Chemically Designed Growth of Monodisperse Iron Oxide

Article pubs.acs.org/crystal

Chemically Designed Growth of Monodisperse Iron Oxide Nanocrystals Christian Cavelius,† Karsten Moh,† and Sanjay Mathur*,‡ †

INM − Leibniz Institute for New Materials, Campus D2 2 Nano- Cell Interactions Group, 66123 Saarbrücken, Germany Institute of Inorganic Chemistry, University of Cologne, Cologne, D-50939 Germany

‡

ABSTRACT: We describe here a chemically controlled pathway for the designed synthesis of iron oxide nanoparticles by thermal decomposition of iron(II) and iron(III) oxalates in high-boiling solvents in the presence of oleylamine and oleic acid acting as capping ligands. The phase composition of the nanocrystals (Fe, FeO, Fe3O4, or α-Fe2O3) could be precisely controlled by adjusting the synthesis conditions or by addition of appropriate oxidants, such as trimethylamine-N-oxide (TMAO), which produced highly monodisperse iron(III) oxide particles in the range of 6−25 nm in good yields. The decomposition behavior of different precursor/ TMAO mixtures was elucidated by differential scanning calorimetry and thermogravimetry, and resulting particles were characterized by comprehensive HR-TEM and XRD analyses.

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(more than) 360 °C under exclusion of air and moisture.8a At higher temperatures (T > 400 °C), the coexistence of metallic (α-Fe) and thermodynamically unstable wüstite (FeO) was observed. Since the fate of the final product depends on the selective or concomitant cleavage of M−O and C−O units in metal oxalates, the decomposition temperature and atmosphere can substantially influence the oxalate-to-oxide transformation chemistry.3 Alternative chemical strategies based on the so-called “heating up” methods and relying on the high-temperature decomposition of metal salts, such as iron oleate,9 or metal− organic compounds, such as iron carbonyls,10 have also been studied to obtain iron oxide nanoparticles. However, the control over particle size and morphology is subject to several factors, including, but not limited to, the purity of the starting materials (e.g., the variation of water of crystallization in iron oleate can influence the particle size and composition),11 the decomposition temperature, and the presence of surface active reagents, that can affect the mass transfer and thus the particle shape and growth. For a homogeneous and single nucleation burst, instantaneous decomposition of precursors is necessary; however, any small change in the chemical composition or structural variation can adversely affect the nucleation stage, leading to polydisperse particles. For instance, Bronstein et. al11 have shown that the particle size and morphology were influenced by the water and oleic acid contents as well as by the formation of bridged complexes, when iron oleates were used as precursors. Since iron carbonyls are, in general, toxic and volatile, the use of carbonyl precursors is restricted to low-scale synthesis. Thus, we chose

INTRODUCTION In the past several decades, metal oxalates [M(C2O4)x/2] (x = valency of the metal) have been used as precursors for the production of oxidic and metallic powders.1 Their frequent use can mainly be attributed to the stability of oxalate salts and the well-investigated thermal decomposition of the oxalate anion, which results in the formation of metal oxides (or metal) and carbon monoxide (or carbon dioxide).2 The final outcome of the decomposition process, however, is subject to the redox potential of the metal and competing chemical reactions, whose course depend on: (i) the free energy of formation of metal oxide from the elements and (ii) the free energy of formation of carbon dioxide from carbon monoxide and oxygen. When the free energy for oxide formation dominates, formation of MOx is favored, while the reduction of metal ions to metals is facilitated in the latter case. For instance, cobalt and nickel oxalate decompose into metallic powders; oxidic phases were obtained from the pyrolysis of zinc and lanthanide oxalates.3 In this context, and given the special redox chemistry of the Fe(II)−Fe(III) couple, the thermal decomposition of iron oxalates is interesting, also, from the viewpoint of the final material as both nanosized iron and its oxides are useful for a variety of applications, such as catalysis,4 energy storage,5 bioimaging,6 and visible light absorber pigments.7 Investigations on the thermal decomposition behavior of iron oxalates were the subject of many previous studies8 because, subject to the decomposition parameters and chemical composition of the starting material, various iron oxide phases, such as maghemite (γ-Fe2O3) and hematite (α-Fe2O3), can be selectively obtained. Under an inert atmosphere (N2, Ar), hematite is the predominant phase, whereas the decomposition in air mostly yielded the maghemite phase. Hermanek et al. have reported on the formation of nanocrystalline magnetite (Fe3O4) particles by heating iron oxalate to © 2012 American Chemical Society

Received: July 8, 2012 Revised: September 10, 2012 Published: October 15, 2012 5948

dx.doi.org/10.1021/cg300936t | Cryst. Growth Des. 2012, 12, 5948−5955

Crystal Growth & Design

Article

diffraction patterns (XRD) were recorded from a D500 diffractometer (Siemens, Germany) on a silicon single crystal using monochromatic Cu Kα1 radiation with a constant incidence angle of 0.5°. Thermal analysis was performed with a Simultaneous Thermal Analyzer STA 449 C Jupiter system (Netzsch, Germany) under a constant argon flow of 50 sscm using aluminum crucibles in the temperature range between 35 and 600 °C (heating rate: 5 K/min). Particle Synthesis. Iron precursors were purchased from SigmaAldrich as salt hydrates (Fe2(C2O4)3·6H2O 1, FeC2O4·2H2O 2). Particle synthesis was typically performed as a heating-up process by decomposing the corresponding iron oxalate salts dispersed in 10 mL of trialkyl-amine (alkyl = n-octyl, n-dodecyl) in the presence of oleylamine and oleic acid as stabilizing ligands. In a typical experiment, 282 mg of oleic acid (1 mmol) and 134 mg of oleylamine (0.5 mmol) are added to a dispersion of 242 mg of Fe2(C2O4)3·6H2O (0.5 mmol) in 10 mL of tri-n-dodecylamine in a 50 mL three-neck flask. After heating to 365 °C, the mixture is decomposed for a further 120 min, during which the suspension changes from orange to black, indicating particle formation. After cooling to room temperature, 5 mL of cyclohexane are added, and the particles are separated from the residual precursor through centrifugation (3000 rscm, 5 min), followed by precipitation in ethanol (50 mL), and another centrifugation step (7000 rscm, 60 min). Trimethylamine-N-oxide (TMAO). Typically, the respective amount of TMAO (Table 2) was directly added to the reaction mixture before starting the decomposition process to influence particle size and shape already at the nucleation stage. The influence of the presence or absence of TMAO on the shape and polydispersity of nanoparticles was systematically investigated, which is exemplarily accounted in the following section. Purification. The as-obtained particles were purified by further precipitation/centrifugation steps using cyclohexane as a nonpolar solvent and ethanol/acetone mixtures for the precipitation step. Typical yields varied depending on the number of purification steps between 50 and 80%.

commercially available iron oxalate hydrates as starting materials that exhibit no acute toxicity before and after decomposition. Despite a large number of reports available on the application of iron oxalates as precursors, for example, in sol−gel, hydrothermal techniques, a systematic investigation on the control of reaction chemistry by the judicious choice of chemical additives (oxidants or reactants) is missing, which motivated us to undertake this work. Herein, we report the preparation of nanoparticles by thermal decomposition of Fe2(C2O4)3·6H2O (1) and FeC2O4·2H2O (2) in high-boiling solvents (Scheme 1). The decomposition temperatures Scheme 1. Schematic Drawing of the Products Obtained after Decomposition of Iron Oxalatea

a

Depending on reaction conditions, such as decomposition temperature and atmosphere, either the Fe−O or the C−O bond can dissociate, leading to different iron and iron oxide phases.

of both Fe(II) and Fe(III) oxalates were determined by thermogravimetry coupled with differential scanning calorimetry (DSC-TG), and the phase composition and particle size in the resulting powders were analyzed by detailed X-ray diffraction studies and transmission electron microscopy. Key parameters for the preparation of different phases are their enthalpies of formation, which can be modulated by temperature variation, and the redox potential of the environment. The latter parameter can be addressed by addition of redox-active compounds or by changing the reaction atmosphere (inert or oxidizing).8a Both iron(II) and iron(III) oxalates were decomposed under a nitrogen atmosphere by heating different amounts of powder dispersed in tri-n-octylamine (TDecomp ≤ 360 °C) or trin-dodecylamine (TDecomp > 360 °C) in the presence of oleylamine and oleic acid as stabilizing ligands. Trimethylamine-N-oxide (TMAO) was chosen as a chemical oxidant to achieve an effective oxidation before and after the decomposition of the precursor oxalates.

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RESULTS AND DISCUSSION Thermal Decomposition of Fe2(C2O4)3·6H2O (1). In the first set of experiments, the decomposition temperature (TDecomp) for Fe2(C2O4)3·6H2O and the stabilizer concentration were varied, keeping the oleylamine/oleic acid ratio constant (Table 1). The obtained solid particles were precipitated with ethanol and redispersed in cyclohexane prior to TEM characterization. For sample A0 (TDecomp = 340 °C), only polydisperse nanoparticles with a broad distribution of d = 10−50 nm, displaying irregular shapes and sizes, were obtained (Figure 1, A0/B0), which indicates an incomplete or uncontrolled decomposition process. At higher stabilizer concentrations (≥0.2 M oleic acid, sample B0), the isolation of particles was not possible, even after 6 h at 380 °C, indicating an inhibition of the decomposition process or formation of soluble iron species. The possible formation of iron oleate could be ruled out since iron oxide particles were obtained under similar conditions by thermal decomposition of the iron oleate.9 Because the decomposition of iron oxalate

EXPERIMENTAL SECTION

Materials and Methods. All reagents were purchased from Sigma-Aldrich (Germany) in the highest purity available with the exception of oleic acid (technical grade, 90%). High-boiling solvents (tri-n-octylamine, TOA; tri-n-dodecylamine, TDA) were predistilled under vacuum and stored over molecular sieves (3 Ǻ ) under exclusion of moisture and light. Thermal decomposition experiments were performed under a nitrogen atmosphere using a modified Schlenk technique.13c Transmission electron micrographs were obtained using a JEM 200 CX transmission electron microscope (Philips). X-ray

Table 1. Experimental Parameters for the Synthesis of Iron Oxide Nanoparticles by Decomposition of 1 sample

precursor

n(Fe2(C2O4)3) [mmol]

solventa

temperature [°C]

time [min]

oleic acid [mmol]

oleylamine [mmol]

A0 B0

Fe2(C2O4)3

A1 A2

Fe2(C2O4)3 Fe2(C2O4)3

0.5 0.5 0.5 0.5 0.5

TOA TOA TOA TOA TDA

340 340 340 365 380

120 120 120 120 120

1 2 6 1 1

0.5 1 3 0.5 0.5

a

TOA: tri-n-octylamine. TDA: tri-n-dodecylamine. Note: All synthesis parameters are related to a solvent volume of 10 mL; iron oxalate salts were used in their hydrated state. 5949



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Figure 2. High-resolution TEM image (a) and electron diffraction pattern (b) of iron oxide nanoparticles obtained after decomposition of iron(III) oxalate at 380 °C. Analysis of the electron diffraction pattern shows mainly reflexes that can be attributed to the crystalline wüstite phase.

cubic crystal system and are not unambiguously identifiable in XRD patterns. At 360 °C, a mixture of Fe3O4 and FeO is evident, whereas stronger peaks were due to the magnetite phase. The XRD pattern of the samples prepared at 380 °C show wüstite to be the main crystalline phase, supporting the results obtained from selective area electron diffraction. Unlike the results reported for the decomposition of iron(III) oleate, no α-Fe phase was found at this temperature (see arrows I and II in Figure 3a, which represent the most prominent peaks of α-Fe).9 On the basis of the (311), (400), and (440) reflexes, the lattice parameter was calculated according to Nelson and Riley. For cubic systems, there is a direct correlation between the lattice constants and the hkl-reflexes derived from the X-ray diffraction pattern. Plotting the lattice constants against cos θ cot θ (θ = scattering angle) and linear fitting yields the corrected lattice parameters after Nelson and Riley. Because the lattice parameters of FexO depend on the Fe/O ratio, an exact calculation of the stoichiometric composition can be perfomed according to the empirical equation, a = 3.856 + 0.478x, reported by McCammon and Liu, where a is the lattice constant of the cubic system and x is the stoichiometric factor.12 For the experimental lattice constant of a = 426.2 pm, a stoichiometric composition of Fe0.84O for the particles synthesized by decomposition of 1 at 380 °C was calculated. From the thermodynamics viewpoint, FeO is only stable at temperatures above 560 °C and decomposes at lower temperatures into α-Fe and Fe3O4. Thus, the fact that no crystalline iron was detected in the X-ray diffraction pattern can be attributed either to a kinetic stabilization13 or to immediate oxidation of the metallic iron to magnetite during the disproportionation of the wüstite phase. Thermal Decomposition of Iron(II) and Iron(III) Oxalates in the Presence of TMAO. The synthesis parameters employed in the decomposition of 1 and 2 performed in the presence of trimethylamine-N-oxide as an additional oxidizing agent are summarized in Table 2. On the basis of the results obtained in the thermolysis of 1, all decompositions were performed at temperatures > 360 °C. Figure 4 shows the TEM images of nanoparticles obtained by the addition of TMAO before (B1) and after (B2) the decomposition of the precursor at 380 °C. When TMAO was present in the solution during heating (B1), the formation of a pale yellow solution at 200 °C was observed, which turned dark at ∼360 °C, indicating the formation of nanoparticles. For in situ oxidized samples (B1), very small crystalline particles with an average size of 6 ± 2.5 nm are observed with a significant polydispersity (∼40%), whereas larger nanocrystals (23 ± 3.5 nm, 15% polydispersity) with a narrow size distribution

Figure 1. TEM images obtained after decomposition of Fe2(C2O4)3·6H2O at temperatures between 340 °C (A0, B0), 365 °C (A1), and 380 °C(A2) and the corresponding histograms (for A1, A2). An oleic acid-to-oleylamine ratio of 2:1 was kept constant with a total amount of 1 mmol of oleic acid per reaction for A0−A2 and 6 mmol for B0. Higher amounts of stabilizer led to incomplete decomposition of the precursor at low temperatures. Note: The histograms on the right side of the figure correspond to the TEM images to the left of them.

powder under inert conditions was observed at temperatures above 350 °C, the in situ formation of the iron oleate complex is quite improbable. Apparently, a higher stabilizer excess (4−12 fold) hinders the formation of stable seeds with sizes above the critical seed radius. This assumption is supported by the formation of nanoparticles in sample A1, after the decomposition of the precursor above the threshold temperature (365 °C), producing well-defined crystals with a mean particle size of 23.0 ± 2.8 nm (Figure 1, A1). All particles were of spherical morphology and exhibited a narrow dispersity, which induced a self-organization tendency to form 2D assemblies. Compared with the powders obtained at 340 °C, a considerable improvement in particle size distribution and crystallinity was observed by increasing TDecomp to 365 °C. Decomposition of 1 at 380 °C yielded crystalline monodisperse iron oxide nanoparticles with a mean diameter of 26 nm and a dispersity of approximately 10%. The electron diffraction pattern (Figure 2b) indicates the formation of the crystalline FeO phase. X-ray diffraction data of samples A0, A1, and A2 exhibit a change of phase structure from the spinel phase (magnetite, maghemite) to wüstite with increasing reaction temperature. At 340 °C, the diffraction pattern can be assigned to the magnetite phase, with a small amount of FeO present in the sample. The presence of the wüstite phase at the particle surface cannot be excluded because both Fe3O4 and FeO crystallize in a 5950



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Figure 4. TEM and HRTEM images of iron oxide nanoparticles obtained after decomposition of iron(III) oxalate at 380 °C. Particles and corresponding histograms are shown for particles isolated upon addition of TMAO before (B2) and after (B1) decomposition of the oxalate precursor. Image analysis yielded a mean particle size of 6 ± 2.5 nm for sample B2, whereas particles with a size of 23 ± 3.5 nm were obtained for B1.

Figure 3. X-ray diffraction pattern (a) and Nelson−Riley plots (b) derived from the XRD profiles of iron oxide nanoparticles obtained at different decomposition temperatures. An increase in temperature is accompanied by phase transformation from the magnetite phase (JCPDS: 75-0449, blue) to the wüstite phase (JCPDS: 79-1667). I and II indicate the main reflexes of cubic (JCPDS: 06-0696) and hexagonal (JCPDS: 34-0529) iron phases.

are obtained in the case of postsynthesis oxidation of iron oxide nanoparticles (B2). The particles exhibited a spherical morphology and show now significant agglomeration effects. Powder X-ray diffraction patterns confirm the presence of magnetite as the crystalline phase, supported by the results of the TEM analysis (Figure 5). Two clearly different diffraction patterns are obtained; the small particle size led to only a short-range order in B1, as evident in the diffraction pattern, whereas, for the postoxidized sample B2, a well-defined diffraction profile was measured. The mean crystallite size of 17.3 nm was calculated using the Scherrer equation14 (from the (220), (311), (222), (400), (511), and (440) peaks), which was in good agreement with the particle diameter of 23 nm observed in TEM images. Because of the very small particle size in sample B1, no crystallite size and detailed phase analysis was possible;

Figure 5. X-ray diffraction patterns obtained for samples prepared by addition of TMAO before (B1) or after thermal decomposition (B2). The B2 spinel phase could be identified from X-ray pattern analysis (blue). Reflex indication was done according to JCPDS File: 89-0691. Profile analysis was performed on smoothed (Savitzky−Golay) curves using a Pearson VII fitting function.

however, broad reflexes at 2θ = 36° and 63° may be attributed to the (311) and (440) lattice planes of magnetite. The TEM and XRD results indicate that different phase formation mechanisms are involved in the nucleation events in samples B1 and B2. The in situ oxidation and the presence of an

Table 2. Reaction Conditions for the Decomposition of 1 and 2 in the Presence of TMAOb sample

precursor

n(precursor) [mmol]

solventa

temperature [°C]

time [min]

TMAO [mmol]

B1 B2 B3 C1 C2 C3 F

Fe2(C2O4)3 Fe2(C2O4)3 Fe2(C2O4)3 FeC2O4 FeC2O4 FeC2O4 Fe2(C2O4)3

0.5 0.5 0.5 0.5 0.5 0.5 0.5

TDA TDA TDA TDA TDA TDA TDA

380 380 380 380 380 380 400

60 60 60 15 40 90 60

1 2 6 1

1

a TOA: tri-n-octylamine. TDA: tri-n-dodecylamine. Note: All synthesis parameters are related to a solvent volume of 10 mL; iron oxalate salts were used in their hydrated state. bOleic acid and oleylamine were chosen as stabilizers at fixed concentrations of 0.1 mM (oleic acid) or 0.05 mM (oleylamine).

