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Insights into the Mechanism Related to the Phase Transition from γ‑Fe2O3 to α‑Fe2O3 Nanoparticles Induced by Thermal Treatment and Laser Irradiation Yassine El Mendili,† Jean-François Bardeau,*,† Nirina Randrianantoandro,† Fabien Grasset,‡ and Jean-Marc Greneche† †

LUNAM Université, Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Université du Maine Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France ‡ Université de Rennes 1, Institut des Sciences Chimiques de Rennes, UMR UR1-CNRS 6226, Bât 10A, Campus de Beaulieu, 35042 Rennes Cedex, France ABSTRACT: The nature of the physical mechanisms related to the γ-Fe2O3 to α-Fe2O3 phase transition under laser irradiation and heat treatment has been investigated using in situ microRaman spectroscopy and X-ray powder diffraction (XRPD) analysis. Measurements were carried out on as-prepared γ-Fe2O3 nanoparticles of about 4 nm in size as a function of laser power and on annealed γ-Fe2O3 particles. Annealing temperature affects the relative fractions of the γ-Fe2O3 and α-Fe2O3 phases, and at 450 °C, the phase transition into α-Fe2O3 becomes complete with apparent crystallite size ⟨D⟩ of about 30 nm. The hematite nanoparticles increase then up to more than 180 nm at 1400 °C. The excellent agreement between evolution of the wavenumbers and bandwidths confirms that the heat treatment and laser irradiation produces the same effects on nanoparticles. Correlations between structure modifications occurring at the nanometric scale during grain coalescence and the evolution of Raman vibrational spectra were quantitatively examined, and a physical mechanism for the γ → α-Fe2O3 phase transition was proposed. as Raman spectroscopy, 57Fe Mössbauer spectrometry, and transmission electron microscopy (TEM), nearly complete mean structural, magnetic, and morphological properties of nanoparticles can be obtained. The precise structural characterization and understanding of phase transformations in nanoparticles is still not well understood due to the lack of welldefined model systems and difficulties in characterization. For this reason, we have chosen in this study iron(III) oxide as a convenient compound for the general study of the structural phase transitions of nanoparticles. The polymorphic nature of Fe2O3 has been identified for a long time, but the existence of amorphous Fe2O3 and four crystalline polymorphs (α, β, γ, and ε) is now well established9,10 while previous studies11−13 have also shown that different routes of preparation lead to different phases or mixtures of phases. Ayyub and Multani14,15 established a correlation between particle size and the iron oxide phase formed. Indeed, their results show that below approximately 5 nm an amorphous structure is obtained, whereas, between 5 and 30 nm and then

1. INTRODUCTION Magnetic nanomaterials have attracted widespread interest in recent years by virtue of their unusual mechanical, electrical, optical, and magnetic properties.1−6 In addition, studies have evidenced that the thermal behavior of nanoscaled materials is quite different from that observed in microsized powders.7 Nanoparticles present a challenge to both experimental and theoretical methods due to their small size and their large surface areas. Structural characterization of nanoparticles becomes thus a multifaceted problem. The finite size of nanoparticles originates broadened features in the experimental data from typical diffraction tools. In addition, the high surface to volume ratio gives rise to greater emphasis on the knowledge of both the atomic structure and coordination number at the surface,8 which remains experimentally difficult to characterize. For theoretical methods, the lack of symmetry and the importance of the surfaces make the study of nanoparticles highly complicated. A combination of experimental and theoretical methods can overcome these problems and enable the determination of atomic structure, kinetic changes, and structural aspects of nanoparticles. X-ray diffraction is a powerful means to study structures of materials. When combined with other tools such © 2012 American Chemical Society

