Photocatalytic performance of magnetically separable Fe, N co-doped TiO2-cobalt ferrite nanocomposite
Graphical abstract
Introduction
Semiconductor photocatalysis has attracted great interest for the degradation of organic pollutants present in water [1]. TiO2-based materials are the most popular ones for this purpose because of their chemical stability, high photocatalytic reactivity, and nontoxicity [2], [3], [4], [5], [6], [7], [8], [9]. In order to utilize TiO2 effectively for the photocatalytic reaction, the bandgap of TiO2 has to be decreased to less than 3 eV. Besides, the diffusion length of photogenerated electrons and holes has to be sufficiently long to reach the surface of the photocatalyst and initiate the redox reactions. Alteration of the bandgap of TiO2 has been done by doping it suitable cations or anions.
Dopants such as Cr, Ni, Mn, Cu, Fe, La, V and W [10], [11], [12], [13], [14], [15] have been used for the modification of the optical and photo-electrochemical properties of TiO2. Kim et al. [16] carried out a systematic investigation of the photocatalytic activity of TiO2 doped with different metal ions. It was found that doping with metal ions may extend the photo-response of TiO2 into the visible region by introducing additional energy levels within the band gap of TiO2. Non-metal dopants like N, F and S [17], [18], [19] are also utilized for modifying the band gap of TiO2 and in all cases, an improved optical absorption and photocatalytic activity are reported for these systems. Xu et al. [20] proposed that absorption edge of TiO2 was shifted to the visible light range by co-doping with both N and B. Transition metals, like Fe3+ ion can minimize the recombination of photogenerated charge carriers and can enhance the photocatalytic activity of TiO2 [21].
Another problem which has to be addressed in the photocatalytic process is the recovery of the photocatalyst after the effluent treatment. As nanomaterials are used commonly for these applications, their recoverability has been one of the challenging jobs. Incorporation of nanocrystalline TiO2 on alumina, cellulose etc. has been done to induce faster sedimentation of the photocatalyst so that it can be removed by decantation. Another method employed is to immobilize TiO2 on rigid substances like glass so that it can be removed easily [22]. However, these methods are either time consuming or the synthesis procedure is not very simple. Reusable magnetic and photocatalytic hybrid nanocomposites are synthesized to overcome this problem [23], [24]. These composites not only ease the separation process but also improve the surface area and optical absorption properties. Composites of cobalt ferrite with TiO2 [25], graphene [26], g-C3N4 [27], silica [28] and PANI [29] have been studied for photocatalytic reactions. In all cases, enhanced photocatalytic activity is reported. The improved activity is attributed to increased optical absorption, increased surface area or increased lifetime of the photogenerated charge carriers.
In the present work, we have investigated the photocatalytic properties of a new combination of magnetically separable photocatalyst, which is Fe, N co-doped TiO2 decorated CoFe2O4. The nanocomposite showed improved visible light absorption and increased photocatalytic activity for the degradation of methyl orange compared to TiO2.
Section snippets
Synthesis
The most common and widely used method for the synthesis of CoFe2O4 is the co-precipitation method using an alkali [30]. In the present work, cobalt ferrite (CoFe2O4, CF) nanoparticles were synthesized by auto combustion method [31]. The precursors used are commercially available Co(NO3)2·6H2O (99%), Fe(NO3)2·6H2O (99%) and NH3 (38%) solution. Stoichiometric amounts of Co (NO3)2·6H2O and Fe(NO3)2 were dissolved in distilled water and to this citric acid solution was added. The molar ratio of
Characterization
Powder X-ray diffraction patterns of TiO2, FeN-TiO2, CF, TiO2-CF and FeN-TiO2-CF are shown in Fig. 1 (in the composite the amount of TiO2/doped TiO2 is 30 wt%).
It is seen that both TiO2 and FeN-TiO2 exist as anatase phase of TiO2 (JCPDS card no. 89-4203) and no peaks corresponding to any other phase is seen. CF could be indexed as the cubic spinel phase (JCPDS card no. 22-1086). XRD patterns of TiO2-CF and FeN-TiO2-CF show peaks corresponding to both TiO2 and CF suggesting that TiO2 and CF exist
Conclusion
Recyclable and magnetic cobalt ferrite coupled with Fe, N co-doped TiO2 exhibit photocatalytic activity for the degradation of methyl orange when irradiated with visible light. Doped TiO2 exists as anatase phase of TiO2 and cobalt ferrite as cubic spinel phase in the composite. Co-doping of TiO2 with Fe and N decreases the band gap of TiO2 and shows increased visible light absorption. The Fe and N co-doped TiO2 shows the highest photocatalytic activity compared to other composites and TiO2. The
Acknowledgements
Authors PPH and PNG are very much thankful to BRNS, Trombay, Mumbai for financial assistance through Major research project no. 2012/37C/58/BRNS/2618 and UGC-BSR faculty fellowship.
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