Thermal Behavior of Gold(I) - American Chemical Society

Aug 15, 2008 - Nanoparticles under Heat Treatment Process. Sang-Ho ... differential scanning calorimetry, spectrofluorophotometer, FT-IR spectroscopy,...
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J. Phys. Chem. C 2008, 112, 13862–13868

Thermal Behavior of Gold(I)-Thiolate Complexes and Their Transformation into Gold Nanoparticles under Heat Treatment Process Sang-Ho Cha, Ki-Hyun Kim, Jong-Uk Kim, Won-Ki Lee, and Jong-Chan Lee* Department of Chemical and Biological Engineering, Seoul National UniVersity, Shilim-9-Dong, Gwanak-Gu, Seoul 151-744, Korea ReceiVed: April 24, 2008; ReVised Manuscript ReceiVed: May 22, 2008

The structural and thermal behavior of photoluminescent Au(I)-alkanethiolate was investigated by using differential scanning calorimetry, spectrofluorophotometer, FT-IR spectroscopy, NMR spectroscopy, X-ray diffraction, and transmission electron microscopy. Upon heating the sample, the conformation of the alkyl groups in Au(I)-alkanethiolate changed from the initial all-trans state at room temperature to the gauche containing disorder state along with the decrease of emission intensity, and then Au(I)-alkanethiolate changed to a mixture of di-n-alkyl disulfide, di-n-alkyl sulfide, and gold nanoparticles stabilized by the n-alkanethiolates. Carboxylic acid and ester functionalized gold nanoparticles can be easily obtained from this simple one-step heating process, using the corresponding functionalized Au(I)-thiolates without any solvent, surfactants, or chemical reagents. Introduction For the past decades, the wide potential applications of metal complexes for useful electrical, magnetic, catalytic, or photoactive materials have made the studies of these hybrid inorganic-organic materials an active area of research.1-3 It is known that the unique characteristics of metal complexes are essentially dependent upon their crystal structures; therefore, great effort has been made to achieve detailed descriptions of metal complex structures. For example, silver(I)-alkanethiolates, Ag(I)-SRs, were found to have a uniformly layered crystal structure consisting of bimolecular assemblies with a quasihexagonal inorganic Ag-S lattice via Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) studies.4-7 Other metal complexes such as copper- and palladium-alkanethiolates, where well-developed bilayer structures and all-trans chain conformations predominate, were also reported.8,9 On the other hand, only a few structural characterizations of gold(I) complexes containing alkanethiolate side chain groups have been established. This is because their poor solubility, which originates from the attractive inter- and intramolecular interaction between the adjacent gold atoms, commonly referred to as aurophilic interaction, has hindered structural characterization.10-12 Very recently, we synthesized luminescent gold(I)-alkanethiolates, Au(I)-SRs, having a remarkably high degree of conformational order and a well-developed lamellar structure by simply mixing gold salts with excess n-alkanethiols in the tetrahydrofuran (THF).13 Au(I)-SRs were found to be used as precursors for gold nanoparticles through electron beam irradiation in transmission electron microscopy (TEM).14 In this study, we examined the temperature-dependent thermal behavior of Au(I)-SRs to gain additional insight into the nature of their crystal structures. Interestingly, Au(I)-SRs were found to have irreversible endothermic transitions from the first heating scans of differential scanning calorimetry (DSC) studies, and these transition temperatures decrease with the increasing length of the alkyl groups. Mostly, thermal transition temperatures * Corresponding author. Phone: +82-2-880-7070. Fax: +82-2-880-8899. E-mail: [email protected].