Received: August 24, 2012 Revised: October 23, 2012 Published: October 23, 2012 23785

dx.doi.org/10.1021/jp308418x | J. Phys. Chem. C 2012, 116, 23785−23792

The Journal of Physical Chemistry C

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wavenumber 180−1000 cm−1 region with an integration time varying between 60 and 600 s. Both the wavenumbers and bandwidths of the vibration modes were determined after a constant baseline correction using the Lorentzian function defined as

beyond 30 nm, stable maghemite (γ-Fe2O3, cubic, spinel structure) and hematite (α-Fe2O3, rhombohedral, corundum structure) phases are formed, respectively. The transformation of maghemite nanoparticles into hematite can be induced in several ways, in particular by heat treatment between 523 and 873 K (depending on the preparation methods and history,16−19 under laser irradiation,20,21 and by application of external pressure).22,23 In this paper, we report the study of the structural modification induced by thermal treatment and under laser irradiation on maghemite nanoparticles with a diameter of about 4 nm and we proposed a mechanism of γ/α-Fe2O3 phase transition at the nanoscale. The evolution of particle size as a function of annealing temperature was studied using X-ray powder diffraction (XRPD), while the correlations between structure modifications occurring at the nanometric scale after laser excitation and thermal treatment were identified after a fine analysis of Raman spectra.

1 Γ/2 π (ν ̅ − ν0̅ )2 + (Γ/2)2 where ν̅0 is the position of the peak maximum and Γ represents the full width at half-maximum (fwhm) of the Lorentzian fitted curve function. L(ν ̅ ) =

3. RESULTS AND DISCUSSION 3.1. Powder X-ray Diffraction Analysis. The structural properties of our samples have been carefully characterized by several techniques. The microstructure of the samples was analyzed by XRPD measurements. Figure 1 illustrates the

2. EXPERIMENTAL SECTION 2.1. Elaboration of γ-Fe2O3 Nanoparticles. γ-Fe2O3 nanoparticles were prepared according to Massart’s method24 with the cationic precursors used in the form of metallic salts soluble in the water. Magnetite powders were first prepared by soft chemistry using coprecipitation of precursor’s cation FeSO4·4H2O (99%, Aldrich) and Fe(NO3)3·6H2O (99%, Acros) (Fe2+/Fe3+ = 1/2). The mixture was dropped into 200 mL of NaOH (2 mol·L−1) solution under vigorous stirring for about 30 min. The precipitate of magnetite (black precipitate immediately formed) was converted into γ-Fe2O3 particles by repeated treatment with HNO3 (2 mol·L−1) and FeNO3 (0.3 mol·L−1) solutions. The acidic precipitate was isolated by decantation on a magnet, separated by centrifugation (6000 rpm), washed in acetone, and dispersed in pure water, to form a stable sol (pH ≈ 2) with minimum aggregation of particles.25 The flocculated powders were further heated in air up to 100, 200, 300, 350, 375, 400, 425, 450, 475, 500, 600, 800, 900, 1200, and 1400 °C successively for 30 min and then cooled down to room temperature. Heating and cooling rates were set at 5 °C/min. 2.2. Structural Characterization. The X-ray diffraction measurements were performed with a Siemens D500 X-ray powder diffractometer using Cu Kα1,2 radiation (λ1 = 1.5406 Å, λ2 = 1.5444 Å). X-ray powder diffraction (XRPD) data were collected with a step size of 0.02° 2θ over the angular range from 20 to 100° 2θ. In this study, we used the MAUD program26 based on the full pattern XRD Rietveld fitting procedure combined with a Fourier analysis to describe the broadening of peaks.27 This method consists of refining the structural parameters from the model of the phases. 2.3. Laser Irradiation and Raman Analysis. The Raman spectra were recorded at room temperature in the backscattering configuration on a T64000 Jobin-Yvon-Horiba spectrometer equipped with the diffraction grating 600 lines/ mm under a microscope (Olympus Bx41) with a 100× objective focusing the 514 nm line from an argon−krypton ion laser (coherent, Innova). The spot size of the laser was estimated at 0.8 μm and the spectral resolution at 2 cm−1. Measurements using different laser output powers between 1 and 600 mW (corresponding to a laser power of 0.08−50 mW on the sample) were carried out consecutively without moving the sample. Single spectra were recorded twice in the

Figure 1. XPRD patterns of γ-Fe2O3 powders as-prepared and heated in the temperature range 300−1400 °C.

