**3. Energy applications of plasmonic-TiO2 nanohybrid**

In this section plasmonic-TiO2 nanostructure-driven energy production applications have been elucidated in detail. This section includes the applicability of plasmonic-TiO2 nanohybrids for photovoltaics applications. This section explains the basic mechanism responsible for the tremendous usage of plasmonic-TiO2 nanohybrid for photovoltaic applications. This section will expect to enhance knowledge and understanding of plasmonic nanostructures-based solar cells.

As we have discussed in the previous sections, the plasmonic nanoparticles can significantly control the charge kinetics in TiO2 nanostructures and can additionally provide visible light absorption ability. By using these outstanding properties, different research groups reported the improved performance of plasmonic nanohybridbased dye-sensitized solar cells (DSSC) [83–85]. After the discovery of TiO2-based DSSC in 1991 by O'Regan and Grätzel the development of DSSC has been made using other semiconductors [86]. DSSC known to be the third generation of photovoltaics has similar working as we observed in the photosynthesis process. Photoanode in plasmonic nanohybrids based DSSC formed by the layer of plasmonic-TiO2 nanohybrid which is further covered by the light-sensitive commercially available dye molecules such as black dye, which help to harvest solar light. Firstly, a light-sensitive dye molecule starts the conversion of light energy into electric energy by using a plasmonic-TiO2 photoanode, electrolyte, and counter electrode. The basic process in Plasmonic-TiO2 based DSSCs has been depicted in **Figure 9** below.

Dissanayake et al. [87] used Ag and Au-decorated TiO2 nanostructures via chemical reduction methods. The fabricated Ag-TiO2 and Au-TiO2 nanohybrids were employed to design the photoanodes of DSSCs. In their study, they highlighted that the obtained PCE values for pure TiO2, Ag-TiO2, and Au-TiO2-based photoanodes were 5.12%, 6.51%, and 6.23%, respectively. In addition, they have concluded that

**Figure 9.**

*Schematic diagram revealing the basic process of plasmonic-TiO2 based photoanodes of DSSC.*

Ag-functionalized TiO2 plasmonic nanohybrids exhibited higher PCE values compared to pure TiO2 and Au-functionalized TiO2 nanohybrids.

Vaghasiya et al. [88] fabricated Ag-TiO2 hybrid thin films and used them as photoanodes in DSSCs. The Ag-functioned TiO2 nanohybrids contained thin films were found to result in superior PCE than bare TiO2-based photoanodes. The computed values of Jsc, Voc, FF, and PCE for Ag-TiO2 thin films were found to be 5.7 mA/cm2 , 0.621 V, 54%, and 1.9%, respectively. The estimated Jsc, Voc, FF, and PCE for pure TiO2 thin films were 4 mA/cm<sup>2</sup> , 0.507 V, 46%, and 1.1%, respectively.

Guo et al. [89] prepared core-shell of Ag@TiO2 nanostructures and employed them to design DSSCs. They have modulated the Ag content for enhanced photocurrent density. In their photovoltaic studies, they have highlighted that the optimum concentration of Ag (0.15 wt %) provides the maximum photo conversion efficiency (PCE = 5.33%) in comparison to bare TiO2−based DSSC (PCE = 3.96%).

Lim et al. [90] successfully prepared Ag-functionalized N-TiO2 nanohybrids by using thermal annealing combined with the chemical method. The obtained nanohybrids were embraced to fabricate the photoanode of DSSC. They varied the Ag loading by varying the concentration from 2.5 to 20 wt% over the surface of N-TiO2 and obtained a significant value of PCE of 8.15%. **Figure 10(a-b)** showing the surface morphology of the Ag-TiO2 nanohybrids while their elemental mapping is presented in **Figure 10(c-d)**. The improvement in the charge separation due to the attachment of Ag nanoparticles over TiO2 is assured by PL spectroscopy (**Figure 10(e)**). They have highlighted that the PCE value is higher for the N-TiO2-based photoanodes as compared to the PCE of photoanodes based upon pure TiO2 (2.19%), N-TiO2 (2.93%) and Ag-TiO2 (4.86%) (**Figure 10(f )**). In their study, they showed that increments in the Ag content significantly improve the PCE of DSSC.

In another study, Wang et al. [91] prepared Ag nanoparticles functionalized TiO2 nanostructures via a hydrothermal process. They have used Ag nanoparticles to functionalize feather-like TiO2 nanostructures. They have varied the Ag content over the

### **Figure 10.**

*(a-b) SEM images of Ag functionalized N-TiO2 nanostructures, (c-d) Elemental mapping of Ag functionalized N-TiO2 nanostructures showing the existence of Ti, O, N, and Ag atoms, (e) Photoluminescence spectra of N-TiO2 and Ag decorated N-TiO2, (f) Current density characteristic of fabricated DSSC using TiO2, N-TiO2, and Ag decorated N-TiO2 nanostructures. Reprinted from the reference [90].*

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

surface of TiO2 varying from 1 to 7.5 wt %. They concluded that the optimum amount of Ag content on TiO2 can remarkably enhance the PCE value of Ag-TiO2-based photoanode. With the modulation of Ag loading on the TiO2 nanostructures, the obtained photo conversion efficiency increased from 6.19% to 6.74%. Bhullar et al. [92] studied the effect of Ag implantation in TiO2 thin films for DSSCs application. They have doped the Ag ions by varying the fluence value from 1013 to 1016 per cm2 . They have highlighted that due to the incorporation of Ag ion, the optical absorption is significantly improved and also controls the recombination rate, which significantly contributed to enhanced the PCE of DSSCs. They demonstrated that at a fluence value of 1014, the Ag-doped photoanode significantly showed 21.82% better efficiency as compared to the bare TiO2-based photoanode of DSSCs.

