Behavior of N-Doped TiO2 and N-Doped ZnO in Photocatalytic Azo Dye Degradation under UV and Visible Light Irradiation: A Preliminary Investigation

Abstract

N-doped TiO2 (N-TiO2) and N-doped ZnO (N-ZnO) were synthesized utilizing ammonia as a dopant source. The chemico-physical characteristics of synthesized samples were studied by Raman spectroscopy, X-ray diffraction, SEM analysis, N2 adsorption–desorption at −196 °C, and diffuse reflectance spectroscopy. Compared to undoped samples, the introduction of nitrogen in the semiconductor lattice resulted in a shift of band-gap energy to a lower value: 3.0 eV for N-ZnO and 2.35 eV for N-TiO2. The photocatalysts were tested for the degradation of Eriochrome Black T (EBT), which was selected as a model azo dye. Both N-doped semiconductors evidenced an improvement in photocatalytic activity under visible light irradiation (62% and 20% EBT discoloration for N-TiO2 and N-ZnO, respectively) in comparison with the undoped samples, which were inactive in the presence of visible light. Different behavior was observed under UV irradiation. Whereas N-TiO2 was more photoactive than commercial undoped TiO2, the introduction of nitrogen in ZnO wurtzite resulted in a drastic reduction in photocatalytic activity, with only 45% EBT discoloration compared to total color removal obtained with the commercial ZnO sample, suggesting intrinsic limitations for doping of this class of semiconductors.

Photocatalytic Degradation of Ceftriaxone Using TiO2 Coupled with ZnO Micronized by Supercritical Antisolvent Route

“…The Raman bands for PC50 were observed at 144, 192, 393, 513, and 636 cm −1 , corresponding to the Eg(1), Eg(2), B1g(1), A1g + B1g(2), and Eg(3) of anatase modes, respectively [35]. The Raman signals observed for ZnO comm and ZnO SAS were ascribed to the active modes of wurtzite ZnO crystal [40]. In particular, the E2 (high) Raman mode observed at about 437 cm −1 and at 332 cm −1 , ascribed to E 2 (H)-E 2 (L) (multi-phonon), was identified [41,42].…”

Tuning the selectivity of visible light-driven hydroxylation of benzene to phenol by using Cu, Fe and V oxides supported on N-doped TiO2

“…The indirect band gap values of samples were obtained through the corresponding Kubelka–Munk function ( F ( R ∞ )) and by plotting [ F ( R ∞ )· hν ] 1/2 vs. hν (eV). 22,23…”

“…The indirect band gap values of samples were obtained through the corresponding Kubelka-Munk function (F(R ∞ )) and by plotting [F(R ∞ )•hν] 1/2 vs. hν (eV). 22,23 X-ray photoelectron spectroscopy (XPS) characterization of the catalysts' surface was performed on a PHI Versaprobe II spectrometer using a monochromatized Al-Kα source (47.6 W) at constant charge neutralization with a 200 µm spot size. Spectra were acquired on at least three points of the samples and processed with ULVAC-PHI MultiPak® (v. 9.9.3) software.…”

Visible Light Absorption Is Not Always Related to N-Doped TiO2 and High Photocatalytic Activity in Materials Synthesized by the Sol–Gel Method Using Urea and Ammonia as Precursors

“…Even today, it continues to appear in the literature studies claiming the synthesis of N-doped TiO 2 by sol–gel method using urea and ammonia as precursors and exhibiting high photocatalytic activity under visible-light irradiation. − For this reason, the main objective of this study is to attempt to demonstrate the formation of heterojunctions, defective TiO 2 surfaces, or surface complexes instead of N-doping in nitrogen-modified-TiO 2 when it is synthesized by the sol–gel method, using ammonia and urea as precursors. For this purpose, deep characterization of materials through techniques such as ATR-FTIR, DRS, XPS, HR-TEM, TGA-FTIR, low temperature (77 K) electron spin resonance spectroscopy (ESR), and XRD with Rietveld refinement was achieved.…”