The Nature of Electron Transport and visible light absorption in Strontium Niobate -- A Plasmonic Water Splitter

TL;DR

This study reveals that visible light absorption in strontium niobate thin films is due to bulk plasmon resonance from high carrier density, enhancing charge carrier lifetime and potentially improving water splitting efficiency.

Contribution

The paper demonstrates that visible light absorption in Sr$_{0.94}$NbO$_{3+{ extdelta}}$ is caused by bulk plasmon resonance, not interband transitions, and links this to improved water splitting performance.

Findings

Carrier density reaches 10^22 cm^-3, close to metals.

Visible light absorption at 1.8 eV is due to plasmon resonance.

Charge carrier lifetime is significantly enhanced by plasmon decay.

Abstract

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 (Sr$_{1-x}$NbO$_3$, 0.03 < x < 0.20) have shown competitive photocatalytic efficiency, opening up the material space available for finding optimizing performance in water-splitting applications. The origin of the visible light absorption in these powders was reported to be due to an interband transition and the charge carrier separation was proposed to be due to the high carrier mobility of this material. In the current work we have prepared epitaxial thin films of Sr$_{0.94}$NbO$_{3+{\delta}}$ and found that the bandgap of this material is ~4.1 eV, which is very large. Surprisingly the carrier density of the conducting phase reaches 10$^{22}$ cm$^{-3}$, which is only one order smaller than that of elemental metals and the carrier mobility is only 2.47 cm$^2$/(V$\cdot$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. Excitation of the plasmonic 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.