Goethite (α-FeOOH) Nanorods as Suitable Antiferromagnetic Substrates

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Goethite (r-FeOOH) Nanorods as Suitable Antiferromagnetic Substrates Rosalía Mari~no-Fernandez,† Sueli Hatsumi Masunaga,‡ Nerio Fontaí~na-Troiti~no,† M. Puerto Morales,§ Jose Rivas,^ and Veronica Salgueirino*,† †

Departamento de Física Aplicada, Universidade de Vigo, 36310, Vigo, Spain Instituto de Física, Universidade de S~ao Paulo, CP 66318, 05315-970, S~ao Paulo, SP, Brazil § Instituto de Ciencias de Materiales de Madrid (ICMM)-CSIC, Madrid, Spain ^ Departamento de Física Aplicada, Universidade de Santiago de Compostela, 15782, Santiago de Compostela, Spain ‡

ABSTRACT: We propose goethite nanorods as suitable antiferromagnetic substrates. The great advantage of using these inorganic nanostructures as building blocks comes from the fact that it permits the design and fabrication of colloidal and supracolloidal assemblies knowing first their magnetic characteristics. As a proof of concept, we have developed mix multifunctional systems, driving on the surface of these AFM substrates, cobalt ferrite nanoparticles (the study of bimagnetic systems opens new degrees of freedom to tailor the overall properties and offers the Meiklejohn
Bean paradigm, but inverted), a silica shell (protection purposes, but also as a tailored spacer that permits controlling magnetic interactions), and metallic gold clusters (seeds that can favor the acquisition of optical or catalytic properties).

’ INTRODUCTION One-dimensional nanostructures of magnetic materials are presently the subject of intensive research, taking into account the considerable attention they have recently received and the few cases reported.1
6 Much of the early work was concerned with exploratory issues, such as establishing an easy axis for typical preparation conditions and the essential involvement of shape anisotropy, as opposed to magnetocrystalline anisotropy. More recently, attention has shifted toward the understanding of magnetization processes because magnetic nanowires have provided a highly successful test ground for understanding the microscopic mechanisms that determine macroscopically important parameters in different applications.7 These building blocks, as in the case of spherical nanoparticles, are at the border between the solid and molecular state, displaying novel effects that, in the case of magnetic materials, are a result of both the intrinsic properties of the small building blocks and the interactions in between.8 Appealing extrinsic novel properties were, for example, encountered in bimagnetic (ferro- or ferrimagnetic (FM or FiM) and antiferromagnetic (AFM)) nanostructures, characterized by parameters, such as enhanced superparamagnetic blocking temperatures or tunable coercivities.9
11 Some of these bimagnetic systems have been obtained from the oxidation of transition-metal r 2011 American Chemical Society

nanoparticles, leading to an FM core and the corresponding AFM or FiM shell. In this type of bimagnetic system, the remanent magnetization of the AFM component and the associated uncompensated moments can have an important role in the exchange bias phenomena. In most of the cases, the largest fraction of remanent magnetization is given by the FM or the FiM component, and the role of the AFM constituent is to keep the moment in a fixed direction. Thus, the AFM component increases the coercivity and the blocking temperature of the nanostructures analyzed but additionally may become involved in other important issues, such as how exchange anisotropy can be established, the role of the AFM uncompensated spins in the hysteresis, or the origin of the magnetic properties of the AFM
FiM interface.12 The main inconvenience when considering these conventional bimagnetic core
shell nanoparticle systems comes from the fact that control of the core and shell dimension distributions is generally difficult with these oxidized particles because, due to the reduced size, the AFM shell usually grows highly disordered. In view of that, recently, “inverse” core/ shell nanostructures with AFM cores and FiM or spin-glass shells Received: February 15, 2011 Revised: June 10, 2011 Published: June 13, 2011 13991

