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log no. MAM00006
Microscopy Microanalysis
Microsc. Microanal. 16, 1–10, 2010 doi:10.1017/S1431927610000061
AND
© MICROSCOPY SOCIETY OF AMERICA 2010
A Transmission Electron Microscopy Study of CoFe2O4 Ferrite Nanoparticles in Silica Aerogel Matrix Using HREM and STEM Imaging and EDX Spectroscopy and EELS Andrea Falqui,1,2 Anna Corrias,1 Peng Wang,3 Etienne Snoeck,4 and Gavin Mountjoy 1, * ,† 1
Dipartimento di Scienze Chimiche and INSTM, Università di Cagliari, S.P. Monserrato-Sestu Km 0.700, I-09042 Monserrato, Cagliari, Italy 2 Istituto Italiano di Technologia, Via Morego 30, 16163 Genova, Italy 3 SuperSTEM, Daresbury Laboratory, Keckwick Lane, Daresbury, Cheshire WA4 4AD, UK 4 CEMES, 29 rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4, France
Abstract: Magnetic nanocomposite materials consisting of 5 and 10 wt% CoFe 2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. For the CoFe2O4-10wt% sample, bright-field scanning transmission electron microscopy ~BF STEM! and high-resolution transmission electron microscopy ~HREM! images showed distinct, rounded CoFe2O4 nanoparticles, with typical diameters of roughly 8 nm. For the CoFe 2O4-5wt% sample, BF STEM images and energy dispersive X-ray ~EDX! measurements showed CoFe2O4 nanoparticles with diameters of roughly 3 6 1 nm. EDX measurements indicate that all nanoparticles consist of stoichiometric CoFe 2O4 , and electron energy-loss spectroscopy measurements from lines crossing nanoparticles in the CoFe2O4-10wt% sample show a uniform composition within nanoparticles, with a precision of at best than 60.5 nm in analysis position. BF STEM images obtained for the CoFe 2O4-10wt% sample showed many “needle-like” nanostructures that typically have a length of ;10 nm and a width of ;1 nm, and frequently appear to be attached to nanoparticles. These needle-like nanostructures are observed to contain layers with interlayer spacing 0.33 6 0.1 nm, which could be consistent with Co silicate hydroxide, a known precursor phase in these nanocomposite materials. Key words: CoFe 2O4 , ferrite, nanoparticles, HREM, EDX, EELS
I NTR ODUCTION The synthesis of nanoparticles with controlled size and composition is a topic of great current technological interest. Nanoparticles present special properties that can be exploited, and this motivates efforts to understand the physics of these smaller structures. Magnetic nanocomposite materials that include ferromagnetic nanoparticles in an insulating matrix have attracted attention because the matrix can stabilize the size and dispersion of the nanoparticles ~Abeles, 1976!. Ferromagnetic nanoparticles exhibit interesting magnetic properties, for example superparamagnetism, which depend on the composition, size, and shape of the nanoparticle and on the volume fraction of nanoparticles in the matrix. The average size of, and distance between, Received June 1, 2009; accepted January 5, 2010 *Corresponding author. E-mail:
[email protected] † Permanent address: School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NH, UK
ferromagnetic nanoparticles is strongly influenced by the density and porosity of the matrix. Hence, to understand the physical properties of these magnetic nanocomposite materials, it is important to carry out a complete structural characterization. This goal is addressed by the present study, which uses advanced electron microscopy techniques. The sol-gel technique ~Brinker & Scherer, 1990! has been increasingly used to prepare magnetic nanocomposite materials, thanks to advances in preparation methods. Accurate control over composition, purity, and homogeneity at a microscopic level is possible using the sol-gel technique, and silica-based nanocomposites with controlled textures ~e.g., Ennas et al., 2001; Moreno et al., 2002! have been prepared by taking advantage of the well-developed sol-gel chemistry of silica. The matrix in silica aerogels has a fractal-like porous structure and offers attractive features such as chemical inertness, low density, transparency, and low dielectric constant ~Husing & Schubert, 1998; Pierre & Pajonk, 2002!. Innovative magnetic nanocomposites such as high coercivity NdFeB-SiO2 aerogels have been prepared ~Gich et al., 2003!. g-Fe2O3-SiO2 aerogel nanocomposites
