Abstract:

NaBiO₃ and Bi(NO₃)₃ were used to synthesize Bi-doped NaTaO₃. The influence of the Bi chemical state on the photocatalytic activity was investigated using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and diffused reflectance spectroscopy to study the structure, chemical state and light absorption characteristics, respectively. The photocatalytic activity was evaluated by the H₂ evolution water splitting reaction. The monoclinic phase of NaTaO₃ remained intact for the two Bi-doped samples, but the Ta-O-Ta bond was distorted from 180° after Bi doping. XPS results indicated that Bi³⁺ was doped into NaTaO₃ with the Bi(NO₃)₃ precursor, while Bi⁵⁺ and Bi³⁺ were doped into NaTaO₃ with the NaBiO₃ precursor. The two samples showed identical light absorption, where doping with Bi extended the light absorption to long wavelength light as expected. However, Bi³⁺ doping did not promote the photocatalytic activity of NaTaO₃, while Bi⁵⁺ and Bi³⁺ doping did. The distorted Ta-O-Ta bond from 180° due to doping with Bi was detrimental for charge carrier transfer in the photocatalytic process. In contrast, the vacancies or defects in the NaTaO₃ lattice induced by Bi doping for charge balance were beneficial for charge carrier separation. The opposing action of these two factors resulted in the activity of the Bi³⁺-doped sample being comparable with pristine NaTaO₃. For Bi⁵⁺- and Bi³⁺-doped NaTaO₃, a high concentration of defects was induced by the high valence Bi⁵⁺ ion and this led to its higher photocatalytic activity. Our results indicated that charge carrier transfer is a priority factor in the photocatalytic process and the doping of a high valence ion in the ABO₃ structure is a way to promote the separation of charge carriers.

Key words: Photocatalyst, Sodium tantalum oxide, Bi doping, Chemical state, Water splitting