Electrodeposition of BaCO3 Coatings on Stainless Steel Substrates M.
Dinamani,†
P. Vishnu
Kamath,*,†
and Ram
Seshadri*,†,‡,#
Department of Chemistry, Central College, Bangalore University, Bangalore 560 001, India, and Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 3 417-423
Received November 14, 2002
ABSTRACT: While cathodic deposits of BaCO3 obtained from a dilute barium bicarbonate bath are unoriented, those obtained from EDTA-stabilized baths have a (200) orientation with respect to the substrate; thick coatings obtained at longer deposition times have a stronger orientation compared to thin coatings. The orienting influence is shown to be due to the combined effect of EDTA and the use of electrochemistry. Introduction Barium carbonate [BaCO3, Pmcn, a ) 5.31 Å; b ) 8.9 Å; c ) 6.43 Å] is an important material whose many applications in the glass and ceramics industry are described in ref 1. Scientific interest in BaCO3 arises from the fact that it is among the few orthorhombic carbonates found in nature and is isostructural with mineral aragonite, an important polymorphic modification of CaCO3.2 While the orthorhombic modification is metastable among the calcium carbonates, it is the thermodynamically most stable phase among the heavier metal carbonates, ACO3 (A ) Sr, Ba, and Pb).3 Consequently, the incorporation of heavy ions such as Ba2+ during CaCO3 crystallization affects the thermodynamic stability, crystal chemistry, as well as crystal morphology of both biotic and abiotic deposits of the latter.4-6 On account of the importance of CaCO3 as an inorganic biomaterial and its close relationship with BaCO3, the latter itself has become an object of many scientific investigations.7-9 We have for some time been interested in the use of electrochemistry to bring about the crystallization of inorganic materials such as BaSO4,10 CaCO3,11 SrSO4,12 and calcium phosphates13smaterials that are associated with biomineralization. In this, we have been motivated by the many parallels between electrochemical synthesis and templated/additive mediated synthesis of the kind practiced in model biomineralization studies. These include the orienting effects of electrodes and morphological control over the deposited crystallites. The effectiveness of electrochemical synthesis has been amply demonstrated by Switzer and co-workers in their preparations of oriented,14 compositionally modulated,15 epitaxial,16 and nanostructured17 films of metal oxides. Our own previous studies have shown that by varying the deposition current and time, crystal orientation with respect to the electrode, and particle morphology can be influenced. However, the ability to predict or obtain a predetermined orientation or morphology by the use of electrochemistry remains elusive. Toward the realiza†
Bangalore University. Indian Institute of Science. Present address: Materials Department, University of California Santa Barbara 93106-5050, USA. Fax: (805) 893 8797. E-mail:
[email protected]. ‡
#
tion of this objective, we have extended our studies to the electrodeposition of BaCO3, which we present here. Experimental Procedures All solutions used in this study were prepared using ionexchanged (Barnstead Easypure) water. Equal (25 cm3) volumes of Ba(NO3)2 (500 ppm) and NaHCO3 (1000 ppm) solutions were mixed and allowed to stand for up to 2 h. The pH of the bath was measured to be 8.8. During this period, no chemical precipitation was observed. A large stoichiometric excess of bicarbonate ions was taken to facilitate adequate carbonate release upon electrolysis. EDTA-stabilized baths were prepared at higher concentrations. Equal (25 cm3) volumes of Ba(NO3)2, EDTA (0.05 M each), and NaHCO3 (0.1 M) were mixed in that order and allowed to stand for up to 24 h. No chemical precipitation was seen in this bath even on prolonged standing. Electrodeposition was carried at two different current densities (10 and 30 mA/cm2, respectively) for different times. The deposition times were chosen to maintain similar values of Q, the charge passed, which is computed as current density × deposition time in mAh/cm2. Low values of Q were obtained by fixing the deposition times at 0.5 h and 10 min, respectively, at low and high currents. This yields a Q value of 5 mAh/cm2. This Q value yielded a BaCO3 coating of a distinctive character, which is fully described in the next section. We refer to this as a “thin” coating. At higher (10-15 mAh/cm2) values