XRPD patterns of the as-prepared maghemite nanoparticles and the samples isothermally annealed at different temperatures (300, 350, 400, 450, 500, 900, 1200, and 1400 °C). The X-ray diffractograms of the untreated and heated at 300 °C maghemite samples are characteristic of the cubic spinel structure of maghemite (corresponding to the P4132 space group, JCPDS 16-629).28 The refined lattice parameter (a = 0.836 ± 0.001 nm, Table 1) for the as-prepared and heated (300 °C) γ-Fe2O3 powders was found to be an intermediate value between those usually found for bulk maghemite (0.834 nm) and of bulk magnetite (0.839 nm). The slight difference observed could be originated from the stoichiometry shift, nanocrystalline size, and ordering of octahedral and tetrahedral vacant sites. The microstructure (crystalline size, strain, and shape) evolution was followed up by XRPD and by transmission electronic microscopy (TEM). By XRPD, the microstructure analysis is based on line broadening analysis of the Bragg peaks. For this purpose, one considers that the line broadening originates from the reducing of crystallite size and the existence of local deformation. The used fitting method allows one to measure the coherence domain length mean value defined as apparent crystallite size ⟨D⟩ and the root-mean-square microstrain value, ⟨ε2⟩1/2, related to the local deformation. The obtained values for different experimental conditions are 23786



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Table 1. Refined Values of Lattice Parameters, Average Diameter, ⟨ε2⟩1/2, and Corresponding Agreement Factors of Maghemite Powders as-Prepared and after-Annealed at Different Temperatures annealed temperature 1400 °C 1200 °C 900 °C 500 °C 450 °C 400 °C 350 °C 300 °C maghemite as-prepared

phases α α α α α α γ α γ γ γ

% 100 100 100 100 100 60 40 44 56 100 100

a (nm) 0.503 0.503 0.503 0.504 0.505 0.505 0.836 0.505 0.836 0.836 0.836

± ± ± ± ± ± ± ± ± ± ±

0.001 0.001 0.001 0.001 0.001 0.001 0.002 0.001 0.002 0.002 0.002

c (nm) 1.375 1.375 1.375 1.375 1.376 1.377

± ± ± ± ± ±

0.001 0.001 0.001 0.001 0.001 0.001

1.377 ± 0.001

reported in Table 1. For the untreated γ-Fe2O3 nanoparticles and up to an annealing temperature of 400 °C, the nanoparticle average diameter remains constant and is evaluated to 4 ± 1 nm. Images of γ-Fe2O3 nanoparticles obtained by TEM, not presented here, illustrate nearly monodisperse and quasispherical particles with an average diameter of the order of 4 nm. The obtained results reported in Table 1 show also that, for the γ-Fe2O3 nanoparticles, the microstrain rms value increases (from 2.9 × 10−3 to 8.73 × 10−3) with the annealing temperature, up to 400 °C. This trend reflects a progressive deliquescence of the lattice. Between 350 and 400 °C, the diffraction Bragg peaks (higher index) corresponding to the α-Fe2O3 phase appear jointly with the vanishing peaks (lower index) of the γ-Fe2O3 phase, and at 450 °C, the transition γ-Fe2O3 to α-Fe2O3 is completely achieved, as all the reflections correspond to the α phase. Between 350 and 400 °C, the X-ray diffraction patterns were fitted with both γ- and α-Fe2O3 phases. The annealing temperature affects the relative fractions of the two phases. The values of lattice parameters, crystallite size, and microstrain determined from the fitting of these diagrams are given in Table 1. Table 1 also summarizes the changes observed in lattice parameters of the α-Fe2O3 phase after heat treatment. Both the “a” and “c” lattice parameters are significantly larger in the annealed sample at 350 °C than those reported for bulk crystalline (a = 5.035 Å, c = 13.748 Å).29,30 The lattice parameters decrease continuously with both increasing annealing temperature up to 900 °C and the formation of αFe2O3 in the polycrystalline state. For samples heated at 1200 and 1400 °C, the calculated Rexp and Rwp factors are respectively larger than 10 and 17, meaning the accuracy of the refined grain size value decreases. Our results also show that, while the refined average grain size value of the hematite phase increases from 14 nm at 350 °C up to more than 180 nm at 1400 °C, the rms microstrain was reduced mainly by thermal activation. Indeed, the heat treatment promotes atomic diffusion and leads to an increase in grain size, a reduction of structural defects, and therefore a better crystallinity of the particles. In addition, transmission 57 Fe Mössbauer spectrometry has been performed to detect the presence of α-Fe2O3 phases at 350 °C (not shown here). According to 57Fe Mö ssbauer spectrometry and XRPD measurements, it was found that the related proportions of hematite and maghemite phases are well consistent.