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The Journal of Physical Chemistry C have been synthesized, leading to a number of novel magnetic properties.13 For example, the AFM component was found to increase the ordering temperature of the FiM component.14 Goethite fine particles behave as an AFM material with a Neel temperature of about 400 K and are usually found to be elongated along the [010] direction,15 although fluctuations of the magnetization directions of the grains have been reported.16 Nanoparticles of AFM materials have nonzero magnetic moments and are, therefore, strictly speaking, not AFM. However, to simplify, the term AFM nanostructures will be used instead of nanostructures of AFM materials, quoting Morup and co-workers.17,18 Numerous magnetization studies of AFM nanoparticles have shown that both the initial susceptibility and the magnetization in large applied fields are considerably larger than in the corresponding bulk materials. Neel justified this result due to the finite number of magnetic atoms in the nanoparticles, which may lead to a difference in the numbers of spins in the two sublattices because of random occupancy of lattice sites.19 This results in an uncompensated magnetic moment, μ, which, in the case of AFM nanostructures without impurities of either FM or FiM materials, comes from a random occupation of surface sites. These AFM nanostructures have rich magnetic behavior that can be quite different from their bulk counterparts, characterized by enhanced magnetic moment and coercivity, exchange bias, or a decrease in the AFM susceptibility with temperature (below the order temperature TN).20 Besides, the uncompensated magnetic moment mentioned superposes to the AFM susceptibility and hinders its determination based on low-field susceptibility measurements since it has an important paramagnetic contribution. These nanostructured AFM materials have important applications in areas related to spin valves and magnetic random access memory (MRAM) devices, mainly related to new types of hard magnetic materials consisting of composites of AFM and FM or FiM nanoparticles, which, depending on the magnetic characteristics of the materials involved (TC for example), can give rise to a number of interesting effects.14,21 In line with that, we propose goethite nanorods as suitable AFM substrates to study this type of effect. The great advantage in terms of using these inorganic nanostructures as building blocks comes from the fact that it permits the design and fabrication of colloidal and supracolloidal assemblies and challenges the development of simple, low-cost, and environmentally friendly approaches. Additionally, because the mix systems are rarely described because measurements of their physical properties are difficult to interpret and various methods of analysis are usually needed to separate and characterize each contributing component, this strategy permits one to know first the magnetic characteristics of the suitable AFM substrates and offers the Meiklejohn
Bean paradigm,22 but inverted. As a proof of concept and once the nanorods were completely characterized, we have driven on their surface cobalt ferrite nanoparticles (the study of bimagnetic systems opens new degrees of freedom to tailor the overall magnetic properties), a silica shell (protection purposes, but also as a tailored spacer that permits controlling magnetic interactions), and metallic gold clusters (seeds that can favor the acquisition of optical or catalytic properties).

’ EXPERIMENTAL SECTION Chemicals. Ammonium hydroxide solution (NH4OH, 28
30% NH3), iron(III) chloride hexahydrate (FeCl3, 97%), and tetramethyl ammonium hydroxide solution (TMAOH)

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were obtained from Fluka. Iron(II) sulfate heptahydrate (FeSO4 3 7H2O, 98%), cobalt(II) chloride hexahydrate (CoCl2 3 6H2O, 98%), iron(III) nitrate nonahydrate (Fe(NO3)3, 98%), nitric acid (HNO3, 65%), (3-aminopropyl)-trimethoxysilane (APS), poly(sodium 4-styrenesulfonate) (PSS) (Mw ≈ 70 000), poly(diallyldimethylamonium chloride) (PDADMAC) (Mw < 200 000), tetrakis(hydroxymethyl) phosphonium chloride solution (THCP, 80% in water), gold(III) chloride hydrate (HAuCl4 3 xH2O, 99.99%), sodium silicate, sodium chloride, and sodium hydroxide were supplied from Sigma-Aldrich. Hydrochloric acid (37 wt %) (HCl) was supplied by Merck. All chemicals were used as received. Goethite Nanorods. Aqueous dispersions of goethite nanorods were synthesized according to a modified Massart method23 based on the coprecipitation of ferrous and ferric ions solutions, but at acid pH (pH ∼ 4). A 20 mL portion of aqueous 1 M FeCl3 and 5 mL of 2 M FeSO4 3 7H2O in 1.3 M HCl were simultaneously injected into 250 mL of 0.4 M NH4OH under rapid mechanical stirring. Stirring was allowed to continue for 30 min, and then the yellow-brown solid product was allowed to precipitate. The sediment was redispersed in 50 mL of distilled water, and subsequently, three 30 mL aliquots of tetramethylammonium hydroxide solution (1 M) in 50 mL of water were added, again with rapid stirring. Finally, water was added to the dispersion up to a total volume of 250 mL. Cobalt Ferrite Nanoparticles and LbL Self-Assembly onto the Goethite Nanorods. The cobalt ferrite synthesis was carried out according to previous work.24 A 5 mL portion of 2 M CoCl2 3 6H2O in HCl 7.4% solution and 40 mL of 0.5 M FeCl3 3 6H2O in Milli Q water solution were prepared. Both solutions were taken to 50 C, mixed, and poured into a boiling solution of 200 mL (1 M) NaOH under vigorous stirring. The boiling is maintained for 30 min, and the solution is cooled to room temperature without stirring. After five water-cleaning stages by magnetic sedimentation and elimination of the supernatant, the ferrofluid is treated with an oxidative reaction to passivate the surface by redispersion in 30 mL of 2 M HNO3 solution with 0.35 M Fe(NO3)3 3 9H2O and heating to 100 C for 45 min with continuous stirring. The resulting product is magnetically sedimented overnight. The supernatant is then decanted and substituted by 100 mL of Milli Q water. Once synthesized, the cobalt ferrite nanoparticles (crystalline structure confirmed by XRD, not shown) were driven onto the goethite nanorods by using the well-established method known as the layer-by-layer (LbL) technique for the assembly of nanoparticles on colloidal templates. This technique permits the stepwise adsorption of various components as the layer growth is governed by their electrostatic attraction and allows the formation of multilayer shells with nanometer precision.25
28 Cationic goethite nanorods were first primed with three layers of polyelectrolytes, PSS/PDADMAC/PSS, as this provides a uniform negatively charged and smooth surface that assists subsequent uniform deposition of the cationic cobalt ferrite nanoparticles synthesized. The polyelectrolyte-coated nanorods were prepared by taking the goethite nanorods precipitated from the 10 mL suspension (4 mM), adding 15 mL of PSS (1 mg mL
1, containing 0.5 M NaCl) solution, waiting 15 min for absorption, and then removing excess PSS by four repeated centrifugation/ wash cycles. The centrifugation was done with a speed of 3800g for 10 min. PDADMAC (1 mg mL
1, containing 0.5 M NaCl) was deposited onto PSS-coated nanorods similarly, using the same conditions, followed by the next PSS layer. CoFe2O4 13992

