Iron(III) Oxide Nanoparticles in the Thermally Induced Oxidative

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Iron(III) Oxide Nanoparticles in the Thermally Induced Oxidative Decomposition of Prussian Blue, Fe4[Fe(CN)6]3 Radek

Zboril,*,†

Libor

Machala,†

Miroslav

Mashlan,†

and Virender

Sharma‡

Departments of Physical Chemistry and Experimental Physics, Palacky University, Svobody 26, 77146 Olomouc, Czech Republic, and Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901 Received July 23, 2004;

CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 6 1317-1325

Revised Manuscript Received September 20, 2004

ABSTRACT: The thermally induced decomposition of Prussian Blue, Fe4[Fe(CN)6]3 (PB), was studied in air at 250 and 350 °C. Amorphous Fe2O3 nanoparticles, cubic bixbyite β- and cubic spinel γ-Fe2O3 (maghemite) polymorphs, have been identified as the products of the decomposition under different reaction conditions. 57Fe Mo¨ssbauer spectroscopy, XRD, AFM, TEM, quasielastic light scattering method (QELS) of particle size analysis, BET surface area, and magnetization measurements were used to understand the influence of the PB particle size and oxidation conditions on the decomposition mechanism at 250 and 350 °C. At a minimum decomposition temperature of 250 °C, amorphous Fe2O3 nanoparticles were formed with the size ranging from 1 to 4 nm and large surface area of 400-200 m2/g in dependence on the PB particle size. Such small amorphous Fe2O3 nanoparticles were obtained by the solid-state route for the first time. At 350 °C, cubic β-Fe2O3 and γ-Fe2O3 polymorphs were identified and their contents were found to be strongly dependent on the initial PB particle size and oxidation-diffusion conditions. Generally, the higher relative content of γ-Fe2O3 was obtained for larger PB particles and in air-limited conditions. 1. Introduction Iron(III) oxide as a unique polymorphic compound gives the varied structural forms with different particle size and morphology depending on the preparation method. Iron(III) oxide is thus an appropriate compound to perform a study of polymorphism, which includes chemically, thermally, or pressure-induced isochemical structural transformations of various polymorphs. The existence of amorphous Fe2O3 and four crystalline polymorphs (alpha (R), beta (β), gamma (γ), epsilon ()) has been known.1 R-Fe2O3, occurring in nature as a mineral hematite, has a rhombohedrally centered hexagonal structure of the corundum type (the space group R3 h c) with a close-packed oxygen lattice in which twothirds of the octahedral sites are occupied by FeIII ions.1-3 β-Fe2O3 has a body-centered cubic “bixbyite” structure with Ia3 h space group, and two nonequivalent octahedral sites of FeIII ions in the crystal lattice. The cubic unit cell contains 32 FeIII ions, 24 of which have a C2 symmetry (d-position) and 8 have a C3i symmetry (b-position).4,5 γ-Fe2O3, which is known as maghemite, is an inverse spinel with a cubic unit cell (space group P4132). It contains cations in tetrahedral A and octahedral B positions, but there are vacancies, usually in octahedral positions.1-3,6 The orthorhombic structure of -Fe2O3 (space group Pna21) is derived from a close packing of four oxygen layers.7 There are three nonequivalent anion and four cation positions. One of the cation positions is tetrahedrally coordinated, while the other three positions are octahedral in -Fe2O3. Amorphous iron(III) oxide is usually formed as very small particles with diameters less than 5 nm. It is generally suggested that FeIII ions in the amorphous Fe2O3 phase * To whom correspondence should be addressed. E-mail: zboril@ prfnw.upol.cz; fax: +420585225737; tel.: +420585634947. † Palacky University. ‡ Florida Institute of Technology.

