Beware of Doping: Ta₂O₅ Nanotube Photocatalyst Using CNTs as Hard Templates

Nanostructuring constitutes a promising strategy to increase efficiency and stability of contemporary photocatalysts. Here, we report on the synthesis of highly crystalline Ta₂O₅ nanotubes (NTs) by using carbon nanotubes (CNTs) as sacrificial hard templates and elucidate the role of residual Fe nanoparticles - often used as catalyst for the CNT growth - on their photocatalytic performance toward H₂ evolution. We show that, when using as grown CNTs, the resulting Ta₂O₅ NTs contained detectable amounts of Fe and possessed negligible photocatalytic activity. When CNTs were, however, purified from Fe by thermally annealing the CNTs at 2100 °C, the same synthetic procedure yielded pure Ta₂O₅ NTs that showed a 40-fold increase in activity compared to the Fe-containing counterpart. A complementary set of analytical techniques in a combination with additional model experiments indicate that the detrimental effect of the residual Fe on the photocatalytic activity originates from atomic doping and formation of a segregated FeO_x phase within the Ta₂O₅ matrix that can both act as efficient electron traps. Our result highlights that the presence of residual catalyst needs to be taken into account when using CNTs as hard templates and generally illustrates a possible effect of unintentional dopants that are often not considered in preparing functional nanostructures.