Microenvironment effects on the electrochemical and photoelectrochemical properties of thionine loaded Nafion^® films

Abstract

The polarity of the microenvironments within a Nafion^® (Nf) film was studied by electrochemical and photoelectrochemical techniques using a phenothiazine dye, thionine (TH) as a probe. When tetrabutylammonium ion-exchanged Nf film (TBA-Nf) is immersed in TH solution, TH partly displaces TBA ions into hydrophobic regions and binds to the less polar ionic cluster region of the Nf film. Although TH is able to enter the TBA-Nf film, the amount of TH incorporated in the TBA-Nf film is lower than the amount of TH incorporated in the Nf film. This is because Nf shows higher selectivity for TBA ions over TH. TH loaded into TBA-Nf film (TBA-Nf/TH) shows a 50 mV negative shift in the E_1/2 value and most of the exchanged TH becomes electroinactive. The observed electrochemical properties of the TBA-Nf/TH film could be attributed to the less polar environment experienced by TH molecules in the dehydrated TBA-Nf film. In the dehydrated TBA-Nf/TH film, the movement of protons is hampered by the lack of water molecules. However, when the TBA-Nf/TH film is immersed in supporting electrolyte solution for long times, the dehydrated TBA-Nf/TH film becomes hydrated due to the entry of ions and water molecules into the TBA-Nf/TH film, which leads to a positive shift in the E_1/2 value with increased electroactive TH species. The less polar environment experienced by TH in TBA-Nf film was also reflected in the photogalvanic cell measurements. In the photogalvanic cell, when the Nf/TH film was exposed to visible light, a cathodic photocurrent was observed whereas an anodic photocurrent was observed for the TBA-Nf/TH film. The observed difference in the polarity of the photocurrent again reiterates the polar and less polar environments experienced by TH in Nf and TBA-Nf films, respectively. The polarity of the photocurrent changes from anodic to cathodic when the TBA-Nf/TH film is soaked in the photogalvanic cell solution for longer times.