including the melting and decomposition temperatures of organometallic complexes or organic compounds increase with the increasing length of alkyl groups due to the increased van der Waals interaction between the longer alkyl groups.15,16 In this paper we investigated the origin of this unusual irreversible transition behavior of Au(I)-SRs using combined applications of spectrofluorophotometer, FT-IR spectroscopy, NMR spectroscopy, X-ray diffraction, and TEM. Experimental Methods Materials. Gold salt, hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 · 3H2O), n-alkanethiols [HS(CH2)nH, n ) 4, 6, 8, 10, 12, 16, and 18], and 11-mercaptoundecanoic acid [HS(CH2)10COOH] were purchased from Aldrich. n-Tetradecylmercaptan was purchased from TCI. THF was dried by refluxing over sodium and benzophenone followed by distillation. All other reagents were used as received. Preparation of Ethyl 11-Mercaptoundecanoate (HS(CH2)10COOCH2CH3). Ethyl 11-mercaptoundecanoate was prepared from 11-mercaptoundecanoic acid and ethanol by the Fischer esterification method.17 Simply, 8.00 g of 11-mercaptoundecanoic acid (36.6 mmol), 120 mL of absolute ethanol, and 2-3 drops of concentrated sulfuric acid were added to a roundbottomed flask fitted with a condenser. The mixtures were then heated with stirring and refluxed overnight. After the ethanol was dried, the reaction mixtures were extracted with methylene chloride (MC) and saturated aqueous sodium bicarbonate. The MC layer was separated and dried over anhydrous magnesium sulfate and concentrated to afford 8.47 g (34.4 mmol) of ethyl 11-mercaptoundecanoate. The yield was 94.0%. 1H NMR (CDCl , TMS) δ 1.20-1.46 (m, 15H, -CH , 3 3 HSCH2CH2-(CH2)6-), 1.56-1.72 (m, 4H, HSCH2-CH2-, -CH2-CH2CO2-), 2.29 (t, 2H, -CH2-CO2CH2CH3), 2.52 (quartet, 2H, HS-CH2-), 4.12 (quartet, 2H, -CO2-CH2CH3). Preparation of Au(I)-SRs. The following procedure, shown for Au(I)-SC18, which has 18 carbons in the alkyl group, was used to synthesize all gold(I)-alkanethiolates by using other

10.1021/jp803583n CCC: $40.75  2008 American Chemical Society Published on Web 08/15/2008

Thermal Behavior of Gold(I)-Thiolate Complexes n-alkanethiols, respectively. A solution of n-octadecanethiol (C18H37SH, 0.5 mmol, 144 mg) in THF (5 mL) was added dropwise into a solution of HAuCl4 · 3H2O (0.1 mmol, 39.4 mg) in THF (5 mL) at room temperature. A brown precipitate formed immediately and then became a white solid after the mixture was stirred for 1 day. The product was purified by washing several times with THF, ethanol, and acetone then dried under vacuum overnight to produce about 39.4 mg of white powdery solid at 82% yield. The more detailed synthetic procedures and characterizations including elemental analysis results of Au(I)SC18 and other Au(I)-SRs have been reported before.13 The functionalized Au(I)-SRs such as Au(I)-SC10COOH and Au(I)SC10COOEt were prepared by using 11-mercaptoundecanoic acid and ethyl 11-mercaptoundecanoate, respectively. The yields were always above 85%. Characterization. DSC was carried out on a TA instruments 2920 differential scanning calorimeter with a 5 deg per min heating rate under nitrogen atmosphere. Temperature-dependent solid state photoluminescence spectra were acquired on a HORIBA FluoroMax-3 spectrofluorometer with KBr pellets. X-ray scattering experiments were performed at the 3C2 beam line at the Pohang Accelerator Laboratory (PAL) at various temperatures. IR spectra were recorded on a JASCO FT/IR200 over the range of 4000-500 cm-1, using a KBr pellet. For TEM experiments, Au(I)-SRs heated to 200 °C with DSC for 3 h were washed several times with a series of solvents (THF, ethanol, and acetone) and then a droplet of the dispersed solution was dropped onto a carbon-coated copper grid. TEM was performed on a Carl Zeiss LIBRA-120 at 120 keV. 1H NMR spectra were obtained on a JEOL LNM-LA 300 spectrometer at 300 MHz with chemical shifts indicated relative to SiMe4 in CDCl3 solution. Results and Disscussion Thermal properties of gold(I)-alkanethiolates, Au(I)-SRs, were investigated by using DSC. Figure 1 shows the DSC heating curves of gold(I)-octadecanethiolate, Au(I)-SC18. In the first heating scan, a sharp endothermic peak is found at 158 °C, which was initially considered as the melting point of polymeric Au(I)-SC18 (Figure 1a). However this peak disappears in the second heating scan, while two new endothermic peaks appear at 31 and 55 °C, respectively (Figure 1b). These two new peaks appear at the same temperatures and have the same enthalpy change (∆H) values at the third and subsequent

