Heterogeneity and Disorder in Ti1âxFeyO2âd Nanocrystal Rutile

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Heterogeneity and Disorder in Ti1-xFeyO2-d Nanocrystal Rutile-Based Flowerlike Aggregates: Detection of Anatase omor,† Philippe Colomban,§ Aleksandar Kremenovic,*,†,‡ Bratislav Antic,† Jovan Blanusa,† Mirjana C || †,^ Leo Mazerolles, and Emil S. Bozin †

Institute of Nuclear Sciences “Vinca”, University of Belgrade, P.O. Box 522, 11001 Belgrade, Serbia Faculty of Mining and Geology, Laboratory for Crystallography, University of Belgrade, Djusina 7, 11000 Belgrade, Serbia § LADIR, UMR 7075 CNRS, and Universite Pierre and Marie Curie, 94230 Thiais, France ICMPE, UMR 7182 CNRS, and Universite Paris Est Creteil, 94320 Thiais, France ^ Condensed Matter Physics and Material Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States

)

‡

ABSTRACT: Here we report results of systematic investigation of heterogeneity and disorder in Ti1-xFeyO2-d nanorod rutilebased flowerlike aggregates. It was found that Ti1-xFeyO2-d aggregates are composed of two crystalline phases: rutile as a dominant and anatase as a minor phase. Flowerlike aggregates were found to grow from an isometric core ca. 5-10 nm in diameter that was built from anatase and rutile nanorods ca. 5 100 nm that were grown on the anatase surface having base plane (001) intergrowth with an anatase plane. The direction of rutile nanorods growth, i.e., direction of the nanorod elongation, was [001]. Highly nonisometric rutile crystals produce anisotropic X-ray powder diffraction line broadening and doubling of vibrational bands in Raman spectra. Both these techniques confirmed nonisometric character of rutile crystals and gave a quantitative measure of crystal shape anisotropy in excellent agreement with high-resolution transmission electron microscopy measurements. In addition, from the atomic pair distribution function and Raman spectral analyses the level of vacancy concentration was determined in rutile and anatase phases of investigated samples.

1. INTRODUCTION Because of its wide range of potential technological applications, such as for pigments, cosmetic products, photovoltaic cells, and catalysis, titanium dioxide, TiO2, has been the focus of intense research.1,2 As a well-known photocatalyst, its photocatalytic characteristics are greatly influenced due to the advent of nanotechnology. With a size reduction to nanoscale, not only the surface area of titanium dioxide particle increases dramatically, which is important for the activity, but also the system exhibits additional effects in optical properties and size quantization. Reduction in size commonly involves modification of the structure and introduces nonstoichiometric conditions. Defect concentration highly depends on the distance from the center of the crystal, with the regions closer to the surface retaining more vacancies. Furthermore, the atoms at the surface tend to react with the atmosphere (protonation, sulfur and nitrogen adsorption, etc.). Another marked effect of the reduced size comes from the increased surface to volume ratio, and the number of atoms near the surface becomes dominant compared to the number of atoms in the bulk.3,4 Consequently, it is of great importance to be able to characterize in as much detail as possible the exact nature r 2011 American Chemical Society

of the nanomaterial, to understand the origin of its properties, and ultimately to gain control over tunability of these properties. Therefore, nano-TiO2 is applicable in a number of branches of traditional industry,5 such as materials for environmental treatment, antimicrobial materials, and self-cleaning materials, and could find applications as UV-block sunscreen and UV-block paint. Most of the unique optical properties of TiO2 are derived from its remarkably high value of refractive index,5 which expresses the ability of a material to bend and scatter the light. From a crystallographic point of view, bulk TiO2 crystallizes in three main forms: anatase (space group I41/amd), rutile (space group P42/mnm), and brookite (space group Pbca). Two other existing structural forms, known as columbite and baddeleyite, are stabilized only under high pressure.6 Quite generally, structure and microstructure (shape, size, surface morphology, and anisotropy) determine the physical and chemical properties of a material. On the other hand, structure and microstructure Received: October 1, 2010 Revised: January 31, 2011 Published: March 01, 2011 4395

