Hydrogen by photocatalysis with nitrogen codoped titanium dioxide

Titanium dioxide TiO2 remains a benchmark photocatalyst with high stability, low toxicity and cost, but it is active only under the UV light. To increase photocatalytic activity, TiO2 is "doped" with metals and nonmetals; nitrogen doped titania N-TiO2 has been extensively investigated since the early 2000s. In the recent decade, an increased attention has been paid to additional dopant aka "codopant" added to N-TiO2 to increase the photocatalytic rate. This focused critical Review covers the research on N-TiO2 codoped with an additional element for the photocatalytic hydrogen generation, namely: (1) mechanistic studies of charge separation aimed to understand and predict photocatalytic activity; (2) nonmetal codoped N-X-TiO2; (3) base metal codoped N-M-TiO2; (4) noble metal codoped N-M-TiO2. Suitability and limitations of experimental methods for characterization of codoped N-TiO2 are discussed. The following mechanisms of photocatalysis with codoped N-TiO2 are reviewed: (a) excitation of TiO2; (b) excitation of N dopant induced states; c) an increased electron hole (e-h) separation; (d) lowering over potential of hydrogen reduction; (e) excitation of the surface plasmon resonance (SPR) in N-TiO2 codoped with nanoparticles (NPs) of noble metals. Temporal stability of codoped N-TiO2 in H-2 generation and transformation pathways of sacrificial electron donors are discussed as well.