2, National University of Singapore, Singapore, , Singapore
Semiconductor compounds are widely used for water splitting applications, where photo-generated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr1-xNbO3, 0.03 < x < 0.20) have shown competitive photocatalytic efficiency, opening up the material space available for finding optimum performance in water-splitting applications1. In one of our previous report, the hot electron and hole carriers excited via Landau damping (during the plasmon decay) are responsible for the photocatalytic property of this material under visible light irradiation2.
To better understand the photocatalytic mechanism of such materials and to design high performance photocatalyst, we prepared high quality single crystal thin films of MNbO3 (M=Ca, Sr, Ba). We found that all MNbO3 are metallic oxides and have a very large bandgap of ~4.0 eV. Surprisingly the carrier densities can exceed 1022 cm-3, which is only one order smaller than that of elemental metals and the carrier mobility is only 2.47 cm2/(V×s). Contrary to earlier reports, the visible light absorption at 1.8 eV (~688 nm) is due to the bulk plasmon resonance, arising from the large carrier density, instead of an interband transition. By fitting the reflection spectra with Drude-Lorentz model, we show that the peaks of plasmon resonance in CaNbO3, SrNbO3 and BaNbO3 were at 693nm, 715nm and 682nm, respectively. The fit can also reproduce the experimentally observed optical absorption peak in the visible for all M values. Excitation of the plasmon resonance results in a multifold enhancement of the lifetime of charge carriers. Thus, we propose that the hot charge carriers generated from decay of plasmons produced by optical absorption is responsible for the water splitting efficiency of this material under visible light irradiation.2 The relative performance of the photocatalytic activity for different M (where the water splitting efficiency of CaNbO3 > SrNbO3 > BaNbO3) showed a strong correlation with the product of the hot carrier lifetime, solar energy absorption and surface area. This work lays the foundation for a novel class of photocatalysts driven by intrinsic plasmonic absorption with significant future promise for the field of sustainable energy sources.