dx.doi.org/10.1021/jp109437b | J. Phys. Chem. C 2011, 115, 4395–4403

The Journal of Physical Chemistry C depend on the preparation conditions. For example, it was found that the electrical conductivity of nanocrystalline TiO2 films exhibits a pronounced dependence on crystallite size. 7-9 Wu et al. sinthesized and used titania films to assist photodegradation of Rhodamine B in water.10-12 They have shown that the efficiency depends on both preparation method and thermal treatment of synthesized titania. The efficiency of TiO2 nanopowders in the photocatalysis was also related to synthesis procedure.13 One of the aim of this study was synthesis of Fedoped TiO2 nanopowders. While microstructure parameters strongly affect properties of nanosized materials, nonstoichiometry can also have a profound influence on some physical properties, such as electronic transport properties.14,15 In TiO2, nonstoichiometry may be due to oxygen vacancies and excess titanium on interstitial sites or Ti vacancies. Ion doping (or partial ion substitution) of TiO2 can influence a deviation from stoichiometry by forming oxygen vacancies and appearance of cation deficit/suficit with a change in cation valence. For example, in TiO2 doped by 3d ions an increase in the photocatalytic activity has been observed.16 By incorporation of Fe3þ in host TiO2 the light absorption of pure TiO2 is significantly shifted from UV toward the visible spectral region.8,17 Crystalline structure, size, and shape of the particles strongly influence the possible application of TiO2. Although for photocatalytic applications anatase displays higher efficiency than rutile, a combination of anatase and rutile seems to be the best.18 However, because of its wide band gap (3.0-3.2 eV)18 which requires UV radiation for excitation, commercial photocatalytical application of TiO2 is still moderate. Further, fast recombination of photogenerated e--hþ pairs lowers the quantum yield below 10%. The photocatalytic efficiency of titanium dioxide could be enhanced by doping with various elements,2,16,17,19-22 e.g., Cr3þ, Ni2þ, or Fe3þ ions. Incorporating these ions in titania crystal structure could shift the absorption threshold above 450 nm, resulting in absorption in the visible part of the spectrum.16 Moreover, the shape of titanium dioxide nanocrystals greatly influences e--hþ recombination rate. For example, in nanorods charge carriers are free to move along the length of the elongated nanocrystal, which is not the case for nanospheres. Therefore, charge carriers delocalization is higher and, consequently, the probability of their recombination is lower.9 This study is focused on synthesis and characterization of elongated TiO2 nanocrystals, nanorods with rutile crystalline structure doped with Fe3þ ions. The aim of the research presented here on pure and Fe-doped TiO2 nanorods is determination of materials composition, structure, and microstructure (particle size, morphology, microstrain, and defects concentration), as they provide important insights for better understanding of their properties, essential for applications. To achieve this, we carried out HRTEM, Raman spectroscopy, X-ray diffraction (Rietveld and atomic pair distribution function (PDF) methods), and SQUID magnetic measurements.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All chemicals used were of p.a. purity and were used without further purification. Triply distilled water was used for aqueous solutions. TiO2 nanopowders doped with iron ions were prepared by a modified synthetic procedure of Abazovic et al.23 An appropriate amount of FeCl3 (Aldrich), in order to get samples with 0.25, 0.50, and 1.00 of iron ions in TiO2, was dissolved in 200 mL of triply distilled water. Then, 5 mL of TiCl4 (Fluka) prechilled to -20 C was added dropwise