are surrounded by oxygen octahedra with the respective symmetry axes randomly orientated in a nonperiodic lattice.8 Iron(III) oxides, particularly in the form of nanoparticles, are being used as catalysts, pigments, gas sensors, contrast agents in the magnetic resonance imaging, magnetic storage media, and furthermore, as basic components in the ferrofluid technologies or in biomagnetic separation processes.1 Due to many industrial applications of Fe2O3 nanoparticles, the novel methods for their synthesis have been developed in the past few years. Nanoparticles of the most common polymorphs, γ-Fe2O3 (maghemite) and R-Fe2O3 (hematite), have been synthesized in different forms including nanocomposites with various matrixes,9-26 pure nanopowders,27-35 coated particles, or colloidal suspensions.36-38 β-Fe2O3 has so far been developed as a thin film by a chemical vapor deposition method with an organometallic compound as a precursor material and as a powder by a thermally induced solid-state reaction of sodium chloride with ferric sulfate in air.1,4 The youngest and rarest of iron(III) oxide polymorphs, -Fe2O3, was obtained as nanoparticles dispersed in silica by the direct sol-gel method or by combining reverse-micelle and sol-gel methods.7,39 Amorphous Fe2O3 phase in the form of powder, thin film, or nanocomposite was prepared by several routes including sonochemical, precipitation, sol-gel, decomposition, or deposition methods.40-51 The thermally induced solid-state decompositions of iron-containing materials represent an important group of the preparation methods.1,34,52-55 However, some solid-state reactions result in the simultaneous formation of different polymorphs under any particular reaction conditions. For example, the thermal conversion of rhombohedral Fe2(SO4)3 yields all Fe2O3 polymorphs (R-, β-, γ-, -), depending on the reaction temperature.1,56 The phase composition of samples (content of individual polymorphs) depends significantly on the size of Fe2-

10.1021/cg049748+ CCC: $27.50 © 2004 American Chemical Society Published on Web 10/05/2004

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(SO4)3 particles.57 This phenomenon was explained by the nonequivalent reaction routes occurring on the surface and in the bulk of Fe2(SO4)3 particles. The reaction routes were related to different conditions under which diffusion and liberation of sulfur trioxide gas occurred. Understanding of the reaction mechanism of such decomposing reactions is thus important to optimize reaction conditions to synthesize a specific polymorph. In the present study, we have performed thermally induced oxidative decomposition of Prussian Blue (Fe4[Fe(CN)6]3, PB). Mo¨ssbauer spectroscopy and other techniques were used to observe formation of various Fe2O3 polymorphs in the decomposition, which enabled us to understand the mechanism of the process. Amorphous Fe2O3 nanoparticles, cubic bixbyite β- and cubic spinel γ-Fe2O3 (maghemite) polymorphs, were formed with the size and contents dependent on the properties of PB. In contrast to ferric sulfate (Fe2(SO4)3), PB showed a low decomposition temperature preventing the secondary thermal transformations of metastable polymorphs to thermodynamically most stable R-Fe2O3. We were thus able to understand the primary steps involved in the mechanism of thermally induced oxidative decomposition of PB. 2. Experimental Section The study was performed on two starting materials of PB powders, prepared through different routes and with different particle size distributions. The sample labeled “W” was prepared by wet route from a diluted solution of potassium ferrocyanide added to an excess of iron(III) chloride.58 The sample labeled “S” was synthesized by solid-state thermal decomposition of ammonium ferrocyanide in air at 160 °C. The residual ammonium-containing phases were removed by dissolution in distilled water and the PB powder was separated by filtration in sintered glass filter and dried at 100 °C. The thermogravimetric (TG) analyses of W and S samples were performed in air to know the chemical stability and purity of the synthesized PB precursors. TG curves indicated a onestep decomposition of the ferrocyanide structure starting at 200 °C and finishing at 270 °C in both samples. The experimental weight losses found in sample W and S were 34.8 and 34.7%, respectively. These losses are similar to the calculated loss of 35% in the transformation of Fe4[Fe(CN)6]3 to Fe2O3. The TG data confirmed the high purity of the synthesized PB samples with a negligible content of external ions (H2O, NH4+). Wet chemical and elemental (C, H, N) analyses gave a chemical composition as Fe4[Fe(CN)6]3. The transmission 57Fe Mo¨ssbauer spectra of 512 channels were collected using a Mo¨ssbauer spectrometer in a constant acceleration mode with a 57Co(Rh) source. Measurements were performed at temperatures ranging from 20 to 300 K. The phase composition of samples was monitored by XRD using a Seifert-FPM diffractometer with CuKR radiation and conventional θ-2θ geometry. Particle size determination was carried out using transmission electron microscope CM12 TEM/STEM (Philips). AFM images were applied using an atomic force microscope (Explorer, ThermoMicroscopes). For AFM measurements, 2 mL of liquid containing the dispersed particles were spread on a preheated mica layer (Structure Probe, Grade V-4, USA) serving as ground with an atomically smooth and clean surface. The water evaporated at 50 °C within 30 min. AFM measurements of the samples were performed in air at room temperature in a noncontact mode with silicon tips of the 1650-00 type with resonance frequencies ranging from 180 to 240 kHz. Particle size distribution of the PB precursors were monitored using quasi-elastic light scattering method (90Plus Particle Sizer, Brookhaven Instruments Corporation)

