Nanostructure-Related Magnetic Properties of Various Mesoporous

Article pubs.acs.org/JPCC

Nanostructure-Related Magnetic Properties of Various Mesoporous Cobalt Oxide and Cobalt Ferrite Spinel Phases Stefanie Haffer,† Till Walther,‡ Roberto Köferstein,‡ Stefan G. Ebbinghaus,*,‡ and Michael Tiemann*,† †

Faculty of Science, Department of Chemistry, University of Paderborn, 33098 Paderborn, Germany Institute of Chemistry, Martin-Luther-University Halle-Wittenberg, 06099 Halle, Germany

‡

S Supporting Information *

ABSTRACT: Nanostructure-related magnetic properties are investigated systematically for various mesoporous cobalt oxide (Co3O4) and cobalt ferrite (CoFe2O4) spinel phases. Synthesis of the materials by nanocasting offers the opportunity to obtain materials which are different from each other with respect to both specific surface area and crystallite size. As a result, the respective contributions of two types of interfaces, namely, “solid−gas” and “solid−solid” interfaces, to the magnetic ordering can be distinguished. Structural characterization of the porous materials by X-ray diffraction, N2 physisorption, and electron microscopy as well as investigation of the magnetic behavior (field-dependent magnetization and temperature-dependent susceptibility) are presented.

■

INTRODUCTION The spinels Co3O4 and CoFe2O4 are of interest because of their broad applications in advanced technologies, e.g., in gas sensors1−3 or as electrode materials in the anodic evolution of oxygen and chlorine.4 CoO/Co3O4 layers have been used in solar absorber surfaces.5 CoFe2O4 is a hard magnetic material with moderate saturation magnetization and good chemical stability and can therefore be applied for magnetic recording applications.6 It also acts as a catalyst, for example in the decomposition of methanol7 and in alkylation reactions.8,9 Co3O4 crystallizes in the normal cubic spinel structure. The Co2+ ions occupy the tetrahedral sites and the Co3+ ions the octahedral sites of the cubic close packed oxygen framework. It is an antiferromagnetic material with a Néel temperature (TNéel) of about 40 K. The octahedrally coordinated Co3+ ions have a total spin of S = 0 due to their (t2g)6 electronic configuration. In contrast, the tetrahedral Co2+ ions ((e)4(t2)3 configuration) bear a spin of S = 3/2. At TNéel, a phase transition from the paramagnetic high-temperature (space group Fd3̅m, no. 227) to the antiferromagnetic low-temperature modification (space group F4̅3m, no. 216) occurs, in which the spins of the 4a (0, 0, 0) and 4c (1/4, 1/4, 1/4) sites order antiparallel.10 The structure of CoFe2O4 lies between a normal spinel and an inverse spinel type and can be described by the formula T (Co1−xFex)O(CoxFe2−x)O4, where T and O represent the tetrahedral and octahedral sites of the spinel structure, respectively. The fraction of octahedral sites occupied by Co2+ ions, expressed by the inversion parameter x, depends on the preparation method. For slowly cooled bulk or submicrometer samples, x is large (≈0.95); in quenched or nanosized samples, x can take much lower values down to ≈0.75.11 CoFe2O4 is a ferrimagnet with a Curie temperature of 793 K12 in which the spins of the two ferromagnetic sublattices (formed © 2013 American Chemical Society

by the A and B sites) are opposed. However, the magnetizations of these two sublattices are not equal, resulting in a remaining magnetization at zero field. Ferrimagnetic materials show hysteresis loops and a saturation of the magnetization at a sufficiently high magnetic field, whereas the magnetization of an antiferromagnet increases linearly with the applied field. However, nanoscale cobalt-based spinel phases, in particular Co3O413−16 and CoFe2O4 (cobalt ferrite),16,17 have been shown to reveal different magnetic behavior than the respective bulk phases because of the increasingly relevant role of the surface. Therefore, we investigated the structure-related magnetic behavior of various mesoporous Co 3O 4 and CoFe2O4 samples in some detail, particularly with respect to their surface-to-volume ratio. Mesoporous Co3O4 and CoFe2O4 materials with variable nanostructural properties were obtained by varying the conditions during their synthesis by nanocasting. Nanocasting is a versatile synthesis method for a large number of ordered mesoporous metal oxides. In this procedure a mesoporous solid material serves as a structure matrix in a replication process.18−23 The pores of the matrix, e.g., silica or carbon, are infiltrated with a suitable precursor species, such as a metal salt, by various techniques;24 this can be accomplished using (more or less) concentrated solutions25−28 or (solventfree) melts29−31 of the respective precursor compound. The precursor is then thermally converted into the desired product. Because the conversion of the metal nitrate to the respective oxide is accompanied by substantial volume shrinkage,32,33 the cycle of impregnation and subsequent oxide formation is usually repeated several times ensuring a more sufficient pore filling in the matrix. Finally, the structural mold is selectively Received: September 10, 2013 Revised: October 25, 2013 Published: October 25, 2013 24471

dx.doi.org/10.1021/jp409058t | J. Phys. Chem. C 2013, 117, 24471−24478

The Journal of Physical Chemistry C

Article

Figure 1. Transmission (left) and scanning (right) electron microscopic images of mesoporous Co3O4 replicated from small-pore KIT-6 silica.

