Lu He

Titanium dioxide (TiO₂) in the form of thin films has attracted enormous attention for photocatalysis. It combines the fundamental properties of TiO₂ as a large bandgap semiconductor with the advantage of thin films, making it competitive with TiO₂ powders for recycling and maintenance in photocatalytic applications. There are many aspects affecting the photocatalytic performance of thin film structures, such as the nanocrystalline size, surface morphology, and phase composition. However, the quantification of each influencing aspect needs to be better studied and correlated. Here, we prepared a series of TiO₂ thin films using a sol-gel process and spin-coated on p-type, (100)-oriented silicon substrates with a native oxide layer. The as-deposited TiO₂ thin films were then annealed at different temperatures from 400 °C to 800 °C for 3 h in an ambient atmosphere. This sample synthesis provided systemic parameter variation regarding the aspects mentioned above. To characterize thin films, several techniques were used. Spectroscopic ellipsometry (SE) was employed for the investigation of the film thickness and the optical properties. The results revealed that an increasing annealing temperature reduced the film thickness with an increase in the refractive index. Atomic force microscopy (AFM) was utilized to examine the surface morphology, revealing an increased surface roughness and grain sizes. X-ray diffractometry (XRD) and UV-Raman spectroscopy were used to study the phase composition and crystallite size. The annealing process initially led to the formation of pure anatase, followed by a transformation from anatase to rutile as the annealing temperature increased. An overall enhancement in crystallinity was also observed. The photocatalytic properties of the thin films were tested using the photocatalytic decomposition of acetone gas in a home-built solid (photocatalyst)–gas (reactant) reactor. The composition of the gas mixture in the reaction chamber was monitored using in situ Fourier transform infrared spectroscopy. Finally, all of the structural and spectroscopic characteristics of the TiO₂ thin films were quantified and correlated with their photocatalytic properties using a correlation matrix. This provided a good overview of which film properties affect the photocatalytic efficiency the most.

Plasmonic nanostructures have attracted growing interest over the last decades due to their efficiency in improving the performance in various application fields such as catalysis, photovoltaics, (opto-)electronic devices, and biomedicine. The behavior of a specific metal plasmonic system depends on many factors such as the material, the size, the shape, and the dielectric environment. The geometry, that is, size and shape of both single plasmonic elements and patterned arrays of plasmonic nanostructures, plays an essential role, and it provides considerable freedom to tune the plasmonic properties of a single plasmonic nanostructure or any combination of nanostructures. This freedom is mainly used in the application fields of surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). In this context, the chapter encompasses how the geometry of the SERS-active plasmonic nanostructures and tips with/without metal substrates used in TERS influences the localized surface plasmon resonances of the plasmonic systems.