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Correlative Analysis of the Crystallization of Sol-Gel Dense and Mesoporous Anatase Titania Films Plinio Innocenzi,*,† Luca Malfatti,† Tongjit Kidchob,† Stefano Enzo,‡ Giancarlo Della Ventura,§ Ulrich Schade,| and Augusto Marcelli⊥ Laboratorio di Scienza dei Materiali e Nanotecnologie, D.A.P., UniVersita` di Sassari, CR-INSTM, Palazzo Pou Salid, Piazza Duomo 6, 07041 Alghero (Sassari), Italy, Dipartimento di Chimica, UniVersita` di Sassari, Via Vienna 2, 07100 Sassari, Italy, Dipartimento di Scienze Geologiche, UniVersita` degli Studi Roma Tre, Largo S. Leonardo Murialdo 1, I-00146 Roma, Italy, Helmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH, Elektronenspeicherring BESSY II, Albert-Einstein-Strasse 15, D-12489 Berlin, Germany, and INFN Laboratori Nazionali di Frascati, Via E. Fermi 40, 00044, Frascati, Italy ReceiVed: May 11, 2010; ReVised Manuscript ReceiVed: July 29, 2010
Mesoporous and dense sol-gel titania films have been synthesized and processed in similar conditions to make a comparative study of crystallization to anatase in porous and dense samples. We have performed a correlative analysis combining THz spectroscopy with synchrotron radiation, variable angle incidence infrared spectroscopy, spectroscopic ellipsometry and grazing incidence X-ray diffraction to characterize the structure of titania films as a function of thermal processing conditions. THz spectra have shown that small anatase crystalline clusters are already present in the as-deposited samples, and both crystallization and growth process are favored in the mesoporous films. Moreover, the analysis by X-ray diffraction and variable angle incidence infrared spectroscopy has allowed identification of the presence of small textures in the samples. Introduction The atomic structural complexity in a titania film grown at room temperature or deposited from a liquid phase, such as in sol-gel film processing, is still an open question. This type of materials, before crystallization to anatase, shows an amorphous structure, with different degrees of local order.1 Even if longrange order is lacking, some crystalline clusters or “ordered domains extending beyond nearest neighbours to the subnanometer or even nanometer level”1 can be present. Controlling the intimate structure is very important because these small clusters can act as seed for crystallization and, therefore, affect the final structure2 and functional properties in titania dense and mesoporous films.3 The detection of nanocrystals or short-range ordered structures is, however, extremely difficult and elusive. Different analytical techniques, such as X-ray diffraction, spectroscopic ellipsometry, Raman spectroscopy, and X-ray absorption near-edge spectroscopy (XANES),4-6 have been used for this purpose. The local structure is very much dependent on synthesis7 and processing parameters of the titania films, especially when they are prepared via liquid phase,8 such as from sol-gel precursor sols. Several sol-gel synthesis routes have been developed as a function of the specific application of the titania films.9,10 A combination of sol-gel and supramolecular chemistry can be also used to obtain mesoporous ordered titania anatase films via evaporation induced self-assembly.3 Mesoporous titania films11,12 have attracted much attention because of their very peculiar properties and potential applications in different fields,13 such as for photocatalysis14 and * To whom correspondence should be addressed. † Laboratorio di Scienza dei Materiali e Nanotecnologie, Universita` di Sassari. ‡ Dipartimento di Chimica, Universita` di Sassari. § Universita` degli Studi Roma Tre. | Helmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH. ⊥ INFN-Laboratori Nazionali di Frascati.