The resulting solid product was filtered off and washed with deionized water. For the removal of P-123 block copolymer the dried product was calcined under air at 823 K for 6 h (heating rate 2.5 K min−1). Mesoporous Cobalt Oxide (Co3O4). Mesoporous cobalt oxide was obtained from cobalt nitrate by structure replication using a silica matrix (SBA-15 or KIT-6, respectively). The matrix was impregnated with a saturated cobalt nitrate solution (15 g of Co(NO3)2·6H2O, Merck, in 10 mL of deionized water); the amount of the aqueous precursor solution corresponded to the pore volume of the utilized structure matrix. After drying at 333 K, the sample was thermally treated in air at 573 K for 2 h (heating rate 2 K min−1) to convert the cobalt nitrate into cobalt oxide. For a more sufficient pore filling, these steps of impregnation, drying, and oxide formation (denoted as an “impregnation−oxide formation cycle” in the following) were repeated one or two times. Finally, the silica matrix was removed by repeated leaching with sodium hydroxide solution (2 mol L−1) for 4 h at room temperature; the sample was finally washed with deionized water. Traces of residual silica were below 1% according to EDX analysis. Mesoporous Cobalt Ferrite (CoFe2O4). Mesoporous cobalt ferrite was synthesized by a similar procedure as described above for cobalt oxide; only the utilized precursor and the thermal treatment for the oxide formation were different. Here, the pores of a KIT-6 silica matrix were infiltrated with a precursor solution (10 mL of deionized water) of cobalt nitrate (8 g of Co(NO3)2·6H2O, Merck) and iron nitrate (22 g of Fe(NO3)3·9H2O, Sigma-Aldrich), with the molar ratio of Co:Fe = 1:2, followed by drying and subsequent conversion into the oxide by thermal treatment in air at 873 K for 2 h (heating rate 1 K min−1). Traces of residual silica were below 1% according to EDX analysis. Characterization Methods. Powder X-ray diffraction (PXRD) was carried out on a Bruker AXS D8 Advance diffractometer using Cu Kα radiation (40 kV, 40 mA). The step size for low-angle measurements (2θ < 5°) was 2θ = 0.0075°; for wide-angle measurements (20° < 2θ < 80°), the step size was 0.02° with 3 s counting time per step in both cases. Nitrogen physisorption analysis was conducted at 77 K with a Quantachrome NOVA 4000e instrument. Prior to measure-

removed, for instance by etching with sodium hydroxide solution or hydrofluoric acid in the case of a silica matrix, or by thermal combustion in the case of a porous carbon matrix. This synthesis approach yields a large variety of mesoporous materials, especially metal oxides, such as Co3O4,3,27,28 In2O3,34,35 SnO2,30,36 or Al2O3,37,38 as well as mesoporous carbon.39,40 Most recently several ternary metal oxides have been prepared by nanocasting, including CoFe 2 O 4 , 17 NiFe2O4,41 and Cu/CeO2.41,42 The products possess ordered mesopore systems, high specific surface areas, and often high thermal stability. In particular, varying the synthesis parameters offers an opportunity to control important structural properties such as pore size, pore wall thickness, and specific surface area, as well as particle size and morphology35,43−46 Here we present a systematic study on the magnetic behavior (field-dependent magnetization and temperature-dependent susceptibility) of various mesoporous spinel materials based on antiferromagnetic Co3O4 and ferrimagnetic CoFe2O4. The samples are different from each other with respect to their nanostructural properties. In particular we show that such terms as “surface area” and “size” of the nanomaterials need to be differentiated with caution, because both the BET surface area (“solid−gas interface”) and the “solid−solid interface” play important individual roles. Likewise, “crystallite size” may not necessarily be the same as “particle size”. These differences must be accounted for when the magnetic behavior of nanostructured materials is assessed.

