Fabrication of patterned thin films for photocatalytic CO2 reduction

The escalating concentration of atmospheric CO₂ driven by fossil fuel combustion demands urgent development of renewable energy technologies capable of converting solar energy directly into chemical fuels. This thesis investigates the fabrication, characterisation, and photoelectrochemical and photocatalytic evaluation of lithographically patterned Cu₂O@BiVO₄ composite thin films as heterojunction photoelectrodes for solar-driven water splitting and CO₂ reduction. BiVO₄ and Cu₂O thin films were synthesised by sequential potentiostatic electrodeposition on FTO substrates. BiVO₄ was deposited from a Bi³⁺/VO₂⁺ nitrate–acetate electrolyte at 1.9 V vs. Ag/AgCl (60 °C, 20 min), annealed at 500 °C, and KOH-etched to yield phase-pure monoclinic scheelite. Cu₂O was deposited from a lactate-complexed Cu²⁺ electrolyte at −0.35 V vs. Ag/AgCl (pH 13.2, 60 °C), producing phase-pure cuprite. Patterning was achieved using the MLA100 maskless laser aligner with AZ 5214E photoresist, resolving circular features down to 2 μm. XRD confirmed coexistence of monoclinic scheelite BiVO₄ and cubic cuprite Cu₂O with no secondary phases. Raman spectroscopy corroborated phase purity, and UV-Vis Tauc analysis yielded an indirect bandgap of 2.42 eV for BiVO₄. Under 450 nm illumination, pristine BiVO₄ yielded only 20–30 μA cm⁻² with a spike-to-steady-state ratio of ~20:1. Photolithographically patterned Cu₂O@BiVO₄ achieved ~920 μA cm⁻² transient photocurrents, stabilising to 100–150 μA cm⁻² — a 5–7× improvement over pristine BiVO₄. Photocatalytic CO₂ reduction yielded exclusive CO production of 5.10 μmol over 240 minutes (1.28 μmol h⁻¹), with no activity from either pristine material alone, confirming synergistic heterojunction charge transfer. Photocatalyst stability over 4 hours was attributed to the protective BiVO₄ overlayer suppressing Cu₂O photocorrosion. These results establish photolithographic patterning as a powerful strategy for activating oxide semiconductor heterostructures.