TiO2 nanoparticles—a complex structural study

TSF-33457; No of Pages 8 Thin Solid Films xxx (2014) xxx–xxx

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Fe2O3/TiO2 nanoparticles—a complex structural study V. Valeš a,⁎, M. Buljan b, V. Janicki b, S. Bernstorff c, S. Mangold d, Z. Siketić b, O. Schneeweiss e, V. Holý a a

Charles University in Prague, Czech Republic Rudjer Boskovic Institute, Zagreb, Croatia ELETTRA Sincrotrone Trieste, Italy d ANKA Synchrotron, Karlsruhe, Germany e Institute of Physics of Materials, ASCR, v. v. i, Brno, Czech Republic b c

a r t i c l e

i n f o

Article history: Received 15 October 2013 Received in revised form 28 March 2014 Accepted 7 May 2014 Available online xxxx Keywords: Ferric oxide Titanium oxide Nanoparticles Multilayer Particle arrangement X-ray absorption spectroscopy X-ray diffraction Grazing incidence small angle scattering

a b s t r a c t We report on the structure and arrangement of particles created in the Fe2O3/TiO2 + SiO2 multilayers. X-ray diffraction and extended X-ray absorption fine structure spectroscopy reveal the presence of crystalline rutile-TiO2 while the iron oxide remains either amorphous or forms very small clusters of Fe2O3. The Fe3+ oxidation state of iron atoms has been confirmed by Mössbauer and X-ray spectroscopy. The degree of the particle ordering has been studied by grazing-incidence small-angle X-ray scattering. It was demonstrated that with increasing temperature partially ordered nanoparticles are created and grow up to a critical temperature when the ordering is destroyed. Both particle sizes and inter-particle distances depend strongly on the thickness of the Ti/Fe containing layer. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Titania (TiO2)-based systems have been very intensively studied because of their photocatalytic activity in the last decades, which found broad commercial applications [1–3]. Functionalized titania composites, especially Fe2O3/TiO2 systems, attracted a lot of attention recently (see Refs. [4–8] among others), since they improve the photocatalytic performance of titania [9]. Fe2O3/TiO2 composite compact thin layer can respond as a photocatalyst to the visible light due to the narrow band-gap of Fe2O3. TiO2/Fe2O3 composites have been tested for photoelectrochemical water splitting process [10]. Both materials are suitable for massive production because of their non-toxicity and low cost. The optical and electronic properties of Fe2O3/TiO2 nanoparticles depend substantially on the width of their size distribution; therefore a way of producing these nanoparticles with uniform sizes is necessary. In our previous work [11] we have shown that for semiconductor nanoparticles a spontaneous ordering of nanoparticles resulted in narrowing of the particle size distribution. Now we use this approach to obtain ordered Fe2O3/TiO2 nanoparticles in silica matrix. We have already studied a similar system [12] with both pure Fe2O3 and Fe2O3/ TiO2 multilayers. The ordered rutile-TiO2 particles have been observed but no crystalline phase of iron oxide could have been detected. In this ⁎ Corresponding author at: Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, Praha, 121 16, Czech Republic. Tel.: +420 221 91 1482. E-mail address: [email protected] (V. Valeš).

work we bring a more detailed investigation of Fe2O3/TiO2 multilayers prepared under different conditions by combining several independent methods (powder X-ray diffraction, grazing-incidence small-angle scattering, extended X-ray absorption fine structure, time-of-flight elastic recoil detection analysis and Mössbauer spectroscopy) in order to clarify the structure of iron atom surroundings and to optimize the processing conditions to obtain well-ordered nanoparticles.

2. Experimental 2.1. Sample growth We have studied (Fe2O3 + SiO2)/(TiO2 + SiO2)/SiO2 periodic multilayers with the thicknesses of both iron oxide and titanium oxide containing layers of 0.6 nm, 1 nm, or 2 nm (Ti/Fe-containing layers) and the SiO2 spacer of 10 nm. The samples were grown by a sequential deposition, in which 10-period multilayers 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 base pressure was kept to 6.6 × 10−4 Pa, the partial pressure of oxygen to 6.6 × 10−3 Pa during evaporation of SiO2 and 1.2 × 10−2 Pa during evaporation of Fe2O3 and TiO2. The mass thickness was controlled by a quartz crystal monitor. The samples have been subsequently annealed for 1 h at various temperatures (from as-grown up to 1000 °C) and in various atmospheres (air, vacuum, forming gas – FG Ar + 4% H2). The most

