Doping can be difficult in aqueous systems because the
dopant species can easily form aquo ions and not be
incorporated into the crystal lattice. This is particularly true
for first-period transition metals, nearly all of which form
stable, inert complexes with water. Recently, transition-metal
doping of ZnO nanowires in aqueous solutions has been
demonstrated by applying a reducing potential to the growth
substrate that favors doping over metal-aquo complex
formation