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Langmuir 1999, 15, 1902-1904
A New Technique for the Spontaneous Growth of Colloidal Nanoparticle Superlattices K. S. Mayya and Murali Sastry* Materials Chemistry Division, National Chemical Laboratory, Pune-411 008, India Received September 25, 1998. In Final Form: January 27, 1999 The spontaneous growth of thin films of carboxylic acid derivatized colloidal gold particles electrostatically immobilized at the hydrosol-organic solution interface onto moistened hydrophilic substrates is demonstrated. Immersion of the substrates up to the interface of the biphasic mixture leads to superlattice formation by an extremely fast climbing mechanism, the process apparently driven by surface tension gradients at the substrate surface (Marangoni growth). This approach shows promise for development in the deposition of superlattice films of different surface-modified colloidal nanoparticles as well as salts of fatty lipids.
Nanoparticles exhibit extremely exciting physicochemical and optoelectronic properties both on an individual scale1 and collectively as two-dimensional organizates.2 The latter aspect has motivated research into the organization of nanoparticles in thin film form. Among the many techniques being investigated, the colloidal route for the synthesis of nanoparticles followed by self-assembly on suitable substrates3,4 has evoked much interest. It has been observed that good quality, close-packed colloidal nanoparticle films are obtained by self-assembly of particles synthesized and capped with suitable surfactants in an organic solvent.4 Recently, Rao and co-workers have shown that colloidal gold, silver, and platinum nanoparticles synthesized in an aqueous medium could be transferred to an organic phase by chemisorbing the nanoparticles with alkanethiols present in the organic solvent.5 This was an important deviation over earlier procedures wherein metal ions were reduced to form particles and simultaneously capped with surfactants in an organic medium.4 Motivated by the report of Rao et al.,5 we investigated whether electrostatic interactions between aqueous, charged colloidal gold particles (charged negatively with chemisorbed carboxylate ions) and octadecylamine molecules would lead to transfer of the * To whom correspondence should be addressed: phone, 0091212-337044; fax, 0091-212-337044; e-mail,
[email protected]. Current address: Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115. Phone, 301-405-6566; fax, 301-314-7136; e-mail,
[email protected]. (1) (a) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaff, T. G.; Khoury, J.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279. (2) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997 277, 1978. (3) (a) Chumanov, G.; Sokolov, K.; Gregory, B. W.; Cotton, T. M. J. Phys. Chem. 1995, 99, 9466. (b) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (c) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (d) Bandyopadhyay, K.; Patil, V.; Vijayamohanan, K.; Sastry, M. Langmuir 1997, 13, 5244. (4) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (c) Badia, A.; Gao, W.; Singh, S.; Demers, L.; Cuccia, L.; Reven, L. Langmuir 1996, 12, 1262. (d) Wang, Z. L.; Harfenist, S. A.; Whetten, R. L.; Bentley, J.; Evans, N. D. J. Phys. Chem. B 1998, 102, 3068. (e) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (f) Wang, Z. L. Adv. Mater. 1998, 10, 13. (g) Connolly, S.; Fullam, S.; Korgel, B.; Fitzamurice, D. J. Am. Chem. Soc. 1998, 120, 2969. (5) Vijaya Sarathy, K.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876.
particles from the aqueous phase to the organic phase containing the surfactant. We observed that during vigorous mixing of the biphasic mixture, the colloidal particles were immobilized at the interface with no transfer to the organic phase. Even more interesting was the observation that the colloidal gold particle film at the interface was spontaneously deposited (by an extremely fast climbing mechanism) on the sides of the beaker containing the biphasic mixture. Presented below are details of the investigation and discussion on the possible mechanism for this unusual observation. The colloidal gold particles were prepared by citrate reduction of chloroauric acid in aqueous medium and capped with an aromatic bifunctional molecule, 4-carboxythiophenol (4-CTP), as described elsewhere.6 The citrate reduced hydrosol was at pH ) 3; the pH was adjusted to 9.0 using ammonia solution prior to capping with 4-CTP. Thiolate bond formation of 4-CTP with the gold particles leads to carboxylic acid derivatization of the colloidal particle surface.6 The carboxylic acid groups are fully ionized at pH 9.0 and electrostatically stabilize the colloidal particles.6 Optical absorption spectroscopy measurements, carried out on a Hewlett-Packard 8452 diode array spectrophotometer, yielded a shift in the surface plasmon resonance from 525 nm for the uncapped gold sol to 530 nm after capping with 4-CTP. The gold particles thus synthesized were of dimensions 130 ( 30 Å. A 1 mg/mL concentrated solution of octadecylamine in toluene was prepared. To 60 mL of the 4-CTP capped gold hydrosol taken in a flat beaker (cross-sectional area ) 79 cm2 ), 20 mL of the toluene solution containing octadecylamine was added. Two immiscible layers of the clear toluene solution on the ruby colored gold sol formed which, after vigorous shaking for 30 s, resulted in the transfer of the gold colloidal particles to the interface. Under certain conditions of the colloidal solution pH and concentration of the amine in the organic medium, almost complete immobilization of the gold particles at the interface could be achieved. Figure 1A shows the optical absorption spectra recorded from the gold sol before (solid line) and after (dashed line) shaking the biphasic solutions while Figure 1B shows the corresponding spectra recorded from the toluene phase. A comparison of parts A and B of Figure 1 shows that while the gold particles have migrated away from the bulk of the hydrosol, they have not been (6) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 3944.
