Scanning Transmission Electron Microscopy Study of the Evolution of

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Scanning Transmission Electron Microscopy Study of the Evolution of Needle-Like Nanostructures in CoFe2O4 and NiFe2O4 Silica Nanocomposite Aerogels Gavin Mountjoy,§,†,* Danilo Loche,† Peng Wang,||,‡ Kasim Sader,^,‡ and Anna Corrias† †

Dipartimento di Scienze Chimiche and INSTM, Universita di Cagliari, S.P. Monserrato-Sestu Km 0.700, I-09042 Monserrato, Cagliari, Italy ‡ SuperSTEM, Daresbury Laboratory, Keckwick Lane, Daresbury, Cheshire, WA4 4AD, United Kingdom ABSTRACT: Magnetic nanocomposite materials consisting of 10 wt % CoFe2O4 or NiFe2O4 nanoparticles in a silica aerogel matrix have been synthesized by the sol-gel method. A 100-kV aberration-corrected scanning transmission electron microscope (STEM) has been used to study these materials, and bright field and high angle annular dark field images show that after heat treatment at both 450 and 900 C, they contain needle-like nanostructures ∼1 nm in width and 10 nm in length. High resolution STEM images show that the needle-like nanostructures have a layered internal structure with typical interlayer spacings of 0.33 ( 0.02 nm. Electron energy loss spectroscopy using a 0.13-nm diameter probe gives information on the composition of these nanostructures. The results presented here for samples heat treated at 450 C are consistent with needle-like nanostructures arising from Co and Ni silicate hydroxides which are separate from the also present Fe-containing phase of ferrihydrite nanoparticles. Samples heat treated at 900 C have previously been shown to contain round ferrite nanoparticles ∼8 nm in diameter. The results presented here are consistent with the needle-like nanostructures being transformed into ferrite-like phases after heat treatment at 900 C, and the needle-like nanostructures are often found attached to round ferrite nanoparticles.

1. INTRODUCTION Metal and metal oxide nanocomposites play a key role in heterogeneous catalysis as well as in electronic and magnetic device fabrication.1-3 For magnetic applications, ferromagnetic nanoparticles exhibit interesting properties such as superparamagnetism. The use of ferrite nanoparticles is of interest4,5 due to applications including magnetic fluids,6 drug delivery,7 and high density magnetic recording.8 For preparing magnetic nanocomposite materials based on ferromagnetic nanoparticles, an insulating matrix can stabilize the size and dispersion of the nanoparticles.9 The properties of such materials can be improved by tuning particle size and shape, particle-support interactions, degree of dispersion of the nanoparticles within the matrix, and texture of the matrix.10-12 Because of its high transparency, low dielectric constant, relative thermal stability, and chemical inertness, silica is widely used as a matrix.13,14 In addition, through well-developed sol-gel routes, the desired porous texture with controlled pore size and shape can be obtained and extremely high porosity can be achieved, such as in aerogels.13 Typical approaches for the preparation of silica-based nanocomposites include deposition-precipitation, pore volume impregnation, and sol-gel routes. In all of these approaches to metal and metal oxide nanocomposites, the final dispersed metal/metal oxide nanocrystals are formed upon post synthesis r 2011 American Chemical Society

treatments, such as calcination under controlled atmosphere. The detailed mechanism and the intermediate species which give rise to the final nanocrystals are often misinterpreted or not fully understood due to the difficulties of characterizing poorly ordered and nonstoichiometic phases. This issue limits how well preparation of nanocomposites can be controlled, since the phases present at early stages affect the final outcome of the synthesis. Moreover, the study of such initial phases is helpful for elucidating the structure and peculiar properties of species which are often neglected, but are commonly found in many hydrolytic processes (including soil science). We have recently developed a urea-assisted sol-gel method15,15b to produce highly porous CoFe2O4-SiO2 and NiFe2O4-SiO2 nanocomposite aerogels. In particular, urea, a base gradually releasing OH- groups, promotes fast gelation while at the same time avoiding the precipitation of metal hydroxides. Suitable precursors are mixed in solution (sol) and aerogels are obtained by supercritical drying. The aerogels are submitted to thermal treatments from 450 to 900 C. The method is of general applicability for supported ferrites and has been extended to other magnetic nanocomposite aerogels, such as FeCo-SiO2.16 Received: November 2, 2010 Revised: January 11, 2011 Published: March 09, 2011 5358

