Highly Visible Light Active TiO_2−xN_x Heterojunction Photocatalysts

Abstract

Nitrogen doped anatase-rutile heterojunctions are successfully synthesized through an ethylenediaminetetraacetic acid (EDTA) modified sol−gel process. An FT-IR study of EDTA modified TiO2 gel confirms the existence of an ionic intermediate (as indicated by a Δν value of 233 cm−1). Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Raman spectroscopy are employed to study the phase evolution, phase purity, and crystallite size of samples. Formations of O−Ti−N and N−Ti−N bonds in calcined samples are confirmed using XPS and FT-IR spectroscopy. All EDTA modified samples show significantly higher visible light photocatalytic activity than the unmodified sample. The most active nitrogen doped heterojunction obtained at 400 °C exhibits 9-fold visible light activity in comparison to the standard photocatalyst Degussa P-25. It is proposed that the photo excited electrons (from the visible midgap level) are effectively transferred from the conduction band of anatase to that of rutile causing effective electron−hole separation, which is responsible for the higher visible light activity and lower photoluminescence (PL) intensity.

Effect of N-doping on the photocatalytic activity of sol–gel TiO2

“…Induced low temperature rutile formation in the presence of nitrogen is contradictory to previous reports by this group [41]. It has been previously observed that using urea as a nitrogen source alters the condensation pathway, extending the temperature at which rutile forms [42]. However, the current study shows that 1,3-diaminopropane stabilises titanium tetraisopropoxide as a monomer which will contribute to the low temperature formation of rutile.…”

“…6, show peaks with binding energies of ∼401 eV which is higher than the typical N1s value but can be attributed to the binding of the 1s electron of N in the environment of O-Ti-N in lattice N-doped TiO 2 . The change in binding energy is due to the N substituting for oxygen in the O-Ti-O structure and the electron density around nitrogen becoming reduced compared to that of a TiN crystal, due to the presence of the oxygen atom [42]. This observation is consistent with literature on the oxidation of TiN surfaces [44].…”

“…However, N-TiO 2 calcined at the same temperature (600 • C) has begun to transform into rutile (46%). Crystallisation results show that inclusion of nitrogen within the lattice reduces the anatase-to-rutile transformation temperature, which is accredited to nitrogen within the lattice acting as nucleation sites for the anatase-to-rutile transformation [42]. Substitutional nitrogen doping was also previously reported to improve the visible light photocatalytic activities of nitrogen doped anatase-rutile heterojunctions.…”

“…The peak is at higher energy than would be expected for an undoped TiO 2 and have previously been assigned to the formation of O-Ti-N and N-Ti-N bond formation. [42] The infrared spectra of the N-TiO 2 calcined at 500 • C shows no peaks in this region, indicating that the Ti-N bond has not formed.…”

Comparative photocatalytic degradation of monoazo and diazo dyes under simulated visible light using Fe3+/C/S doped-TiO2 nanoparticles

“…In all the spectra, the strong and broad band below 1000 cm -1 is assigned to the combined bands of Ti-O-Ti, Fe-O-Ti, S-O-Ti and O-Ti-C crystal vibrations, 24 and the absorption bands at 1628 cm -1 and around 3500 cm -1 are due to -OH bending and stretching vibrations respectively as a result of absorbed water molecules. For the C/S-doped Ti-O 2 and Fe 3+ /C/S-doped TiO 2 photocatalysts (Figure 2b, 2c, 2d and 2e), the peak located at 1065 cm -1 may be attributed to bidentate sulphate ions (SO 4 2-) co-ordinated to metal ions such as T 4+ .…”