On the Solid-State Speciation of Fe-TiO2 for Visible-Light Photocatalysis

  • Dr Alexander Kokorin, Institute of Chemical Physics RAS, Kosygin st. 4, Moscow 119991, Russia
  • Dr Wey Yang Teoh, ARC Centre of Excellence for Functional Nanomaterials, The University of New South Wales, Sydney, NSW 2052, Australia
  • Prof Rose Amal, ARC Centre of Excellence for Functional Nanomaterials, The University of New South Wales, Sydney, NSW 2052, Australia

    • Substitutional doping of TiO2 with Fe(III) via Flame Spray Pyrolysis (FSP) was recently shown to effectively extend the photocatalytic response to visible light [1]. Nevertheless, little information regarding the solid-state speciation of Fe and its role in photocatalysis is currently understood. Hence the work investigates by EPR the multi-speciation of Fe(III) in TiO2 matrix.

    At low concentrations (0.5 at.%<[Fe]<5 at.%), two groups of spectral lines were seen: g~2.0 and at g ~ 4.2. Both groups are complex, e.g., several different isolated paramagnetic centers, attributed to the high-spin Fe(III) ions (S=5/2) in rhombic ligand fields state [2,3]. Increasing Fe content resulted in noticeable line broadening of both groups up to [Fe]=5 at.%. At [Fe]=10 at.% the spectrum became very broad and weak, and at [Fe]>10 at.% it transformed to a broad (line width ΔB~200 mT) single line at g~2.07, typical for Fe(III) ferromagnetic clusters.

    Interesting correlations were obtained when comparing the EPR results with the activity of Fe(III)-TiO2. From 0.5 to 5 at.% Fe content, the photocatalytic mineralisation of oxalic acid increased approximately 3.5 times [1]. This is in strong agreement with the monotonous increase of isolated Fe(III) ions concentration. At [Fe]>5 at.%, ferromagnetic Fe clusters started forming at the expense of isolated ions, and resulting in the loss of visible light photocatalytic activity.

    1. Teoh W.Y. et al., Catal. Today, 120, 203-213 (2007).
    2. Abragam A., Bleaney B. Electron Paramagnetic Resonance of Transition Ions. Oxford Univ. Press, New York, 1970.
    3 Walker F.A. Coord. Chem. Rev., 185-186, 471-534 (1999).