J. Phys. Chem. B 1997, 101, 5909-5912
5909
Plasma-Electrochemical Growth of AgBr Layers on AgCl Substrates J. Janek* and C. Rosenkranz Institut fu¨ r Physikalische Chemie und Elektrochemie, UniVersita¨ t HannoVer, Callinstrasse 3-3A, 30167 HannoVer, Germany ReceiVed: April 8, 1997; In Final Form: June 11, 1997X
By applying a glow discharge in bromine gas, we grew thin silver bromide layers on single crystalline silver chloride substrates. The growth process is based on a reaction between (a) silver cations that are transferred from the silver chloride substrate to the reacting surface from a silver anode and (b) bromine (and electrons) from the plasma. Essentially, the bromine plasma acts as a fluid electrolyte and supplies electrons and bromine to the silver chloride surface, which works as the anode for the glow discharge. The chemical and morphological identity of the product layers is analyzed. The growth process itself is discussed with respect to further applications for the growth of ion-conducting layers.
1. Introduction Plasma chemistry is an extensive field with numerous applications in material science, mainly for surface modification in semiconductor technology.1 In electrochemistry, plasmas have been applied from the very beginning in the electrolysis of liquid electrolytes. Thus, the processes at interfaces between glow discharges and liquid electrolytes have been studied intensively during the first decades of this century (“glow discharge electrolysis”, cf. refs 2 and 3). Equivalent processes, leading to the formation of reaction products on the substrate surface, should take place at the interface between a glow discharge and a solid electrolyte substrate.4 However, the use of plasmas for controlled electrode reactions in solid state ionics was not studied until recently.5,6 The controlled growth of ion-conducting layers is of interest both for technical applications and in basic research. In particular, the growth of one ionic conductor onto a different ion-conducting substrate under well-controlled conditions is a possible way to produce clean and reproducible interfaces. In the present study we report on the first results of our attempts to produce silver ion-conducting layers of AgBr on single crystals of AgCl by means of plasma-electrochemical growth. The silver halides are ionic crystals with Frenkel disorder in the cation sublattice, showing high intrinsic silver ion conductivity and a small transference number of electronic charge carriers at elevated temperatures. The reactive formation of a silver halide layer AgY (Y ) halogen) on a silver halide substrate AgX (X ) halogen) can in principle be obtained from two different processes, as shown in Figure 1. (a) Chemical deposition of AgY on AgX: By exposing a silver halide crystal AgX with one of its crystal surfaces to the parent metal Ag and the opposite site of the crystal to an atmosphere of a gaseous halide Y2, a gradient of the chemical potential µAg of silver metal within the halide is built up. The ionic and electronic charge carriers (Ag+, e-) are driven by the gradients of their electrochemical potentials, µ˜ i, toward the AgX|Y2(gas) interface. Essentially, a tarnishing layer of AgY on AgX is produced by chemical deposition. Due to flux coupling by the electroneutrality condition (jAg+ ) je-), the silver flux jAg through the substrate (AgX) and the growing product (AgY) is limited by the electronic conductivity of the silver halides if the transport * Corresponding author: tel, (0511) 762-5298; fax, (0511) 762-4009; e-mail,
[email protected]. X Abstract published in AdVance ACS Abstracts, July 15, 1997.
S1089-5647(97)01205-4 CCC: $14.00
Figure 1. Particle fluxes and driving forces in (a) the formation of a tarnishing layer of AgBr on AgCl and (b) the plasma-electrochemical deposition of AgBr on AgCl.
of neutral silver along internal surfaces can be neglected. Since the electronic conductivity is always small, the growth process occurs very slowly (∆x/∆t ≈ 3 × 10-14 cm s-1 at T ) 300 °C), and the diffusion control leads to a stable surface of the growing layer. Corresponding experiments confirm this slow chemical growth process. (b) In the plasma-electrochemical deposition of AgY on AgX, electric charges are mainly driven by the local gradients of the electric potential φ across the cell:
-Ag|Y2(discharge)|AgX|Ag+
(1)
Silver ions are driven from the silver anode through the ion conducting AgX toward the AgX|Y2(discharge) interface. At the interface the silver ions react with the charged (or neutral) plasma particles (electrons, bromine anions, and molecular bromine), forming AgY. As electrons are now supplied by the © 1997 American Chemical Society
5910 J. Phys. Chem. B, Vol. 101, No. 31, 1997
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Figure 2. Experimental setup for plasma-electrochemical deposition of AgBr on AgCl in a dc glow discharge.
