Volume 55, Number 8, 2001 APPLIED SPECTROSCOPY 10850003-7028 / 01 / 5508-1085$2.00 / 0q 2001 Society for Applied Spectroscopy

Temperature-Dependent FT-IR Spectroscopy Study of Silver1,9-Nonanedithiolate

HYOUK JIN CHOI, SANG WOO HAN, SEUNG JOON LEE, and KWAN KIM*Laboratory of Intelligent Interface, School of Chemistry and Molecular Engineering and Center for Molecular Catalysis, SeoulNational University, Seoul 151-742, Korea

The structure and thermal behavior of silver 1,9-nonanedithiolate(AgNDT) is investigated by means of diffuse re� ectance infraredFourier transform (DRIFT) spectroscopy, X-ray diffraction (XRD),thermogravimetric analysis (TGA), differential scanning calorime-try (DSC), and elemental analysis. The XRD data suggests thatAgNDT has a layered structure but with poor registry between lay-ers. The poor registry seems to be associated with the de� ciency ofAg by 20% in binding to the available sulfur atoms. DRIFT spectraldata nonetheless indicates that the carbon chains take nearly all-trans conformation. The temperature-dependent DRIFT spectraldata suggests that AgNDT should degrade at .530 K without anythermodynamic phase transition, in agreement with the TGA andDSC analysis. The � nding that AgNDT does not exhibit liquid-crys-talline behavior upon melting is attributed to the formation of twoAg–S bonds (as a , v -dithiolate) in AgNDT, even though each Agatom is trigonally bonded to sulfur, as in silver monothiolate(AgSR).

Index Headings: Silver dithiolate; Silver 1,9-nonanedithiolate;Thermal behavior; Infrared spectroscopy; DRIFT.

INTRODUCTION

Mixing of solutions containing Ag1 and primary, un-branched organic thiols [CH3(CH2)nSH] typically leadsrapidly to insoluble precipitates. The resulting solids ex-hibit an in� nite-sheet, two-dimensional, nonmolecularlayered structure.1–7 These layered AgSR species showliquid-crystalline behavior upon melting.2 The latter phe-nomenon is associated with the two structural motifs thatthe coordination of Ag to thiolates changes from trigonalto diagonal and that the interlayer CH3–CH3 contacts dis-rupt to form stacked-disk micellar structures. The struc-ture of AgSR species is also of interest as a 3D analogof 2D self-assembled monolayers (SAMs) of thiols on� at Ag surfaces.8 The alkyl chains in these systems pos-sess fully-extended all-trans conformation. The S–S spac-ing in 3D AgSR is very similar to that observed in close-packed 2D organothiol SAMs on Ag, even though thethiol occupancy (above the Ag layer) in the 3D materialis only 50% of that in the 2D SAMs.3

Natan and colleagues recently reported that a layeredcompound was also formed by mixing Ag1 and 1,5-pen-tanedithiol.9 However, unlike AgSR, gauche conformerswere present in silver 1,5-pentanedithiolate. In addition,the compound did not exhibit liquid-crystalline behaviorupon melting, probably due to the covalent attachmentbetween layers. It is not clear yet whether the presenceof gauche conformers is intrinsic to the dithiolate speciesor whether they simply derive from the relatively shortalkane chains.

Received 5 December 2000; accepted 20 March 2001.* Author to whom correspondence should be sent.

The structural characteristics of silver dithiolate are ofgreat concern in conjunction with the structures of SAMsof organic dithiols on Ag and Au. Dithiols are known toadsorb on Au as monothiolates.10 We recently demon-strated by observing the distinct S–S stretching band thateven multilayered � lms could be assembled by aliphaticdithiols on a gold surface.11,12 In contrast, in our earliersurface-enhanced Raman scattering (SERS) study inaqueous Ag sol, it was concluded that dithiol moleculesadsorbed on Ag as dithiolates by forming two Ag–Sbonds.13,14 Very recently it was found that multilayered� lm appeared to form even on Ag by aliphatic dithiols;however, they did so speci� cally in a nonpolar medium.15

Recently, organic-inorganic heterostructures have at-tracted interest because technologically relevant materialswith speci� c properties should be readily prepared bysystematic variation in the structure and properties of theorganic and inorganic constituents at the molecular lev-el.16 In conjunction with all of these implications, wehave attempted to deduce the structure and thermal be-havior of silver 1,9-nonanedithiolate (AgNDT) by meansof temperature-dependent diffuse re� ectance infraredFourier transform (DRIFT) spectroscopy.