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into solution containing FeCl3 under stirring. After 2 h of stirring at room temperature, the obtained dispersions were heated and kept at 60-70 C for 24 h with continuous stirring. The resulting precipitates were dialyzed against water until test reaction for Clions was negative. After centrifugation of dispersions, transparent solutions were decanted and precipitates were dried in vacuum at room temperature. Pure TiO2 powder was synthesized in the same manner, without FeCl3 in the reaction solution. Concentration of Fe3þ ions was determined by inductively coupled plasma optical emission spectroscopy (Spectroflame ICP, 2.5 kW, 27 MHz). ICP-OES measurements were performed by measuring the intensity of radiation at the specific wavelengths emitted by each element. Obtained concentrations were 0.22, 0.47, and 1.05 at. % of Fe3þ in TiO2 matrix. 2.2. Characterization Techniques. High-resolution transmission electron microscopy (HRTEM) measurements of the samples were carried out using a TOPCON 002B electron microscope operating at 200 kV. The samples were prepared by ultrasonication in ethanol and deposition on a conventional carbon-covered copper HRTEM grid. After drying, the samples were examined by HRTEM. Synchrotron X-ray experiments were performed at room temperature at the 6-ID-D beamline at the Advanced Photon Source at Argonne National Laboratory, Argonne, IL. Diffraction data were collected using the rapid acquisition pair distribution function (RAPDF) technique24 that benefits from two-dimensional (2D) data collection. The powder samples were packed in Kapton capillaries with diameter of 1.0 mm, sealed at both ends. The data were collected at room temperature with an X-ray energy of 98.65 keV (λ = 0.126 Å) selected using Si (331) monochromator. Incident beam size was 0.5 0.5 mm. An image plate detector (General Electrics)25 with active area of 410 mm 410 mm was mounted orthogonally to the beam path with a sample-to-detector distance of 268.62 mm, as calibrated by using ceria standard sample.24 Each sample was exposed for 10 s, and this was repeated six times for a total data collection time of 60 s to obtain good counting statistics. The RAPDF approach is described in detail elsewhere.24 The data were corrected and normalized26 using the program PDFgetX227 to obtain the total scattering structure function, F(Q), and its sine Fourier transform, the atomic PDF, G(r). The dc magnetization was measured in the temperature region of 1.8-300 K, and in an applied field of 500 Oe using an MPMS XL-5 SQUID magnetometer. Hysteresis loops were measured in zero-field-cooled regime over (-50 kOe, 50 kOe) at temperatures of 10 K. A high-sensitivity multichannel notch-filtered INFINITY spectrograph (Jobin-Yvon-Horiba SAS, Longjumeau, France) equipped with a Peltier cooled CCD matrix detector was used to record Raman spectra between ∼150 and 2000 cm-1, using 532 and 632 nm exciting lines (YAG and He-Ne lasers). Backscattering illumination and collection of the scattered light were made through an Olympus confocal microscope (long focus Olympus 10 and 50 objective, total magnification 100 and 500). Special attention was paid to the power of illumination used (0.1-2 mW). The examination was carried out on small particle aggregates, in a procedure preferentially used for dark material.

3. RESULTS AND DISCUSSION 3.1. Morphology and Microstrucutre by HRTEM. Electron transmission microscopy was used in order to characterize size 4396

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The Journal of Physical Chemistry C

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Figure 1. HRTEM images (a-c) and SAED pattern (d) of TiO2 doped with 0.22 at. % of Fe.

Figure 2. HRTEM images (Fourier transform in the inset) of longitudinal (a) and transversal (b) sections of a nanorod for TiO2 doped with 0.22 at. % of Fe.

and morphology of nanocrystallites. Rodlike shapes of nanocrystallites were observed (Figure 1a), typical for rutile phase of TiO2. High-resolution electron microscopy is a very good method for revealing crystal defects such as stacking faults. Our observations on different areas (Figure 1a and Figure 2 are examples) revealed neither dislocations nor stacking faults inside the crystal structure of nanorods. The average length of nanorods was ∼80 ( 20 nm, with average diameter of ∼7 ( 2 nm for samples containing 0.22 at. % of Fe. Similar results were obtained for the other Fe concentrations studied, indicating that in the investigated region Fe content did not significantly modify the crystalline morphology and nanorod dimensions. Nanorods of rutile samples display a flowerlike structure (Figure 1b,c). The selected area electron diffraction (SAED) pattern (Figure 1d) indicated that nano-objects are crystalline. Here, it is worth mentioning results of Wu et al. on synthesis of titania thin films with a dual structure, that is, flowerlike rutile aggregates sitting on

top of an anatase layer, that were fabricated simply by oxidizing metallic Ti plates.10 The interplanar distance of ∼3 Å corresponds to (001) planes of rutile structure, which indicates that the preferred particle growth direction is [001], in agreement with results of Rietveld analysis (for details see results in section 3.2). Theoretical calculation by Oliver et al. showed that the (001) surface has the highest surface energy; the [001] is favored growth direction.28 The same conclusion for rutile nanocrystals was drawn from experimental results of Zhang et al.29 No crystalline or amorphous phase containing Fe (pure Fe or its oxide) was detected from the HRTEM examinations. The HRTEM image in Figure 2a and its Fourier transform (similar to a local microdiffraction) in the inset show that these needles are grown parallel to the c-axis of the rutile structure. Observation of a transverse section of these needles (Figure 2b) reveals facets corresponding to (110) planes. 4397



