**1. Introduction**

The optical, magnetic, and photocatalytic properties of wide bandgap metal oxide semiconductors (MOS) are easily tunable by adjusting the defect concentration, attaining great attention in the scientific research community [1, 2]. The position of the defect levels significantly influences the photons of various absorption and emission energies, and the intensity of intrinsic magnetism is also affected by the number of unpaired electron spins created by the defect levels in MOS compounds [3]. Therefore, tuning the magnetic properties of the MOS nanoparticles by defect

engineering could be directly correlated with the optical as-well-as photocatalytic properties [1, 2]. The tuning of the absorption spectra by the defects of varying charge states helps prepare light-emitting diodes, optic-magnetic-based devices, or optically writable oxides by the d0 -magnetism various wavelengths of light [4, 5]. The nature of MOS and their recent research on n-type and p-type models were remarkable in many applications [6].

The MOS nanoparticles with a unique combination of magnetic and charge transport properties such as TiO2, ZnO, and SnO2 are attracting substantial attention from the academic and industrial community. From all these various MOS materials, TiO2 gains special attention due to its solid photocatalytic behavior and several other advantages like low cost, chemical and thermal stability, innocuity, and high refractive index [7, 8]. However, this wide-bandgap TiO2 semiconductor is activated to perform photocatalysis only under irradiation of ultraviolet (UV) light, which needs to improve for practical applications. Many investigations have been reported and strategies to enhance TiO2 photo-absorption capability [9–13]. Various strategies to improve photo-absorption, doping, co-doing, surface grafting, the combination of surface grafting and doping are efficient and established routes [14–18]. Suppose MOS nanoparticles are sitting in the core. In that case, the structure of MOS composite nanomaterials could be divided into four forms: core-shell, matrix-dispersed, Janus, and shell-core-shell structures, as shown in **Figure 1**.

For example, metal-doped TiO2 nanoparticles improve the bandgap from the range of wide to mid-level electronic states, which imparts enhancement in charge migration or produces a strong redshift in the photo-absorption spectrum. More

**Figure 1.**

*Various structures of magnetic MOS composite materials. Blue spheres indicate the magnetic MOS nanoparticles, and the non-magnetic matrix and secondary materials are shown in another color [19].*

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

emphasis has been explained in recent years on the [SnxTi1 − xO2] system by coupling TiO2 with SnO2 oxide. It is highly acceptable that these new nanocomposites exhibit high photocatalytic activity compared to pure TiO2 [20]. The simple hydrothermal synthesis route will produce SnO2-TiO2 nanocomposites; however, a small variation in the synthesis condition could lead to the formation of distinct secondary phases [21]. Cao *et al*. reported that annealing temperature strongly influences the Sn4+ ions doping into TiO2 lattice, depends on temperature, which may substitute in lattice and exist as secondary phases like SnClx or SnO2 [22]. Sn-doped TiO2 nanoparticles showed significant enhancement in performance as components of active visible light photocatalyst [23, 24], lithium-ion batteries [25], antibacterial activity [26], dyesensitized solar cells [27], photo-electrochemical conversion [28] and water splitting [29] has been reported. It is important to find a reliable way to synthesize Sn-doped TiO2 nanostructures, as TiO2 and SnO2 are environmentally benign, highly stable, and strong oxide materials [30, 31]. We developed a simple hydrothermal method to synthesize Sn-TiO2 nanocrystals with sufficient oxygen vacancies, in this nanocrystal with different concentrations of Sn observed ferromagnetism and excellent photocatalytic activity [32, 33]. Wang *et al.* reported that Sn doping and Sn-Fe co-doping in TiO2 showed a strong red-shift in the optical absorption spectrum [34]. The reason for this shift in absorption spectrum in the Sn-doped TiO2 system comes from the most of the Sn 5 s states are located at the bottom of the conduction band where Ti 3d states are present and mixed with them.

The combination of non-transition metal and non-metal co-doping improves the visible-light activities of MOS materials. The non-metal doping in TiO2 can make the new extra valance band and non-transition metal doping create the additional charge carrier traps, which improve the separation efficiency of photo-generated electron–hole pairs, reducing the bandgap width, and broadening the photo-absorption limit [35, 36]. Therefore, the combination of metal and non-metal co-doping will be applied to drastically enhance the visible-light photocatalytic performance of TiO2. Among the various non-metals, nitrogen is an effective and promising candidate because N doping modifies the charge transport properties of TiO2 along with which also induces the oxygen-defect sites, therefore improving the photocatalytic performance [37]. The substitutional nitrogen doping on TiO2 showed an effective reduction in the bandgap width [38]. The nitrogen atoms were successfully substituted by either titanium or oxygen vacant atomic sites in the lattice of TiO2 lattice. Asahi *et al*. reported that nitrogen atoms successfully replaced the oxygen lattice sites and reduced the bandgap width by mixing N 2*p* and O 2*p* states [39]. Wang *et al*. have studied that the TiO2 nanocrystals were compacted closely together to form the solid TiO2. By doping nitrogen, some extra impurity levels were distributed on the surface of the TiO2 [40], as shown in **Figure 2a**. The solid TiO2 with a close packing structure creates the difficulty of nitrogen doping into the bulk structure of TiO2 and makes the diffusion of nitrogen difficult. However, the addition of the dodecyl tri-methyl ammonium bromide (CTAB) to TiO2 nanocrystals produces a loose packing mesoporous structure, which is conducive for TiO2 to take up ammonia into the interspaces.

Compared to undoped mesoporous TiO2, the nitrogen-doped mesoporous TiO2 with uniform distribution from the inside out produced successive energy levels from the bulk to the surface (**Figure 2b**). This subsequent impurity energy-band level formed by nitrogen doping are located above the valence band and successfully reduces the bandgap of the mesoporous TiO2, which is the primary attribution for the improved photocatalytic activity throughout the visible-light range. Zhuang *et al*. have reported that the facile sol–gel method prepared Sn and N co-doped TiO2 (SNT)

### **Figure 2.**

*Schematic diagrams depicting the band structures of (a) solid and (b) mesoporous TiO2 before and after doping on N [41, 42].*

photocatalysts. The post-nitridation treatment enhances the photocatalytic performance of co-doped TiO2 under visible light or simulated solar light irradiation [43]. However, more studies are required to clearly understand the effect of doping on the physical, chemical and catalytic properties of SNT microspheres.
