Fe₂O₃/TiO₂ nanoparticles – a complex structural study

V. Valeš¹, M. Buljan², S. Bernstorff³, S. Mangold⁴, V. Holý¹

¹Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Praha, Czech Republic

²Ruder Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia

³ Elettra – Sinctrotrone Trieste S. C. p. A., Strada Statale 14, 34149 Basovizza, Italy

⁴ ANKA Synchrotron Radiation Facility, KIT, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany

vales@mag.mff.cuni.cz

Titania (TiO₂)-based systems have been very intensively studied in last decades because of their photocatalytic activity, which found broad commercial applications [1]. Functionalized titania composites, especially Fe₂O₃/TiO₂ systems attracted a lot of attention recently, since they make it possible to improve the photocatalytic performance of titania [2]. The ε-phase of Fe₂O₃ exhibits a very large magnetic coercivity at room temperature so that Fe₂O₃/TiO₂ in solutions can easily be manipulated by external magnetic field. Fe₂O₃/TiO₂ compact thin layer composites as a photocatalyst can respond to visible light due to the narrow band-gap of Fe₂O₃. The optical and electronic parameters of Fe₂O₃/TiO₂ nanoparticles substantially depend on the width of their size distribution.

In our previous work [3] we dealt with semiconductor nanoparticles in amorphous silica matrix and we demonstrated that a self-ordering mechanism of the nanoparticles occurs during the deposition of multilayers. A spontaneous ordering of nanoparticles resulted in narrowing of the particle size distribution. In this work we use this approach for the improvement of the structure of Fe₂O₃/TiO₂ nanoparticle systems, namely we study the growth of (Fe₂O₃+TiO₂)/SiO₂ multilayers and the crystallization of the mentioned nanoparticles during post-growth annealing. We investigated the ordering and the size distribution of the particles using grazing-incidence small-angle x-ray scattering (GISAXS) and the inner crystalline structure by x-ray diffraction (XRD) and x-ray absorption spectroscopy (EXAFS).

The studied (Fe₂O₃ + SiO₂)/(TiO₂ + SiO₂)/SiO₂ samples were grown by a sequential deposition, in which 10-period multilayers (with a layers thickness 0.6, 1 or 2 nm and the SiO₂ spacer thickness of 10 nm) were deposited by electron beam evaporation onto rotating Si substrates at room temperature. The deposition rates were 10 Å/s for SiO₂ and 1 Å/s for both Fe₂O₃ and TiO₂. The samples have been subsequently annealed for 1 h at various temperatures in vacuum, air or forming gas (FG – Ar + 4% H₂). The partial results have already been published [4].

The powder xrd curves have been measured by a laboratory diffractometer with a standard x-ray tube (CuKα, 1.4 kW). We used a parallel-beam setup with a parabolic multilayer mirror in the primary beam and a parallel-plate collimator and a flat graphite monochromator (to reduce the fluorescence signal from Fe atoms) in the diffracted beam. The angle of incidence of the primary beam was kept constant at 0.5 deg to suppress the substrate signal. Small angle grazing incidence x-ray scattering has been carried out at ELETTRA synchrotron at the SAXS beamline with the photon energy of 8 keV. The incidence angle was a few tenths deg above the critical angle of total external reflection. The scattered radiation was recorded by MAR image plate (2000 × 2000 pixels). The necessary angular resolution was achieved by a large sample-detector distance of about 1.9 m; the air scattering was suppressed by an evacuated flight-tube. The x-ray absorption spectroscopy has been performed at XAS beamline at ANKA synchrotron. We measured the spectra in the range 150 eV below and 650 eV above the absorption edge of both Fe and Ti in the fluorescence mode. The measured data have been processed and analyzed by the standard software package of the programs Athena and Artemis.

The xrd data show clearly visible peaks corresponding to the rutile-TiO₂. An example of the evolution of the crystallinity with annealing temperature of the samples with the layer thickness of 2 nm annealed in the air is displayed in the Fig. 1(a). The crystallite size obtained by the Sherrer equation applied to the 110 rutile peak exhibits a systematic growth with annealing temperature ranging from 4 nm at 700 °C to 13 nm at 1000 °C. For the sample annealed at the highest temperature, additional peak, which may correspond to hematite-Fe₂O₃, arises.

The findings from the xrd data have been validated by the EXAFS measurements. The absorption spectra around Ti edge revealed the presence of rutile-TiO₂ as expected. From the measurements around Fe edge we supposed to get information on the local structure around Fe atoms that we did not obtain form the xrd. The data were simulated with a model assuming that the sample consists of two components. The first component is one shell of octahedrally coordinated oxygen atoms around an iron atom and the second one is two-shell hematite structure. We observed a systematic increase of the two-shell component at the expense of simple octahedrons.

The measured GISAXS maps have been compared to the simulations carried out by our software using a model of disordered particles in a hexagonal two-dimensional array including interface roughness. Individual layers are expected to be laterally fully disordered due to the fact that the particles grow during annealing after multilayer growth. An example of nicely ordered particles (expressed by well-defined lateral maxima in the GISAXS map) is displayed in the Figure 1(b). The results from the GISAXS maps simulations show that with increasing annealing temperature particle size increases as well as the degree of ordering. At the temperature of 1000 ºC the particle arrangement as well as inter-layer structure is broken, probably due to coalescence of particles from different layers. The particle sizes obtained from GISAXS are in the agreement with those obtained from xrd.

Figure 1. The evolution of diffraction profiles of samples annealed in the air (a) with diffraction peaks corresponding to rutile (the dashed lines, the grey scale corresponds to the relative peak intensity). The arrow denotes an additional peak corresponding to hematite-Fe₂O₃. In the panel (b) the GISAXS map in the logarithmic scale with the contour step of 10^0.15 from sample annealed in the air at 900 ºC is displayed.

From the analysis of all the data from all experiments we found out that titanium oxide tends to form ordered rutile particles from the annealing temperature of 700 ºC with increasing crystallinity and better ordering for higher temperatures. The particles are smaller and closer together for thinner samples and vice versa. The multilayer structure is preserved up to temperatures around 900 ºC. On the other hand, iron atoms form only very small particles that are not recognizable by xrd and most of the atoms is surrounded only by octehdrally coordinated oxygen atoms. The best arrangement of (rutile) particles has been achieved for the sample annealed at 900 ºC in the air.

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