**2. Diluted magnetic semiconductors**

Diluted magnetic semiconductors (DMS), referred to as doping of magnetic impurities in bulk semiconductors, also called "semi-magnetic semiconductors", have been studied. This concept has had a particular interest in the research community for the past few years because ferromagnetism in diluted magnetic semiconductors (DMS) has been another important subject that can manipulate the carrier-associated charge and spin-based parameters [44, 45]. Especially, DMS with room temperature ferromagnetic oxides gained particular attention in the applications of magnetic fluids, biomedical, magnetic resonance imaging, catalysis, and environmental remediation [46, 47]. Wang *et al*. developed a facile method to synthesize ZnO crystals with Zn vacancies, and these doped Zn vacancies created p-type conductivity, room-temperature ferromagnetism, and excellent photocatalytic performance [48]. The recent development of ferromagnetic ordering in photo-induced transition metal-doped TiO2 nanoparticles can be justified by creating defects in the samples [15]. However, the actual role of dopants (e.g. transition metals) at the room temperature ferromagnetism in TiO2 nanoparticles is still an unclarified problem [49]. In one of our recent papers, our group proposed a new model for combined mechanics of ferromagnetism

### *Tuning the Magnetic and Photocatalytic Properties of Wide Bandgap Metal Oxide… DOI: http://dx.doi.org/10.5772/intechopen.110422*

and their photocatalytic activity in wide-band-gap metal oxide-associated nanocomposites [32]. The study of ferromagnetism and photocatalytic activity on synthesized metal oxide-based nanocomposites suggesting a significant role of oxygen vacancies present on the surface and improved charge carrier concentration on magnetism and photocatalytic performance [50]. Charanpahari *et al*. reported room-temperature ferromagnetic nanocomposites showing better photocatalytic performance compared to commercially available diamagnetic photocatalysts under visible light irradiation [51]. Doping and co-doping have the advantage of high activity in semiconductor nanocomposites, which imparts the concept of magnetic photocatalysts with charge carrier and separation function was raised [51, 52]. Hence, in the research of photocatalytic activity today researchers are focusing on the development of photocatalyst possessing ferromagnetic property and visible-light activity.

DMS with room temperature ferromagnetism has been extensively studied for the applications of spin-based field-effect transistors, spin-based light-emitting diodes (LEDs), and non-volatile memory devices [53, 54]. In DMS materials are due to the coupling of magnetic ordering with one of the other types of ferroic ordering parameters like ferroelasticity or ferroelectricity, which are very interesting from the standpoint of device applications in fields such as spintronic and magneto-optics. Therefore, DMS offering certainly promising immense opportunities for new next-generation applications [55]. Theoretical and experimental studies on these metal oxides have shown improved ferromagnetism by the presence of defects or lightweight doping elements like C, N, and Li [56]. The addition of light elements in DMS can develop magnetism and significantly stabilizes the intrinsic defects in the oxide materials [56]. In these systems, the improved ferromagnetism is mainly attributed to the following mechanisms (i) the concentration of the oxygen vacancies (VO) and defects sites and (ii) the substitution of an oxygen atom with the doping element and associated formation of spin-polarized states in the bandgap and (iii) the change of titanium oxidation state (Ti3+) in the occurrence of ferromagnetic order. Therefore, defect engineering is a powerful tool to tune or improve the functional properties of the metal oxides like their electronic band structure, charge carrier transport, and catalytic performance [48]. The photocatalytic performance of TiO2 significantly depends on their electrical and optical properties, which are primarily determined and altered by the crystal structure, optimized concentration of dopants, and defects [57].

