New results on the synthesis of nitrogen-doped TiO2 and their application in heterogeneous photocatalysis under solar irradiation

Abstract

Regarding nitrogen doping of TiO₂, there is now more information in the literature that points to the development of more photosensitive materials for photocatalytic applications. To develop better photocatalysts, pure and nitrogen-doped TiO₂ (TiO₂:N) nanoparticles have been synthesized and widely studied. First, the bandgap value of both semiconductors was determined, finding that doping with N atoms decreases the bandgap value relative to the pure material, allowing the doped semiconductor to be excited with visible light below 403 nm. An important property of N-doped TiO₂ is that it has on average a smaller crystal size (15.2 nm) and a greater BET surface area (79.6 m²/g) than the commercial photocatalyst Degussa P-25 (23 nm and 48.6 m²/g, respectively), a fact that influences having a greater availability of active sites as a photocatalyst. Doping of TiO₂ with N caused the (101) and (200) diffraction peaks to shift towards higher 2$\theta$ values, which led to a slight decrease in the interplanar distances d₁₀₁ and d₂₀₀. After that, combining the experimental results obtained in this work and others already reported in the literature, it was possible to conclude N doping was achieved through a substitutional incorporation of N into the TiO₂ crystal structure rather than through an interstitial location. As an application of TiO₂ and TiO₂:N as photocatalysts, the treatment of the analgesic acetaminophen (ACT) in the aqueous phase was carried out by heterogeneous photocatalysis in a photocatalytic reactor using concentrated solar radiation. In comparison, the same degradation experiments were carried out but using the commercial photocatalyst Degussa P-25. The degradation percentages were as follows: the best were obtained with the TiO₂:N nanoparticles (90 and 95%), followed by pure TiO₂ (83 and 91%), and finally, those obtained with the Degussa P-25 photocatalyst (80 and 83%), all of them characterized by their COD and TOC, respectively.

Highlights

1. Doping of TiO2 with nitrogen (1%) had a positive influence on the crystal structure of the semiconductor, achieving a decrease in the forbidden energy band from 3.4 to 3.07 eV.

2.  Some diffraction peaks in the X-ray pattern experience a shift in the 2θ axis towards larger angles, indicating that the d_hkl distance corresponding to these crystal planes has decreased slightly. This shift is due to the electrostatic interaction between Ti⁴⁺⁺, N³⁻ and O²⁻ ions.

3. N atoms enter the TiO₂ crystal cell in a substitutional manner at the oxygen sites.

4. Pure TiO₂ and TiO₂:N semiconductors synthesized by the sol-gel/solvothermal method crystallize in an anatase phase and have a crystal size of 13.3 and 15.2 nm, respectively.

5. The rate constant of ACT degradation using different photocatalysts follows the order: 0.07231 (TiO₂:N) ˃ 0.02582 (pure TiO₂) ˃ 0.01425 (TiO₂ P25) min−¹.

6. N doping of TiO₂ allowed for achieving a higher ACT photodegradation efficiency, a reduction of photogenerated electron (e −) and hole (h +) recombination processes, and triggering an optimal photocatalytic activity compared to commercial TiO₂ P-25.