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have been obtained by impregnation from solution of a preformed silica aerogel ~Casas et al., 2002! or cogelation of iron oxide and silica precursors ~Cannas et al., 2001!. The present study concerns magnetic nanocomposites of ferromagnetic cobalt ferrite, CoFe 2O4 , nanoparticles in silica aerogel matrix. The use of ferrite nanoparticles in magnetic nanocomposites is of interest ~Hutlova et al., 2003; Congiu et al., 2004! due to the broad range of applications of ferrites. These include magnetic fluids ~Raj et al., 1995!, drug delivery ~Haefeli et al., 1997!, and high density magnetic recording ~Kryder, 1996!. Ferrites have composition MFe2O4 , where M is a cation of oxidation state 2⫹ and Fe has oxidation state 3⫹. The ferrite structure consists of a close-packed cubic lattice of oxygen ions with 32 oxygen ions per unit cell, and with cations occupying 16 out of 32 octahedral interstitial sites and 8 out of 64 tetrahedral interstitial sites. Cobalt ferrite-silica nanocomposites are of significant interest because of the striking properties of bulk cobalt ferrite, CoFe2O4 : chemical stability, mechanical hardness, large anisotropy, high saturation magnetization, and high coercivity ~Ammar et al., 2001!. In the present study, advanced electron microscopy techniques are used to study cobalt ferrite nanoparticles in a highly porous silica aerogel matrix, which have been obtained by using a sol-gel method involving the use of urea as a gelation agent ~Carta et al., 2007b!. Cobalt ferrite, CoFe2O4 , nanoparticles in a SiO2 aerogel matrix have previously been characterized using standard transmission electron microscopy ~TEM! and X-ray diffraction ~XRD! ~Casu et al., 2007!, which showed the dispersed and nanocrystalline nature of nanoparticles. In a sample with 10 wt% CoFe2O4 , the nanoparticle diameters were estimated from TEM, XRD line broadening, and magnetic measurements to be 4–7 nm, 6 6 1 nm, and 9 nm, respectively ~Casu et al., 2007!. Detailed information about atomic structure has been obtained from X-ray absorption spectroscopy ~Carta et al., 2007b!, but this information is necessarily averaged over all nanoparticles. Details about structure and composition of individual CoFe2O4 nanoparticles are directly relevant to modeling the magnetic properties of these magnetic nanocomposite materials ~Casu et al., 2007! but has been lacking in previous studies. One high-resolution electron microscopy ~HREM! image of a sample with 10 wt% CoFe2O4 has previously been reported ~Carta et al., 2007a!. This is the first study, to our knowledge, to present HREM and scanning transmission electron microscopy ~STEM! images, and energy dispersive X-ray ~EDX! and electron energy loss spectroscopy ~EELS! spectra for CoFe2O4-SiO2 nanocomposite materials.
M ATERIALS
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M ETHODS
The sample preparation followed the sol-gel procedure reported in Carta et al. ~2007b!. Tetraethoxysilane @Si~OC2H5 !4 ,
Aldrich 98%, TEOS#, and iron ~III! and cobalt ~II! nitrates @Fe~NO3 !3{9H2O, Aldrich, 98%, and Co~NO3 !2{6H2O, Aldrich, 98%#, were used as precursors for the silica and for the CoFe2O4 phase, respectively; absolute ethanol ~purchased from Fluka! was used as mutual solvent. The alcogel was obtained by a two-step procedure, including the mixing of the ethanolic solution of the iron and cobalt nitrates into the TEOS, prehydrolyzed under acidic conditions, followed by the addition of urea ~NH 2CONH 2 , Sigma-Aldrich, .99.0%! under reflux for 2 h at 858C. Urea enhances both the successful cogelation of the silica and Fe and Co precursors, and the formation of pore structure in the gel ~Casula et al., 2007!. Gelation was performed at 408C in a closed container and occurred in less than 2 days. The alcogels were submitted to high-temperature supercritical drying in an autoclave ~manufactured by Parr, 0.3 L capacity!. The autoclave was filled with an appropriate amount of ethanol and flushed with N2 , closed, and then heated up to 3308C when the pressure reaches 70 atm ensuring the solvent is in the supercritical state ~critical parameters for ethanol being Tc ⫽ 2438C and Pc ⫽ 63 atm!. The autoclave was then vented, and highly porous aerogel samples were obtained. After supercritical drying, the samples were powdered and calcined at 4508C in static air for 1 h to eliminate the organics. Samples containing nanocrystalline CoFe 2O4 in highly porous SiO2 aerogel matrix were obtained by calcining samples in air at 9008C for 1 h. Samples