Preparation, characterization and application in cobalt ion adsorption using nanoparticle films of hybrid copper–nickel hexacyanoferrate

Abstract

Different mole ratios (<i>n</i> _Cu : <i>n</i> _Ni = <i>x</i> : <i>y</i>) of hybrid copper-nickel metal hexacyanoferrates (Cu _<i>x</i> Ni _<i>y</i> HCFs) were prepared to explore their morphologies, structure, electrochemical properties and the feasibility of electrochemical adsorption of cobalt ions. Cyclic voltammetry (CV), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) indicated that the <i>x</i> : <i>y</i> ratio of Cu _<i>x</i> Ni _<i>y</i> HCF nanoparticles can be easily controlled as designed using a wet chemical coprecipitation method. The crystallite size and formal potential of Cu _<i>x</i> Ni _<i>y</i> HCF films showed an insignificant change when 0 ≤ <i>x</i> : <i>y</i> < 0.3. Given the shape of the CV curves, this might be due to Cu²⁺ ions being inserted into the NiHCF framework as countercations to maintain the electrical neutrality of the structure. On the other hand, crystallite size depended linearly on the <i>x</i> : <i>y</i> ratio when <i>x</i> : <i>y</i> > 0.3. This is because Cu tended to replace Ni sites in the lattice structure at higher molar ratios of <i>x</i> : <i>y</i>. Cu _<i>x</i> Ni _<i>y</i> HCF films inherited good electrochemical reversibility from the CuHCFs, in view of the cyclic voltammograms; in particular, Cu₁Ni₂HCF exhibited long-term cycling stability and high surface coverage. The adsorption of Co²⁺ fitted the Langmuir isotherm model well, and the kinetic data can be well described by a pseudo-second order model, which may imply that Co²⁺ adsorption is controlled by chemical adsorption. The diffusion process was dominated by both intraparticle diffusion and surface diffusion.