**1. Introduction**

Nanostructured semiconductor-based photocatalysts have proven tremendous candidates after the outstanding water-splitting discovery by using TiO2-based electrodes in 1972 [1]. Owing to the band structure properties, semiconductors have the ability to perform and enhance the redox reaction with different light exposure [2]. The nontoxicity, ease of availability, and cost-effectiveness properties of semiconductors make them fascinating for environmental and energy applications [3–5]. Several nanostructured semiconductors such as ZnO, CuO and TiO2, WO3, and V2O5 have been employed for different energy and environmental applications [6–10].

In comparison to other semiconductors, TiO2 has been found more beneficial for photocatalytic and energy generation devices due to its high photostability, unique band gap, lower cost, and non-toxic nature [11]. TiO2 is known to be an n-type semiconductor and is the most explored towards energy harvesting and energy generation applications owing to its fascinating electrical and optical behavior [12]. TiO2 contains three crystalline phases with wide bandgap; anatase rutile (3 eV), brookite (3.1 eV), and (3.2 eV) [13]. Among these phases, the rutile phase is more stable as compared to the brookite and anatase phases. Brookite and anatase phases are stable only at low temperatures and can be transformed into rutile phases by using high-temperature thermal annealing. The anatase phase is found to be more efficient as a photocatalyst as compared to both other phases of TiO2. Rutile TiO2 is the most stable phase, while. As compared to the rutile and brookite phases, anatase TiO2 is more suitable for energy-harvesting reaction processes [13]. The formation of the biphasic TiO2 is also one of the effective approaches to improving the charge separation in TiO2 without any external modification. Several research groups have demonstrated the improved photodecomposition ability of mixed-phase TiO2 in comparison to single-phase TiO2 [14–17]. Singh et al. [13] used the hydrothermal method and synthesized mixed-phasic TiO2 nanoflowers and used them as photocatalysts for the water remediation applicators. They have shown that mixed-phase TiO2 nanoflowers exhibited high photocatalytic activity owing to the creation of the heterojunction interfaces among the rutile and anatase phases of TiO2.

As compared to the bulk, nanostructured TiO2 contains superior photocatalytic efficiency due to their effective active sites and high surface-to-volume ratio, which provides a strong tendency for molecular interactions. TiO2 with a wide band gap (3.2 eV) absorbs ultra-violet light, following the charge separation, yielding the photoinduced electrons in the conduction band and the complementary holes in the valance band. These photo-generated carriers are short-lived, so they quickly recombine and result in diminishing photocatalytic efficiency. In order to resolve these issues, several methods have been adopted by various research groups, such as doping with metals [18], non-metals [19], and, more recently, through surface modification by noble metal nanoparticles [20–22]. Metal nanoparticles functionalizing TiO2 duplicate as an electron sink capturing electrons from TiO2 and also help to furnish more charged carriers using its localized electric field or Surface Plasmon Resonance (SPR) [20, 21]. With the attachment of plasmonic metal nanoparticles (MNP), the reduction in the recombination rate takes place by the migration of electrons from the conduction band of TiO2 to MNP. In addition, the electromagnetic field generated by the plasmonic nanoparticles attached over the TiO2 surface under electromagnetic radiation also helps to reduce the recombination rate. Owing to the SPR effect of plasmonic nanoparticles, the plasmonic nanoparticles modified TiO2 enable visible light adsorption. Thus plasmonic nanoparticles functionalized TiO2 nanostructures are expected to exhibit superior energy harvesting efficiency as compared to bare nanostructured TiO2 [23]. **Figure 1** reveals the working mechanism of the plasmonic nanoparticles functionalized TiO2 nanostructures. Bare TiO2 nanostructures contain a high probability of charge recombination. With the modification with plasmonic nanoparticles, efficient charge transfer occurs; consequently, a reduction in the recombination rate takes place. A high density of electrons in the conduction band produces superoxide radicals by reacting with the surface oxygen, while holes transform the water molecule into hydroxyl radicals [23]. These two unsaturated radicals effectively control the different energy generation and environmental applications, such as H2 production and water purification, respectively. Plasmonic nanoparticles functionalized TiO2 nanostructures

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*Plasmonic-TiO2 Nanohybrid for Environmental and Energy Applications DOI: http://dx.doi.org/10.5772/intechopen.111524*

### **Figure 1.**

*Scheme reveals the efficient charge transfer mechanism in TiO2 using plasmonic noble metal nanoparticles.*

significantly improve its efficiency in various applications such as sensors [24], solar cells [25], photocatalytic activity [26], energy storage [27], and energy production [28]. Various physical and chemical techniques such as sol-gel [29], impregnation method [30], sputtering [31] pulse laser deposition method [32], and photo-deposition method [33] have been embraced by different research groups for the fabrication of plasmonic nanocomposites such as Ag-TiO2 and Au-TiO2.
