Visible-light photocatalytic activity of NH4NO3 ion-exchanged nitrogen-doped titanate and TiO2 nanotubes

•Nitrogen-doped titanate nanotubes and TiO2 nanotubes were obtained via the hydrothermal treatment.•The average diameter of both types of nanotubes was 11 nm, and the length varied with the NH4NO3 concentration.•The EPR spectra revealed that the nitrogen doping in TiO2 nanotubes induces NO and self-electron-trapped oxygen vacancies.•Under visible light, superoxide radicals are generated from the self-electron-trapped oxygen vacancies.

Titanium dioxide and titanate nanotubes (TiNTs) have attracted much attention because of their unique properties, which allow their application in energy conversion and storage devices, magnetic materials, electrocatalysis, and photocatalysis. However, these materials can only absorb UV radiation, which is approximately 5% of the incident solar radiation on Earth. The doping of TiNTs with metals or nonmetals, such as nitrogen, is one strategy that is used to make these materials sensitive to visible light. Here, we obtained TiNTs by hydrothermally treating anatase powder in a NaOH aqueous solution. The TiNTs were subsequently doped with nitrogen via an ion exchange process using different concentrations of NH4NO3 (1.0, 1.5, 2.0, 2.5, or 3.0 mol L−1) as the nitrogen source. After the ion-exchange process, the samples were calcined at 200 and 400 °C, which produced nitrogen-doped titanate nanotubes (NTiNTs) and nitrogen-doped TiO2 nanotubes (NTiO2NTs), respectively. All of the samples were characterized by X-ray diffraction, electron microscopy, and various spectroscopic techniques. Electron paramagnetic resonance (EPR) was used to identify and quantify the defects created in the nanotube structures from doping and the calcination process. Our analysis revealed that the number of defects in the NTiO2NTs depended on the nominal NH4NO3 concentration, which formed paramagnetic NO species and single electron-trapped oxygen vacancies (SETOVs). As proof of this concept, the nanotubes were used as photocatalysts with visible light for the degradation of methylene blue (MB). The degradation rate significantly improved depending on the NH4NO3 concentration when the NTiO2NTs were used as the photocatalyst.

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