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Please cite this article as: V. Valeš, et al., Fe2O3/TiO2 nanoparticles—a complex structural study, Thin Solid Films (2014), http://dx.doi.org/10.1016/ j.tsf.2014.05.016

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Table 1 Parameters of the sample preparation and the particle parameters following from the GISAXS simulations (base vector of the ordering a, thickness of one period of the multilayer c, lateral and vertical root mean square deviations σL and σV, lateral and vertical radius of the particle rL, rV) and XRD analysis (particle radius rXRD). The * sign denotes that only ratio of lateral and vertical radii could have been obtained. The parameters denoted by “–” could not be fitted, the parameters denoted by “/” have not been measured. The errors of the values are discussed in the text. The table does not contain the non-annealed samples and the samples annealed at 300 °C. Sample 700 °C air

700 °C FG

700 °C vacuum

800 °C air

800 °C FG

800 °C vacuum

900 °C air

900 °C FG

900 °C vacuum

1000 °C air

1000 °C FG

0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm 0.6 nm 1 nm 2 nm

a (nm)

c (nm)

σL (nm)

σV (nm)

rV (nm)

rL (nm)

rxrd (nm)

– 18 28 7 30 30 8 12 – 9 15 28 7 15 30 8 12 25 12 15 26 10 12 28 – –

– 12.5 14.5 11.8 13.7 13.7 11 11.5 13 11 11.5 11.7 10.7 11.5 13.3 10.8 10.8 11.8 11.7 11.2 13.2 10.5 10.5 12.5 – – – – – – – – –

– 9 10 2.5 20 20 3.2 4.8 – 3.2 5.4 10 4.7 7.5 20 4.0 6.0 12.5 4.3 5.4 9.3 6.7 6.0 18.7 – – – – – – 7 10 15

– 0.5 0.5 0.5 0.5 0.5 0.4 0.5 – 0.4 0.5 0.5 0.4 0.5 0.5 0.4 0.4 0.5 0.5 0.4 0.5 0.4 0.4 0.5 – – – – – – 3.7 3.7 3.7

– 1.5 1.5 1.2 1.7 1.7 1.6 1.6 1.8 1.3 1.7 2.2 1.4 1.8 1.8 1.5 2.0 2.4 1.6 2.0 3.0 1.6 1.8 2.2 1.6* 2.2* 2.7* 3.0* 3.0* 3.0* 1.4 1.8 2.5

– 1.3 2.2 1.1 2.5 2.5 1.3 1.6 2.5 1.1 2.0 2.8 1.4 1.8 3.0 1.5 1.9 3.0 1.7 2.2 4.5 1.3 2.0 3.0 1.5* 2.0* 2.7* 3.0* 3.0* 3.5* 1.6 1.8 2.5

/ / 2.1 / / 2.9 / / 2.4 / / 2.4 / / 2.7 / / 4.8 / / 3.6 / / – / / 7.1 / / 6.6 / / 4.4

– – – 10 15 30

important samples (i.e. annealed at temperatures of 700 °C and higher) are listed in Table 1. 2.2. Characterization methods X-ray diffraction (XRD) curves of the samples 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° to suppress the substrate signal. Small-angle grazing-incidence X-ray scattering (GISAXS) 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 of degree, i.e. just above the critical angle of external reflection. The scattered radiation was recorded by a 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 measured GISAXS maps have been compared to the simulations carried out by our software [11,13] using a model of disordered ellipsoidal particles in a hexagonal two-dimensional array, where the individual layers are expected to be randomly laterally shifted with respect to the following one. Since the particles in the layers grow only during annealing after deposition (separated by thick SiO2 spacers), there is no reason to expect any correlation of particle positions in different layers. Therefore, for the purpose of calculation we assumed that all particles (titanium and iron oxide) occur in a single layer, denoted as Ti/Fe containing layer, which is separated by the SiO2 spacer from the following one. Since the positions of particles in different layers are statistically independent, the X-ray waves scattered by various layers have

