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J.

Phys. D: Appl. Phys. 24 (1991) 217-225. Prinled in the UK

Electrocrystallization and electrochemical control of crystal growth: fundamental considerations and electrodeposition of metals
Frank C Walsht and Maura E Herront

t

*

Chemistry Department, Portsmouth Polytechnic, White Swan Road, Portsmouth PO1 ZDT, UK Chemistly Department, Southampton University, Highfield, SouthamptonSO9

5 N H , UK

Received 17 July 1990, in final form 13 November 1990
Abstract. The fundamental steps involved in an electrochemical reaction are reviewed with respect to their role in crystallization at the electrode surface. The reaction system is seen to be sensitive to the flow environment and the nature of

the electrode surface. The electrode potential can influence many featuresincluding reaction rate, chemical and phase composition, the extent of adsorption, orientation and texture. Aspects of electrocrystallization are illustrated by considering cases of metal deposition. Particular emphasis i placed upon the s development of surface roughness and the formation o metal powders during f copper deposition from aqueous acid sulphate solutions.
1. Introduction

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The term ‘electrocrystallization’ was coined by Fischer [l] in the 1940s to describe a crystallization process in which mass transfer is accompanied by charge transfer. The early development of the subject has been summarized by Bockris and Razumney [ 2 ] . In the present paper, electrocrystallization may be broadly defined as theprocess (or result) of a direct or indirect electrochemical influence on crystallization. (i) A direct electrochemical influence may be exerted if the value of the electrode potential dominates the type of nucleation and the growth kinetics for electrodeposition of a metal. (ii) Electrochemical reactions may indirectly alter the local reaction environment (e.g. the pH) and hence the nature of thereaction product. For example, in an unbuffered electrolyte the occurrence of hydrogen evolution as a side reaction at the cathode may result in the co-deposition of metal oxides/hydroxides in the metal deposit.

In many processes, both of these influences are experienced to some extent, i.e. there is a significant coupling between electrochemical and chemical steps in the overall reactionsequence. In order to appreciate some of the steps involved in the overall process of electrocrystallization, an
0022-37271911020217 + 09 $03.500 1991 IOP Publishing Lld

(1) As shown schematically in figure 1, this heterogeneous charge transfer reaction takes place at the interface between an electronic conductor (or semiconductor) and a n ionicconductor. Extensive treatments of the interfaceregion are available elsewhere and consider charge separation resulting in a potential distribution over the electrical ‘double layer’ (e.g. [3]), as shown in figure Z(a). Additionally, a concentration profile will exist for each of the eiectrolyte species near the electrode surface, due to convective diffusion (e.g. [4, 51). This situation arises due to depletion or build-up of species; in figure 2 (b ) ,reactant depletion is considered. At the limiting current, the reaction is under complete mass transport control and the surface concentration bas reachedzero. Assuming a (fictitious) linear profile allows the Nernstian diffusion layer thickness to he defined, BN isdirectly relatedto themasstransport coefficient, k,and the diffusion coefficient, D by the equation k , = D/B,. A third layer,the fluid velocity boundary layer (figure 2 ( c ) ) , develops due to localized differences in electrolyte convection. The hulk velocity is increasingly retarded as the electrode surfaceis approached. At the surface itself, the fluid is stationary relative to the electrode. Assumption of a linear profile allows the Prandtl (hydrodynamic) boundary layer, B,, to be defined.
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