Complex ions, deposition potentials stability constant

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

This jndicions nse of a complexant to allow codeposition of two cations with widely differing valnes of K p is, nnfortnnately, not always nsefnl. An example is the deposition of (Cd,Zn)S—a material of interest particnlarly becanse of its potential nse in photovoltaic cells (see Chapter 9). The stability constants of Cd and Zn ions are in most cases very similar for any particnlar complex (although, of conrse, they vary greatly from one complex to another). This reflects the similarity of the chemistry of these two ions (and the difference between them and the Hg ion). Therefore complexation is very limited as a means to control the concentration of one of these ions relative to the other. 

When a metal ion is complexed, its standard potential is shifted cathodically by (2.3RT/nF)logK, where K is the stability constant of the complex formed. As a result, hydrogen evolution can occur along with metal deposition. The faradaic efficiency, which is the fraction of the current consumed to deposit the metal, may decrease. This quantity is defined by... 

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