Low dimensional titanium dioxide nanostructured devices for catalytic and hot electron driven solar energy conversion

Abstract

We investigated the effect of the support on catalytic activity for CO oxidation using platinum nanoparticles on doped and undoped titanium dioxide ($TiO_2$). As a support, the undoped $TiO_2$ was synthesized via the sol-gel process. The thin films were then chemically doped with non-metal anions, such as nitrogen (N) and fluorine (F). Thin films were prepared using the spin coating technique

characterization techniques in particular, XPS analysis of the doped $TiO_2$ thin films revealed that the nitrogen sites were interstitial whereas fluorine was doped substitutionally into $TiO_2$ lattice. The Pt/N-, Pt/F-, and Pt/undoped TiO2 catalysts were fabricated by depositing platinum nanoparticles on N-, F-, and undoped $TiO_2$ thin films using the arc plasma deposition (APD) technique. CO oxidation was carried out to elucidate the catalytic activity of the Pt nanoparticles. The turnover rates of Pt/N-, and Pt/F-doped $TiO_2$ were a factor of ~ 2.5 higher than that of the Pt/undoped $TiO_2$. We attribute the enhanced catalytic activity to oxygen vacancies formed during the doping process and the facile charge transfer at the metal-oxide interfaces. In the third chapter we fabricated the $TiO_2$ nanotube based Schottky nanodiode for solar energy conversion applications. We controlled and harvested the different wavelengths of the spectrum by tuning the size of metal/ $TiO_2$ nanotube (TNA). TNA with vertically aligned array structures shows substantial advantages in solar cells as electron transport material that offers a large surface area with charge transport along with the length of the nanotubes. Integrating this one-dimensional (1D) semiconductor material with plasmonic metal can influence the solar energy conversion utilizing the generated hot electrons. Here, we fabricated the nanotubes by potentiostatic anodization and devised the plasmonic metal (Au or Ag)/ $TiO_2$ nanodiode architecture. Hot electrons were measured with the plasmonic metal/ $TiO_2$ Schottky diode electrically connected to ohmic contact pads, ensuring a continuous flow of hot electrons. The electrical characteristics of the nanodiode were measured using the I-V curves, photocurrent and incident photon to current conversion efficiency (IPCE) of the diode was obtained as the function of photon energy for detection of the hot electrons. The enhancement of IPCE observed at 2.12 eV was attributed to the hot electrons generated by plasmonic Au on TNA. This was confirmed by the calculated absorbance of Au/TNA at 2.138 eV which is in proximity with the peak obtained from IPCE. This observation represents the enhancement of photocurrent on metal/TNA nanodiode majorly via plasmon induced hot electrons. The quantum efficiencies results of Ag based diodes were more enhanced when compared to the Au based plasmonic diodes. Around tenfold times better efficiencies obtain from the fact that the general criteria of intrinsic near field properties in silver is far superior to the gold plasmonics. Due to the ease of fabrication, large surface area, and stability, plasmonic diodes based on nanotube array can open up a window for efficient and practical applications of solar energy conversion.