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Dialkyl Sulfides: Novel Passivating Agents for Gold Nanoparticles Elwyn J. Shelley,† Declan Ryan,§ Simon R. Johnson,† Martin Couillard,‡ Donald Fitzmaurice,§ Peter D. Nellist,‡ Yu Chen,‡ Richard E. Palmer,‡ and Jon A. Preece*,† School of Chemical Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K., Nanoscale Physics Research Laboratory, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, U.K., and Nanochemistry Group, Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland Received June 19, 2001. In Final Form: October 2, 2001 Gold nanoparticles passivated with symmetrical and unsymmetrical dialkyl sulfides (H21C10SC10H21 and H37C18SC10H21) have been synthesized via the borohydride reduction of HAuCl4 and characterized by 1H NMR, FTIR, UV-vis, Auger Electron, XPS spectroscopies, and TEM. Under equivalent conditions of formation, the size and polydispersity of the gold cores obtained was greater for dialkyl sulfide ligands (d(C10SC10) ) 5.3 ( 0.8 nm; d(C18SC10) ) 6.3 ( 1.1 nm) than alkanethiol ligands (d(C10H22SH) ) 2.2 ( 0.1 nm). Edge-edge interparticle spacing of 2-D arrays of the nanoparticles is found to be dependent on the length of the longest alkyl chain passivating the nanoparticles and is independent of the asymmetry of the alkyl chains in the dialkyl sulfide.
Introduction Inorganic/organic hybrid nanoparticle assemblies represent a rapidly emerging field of immense fundamental scientific and speculative interest in terms of the technologies, such as Nanotechnology,1 that may evolve from them. Films of nanoparticles can show interesting physical properties such as single electron tunneling behavior2 and can also be modified with an STM.3 While alkanethiols (RSH) are the most popular passivant4 for gold nanoparticles and the use of dialkyl disulfides (RSSR′) has more recently been reported,5 the use of dialkyl sulfides (RSR′) has been reported in just two papers,6 despite extensive investigation of self-assembled monolayers (SAMs) of dialkyl sulfides (RSR′) on planar gold surfaces.7 We are * To whom correspondence should be addressed. Dr Jon A. Preece, School of Chemical Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT,U.K. Tel: +44 121 414 3528 Fax: +44 121 414 4403 Email:
[email protected]. † School of Chemical Science, University of Birmingham. ‡ School of Physics and Astromony, University of Birmingham. § Department of Chemistry, University College Dublin. (1) a) Heath, J. R. Acc. Chem. Res. 1999, 32, 388. b) Korgel, B. A.; Fitzmaurice, D. Adv. Mater. 1998, 10, 661. (2) Osman, H.; Schmidt, J.; Svensson, K.; Palmer, R. E.; Shigeta, Y.; Wilcoxon, J. P. Chem. Phys. Lett. 2000, 330, 1-6. (3) Durston, P. J.; Palmer, R. E.; Wilcoxon, J. P. Appl. Phys. Lett. 1998, 72, 176. (4) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (5) Porter, L. A.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halea, N. J.; Lee, T. R. Langmuir 1998, 14, 7378. (6) A communication reporting the synthesis of gold nanoparticles using benzylic akyl disulfides has been reported, see (a) Pankau, W. M.; Verbist, K.; von Kiedrowski, G. Chem. Commun. 2001, 519. (b) A full paper appeared in the literature after the submission of this manuscript detailing the synthesis of gold nanoparticles passivated with dialkylsulfides and their characterisation by 1H NMR and TEM. See Inoue, K.; Shinkai, S.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2001, 11, 1919. (7) a) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. b) Takiguchi, H.; Sato, K.; Ishida, T.; Abe, K.; Yase, K.; Tamada, K. Langmuir 2000, 16, 1703. c) Jung, Ch.; Dannenberger, O.; Xu, Y.; Buck, M.; Grunze, M. Langmuir 1998, 14, 1103. d) Beulen, M. W. J.; Huisman, B.-H.; van der Heijden, P. A.; van Veggel, F. C. J. M.; Simons, M. G.; Biemond, E. M. E. F.; de Lange, P. J.; Reinhoudt, D. N. Langmuir 1996, 12, 6170.