10.1021/la981335l CCC: $18.00 © 1999 American Chemical Society Published on Web 02/17/1999
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Figure 1. (A) Optical absorption spectra recorded from the 4-CTP derivatized gold sol before (solid line) and after shaking with toluene containing the octadecylamine surfactant (see text for details). (B) Optical absorption spectra recorded the toluene phase before (solid line) and after shaking with the 4-CTP capped gold hydrosol (see text for details). Scheme 1. Chart Illustrating the Beaker-Based Experimental Procedure for Spontaneous Growth of Gold Colloidal Particles Electrostatically Immobilized at the Hydrosol-Toluene Interface
transferred into the organic phase but are immobilized at the interface. This is different from the work of Rao et al.,5 where the gold particles were transferred into the bulk of the organic phase. However, the most interesting observation was the almost instantaneous deposition of the gold colloidal particle films on the sides of the beaker which occurred once the biphasic solution settled down after the shaking process. A violet colored film could clearly be seen climbing up the sides of the beaker from the hydrosoltoluene interface (Scheme 1 and Figure 1 (Supporting Information)). The gold nanoparticle climbing process was extremely faststhe film scaled ca. 8 cm of the beaker wall in 5 s. To quantify the colloidal particle superlattice growth, a glass slide of dimensions 7.5 cm × 2.5 cm was introduced into the biphasic solution up to the interface as shown in Schematic 1 and the resulting film was characterized using optical absorption spectroscopy. It was observed that the formation of the colloidal particle films was considerably faster if the glass surface was wet. The substrates were therefore moistened prior to immersion.
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Figure 2. Optical absorption spectra recorded from spontaneously grown gold superlattice films on to moistened glass substrates as a function of the hydrosol pH (pH indicated next to the respective curves). The inset shows the QCM mass uptake of similarly formed gold nanoparticle superlattices as a function of the hydrosol pH.
The cluster concentration in the superlattice films was estimated using quartz crystal microgravimetry (QCM).7 It is clear that there is fractionation of the amphiphilic octadecylamine molecules at the hydrosol-toluene interface. At pH ) 9.0, the amine groups would be ionized (-NH3+) and the attractive electrostatic interaction between the negatively charged gold colloidal particles and the amine groups would lead to the immobilization observed. The considerable hydrophobic component arising from the octadecyl-hydrocarbon tail lends stability to the colloidal particle-amine monolayer at the hydrosoltoluene interface and contributes to the formation of a stable superlattice of colloidal particles after transfer to a solid substrate (to be discussed below). This process is conceptually similar to the electrostatic process used by us to immobilize colloidal particles at the air-hydrosol interface.8 The shaking process used in this study considerably accelerates the electrostatic immobilization process at the hydrosol-toluene interface, which is extremely slow for colloidal particles at the air-hydrosol interface.8d Since the electrostatic immobilization of the clusters should be driven by the extent of ionization of the amine and carboxylic acid groups, a strong pH dependence of the transfer to the interface is expected. Figure 2 shows the optical absorption spectra recorded from the spontaneously grown gold nanoparticle films on moistened glass as a function of colloidal solution pH (pH adjusted using dilute HCl). It is observed that the cluster density is maximum at pH ) 6 and monotonically decreases as the hydrosol pH increases. As the cluster density in the film increases, the surface plasmon resonance shifts to the red. This is a consequence of enhanced coupling of the (7) QCM measurements were made on a gold-coated 6 MHz AT-cut quartz crystal. The gold electrodes were coated with aluminum to render the surface hydrophilic. The frequency changes were recorded on an Edwards FTM5 frequency meter which had a resolution and stability of 1 Hz (mass resolution of 12 ng/cm2). The frequency changes were converted to mass loading using the Sauerbrey formula (Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206). (8) (a) Mayya, K. S.; Patil, V.; Sastry, M. Langmuir 1997, 13, 2575. (b) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B 1997, 101, 4954. (c) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281. (d) Mayya, K. S.; Sastry, M. J. Phys. Chem. B 1997, 101, 9790. (e) Mayya, K. S.; Patil, V.; Sastry, M. J. Chem. Soc., Faraday Trans. 1997, 93, 3377. (f) Mayya, K. S.; Sastry, M. Langmuir 1998, 14, 74. (g) Mayya, K. S.; Patil, V.; Kumar, M.; Sastry, M. Thin Solid Films 1998, 312, 308.