Figure 1. DSC curves of Au(I)-SC18 on a heating scan at 5 deg min-1 (a) in the first heating, (b) in the second heating, and (c) in the third heating scan.

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Figure 2. (a) Lowest onset decomposition temperatures taken from ref 13, (b) peak maximum temperatures of irreversible transitions in the first heating scan, and (c) highest peak maximum temperatures of reversible transitions in the second heating scan of DSC as a function of the number of carbons in Au(I)-SRs.

DSC heating scans (Figure 1c). Therefore the DSC peak observed from the first heating scan originated from an irreversible endothermic transition, while the two peaks observed from the second and further scans are reversible ones. The thermal gravity analysis (TGA) result of Au(I)-SC18 showed that the lowest onset decomposition temperature, Tonset, was about 295 °C, higher than the irreversible transition temperature. Other Au(I)-SRs with different alkyl lengths [R ) (CH2)nH, n ) 4, 6, 8, 10, 12, 14, and 16] also show similar thermal phase behaviors where irreversible and reversible transitions were observed in the first and second heating scans of DSC, respectively, and were observed below their decomposition temperatures. The thermal decomposition and reversible transition temperatures of Au(I)-SRs increase with the increasing length of the alkyl groups, while the irreversible transition temperatures show an opposite trend (Figure 2).18 DSC curves for all Au(I)-SRs showing the irreversible and reversible transitions are given in the Supporting Information. This result implies that the thermal decomposition and reversible transition temperature of Au(I)-SRs are strongly related to the van der Waals attraction between the alkyl chains because they increase with the length of the alkyl groups,15,16 while the irreversible transitions are not related to the interaction between the alkyl groups. We also measured the ∆H at the reversible and irreversible transition temperatures from the DSC curves and both of them increase as the length of the alkyl group increases, as shown in the Supporting Information. For example, the ∆H values at reversible transition temperatures for Au(I)-SC8, Au(I)SC10, Au(I)-SC12, AuS(I)-SC14, Au(I)-SC16, and Au(I)-SC18 are 15.0, 18.9, 21.2, 23.1, 51.3, and 53.7 cal g-1, respectively, and those at irreversible transition temperatures are 38.0, 40.3, 44.9, 46.4, 53.1, and 61.9 cal g-1, respectively. More detailed studies, so as to verify the origins of these anomalous trends for the thermal phase transitions of the Au(I)-SRs, were performed by using various analytical tools and they are shown in the next parts of this article. We found that the luminescent property of Au(I)-SRs disappears at their irreversible temperatures upon heating from room temperature. For example, when Au(I)-SC18 was heated from room temperature, the intensity of the luminescence decreased continually, and then it disappeared completely when the temperature reached about 158 °C (the irreversible transition temperature). This was confirmed by the temperature-dependent emission spectra of Au(I)-SC18 in a KBr pellet upon excitation

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Figure 3. Temperature dependence of the photoluminescence spectra of Au(I)-SC18 in a KBr pellet.

Figure 4. Representative powder XRD patterns of Au(I)-SC18 at various temperatures.