interested in manipulating the way in which gold and silver nanoparticles assemble into arrays8 by using dialkyl sulfides as passivating agents. This manipulation will be achieved principally by noncovalent bonding interactions9 and in particular the designed interdigitation10 of unsymmetrical alkyl chains. We have prepared two dialkyl sulfides: didecyl sulfide (H21C10SC10H21, referred to as C10SC10) and decylthiooctadecane (H37C18SC10H21, referred to as C18SC10) shown in Figure 1, and report here the synthesis and characterization of gold nanoparticles passivated with them. Comparisons with decanethiol (C10H21SH, referred to as C10SH) passivated nanoparticles prepared in an identical way to the dialkyl sulfide passivated nanoparticles are made. Experimental Section Materials. Decanethiol was purchased from the Fluka Chemical Company and used as received. All other chemicals used for the synthesis and purification of didecyl sulfide and decylthiooctadecane were purchased from the Aldrich Chemical Co. Ultrapure water with a resistivity of 18 MΩ cm-1 (USF-Elga) was used during synthesis of the nanoparticles. Synthesis of Dialkyl Sulfides. The dialkyl sulfides were prepared by the 9-borobicyclo[3.3.1]nonane catalyzed radical coupling11 of an alkanethiol and an alk-1-ene i.e. starting from decanethiol (for C10SC10) or octadecanethiol (8) Durston, P. J.; Schmidt, J.; Palmer, R. E.; Wilcoxon, J. P. Appl. Phys. Lett. 1997, 71, 2940. (9) a) Fitzmaurice, D.; Rao, S. N.; Preece, J. A.; Stoddart, J. F.; Wenger, S.; Zaccheroni, N. Angew. Chem., Int. Ed. 1999, 38, 1147. b) Ryan, D.; Rao, S. N.; Rensmo, H.; Fitzmaurice, D.; Preece, J. A.; Wenger, S.; Stoddart, J. F.; Zaccheroni, N. J. Am. Chem. Soc. 2000, 122, 6252. (10) For an investigation of the use of the interdigitation of alkyl chains as an ordering element via a chemical force microscopy study of the fundamental interaction see (a) Van der Vegte, E. W.; Subbotin, A.; Hadziioannou, G.; Ashton, P. R.; Preece, J. A. Langmuir 2000, 16, 3249. Such an interaction can also be exploited as an ordering element in triphenylene liquid crystals. (b) Allen, M. T.; Harris, K. D. M.; Kariuki, B. M.; Preece, J. A.; Kumari, N.; Diele, S.; Lose, D.; Hegmann, T.; Tschierske, C. Liq. Cryst. 2000, 27, 689. (c) Allen, M. T.; Harris, K. D. M.; Kariuki, B. M.; Preece, J. A.; Diele, S.; Lose, D.; Hegmann, T.; Tschierske, C. J. Mater. Chem. 2001, 11, 302.
10.1021/la0109260 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/08/2002
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Figure 1. Molecular structure of the compounds used to passivate gold nanoparticles.
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UV/Vis spectra were recorded in CHCl3 using a HewlettPackard 8452A scanning spectrophotometer equipped with a 10 mm path length quartz cell. Elemental Microanalysis (EA) was carried out using an Exeter Analytical CE440 instrument. 1 H Nuclear Magnetic Resonance Spectroscopy (NMR) was carried out on a Bruker instrument operating at 300 MHz and referenced to the residual CHCl3 in CDCl3 at 7.26 ppm. Transmission Electron Microscopy (TEM) was carried out using a JEOL JEL-2000EX operating at an accelerating voltage of 80 keV. The samples were prepared by depositing a drop of a dilute ( 1 mg mL-1) was deposited onto a freshly cleaved (highly oriented pyrolytic) graphite substrate, such that a thin film was formed on the substrate. The sample was then introduced into the UHV system through a load-lock chamber.
