups, leaving the other -SH group free. This free -SH group is oxidized to disulfide at two different potentials in 0.20 M PB solution (Figure 2A). Further, the observed oxidation waves in Figure 2A clearly suggest that the adsorbed DMT does not undergo tautomerism as it does in solution.6,7 The cause for the oxidation of -SH at two different potentials may be due to the adsorption of DMT at different crystallographic planes of the Au surface.44-46 When DMT was adsorbed from AN for 40 h, it is observed that, except a small increase in the oxidation peak currents, no significant change in the oxidation behavior occurred (curve c). To confirm the aforesaid oxidation behavior of DMT adsorbed on Au surface, we have studied the electrochemical behavior of two thiadiazole derivatives, viz., 5-methyl-2-mercapto-1,3,4-thiadiazole (MMT) and 5-amino-2-mercapto-1,3,4-thiadiazole (AMT), adsorbed on Au electrodes from AN solution. No oxidation wave was observed for either MMT or AMT within the potential region of -0.10 to +0.70 V, indicating that the observed oxidation waves for DMT (Figure 2A, curves b and c) correspond to the -SH oxidation. The LSVs recorded for the reductive desorption of DMT adsorbed from AN for 40 h in 0.10 M KOH “before” and “after” electrochemical oxidation in 0.20 M PB solution in the potential window of -0.10 V to +0.70 V are shown in Figure 2B. DMT adsorbed from AN shows a reductive desorp-

tion wave at -0.35 V along with a shoulder wave around -0.90 V (curve a). The reductive desorption of DMT, after electrochemical oxidation in 0.20 M PB solution, shows an identical desorption wave (curve b) as observed for directly desorbed DMT, except for a slight increase in the desorption current at -0.35 V. That is, the reductive desorption behavior was the same for both direct desorption and desorption after electrochemical oxidation. This indicates that the adsorbed DMT layer was stable and not desorbed from the electrode surface after electrochemical oxidation. The appearance of two waves in Figure 2B was attributed to the desorption of DMT adsorbed on different crystallographic domains of the polycrystalline Au electrode.44-46 It is well-known that polycrystalline Au surface is composed of three low-index crystallographic faces.47 The charge under the reductive desorption waves was used to calculate the surface coverage. Integration of reductive desorption waves in Figure 2B (curve b) based on one-electron reduction gives a charge density of 36 µC cm-2 without subtracting the roughness factor. This yields the surface coverage of 4.70 × 10-10 mol cm-2 for DMT, which is in the submonolayer range. Electrochemical Oxidation and Reductive Desorption of DMT Adsorbed on Au Electrodes from Aqueous Solution. Figure 3A shows the first run of LSVs obtained for DMT adsorbed on Au electrodes from a completely deaerated aqueous solution at various time intervals. DMT adsorbed on Au electrode for 15 min shows an oxidation wave at 0.47 V (curve a). In addition, one more oxidation wave also appeared at 0.58 V for DMT adsorbed for 3 h on Au electrode (curve b) and which was shifted to 0.62 V when adsorption of DMT was increased on Au electrodes (curves c-f). Further, the oxidation peak current increases when the adsorption of DMT on Au electrode increases. However, it started to decrease after 40 h adsorption. The observed decrease in the oxidation peak current is likely in that the conductivity of the DMT multilayer may decrease due to the formation of thick film after 40 h of adsorption. Unlike DMT adsorbed from ethanol, DMSO, AN, and chloroform, the observed increase in the oxidation peak currents in aqueous solution (Figure 3A) indicates that the adsorption of DMT increases on Au electrodes with an increase in the soaking time. The charges calculated for DMT adsorbed on Au electrodes at different time intervals due to the oxidation of -SH are given in Table 1. The increase in the charges suggests the formation of DMT multilayer on Au electrode. Although DMT adsorbed from ethanol, AN, DMSO, and chloroform solutions exhibit oxidation waves for -SH, we have not observed any significant

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Figure 3. (A) LSVs obtained for DMT adsorbed on Au electrodes from 1 mM aqueous solution of DMT at different time intervals (a) 0.15 (b) 3 (c) 6 (d) 14 (e) 24, and (f) 40 h in 0.2 M PB solution at a scan rate of 0.05 V s-1. (B) LSVs obtained for the reductive desorption of electrodes used in part A (a-f) and direct desorption of DMT adsorbed for 6 h on Au electrode (g) in 0.1 M KOH at a scan rate of 0.1 V s-1.