photovoltaic15 devices and in nanobiotechnologies.16 In particular, crystallization of mesoporous titania films has shown some distinctive properties with respect to dense titania films.2,17,18 The crystallization depends on many parameters such as the nature of the substrate, the film thickness, the presence of water, and the thermal ramp during firing. The activation energy for the crystallization of anatase was found to be higher in the presence of porosity. Crystalline titania at room temperature and atmospheric pressure is observed in the three phases, the thermodynamically stable rutile and the metastable phases (below ∼800 °C) anatase and brookite. The crystalline structure observed in thin films is very much depending on the deposition and processing conditions. The anatase phase is the most interesting for applications, and controlling the crystalline structure is an important point. Characterization of crystalline transition metal oxides19,20 and in particular anatase by infrared (IR) spectroscopy in the farIR range (THz spectroscopy21) has attracted very much attention as a basic, fast, and simple analytical tool.22 This technique has shown to be very effective also for characterization of thin films; in particular it can be successfully applied to anatase titania which has a tetragonal structure with two TiO2 formula units and six atoms per primitive cell and the space group is I41/ amd. This structure gives 15 optical modes23 with the following irreproducible representation of normal vibrations 1A1g + 1A2u + 2B1g + 1B2u + 3Eg + 2Eu. The A2u mode is active for light polarized parallel to the c axis, while the Eu modes are active for light polarized normal (E⊥c) to the c axis. The IR spectra of single-crystal anatase have been determined theoretically and experimentally by Gonzalez et al.,24 who found that an electric field parallel to the crystallographic vector, (E|c) give a pair of A2u modes, the transverse optical (TO) phonon mode in the THz region at 367 cm-1 and a longitudinal optical (LO) phonon mode in the middle IR (mid-IR) region at 755 cm-1; the electric field normal to c, (E⊥c), gives instead two pairs (TO-LO) of Eu
10.1021/jp1042766 2010 American Chemical Society Published on Web 12/08/2010
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modes: in the THz region at 262 (TO) - 366 (LO) cm-1 and in the mid-IR region at 435 (TO) - 876 (LO) cm-1. In noncrystalline and polycrystalline samples,25 however, not all of these modes are observed because of the orientational disorder of Ti-O units and the dielectric function is described by a directional average of the dielectric functions for the E⊥c and E|c orientations:
ε(υ) ) [2ε⊥(υ) + 2ε|(υ)]/3
(1)
with ε⊥(ν) and ε|(ν) the dielectric functions for the E⊥c and E|c orientations, respectively. This means that IR analysis performed with unpolarized light is not able to observe LO modes in polycrystalline films. However, in thin films where the thickness, t, is much lower than the wavelength of the incident IR radiation, t , λ (IR) the Berreman effect34 can be used to excite at variable incidence angles LO modes. IR analysis changing the angle of incidence has been applied to study the structure of sol-gel anatase titania films26 and the texture of plasma-enhanced chemical vapor deposition (PECVD) anatase titania films.27,28 We have prepared “dense” and mesoporous titania films to clarify how the differences of structure and processing can affect the formation and the intimate structure of titania nanocrystals. We have used transmission THz spectroscopy, with a synchrotron light source, and variable-incidence IR analysis to study the morphology of the titania anatase films which have been processed via sol-gel. Experimental Section Titanium tetrachloride (TiCl4, 99.9%, Aldrich), triblock copolymer Pluronic F127 (PEO106-PPO70-PEO106, Aldrich), ethanol (EtOH) (Carlo Erba), and bidistilled water were used as precursors without further purification; silicon wafers [(100), p-type, boron doped, (Jocam)] were used as substrates. Preparation of the Precursor Solution for Titania Films. The titania sol has been prepared by dissolving TiCl4 in ethanol and water to obtain the final molar ratios TiCl4:EtOH:H2O ) 1:40:10. For the preparation of mesoporous films, Pluronic F127 has been added to the titania sol to reach a molar ratio of s ) 5 × 10-3 where s ) [F127/TiCl4]. Film Processing. The films have been deposited on silicon wafers by dip-coating in controlled conditions of relative humidity and temperature; the relative humidity (RH) inside the deposition chamber has been maintained around 20% RH and the temperature at 25 °C. A withdrawal rate of 15 mm×min-1 has been used; we have used single depositions without preparing multilayer films. After deposition, the films have been fired in air at increasing temperatures from 25 up to 600 °C for 10 min. For each firing temperature, a mesoporous film and a dense one have been placed in the oven at the same experimental conditions; this procedure increases the reproducibility and allows a reliable comparison between the two types of samples. IR and THz Analysis. IR analysis in the mid-IR range was performed using a Bruker Vertex 70 V spectrophotometer. The optical bench and the sample compartment have been kept in vacuum during the measures; a pressure lower than 0.5 hPa in the two compartments was used. The measurements have been done using a Globar source, a KBr beamsplitter, and a RTDTGS detector. The spectra have been recorded in transmission, in the 4000-370 cm-1 range by averaging 256 scans with 4 cm-1 of resolution. A silicon wafer has been used as substrate
