Photoelectrochemical Performance of Catalyst Systems Based on Pd deposited TiO2 and Ag incorporated BiFeO3

Abstract

Since the discovery of titanium dioxide’s (TiO2) capability for water-splitting and photocatalytic degradation of organic compounds, semiconductor photocatalysis has received great attention and promises an environmentally clean and sustainable solution by solar hydrogen production and waste water treatment.1,2 In this thesis, the photocatalytic performance of two different photocatalyst systems based on Pd nanoparticle decorated n-type TiO2 nanorods and Ag incorporated ptype BiFeO3 thin films were investigated for solar hydrogen and oxygen production and photodecolourisation of a common textile dye, Rhodamine B. High surface area TiO2 nanorods were grown on glass fibre substrates by a hydrothermal method to produce a mechanically robust photocatalytic filter.Metallic Pd nanoparticles were deposited onto TiO2 nanorods via a photochemical method. It was found that the hybrid Pd/TiO2 catalyst system showed higher photoactivity with a doubled kinetic rate for the photodecolourisation of RhB. Full decolourisation has been achieved in 180 minutes with as-grown TiO2 nanorods whereas this time was reduced to only 90 minutes for the Pd/TiO2 hybrid catalyst. This enhancement was associated with the localised surface plasmon resonance (LSPR) effect due to the interaction of Pd with visible light and the electron scavenging role of Pd for efficient charge separation. The same hybrid Pd/TiO2 photocatalyst system was then developed on FTO coated glass substrates so that the photoelectrochemical experiments can be carried out using a potentiostat. Mott-Schottky curves demonstrated a positive shift in flat band potential and an increased charge carrier density after Pd deposition. The facilitated charge transfer at the interface of Pd and TiO2 was shown by EIS data with a smaller arc size for Pd/TiO2 in Nyquist plots. The photoelectrochemical performance of the bare TiO2 and hybrid Pd/TiO2 samples were compared through the photoelectrocatalysis of Rhodamine B (RhB) and solar hydrogen production in different electrolyte solutions at various applied voltage values. A higher amount of hydrogen by Pd/TiO2 was photogenerated in methanol solution whereas bare TiO2 produced a higher amount of hydrogen in 0.01M Na2SO4 and pure deionised water under the same conditions. The results were discussed by proposing possible reaction mechanisms with an emphasis on the charge trapping role of Pd nanoparticles. P-type BiFeO3 (BFO) thin films were deposited on large scale FTO coated glass substrates by a sol-gel method. A photocurrent density of -0.004mA/cm2 was achieved at 0V vs NHE under AM1.5 G illumination and 1.2μmol of O2 was produced in 2h at an external bias of -0.5V vs Ag/AgCl. These values were significantly increased upon the incorporation of Ag into the BFO matrix. Ag was incorporated into the BiFeO3 matrix at different concentrations as metallic Ag structures and Ag nanowires. The enhancement by Ag modification was attributed to enhanced light absorption due to light scattering effect and efficient charge separation by Ag as they act as electron sinks. These explanations were supported by shifts in flat band and onset potentials after Ag modification in detailed measurements of Mott-Schottky plots and j-v curves.