■

EXPERIMENTAL SECTION Synthesis Procedures. Mesoporous Silica. Mesoporous SBA-15 and KIT-6 silica phases serving as structure matrices for the synthesis of mesoporous metal oxides were prepared by a modified literature procedure:47,48 Pluronic P-123 triblock copolymer (Sigma-Aldrich; 8.0 g) was dissolved in a mixture of deionized water (240 mL) and hydrochloric acid (32%; 24.6 mL) by stirring at 308 K for 24 h. In the case of KIT-6, nbutanol (9.9 mL) was added, followed by further stirring for one hour. After addition of tetraethyl orthosilicate (≥99.0%, Merck; 17.0 mL), the mixture was stirred at 308 K for another 24 h. The resulting gel was transferred to a Teflon-lined autoclave and kept for 24 h at 353 K and at 413 K, respectively. 24472



Article

Figure 2. Transmission (left) and scanning (right) electron microscopic images of mesoporous Co3O4 replicated from large-pore SBA-15 silica.

ment the samples were degassed at 393 K for 24 h. Pore size evaluation was carried out by nonlocal density functional theory (NLDFT)-based analysis. Transmission electron microscopy was performed with a Philips CM30-ST microscope. Scanning electron micrographs were recorded with a HREM EDX Leo Gemini as well as a Zeiss Neon 40 with CrossBeam. Magnetic measurements were carried out on a Quantum Design PPMS 9 system. The samples were enclosed in gel capsules whose small contribution to the measured magnetic moment was subtracted before data evaluation. The field-dependent magnetization was measured with a magnetic field cycling between −90 and +90 kOe. For the temperature-dependent magnetization the samples were cooled to 10 K under zero-field conditions before the magnetization was measured in the range from 10 to 150 K at an applied field of 500 Oe.

Figure 3. Field-dependent magnetization (T = 10 K) of four mesoporous Co3O4 samples replicated from various silica matrices, as well as bulk-phase Co3O4. The inset shows the overall measurement range, displaying slight differences in the slopes of the curves.

■

RESULTS AND DISCUSSION Four different mesoporous silica materials were used as structural molds for the synthesis of mesoporous Co3O4. Two of these silica phases possess the same average pore diameter of 7.0 nm, the other two have pores of 9.8 nm average diameter. For each of these two pore sizes two distinct pore systems were accomplished: in the case of SBA-15 silica materials linear mesopores are arranged in a two-dimensional hexagonal symmetry (p6mm),47 while KIT-6 materials have strongly branched mesopore systems in a cubic arrangement (Ia3d̅ ).48 As a result, the four Co3O4 replicas are different from each other in a similar way. Electron microscopic images of two representative products (with both pore symmetries) are shown in Figures 1 and 2. The structural parameters of all samples (all four silica matrices and respective Co3O4 replicas), including powder X-ray diffraction and nitrogen physisorption analysis data of the Co3O4 replicas, are shown in the Supporting Information section (see Figures S1−S4 and Table S1). Figure 3 shows the specific magnetization versus applied magnetic field (T = 10 K) of the four mesoporous Co3O4 samples, as well as bulk Co3O4 (Alfa Aesar), respectively. The four porous samples were prepared from variable silica structure matrices by the same procedure, resulting in deviating BET surface areas, as listed in Table 1. Slight differences in the slope of the magnetization curves occur, as visible in the inset of Figure 3. The differences are correlated with the respective

Table 1. BET Surface Areas and Crystallite Sizes for the Mesoporous Co3O4 Materials Replicated from Various Silica Matrices and for Bulk-Phase Co3O4 Co3O4 nanocast from:

BET surface area (m2 g−1)

crystallite size (nm)

small-pore KIT-6 silica small-pore SBA-15 silica large-pore KIT-6 silica large-pore SBA-15 silica (bulk Co3O4)

130 101 73 80 10

17 19 15 16 57

BET surface area: for larger surface areas steeper slopes, corresponding to higher magnetic moments, are observed. This trend is confirmed by the fact that the bulk-phase sample (exhibiting the smallest surface area) shows the lowest slope. This correlation stands to reason because a disruption of the antiferromagnetic ordering is to be expected in the surface-near region (also denoted as “deadlayer”), as has been discussed for various nanostructured Co3O4 materials,13 including mesoporous ones.14,16 An antiferromagnet can be described by two magnetic sublattices with opposed spins resulting in a complete magnetic compensation. However, the antiferromagnetic compensation remains incomplete at the surface, where the neighbors with opposite spins are missing. In materials with 24473



Article

antiferromagnetic ordering just as much as the BET surface areas (i.e., solid−gas interfaces). Indeed, the solid−solid interfaces, if normalized to the sample masses, are approximately of the same order of magnitude. (This was derived by a simple geometric model assuming spherical shape of nonporous crystallites with a bulk-phase density of 6.1 g cm−3.) These differences may have an impact on the magnetic behavior in addition to the differences in BET surface area, although the latter seem to be more dominant for these measurements. Still, the contribution of the solid−solid interfaces should be investigated in more detail, which will be attempted in the following. To study this impact we have chosen the porous Co3O4 sample prepared from cubic KIT-6 silica with small mesopores (7.0 nm); this Co3O4 material has a BET surface area of 130 m2 g−1 (see above). In addition, we prepared two more samples from the same silica matrix but under different experimental conditions, namely by varying the number of impregnation− oxide formation cycles in the same way as was described before.35 The structural parameters for these samples including the structure matrix are shown in the Supporting Information (see Figures S5 and S6 and Table S2). As a result, these two samples have approximately the same BET surface area. However, the crystallite sizes are different (according to Scherrer analysis), as shown in Table 2. Figure 5 shows the

