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Langmuir 2006, 22, 4100-4103
Growth of Gold Nanoparticle Films Driven by the Coalescence of Particle-Stabilized Emulsion Drops B. P. Binks,‡ J. H. Clint,‡ P. D. I. Fletcher,*,‡ T. J. G. Lees,‡ and P. Taylor† Surfactant & Colloid Group, Department of Chemistry, UniVersity of Hull, Hull HU6 7RX, UK, and Syngenta, Jealott’s Hill International Research Station, Bracknell, Berkshire, RG42 6EY, UK ReceiVed October 12, 2005. In Final Form: January 3, 2006 We have investigated the mechanism of the spontaneous growth of a gold nanoparticle film on a container wall when an aqueous dispersion of gold nanoparticles is shaken with an oil phase containing octadecylamine, as first described by Mayya and Sastry (Mayya, K. S.; Sastry, M. Langmuir 1999, 15, 1902.). Experimental evidence is described, which shows that the film growth is driven by the coalescence of particle-coated emulsion drops with the flat oil-water interface separating the oil and water phases.
The adsorption of nanoparticles at liquid-liquid interfaces can be very strong if the nanoparticle is partially wetted by both liquids, that is, the contact angle between the interface and the particle surface is not too far from 90°.1 For particles with diameters larger than about 10 nm, the energy of adsorption is very often orders of magnitude larger than the thermal energy, and so particle adsorption is expected to be irreversible. This strong adsorption finds application in the use of particles to stabilize emulsions2 and is also of interest as a basis for methods to manipulate and assemble nanoparticles into various structures such as two-dimensional (2D) films on solids3,4 and threedimensional (3D) porous solids.5 In 1999, Mayya and Sastry3 reported a fascinating new technique for the spontaneous growth of colloidal nanoparticle superlattices onto a glass surface from an aqueous dispersion. They took an aqueous dispersion of 13 nm diameter gold particles capped with 4-carboxythiophenol and added an immiscible layer of toluene containing a low concentration of octadecylamine (ODA). After vigorous shaking for 30 s and “once the biphasic solution settled down after the shaking process”, a violet-colored film of gold particles rapidly climbed the inner wall of the container starting from the oil-water interface. They were able to immerse a prewetted glass slide in the oil-water interface, causing the films to grow up the slide so that they could then remove it and characterize the deposited superlattices of gold nanoparticles. They ascribed the film growth to surface-tension-driven hydrodynamic flows (a Marangoni effect) but noted that the reason behind the spontaneous film growth rate being so rapid (approximately 16 mm s-1) compared to the Marangoni velocities observed in other work remained to be understood. We have investigated this process of gold nanoparticle film growth from an oil-water interface further with a view to elucidate the mechanism. Experimental Section Hydrogen tetrachloroaurate(III) trihydrate (Aldrich, 99.9%), sodium citrate dihydrate (Aldrich, ACS reagent grade), octadecyl* Corresponding author. Address: Surfactant & Colloid Group, Department of Chemistry, University of Hull, Hull HU6 7RX, UK. E-mail:
[email protected]. ‡ University of Hull. † Syngenta. (1) Binks, B. P. Curr. Opin. Colloid Interface Sci. 2002, 7, 21. (2) Aveyard, R.; Binks, B. P.; Clint, J. H. AdV. Colloid Interface Sci. 2003, 100-102, 503. (3) Mayya, K. S.; Sastry, M. Langmuir 1999, 15, 1902. (4) Li, D.-G.; Chen, S.-H.; Zhao, S.-Y.; Hou X-M.; Ma, H.-Y.; Yang, X.-G, Appl. Surf. Sci. 2002, 200, 62.
