An In-Depth Study of the Crystal Structure, Oxidation States, Defects, and Photocatalytic Activity of Fe-Doped WO ₃ Nanomaterials

WO3 is a semiconductor with a suitable valence band maximum (VBM) for the oxidation of water or hydroxide ions to generate hydroxyl radicals, which can be used to degrade organic compounds nonselectively. However, its poor crystallinity, rapid recombination of photogenerated electrons and holes, and an unfavorable conduction band minimum (CBM) for the single-electron reduction of O2 limit the practical application of this semiconductor. To ameliorate these drawbacks, Fe3+-doped WO3 nanomaterials were fabricated by a hydrothermal method and investigated using density functional theory (DFT) calculations to achieve a comprehensive understanding of how Fe3+ doping affects the properties of WO3. The crystallinity of WO3 was significantly improved, with phase transformation from monoclinic-WO3 to hexagonal-WO3 occurring in the presence of Fe3+. This transition is evidenced by the increase in the hexagonal phase fraction (φh) from 32% in WO3 to 61% in 7% Fe:WO3. In addition, the presence of Fe3+ at optimal levels created the intermediate energy states and oxygen vacancies on the surface to enhance electron–hole separation, reduce recombination, and improve the tetracycline (TC) adsorption capacity. As a result, the 7% Fe:WO3 demonstrated a photodegradation efficiency of 31.6% (k = 0.002 min–1), a 1.6-fold enhancement over the WO3. Furthermore, h+ and OH· were the main oxidizing agents, and the presence of the Fe3+ ion allowed the photocatalyst to utilize O2 for producing O2·, thus overcoming the disadvantages of WO3. Moreover, Fe3+ doping introduces shallow trap states in both the bulk and at the surface, which effectively suppress photogenerated electron–hole recombination and consequently facilitate the OH·/H2O redox process.