of Q, the nature of the deposit was different and we refer to this as a “thick” coating. The designation of “thin” and “thick” is purely qualitative and refers to the range of Q values employed rather than to the actual thickness. In plain barium bicarbonate baths, however, the nature of the coating did not differ at the different Q values. Electrodeposition was carried out using a EG&G PARC Versastat IIA potentiostat operating in the galvanostatic mode using a stainless steel (SS304) flag cathode (surface area 4.5 cm2), a cylindrical Pt mesh anode (geometric area 28 cm2) and a SCE as reference. By weighing the cathode before and after deposition, the weight of the coating could be determined. Typical growth curves for the coating obtained from the EDTAstabilized bath are shown in Figure 1. Such curves show that the coating can be grown under controlled conditions and that it is possible to obtain a coating of predetermined weight by suitable choice of deposition conditions. From these curves, it is possible to estimate the mass of thin and thick coatings to be 1.2 ( 0.1 and 4.2 ( 0.1 mg/cm2, respectively. A similar measurement on deposits obtained from plain barium bicarbonate baths was not attempted, owing to the very low coating weights. Prior to electrodeposition, all electrodes were cleaned with detergent and electrochemically polished as described elsewhere.18 Mass loss of the SS304 flag due to corrosion during the electrochemical cleaning has been suitably accounted for in the measurement of the coating mass.
10.1021/cg025608t CCC: $25.00 © 2003 American Chemical Society Published on Web 03/07/2003
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Figure 2. XRD profiles of the BaCO3 deposit obtained from a barium bicarbonate bath. (a) Observed, (b) refined, and (c) the difference profile. Figure 1. Plot of (a) coating mass as a function of time for a fixed current density of 10 mA/cm2. (b) Coating mass as a function of current density after a fixed time (t ) 30 min). The coatings are grown from an EDTA-stabilized bath. Electroless coatings were obtained as controls from the plain barium bicarbonate bath by allowing it to stand for 24 h. A SS304 flag was placed at the bottom of the beaker to collect the deposit. In EDTA-stabilized baths, electroless coatings could not be obtained until the EDTA concentration was reduced to one-tenth the concentration of Ba. At this concentration, the deposit was collected on the SS304 flag as earlier, after 24 h of standing. All coatings were characterized by ex-situ powder X-ray diffractometry (XRD) by directly mounting the electrode on a Seimens D5005 diffractometer operated in reflection geometry. Data were collected with CuKR radiation using a continuous scan rate of 1° 2θ per minute or slower and were then rebinned into 2θ steps of 0.02° or 0.05°. At least three coatings were prepared and examined by XRD under each deposition condition to ensure reproducibility of the XRD patterns. All XRD profiles were fitted by the Rietveld method as implemented in the XND Rietveld code (version 1.20)19 using the published Pmcn structure of BaCO3.3 As the coatings obtained from EDTA-stabilized baths have a very high degree of orientation, satisfactory fits could not be obtained by the incorporation of orientation into the Rietveld refinements. Therefore, we examined the residual intensities in the difference profile after the best possible fit had been obtained without the incorporation of any orientation in the fitting procedure. The reflections that survived in the difference profile were interpreted to be indicative of the orientation direction. Scanning electron micrographs were obtained using a Jeol JSM 5600 LV microscope directly from the coating on the substrate by mounting small pieces of the electrode on conducting carbon tape and sputter coating gold to improve conductivity.
Results and Discussion The successful electrochemical synthesis of an inorganic material depends on (i) the availability of a suitable bath in which the reactant ions or their precursors can be retained in solution without the occurrence of a chemical reaction and (ii) the carrying out of a suitable electrochemical reaction that leads to the precipitation of the desired phase from the bath on the passage of electric current.
Figure 3. XRD profiles of the electroless BaCO3 deposit obtained from a barium bicarbonate bath. (a) Observed, (b) refined, and (c) the difference profile.
