Conclusions

The aim of this work was to investigate the photocatalytic performance of two different systems based on n-type TiO2 nanorods and p-type BiFeO3 thin films. The

photoactivity of the systems were analysed through photodecolourisation of a common textile dye Rhodamine B and photogeneration of hydrogen and oxygen under solar simulator. The influence of metal loading on photoactivity, where Pd was deposited onto TiO2 nanorods and Ag was incorporated into BiFeO3 thin films, was

investigated by a variety of photoelectrochemical tests.

The first stage was to develop a photocatalyst system with high surface area TiO2

nanorods grown on glass fibre substrates and to investigate the effect of annealing and Pd nanoparticle loading on photoactivity. The aim was to produce a mechanically robust photocatalyst system as a photocatalytic filter alternative to powder photocatalyst systems used for waste water treatment. The first trial for the production of the catalyst was an aqueous method whereby ZnO nanorods previously grown on glass fibres were converted to TiO2 nanorods. Although the full chemical

conversion was successful, the photocatalyst failed to show a good photocatalytic activity for the photodecolourisation of Rhodamine B due to insufficient crystallinity and surface area. The production method was then changed to hydrothermal

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synthesis where rutile TiO2 nanorods were directly grown on glass fibre substrates.

The nanorods formed were in rutile phase with high crystallinity.

The photoactivity of the catalyst system was studied through decolourisation of a common textile dye – Rhodamine B (RhB). Full photodecolourisation of RhB (10 ppm) was achieved within 180 minutes with as-grown TiO2 nanorods. The annealing

of the TiO2 nanorods did not have any effect on photoactivity of the system as the

crystallinity was not immensely improved by annealing. The influence of Pd metal loading on TiO2’s photoactivity was further investigated. Pd nanoparticles were

deposited onto TiO2 nanorods via photochemical method which was successfully

confirmed by TEM and XPS. It was found that the hybrid Pd/TiO2 catalyst system

showed higher photoactivity where full decolourisation occurred in 90 minutes. The credit in enhancement of photoactivity by Pd loading was given to localised surface plasmon resonance (LSPR) effect due to interaction of Pd with visible light and the electron scavenging role of Pd for efficient charge separation to surpass recombination which was explained by the mechanism. The recyclability tests showed that the reused hybrid Pd/TiO2 catalyst system had a good catalytic

performance that is similar to the original samples.

The next stage was to replicate the same photocatalyst system on a conductive substrate so that photoelectrochemicals experiments could be carried out. In this respect, the Pd/TiO2 photocatalyst system was produced on FTO coated glass

substrate. The production method was the same with the photocatalyst system on the glass fibres substrates where metallic Pd nanoparticles were deposited by a photochemical method onto TiO2 nanorods that were hydrothermally grown on the

substrate. The samples were photoelectrochemically characterised by a potentiostat with tests for i-v and i-t measurements under dark and illumination, Mott-Schottky

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curves for estimation of flat-band potential and electrochemical impedance spectroscopy (EIS) to understand the charge transfer kinetics at the Pd/TiO2/electrolyte interface. The flat band potential of TiO2 was estimated to be -

0.45V (vs NHE, pH = 7) and there was a positive shift in flat band potential when Pd was loaded onto TiO2 due to the relative position of Pd’s Fermi level to the

conduction band of TiO2. Pd loading induced a decrease in band bending which

implies a facilitated electron transfer between Pd and TiO2. Nyquist plots from EIS

data confirmed this by a smaller arc size measure for Pd/TiO2 which indicates a

lower resistance against as the charge transfer.

The photoelectrochemical performance of the bare TiO2 and hybrid Pd/TiO2 samples

were then compared depending on their photoactivities on the decolourisation of a standard commercial textile dye Rhodamine B (RhB) and solar hydrogen production in different electrolyte solutions at various applied voltage values. Hybrid photocatalyst Pd/TiO2 showed enhanced photoelectrocatalytic activity in

decolourisation RhB in aqueous solution. A higher amount of hydrogen by Pd/TiO2

was photogenerated in methanol solution. However, bare TiO2 samples produced a

higher amount of hydrogen in 0.01M Na2SO4 and pure deionised water under same

conditions. The role of methanol as a hole scavenger helped the efficient charge separation so that the electrons trapped by Pd could contribute to the hydrogen reduction. The results were explained by possible mechanisms of photoelectrochemistry proposed for both dye degradation and solar hydrogen production.

Finally, the photoelectrochemical activity of a p-type photocatalyst based on BiFeO3

was investigated. BiFeO3 thin films were deposited on large scale FTO coated glass

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Ag modification. Ag was incorporated into the BiFeO3 matrix at different

concentrations as metallic Ag structures and Ag nanowires. The light absorption was enhanced by the addition of Ag which was due to the light scattering between the Ag and BiFeO3 matrix. An indirect band gap of 2.1eV and direct band gap of 2.65eV

were estimated for pure BiFeO3 thin films with small variations after Ag

modification. All of the films showed p-type behaviour with a flat band potential of 1.15V (vs NHE, pH=7) for pure BiFeO3. Ag modification induced a small negative

shift in flat band potential due to the reduced band bending.

Photocurrents of BiFeO3 with different Ag concentration were investigated.

Photocurrent density of -0.004mA/cm2 was achieved by pure BiFeO3 thin films at 0V

vs NHE under AM1.5 G illumination. For the Ag modified BiFeO3 samples, the

highest photocurrent was achieved for BiFeO3 with 2molar% metallic Ag structures

and BiFeO3 with 0.1vol% Ag nanowires with photocurrents of -0.07mA/cm2 and -

0.05mA/cm2, respectively. Photoelectrocatalytic water splitting of pure deionised water with these three samples was carried out at an applied voltage of -0.5V (vs Ag/AgCl). Only half cell reaction was successful and only oxygen evolution occurred whereas hydrogen evolution was not detected. The oxygen evolution amount was immensely enhanced by Ag modified BiFeO3 compared to the pure

BiFeO3 thin films. The significant increase in photocurrent and corresponding

amount of photogenerated O2 gas were attributed to reduced overpotential losses and

efficient charge separation by the Ag particles.