Effect of Fe Doping on NiTiO3 Structure for Hydrogen Generation by Photocatalysis

Abstract

Iron is one of the most widely used dopants for enhancing the photocatalytic properties of oxide semiconductors. In this work, nickel titanium ilmenite was synthesized by the sol–gel method and doped with small amounts of iron substituting for nickel to determine the dopant’s influence on its structural, optical, and photocatalytic properties. The materials were characterized by XRD with Rietveld refinement, Raman spectroscopy, FTIR, SEM, TEM, UV-Vis diffuse reflectance, XPS, and steady-state and time-resolved photoluminescence, and were evaluated for photocatalytic hydrogen generation, including reuse tests. The characterization confirmed a pure ilmenite phase characteristic of NiTiO3, with iron incorporated substitutionally and no secondary iron-containing phase. Raman deconvolution quantifies the resulting lattice disorder: the Ag mode at 710 cm−1 broadens from 44.8 to 60.4 cm−1 and loses more than 90% of its integrated area across the series. UV-Vis diffuse reflectance shows a systematic red shift in the apparent optical absorption edge, from 2.25 to 1.88 eV; the Urbach energy increases from 118 to 365 meV, a magnitude comparable to the shift itself, indicating that the red shift is dominated by disorder-induced band tailing rather than by a rigid displacement of the band edges. Time-resolved photoluminescence shows that the photogenerated carrier lifetime increases from 1.78 ± 0.01 ns in pristine NiTiO3 to 2.14 ± 0.02 ns in Ni0.97Fe0.03TiO3 and decreases again at higher dopant contents, reproducing the volcano-shaped dependence of the photocatalytic activity. The Ni0.97Fe0.03TiO3 sample exhibited the highest hydrogen production rate in the presence of ethanol as a sacrificial agent, 378.3 ± 15.6 μmol∙g−1h−1, a 31% improvement compared to pristine NiTiO3 (288.8 ± 10.8 μmol∙g−1h−1), achieved without any co-catalyst. Across the series, activity correlates strongly with carrier lifetime (r = 0.85) but not with the absorption edge (r = −0.33), identifying the regulation of charge carrier dynamics by Fe3+ trap states as the origin of the enhancement. The ilmenite structure was preserved after four consecutive reuse cycles. These results show that iron is an effective dopant for NiTiO3 and identify the optimum composition for subsequent development.