Correlation between the H2 response and its oxidation over TiO2 and N doped TiO2 under UV irradiation induced by Fermi level

Abstract

In the previous work, we have ever found that the photocatalytic oxidation of a reactant gas over TiO2 would be somewhat dependent on the electron transfer behavior between the adsorbed gas and TiO2 under UV irradiation. To further confirm the viewpoint, in this work, a TiO2 (in-situ) sample was prepared by an in-situ method, and was tested its gas-sensing performance to H2 and its photocatalytic performance of oxidizing H2 as compared to a TiO2 (commercial) sample. It was found that TiO2 (in-situ) would increase the conductivity with the introduction of H2 under UV irradiation, but TiO2 (commercial) would decrease the conductivity in the same case. Based on the surface structural and electrochemical characteristics of samples, it was proposed that the existence of surface defects over TiO2 (in-situ) would decrease the Fermi level (EF), resulting in the electron transfer from the adsorbed H2 to TiO2, while the adsorbed H2 accepted electrons from TiO2 (commercial) due to its higher EF. Moreover, the adsorbed H2 on TiO2 (in-situ) could be oxidized under UV irradiation but that on TiO2 (commercial) could be hardly. This indicated that the photocatalytic oxidation of H2 over TiO2 would be dependent on the electron transfer direction between the adsorbed H2 and TiO2, i.e., the electron-donated H2 could be oxidized, while the electron-accepted H2 could be not. This above effect induced by the surface defects could be further demonstrated by a N-doped TiO2 (N-TiO2) sample. This N-TiO2 owned a lower EF than TiO2 (in-situ) due to the introduction of a more impurity defects, resulting in a more electron transfer from the adsorbed H2 to N-TiO2 and then the oxidation of more H2. This study also indicated that the adjustment of EF could improve the photocatalytic activity of oxidizing H2 by changing the adsorbed behavior of H2 over TiO2, which may be applicable for investigating other reactants’ oxidation behaviors over other semiconductor photocatalysts.