Composite Tin and Zinc oxide nanocrystalline particles for enhanced charge separation in sensitized degradation of dyes J. Bandara * , K. Tennakone, P.P.B. Jayatilaka Institute of Fundamental Studies, Hantana Road, Kandy, Sri Lanka Received 1 May 2001; received in revised form 10 June 2002; accepted 10 June 2002 Abstract Composite ZnO/SnO 2 catalyst has been studied for the sensitized degradation of dyes e.g. Eosin Y (2 0 ; 4 0 ; 5 0 ; 7 0 - tetrabromofluorescein disodium salt) in relation to efficient charge separation properties of the catalyst. Improved photocatalytic activity was observed in the case of ZnO/SnO 2 composite catalyst compared to the catalytic activity of ZnO, SnO 2 or TiO 2 powder. The suppression of charge recombination in the composite ZnO/SnO 2 catalyst led to higher catalytic activity for the degradation of Eosin Y. Degradation of Eosin follows concomitant formation of CO 2 and formation of CO 2 followed a pseudo-first-order rate. Photoelectrochemical cells constructed using SnO 2 , ZnO, ZnO/SnO 2 sensitized with Eosin Y showed V oc of 175, 306, 512 mV/cm 2 and I sc of 50, 70, 200 lA/cm 2 respectively. A higher irreversible degradation of Eosin Y and higher V oc observed on composite ZnO/SnO 2 than ZnO and SnO 2 separately can be considered as a proof of enhanced charge separation of ZnO/SnO 2 catalyst. Eosin Y showed a higher emission decreases on ZnO/SnO 2 composite than on individual ZnO, SnO 2 or TiO 2 indicating dominance of the charge injection process. Photoinjected electrons are tunneled from ZnO to SnO 2 particles accumulating injected electrons in the conduction bands allowing wider separation of excited carriers. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Charge separation; Sensitized degradation; Tin oxide; Zinc oxide 1. Introduction For several years TiO 2 has been used as an active photocatalyst in the chemical treatment of organic pol- lutant (Ollis and Al-Ekabi, 1993) in aqueous phase un- der UV light irradiation. TiO 2 and other wide band gap semiconductors are found to be the most suitable photocatalysts owing to their photostability. Although much research has been carried out on use of wide band gap semiconductors as photocatalysts (Augugliaro et al., 1991; Tegner, 1992; Abe et al., 1999), their response to visible light is very poor and hence cannot be utilized for direct solar illumination. Sensitization of wide gap semiconductors such as TiO 2 (Ryan et al., 1989), ZnO (Patrick and Kamat, 1992) and SnO 2 (Vogel et al., 1994) has gained significant attention largely because of the demonstration of dye sensitized solar energy cells with high conversion efficiencies. The dye sensitization pro- cess involves the excitation of dye molecules with visible light and the subsequent electron injection into con- duction band (CB) of a semiconductor (1) and (2), fol- lowed by a back reaction (3): dye ! hm dye  ð1Þ Chemosphere 49 (2002) 439–445 www.elsevier.com/locate/chemosphere * Corresponding author. Present address: Department of Chemistry, Pearson Chemistry Laboratory, Tufts University, Medford, MA 02155, USA. Tel.: +1-94-8-232002; fax: +1-94-8- 232131. E-mail addresses: jayasundera@yahoo.com, jayasund@ifs. ac.lk (J. Bandara). 0045-6535/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0045-6535(02)00306-5