plasma, the deposition rate of AgY is determined not by the chemical diffusion of silver metal through AgX but rather by the conductivity of the plasma, i.e. the least conductive medium in the plasma-electrochemical cell. In effect, any appreciable growth of a new ion-conducting layer in a halogen plasma can only proceed if a supply of silver ions in an electric field is given. An additional chemical reaction may occur by the exchange of the halogen contained in AgX with the gaseous halogen Y2:
2AgX(s) + Y2 f 2AgY(s) + X2
(2)
However, silver halides show no significant anionic mobility at the temperature we chose. So the exchange of the halogen in reaction 2 would only lead to a very thin layer of AgY on AgX and should not influence the study of a plasma-controlled reaction. 2. Experimental Section Preparation of AgCl Crystals. Silver chloride single crystals (Korth Kristallhandel, Kiel/Germany) were machined into rectangular shape (8 × 6 × 3 mm3) and polished both mechanically and chemically, using an aqueous solution of Na2S2O3. The Plasma Experiment. In a cylindrical shaped plasma reaction chamber (see Figure 2), we placed two flat silver electrodes (8 × 5 mm2) at a distance of 4 cm. A silver chloride single crystal was fixed onto the anode and covered with an alumina mask with a hole of 4 × 4 mm2 to focus the glow discharge. The mask was pressed to the assembly by a springloaded glass rod. The reaction chamber was evacuated to a pressure of 260 Pa and heated by a resistance furnace to 300 °C. A continuous flux of bromine (800 cm3/h) was arranged. It was checked in an independent experiment that no chemical reaction between the silver chloride crystal and the bromine gas took place. A voltage of approximately 800 V was applied between the electrodes to ignite the glow discharge. A voltage of 600 V was sufficient to stabilize the discharge after the ignition. The current across the cell
-Ag|Br2(discharge)|AgBr|AgCl|Ag+
(3)
was in the range of 0.5 mA-1 mA, depending on the geometry of the reactor (electrode distance). A typical glow discharge in our experiments was maintained by a dc voltage of 700 V and a current of approximately 1 mA (0.7 W). The bromine discharge appeared yellowish green in the anode region and
Figure 3. Surface of a silver chloride single crystal (dark areas) showing different morphologies of the plasma-electrochemically deposited AgBr (light areas); SEM picture.
reddish blue near the cathode with the Faraday dark zone in between. Usually we chose a reaction time between 10 and 30 min. 3. Results As stated, we proved that the silver chloride substrate shows no substantial reactivity against bromine. So the formation of the product layer on the silver chloride substrate is caused exclusively by plasma-electrochemical deposition. The deposited silver bromide crystallizes in form of a pale-yellow spot on the surface of the AgCl single crystal. The product spot was identified as AgBr by X-ray diffraction measurements and electron microprobe analysis. A SEM picture in Figure 3 shows the surface of the silver chloride substrate in an early state of the plasma deposition. The picture reveals also information on the topology of the different stages during the deposition of AgBr: (i) The product nucleates on the silver chloride surface in separated spots. (ii) Starting from these nucleation spots, a complete and denselooking layer of AgBr grows. The further deposition of AgBr leads to a roughening of the growing surface with a (iii) canyonlike morphology and finally (iv) results in the growth of separated columns of AgBr. Figure 4 shows the cross section of the silver chloride substrate (right phase) and the product layer of silver bromide (left phase). The silver bromide and chloride can be distinguished by their different secondary-electron energies colored in the SEM picture. In this particular experiment, the thickness of the plasma-deposited AgBr layer equaled approximately 40 µm. A 30 µm thick layer of AgBr would have been expected, according to the applied current of 0.5 mA and the reaction time of 82 min. This difference is caused by the high porosity of the columnar structure of the product layer. The surface of the substrate remained flat and was not affected by electron bombardment from the discharge. There is no experimental evidence for a shift of the original AgCl surface, i.e. for a mixing of both halides. 4. Discussion The experimental results prove that ion-conducting layers can be grown by means of a plasma-assisted electrochemical process
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J. Phys. Chem. B, Vol. 101, No. 31, 1997 5911
Figure 4. Cross section of the single crystalline silver chloride substrate (right phase) covered with plasma-electrochemically deposited silver bromide layer (left dendritic phase, black color indicates the embedding material); SEM picture.
