J. Phys. Chem. B 1998, 102, 1397-1403
1397
Electrochemical Formation of Luminescent Convective Patterns in Thin-Layer Cells Marek Orlik,† Ju1 rgen Rosenmund, Karl Doblhofer,* and Gerhard Ertl Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: June 11, 1997; In Final Form: December 3, 1997
The electrolysis of rubrene in a thin-layer electrochemical cell leads to formation of electrohydrodynamic (EHD) convection patterns. The well-known electrochemiluminescence produced in this process renders the convective structures clearly visible. The electrochemical mechanism leading to the EHD convection is analyzed. In particular, the importance of ionic salt concentration is elucidated. Example structures are shown, and the basic mechanism of their formation by convective flows is proposed.
1. Introduction Investigations of the principles that lead to self-organization of previously homogeneous matter have become a subject of great interest.1-4 Such knowledge constitutes a basis for a better understanding of processes in living systems; it may also provide completely new ways for controlling technical processes. The subject of this work is hydrodynamic instabilities, leading to convective patterns.2-10 In liquids subject to thermal convection, realized in a thin layer of fluid exposed to a temperature gradient, convective hexagonal cells (resembling honeycombs), as reported first by Be´nard,5 or rolls with different degrees of complexity2-4,6,10 can be observed. The basic cause for these classical Rayleigh instabilities7,8 is density gradients of the fluid. Such density gradients can result also under isothermal conditions from concentration gradients in a solution, e.g., during electrolysis between two electrodes.4,11-15 With a free fluid surface Marangoni instabilities,3,4 involving gradients of a surface tension, may interfere with Rayleigh instabilities. Under certain conditions, these instabilities can even give rise to chaotic patterns.3 Electrohydrodynamic (EHD) conVection is observed in charged fluids under the influence of an electrical field.16-31 The driving force of such convection, sometimes called the Felici instability,30,31 is the interaction of the electric field with an uncompensated space charge which can be transported by the moving fluid. An example of electrohydrodynamic convection are patterns of instabilities (e.g., Williams domains) observed in thin layers of nematic liquid crystals exposed to dc or ac field.32-36 An interplay of electrical and photochemical phenomena leads to patterns of photoelectrohydrodynamic convection.37 A particularly remarkable variant of an EHD convection experiment has been described by Schaper, Schnedler, and Ko¨stlin.21-25 A constant voltage was applied across a thin-layer (10-100 µm) electrochemical cell containing a solution of rubrene (5,6,11,12-tetraphenylnaphthacene) in 1,2-dimethoxyethane (1,2-DME). The electrolysis of rubrene produces charged radicals migrating in the electrical field (∼103 V cm-1). As the reaction between cationic and anionic radicals of rubrene leads to chemiluminescence,38,39 the evolution of convective * Corresponding author: fax (+ 030) 8413-3101, e-mail
[email protected]. † Permanent address: University of Warsaw, Department of Chemistry, ul. Pasteura 1, 02-093 Warsaw, Poland.
Figure 1. Scheme of the laboratory equipment for observing convective structures of electrochemiluminescence of rubrene in a thin-layer electrochemical cell: A, Vigreux column; B, cooler; C, microsyringe.