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The Journal of Physical Chemistry C nanoparticles were deposited on the goethite nanorods by adding 400 μL of the CoFe2O4 sol (∼0.05 M) to the precoated nanorods (as prepared and dispersed in 10 mL of water) and redispersed up to 70.4 mL. The nanoparticles were allowed to adsorb for 20 min, and excess nanoparticles were removed by five repeated centrifugation (1000g, 10 min)/wash cycles. The final sample was dispersed in 25 mL of water. Silica Coating and Gold Functionalization. A 1.2 mL portion of the goethite nanorod initial solution was diluted up to 250 mL, and 1.25 mL of APS (10
3 M) was added under continuous stirring and left for 1 h. Subsequently, 10 mL of the sodium silicate solution (1% wt) and 50 μL of HCl (10
3 M) were added (dropwise). This mixture was left for 24 h and consequently washed and redispersed in 15 mL of ethanol. Amino groups were attached to the surface of the silica-coated goethite nanorods by refluxing them in ethanol in the presence of (3-aminopropyl)trymethoxysilane (APS). A 300 μL portion of APS was added to the 15 mL solution containing the silica-coated goethite nanorods (from the previous synthetic step) and diluted up to 100 mL of ethanol. After that, the solution was refluxed for 60 min. The sample was centrifuged and dispersed in 40 mL of ethanol twice and finally centrifuged and dispersed in 20 mL of aqueous solution. The 2
3 nm gold clusters were synthesized following the method published by Duff et al., mixing 1.5 mL of NaOH (0.2 M) and 1 mL of tetrakis(hydroxymethyl) phosphonium chloride solution (THCP) (0.067 M) in 45.5 mL of water under magnetic stirring.29 Subsequently, 2 mL of an aqueous solution of HAuCl4 (2.5  10
4 M) were added while vigorously stirring. A 20 mL portion of silica-coated goethite nanorods (amino-functionalized) (as prepared) was mixed with 20 mL of the freshly prepared gold cluster solution. An adsorption time of 20 min was allowed in order to guarantee the gold cluster deposition. The excess of gold seeds was removed by three repeated centrifugation/wash cycles, and the composites were redispersed in 2 mL of pure water. Characterization. Samples for TEM were prepared by depositing them upon a carbon-coated copper grid. TEM and HRTEM measurements were performed on a Philips CM20 microscope operating at 100 kV and on a JEOL 1010 operating at 200 kV. X-ray diffraction patterns were collected using a Bruker SMART CCD 1000 diffractometer. Magnetic measurements were performed using Quantum Design SQUID Magnetometry. The ζ potential was measured in a ZETASIZER NANO-ZS device (Malvern Instruments).

