Electrodeposition and dissolution of yttrium-hexacyanoferrate layers

The electrodeposition and dissolution of yttrium-hexacyanoferrate [YHCNFe(II)] were investigated by electrochemical quartz crystal microbalance technique (EQCM). The electrodeposition was carried out by potential cycling or stepping from solutions of Y(NO3)3 and K3[FeIII(CN)6] of different concentrations. The ratio of the reactants was also varied. No deposition was found in dilute solutions (c < 10−3 mol dm−3). The increase of concentrations led to an intense deposition of YHCNFe(II) in the course of reduction of [FeIII(CN)6]3−. At high concentrations of the reactants a coagulation deposition of YHCNFe(III) at open-circuit has also been detected. During the reduction the first phase is the nucleation which requires saturation or oversaturation in respect to the reacting species near the gold surface. The growth phase is much faster than the formation of nuclei, and its rate depends on the concentration and the concentration ratio of the species. The composition of the deposits has been determined by total reflection X-ray fluorescence (TXRF) spectrometry. From the molar ratio of atomic constituents (K, Y and Fe) of the slightly soluble deposit (solubility: 5 × 10−5 mol dm−3) formed after reduction of Fe(III) a formula K0.46Y1.18[FeII(CN)6] can be derived. This value is in good accordance with the molar mass calculated from the results of EQCM experiments which also revealed that the deposit contains ca. 2 mol H2O/mol YHCNFe(II). The solubility of YHCNFe(III) is substantially higher (s = 2 × 10−3 mol dm−3), and according to the results of TXRF measurements, its composition is Y[FeIII(CN)6]. The reoxidation of YHCNFe(II) takes place in two steps. The first one is a partial oxidation which is accompanied by the desorption of K+ ions from the layer. During further oxidation a fast dissolution occurs due to the high solubility of YHCNFe(III).