In this dissertation, we investigated plasmon enhanced optical properties and photocatalyticrnapplications of composite nanostructures by using noble materials fromrntheoretical and experimental perspectives.rnThe theoretical investigation mainly focused on the optical and plasmonic propertiesrnof spherical and spheroidal ZnO@Ag core-shell nanocomposite embedded in nonabsorptivernmedium. The e_ective dielectrics function, polarizability, refractive indexrnand optical absorbance for spherical as well as absorption cross-section and radiationrne_ciency for spheroidal nanocomposite are studied. These optical and plasmonicrnproperties are tuned when size of core and/or composite, shell thickness, shape ofrncore and/or shell, and refractive index of the host-medium is changed. In sphericalrnnanocomposite, there are two resonances associated with interfaces while in spheroidalrnnanocomposite four plasmonic resonances are appeared that correspond to interfacesrnand the bonding/antibonding pairs. The tunable of plasmonic resonances from UVrnto infrared spectral regions of such composite enables it to exhibit very interestingrnmaterial properties in a variety of applications.rnExperimentally, we emphasized the optical properties and photocatalytic applicationsrnof ZnO nanoparticles, triple layered ZnO@SiO2@Ag core-shell composite nanostructuresrnand Bi doped/undoped TiO2 nanostructures. In particular, we synthesizedrnZnO nanoparticles using three di_erent methods as presented in the experimentalrnsection of the study. Due to di_erent synthesis methods and procedures as well asrndi_erent surfactant/stablizing agent, the synthesized ZnO nanoparticles have di_erentrnmorphology, band gap energy, varying quantum size e_ect and photocatalyticrnresponses. The photocatalytic performance of ZnO nanoparticles were evaluated byrnusing methyl orange dye as a pollutant under UV-light irradiation. Triple layeredrnZnO@SiO2@Ag core-shell nanocomposites are also fabricated at 400 oC for di_erentrncalcination time. The photocatalytic activity and stability of the as-synthesized samplesrnwere evaluated by photo-degradation of organic methylene blue under ultravioletrnlight irradiation. When ZnO@SiO2 is coated with Ag, the stability and photocatalyticrnperformance of the composite ZnO@SiO2@Ag photocatalysts were highlyrnenhanced. In addition, Bi- doped/undoped TiO2 nanoparticles are synthesized byrngreen synthesis method using zingiber o_cinale extract solution. After doping Bi,rnthe band gap energy is decreased from 3:00 eV to 2:76 eV , absorption region is shiftedrnto visible range of solar spectrum, and surface function is modi_ed. The photocatalyticrnactivity of the sample is analyzed by UV-Vis spectroscopy using trypan bluerndye. The photo-degradation e_ciency of chemical synthesis TiO2 nanoparticles is enhancedrnto 98:34 % by doping basic metal Bi and using ginger extraction solution. Thernsynthesized samples were analyzed using a characterization techniques such as X-rayrndi_raction (XRD) spectroscopy, scanning electron microscope (SEM), Transmissionrnelectron microscope (TEM), Ultraviolet visible (UV-Vis) spectroscopy, Fourier transformrninfrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS).