Facile, sustainable and unassisted plain water oxidation on Au/Ce0.9Ti0.1O2 nanorods in direct sunlight

Impressive rate of solar water oxidation to molecular oxygen (O₂) has been demonstrated on nanorods (NRs) of Ce_0.9Ti_0.1O₂ (CT-NR) and Au-deposited CT-NR (Au-CT-NR) photocatalysts with a sacrificial agent (Fe³⁺) and in plain water in one sun condition, direct sunlight and with λ ≥ 455 nm. Probably the highest O₂ yield of 11 mmol/h.g was observed with Au-CT-NR thin film in plain water in direct sunlight, with no sacrificial agent or applied potential. Photoelectrochemical measurements demonstrate a marked reduction in oxidation onset potential of Au-CT-NR by 150 mV with stable photocurrent (0.75 mA/cm²), compared to CT-NR (0.23 mA/cm²), indicating the operative of plasmon-induced resonant energy transfer (PIRET) process. Effective electron quenching by nanogold and hence low recombination in the depletion region is a critical step for the observation of a high rate of oxygen evolution. In addition to this, a predominant change in the nature of the valence band from O-2p dominated on CeO₂ to Ce-4f dominated with CT-NR (due to Ti⁴⁺ introduction in CeO₂), the efficient light absorption of photocatalysts in thin-film form, functional and effective PIRET process, and facile E_F alignment, enhances the oxygen evolution with Au-CT-NR in direct sunlight and make it highly sustainable. A possible mechanism of water oxidation is proposed from the observed experimental findings.

Graphical abstract

Extraordinary water oxidation capacity to O₂ in sunlight was achieved with a thin-film form of Au-CeTiO₂-NR by efficient light absorption followed by effective electron trapping in gold. This minimizes charge recombination and hence minority carriers were efficiently utilized. Reduction in overpotential, Fermi level equilibration and PIRET process due to nano-gold significantly improve the water oxidation capacity in a sustainable manner.