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Electrochemical growth of crystals from electrolyte solutions
Journal of Crystal Growth 34 (1976) 245—247 © North-Holland Publishing Company
ELECTROCHEMICAL GROWTH OF CRYSTALS FROM ELECTROLYTE SOLUTIONS A.D. FRANKLIN Institute for Materials Research, National Bureau of Standards, Washington, DC 20234, USA Received 7 November 1975; revised manuscript received 19 February 1976
An electrochemical crystal-growth technique is described in which an electrolyte reaction involving one ion in the desired crystal is used to create a concentration gradient in that ion across the cell. Nutrient material held near the consuming electrode is dissolved, and crystal growth near the producing electrode occurs at a rate which can be controlled by the current through the cell. The technique has been demonstrated with the growth of Ca(OH) 2 crystals from aqueous solutions of CaC12 plus KC1.
Electrochemical reactions are often used to deposit chemical species which, under appropriate conditions, grow as large grains or single crystals on an electrode. In electrodeposition, the crystallizing species is produced by the electrochemical reaction, the rate of which in turn depends upon the current through the cell. Because the latter can be controlled with great sensitivity, the method in principle allows delicate control over the rate of crystallization and therefore over the growth process. Kaspar et al. [l} and Barta et al. [21developed a variant for the growth of CaCO3 crystals in which growth occurred as a result of interdiffusion of CaCl2 and NaHCO3 solutions in the anode and cathode compartments, respectively. Electrolysis of the solution resulted in a rise in pH that allowed precipitation as CaCO3. In the work described here, we report a somewhat similar technique, in which the passage of current results in a concentration gradient of some critical ionic species. Nutrient material is dissolved from one electrode compartment and transported by diffusion to the other, where crystallization occurs. The method has been demonstrated by the growth of Ca(OH)2 crystals. A series of electrolytic cells, shown in fig. 1, was set up in which the electrolyte was an aqueous solution of KC1 and CaCl2 in a large test tube (35 mm diameter, 20 cm deep). The cathode was a Cu wire in a 10mm diameter glass tube about 15 cm long, the bottom of which was open to the solution in the cell. The anode was a Pt wire in a similar tube, the tube
terminating in a coarse glass frit immersed in the electrolyte of the cell. Powdered Ca(OH)2 was held in the anode compartment by the frit, but ions were free to
+
—
H2
~-
o.~
o
3 U
0
Fig. 1. Electrolytic cell. (1) Common electrolyte solution, (2) glass frit containing nutrient Ca(OH)2, and (3) open glass tube with Ca(OH)2 crystals growing on inner wall. 245
246
AD. Franklin
/ Electrochemical growth of crystals of electrolyte solution
travel between compartments. Several current levels from 0.1 to 3 mA were explored, the currents being kept constant by the simple expedient of a large resistance in series with the cell and a battery with sufficient voltage to produce the desired current. No special temperature control was used, but the laboratory generally is held to 22 ±1—2°C.The voltage drop across the cell was at least 1 .5 V. Several levels of Ca2+ ion concentration were explored, the total cation content being kept near 1 molar. Growth of Ca(OH) 2 crystals observed 2+ concentrations ranging fromwas 0.05 to 0.5 with molar Ca and with currents of 1 mA or larger. Good results were obtained using 0.5 molar KC1, 0.5 molar CaC1 2, and I mA current, and the behavior will be described in terms of this combination as an example. Nucleation of crystals occurred on the inside walls of the glass tube surrounding the Cu electrode after about 3 days. The nucleation began on the wall opposite the electrode, and was quite dense. As time passed the band of nucleation moved down the tube, away
from the cathode, and the crystals became fewer and larger the further they were from the electrode. After about 2 weeks the nucleation had proceeded down the approximately 12 cm of the glass tube to its mouth, and in the lower region scattered crystals several mm across (fig. 2) had grown. Subsequently, a fine precipitate appeared in the electrolyte outside the cathode tube. This behavior can be understood qualitatively in terms of a simple model that should apply for sufficiently small currents. The electric fieldbyis probably screened by the space charge produced the KC1 present to the extent that mass transport in the body of the cell is dominated by diffusion (disregarding convection produced by thermal and concentration gradients). Hydroxyl ions are produced in the cathode (the pH was about 11) and consumed at the anode (pH about 1 to 3). This concentration gradient produced a flux of hydroxyl ions (and a counter-flux of hydrogen ions) from cathode to anode. In the anode compartment the powdered Ca(OH)2 dissolved be-
~
1MM
Fig. 2. Ca(OH)2 crystals as grown.