to measure the background; the baseline has been calculated by a rubberband algorithm (OPUS 7 software) while no smoothing on the data has been performed. Variable incidence IR analysis has been performed in transmission by changing the angle of incidence of the IR beam with respect the sample surface; the angles of 45 and 60° have been used. The background was recorded using a silicon substrate set at the same incidence angle of the sample; we also kept constant the orientation of the silicon used for the sample and silicon used for the background. Mismatch in orientation or background recorded with silicon at different angles can be a source of artifacts in the transmission spectra. The experiments in the THz energy domain21 were performed at the IRIS beamline of the BESSY synchrotron radiation facility (Berlin) using synchrotron radiation as the IR source, a Bruker IF-66v/S as interferometer, a Mylar beamsplitter, and a liquid helium cooled bolometer as detector. The spectral resolution was 8 cm-1, and each spectrum has been obtained by averaging 16 scans at a scanner velocity of 0.633 cm × s-1. As reference spectrum we used the average of 150 scans collected measuring the silicon substrate. X-ray Diffraction (XRD) Analysis. We have used a Bruker D8 diffractometer with an X-ray generator working at a power of 40 kV and 40 mA; the goniometer has been equipped with a graphite monochromator in the diffracted beam. The Cu KR radiation (λ ) 1.5418 Å) has been used to perform 2θ/θ scans in an angular range from 15 to 90° with angle step size of 0.02° and 10 s of exposition time per step. The XRD data were analyzed with the MAUD software according to the Rietveld method;29 the structure data were compared in terms of lattice parameters with those of well crystallized anatase reported in the literature.24 According to the space group I41/amd of anatase, only the ZO fractional coordinate of oxygen atoms is permitted to vary, the other being special coordinates. The texture parameter of each specimen was determined using a modified March-Dollase function applied to the (004) direction. Its insertion was justified “a posteriori” with an improvement of the fit quality. Average crystallite size and lattice strain were separated from the total broadening assuming a dependence of microstrain from the reflection order following an isotropic model. Spectroscopic Ellipsometry. The film thickness and refractive index at 633 nm have been estimated by an R-SETM Wollam spectroscopic ellipsometry using a Cauchy film as a fitting model. Plots of Ψ and ∆ as a function of incident wavelength from 400 and 900 nm have been simulated using “CompleteEASE v. 4.2” program from Wollam. The results of the fits have been evaluated on the basis of the mean squared error (MSE), which was maintained below 9. Results and Discussion After deposition and processing, the dense sol-gel and mesoporous titania films appear optically transparent and crack free; the samples have been processed in the same conditions, put simultaneously in the same oven, and characterized immediately after. The preparation of mesoporous titania films requires a surfactant to template the mesopores; sol-gel dense titania films have been prepared using exactly the same chemistry with the only exception of the addition of the block copolymer. This has been done with the specific purpose of investigating how the presence of a porous texture in the film can affect the crystallization in the same type of material. To distinguish the two samples, we have used the term “dense” for the sol-gel titania films prepared without surfactant, even
Sol-Gel Dense and Mesoporous Anatase Titania Films
Figure 1. (a) THz absorption spectra in the 600-200 cm-1 range of dense titania sol-gel films treated at different temperatures. (b) Temperature (y axis)-wavenumber (x axis)-absorbance (false color scale, right y axis) graph in 600-200 cm-1 range of dense titania sol-gel films.
if they can be considered fully dense only at the highest processing temperatures. THz Spectroscopy. Figure 1a shows the THz absorption spectra in the 600-200 cm-1 range of the titania dense films treated at different temperatures. Two intense absorption bands at 260 and 460 cm-1 are observed in the samples treated at temperatures higher than 350 °C; they are assigned to Eu TO mode of the titania anatase.24 The appearance of the Eu bands indicates that the sample is crystallized into the anatase phase. The spectrum of the sample treated at 350 °C, even if it does not show the clear signature of the Eu modes, increases in intensity with respect to the spectra of the samples fired at lower temperatures. The low-temperature spectra show two featureless bands of weak intensity peaking around 260 and 420 cm-1. We have also used a temperature (y axis)-wavenumber (x axis)-absorbance (false color scale, right y axis) graph (Figure 1b) to enhance the visualization of the changes in the spectra as a function of the temperature. The graph shows that the phase transition to anatase is quite sharp and is observed between 350 and 400 °C. We have used the same experimental protocol to make a comparative study of the crystallization in mesoporous titania films as a function of temperature. Figure 2a shows the IR absorption spectra in the 600-200 cm-1 range of the titania mesoporous films treated at different temperatures. The spectra of the samples treated at temperatures higher than 400 °C show the typical signature of the titania anatase. In comparison with the dense titania film, the crystallization process appears not so sharp, and the band at 270 cm-1 increases in intensity with the
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Figure 2. (a) THz absorption spectra in the 600-200 cm-1 range of mesoporous titania sol-gel films treated at different temperatures. (b) Temperature (y axis)-wavenumber (x axis)-absorbance (false color scale, right y axis) graph in the 600-200 cm-1 range of mesoporous titania sol-gel films.