Zboril et al. Table 1. Mo1 ssbauer Parameters of W and S, Obtained by Fitting the Spectra Taken at 300 and 77 Ka sample

T [K]

Fe cation

δFe [mm/s]

W

300

Fe2+ Fe3+ Fe2+ Fe3+ Fe2+ Fe3+ Fe2+ Fe3+

-0.14 0.41 -0.09 0.48 -0.14 0.42 -0.09 0.47

77 S

300 77

∆EQ [mm/s] 0.22 0.33 0.32 0.42

RA [%] 47 53 43 57 47 53 43 57

a

δFe, isomer shift; ∆EQ, quadrupole splitting; RA, relative spectrum area.

with the MasOption software allowing both the log-normal and the multimodal size distribution analyses. The specific surface area of Fe2O3 powders was determined using a BET surface area analyzer, Coulter SA 3100. DTA and TG curves were recorded in air with a temperature increase of 2.5 °C/min in the range of 25-500 °C using EXSTAR 6000 instrument (Seiko Instruments Inc.). Elemental (C, H, N) analyses were performed with an EA Flash 1112 Series instrument (Thermofinnigan). The field dependence (up to 10 T) of magnetization and field-cooled (FC)/zero-field-cooled (ZFC) magnetization curves were measured by a VSM magnetometer (Oxford Instruments VSM 3001) in the temperature range of 5-300 K. Field-cooled magnetization curves were collected by cooling the sample in the field of 300 Oe. Zerofield-cooled magnetization curves were measured after cooling the sample in zero field and then at increasing temperature with an applied field of 300 Oe.

3. Results and Discussion 3.1 Characterization of the Prussian Blue Samples. RT Mo¨ssbauer spectra of W and S samples contained a doublet and a singlet of Fe3+ and Fe2+ cations in the cubic PB structure, respectively (Figure 1). In the structure of PB, the low-spin Fe2+ (δ ) -0.14 mm/s) was coordinated to C atoms and the high-spin Fe3+ (δ ) 0.41-0.42 mm/s) was similarly coordinated to N atoms of six (CN)- ligands.23 A symmetrical octahedral environment of the Fe2+ cations in ferrocyanides gave the singlet subspectrum with the zero value of ∆EQ.59-61 The regular vacancies in the sites of ferrocyanide anions produced an asymmetry in the Fe3+ coordination sphere giving a doublet subspectrum.23 Mo¨ssbauer parameters of the samples W and S measured at 300 and 77 K are listed in Table 1. Similar values of the hyperfine parameters in both iron sites suggest the structural similarity of samples W and S. At 77 K, the spectral area ratio of Fe2+/Fe3+ was close to the ideal value of 3/4, corresponding to the ratio of divalent to trivalent iron atoms in the crystal lattice. On the basis of the XRD patterns of samples, W and S showed only the lines corresponding to the structure of PB62 with no indication of the presence of any other phases (Figure 2). Although the line positions were almost the same in both samples, broader lines in sample S reflect a smaller coherence length of particles prepared by the solid-state route. The mean particle diameters obtained from XRD using the Scherrer’s equation are of 68 nm for sample W and 31 nm for sample S. This observation was supported by the TEM analysis of the samples, where sample W showed a lateral dimension range of 60-70 nm due to more crystalline particles than sample S whose lateral dimension was within the range of 20-35 nm. Multimodal

Oxidative Decomposition of Prussian Blue

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Figure 2. XRD patterns of the Prussian Blue precursors.

