A Comprehensive Study of the Mechanism of Formation of Polyol

Article pubs.acs.org/JPCC

A Comprehensive Study of the Mechanism of Formation of Polyol-Made Hausmannite Nanoparticles: From Molecular Species to Solid Precipitation Tarik Rhadfi,†,‡ Lorette Sicard,*,† Fabienne Testard,§ Olivier Taché,§ Ahmed Atlamsani,‡ Elodie Anxolabéhère-Mallart,∥ Yann Le Du,⊥ Laurent Binet,⊥ and Jean-Yves Piquemal† †

Univ. Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR CNRS 7086, 15 rue J.-A. de Baïf, 75205 Paris Cedex 13, France Laboratoire de Physico-Chimie des Interfaces et Environnement, Faculté des Sciences, Univ. Abdelmalek Essaâdi, BP2121, 93000 Tétouan, Morocco § LIONS, Service de Chimie Moléculaire, CEA Saclay, Bât 125, 91191 Gif-sur-Yvette, France ∥ Univ. Paris Diderot, Sorbonne Paris Cité, LEM, UMR CNRS 7591, 15 rue J.-A. de Baïf, 75205 Paris Cedex 13, France ⊥ Univ. Pierre et Marie Curie, ENSCP, UMR CNRS 7574, 11, rue Pierre et Marie Curie 75231 PARIS Cedex 05, France ‡

S Supporting Information *

ABSTRACT: This study aims at achieving a better understanding of the mechanisms of formation of Mn3O4 nanoparticles prepared by the polyol process. The role of each reactant is studied, and a possible scheme of reaction is proposed, involving the activation of dioxygen by Mn(II) species. The growth of the particles (evolution of the size and concentration of particles) has been followed in solution by SAXS, and the results have been compared to those obtained by other techniques on dried powders. The results indicate a decrease of the number of particles in solution with time together with their enlargement. A stabilization of the size and number of particles is reached after a few hours. The shape of the particles then evolves into a truncated ditetragonal-dipyramidal polyhedron.

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INTRODUCTION A deep comprehension of the mechanisms of formation of nanoparticles is of major importance in order to really tailor their morphology: form, size and size dispersity.1−3 Lamer and Dinegar4 have claimed that a separation between nucleation and growth is required to obtain very monodisperse nanoparticles (σ/L ≤ 10%, with σ the standard deviation and L the mean size). The first step involves the formation of monomers until it reaches a supersaturation threshold, followed by a brief outburst of nuclei. The second step is the growth of these nuclei by incorporation of additional monomers from the reaction medium. Den Ouden and Thompson5 have shown that monodisperse particles can be obtained even if the nucleation and growth steps are not separated. In this case, the key point is a small growth rate relative to the nucleation rate. Several studies have been devoted to the establishment of a formation mechanism for nanoparticles. They have first relied on electron microscopy observations of the size and shape of the objects.6,7 In some favorable cases, for example, with quantum dots, these characterizations have been coupled with UV−visible spectroscopy to follow the morphology of the objects.8−10 However, it is not adapted for all kinds of inorganic nanoparticles contrary to small-angle X-ray scattering (SAXS). © 2012 American Chemical Society

This powerful technique permits one to characterize the morphology of powders or to follow in situ the shape, size, size dispersity, and concentration of any stable suspension.11−13 It has been applied to the study of sol−gel systems in the 80s,14 silica materials15−18 and gold nanoparticles.19−21 As far as the so-called polyol process is concerned, a few researchers have tried to elucidate the mechanistic aspects. In some studies, intermediate species, such as alkoxides or hydroxides,22,23 have been recovered from the reaction mixture and characterized. They were proposed to act as monomer reservoirs, leading to a kinetic control of the growth step. The evolution of the habitus with time and reaction conditions has also been studied by transmission electron microscopy (TEM).24−31 However, when the characterization is performed on dried powders, the drying step could affect the final morphology due to the surface tension of the solvent when evaporating (capillary forces). Moreover, centrifugation is very often used to recover the materials and could lead to a nondesired separation of the possible different-sized particles Received: December 24, 2011 Revised: February 15, 2012 Published: February 16, 2012 5516

dx.doi.org/10.1021/jp212454m | J. Phys. Chem. C 2012, 116, 5516−5523

The Journal of Physical Chemistry C

Article

A model of polydisperse spheres with a Gaussian distribution and a cut-off limit for the minimum size of the particles was used to fit the data using a model developed in Python.20,35 The absolute scattering intensity of noninteracting polydisperse particles is given by the formula

obtained during the synthesis and, consequently, to a restricted appreciation of the whole system. By opposition, SAXS ensures obtaining a global analysis of the suspension under study. We have recently developed a synthesis procedure based on the so-called polyol process, which allows obtaining welldispersed Mn3O4 nanoparticles with a low dispersity in size (standard deviation over diameter σ/d = 13−14%).32 We reported that, depending on the synthesis duration, we were able to vary the morphology from 5 nm quasi-spherical-shaped to 12 nm rhombohedron-like particles. These materials display interesting catalytic activity for Fenton-like chemistry.33 To better control the morphology and size of the particles and thus their properties, a good understanding of the formation mechanisms is required. Thus, the nucleation and growth of the nanoparticles have been studied by SAXS carried out on aliquots of the colloidal solution at different temperatures and reaction times. The volume fraction of solid, the specific surface area, the size, and concentration of particles have been deduced. The results were compared to those obtained by transmission electron microscopy and nitrogen adsorption−desorption experiments and allowed us to propose a formation mechanism.

∞ 2 (Δρ)2 ∫ G(R2 ,r ,σ)NVpart (r )P(q ,r ) dr

r min (2)

with Vpart the volume of one particule, N the concentration of particles (equal to the number of particles divided by the volume of solution), Δρ the electronic density contrast between Mn3O4 (ρ = 3.835 × 1015 m−2) and the solvent (ρ = 1.0166 × 1015 m−2), and P(q,r) the form factor of a sphere ⎡ sin(qr ) − (qr )cos(qr ) ⎤2 2 2 2 ⎥ P(q ,r2) = ⎢3 3 ⎢⎣ ⎥⎦ (qr2)

(3)

and G(R2,r,σ) the Gaussian function

EXPERIMENTAL METHODS Synthesis. Compared with the synthesis described previously,30 a lower manganese concentration has been used in this work: this leads to a lower particle concentration and thus to an improved stability of the suspension. This is an important point for the SAXS experiments. In a typical experiment, 900 mg of manganese(II) diacetate tetrahydrate (≥99%, Aldrich) is introduced in a three-neck flask with 30 mL of deionized water and 375 mL of diethylene glycol (DEG, 99%, Acros). Unless stated otherwise, the syntheses were conducted under an air atmosphere. The solution is stirred mechanically and heated to 100 °C at a rate of 10 °C min−1. Aliquots of the solution (20 mL) are taken when the solution reaches 70, 80, and 90 °C and when the solution is at 100 °C for a time t = 0 to 1740 min. The aliquots are immediately quenched in a water bath at ambient temperature. It is noteworthy that control experiments have been realized for the different reaction times indicated above. TEM analyses have confirmed the results obtained with the aliquots. The colloidal solutions are studied by SAXS (vide infra). The powders were also recovered by centrifugation adding acetone (CH3COCH3/DEG = 2/1 vol/vol) and characterized by transmission electron microscopy (TEM) and nitrogen adsorption−desorption experiments. SAXS Measurements. The raw data were obtained on a home-built apparatus equipped with a rotating copper anode and collimating optics.34 The colloidal solution is introduced in a cell closed by two kapton windows. The intensity scattered during 1 h is obtained on a 2-D detector after radial averaging. The intensity is calibrated with a standard sample, giving access to absolute intensity per thickness of sample, which is plotted as a function of the scattering vector (eq 1), using the classical procedures 4π sin(θ/2) λ

∞

∫ G(R2 ,r ,σ) dr

■

q=

r min

Ipoly(q) =

⎛ (r − R )2 ⎞ 1 2 ⎟ exp⎜⎜ − ⎟ σ 2π 2σ2 ⎠ ⎝

G(R2 ,r ,σ) =

(4)

with σ the half-width of the Gaussian function. The volume fraction of particles (Φv equal to the total volume of solid divided by the volume of solution) is calculated using the invariant equation: ∞

∫

I(q)q2 dq = 2π 2(Δρ)2 Φv (1 − Φv ) (5)

0 2

The specific surface area (Σ) (m /g) is deduced from the Porod law at high q values Σ=

1 2π(Δρ)2 d m Φv

limq →∞ I(q)q4

(6)

with dm the relative density of Mn3O4. This value represents the surface per gram of materials and can be compared to the BET surface obtained on the dried powder. Both Φv and Σ are independent of the model used to fit the curves. Powder Characterization. Elemental analyses (Mn, C, H) were carried out at the Service Central d’Analyses (CNRSLyon) by inductively coupled plasma atomic emission spectroscopy (ICP-AES) after alkaline fusion with Li2B4O7. Thermal analyses (TGA-DTA) were performed under an air flow on a Setaram TG 92-12 apparatus from ambient temperature to 1073 K with a heating rate of 5 K min−1. UV−vis-NIR spectra were recorded using a PerkinElmer Lambda 1050 spectrometer equipped with a PMT-InGaAs-PbS three-detector module. The X-ray diffraction (XRD) patterns were obtained using a Panalytical X’Pert Pro diffractometer equipped with a PIXcel detector using Ni-filtered Cu Kα radiation. The data were collected at room temperature with a 0.026° step size in 2θ between 20 and 70°. The crystallite sizes were determined using MAUD software.36 TEM and high-resolution TEM (HRTEM) experiments were performed using a JEOLl JEM 100CXII

(1)

where θ is the scattering angle and λ the X-ray wavelength. For the analysis, the contribution of the solvent is systematically subtracted. 5517



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Mn2p3/2 and Mn2p1/2 were very similar to that of bulk Mn3O4. All these results indicate a high purity of the sample. The morphology of the particles was characterized by TEM. The images are given in Figure 2. After 3 min, they are quasispherical with a mean size (d) of 6.6 nm. They present a standard deviation over size ratio, σ/d, equal to 17%. At 30 min, the particles are still quasi-spherical in their majority with some faceted particles. Their size is 8.7 nm, and σ/d = 14%. Finally, the objects are mostly faceted after 1 h at 100 °C. At 6 h, d = 12.9 nm and σ/d = 14%. The good agreement between the values obtained by TEM and XRD prove that the particles are single crystals. HRTEM images of these samples are also presented in Figure 2. All the particles, even after 3 min, are very well crystallized. Numerical electron diffraction patterns could be inferred from the images. In all cases, they were indexed within the I41/amd crystal system. The morphology of the faceted particles could also be deduced: as shown in Figure 2g, they correspond to truncated ditetragonal-dipyramidal polyhedra. This crystal habitus is in agreement with the theoretical one calculated by Vazquez-Olmos et al.37 Conditions for the Precipitation of Hausmannite. In a typical experiment, after dissolution of the manganese precursor, the solution is nearly colorless at room temperature. As the temperature increases, the solution becomes brown, which is a good indication of the oxidation of MnII into MnIII and/or MnIV ions. For a parallel experiment realized at room temperature without heating, the coloration of the polyol− water solution also turned progressively to brown with time, indicating the progressive oxidation of MnII. By opposition, when the reaction is performed in pure water (9.1 mM of MnII aqueous solution, pH 5.5), the UV−vis spectrum showed no change over a period of 24 h (not shown). UV−vis spectroscopy is indeed a convenient tool to monitor the progressive oxidation of the MnII species in the diethylene glycol−water mixture (9.1 mM), as shown in Figure 3. After 30 min at room temperature, a broad peak at about 420 nm appears as a shoulder of a strong absorption at lower wavelength, which probably corresponds to charge-transfer transitions. For MnIII complexes in 95% aqueous acetic acid, a broad absorption located at about 460 nm has been observed.38 We tentatively attribute the peak at 420 nm to MnIII species; the observed blue shift could be explained by the different nature of the ligands present in the coordination sphere of the manganese ion. A red shift of the charge-transfer band together with an increase of the intensity of the peak at 420 nm is observed when the reaction time is increased (see Figure 3), although after 8 h at room temperature, the absorbance at a given wavelength is significantly lower than what is obtained with an authentic 0.5 mM solution of MnIII(OAc)2. All these results indicate that, even at room temperature, oxidation of MnII species occurs, even if it is obviously not complete. To ascertain the essential role of dioxygen in the formation of Mn3O4 in the diethylene glycol−water mixture, syntheses were carried out under an argon flow at 100 °C. The solution remained colorless and limpid for one night. The introduction of O2 after this period of time resulted into the immediate precipitation of Mn3O4 hausmannite, which means that O2 is involved in the reaction. Moreover, when the synthesis was carried out under air in pure DEG, that is, in the absence of water, using anhydrous MnII(OAc)2, Mn3O4 could not be obtained either and a white powder was recovered.

transmission electron microscope operating at 100 kV. The as-produced particles are first dispersed in ethanol, and one drop of this suspension is deposited on the carbon membrane of the microscope grid for the observations. The particle size distribution is obtained from the transmission electron microscopy (TEM) images using a digital camera and the SAISAM software (Microvision Instruments), calculating the surfaceaverage particle diameter, d, from eq 7

∑ d= i

∑ i

nidi2 nidi (7)

where ni is the number of particles with di diameter. The statistical result of the particle size was obtained by counting about 200 particles. d is given in nanometers. Adsorption−desorption nitrogen isotherms were obtained at 77 K using a Belsorp Max apparatus. The samples were outgassed at 423 K and 0.1 Pa for 12 h before measurements. Specific surface area (SBET) values are calculated with the Brunauer−Emmett−Teller equation with relative pressures in the range of 0.05−0.20. Raman spectra were obtained at room temperature with a Dilor XY microspectrometer equipped with a 1024 × 256 pixel thermoelectrically cooled Jobin-Yvon CCD detector using an excitation line at 514.5 nm from a Spectra Physics model 165 argon-ion laser (power: 200 mW). With the lens used (100×), the size of the illuminated area is approximately 1 mm2.

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RESULTS Characterization of the Mn3O4 Nanoparticles. The particles obtained at 100 °C after different reaction times were characterized by XRD. The patterns recorded at 5 min, 30 min, and 16 h are given in Figure 1. Even after 5 min of reaction, the

Figure 1. XRD patterns of the powders obtained after 5 min (a), 30 min (b), and 16 h (c) of reaction. The solid lines correspond to the hausmannite phase (PDF no. 00-024-0734).

pattern is characteristic of well-crystallized hausmannite particles with the I41/amd space group. No crystalline impurity is detected whatever the reaction time. The peaks are broad and become relatively thinner as the reaction time increases, showing an increase of the particle size with time: the crystallite size is equal to 6.6, 8.7, and 12.3 nm for 5 min, 30 min, and 16 h, respectively. Moreover, in a previous paper,32 we reported the X-ray photoelectron spectroscopy characterization of the same sample and we showed that the binding values obtained for 5518



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Figure 2. TEM (a−c) and HRTEM (d−f) images and corresponding numerical electron diffraction patterns of the particles obtained at 100 °C after (a, d) 3 min, (b, e) 30 min (zone axis: [531̅]), and (c, f) 6 h (zone axis: [100]). (g) Crystal habitus for the particles corresponding to (f).

To conclude, all these experiments point out the essential and interdependent role of O2, water, and acetate counterions for the formation of Mn3O4. Determination of the Mechanism of Formation of the Particles. Raman spectroscopy was carried out in order to further characterize the reaction mixture during the heating step. Several aliquots were recovered for different reaction times and temperatures. The corresponding Raman spectra are presented in Figure 4 together with that of the diethylene glycol

Figure 3. Evolution of the UV−vis spectrum of the MnII(OAc)2 solution with time at room temperature: (■) t0; (○) 30 min; (▲) 1 h; (◇) 3 h; and (□) 8 h; (▼) mixture of DEG and water and (●) MnIII(OAc)3 (0.5 mM) in DEG−water solution. The concentration of MnII(OAc)2 and MnIII(OAc)3 was 9.1 mM. DEG/H2O = 125/10 vol/vol.

Elemental analyses (experimental wt %: Mn = 32.0, C = 25.0, and H = 3.0%) gave results close to the formula MnC4H6O4 (calculated wt %: Mn = 31.8, C = 27.8, and H = 3.5%). Thermal analysis of the white product showed a single weight loss (56.0%) at about 325 °C associated with an exothermic peak, corresponding to the decomposition of the compound. The final product of decomposition at 1273 K was the oxide Mn3O4, as inferred from X-ray diffraction. The formula weight calculated from this loss, 173.3 g mol−1, is very close to the formula weight M(MnC4H6O4) = 173.0 g mol−1. Finally, the use of manganese(II) chloride did not permit the precipitation of a solid without the joint addition of NaOH. This indicates the important role of the acetate ions probably allowing a control of the basicity of the medium and/or acting as bridging ligands, allowing condensation to occur. Chloride ions could also be responsible for the partial blocking of coordination sites, hindering the further condensation of the manganese species.

