The phase identification of the TiO2and Au:TiO2 was evaluated using Raman spectroscopy performed in therange of 100-800 cm-1, and the resulting spectrum is shown in Figure3.4. The anatase TiO2 phase was observed by the peaks145, 197, 396, 516 and an overtone 639 cm-1, whereas the rutile TiO2phase was detected by the peak at 446 cm-1.
This clearly designatethat the TiO2 and Au:TiO2 nanoparticles contained amixture of anatase and rutile phases. There is no signals related to Auparticles were identified because of the relatively lower concentration of Aumixed with TiO2 and its weak Raman scattering. Thereduction in the peak intensities and the broadening of the Raman peakscorrespond to the Au:TiO2 nanoparticles is observed compared to thatfor the TiO2 nanoparticles. The reduction in peak intensity and broadening of the peaks suggest anincrease in crystalline defects in the framework after mixing of gold. Suchcrystalline defects serve as additional photoelectron trapping centres in thesolar cells which will reduce the charge recombination (Naphade et al., 2014; Lim et al., 2015).TheUV-vis-NIR absorbance spectrum of colloidal AuNPsis shown in Figure 3.
5(a). Thecharacteristic absorbance peak of AuNPs at 537 nm exhibits the surface plasmonresonance (SPR) property (Li et al., 2013).
In a dye-sensitizedsolar cell, N719 dye molecules are adsorbed onTiO2 film. In order toestimatethe effect of AuNPs on the performance of DSSC, we prepared TiO2-N719and Au:TiO2-N719 electrodes by doctor blade method. The resultingelectrodes were soaked in N719 dye solution for 24 h. The electrode with pureTiO2 slurry was prepared by samedoctor blade method and was used asa reference for the UV-vis-NIR study.
The absorption spectra of pureTiO2, TiO2-N719 and Au:TiO2-N719 films are shown in Figures 3.5(b-d).Thepeak at 368 nm is ascribed to the characterstic absorption of pure TiO2 asshown in Figure 3.5(b), represents the intrinsicoptical absorption of TiO2 (Kochuveedu et al., 2012).
The pure TiO2 film,due to its wide bandgap, does not absorb the light in the visible region. Theabsorption spectra of N719-dye molecules adsorbed onto a pure TiO2film and Au:TiO2nanocomposite film exhibits similar absorbancefeatures with peak maximum occur at 536 nm, as seen in Figures 3.5(c and d).However Au:TiO2-N719 nanocomposite film exhibits higher absorptioncompared to that of TiO2-N719 film. Despite the same duration of dyeloading in these films, the presence of Au in TiO2 seems to have anoticeable effect on the absorption properties.
The increased light absorptionof the dye arises from the contributions of resonant energy transfer ornear-field coupling between the surface plasmon and the dye-excited state (Cushinget al., 2012; Zhou et al., 2015; Zarick et al., 2014; Choi et al., 2012).
Thiscan further effectively lead to the charge separation by transferringphoto-excited electrons to the conduction band of TiO2 (Kochuveedu et al., 2012).