structures can be visualized as pattern of bright luminescence. Quasi-hexagonal arrangements of spots, resembling Be´nard structures, were reported. Later Jaguiro, Smirnov, and Zhivnov26-29 continued these studies. A simple model of observed motions was proposed,26 and possible practical applications of the luminescent EHD in liquid pumps, dynamic light scattering, and electrochemiluminescence displays26,27 were suggested. Luminescent EHD patterns can be studied more thoroughly with the modern tools of image recording and processing. It is considered that such studies will lead to new valuable insight into the underlying electrochemical processes and the complex nonlinear dynamics. In particular, it appears important to elucidate the role of ionic contaminants21-24 or pretreatment of the cell with relatively high voltage (up to 50 V),40 which have been suggested as a precondition for formation of the EHD structures. In this paper we present the results of such studies, the particular aim of which was: (i) to analyze the state of the system that leads to the electroconvective motion of liquid and (ii) to describe the basic geometrical features of luminescent patterns. 2. Experimental Section The experimental setup is sketched in Figure 1 and was designed to warrant extremely pure reagents. A solution of rubrene in dimethoxyethane (distilled under argon and then kept
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1398 J. Phys. Chem. B, Vol. 102, No. 8, 1998
Orlik et al. can be fixed. The front circular surface of this electrode was polished flat down to 1 µm. The transparent electrode was made by coating the glass plates (diameter 3 cm) with an ITO layer (In2O3:Sn ) of about 200 nm thickness by sputtering. To avoid the electrochemical decomposition of the copper electrode during the experiments, it was always polarized cathodically with respect to the ITO electrode. Gold and platinum electrodes worked both as a cathode and an anode. The interelectrode distance d in the electrolytic cell was determined from capacitance measurements:
Cel ) 0
Figure 2. Simplified cross section of the thin-layer electrochemical cell: A, copper (gold, platinum) electrode; B, KEL-F insulating layer; C, metallic (bronze and steel) movable block; D, metallic (bronze) fixed block connected with part C by the thread; E, steel ring for fixing the glass electrode; F, transparent electrode (glass plate coated with ITO); G, sealing O-ring; H, copper rod (for copper electrode A and H denote a single copper rod). For clarity, several O-rings, threads, and supply and outlet lines for the solution and the inert gas (argon) are omitted.
over molecular sieves 4A) is prepared in a closed system under exclusion of oxygen and water. This solution may then be enriched with small amounts of a supporting electrolyte (tetrabutylammonium hexafluorophosphate) and transferred under pressure of argon to the two-electrode electrolytic cell with one electrode metallic (Cu, Au, or Pt) and the one transparent (conducting glass surface). Patterns of luminescence evolved after application of the dc voltage between these electrodes were observed on the monochrome monitor (with about 80 times magnification) connected to the CCD (charge coupled device) camera and recorded on a video tape. For each experiment also the current-voltage characteristics of the system were determined. As the interelectrode distance is an important control factor, it was determined from measurements of the capacitance of the cell, as described below. As a source of dc voltage, a homemade sweep generator was used. A digital multimeter with an analog signal output (Keithley 177 Microvolt DMM) was used to measure the current. The current-voltage characteristics of the system were recorded by an XY recorder BD 91 (Kipp & Zonen). The capacitance measurements of the electrochemical cell were performed with a lock-in-amplifier (model 5210, EG&G). A monochrome CCD camera, Fujitsu model TCZ-230 EA, was used, together with an SVHS video recorder NV-HS1000 EGC (Panasonic), for recording the luminescent patterns on the SVHS-SE-180 video tape (Fuji). The images were digitized with a computerized setup, consisting of a digital video recorder and a Sun workstation. The construction of the electrochemical cell is shown in Figure 2. It consists of metallic (bronze and steel) blocks, interconnected with a fine thread (500 µm for one full turn, with a division marked every 2°), allowing the precise adjustment of the interelectrode distance. The cylindrical copper rod is isolated from the rest of the cell through a layer of the KEL-F plastic. To the front of that rod gold or platinum electrodes
Amet Amet ) 0 d dth + ∆d
(1)
where Cel is the capacitance of the system metal electrodeITO electrode, Amet ) 2.011 cm2 is the surface area of a metal (Cu, Au, Pt) electrode (smaller than that of the counter electrode, A ) 7.070 cm2), is the relative dielectric constant of the medium, 0 ) 8.85419 × 10-12 F m-1 is the permittivity of a vaccum, and dth is the interelectrode distance read from the characteristics of the thread with an uncertainty (offset) ∆d originating from limited mechanical precision of the thread, certain roughness of the electrode surface, and deviations of parallel position of two electrode surfaces. ∆d was typically of the order 6-9 µm as derived by adjusting Cel to (dth + ∆d)-1 to a linear relationship. For comparison, interelectrode distances were determined from capacity measurements of the cell filled with carbon tetrachloride ( ) 2.238 at 20 °C41) and led to practically the same results. The interelectrode distances were checked before each series of experiments. Commercially available rubrene (for chemiluminescence, Fluka), 1,2-dimethoxyethane (puriss., Fluka,