random phases and their superposition can be calculated incoherently, i.e. we add the intensities stemming from individual layers of particles. The shape of the particles is described by their lateral and vertical radii rL, rV and by the order of their Γ-distribution mR, which is connected to the pffiffiffiffiffiffiffi dispersion σ of radius r by σ ¼ r= mR. The particle array in a single layer is constructed assuming a two-dimensional short-range order model [14] described by a disordered hexagonal array with mean basis vectors a1,2 forming the angle of 120° and the same length a. The actual vector connecting the centres of the neighbouring particles L is random with the mean 〈L〉 = a. The inter-particle distance is therefore described by its mean value (a) and by its lateral (σL) and vertical (σV) root mean ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi rD   E Ljj −a2 ; σ V ¼ square deviations for which we can write σ L ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D Effi jL⊥ j2 ; where L|| and L⊥ are the components of the vector L parallel and perpendicular to the array plane, respectively. As we show later, not all Ti and/or atoms create the particles and a non-negligible amount of these atoms is still dispersed in the layers between the particles. This fact is the reason for a contrast in the refraction indexes of the Ti/Fe containing layers between the particles and the SiO2 spacers. The schematic picture of the multilayer system used in our simulations is depicted in Fig. 1. Since the interfaces of the layers are rough, and the roughness profiles of different interfaces are correlated, they give rise to diffuse Xray scattering that has to be included in the simulations. The simulation program includes also X-ray refraction and absorption effects in the SiO2 matrix by using distorted-wave Born approximation method (see Ref. [15] for details). The extended X-ray absorption fine structure (EXAFS) spectroscopy has been performed at the XAS beamline at ANKA synchrotron source.

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therefore they are not shown here. The lattice parameters of the rutile phase correspond to the nominal values a = 4.593 Å, c = 2.959 Å [19]. The formation of rutile particles is therefore evident. Since the diffraction signal is rather weak and only the most intense diffraction maximum can be clearly observed for all annealing temperatures (diffraction 110 at 2θ ≈ 27.4°), we just investigated the particle size evolution with the temperature by means of the Scherer equation [20] d¼

Fig. 1. A schematic picture of the multilayer system. The light grey colour denotes the SiO2 spacer, the dark grey colour denotes the Ti-Fe-containing layers where the particles form. The particle lateral radius rL together with the basis vectors a1,2, lateral and vertical root mean square deviations σL, σV and the actual vector between two particles L are depicted in the topmost layer.

We measured the EXAFS spectra in the range from 150 eV below up to 650 eV above the absorption edge of both Fe and Ti in the fluorescence mode. The measurement step was 0.5 eV in the vicinity of the absorption edge and 1 eV elsewhere. The measured data have been processed and analysed by the standard software package of the programs Athena and Artemis [16,17]. The X-ray absorption near edge structure (XANES) signal has been simulated using the FDMNES software [18] by the finite difference method. The calculations have been performed for the particle radius of 6 Å. 57 Fe Mössbauer spectra were measured at room temperature using ~50 mCi 57Co(Rh) radioactive source, the detection of conversion electrons (CEMS) and calibrated against α-Fe as the standard. The values of the isomer shift are related to α-Fe at room temperature. The computer processing of the spectra yielded the values of the relative spectrum area I and values of the hyperfine parameters including isomer shift δ and a quadropole splitting Δ. The time of flight–elastic recoil detection analysis (ToF-ERDA) was measured using 127I ions with the energy of 15 MeV. The incidence angle towards the sample surface was 20° and the scattering angle towards the beam direction was 37.5°. 3. Results 3.1. X-ray diffraction For the XRD studies we chose the samples with the nominally thickest Ti/Fe-containing layer since these samples consist of the most material and therefore their diffraction signal is stronger than from the thinner ones. From the XRD data it can be seen that diffraction maxima corresponding to rutile-TiO2 appear for all annealing atmospheres at least for higher temperatures (Fig. 2). All the samples annealed at lower temperatures than 700 °C do not exhibit any diffraction maxima;