on an ion-conducting substrate, even in systems like the silver halides, which are sensitive to decompostion by UV/vis radiation or highly energetic electron bombardment. An essential feature of the present experiments is the morphological instability of the product layers. In principle, different mechanisms may cause this instability. (a) Morphological Instability Criterion of a Moving Phase Boundary. The electrical (ionic) conductivity of both the AgBr product and the AgCl substrate is approximately 100 times higher than the electrical conductivity in the anode region of the Br2 plasma. Hence the local gradient of the electrical potential φ in the circuit-Ag|Br2(discharge)|AgBr|AgCl|Ag+ will be higher in the discharge region than within the solid electrolytes (see Figure 1b). As the substrate and the product show a pronounced Frenkel disorder, the gradient of the chemical potential of ions is negligible (∇µAg+ = 0), and the only driving force for the ionic flux is the local electric field:
∇µ˜ Ag+ = + F∇φ
(4)
The transport properties change discontinuously at the moving phase boundary of the growing silver bromide layer. As shown formally by Martin,7 the growth of a conducting phase into another conducting phase with lower conductivity should always lead to a morphological unstable phase boundary, once the conductivity difference is large enough. In the present case the conductivity difference is relatively large, and a small disturbance of the moving and originally plane Br2(discharge)|AgBr interface during the growth process leads to a growing instability of the phase boundary. The rough and columnar morphology of the product layer results from this increasing instability. (b) Decomposition by Electron Bombardment and UV/ vis Radiation. Another kinetic effect that might influence the morphology of the AgBr layer is decomposition of the product layer by electron bombardment from the glow discharge. The growing silver bromide layer shows the bright yellow color of unreduced material, which is a very sensitive indication for the negligible content of silver metal. Nevertheless, the kinetic energy of electrons in the glow discharge can reach up to 1-10
eV.1 The dissociation energy for molecular AgBr has been determined to 2.6-3.0 eV.8 Thus newly formed AgBr can simply be decomposed by electron bombardment. However, the degree of electron bombardment depends on the local electric field. Since the electric field is stronger at curved parts of the surface, this effect should only lead to a smoothing of the product layer. UV/vis radiation is also an unavoidable side product of a lowtemperature plasma, which may have an impact on the silver halides (photographic materials). However, against the partial decomposition of the growing silver bromide layer at its surface the same arguments as mentioned above for the decomposition of the product layer by electron bombardment are valid. In addition, any silver metal produced by decomposition is subject to reoxidation by the halogen in the plasma. (c) Anion Sputtering. Dependent on the degree of ionization in the plasma and on the magnitude of the anode potential fall, the bromine anions are accelerated in the direction of the anodic AgCl surface. Like the well-known sputtering of cathodes by accelerated cations, sputtering of the substrate and the product layer should take place. However, as for the electron bombardment, any sputter effect should focus on the convex irregularities of the anode surface. Again, we expect rather a smoothing than a roughening of the product layer as the result of this unavoidable effect. (d) Ion-Exchange. Since the substrate (AgCl) is a single crystal with an extremely low mobility of halide ions, no serious anion exchange can take place (cf. Buck9 for chloride/bromide exchange in surface layers at low temperatures). Previous work on plasma-electrochemical deposition of ionconducting layers has focused on the creation of oxygenconducting