TABLE 1: Electrochemical Oxidation and Reductive Desorption Charges of DMT Adsorbed on Au Electrodes from an Aqueous Solution at Various Time Intervals charges (106 C/cm2) soaking time (h)

electrochemical oxidationa

reductive desorptionb (106 C/cm2)

0.15 3.0 6.0 6.0c 14.0 24.0 40.0

34 68 278

47 84 284 48c 1218 1921 2119

1201 1907 2104

a

Based on two-electron oxidation. b Based on two-electron reduction. c Charge obtained from direct reductive desorption.

increase in the peak current when Au electrode was soaked in these solvents, even for a longer time. This indicates that medium plays an important role in the formation of DMT multilayer on Au electrode. As spelled out earlier, since water is a relatively weak proton acceptor, the dithiol form of DMT may present predominantly and thus DMT gets adsorbed on Au electrode via one of the two -SH groups, leaving the other -SH group free from binding. Thus, the surface pKa of the adsorbed DMT was different from that in solution. It is most likely that the hydrogen bonding between water molecules and the free -SH groups of the adsorbed DMT are responsible for the formation of a multilayer. The involvement of hydrogen bonding between water molecules and the -SH groups was confirmed by ATR FT-IR and XPS studies (vide infra). Further, the formation of multilayer via -SH group was also confirmed from the decreased oxidation peak current due to -SH oxidation when the multilayer electrode was immersed in 0.10 M NaOH (see Supporting Information, Figure S2). When DMT multilayer was immersed in NaOH solution, the hydrogen bonding is lost due to the dissociation of the -SH group. This leads to the removal of DMT from the multilayer, and thus, the oxidation current due to S-H oxidation was observed to decrease. Since our main intention is to show the involvement of S-H groups in the formation of multilayer, we have restricted the potential window between -0.10 and +0.80 V (Figure 3A). Actually, we observed a reduction wave at -0.40 V due to the reduction of S-S when we extend the potential to -0.60 V (see Supporting Information, Figure S3). Further, the electrochemical oxidation of S-H to S-S and the absence of S-S linkage in the multilayer formation were

confirmed from the observed huge differences in the reductive desorption charges of DMT multilayer in 0.10 M KOH before and after electrochemical oxidation. After electrochemical oxidation in the potential region of -0.10 to +0.80 V in 0.20 M PB solution, the multilayer electrodes were immediately transferred to 0.1 M KOH for reductive desorption measurements, and the corresponding LSVs are shown in Figure 3B. As mentioned earlier, immersion of multilayer electrode in an alkali solution leads to the removal of DMT molecules from the multilayer. Therefore, we have carried out the electrochemical reductive desorption measurements immediately in 0.10 M KOH. A reductive desorption wave around -0.40 V along with a shoulder wave at -0.84 V were observed for DMT adsorbed for 15 min on Au electrode (curve a), which is similar to the reductive desorption of DMT adsorbed from AN (Figure 2B). Unlike AN, the potential of the reductive desorption waves was shifted to more negative potentials, and their currents were increased as the adsorption of DMT from aqueous solution increases (curves b-f). For DMT adsorbed for 40 h, the reductive desorption waves were observed at -0.47 and -1.10 V (curve f). The reductive desorption charges calculated at different time intervals after electrochemical oxidation are given in Table 1. The identical charges for the reductive desorption waves corresponding to monolayer and multilayer of directly desorbed DMT in 0.1 M KOH indicates that the amount of thiolate chemisorbed on Au electrode is the same for both of them. For example, direct desorption of DMT adsorbed for 6 h on Au electrode shows the same reductive desorption waves (curve g) as observed for DMT adsorbed for 15 min (curve a). This indicates that in both the cases the consumed charge corresponds to the reduction of Au-S. However, the reductive desorption of multilayer in 0.1 M KOH after electrochemical oxidation in 0.2 M PB solution shows higher charges than the directly desorbed DMT multilayer. For example, the reductive desorption of DMT adsorbed for 6 h on Au electrode after electrochemical oxidation shows much enhanced reductive desorption waves (curve c) than the directly desorbed DMT prepared under identical conditions (curve g). The charge estimated from the reductive desorption waves after electrochemical oxidation was nearly 6-fold higher than that obtained for the directly desorbed DMT on Au electrode (Table 1). After electrochemical oxidation, the -SH groups present in the multilayer were oxidized to S-S in 0.2 M PB solution and reductively desorbed in 0.1 M KOH at the same potential where Au-S reduction was observed. These results once again

