Adsorption thermodynamics and kinetics study for the self-assembly of 1,8,15,22-tetraaminophthalocyanatocobalt(II) on glassy carbon surface

Abstract

Spontaneous adsorption of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4α-Co IITAPc) on glassy carbon (GC) electrode leads to the formation of a stable self-assembled monolayer (SAM). Since the SAM of 4α-Co IITAPc is redox active, its adsorption on GC electrode was followed by cyclic voltammetry. SAM of 4α-Co IITAPc on GC electrode shows two pairs of well-defined redox peaks corresponding to Co III/Co II and Co IIIPc −1/Co IIIPc −2. The surface coverage ( Γ) value, calculated by integrating the charge under Co II oxidation, was used to study the adsorption thermodynamics and kinetics of 4α-Co IITAPc on GC surface. Cyclic voltammetric studies show that the adsorption of 4α-Co IITAPc on GC electrode has reached the saturation coverage ( Γ s) within 3 h. The Γ s value for the SAM of 4α-Co IITAPc on GC electrode was found to be 2.37 × 10 −10 mol cm −2. Gibbs free energy (Δ G ads) and adsorption rate constant ( k ad) for the adsorption of 4α-Co IITAPc on GC surface were found to be −16.76 kJ mol −1 and 7.1 M −1 s −1, respectively. The possible mechanism for the self-assembly of 4α-Co IITAPc on GC surface is through the addition of nucleophilic amines to the olefinic bond on the GC surface in addition to a meager contribution from π stacking. The contribution of π stacking was confirmed from the adsorption of unsubstituted phthalocyanatocobalt(II) (CoPc) on GC electrode. Raman spectra for the SAM of 4α-Co IITAPc on carbon surface shows strong stretching and breathing bands of Pc macrocycle, pyrrole ring and isoindole ring. Raman and CV studies suggest that 4α-Co IITAPc is adopting nearly a flat orientation or little bit tilted orientation.