Broadband photon absorption in self-assembled networks of multiphase oxides of titanium

Abstract

Titanium -dioxide (TiO2) has garnered immense interest as a potential photon absorber after the discovery of its photocatalytic properties. However, its absorption is limited to the ultraviolet region of the solar spectrum. Despite numerous efforts being made, the challenge to extend its absorption capability to the entire visible and near infrared region (vis-NIR) still exists, which together constitute about 90 % of the solar spectrum. In this dissertation, a multiphase nano TiOx network, rich in defects and oxygen vacancies, has been presented which can absorb photons over a broader range of the solar spectrum. Experimental studies were initially conducted to phase functionalise titanium towards enhanced photon absorption via a single step, ultrashort laser pulse material interaction process. This phase functionalised titanium, characterised to be uniquely composed of multiple oxide phases of titanium, can effectively absorb photons in the vis-NIR region. Using the above study as a template, a complex three-dimensional self-assembled nano network composed of similar multiphase titanium oxides, was then synthesised. Free of any external dopants, it exhibits a remarkable absorption of photons ranging from 300-1000 nm. To further improve the absorptive properties of this ‘multiphase nano TiOx network’, particularly in the lower visible range, the phenomenon of Surface Plasmon Resonance was utilised via its hybridisation with gold and gold/palladium alloy. This successfully resulted in further optimisation of its absorption. The final study of the multiphase nano TiOx was done to understand the fundamental physics behind its broadened photon absorptive behaviour. The condition of synthesis was varied by introducing various contrasting plasma environments. Pronounced disorders and oxygen defects of varying degrees within the crystalline structure were observed. The enhanced and broadened absorption spectrum achieved was attributed to such defects and disorders. The research done in this thesis demonstrates a unique nanomaterial based on multiple oxides phases of titanium that is capable of absorbing photons both in the visible and NIR regions. The contribution made towards the synthesis, investigation and subsequent manipulation of the self-assembled multiphase nano TiOx network can thus be exploited in various photon harvesting applications like photovoltaics and photo catalysis, where such a broadband photon absorption is desirable.