dx.doi.org/10.1021/jp110472d | J. Phys. Chem. C 2011, 115, 5358–5365

The Journal of Physical Chemistry C The structure of the Co and Ni ferrite nanocomposite aerogels has been thoroughly studied at all stages of heat treatment using complementary structural techniques, including X-ray diffraction (XRD), transmission electron microscopy (TEM),17-20 and X-ray absorption spectroscopy (XAS).19,21 Evidence was found that after initial heat treatment at 450 C, there are two separate dispersed phases,19,21 one being the Fe-containing phase ferrihydrite, and the other being a phyllosilicate hydroxide compound containing Co (or Ni). There is a clear evolution upon further heat treatment up to 900 C, where the samples are found to contain round CoFe2O4 (or NiFe2O4) ferrite nanoparticles with approximate diameters of 8 nm.18,22 Interestingly, the most recent studies of the Co and Ni ferrite nanocomposite aerogels20,22 revealed the presence of additional anistotropic, needle-like nanostructures. Crucial in revealing this was the technique of aberration corrected scanning transmission electron microscopy (STEM)23 because aberration correction provides the ability to obtain high resolution images at low electron accelerating voltage of 100 kV. (The structures were not previously observed in images from 200 kV microscopes.17,22) Needle-like nanostructures were shown to be present in CoFe2O4-SiO2 and NiFe2O4-SiO2 nanocomposite aerogels after heat treatment at 450 C20 and were suggested to be associated with Co (or Ni) silicate hydroxide phases. Needle-like nanostructures were also seen in CoFe2O4-SiO2 nanocomposite aerogels after heat treatment at 900 C, but the phase(s) associated with these remained unidentified.22 The purpose of the present work is to further study needle-like nanostructures in CoFe2O4-SiO2 and NiFe2O4-SiO2 nanocomposite aerogels, and to clarify their relation to other phases present in these nanocomposite materials. This has been done by using the aberration-corrected STEM technique to obtain high resolution images and localized electron energy loss spectra (EELS) from samples of CoFe2O4-SiO2 and NiFe2O4-SiO2 nanocomposite aerogels after heat treatments at 450 and 900 C.

2. MATERIALS AND METHODS 2.1. Synthesis. The synthesis was carried out by a two-step sol-gel process which uses urea as basic gelation agent, which has shown to be able to produce nanocomposite aerogels with very high porosity. In particular, the synthesis was carried out by the sol-gel process using tetraethoxysilane ((Si(OC2H5)4, Aldrich 98%, TEOS) as a precursor for silica, iron(III), and cobalt(II) or nickel(II) nitrates (Fe(NO3)3 3 9H2O, Aldrich, 98%, Co(NO3)2 3 6H2O, Ni(NO3)2 3 6H2O Aldrich, 98%) as precursors for the cobalt and nickel ferrite nanoparticles, respectively, which in the final nanocomposites are dispersed in the silica matrix, and absolute ethanol (EtOH, Fluka) as mutual solvent.15 The precursors were added in such a way to obtain final nanocomposites containing a nominal ratio of 10 wt % CoFe2O4/(CoFe2O4þSiO2) and NiFe2O4/ (NiFe2O4þSiO2). The ethanolic solution of the metal salts was added into the prehydrolyzed TEOS under acidic catalysis. Urea (NH2CONH2, Aldrich, >99.0%) was then added under reflux for 2 h at 85 C as basic gelation agent. The sols were left in a closed container at 40 C; gelation occurred in less than 2 days. The alcogels were submitted to high temperature supercritical drying in an autoclave (Parr, 300 cm3). The autoclave was filled with an appropriate amount of ethanol and flushed with N2 before being

