Adsorption of cobalt ferrite nanoparticles within layer-by-layer films: a kinetic study carried out using quartz crystal microbalance

Abstract

The paper reports on the successful use of the quartz crystal microbalance technique to assess accurate kinetics and equilibrium parameters regarding the investigation of in situ adsorption of nanosized cobalt ferrite particles (CoFe₂O₄—10.5 nm-diameter) onto two different surfaces. Firstly, a single layer of nanoparticles was deposited onto the surface provided by the gold-coated quartz resonator functionalized with sodium 3-mercapto propanesulfonate (3-MPS). Secondly, the layer-by-layer (LbL) technique was used to build multilayers in which the CoFe₂O₄ nanoparticle-based layer alternates with the sodium sulfonated polystyrene (PSS) layer. The adsorption experiments were conducted by modulating the number of adsorbed CoFe₂O₄/PSS bilayers (n) and/or by changing the CoFe₂O₄ nanoparticle concentration while suspended as a stable colloidal dispersion. Adsorption of CoFe₂O₄ nanoparticles onto the 3-MPS-functionalized surface follows perfectly a first order kinetic process in a wide range (two orders of magnitude) of nanoparticle concentrations. These data were used to assess the equilibrium constant and the adsorption free energy. Alternatively, the Langmuir adsorption constant was obtained while analyzing the isotherm data at the equilibrium. Adsorption of CoFe₂O₄ nanoparticles while growing multilayers of CoFe₂O₄/PSS was conducted using colloidal suspensions with CoFe₂O₄ concentration in the range of 10⁻⁸ to 10⁻⁶ (moles of cobalt ferrite per litre) and for different numbers of cycles n = 1, 3, 5, and 10. We found the adsorption of CoFe₂O₄ nanoparticles within the CoFe₂O₄/PSS bilayers perfectly following a first order kinetic process, with the characteristic rate constant growing with the increase of CoFe₂O₄ nanoparticle concentration and decreasing with the rise of the number of LbL cycles (n). Additionally, atomic force microscopy was employed for assessing the LbL film roughness and thickness. We found the film thickness increasing from about 20 to 120 nm while shifting from 3 to 10 CoFe₂O₄/PSS bilayers, using the 8.9 × 10⁻⁶ (moles of cobalt ferrite per litre) suspension.