EXPERIMENTAL

Preparation of Silver 1,9-Nonanedithiolate. Silvernitrate (99.8%, Duksan), 1,9-nonanedithiol (97%, Lan-caster), and ethanol (.99.9%, Hayman) were used as re-ceived. Unless speci� ed, other chemicals were reagentgrade, and triply-distilled water (resistivity greater than18 MV·cm) was used throughout. All � asks were cleanedin KOH (1 kg) solution in an isopropanol (18 L) andwater (1 L) mixture. Sample preparation was carried outin Al foil-wrapped � asks to minimize light exposure. Asolution of AgNO3 (1 mmol) in 20 mL of ethanol wasadded dropwise to equimolar dithiol, also in 20 mL ofethanol. After the mixture was stirred for 1 h, the result-ing precipitate was � ltered and then washed with ethanoland acetone. The � nal product was dried for 3 h undervacuum.

Characterization. X-ray diffraction (XRD) patternswere obtained on a Rigaku Dmax-3C diffractometer fora 2u range of 58 to 508 at an angular resolution of 0.058using Cu Ka (1.5418 AÊ ) radiation. Thermogravimetricanalysis (TGA) and differential scanning calorimetry(DSC) analysis were performed on a Rigaku TAS-100thermal analyzer; the analysis was conducted in a nitro-gen atmosphere between 25 and 500 8C at a heating rateof 5 8C/min. Elemental analysis was carried out using aGmbH Vario EL analyzer at the Korea Basic ScienceInstitute. Infrared spectra were measured using a Bruker

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FIG. 1. XRD pattern of AgNDT. SCHEME I. A schematic depiction of the self-assembly process to formAgNDT.

IFS 113v FT-IR spectrometer equipped with a globarlight source and a liquid N2-cooled wide-band mercurycadmium telluride detector. To record the DRIFT spectra,a diffuse re� ection attachment (Harrick Model DRA-2CO) designed to use 6:1, 908 off-axis ellipsoidal mirrorssubtending 20% of the 4p-solid angle was � tted to thesampling compartment of the FT-IR spectrometer. Thesample diluted with KBr was transferred to a 4 mm di-ameter cup without compression and leveled by a gentletap. A reaction chamber, made of stainless steel (HarrickModel HVC-DR2) and loaded with the sample, was lo-cated inside the re� ection attachment. CaF2 crystals wereused as the infrared transparent windows. The tempera-ture of the sampling cup was regulated by a home-madetemperature controller, and the chamber was � ushed con-tinuously with dry nitrogen (;10 mL/min) during themeasurement of DRIFT spectra. A total of 32 scans wasmeasured in the range 3500–1000 cm21 at a resolutionof 4 cm21 using previously scanned pure KBr as the back-ground. The temperature of the sampling cup was raisedat a rate of 10 K/min and kept for 5 min at each speci� edtemperature for recording the DRIFT spectra. The Happ–Genzel appodization function was used in Fourier trans-forming all the interferograms. The DRIFT spectra arereported as 2log(R /Ro), where R and Ro are the re� ec-tance of the sample and of the pure KBr; the DRIFTspectral pattern presented in units of 2log(R /Ro) was,however, barely different from that rendered using theKubelka–Munk equation.

RESULTS AND DISCUSSION

Figure 1 shows the XRD data for AgNDT. Only a fewpeaks are identi� ed as in the XRD data for silver 1,5-pentanedithiolate. Their presence nonetheless suggeststhat AgNDT has a layered structure but with poor registrybetween layers. Elemental analysis showed the atomicratio of sulfur and silver to be 1.24:1.00 (Anal. Calc’dfor C9H18S2Ag2:C, 26.61; H, 4.47; S, 15.79; Ag, 53.13.Found: C, 29.12; H, 4.71; S, 17.83; Ag, 48.34); the ratiowas hardly affected by the variation of the molar ratio ofthe starting materials in synthesis. Since silver is de� cientin concentration with respect to sulfur, the Ag–S slabs inAgNDT will be far more irregular than those in AgSR.

