  Two deep level defects (2.25 and 2.03 eV) associated with oxygen vacancies
(V$_o$) were identified in ZnO nanorods (NRs) grown by low cost chemical bath
deposition. A transient behaviour in the photoluminescence (PL) intensity of
the two V$_o$ states was found to be sensitive to the ambient environment and
to NR post-growth treatment. The largest transient was found in samples dried
on a hot plate with a PL intensity decay time, in air only, of 23 and 80 s for
the 2.25 and 2.03 eV peaks, respectively. Resistance measurements under UV
exposure exhibited a transient behaviour in full agreement with the PL
transient indicating a clear role of atmospheric O$_2$ on the surface defect
states. A model for surface defect transient behaviour due to band bending with
respect to the Fermi level is proposed. The results have implications for a
variety of sensing and photovoltaic applications of ZnO NRs.