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plasmon vibrational modes between the colloidal gold particles as has been observed for silver particles by Pileni and co-workers.9 The variation in the cluster density in the films with hydrosol pH is corroborated by the QCM mass uptake data shown in the inset of Figure 2. It can be shown that the mass uptake at pH ) 6 corresponds to the transfer of roughly 1.5 monolayers of close-packed gold colloidal particles (particle size ) 130 Å, mass/particle ) 20 × 10-9 ng). The cluster density in these films is considerably higher than that observed for colloidal particles immobilized at the air-water interface.8e We would also like to point out that the pH of maximum cluster complexation (pH ) 6) is different from that observed for octadecylamine monolayers at the air-water interface (pH ) 8).8e This may be a consequence of the different dielectric constants of air and toluene in contact with the hydrosol phase in the two cases, a factor which is known to affect the energetics of the electrostatic interaction process in such systems.10 The mass uptake during separate depositions as well as the optical density over the substrate surface was uniform, indicating that the mode of film formation is highly reproducible. The films grown spontaneously on moistened glass and quartz substrates as described above rendered the surface hydrophobic (contact angle of a sessile water drop >110°). This indicates that the growth mode is rather similar to that used in the conventional Langmuir-Blodgett (LB) technique.11 Fourier transform infrared spectroscopy (FTIR) measurements of a gold particle film grown spontaneously onto moistened Si(111) substrates showed the methylene antisymmetric and symmetric vibration bands at 2920 and 2850 cm-1, respectively (supplementary information, Figure 2).12 This indicates close packing of the hydrocarbon chains.13a The NH2 antisymmetric vibration was rather broad and centered around 3300 cm-1 . Unlike in the amine salts where this band shifts to ca. 3200 cm-1,13b no shift is observed for the gold cluster films, indicating rather weak coupling of the clusters to the amine groups. This behavior is similar to that observed for silver cluster films electrostatically incorporated by diffusion into thermally evaporated fatty amine.13c Sub-
sequent immersion of the colloidal-particle-covered films up to the interface of the biphasic mixture did not result in further deposition of the colloidal particle film. This fact together with the observation that a thin layer of water on the substrate is important for spontaneous growth of the film clearly indicates the importance of a surface tension gradient at the point of contact of the substrate at the hydrosol-toluene interface. Such a process may be identified with the well-known Marangoni effect, i.e., surface tension driven hydrodynamic flows.14 The solutal Marangoni effect has been shown to lead to the spreading of liquid films on glass, the movement being driven by surface tension gradients arising due to evaporation from an ethanol/water two-component system.14d It is possible that a similar mechanism is operating in the deposition of the gold colloidal particle superlattice films of this study as well. It remains to be understood as to why the spontaneous film growth process is so rapid for the colloidal particles of this study when compared to the Marangoni velocities observed earlier.14d It is clear that identification of the exact mechanism requires further study taking into account other factors such as the nature of the surfactant at the interface, dielectric properties of the organic phase, etc. In conclusion, it has been shown that electrostatically immobilized gold colloidal particle films at the hydrosoltoluene interface grow spontaneously on hydrophilic substrates. Both the cluster immobilization as well as the superlattice growth processes are extremely fast and thus represent a major (advantageous) difference over the LB approach, which in addition to the slow time scales, requires expensive instrumentation. This simple beakerbased superlattice growth technique shows promise for extension to the growth of other surface modified nanoparticle systems as well as films of salts of fatty lipids.
(9) Taleb, A.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 2214. (10) Tamagawa, H.; Sakurai, M.; Inoue, Y.; Ariga, K.; Kunitake, T. J. Phys. Chem. B 1997, 101, 4817. (11) Petty, M. C. Langmuir-Blodgett FilmssAn Introduction; Cambridge University Press: Cambridge, 1996. (12) FTIR measurements were made on a UNICAM GENESIS spectrometer at 2 cm-1 resolution. Since only 1.5 monolayers of the amine film were transferred during the spontaneous growth process, the signal-to-noise ratio in the FTIR spectra was not very high. (13) (a) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (b) Bardosova, M.; Tregold, R. H.; Ali-Adib, Z. Langmuir 1995, 11, 1273. (c) Sastry, M.; Patil, V.; Mayya, K. S. Langmuir 1997, 13, 4490.
Supporting Information Available: Pictures showing a conical flask before and after shaking the 4-CTP capped gold hydrosol and toluene containing octadecylamine solution mixtures as well as FTIR spectrum recorded from a spontaneously grown gold particle film on Si(111). This material is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. K.S.M. thanks the Department of Science and Technology, Government of India, for a research fellowship. This work was funded through a grant to M.S. by the Council of Scientific and Industrial Research (CSIR), Government of India.
LA981335L (14) (a) Bois, A. G.; Ivanova, M. G.; Panaiotov, I. I. Langmuir 1987, 3, 215. (b) Walters, D. A. Langmuir 1990, 6, 991. (c) Ramadane, O. O.; Quere, D. Langmuir 1997, 13, 2911. (d) Fanton, X.; Cazabat, A. M. Langmuir 1998, 14, 2554.