⟨D⟩ (nm) >180 >180 177 ± 10 35 ± 5 30 ± 5 18 ± 2 4±1 14 ± 2 4±1 4±1 4±1

⟨ε2⟩1/2 3.70 3.73 4.80 2.21 2.87 2.62 8.73 2.44 4.90 3.87 2.9

× × × × × × × × × × ×

−4

10 10−4 10−4 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

Rexp

Rwp

χ2

11.97 10.16 8.8 8.36 5.99 5.73

22.24 17.78 16.45 10.74 9.68 7.15

3.45 3.06 3.49 1.65 2.60 1.55

5.22

6.39

1.49

5.55 8.05

6.04 10.75

1.18 1.77

3.2. Raman Investigation. Under laser irradiation, significant heat generation was expected on γ-Fe2O3 nanoparticles21 and profile intensity changes were recorded in the Raman spectra. The laser power increase leads to changes in both width and intensity of Raman bands. Considering that the output laser power can vary from a few milliwatts to several hundreds of milliwatts, the beam focused through a microscope objective can easily alter or destroy the samples, or transform it into a different chemical phase. Raman experiments under laser irradiation have been performed under a microscope with a 100× objective (spot size estimated to 0.8 μm) with laser powers from 1 to 600 mW (corresponding to 0.08 to 48 mW on the sample). More attention has been made during Raman measurements on as-prepared maghemite nanoparticles and powdered samples heated in a furnace at different temperatures to minimize the possibility of phase transition with operating time.31,32 For this reason, the Raman spectra were systematically recorded for these compounds at 6 mW (corresponding to 0.5 mW on the sample). Figure 2 compares the Raman spectra of γ-Fe2O3 nanoparticles versus the laser power ranging from 1 to 600 mW (Figure 2a) with as-prepared γ-Fe2O3 nanoparticles and powdered samples heated in a furnace at different temperatures (300, 350, 400, 450, 500, 900, 1200, and 1400 °C) (Figure 2b). Only three broad peaks at 350 (T1), 500 (E), and 720 cm−1 (A1) characteristic of maghemite are clearly evidenced in Figure 2a (at 1 and 5 mW) and in Figure 2b (room temperature and 300 °C).33−36 With increasing laser power or thermal treatment, the vibration modes of maghemite gradually vanish, while at 15 mW and 350 °C low intensity bands (119, 160, 220, 238, 285, 315, 400, 432, 485, 598, 659, and 800 cm−1) attributed to the hematite phase appear in the Raman spectra. These vibration modes become stronger with increase of the laser power and the thermal treatment. In order to better understand the physical mechanisms responsible for the γ to α phase transition under both laser irradiation and heat treatment, several spectral variables have been investigated such as peak position, changes in bandwidth, and changes in background intensity using in this case a baseline profile analysis approach based on the difference between the intensity of two reference-like lines LMIN and LMAX centered at 340 and 850 cm−1, respectively.21 From Figure 2, we can conclude that most of the wavenumbers and the bandwidths (full width at half-maximum, fwhm) of the Raman modes appear dependently on the laser power and on the temperature of treatment. Chernyshova et al.37 pointed out that the size-induced increase in the line 23787



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Figure 2. (a) Raman spectra of maghemite powder as laser power increases. Reference-like lines (dashed lines) LMIN and LMAX are centered, respectively, at 340 and 850 cm−1. (b) Raman spectra of asprepared maghemite nanoparticles and powdered samples heated in a furnace at different temperatures (100, 200, 300, 350, 375, 400, 425, 450, 500, 600, 800, 900, 1200, and 1400 °C). The Raman spectra were recorded at 0.5 mW on the sample.