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Table 1. Unit Cell Parameters, Average Strain, and Crystallite Size for Pure and Fe-Doped TiO2 Nanocrystals unit cell/strain/size a [Å]

pure

0.22 at. % Fe 0.47 at. % Fe 1.05 at. % Fe

4.6055 (3) 4.6149 (3)

4.6139 (3)

4.6078 (3)

c [Å] 2.9543 (3) 2.9536 (3) average strain 10-4 20.26 (2) 25.36 (3)

2.9545 (2) 28.1 (2)

2.9569 (2) 30.842 (2)

size [Å] along [110]

102

51

53

68

along [001]

23000a

16000a

22000a

12000a

along [103]

488

245

246

326

a

Resolution limited. These micrometer-scale values are out of XRD measurement scope and should not be regarded as reliable.

Figure 3. Part of synchrotron X-ray diffraction patterns of pure TiO2 and Fe-doped TiO2 samples (A denotes anatase contribution).

Figure 5. Rietveld fit for TiO2 doped with 1.05 at. % of Fe (Rwp ∼ 3%). The difference (bottom) curve between experimental and calculated values is offset for clarity. Reflection positions are represented by vertical ticks (first row denotes rutile phase and second row anatase phase).

Figure 4. Rietveld fit for TiO2 doped with 0.22 at. % of Fe (Rwp ∼ 3%). The difference (bottom) curve between experimental and calculated values is offset for clarity. Ticks mark reflections for rutile.

3.2. Structure and Microstructure Analysis in Reciprocal Space: Rietveld and Size-Strain Analysis. The analysis of

obtained XRPD patterns (Figure 3) shows that all samples predominantly crystallized in rutile form with a small amount of anatase phase that could be noticed only in pure TiO2 and TiO2 doped with 1.05 at. % of Fe. The patterns were analyzed by FullProf program 30 started initially with standard Rietveld refinement procedure. However, an attempt to fully implement Rietveld refinement in order to refine all crystal structure parameters was unsuccessful in this case; this was attributed to the difficulties in integrated intensity calculations for highly anisotropic particles. To avoid these problems, further analysis was done by using FullProf in profile matching mode with the chosen “needlelike” crystallite size line broadening model.30 The refinement based on this model has shown very good agreement with experiment (Figure 4). The profile of diffraction peaks was described by Thompson-Cox-Hastings approximation of Voigt function comprising the size and strain calculations based on integral breadths.31,32 The instrumental full width at halfmaximum (fwhm) was derived from XRPD pattern of standard CeO2 powder sample for microstructure analysis.33

The results obtained for cell parameters and microstructure (crystallite size and average microstrain) are summarized in Table 1. It was not possible to calculate average particle size due to the fact that fwhms of certain reflections were similar to the instrumental fwhm, so in these directions (close to [001]) it was not possible to extract the actual particle size (these values are indicated in Table 1 as “resolution limited”). Thus, instead of the average size values, dimensions along three directions are stated in Table 1: [110] direction, for which the smallest size value was found in all samples, indicates the direction corresponding to the cylinder diameter (assuming cylindrical particle shape as observed by TEM); the second one is size along direction [001], the value of which was too large to be correctly evaluated by XRD, indicating the preferential particle growth along [001] direction; the third direction listed in Table 1 is [103], for which the size value was found to be the largest among all nonorthogonal directions to [110]. This value can be used as an indicator of crystallite length along [001] since the real value along [001] cannot be precisely determined. Table 1 shows that the isotropic microstrain increases uniformly with the iron content increase. However, crystallite size changes nonuniformly with the Fe content increase. It was shown that smallest crystallites were formed in samples doped with 0.22 and 0.47 at. % of iron. As expected, the largest crystallite size was found in pure TiO2. However, further increase of iron content above 0.47 at. % did not induce further decrease of size; on the 4398



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Figure 6. (a) Representative experimental total scattering function, F(Q), and (b) corresponding atomic PDF, G(r) (open symbols), with rutile structure model (solid line). Difference curve is offset for clarity. The data are shown for pure TiO2 nanocrystal at room temperature, with Qmax = 24 Å-1 used in Fourier transform.