Kλ ; B cosðθÞ

where d is the mean crystallite size, K = 0.89 is the shape factor of a spherical particle, λ is the radiation wavelength, B the full width at half maximum and θ is the diffraction angle. The results are summarized in Table 1 where it can be seen that the crystallite sizes increase with increasing annealing temperature. The uncertainty of the crystallite size determination is roughly 10%. No diffraction maximum corresponding to anatase-TiO2 has been observed for any sample, which suggests that at the temperature around 700 °C rutile starts to form directly from the amorphous titanium oxide or very small (non-detectable) anatase particles. This observation is supported by the literature data from sol-gel prepared samples [21], where anatase diffraction peaks get broader with increasing amount of SiO2 in the sample. Therefore, possible anatase diffraction peaks are hardly recognizable in our case of few nanometres thick multilayers. The presence of crystalline iron oxide particles is not proved by XRD, however for the sample annealed at 1000 °C in the air a peak that might correspond to the most intense (104) peak of hematiteFe2O3 is observed. The peak is denoted by an arrow in Fig. 2(a). Similar observation regarding the presence of silica has been reported [22] for samples prepared by spin-coating technique. The authors obtained nice hematite diffraction maxima for pure iron oxide samples, while samples with any content of silica seemed to be fully amorphous up to 800 °C in XRD. The nature of the Fe atoms has been studied further by EXAFS. 3.2. Grazing-incidence small-angle X-ray scattering The measurement in the GISAXS geometry is not sensitive to the crystal structure of the studied material but to the features in bigger scale like particle shape and their ordering [15]. This means that we are not able to determine the nature of particles giving rise to the GISAXS signal (Ti or Fe oxides). Three typical examples of the measured maps and their fits are displayed in Fig. 3. In panel (b) there is a fit of the measured map (a) of the sample annealed at 700 °C in the vacuum with the Ti/Fe-containing layer thickness of 1 nm. This example illustrates the behaviour of the samples annealed at low temperatures. The denoted sections of the measured and simulated data are displayed in Fig. 4. Obvious sheets come from the scattering on the multilayer system (Fig. 4(b)), indicating that a significant part of Ti/Fe atoms still remains homogeneously dispersed in the SiO2 matrix. Subtle lateral maxima in Fig. 4(a) indicate the beginning of formation of ordered particles. Samples annealed at lower temperatures are characterized by even more pronounced sheets and no lateral maxima. It means that the data do

Fig. 2. The evolution of X-ray diffraction curves with annealing temperature for the samples with the thickness of Ti/Fe-containing layer of 2 nm annealed in the air (a), forming gas (b) and vacuum (c). The lines below the graphs indicate the positions of rutile-TiO2 diffraction peaks. The position of the strongest diffraction peak of hematite-Fe2O3 is indicated with an arrow.

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Fig. 3. The measured (a, c, e) and simulated (b, d, f) GISAXS maps of the samples annealed in the vacuum at 700 °C (a, b), in the air at 900 °C (c, d), and in the forming gas at 1000 °C (e, f). The maps are in a logarithmic scale with the contour step of 100.15. The denoted sections are displayed in the Fig. 4. The disturbances denoted by arrows come from the dynamical multiple scattering and they have not been included in the simulations.

not contain any particle-related information and therefore they are not shown here. In Fig. 3(d) we plotted a fit of the measured map (c) of the sample annealed at 900 °C in the air with the layer thickness of 1 nm. In this case we still can see the sheets but most of the material has transformed into particles. The particles are ordered as can be seen from very nicely pronounced lateral maxima. All these features can be seen in the sections in Fig. 4(c, d). Sample annealed at 1000 °C in the forming gas with the layer thickness of 1 nm (the measured map is displayed in Fig. 3(e), its fit in panel (f)) practically does not exhibit the sheets (Fig. 4(e)). It means that the refractive index of the effective Ti/Fe-containing layer is almost the same as of the SiO2 spacer and all the material (iron and titanium oxide) has formed particles. The particles are even not ordered—only very small indication of lateral maxima can be seen (Fig. 4(f)). This can be explained by the fact that as the particles grow, they agglomerate together which destroys the original arrangement. The results of the numerical analysis of GISAXS maps (particle size, mean inter-particle distance and its root mean square deviations) are listed in Table 1 and in Fig. 5. Several observations and systematic tendencies are obvious from these data. The thicknesses of one period are

in a good agreement to the nominal values. The errors of determination of these values are estimated to be ~5%, since the presence of multiple sheets enable reliable determination of the period thickness. The particle sizes (the uncertainty of their determination is around 10%) slightly increase with the annealing temperature in all samples. Their values, however, depend mainly on the layer thickness. At 800 °C the mean particle radius for the Ti/Fe-containing layer thickness of 2 nm is around 3 nm, for the layer thickness of 1 nm around 2 nm, and in the case of 0.6 nm thick layers around 1.5 nm. No systematic differences with the annealing atmosphere have been observed (Fig. 5(a–c)). The simulations are not much sensitive to order of the Γ-distribution of the particles sizes, therefore its value has been fixed to mR = 5. The evolution of the mean inter-particle distance a is shown in Fig. 5(d–f). Generally, the inter-particle distances remain the same depending on annealing temperature, within the uncertainty, which is larger for thin and low-temperature annealed samples because of the bigger noise. Again, various atmospheres do not cause any distinct systematic change. The mean inter-particle distance, however, changes with the layer thickness: from ~28 nm for the layer thickness of 2 nm, ~18 nm for the layer thickness of 1 nm, to ~9 nm for the layer thickness of 0.6 nm. The root mean square deviations that describe the degree of