A similar de� ciency in silver was found previously forother silver alkanedithiolates such as 1,4-butanedithiolate,1,5-pentanedithiolate, and 1,6-hexanedithiolate.3,9,17 Forinstance, elemental analysis repeatedly yielded a S:Agratio slightly greater than 1 (1.05–1.1) for silver 1,5-pen-tanedithiolate.9 The broadness of the XRD peaks in Fig.1 implies disorder resulting from variations in layer spac-ing. The interlayer spacing estimated using Bragg’s law,;15.7 AÊ , is, however, rather close to the calculated value,15.2 AÊ , obtained by assuming that the carbon chain isall-trans and the Ag–S slab is 1.0 AÊ thick.9 Along withthis, the intense yellow color of AgNDT indicates thatAg1 is trigonally bonded to thiol as in AgSR.18

Unlike silver alkanedithiolate, AgSR is known to formvery ordered materials. This suggests that the self-assem-bly process of forming AgNDT is different from that offorming AgSR. It has been proposed that AgSR is formedthrough a two-step crystallization mechanism.6 The pri-mary self-assembly process, wherein the puckered quasi-2D sheets of the Ag–S lattice are formed, occurs by di-rected assembly of Ag1 ions with the –SR (deprotonatedthiols). The strong triple coordination between Ag and Sis supposed to drive this assembly. The pseudo-2D build-ing blocks thus formed stack in the third dimension viaa secondary self-assembly process wherein the transla-tionally related registry between the 2D Ag–S sheets isdeveloped by simple van der Waals interactions betweenthe methyl groups. Such a two-step mechanism will notbe applied, however, to the formation of AgNDT sincedithiols have two terminal SH groups that can coordinatewith Ag1 ions. In this sense, regarding the self-assemblyprocess of AgNDT, we propose that small-sized AgNDTpatches are formed at the beginning, and then thesepatches stack together in three dimensions, as schemati-cally drawn in Scheme I. These processes certainly leadto both Ag de� ciency and poor registry in AgNDT. None-theless, as illustrated in Scheme II (i.e., top view repre-sentation of the structure of AgNDT), all the Ag atomsare supposed to be trigonally bonded with S atoms; Agde� ciency leads only to the reduction of the coordinationnumber of a few S atoms from 3 to 2. The structure ofAgNDT supposed herein is similar to that of silver 1,5-pentanedithiolate proposed by Natan et al.9

APPLIED SPECTROSCOPY 1087

SCHEME II. Top view representation of AgNDT.

FIG. 2. DRIFT spectrum of AgNDT at room temperature. The insetshows the DRIFT spectrum of silver 1,5-pentanedithiolate in the regionof 600–780 cm21.

TABLE I. Vibrational assignment of AgNDT.

Assignmenta

Peakposition(cm21)b

CH2 asymmetric stretching (nas(CH 2), d2)CH2 symmetric stretching (ns(CH 2), d1)CH2 (all-trans) scissoring (d(CH 2))CH2 (next to gauche) scissoringCH2 (adjacent to S) scissoring (ds)CH2 wagging for kink defect (GTG)CH2 wagging for double-gauche defect (GG)

2918 vs2847 vs1467 m1451 vw1426 m1366 sh1349 vw

CH2 wagging (W x)CH2 wagging (W x)CH2 wagging (W x)CH2 wagging (W x)CH2 wagging (W x)CH2 wagging (W x)CCC stretching (Rx)CCC stretching (Rx)CH2 rocking (Px)CH2 rocking (Px)CH2 rocking (Px)CH2 rocking (Px), (trans) C–S stretching (n(CS)T)(gauche) C–S stretching (n(CS)G)

1327 vw1298 m1266 m1232 m1202 m1180 vw1093 vw1056 m986 vw924 w793 w720 s647 vw

a See text for details.b Vs, very strong; s, strong; m, medium; w, weak; vw, very weak; sh,

shoulder.