(for C18SC10) and 1-decene as reported by us previously.10a It has been shown7,12 that SAM formation on planar gold from dialkyl sulfide solutions doped with 1% of an alkanethiol result in SAMs which are disproportionately rich in the alkanethiol. The compounds were thus analyzed by GC-EIMS to ensure that no decanethiol or octadecanethiol remained after purification.10a Synthesis of Thiol and Dialkyl Sulfide Passivated Gold Nanoparticles. C10SH, C10SC10 and C18SC10 passivated gold nanoparticles were prepared using the twophase method reported by Brust et al.4 To a vigorously stirred solution of HAuCl4‚3H2O (0.51 mmol) in H2O (30 mL) was added a solution of tetraoctylammonium bromide (1.12 mmol) in toluene (30 mL). After 45 min the organic phase was separated from the aqueous phase. To the organic phase was added a solution of C10SH, C10SC10 or C18SC10 (0.51 mmol) in toluene (30 mL), followed by a freshly prepared solution of NaBH4 (5.59 mmol) in H2O (30 mL) with vigorous stirring. Upon addition of the NaBH4 solution, the color of the organic phase changed immediately from reddish orange to black. The mixture was stirred vigorously for a further 3 h before the organic phase was separated. The nanoparticles were purified by precipitation from the PhMe solution with acetonitrile followed by centrifugation of the suspension. The supernatant was discarded. The condensed phase was redissolved in chloroform (1 mL) and precipitated with acetonitrile. This process was repeated three more times after which the condensed phase was left to dry in the air for 30 min before being dissolved in deuterated chloroform for FTIR and 1H NMR analysis. No size selection procedure was performed on the nanoparticles to enable analysis of the distribution of nanoparticle sizes from this preparation. Gas Chromatography-Electron Impact Mass Spectrometry (GC-EIMS) was performed with a VG ProSpec mass spectrometer equipped with a Fisons Instruments GC 8000 series gas chromatograph using helium as the mobile phase. Fourier Transform Infrared Spectroscopy (FTIR) of nanoparticle solutions (CDCl3) was performed using a Mattson Infinity FT equipped with a 0.200 mm path length CaF2 cell.
In both cases the C10SC10 and C18SC10 nanoparticles were soluble in chloroform and toluene. The stability of the nanoparticles in solution was found to be concentration dependent. At low concentrations (0.1 mg mL-1) the nanoparticle cores aggregated and precipitated from solution in a matter of hours, while a relatively concentrated solution (1 mg mL-1) was stable indefinitely. We believe that the relatively weak gold-sulfide bond14 results in an equilibrium in dilute solutions favoring unbound sulfide, which allows the cores to aggregate. Additionally, the nanoparticles were prone to depassivation if left desolvated for more than an hour. While some of the solid could be redissolved the majority remained insoluble and aggregation of the gold cores was observed: a gold film appeared on the containers’ sides. Reinhoudt et al. also noted6b the short-term stability of gold nanoparticles passivated with sulfides. FTIR spectra of the nanoparticles in CDCl3 showed absorbances resulting from C-H and C-C vibrations at 2800-3000 cm-1 and 700-1500 cm-1 for both C10SC10 and C18SC10 as would be expected for the liquid like packing of the sulfides on planar gold surfaces reported7 by Whitesides et al. UV/Vis. The UV/vis spectrum of a solution of the didecyl sulfide passivated nanoparticles, Figure 2, is typical of those previously reported for gold nanoparticles passivated
(11) Masuda, Y.; Hoshi, M.; Nunokawa, A.; Arase, A. J. Chem. Soc., Chem. Commun. 1991, 1444. (12) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
(13) Chen, Y.; Schmidt, J.; Siller, L.; Barnard, J. C.; Palmer, R. E. Phys. Rev. B. 1998, 58, 1177. (14) Lavrich, D. J.; Wetterer, S. M.; Bernasek, S. L.; Scoles, G. J. Phys. Chem. B 1998, 102, 3456.
