Reduced titanium dioxide nanotubes for photoelectrochemical sensing and energy storage

Titanium dioxide (TiO₂) is a widely studied semiconductor known for its chemical stability, and versatile photocatalytic and electronic properties. Among its various morphologies, TiO₂ nanotubes (NTs) are particularly promising due to their high surface area, one-dimensional charge transport pathways, and well-ordered tubular architecture. Despite these advantages, pristine TiO₂ NTs face limitations, including a wide band gap restricting visible light absorption and low electrical conductivity, both of which limit their efficiency in light-driven and charge-transfer-dependent applications. To overcome these limitations, controlled electrochemical reduction has emerged as an effective strategy to introduce oxygen vacancies and Ti³⁺ states into the TiO₂ lattice (i.e. defect engineering).The work conducted in the PhD thesis aims to establish a reproducible fabrication method for the oxygen vacancy induced reduced TiO₂ NTs (R-TiO₂ NTs), while providing insight into the material properties such as its electrical conductivity, capacitance, charge carrier generation and separation efficiency, and optical absorption. This was achieved by coupling chemical, morphological, and in situ structural characterization methods with PEC analyses. Their ability to combine enhanced light absorption, efficient charge transport, and high surface area renders them highly promising for a variety of applications. As a result, this knowledge enabled the fabrication of R-TiO₂ NTs suitable for applications in (i) phenolic compound sensing and (ii) energy storage. A PEC sensing platform for the detection of toxic, endocrine-disrupting hydroquinone (HQ)/benzoquinone (BQ) redox couple was successfully developed only using reduced TiO₂ NTs. The advanced charge transfer and photoactivity of the R-TiO₂ NTs, combined with well-established sensing parameters, provided high sensitivities to detect both HQ and BQ compounds below the reported toxicity levels that cause ecotoxicity in microbial communities in water resources. In the energy storage domain, a supercapacitor electrode was fabricated using R-TiO₂ NTs. Improved electrical properties of the R-TiO₂ NTs, including enhanced conductivity and capacitance, allowed the design of a pseudocapacitive supercapacitor electrode based on Ni(OH)2 nanosphere (NSs) decoration, exhibiting high capacitance values with good cyclic stability. Finally development of a R-TiO₂ NTs-based supercapacitor device was conducted. A symmetric thin-film supercapacitor was fabricated using two R-TiO₂ NTs/Ni(OH)2 NSs electrodes. Additionally, an asymmetric thin-film supercapacitor was developed by fabricating and optimizing a graphene-based counter electrode. The fabricated asymmetric supercapacitor delivered high energy density values. The fabricated device stands out among thin-film supercapacitors, while being the first of its example as an R-TiO₂ NTs-based supercapacitor.