increase of the thermal treatment temperature, even if the typical signature of the titania anatase band is observed only above 400 °C. The temperature (y axis)-wavenumber (x axis)absorbance (false color scale, right y axis) graph (Figure 2b) shows, in fact, that the phase transition to anatase is smoother with respect to the dense titania films. The crystallization process starts at low temperatures, around 200 °C a significant increase of the 270 cm-1 band is already observed. It is interesting to observe that this band appears more sensitive to changes of the structure as a function of the temperature with respect to the other vibrational mode at 420 cm-1, which shows no changes in the spectra of the low temperature samples. The nature of the two small intensity absorption bands around 260 and 420 cm-1 also needs a specific discussion. We have used a rubberband algorithm (OPUS 7 software) to calculate the baseline between 600 and 200 cm-1 but no smoothing or postzero filling of the data has been performed; we have changed the conditions of the calculation of baseline, and we have concluded that the two bands are not a measure artifact. The trend of the measures also supports this finding, because the bands increase in intensity with the increase of the thermal treatment temperature. We assign, therefore, these bands to the presence of small ordered clusters that are formed immediately after the film deposition. These clusters act as nucleation centers and grow around the critical crystallization temperature of 350 °C. This process has been well described by Soler Illia et al. in mesoporous titania films and has been probed by XANES in different Ti(IV) environments to assess the amorphous-anatase transition.30 They have found that significant anatase formation takes place in films processed at temperatures of 100-150 °C
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Figure 3. (a) Shift of absorption peak (260 cm Eu TO mode of the titania anatase) as a function of the temperature for the dense and mesoporous films. (b) Normalized absorption bands of the dense and mesoporous titania films treated at 600 °C.
for films deposited on crystalline Si. Analysis by IR spectroscopy allows, therefore, at a first glance to assess the presence of crystalline anatase clusters even in very thin films. On the other hand, crystallization in mesoporous films takes places at lower temperatures in comparison with dense films. If the formation of crystalline clusters appears similar at the early processing stages, the presence of a porous structure can favor the growth process. The spectra of both samples, dense and mesoporous titania films, show that with the increase of the thermal treatment temperature the absorption bands exhibit a shift to lower wavenumbers and an increase of the full width half-maximum (fwhm). We have reported in Figure 3a the shift of absorption peak (260 cm-1 Eu TO mode of the titania anatase) as a function of the temperature for the dense and mesoporous films. The data show a similar trend: there are two different steps, at about 150 and at 400 °C (continuous lines are inserted in the Figure 3a as a guide for eyes). However, in the dense films the first shift is more pronounced. This can be clearly seen in Figure 3b, which shows the normalized absorption bands of the dense and mesoporous titania films treated at 600 °C. In general, a line broadening is due to a higher orientational disorder due to local defects and or local strains, and the comparison between the mesoporous and dense sol-gel film indicates that the mesoporous samples exhibit a higher disorder with respect to the sol-gel dense films. The shift to lower wavenumbers with the increase of the thermal treatment temperature is due to a progressive densification of the titania network. These data show, therefore, that in the mesoporous samples the titania network has a higher orientational disorder and is less dense in comparison with the other sol-gel films. Spectroscopic Ellipsometry. The effect of the thermal treatment on sol-gel films is generally a decrease of thickness,
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Figure 4. Variation of thickness (a) and refractive index, measured at 633 nm, (b) as a function of thermal treatment measured by spectroscopic ellipsometry in sol-gel dense (black line) and mesoporous (red line) titania films.