trichlorosilane (OTS; Aldrich, 90+%), and ODA (Fluka, >99%) were used without further purification. n-Dodecane (Aldrich, 99+%) and toluene (Fisher, Analytical grade) were passed three times through neutral chromatographic alumina to remove traces of polar compounds before use. Colloidal gold was formed by the citrate reduction of the gold salt hydrogen tetrachloroaurate(III) trihydrate using a modification of the method described by Chow and Zukoski.6 An aqueous citrate solution (20 cm3) was added to an aqueous gold salt solution (180 cm3) at 70 °C, and the mixture was held at 70 °C, with rapid stirring, until the solution turned a deep wine red (approximately 10 min). The concentrations of hydrogen tetrachloroaurate(III) trihydrate and citrate after mixing but before reaction were 2.68 and 5.36 mM, respectively. These concentrations were used to obtain the maximum gold content in a nonflocculated colloidal dispersion. It was found that higher reagent concentrations produced a flocculated gold colloid, presumably because the final ionic strength exceeded the critical flocculation concentration. Higher gold concentrations can be achieved only by additional treatment to remove the unwanted reaction byproducts. The particle diameter distribution, determined using transmission electron microscopy, showed a single peak centered at 30 nm with a half width at half-maximum of 9 nm. Except where otherwise stated, the standard gold film spreading conditions were as follows. A 5 cm3 portion of the gold dispersion containing 2.68 mM Au with a pH of approximately 4 and 1 cm3 of dodecane containing 0.08 mM ODA were contained within a stoppered glass test tube with an internal diameter of 13 mm. The aqueous phase was wine red, and the oil phase was transparent. Hand shaking for 20 s caused a visible, gold metallic-colored film to rise to the top of the test tube (approximately 70 mm above the oil-air interface) within a few seconds. When required, the pH of the aqueous phase was altered by the addition of small volumes of 1 M NaOH. For some experiments, the inner walls of the test tubes were hydrophobically modified using a 1 wt % solution of OTS in toluene under a nitrogen atmosphere for different set times. The wettabilities of the modified surfaces were characterized by measuring the static contact angles (using a Kru¨ss DSA10 instrument) of drops of the aqueous gold colloid under dodecane containing 0.08 mM ODA on glass slides modified under identical conditions. The aqueous-phase drops were first added to the surface followed by the addition of the oil phase. Prior to the contact-angle measurements, all solutions were preequilibrated together under conditions identical to those used in the film-growth observations. This preequilibration was found to affect the contact angle strongly, presumably as a result of changes in the equilibrium ODA concentration in the dodecane phase as a result of adsorption on the gold particles. (5) Binks, B. P. AdV. Mater. 2002, 14, 1824. (6) Chow, M. K.; Zukoski, C. F. J. Colloid Interface Sci. 1994, 165, 97.
10.1021/la052752i CCC: $33.50 © 2006 American Chemical Society Published on Web 03/22/2006
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All measurements were made at a room temperature of 20 ( 2 °C.
Results and Discussion The system studied here (detailed in the Experimental Section) was similar to but not identical with that of Mayya and Sastry. The gold particles used here were larger (30 nm compared with the 13 nm average diameter) and were stabilized by citrate alone, without the use of the 4-carboxythiophenol capping agent used by them. Dodecane was used here instead of toluene, and the final gold concentration was higher (2.68 mM compared with 1.26 mM). However, the systems were similar in that the gold particles were either in the bulk aqueous phase or the interface; transfer to the oil phase did not occur. The following key observations of the film climbing phenomenon were made. First, following shaking of a two-phase system, film climbing is always associated with the coalescence of the unstable emulsion drops with the oil-water interface separating the bulk phases. It appears that the film is only formed as the emulsion drop surface is lost by coalescence. Second, the film adhering to the inner wall of the container remains stable until the tube is reshaken, creating more emulsion drops, that is, it appears that the film is only lost when a new emulsion drop surface is created. Third, experiments were made in which film climbing was observed in a larger cylindrical glass tube with an internal diameter of 23 mm instead of the 13 mm for the test tubes used in the