**6. Magnetic particles deposited TiO2 photocatalyst**

Though these noble metals dopant/hybrid had a significant contribution on the photocatalytic mechanism of TiO2, they are highly expensive and further increasing the cost of the prepared photocatalyst. Most of the modification techniques solve the issues of photocatalytic efficiency however, leaving behind the separation difficulty. Such separation could make them reuse and contribute for economy of the treatment cost. Most commonly these photocatalysts are nanoparticles and requires high speed centrifugation or membrane filtration. However, adopting these techniques further burdens the economy of the treatment. Hence doping/hybridizing the photocatalyst with a ferromagnetic material could be a best alternative.

Hence coating the photocatalyst with magnetic particles emerges to be a promising method to separate the photocatalyst from treated stream [76]. Nanoparticles combine with magnetic core and photoactive shell using magnetic granules and semiconductor photocatalyst is reported to possess magnetic property and separation could be achieved easily by applying simple magnetic field [77]. For these purpose magnetic cores such as magnetite (Fe3O4), maghemite (Fe2O3), nickel ferrite (NiFe2O4), etc. were chosen. Though separation of photocatalysts was achieved their photocatalytic activity was found to decrease than that of pure TiO2 [8, 78, 79].

Beydoun et al. prepared the magnetic photocatalyst by coating TiO2 particles onto Fe3O4 particles. They observed that the magnetic core was easily oxidized and suppressed the photoactivity of the TiO2 [76, 80, 81]. Alternatively, Chen et al., 2001 prepared the magnetically separable photocatalyst by coating TiO2 particles onto γ-Fe2O3 particles. Their preparation method transformed ferromagnetic γ-Fe2O3 to α-Fe2O3 paramagnetic material and resulted in poor separation efficiency [79]. Such phase transformation from γ-Fe2O3 to α-Fe2O3 was triggered by annealing temperature. Therefore, difficulties arise to synthesize TiO2 coated particles with high photoactivity without loss of magnetic property by an iron oxide phase transition, as well as of high crystallinity without agglomeration, or formation of impurities by solid diffusion [9].

Chung et al., 2004 reported a TiO2-coated NiFe2O4 photocatalyst by multi-step ultrasonic spray pyrolysis method. Their complicated synthesize method resulted the photocatalyst in micron size. Owing to the micron size of the photocatalyst the activity and the separation efficiency declined drastically [4]. Similarly, Xu et al., 2007 prepared a magnetically separable nitrogen-doped photocatalyst, TiO2xNx/ SiO2/NiFe2O4 (TSN) by a simple method, which consists of a NiFe2O4 as magnetic core, a SiO2 as magnetic barrier and nitrogen as visible-light-active dopant. Their

prepared TSN was found to possess a great photocatalytic activity by removing MO in the presence of artificial UV and visible light illumination [9].

In recent years the M type hexaferrites, MFe12O19 (M = Ba, Sr., Pb, etc) gained interest over the spinel ferrite (NiFe2O4), since the magnetic properties of M type hexaferrites allow them to serve as highly stable permanent magnet. One such M type hexaferrites, strontium ferrite (SrFe12O19), is regarded as an excellent magnetic material [82]. There was so far no report discussed about nanoscale hexaferrites as carriers for magnetic photocatalyst before Fu et al., 2006 synthesized TiO2/ SrFe12O19 composite nanoparticles with core-shell structure. Despite the fact that the photocatalytic activity of the composite is slightly lower than that of Degussa P25, the separation of composite particles was well achieved with an external magnetic field, thus proved the separation incapability of commercial photocatalyst Degussa P25 [82].

Researches on protective layer-coated permanent magnets nanoparticles have been studied for both fundamental magnetic investigations and practical engineering applications. In such investigations, coated nanoparticles attracted the attention as the coating hinders the nanocomposites from coarsening and agglomeration. In practical engineering applications, coating works well in magnetic applications as an insulate phase to achieve high electric resistivity and behaves as a binder to ease the consolidation of the nanoparticles [18, 19].

Coating magnetic nanoparticles with silica (SiO2) is becoming a significant topic in the research of magnetic nanocomposites. The formation of SiO2 interlayer on the surface of magnetic nanoparticles helps to screen the magnetic dipolar attraction between magnetic nanoparticles. It also protects from leaching of the core magnetic materials during the dispersion in the aqueous phase. Moreover, SiO2 coating could be easily activated to provide its surface with various functional groups due to the presence of abundant silanol groups in it. Finally, SiO2 interlayer plays a very significant role in providing a chemically inert surface for magnetic layer. Hence inclusion of a protective SiO2 coating will suppress the electron–hole recombination rate that occurs in the photocatalyst and benefits both the photo and magnetic activity.
