**4. Ferromagnetic TiO2-based photocatalyst**

In our previous reports, we worked on various concentrations of Sn doping to improve the structural, electronic, magnetic, and photocatalytic properties of TiO2 nanoparticles [32, 33, 85, 86]. Significantly, the study of room temperature photocatalytic and ferromagnetic performance in the Sn-doped TiO2 nanoparticles is one of the most emerging and fascinating fields in environmental remediation. Adding various concentrations of SnCl4 in Ti(NO3)4 aqueous solutions produced any one of the anatase, a mixture of anatase-rutile and rutile phases of TiO2 nanoparticles with the added Sn atoms, which are synthesized using the facile hydrothermal method. To study the photocatalytic performance of the synthesized Sn-TiO2 nanoparticles, both methyl orange (MO) and *R*PhOH (where PhOH is phenol group and *R* is 3-NH2, H, and 4-Cl) in water were chosen as model pollutants under both the illumination of visible light and UV light irradiation. Light irradiation showed a significant relationship between the Hammett substitution constant (σ) of RPhOH and the photocatalytic degradation efficiency of Sn-TiO2 nanoparticles. The concentration of Sn doping significantly affected the structural, electronic, magnetic and, photocatalytic properties of the TiO2 nanoparticles. Even after decade-long research, the actual mechanism of ferromagnetism combined with photocatalytic behavior in these materials is still not understood. However, hints about some of the critical factors that contribute to magnetism have been revealed. It is believed that oxygen vacancies, phase changes,

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

and doping level play a significant role in the RTFM of semiconductor oxides; however, demonstration of a direct correlation between the magnetism, dopant concentration, oxygen vacancies, and photocatalytic activity has been strenuous. Because of these reasons, in this work, we made an effort to investigate the essential role of Sn4+ ions on the above properties of TiO2 nanoparticles.

In another report, we first follow the facile hydrothermal synthesis route for preparing ST microspheres, followed by nitriding treatment by flowing an ammonia gas to successfully fabricate hierarchical SNT microspheres with VO [64]. The fabricated as-prepared samples are characterized by the conventional analytical techniques and 119Sn Mössbauer spectroscopy to understand the structure, magnetism, and photocatalytic performance. The main objective of this study is to improve the photocatalytic performance and RTFM of TiO2 by the co-doping of Sn and N atoms. As compared to pristine and Sn doped TiO2 nanoparticles, SNT microspheres showed significant absorption of visible light for photocatalytic activity is observed. Then we have further studied the photocatalytic movement of Rhodamine B (RhB) degradation under the illumination of visible light irradiation on pristine TiO2, P25, ST, and SNT microspheres and observed vigorous photocatalytic activity in SNT microspheres. However, until now, no one reported magnetic studies on the SNT microspheres. Suppose, if the photocatalysts exhibit RTFM, the phenomenon may insist on the electrons trapped in VO or structural defects. In this aspect, we can believe that this study can be implemented in the various other types of facile designing semiconductors to obtain an insight into the role of the visible light photocatalytic performance, RTFM behavior, and combined performance enhancement. In addition, we also studied the photovoltaic performance of ST and SNT microspheres in the applications of Perovskite solar cells. The combined mental and nonmetal doped TiO2 nanoparticles with other structural defect sites represent a new kind of semiconductor materials and provide novel opportunities for TiO2-based materials.

For the first time, we have reported a facile hydrothermal synthesis route to successfully fabricate hierarchical AgCl in Sn-TiO2 (AST) microspheres using post-calcination treated with different temperature samples [66, 87]. The primary objective of this study is to modify Sn doped TiO2 by loading AgCl nanoparticles to enhance photocatalytic performance. Improved visible light absorption capability was observed in the AST microspheres compared to Sn-TiO2, AgCl, Ag/AgCl, and commercial Degussa P25 photocatalysts. To check the photocatalytic performance of the as-synthesized AST microspheres, the rhodamine B (RhB) and 3-nitrophenol aqueous solutions were used as the model systems under visible light (λ ≥ 420 nm). The obtained results indicate that the hierarchical AST microsphere photocatalysts showed a higher photodegradation rate than Ag/AgCl, AgCl, Sn-TiO2, and the commercial TiO2 (P25) materials. However, the study on various concentrations of AgCl in the AST microsphere is crucial to understand the optimized amount needed to obtain the best photocatalytic performance. To the best of our knowledge, for the first time, we reported the facile preparation route, high visible-light photocatalytic performance in hierarchical AST microspheres, and the magnetic behavior of these photocatalysts characterized by the 119Sn Mössbauer technique. The new semiconductor family of noble metal halide and metal-doped TiO2 nanoparticles opens up novel opportunities for TiO2-based materials.