standard system. Solution volumes and concentrations and shaking time were kept constant. For the larger tube, the film climbing was slower and stopped at a height of 35 mm (compared with climbing to the top of the test tube, 70 mm). All these observations suggest that, following the first cycle of shaking the system and film climbing, the total amount of the gold-particle-covered surface (i.e., oil-water interface plus film plus emulsion drop surfaces) is either fully or partially conserved. Film climbing or loss is associated with a loss or gain in emulsion drop surface area. Control experiments in which either the gold particles or the ODA were absent showed no film climbing. In the absence of ODA, the citrate-stabilized gold particles are presumably too hydrophilic such that they remain in the water phase and do not adsorb sufficiently at the oil-water interface. Although ODA is expected to be surface-active at the oil-water interface, at a concentration of 0.08 mM, it does not stabilize emulsions or drive film growth in the absence of the gold colloid. At higher concentrations (1.3 mM), the ODA caused flocculation and sedimentation of the gold colloid but did not lead to transfer of the gold particles from the aqueous to the dodecane phase. Figure 1 shows the postulated mechanism of film growth which appears to be consistent with all experimental observations. Shaking the initial two-phase mixture forms emulsion drops. We hypothesize that these emulsion drops are unstable with respect to coalescence with the flat oil-water interface because the adsorption of gold particles at the curved and flat oil-water interfaces is too low to prevent coalescence. However, the particles at low density in the surface are likely to be adsorbed irreversibly with an adsorption energy greatly in excess of the thermal energy. In this situation, drop coalescence reduces the total oil-water interfacial area, increases the surface concentration of the adsorbed gold particles, and hence increases their surface pressure in the oil-water interface, separating the bulk oil and water phases. This increased surface pressure drives the film climbing and corresponds to the Marangoni process noted by Mayya and Sastry. Drop coalescence will stop when the surface concentration of adsorbed gold particles is sufficient to stabilize the surfaces against
Figure 1. Schematic illustration of how the coalescence of particlestabilized emulsion drops with the flat oil-water interface causes spontaneous spreading of a particle-stabilized thin water film up the container walls. For simplicity, we have shown emulsion drops formed only in the water phase.
coalescence, a feature that has many similarities with the phenomenon of limited coalescence observed in emulsions stabilized solely by particles.7 Film growth will stop when the surface pressure of the adsorbed particles has reduced below a critical level as a result of the area expansion. Within this mechanism, we would expect that the process of emulsion loss with film growth and the reverse process of emulsion formation with film loss would be repeatedly reversible by shaking, as is observed experimentally. As discussed in a recent review,2 many types of nanoparticles, including silica, clay, and polymer species, are very effective as emulsion stabilizers in the absence of any other surface-active species when the particle wettability is suitable. It is therefore relevant to question why the gold particles used here do not stabilize the emulsion drops in the present system for more than a minute or so, even when they are clearly observed to adsorb at the oil-water interface. For most nanoparticle-stabilized emulsion systems reported in the literature, the nanoparticle concentration must be on the order of 1 wt % to provide good long-term emulsion stability.1,2 In the system studied here, the concentration of gold particles is only 0.053 wt %, which may simply be too low to confer long-term emulsion stability against coalescence. As noted in the Experimental Section, higher gold concentrations than that used here are not possible without additional treatment of the colloids to remove the electrolyte products of the reaction, which otherwise cause flocculation of the gold colloid. A further factor that may contribute to the poor emulsion stabilization by gold particles is that the Hamaker constant for gold surfaces interacting across water or hydrocarbons is 40 × 10-20 J. This indicates that the van der Waals attractive (7) Arditty, S.; Whitby, C. P.; Binks, B. P.; Schmitt, V.; Leal-Calderon, F. Eur. Phys. J. E 2003, 11, 273.
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Binks et al.