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confirmed that the DMT multilayer was formed via S-H groups. Parallel observation was made recently in the study of monolayer and multilayer formation of mercaptoferrocene derivatives on Au(111) electrode. It is reported that identical reductive desorption charges are obtained for the monolayer and multilayer of mercaptoferrocene, suggesting that the amount of thiolate chemically bonded to Au electrode for a multilayer and monolayer is the same.48 The reductive desorption of electrochemically generated bilayers of 1,4-benzenedimethanethiol (BDMT) on Au electrode has been reported.49 It is shown that Au-S bond is reduced before the S-S bond, leading to direct desorption of the bilayer, and this is supported by the identical reductive desorption charges obtained for the monolayer and bilayer of BDMT.49 For DMT multilayer, in the present study, both Au-S and S-S reductions occur at the same potential and thus we observed relatively higher reductive desorption charges. Very recently, the reductive desorption behavior of 1-octanethiol and 1,8octanedithiol on Au(111) electrode has been reported.50 It was found that the reductive desorption charge for 1,8-octanedithiol is greater by 32% than that for 1-octanethiol, implying that for a fraction of dithiol molecules both sulfur atoms are involved in the reduction process.50 In the present study, the contribution of S-S charge was confirmed from the observed higher reductive desorption charge for DMT multilayer in 0.1 M KOH after electrochemical oxidation, in contrast to the directly desorbed multilayer. The role of -SH in the formation of DMT multilayer was also verified from the differences in the electrochemical oxidation behavior of multilayer before and after exposure to air. The DMT adsorbed for 6 h shows two oxidation waves at 0.50 and 0.59 V before exposure to air (Figure S4A, curve a in Supporting Information). However, for the electrodes prepared under identical conditions, when exposed to air for 10 and 20 min, the oxidation peak currents were decreased (curves b and c). It is likely that the aerial oxidation of S-H leads to the formation of S-S, and thus a decrease in the oxidation peak current was observed. The formation of S-S after aerial oxidation was confirmed from the observed stretching frequency at 537 cm-1 in Raman spectrum (vide infra). The reductive desorption of DMT multilayer in 0.1 M KOH before and after exposure to air for 10 and 20 min is shown in Figure S4B (Supporting Information). The direct desorption of DMT shows a reductive desorption peak at -0.42 V along with a shoulder wave at -0.84 V (curve a). However, when the electrodes were exposed to air, the reductive desorption currents were significantly increased (curves c and d), similar to desorption of electrochemically oxidized DMT multilayer (curve b). The observed identical reductive desorption characteristics for both aerial and electrochemically oxidized DMT multilayer suggest that both aerial and electrochemical oxidation of DMT multilayer leads to the formation of S-S. SEM Images of DMT Multilayer Assemblies on Au Surface. The morphology of the DMT multilayer formed on Au surface was examined by SEM. The SEM images of DMT adsorbed on Au surface from aqueous solution for 6 and 40 h are shown in parts A and B of Figure 4, respectively. It is interesting to note that DMT adsorbed for 6 h shows a leaflike structure with island growth. The thickness of each leaf was found to be ∼0.8-1.0 µm. As the adsorption time increases from 6 to 40 h, densely packed leaves were formed with a thickness of 2-2.5 µm. The SEM images support that adsorption of DMT on Au surface from an aqueous solution leads to the formation of multilayer assembly with a leaflike structure.

Kalimuthu et al.

Figure 4. SEM images obtained for DMT adsorbed on Au surface for (A) 6 h and (B) 40 h.

