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

WO 3 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 O 2 limit the practical application of this semiconductor. To ameliorate these drawbacks, Fe 3+ -doped WO 3 nanomaterials were fabricated by a hydrothermal method and investigated using density functional theory (DFT) calculations to achieve a comprehensive understanding of how Fe 3+ doping affects the properties of WO 3 . The crystallinity of WO 3 was significantly improved, with phase transformation from monoclinic-WO 3 to hexagonal-WO 3 occurring in the presence of Fe 3+ . This transition is evidenced by the increase in the hexagonal phase fraction (φ h ) from 32% in WO 3 to 61% in 7% Fe:WO 3 . In addition, the presence of Fe 3+ 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:WO 3 demonstrated a photodegradation efficiency of 31.6% ( k = 0.002 min –1 ), a 1.6-fold enhancement over the WO 3 . Furthermore, h + and OH · were the main oxidizing agents, and the presence of the Fe 3+ ion allowed the photocatalyst to utilize O 2 for producing O 2 ·, thus overcoming the disadvantages of WO 3 . Moreover, Fe 3+ 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 · /H 2 O redox process.