with ~CoFe2O4 !:~CoFe2O4 ⫹ SiO2 ! ratios of 5 and 10 wt% were prepared and are hereafter referred to as CoFe 2O4-5wt% and CoFe2O4-10wt%, respectively. The XRD patterns were recorded on a X3000 Seifert diffractometer equipped with a graphite monochromator on the diffracted beam. The scans were collected within the range of 5–458 2u using Mo-Ka radiation. Electron microscopy observations were carried out using three different electron microscopes. Samples for electron microscopy were prepared by suspending powders in absolute ethanol and dropping onto 3 mm 400 mesh copper microscope grids covered with holey carbon film ~SPI Supplies!. The HREM results were obtained using two 200 kV TEM microscopes, both equipped with field emission guns: ~1! a FEI F20 and ~2! a JEOL 2100F. The latter microscope ~2! can also operate in STEM mode, when the electron beam forms a 1.5 nm diameter probe, and this mode was used with scanning of the probe to collect bright-field ~BF! STEM images ~with an objective aperture! and EDX spectra ~without an objective aperture!. Note that it was not possible to simultaneously collect both BF and dark-field ~DF! images with this microscope because it had only one HAADF detector that required very different camera lengths to detect BF and DF signals ~long and short camera lengths, respectively!. For EDX spectra the typical probe current was ;2 nA, the typical collection time was 30 s. The EDX detector was a Princeton Gammatech LS30135 Si~Li! detector with ultrathin window and had a uniform response in the range 3 to 20 keV and a resolution of 135 eV at 5.9 keV.
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In the range 6.0 to 9.5 keV, electron generated X-ray emission peaks occur in the following sequence: Fe Ka at 6.404 keV, Co Ka at 6.9303 keV ~overlapping with! Fe Kb at 7.058 keV, and Co Kb at 7.649 keV. Copper is a background signal ~Cu Ka at 8.048 keV and Cu Kb at 8.904 keV! due to the use of Cu microscope grids. The EDX spectra were fitted quantitatively using the PYMCA software distributed by the European Synchrotron Radiation Facility. The background was fitted with a linear polynomial of order 5, and peaks were fitted with the Hypermet function. The EDX signals from Fe and Co originate from cobalt ferrite nanoparticles with thickness not greater than approximately 10 nm, and so the absorption and fluorescence corrections in the Cliff-Lorimer method ~Cliff & Lorimer, 1975! can be neglected without significant error. The Cliff-Lorimer ratio of kCoFe ⫽ 1.04 6 0.19 ~relative uncertainty of 18%! was estimated from kFeSi ⫽ 1.35 6 0.16 and kCoSi ⫽ 1.41 6 0.20 for 200 keV electrons with the relative uncertainties being added in quadrature ~Williams & Carter, 1996!. The Cliff-Lorimer k-factor kCoFe was used to convert X-ray intensity I to wt% concentration C using CCo CFe
⫽ k CoFe
ICo IFe
.
~1!
The values of ICo and IFe were typically 1,000 counts, with relative uncertainty being 10% ~three standard deviations!. The relative uncertainty in CCo /CFe is the sum in quadrature of the relative uncertainties in ICo , IFe , and kCoFe . Note that ICo and IFe are different for each measurement, but the same value of kCoFe is used for all measurements. A value of x representing Fe:Co ratio, expressed as Fe x :Co1⫺x , was then obtained using
冉 冊
x ⫽ 1⫹
* CCo * CFe
3
Figure 1. XRD patterns for ~a! CoFe2O4-5wt% and ~b! CoFe2O410wt% samples. Asterisks show Bragg peak positions for strongly scattering ~220!, ~311!, and ~400! lattice planes in the spinel structure ~space group Fd3m!. ~These planes appear in HREM images.! Note the broad band at ;108 is due to the amorphous silica aerogel matrix.
a Gatan Enfina parallel EELS spectrometer. The EELS spectra were obtained with a dispersion of 0.3 eV/channel, and an energy resolution of 0.6 eV ~the full-width at halfmaximum of the zero-loss peak!. The energy range used includes the O K-edge and Fe and Co L2,3-edges at 532, 708, and 780 eV, respectively. Information about the concentrations of Fe and Co were obtained by quantitative analysis of the EELS spectra using the Gatan DigitalMicrograph software distributed by Gatan Incorporated. This includes calculation of the partial ionization cross sections, as described in Egerton ~1996!. ~The values of convergence semiangle and collection semiangle were 24 and 8 mrad, respectively, and the widths of the background and signal windows were 60 and 40 eV, respectively.!
