Synthesis, Characterization and Engineering of Photocatalytic Materials for Water Splitting Under Visible Light

Abstract

Hydrogen is considered a renewable large source of energy and as such has attracted considerable interests. Water molecules can be split into hydrogen and oxygen using a number of methods (e.g., electrolysis) but the ultimate route (outside of the carbon cycle) is by using the solar energy (i.e., light) impacting suitable semiconductor (SC) photocatalysts. In this work, several materials were synthesized: (i) Orthorhombic WO3.H2O precursor (tungstite) with a phase transformation at relatively low temperature (upper than 300°C) to the monoclinic WO3 (tungsten oxide). For the first time, an environmentally benign method capable of producing large quantities of tungstite nanoribbons (NRs) and leaf-shaped nano-platelets (LNPs) was reported. The heat-treatment leading to the phase transition was carefully investigated in situ in the transmission electron microscope and Raman spectrometer. The obtained WO3 NRs and LNPs materials were n-type SC and can be used as photoanodes in two-step (Z-scheme) for water splitting. (ii) SCs can also be engineered to adjust their band-gap in order to use them or make them more efficient as photocatalysts in the visible light region. Here, a widely available commercial material was used: TiO2 nanoparticles. It was doped by vanadium and the band gap dropped by a significant 0.5 eV, making it more suitable in the visible light spectrum. (iii) Functionalizing the SCs surfaces can reduce among others the overvoltage potentials for the surface reactions to proceed with increased efficiency (i.e., the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER)). Several schemes were tested. The surfaces of WO3 NRs were grafted with nanoparticles of platinum (Pt), palladium (Pd), silver (Ag) and ruthenium (Ru), using a relatively simple photodeposition technique. Moreover, Pt was also grafted to V-doped TiO2 NPs.