Exploring the electronic structure of nitrogen-modified TiO2 photocatalysts through photocurrent and surface photovoltage studies

Wavelength resolved surface photovoltage (SPV) and photocurrent measurements in the presence of various reducing agents were conducted to obtain experimental evidence for the electronic structure of the valence band region in novel nitrogen modified anatase powders containing 0.5, 1.0, and 11.7 wt% of nitrogen. In the presence of air and absence of a hole scavenger a weak SPV signal was observable already in the visible region but completely vanished in the UV above 3.40 eV suggesting complete charge recombination. Whereas in the presence of formic acid only upon UV excitation the signal was increased, this occurred already upon Vis light irradiation when iodide was the hole scavenger. Similarly, in the photocurrent measurements visible light irradiation induced an enhancement only in the presence of iodide or hydroquinone, whereas water, thiocyanate, and formate were not oxidized efficiently since their redox potential is more positive than that of holes trapped in the region of the upper valence band edge. These findings were rationalized by assuming a strong electronic coupling of N 2p states with titania O 2p levels that generates a novel valence band with a red-shifted band edge. It is therefore expected that holes produced in this band will efficiently relax to the band edge followed by recombination with conduction band electrons. Depending on the reduction potential of the hole scavenger, interfacial electron transfer may successfully compete with this process.