forces destabilizing the gold particle emulsions are higher than those for silica or polymers for which the corresponding Hamaker constants are in the range of 0.5-0.8 × 10-20 J.8 It is relevant to discuss the possible origins of the surface pressure arising from the compression of the adsorbed particle monolayer as a result of emulsion drop coalescence. As discussed in ref 1, the surface pressure Π exerted by a monolayer of particles with area per particle A and behaving as an ideal 2D gas is given by
ΠA ) kT
(1)
where k is the Boltzmann constant, and T is the absolute temperature. For typical nanoparticle sizes, the minimum A is always large relative to the molecular areas, and hence the maximum Π is predicted to be relatively very small. However, adsorbed nanoparticles will have an excluded area Ao and, in addition, may possess significant longer range interactions. A more realistic surface equation of state incorporating these nonideal factors is
(Π + R/A2)(A - Ao) ) kT
(2)
where R is a parameter reflecting the lateral interactions between adsorbed particles, which is positive for attraction and negative for repulsion. In the case of hard-sphere particles (i.e., R ) 0), the surface pressure approaches infinity as A approaches Ao. In the case of particles showing significant repulsion, the surface pressure is predicted to be correspondingly larger than that in the ideal 2D gas case for A > Ao. Hence, the surface pressure exerted by adsorbed particles results mainly from repulsive interactions that are either solely short range (expressed by the excluded area Ao) or include additional long-range repulsion when R is negative. For the case of the gold particles used in this work, increased surface pressure is expected for an increased overall particle charge associated with either the citrate or the ODA adsorbed at the gold particle surfaces. Increased particle charge, and hence repulsion, leading to the increased particle surface pressure driving the film growth, might be a possible explanation for the increase in the film climbing rate observed as the pH is decreased (see Figure 4). A series of experiments has been made to further test the postulated mechanism and to characterize the film formation process. In the first, the effect of hydrophobizing the inner wall of the test tube was examined. The schematic of Figure 1 shows the situation in which the test tube inner wall is preferentially wetted by the aqueous phase, that is, the glass-water-phaseoil-phase contact angle (as measured through the water phase) is less than 90°. The meniscus curvature of the oil-water interface is such that growth of the film from this interface is directed upward. Hydrophobization of the glass wall such that the glasswater-oil contact angle is greater than 90° should direct the film downward. Figure 2 shows the appearance of the films in a series of test tubes progressively made more hydrophobic by silanization with OTS for different reaction times. The glass-water-phaseoil-phase contact angles, measured for preequilibrated phases with compositions identical to those used in the film-growth observations, are noted for each tube. As expected, for tubes with glass-water-phase-dodecane-phase contact angles of 2466°, the film growth is clearly upward, whereas it is downward for tubes with contact angles of 115 and 130°. The film formed (8) Israelachvili, J. Intermolecular and Surface Forces, 2nd edition; Academic Press: London, 1992.
Figure 2. Appearance of test tubes immediately after shaking and film growth showing the effect of hydrophobization of the inner walls of the test tubes. From left to right, the glass-water-phasedodecane-phase contact angles ((10°) are 24, 66, 90, 115, and 130°. Film growth is clearly upward in the first two tubes and downward in the last two. The middle tube shows a small extent of film growth upward.
for the tube with a contact angle of around 90° appears to be less strongly developed and less uniform than those with angles further from 90°. In the second set of experiments, we used a long hydrophilic glass tube with a length of 900 mm and an inner diameter of 9 mm to make direct observations of the impact of both water and oil drops on the oil-water interface without forming emulsion drops by shaking. In the first setup, oil drops containing 0.08 mM ODA were injected at the bottom of the tube, which was partially filled with aqueous gold colloid with a gold concentration of 2.68 mM. The oil drops were made only slightly smaller than the tube diameter in order to increase the rise time of the drops, which took approximately 25 s to reach the top of the aqueous gold dispersion. As they rose, the drops developed a metallic gold surface appearance by the time they reached the top (Figure 3), indicating that they accumulated gold particles at their surfaces. The first coalescence event occurred promptly as the first drop hit the air-water surface and produced a very faintly colored film which moved up the tube wall. Subsequent drops coalesced promptly with the oil-water interface and produced both an increased film height and a progressive darkening in the color of the film, presumably associated with an increased gold content of the film following repeated drop coalescence and film rise events. We speculate here that the repeated overlaying of films following drop coalescence in this geometry may result in multilayers of gold particles. It was clear that the film growth was linked to the drop coalescence events. In the second setup, water drops containing gold colloid with 2.68 mM Au were injected into the top of the tube filled with dodecane containing 0.08 mM ODA and allowed to sediment to the bottom. The drops were again only slightly smaller than the tube diameter such that the sedimentation took approximately 25 s. In this case, the drop appearance did not change with passage down the tube. Although the drops were colored as a result of being filled with gold colloid, their visual appearance suggested that gold particles did not accumulate at the drop surfaces, probably because the gold particles were moving within the water drops and the water drops were not moving through the gold dispersion, as was the case for the oil drops. The first drop contacting the bottom of the tube spread over the glass surface within 5 s or so. Subsequent drops coalesced with the water phase within a few seconds, with no evidence of film formation up the tube walls. Some additional observations were made with
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Figure 4. Variation of the film climbing rate with the pH of the aqueous phase for hydrophilic test tubes containing 5 cm3 of gold colloid (2.68 mM Au) plus 1 cm3 of dodecane containing 0.08 mM ODA.