Figure 5. Representative infrared spectra of Au(I)-SC18 at various temperatures (a) in the high frequency (2700-3100 cm-1) region indicating the symmetric (d+) and antisymmetric (d-) bands from the CH2 C-H stretching and (b) in the low frequency (1100-1600 cm-1) region containing the methylene wagging (Wx) progressions.

at 310 nm as shown in Figure 3. Regarding the dependence of photoluminescent properties of gold(I) complexes upon their structures, the temperature-induced change in structure for Au(I)SC18 could be expected.19,20 Another apparent change observed at the irreversible transition temperature of Au(I)-SC18 is the color change of the sample: the white powdery sample as synthesized becomes dark brown when heated above 158 °C. Therefore it appears reasonable to suggest that the structure of Au(I)-SC18 changed and/or new materials were formed above the irreversible transition temperature. The exact same phenomena involving both the disappearance of luminescence and a color change were also observed in other Au(I)-SRs around their irreversible transition temperature. The structure change of Au(I)-SRs during the first heating process was studied further by using XRD measurements at variable temperatures. We selected Au(I)-SC18 as the model compound again. Figure 4 shows the X-ray curves of Au(I)SC18 obtained upon heating of the white powdery sample. A series of small-angle reflections were observed below the irreversible transition temperature (158 °C), two new peaks appeared at 38.10° and 44.35° and all the small angle reflections disappeared. These new wide angle peaks are in complete agreement with diffraction from the (111) and (200) planes of face-centered cubic (fcc) gold,21 indicating that the ordered lamellar structure of the original sample turned into gold crystals above the irreversible transition temperature. Then, the large endothermic peak observed in the first heating scan of the DSC could be ascribed to the thermal reduction process of Au(I)-SC18 to Au(0). Additionally, the relative diffraction intensity of (111) to (200) diffraction, approximately 2.6 in our case, is larger than the 1.9 value obtained for standard diffraction of gold powders, indicating that the metallic gold from the thermal reduction of Au(I)-SC18 is primarily dominated by (111) facets.22 Similarly, other Au(I)-SRs also show an ordered lamellar structure below their transition temperatures,13 and they turned into gold crystals having 38.10° and 44.35° peaks from X-ray curves when heated above their irreversible transition temperatures. IR spectroscopy was used to study the structure changes, especially the conformation changes of the alkyl side chain of Au(I)-SC18 during the first heating process.23 It is well-known that the strong symmetric (d+) and antisymmetric (d-) CH2 stretching bands with peak maxima around 2847 and 2916 cm-1 in FT-IR are correlated to an extremely high percentage of alltrans conformation.24 When the population of gauche conformation in alkyl chains increases, these two bands at 2847 and 2916 cm-1 shift to around 2855 and 2926 cm-1, respectively.25 Figure

Thermal Behavior of Gold(I)-Thiolate Complexes

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Figure 6. (a) TEM image (b) infrared spectrum and (c) heating and cooling DSC curves on a heating scan at 5 deg min-1 of gold nanoparticles formed from Au(I)-SC18 by the heat treatment process. (d) TEM image of gold clusters obtained through the thermal decomposition of Au(I)SC18. The inset of panel a shows the corresponding selected area electron diffraction (SAED) pattern of gold nanoparticles.