Figure 4. Raman spectra of diethylene glycol (DEG), aliquots of the reaction mixture for different temperatures and reaction times, and of the Mn3O4 nanoparticles. Inset: enlargement in the 500−800 cm−1 region.

solvent and of the Mn3O4 nanoparticles. For the hausmannite phase, which has a distorted spinel structure with tetragonal symmetry (I41/amd space group), 10 allowed phonon modes (2A1g + 3B1g + B2g + 4Eg) are expected, but only five peaks are generally reported,39 giving spectra similar to those observed for cubic spinels. These phonon modes are located at about 290, 320, 375, and 480 cm−1 as well as an intense peak at about 660 cm−1.40 Moreover, it has been reported that the Raman 5519



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shift and the bandwidth were dependent on the crystallite size of the material: the peak at about 660 cm−1 was found to be shifted to lower wavenumber values, and the peaks were broadened when the crystal size was decreased.41 For instance, with Mn3O4 material with a 13 nm mean crystal size, only the most intense peak near 650 cm−1 was visible. In our work, a well-defined peak at 654 cm−1 and two small peaks at 364 and 312 cm−1 are clearly indicative of the formation of the hausmannite phase (see Figure 4). The Raman spectra of the different aliquots recovered at different reaction times and temperatures are dominated by the vibrations corresponding to the polyol (see Figure 4). In particular, it was not possible to clearly identify manganesebased molecular species, but the development of a very small peak was observed at about 655 cm−1 once the temperature reached 100 °C. Moreover, its intensity increases after a reaction time of 1 h (see Figure 4, inset). This suggests that the hausmannite phase is generated when the reaction temperature reached 100 °C. To confirm this observation, the first steps of the formation mechanisms have been studied by SAXS. The curves obtained after subtraction of the solvent contribution are given in Figure 5. From a qualitative point of view, three conclusions can be drawn:

Figure 6. Volume fraction, Φv (□), and specific surface area (⧫) of the particles versus time of reaction. The dashed line corresponds to the maximum value for Φv, i.e., considering that all the manganese ions have precipitated as Mn3O4.

is about 140 m2 g−1, whereas the yield of precipitated hausmannite can be considered as quantitative since the volume fraction reaches its maximum value corresponding to the precipitation of all the manganese ions into Mn3O4. Note that, after 30 min, the yield is already very high, about 87%. All the SAXS curves were fitted with a model of polyGaussian distribution of spheres (see Figure 5). The obtained mean diameters of the particles have been plotted in Figure 7

Figure 5. SAXS patterns (scattered plot) as a function of the scattering vector after subtraction of the solvent contribution at different temperatures between 70 and 90 °C and at 100 °C for different reaction times. Lines represent the fits obtained with a poly-Gaussian distribution of spheres for the different reaction times. Figure 7. Mean diameter, d, of the particles deduced from SAXS analysis by fitting the data (red circle), and using the specific surface area Σ (red θ) and from TEM images (red solid circle); concentration of particles N (blue solid square) as a function of time.

(i) At 70, 80, and 90 °C, no particles are formed yet as the intensity is very low and nearly constant, whereas at anytime at 100 °C, the signal at large q follows a Porod law, characteristic of well-defined small particles. (ii) The intensity at low q values tends toward a constant for times shorter than 1−2 h, indicating that the particles are not aggregated; after this period of time, a rise of the intensity is observed at low q values, due to aggregation and/or change of form (to faceted objects). (iii) The intensity becomes higher with the reaction time, showing that the volume of the synthesized nanoparticles increases. The experimental data have been further treated from a quantitative point of view. The volume fraction (Φv) and the specific surface area (Σ) of the particles are plotted as a function of time in Figure 6. A rapid increase of Φv is observed during the first 30 min in parallel with a rapid decrease of Σ. The values then tend toward a constant. After 19 h, the specific surface area

together with the particle concentration. An increase of the size of the particles is observed from 3.8 nm at t = 0 to 8.6 nm at 29 h, whereas the number of particles decreases. The values tend to stabilize after 30 min. The fits gave, for times ≥ 5 min, values of dispersity σ/d = 15−20%, close to that obtained from TEM. At lower times, the polydispersity is higher. The diameter values fit well with that inferred from the specific surface area using the formula d=

6 Σd m

(8)

The results obtained from SAXS have been compared to those obtained by other techniques. First, the mean values of 5520



