J. Phys. Chem. 1993,97, 6334-6336
6334
Formation of Ordered Two-Dimensional Gold Colloid Lattices by Electrophoretic Deposition M. Giersig and P. Mulvaney'lt Hahn-Meitner Institut, Abteilung Photochemie, W-1000, Berlin, 39, Germany Received: March 5, 1993; In Final Form: April 29, 1993 The preparation of ordered two-dimensional (2D)gold colloid lattices using an electrophoretic technique is reported. The lattices can be readily observed by electron microscope and the lattice constants determined by electron diffraction. Using image processing, it can be shown that the gold particles condense into a hexagonal lattice and also that the crystallographic axes of the individual gold particles are randomly oriented. The equilibrium distance between the particles corresponds approximately to the size of the adsorbed stabilizers. The structures are therefore stabilized by short-range steric repulsion and not by a diffuse layer electrostatic barrier, as is normally the case for latex sols. The method is of general interest as a means to prepare monolayer films of nanosized metal or semiconductor particles.
Introduction The essential validity of the DLVO theory is beautifully demonstrated by concentrated latex sols, where the delicate balance between the van der Waals attractive force and the diffuse layer repulsion leads to ordered structures in the sols, which makes possible vivid optical effects such as the diffraction of light.192 Ordering is possible because, for micron-sized particles, the van der Waals attraction between particles is sufficiently strong to create a secondary minimum in the potential energy-distance curve between parti~1es.l.~ Onoda and Pieranski have also shown that micron-sized latex particles form ordered 2D crystals on liquid surfaces, again with an interparticle spacing consistent with electrostatic rep~lsion.~ For very small particles, this type of ordering should not be possible because the secondaryminimum is too shallow at room temperature to allow ordering. Small particles are therefore less stable, and coalescence should lead to collapse into the primary minimum, Le., toirreversible coagulation. In this communication, we show that nanosized gold particles (mean diameter 141A) can form ordered two-dimensional lattices on carbon-coated electron microscope grids. The interparticle spacing in these lattices is not consistent with electrostatic stabilizationbut rather with stabilization afforded by short-range steric repulsion. This result is particularly surprising since it is well-known that these gold sols normally undergodiffusion-limited aggregation processes when destabilized, which result in characteristic'fractal" structures.s Colloidal gold was chosen because it can be readily prepared with a narrow size distribution. Furthermore, Kreibig and co-workers have previously shown that gold particles can form densely packed monolayerson microscope grids if the surface is capped with a chemically bound adsorbate.697 Alternating current conductivitymeasurements on densely packed monolayers have already demonstrated that electron transport between particles is possible, the activation energy being due to Coulombic charging8
Experimental Section Colloidal gold was prepared by the method of Turkevich et al. using citrate to reduce A U C ~ ~ The - . ~ mean particle size as determined by electron microscopicanalysis of 250 particles was 141 A. The standard deviation was 10%. Electrophoresis was carried out in a small perspex cell containing 5 mL of gold sol. A dc voltage of 10-100 mV was applied to a carbon-coated (100A) copper microscope grid, using AI foil (3 cm2) as the cathode. Citrate-stabilized gold sol particles move in an electric field with a mobility of about -4 X 10-4 cm2 V-1 s-1 at pH 7, due to the Present address: Department of Physical Chemistry, University of Melbourne, Victoria, 3052, Australia.
negative charge on the adsorbed citrate ions. The lattices were examined using a Philip CM12 electron microscope.