**Figure 3**(**A**) showing the schematic diagram of the magnetic orientation of Fe doped TiO2 nanoparticles, which are annealed under vacuum. It shows the possible paramagnetic species, their distribution in the nanoparticles lattice, surface, and interfacial boundary, and the potential interaction with ferromagnetic or antiferromagnetic species. The red circles inside the nanoparticles representing the magnetic polaron and overlapped magnetic polarons form BMPs. Along with BMPs, coupled F+ centres on the surface and interface also contribute towards ferromagnetism. However, F2+ without any electrons and F Centre with two trapped electrons are not likely to contribute towards ferromagnetism [58]. In vacuum annealed pristine TiO2 nanoparticles, the total magnetization is contributed from the surface and interfacial oxygen vacancies, i.e. Mtotal = Msurface + Minterface. However, an extra BMP factor is added in the Fe doped vacuum annealed TiO2 nanoparticles; therefore, the total magnetization is written as Mtotal = MBMP + Msurface + Minterface. These observations of paramagnetic behavior in Fe doped TiO2 nanoparticles suggest that the density of oxygen vacancies is possibly insufficient to generate solid ferromagnetic coupling with the nearest lattice site of Fe3+ ions. To improve the magnetization in pure and 2%

### **Figure 3.**

*(A) Diagram represents various possible magnetic species, their distribution, and interaction [58]. (B) M–H curves of vacuum annealed nanoparticles of (a) pristine TiO2 and, (b) 2% Fe doped TiO2 at room temperature, (c) 2% Fe doped TiO2 at 20 K and, (d) paramagnetic M–H curve of vacuum annealed 2% Fe doped TiO2 after reheating in the air at 450°C [58].*

Fe doped TiO2, vacuum annealed at 200°C for 3 h, generating donor carrier or oxygen vacancies. M–H measurements are carried out after the annealing on the samples, and as plotted in **Figure 3**(**B**), initially diamagnetic pristine TiO2 and paramagnetic Fe doped TiO2 nanoparticles both have exhibited ferromagnetism. The observed ferromagnetism in pure TiO2 nanoparticles could be attained from either Ti3+ ions or the presence of oxygen vacancies on the lattice site or the surface. Even though pristine and Fe doped TiO2 showed ferromagnetically, the saturation magnetization of pure TiO2 is less than that of Fe doped TiO2 nanoparticles. The enhanced magnetization in Fe doped samples could be due to the extra magnetic interaction generated by both Fe dopants and defects in the ferromagnetic exchange coupling. The ferromagnetism is again switched back to paramagnetic for reheated vacuum annealed Fe doped TiO2 in the air at 450°C samples as shown in **Figure 3**(**B**)**d**. The above results support that the oxygen vacancies possibly play the driving role in switching the magnetic ordering from paramagnetic to ferromagnetic and then back to paramagnetic in Fe doped TiO2 nanoparticles. Just simple doping of Fe may not be sufficient to induce ferromagnetic solid exchange interaction. Only, when a high concentration of oxygen vacancies and Fe doping combining may participate in ferromagnetic exchange interaction.

Irradiation of various energy ion beams is one of the sophisticated techniques for incorporating the defects (i.e., vacancies, interstitials, etc.) into transition metaldoped metal oxide semiconductor matrix materials. Many researchers have studied that ion beam irradiation could improve the structural complexity of the ZnO nanoparticles by dissolving the secondary impurity phases, helps in substitutional incorporation of Mn2+ at the Zn2+ site (Mn and Zn) and improves the ferromagnetic property of the samples [59–61]. To avoid the segregation of nano-dimensional doped transition metal or its oxide clusters and to induce intrinsic structural defects in the host material in a controlled fashion, irradiation of a low energy ion beam using inert gases such as Xe or Ar is the best option which also eradicates the complexities arising from the chemical reactivity of the ion beams [60]. A multilayer coating and hightemperature calcination, thus affecting the photocatalytic efficiency, often influence the magnetic properties [62]. Therefore, a novel and facile approach to the low-cost

*Tuning the Magnetic and Photocatalytic Properties of Wide Bandgap Metal Oxide… DOI: http://dx.doi.org/10.5772/intechopen.110422*

preparation of the ferromagnetic and photocatalytic TiO2 nanocomposite at relatively low temperatures is highly recommended. We have reported several research articles related to the photocatalytic performance and magnetic properties of TiO2-based photocatalysts such as various metal (Sn, Cu and, Fe) oxide coupled TiO2 [32], Sn doped TiO2 [33], Fe2O3 coupled. Doped TiO2 [63], nickel(II)-imidazole doped TiO2 [64], hierarchical Sn and N co-doped TiO2 [65] and hierarchical AgCl loaded Sn doped TiO2 [66].