Figure 2 shows the DRIFT spectrum of AgNDT atroom temperature. The detailed vibrational assignmentsare provided in Table I. The high frequency region of2750–3050 cm21 reveals the C–H stretching modes of themethylene groups of AgNDT. Speci� cally, the two in-tense bands at 2847 and 2918 cm21 are assigned, respec-tively, to the symmetric (ns(CH2), d 1) and the antisym-metric (nas(CH2), d 2) stretching vibrations of the methy-lene groups. It has been well established that the d 1 andd 2 modes are strong indicators of the chain conformation.The d 1 and d 2 modes usually lie in the narrow ranges of2846–2850 and 2915–2918 cm21, respectively, for all-trans extended chains19 and in the distinctly differentranges of 2854–2856 and 2924–2928 cm21 for disorderedchains characterized by a signi� cant presence of gaucheconformers.20 On this basis, the observed peak frequen-cies of 2847 and 2918 cm21 suggest that the alkyl chainsin AgNDT are nearly in an all-trans conformational state.It is noteworthy that the peak positions of the d 1 and d 2

modes in AgNDT are little different from those in silvermonothiolate. For the SAMs of 1,9-nonanedithiol on vac-uum evaporated silver substrate, however, the d 1 and d 2

peaks appeared at 2853 and 2927 cm21, respectively, im-plying that the SAMs consisted of disordered chains char-acterized by a signi� cant presence of gauche conform-ers.15 We have to mention also that the S–H stretchingband is completely absent in the DRIFT spectrum ofAgNDT; for pure 1,9-NDT, the band is clearly seen at2558 cm21. This supports the proposition that all the Satoms in AgNDT are coordinated with Ag atoms as de-picted in Scheme II.

The low frequency region (600–1500 cm21) in Fig. 2provides additional structural information regarding

AgNDT. The peaks appearing in the region are associatedwith the scissoring, rocking, wagging, and twistingmodes of the methylene groups. The peak at 1426 cm21

is assigned to the CH2 scissoring mode associated withthe carbon atoms bonded to sulfur atoms;21 the band notobserved in n-alkanes is labeled as ds in Table I. A sharppeak at 1467 cm21 can be attributed to the scissoringvibration of the remaining methylene groups (d(CH2)). Itis remarkable that its exact shape, including the peak po-sition, width, and the number of components, is well suit-ed to re� ecting the packing arrangement of the alkylchain assemblies.22 For instance, the appearance of a sin-gle narrow peak at 1473 or 1467 cm21 has been attributedto triclinic or hexagonal subcell packing, respectively.

1088 Volume 55, Number 8, 2001

FIG. 3. DRIFT spectra of AgNDT taken as a function of temperature.

The appearance of a well-resolved doublet with two dis-tinct components is known to occur due either to inter-molecular vibrational coupling caused by crystal-� eldsplitting in orthorhombic or monoclinic packing or to thecoexistence of triclinic and hexagonal packing in the ma-terial. In addition, the peak is known to be broad whenthe alkyl chains assume disordered conformation. Thefact that a single narrow band (full width at half maxi-mum ;5.9 cm21) is observed at 1467 cm21 suggests thatAgNDT exists in a hexagonal subcell packing as in silvermonothiolate. Bands appearing in the region of 720–930cm21 are due to the CH2 rocking bands (Px), and theirobserved peak positions are also, after consulting the lit-erature data, consistent with the hexagonal subcell pack-ing.22

The progression bands in the region of 1150–1330cm21 in Fig. 2 can be attributed to the wagging vibration(W x) of the CH2 groups. Their presence is known to in-dicate that the alkyl chains assume all-trans conformationas concluded by the peak positions of the d 1 and d 2

modes.23 It has to be mentioned, however, that the inter-band separation of the Wx modes in AgNDT is slightlymore irregular than that in silver monothiolate. This sug-gests that the alkyl chains in AgNDT are less orderedthan those in silver monothiolate. In fact, the existenceof a weak peak at 1451 cm21 in Fig. 2, assigned to thescissoring motion of the methylene group next to agauche bond,21 reveals that the packing of alkyl chains ispartially disordered. We can also observe the W x bandsof the kink (GTG) and the double-gauche (GG) defects,24

although weak, at 1366 and 1349 cm21, respectively, inFig. 2. In addition, a very weak peak at 647 cm21 shouldarise from the C–S stretching vibration of a gauche de-fect, i.e., n(CS)G. Its intensity is nonetheless much weakerthan that observed for silver 1,5-pentanedithiolate;† (seethe DRIFT spectrum of silver 1 ,5-pentanedithiolateshown in the inset of Fig. 2). This may indicate that theextent of the gauche defect decreases as the chain lengthincreases.