Results and Discussion It has previously been shown that when colloidal gold is protected by (C6H5)2PC6H,$03Na ligands, fractal structures are no longer observed by electronmicroscopy, even at high particle concentrations.6~~Instead, the particles remain well separated and form densely packed layers. Powders of such colloidal gold particles also redissolve in water.1° The packing of these particles to form ordered structures is limited by the fact that when the colloidal gold sols are allowed to dry on microscope grids, the evaporation of the solvent leads to the creation of circular voids, from which the particles are excluded.' To obviate this problem, we have used electrophoresis to deposit gold sols onto the grids, because the films are then generated while the grid is still immersed. In Figure 1, electron micrographs are shown of 2D gold colloid lattices at two different magnifications. The first micrograph shows the general morphology of the films over an area of several hundred square microns. It can be seen that fairly homogeneous coating of the carbon support has been achieved with only a few percent of the area being coated with multilayers. Furthermore, it is clear from the shape of the tears that the gold film itself was originally much larger but has broken following the preparation. This is apparently due to the subsequent solvent evaporation. The individual rafts contain in many cases several hundred thousand colloid particles. In the channels between the film, individual gold colloid particles can still be readily discerned. This shows that the carbon support itself was not broken, only the 2D gold lattice deposited onto it. In fact, at present, the primary factor limiting the domain size we can achieve is the tendency of the gold lattices to tear as the grids are removed from the solution. The second micrograph (Figure lb) shows the same film at a higher magnification,where theordering of the individualparticles can be easily recognized. It is clear that the film consists of a monolayer of gold particles. However, it also reveals that the domains are not single 2D colloid crystals but are made up of myriad smaller domains which apparently grow together during the deposition process. In a more detailed report, we will show that this is indeed the case." Two-dimensional nucleation of the islands takes place at low coverages all over the grids, and these domains fuse at higher coverages but are unable to realign in order to form a single monocrystalline film. The time, T , needed to attain monolayer coverage of the electrodewas estimated using wherepis theparticlemobility, E theappliedfieldstrength [Au],"
0022-3654/93/2097-6334$04.00/0 0 1993 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 6335 -
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i
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Figure 1. Electron micrographs of 2D gold colloid lattices prepared on carbon-coated copper grids (coating thickness 100 8)by electrophoresis of a 0.5 mM citrate-stabilized Au sol, using an AI foil reference electrode (surface area 3 cm2) at an applied positive voltage of 50 mV with an electrode spacing of 2 mm. Note that the carbon coating on the grids is itself conducting.
the concentration of gold colloid particles (mol cm-9, and a the particle radius. During the deposition, the colloids are depleted from up to a distance d = rpE from the electrode, which in our case is about 200 pm. The electrode spacing must be kept larger than this. Under our experimental conditions, the time required for a monolayer to form was calculated to be about 300 s using eq 1. Generally we found, however, that satisfactory results were obtained with electrophoresistimes of 60-1 20 s. At much longer times or at significantly higher applied voltages, multilayer coverage was observed. An advantage of the lower potentials is that electrolyticoxidation of adsorbed gold colloid particles does not take place. This is partly due to the high overpotential for electrochemical oxidation on carbon. In no case did we find any significant ordering if the grid was placed in the gold sol without an applied anodic bias. Indeed if, after electron microscopy, a grid was placed back in the sol and a cathodic bias of -50 mV applied, the gold colloidsdesorbed. Thus, the films are definitely formed via electrophoretic deposition and not by adsorption. In Figure 2a, a high-resolution micrograph of part of a single domain is shown, from which it is readily discerned that the interparticle spacing is 10 A and that the facetting of the gold particles itself facilitates the formation of the lattice. “Grain boundaries” tend to form at particles which are either aspherical or too large to fit into the lattice. The equilibrium distance is far too small to be explained by diffuse layer repulsion. Instead, the particles are stabilized by the presence of adsorbed stabilizer molecules, in this case citrate ions. These ions remain adsorbed during the deposition. The exact thickness of the adsorbed layer created by the citrate ions is not clear. The ions must be neutralized by sodium ions once the grid is removed from the solvent. Computer models yield values between 5 and 6 A if the citrate is assumed to be lying flat on the surface, consistent with the observed interparticle spacing of 10 A. Pieranski found that micron-sized latex particles formed 2D crystals with an interparticle spacing of several particle diameter~.~a With the microscope in diffraction mode, it was possible to obtain weak diffraction of the electron beam. However, because of the unusually large lattice size, accuratelattice constantscould not be obtained. In order to examine the ordering of the particles more statistically, the best of a series of micrographs, as selected by light-optical diffraction, were digitized using a DATACOPY densitometer. A sampling distance of 6.5 A was employed. The data were then processed on a microVAX computer using the SEMPER software package.’* In Figure 2b, the results obtained
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Figure 2. (a) Electron micrograph of part of a single 2D domain. Scale bar is 2WA. (b) Power spectrum of (a) showing hexagonal close packing with the first-, second-, and third-order reflexes. The first reflex, which also indicates the size of the lattice unit cell, amounts to 1/151 A-l, Le., to a core-to-core separation of 15 1 A. The average diameter of the gold particles in the sol was 141 A.