’ RESULTS AND DISCUSSION The goethite (R-FeOOH) structure consists of an hcp array of anions (O2
and OH
) stacked along the [010] direction with Fe(III) cations occupying half the octahedral interstices within a layer. These Fe cations are arranged in double rows separated by empty sites also positioned in double rows that appear as grooves at the crystal surfaces.15 For our goethite nanoparticles, the major morphological feature corresponds to the rodlike shape with relatively low polidispersity (Figure 1a,b). The average size distribution corresponds to 118 ( 20 nm (length) and 12 ( 4 nm (diameter). Figure 1c shows X-ray diffraction pattern (XRD) data of the goethite nanorods analyzed. All peaks correspond to those expected for goethite, and no other phases are observed. Additionally, HRTEM analysis and selected area electron diffraction (SAED) (Figure 1d and inset, respectively) confirmed the R-FeOOH crystalline structure. The orthorhombic standard

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values of the unit cell dimensions, a = 0.99 nm, b = 0.30 nm, and c = 0.46 nm, were used,15 and interplanar distances were determined by Fourier transform analysis, obtaining d[101] = 0.41 nm, d[010] = 0.30 nm, and d[111] = 0.24 nm. These goethite nanostructures were prepared by modifying the well-known synthesis of magnetite nanoparticles, reported by Massart,23 based on the precipitation of a mixture of Fe(II)/ Fe(III) solutions with alkali, followed by oxidation. There are several parameters whose influence becomes very important in the production of anisotropic nanostructures. In the case herein reported, we were concerned about the pH at which the coprecipitation takes place. The process mentioned serves to produce yellow, orange, black, and red iron oxides of different compositions, as a function of the Fe(II)/Fe(III) molar ratio and the pH.15 For example, if the pH falls toward 4 or rises toward 12, goethite (R-FeOOH) is predominant. However, below 4 and above 14, hematite (R-Fe2O3) can appear. pHs between 9 and 12 lead to the formation of magnetite (Fe3O4). In this experiment, the Fe(II)/Fe(III) molar ratio is not changed but the pH falls toward 4 as the NH4OH molarity is reduced from 0.7 (Massart’s method) to 0.4. Although the concentration of hydrochloric acid has also been reduced (from 2 to 1.3 M in the initial FeSO4 solution), its effect is not that pronounced in the final medium where precipitation occurs. In these conditions, the formation of intermediate greenishblue, mixed Fe(II)
Fe(III) phases, called green rust, predominates since oxidation takes place under slightly acidic conditions. This solid green rust corresponds to double layer hydroxide salts in which positively charged octahedral Fe hydroxy layers are linked by interlayer anions (in this case, Cl
and/or SO42-). They form either by direct precipitation from the Fe(II) salt solution upon oxidation once their solubility product is exceeded or by interaction between 2-line ferrihydrite precipitated initially and Fe(II) in solution. This reaction is accompanied by production of an equivalent amount of protons and the loss of Fe(II) and the respective anions (Cl
and SO42
) from solution. Once the [Fe2+] falls below a critical level, further oxidation leads to the decomposition of green rust and the formation, in this case, of goethite as the unique crystalline phase.15 Indeed, the conditions concerning low temperatures, low oxidation rates, and the presence of sulfate ions have a goethite-promoting effect. The anisotropic shape of the goethite particles is governed by the symmetry of the unit cell (orthorhombic) that always leads to elongated crystals along the [010] direction (c axis). This nanoparticle growth follows the crystal habit, as it is the case for most particles prepared in solution and in the absence of molecules with preferential absorption on some crystal faces. The particle size and acicular or rodlike shape morphology depend on the rate at which the different faces grow. This is controlled by the parent iron solutions (Fe(II) or Fe(III) or a mixture) and the pH media. In general, goethite nanostructures obtained at neutral or acid pH are always smaller than those obtained at high pH. At high pH, acicularity is well expressed due to a rapid preferential growth along the needle axis. In this work, goethite nanorods were favored due to a slower growth at slightly acid pH where hydrolysis is hindered. Moreover, sulfate anions also control the rate of release of Fe species from the Fe
SO4 complex, as in the case of carbonate anions.30
32 Magnetic Properties. Goethite bulk has a Neel temperature of 400 K and is, therefore, antiferromagnetically ordered at room temperature. The ordering behavior is influenced by its comparatively low anisotropy constant as well as by size and shape in 13993

