Photocatalytic degradation of methyl-red by immobilised nanoparticles of TiO 2 and ZnO R. Comparelli*, P.D. Cozzoli*, M.L. Curri**, A. Agostiano* , **, G. Mascolo*** and G. Lovecchio*** * Dipartimento di Chimica, Università di Bari, via Orabona 4, I-70126, Bari, Italy (E-mail: csilrc30@area.ba.cnr.it; csildc28@area.ba.cnr.it; agostiano@area.ba.cnr.it) ** CNR IPCF – Bari Division c/o Dipartimento di Chimica, Università di Bari, via Orabona 4, I-70126 Bari, Italy (E-mail: csilmc20@area.ba.cnr.it) *** Istituto di Ricerca Sulle Acque, Consiglio Nazionale delle Ricerche, Via F. De Blasio 5, I-70123 Bari, Italy (E-mail: mascolo@area.ba.cnr.it; giangilov@libero.it) Abstract In this work, we report on the degradation of methyl-red (2-(4-Dimethylamino-phenylazo)-benzoic acid – C.I. 13020) under UV irradiation in the presence of nanosized ZnO and TiO 2. Oxide nanocrystals with controlled size were synthesised by using non-hydrolytic approaches and tested for the photocatalysed degradation. The performances of the immobilised nanoparticles were compared with their commercial counterparts after immobilization onto a solid support. The influence of some experimental conditions, namely pH and dye concentration, were investigated by monitoring the dye decoloration spectrophotometrically. Several intermediate by-products were identified by HPLC-MS, showing that two different mechanisms were operative during the photocatalytic oxidation. Keywords Colloidal nanocrystals; degradation of organic pollutants; HPLC-MS; photocatalysis Introduction It is well established that semiconductor assisted-photodegradation of organic compounds represents a powerful tool to remove water pollutants. In order to circumvent difficulties associated with the photocatalyst recovery from the treated streams, a common approach has been based on the immobilization of the photocatalyst onto a suitable transparent sup- port (e.g. quartz glass) (Neppolian et al., 2002; Guillard et al., 2002). In most cases, howev- er, such an immobilization has resulted in a loss of degradation efficiency due to the consequent decrease of the active surface area. In this respect, nanosized semiconductor materials have been proved to be particularly effective for degrading contaminants under UV irradiation due to the increasing surface-to-volume ratio with decreasing nanocrystal size. Moreover, as the bandgap of nanostructured semiconductors depends on their dimen- sion, the electron and hole redox potentials can be eventually tuned by modulating the size of the crystals. In the present paper, highly crystalline, surfactant-capped ZnO and TiO 2 nanoparticles were synthesised by colloidal non-hydrolytic routes and immobilised onto a glass substrate to be tested as photocatalysts under UV irradiation. The degradation of a model pollutant (the organic dye methyl-red [2-(4-Dimethylamino-phenylazo)-benzoic acid) – C.I. 13020]) was followed to explore the potential of the catalytic system. The influence of pH and of the initial dye concentration was investigated with reference to their commercial counterparts. Several organic by-products were also identified by HPLC-MS. The results were encouraging as they indicated that immobilised nanosized ZnO and TiO 2 were catalytically active even in the presence of the organic-passivating layer at their surface. Water Science and Technology Vol 49 No 4 pp 183–188 © IWA Publishing 2004 183