In order to obtain information on the stability and thepossible phase transition in AgNDT, we have recordedthe DRIFT spectra as a function of temperature. Figure3 shows a series of DRIFT spectra obtained in the tem-perature region of 298–600 K. All spectra were measuredat the temperatures indicated, with the temperature heldconstant to 6 1 K for 5 min while the spectra were beingrecorded; the temperature was raised in 10 K steps. Thechanges in frequency and intensity across the variationof temperature provide signi� cant information about thestructural properties of the material.

For clearer presentation, the temperature dependenceof the peak positions of the CH2 stretching, scissoring,and wagging modes are plotted in Fig. 4. Each data pointcorresponds to the average of three independent mea-surements, with their standard deviations being repre-sented as uncertainties. The peak positions of the d 1 andd 2 modes are rather invariant up to about 520 K, albeitthat their intensities are gradually diminished upon heattreatment. The Wx progression bands also appear at thesame positions from room temperature to 520 K, al-

† Silver 1,5-pentanedithiolate was prepared following the procedure de-scribed in Ref. 9 except that ethanol was used as a solvent.

though their intensities become weaker upon heat treat-ment than those at 298 K. The amount of the peak shiftof the d(CH2) mode is also marginal between 298 and520 K. However, as the temperature reaches ;530 K, allof the bands are subjected to dramatic changes. The d 1

and d2 peaks suddenly shift upwards by 6–7 cm21 as thetemperature is raised from 520 to 530 K. The d(CH2)band, on the other hand, downshifts by 3 cm21. The scis-soring vibration of the methylene group adjacent to sulfuralso shows a subtle decrease in peak frequency. More-over, the W x progression bands weaken abruptly at ;530K. The intensities of the d 1, d 2, and d(CH2) bands arealso lowered substantially in the temperature region of520–530 K. All of these clearly indicate that a certainstructural change takes place for AgNDT at ;530 K.Similar peak shift and intensity lowering are known tooccur during the phase transition of n-alkanes and poly-ethylene.20 Along with the upward shift of the d 1 and d 2

modes, the abrupt weakening of the W x bands implies theaccumulation of gauche conformers. The substantialdownshift of the d(CH2) mode indicates the disruption ofthe hexagonal chain packing. As expected, the intensityof the next-to-gauche CH2 scissoring band was less sub-ject to change than that of the all-trans CH2 scissoringband.

The relative intensity ratio I(d 2)/I(d 1) has frequentlybeen taken as a measure of disorder. Increasing in con-formational disorder, the intensity ratio increases; the in-terchain coupling is reduced along with the increase inthe twisting rotational mobility of the individual alkylchains.25 Shown in Fig. 5, the ratio for AgNDT is ratherinvariant from 298 K to 520 K but sharply increases near530 K. This supports the contention that a certain struc-tural change takes place for AgNDT at ;530 K.

As mentioned previously, although weak, the W x bandsof the kink (GTG) and the double-gauche (GG) defectswere identi� ed at 1366 and 1349 cm21, respectively, evenin the room temperature DRIFT spectrum (see Fig. 2).These bands were in fact intensi� ed upon temperatureincrease. In particular, as the temperature reached 490 K,

APPLIED SPECTROSCOPY 1089

FIG . 4. Temperature dependence of the DRIFT peak frequencies for (a) nas(CH2), (b) ns(CH2), (c) d(CH2), and (d ) Wx modes.

FIG. 5. Intensity ratio of the nas(CH2) and ns(CH2) bands.

a new peak due to a gauche defect developed at 1357cm21. As the temperature was increased, another newpeak also developed at ;1380 cm21 but its assignmentwas uncertain.

