Insights of Nanostructured Ferberite as Photocatalyst, Growth Mechanism and Photodegradation Under H2O2-Assisted Sunlight

Abstract

In this study, nanostructured ferberites (FeWO₄) were synthesized via hydrothermal routes in an acidic medium. It was then investigated as an efficient photocatalyst for degrading organic dye molecules, with methylene blue (MB) as a model pollutant. The formation mechanism of ferberite revealed that the physical form of the precursor, FeSO₄·7H₂O, acts as a decisive factor in morphological evolution. Depending on whether it is in a solid or dilute solution form, two distinct nanostructures are produced: nanoplatelets and self-organized microspheres. Both structures are composed of stoichiometric FeWO₄ (Fe: 49%, W: 51%) in a single monoclinic phase (space group P2/c:1) with high purity and crystallinity. The p-type semiconductor behavior was confirmed using Mott-Schottky model and the optical analysis, resulting in small band gap energies (≈1.7 eV) favoring visible absorption light. Photocatalytic tests under simulated solar irradiation revealed rapid and efficient degradation in less than 10 min under near-industrial conditions (pH 5). This was achieved using only a ferberite catalyst and a low concentration of H₂O₂ (4 mM) without additives, dopants, or artificial light sources. Advanced studies based on photocurrent measurements, trapping and stability tests were carried out to identify the main reactive species involved in the photocatalytic process and better understanding of photodegradation mechanisms. These results demonstrate the potential of nanostructured FeWO₄ as a sustainable and effective photocatalyst for water purification applications.

Keywords: advanced water treatment; ferberite; hydrothermal synthesis; methylene blue degradation; nanostructured materials; photocatalysis; simulated sunlight irradiation.

Figure 1

X-ray diffraction patterns of synthesized ferberite: (a) self-organized morphology and (b) nanoplatelets. (c) Coordination polyhedral (WO₆ and FeO_{6 o}ctahedral units) and (d) three-dimensional representations of the monoclinic unit cell of FeWO₄.

Figure 2

SEM/TEM of ferberite (a,b) self-organized and (c,d) platelets.

Figure 3

FTIR spectra corresponding to (a) stage 1 (acidification of a sodium tungstate solution to pH 1 with 3 M HCl), (a’) zoom section of spectra (a,b) stage 2 (addition of oxalic acid), (c) stage 3 (addition of iron sulfate heptahydrate) of the synthesis methodology and (c’) zoom section of spectra (c).

Figure 4

TEM images at different stages of ferberite growth (a) after mixing the precursors corresponding to step 3. (b) An amorphous zone of the sample. (c) Disk-forming stage and (d) growth form and formation of platelets. (e) Goethite-like platelets formation only with iron precursor.

Scheme 1

Growth mechanism of ferberite. (a₁) Tungstate oxalate. (a₂) Tungstate hydrate. (b₁) Iron–tungsten oxalic complexation. (b₂) Iron–tungsten hydrate and (c) final ferberite formation step. Reproduced with permission from Ref. [33]. Copyright Elsevier. 2011.

Figure 5

N₂ adsorption/desorption isotherm of ferberite self-organized and platelets.

Figure 6

Tauc plots and absorption spectra of FeWO₄: (a) nanoplatelets and (b) self-organized.

Figure 7

Catalytic activities of FeWO₄ under various conditions for MB dye photodegradation: photolysis without catalyst, with H₂O₂ addition, with ferberite and ferberite/H₂O₂ system. Self-organized (a–c) and platelets (d–f), at pH 3, 5 and 10, respectively.

Figure 8

Effect of pH on photodegradation of MB with and without H₂O₂. (a) self-organized, (b) platelets, and (c) pH_pzc of self-organized and platelets with MB molecule.

Figure 9

Photocatalytic degradation of MB at pH 5 with 4 mM H₂O₂: (a) self-organized microstructures, and (b) platelet morphology. Inserts: corresponding kinetic model fits, respectively, for self-organized (insert (a)) and platelet morphology (insert (b)). (c) Effect of H₂O₂ concentration on MB degradation. (d) Photograph color change in MB solution, after adsorption–desorption equilibrium and after 30 min of irradiation recyclability tests of (e) self-organized and (f) platelets samples.

Figure 10

Radicals trapping test by ferberite (self-organized and platelet morphologies) (measurements after 30 min).

Figure 11

The transient photocurrent curves of self-organized and platelet-shaped FeWO₄.

Figure 12

(a) Mott–Schottky plots (1/C² vs. V) of both FeWO₄ electrodes measured at 300 Hz. (b) Energy band diagram with VB and CB edge positions in the NHE scale with the key ROS redox potentials involved in photodegradation mechanism.