due to a uniaxial shrinkage with the progress of condensation reactions, which is accompanied by an increase in the refractive index. Figure 4a shows the variation of thickness as a function of thermal treatment; the mesoporous films show the highest change, the thickness decreases from 450 nm in the as deposited film to 200 nm in the sample treated at 600 °C. This high variation is due to the higher shrinkage which is observed in mesoporous films; with the increase of the temperature of the treatment the pores collapse and the film shrinks. We have used two dot lines in Figure 4a in correspondence of the temperatures (150 and 400 °C) which are the critical temperatures pointed out in the THz spectra (see Figure 3a). The mesoporous samples are thicker in comparison to the dense films and in principle questions could arise about the thickness effect on the crystallization process and if the two samples can be really compared. However, to obtain the same thickness we should have modified the chemistry and/or the film processing conditions. We have intentionally chosen, therefore, to use the same chemistry for dense and mesoporous films with the exception of the addition of the surfactant and the same processing conditions which means the same withdrawal rate for dip-coating. It has been demonstrated in a previous work that the thickness only slightly affects the crystallization of dense films and mesoporous films are not affected at all.17 Furthermore, a change in the evaporation rate produces a variation in the structure of the as-deposited films; therefore, even if the film thickness is the same, several changing parameters have been introduced which makes a comparison very difficult. Figure 4b shows the change of the refractive index, measured at 633 nm, as a function of temperature for the dense and mesoporous titania samples. The two samples have different trends, while the refractive index of the dense titania film increases continuously from as deposited (n ) 1.85) to 600 °C,
Sol-Gel Dense and Mesoporous Anatase Titania Films
J. Phys. Chem. C, Vol. 114, No. 51, 2010 22389 the orientation. The texture coefficients suggest that the two samples have a preferential orientation, which is normal to the substrate for the mesoporous films and parallel for the dense films. The amount of disorder extracted from the line broadening is huge, the figures being interpreted in relative terms as 1.5-1.6 defects in average out of 100 cases examined. Of course from the physical point of view these figures must be treated with caution since we do not know neither the disorder type nor their distribution. Moreover, a large amount of broadening is also caused by the reduced size of the crystallites, as testified by the values of 22 and 13 nm, which we have calculated for the dense and mesoporous samples, respectively. This finding is well in agreement with what reported in literature; previous kinetic studies have shown that in mesoporous films the growth of anatase crystallites is limited by the pore wall dimension.31,32 This explains why smaller crystals are obtained in mesoporous samples in comparison to dense titania films. It should be noted that the volume expansion of the unit cell implies an extension of the Ti-O bonds that can be calculated from the location of atoms in the unit cell. The fractional coordinate of oxygen atoms did not change with respect to the original figure. The Ti-O distance distribution is a distorted octahedron of oxygen atoms around the titanium, with four “equatorial” and two “axial” Ti-O distances, that slightly increase for the deposited specimens. The mesoporous anatase titania film seems to maintain a more symmetric distance distribution with respect to the dense sample. In any case, the relative changes of the Ti-O distances are expected to slightly modify the vibrational properties of these materials. Variable Incidence IR Spectroscopy. We have performed our IR analysis on anatase titania thin films using unpolarized light; in this condition only Eu (TO) modes in the THz region can be detected in polycrystalline films.1 LO modes can be observed with polarized light27,28 or in variable incidence conditions26,25,33 which allow exciting not only the TO modes that are activated at normal incidence but also the LO modes. Performing the analysis in oblique incidence on thin films has the advantage of using the Berreman effect34 to excite the LO vibrations and getting additional information on the absorption spectra. We have done the experiments changing the incidence angle of the IR light on the titania dense and mesoporous titania films treated at 600 °C. The analysis in the mid-IR range has been performed using a conventional light source and two different incidence angles, 45 and 60°. We performed the analysis in the mid-IR because the Eu (LO) modes fall in this range (the calculated frequencies fall at 876 and 755 cm-1), and the analysis of these vibrations could give in principle additional information on the structural texture of the material.27,28 Figure 6 shows the variable incidence angle FTIR absorption spectra in the 900-365 cm-1 range of the titania dense film treated at 600 °C. The spectra show an intense band at 437 cm-1, which is due to the Eu(TO) mode (see Figure 1) in the anatase titania; this band is not affected by the change of the incidence angle. Several bands due to the silicon substrate are observed in the spectra and are specifically labeled in Figure 6. The spectrum in normal
Figure 5. Grazing incidence XRD patterns of the titania mesoporous (top curve) and dense sol-gel films (bottom curve) after firing at 600 °C. The blue dots are the experimental data and the red continuous line the Rietveld fit.