Figure 1. RT Mo¨ssbauer spectra of the Prussian Blue powders.

size distributions obtained by QELS (Figure 3) confirmed the difference in particle size of the PB powders. The size ranges found in both samples (W: 56-66 nm, S: 18-29 nm) also corresponded with observations made by TEM and XRD. In summary, two dissimilar preparation routes of samples W and S resulted in high-purity PB powders with the same crystal structure having a different particle size. Both samples W and S were thermally treated in air at 250 and 350 °C to see the influence of the PB particle size on the mechanism of the oxidationdecomposition process. The results are summarized below. 3.2 Effect of Prussian Blue Particle Size on the Mechanism of Decomposition. The role of the PB particle size on the mechanism of thermally induced

Figure 3. Particle size distributions in samples S and W as obtained from QELS analysis.

decomposition of powders S and W was studied by heating the samples in air at 250 and 350 °C for 1 h. 3.2.1 250 °C - Formation of Amorphous Fe2O3 Nanopowders. Heating of S and W samples at 250 °C gave iron(III) oxide nanopowders, which were assigned as S250 and W250, respectively. XRD measurements of both samples suggest amorphous phase without any crystalline phases (Figure 4). The total contents of C and N were lower than 0.5 wt % in both samples. The

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Figure 4. XRD patterns of amorphous Fe2O3 nanopowders synthesized from Prussian Blue.

Figure 5. TEM micrographs of agglomerates observed in sample W250.

differential thermal analysis (DTA) curves of samples S250 and W250 gave two well-resolved exothermic peaks. The first peak at ∼300 °C could be ascribed to the process of crystallization of amorphous phase to maghemite. The other exothermic peak at ∼410 °C

Zboril et al.

corresponds to the structural transformation of maghemite to hematite. These results are similar to the nature of amorphous (5 nm) particles prepared by a microemulsion technique.63 TEM and AFM images of W250 samples demonstrated spongelike agglomerates between 30 and 120 nm. AFM images showed that the circular cross-sections and the ratio of lateral to vertical dimension was nearly 1. Spongelike agglomerates of the sample was similar to amorphous Fe2O3 powders, prepared by microwave irradiation of aqueous solution of FeCl3‚6H2O containing poly(ethylene glycol) and urea49 and by sonochemical synthesis from Fe(CO)5.48 For illustration, agglomerates observed in sample W250 and comprising of ultrasmall amorphous Fe2O3 particles are shown in Figure 5. To observe the size of particles creating agglomerates with AFM, a small amount of a powder was dispersed in water by using the ultrasound at 60 °C for 3 min. Thus, very small particles were observed in AFM images of samples (Figure 6). The modes of vertical dimensions were 1.5 and 3 nm, respectively (see histograms in Figure 6). These values were supported by TEM images of the samples showing particles in the range of 2-3 nm for sample S250 and 3-4 nm for W250. The local electron diffraction performed with TEM confirmed the amorphous nature of Fe2O3 particles prepared at 250 °C unambiguously. The specific surface areas were 415 and 210 m2/g, for samples S250 and W250, respectively, which were extremely large. Amorphous iron(III) oxide particles with such a large surface area are not known in the literature. The complementary AFM and BET surface area analyses indicate an inverse relation between particle dimensions and surface area, which was expected for the spherical particles. A double increase in particle dimensions resulted in approximately a 2-fold decrease in the specific surface area of particles. The larger the PB particles are, the larger the size of the formed particles of amorphous Fe2O3 is.

Figure 6. AFM images of nanoparticles in samples S250 (left) and W250 (right) with corresponding distributions of vertical dimensions.

Oxidative Decomposition of Prussian Blue

Figure 7. Mo¨ssbauer spectra of samples S250 and W250 taken at temperatures between 20 and 300 K.

RT Mo¨ssbauer spectra of samples S250 and W250 (upper curves in Figure 7) gave a slightly broadened doublet of the non-Lorentzian shape indicating a quadrupole splitting distribution arising from the electric field gradient distribution encountered in amorphous iron(III) oxides.8,43,44 The widths of the spectral lines (Γ1/2 ) 0.52 mm/s, both in S250 and in W250 samples) suggest the narrower distribution of the electric field gradients than in other amorphous Fe2O3 powders (Γ1/2 ) 0.60-0.70 mm/s).8,44,45,48 The same isomer shift value for both samples, δ ) 0.33 mm/s, corresponds to highspin Fe3+ in octahedral coordination. The symmetry of the iron environment was generally lower in sample S250 as documented by the higher ∆EQ value (0.84 mm/ s) in comparison with sample W250 (0.76 mm/s). The effect of temperature on the Mo¨ssbauer spectrum was conducted and Figure 7 illustrated the temperature

