*3.1. Characterization of Nano-Photocatalyst*

UV-visible spectra of the aqueous dispersion of Ag nanoparticles, and Ag@ZnO core-shell nanoparticles synthesized by ultrasonic hydrolysis of zinc nitrate hexahydrate are shown in Figure 1a. Aqueous dispersion of Ag nanoparticles showed a clear SPR band at 391 nm which showed a distinct red shift of about 22 nm immediately after addition of aqueous sodium zincate sol. This is attributed to an immediate change in the chemical environment around Ag nanoparticles. With increase in the ultrasonic irradiation time SPR band has shown a red shift to 397 nm with development of a shoulder peak at 487 nm. It is expected that during the formation of core-shell nanoparticles, Ag nanoparticles may have aggregated slightly to form large clusters. This may have caused dipole coupling between closely interacting metal nanoparticles. This hypothesis is supported by the electron microscopy images.

Results of our XRD study is shown in Figure 1b. For the synthesized nanoparticles, three distinct peaks at 2θ = 38.2, 44.9 and 64.8 corresponding to (111), (200) and (220) planes of metallic Ag with face-centered cubic structure (JCPDS Card No. 04-0783) is observed. Similarly three major peaks of ZnO at 2θ = 31.99, 34.63, and 46.51 corresponding to (100), (002), and (102) planes of synthetic ZnO with hexagonal wurtzite structure (JCPDS Card No. 36-1451) are obtained. Any peak corresponding to other Ag/Zn compounds was not obtained. This suggests that no alloy or solid solution is formed. Mean crystallite diameter (MCD) was found to be ≈ 15 and 25 nm for Ag and ZnO nanoparticles respectively. It is also observed that the crystal structure and phase remained unchanged after heat treatment (at 200 and 400 ◦C). However the MCD and crystalinity have increased slightly after heat treatment. The results of FTIR spectroscopy are shown in Figure 1c. The broad band around 3400 cm−<sup>1</sup> may correspond to O–H stretching mode of hydroxyl groups whereas the strong peak at 2345 cm−<sup>1</sup> resembles to the stretching mode of acidic O–H group, which arises in the range of 2400–3300 cm<sup>−</sup>1. The small vibration appearing at 1630 cm−<sup>1</sup> may belong to the stretching peak of C=O group [11]. Vibration peaks at 1500 and 1280 cm−<sup>1</sup> corresponds to C–H bending and C–O stretching mode respectively [11,14]. The peaks at 1630 and 637 cm−<sup>1</sup> may correspond to Zn–O stretching and deformation vibration, respectively [14].

**Figure 1.** UV-Visible spectra of aqueous dispersion of Ag and Ag@ZnO core-shell nanoparticles (**a**), XRD pattern (**b**) and FTIR spectrum (**c**) of Ag@ZnO core-shell nanoparticles, (**d**) Nitrogen adsorption/desorption isotherms obtained at 77 K and inset shows the pore size distribution of the as-synthesized Ag@ZnO NCs synthesized by the sonochemical technique and dried at 80 ◦C for 2 h.

The adsorption-desorption isotherm plot for the nitrogen sorption (77 K) of the Ag@ZnO nanoparticles sample that was synthesized by sonochemical technique and dried at 80 ◦C for 2 h shows typical "type IV" isotherm in the Brunauer classification (Figure 1d). The sample exhibited average pore size in the range of 5–20 nm indicating the porous nature of the material. The specific surface area of Ag@ZnO core-shell nanoparticles was evaluated to be 65.5 m2/g based on the BET result. This high surface area and porous nature are expected to be very beneficial for photocatalytic applications [12,14].

Morphology of the Ag@ZnO synthesized by the sonochemical technique were investigated by TEM. TEM samples were prepared by dipping the TEM grid in aqueous colloidal dispersion of NC followed by freeze drying for 12 h. Figure 2 shows TEM and HRTEM images of core-shell Ag@ZnO nanoparticles. Core-shell structure is observed for the materials. However, multiple silver nanoparticles were encapsulated within a single zinc oxide shell. Similar situation was also observed by Tripathy et al. [11]. A broad size distribution is observed for synthesized nano-Ag particles. The size of the Ag is found to be in the range of 10–30 nm and that of ZnO shell is about 5 to 10 nm. Metal core was found to have inter planar spacing of ~0.23 nm which corresponds to the (111) plane of the metallic silver with face-centered cubic structure. In ZnO shell, the spacing between adjacent lattice fringes is 0.16 nm, which is close to the *d-*spacing of the (110) plane of hexagonal ZnO (exact value is 0.168 nm).

**Figure 2.** TEM (**a**) and HRTEM (**b**) images of Ag@ZnO core-shell nanoparticles synthesized by the sonochemical technique and dried at 80 ◦C for 2 h.