zirconia layers. Ogumi et al.5 obtained a thin layer of yttria-stabilized zirconia (YSZ, 7%-9% yttria) with a plane surface on platinum by means of plasma-electrochemical deposition from YCl3 and ZrCl4 in the gas phase. Using a microwave-induced argon discharge (100 W, 13.56 MHz RF), the deposition of YSZ is enhanced in comparison to the pure chemical deposition of the oxide from the two metal halides. The charge carriers of the plasma are driven in a dc bias of 65 V to the cathode where the reaction of ZrCl4, YCl3, and O2 to YSZ takes place. The difference in the electric transport properties of the argon plasma and the solid electrolyte YSZ is comparable to that of the system Br2(discharge)|AgBr|AgCl. The stability of the moving YSZ|YCl3,ZrCl4(discharge)-phase boundary compared to the morphological instability of the growing AgBr layer is probably caused by the different conductivities in the discharge regions. While the microwave discharge is maintained by an electric power of 100 W, a typical glow discharge in our experiments was maintained by a dc voltage of 700 V and a current of approximately 1 mA (0.7 W). Consequently, the density of charge carriers in a microwave discharge is much higher compared to the density of charge carriers in the dc discharge (by at least a factor of 103). Thus, in contrast to our experiments, the ion-conducting YSZ as the phase with the lower conductivity grows into the higherconducting discharge region. Any disturbance in the plane geometry of the moving phase boundary decreases, resulting in a plane surface. A microwave-induced discharge of gasous iodine (12 W, 10 kHz) has been applied by Ogumi et al.6 to grow an orientated β-AgI layer on a silver ion-conducting glass. The bulk conductivity of β-AgI is anisotropic, i.e. the sphalerite type structure of β-AgI favors silver ion transport perpendicular to the c-axis. The experiments of Ogumi et al. resulted in a silver iodide layer with its growth direction perpendicular to the c-axis.
5912 J. Phys. Chem. B, Vol. 101, No. 31, 1997 5. Conclusions The use of plasmas as fluid “mixed conductors” appears as a new concept in solid state ionics. The voltage-controlled reaction between two educts, one being supplied by a solid electrolyte and one being supplied by the plasma, provides interesting possibilities for the growth of ion-conducting layers. Thus, to improve the facilities for the preparation of well-defined interfaces between ionic conductors, we will continue the experiments, particularly with other plasma sources (radio frequency or microwave-induced discharges). As is shown by the present results, the plasma properties (conductivity, etc.) have to be improved in order to obtain better morphologies of the product layers. In dc glow discharges, the anodic growth of cation-conducting layers may be subject to morphological instability. Acknowledgment. We thank Prof. H. Schmalzried for his suggestion to apply glow discharges to electrochemical processes
Letters in solid electrolytes. One of us (J.J.) is grateful to the Fonds der Chemischen Industrie (Germany) for financial support. Financial support by the DFG (Janek 648/2-1) in the framework of the program “Reactivity of Solids” is also gratefully acknowledged. References and Notes (1) Boenig, H. v. Fundamentals of Plasma Chemistry and Technology; Technomic Pub. Co., Lancaster, PA, 1988. (2) Klemenc, A.; Hohn, H. F. Z. Phys. Chem. A 1931, 154, 385. (3) Kortu¨m, G. Lehrbuch der Elektrochemie; Verlag Chemie: Weinheim, 1957; p 480. (4) Schmalzried, H. Personal communication. (5) Ogumi, Z.; Uchimoto, Y.; Takehara, Z. Solid State Ionics 1992, 58, 345. (6) Uchimoto, Y.; Okada, T.; Ogumi, Z.; Takehara, Z. J. Chem. Soc., Chem. Commun. 1994, 585. (7) Martin, M. et al. Solid State Ionics 1995, 75, 219. (8) Herzberg, G. Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules, 2nd ed.; Prentice Hall: New York, 1950; p 502. (9) Rhodes, R. K.; Buck, R. P. Anal. Chim. Acta 1980, 113, 67-78.
