  The incorporation of impurities during the chemical synthesis of
nanomaterials is usually uncontrolled and rarely reported because of the
formidable challenge that constitutes measuring trace amounts of often light
elements with sub nanometre spatial resolution. Yet these foreign elements
influence functional properties, by e.g. doping. Here we demonstrate how the
synthesis and partial reduction reaction on hollow TiO2 nanowires leads to the
introduction of parts-per-millions of boron, sodium, and nitrogen from the
reduction reaction with sodium borohydride at the surface of the TiO2 nanowire.
This doping explains the presence of oxygen vacancies at the surface that
enhance the activity. Our results obtained on model metal-oxide nanomaterials
shed light on the general process leading to the uncontrolled incorporation of
trace impurities that can have a dramatic effect on their potential use in
energy-harvesting applications.