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Figure 1. TEM images (a, b), XRD (c), and HRTEM analysis (d, inset: selected area electron diffraction (SAED)) of the goethite nanorods synthesized.

the case of nanostructures. AFM fine particles are characterized by a very small net magnetic moment, and the demagnetizing effect and hence the shape anisotropy are very small compared with the magnetocrystalline anisotropy.33 Additionally, in AFM nanostructures, the surface is invoked as the origin of uncompensated moments,12 as is indicated in the model proposed by Neel based on the presence of two sublattices, one with spins “up” and the other with spins “down”. The imbalance in the number of both types of spins at the surface is the origin of the net magnetic moment.19 Another theoretical explanation for the presence of a spontaneous magnetization was provided by Dzialoshinskii, who proposed a tilting of magnetic moments toward one another with an identical result.34 This (paramagnetic) contribution due to the noncompensation of the two sublattices can dominate over the AFM contribution itself in the case of nanostructures (as increasing the relative number of surface atoms) and because of the structural disorder that favors the surface spin to be more easily deviated from the AFM alignment by a magnetic field. Figure 2a shows the temperature dependence of the magnetization of the goethite nanorods under zero-field-cooled (ZFC) and field-cooled (FC) conditions, applying fields of 100 and 500 Oe. Irreversibility (i.e., splitting between the FC and ZFC curves) is observed below a field-dependent temperature, which decreases with increasing magnetic field (although, in this case,

the decrease is almost imperceptible). The ZFC and FC curves are irreversible even at temperatures well above the maxima shown in the ZFC, probably due to the (relatively large) size distribution of the nanorods. These ZFC curve rounded maxima are related to the average blocking temperature TB, which shifts to lower temperatures with increasing the field applied (from 45 to 35 K). Essentially, the applied field is lowering the KV energy barrier so that superparamagnetism (SPM) begins at lower T. Another characteristic to be underlined is the fact that the FC curve below TB increases as the temperature decreases, which can be associated with a weakly interacting system of nanorods. It has been inferred that SPM of AFM nanostructures is due to the random arrangement of uncompensated spins, in accordance with the model proposed by Neel, as previously introduced.19 An interesting aspect of the SPM behavior in nanostructures is the nonlinear variation of 1/χ with T (TB < T < TN in the case of an AFM material), where χ is the low-field magnetic susceptibility. Generally, this nonlinear variation in FM materials would be attributed to interparticle interactions, and the particle size distribution would provide additional deviations. In this case, however, due to the very small magnetic moments of the goethite nanorods, interparticle interactions, as reflected by the FC curve below the TB, may be negligible. Figure 2b shows the inverse of the susceptibility (1/χ) that fits in the high-T range the paramagnetic Curie
Weiss law with 13994

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Figure 2. Plots of ZFC
FC measurements (a) and inverse susceptibility 1/χ (b) as a function of temperature and low-field applied.

Figure 3. Hysteresis loops below (5, 10, and 50 K) (a) and above (100, 200, and 250 K) (b) the average blocking temperature TB.

negative θ (θ100 Oe =
170 K, θ500 Oe =
228 K, characteristic of AFM materials). χ of these AFM nanostructures has two sources corresponding to the bulk susceptibility of the randomly oriented goethite nanorods and to the second one that develops below the blocking temperature. It arises from the couple exerted by the applied field on the AFM nanostructures with even numbers of ferromagnetic spin planes. The result is a progressive rotation of the AFM axis from one end (edge) of the nanorod to another.35 This second contribution is strongly dependent on temperature, but since fitting 1/χ in the high-temperature range, it becomes negligible, allowing the AFM bulk contribution to become the predominant one. These features can be checked on the goethite nanorods because the paramagnetic surface component appears superimposed on the AFM behavior. SPM behavior has been reported in certain systems of AFM nanostructures.36 In such systems, the magnetization M for TB > T > TN usually follows the modified Langevin function (L)   μ H ð1Þ M ¼ M0 L R + χa H kB T where M0 is the saturation magnetization, μR is the magnetic moment per nanostructure, H is the magnetic field, kB is the Boltzmann constant, χa is the AFM susceptibility of the core, and L(x) = coth x
1/x is the Langevin function (x = μRH/kBT).

