TiO2 based long-range ordered crystals (LROCs) for recyclable SERS applications

Semiconductors have established a great potential for applications requiring photocatalytic degradation of organic materials due to their ability to generate electron-hole pairs under photo-excitation and form radicals. These radicals can attack organic molecules and degrade them into smaller molecules, in some instances, gas species such as CO2 and H2O. This property of semiconductors has been used to introduce self-cleaning functionality into chemical sensors to clean them from any organic pollutants or analyte residues thus making the sensors reusable and safe for storage and disposal. Among the inorganic semiconductor materials, titanium dioxide (TiO2 or titania) is the most widely used photocatalyst due to its relatively high photocatalytic efficiency, facile fabrication processes, being cheap and having a low toxicity. Furthermore, the wide bandgap nature of TiO2 allows for photoexcitation to occur at UV wavelengths, away from the visible or IR range of the electromagnetic spectrum. However, pure TiO2 has a short electron-hole lifetime that reduces the radical formation kinetics and subsequently decreases the photocatalytic reaction performance. The objective of this project was to develop TiO2 based multifunctional substrates through silver decoration, while simultaneously enhancing the photocatalytic performance of TiO2 in the substrate. Employing Ag nanoparticles introduced additional functionality to the system, namely surface-enhanced Raman scattering (SERS), thus enabling the detection of trace amounts of organic molecules prior to their degradation, all with the same substrate. Furthermore, Ag also formed Schottky junctions with TiO2 thus increasing the electron-hole lifetime, as evidenced by the relatively higher photocatalytic activity of the composites. However, since both functionalities deal with light-matter interaction, long-range ordered crystals (LROCs) needed to be employed in order to enable the same response due to a light stimulus from the different regions of the substrate. LROCs were formed by using monodispersed polystyrene colloidal crystals as the template thus enabling to form reproducible and uniform TiO2 nanostructures using a cheap and easy method. The colloidal crystal nature of the TiO2 substrate enabled the control of the SERS active Ag nanoparticle size and deposition sites. Such control, while undergoing electroless deposition of Ag, was the key to forming a multifunctional substrate (hence, regenerable SERS substrate) that can achieve both high photocatalytic and SERS functionalities.