ATR-FT-IR and Raman Spectral Studies of DMT Adsorbed on Au Surface. Figure 5A shows the ATR-FT-IR spectra for DMT adsorbed on Au surface from 1 mM aqueous solution for 3, 12, and 40 h. For comparison, the FT-IR spectrum for solid DMT is shown in Figure 5B. The various vibrational frequencies observed for DMT adsorbed on Au surface and solid DMT and their assignments are given in Table 2. It can be seen from Figure 5A that the signals of all the stretching bands were increased when the adsorption of DMT on Au surface increases, indicating that adsorption of DMT increases when the soaking time increases. It is well-known that O-H stretching frequency is very sensitive to hydrogen bonding.51,52 When O-H was not involved in hydrogen bonding, a sharp band will be observed around 3600 cm-1.51-53 The involvement of water molecules in the formation of multilayer assembly through intermolecular hydrogen bonding with free SH groups of DMT can be easily identified from the appearance of a broad band in the region from 3400 to 3100 cm-1, corresponding to O-H stretching. A significant increase in the intensity of the broad band corresponding to hydrogen bonded O-H with respect to increased adsorption time clearly shows that water molecules are responsible for the formation of DMT multilayer. The increased intensity of the O-H band (Figure 5A) does not result from the adsorbed water molecules, as the adsorption of DMT on Au surface makes the film become more and more hydrophobic. As can be seen from the inset of Figure 5A (2550-2400 cm-1), the intensity of the stretching band due to SH at 2478 cm-1 was also increased as the adsorption time increases from 3 to 40 h. Further, the stretching bands observed for solid DMT are retained for DMT adsorbed on Au surface with small shifts in their frequencies. On the basis of the earlier reports, we assigned the strong signals observed between 2950 and 2800 cm-1 to overtones and Fermi resonances.54-56 A band observed at 1538 cm-1 corresponds to CdN stretching and the band at 1225 cm-1 to C-N stretching frequency. The bands at 1160 and 1053 cm-1 are assigned to N-N and C-S stretching frequencies, respectively.13,39,51,55,56 Raman spectroscopy is one of the sensitive techniques to identify disulfide species, because it shows a strong signal around 540 cm-1 for disulfide.38 The Raman spectra obtained for DMT multilayer prepared by 6 h adsorption on Au surface before exposure to air are shown in Figure 6a. The stretching frequencies at 1418, 1043, 737, and 675 cm-1 correspond to ν(CdN), ν(N-N), νas(C-S-C endocyclic), and νs(C-S-C endocyclic), respectively.13 The absence of a band around 540 cm-1, characteristic of the S-S stretching mode, confirms that the DMT multilayer assembly is not formed by S-S linkage. However, the appearance of a band at 537 cm-1 for the same multilayer after exposure to air suggests that only aerial oxidation of DMT multilayer leads to the formation of S-S (Figure 6b).

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Figure 5. (A) ATR spectra obtained for DMT adsorbed on Au surface from 1 mM aqueous solution for (a) 3 h, (b) 12 h, and (c) 40 h. Inset: Magnified spectra in the region of 2550-2400 and 1100-980 cm-1. (B) FT-IR spectrum recorded for solid DMT in KBr pellet.

TABLE 2: Vibrational Data (cm-1) and Their Assignments for DMT/KBr and DMT Adsorbed on Au Surface for 40 h DMT/KBr

ATR DMT/Au

assignment

marked in the spectra

3058 2480 1498 1444 1256 1112 1052

3300 2921 2478 1538 1460 1225 1160 1053

O-H ν(N-H) ν(S-H) ν(CdN) ν(N-H) ν(C-N) ν(N-N) ν(C-S)