Crystal Growth & Design, Vol. 4, No. 6, 2004 1321

dependence spectral changes of the samples. Mo¨ssbauer spectra from 20 to 60 K could be modeled entirely using the magnetic hyperfine field distribution. This reveals the gradual decrease in the average field with temperature. Between 60 and 80 K, low fields gave magnetically unsplit components, which could be modeled by a broadened singlet and/or quadrupole doublet. Their relative areas varied in ratio with temperature and only the doublet remained above 80 K. The evolution of the singlet and doublet relative areas at a temperature more than 60 K means that there was up-down relaxation with a distribution of the relaxation time (τ). This is a common phenomenon in the pure superparamagnetic regime. However, the τ distribution was narrower than for the systems of noninteracting or weakly interacting particles, which have been shown giving unusual fast temperature variation of the superparamagnetic fraction. This indicates that the transition to superparamagnetism keeps the memory of the collective state.64 From Figure 7, we could estimate the Mo¨ssbauer blocking temperature of sample (TB). At TB, the area of the superparamagnetic fractions is equal to the sextet fraction. The TB values were 69 and 65 K for W250 and S250, respectively. This difference is very small although both samples have different particle size. Figure 7 also showed drastic changes in spectral lines in a narrow temperature interval at nearly the same Mo¨ssbauer transition temperature. The changes were independent of the particle size, which can be ascribed to the interparticle interactions. Magnetization measurements were performed as a function of the magnetic field at different temperatures. Generally, magnetization curves of samples W250 and S250 had the same character. At a temperature above TB, the magnetization curves of both S250 and W250 samples were not hysteretic. Below TB, the superparamagnetic transition was blocked, the magnetization could not relax during the measurement, and a hysteretic behavior occurred. This is illustrated in the curves of magnetization versus the applied field including the virgin magnetization curves at 5 K (Figure 8). Evidently, magnetization was not saturated even at the maximum field value of 10 T. Such type of nonsaturation behavior of magnetization under a high field is usually found in spin glass/cluster-spin glass systems with competing exchange interactions below the spin freezing temperature.65,66 The absence of the saturation would be also explained by the extremely small particle size, so that the Langevin variable can reach a large enough value only for very large fields as in the nanoparticle systems with long-range dipole-dipole interactions. Nevertheless, the spin-glass analogy seems to be more correct especially with respect to the results of the FC/ZFC measurements (see below). In our samples, the maximum values of magnetization at 10 T were 19.2 and 15.8 emu/g for W250 and S250, respectively. These values were significantly lower than the values for the nanocrystalline γ-Fe2O3 systems. In another study, the saturation magnetization of 3 nm maghemite nanoparticles in the field of 5.5 T at 5 K reached a quite high value of 48 emu/g.67 The lowtemperature coercive field in our smaller particle sample S250 (HC ) 4150 Oe) was surprisingly slightly larger than W250 sample (HC ) 3700 Oe). Although the finite-

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Zboril et al.

Figure 8. M vs H curves taken at 5 K and ZFC/FC curves of samples W250 and S250.

size effects usually reduce the coercive field, they are often superimposed by the surface effects in very small particles.18,67,68 Thus, the observed fairly high coercivity in both samples at 5 K and its increase with decreasing size can be attributed to the extra energy required for the switching of the core spins that are pinned by the exchange interactions with the frozen spin glasslike surface layer. The virgin magnetization curve for the sample W250 was below the loop for a certain field range. Such behavior was probably due to a higher stability of the nanoparticle system against the application of the magnetic field. The observed features of virgin magnetization curve are related to enhanced interparticle interactions.67 The ZFC and FC magnetization measurements give information on the influence of interparticle interactions on magnetic properties of amorphous Fe2O3. The samples in our study represent the strongly interacting systems, with a maximum at Tmax ) 50 K in ZFC curves (see Figure 8). The maximum of the ZFC curve for the sample S250 is narrower than the W250 sample. This indicates a narrower particle size distribution in S250 (compare with Figure 6). The FC curve remained practically constant below Tmax, similar to behavior observed in spin glasses, characterized by a random cooperative freezing of spins or spin clusters at a welldefined transition temperature.18,67 On the other hand, the FC curve steeply ascends below irreversibility temperature and Tmax was shifted to a much lower value in nanoparticle systems with suppressed interparticle interactions such as amorphous Fe2O3 dispersed in SiO2 matrix.69,70 The another possible interpretation of the downturn in FC data of samples S250 and W250 by the antiferromagnetic behavior of the R-Fe2O3 admixture is out of record with the low-temperature Mo¨ssbauer spectra, where no traces of hematite were identified. 3.2.2 350 °C: Formation of Nanocrystalline Cubic β- and γ-Fe2O3 Nanopowders. Both samples, S