Since BaCO3 has a low solubility product (8 × 10-9), a bath comprising Ba2+ and HCO3- ions was employed. The crystallization of BaCO3 from this solution was achieved by the decomposition of HCO3- ions at high pH according to the reaction20:
HCO3- + OH- f CO32- + H2O The hydroxyl ions required for this reaction were obtained by electrogeneration of base21 at the cathode as a result of the electrolysis of water
2H2O + 2e f H2 + 2OHUpon generation of CO32- ions near the cathode, BaCO3 deposition takes place according to the reaction
Ba2+ + CO32- f BaCO3 V Since the pH changes take place close to the cathode,
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Figure 4. SEM images of BaCO3. (a,b) Electrodeposit corresponding to Figure 2. (c,d) Electroless precipitate corresponding to Figure 3.
BaCO3 is deposited on it as a coating, and no bulk precipitation is observed. After 10-30 min of deposition at a current of 10-30 mA/cm2, the cathode flag reveals upon optical microscopic examination the outward growth of tiny white crystallites. The PXRD pattern of this as-prepared coating reveals a prominent diffraction maximum at 2θ ) 23.9°, corresponding to the (111) reflection of BaCO3 (PDF: 05-0378). Other weak features are seen at 2θ values, 19.5, 34.1, and 34.6, respectively. To confirm that this pattern is indeed due to BaCO3, a Rietveld fit of the experimental profile was carried out using the known structure of BaCO3. The results are shown in Figure 2. The fit is satisfactory showing that the electrodeposit is indeed an unoriented coating of BaCO3. Fugure 3 shows the PXRD pattern of a typical electroless coating of BaCO3 obtained by chemical precipitation. Once again, the Rietveld fit reveals the growth of an unoriented BaCO3 coating. The low signal-to-noise ratio of the observed patterns is due to the very small quantity of the deposit owing to the low bath concentration. In Figure 4 the scanning electron micrographs of the electrodeposited BaCO3 coating are compared with those of the electroless coating. In both cases, the coatings are not uniform with only a few crystallites seen on the otherwise bare stainless steel surface. The electrodeposit
Figure 5. XRD profiles of the thin BaCO3 deposit obtained from an EDTA-stabilized bath (10 mA/cm2, 30 min). (a) Observed, (b) refined, and (c) the difference profile.
comprises well-facetted needles with 10-20 mµ long stems that split into two or more similarly well-facetted branches. The branches display six sides. The six-sided morphology arises from the trilling (like a twinning, except between three crystals rather than two) of BaCO3
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Figure 6. XRD profiles of the thick BaCO3 deposit obtained from an EDTA-stabilized bath (10 mA/cm2, 60 min). (a) Observed, (b) refined, and (c) the difference profile.
Figure 7. XRD profiles of the BaCO3 deposit obtained from an EDTA-stabilized bath (30 mA/cm2). (a) Thin deposit (10 min) and (b) thick deposit (30 min).
around the 001 axis which is pseudohexagonal.22 Such trilling in aragonite and strontianite crystals grown on SAM substrates has been previously described.23 The electroless deposit has particles with a poorly defined irregularly shaped morphology which does not reveal any special features even at high magnification. This is probably reflective of uncontrolled chemical precipitation. To make thicker and uniform deposits, it is necessary to use a bath with a higher Ba2+ concentration. To prevent chemical precipitation, EDTA was used as a stabilizing agent. EDTA complexes with Ba, due to which free Ba2+ ions are unavailable for chemical precipitation. The Ba/EDTA ratio was maintained at 1 in keeping with the stoichiometry of the Ba[EDTA] complex.24 Electrolysis (current density 10 mA/cm2) of the EDTA-stabilized barium bicarbonate bath yielded dense, uniform coatings, whose PXRD patterns obtained at low (30 min) and high (60 min) deposition times are shown in Figures 5 and 6, respectively. These are the “thin” and “thick” coatings, respectively. While the posi-
Dinamani et al.
Figure 8. XRD profiles of the BaCO3 deposit obtained from an EDTA-stabilized bath (10 mA/cm2, 60 min). (a) Thin deposit ([Ba2+]/[EDTA] ) 0.67) and (b) thick deposit ([Ba2+]/[EDTA] ) 2]).