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heated in such a way to take the solvent to the supercritical state (i.e., 330 C, 70 atm). The autoclave was then vented and highly porous aerogel samples were obtained. The samples investigated in the present work were then calcined at 450 C in static air for 1 h. In order to obtain the final CoFe2O4-SiO2 and NiFe2O4-SiO2 nanocomposites, additional calcination at 900 C in static air for 1 h is required. The two aerogels calcined at 450 C will hereafter be called AFeCo450 and AFeNi450, respectively, while those calcined at 900 C will be referred to as AFeCo900 and AFeNi900. 2.2. STEM. 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). High resolution bright field STEM images 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, United Kingdom. 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 also used to collect high angle annular dark field (HAADF) STEM images. EELS spectra were collected using a Gatan Enfina parallel EELS spectrometer. To collect EELS from such small and delicate nanostructures, we used a new technique of “smart acquisition”.23b Smart acquisition allows independent control of probe scanning procedures while simultaneously acquiring EELS. It enables rapid manual scanning of the STEM probe over an area of interest while collecting a single or small number of EELS spectra. This makes it possible to control the electron dose experienced by beam-sensitive specimens while maintaining a sufficiently high signal-to-noise ratio in the EELS. Alternative methods of collecting spectra from a single point (which results in a higher dose) and scanning the probe over an area including the structure of interest (which includes material of no interest) did not produce usable EELS spectra. The EELS spectra were obtained with a dispersion of 0.3e V/ channel, and an energy resolution of 0.9 eV (the full width halfmaximum of the zero-loss peak). The energy range used includes the O K-edge at 532 eV and Fe, Co and Ni L2,3-edges at 708, 780, and 852 eV, respectively. The acquisition time was 15-20 s per spectrum. Information about the concentrations of Fe, Ni, 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 by Eggerton.24 (The values of convergence semiangle and collection semiangle were 24 and 8 mrad, respectively, and the widths of the background and signal windows were 50 and 40 eV, respectively). This analysis produces results for the relative concentrations of Fe, Co, or Ni atoms present in the beam CM (where M = Fe, Co, or Ni), and the corresponding uncertainties due to counting statistics. Here we report the Fe:Co and Fe:Ni ratios as FexCo1-x and FexNi1-x respectively, where (for example), CCo -1 x ¼ 1þ CFe The CoFe2O4 ferrite phase has x = 0.67. The relative uncertainty in x is equal to the relative uncertainty in (1 þ CFe/CCo). 5359

dx.doi.org/10.1021/jp110472d |J. Phys. Chem. C 2011, 115, 5358–5365

The Journal of Physical Chemistry C

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Figure 1. Low magnification STEM bright field (left) and HAADF (right) images for samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900, and (d) AFeNi900. (Asterisks illustrate the presence of needle-like nanostructures).

3. RESULTS 3.1. STEM Images. Figure 1 shows low magnification STEM bright field (left) and HAADF (right) images (collected simultaneously) for samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900, and (d) AFeNi900. The materials are dominated by an amorphous silica matrix. Previous work18,22 has confirmed that after heat treatment at 900 C the matrix contains round MFe2O4 ferrite nanoparticles (M = Co or Ni) with diameter of ∼8 nm, and such nanoparticles, with strong scattering contrast, are clearly visible in Figure 1c,d for samples AFeCo900 and AFeNi900, respectively. The images presented here also show that needle-like nanostructures are observed (see asterisks in Figure 1), with approximate dimensions of 1 nm in width and 10 nm in length. Given that TEM images show a projection of the three-dimensional structure, the appearance of one-dimensional 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,25 and for this reason, it is much more likely that they are needles. Furthermore, in samples AFeCo900 and AFeNi900, the needle-like nanostructures can be found to be attached to round ferrite nanoparticles. (This was previously reported for the AFeCo900 sample in.22) Figure 2 shows selected high resolution bright field STEM images of needle-like nanostructures from samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900 and (d) AFeNi900, respectively. (The holey carbon support film is seen in the background of Figure 2). The bright field images were of significantly higher quality than HAADF images, probably because bright field imaging is more efficient in its use of electrons.25b The high resolution bright field images appear to show that the needle-like nanostructures have a layer-like structure parallel to the long axis. (Figure 2a,b were previously reported in ref 20 and Figure 2c was previously reported in ref 22). Many other attempts to obtain