Figure 3. Temperature (■) and laser power (▲) dependence of the wavenumbers (a) and the bandwidths (b) of the A1 (∼225 cm−1) and 2Eg (∼290 cm−1) lines of hematite.

originally from the disorder-induced breaking of the symmetry properties of the T1(LO) phonon which may be caused by the defects in the materials. In this case, the structural disorders may be due to a strong resonance on the surface of the nanoparticles, and the structure defects (oxygen vacancies).37,38 The shift of Raman vibrational modes of hematite has been directly attributed in the literature to three major effects:36,39−42 (i) thermal expansion (or volume change) and (ii) stresses in the structure resulting from the increase in local temperature and (iii) structural relaxation. Indeed, the progressive transformation of maghemite into hematite does induce some stresses, yielding changes of lattice parameters and consequently a change of the corresponding wavenumbers. The softening and broadening of Raman vibrational modes can thus be attributed to a distribution of the particle size, crystallinity, packing, and shape (the so-called inhomogeneous broadening) and the finite lifetime of the vibrational states (anharmonicity).43 A recent work allowed the conclusion that phonon confinement, strain, size distribution, defects, and variations in phonon relaxation with particle size contribute to the changes in the Raman peak position and linewidth as the laser power increases. Then, between 100 and 330 mW (similarly between 400 and 600 °C), the Raman line shape of A1 and 2Eg vibrational modes of hematite exhibits, respectively, a strong red shift of 10 and 16 cm−1 and a progressive narrowing of the

fwhm's was not similar for modes associated with the movement of iron ions and those related to movements of the oxygen atoms. Indeed, in the case of thermal treatment, we observed that the fwhm for the well-resolved mode Eg around 400 cm−1 was constant (11−14 cm−1) despite the increase of the nanoparticle sizes. Accordingly, we decided to focus our study on the stronger A1 and 2Eg vibrational modes of hematite. The evolution of the wavenumber and bandwidth of these two modes as a function of thermal treatment and under laser power are compared in Figure 3. The excellent agreement between evolution of the wavenumbers and bandwidths confirms that the heat treatment and laser irradiation produces the same effects on nanoparticles.21 In particular, between 25 and 100 mW, the Raman line shape of A1 and 2Eg modes exhibits clearly a blueshift of 4 and 8 cm−1, respectively. Similar decreases were noticed between 350 and 400 °C with respectively 4 and 6 cm−1. At 100 mW, A1 and 2Eg modes were observed at about 219 and 279 cm−1. Figure 3b reveals that, whatever the treatments we used, the Raman shifts are systematically accompanied by an increase in linewidths of ca. 3.5 cm−1. Thus, at 100 mW, bandwidths of 19 and 22 cm−1 were observed respectively for A1 and 2Eg modes. As the Raman spectra show the coexistence of hematite and maghemite phases, it is reasonable to assume that the mechanism of activation of the hematite phase has arisen 23788



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bandwidths in addition to the extinction of the vibration modes of maghemite. These changes have been previously discussed21 and were mainly attributed to a sudden increase of hematite grains and structural relaxation phenomena. Above 600 °C, the wavenumbers of the A1 and 2Eg modes evolve almost over with the temperature and reach limit values, respectively, at about 228 and 294 cm−1. However, we note that the linewidths still decrease with the laser power. This observed phenomenon can be explained by an increase of grain size and a gain in crystallinity during annealing. This assumption is confirmed by data analysis of X-ray diffraction. Indeed, the average crystallite size, reported in Table 1, shows an increase from 35 to 177 nm when the temperature increases from 500 to 900 °C. Above 900 °C, while the A1 mode remains unchanged, the bandwidth of the 2Eg mode continues to decline, and this was probably due to a progressive crystallization of particles and the increase of grain sizes in a polycrystalline state as previously reported in the literature.20,41,44 These results are in agreement with those reported on hematite nanoparticles by Chernyshova et al.37 Indeed, these authors show that the vibration modes shift to lower wavenumbers when the particle size increases from 7 to 39 nm and then shift toward high wavenumbers from 39 to 120 nm. In addition, they observed a broadening of the line width of these vibrational modes when the particle size decreases. Conversely, it has been reported37 that the fwhm of the Eg mode around 400 cm−1 (associated with movements of the oxygen atoms) was independent of the particle sizes. However, in Figure 2a, we can also observe at 15 mW that the Eg mode is broader compared to the Eg mode for the heat-treated samples (Figure 2b) when modes characteristic of the hematite phase appear in the Raman spectra. Where the laser light is focused on the micrometer-size area, the laser energy density can be very high in a very short time: consequently, the temperature gradient present in the heated area can lead to an inhomogeneous grain growth, an increasing lattice disorder, and an inhomogeneous oxidation content resulting from the competition between laser energy density and oxygen pressure. In the case of thermal treatment, the process is ideally suited to perform nonspecific localized area treatment and to provide uniformity in grain size and composition. Accordingly, it is not so surprising to observe such a difference in Raman spectra for phenomena underlying oxidation mechanisms with experiments carried out under laser and thermal treatment. The phase transition γ/α was also investigated in more detail using a baseline profile analysis approach based on the difference between the intensities of two reference-like lines LMIN and LMAX. Previous investigation of maghemite (ρr = 1)21 shows that the evolution of (I(LMIN) − I(LMAX)) is directly linked on one hand to structural and morphological modification of nanoparticles and on the other hand to the absorption of particles. As the maghemite powder is brown and the hematite one is red, the light absorption at 514.5 nm (radiation used for Raman experiments) of hematite nanoparticles is higher than that of maghemite nanoparticles.45 For this reason, we plot in Figure 4 the evolution of the calculated intensity (I(LMIN) − I(LMAX)) as a function of laser power before and after obtaining the hematite and we compare the effect generated both by temperature and under laser irradiation. Figure 4a illustrates how the intensity of the baseline (I(LMIN) − I(LMAX)) increases between 1 and 15 mW. This