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the particle size obtained by TEM are plotted in Figure 7 together with the SAXS results. The values are close to each other. The specific surface area of the dried particles obtained after 2 h deduced from nitrogen adsorption−desorption measurements, 121 m2 g−1, correlates also pretty well with that obtained in solution from SAXS measurements (144 m2 g−1). It is noteworthy that SAXS measurements are made on suspensions, whereas physisorption is carried out on dried powders obtained by centrifugation and degassed at 150 °C under vacuum. In this last case, a partial aggregation of the particles can occur. Indeed, the isotherm displays a hysteresis that has been already reported in a previous paper33 and which was explained by the existence of interparticle porosity. For particles isolated after 16 h reaction time, the specific surface area obtained using the BET equation is lower, that is, 112 m2 g−1, indicating a more aggregated state for longer reaction times.

Figure 8. Proposed mechanism of formation for the hausmannite phase from manganese molecular species and dioxygen. [Mn] denotes the Mn complex, including ligands present in the coordination sphere of the metal.

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DISCUSSION In Mn3O4 with the distorted normal spinel structure, MnII cations are located in tetrahedral sites, whereas MnIII are in octahedral sites. It is noteworthy that, in this work, only MnII species are introduced in the reaction mixture, indicating that partial oxidation of divalent cations is necessary to precipitate the hausmannite phase. Given that the precipitation of the oxide phase could not be obtained under an argon atmosphere, dioxygen is likely to be involved in the overall reaction process. The activation of dioxygen by transition-metal complexes, and particularly by MnII complexes, has been the subject of intense research these last years. This can be partly explained considering that the resulting peroxo and/or oxo complexes are proposed to be involved in very important biological processes, such as the oxidation of water to dioxygen within the photosystem II. Recently, Borovik et al. have reported the formation of a monomeric MnIII peroxo compound derived directly from the reaction of MnII acetate and dioxygen in the presence of a reducing agent, diphenylhydrazine.42 This compound displays a side-on (or η2) peroxo ligand and is an intermediate in the catalytic oxidation of water to dioxygen at room temperature.43 A dinuclear MnIV complex displaying a μ-peroxo ligand was also obtained under aerobic conditions from a MnII precursor.44 Dinuclear mixed-valent MnIIMnIII complexes have also been generated under dioxygen and characterized by single-crystal X-ray diffraction.45 Given the precursor used in the synthesis, MnII(ClO4)2, the formation of the mixed-valent MnIIMnIII dinuclear species implies a oneelectron transfer process. The authors have underlined that the reaction can only occur in the presence of dioxygen and that a partial oxidation of the alcohol solvent occurs during the reaction, generating an aldehyde.46 On the basis of these reported previous studies and on our observations, we tentatively propose the following mechanism, presented in Figure 8, to explain (i) the partial oxidation of [MnII] into [MnIII] in the water−polyol mixture ([Mn] stands for the metal and the ligands present in the coordination sphere) and (ii) the subsequent precipitation of the solid phase. In the presence of the reducing polyol, dioxygen would be first activated, leading to the formation of [MnIII]−O−O− peroxo end-on species. [MnII] precursors could react in a second step with these species to give [MnIII]−O−O−[MnIII] dinuclear complexes, with a μ-peroxo ligand bridging the two manganese centers, in the same manner than what is found in the complex [L2Mn2(μ-O)2(μ-O2)](ClO4)2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane).47 Parallel polarization mode EPR experiments