Figure 9. XRD profiles of the electroless BaCO3 deposit obtained from an EDTA-stabilized bath. (a) Observed, (b) refined, and (c) the difference profile.
tions of the diffraction maxima agree with those expected for BaCO3, the relative intensities of the various reflections are completely different. In the former, the most intense peak is due to the (111) reflection, while in the latter, the most intense peak is due to the (200) reflection. However, in both cases, the peak due to the (200) reflection has gained in intensity compared to the PXRD pattern of the unoriented coating indicating strong orientational effects. To determine the orientation, we attempted a Rietveld fit of the observed profiles and examined the residual intensities in the difference profile after the best possible fit of minimum error had been achieved. In both cases, the difference profiles revealed considerable residual intensity under the (200) reflection showing that both types of coatings have an a-axis orientation, the thick coating being more oriented than the thin coating. At higher (30 mA/cm2) currents too, similar patterns were obtained at low (10 min) and high (30 min) deposition times (see Figure 7). It appears that the nature of the PXRD pattern and the extent of
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Figure 10. SEM images of BaCO3 electrodeposits. (a,b) Thin coating corresponding to Figure 5. (c,d) Thick coating corresponding to Figure 6.
orientation is independent of the deposition current and is dependent only on the total charge passed. The latter being reflective of the quantity of the deposit, we conclude, that orientation effects are purely related to the thickness of the deposit, the thicker deposit being oriented more strongly. This observation is in opposition to that made on the CaCO3 system,25 where thin calcite deposits were oriented and thick deposits were not. The latter observation is indicative of the effect of the substrate, while our observation in the present system indicates the orienting effect of an active ingredient in the bath. The active ingredient in the present instance is the EDTA. EDTA can significantly affect BaCO3 crystallization by two specific mechanisms. The EDTA anion has a planar geometry similar to that of the carbonate anion and is known to bind to specific crystal faces of inorganic carbonates by forces of molecular recognition and thereby promote crystal growth along other directions.26 EDTA can control the degree of supersaturation of the metal ion and thereby affect the kinetics of crystallization as is seen in the deposition of CaCO3 in the presence of Mg2+.27 Both these mechanisms can accentuate orientational effects in thick compared to thin coatings. To verify which of the two effects are actually operative, we carried out depositions at high Q (10 mA/cm2, 60 min;
conditions that yield “thick” coatings) by varying the EDTA concentration of the bath. The EDTA-rich bath had a Ba/EDTA ratio 0.67, while the EDTA-deficient bath had a Ba/EDTA ratio 2. The latter has a higher degree of supersaturation compared to the former. Expectedly, the EDTA-rich bath yielded a “thin” deposit, whereas the EDTA-deficient bath yielded a “thick” deposit at the same Q value (see Figure 8). This observation strengthens our surmise that supersaturation has a greater role to play than is generally accepted in directing crystal growth. The results reported in Figures 5-8 have to be examined in the light of the PXRD data shown in Figure 9, which are obtained from an electroless coating formed in the presence of EDTA. The difference profile obtained after a Rietveld fit reveals a coating with a mild c-axis orientation, which can in fact be ignored compared to the strong orientational effects seen in the electrodeposited coatings. Clearly, the use of electrochemistry or of EDTA (but not together) leads to unoriented coatings. The simultaneous use of both, on the other hand, leads to oriented coatings. In Figure 10 are shown the scanning electron micrographs of thin and thick BaCO3 coatings obtained at 10 mA/cm2. The thin coating is continuous with small micron-sized extrusions emerging from the surface. An examination of the regions where the continuous film
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Figure 11. SEM images of BaCO3 electrodeposits. (a,b) Thin coating corresponding to Figure 7a. (c,d) Thick coating corresponding to Figure 7b.
that obtained at low current, whereas the thick coatings comprise needles up to 10 µm in length, lying with their long axes parallel to the substrate. The needles are not facetted. Beneath the needles, the continuous base coat is seen. Accretion of the material on the nuclei provided by the thin coatings takes place perpendicular and parallel to the substrate at low and high currents, respectively. These growth patterns are shown schematically in Figure 12.
Figure 12. A schematic of the growth patterns of the BaCO3 electrodeposit at low (a) and high (b) currents.
