Fundamentals of doping engineering strategies of metal oxide semiconductors and various charge transfer processes for emerging SERS applications are discussed.

We report on the synthesis of multifunctional Ag–TiO ₂ nanocomposites and their optical, physio-chemical, surface enhanced Raman scattering (SERS) and antibacterial properties. A series of Ag–TiO ₂ nanocomposites were synthesized by sol–gel technique and characterized by x-ray diffraction, scanning and transmission electron microscopy, energy-dispersed x-ray analysis, photoluminescence, UV–vis, x-ray photoelectron and Raman spectroscopy and Brunauer–Emmett–Teller method. The Ag nanoparticles (NPs) (7–20 nm) were found to be uniformly distributed around and strongly attached to TiO ₂ NPs. The novel optical responses of the nanocomposites are due to the strong electric field from the localized surface plasmon (LSP) excitation of the Ag NPs and decreased recombination of photo-induced electrons and holes at Ag–TiO ₂ interface providing potential materials for photocatalysis. The nanocomposites show enhancement in the SERS signals of methyl orange (MO) molecules with increasing Ag content attributed to the long-range electromagnetic enhancement from the excited LSP of the Ag NPs. To further understand the SERS activity, molecular mechanics and molecular dynamics simulations were used to study the geometries and SERS enhancement of MO adsorbed onto Ag–TiO ₂ respectively. Simulation results indicate that number of ligands (MO) that adsorb onto the Ag NPs as well as binding energy per ligand increases with increasing NP density and molecule-to-surface orientation is mainly flat resulting in strong bond strength between MO and Ag NP surface and enhanced SERS signals. The antimicrobial activity of the Ag–TiO ₂ nanocomposites was tested against the bacterium Staphylococcus aureus and enhanced antibacterial effect was observed with increasing Ag content explained by contact killing action mechanism. These results foresee promising applications of the plasmonic metal−semiconductor based nano-biocomposites for both chemical and biological samples.

Semiconductor photocatalyst nanomaterials have been extensively studied for the last few decades due to their great potential in solving energy and environmental problems on the earth by harnessing solar light. Cadmium sulfide (CdS) based nanomaterials have emerged as promising photocatalyst materials due to their visible light absorption and physio-chemical properties suitable for high-performance photocatalytic activities in terms of solar-fuel generation and environmental/water remediation. CdS photocatalysts have been reported in several morphologies from 0-dimensional to 3-dimensional (0-3D) nano/micro-architectures. However, regarding CdS photocatalysts with specific nanostructures, 3D nanostructures (nanoflowers, self-assembled -hierarchical, hollow, tetrapod, etc.) have shown special structural features with high surface/volume ratio and significant improvement in their photocatalytic properties for various energy and environment applications. This review deals with CdS-based 3D nano/micro-architectures (sole CdS and nanocomposites with various functional nanomaterials), their formation mechanism and tailoring of properties for visible light induced photocatalytic activities in energy and environmental applications. Particularly, it includes the emerging applications of 3D CdS-based photocatalysts in photocatalytic H₂ production, photocatalytic CO₂ reduction and photodegradation of organic pollutants with an emphasis on the mechanism as well as the role of functional nanomaterials in boosting photocatalytic activities of CdS. Moreover, various challenges and future prospects in the research of CdS-based photocatalysts have also been discussed.