1 2 3 4 5 6 7 8

XPS of DMT Multilayer on Au Surface. XPS has been extensively used to characterize SAMs formed from organosulfur compounds terminated with different functional groups.57-59 It is considered as one of the most sensitive methods to provide information about the oxidation states of adsorbed species.58 Figure 7 depicts the deconvoluted high-resolution XPS obtained for DMT adsorbed for 3 and 6 h on Au surfaces in the S 2p, N 1s, and O 1s core level regions. The peak-fitting results and their assignments for DMT adsorbed for 3 h are given in Table 3. It was found that the intensities of S 2p, N 1s, and O 1s core level regions were increased from 500.1, 300.0, and 470.2 (a, c, and e) to 600.0, 490.1, and 1880 (b, d, and f), respectively, while increasing the adsorption time from 3 to 6 h. We have calculated the thickness (d) of the DMT multilayer formed from 3 and 6 h adsorption using relative intensities (I) of the Au (4f) and C (1s) peaks with the corresponding Scofield factors employing the following equation.60 It was assumed the same photoelectron escape depths of gold and carbon for DMT multilayer film were λAu ) 45 Å at 1400 eV and λC ) 35 Å at 1202 eV. From the XPS spectrum, we obtained IAu values of

σC λCEC[1 - exp(-d/λCEC)] IC ) IAu σAu λAuEAu exp(-d/λAuEAu) 48 × 103 and 50 × 103 and IC values of 120 × 102 and 300 × 102 for the DMT multilayer adsorbed from 3 and 6 h soaking time. Thickness (d) values of 7.63 and 12.44 µm were obtained for DMT adsorbed for 3 and 6 h, respectively. On the basis of XPS study, Woll and co-workers have reported that DMT forms a very thin layer (20.9 Å) on Au substrate when it was adsorbed for 15 h in ethanol solution.39 However, the observed higher thickness for DMT, in the present study, clearly suggests that DMT forms a multilayer assembly on the Au electrode when it was adsorbed from an aqueous solution. Inspection of the S 2p spectrum (Figure 7a) reveals four distinct sulfur species with different intensities located at 162, 162.9, 163.7, and 164.5 eV. It has been reported that the binding energies at 162.0 and 162.9

Figure 6. Raman spectra obtained for DMT adsorbed on Au surface from 1 mM aqueous solution for 6 h (a) before and (b) after exposure to air.

eV were attributed to the spin-orbit split doublet of S(2p3/2) and S(2p1/2) and were assigned to the gold-bound sulfur. The binding energies at 163.7 and 164.5 eV, corresponding to the spin-orbit split doublet of S(2p3/2) and S(2p1/2), were assigned to the unbound sulfur.33,59,61 The N 1s spectrum of DMT multilayer was deconvoluted into three component peaks at 397.1, 399.2, and 400.6 eV and were attributed to -Nd, a weakly hydrogen accepting nitrogen, and a strongly hydrogen accepting nitrogen, respectively (Figure 7c).62 The observed binding energies at 399.2 and 400.6 eV suggest that nitrogen atoms may also be involved in the hydrogen bonding with water molecules for the formation of DMT multilayer. Further, the O1s region was recorded to reveal the involvement of water molecules in the formation of DMT multilayer on Au surface. The O 1s spectrum exhibits two components at 532.1 and 533.4 eV (Figure 7e). Adsorption of water on various metal oxide surfaces and their characterization by XPS have been extensively investigated.62-65 It has been reported that adsorbed water molecules in their native environment are typically observed above 534 eV, while the dissociated products of hydroxyl and hydrogen-donating water molecules show peaks at lower binding region.62-64 For example, in the case of TiO2 anatase (101) surface, the O 1s peak due to adsorbed water is observed around 534.5 eV, and when the coverage of water on the surface increases, the peak was shifted to lower binding energies by ∼1 eV.63,65 Recently, the binding energy of 533-533.5 eV was reported for the hydrogen-donating water molecules to the nitrogen atom of pyridine.62 Hence, we assigned the binding energy of 533.4 eV observed in the present work to the hydrogen-donating water molecules binding to the sulfur atom of DMT.62,65-69 The -OH group of the water molecule shows a binding energy around 531.7-532.2 eV.62 Thus, we assigned the binding energy at 532.1 eV in the O 1s region to the OH

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Figure 7. High-resolution XPS obtained for DMT adsorbed on Au surface for 3 and 6 h in the S 2p (a and b), N 1s (c and d), and O 1s (e and f) regions.