Table 2. Structure Parameters from PDF Refinements of Rutile P42/mnm Model over 1.2-20 Å Rangea 0.22 at.

0.47 at.

1.05 at.

pure

% Fe

% Fe

% Fe

a [Å]

4.6058 (4)

4.6076 (4)

4.6063 (4)

4.6037 (4)

c [Å]

2.9559 (5)

2.9549 (4)

2.9558 (4)

2.9568 (4)

x(O)

0.3068 (2)

0.3067 (2)

0.3069 (2)

0.3056 (2)

Uiso(Ti) [Å2]

0.00746 (5) 0.00627 (5) 0.00604 (5) 0.00588 (5)

Uiso(O) [Å2] occupancy (O)

0.01319 (4) 0.01304 (4) 0.01339 (4) 0.01649 (4) 0.90 (1) 0.98 (1) 0.98 (1) 0.95 (1)

Rw [%]

14.0

parameter

14.8

14.6

14.5

with anatase Rw [%] 13.0

13.2

anatase fraction [%] 5 (3)

6 (3)

a Ti is at 2a (0,0,0) position while O is at 4f (x, x, 0). Anatase I41/amd phase was used as a secondary phase to fit data for pure and 1.05 at. % Fe-doped samples.

contrary, 1.05 at. % Fe doped sample showed the largest particle size among all three doped samples. The amount of anatase phase was also found not to be in regular correspondence with the iron content. The diffraction patterns of samples doped with 0.22 and 0.47 at. % of iron did not show the presence of anatase detectable by X-ray diffraction, and these were refined as single phase patterns (Figure 4). On the other hand, pure TiO2 and 1.05 at. % Fe doped samples gave diffraction patterns with notable presence of anatase phase. They were refined as two-phase patterns (for illustration, refined pattern of 1.05 at. % doped sample is shown in Figure 5), and the amount of anatase ratio is estimated to 4.6% and 6% for pure and 1.05 at. % Fe-doped sample, respectively. The crystal structures of rutile and anatase are in agreement with literature.6 Because of very small amount of Fe, it was not possible to refine reliably Fe content and its distribution in doped samples. Irregular unit cell parameter change with Fe concentration increase could indicate large crystallite macrostrain or random substitution of Ti by Fe. Large crystallite size anisotropy is in accord with HRTEM results. Rather small strain anisotropy indicates absence of dislocations and stacking faults, again in

Figure 7. Simulated PDF profiles based on room temperature rutile (top) and anatase (bottom) models. Specific pair contributions are labeled for reference close to peaks that they contribute to. After the second well-pronounced PDF peak, higher-r peaks for the two structures are completely out of registry. Notably, the second pronounced PDF peaks in both profiles are very close in position, with offset indicated by arrows and dashed lines. The largest discrepancy between the rutile model and the data, observable from the difference curve in Figure 6b, is precisely in this range.

accord with HRTEM results. However, the presence of point defects in Fe-doped TiO2 could be expected. 3.3. Structure Parameters from PDF Analysis in Direct Space. Total X-ray scattering data of all the samples were considered in direct space via the PDF approach, using the program PDFgui.34 Total scattering structure F(Q) is Q-weighted structure function that enhances the signal at higher momentum transfer, Q, compared to low Q (see Figure 6a). When Fourier transformed, these data can be fit using the least-squares approach (Figure 6b). This allowed overcoming the difficulties of determining structural parameters using the Rietveld method. Data for all the samples are consistent with rutile structure, and refined parameters are reported in Table 2. The starting values for lattice parameters were those obtained from Rietveld refinement, and these were allowed to further vary in the PDF modeling procedure. Small but observable discrepancies between the lattice parameters observed by Rietveld and PDF methods can be attributed to the fact that the former method depends on Bragg intensities alone, while the later utilizes both Bragg and diffuse scattering components and essentially yields different structural information. Oxygen occupancy is also refined, indicating that the pure and 1.05 at.% Fe samples are slightly oxygen deficient (ca. 10% for pure and 5% for 1.05 at. % Fe-doped TiO2; see Table 2), while 0.22 at. % Fe and 0.47 at. % Fe samples are nearly stoichiometric. In addition, since some presence of anatase was observed in pure sample and sample with 1.05 at. % Fe, as mentioned earlier, after completing the refinement of the rutile model refinement, anatase was added as a second phase using initial value for parameters from the literature,35,36 and phase fraction was estimated, which is consistent with the results of Rietveld refinement, and contributed to a small but observable improvement of Rw values. The same wide-range modeling was attempted for the other two Fe-doped samples, but this gave unstable fits with larger value of Rw than the single-phase rutile model. 4399