5 shows a series of FT-IR spectra, obtained as a function of temperature, for Au(I)-SC18. The peak positions of d+ and dbands do not change below 120 °C, indicating that the initial all-trans conformation is preserved. On the other hand, those observed above 120 °C begin to shift gradually to 2854 and 2925 cm-1, respectively, implying an increase in the population of gauche conformation. The upward shift of d+ and d- bands between 120 and 140 °C, below the irreversible transition temperature, reveals that the premelting event originating from the gradual accumulation of gauche conformation occurs in a small but nonvanishing population.26 Almost no change of those band intensities above 160 °C is seen, if any. Such a conformation change, from a highly ordered chain state to a disordered chain state with increasing temperature, was further studied by the observation of wagging vibrations (Wx) in the low-frequency region (Figure 5b). The well-resolved progression bands between 1175 and 1350 cm-1 are attributed to wagging vibrations and are used as markers of trans and gauche bond populations in the alkyl chains.27 From the equation ∆ν ) 326/(m + 1), which shows the correlation between the precise separation in the wagging mode peaks and the average number of trans units m, the all-trans conformation of alkyl chain in Au(I)-SC18 at room temperature was previously confirmed: ∆ν ) 17.7, m + 1 ) 18.4, n ) 18.13 Above 160 °C, the peaks could no longer be consistently discerned, indicating the existence of a significant population of gauche conformations in the alkyl side chain. From the gradual diminution of the wagging mode intensities between 120 and 140 °C, the premelting event also can be confirmed.4

The sharp singlet peak at 1467 cm-1, which can be assigned to the CH2 scissoring mode (δ), reveals that the methylenes in the alkyl chains have an all-trans conformation and the unit cell is composed of only one chain with the hexagonal subcell packing at room temperature.28 The broadening of this peak with increasing temperature implies the disruption of alkyl chain packing in the unit cell. In addition, the intensity of the 1416 cm-1 band, which is associated with scissoring of a methylene group adjacent to the Au-S bonds (δs), decreases above 160 °C while the peak position does not change,29 indicating that the number of this Au-S bond decreased above the irreversible transition. The ratio of sulfur to gold atoms is one for Au(I)SC18, while that of gold crystals including nanoparticles stabilized by n-alkanethiolate should be much smaller than one.30,31 Therefore the intensity decrease in the 1416 cm-1 band above the irreversible transition temperature should be another strong indication of the formation of gold crystals or nanoparticles capped by n-alkanethiolate. The formation of nanoparticles from Au(I)-SRs above the irreversible transition temperature was clearly observed from TEM analysis. For TEM sample preparation, Au(I)-SC18 was heated to 200 °C for 3 h to make sure all the sample was reduced with use of DSC. Then, the sample in the DSC pan was collected and washed several times with THF, ethanol, and acetone to ensure the removal of any organic byproducts. The washed product was dispersed in THF and a drop was placed onto a carbon-coated copper grid for TEM measurement. The byproducts dissolved in the organic solvents in the above purification process were

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Figure 7. 1H NMR spectrum of the byproducts separated from gold naonparticles formed from the first heating scan of Au(I)-SC18 in DSC.

Figure 8. TEM images of gold nanoparticles formed by the heat treatment process (a) from Au(I)-SC10COOH and (b) Au(I)SC10COOEt.

separated by distillation for further characterization, such as 1H NMR as shown in the next paragraph. Figure 6a shows gold nanoparticles with an average diameter of 8.7 ( 1.7 nm (calculated for a sample of 200 nanoparticle diameters) were obtained from

Cha et al.

Figure 9. Infrared spectra of (a) 11-mercaptoundecanoic acid, (b) gold nanoparticles from Au(I)-SC10COOH, (c) ethyl 11-mercaptoundecanoate, and (d) gold nanoparticles from Au(I)-SC10COOEt.