The hysteresis loops included in Figure 3 (at temperatures of T = 5, 10, and 50 K, below the average blocking temperature TB and at T = 100, 200, and 250 K, above) reflect the two contributions: the one responsible for the large magnetization values observed at low temperature due to the noncompensation of surface spins, which is superimposed on the other common AFM contribution (Ma = χaH), responsible for the nonsaturation of the magnetization at high field values (bulk susceptibility of the randomly oriented AFM nanorods). Because of the reduced nanometer size of the goethite nanorods, the paramagnetic surface contribution (that develops below the blocking temperature) becomes more important while the contribution of the AFM core diminishes, underlining the structural disorder at the surface that determines the paramagnetic behavior observed in the magnetization curves. Figure 3a includes the hysteresis loops obtained at temperatures of T = 5, 10, and 50 K, below TB. The coercive field HC decreases with increasing temperature and becomes almost negligible above 50 K, in agreement with the blocking temperature inferred from ZFC curves shown previously. The coercive field is also an indication of the extent of mentioned imbalance of spins occurring at the surface, since the number of atomic moments uncompensated is not negligible. The variation of coercivity with temperature is also indicative of the distribution of sizes. An assembly of noninteracting, identical, and aligned uniaxial goethite nanorods with a volume V and an anisotropy 13995

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Figure 4. (a) Plots of [(M
χaH)/M0] vs H/T for the goethite nanorods at temperatures of T = 100, 200, and 250 K in the range between TB and TN. (b) Magnetic hysteresis loops at T = 6 K (zero-field-cooled (ZFC) and field-cooled (FC) (5T)).

Table 1. Temperature-Dependent Parameters M0, μR, and χa Fitted from eq 1 T (K)

M0 (emu/g)

χa (10
5 emu/g Oe)

μR (103 μB)

100

0.57

0.99

5.5

200

0.55

0.84

7.9

250

0.54

0.78

8.4

K would have zero coercivity above the blocking temperature TB (TB = 45 K (H = 100 Oe), TB = 35 K (H = 500 Oe)). In contrast, in the sample, the spread in K and V leads to a range of TB, and the existence of a finite coercivity at 50 K is instead due to the presence of the small number of larger volumes within the distribution. The magnetization of the nanorods, measured at 100, 200, and 250 K (all temperatures in the range between TB and TN), was, therefore, modeled according to the modified Langevin law (eq 1), which accounts for the uncompensated spins at the surface superposed to the AFM susceptibility. The AFM contribution (Ma = χaH) was linearly fitted in the large magnetic field region, and it was subtracted so that the magnetic behavior associated with the uncompensated surface spins can be shown. Neel predicted that small enough AFM particles should exhibit SPM. For superparamagnetic samples, the magnetization should be given by the Langevin equation,37 but the magnetization curves versus H/T do not superimpose, as is usually observed for very small FM and FiM nanoparticles. Subtracting, however, the linear portions and normalizing by M0, in order to plot [(M
χaH)/M0] versus H/T data fit on a universal curve, as shown in Figure 4a, at the temperatures indicated in the range between TB and TN. This points to an SPM behavior at these temperatures above the peak ordering temperature and implies again that interparticle interactions are negligible. The reduced magnetization curves do not perfectly superimpose when plotted as a function of the reduced variable H/T, but the nanorod size distribution can be the reason for the nonscaling. However, from these temperatures and magnetic field variations of the magnetization, the fitted parameters, M0, μ (in our case, μR), and χa, from eq 1 were found to be temperaturedependent parameters (see Table 1). Both M0 and χa decreased and μR increased, as T increased.36 These temperature variations or dependencies suggest, therefore, that the SPM of AFM