at 5 K have been carried out in order to assess the possible formation of MnIII species. Unfortunately, no MnIII species were detected, but it is well known that the MnIII complexes can be EPR-silent, depending on the geometry around the manganese center (see the Supporting Information). Moreover, the determination of the molecular structure of the species solvated in the polyol solvent is not straightforward. Some crystal structures have been described for manganese complexes with polyols, such as triethyleneglycol with the [MnII{HO(CH2CH2O)3H}2]3+ cation45 or with ethylene glycol with the polymeric {[MnII(C14H6O6S)(C2H6O2)(H2O)2]·C4H9NO}n two-dimensional metal−organic network.46 In our system, acetate ions as well as diethylene glycol and water molecules can be present in the coordination sphere of the manganese ion. Moreover, note that the structure of MnIII acetate is not well-established. The crystals resulting from the dehydration of Mn(OAc)3·2H2O have the [Mn3O(OAc)6(HOAc)(OAc)]n formula, and the three metal atoms are connected by three pairs of acetate bridges.48 In the MnIIMnIII complexes obtained by Hendrickson et al., two acetate ions are bridged to the two manganese centers.49,50 These dinuclear [MnIII]−O−O−[MnIII] species would be very reactive and could abstract protons from water or from the polyol and lead, after homolytic cleavage, to the generation of species with hydroxo ligands. Using a ligand that prevents the condensation of MnIII species, Borovik et al. have been able to isolate a MnIII complex displaying a terminal hydroxo ligand.51 The formation of a 1,2-μ-peroxo bridge was postulated, which, after homolysis, leads to monomeric MnIII(OH) species. It was proposed that the reactive hydroxo groups are sufficiently buried inside the complex cavity structure and cannot undergo condensation to lead to the formation of oxo-bridged clusters.51 In the present study, the MnIII(OH) species are able to react with either other MnIII species and/or MnII species via olation/ oxolation reactions, owing to the presence of water in the coordination sphere of the metal species. This results in the formation of manganese-oxo clusters that can be considered as nuclei for the growth of the hausmannite phase. Considering the formation of the solid phase, all the results converge: the nucleation begins during the heating period between 90 and 100 °C. This nucleation occurs when the concentration of the intermediate species of MnIII reaches a critical threshold. Within the experimental parameters, it is reached at the synthesis temperature. However, it is noteworthy 5521



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polyhedron is then observed. The mechanism of growth has been identified as Ostwald ripening rather than coalescence. Further works are in progress with other oxide solids, such as Co3O4 or Fe3O4, which are under study in our group.

that nucleation was also observed at a lower temperature (70 °C) with a delay because of a slower kinetics. With time, a decrease of the number of particles together with an increase of their size is observed. The evolution rate is rapid during the first 30 min, then slows down until all the manganese species are incorporated in the solid. Such a decrease of the concentration of particles in solution has already been observed in other systems and can be interpreted in two ways: either by Ostwald ripening15 or by coalescence.19 Considering the system studied, the particle size evolves progressively, with a distribution of size σ/d of about 15%, and even more during the first minutes. A possible aggregation of primary particles is not observed by SAXS for the first hour. Moreover, twinned particles have never been seen by TEM. Thus, a mechanism of dissolution and reprecipitation is more likely. It was previously reported that the cube of the radius r3 follows a linear law as a function of times in case of Ostwald ripening mechanisms.21 Figure 9

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ASSOCIATED CONTENT

S Supporting Information *

Simulation of EPR spectra of Mn(III) in parallel mode EPR. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 33(0) 1 57 27 87 59. Fax: 33(0) 1 57 27 72 63. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Frédéric Herbst for TEM studies, S. Lau for Raman experiments, and Prof. Nicolas Menguy for his help concerning the interpretation of TEM images. We also gratefully acknowledge the French network of EPR facilities, TGE-Renard, supported by CNRS for the complementary EPR experiments.

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REFERENCES

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Figure 9. Cube of the particle radius of Mn3O4 nanocrystals as a function of the reaction time.

confirms that r3 = f(t) gives a straight line until 15 min. From 20 min, the radius then becomes stable as a thermodynamic state is reached. This means that a dissolution−recondensation processes take place: the smallest particles dissolve, and the species condense at the surface of the largest particles. This process can also explain the evolution of the morphology of the particles into faceted objects, thermodynamically favored. The polyol molecules and acetate ions that are likely to adsorb at the surface of the particles52 can explain the mechanism of ripening rather than coalescence.

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CONCLUSION This study focused on the mechanism of formation of oxide nanoparticles prepared with the polyol process, from the molecular processes taking place in solution to the growth processes. First, the roles of dioxygen, water, acetate ions, and solvent in the precipitation of Mn3O4 have been examined and discussed. On the basis of these results and on the existing literature, a possible mechanism of precipitation has been proposed: it involves the activation of dioxygen by Mn(II) species, leading to the possible formation of manganese peroxo species, followed by their hydrolysis and condensation by olation/oxolation to give the final oxide phase. Second, the evolution of the particle concentration in relation with their size and morphology was followed by SAXS. Initially, the particles are quasi spherical and their size increases progressively for the 30 min, whereas their number decreases. An evolution of the habitus of the particles into a truncated ditetragonal-dipyramidal 5522



Article

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