**3. Visible light photocatalysts**

Progressive research towards solar power-based energy conversion, wastewater treatment, and efficient photocatalysts attracting great attention [67–70]. Photocatalytic and photovoltaic solar cells convert solar-based light energy into chemical reaction and electrical power generation. Consequently, improving the stabilizations of photo-induced charge carrier transportation is the critical factor for light-harvesting systems. TiO2-based materials are widely used in environmental and energy-related applications like photocatalysis, photovoltaics, artificial photosynthesis, and spintronic, which have been often foreseen. For better performance, TiO2 is usually employed as nanocrystals or nanostructures [71–73]. However, the efficiency of photocatalytic activity of TiO2 needs to improve to induce charge carrier activity using visible light or sunlight. Noble metal (Pt, Pd, Rh, and Au) doped and modified TiO2 photocatalysts have been attracted great attention towards efficiency enhancement [74–76]. Especially in this context of an investigation, Ag-loaded TiO2 that is Ag cluster-incorporated AgBr nanoparticles [77], Ag nanoparticles and CuO nanoclusters [78], and Ag/AgCl [79] in TiO2 photocatalysts are undoubtedly intriguing to attain high performance [80]. The interfacial heterojunction between TiO2 and SnO2 particles can have a synergetic effect on photo activity [24]. Furthermore, any agglomeration in TiO2/Ag/AgCl system due to the nature of the materials process used can influence the observed photocatalytic activity given that Ag/AgCl is a plasmonic system.

Therefore to improve the photocatalytic performance of metal oxide nanoparticles by expanding the range of photo-response and increasing the efficiency of electron– hole carrier separation, the hierarchical assembly of nanoscale building photocatalytic blocks with a tunable dimensionality and structural complexity offers a practical strategy towards the realization of multi-functionality of nanomaterials [81]. In general, hierarchical heterostructures are formed by connecting two different lowdimensional nanostructure materials; this type of structure provides the ultrahigh specific surface area and a network system consisting of parallel connective paths and provides interconnection of various functional components [82].

Liu et al., in their work, explained the photocatalytic mechanisms operating in the Fe(III)-FexTi1−xO2 system as illustrated in **Figure 4**. are discussed [17, 18]. They are owing to the wide bandgap of pristine TiO2, which is inactive under the illumination of visible or sunlight. However, by the selected surface grafting and bulk doping of Fe(III) ions, which have band energy levels identical to TiO2, the visible-light absorption of TiO2 is drastically improved by the bulk-doped Fe(III) ions. The QE was unaffected because of the efficient transfer of electrons between doped Fe(III) and surface Fe(III). Moreover, a good interface junction between surface-grafted and bulk-doped Fe(III) ions is needed for efficient charge carrier transfer. Notably, the visible-light activity reaction was markedly reduced by introducing a thin layer between the

**Figure 4.**

*(A) Proposed photocatalysis process. (B) Change in bandgap and photo-activity by Fe doping [17, 18].*

surface Fe(III) ions and doped TiO2. The photo-generated charge carriers are effectively transferred to the surface of Fe(III) doped TiO2, which acts as an efficient co-catalyst for multi-electron reduction reactions. In photocatalysis by Fe(III) doped TiO2, holes with high oxidation potential are kept in the deep level of the valance band and effectively decompose the organic compounds. Therefore, efficient visible-light photocatalysts with high R is achieved.

The conceptual ferromagnetic photocatalysts show a better charge carrier separation function to take advantage of high activity in the couple, doped, surface modified, or co-doped semiconductor nanocomposites. However, furthermore development in these TiO2-based photocatalysts requires other strategies to improve photocatalytic efficiency. In today's research, one of the effective strategies is AgCl nanoparticles loaded in Sn-doped TiO2 microsphere to enhance the visible-light activity have become an essential outcome in the photocatalytic and photovoltaic applications [83, 84].
