Materials Chemistry and Physics 129 (2011) 83–89 Contents lists available at ScienceDirect Materials Chemistry and Physics j ourna l ho me pag e: www.elsevier.com/locate/matchemphys Magnetically recoverable photocatalytic nanocomposite particles for water treatment Darko Makovec a,∗ , Marjan Sajko b , Aljaˇ z Seliˇ snik c , Miha Drofenik d,a a Department for Materials Synthesis, Joˇ zef Stefan Institute, Ljubljana, SI-1000, Slovenia b Environmental Protection Institute, Public Health Institute, Maribor, SI-2000, Slovenia c Cinkarna Celje, Celje, SI-3001, Slovenia d Faculty of Chemistry and Chemical Technology, University of Maribor, Maribor, SI-2000, Slovenia a r t i c l e i n f o Article history: Received 12 January 2011 Received in revised form 7 March 2011 Accepted 23 March 2011 Keywords: Magnetic photocatalyst Anatase Nanocomposite Magnetic separation Photocatalytic activity a b s t r a c t Magnetically recoverable, photocatalytic, nanocomposite particles were prepared by the simple het- eroagglomeration of previously synthesized anatase nanoparticles and superparamagnetic maghemite nanoparticles in an aqueous suspension by applying attractive electrostatic forces between the nanopar- ticles with an opposite electric surface charge. The anatase nanoparticles, with sizes between 3 and 5 nm, were synthesized using a simple hydrolysis of aqueous TiOSO 4 , and the maghemite nanoparticles, with size approximately 13 nm, were synthesized with a simple co-precipitation from an aqueous solution of Fe 2+ /Fe 3+ using ammonia. The surface charge of the maghemite nanoparticles was modified by graft- ing aminopropyl triethoxy silane (APS) onto their surfaces. The nanocomposite particles formed in the suspension at neutral pH, where the as-synthesized anatase nanoparticles display a negative charge and the APS-grafted maghemite nanoparticles display a positive charge, and consequently the two types of nanoparticles heteroagglomerate. Testing the nanocomposite particles for magnetic separation using a laboratory separator showed that they can be effectively separated from a water suspension, even when they contain only a small amount, i.e., 10% of magnetic phase. Their photocatalytic activity was tested in the decomposition of formic acid as a model compound. It appeared that the activity of the anatase nanoparticles increased when they were combined with the maghemite nanoparticles in the nanocomposite, providing the content of maghemite was below ∼50%. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The photocatalytic decomposition of different pollutants using anatase (TiO 2 ) as the photocatalyst can be effectively employed in the purification of water [1,2]. However, in the process of pho- tocatalysis, a large surface area is required for the photocatalyst. This can be easily achieved when the photocatalyst is prepared in the form of nanoparticles and dispersed in the polluted water. However, there is a problem related to the difficulty in completely eliminating the nanoparticles from the water after the purification, which can result in leaching of the nanoparticles into the envi- ronment. For these reasons, developments have mainly focused on mobilizing the photocatalyst onto different solid substrates, such as glass beads, glass fibers, zeolites, etc. [3]. Unfortunately, by immobi- lization of the photocatalyst on solid substrates, the photocatalytic nanoparticles lose a large part of their effective surface area. ∗ Corresponding author. Tel.: +386 1 4773 579; fax: +386 1 2519 385. E-mail address: darko.makovec@ijs.si (D. Makovec). An alternative solution to the problem of the separation and reuse of photocatalytic nanoparticles involves their immobiliza- tion on magnetic carriers, which allows them to be eliminated from the water suspension after the treatment using an external magnetic field [4–22]. Simple magnetic iron oxides (maghemite or magnetite) and different ferrites have been used as magnetic materials for the carriers. The use of superparamagnetic nanoparti- cles is beneficial, because they do not show spontaneous magnetic moments, and, in contrast to larger ferri/ferromagnetic particles, they do not agglomerate due to magnetic dipole–dipole inter- actions. The size where the nanoparticles transform into the superparamagnetic state is relatively low, below approximately 20 nm for soft-magnetic materials such as maghemite. Due to the small size, the forces acting on the individual superparam- agnetic nanoparticle in the magnetic-field gradient are generally too weak to enable their efficient, industrial, high-gradient mag- netic separation (HGMS) [23]. The magnetic separation is much more effective for clusters of superparamagnetic nanoparticles. Such clusters retain the superparamagnetic nature of the individ- ual nanoparticles, whereas their relatively large volume enables their effective HGMS [23]. As carriers for the photocatalyst, 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.03.059