high resolution images showing internal structure were unsuccessful due to the very fragile nature of the needle-like nanostructures. The high resolution bright field images have been analyzed to obtain estimates of the interlayer spacing in the needle-like nanostructures. This was done using the average intensity profile perpendicular to the long axis (see boxes indicated on images). Values were obtained from individual needle-like nanostructures by measuring the distance for three fringes (e.g., three interlayer spacings), and assuming a minimum uncertainty of (1 pixel gives an uncertainty of (0.01 nm in the calculated interlayer spacing. For each sample, the results for two needle-like nanostructures were averaged. The estimated interlayer spacings for needle-like nanostructures in the samples AFeCo450, AFeNi450, AFeCo900, and AFeNi900 are 0.33 ( 0.02, 0.31 ( 0.02, 0.33 ( 0.01, and 0.33 ( 0.01 nm, respectively. In addition, another, shorter, interlayer spacing of 0.24 ( 0.01 nm was observed transverse to the long axis of needlelike nanostructure in sample AFeNi900 (see Figure 2d). 3.2. EELS Spectra. EELS spectra were collected by using smart acquisition mode to position the electron beam over a needle-like nanostructure in an HAADF image of the region of interest. Figure 3 shows bright field STEM images of needle-like nanostructures for which EELS spectra were collected in the samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900, and (d) AFeNi900 (in selected cases HAADF images are also shown to confirm the presence of a needle-like nanostructure). For each sample, the results for two EELS spectra are reported, taken from nanostructures labeled “A” and “B” (see Figure 3). Figure 4 shows an example EELS spectra from a needle-like nanostructure in sample AFeCo900 (labeled “A” in Figure 3c). The EELS spectra include the O K-edge at 532 eV, and the Fe, Co, and Ni L-edges at 708, 780, and 852 eV, respectively, and were analyzed to obtain the Fe:Co or Fe:Ni atomic ratios, which we express as FexCo1-x or FexNi1-x, respectively. Note that since the supporting aerogel matrix is silica, the oxygen content of nanostructures cannot be separately extracted from the EELS spectra. Figure 5 5360



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Figure 2. high resolution STEM bright field images of needle-like nanostructures from samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900, and (d) AFeNi900, respectively.

shows the values of x obtained (with error bars at two standard deviations) for needle-like nanostructures labeled “A” and “B” in the samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900, and (d) AFeNi900. EELS spectra were also collected from nanoparticles in the same samples. This was to provide a comparison with the needlelike nanostructures, and to provide a reference value for x using the ferrite nanoparticles in samples AFeCo900 and AFeNi900 (ferrites MFe2O4 with M = Co or Ni are expected to have x = 0.67). Figure 6 shows bright field STEM images of nanoparticles from the samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900 and (d) AFeNi900 for which EELS spectra were collected. For each sample the results for two EELS spectra are reported, taken from nanoparticles labeled “C” and “D”. We used “spot” mode to collect EELS spectra from the center of each nanoparticle because the nanoparticles were structurally stable in the electron beam. Figure 5 shows the values of x obtained (with error bars at two standard deviations) for nanoparticles labeled “C” and “D” in the samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900, and (d) AFeNi900. These values of x can be compared with those obtained for needlelike nanostructures from the same samples (labeled “A” and “B”), which are also shown in Figure 5.

4. DISCUSSION The purpose of the current work is to provide new results which reinforce recent observations that needle-like nanostructures are found in CoFe2O4 and NiFe2O4 silica aerogel nanocomposites. In addition, the current work provides further information useful for understanding the relation between these needle-like nanostructures and other phase(s) present in these nanocomposite materials. The high resolution bright field STEM images presented in Section 3.1 give information about the internal structure of the needle-like nanostructures. The estimates obtained for interlayer spacings in the samples AFeCo450, AFeNi450, AFeCo900, and AFeNi900, are 0.33 ( 0.02, 0.31 ( 0.02, 0.33 ( 0.01, and 0.33 ( 0.01 nm, respectively. In addition, another short interlayer spacing of 0.24 ( 0.01 nm was observed for the sample AFeNi900. These can be compared with interplanar spacings in possible Feand Co- or Ni-containing phases relevant to these materials, as shown in Table 1. Such comparison shows that the needle-like nanostructures in samples AFeCo450 and AFeNi450 cannot correspond to the ferrihydrite phase which is known to be present in these samples19-21 because the largest (110) interplanar spacing in ferrihydrite is 0.252 nm. For these samples, the typical interlayer spacings of 0.32 ( 0.02 nm in the needle-like 5361



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Figure 3. Bright field STEM images of needle-like nanostructures for which EELS spectra were collected in the samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900, and (d) AFeNi900 (in selected cases HAADF images are also shown). For each sample, the results for two spectra are reported, taken from nanostructures labeled “A” and “B”. (Results from EELS spectra are reported in Figure 5.).