AD. Franklin
/ Electrochemical growth of crystals of electrolyte solution
cause of the low pH, while crystal growth occurred in the cathode compartment where the pH was high. There was therefore a concentration gradient and flux of calcium ions from anode to cathode. The result was to transfer Ca(OH)2 from anode to cathode compartment, where the redeposition took place as crystal growth, at a steady state rate which should be controlled by the current, Brown [3] has interpreted experiments [4,5] in which electric currents appear to influence bone growth, as demonstrating an analogous process, in which calcium hydroxyapatite dissolves at the anode and deposits at the cathode. A simple linearized diffusion model neglecting all convection effects suggests that steady state should not be achieved until the “zone of deposition” has moved down the wall of the cathode compartment to a position determined primarily by the geometry of the cell and only somewhat sensitive to the magnitude of the current. In these preliminary experiments, apparently, the cathode compartment was not long enough enough for true steady state to have occurred. Finely-controlled crystal growth should be possible in a linear cell of uniform cross section sufficiently long for true steady state to be achieved. Once the limit to the zone of deposition has been established, crystal growth of those crystals on the edge of this zone should proceed. Reducing the current after the nucleation at this limit has occurred might allow fine tuning of the steady state growth rate to produce controlled growth. Experiments of this kind are now
247
being carried out in a cell 185 cm long, with the glass frit about 33 cm from the anode. The steady state is expected to produce a situation roughly symmetrical about the midpoint of the cell, with the zone of deposition extending 30—35 cm from the cathode. The method should be applicable wherever an electrochemical reaction can be used to establish a gradient in the activity of a chemical species which is involved in the solution or deposition of the chosen crystal, and where the solubility of the crystal is sufficient to maintain a reasonable growth rate. The author would like to acknowledge the assistance of Gregory Besio and Judith Nelson in performing the experiments, and to the National Institute of Dental Research for partial support during the course of the work.
References Lii
J. Kasper, C. Barta and J. Nigrinova, in: Rost Kristallov, VoL VI (Nauka, Moscow, 1965) p. 5; translation appears as Growth of Crystals, Vol. V1B, Ed. J.E.S. Bradley (Consultants Bureau, New York, 1968) p. 3. [2] C. Barta and J. Zemlicka, Silikaty 10 (1966) 275. [3] W.E. Brown, Colloques Internationnaux CNRS No. 230, Physicochimie et Cristallographie des Apatites d’Interét Biologique. [4] C.A.L. Bassett, R.J. Pawluk and R.O. Becker, Nature 204 (1964) 652.
~sjB.T. O’Connor, H.M. Charlton, J.D. Currey, D.R.S. Kirby and C. Woods, Nature 222 (1969) 163.
ELECTROCHEMICAL GROWTH OF CRYSTALS FROM ELECTROLYTE SOLUTIONS A.D. FRANKLIN Institute for Materials Research, National Bureau of Standards, Washington, DC 20234, USA Received 7 November 1975; revised manuscript received 19 February 1976
An electrochemical crystal-growth technique is described in which an electrolyte reaction involving one ion in the desired crystal is used to create a concentration gradient in that ion across the cell. Nutrient material held near the consuming electrode is dissolved, and crystal growth near the producing electrode occurs at a rate which can be controlled by the current through the cell. The technique has been demonstrated with the growth of Ca(OH) 2 crystals from aqueous solutions of CaC12 plus KC1.
Electrochemical reactions are often used to deposit chemical species which, under appropriate conditions, grow as large grains or single crystals on an electrode. In electrodeposition, the crystallizing species is produced by the electrochemical reaction, the rate of which in turn depends upon the current through the cell. Because the latter can be controlled with great sensitivity, the method in principle allows delicate control over the rate of crystallization and therefore over the growth process. Kaspar et al. [l} and Barta et al. [21developed a variant for the growth of CaCO3 crystals in which growth occurred as a result of interdiffusion of CaCl2 and NaHCO3 solutions in the anode and cathode compartments, respectively. Electrolysis of the solution resulted in a rise in pH that allowed precipitation as CaCO3. In the work described here, we report a somewhat similar technique, in which the passage of current results in a concentration gradient of some critical ionic species. Nutrient material is dissolved from one electrode compartment and transported by diffusion to the other, where crystallization occurs. The method has been demonstrated by the growth of Ca(OH)2 crystals. A series of electrolytic cells, shown in fig. 1, was set up in which the electrolyte was an aqueous solution of KC1 and CaCl2 in a large test tube (35 mm diameter, 20 cm deep). The cathode was a Cu wire in a 10mm diameter glass tube about 15 cm long, the bottom of which was open to the solution in the cell. The anode was a Pt wire in a similar tube, the tube
terminating in a coarse glass frit immersed in the electrolyte of the cell. Powdered Ca(OH)2 was held in the anode compartment by the frit, but ions were free to
