| Summary: | The doping of Au/TiO<sub>2</sub> with FeO<sub>x</sub> is shown to result in a strong enhancement of its photocatalytic activity in the degradation of the azo dye Orange II. In order to examine the source of this enhancement, Au-FeO<sub>x</sub>/TiO<sub>2</sub> nanocomposites containing different molar ratios of Au:Fe were synthesized, and X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and high-resolution transmission electron microscope (HRTEM) analyses indicated that the TiO<sub>2</sub>-supported Au nanoparticles were partially covered with an amorphous layer of FeO<sub>x</sub> species, in which the iron was present as Fe<sup>2+</sup> and Fe<sup>3+</sup>. The metal-semiconductor system, i.e., Au/TiO<sub>2</sub>, showed only a moderate degradation rate, whereas doping with FeO<sub>x</sub> strongly enhanced the degradation activity. The bandgap energy decreased gradually from Au/TiO<sub>2</sub> (3.13 eV) to the catalyst with the highest FeO<sub>x</sub> loading Au-FeO<sub>x</sub> (1:2)/TiO<sub>2</sub> (2.23 eV), and this decrease was accompanied by a steady increase in the degradation activity of the catalysts. XPS analyses revealed that compared to Au/TiO<sub>2</sub>, on Au-FeO<sub>x</sub>/TiO<sub>2</sub> a much higher population density of chemisorbed and/or dissociated oxygen species was generated, which together with the decreased bandgap resulted in the highest photocatalytic activity observed with Au-FeO<sub>x</sub> (1:2)/TiO<sub>2</sub>. The processes occurring during reaction on the catalyst surface and in the bulk liquid phase were investigated using operando attenuated total reflection IR spectroscopy (ATR-IR) combined with modulation excitation spectroscopy (MES), which showed that the doping of Au/TiO<sub>2</sub> with FeO<sub>x</sub> weakens the interaction of the dye with the catalyst surface and strongly enhances the cleavage of the azo bond.
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