high surface-to-volume ratios this effect becomes much more significant. The remaining spins at the surface result in a paramagnetic moment or even in weak ferromagnetism.49,50 In fact, a closer inspection of the mesoporous Co3O4 samples shows small hysteresis loops with coercivities between 188 and 329 Oe as a result of the ferromagnetic ordering at the surface. In contrast, bulk-phase Co3O4 exhibit coercivities of only 70 Oe (see Figure S10 of the Supporting Information). The formation of small hysteresis loops was also found in nanoscale Co3O4 samples.50,51 Here we find that the degree of deviation from bulk-like behavior is indeed directly related to the surface-tovolume ratio of the respective sample. In addition, it seems generally reasonable to assume a different chemical composition at the surface, such as hydroxyl groups, originating, e.g., from the synthesis method, such as etching with NaOH. The same clear trend is observed for the Néel temperatures of said samples. This temperature marks the transition from antiferromagnetic to normal paramagnetic behavior, as mentioned above. For the bulk-phase Co3O4, a value of TNéel = 40 K is indicated in the literature,10 although some authors have reported lower values.13 Figure 4 shows the temperature-

Table 2. BET Surface Areas and Crystallite Sizes for the Co3O4 Replica Materials Obtained by One, Two, and Three Impregnation−Oxide Formation Cycles and for Bulk-Phase Co3O4 Co3O4 nanocast from small-pore KIT-6 by: 1 impregnation−oxide formation cycle35 2 cycles 3 cycles (bulk Co3O4)

Figure 4. Temperature-dependent molar susceptibility for the same Co3O4 samples as in Figure 3 (see legend there). All samples were measured under zero-field-cooled (ZFC) and field-cooled (FC) conditions (H = 500 Oe). For reasons of clarity the curves are shifted. Correlation of the Néel temperature and the BET surface area are shown in the inset.



126

12

126 130 10

14 17 57

magnetization of these three mesoporous Co3O4 samples. Again, the bulk-phase sample is shown for comparison. Also for these samples, the slopes of the magnetization curves are different. Here, contrary to the former samples (discussed above), the difference cannot be attributed to the BET surface

dependent molar susceptibility for all samples measured under both zero-field-cooled (ZFC) and field-cooled (FC) conditions (H = 500 Oe). The relative maximum value in each curve (more pronounced for ZFC than for FC measurements) corresponds to the Néel temperature of the respective sample. The obtained values are plotted in the inset of Figure 4 as a function of BET surface area. Again, a clear correlation is observed. The larger the surface-to-volume ratio, the lower the Néel temperature, consistent with what has been described qualitatively before.52 Upon closer inspection, the BET surface area is not necessarily the only structural difference between the samples. The Scherrer analysis of the wide-angle powder X-ray diffraction data (311 peak) indicates slight differences in the coherent scattering domains (denoted in the following as “crystallite size”); between 15 and 19 nm for the porous samples and 57 nm for the bulk material (Table 1). Hence, the samples are different from each other also with respect to the interfaces between adjacent crystallites (solid−solid interfaces). These will contribute to the above-mentioned disruption in

Figure 5. Field-dependent magnetization (T = 10 K) of three mesoporous Co3O4 samples replicated from small-pore KIT-6 by one, two, and three impregnation−oxide formation cycles, as well as bulk Co3O4. The inset shows the overall measurement range, displaying slight differences in the slopes of the curves. 24474



Article

thorough discrimination between “crystallite size” and “particle size” may be compulsory in many situations, including interpretation of magnetic behavior such as in this study. These two discriminations (solid−gas interface vs solid−solid interface; crystallite size vs particle size) are illustrated in Scheme 1.

areas. Instead, the differences in crystallite sizes are obviously the dominating factor in this case. As described before, smaller crystallites result in larger solid−solid interfaces, which contribute to the disruption of antiferromagnetic ordering just as much as the BET surface area (solid−gas interface). This is explicitly confirmed by the data depicted in Figure 5 because all samples have nearly identical BET surface areas (within the limit of accuracy of the BET measurements). For completeness, the temperature-dependent plots of the susceptibilities are shown in Figure 6; the variation of the Néel temperature is consistent with the above-made considerations.