Figure 3. Appearance of a dodecane drop rising through a tube (inner diameter 9 mm) filled with gold particles (left photo) and drops of aqueous gold colloid sedimented at the bottom of the tube filled with dodecane containing 0.8 mM ODA (right photo).
an increased concentration (0.8 mM) of ODA in the dodecane. Presumably because of higher ODA adsorption at the oil-water interface, this higher ODA concentration caused the water drops to be stable with respect to coalescence over several hours, which enabled images to be taken following their arrival at the bottom of the tube (Figure 3). The two images of Figure 3 clearly show the different surface appearances of the rising oil drops, where gold particles adsorb, and falling water drops, where particle adsorption does not occur. In the final set of experiments, the effect of aqueous-phase pH on the climbing films in the standard shaken hydrophilic test tube was measured. As seen in Figure 4, increasing the pH decreases the film climbing speed. The fastest growth measured here (35 mm s-1) is even faster than the speed of 16 mm s-1 noted by Mayya and Sastry and serves to further highlight the difference they noted between the speed of this type of film growth and the Marangoni velocities reported in the literature.3 It was also observed that the films were less colored with increasing pH and were transparent above pH 9.4. No films were obtained for pH values higher than about 11.2. These observations suggest that gold particle adsorption at the oil-water interface is decreased at higher pH, probably as a consequence of the change in charge of the ODA from cationic at low pH to uncharged at high pH (pKa ) 10.7). Cationic ODA is expected to bind strongly to the negatively charged citrate-stabilized gold particles, rendering them sufficiently hydrophobic so as to adsorb at the oil-water interface. At high pH, ODA adsorption will be suppressed, and the gold particles will remain hydrophilic and nonadsorbing at the oil-water interface. As noted earlier, changes in the charge of the gold particles with pH are expected to affect
the repulsive interactions between the gold particles and, hence, the surface pressure of the adsorbed film. Such surface pressure changes may also affect the film climbing rate. On the basis of direct observations in different geometries and the fact that the direction of film growth is reversed in hydrophobic tubes, the key conclusion of this study is that the very rapid film growth observed here is induced by the coalescence of particlestabilized emulsion drops with the flat oil-water interface separating the bulk oil and water phases. The film of irreversibly adsorbed gold particles is transferred from the curved emulsion drop surfaces to the flat interface and drives the growth of the film outward from this interface and either up or down the container wall. Similar film growth has been observed in systems containing silver nanoparticles stabilized by sodium oleate in water-toluene mixtures following vigorous shaking.4 Although not suggested by the authors, the emulsion drop coalescence mechanism shown in Figure 1 may also apply in this system. There is also one example of particle film growth from an oilwater interface that is induced by alcohol addition rather than shaking and emulsion drop coalescence. Reincke et al. prepared a two-phase system comprising an aqueous gold colloid with heptane.9 Addition of ethanol to the aqueous dispersion of gold particles is shown to make them more hydrophobic. This increases their adsorption at the water-heptane interface and drives the growth of a particle film up the tube wall. No shaking of the system to produce transient emulsion drops was required in this case. In addition to their applications in conventional emulsion science, it is worth noting that nanoparticle-stabilized emulsions can form a convenient vehicle for handling and manipulating bulk quantities of nanoparticles. It is therefore important to understand more about the mechanisms whereby nanoparticles can be spontaneously and rapidly transferred from such emulsions to deposited 2D films to assemble nanoparticle-based devices such as sensors. Acknowledgment. We thank the Engineering & Physical Science Research Council, UK, and Syngenta Ltd. for their financial support. LA052752I (9) Reincke, F.; Hickey, S. G.; Kegel, W. K.; Vanmaekelbergh, D. Angew. Chem., Int. Ed. 2004, 43, 458.