materials (also termed superantiferromagnetism35) at the nanoscale is more complicated than that observed in FM or FiM materials. Additionally, one should also note that the estimation of χa was obtained by fitting the experimental data of M versus H, which have some inherent uncertainty because the system is far from saturation. This point was indeed emphasized in a recent paper by Silva et al.20 Exchange bias is a magnetic effect that manifests itself as a shift along the field axis and an increase in loop width. Usually, at least two material components of different magnetic properties are combined. In this case, the structural disorder prevalent at the surface of the goethite nanorods strongly influences the magnetic properties, as already seen, and appears to be the main source of exchange bias. Figure 4b shows hysteresis curves collected at 6 K (zero-field-cooled (ZFC) and field-cooled (FC) (5 T)) displaying this shift along the direction of the cooling field with coercivities of HC = |HC1
HC2|/2 = 159 Oe (ZFC) and HC = 159.4 Oe (FC) and an exchange bias field of HE =
(HC1 + HC2)/2 = 30 Oe (in the FC curve). This feature can again be explained by the exchange coupling between the uncompensated spins at the surface and the AFM cores. Suitable AFM Substrates. Nanoscience and nanotechnology have reached the stage of development where the subject of most investigations is not individual nanoparticles or nanorods, but rather, systems of much greater complexity. In many cases, the main challenge is the assembly of nanomaterials into a complex hierarchical or supramolecular system such that the new organization of the nanostructures into complex assemblies results in a substantial improvement in their functional properties because many processes are strongly dependent on the organization, complexity, and interconnectivity of nanoscale components of superstructures.38 Consequently, we report three examples in which the suitability of the goethite nanorods is demonstrated, in order to improve and/or modify their magnetic behavior, in order to isolate the rods to study the exchange interaction effects in the final magnetic properties, or in order to address a second functionality (Figure 5). In the first case and as indicated in the Introduction, FM or FiM nanoparticles attached to an AFM material have been shown to improve hierarchically the magnetic properties of the final composite. In line with that, we have driven cobalt ferrite nanoparticles on the surface of uniform and negatively charged goethite nanorods, exploiting electrostatic interactions. For that, 13996

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Figure 5. TEM images of the goethite nanorods as suitable AFM substrates onto which cobalt ferrite nanoparticles (a), a uniform shell of silicon oxide (b, c), and homogenously distributed gold clusters (c, d) were deposited.

the goethite nanorods were first coated with three layers of polyelectrolytes that ensure this electrostatic self-assembly. Figure 5a shows a TEM image of the successful coating, attained after the optimization process in terms of the cobalt ferrite/ goethite nanorod ratio. The control and tuning of the magnetic properties in the final composites depends on the distance in between the two types of magnetic materials, controlled by the number of layers of polyelectrolytes used. In the case included in Figure 5a, there are three layers of polyelectrolyte between the FiM cobalt ferrite nanoparticles and the AFM goethite nanorods. Concerning the second example included, the superparamagnetic relaxation of nanoparticles is very sensitive to interparticle interactions. The magnetic moments of AFM nanostructures are typically much smaller than those of FM or FiM nanostructures, and therefore, the dipole interactions are insufficient to significantly affect the superparamagnetic relaxation.39 In fact, the dipole interactions between AFM nanoparticles in close proximity are typically so small that the related critical temperature is well below 1 K. Nevertheless, samples of uncoated AFM nanoparticles were found to change their magnetic dynamics drastically in case of aggregation, increasing the temperature at which the particles become superparamagnetic more than

100 K.17 It has been concluded that exchange interaction between surface ions of neighboring particles is responsible for the effect. This implies that the particles are in such a close proximity that the electronic wave functions of atoms at the interfaces overlap. As already introduced, exploiting exchange interactions between nanostructures of different magnetic materials was demonstrated to be a potential method for controlling the final properties, but establishing previously the strong exchange coupling between nanoparticles becomes, therefore, the major challenge.40 Consequently, silica-coated goethite nanorods provide a great approach to isolate the nanorods, in order to check the differences in the magnetic properties that stem from exchange coupling. On the basis of this statement, the nanorods were coated with an outer layer of silica (∼10 nm average thickness), additionally improving their colloidal and chemical stability. Figure 5b reflects this homogeneous coating of goethite nanorods with silica. Moreover, evolution of the ζ potential versus pH was measured, with a 0.01 M concentration of KNO3 at different pH values between 2 and 11. The isoelectric or zero point of charge of the goethite suspension shifted from 6.7 (without silica) down to 3.6 in the case of the silica-coated goethite nanorods, close to the isoelectric point of silica 13997

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Figure 6. Hysteresis loops (T = 5 K) of the cobalt ferrite nanoparticles, the goethite nanorods, and the composites obtained by assembling the metal transition ferrite nanoparticles onto the AFM nanorods (a). ZFC
FC measurements of the goethite nanorods before and after being coated with silica (H = 100 Oe) (b).

