Deposition of oriented SrSO4 coatings by electrogeneration of acid
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Introduction
An important contemporary concern of materials chemistry is the development of techniques which permit delicate control over the whole crystallization process whereby polymorphism, crystal habit, texture and morphology can be engineered to meet specific predetermined goals. These are often achieved by employing molecular recognition processes through the use of (i) additives that bind to specific crystal faces [1], (ii) templates that direct crystal growth [2] and (iii) substrates that nucleate epitaxial growth [3]. Recent years have seen the increasing use of electrochemistry to achieve similar ends.
Electrochemistry has several features in common with template/additive mediated synthesis:
- (i)
The electrode itself functions as a template;
- (ii)
The potential gradient within the double layer can in principle direct crystal growth;
- (iii)
The deposition current can be varied to control the kinetics of crystallization.
In the light of these developments and our own earlier work on the electrodeposition of inorganic biomaterials such as CaCO3 [7], BaSO4 [8] and certain calcium phosphates [9], we report in this paper the electrodeposition of oriented SrSO4 coatings.
Strontium sulfate (SrSO4) in the celestite mineral modification (its common form) crystallizes in the orthorhombic Pnma structure. It is isostructural with BaSO4 and co-crystallizes with scales of the latter in industrial water treatment systems. Since the deposition of insoluble scales in water pipes is a major problem, the crystallization of SrSO4 by itself and in association with BaSO4 or other salts has been widely studied [10], [11]. SrSO4 precipitation is also studied as a model to test the many theories of precipitation reactions under static as well as flow conditions [12], [13]. SrSO4 of biotic origin is known to form the skeletons of certain Acantharian protozoa. The micron sized SrSO4 spicules found therein are single crystalline in nature [14], evocative of the skeletal plates of echinoids (sea-urchins), which comprise single-crystalline calcite [15]. Moreover, biotic SrSO4 spicules display crystal habits that are uncharacteristic of both mineral celestite deposits as well as synthetic SrSO4 preparations [14].
Our studies show that by varying the cell current and time, the orientation of the SrSO4 coating electrodeposited on a polycrystalline stainless steel substrate can be switched towards different crystallographic directions. Other thin film/coating deposition techniques such as pulsed laser deposition, chemical vapour deposition and rf sputtering are not only energy and capital intensive, but also do not offer such control over deposit orientation in a one-pot synthesis.
Section snippets
Experimental section
SrSO4 has a low solubility product (2.8×10−7). Consequently, Sr2+ and SO2−4 ions cannot be stabilized in a bath in the absence of a complexing agent. The complexing agent chosen was the disodium salt of EDTA (H2Y2−). Not having any tetrahedral moieties, the H2Y2− ion is not expected to exercise any growth modifying influence on sulfates [16], but only serves to control the degree of supersaturation.
All solutions were made using ion-exchanged (Barnstead EasypureTM) water with specific resistance
Results and discussion
Preparation of inorganic materials from aqueous solution requires dehydration of the solvation spheres around the cations. Electrosynthetic techniques involve the use of electrochemistry to achieve this. In situations requiring the use of anodic oxidation to produce cations in high oxidation states, the dehydration of the solvation sphere is instantaneously driven by the highly polarizing nature of the Mn+ (n⩾3) cation which leads to oxide formation by the scission of the OH bonds of the
Acknowledgements
The work reported here has been supported by the Department of Science and Technology.
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