⫺1
,
~2!
where C * is in at.% concentration. A value of x ⫽ 0.67 is expected for CoFe2O4 . From equation ~2! it can be seen that the relative uncertainty in x is equal to the relative uncer* * /CFe !. The typical relative uncertainty in x tainty in ~1 ⫹ CCo is about 8%, but this includes the contribution from the relative uncertainty in kCoFe that affects all values of x in the same way ~e.g., if kCoFe is too large, then all values of x will be too small!. The EELS results were obtained using a 100 kV VG501 STEM microscope, equipped with a cold-field emission gun and a Nion spherical aberration corrector located at SuperSTEM, Daresbury Laboratory, Daresbury, U.K. In this microscope the electron beam forms a 0.13 nm diameter probe that is scanned. The typical probe current was ;0.1 nA. This microscope was used to collect high-angle annular dark-field ~HAADF! images and EELS spectra using
R ESULTS XRD Figure 1 shows XRD patterns of the CoFe 2O4-5wt% and CoFe2O4-10wt% samples, which were previously reported ~Casu et al., 2007!. The XRD pattern for the CoFe 2O410wt% sample shows Bragg peaks due to CoFe2O4 ~PDF-2 card 22-1086!,a which are superimposed on the typical halo due to the amorphous silica aerogel matrix. The average crystallite size, estimated by diffraction line broadening, is 6 6 1 nm ~Casu et al., 2007!. It has previously been assumed that nanoparticles consist of single crystal grains, so that these values for crystallite size correspond to nanoparticle diameters. The XRD pattern for the CoFe 2O4-5wt% sample
a
The PDF-2 database was released in 1998 by the JCPDS International Centre for Diffraction Data, 1601 Park Lane, Swarthmore, PA.
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Figure 2. BF STEM images of ~a! CoFe2O4-5wt% and ~b! CoFe2O4-10wt% samples.
shows extremely broad and weak peaks that can be assigned to very small nanocrystals of CoFe 2O4 but do not provide a reliable estimate of crystallite size.
BF STEM Images from the TEM Microscope Figure 2 shows BF STEM images of the CoFe2O4-5wt% and CoFe2O4-10wt% samples obtained from the JEOL 2100F TEM microscope operating in STEM mode. The BF STEM shows contrast for objects with high atomic number compared to the matrix, and this provides a method of detecting CoFe 2O 4 nanoparticles within an amorphous silica aerogel matrix. For each sample the average diameter of CoFe2O4 nanoparticles was estimated by surveying 32 nanoparticles. The average diameter of nanoparticles in the CoFe2O4-5wt% samples ~see Fig. 2a! is 2.5 6 1.0 nm ~two standard deviations!. Those in the CoFe2O4-10wt% sample ~see Fig. 2b! are significantly larger, with average diameters of 8.7 6 4.2 nm ~two standard deviations!. These observations are reasonable when compared with the XRD patterns. The BF STEM images also show the very high porosity of the silica aerogel matrix.
