Photoelectrochemical Properties of a Dinitrogen-Fixing Iron Titanate Thin Film Olga Rusina, ² Wojciech Macyk, ‡ and Horst Kisch* ,§ Department of Material Science and Engineering (Glass and Ceramics), UniVersity of Erlangen-Nu¨rnberg, Martensstr. 5, 91058 Erlangen, Germany, Faculty of Chemistry, Jagiellonian UniVersity, ul. Ingardena 3, 30-060 Krako´ w, Poland, and Institute of Inorganic Chemistry, UniVersity of Erlangen-Nu¨rnberg, Egerlandstr. 1, D-91058 Erlangen, Germany ReceiVed: January 19, 2005; In Final Form: March 17, 2005 The band edge positions of a nitrogen-fixing nanostructured semiconductor thin film are determined both in the dark through spectroelectrochemistry and under irradiation by photovoltage measurements. Both methods afford the same result indicating that the film in addition to the dinitrogen-fixing phase Fe 2 Ti 2 O 7 also contains titanium dioxide. Thus, both methods enable the analysis of a mixture of semiconducting thin films. For pH 7, values of -0.4 and +1.6 V were estimated for the conduction and valence band edge of the iron titanate film, respectively. A 3-fold photocurrent increase by methanol was observed only when the film was calcined at 600 °C but not below or above this temperature; the films calcined at temperatures other than 600 °C were also inactive in the photoreduction of dinitrogen. For a matter of comparison, an iron(III) oxide film was characterized analogously. Introduction Recently, we have found that a nanocrystalline iron titanate film photocatalyzes the fixation of dinitrogen into ammonia and nitrate. 1,2 The thin film was prepared by dip-coating of glass slides into an alcoholic solution of iron(III) chloride and titanium tetraisopropoxide in the ratio of Fe/Ti ) 1:1 followed by hydrolysis in humid air and annealing at 600 °C. Electron microscopy indicated the presence of a nanostructured matrix of about 300 nm in thickness. It contains cubic crystals with an average diameter of 150 nm. The ratio of Fe/Ti/O as determined by energy dispersive X-ray (EDX) analysis was 1:1:3.5 for both the matrix and the crystals. 1 This suggested that the film has the composition Fe 2 Ti 2 O 7 as also previously reported for an intermediary phase obtained by heating ilmenite minerals (FeTiO 3 ) in oxygen atmosphere at 700 °C. 3 This assignment was corroborated by the good agreement between published and measured X-ray diffraction (XRD) spectra. In addition, traces of the pseudobrookite phase Fe 2 TiO 5 and of anatase were found. In the Mo¨ssbauer spectrum, a doublet at the chemical shift of δ (relative to R-Fe) ) 0.462 mm s -1 , ΔE Q ) 0.910 mm s -1 (line widths of 0.294 mm s -1 ) revealed the presence of hexacoordinated iron(III). 2 For the sake of simplicity, this film is written as Fe 2 Ti 2 O 7 throughout this paper. Irradiation with visible or ultraviolet light of the thin film in the presence of dinitrogen, traces of oxygen, and ethanol or humic acid afforded ammonia and nitrate. 1,2 Mechanistic investigations revealed that in the first observable reaction step hydrazine is produced, which then undergoes further photo- reduction to ammonia. Oxidation of the latter by oxygen affords nitrate as the final product. Since the reaction occurred also in the presence of air and humic acids and since the iron titanate phase may be formed by weathering of ilmenite minerals, it may be a model for a presently unknown nonenzymatic nitrogen fixation in nature. 2 To rationalize the experimental findings, the following semiconductor mechanism was postulated for the ammonia formation. 1,2 Absorption of a photon by the Fe 2 Ti 2 O 7 phase (SC) generates a reactive electron-hole pair that is trapped at the surface (eq 1). Oxidation of ethanol by a valence band hole affords the hydroxyethyl radical (eq 2), which then injects an electron into the conduction band, a process known in photo- electrochemistry as “photocurrent doubling” (eq 3). The resulting two electrons in the conduction band reduce N 2 to diazene as a first plausible intermediate (eq 4). Thus, due to the current- doubling effect of ethanol, the absorption of only one quantum of light affords two electrons, which are required for the first reduction step. The faster the reactions steps according to eqs 2 and 4, the less efficient the undesired recombination of the reactive electron-hole pair (eq 5). The conversion of diazene to hydrazine may occur through disproportionation or photo- reduction. Subsequent reduction to ammonia is analogous to reactions according to eqs 1-4. To support the postulate that a semiconductor photocatalysis mechanism is operating, we investigated the basic photoelec- trochemical properties of this iron titanate film. The main objective was to determine the band edge positions and to find * Author to whom correspondence should be addressed. Phone/Fax: (+49) 9131 8527363. E-mail: kisch@chemie.uni-erlangen.de. ² Department of Material Science and Engineering (Glass and Ceramics), University of Erlangen-Nu¨rnberg. ‡ Faculty of Chemistry, Jagiellonian University. § Institute of Inorganic Chemistry, University of Erlangen-Nu¨rnberg. SC + hν f SC(e r - ,h r + ) (1) SC(e r - ,h r + ) + CH 3 CH 2 OH f SC(e r - ) + CH 3 C‚HOH + H + (2) SC + CH 3 C‚HOH f SC(e r - ) + CH 3 CHO + H + (3) 2 SC(e r - ) + 2H 2 O + N 2 f 2SC + N 2 H 2 + 2 OH - (4) SC(e r - ,h r + ) f SC + heat (5) 10858 J. Phys. Chem. B 2005, 109, 10858-10862 10.1021/jp0503314 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/05/2005