For the case of AgSR, two or more phase changes are

known to take place as the temperature is increased.2,7

The temperature-dependent DRIFT spectral feature sug-gests that any distinct phase transition does not take placefor the case of AgNDT, however. Rather, AgNDT ap-peared to degrade at .530 K. The present spectral dataare consistent with the TGA and DSC data. The TGAcurve shown in Fig. 6a illustrates that mass loss takesplace in the temperature region of 550–640 K; the massloss after 640 K is about 70%. The DSC curve shown inFig. 6b reveals that a sharp structural change occurs at568 K. The latter temperature is about 30 K higher thanthat showing dramatic DRIFT spectral changes, but thismay simply indicate that vibrational spectroscopy is a farmore sensitive means than calorimetry when probingsubtle structural changes occurring in AgNDT by tem-perature variation. We have also con� rmed from theDRIFT spectra taken after cyclic thermal treatment thatAgNDT does not exhibit liquid-crystalline behavior uponmelting.2 Some of the results are shown in Fig. 7, wherethe sample was repeatedly raised to elevated tempera-tures, its DRIFT spectrum recorded, and then lowered toroom temperature and a subsequent DRIFT spectrum ob-tained. The spectral pattern is seen to be quite reversiblebelow 520 K, but it becomes irreversible once the tem-perature is raised above 530 K. For instance, the peak

1090 Volume 55, Number 8, 2001

FIG. 6. (a) TGA and (b) DSC data of AgNDT.

FIG. 7. DRIFT spectrum of AgNDT after cyclic thermal treatment for(a) nas(CH2) and ns(CH2) and (b) Wx modes.

frequencies of the d 2 and d 1 modes are invariant duringthe 298–520 K thermal cycle. The values increase from2918 and 2847 cm21 to 2925 and 2852 cm21, respectively,upon cycling to above 530 K. The normalized intensitiesof the d 2 and d 1 bands are also hardly subjected tochange during the 298–520 K cycle but abruptly decreaseupon cycling to above 530 K. It should also be mentionedthat the W x progression bands are no longer observedafter cycling to above 530 K, as shown in Fig. 7b. Alongwith the TGA and DSC data, these observations suggestthat AgNDT is irreversibly degraded above 530 K. Thisimplies that silver dithiolate decomposes immediately af-ter melting without any reversible mesophase transitionas seen in silver monothiolate. The representative type ofthermal decomposition occurrence must be the dehydro-genation of hydrocarbon chains along with the dissocia-tion of the Ag–S bonds. When AgNDT was heated up to;700 K under nitrogen atmosphere and then cooleddown to room temperature, numerous peaks were iden-ti� ed in the DRIFT spectrum that could be attributed tothe presence of the vinyl group (at 991 and 909 cm21),the trans-vinylene group (at 966 cm21), and the C5C

bond (at 1640 cm21). 26 From the XRD data, we couldalso identify the presence of metallic silver as far asAgNDT was heated up to ;550 K; XRD peaks at 37.8,43.9, 64.0, and 76.88 clearly indicate the formation ofAg(0).27 All these types of behavior exhibited by AgNDTand differing from that of AgSR must be associated withthe formation of two Ag–S bonds in AgNDT.

CONCLUSION

The structure and thermal behavior of AgNDT havebeen investigated by DRIFT spectroscopy along withXRD, TGA, DSC, and elemental analyses. XRD datasuggested that AgNDT had a layered structure but withpoor registry between layers. From the elemental analy-sis, the latter poor registry seemed to arise from the de-� ciency of Ag by 20% in binding to the available sulfuratoms. Nonetheless, DRIFT spectral data clearly indicat-ed that the carbon chains took nearly all-trans confor-mation. The temperature-dependent DRIFT spectral datasuggested that AgNDT should degrade, without any ther-modynamic phase transition, at .530 K. This could bealso con� rmed from the TGA and DSC analysis. A seriesof DRIFT spectra taken after cyclic thermal treatment ledus to con� rm that AgNDT did not exhibit liquid-crystal-line behavior upon melting. This must be associated withthe formation of two Ag–S bonds (as a,v-dithiolate) inAgNDT, albeit that each Ag atom is trigonally bonded tosulfur as in AgSR.

ACKNOWLEDGMENT

K.K. was supported by the Korea Research Foundation (KRF, 042-D00073) and by the Korea Science and Engineering Foundation (KO-SEF, 1999-2-121-001-5). S.W.H. was supported by KOSEF through theCenter for Molecular Catalysis at Seoul National University. H.J.C. andS.J.L. acknowledge the KRF for providing the BK21 fellowship.

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