Scheme 1. Schematic Drawing to Illustrate the Differentiation of “Solid−Solid Interface” vs “Solid-Gas Interface” and “Particle Size” vs “Crystallite Size”

On the basis of the results obtained for mesoporous cobalt oxide with antiferromagnetic properties, we have conducted a similar study with mesoporous cobalt ferrite. Bulk-phase cobalt ferrite is a ferrimagnetic material,12 as mentioned in the Introduction. The mesoporous CoFe 2O4 samples were obtained by a similar structure replication procedure as has been described above for the binary cobalt oxide. To study the impact of the nanostructural parameters on the magnetization, two different mesoporous CoFe2O4 samples were prepared from small- and large-pore KIT-6 silica in a way analogous to that used in the above-described investigations. Figure 7 depicts two electron microscopic images of a representative mesoporous CoFe2O4 sample replicated from small-pore KIT-6 silica. The structural parameters for all samples including the structure matrices are shown in the Supporting Information (see Figures S7 and S8 and Table S3). Figure 8 shows the field-dependent magnetization measurements of the respective mesoporous samples as well as a bulk CoFe2O4 sample for comparison. For all three samples, pronounced and symmetric hysteresis loops are observed at 10 K. Actually, the presence of the hysteresis is a clear indication that the synthesis of the ternary CoFe2O4 system was successful. If, instead substantial amounts of by-phases had formed, then these would affect the magnetic behavior. This is demonstrated in Figure S9 of the Supporting Information, which shows the magnetization of a sample with significant contributions of nonferrimagnetic phase(s), apparent from weaker hysteresis and the absence of saturation. Most likely the byproduct is Co3O4, which is difficult to distinguish from CoFe2O4 by X-ray diffraction. The coercivity fields of our CoFe2O4 samples differ only slightly, ranging from 12.0 to 13.5 kOe for CoFe2O4 from small- and large-pore KIT-6 silica, respectively, while for the bulk material an intermediate value of 12.3 kOe was found. In contrast, clear differences were observed for the saturation magnetization and the remanence. All characteristic magnetic parameters are listed in Table 3 with the values of specific BET surface areas and crystallite sizes. The saturation magnetizations (MS) were calculated by a linear fit of M versus 1/H in the high-field regions (+50 to +90 kOe and −50 to −90 kOe), extrapolation to 1/H = 0 (i.e., H = ∞), and averaging.53 The bulk-phase sample exhibits the highest MS value of 3.78 μ B mol −1 (89.9 emu g −1 ). From the antiferromagnetic coupling of the two magnetic sublattices

Figure 6. Temperature-dependent molar susceptibility for the same Co3O4 samples as in Figure 5 (see legend there). All samples were measured under zero-field-cooled (ZFC) and field-cooled (FC) conditions (H = 500 Oe). For reasons of clarity the curves are shifted. Correlation of the Néel temperature and the BET surface area are shown in the inset.

In summary, the data shown so far confirm that the magnetic behavior in nanostructured Co3O4 is different from that in bulk Co3O4. In particular, (i) the slope of the field-dependent magnetization curve is steeper, indicating higher magnetic moments; (ii) the Néel temperature is shifted to lower values; and (iii) a small ferromagnetic contribution evolves. These findings can be explained by disruptions in the antiferromagnetic ordering due to the nanostructural size confinement, as has been reported in earlier studies.13,14,16 Our data corroborate these phenomena by a systematic variation of the nanostructural parameters by fine-tuning the synthesis conditions. Most importantly, it turns out that it is mandatory to distinguish between several aspects of “nanostructure”: as we have shown, the terms “surface” and “size” in nanostructured materials are both ambiguous. The term surface of a nanostructured (e.g., nanoporous) material may either refer to the accessible solid−gas interface (BET surface area) or include the solid−solid interface (between adjacent single-crystalline domains). For many applications, the latter contribution may not play any major role (such as in gas sensing or other sorption or surface-chemistry-based fields). Here, however, solid−solid boundaries must be taken into account just as much as the BET surface area because the magnetic behavior depends on ordering disruption at both kinds of interfaces. Likewise, the size in a nanostructured material may either refer to the size of individual single-crystalline domains (crystallite size) or to the overall size of an entire particle which may or may not consist of more than one crystallite. In other words, each particle may be single- or oligocrystalline. The latter type of particle size is often referred to as “grain size”. It needs to be stressed that 24475



Article

Figure 7. Transmission (left) and scanning (right) electron microscopic images of mesoporous CoFe2O4 replicated from small-pore KIT-6 silica.