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self-assemble nanoparticles.16 Because all other parameters were kept constant, the influence of the TMAO concentration was quite obvious and is suggestive of the formation of an intermediate with a single decomposition temperature. Figure 6 shows a high-resolution TEM image and the corresponding electron diffraction pattern for sample B3. The diffraction peaks could be attributed to the magnetite phase (JCPDS: 19-0629). No FeO phase could be identified, which is, however, not expected as no primary reducing agents were present in solutions with a high content of TMAO. The observations made in the decomposition of 1 were applied to the pyrolysis of Fe(II) oxalate 2, which, in general, decomposes at higher temperatures than the corresponding Fe(III) oxalate 1. Fe(II) oxalate was chosen due to its low price and high storage stability and ease of synthesis by directly reacting Fe(II) salts with oxalic acid. The TMAO content was chosen to be two-thirds of the amount necessary to oxidize all oxalate groups in solution. Typical reaction parameters are given in Table 2. The reaction temperature was kept at 380 °C, and the evolution of particle size as a function of the reaction time was followed by TEM studies. After 15 min of reaction time, nanoparticles with a bimodal size distribution were obtained (Figure 7, C1). Additionally, larger agglomerates were

oxidizing agent during the decomposition reaction facilitate the oxidation of carbon monoxide (formed as the decomposition product in the thermolysis of oxalate anion), which would essentially reduce the oxide to metal, thereby causing two competing reactions (reduction of FeOx to metal due to the oxidation of CO to CO2 and oxidation of formed Fe(0) into oxide by TMAO) with different kinetics. This results in a lessdefined nucleation and growth (crystallization) process, as evident in the large size variance. Referring to the observation that the precursor dissolved completely in the presence of TMAO, its coordination to the metal center can be envisaged, as known for other N-containing ligands, such as hydrazine, that have been used to influence the decomposition reactions of iron oxalates.15 Because the solvents used in this study (tri-noctylamine/tri-n-dodecylamine) are polar in nature, only lipophilic groups can render the precursor compound more soluble in the medium. Because the oleate anion carries a longchain alkyl group, a ligand exchange, in addition to the coordination of TMAO, appears likely. However, the amount of TMAO (0.166 mmol) was not enough to completely oxidize all oxalate groups present in the weighed amount of 1 (0.5 mmol). In a following synsthesis, the amount of TMAO was increased up to 1.5 mmol in the reaction mixture, keeping other parameters constant, to achieve a more complete oxidation of the oxalate present in the precursor (sample B3). Figure 6

Figure 6. TEM images (a−e) of iron oxide nanoparticles obtained after thermal decomposition of Fe2(C2O4)3·6H2O at 380 °C in the presence of TMAO (1.5 mmol)(sample B3). The particles exhibit a mean particle size of 13.1 ± 0.54 nm (4.2% dispersity) and selfassemble in 2D and 3D superstructures. The inset of (c) shows the electron diffraction pattern of the particles, which can be attributed to the magnetite phase (JCPDS file: 19-0629).

shows the TEM images of the particles prepared by this route. Compared with sample B1, which contains only small amounts of TMAO at the beginning of the reaction, the particles in sample B3 were larger and monodisperse, which results in the formation of 2D and 3D superlattices. The mean particle size calculated from the TEM images was found to be around 13 nm with a relative dispersity of 4.2%, which is below the maximum dispersity level of 10% necessary to effectively

Figure 7. TEM images of iron oxide nanoparticles obtained after a decomposition time of 15, 40, and 60 min at fixed decomposition conditions (0.333 mmol of TMAO, 1 mmol of oleic acid, 0.5 mmol of oleylamine, TDecomp = 368 °C). Nondecomposed educt is mainly found after 15 min (black arrow, C1) and 40 min decomposition times.

observed, which can be attributed to residual undecomposed oxalate precursor. For samples C2 and C3, the content of 5952



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For each synthesis time, the cubic iron oxide phase could be identified with an increasing content of oxide phase upon prolonged decomposition periods, indicating that its existence is thermodynamically favored. After 15 min of decomposition, crystalline oxalate phases are the main crystalline material in the sample. As mentioned above, this observation is in agreement with the TEM results (Figures 7 and 8). The mean crystallite size of the iron oxide phase was calculated for the strong (311) and (400) reflexes for samples C2 and C3. The values of = 11.3 nm and

Chemically Designed Growth of Monodisperse Iron Oxide

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