Figure 9. RT Mo¨ssbauer spectra of samples W 350 (left) and S350 (right).

and W, were heated to 350 °C and the resulted samples were assigned as S350 and W350, respectively. The results manifested again the strong influence of the PB particle size on the molar ratio of both polymorphs and size of ferric oxide particles. XRD patterns of samples displayed exclusively the lines corresponding to cubic “bixbyite” β-Fe2O3 structure and cubic spinel γ-Fe2O3 (or Fe3O4) structure. The widths and intensities of diffraction lines indicate the larger particles and lower content of β-Fe2O3 in sample W350. RT Mo¨ssbauer spectra shown in Figure 9 exclude the presence of Fe3O4 in the samples and exhibit the superposition of (super)paramagnetic doublet phase and broad sextet corresponding to magnetic hyperfine field distribution. Magnetically split components in the spectra can be ascribed to larger maghemite particles with size distribution, while paramagnetic β-Fe2O3 particles and superparamagnetic maghemite nanoparticles can contribute commonly to the doublet spectrum. Due to the possible fraction of γ-Fe2O3 in superparamagnetic state at room temperature, quantification of reaction products was performed from Mo¨ssbauer spectra taken at 150 K (Figure 10). This temperature is above Neel

Oxidative Decomposition of Prussian Blue

Crystal Growth & Design, Vol. 4, No. 6, 2004 1323 Table 2. Mo1 ssbauer Parameters of W350 and S350 Obtained by Fitting the Spectra at 300 and 150 Ka sample W350

S350

T [K]

Fe component

300 Fe3+ doublet Fe3+ sextet 150 β-Fe2O3 doublet γ-Fe2O3 sextet 300 Fe3+ doublet Fe3+ sextet 150 β-Fe2O3 doublet γ-Fe2O3 sextet

δFe ∆EQ [mm/s] (Q) [mm/s] 0.36 0.33 0.43 0.40 0.35 0.32 0.42 0.39

0.85 0.00 0.77 -0.01 0.83 0.01 0.76 -0.02

Bav [T] 48.1 49.8 46.6 48.2

RA [%] 64 36 49 51 85 15 67 33

a

Q, average quadrupole shift; Bav, average hyperfine magnetic field.

Figure 10. Mo¨ssbauer spectra of samples W350 (left) and S350 (right) taken at 150 K.

Figure 11. 150 K Mo¨ssbauer spectrum of the sample W pressed to the pellet and heated at 350 °C for 1 h.

magnetic transition temperature of β-Fe2O3 (110 K), where particles are still paramagnetic (doublet). However, maghemite particles below blocking temperature gave magnetically split spectrum (sextet). Mo¨ssbauer parameters of the spectra taken at 300 and 150 K for samples S350 and W350 are listed in Table 2. Temperature decrease from 300 to 150 K resulted in the lowering of the quadrupole splitting parameter of the doublet phase, 0.77 mm/s, a value typical for β-Fe2O3. Decrease in the temperature also increased the spectrum area of the sextet components. These two effects confirmed the assumption that some fraction of maghemite particles occurred in the superparamagnetic state at room temperature.

The average hyperfine magnetic fields values in the sample W350 suggest the larger maghemite particles than that of sample S350. This finding was in agreement with XRD measurements of these samples. The molar ratio of β-Fe2O3 to γ-Fe2O3 in S350 obtained from the spectra was approximately double to that in W350 at 150 K. This indicates the importance of PB particle size on the phase composition of the formed ferric oxide phase. Such an effect may be related to different conditions for the oxidation and liberation of the gaseous reaction product on the surface and in the bulk of the PB particles. It appears that β-Fe2O3 forms preferentially from the surface layer of the PB particles, while the worse diffusion and oxidation conditions in the bulk result in the formation of a maghemite phase. Similar phenomenon was previously observed in our laboratory for thermal decomposition of ferric sulfate in air.56 3.3 Effect of Oxidation and Diffusion Conditions. Experiments were conducted to see the role of oxidation and diffusion conditions on the polymorphous composition of the ferric oxide phase formed by oxidative decomposition of PB. In this experimental setup, sample W was pressed to the pellet (4 mm thickness), which represented the system with the worse conditions for the air-oxygen access and liberation of gaseous (CN)2. This sample was heated at 350 °C for 1 h and analyzed by Mo¨ssbauer spectroscopy at 150 K (Figure 11). The results can be compared with the narrow nonpressed powder layer (1 mm) heated under same conditions (see W350 in Figure 10). The pressed sample showed dominance of maghemite over β-Fe2O3 in the Mo¨ssbauer spectrum. The molar ratio of γ-Fe2O3 to β-Fe2O3 was found to be 9:1, which

Figure 12. XRD pattern of the sample W heated at 350 °C for 1 h. (a) Narrow powder layer; (b) the pressed pellet.