solid−gas interface) and the crystallite size (which is related to the solid−solid interface) clearly increase in the order of bulk sample < mesoporous sample from large-pore matrix < mesoporous sample from small-pore matrix. The pronounced correlation between magnetic behavior and surface-to-volume ratio can again be attributed to disruption of the magnetic ordering in the surface-near regions. For CoFe2O4, the reduced magnetic ordering results in lower saturation magnetization. This effect cannot be explained by a lack of interacting neighbors because for a given ion at the surface (e.g., Fe3+) both the ferromagnetically arranged moment (spin of another Fe3+-ion) and the antiferromagnetically arranged moment (spin of a Co2+-ion) are missing. Indeed, it is not the lack but rather the orientation of magnetic moments near the surface that affects the magnetization. A noncollinear spin orientation near the surface of CoFe2O4 particles was reported in the literature.55 This spin-canting at the surface (also called “deadlayer”) causes a progressive reduction of the saturation magnetization.56 In summary, the increasing distortion of the magnetic ordering near the surface with decreasing crystallite and/or particle sizes leads to distinct modifications of the magnetic properties. Depending on the characteristics of the magnetic sublattices, these distortions can lead to either enhanced or reduced magnetic moments. As demonstrated in this work, nanocasting using silica matrices of different pore system symmetries is a highly versatile route for tailoring the magnetic behavior of spinel-type oxides.

Figure 8. Field-dependent magnetization (T = 10 K) of two mesoporous CoFe2O4 samples replicated from small- and large-pore KIT-6 silica matrices, as well as bulk CoFe2O4.

corresponding to T(Co2+) and O(Fe3+), a saturation magnetization of 3 μB mol−1 (71.4 emu g−1) is expected. Typically, slightly larger saturation magnetizations are found and explained by an incompletely inverse spinel structure for CoFe2O4.54 When these results are compared with those of the mesoporous materials, a clear trend can be observed. Highest saturation magnetization and remanence is found for the bulk, a clear reduction occurs for CoFe2O4 from large-pore KIT-6, whereas the lowest value is measured for the mesoporous samples replicated from small-pore KIT-6 silica (Table 3). This corresponds to the respective surface-to-volume ratio. As for Co3O4, it would be desirable to distinguish between solid−gas and solid−solid interfaces, respectively. However, for the three CoFe2O4 samples this distinction is not possible in a straightforward way because both the BET surface area (i.e.,

■

CONCLUSION Our study shows that the magnetic ordering of mesoporous spinel-type oxides Co3O4 and CoFe2O4 is clearly affected by their interfaces: the stronger the contribution of surface-near

Table 3. BET Surface Areas, Crystallite Sizes, and Magnetic Parameters for the Mesoporous CoFe2O4 Materials Replicated from Small- and Large-Pore KIT-6 As Well As for Bulk-Phase CoFe2O4 CoFe2O4 nanocast from:



coercivity (kOe)

saturation magnetization (μB mol−1)

remanence (μB mol−1)

small-pore KIT-6 silica large-pore KIT-6 silica (bulk CoFe2O4)