Figure 4. An example EELS spectra from a needle-like nanostructure in sample AFeCo900 (labeled “A” in Figure 3c). The EELS spectra include the O K-edge at 532 eV, and the Fe, Co, and Ni L-edges at 708, 780, and 852 eV, respectively.

nanostructures are somewhat similar to the (002) and (004) interplanar spacings of 0.360 and 0.366 nm in Co and Ni silicate hydroxide structures Co3Si2O5(OH)4 and Ni3Si2O5(OH)4, respectively. This association is reinforced by the fact that needlelike nanostructures were observed in low magnification, 100 kv, conventional TEM images of samples prepared with no Fe (otherwise using the same synthesis) which are known to contain Co and Ni silicate hydroxide phases.20 The EELS results presented in Section 3.2 give some information about the composition of needle-like nanostructures and nanoparticles. Figure 5a,b shows that AFeCo450 and AFeNi450 samples contain nanoparticles with predominantly Fe composition, consistent with the ferrihydrite phase identified in previous work.19-21 In contrast, these samples contain needle-like nanostructures with a low Fe content, consistent with the interpretation that the corresponding phases are similar to Co or Ni silicate hydroxides.20

Figure 5. EELS spectra were analyzed to obtain the Fe:Co or Fe:Ni atomic ratios, which we express as FexCo1-x or FexNi1-x, respectively. Bar charts show values of x obtained (with error bars at two standard deviations) for needle-like nanostructures labeled “A” and “B”, and nanoparticles labeled “C” and “D” in the samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900, and (d) AFeNi900. (See Figures 3 and 6 for the locations from which EELS spectra were collected.).

For the needle-like nanostructures in samples AFeCo900 and AFeNi900, the STEM images gave estimated interlayer spacings of 0.33 ( 0.01 nm that is somewhat similar to the (220) interplanar spacing of 0.295 nm in the Co ferrite structure (the 5362



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Figure 6. Bright field STEM images of nanoparticles from the samples (a) AFeCo450, (b) AFeNi450, (c) AFeCo900 and (d) AFeNi900 for which EELS spectra were collected (in selected cases HAADF images are also shown). For each sample the results for two EELS spectra are reported, taken from nanoparticles labeled “C” and “D”. (Results from EELS spectra are reported in Figure 5.).

Table 1. Interplanar Spacings d (nm) and Corresponding Miller Indices (h,k,l) for Crystal Structures26 of Ferrihydrite, Co and Ni Silicate Hydroxides, and Ferritesa 6-line ferrihydrite FeO(OH) hexagonal PDF 46-1315

a

Ni silicate hydroxide Ni3Si2O5(OH)4 monoclinic PDF 22-0754

Co silicate hdyroxide Co3Si2O5(OH)4 orthorhombic PDF 21-0872 0.726

(0,0,1)

0.743

(0,0,2)

0.461

(0,2,0)

0.450

(1,1,0)

0.360

(0,0,2)

0.366

(0,0,4)

(2,0,0) (1,2,2)

0.262 0.245

(2,0,0) (2,0,2)

0.252

(1,1,0)

0.268 0.252

0.223

(1,1,2)

0.213

(1,4,0)

0.196

(1,1,3)

0.172

(1,1,4)

0.178

(0,1,4)

0.151/0.148

(1,1,5)/(3,0,0)

0.156/0.154

(0,3,4)/(0,6,0)

0.153

(2,0,8)

Co ferrite CoFe2O4 cubic PDF 22-1086

0.485

(1,1,1)

0.297

(2,2,0)

0.253/0.242

(3,1,1)/(2,2,2)

0.210

(4,0,0)

0.171/0.162

(4,2,2)/(5,1,1)

0.148

(4,4,0)

Note that spacings on the same row differ by less than 3%.

Ni ferrite structure is the same). In addition, another short interlayer spacing of 0.24 ( 0.01 nm was observed for a needle-like nanostructure in sample AFeNi900, and this is similar to the (222) interplanar spacing of 0.242 nm in the ferrite structure. We recall that after heat treatment at 900 C, the needle-like nanostructures are often found to be attached to round ferrite nanoparticles. In fact, Figure 2c,d shows just such arrangements. It may be that needle-like nanostructures of Co or Ni silicate hydroxide and very small ferrihydrite nanoparticles (which are both present at 450 C) react during high temperature treatment up to 900 C to form larger, round nanoparticles of ferrite. Sometimes these round nanoparticles are attached to the needle-like nanostructures, where the latter have changed their composition but retained their morphology. The interlayer spacings of the needle-like nanostructures show some alignment, but they are