Figure 8. Magnetization versus temperature for TiO2:Fe samples measure after zero field cooling. Inset: magnetization versus magnetic field.

Typical fit of rutile model is shown in Figure 6b. The model works rather well, with only a noticeable mismatch in the low-r region, around 3 Å. This type of discrepancy is often observed in cases of nanocrystals when there are appreciable surface reconstruction effects. However, another possible source of the discrepancy could be the presence of highly amorphous anatase phase. If we consider the second PDF peak, the discrepancy in the fit, Figure 6b, is such that the data PDF has extra intensity at the high-r side of the peak, with respect to the model PDF. Interestingly, the same type of mismatch is observed in the second PDF peak of model profiles calculated for rutile and anatase, based on literature values for the two crystallographic phases of TiO2 (Figure 7). To further test whether this is consistent with the data, we have carried out a two phase refinement over a narrow r-range from 1.2 to 4.0 Å where this discrepancy is observed. While parameters for rutile phase were fixed to values obtained from single-phase fits (Table 2), anatase parameters were varied. The results are summarized in section 3.6. 3.4. Fe Ion Distribution. From the magnetic point of view Fe-doped or substituted TiO2 belongs to diluted magnetic semiconductors (DMS). Among different DMS, the 3d ions ZnO and TiO2 in hosts are of importance as potential materials for spintronics.37 However, to be applicable in this sense, they need to show the ferromagnetic behavior at room temperature. A number of recent publications deal with magnetic ion-doped TiO2 and ZnO.38 There are some published experimental results regarding room temperature magnetism for these materials, followed by many propositions on causes of high-temperature ferromagnetism in a magnetic ion-doped TiO2 and ZnO.39 However, our magnetization measurements versus temperature and field have shown characteristics of typical paramagnetic behavior for all Fe-doped samples (Figure 8). The presence of Fe ion clusters was not detected, which is not completely surprising having in mind very low Fe concentration in the samples studied. 3.5. Raman Spectroscopy. From Figure 9 it is evident that all samples studied were composed only from rutile and anatase (although the latter in small fraction).40-44 Evidently, anatase/ rutile ratio is highest in pure TiO2 and TiO2 with 1.05 at. % of Fe, in agreement with the results of Rietveld and PDF refinements. Although the anatase content is similar for pure and 1.05% Fe batch (∼5-6% in volume, Table 2), strong differences are

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Figure 9. Raman spectra of pure and iron-doped TiO2 nanocrystals (A = anatase; R = rutile).

observed in their Raman spectra: The spectrum of the latter compound is dominated by the anatase signature, but the main peak is broadened (fwhm = 33 cm-1 instead of 11 cm-1) and shifted from 146 to 159 cm-1. A possible explanation of this observation is that the broad and diffuse background-like X-ray scattering observed below 3 2θ in Figure 3 for pure TiO2 corresponds to amorphous anatase and that the true anatase content is in fact much higher than reported in Table 1. By comparison with pure TiO2 where standard crystalline anatase signature is observed, we can conclude that anatase content in 0.22 and 0.47% Fe:TiO2 is less than 1% in volume. Careful examination of the Raman signature of these two compounds shows unusual features: Rayleigh wings extend at least up to ∼400 and 800 cm-1. Such Raman feature is observed for disordered compounds such as substituted zirconia, especially those obtained from sol-gel route,45 Al2TiO5 nanopowders prepared by high-energy ball milling,46 and in glassy silicates (the so-called “boson peak”).4 The common feature of these materials is the presence of defects (oxygen vacancies and substituted atoms, stacking faults and Fe2þ/Fe3þ distribution, compensating cations and depolymerization, etc.) which breaks the periodicity and the propagation of the phonons and hence make active at the zone center modes of the whole Brillouin zone. In our case the small needle section, the Ti/Fe substitution, and presence of oxygen vacancies can hinder the propagation of the phonons and induce a vibrational distribution of state signature. The size effect is expected to be the dominant effect. By comparing fwhm of the Raman bands, it is evident that fwhm increase as Fe content increases. This is likely due to the phonon breaking by Fe ions and vacancies that are formed during the replacement of Ti4þ by Fe. Additionally, increase of the background intensity with increasing Fe content is evident. This could be ascribed again to the phonon breaking by Fe ions and vacancies that are formed during replacement of Ti4þ by Fe3þ or Fe2þ. In order to make appropriate fits of Raman spectra, not only the bands of vibrations belonging to anatase and rutile were fit by pure Lorentzian profiles but also fit of a “boson-like” peak was added in order to take into account vibrations that represent a disordered part of the material (Figure 10).47 Evidently, bosonlike peak intensity increases with the increase of the Fe content, indicating that disordered part of the structure increases as the Fe concentration increases. However, the rutile/anatase ratio did not change systematically with change of the Fe content. This 4400