HREM Images Figures 3 and 4 show a selection of HREM images of nanoparticles in the CoFe2O4-5wt% and CoFe2O4-10wt% samples, respectively. In each inset the corresponding twodimensional ~2D! fast Fourier transform ~FFT! is reported. Figure 3a is typical of the HREM results obtained for the CoFe2O4-5wt% sample because nearly all attempts to image lattice planes in the nanoparticles were unsuccessful. Note that this is despite the clear evidence of nanoparticles in BF STEM images ~e.g., see Fig. 2a! and in EDX results ~see the EDX Spectra section!. The difficulty of imaging lattice
planes might be due to the very small size of nanoparticles in the CoFe2O4-5wt% sample ~average diameter of 2.5 nm!, which are obscured by the surrounding silica aerogel matrix. In only one case were lattice planes observed, and this was for a relatively large nanoparticle at the edge of the aerogel matrix. This is shown in Figure 3b, where ~311! and ~400! planes occur because the nanoparticle is oriented on the @013# axis. Note that a previous HREM study of Fe0.5Co0.5 alloy nanoparticles in a silica aerogel matrix ~Falqui et al., 2009! was able to image lattice plans of nanoparticles only 4 nm in diameter, but that was possible due to the greater contrast between the metal alloy phase and the silica aerogel matrix, which does not occur in the present samples containing only oxide phases. An alternative explanation may be that very small nanoparticles suffer beam damage in a 200 keV microscope, and this disrupts the lattice fringes. Lattice planes could clearly be imaged for the CoFe2O410wt% sample, as shown in Figures 4 and 5. The CoFe2O4 crystal has a spinel structure ~space group Fd3m!, with strong scattering from ~220!, ~311!, and ~400! planes ~as shown in the XRD pattern in Fig. 1b!. These planes are prominent in the HREM images, and combinations of ~hkl ! planes occur when nanoparticles are oriented on particular @hkl # zone axes: ~220! planes for @111# axis ~Fig. 4b!, ~220! and ~311! planes for @112# axis ~Fig. 5c! and @114# axes ~Figs. 4a,b, 5b!, ~220! and ~400! planes for @001# axis ~Fig. 4c!, and ~311! and ~400! planes for @013# axis ~Figs. 3b, 4d!. The HREM results clearly show the CoFe2O4 nanoparticles are larger in the CoFe 2O4-10wt% sample, as noted previously from XRD patterns and BF STEM images. The nanoparticles seen in HREM images typically consist of single crystal grains and have diameters of 5–8 nm, which supports the previous interpretation of XRD patterns.
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Figure 3. HREM images of the CoFe 2O4-5wt% sample: ~a! dark region that contains a CoFe2O4 nanoparticle, but no lattice fringes are observed; ~b! a CoFe2O4 nanoparticle oriented on @013# zone axis. ~Inset shows the corresponding 2D FFT.!
EDX Spectra Figure 6 shows EDX spectra that were obtained from the JEOL 2100F TEM microscope operating in STEM mode. Figure 6a,b shows EDX spectra obtained with the 1.5 nm probe positioned at the center of individual nanoparticles in the CoFe2O4-5wt% and CoFe2O4-10wt% samples. The first two peaks in the EDX spectra correspond to Fe K a at 6.404 keV and Co Ka at 6.9303 keV ~overlapping Fe Kb at 7.058 keV!, and the third peak corresponds to Co Kb at 7.649 keV. ~Additional peaks corresponding to Cu K a at 8.048 keV and Cu K b at 8.904 keV occur due to the use of Cu microscope grids.! The Fe and Co X-ray emission intensity IFe and ICo were extracted by quantitative fitting of the EDX spectra and were then used to calculate the value of x, where the Fe:Co ratio is expressed as Fe x :Co1⫺x and x ⫽ 0.67 is expected for CoFe2O4 ~as discussed in the Materials and Methods section!. The EDX spectra shown in Figure 6 for individual nanoparticles in the CoFe2O4-5wt% and CoFe 2O4-10wt% samples give values of x ⫽ 0.66 6 0.08 and 0.63 6 0.05, respectively. Such EDX spectra were collected from 16 and 19 individual nanoparticles in the CoFe2O4-5wt% and CoFe2O4-10wt% samples, respectively. ~As previously commented, this confirms that nanoparticles in the CoFe2O4-5wt% sample could be identified in BF STEM images, despite the absence of lattice planes in HREM images from that sample.! The mean ^x& and standard deviation sx of the values of x obtained were ^x& ⫽ 0.64 6 0.02 and sx ⫽ 0.03 for the CoFe2O4-5wt% sample, and ^x& ⫽ 0.64 6 0.02 and sx ⫽ 0.03 for the CoFe2O4-10wt% sample @where the error in the mean is taken as 3sx /~N ⫺ 1!1/2 #. In addition, EDX spectra ~not
shown here! were obtained while the probe was scanned over a 220 ⫻ 180 nm 2 area of sample and gave values of x ⫽ 0.66 6 0.04 and 0.66 6 0.04 for the CoFe2O4-5wt% and CoFe2O4-10wt% samples, respectively.
