Microenvironment effects on the electrochemical and photoelectrochemical properties of thionine loaded Nafion Ò films S. Abraham John a , R. Ramaraj b, * a Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624 302, India b Centre for Photoelectrochemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India Received 9 December 2002; received in revised form 30 May 2003; accepted 2 July 2003 Abstract The polarity of the microenvironments within a Nafion Ò (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 dif- ference 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. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Nafion Ò ; Thionine; Tetrabutylammonium ion; Photoelectrochemistry; Photogalvanic cell 1. Introduction Nafion Ò , a perflurosulfonate ion-exchange polymer, is one of the best polymers used in the past two decades for the modification of electrode surfaces [1–4]. Several research groups have exploited the versatile properties of Nafion Ò (Nf) film for a variety of electrochemical applications [5–9]. Extensive studies have also been carried out to understand the multiphase structure of Nf film [10–15]. It has been suggested that three phases exist within Nf; a hydrophobic fluorocarbon phase, inter- connected ionic clusters and an interfacial region be- tween the two (Fig. 1) [10]. Nf films have remarkable selectivity for cations of hydrophobic nature [12]. It was shown [10] that the water content of the ionic clusters decreases as the size of the counter ion increases. Thus large counter ions would produce ionic clusters of very low water content. Water is believed to act as a plasti- cizer for the ionic cluster region and therefore as the water content of the cluster decreases the polymer chain material should play an increasingly important role in the chemical environment of the polymer [10]. Very re- cently, substantial research has been devoted to study the dehydration of Nf film by different methods after exchanging several cations into Nf film [18–21]. Leddy and co-workers [18] have shown that the electrostatic Journal of Electroanalytical Chemistry 561 (2004) 119–126 www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry * Corresponding author. E-mail address: ramarajr@yahoo.com (R. Ramaraj). 0022-0728/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2003.07.025