Discussion
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Figure 2. UV/vis spectrum of C18SC10 passivated nanoparticles. Table 1. Comparison of Molecular Formulas of the Pure Ligands with Molecular Formulas Derived from Elemental Analysis of Each Type of Nanoparticle Prepared ligand
molecular formula (pure ligand)
C10SH C10SC10 C18SC10
C10H22S C20H42S C28H58S
molecular formula (ligand on gold) C10H21S C20H39S0.25 C28H58S
by alkanethiols.15 The magnitude of the absorption at 520 nm is concurrent with chloroform solutions of nanoparticles in the 5-6 nm size range,16,17as well as the data obtained by Reinhoudt et al. 6b Elemental Analysis of the passivated gold nanoparticles reveals the empirical formula of the organic species passivating the gold nanoparticle. This formula should be concurrent with that of the pure organic ligand. Table 1 shows the results obtained from all the nanoparticles prepared. With the exception of the C10SC10 nanoparticles, the other nanoparticles’ molecular formulas determined by elemental analysis are in good agreement with the pure organic ligand. The origin of this discrepancy is not clear to the authors as all nanoparticles were prepared in similar manners, avoiding contamination or impurity. 1H NMR. Comparison of the spectra from pure C SC 10 10 and the gold nanoparticles passivated with C10SC10 (Figures 3(a) and (b)) reveals substantial broadening of all the signals from C10SC10 when passivating the gold nanoparticles. In particular, the resonances of the protons on the carbons R to the sulfur are broadened to such an extent that they are no longer visible. A similar situation is seen when gold is passivated by alkanethiols and dialkyl disulfides.18 Upon heating the NMR sample at 50 °C for 4 h, the gold was seen to aggregate and the color of the CDCl3 solution changed from dark purple to light yellow. C10SH capped gold nanoparticles prepared in the same way were stable when heated in the same way.19 Pemberton20a et al. and Freund20b et al. have separately reported the oxidation of alkanethiols to the corresponding sulfonate, sulfonite, sulfate, and sulfite species on planar gold surfaces after a few hours of exposure to air. In the case of the sulfides presented in this paper, such an oxidation could lead to a further weakening of the Au-S (15) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (16) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (17) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 4212. (18) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359. (19) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. Adv. Mater. 1996, 8, 428. (20) a) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. b) Lee, M.-T.; Hsueh, C.-C, Freund, M. S.; Ferguson, G. S. Langmuir, 1988, 14, 6419.
Figure 3. 1H NMR spectra. a) Unbound C10SC10. b) C10SC10 passivated nanoparticles. c) Organic residue from depassivation.
bond and allow aggregation of the gold cores. This oxidation involving the nanoparticles was thus investigated by 1H NMR and FTIR spectroscopies. The 1H NMR spectrum of the heated CDCl3 nanoparticle solution changes (Figure 3(c)) from that of the passivated nanoparticles’ but does not revert to that of the pure C10SC10 (Figure 3(a)). The spectrum retains the primary characteristics of pure C10SC10, with the exceptions that the R-proton resonance shifts 0.44 ppm downfield to 2.94 ppm, indicative of sulfoxide formation. The β and γ-proton resonances also shift downfield by 0.24 and 0.14 ppm, respectively. The same chemical shift changes are seen in the 1H NMR spectrum of synthetically oxidized (periodate) C10SC10. An FTIR spectrum of the residue shows the expected absorbances attributable to C-C and C-H stretches also present in the unreacted dialkyl sulfide but also a new absorbance at 1265 cm-1 which is indicative of an SdO group stretching vibration. These FTIR results were reproduced with C18SC10. AES/XPS. An Auger electron spectrum (Figure 4(a)) of the nanoparticles assembled on graphite typically shows Auger peaks corresponding to C, Au, S, and O. The presence of the O peak suggests oxidation of the organic ligands/nanoparticles. Quantitative analysis of the Auger electron spectrum by the sensitivity factor method22shows a large O:S atomic concentration ratio (∼5:1) which implies multiple oxidation states within the film. These could arise from oxygen at the Au-ligand interface or from oxygen physically trapped within the film. The AES results are supported by XPS measurement (Figure 4(b)), where a signal from the O1s line is observed, together with signals from Au4f, Au4d, Au4p, C1s, and S2p (weak), with the sample surface perpendicular to the analyzer. The binding energies are referenced to the Au4f7/2 line at 84 eV. The O1s peak (533 eV, fwhm ) 1.22 eV) suggests the existence of R2SO (∼531.6 eV) and R2SO2 (∼531.8 eV) species in the (21) Martin, J. E.; Wilcoxon J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475. (22) Handbook of Auger Electron Spectroscopy, 3rd Ed; Hedberg, C. L. Ed.; Physical Electronics Inc, 1995.