Au(I)-SC18. The corresponding selected area electron diffraction (SAED) pattern (see inset in Figure 6a) shows an fcc packing arrangement, which is consistent with the XRD results presented in this article. FT-IR spectroscopy of this sample was performed to analyze the structure of the organic moiety attached to the nanoparticles, possibly n-octadecanethiolate that stabilizes the nanoparticles from aggregation. As shown in Figure 6b, the FTIR spectrum for gold nanoparticles from thermal reduction of Au(I)SC18 is almost identical with that of gold nanoparticles stabilized by n-octadecanethiolate prepared by others using the two-phase synthetic method.32 For example, the strong symmetric (d+) and antisymmetric (d-) CH2 stretching bands with a peak maxima around 2847 and 2916 cm-1 and the well-resolved progression bands, wagging vibrations, between 1175 and 1350 cm-1, indicate that the conformation of alkyl chains on the surface of gold nanoparticles is a highly ordered all-trans one: ∆ν ) 18.3, m + 1 ) 17.8, n ) 18. The DSC curve of gold nanoparticles shows one broad endothermic peak at around 52 °C (∆H ) 4.8 cal g-1) from a first heating scan and one broad exothermic peak at around 49 °C (∆H ) 4.5 cal g-1) from a first cooling scan after heating to 140 °C (Figure 6c). The same thermal behavior was observed from the second and subsequent DSC scans indicating that this behavior is reversible. Similar reversible thermal behaviors of gold nanoparticles stabilized by n-octadecanethiolates were also previously reported.33 On this basis, it can be concluded that the heat treatment process reduces the Au(I)-SC18 into gold nanoparticles stabilized by n-octadecanethiolates. When Au(I)-SC18 was heated above the decomposition temperature, about 300 °C, gold clusters having irregular shapes were obtained (Figure 6d): gold nanoparticles coagulated with each other by the decomposition of n-alkanethiolate as reported by others.34 We also found that other Au(I)-SRs could be converted into n-alkanethiolate stabilized gold nanoparticles by heating them above their irreversible transition temperature and that gold clusters were obtained when the temperature was increased to above their decomposition temperatures. Obviously, the size and the size distribution of the gold nanoparticles from Au(I)-SRs were found to be dependent on temperature, heating time and rate, and the length of the alkyl groups. For example, when Au(I)-SC12 was heated to 200 °C for 3 h, gold nanoparticles with an average diameter of 6.7 nm were obtained. Detailed studies on the formation of gold nanoparticles from Au(I)-SRs, using different heating conditions, including the development of large scale synthesis, are currently in progress. Since the gold nanoparticles from Au(I)-SC18 show a broad peak at around 52 °C with a small ∆H value of 4.8 cal g-1, the

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Figure 10. Schematic diagram of the thermal phase transitions for Au(I)-SRs.

two sharp reversible endothermic peaks observed at 31 and 55 °C in the second heating scan of the DSC experiments should have originated from the byproducts produced along with the gold nanoparticles, and the broad peak from the gold nanoparticles should be buried or overlapped with the two larger peaks. Gold nanoparticles stabilized by n-alkanethiolate have more than one gold atom per n-alkanethiolate group, while Au(I)-SRs have one n-alkanethiolate group per gold atom.30,31 Therefore the byproducts from the thermal reduction process of Au(I)-SRs should be generated from the free n-alkanethiolate that is not attached to the gold surface. To verify the chemical structure of the byproduct, the 1H NMR spectrum of the byproduct obtained from the heat treatment process of Au(I)-SC18 was observed. Di-n-octadecyl disulfide, n-octadecanethiol, and din-octadecyl sulfide are thought to be possible candidates produced from extra n-octadecanethiolates of Au(I)-SC18. Figure 7 shows that the 1H NMR spectrum of the organic soluble byproducts separated from gold naonparticles formed after the first heating scan of Au(I)-SC18. The triplet peak found at 2.7 ppm, ascribed to the protons adjacent to sulfur in -CH2-CH2S-S-, indicates the formation of di-n-octadecyl disulfide.35 Furthermore, the DSC heating curve of pure di-n-octadecyl disulfide prepared by using a known chemical reaction36 shows two endothermic peaks at around 31 and 60 °C, respectively, and its 1H NMR spectrum clearly shows a triplet peak at 2.7 ppm (see the Supporting Information). The triplet peak observed at 2.5 ppm indicates that n-octadecanethiol is not a component of the byproduct because the protons of methylene in -CH2-CH2-SH give rise to a quartet peak at around 2.5 ppm as reported by others and from our own experiments.13,37 Therefore the triplet peak at 2.5 ppm was ascribed to the protons adjacent to sulfur in -CH2-CH2-S-CH2-CH2-. Also the 1H NMR spectrum of pure di-n-octadecyl sulfide (see the Supporting Information) matches very well with the corresponding peaks in Figure 7. Although the detailed formation mechanism of din-octadecyl sulfide in this study is not yet completely understood, it seems that a combination between the n-octyl free radical generated by thermal cleavage of C-S bond and n-octadecanethiolate might produce di-n-octadecyl sulfide as previously reported.38,39 The melting point (Tm) of di-n-octadecyl sulfide is known to be about 60 °C.40 Thus, the endothermic peak at 55 °C in the second DSC heating curve in Figure 1 should have originated from the combination of the three endothermic peaks of di-n-octadecyl disulfide (Tm ≈ 60 °C), di-n-octadecyl sulfide (Tm ≈ 60 °C), and gold nanoparticles stabilized by n-octadecanethiolates (Tm ≈ 52 °C) and the small endothermic peak at 31 °C is from the peak of di-n-octadecyl disulfide.41 Therefore the overall reaction of thermal reduction for Au(I)-SC18 can be expressed (in outline form) as

