© Copyright 1996 American Chemical Society
NOVEMBER 27, 1996 VOLUME 12, NUMBER 24
Letters In Situ Observation of 2-Dimensional Clustering during Electrophoretic Deposition Marcel Bo¨hmer* Philips Research Laboratories Eindhoven, Professor Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received February 28, 1996. In Final Form: September 30, 1996X
Electrophoretic deposition of micron-sized polystyrene latex particles on an indium tin oxide (ITO) electrode was studied in situ using optical microscopy. Strong 2-dimensional clustering of the particles on the electrode surface was observed upon application of a potential. The clustering decreases somewhat with increasing salt concentration. Upon reversal of the direction of the field, the clusters broke up. The aggregation on the electrode cannot be explained by DLVO or dipole interactions. A possible interpretation may be formulated in terms of electro-osmotic flow.
Introduction Electrophoretic deposition (EPD) of particles on conducting surfaces can give very uniform layers. EPD offers possibilities for layers of nanoparticles. In some cases these layers, applied at a low voltage, show a surprising degree of ordering, as found by Giersig and Mulvaney1,2 for nanosized gold particles on a carbon electrode. Early experiments on EPD are described by Hamaker,3 and a theory has been put forward by Koelmans and Overbeek.4 Koelmans and Overbeek concluded that the particle transport toward the electrode is governed by the applied field while the adhesion of the deposit depends on the reaction at the electrode surface in which e.g. H+ may be produced to flocculate the system near the electrode. An additional mechanism has been proposed by Estrelia* E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408 (2) Giersig, M.; Mulvaney, P. J. Phys. Chem. 1993, 97, 6334 (3) Hamaker, H. C. Trans. Faraday Soc. 1940, 36, 279 (4) Koelmans, H.; Overbeek, J. Th. G. Discuss. Faraday Soc. 1954, 18, 52
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Lopez et al.5 and Lavrov and Smirnov,6 who interpret the flocculation of the suspension at the electrode not only in terms of irreversible flocculation due to the products of the electrode reaction but also by dipole interaction. The latter mechanism can account for reversible chain formation perpendicular to the electrode and can in principle lead to more densely packed layers.5 In most studies the structure of the electrophoretically deposited layer is studied after drying. As it is known that during drying strong clustering can occur, and even offers a way to prepare very uniform layers,7-9 we decided to study the deposition mechanism in situ. Negatively charged monodispersed polystyrene latex particles were deposited on a transparent indium tin oxide (ITO) electrode. Qualitative aspects of the aggregation are presented, and a quantitative analysis of the initial stages of the process is given for different salt concentrations. In the Discussion a possible explanation is suggested. (5) Estrelia-Lopez, V. R.; Ul’berg, Z. R.; Koniashvili, S. A. Kolloidn. Zh. 1982, 44, 74 (6) Lavrov, I. S.; Smirnov, O. V. J. Appl. Chem. USSR 1969, 42, 1459 (7) Kralchevski, P. A.; Nagayama, K. Langmuir 1994, 10, 23 (8) Denkov, N. D.; Velev, O. D.; Kralchevski, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Langmuir 1992, 8, 3183 (9) Micheletto, R.; Fukuda, H.; Ohtsu, M. Langmuir 1995, 11, 3333
© 1996 American Chemical Society
5748 Langmuir, Vol. 12, No. 24, 1996
Letters
Experimental Section A glass slide with a 270 nm ITO layer, with a sheet resistance of 8.2 Ω (per square), is placed in a cylindrical Teflon sample holder with an inner diameter of 4 mm. On top of the cylinder a Pt counter electrode is placed. The distance between ITO and the counter electrode is 6 mm. For the system studied, voltages up to 2.5 V could be applied across this cell without visible O2 evolution at the ITO. The deposition of 4 and 10 µm of negatively charged polystyrene latex particles (Duke Scientific) was investigated. Just prior to the measurement, the 4 µm latex was diluted 10 times with an aqueous salt solution to give a concentration of 1.0 × 107 particles per milliliter of suspension. Using a Zeiss metallurgical microscope with a 20× dark field objective and a Philips LDH0703 CCD camera, the deposition was followed through the ITO-coated glass substrate. Images were stored on videotape, and selected images were digitized afterward. The total number of particles, the number of particles in aggregates, and the average aggregate size were counted using image analysis software (Image-Pro-Plus). The area of the objects and the ratio of the minimum and maximum diameter were measured.
Results Qualitative Aspects of Aggregation on the Electrode. As the particles arrive on the surface they maintain their brownian motion, i.e., they do not become immobilized on the surface. Only at higher voltages (>3 V) are particles seen to become fixed on the surface. These high voltages can only be applied for short periods of time because gas evolution starts destroying the layer. At e.g. 2 V (see Figure 1), strong surface aggregation of particles takes place. Interactions between particles are seen over distances of several particle diameters, especially the interactions between single particles and clusters. Even cluster-cluster aggregation phenomena are observed. Once clusters are formed, they rapidly grow and the distance over which they attract newly arrived particles on the surface increases. Particles arriving at a cluster move around its periphery until they reach their most favorable position, i.e. a position where they may be incorporated into the already existing lattice. The features of the process are very similar to those described by Onoda10 for clustering of latex particles at the air-water interface. In clusters, the particles still show brownian motion. When the electric field is switched off, the clusters become less ordered and slowly break up. When the field is reversed, the clusters break up faster. If a small voltage (-1 V) is chosen to achieve this, the particles remain on the electrode during this process. A time series is given in Figure 2. In this experiment 10 µm particles are used, because they are less subject to brownian motion. In Figure 2 it is seen that all particles in the cluster move away from each other with approximately the same velocity. For bigger agglomerates the disintegration starts at the boundaries because the particles at the interior are restricted in their motion by neighboring particles. Upon switching to a positive potential again, the particles reorder into big crystalline structures. Quantification of Aggregation in the Initial Stage of EPD. The coverage of the electrode as a function of time at an applied potential of 2 V is given in Figure 3a for different salt concentrations. The particles sediment slowly; the coverage at 0 V is included in the figure. The reproducibility is around 5% except for short deposition times (