161 112 5

9 12 27

12.0 13.5 12.3

1.94 2.92 3.78

0.82 1.73 2.85

24476



Article

(10) Roth, W. L. The Magnetic Structure of Co3O4. J. Phys. Chem. Solids 1964, 25, 1−10. (11) Sawatzky, G. A.; van der Woude, F.; Morrish, A. H. Mössbauer Study of Several Ferrimagnetic Spinels. Phys. Rev. 1969, 187, 747−757. (12) Morrish, A. H. The Physical Principle of Magnetism; R. E. Krieger Publishing Company, Inc.: Huntington, NY, 1980. (13) Dutta, P.; Seehra, M. S.; Thota, S.; Kumar, J. A Comparative Study of the Magnetic Properties of Bulk and Nanocrystalline Co3O4. J. Phys.: Condens. Matter 2008, 20, 1−8. (14) Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F.; Schüth, F. Exchange Anisotropy in Nanocasted Co3O4 Nanowires. Nano Lett. 2006, 6, 2977−2981. (15) Cabo, M.; Pellicer, E.; Rossinyol, E.; Estrader, M.; LópezOrtega, A.; Nogués, J.; Castell, O.; Surinach, S.; Baró, M. D. Synthesis of Compositionally Graded Nanocast NiO/NiCo2O4/Co3O4 Mesoporous Composites with Tunable Magnetic Properties. J. Mater. Chem. 2010, 20, 7021−7028. (16) Tüysüz, H.; Salabas, E. L.; Bill, E.; Bongard, H.; Spliethoff, B.; Lehmann, C. W.; Schüth, F. Synthesis of Hard Magnetic Ordered Mesoporous Co3O4/CoFe2O4 Nanocomposites. Chem. Mater. 2012, 24, 2493−2500. (17) Sun, Y.; Ji, G.; Zheng, M.; Chang, X.; Li, S.; Zhang, Y. Synthesis and Magnetic Properties of Crystalline Mesoporous CoFe2O4 with Large Specific Surface Area. J. Mater. Chem. 2010, 20, 945−952. (18) Tiemann, M. Repeated Templating. Chem. Mater. 2008, 20, 961−971. (19) Lu, A.-H.; Schüth, F. Nanocasting: A Versatile Strategy for Creating Nanostructured Porous Materials. Adv. Mater. 2006, 18, 1793−1805. (20) Lu, A.-H.; Zhao, D.; Wan, Y. Nanocasting: A Versatile Strategy for Creating Nanostructured Porous Materials; The Royal Society of Chemistry: Cambridge, UK, 2010. (21) Yang, H.; Zhao, D. Synthesis of Replica Mesostructures by the Nanocasting Strategy. J. Mater. Chem. 2005, 15, 1217−1231. (22) Ren, Y.; Ma, Z.; Bruce, P. G. Ordered Mesoporous Metal Oxides: Synthesis and Applications. Chem. Soc. Rev. 2012, 41, 4909− 4927. (23) Schüth, F. Endo- and Exotemplating to Create High-SurfaceArea Inorganic Materials. Angew. Chem., Int. Ed. 2003, 42, 3604−3622. (24) Wagner, T.; Haffer, S.; Weinberger, C.; Klaus, D.; Tiemann, M. Mesoporous Materials as Gas Sensors. Chem. Soc. Rev. 2013, 42, 4036−4053. (25) Tian, B.; Liu, X.; Yang, H.; Xie, S.; Yu, C.; Tu, B.; Zhao, D. General Synthesis of Ordered Crystallized Metal Oxide Nanoarrays Replicated by Microwave-Digested Mesoporous Silica. Adv. Mater. 2003, 15, 1370−1374. (26) Tian, B.; Liu, X.; Solovyov, L. A.; Liu, Z.; Yang, H.; Zhang, Z.; Xie, S.; Zhang, F.; Tu, B.; Yu, C.; Terasaki, O.; Zhao, D. Facile Synthesis and Characterization of Novel Mesoporous and Mesorelief Oxides with Gyroidal Structures. J. Am. Chem. Soc. 2004, 126, 865− 875. (27) Dickinson, C.; Zhou, W.; Hodgkins, R. P.; Shi, Y.; Zhao, D.; He, H. Formation Mechanism of Porous Single-Crystal Cr2O3 and Co3O4 Templated by Mesoporous Silica. Chem. Mater. 2006, 18, 3088−3095. (28) Rumplecker, A.; Kleitz, F.; Salabas, E.-L.; Schüth, F. Hard Templating Pathways for the Synthesis of Nanostructured Porous Co3O4. Chem. Mater. 2007, 19, 485−496. (29) Yue, W.; Zhou, W. Synthesis of Porous Single Crystals of Metal Oxides via a Solid-Liquid Route. Chem. Mater. 2007, 19, 2359−2363. (30) Shon, J. K.; Kong, S. S.; Kim, Y. S.; Lee, J.-H.; Park, W. K.; Park, S. C.; Kim, J. M. Solvent-free Infiltration Method for Mesoporous SnO2 Using Mesoporous Silica Templates. Microporous Mesoporous Mater. 2009, 120, 441−446. (31) Wang, Y. M.; Wu, Z. Y.; Shi, L. Y.; Zhu, J. H. Rapid Functionalization of Mesoporous Materials: Directly Dispersing Metal Oxides into As-Prepared SBA-15 Occluded with Template. Adv. Mater. 2005, 17, 323−327. (32) Lepoutre, S.; Smått, J.-H.; Laberty, C.; Amenitsch, H.; Grosso, D.; Lindén, M. Detailed Study of the Pore-filling Processes during

regions in the porous materials becomes, the more significantly the magnetic ordering is disturbed. This applies to both types of interfaces, i.e., solid−gas (BET surface area) and solid−solid (between adjacent single-crystalline domains) interfaces. Depending on details of the spin-arrangements, the distortion of the magnetic ordering may result in enhanced or reduced magnetic moments. Nanocasting of such magnetic materials using mesoporous silica with different pore sizes and/or connectivities thus turn out to be an effective tool for tailoring both the morphological and magnetic properties of functional oxides.

■

ASSOCIATED CONTENT

S Supporting Information *

Structural characterization data of all presented materials by XRD and N2 physisorption. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. Roland Marschall (University of Bochum, Germany) for providing bulk CoFe2O4. Raphael Geissinger is thanked for valuable help in the synthesis work. Financial support by the DFG through SFB 762 is highly acknowledged.