[Au(I)-SC18H37]n f Au nanoparticles + C18H37-S-S-C18H37 + C18H37-S-C18H37 (1) Functionalized gold nanoparticles stabilized by ω-substituted thiolates containing ester, amide, carboxylic acid, and so on are

known to be very difficult to prepare directly from the chemical reducing method using gold salts, reducing reagents, capping agent, and/or phase-transfer catalyst for several reasons.42-44 Therefore such functionalized gold nanoparticles could be prepared from ligand exchange procedures by using gold nanoparticles made from the chemical reducing method.45 We found that our thermal reduction method of Au(I)-SRs can be used to prepare functionalized gold nanoparticles directly without any ligand exchange step. For the preparation of carboxylic acid and ester functionalized gold nanoparticles, functionalized Au(I)-SRs were first synthesized by using 11mercaptoundecanoic acid [Au(I)-SC10COOH] and ethyl 11mercaptoundecanoate [Au(I)-SC10COOEt], respectively, instead of n-octadecanethiol. Thermal behavior of these functionalized Au(I)-SRs was found to be similar to that of Au(I)-SRs composed of n-alkanethiolate: irreversible and reversible transitions occurred in the first and second heating scan, respectively. The irreversible transition temperatures of Au(I)-SC10COOH and Au(I)-SC10COOEt are 182 and 150 °C, respectively, and the Tonset are about 269 °C for Au(I)-SC10COOH and 304 °C for Au(I)-SC10COOEt, respectively. Figure 8 shows TEM images of the gold nanoparticles prepared from the thermal reduction process of Au(I)-SC10COOH and Au(I)-SC10COOEt. A particle size of 12.4 ( 1.2 nm for Au(I)-SC10COOH and 8.9 ( 1.8 nm for Au(I)-SC10COOEt was obtained, respectively. Each corresponding selected area electron diffraction pattern (not shown in here) shows that gold nanoparticles have an fcc crystal structure. To confirm the functional groups on the surface of gold nanoparticles, FT-IR characterization was performed as shown in Figure 9. The FT-IR spectra of gold nanoparticles from the Au(I)-SC10COOH and Au(I)-SC10COOEt are very similar to those of pure 11-mercaptoundecanoic acid and ethyl 11-mercaptoundecanoate, where the characteristic carbonyl stretching bands at 1695 cm-1 of carboxylic acid and at 1735 cm-1 of ester are observed, respectively, indicating that the functionalized gold nanoparticles are successfully synthesized. Conclusions In this study, the structural and thermal behavior of the Au(I)SRs was investigated. DSC analysis indicated that Au(I)-SRs were subjected to irreversible transitions below the thermal decomposition temperature in the first heating scan, while new reversible transitions were observed from the second heating scan. As the number of carbons in the Au(I)-SRs increases, the thermal decomposition and reversible transition temperatures increase due to the increased van der Waals attraction between the longer alkyl chains, while the irreversible transition temperature decreases, indicating that the irreversible transition is not related to the interaction between the alkyl groups. From the combined XRD, TEM, and FT-IR studies, we found that reduction of Au(I)-SRs to gold nanoparticles stabilized by n-alkanethiolate occurs during this endothermic irreversible transition. The overall structural and thermal behavior of Au(I)SRs is depicted in Figure 10.