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was much larger than the 2:1 ratio for powder layer. These results were supported by XRD data, which confirmed the noticeable ascendancy of maghemite phase in pressed sample (Figure 12). In contrast to the narrow powder layer, both polymorphs formed larger particles. The higher peak intensities in XRD pattern of the pressed sample indicate faster sintering process responsible for increase of particle size. 4. Conclusions The oxidative conversion of PB in air at a minimum decomposition temperature of 250 °C allowed the synthesis of amorphous Fe2O3 nanoparticles with controlled size. The nanoparticles exhibited unique properties including a very low particle size, uniform size distribution, an extremely large surface area, and the specific magnetic behavior. At 350 °C, nanocrystalline cubic βand γ-Fe2O3 polymorphs were identified as the primary decomposition products, and their molar ratio was found to be strongly dependent on the reaction conditions. Clearly, the relative content of γ-Fe2O3 increased with increasing PB particle size. The influence of the PB particle size on the polymorphous composition of ferric oxide was explained by the preferential formation of β-Fe2O3 from the surface layer of the PB particles. The diffusion and oxidation conditions in the bulk of particles result in the formation of maghemite phase. Acknowledgment. Sincere gratitude to R. Mu¨ller (IPHT Jena, Germany) for the magnetic measurements, N. Pizu´rova´ (IPM ASCR Brno, Czech Republic) for TEM measurements, and M. Vu˚jtek (Palacky University, Olomouc, Czech Republic) for AFM measurements. The authors are deeply grateful also to D. Petritidis (National Center for Scientific Research “Demokritos”, Athens) for the manuscript review. This work has been supported by the Grant Agency of the Czech Republic (202/03/P099) and by grant CZE03/013 of the Ministry of Education of the Czech Republic. References (1) Zboril, R.; Mashlan, M.; Petridis, D. Chem. Mater. 2002, 14, 969. (2) Cornel, R. M.; Schwertmann, U. The Iron Oxides. Structure, Properties, Reactions and Uses; VCH: Weinheim, 1996. (3) Mitra, S. Applied Mo¨ ssbauer Spectroscopy, Series: Physics and Chemistry of the Earth; Pergamon: Oxford, 1992. (4) Bauminger, E. R.; Ben-Dor, L.; Felner, I.; Fischbein, E.; Nowik, I.; Ofer, S. Physica B 1977, 86-88, 910. (5) Ben-Dor, L.; Fischbein, E. Acta Crystallogr. B 1976, 32, 667. (6) Vandenberghe, R. E.; de Grave, E.; Landuydt, C.; Bowen, L. H. Hyperfine Interact. 1990, 53, 175. (7) Tronc, E.; Chane´ac, C.; Jolivet, J. P. Solid State Chem. 1998, 139, 93. (8) van Diepen, A. M.; Popma, T. J. A. J. Phys. Colloq. C6 1976, 37, 755. (9) Bourlinos, A.; Zboril, R.; Petridis, D. Microporous Mesoporous Mater. 2003, 58, 155. (10) Cannas, C.; Concas, G.; Gatteschi, D.; Musinu, A.; Piccaluga, G.; Sangregorio, C. J. Mater. Chem. 2002, 12, 3141. (11) Casula, M. F.; Corrias, A.; Paschina, G. J. Non-Cryst. Solids 2001, 293, 25. (12) Chatterjee, J.; Haik, Y.; Chen, C. J. J. Magn. Magn. Mater. 2002, 246, 382. (13) Xue, J. M.; Zhou, Z. H.; Wang, J. J. Am. Ceram. Soc. 2002, 85, 807. (14) Tartaj, P.; Gonzales-Carreno, T.; Serna, C. J. Adv. Mater. 2001, 13, 1620. (15) Szabo, D. V.; Vollath, D. Adv. Mater. 1999, 11, 1313.

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