High-Resolution BF STEM and HAADF Images from the STEM Microscope The VG501 STEM microscope was used to collect highresolution BF and HAADF images from the CoFe 2O 410wt% sample using a 0.13 nm diameter probe. Figure 5a shows low magnification BF and HAADF images in which many nanoparticles are visible, including two nanoparticles ~circled! which are shown in the high-resolution BF and HAADF images in Figure 5b,c. Figure 5b,c shows nanoparticles with ~220! and ~311! planes that are oriented on @114# and @112# axes, respectively, and appear to consist of a single crystal grain. Figure 5c shows a very unusual, highly anisotropic, “needle-like” nanostructure. Many instances of such structures were found, and further examples are shown in Figure 5a ~see arrows! and Figure 7a,b. The needle-like structures are clearly not part of the amorphous silica aerogel matrix, and they show dark contrast ~in BF images! expected due to Co and/or Fe content. The needle-like nanostructures are typically 10 nm long and 1 nm wide and appear to be physically connected with CoFe2O4 nanoparticles. All of the aforementioned images were obtained using the VG501 STEM microscope with an accelerating voltage of 100 kV. The needle-like structures were very fragile under the electron beam and were absent from microscopy images obtained using microscopes with an
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Figure 4. HREM images of CoFe 2O4 nanoparticles in the CoFe2O4-10wt% sample. Nanoparticles oriented on ~a! @114# zone axis, ~b! @114# and @111# zone axes ~top and bottom, respectively!, ~c! @001# zone axis, and ~d! @013# zone axis @in addition, top nanoparticle shows ~400! planes#. ~Inset shows the corresponding 2D FFT.!
accelerating voltage of 200 kV ~discussed earlier in the BF STEM Images from the TEM Microscope and the HREM Images sections!. Given that TEM images show a projection of the three-dimensional structure, the appearance of onedimensional nanostructures may be considered to result from either “needles” or disks in three dimensions. However, it seems very unlikely that the short axis of the disks would always be oriented perpendicular to the electron beam ~Wang et al., 2006!, and for this reason it is much more likely that they are needles. Figure 5c shows the nanostructure of the needle to consist of layers, and the
spacing of these layers was estimated to be 0.33 6 0.01 nm by analyzing the profile of image intensity in the direction perpendicular to the axis of the needle. This spacing is of the order of the spacing found between layers of cation polyhedra ~e.g., octahedra! in oxides containing Fe and Co ~see the Discussion section!.
EELS Spectra The VG501 STEM microscope was used to collect EELS spectra from nanoparticles in the CoFe 2O4-10wt% sample using a 0.13 nm diameter probe. Figure 7a,b shows low-
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Figure 6. Quantitatively analyzed EDX spectra obtained with the 1.5 nm probe positioned at the center of individual nanoparticles in ~a! the CoFe2O4-5wt% sample and ~b! the CoFe2O4-10wt% sample.
Figure 5. STEM images: ~left! bright-field and ~right! HAADF images of the CoFe 2O4-10wt% sample. ~a! Low magnification, and ~b,c! high-resolution images and of CoFe2O4 nanoparticles oriented on @114# and @112# zone axes ~locations shown by circles in panel a!. ~Inset shows the corresponding 2D FFT.!
resolution HAADF images of two nanoparticles that were studied using EELS. EELS spectra were collected in “spot” mode from the nanoparticles in Figure 7a,b. EELS spectra collected in “spot” mode for a total of four different nanoparticles gave values for x of 0.64, 0.67, 0.68, and 0.69 ~all
60.05!, with an average of ^x& ⫽ 0.67 and standard deviation of sx ⫽ 0.02. Figure 7 shows two nanoparticles that were studied by collecting EELS spectra from lines crossing the nanoparticles ~as illustrated!. Figure 8 shows a representative EELS spectrum from a single probe position at the center of the analysis line of Figure 7a. At each probe position on the line, the EELS spectra were analyzed quantitatively to obtain the relative concentrations of oxygen, Fe, and Co, and Figure 9 shows the results along lines crossing the two nanoparticles. As expected, clear Fe and Co signals are obtained only from the nanoparticles, whereas the Fe and Co signals fluctuate around zero in the surrounding matrix where there is no Fe or Co. Oxygen is present both in the nanoparticles and in the surrounding silica aerogel matrix. The EELS results show a distribution of Fe and Co that is approximately homogeneous and has an average Fe:Co ratio ~expressed as Fe x :Co1⫺x ! with approximate x value of 0.65. More detailed conclusions are not possible given the statistical fluctuations, which have a precision of at best 60.1 in x and 60.5 nm in analysis position. Note that the nanoparticles shown in Figure 7a,b both have “needle-like” nanostructures attached, as can be seen
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Figure 8. Raw EELS spectrum from midpoint of line crossing CoFe2O4 nanoparticle in Figure 7a. The sequence of edges is oxygen K at 532 eV, Fe L2,3 at 708 eV, and Co L2,3 at 780 eV. ~Note that Si is present due to the silica matrix, but the Si L2,3 edge at 100 eV is outside the range of the EELS spectrum.!