13868 J. Phys. Chem. C, Vol. 112, No. 36, 2008 Acknowledgment. Experiments at Pohang Accelerator Laboratory were supported in part by MOST and POSTECH. Financial support for this work was provided by the Basic Research Program from Korea Science and Engineering Foundation (No. R01-2006-000-10749-0) and Korea Atomic Energy Research Institute (KAERI). Supporting Information Available: DSC heating curves of Au(I)-SRs series, pure di-n-octadecyl disulfide, and di-noctadecyl sulfide and 1H NMR spectra of pure di-n-octadecyl disulfide and di-n-octadecyl sulfide. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gorin, D. J.; Toste, F. D. Nature 2007, 446, 395. (2) Robin, A. Y.; Fromm, K. M. Coord. Chem. ReV. 2006, 250, 2127. (3) Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W.-S.; Withersby, M. A.; Schro¨der, M. Coord. Chem. ReV. 1999, 183, 117. (4) Bardeau, J.-F.; Parikh, A. N.; Beers, J. D.; Swanson, B. I. J. Phys. Chem. B 2000, 104, 627. (5) Fijolek, H. G.; Grohal, J. R.; Sample, J. L.; Natan, M. J. Inorg. Chem. 1997, 3, 6–622. (6) Parikh, A. N.; Gillmor, S. D.; Beers, J. D.; Beardmore, K. M.; Cutts, R. W.; Swanson, B. I. J. Phys. Chem. B 1999, 103, 2850. (7) Bensebaa, F.; Ellis, T. H.; Kruus, E.; Voicu, R.; Zhou, Y. Langmuir 1998, 1, 4–6579. (8) Sandhyarani, N.; Pradeep, T. J. Mater. Chem. 2001, 11, 1294. (9) John, N. S.; Thomas, P. J.; Kulkarni, G. U. J. Phys. Chem. B 2003, 107, 11376. (10) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412. (11) Corbierre, M. K.; Lennox, R. B. Chem. Mater. 2005, 17, 5691. (12) Hunks, W. J.; Jennings, M. C.; Puddephatt, R. J. Inorg. Chim. Acta 2006, 359, 3605. (13) Cha, S.-H.; Kim, J.-U.; Kim, K.-H.; Lee, J.-C. Chem. Mater. 2007, 19, 6297. (14) Kim, J.-U.; Cha, S.-H.; Shin, K.; Jho, J. Y.; Lee, J.-C. J. Am. Chem. Soc. 2005, 127, 9962. (15) Wu, Y.; Li, Y.; Liu, P.; Gardner, S.; Ong, B. S. Chem. Mater. 2006, 18, 4627. (16) Burch, K. J.; Whitehead, E. G., Jr J. Chem. Eng. Data 2004, 49, 858. (17) Moumne, R.; Lavielle, S.; Karoyan, P. J. Org. Chem. 2006, 71, 3332. (18) Since the reversible transitions showed multiple endothermic peaks originated from mixtures of gold nanoparticles and byproducts (sulfides and disulfides), the highest peak maximum temperatures are selected in the second heating scan of DSC. The data for the reversible transitions temperatures of Au(I)-SC4 and Au(I)SC6 are not included in Figure 2c because any reversible transitions were not observed in the second heating

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