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Figure 10. Raman spectra of pure TiO2 nanocrystals and TiO2 nanocrystals doped with 0.22 at. % of Fe (A = anatase contribution, R = rutile contribution, B = “boson-like” contribution).

suggests that the range of Fe concentrations between 0.2 and 0.5 at. % could be optimal for preparation of pure rutile specimen (or maximizes the rutile contribution to the structure). By comparing our results with recent results of Zukerman et al.48 and Swamy et al.49 in regard to crystallite size measured from Raman spectroscopy, it could be found that our result for pure anatase is in agreement with their predictions giving ca. 510 nm crystallite size on average. This is also in line with the results obtained from PDF analysis (for details see sections 3.3 and 3.6). The investigated rutile nanorods are elongated in one direction, and the average crystallite size and form are presumably too complex to be defined by Raman spectroscopy. Notably, Mazza et al.44 demonstrated that the phonon confinement model is not adequate for explaining the features of the lattice dynamics of rutile nanocrystals of any form. However, a simple and effective procedure can be applied for this purpose if calibration diagrams produced by Swamy42 (see Figures 4 and 5 in ref 42) are utilized properly. Considering a highly anisotropic object, we could expect that Raman spectrum for such an object is composed of two convoluted bands originating from two different directions each. These bands should be close to each other, different in fwhm, and with equal intensity. Results of fits presented in Figure 10 for pure TiO2 are in good agreement with this prediction for two most intense rutile bands centered at ca. 445 and 605 cm-1. For both vibration bands two peaks with equal intensity within experimental error were obtained (Table 3). Therefore, the results of our fit indicate that investigated rutile crystals are highly anisotropic, with two dominant dimensions of ca. 5 and 100 nm. For iron-doped samples the calibration curves could not be used since both position and fwhm of the used Raman mode depend on the iron content as well as on the vacancy concentration.44 This issue is more pronounced for highly anisotropic crystals, such as these that we investigated. Notably, Wang et al.1 recorded Raman spectra on a mixed anatase/rutile nanopowders, both pure and doped with iron. In 1% Fe-doped and 2% Fedoped specimens they found a shift in position of the main anatase band with respect to pure compounds from 146.2 to 149.7 cm-1 and to 155.6 cm-1, respectively. This is an indication that anatase in our specimen, considering the position of main vibrational band at 159 cm-1, contains between 2 and 3% of iron. We speculate that in the synthesis process Fe from the reaction volume prefers to be taken by anatase, rather than rutile.

Table 3. Results of Fit Presented in Figure 10 (Errors in Parentheses) center of gravity [cm-1] 0% Fe

0.22% Fe

fwhm [cm-1] 0% Fe

0.22% Fe

448.1 (6)

448 (2)

18 (1)

24 (4)

432 (4)

425 (8)

27 (3)

37 (7)

610.5 (2)

611.7 (6)

>20.3 (9)

26 (3)

603 (2)

570 (9)

53 (6)

65 (13)