Acknowledgment. The authors thank the Department of Science and Technology, India, for financial support. M.D. thanks the Council of Scientific and Industrial Research, India, for the award of a Senior Research Fellowship. References
is broken reveals the underlying needle shaped crystallites. When these needle-shaped crystallites grow with their long axes perpendicular to the substrate, they protrude from the continuous coating. These protrusions appear to act as nuclei and spiral outward into pillars several tens of micrometers in length, in the thick coatings. Representative micrographs taken from several regions reveal such spiral pillars of different sizes representing different stages of growth. The thin and thick coatings obtained at 30 mA/cm2 are shown in Figure 11. The thin coating is identical to
(1) Gutmann, B.; Chalup, A. Am. Ceram. Soc. Bull. August 2000, 54. (2) Speer, J. A. Rev. Mineral. 1983, 11, 145. (3) DeVilliers, J. P. R. Am. Mineral. 1971, 56, 759. (4) Astilleros, J. M.; Pina, C. M.; Fernandez-Diaz, L.; Putnis, A. Geochim. Cosmochim. Acta 2000, 64, 2965. (5) Bath, G. E.; Thorrold, S. R.; Jones, C. M.; Campana, S. E.; McLaren, J. W.; Lam, J. W. H. Geochim. Cosmochim. Acta 2000, 64, 1705. (6) Pina, C. M.; Putnis, A. Geochim. Cosmochim. Acta 2002, 66, 185. (7) Kon-no, K.; Koide, M.; Kitahara, A. J. Chem. Soc. Jpn. 1984, 6, 815.
Electrodeposition of BaCO3 Coatings (8) Kandori, K.; Kon-no, K.; Kitahara, A. J. Disp. Sci. Technol. 1988, 9, 61. (9) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. J. Phys. Chem. B 1997, 101, 3460. (10) Dinamani, M.; Kamath, P. V.; Seshadri, R. Chem. Mater. 2001, 13, 3981. (11) Dinamani, M.; Kamath, P. V.; Seshadri, R. Mater. Res. Bull. 2002, 37, 661. (12) Dinamani, M.; Kamath, P. V.; Seshadri, R. Solid State Sci., accepted. (13) Therese, G. H. A.; Kamath, P. V.; Subbanna, G. N. J. Mater. Chem. 1998, 8, 405. (14) Switzer, J. A. Am. Ceram. Soc. Bull. 1987, 66, 1521. (15) Switzer, J. A.; Shane, M. J.; Phillips, R. J. Science 1990, 247, 444. (16) Breyfogle, B. E.; Phillips, R. J.; Switzer, J. A. Chem. Mater. 1992, 4, 1356. (17) Zhou, Y.; Phillips, R. J.; Switzer, J. A. J. Am. Ceram. Soc. 1995, 78, 981. (18) Corrigan, D. A.; Bendert, R. M. J. Electrochem. Soc. 1989, 136, 723.
Crystal Growth & Design, Vol. 3, No. 3, 2003 423 (19) Program XND, Be´rar, J.-F. NIST Spec. Publ. 1992, 846, 212. (20) Gabrielli, C.; Keddam, M.; Perrot, H.; Khalil, A.; Rosset, R.; Zidoune, M. J. Appl. Electrochem. 1996, 26, 1125. (21) Therese, G. H. A.; Kamath, P. V. Chem. Mater. 2000, 12, 1195; Zhitomirsky, I. Am. Ceram. Soc. Bull. September 2000, 57. (22) Megaw, H. D. Crystal Structures: A Working Approach; W. B. Saunders: Philadelphia, 1973. (23) Ku¨ther, J.; Nelles, G.; Seshadri, R.; Schaub, M.; Butt, H.J.; Tremel, W. Chem. Eur. J. 1998, 4, 1834. (24) Hodgson, S. N. B.; Shen, X.; Sale, F. R. J. Mater. Sci. 2000, 35, 5275. (25) Xu, S.; Melendres, C. A.; Park, J. A.; Kamrath, M. A. J. Electrochem. Soc. 1999, 146, 3315. (26) Mann, S.; Didymus, J. M.; Sanderson, N. P.; Heywood, B. R.; Aso Samper, E. J. J. Chem. Soc., Faraday Trans. 1990, 86, 1873. (27) Devery, D. M.; Ehlmann, A. J. Am. Mineral. 1981, 66, 592.
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