i.e., the fired sample (n ) 2.05); in the mesoporous films it remains almost constant. In the latter sample, in fact, the refractive index is much lower, due to the presence of mesopores; the small decrease of refractive index in the samples treated between 200° and 350 °C, is attributed to the removal of the surfactant template. After firing at 600 °C the refractive index is 1.65 and 2.05 in the dense and mesoporous films, respectively, which also in the case of the most dense film is still lower than the refractive index of the standard dense anatase (n ) 2.488). XRD Analysis. We have characterized the titania samples fired at 600 °C by grazing incidence XRD and simulated the XRD patterns by Rietveld method to obtain the structural parameters. Figure 5 shows the XRD patterns of the titania mesoporous (top curve) and dense films (bottom curve) after firing at 600 °C; the typical signature of the titania anatase (JCPDS 84-1286) is observed with the diffraction peaks (101), (004), (200), (105), (204), (116), (215), (224) that are detected in the 15-90° in the 2θ interval. The axes are I(Q) × Q, vs Q where Q ) 4π/λ sin(θ); this graphical representation allows an appropriate highlighting of the weak peaks with high hkl orders. Moreover, these peaks are essential for a correct separation of the total peak broadening in terms of average crystallite size from lattice distortions. In Figure 5 blue dots are experimental data and the red continuous line the Rietveld fit; the sample parameters deduced from the pattern simulation are reported in Table 1. The lattice parameters of both specimens are higher in comparison to the correspondent literature values. In fact, the unit cell volumes vary from the literature equilibrium value of 136.25 Å3 to 137.84 Å3 and 138.13 Å3 for the dense and mesoporous specimens respectively, with an expansion similar to that one recorded in an anatase powder fired at 600 °C. The texture coefficients, which have been calculated with respect to the (004) planes, are reported in Table 1; a value higher than one corresponds to a preferential orientation of the (004) planes to a direction normal to the sample surface, while a texture value lower than one shows a preferential orientation parallel to the substrate and the higher the difference from one the stronger
TABLE 1: Structural Parameters Calculated from XRD Data of the Mesoporous and Dense Titania Films after Thermal Treatment at 600 °C sample
c/Å
a/Å
texture coefficient
lattice disorder
d/nm
ZO
Ti-O bond length/Å
polycrystalline titania (reference) dense titania film mesoporous titania film
9.514 9.546 9.536
3.785 3.800 3.806
1 0.93 1.09
0.0005 0.016 0.015
>200 22 13
0.21(1) 0.21(1) 0.21(1)
4(1.934) 2(1.980) 4(1.942) 2(1.987) 4(1.944) 2(1.984)
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Figure 6. Variable incidence angle FTIR absorption spectra in the 900-365 cm-1 range of the titania dense film treated at 600 °C. The measure has been done at normal incidence, 0 (black), at 45 (red), and 60° (green). The signal from the silicon substrate is indicated in the figure with dash black lines.
Figure 7. Variable incidence angle FTIR absorption spectra in the 900-365 cm-1 range of the titania mesoporous film treated at 600 °C. The measure has been done at normal incidence, 0 (black), at 45 (red), and 60° (green). The signal from the silicon substrate is indicated in the figure with dash black lines.
conditions (incident light perpendicular to the film substrate), beside the intense Eu(TO) mode at 437 cm-1, does not show any other absorption band. A new band peaking at 844 cm-1 is observed, however, with the increase of the incidence angle; this band, partially overlapped with a signal from the silicon substrate, is assigned to the Eu(LO) mode, which is the pair mode of the Eu(TO) vibration at 437 cm-1. This vibration is observed, therefore, only changing the incidence angle and is typically observed in dense titania films in similar conditions of measure, i.e., variable incidence angle in the mid-IR.25,26 The band peaking around 840 cm-1 broadens with the increase of the incidence angle; this is due to the appearance of another LO mode, A2u(LO), around 780 cm-1, which is the pair of the A2u(TO) vibration at 367 cm-1. This band is weaker than the 844 cm-1 Eu(LO) mode and is responsible of the band broadening in the 900-750 cm-1 interval in the 60° spectrum. We have done a similar analysis in the titania mesoporous films; the results are shown in Figure 7; the variable incidence angle FTIR absorption spectra in the 900-365 cm-1 range of the titania