not exactly coherent, with the interplanar spacings visible in the round ferrite nanoparticles. The exception is in Figure 2d for the AFeNi900 sample where a needle-like nanostructure has an interlayer spacing of 0.24 ( 0.01 nm, which is coherent with the (222) interplanar spacing of 0.242 nm visible in the round ferrite nanoparticle. The EELS results for samples AFeCo900 and AFeNi900 are shown in Figure 5c,d, respectively. This shows that nanoparticles in these samples have the ferrite composition, consistent with previous X-ray absorption spectroscopy19,21 and high resolution TEM18,22 studies that identified ferrite nanoparticles in these samples. Interestingly, the EELS results for needle-like nanostructures in these samples also show compositions similar to the ferrite composition. As discussed in the previous paragraph, there are some cases in which the needle-like nanostructures are 5363



5. CONCLUSIONS An aberration-corrected STEM microscope has proven extremely useful to study fragile needle-like nanostructures found in CoFe2O4-SiO2 and NiFe2O4-SiO2 nanocomposite aerogels. Using a 100 kV electron beam, images and compositional information with high spatial resolution can be obtained, where previous electron microscopy studies at 200 kV were unsuccessful. The results confirm that an intrinsic component of these nanocomposite materials are needle-like nanostructures with a typical morphology of ∼1 nm in width and 10 nm in length. They have an internal structure of layers parallel to the long axis with a typical interlayer spacing of 0.33 ( 0.02 nm, which is similar to a 0.36 nm interplanar spacing in silicate hydroxides and a 0.30 nm spacing in ferrites. EELS results show the needle-like nanostructures to have high Co or Ni contents and low Fe contents after heat treatment at 450 C, consistent with phases similar to Co or Ni silicate hydroxides. After heat treatment at 900 C, the composition of the needle-like nanostructures is similar to Co or Ni ferrite. In fact, the needle-like nanostructures are often found attached to round ferrite nanoparticles, and in one case, the structures were observed to be coherent. The current results suggest structural evolution, which takes place via ion migration involving exchange of Si and Fe ions. ’ AUTHOR INFORMATION Corresponding Author

* Tel: 0044 1227 823 228. Fax: 0044 1227 827 558. E-mail: g. [email protected]. Present Addresses §

School of Physical Sciences, University of Kent, Canterbury, Kent, CT2 7NH, United Kingdom.

)

attached to ferrite nanoparticles (see Figure 2c,d), and one case for the ANiFe900 sample where the needle-like nanostructure is coherent with the lattice planes in the ferrite nanoparticle (see Figure 2d). The similar (ferrite) composition and the spatial proximity of needle-like nanostructures and round ferrite nanoparticles should be interpreted as related phenomena. The present results point to a structural evolution of the needle-like nanoparticles, which after heat treatment at 450 C appear to be similar to Ni and Co silicate hydroxide phases (with low Fe content), and after heat treatment at 900 C similar to ferrite phases (with high Fe content). This structural evolution mirrors the sequence of phases previously observed in X-ray absorption spectroscopy studies19,21 of these nanocomposite materials. Since the needle-like nanostructures are visible at both 450 and 900 C, it is plausible to assume that individual needlelike nanostructures maintain their morphology during the heat treatment. The simplest explanation of these changes would be if the Co or Ni silicate hydroxide phases lose Si and gain Fe thereby turning into ferrite phases. This would involve ion migration. Indeed, the observed attachment of needle-like nanostructures to ferrite nanoparticles (see Figure 2c,d) may be associated with the migration of Fe. Nevertheless, the typical interlayer spacings of 0.33 ( 0.02 nm observed in the needle-like nanostructures are not equal to the 0.36 nm interplanar spacings in Co and Ni silicate hydroxides, or the 0.30 nm interplanar spacing in Co ferrite. Hence, it is likely that structural evolution from phases similar to Co or Ni silicate hydroxides to ferrite structures involves structural rearrangement in addition to ion migration.

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Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, United Kingdom. ^ Division of Physical Biochemistry, National Institutes for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom.

