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Title: Surface modifications and growth of titanium dioxide for photo-electrochemical water splitting
Author: Alexander, John Callum
ISNI:       0000 0004 5349 7567
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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This study investigates photo-anodes based on titanium dioxide (TiO2) that can be used to produce hydrogen by the photo-electrochemical decomposition of water. TiO2 is a wide band gap semiconductor that absorbs only the UV region of the solar spectrum. Sensitization of TiO2 to visible light by the addition of gold nanoparticles (AuNPs) was studied. AuNPs sustain localized surface plasmon resonance (LSPR) that results in the absorption of light at the resonant energy. The evidence for water splitting by Au-TiO2 systems is discussed critically. Fabrication of arrays of AuNPs was done by; annealing sputtered gold thin films, micellar nanolithography, and nano-sphere lithography. The optical characteristics and photo-electrochemical 'water splitting' performance of AuNP coated rutile (110) electrodes were determined. Nb-doped crystals coated in AuNPs of ca. 20 nm exhibited a small photocurrent that was not present with the bare rutile electrode. Reduced un-doped rutile (110) with AuNPs did not exhibit the 'plasmonic photocurrent'. Some Nb-doped electrodes did not exhibit an effect. Batches of Nb-doped and reduced rutile were examined using voltammetry and impedance spectroscopy and it was found that the 'inactive' Nb-doped TiO2 was partially reduced. Thin films of TiO2 were fabricated by pulsed laser deposition (PLD) onto amorphous and single crystal substrates. The effect of growth conditions on the phase and orientation of the film were studied, and procedures to grow anatase films oriented with (100), (001), and (101) were developed. The temperature and heating regime of TiO2 films fused silica affected the orientation of film growing. Nb doping of the films also affected the temperature of the anatase-rutile phase transition and the orientation of the films, acting to stabilize anatase at higher temperatures. Surprisingly, highly doped films were found to be non-conductive. The importance of the oxygen partial pressure in producing conductive films for use as electrodes is discussed.
Supervisor: Alford, Neil; Kelsall, Geoffrey Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral