Article pubs.acs.org/crystal
Size and Size Distribution Control of γ‑Fe2O3 Nanocrystallites: An in Situ Study Henrik L. Andersen, Kirsten M. Ø. Jensen, Christoffer Tyrsted, Espen D. Bøjesen, and Mogens Christensen* Department of Chemistry and iNANO, Center for Materials Crystallography, Langelandsgade 140, DK-8000 Aarhus C, Denmark
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S Supporting Information *
ABSTRACT: The formation and growth of maghemite (γ-Fe2O3) nanocrystals during the hydrothermal synthesis from aqueous solutions of ammonium iron(III) citrate (C6H8O7·xFe(III)·yNH3) have been studied by in situ powder X-ray diffraction (PXRD). Data analysis by Rietveld refinement and whole powder pattern modeling (WPPM) reveals that the crystallite size and size distribution can be precisely tuned through simple adjustments of the reaction temperature and time. Increasing the reaction temperature causes faster growth and results in larger crystallites while the size distribution broaden as reaction times increase, regardless of temperature. The crystallization kinetics were investigated by fitting the Johnson−Mehl−Avrami−Kolmogorov (JMAK) kinetic model to the growth curves. The activation energy was found to be 67(15) kJ/mol, and the limiting mechanisms of crystallite formation were determined. The growth was studied with various precursor compositions of ammonium iron(III) citrate and Fe(NO3)3·9H2O or FeCl3·6H2O. Increasing the fraction of Fe(NO3)3·9H2O in the precursor results in larger maghemite nanocrystallites. The addition of Fe(NO3)3·9H2O also results in the formation of the thermodynamically more stable hematite (α-Fe2O3) phase as a byproduct.
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INTRODUCTION Magnetic iron oxide nanoparticles are of key importance in a broad range of scientific and technological applications, including permanent magnets, magnetic fluids, biomedicine, data storage, and catalysis.1−4 Maghemite in the bulk form is ferrimagnetic at room temperature.5 However, reducing the size of the maghemite nanoparticles below the superparamagnetic limit results in zero magnetization at room temperature, due to random spin reorientation in zero field conditions. The properties of maghemite are highly dependent on particle size, size distribution, morphology, and particle aggregation. In recent years, an increasing effort has been put into developing synthesis methods in which these characteristics are tunable to meet the requirements of the various applications.1,6−8 The hydrothermal method has proven itself a promising simple, cheap, and energy efficient synthesis pathway for synthesizing functional materials. The particle characteristics can often be altered by simple adjustments of reaction parameters such as pH, temperature, pressure, precursor concentration, and reaction time.9−11 However, in order to truly tailor the characteristics and properties of functional materials, it is necessary to understand how the nanocrystallites form and grow under various conditions. In situ characterization methods can reveal the formation, growth, and phase evolution behavior of the nanocrystallites and allow identification of optimal synthesis parameters for specific particle characteristics. Powder diffraction is a very efficient technique for in situ characterization, as shown by several recent studies.12−18 © 2014 American Chemical Society
In the present study, in situ powder X-ray diffraction (PXRD) data reveal the evolution of both the size and size distribution of the crystallites during hydrothermal synthesis of maghemite. We meticulously distinguish between volume weighted and number-averaged sizes and size distributions. We have used the volume-weighted PXRD data to extract number-averaged size distributions, which are comparable to sizes obtained from electron microscopy. The effects of reaction temperature, reaction time, and precursor composition are studied. The activation energy and limiting mechanism of crystallite formation are identified using the Johnson−Mehl− Avrami−Kolmogorov (JMAK) kinetic model.
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EXPERIMENTAL SECTION
Precursor Preparation. For the investigation of optimum synthesis conditions, γ-Fe2O3 precursor solutions of pure ammonium iron(III) citrate were prepared by dissolving ammonium iron(III) citrate (C6H8O7·xFe(III)·yNH3) (Sigma-Aldrich, reagent grade, 265.0 g/mol) in deionized water to obtain a 2.0 M aqueous solution. An additional series of experiments was performed to investigate the dependence on precursor composition. Here, precursors of 2.0 M ammonium iron(III) citrate mixed with 2.0 M Fe(NO3)3·9H2O (Riedel-de-Häen, reagent grade) or 2.0 M FeCl3·6H2O (SigmaAldrich, reagent grade) in the ratios of 1:2, 1:1, and 2:1 were prepared. Magnetic stirring was used to ensure a thorough mixing of the Received: December 5, 2013 Revised: January 14, 2014 Published: February 4, 2014 1307
dx.doi.org/10.1021/cg401815a | Cryst. Growth Des. 2014, 14, 1307−1313
Crystal Growth & Design
Article
Prior to the size analysis, the peak broadening arising from the sample was found by correcting the total broadening for the instrumental contribution, using data obtained from a NIST LaB6 standard. The remaining sample broadening can arise from both crystallite domain size and microstrain in the crystal lattice; however in the present case, the latter contribution is neglected. By doing so, the sample broadening can be described by the Scherrer formula, fwhm = (K·λ)/[⟨D⟩·cos(θ)], where ⟨D⟩ is the volume-weighted mean size of coherently scattering crystalline domains, λ the wavelength, fwhm the peak broadening (i.e., full width at half the maximum intensity), K the shape factor (here 0.94, assuming isotropic crystallite morphology), and θ the Bragg angle.24 In addition to the Rietveld analysis, whole powder pattern modeling (WPPM) of selected diffraction patterns were performed using the program PM2K.22 The instrumental profile components were determined by fitting the Caglioti function to the NIST LaB6 diffraction patterns.25 The size contribution to the peak broadening was implemented as originating from a log-normal size distribution of nanoscale scattering domains, g(D) = [1/(Dσ (2π)1/2)] exp{−(1/2) [(lnD-μ)/σ]2}.26 The volume-averaged crystallite sizes (comparable to those obtained in Rietveld refinement) were then extracted from the size distributions using the volume-averaged size formula, Dvolume av = (3/4) exp[μ + (7/2) σ2].26,27 Again, the contribution from strain to the peak broadening was assumed negligible. The background was described using a Chebyshev polynomial. Examples of integrated data frames with representative WPPM fits are found in the Supporting Information. Transmission Electron Microscopy Measurements. Samples for TEM measurements were prepared in-house using the same setup and conditions as in the in situ studies. A few drops of the collected product was suspended in approximately 5 mL of ethanol and sonicated for 1 h and subsequently evaporated onto a TEM grid at room temperature. The measurements were performed on a Philips CM20 running at 200 kV.
solutions. In the case of the ammonium iron(III) citrate and Fe(NO3)3·9H2O mixtures, an amorphous gel slowly formed. In situ Powder X-ray Diffraction Measurements. The precursor solution was injected into a single crystal sapphire capillary reactor with an inner and outer diameter of 0.7 mm and 1.3 mm, respectively. The sapphire tube was mounted using Swagelok fittings (see Figure 1A).19 The capillary was then pressurized with deionized
Figure 1. (A) Experimental setup. The reactor is heated by a jet of hot air and pressurized by water. As the reaction takes place and nanoparticles are formed, the monochromatized synchrotron X-ray beam diffracts from the sample onto a 2D area detector. (B) Timeresolved PXRD data from the 295 °C experiment. water and subsequently heated by a jet of hot air. The small sample volume and the heater efficiency allow the desired temperature to be reached within seconds. Sequential X-ray exposures were initiated simultaneously with the onset of heating. The in situ data were collected with an Oxford Diffraction Titan CCD detector with a diameter of 16.5 cm at the beamline I711, MAX-II (MAX-lab, Lund, Sweden). Experiments were conducted at three different beamtimes with wavelengths around 1.0 Å, and the detector positioned ∼89 mm behind the sample. The exact wavelength and detector distance, as well as the instrumental contribution to the total peak broadening at the given beamtime were determined by calibration with a NIST LaB6 standard using the program Fit2D.20 The time resolution in all the experiments was 5 s with an exposure time of 4 s and a readout time of 1 s. For all experiments, the pressure was set to 250 bar, whereas the set temperature was varied between 270 and 420 °C. The actual temperature inside the capillary is slightly lower than the set temperature. Heating profiles of the in situ PXRD setup may be found in the Supporting Information. Data Analysis. The raw data frames were integrated in Fit2D20 and subsequently analyzed by both Rietveld refinement using FullProf Suite21 and whole powder pattern modeling using PM2K.22 In Fullprof, the data were analyzed using sequential Rietveld refinement, allowing extraction of phase composition, crystallite size, and unit cell parameters as a function of time. The refinements of maghemite were done based on the structure of γ-Fe2O3 in the Fd3̅m space group. Ordering of vacancies in the spinel structure is known to lower the symmetry;23 however, a random ordering of the vacancies sufficiently describes the in situ PXRD data as the weak superstructure peaks are not observed. The refinement of the hematite phase was done based on the structure of α-Fe2O3 in the R3̅c space group. The background was modeled using a linear interpolation between a set of background points with refinable intensity. The Thompson−Cox− Hasting formulation of the pseudo-Voigt function was applied to describe the peak profiles, from which the broadening was used to determine crystallite sizes. In the refinements, the thermal parameters, occupancy, and atomic positions were held fixed, while the scale factor, unit cell parameters, peak profile parameters related to size [Lorentzian (Y) and Gaussian (IG) contribution], and background were refined. Further details can be found in the Supporting Information.