(pH(I) ∼ 3),41 confirming again the homogeneous coating. The ζ potential of these water-stable suspensions is negative at pH = 7 and allowed the calculation of the surface charge to be
30 mV, assuring long-term stability. The third example of the suitability of the goethite nanorods involves a very useful strategy for imparting optical or catalytic properties at the nanoscale that implies the integration of a noble metal into the final structure. Silica-coated goethite nanorods became functionalized by attaching 2
3 nm gold clusters on their surface through an electrostatic binding with amino groups previously anchored.42 Smaller clusters instead of gold nanoparticles characterized by their associated localized surface plasmon band were chosen to be deposited because they provide a more practical and highly general approach for adding catalytic or optical addressability by virtually any type of gold nanostructure that can be grown starting from these metallic clusters. The metallic loading onto the magnetic composites directly depends on the initial quantities of APS used for the amino groups functionalization (see the Experimental Section), which guarantees the homogeneous distribution of the gold clusters, as can be seen in the TEM images included in Figure 5c,d. The closer view in Figure 5d shows the uniform attachment of clusters. Figure 6 includes the initial magnetic characterization of the first two cases, goethite
cobalt ferrite nanocomposites and silica-coated goethite nanorods. Figure 6a includes the hysteresis loop obtained at T = 5 K for the goethite
cobalt ferrite composites, compared with the goethite nanorods and cobalt ferrite nanoparticles separately. Although the complete magnetic characterization and the determination of the exact magnetismrelated behavior are left for future research, one can appreciate in this comparison the increase in coercivity, directly related to the established AFM
FiM interface. This effect can be definitively improved and controlled by considering and tuning the distance between both types of magnetic materials, in the present case given by the three layers of polyelectrolyte. Figure 6b compares the temperature dependence of the magnetization of the goethite nanorods, before and after being coated with the ∼10 nm thick silica shell. As previously pointed out, the magnetic moments of AFM nanostructures are typically much smaller than those of FM or FiM ones, and therefore, the dipole interactions are insufficient to significantly affect the superparamagnetic relaxation.39 In the case of superparamagnetic nanoparticles of FM or FiM materials, the maxima of the

ZFC curves related to the mean blocking temperatures (TB), as well as the splitting points (Tirr) between the ZFC and FC curves, were reported to shift toward lower temperatures. Simultaneously, the FC magnetization curve below TB increases its slope, quantitatively indicating the decrease in strength of the interparticle interaction.43,44 However, the direction of the TB shift toward lower or higher temperature has implied certain controversy. Although most experimental studies have reported a shift toward lower temperature as the distance between the nanoparticles is increased, Morup and Tronc, by means of M€ossbauer spectroscopy, reported a shift toward higher temperature in a regime of weak dipolar interparticle interactions.45 Analogous, in this case of very weakly interacting goethite nanorods, the TB shifts to higher temperature (indicated by black arrows) as the distance between nanorods is increased from close to 0 to up to ∼20 nm. A change in the average size distribution of the nanorods cannot justify this shift because both naked and coated goethite nanorods come from the same synthetic batch, neglecting this possible contribution. Dipolar interactions were not particularly important in the case of goethite nanorods nor in the case of the coated ones, and therefore, this behavior must be related to the paramagnetic contribution at the surface and their interactions with the surroundings.

’ CONCLUSION Goethite nanorods can be produced by wet-chemistry methods in order to obtain suitable antiferromagnetic susbstrates. Taking into account their characteristic magnetic behavior, these R-FeOOH nanorods offer a net magnetic moment at the surface due to the large surface/volume ratio. As a proof of concept, we have developed mix multifunctional systems to study the promising interactions of these AFM materials at the nanoscale. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT V.S. acknowledges the financial support from the Ramon y Cajal (Ministerio de Ciencia e Innovacion, Spain) Program. The 13998

dx.doi.org/10.1021/jp201490j |J. Phys. Chem. C 2011, 115, 13991–13999

The Journal of Physical Chemistry C Spanish Ministerio de Ciencia e Innovacion, the Xunta de Galicia, and the Universidade de Vigo have supported this work under projects MAT2008-06126, 10PXIB312260PR and 2010/ 78 (Modalidade Emerxentes), and the RyC Startup funds, respectively. S.H.M. acknowledges financial support from the Brazilian agencies CAPES, CNPq, and FAPESP. J.R. also acknowledges the partial financial support from the Large Collaborative Project FP7-214685-2 MAGISTER
MAGnetIc Scaffolds for in vivo Tissue EngineeRing. The authors are indebted to B. Rodríguez-Gonzalez for HRTEM analysis.

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