mesoporous film treated at 600 °C are reported. The spectra show again the intense Eu(TO) mode at 437 cm-1 and the rise of Eu(LO) and A2u(LO) modes with the increase of the incidence angle. In the spectrum recorded at 60° of incident angle we have observed, however, a reverse situation with respect to the dense titania film. The most intense band is, in fact, the A2u(LO) mode around 780 cm-1, while the 844 cm-1 Eu mode appears only as a weak shoulder. The spectra recorded at variable incidence show, therefore, that in the dense titania film the LO mode at 844 cm-1 is more intense with respect to that one at 780 cm-1; the situation is reversed in the mesoporous films. These data can be used to obtain information on the structural texture of the two materials. It has been shown, in fact, that the A2u(LO) mode is correlated with the crystallographic c axes of the anatase crystallites that lies preferentially
Innocenzi et al. on a direction parallel to the substrate, while the Eu(LO) mode to the crystallographic c axis lies normal to the substrate surface.27,28 We can, therefore, deduce from the experimental data that differences in the crystallographic orientations are present between dense and mesoporous titania films. This is well supported by XRD data that have shown the presence of a small texture in the samples. In particular, on the base of IR and XRD analysis it appears that the dense films are oriented preferentially in a direction parallel to the substrate while the mesoporous titania film have an orientation which is preferentially normal to the substrate. The presence of this textural difference is correlated with the synthesis and processing conditions of films deposited via sol-gel. The thermal treatment, which is necessary to condense the film after the deposition, induces a uniaxial shrinkage in a direction which is normal to the substrate; in the direction parallel to the substrate the film can not shrink and a tensile stress arises with the thermal treatment. In the case of mesoporous films, this stress, because of the less dense structure of the film which contains mesopores, is smaller. The different tensile stress in the two samples, favors, therefore the preferential formation of a texture; the presence of a stress parallel to the substrate favors the formation of crystallites with c-axis parallel to the surface, while in the case of the mesoporous films the crystal growth is more epitaxiallike with the c axis preferentially normally oriented with respect to the substrate. Conclusions We have synthesized titania dense and mesoporous films in similar conditions for a comparative study of the crystallization with thermal processing. We have used for the study a combination of different techniques: THz spectroscopy with a synchrotron radiation source, variable angle incidence mid-IR spectroscopy, spectroscopic ellipsometry, and grazing incidence X-ray diffraction. IR spectroscopy has shown that crystalline titania clusters, in accordance with the literature, are present even in the samples processed at low temperatures. Around 350-400 °C the samples crystallize to anatase, but in the mesoporous films this transformation is favored, and is observed at lower temperatures with respect to dense titania samples. The XRD analysis in combination with variable angle incidence IR spectroscopy have shown preferential orientation along the (002) direction; preferential orientation in a direction parallel to substrate is favored in dense sol-gel films, while in mesoporous films the crystallite show a small orientation normally to the substrate. Acknowledgment. We acknowledge the Helmholtz-Zentrum Berlin - Electron storage ring BESSY II for provision of infrared synchrotron radiation at the IRIS beamline. The research leading to these results has partly received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement n.°226716. References and Notes (1) Scarel, G.; Hirschmugl, C. J.; Yakovlev, V. V.; Sorbello, R. S.; Aita, C. R.; Tanaka, H.; Hisano, K. J. Appl. Phys. 2002, 91, 1118. (2) Angelome´, P. C.; Andrini, L. E.; Calvo, M. E.; Requejo, F. G.; Bilmes, S. A.; Soler-Illia, G. J. A. A. J. Phys. Chem. C 2007, 111, 10886. (3) (a) Crepaldi, E. L.; Soler-Illia, G. J. A. A.; Grosso, D.; Cagnol, F.; Ribot, F.; Sanchez, C. J. Am. Chem. Soc. 2003, 125, 9770. (b) Grosso, D.; Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Cagnol, F.; Sinturel, C.; Bourgeois, A.; Brunet-Bruneau, A.; Amenitsch, H.; Albouy, P. A.; Sanchez, C. Chem. Mater. 2003, 15, 4562. (4) Luca, V.; Djajanti, S.; Howe, R. F. J. Phys. Chem. B 1998, 102, 10650.