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RESULTS AND DISCUSSION Crystallite Formation and Growth. Examples of integrated data frames obtained after heating a 2.0 M ammonium iron(III) citrate solution at 320 °C for 2, 5, and 25 min with representative Rietveld fits are shown in Figure 2. Sharpening and increasing intensity of the Bragg peaks are observed as the reaction progresses and the crystallites grow. The real-time development of scale factor and crystallite size extracted from the sequential Rietveld refinements of the in situ synchrotron PXRD data are shown in Figure 3. The evolution of the scale factor shows a clear temperature dependency of the onset of crystallization and rate of crystallite formation. In the high temperature experiments (370 and 420 °C), the entire precursor crystallizes immediately. Gas bubble formation in the initial state of the reaction may cause slight oscillations in the scale factor.28 In the 320 °C experiment, there is a delay in the onset of crystallization, after which all the precursor converts almost immediately. For the low temperature experiments (270 and 295 °C), a gradual change from precursor to product is observed. As a result of the very slow progression of the low temperature experiments, the measurements were stopped before the entire precursor had crystallized; the data thus show that a full reaction takes more than 30 min. Figure 3B shows the time-dependent crystallite size evolution and reveals that varying the reaction temperature offers great control of the resulting size of the product. For the three highest temperatures (T = 320, 370, and 420 °C), the crystallites almost stop growing after reaching a temperaturedependent equilibrium size. The largest crystallites with an average size of around 19 nm are obtained at 420 °C [i.e., supercritical synthesis above the supercritical point (Tc = 374 1308
dx.doi.org/10.1021/cg401815a | Cryst. Growth Des. 2014, 14, 1307−1313
Crystal Growth & Design
Article
°C, pc = 220 bar]. For the lowest temperatures, the growth curves hint an approach toward a stable size; however, this was not reached within the span of the experiment. The hydrothermal synthesis of γ-Fe2O3 nanoparticles in critical and near critical water is clearly a fast and efficient way to obtain crystalline nanoparticles of around 16−20 nm. Performing the synthesis at lower temperatures gives smaller crystallites, but at slower reaction times, and possibly only partially crystallized. However, a high yield of smaller nanocrystallites can be achieved by performing the synthesis at an intermediate temperature (320 °C) and ending the experiment immediately after all of the precursor has been converted as illustrated by the dashed line in Figure 3. Evolution of Crystallite Size Distribution. The evolution in the crystallite size distributions based on WPPM of data recorded at various times in the 320 and 370 °C experiments is shown in Figure 4 (panels A and B). The figure shows that
Figure 2. Examples of integrated PXRD data obtained after (A) 2, (B) 5, and (C) 25 min in the 320 °C experiment, fitted by the Rietveld method. The black points show the measured data; the red curve is the model, and the blue is the difference between the two (Iobs − Icalc).
Figure 4. number-averaged log-normal size distributions from various reaction times of the (A) 320 °C and (B) 370 °C experiments. The two shortest reaction times (0.5 and 1 min) have been omitted from (A), since no product had formed and therefore no Bragg peaks were present in the data.
prolonged synthesis time results in a broadening of the size distribution and an increase in the average size. To control the magnetic properties, it is often crucial to have monodisperse crystallites; therefore short synthesis times are preferred. This is illustrated in Figure 5A, where two number-averaged size distributions having almost the same volume-averaged size from different experiments are shown, namely, 320 °C for 25 min and 370 °C for 2 min. The size distribution obtained at the higher temperature and shorter synthesis time is narrower, with the mode (global maximum) at an unmistakably lower value. This makes it ideal to opt for short reaction times at high temperatures in order to obtain a narrow size distribution in addition to a given average crystallite size. Figure 5B shows a comparison between the volume-averaged sizes obtained from Rietveld refinement and from the whole powder pattern modeling. A good agreement is observed, indicating robustness of the size analysis.
Figure 3. (A) Normalized scale factor as a function of time. (B) Crystallite diameters as a function of time. The dashed line indicates the ideal reaction time for a high yield of small nanoparticles (