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Figure 4. a) Auger Electron Spectrum for C10SC10 passivated nanoparticles. b) X-ray Photoelectron Spectrum for C10SC10 passivated nanoparticles.
film, although it is possible that contributions from H2O (∼532.9 eV) are present. TEM. Analysis of the TEM micrographs of the nanoparticles (Figure 5(a)) shows that the metal cores have diameters of 5.3 ( 0.8 nm (C10SC10) and 6.3 ( 1.1 nm (C18SC10). The increase in particle size with increasing passivating chain length has also been reported when alkanethiol ligands are used.21 When decanethiol is used in the same proportions and under the same formation conditions as the passivating agent, nanoparticles of 2.2 ( 0.1 nm in diameter are obtained. Reinhoudt et al. synthesized6b nanoparticles of diameter between 2 and 3 nm with comparative polydisperties to those reported here. The difference in size between the nanoparticles presented in this paper and the work of Reinhoudt is attributable to the fact that they used a Au:S of 1:3, while we used a ratio of 1:1, which will favor prolonged growing times and hence larger nanoparticles. Measurement of 170 edgeedge interparticle distances reveals separations of 1.2 ( 0.3 nm (C10SC10) and 2.7 ( 0.7 nm (C18SC10). Assuming23 a carbon-carbon bond distance of 0.127 nm in an alltrans conformation of the alkyl chain, these separations correspond to spacings of, respectively, ∼10 and 18 carbon atoms. Thus, the nanoparticles are maximizing the density of packing to approach a solid-state density of the alkyl chains. We envisage that this is achieved by interdigitation as shown in Figure 5d. However, we have no proof for this packing motif other than our previous study on interdigitation by CFM.10a This behavior is also observed when alkanethiol ligands (Figure 5(c)) are used.24a,b Conclusions In conclusion, we have demonstrated the use of dialkyl sulfides as passivants for gold nanoparticles. The nano(23) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321.
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Figure 5. a) TEM micrographs of C10SC10 (left) and C18SC10 (right) passivated nanoparticles. b) Scheme of designed interdigitation mode for C18SC10. c) Scheme of interdigitation mode for alkanethiol passivated nanoparticles. d) Scheme of proposed interdigitation mode found in all dialkyl sulfide passivated nanoparticles.
particles are stable while in relatively concentrated solutions (>1 mg mL-1) but unstable when either desolvated or in more dilute solutions (0.1 mg mL-1). Our data suggests that, compared with alkanethiols, the weaker gold-sulfur bond in the dialkyl sulfides results in (i) slower passivation of the gold surface which allows nucleation and growth processes to continue for longer periods (ii) increased size polydispersity of the nanoparticles obtained and (iii) oxidation of the passivating ligand as a result of the passivation and/or depassivation of the nanoparticles. The interparticle spacing is due to the length of the longest alkyl chain passivating the nanoparticles and not the interdigitation scheme depicted in Figure 5(b). Dialkyl sulfides (RSR′) have two main advantages over thiols (RSH) and dialkyl disulfides (RSSR′) as passivants: (i) R and R′ can be varied independently and with ease, (ii) the issue of phase separation25 when SAMs are formed from dialkyl sulfides or binary solutions of thiols is eliminated. However, the instability of the nanoparticles passivated with these ligands presents some technical challenges for their practical use. It should be noted that Reinhoudt6b et al. have cleverly increased the stability of dialkylsulfide passivated nanoparticles by making di-, tri-, and tetravalent dialkysulfide derivatives. We envisage using the independent variation of R and R′ as a structural ordering element26 of gold and silver nanoparticles via i) (24) a) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. b) Wang, Z. L.; Harfenist, S. A.; Whetten, R. L.; Bentley, J.; Evans, N. D. J. Phys. Chem. B 1998, 102, 3068. (25) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (26) Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379.
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π-stacking of electron rich and poor aromatic units and ii) hydrogen bonding.27 Acknowledgment. This research has been supported by the EPSRC, including a studentship for E.J.S., an (27) Connolly, S.; Fitzmaurice, D. Adv. Mater. 1999, 11, 1202.
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EPSRC-ROPA (GR/N12657/01) award (Y.C.) and has additionally been supported by the EU via the fifth framework (HPRN-CT-2000-00028). D.R. is supported from discretionary funds available to the Nanochemistry Group at University College Dublin. LA0109260