TABLE 3: Peak Fitting Results for Positions (eV) and Their Assignments of the XPS Obtained for DMT Adsorbed on Au Substrate for 3 h components

binding energy (eV)

assignment

S 2p

162.0, 162.9

N 1s

163.7, 164.5 397.1 399.2

gold bound sulfur unbound sulfur -Nd weakly hydrogen accepting nitrogen strongly hydrogen accepting nitrogen -OH H-donating H2O

400.6 O 1s

532.1 533.4

group of water molecules. XPS studies strongly support the important role played by the S-H groups and water molecules for the formation of the multilayer assembly of DMT on Au surface. On the basis of CV, ATR-FT-IR, Raman spectroscopy, and XPS results, we proposed the structure for DMT multilayer assembly on Au surface shown in Scheme 2. When Au electrode was immersed in an aqueous solution of DMT, one S-H group of DMT was chemisorbed on Au electrode while the other S-H is pointing away from the electrode surface. The presence of free S-H groups on the Au surface was confirmed from the

CV, XPS, and ATR-FT-IR measurements (vide supra). These free S-H groups on the Au surface form a hydrogen bond with the water molecules in solution. Subsequently, DMT molecules in solution form a hydrogen bond through one of its S-H groups with the water molecules already hydrogen bonded with DMT molecules, while the other S-H group of DMT forms a hydrogen bond with the water molecules in solution, and this type of intermolecular hydrogen bonding network goes on increasing when the adsorption time of Au electrode in an aqueous solution of DMT increases. The hydrogen bonding of thiol hydrogen with the oxygen atom of water molecule has been reported earlier.70 As evidenced from the XPS results, in addition to thiol, the nitrogen atoms of DMT were also involved in hydrogen bonding with water molecules. Conclusions The spontaneous formation of a self-assembled multilayer assembly of DMT through hydrogen bonding with water molecules was demonstrated. CV and XPS studies showed that DMT has been adsorbed on Au surface through one of its two S-H groups, leaving the other S-H group free and projecting away from the electrode surface. On the basis of CV, XPS, and ATR-FT-IR results, we propose that the free S-H groups on Au surface forms hydrogen bonds with water molecules, and subsequently, DMT in solution is attached to those water

Multilayer Assembly of Dimercaptothiadiazole on Au SCHEME 2: Structure for DMT Multilayer Assembly on Au Surface

molecules through H-bonding, and so on. This type of hydrogen bonding network goes on increasing when the soaking time of Au surface in aqueous solution increases. The involvement of water molecules in the formation of DMT multilayer was confirmed from the broad stretching band observed at 3300 cm-1 corresponding to hydrogen bonded O-H in the ATR-FT-IR studies and the observed binding energy at 533.4 eV in the O 1s region from the XPS studies. The electrochemical and aerial oxidation of multilayer leads to the formation of S-S, which was confirmed from the characteristic stretching frequency at 537 cm-1 in the Raman spectrum. SEM images have shown that DMT forms a leaflike structured multilayer assembly on Au substrate. Acknowledgment. Palraj Kalimuthu thanks CSIR (09/ 715(0008)/2008/EMR-I) and Palanisamy Kalimuthu thanks UGC for the award of Senior Research fellowship. S.A.J. thanks the Department of Science and Technology (DST), New Delhi for the financial support under Fast Track Scheme for Young Scientists (No. ST/FTP/CSA-09/2002). Helpful suggestions and comments given by Dr. K. P. Elango, Department of Chemistry, are greatly appreciated. Supporting Information Available: UV-visible spectra of DMT in the presence of various concentration of HCl. Electrochemical oxidation of DMT multilayer in 0.20 M phosphate buffer solution after immersed different time intervals in 0.10 M NaOH. Redox behavior of DMT multilayer on Au electrode in 0.20 M phosphate buffer solution. Electrochemical oxidation and reduction desorption of DMT multilayer after exposed to air. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ray, P.; Gupta, J. J. Indian Chem. Soc. 1934, 11, 403. (2) Siddiqi, K. S.; Islam, V.; Khan, P.; Zaidi, F. R.; Siddiqi, Z. A.; Zaidi, S. A. A. Synth. React. Inorg. Met.-Org. Chem. 1980, 10, 41. (3) Zaidi, S. A. A.; Varshney, D. K. J. Inorg. Nucl. Chem. 1975, 37, 1806. (4) Oyama, N.; Tatsuma, T.; Sato, T.; Sotomura, T. Nature 1995, 373, 598.

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