+
—
H2
~-
o.~
o
3 U
0
Fig. 1. Electrolytic cell. (1) Common electrolyte solution, (2) glass frit containing nutrient Ca(OH)2, and (3) open glass tube with Ca(OH)2 crystals growing on inner wall. 245
246
AD. Franklin
/ Electrochemical growth of crystals of electrolyte solution
travel between compartments. Several current levels from 0.1 to 3 mA were explored, the currents being kept constant by the simple expedient of a large resistance in series with the cell and a battery with sufficient voltage to produce the desired current. No special temperature control was used, but the laboratory generally is held to 22 ±1—2°C.The voltage drop across the cell was at least 1 .5 V. Several levels of Ca2+ ion concentration were explored, the total cation content being kept near 1 molar. Growth of Ca(OH) 2 crystals observed 2+ concentrations ranging fromwas 0.05 to 0.5 with molar Ca and with currents of 1 mA or larger. Good results were obtained using 0.5 molar KC1, 0.5 molar CaC1 2, and I mA current, and the behavior will be described in terms of this combination as an example. Nucleation of crystals occurred on the inside walls of the glass tube surrounding the Cu electrode after about 3 days. The nucleation began on the wall opposite the electrode, and was quite dense. As time passed the band of nucleation moved down the tube, away
from the cathode, and the crystals became fewer and larger the further they were from the electrode. After about 2 weeks the nucleation had proceeded down the approximately 12 cm of the glass tube to its mouth, and in the lower region scattered crystals several mm across (fig. 2) had grown. Subsequently, a fine precipitate appeared in the electrolyte outside the cathode tube. This behavior can be understood qualitatively in terms of a simple model that should apply for sufficiently small currents. The electric fieldbyis probably screened by the space charge produced the KC1 present to the extent that mass transport in the body of the cell is dominated by diffusion (disregarding convection produced by thermal and concentration gradients). Hydroxyl ions are produced in the cathode (the pH was about 11) and consumed at the anode (pH about 1 to 3). This concentration gradient produced a flux of hydroxyl ions (and a counter-flux of hydrogen ions) from cathode to anode. In the anode compartment the powdered Ca(OH)2 dissolved be-
~
1MM
Fig. 2. Ca(OH)2 crystals as grown.
AD. Franklin
/ Electrochemical growth of crystals of electrolyte solution
cause of the low pH, while crystal growth occurred in the cathode compartment where the pH was high. There was therefore a concentration gradient and flux of calcium ions from anode to cathode. The result was to transfer Ca(OH)2 from anode to cathode compartment, where the redeposition took place as crystal growth, at a steady state rate which should be controlled by the current, Brown [3] has interpreted experiments [4,5] in which electric currents appear to influence bone growth, as demonstrating an analogous process, in which calcium hydroxyapatite dissolves at the anode and deposits at the cathode. A simple linearized diffusion model neglecting all convection effects suggests that steady state should not be achieved until the “zone of deposition” has moved down the wall of the cathode compartment to a position determined primarily by the geometry of the cell and only somewhat sensitive to the magnitude of the current. In these preliminary experiments, apparently, the cathode compartment was not long enough enough for true steady state to have occurred. Finely-controlled crystal growth should be possible in a linear cell of uniform cross section sufficiently long for true steady state to be achieved. Once the limit to the zone of deposition has been established, crystal growth of those crystals on the edge of this zone should proceed. Reducing the current after the nucleation at this limit has occurred might allow fine tuning of the steady state growth rate to produce controlled growth. Experiments of this kind are now
247
being carried out in a cell 185 cm long, with the glass frit about 33 cm from the anode. The steady state is expected to produce a situation roughly symmetrical about the midpoint of the cell, with the zone of deposition extending 30—35 cm from the cathode. The method should be applicable wherever an electrochemical reaction can be used to establish a gradient in the activity of a chemical species which is involved in the solution or deposition of the chosen crystal, and where the solubility of the crystal is sufficient to maintain a reasonable growth rate. The author would like to acknowledge the assistance of Gregory Besio and Judith Nelson in performing the experiments, and to the National Institute of Dental Research for partial support during the course of the work.
References Lii
J. Kasper, C. Barta and J. Nigrinova, in: Rost Kristallov, VoL VI (Nauka, Moscow, 1965) p. 5; translation appears as Growth of Crystals, Vol. V1B, Ed. J.E.S. Bradley (Consultants Bureau, New York, 1968) p. 3. [2] C. Barta and J. Zemlicka, Silikaty 10 (1966) 275. [3] W.E. Brown, Colloques Internationnaux CNRS No. 230, Physicochimie et Cristallographie des Apatites d’Interét Biologique. [4] C.A.L. Bassett, R.J. Pawluk and R.O. Becker, Nature 204 (1964) 652.
~sjB.T. O’Connor, H.M. Charlton, J.D. Currey, D.R.S. Kirby and C. Woods, Nature 222 (1969) 163.