Fig. 4. The horizontal and vertical sections of the measured and simulated maps displayed in Fig. 4 of the samples annealed in the vacuum at 700 °C (a, b), in the air at 900 °C (c, d), and in the forming gas at 1000 °C (e, f). The panels (a, c, e) show the horizontal sections while the panels (b, d, f) show the vertical ones. The side maxima observed in the horizontal section in the panels (a, c) come from the arrangement of the particles and are denoted as lateral maxima. The peaks observed in the vertical section (b) stem from crossing the sheets that come from the scattering on the rough multilayer system. The decrease of the measured intensity around 0 in the horizontal sections is caused by shadowing of the beam stop.

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Fig. 5. The graphical depiction of the selected results. In panels (a–c) there are radii of a sphere with the same volume as the ellipsoidal particle obtained from GISAXS. Panels (d–f) show the evolution of the mean inter-particle distance as a function of the thickness of Ti/Fe-containing layer. The dotted black line stands for samples annealed in the air, dashed grey for the samples annealed in the forming gas and the solid light grey line stands for samples annealed in the vacuum. The annealing temperatures are 900 °C (a, d), 800 °C (b, e), and 700 °C (c, f). The error of mean inter-particle distances and particle sizes is 10%.

the particle ordering are determined with the uncertainty of 20% (Table 1). The particle sizes are correlated with the mean inter-particle distances for different thicknesses of Ti/Fe containing layers. As in the thicker layer bigger particles are created they consume more material from their surroundings and therefore the neighbouring particle has to be formed farther compared to the thinner layers. The GISAXS simulations have not been much sensitive to the roughness parameters. The signal coming from the interface roughness is caused by the Ti/Fe atoms dispersed in the silica matrix among the particles and not from the atoms gathered in the particles already. For the study of the particles this signal is therefore not essentially important. The lateral and vertical correlation lengths did not differ systematically and their values were in the range: LL = (3–6) nm, and LV = (20–100) nm. The best particle ordering (expressed by the lowest relative lateral root mean square deviation σL) is obtained for the samples annealed in the air at 800 °C and 900 °C for all the investigated thicknesses of the Ti/Fe-containing layers. The samples annealed in the air and in the forming gas preserve particle ordering up to 900 °C (at 1000 °C the ordering is already destroyed) while the ordering in the samples annealed in vacuum is destroyed already at 900 °C. The temperature just below the critical temperature exhibits the highest degree of ordering for the specific annealing atmosphere as the most atoms from the layer constitute the particles. In some cases (for high annealing temperatures) only ratio of lateral and vertical radii was obtained. This happened where there were no residues of lateral maxima coming from particle ordering present. Then the scattering feature corresponds to the particle shape but there is no measure to determine the absolute sizes.

representative samples is displayed in Fig. 6. Panel (a) displays k2weighted measured spectra after edge subtraction from the samples annealed at 300 °C, 800 °C and 900 °C. The curves are rather similar, i.e. no dramatic structure changes happened during annealing at different conditions. Subtle changes however can be seen—especially the appearance of a peak around 5 Å−1 (denoted by an arrow). Changes in the intensity of this peak are represented in the real space by changes in the second most intense peak above 2 Å (Fig. 6(b)). The parameters of the data processing were set as follows: the Fourier transform was performed in the range 2 Å−1 to 8 Å−1 and k, k2, and k3-weighted EXAFS signals were used for the data fitting. Samples annealed at low temperatures do not show the mentioned peak and the spectrum can be fully described by a Fe atom octahedrally surrounded with oxygen atoms with Fe–O distance of (1.97 ± 0.01) Å which is in agreement with the length of Fe–O bonds in iron (III) oxides [23]. Therefore this model corresponds to amorphous

3.3. X-ray absorption spectroscopy The EXAFS spectra were measured in order to support the XRD results as for the structure of TiO2 particles and to determine the atomic ordering of iron oxide. We focused mainly on the energy region around the FeK edge since diffraction gave us very little information on the coordination of iron atoms. We measured the spectra of samples annealed at various temperatures and atmospheres (from as-grown up to samples annealed at 1000 °C). An example of EXAFS spectra from

Fig. 6. The EXAFS signal at the Fe-edge in the k-space (a) of samples annealed at 300 °C, 800 °C, and 900 °C and its transformation to the R-space (b). The additional peak that is arising at higher temperatures is denoted by an arrow.

