Utilização de microscopia de varredura eletroquímica no estudo de transferência de elétrons do íon complexo [Ru(bpy)3]2+ induzida por radiação nas regiões do ultravioleta e visível

Abstract

The electron transfer kinetics of [Ru(bpy)3]2+ was studied by cyclic voltammetry and electrochemical impedance by using the following transparent conductive substrates: non-modified FTO (fluorine dopped SnO) and FTO modified upon electrophoretic deposition of TiO2 thus forming FTO/TiO2. The substrate FTO/TiO2, in turn, was submitted to a sintering process and the studies were performed with substrates containing anatase (non sintered) and rutile (sintered) phases of TiO2 films. The voltammetric curves indicated a quasi-reversible behavior for [Ru(bpy)3]2+ when using the substrates FTO (k0 = 2.74 x 10-3 cm s-1), FTO/TiO2-anatase (k0 = 1.56 x 10-3 cm s-1) and FTO/TiO2-rutile (k0 = 9.6 x 10-4 cm s-1); indicating a sluggish kinetics for FTO/TiO2-rutile. Based on the electrochemical impedance technique, the values of the apparent heterogeneous rate constant, kapp, were determined as 5.14 x 10-3, 2.44 x 10-3 and 0.94 x 10-3 cm s-1 for the substrates FTO, FTO/TiO2-rutile and FTO/TiO2-anatase, respectively. The effect of the electromagnetic radiation on the electron transfer reaction of [Ru(bpy)3]2+ was studied by means of scanning electrochemical microscopy (SECM) by using the substrate FTO/TiO2-anatase, since it presented the best kinetics results. A positive feedback current was observed under radiation at 260 nm indicating an increase of the electron density within the valence band of the semiconductor. In such condition, the apparent charge transfer constant was estimated as 1.43 cm s-1. Under radiation in the visible region (450 nm), a different behavior was observed with a negative current (kapp = 0.0028 cm s-1) indicating the substrate recover the oxidized redox species produced slowly at the microelectrode. Such behavior was assigned to a concurrent mechanism of electron transfer in which the redox mediator in the reduced state absorbs the incident radiation (resonance with the charge transfer transition) and emit an electron, which is, then, transferred to the valence band of the semiconductor. This competitive mechanism results in the diminution of the current response of the TIP and, therefore, in a negative feedback current.