Sol-Gel Dense and Mesoporous Anatase Titania Films (5) Almeida, R. M.; Marques, M. I. D.; Orignac, X. J. Sol-Gel Sci. Technol. 1997, 8, 293. (6) Hanley, T. L.; Luca, V.; Pickering, I.; Howe, R. F. J. Phys. Chem. B 2002, 106, 1153. (7) Simonsen, M. E.; Søgaard, E. G. J. Sol-Gel Sci. Technol. 2010, 53, 485. (8) Innocenzi, P.; Kidchob, T.; Mio Bertolo, J.; Piccinini, M.; Cestelli Guidi, M.; Marcelli, A. J. Phys. Chem. B 2006, 110, 10837. (9) Keddie, J. L.; Braun, P. V.; Giannelis, E. P. J. Am. Ceram. Soc. 1994, 77, 1592. (10) Toshikazu, N.; Takayuki, Y.; Nobuyoshi, M.; Makiko, S. Thin Solid Films 2004, 467, 43. (11) Malfatti, L.; Bellino, M. G.; Innocenzi, P.; Soler-Illia, G. J. A. A. Chem. Mater. 2009, 21, 2763. (12) Yu, J. C.; Wang, X. C.; Fu, X. Z. Chem. Mater. 2004, 16, 1523. (13) Aiello, D.; Testa, F.; Aiello, R.; Malfatti, L.; Kidchob, T.; Innocenzi, P.; Aiello, I.; Martino, T.; Ghedini, M.; La Deda, M. Inorg. Chem. Commun. 2009, 12, 237. (14) Peiro, A. M.; Peral, J.; Domingo, C.; Domenech, X.; Ayllon, J. A. Chem. Mater. 2001, 13, 2567. (15) Malfatti, L.; Falcaro, P.; Amenitsch, H.; Caramori, S.; Argazzi, R.; Bignozzi, A. C.; Enzo, S.; Maggini, M.; Innocenzi, P. Microp. Mesop. Mater. 2006, 88, 304. (16) Bass, J. D.; Belamie, E.; Grosso, D.; Boissie`re, C.; Coradin, T.; Sanchez, C. J. Biomed. Mater. Res. A 2009, 96. (17) Bass, J. D.; Grosso, D.; Boissie`re, C.; Sanchez, C. J. Am. Chem. Soc. 2008, 130, 7882. (18) Smarsly, B.; Grosso, D.; Brezesinski, T.; Pinna, N.; Boissie`re, C.; Antonietti, M.; Sanchez, C. Chem. Mater. 2004, 16, 2948.
J. Phys. Chem. C, Vol. 114, No. 51, 2010 22391 (19) Kidchob, T.; Malfatti, L.; Serra, F.; Falcaro, P.; Enzo, S.; Innocenzi, P. J. Sol-Gel Sci. Technol. 2007, 42, 89. (20) Kidchob, T.; Malfatti, L.; Marongiu, D.; Enzo, S.; Innocenzi, P. J. Sol-Gel Sci. Technol. 2009, 52, 356. (21) The THz spectroscopy conventionally covers the spectral range from about 3 cm-1 to about 600 cm-1; this frequency interval is also indicated as the far-IR domain. (1 THz is 33.33 cm-1, and the THz domain ideally falls within the ∼0.1-20 THz interval.) (22) Heilweil, E. J.; Maslar, J. E.; Kimes, W. A.; Nabil, D.; Bassim; Schenck, P. K. Opt. Lett. 2009, 9, 1360. (23) Ohsaka, T.; Izumi, F.; Fujiki, Y. J. Raman Spectrosc. 1978, 7, 321. (24) Gonzalez, R. J.; Zallen, R.; Berger, H. Phys. ReV. B 1997, 55, 7014. (25) Wang, D.; Zhang, X.; Wu, K.; Xu, S. Chem. Lett. 2006, 35, 884. (26) Portal, S.; Almeida, R. Phys. Stat. Sol. A 2004, 13, 2941. (27) Pecharroman, C.; Gracia, F.; Holgado, J. P.; Ocana, M.; GonzalezElipe, A. R.; Bassas, J.; Santiso, J.; Figueras, A. J. Appl. Phys. 2003, 93, 4634. (28) Ocana, M.; Pecharroman, C.; Gracia, F.; Holgado, J. P.; GonzalezElipe, A. R. Thin Solids Films 2006, 515, 1585. (29) Lutterotti, L. Nuclear Inst. Methods Phys. Res. B 2010, 268, 334. (30) Angelome´, P. C.; Andrini, L.; Calvo, M. E.; Requejo, F. G.; Bilmes, S. A.; Soler-Illia, G. J. A. A. J. Phys. Chem. C 2007, 111, 10886. (31) Choi, S. Y.; Mamak, M.; Speakman, S.; Chopra, N.; Ozin, G. A. Small 2005, 2, 226. (32) Kirsch, B. L.; Richman, E. K.; Riley, A. E.; Tolbert, S. H. J. Phys. Chem. B 2004, 108, 12698. (33) Busani, T.; Devine, R. A. B. Sem. Sci. Technol. 2005, 80, 870. (34) Berreman, D. W.; Unterwald, F. C. Phys. ReV. 1968, 174, 791.
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