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iron oxide. The presence of the additional peak is connected to one more shell of the iron oxide structure. In order to fit the data a model containing of two structures—hematite–Fe2O3 and Fe–O octahedron—was supposed. Hematite phase of Fe2O3 was used because possible presence of this phase at high temperatures has been suggested by XRD (Fig. 1). From the EXAFS analysis it is however not possible to distinguish between other polymorphs of Fe2O3 since the distances between closest and second closest atoms are very similar. An example of such fit for the sample annealed at 900 °C in the air is displayed in Fig. 7(a). We obtained a relative content of both considered structures for all the samples and the results are summarized in Fig. 8. The spectra at the Ti edge were measured only for two samples— non-annealed and annealed at 900 °C in the air. A clear difference of both spectra was observed. The non-annealed sample can be fully described by a Ti–O tetrahedral model, while the annealed one corresponds to the rutile-TiO2 (Fig. 7(b)). This observation is in agreement with the XRD results. We investigated also the XANES region of the spectra in order to determine the oxidation state of Fe atoms. An increase of oxidation state is generally represented by a shift of the absorption edge to higher energies [24]. Since the shape of the absorption edge was similar for all the samples, for the analysis we selected the one annealed in the air at 900 °C. Apart from the analysed sample a bcc-Fe has been measured as a reference sample. The edge position was taken as the energy at half maximum. The measured energy shift between bcc-Fe and iron oxide sample was (6.0 ± 0.5) eV. The simulations of XANES spectrum for hematite-Fe2O3 (Fe3+) and bcc-Fe exhibit an energy shift of 4.9 eV and they are compared to the measured data (Fig. 9). The calculated spectra are shifted from their proper values due to unknown value of the Fermi level and therefore they have been shifted by 6.3 eV to higher values to match the measured spectra. The reported value of the edge shift between bcc-Fe and Fe2O3 is 4.8 eV [24], which corresponds to the calculated value, our measured shift is, however, larger. Since iron atoms are in all iron oxides in the form of Fe3+ or Fe2+ [25] higher-energy shift in the measured data is attributed to experimental artefacts rather than to even higher oxidation state of iron atoms than Fe3+. Furthermore, the measured shapes of the absorption edges are for both bcc-Fe and iron oxide described well by the calculated spectra suggesting the presence of Fe3+. 3.4. Mössbauer spectroscopy The measured CEMS spectra of both studied samples (Ti/Fe-containing layer thickness of 2 nm, one annealed at 700 °C in FG, the other at

Fig. 8. The relative content of hematite-Fe2O3 obtained from EXAFS (the rest is Fe–O octahedron) as a function of temperature. The annealing atmospheres are denoted as follows: A, air; V, vacuum; F, forming gas.

900 °C in the air, respectively) are shown in Fig. 10. The experimental data were fitted by one doublet satisfactorily with δ = 0.34 ± 0.01 mm/s, Δ = 0.80 ± 0.02 mm/s and δ = 0.33 ± 0.01 mm/s, Δ = 0.68 ± 0.02 mm/s, respectively. The values of isomer shift and quadrupole splitting correspond to Fe3+ in nanocrystalline Fe2O3 [26,27] which is consistent to the XANES results. A slight contribution to the high value of quadrupole splitting can also arise from a pressure effect of the surrounding TiO2 and SiO2 components due to their shorter unit cells in both amorphous and crystalline states [28]. The decrease of Δ between both samples can be explained by the stress relaxation in the film that has been annealed at higher temperature. 3.5. ToF-ERDA The ToF-ERDA measurements were performed in order to confirm the total molar composition of the layers and to correlate the obtained results with the GISAXS calculations. We measured a series of all three thicknesses of Ti/Fe-containing layers (0.6, 1, and 2 nm) annealed at 900 °C in the air. An illustrative example of a ToF-ERDA profile is displayed in Fig. 11. The depth resolution is not sufficient to observe oscillations of Ti and Fe content between individual layers however the global characteristics can be seen. The content of Ti, Fe and O decreases as the depth reaches the Si substrate, while the content of Si (which is present in the multilayer in the form of SiO2) increases. The obtained results are listed in Table 2, where an increasing relative molar content of both Ti and Fe with increasing Ti/Fe-containing layer thickness can be seen. The molar content of Fe which is approximately twice lower than the content of Ti corresponds to roughly twice higher molar volume of hematite then of rutile (these two phases were assumed as examples of TiO2 and Fe2O3, supported by the XRD results). Assuming the presence of TiO2, Fe2O3 and SiO2 oxides, we calculated the content of oxygen from the content of individual elements (Table 2). The calculated oxygen content corresponds to the measured one within a few percent. 4. Discussion The investigation of the structure of particles created in the Fe2O3 + TiO 2/SiO 2 multilayers during annealing revealed the presence of