Figure 4. Evolution of the calculated intensity (I(LMIN) − I(LMAX)) as a function of laser power and temperature: (a) before and (b) after the appearance of the hematite phase on Raman spectra.

significant increase suggests that these low laser powers are sufficiently high to induce immediately the structural modifications at the surface of maghemite nanoparticles. At 15 mW, we obtain a maximum value of (I(LMIN) − I(LMAX)) which corresponds exactly to the power required to observe for the first time the hematite phase (vibrational bands A1 and 2Eg) on the spectrum. Above 15 mW (Figure 4b), the intensity (I(LMIN) − I(LMAX)) decreases approximately exponentially with increasing laser power. This decrease is probably due to a structural reorganization within the grains. Indeed, this behavior is related to the disappearance of the vibration modes of maghemite at the expense of the appearance of those of hematite modes, suggesting thus the formation of hematite particles. The value of the intensity (I(LMIN) − I(LMAX)) reaches finally similar values around 400 mW synonymous with structural stability achieved due to a progressive crystallization of hematite particles and the increase of grain sizes in a polycrystalline state. Similar behaviors were observed for the heated powdered samples. Between 25 and 350 °C, the intensity of the baseline (I(LMIN) − I(LMAX)) increases and above 350 °C the curve decreases up to 700 °C to reach approximately a constant value. Therefore, the baseline profile analysis (Figure 4) allows addressing directly and quantitatively the correspondence between the effects induced by the output laser power and the temperature of thermal treatment for α-Fe2O3 and γ-Fe2O3 23789



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Figure 5. A physical mechanism assumption for the γ → α-Fe2O3 phase transition occurring at the nanometric scale under laser irradiation and thermal treatment. The inset represents the respective evolutions of the average particle diameters, ⟨D⟩, estimated from the adjustment of the X-ray diffraction pattern (Table 1) for the γ-Fe2O3 and α-Fe2O3 phases as a function of annealed temperature.

two particles (Figure 5, zone 1) in order to reduce the energy at the grain boundary. Experimentally, we confirmed that both low energy laser irradiation and heat treatment induce important defects at the surface of the maghemite nanoparticles. Actually, the structural modification of the surface was directly revealed (Figure 4a) on Raman spectra by the increase of the intensity of the baseline (I(LMIN) − I(LMAX)). Zone 2 (15−100 mW; 300−400 °C) is related to the germination of the hematite phase at the nanoparticle interfaces, which grows toward the core of nanoparticles. Indeed, as the process continues, the heat should permit the movement of the atoms at the interface of nanoparticles and thus the diffusion of Fe3+ cations. Therefore, we can assume that the reorganization of the atoms favors the formation of hematite germs between both nanoparticles at the region where they touch each other (Figure 5, zone 2) and slowly induces a crystallization toward the core of the nanoparticles until the structural transformation is completely achieved. This type of assumption was actually supported by previous results reported on the transition (spinel) γ/α (corundum) alumina Al2O3,56−58 where it was found that the nucleation of the α-Al2O3 phase originates preferentially from the surface of γ-Al2O3 particles or from the reversal of curvature when two particles are juxtaposed or welded. Thus, knowing that the surface of Fe 2 O 3 nanocrystals has a unique type of high energy sites, the most probable scenario is thus that germination (Figure 5, zone 2) takes place at the surface of particles, after the neck formation between packed particles. Zone 3 (100−600 mW; 400−900 °C) is mostly related to the formation and crystallization of hematite particles. Indeed, from Raman spectra, one concludes to the disappearance of the maghemite phase and the decrease of the vibrational band widths of the hematite phase due to a progressive crystallization. The molecular dynamics simulations (MD) suggest that packed particles will transform into a spherule,54,55 which slowly evolves to a sphere with the thermal process. So, in zone 3, disordered particles can still be present, but they will