after Fourier transformation of the micrograph in Figure 2a are shown. The colloid particles crystallize into a hexagonal closepacked lattice with an average interparticle separation (coreto-core) of 15 1 A. Second- and third-order reflexes are evident, which confirms the strong degree of ordering in the film. The absenceof higher ordering at lattice constants of about 5 A shows
6336 The Journal of Physical Chemistry, Vol. 97, No. 24, 1993 that the individual gold particles are randomly oriented within the hexagonal lattice. High-resolution micrographs in which the lattice planes of neighboring particles could be discerned also showed little evidence of alignment. The ordering of the particles is determined by the surface adsorbates, and the crystallographic orientation of the colloids plays no significant role. If the colloids were randomly deposited on the grid by the applied field, then such large monolayers would not be likely without significant multilayer formation, which is not observed. The formation of large monolayer domains can only be rationalized by assuming that the gold particles migrate over the grid surface (and over each other) until they can find sites with the highest positive electrostatic potential, since this leads to the greatest free energy gain for the colloid particles. So incoming sol particles are deflected to unoccupied surface sites and do not induce multilayer condensation of the colloid. Most of the adsorbed water is displaced by the sol particles prior to removal of the grid from the solution, and both the emersion from the solution and the subsequent evaporation of the remaining solvent on the grids no longer drastically affect the development of ordered domains, but do lead to some tearing of the particle lattices.
Conclusion The results presented here demonstrate that nanosized gold particles can form well-ordered 2D lattices with almost macroscopic dimensions by the use of electrophoretic deposition. The ordering was demonstrated for the first time by diffraction of the electron beam used to image the lattices. The lattice spacing found by the diffractionmeasurements showsthat the interparticle
Letters spacing corresponds to the thickness of the adsorbed citrate ions. A similar conclusion has already been reached by Schmid et al. for triphenylphosphine-cappedmonolayers? The ions do not desorb during electrophoretic deposition. The effectsof adsorbed alkanethiols and other stabilizers on the interparticle spacing will be reported elsewhere.II
Acknowledgment. We thank Prof. E. Zeitler of the FritzHaber Institut in Berlin for placing the image processing facilities in his laboratory at our disposal and Prof. A. Henglein of the Hahn-Meitner Institut for valuable discussions and support. References and Notes (1) Ottewill, R. H. Lungmuir 1989, 5, 4. (2) Ottewill, R. H. In Colloidal Dispersions;Goodwin, J. W . ,Ed.;Royal Society of Chemistry: London, 1981. (3) Hunter, R. J. Foundutions of Colloid Science; Clarendon Press: Oxford. 1985: Vols. 1. 2. (4)' (a) P&anski,'P. Phys. Reo. Lett. 1980,45, 569. (b) Onoda, G. Y. Phys. Rev. Lett. 1985, 55, 226. (5) (a) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Sur/. Sci. 1982, 120,435. (b) Dimon, P.; Sinha, S. K.; Weitz, D. A.; Safinya, C. R.; Smith. G. S.: Varadv. W. A.: Lindsav. H. M. Phvs. Rev. Lett. 1986.57.595. (c) Weitz. D. A.: Lin. M. Y.: Sandioff. C. J. Sir[ Sci. 1985. 158. 147. (6) Dusemund, B.; Hoffmann, A.; Sa1zmann:T.; Kreibig; U.; Schmid, G.; 2.Phys. D 1991, 20, 305. (7) Schmid, G.; Lehnert, A.; Kreibig, U.; Adamczyk, Z.; Belouschek, P. Z . Naturforsch. 1990, 456, 989. (8) Kreibia.U.: Fauth. K.:Granavist. . . C.-G.: Schmid. G .Z . Phvs. Chem.
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(Neue'Folge) fh, 169, '1'1. . (9) Eniistiin, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317. (10) Lehnert, A.; Schmid, G. Angew. Chem. 1989,101, 773. (1 1) Giersig, M.; Mulvaney, P., submitted for publication. (12) Saxton, W. 0.;Pitt, T. J.; Horner, M. Ultrumicroscopy1979,4,343.