Fig. 7. The measured EXAFS signal and its simulation at the Fe edge (a) and at the Ti edge (b) of the sample annealed in the air at 900 °C.

Fig. 9. The measured XANES spectra of sample annealed in the air at 900 °C (full circles) and the reference bcc-Fe sample (empty circles). The dashed lines represent the positions of absorption edges as the energy at half maximum. Both corresponding calculated spectra (black lines) of bcc-Fe and hematite have been shifted by 6.3 eV to higher values.

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Table 2 The ToF-ERDA results of relative molar composition of the samples annealed at 900 °C in the air. The oxygen content is compared to the one calculated from the corresponding oxides.

Fig. 10. The measured room temperature conversion electron Mössbauer spectra of the sample annealed at 900 °C in the air (empty circles, vertically shifted by 0.005 for clarity) and at 700 °C in the forming gas (FG, full circles) and their fits (black lines).

rutile-TiO2 by both XRD and EXAFS. The nature of the structure of the Fecontaining oxide that was not fully explained only by XRD follows from the EXAFS results. Consistent to the XRD results, the changes of the local surroundings of a Fe atom throughout various annealing conditions are very tiny. Slight changes that are observed correspond to the change from genuinely amorphous state (octahedrally coordinated oxygen atoms around the Fe atom) to atoms ordered up to the second shell of the iron oxide structure. The inhibition of Fe2O3 crystallization in the presence of silica has been already reported [22]. On the other hand, Ti atoms form rutile particles from the annealing temperature of 700 °C as confirmed both by XRD and EXAFS. At the temperature of around 700 °C, the formation of ordered particles from GISAXS is observed as well. The particle sizes obtained from GISAXS are in a good agreement with those obtained from XRD (apart from the highest annealing temperatures when the assumed multilayer model for GISAXS is not valid any more) which confirms the assumption that the particles observed in GISAXS are rutile-TiO2 (Table 1). Since the ToF-ERDA measurements provide information on the absolute number of atoms it is possible to compare the areal concentration of atoms obtained by ToF-ERDA to the areal concentration of atoms calculated from the knowledge of the particle sizes and the parameters of the particle ordering from GISAXS if the crystal structure of the particles is known. We assumed that the particles observed in GISAXS are crystalline rutile particles (as suggested by XRD and EXAFS), calculated the areal concentration of Ti atoms throughout the whole multilayer and compared it to the areal concentration of Ti atoms obtained from ToFERDA measurements. The ratio of the number of Ti atoms obtained from GISAXS is for all the three samples (annealed at 900 °C in the air, with Ti/Fe-containing layer thicknesses 0.6, 1, and 2 nm) in the range 25–27% of the Ti atoms obtained by ToF-ERDA. These results are reasonable since still certain amount of Ti atoms remains dispersed in the layers because vertical maxima in the GISAXS maps can be seen. These maxima come from the scattering on the interfaces between layers with different refraction indexes. However one has to keep in mind that dispersed Fe atoms contribute to the difference in refraction index as well. The relevant changes of the particles sizes and mean inter-particle distances were observed as a function of Ti/Fe-containing layer

Fig. 11. An example of the ToF-ERDA spectrum for the sample with the Ti/Fe-containing layer thickness of 1 nm annealed in the air at 900 °C. The channel number is proportional to the depth under the sample surface.