Muduli et al. [93] informed the formation of Au-TiO2 nanohybrid by using the hydrothermal process. The prepared Au nanoparticles functionalized TiO2 nanostructures were embraced for the fabrication of photoanodes of DSSCs. They observed the superior PCE for Au-TiO2-based photoanodes as compared to the pristine TiO2-based photoanode. They have concluded that due to the Schottky junction creation among Au and TiO2 nanostructures, the quenching in the recombination rate takes place, and consequently, the enhancement in the PCE of Au-TiO2-based photoanodes takes place as compared to bare TiO2. Alamu et al. [94] prepared Ag-decorated TiO2 nanohybrids and applied them for the fabrication of photoanodes of DSSCs. They have used natural plant extract of *Azadirachta indica* and *Lawsonia inermis* with the commercially available dye N719. They have revealed that the modification of Ag nanoparticles over TiO2 particles enables effective band gap narrowing and improvement in the optical absorption in the visible region. In their study, they concluded that Ag-TiO2-based photoanode with the usage of natural dyes exhibited tremendous photoconversion efficiency. The combined contribution of plasmonic nanoparticles with natural dye sensitizer effectively enhances the PCE for bare TiO2-based photoanode as compared to bare TiO2 nanoparticles based photoanodes.

Ran et al. [95] decorated the TiO2 nanostructures with Ag, Au nanoparticles and Ag, Au nanowires for application in DSSCs. They have highlighted that the improved PCE could correspond to the high mobility of plasmonic nanostructures and the SPR effect of Ag and Au nanostructures. Plasmonic nanoparticles functionalized TiO2-based photoanodes attained a higher PCE value as compared to the pristine TiO2-based photoanodes. Under sun simulator exposure, a PCE of 5.74% was found in the Ag nanowires decorated TiO2. The obtained efficiency of Ag-TiO2-based photoanode attained a 25.3% improvement in comparison to photoanodes prepared by using pure TiO2 film (4.58%). Improved electron mobility properties of Ag nanowires with enhanced optical absorption majorly contributed and enhanced the PCE of photoanodes. In addition, Ag nanowire also provides an enhancement in the light scattering, which is also favorable to improve the PCE of DSSCs.

Nbelayim et al. [96] reported the preparation of Ag@TiO2 core-shell with variations in the Ag content from 0.1 to 1% by wet chemical synthesis process. The synthesized Ag@TiO2 with varied Ag loading was used to fabricate the photoanode of DSSCs. In their study, they optimized the Ag doping concentration for the enhanced PCE of the DSSCs and concluded that 1% of Ag doping showed the maximum PCE. They have concluded that 1% Ag doping provides the optimum band alignment for injection from the Ag nanoparticles to TiO2. In addition, the optimum concentration of Ag nanoparticles can effectively control the recombination rate in TiO2, which majorly contributed to enhancing the PCE of the fabricated DSSCs. The PCE value of the Ag@TiO2 and Ag-doped TiO2 was found to be significantly enhanced as compared to the pristine sample. Song et al. [97] informed the fabrication of different morphology of Ag nanostructures (spherical and multi-shaped) and Au nanoparticles and

explored their effect on the PCE of the DSSCs. In order to fabricate photo anodes of DSSCs, individual Ag, Au nanoparticles, multi-shaped Ag, Au, and their mixture were employed. Each plasmonic nanostructure and its combination were mixed with the mesoporous TiO2 and were employed to fabricate photo anodes of DSSCs. Among all samples, a photoanode containing a multishape of Ag and Au with TiO2 was found to be the most efficient for the photovoltaics performance. The enhanced PCE performance of multishaped Ag and Au nanostructures can be assigned to the wider optical absorption as compared to the spherical Ag and Au nanoparticles.

Villanueva-Cab et al. [98] prepared Au-decorated TiO2-based DSSCs and adjusted the concentration of Au nanoparticles on TiO2 for efficiently enhanced PCE. In their study, they have highlighted several parameters, such as charge collection efficiency, light absorption efficiency, and charge injection efficiency of dye majorly influenced the PCE of DSSC. Moreover, they showed that the existence of various Au content on the TiO2 surface majorly influences the collection efficiency and consequently, PCE of DSSC varied.

We have successfully elaborated the importance of various parameters which majorly affect the photo-conversion efficiency of plasmonic nanostructures-based DSSCs. Plasmonic nanohybrids with their fascinating optical and electronic properties largely increase the PCE of the DSSCs. Plasmonic nanoparticles with TiO2 tune the band alignment and significantly control the charge injection properties. Apart from this, plasmonic nanostructures over the TiO2 surface also quench the rate of recombination and increase the lifetime of the charge carriers. Moreover, plasmonic nanoparticles also enhance the charge conduction properties of photoanodes which is also beneficial for the improvement in the PCE value of DSSCs. Thus this chapter explains the preparation and characterization of plasmonic-TiO2 nanohybrids and their usage for water purification, SERS-based detection, and photovoltaic applications. This chapter provides a wide overview related to the preparation and employment of plasmonic nanostructures for different environmental and energy applications. This chapter also includes the importance of specific parameters of plasmonic-TiO2 nanohybrids, which largely influence their performance for different energy and environmental applications and thus provide a better understanding to the readers.