3.6. Formation, Location, Structure, and Morphology of Anatase. Structural and Raman spectroscopy results showed

that samples crystallized predominantly in rutile form. It was shown that at the nanoscale anatase phase is thermodynamically more stable than rutile, while in bulk form rutile is the more stable phase.50,51 Consequently, the applied synthesis method enables one to obtain rutile—thermodynamically stable TiO2— even in nanoprticle form. It should also be noted that TiO2 doped with Fe favors rutile formation. As it is evident that even pure TiO2 crystallized predominantly in rutile form, this indicates that doping is not the dominant factor in the crystallization process. Through HRTEM assessment of the structure no anatase was observed. Both Rietveld and intermediate range PDF analyses showed that anatase could only be observed if its quantity is greater than ca. 5% (Table 3). Further, results of PDF refinements over a narrow r-range are consistent with presence of small fraction of nearly amorphous anatase in all investigated samples. The total scattering-based PDF approach, which includes both Bragg and diffuse scattering components into consideration, has the advantage over Rietveld approach, in that it can probe both intermediate and short-range order in a material, i.e., well-crystalline and “amorphous-like” material. The results of short-range PDF analysis point to a conclusion that small amount of anatase observed in the nanocrystal system is poorly crystalline. Higher content of anatase in predominantly rutile structure is systematically obtained when PDF refinement was performed over a narrow range from 1.2 to 4.0 Å than over a range from 1.2 to 20.0 Å. Although we cannot exclude a possibility of the presence of anatase at the surface,52 a more plausible explanation could be that anatase is located in the flower center. This position of poorly crystalline anatase is consistent with its low quantity and also explains why it could not be observed by HRTEM. On the 4401



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Table 4. Anatase Content (%) in Pure and Fe-Doped TiO2 Nanocrystals PDF % Fe

a

HRTEM

Rietveld

1.2-4.0 Å

1.2-20.0 Å

0

n.o.a

4.6 (7)

9 (3)

5 (3)

0.22

n.o.

n.o.

4 (3)

n.o.

0.47 1.05

n.o. n.o.

n.o. 6.1 (6)

5 (3) 8 (3)

n.o. 6 (3)

n.o. = not observed.

other hand, it is well-known that TiO2—especially anatase structure in the form of nanorods or nanotubes—may contain few % of protons.52 If we consider that proton stabilizes anatase, this would suggest that the amorphous anatase is actually located at the surface of the rutile needles, since protonation is easier at the surface than in the core of the rod. However, HRTEM measurements did not reveal presence of anatase on rutile nanorod surface, and the samples were properly dried in vacuum at room temperature to a constant weight before all the experiments were carried out. The fraction of anatase obtained from PDF refinements over the range between 1.2 and 20.0 Å is in agreement with the Rietveld refinement results, as expected, since intermediate range order dominates the PDF in this case. With all these considerations in mind, we speculate that the most plausible scenario is the one in which small particles of anatase with low degree of crystallinity form initially, followed by further crystallization of rutile nanorods, finally resulting in formation of well-crystalline flowerlike aggregates. Iron concentration appears not to play role neither for the crystal morphology nor the rutile/ anatase stability.

4. CONCLUSION Industrial applicability of TiO2 and Fe-doped TiO2 nanocrystals critically depends on detailed knowledge of their size, strain, size distribution, particle morphology, cation distribution, and presence of defects. This motivated the study of Fe-doped TiO2 nanocrystals to determine details of their structure and morphology. Rietveld and atomic PDF methods were applied on X-ray powder diffraction data. The analysis of the obtained XRPD patterns and Raman spectra revealed that synthesized samples with 0, 0.22, 0.47, and 1.05 at. % of iron ions in TiO2 predominantly crystallized in rutile form, with a small contribution of anatase phase. High-resolution transmission electron microscopy and Raman spectroscopy revealed specific structure of flowerlike aggregates composed from rodlike shapes of rutile nanocrystallites, while SQUID magnetic measurements showed that Fe clusters were not formed. All techniques used are complementary, and when used in combination, they reveal useful information and provide a more complete picture about heterogeneity and disorder in Ti1-xFeyO2-d nanocrystal rutile-based flowerlike aggregates. The core of the nanocrystals (probably isometric) is made of anatase (presumably iron-rich), while the needlelike nanorods with rutile structure grow on the initially formed anatase, thus forming the rest of the flowerlike aggregate. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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Heterogeneity and Disorder in Ti1âxFeyO2âd Nanocrystal Rutile

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