We have option [Fe(III)(bipy)2Cl2)]+ [Fe(III)Cl4] − ionic salt-like complex as precursor complex [73]. The aqueous solution of precursor complex could behave like electrolytes. While the reduction potential from free Fe(III) to free Fe(II) is 0.77 V, that of photo-reduction from [FeIIICl4] − to [FeIICl3] − is 0.34 V which indicates that photoreduction of the [FeCl4] − ion is easier than the normal chemical reduction of free ferric ions [12]. Hence chosen iron(III) complex interacts with n-type TiO2 semiconductors.

It reduces Fe(III) to Fe(II) *via* interfacial electron transfer dynamics under dark (poor efficiency), near-UV (good efficiency), and visible light (moderate efficiency) irradiation systems. At the same time, the precursor complex is adsorbed on the TiO2 surface to form a surface complex; it acts as a co-catalyst for the reduction of Fe(III) to Fe(II) with TiO2. However, there are no reports on the study of photosensitized *via* IFET dynamics between Fe(III)-bipy complex (bipy without -OH or -COOH groups) and titania semiconductor interface until now. Hence, we report the near-UV and visible-light-induced IFET process on [FeIII(bipy)2Cl2][FeIIICl4] (precursor complex) with TiO2 NPs, and the photochemical product was mainly characterized by electronic absorption, Fe K-edge X-ray absorption fine structure (XAFS), electron paramagnetic resonance (EPR) and 57Fe Mössbauer spectroscopes method. In addition, electron transfer was confirmed by cyclic voltammetric and photoluminescence measurements. However, the following factors control the IFET reaction, those are (i) the presence of TiO2 nanoparticles, (ii) the irradiation time-lapse, (iii) light source with various wavelengths (380 ≤ λ ≤ 520 nm), and (iv) different types of TiO2 nanoparticles.

In one of our works, nickel(II)-imidazole-anatase nanocomposites prepared by a simple adsorption method showed room-temperature ferromagnetism and good photocatalytic performance, which were designed by mixing of [Ni(1-MeIm)6] Cl2H2O complex and anatase TiO2 starting materials in an aqueous medium [64]. Various conventional techniques as adsorption already elucidated the deposition of the surface species. We observed the ferromagnetic behavior in the composite sample under the vibrating sample magnetometer at room temperature. This Ni-dopedTiO2 nanocomposite has good visible light absorption ability than pristine TiO2. To understand and evaluate the adsorption and photocatalytic activity of the Ni-doped TiO2 nanocomposite, selected methylene blue (MB) as an organic pollutant illuminating under visible light irradiation. We first reported the Ni(II)-imidazole complex deposited on the anatase (TiO2) semiconductor with good photocatalytic and magnetic properties prepared by a simple adsorption method. The research of metal oxidebased photocatalysis is expected to open up a general method for synthesizing other transition metal-loaded metal oxide semiconductor photocatalysts.

In all of our previous reports covers the studies related to Mössbauer spectroscopic, photocatalytic and magnetic investigations of Sn and Fe doped TiO2 nanocomposites [32, 33, 63–66, 73, 85–87]. Using the facile hydrothermal synthesizing route, we prepared Sn-based TiO2. For structural and magnetic characterization, Mössbauer spectroscopy has unique advantages to mature into one of the classical techniques for Sn or Fe-based TiO2 nanoparticles. Mössbauer spectroscopic results provided a strong understanding and evidence of the relationship between the structural, photocatalytic, and magnetic properties of Sn or Fe-based TiO2 nanoparticles. The Sn or Fe-doped TiO2 nanocomposites have promising applications in photocatalysis for water purification by degrading organic pollutants using efficient visible light absorption to produce strong stability and high photocatalytic activity. This review helps in the fundamental understanding of structural and magnetic properties of Sn or Fe-doped TiO2 nanocomposites and their contribution towards environmental remediation by visible-light photocatalysis.