Figure 7. HAADF images of two CoFe 2O4 nanoparticles in the CoFe 2O4-10wt% sample. Crosses and dashed lines show locations from which EELS spectra were collected using the 0.13 nm probe.
in Figure 5a, and this demonstrates that such structures are common. Attempts were made to obtain EELS spectra from the needle-like nanostructures, with the 0.13 nm diameter probe held in a fixed position ~due to the small width of the needle-like structures!. However, due to the fragile nature of the needle-like nanostructures, this lead to significant beam damage after only a few seconds.
Figure 9. Quantitative EELS measurement of relative concentration of oxygen ~gray line!, Fe ~solid circles!, and Co ~empty circles! in cross sections of CoFe 2O4 nanoparticles from lines shown in Figure 7a,b. Triangles show values of x where Fe:Co ratio expressed as Co1⫺x :Fex and x ⫽ 0.67 is expected for CoFe2O4 .
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D ISCUSSION Understanding the structure of these recently synthesized materials is greatly assisted by the electron microscopy results presented here for CoFe2O4-5wt% and CoFe2O410wt% samples. In the CoFe2O4-5wt% sample, CoFe2O4 nanoparticles with diameters of roughly 3 6 1 nm could be detected using BF STEM images and EDX measurements. Previous XRD and TEM measurements on this sample were not able to provide an estimate of nanoparticle diameter, but an estimate of 4 nm was obtained from previous magnetic measurements ~Casu et al., 2007!. In the CoFe2O410wt% sample, distinct, rounded CoFe 2O4 nanoparticles with typical diameters of roughly 8 nm could be detected in the BF STEM and HREM images. The HREM images from this sample show that most nanoparticles consist of single grain CoFe2O4 nanocrystals, and hence the average crystal grain size is indicative of the average nanoparticle size, as previously assumed in the interpretation of XRD and magnetic measurements. The images also show some nanoparticles with less rounded shapes, including elongated ~e.g., Fig. 5b!, rhomboid ~e.g., Fig. 5c!, and with some very small adjoining crystal grains ~e.g., Fig. 4c,d!. These details of nanoparticle shape may be important for accurate modeling of the nanoparticle properties. Due to the closeness in atomic number of Fe and Co, the methods of XRD, BF STEM, and HREM imaging are not sensitive to compositional effects within the nanoparticles, and hence chemically sensitive techniques are needed. The only previous detailed structural characterization of these materials was carried out using the chemically sensitive technique of X-ray absorption spectroscopy @extended X-ray absorption fine structure ~EXAFS! and X-ray absorption near edge structure# at the Fe and Co K-edges ~Carta et al., 2007b!. The results confirmed that the nanoparticles were nm-scale CoFe 2O4 crystals, with a larger diameter for the CoFe 2O410wt% sample compared to the CoFe2O4-5wt% sample. The size effect was indicated from the known effect of nanocrystal size on the Debye-Waller factors obtained from EXAFS. Despite being a chemically sensitive technique, X-ray absorption spectroscopy nevertheless measures an average of the entire sample. Additional techniques that are sensitive to composition on the scale of individual nanoparticles are especially useful for the study of these materials. The EDX and EELS results presented here are of great utility because they provide compositional information at the nm-scale. The 1.5 nm probe of the JEOL 2100F TEM microscope was used to obtain EDX spectra from nanoparticles in both CoFe2O4-5wt% and CoFe2O4-10wt% samples. The EDX measurements of many different nanoparticles in both samples gave mean values of ^x& ⫽ 0.64 and standard deviations of sx ⫽ 0.03. This result indicating that nanoparticles are stoichiometric CoFe2O4 ~with x ⫽ 0.67! is not surprising, but it should be remembered that cobalt ferrite structures can exist with a range of Fe and Co contents ~in