phases. Consequently, our result also gives evidence that the local temperature of the sample heated under laser excitation (at the spot laser) could be directly estimated from the bandwidths of Raman vibrational modes. Such a tool could be very useful when the intensity ratio of anti-Stokes to Stokes Raman-scattering lines cannot be accurately estimated (due to anti-Stokes Raman-scattering lines too weak or absent). 3.3. Physical Mechanism Assumption for the γ → αFe2O3 Phase Transition. Both the temperature and the mechanism of the structural transformation are dependent on experimental conditions and particularly on the size of the maghemite particles.46 It was reported in the literature that several parameters can influence the γ/α transition such as intergranular diffusion,47,48 specific surface area,49 and critical size of the system.13,50 Lehtinen et al.51−53 discussed the coalescence process in nanoparticles. Their work indicates that the loss in surface energy resulting from the reduction of the surface area is the main driving force for coalescence. According to our investigations, the γ → α-Fe2O3 phase transition induced under laser irradiation and after heat treatment both lead to an increase of the nanoparticle sizes. Figure 5 illustrates the evolution of the average particle diameters, ⟨D⟩, estimated from the refinement of X-ray diffraction pattern (Table 1) for the γ-Fe2O3 and α-Fe2O3 phases as a function of annealed temperature. From analysis of X-ray diffraction measurements and Raman spectra, a four-step mechanism is then proposed to explain the γ → α-Fe2O3 phase transition occurring at the nanometric scale. Zone 1 (0−14 mW; room temperature 300 °C) is mainly related to the creation of surface defects. It is well established that the coalescence process starts with contact and initial fusion due to the mobility of the surface atoms. Reorganization at the coalescing nanoparticle interface was predicted by molecular dynamics simulations (MD) on similar grain sizes (4.4−10.0 nm).54,55 It has been reported that, in the case of crystalline nanoparticles under low energy laser heating, the coalescence process yields to the formation of a neck between 23790



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be gradually substituted by more ordered clusters in zone 4. In our schematic representation (Figure 5, zones 3 and 4), the two original nanoparticles coalesce, originating thus one large spherical hematite nanoparticle which is thermally stable. Finally, according to our XRPD analysis, heating of the samples up to higher temperatures (T > 900 °C) gives rise to a small increase of the particle size with a continuous crystallization process.

4. CONCLUSION It was confirmed that the γ → α-Fe2O3 phase transition is associated with an increase of the nanoparticle sizes with a reduction of the microstrain by thermal activation. We also demonstrated that the structural modifications observed for γFe2O3 nanoparticles under laser irradiation and heat treatment are identical. The evolution of the wavenumbers, bandwidths, and the quantitative intensity baseline profile analysis evidenced a mechanism responsible for the γ/α-Fe2O3 phase transition occurring at the nanometric scale with four main steps. A mechanism based on the creation of surface defects followed by the unstable neck formation and surface germination while the coalescence of nanoparticles appears then to be the most appropriate to explain the significant decrease of the intensity of the baseline in Raman spectra, the broadening and red shift of vibration modes, and then the increase of the nanoparticle sizes. The physical mechanism for the γ → α-Fe2O3 phase transition can be dependent on the external conditions while the role of oxygen atoms during the coalescence remains an open question for further investigation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33-2 43 83 35 69. Fax: 33-2 43 83 35 18. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Collectivités Locales, Le Mans Métropole − Département de la Sarthe and by ISCR transversal Project 2010. The Centre de Compétence C’Nano Nord-Ouest is also gratefully acknowledged for financial support.

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REFERENCES

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