’ ACKNOWLEDGMENT This work was supported by the EPSRC UK for access to the SuperSTEM facility, by the European Community Sixth Framework Programme under the Marie Curie Intra-European Fellowship (Contract MEIF-CT-2005-024995), and by the Italian Institute of Technology (IIT) through the SEED project “NANOCAT”. D.L. thanks the Regione Autonoma della Sardegna for the cofinanced research grant funded through POR Sardegna FSE 2007-2013, L.R.7/2007 “Promozione della ricerca scientifica e dell’innovazione tecnologica in Sardegna”. ’ REFERENCES (1) Yi, D. K.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006, 18, 2459. (2) Buchanan, K. S.; Zhu, X.; Meldrum, A.; Freeman, M. R. NanoLett 2005, 5, 383. (3) Haque, S. A.; Koops, S.; Tokmoldin, N.; Durrant, J. R.; Huang, J. S.; Bradley, D. D. C.; Palomares, E. Adv. Mater. 2007, 19, 683. (4) Hutlova, A.; Niznansky, D.; Rehspringer, J.-L.; Estournes, C.; Kurmoo, M. Adv. Mater. 2003, 15, 1622. (5) Congiu, F.; Concas, G.; Ennas, G.; Falqui, A.; Fiorani, D.; Marongiu, G.; Marras, S.; Spano, G.; Testa, A. M. J. Magn. Magn. Mater. 2004, 272-276, 1561. (6) Raj, K.; Moskowitz, B.; Casciari, R. J. Magn. Magn. Mater. 1995, 149, 174. (7) Haefeli, U.; Schuett, W.; Teller, J.; Zborowski, M. Scientific and Clinical Applications of Magnetic Carriers; Plenum Press: New York, USA, 1997. (8) Kryder, M. H. MRS Bull. 1996, 21, 17. (9) Abeles, B. In Applied Solid State Science; Wolfe, R., Ed.; Academic Press: New York, USA, 1976; p 1. (10) Yang, C.-M.; Lin, H.-A.; Zibrowius, B.; Spliethoff, B.; Schuth, F.; Liou, S.-C.; Chu, M.-W.; Chen, C.-H. Chem. Mater. 2007, 19, 3205. (11) Mohanan, J. L.; Brock, S. L. Chem. Mater. 2003, 15, 2567. (12) Gross, A. F; Diehl, M. R.; Beverly, K. C.; Richman, E. K.; Tolbert, S. H. J. Phys. Chem. B 2003, 107, 5475. (13) Schubert, U.; Husing, N. Synthesis of Inorganic Materials; WileyVCH: Manheim, Germany, 2000; ch. 6. (14) Pierre, A. C.; Pajonk, G. M. Chem. Rev. 2002, 102, 4243. (15) Casula, M. F.; Loche, D.; Marras, S.; Paschina, G.; Corrias, A. Langmuir 2007, 23, 3509. (b) Loche, D.; Casula, M. F.; Falqui, A.; Marras, S.; Corrias, A. J. Nanosci. Nanotech. 2010, 10, 1008. (16) Casu, A.; Casula, M. F.; Corrias, A.; Falqui, A.; Loche, D.; Marras, S.; Sangregorio, C. Phys. Chem. Chem. Phys. 2008, 10, 1043. (17) Casu, A.; Casula, M. F.; Corrias, A.; Falqui, A.; Loche, D.; Marras, S. J. Phys. Chem. C 2007, 111, 916. (18) Carta, D.; Casula, M. F.; Falqui, A.; Loche, D.; Mountjoy, G.; Sangregorio, C.; Corrias, A. J. Phys. Chem. C 2009, 113, 8606. (19) Carta, D.; Loche, D.; Mountjoy, G.; Navarra, G.; Corrias, A. J. Phys. Chem. C 2008, 112, 15623. (20) Carta, D.; Casula, M. F.; Corrias, A.; Falqui, A.; Loche, D.; Mountjoy, G.; Wang, P. Chem. Mater. 2009, 21, 945. (21) Carta, D.; Mountjoy, G.; Navarra, G.; Casula, M. F.; Loche, D.; Marras, S.; Corrias, A. J. Phys. Chem. C 2007, 111, 6308. (22) Falqui, A.; Corrias, A.; Wang, P.; Snoeck, E.; Mountjoy, G. Microsc. Microanal. 2010, 16, 209. (23) Falke, U.; Bleloch, A.; Falke, M.; Teichert, S. Phys. Rev. Lett. 2004, 92, 116103. (b) Sader, K.; Schaffer, B.; Vaughan, G.; Brydson, R.; Brown, A.; Bleloch, A. Ultramicroscopy 2010, 110, 998. 5364



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