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fact, Co3O4 and Fe3O4 both form with the same structure as ferrites!. The 0.13 nm probe of the VG501 STEM microscope was used to obtain EELS spectra from positions within four individual nanoparticles in the CoFe2O4-10wt% sample, and the results were ^x& ⫽ 0.67 and sx ⫽ 0.02. The results from lines crossing nanoparticles were consistent with a uniform composition of CoFe 2O4 across the nanoparticles, within a precision of at best 60.1 in x and 60.5 nm in position. Within this precision, the nm-scale compositional information obtained confirms that the nanoparticles are uniformly CoFe2O4 . In comparison, a recent EDX study of Fe0.67Co0.33 alloy nanoparticles in silica aerogel matrix ~Falqui et al., 2009! showed a greater variation in composition Fex Co1⫺x with values of ^x& ⫽ 0.63 and sx ⫽ 0.09. The present study shows for the first time that the CoFe2O4-10wt% sample contains highly anisotropic, “needlelike” nanostructures associated with CoFe 2O4 nanoparticles. The explanation for these structures should be related to the process by which CoFe 2O4 nanoparticles are formed from the sol-gel precursor. Previous studies of these materials ~Casu et al., 2007; Carta et al., 2007b, 2009! have shown that after initial heat treatment at 4508C the Co is present as Co silicate hydroxide and the Fe is present as ferrihydrite, whereas after heat treatment at 9008C, the Co and Fe are present as CoFe2O4 , presumably due to the merging and reaction of the previous two phases. It can be supposed that the needle-like structures are a by-product of one of the two previous phases. The most recent study ~Carta et al., 2009! presented TEM images of the CoFe2O4-10wt% sample after initial heat treatment at 4508C, which showed the presence of highly anisotropic, “needle-like” nanostructures. In addition, it showed high-resolution BF STEM images giving evidence of layers within the anisotropic nanostructures, with estimated interlayer spacing of 0.33 6 0.02 nm. Figure 5c is a high-resolution BF STEM image of the CoFe 2O 4-10wt% sample after heat treatment at 9008C and shows the needle to contain layers with a spacing of 0.33 6 0.01 nm. This is not similar to planar spacings reported for “6-line” ferrihydrite ~PDF Card 29-712! but is similar to the ~002! planar spacing of 0.36 nm reported for Co3Si2O5 ~OH!4 ~PDF-2 card 21-0872!. Since the needle-like nanostructures appear to be attached to CoFe 2O4 nanoparticles, they may instead consist of CoFe 2O4 ~PDF-2 Card 22-1086!; however, the nearest planar spacing in the ferrite structure is 0.30 nm for ~220! planes. Further studies are in progress to better identify the nature of these needle-like nanostructures.
C ONCLUSIONS For the CoFe2O4-5wt% sample, BF STEM images and EDX measurements showed CoFe2O4 nanoparticles with diameters of roughly 3 6 1 nm. For the CoFe2O4-10wt% sample,
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BF STEM and HREM images showed distinct, rounded CoFe2O4 nanoparticles, having typical diameters of roughly 8 nm. These results support previous interpretations of XRD and magnetic measurements. The images also show some nanoparticles with less rounded shapes, and with some very small adjoining crystal grains. EDX measurements of many individual nanoparticles in both CoFe2O45wt% and CoFe 2O4-10wt% samples are consistent with stoichiometric CoFe2O4 . EELS measurements from lines crossing nanoparticles in the CoFe2O4-10wt% sample show a uniform composition within nanoparticles, within a precision of at best 60.1 in x and 60.5 nm in analysis position. BF STEM images obtained using an accelerating voltage of 100 kV showed that the CoFe2O4-10wt% sample contains many “needle-like” nanostructures that appear to be attached to nanoparticles. They typically have length ;10 nm and width ;1 nm, and an internal layer structure has been observed with interlayer spacing of 0.33 6 0.01 nm.
A CKNOWLEDGMENTS We thank D. Loche for preparation of samples and the Engineering and Physical Sciences Research Council ~U.K.! for access to the SuperSTEM facility. This work was supported by the European Community Sixth Framework Programme under Marie Curie Intra-European Fellowship ~Contract MEIF-CT-2005-024995!. The authors acknowledge financial support from the European Union under the Framework 6 program under a contract for an Integrated Infrastructure Initiative ~Reference 026019 ESTEEM!. The help of Y. Lefrais is gratefully acknowledged.
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