Thickness (nm)

Fe (%)

Ti (%)

Si (%)

O (%)

Ocalc (%)

0.6 1 2

1.2 1.9 3.6

3.0 3.9 6.8

29.0 26.7 23.9

66.8 67.5 65.8

65.7 64.1 66.6

thicknesses—the particles in thicker layer were bigger and ordered at larger distances compared to the samples with thinner layers. The evolution of the particle structure with the annealing temperature was similar for all atmospheres. The destruction of the ordering, however, happened for the samples annealed in vacuum at lower temperature (at 900 °C compared to 1000 °C in the other atmospheres). This discrepancy may be caused by slightly different annealing conditions. The annealing was performed in an oven where the sample was placed in a tube and the thermocouple that measures the temperatures was situated outside the tube. When annealing in vacuum, the tube was evacuated and sealed while annealing in an atmosphere (both air and forming gas) was performed by a flow of the gas (initially at room temperature) through the opened end of the tube. Therefore the gas could have decreased the actual temperature of the sample compared to the temperature of the thermocouple. 5. Conclusion We have studied a series of Fe2O3 + TiO2/SiO2 multilayers annealed at different temperatures in the air, vacuum and forming gas. We found out that Ti atoms crystallize into rutile-TiO2 while Fe3+ atoms remain oxidized in amorphous state or form very small particles. An increase of the particle sizes and the mean inter-particle distances have been observed for increasing thickness of the Ti/Fe-containing layer and therefore these parameters can be easily tuned. The best (rutile) particle ordering is achieved when annealing in the air at 800–900 °C. Acknowledgements The work has been supported by the Czech Science Foundation (project P204/11/0785). MB acknowledges Ministry of Science, Education and Sports, Croatia (project no. 098-0982886-2859). References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (5358) (1972) 37. [2] K. Hashimoto, H. Irie, A. Fujishima, TiO2 photocatalysis: Photocatalysis: A historical overview and future prospects, Jpn. J. Appl. Phys. 44 (12) (Dec. 2005) 8269. [3] M. Hoffmann, S. Martin, W. Choi, D. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev. 95 (1995) 69. [4] H. Cui, W. Ren, W. Wang, Highly transparent UV absorption TiO2-SiO2-Fe2O3 films without oxidation catalytic activity prepared by a room temperature sol-gel route, J. Sol-Gel Sci. Technol. 58 (2) (Feb. 2011) 476. [5] M. Esfandarani, L. Minggu, W. Daud, M. Kassim, Synthesis and characterization of Fe2O3/SiO2/TiO2 composite thin film on different substrates for water splitting, J. New Mater. Electrochem. Syst. 13 (2010) 333. [6] V. Tyrpekl, J.P. Vejpravová, A.G. Roca, N. Murafa, L. Szatmary, D. Niznansky, Magnetically separable photocatalytic composite γ-Fe2O3@TiO2 synthesized by heterogeneous precipitation, Appl. Surf. Sci. 257 (11) (Mar. 2011) 4844. [7] V. Valeš, J. Poltierová-Vejpravová, V. Holý, V. Tyrpekl, P. Brázda, S. Doyle, Study of the phase composition of Fe2O3 and Fe2O3/TiO2 nanoparticles using X-ray diffraction and Debye formula, Phys. Status Solidi C 7 (5) (Apr. 2010) 1399. [8] J.a. Glasscock, P.R.F. Barnes, I.C. Plumb, N. Savvides, Enhancement of photoelectrochemical hydrogen production from hematite thin films by the introduction of Ti and Si, J. Phys. Chem. C 111 (44) (2007) 16477. [9] S. Kuang, L. Yang, S. Luo, Q. Cai, Fabrication, characterization and photoelectrochemical properties of Fe2O3 modified TiO2 nanotube arrays, Appl. Surf. Sci. 255 (16) (May 2009) 7385. [10] B.-R. Kim, H.-J. Oh, K.-S. Yun, S.-C. Jung, W. Kang, S.-J. Kim, Effect of TiO2 supporting layer on Fe2O3 photoanode for efficient water splitting, Prog. Org. Coat. 76 (12) (Dec. 2013) 1869. [11] M. Buljan, U. Desnica, M. Ivanda, N. Radić, P. Dubček, G. Dražić, K. Salamon, S. Bernstorff, V. Holý, Formation of three-dimensional quantum-dot superlattices in

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Please cite this article as: V. Valeš, et al., Fe2O3/TiO2 nanoparticles—a complex structural study, Thin Solid Films (2014), http://dx.doi.org/10.1016/ j.tsf.2014.05.016