Combined role of H2O and O2 adsorbates on the persistent UV photoconductivity of perfectly-square SnO2 nanotubes

Abstract

The UV radiation induced persistent photoconductivity (PPC) of single-crystal square SnO₂ nanotubes was investigated in a range of different O₂ and H₂O containing, and inert N₂ and Argon, ambients. Scanning tunnelling microscopy was used to characterize the atomically-smooth ‘single terrace’ and lamellar-like ‘step terrace’ regions of the SnO₂(110) nanotube sidewalls. The ends of the nanotubes were then coated with thick Ti/Au contact layers so that as much of the atomically-smooth SnO₂(110) surface as possible was exposed. The PPC relaxation time (τ) following exposure to above bandgap 285 nm UV radiation was found to be significantly lower in combined O₂ and H₂O ambients compared to those in which O₂ or H₂O were present on their own, suggesting a co-operative mechanism. This was confirmed by a new effect in which τ could be dramatically reduced and the PPC almost eliminated by a combination of O₂ and high humidity. A model is introduced in which this is explained by the formation of a strongly hydrogen bonded O₂⁻/H₂O network at high coverages that significantly inhibits the UV-induced desorption of superoxide O₂⁻ ions from the hydroxylated SnO₂(110) surface.