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

The UV radiation-induced persistent photoconductivity (PPC) of single-crystal perfectly square SnO₂ nanotubes was investigated in a range of different O₂ and H₂O containing, and inert N₂ and argon, atmospheres. 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 (<i>τ</i>) was found to be significantly lower in combined O₂ and H₂O atmospheres compared to those in which either O₂ or H₂O was present on its own, suggesting a co-operative mechanism. This was confirmed by a new effect in which <i>τ</i> 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.