■

REFERENCES

(1) Yamaura, H.; Moriya, K.; Miura, N.; Yamazoe, N. Mechanism of Sensitivity Promotion in CO Sensor Using Indium Oxide and Cobalt Oxide. Sens. Actuators, B 2000, 65, 39−41. (2) Wö llenstein, J.; Burgmair, M.; Plescher, G.; Sulima, T.; Hildenbrand, J.; Böttner, H.; Eisele, I. Cobalt Oxide Based Gas Sensors on Silicon Substrate for Operation at Low Temperatures. Sens. Actuators, B 2003, 93, 442−448. (3) Wagner, T.; Roggenbuck, J.; Kohl, C.-D.; Fröba, M.; Tiemann, M. Gas-Sensing Properties of Ordered Mesoporous Co3O4 Synthesized by Replication of SBA-15 Silica. Stud. Surf. Sci. Catal. 2006, 165, 347−350. (4) Trasatti, S. Electrocatalysis in the Anodic Evolution of Oxygen and Chlorine. Electrochim. Acta 1984, 29, 1503−1512. (5) Hutchins, M. G.; Wright, P. J.; Grebenik, P. D. Comparison of Different Forms of Black Cobalt Selective Solar Absorber Surfaces. Solar Energy Mater. 1987, 16, 113−131. (6) Maaz, K.; Mumtaz, A.; Hasanain, S. K.; Ceylan, A. Synthesis and Magnetic Properties of Cobalt Ferrite (CoFe2O4) Nanoparticles Prepared by Wet Chemical Route. J. Magn. Magn. Mater. 2007, 308, 289−295. (7) Manova, E.; Tsoncheva, T.; Paneva, D.; Mitov, I.; Tenchev, K.; Petrov, L. Mechanochemically Synthesized Nano-Dimensional IronCobalt Spinel Oxides as Catalysts for Methanol Decomposition. Appl. Catal., A 2004, 277, 119−127. (8) Sreekumar, K.; Sugunan, S. Ferrospinels Based on Co and Ni Prepared via a Low Temperature Route as Efficient Catalysts for the Selective Synthesis of o-Cresol and 2,6-Xylenol from Phenol and Methanol. J. Mol. Catal. A: Chem. 2002, 185, 259−268. (9) Sreekumar, K.; Mathew, T.; Rajgopal, R.; Vetrivel, R.; Rao, B. S. Selective Synthesis of 3-Picoline via the Vapor-phase Methylation of Pyridine with Methanol over Ni1−xCoxFe2O4 (x = 0, 0.2, 0.5, 0.8 and 1.0) Type Ferrites. Catal. Lett. 2000, 65, 99−105. 24477



Article

(53) Chikazumi, S. Physics of Ferromagnetism; Oxford University Press: Oxford, U.K., 2005. (54) Sawatzky, G. A.; van der Woude, F.; Morrish, A. H. Cation Distributions in Octahedral and Tetrahedral Sites of the Ferrimagnetic Spinel CoFe2O4. J. Appl. Phys. 1968, 39, 1204−1205. (55) Haneda, K.; Morrish, A. H. Noncollinear Magnetic Structure of CoFe2O4 Small Particles. J. Appl. Phys. 1988, 63, 4258−4260. (56) Chen, J. P.; Sorensen, C. M.; Klabunde, K. J.; Hadjipanayis, G. C.; Devlin, E.; Kostikas, A. Size-dependent Magnetic Properties of MnFe2O4 Fine Particles Synthesized by Coprecipitation. Phys. Rev. 1996, 54, 9288−9296.

Nanocasting of Mesoporous Films Using SnO2/SiO2 as a Model System. Microporous Mesoporous Mater. 2009, 123, 185−192. (33) Sun, X.; Shi, Y.; Zhang, P.; Zheng, C.; Zheng, X.; Zhang, F.; Zhang, Y.; Guan, N.; Zhao, D.; Stucky, D. Container Effect in Nanocasting Synthesis of Mesoporous Metal Oxides. J. Am. Chem. Soc. 2011, 133, 14542−14545. (34) Waitz, T.; Wagner, T.; Sauerwald, T.; Kohl, C.-D.; Tiemann, M. Ordered Mesoporous In2O3: Synthesis by Structure Replication and Application as a Methane Gas Sensor. Adv. Funct. Mater. 2009, 19, 653−661. (35) Haffer, S.; Waitz, T.; Tiemann, M. Mesoporous In2O3 with Regular Morphology by Nanocasting: A Simple Relation between Defined Particle Shape and Growth Mechanism. J. Phys. Chem. C 2010, 114, 2075−2081. (36) Waitz, T.; Becker, B.; Wagner, T.; Sauerwald, T.; Kohl, C. D.; Tiemann, M. Ordered Nanoporous SnO2 Gas Sensors with High Thermal Stability. Sens. Actuators, B 2010, 150, 788−793. (37) Haffer, S.; Weinberger, C.; Tiemann, M. Mesoporous Al2O3 by Nanocasting: Relationship between Crystallinity and Mesoscopic Order. Eur. J. Inorg. Chem. 2012, 3283−3288. (38) Liu, Q.; Wang, A.; Wang, X.; Xu, J.; Zhang, Y.; Wang, X.; Zhang, T. Preparation of Ordered Mesoporous Crystalline Alumina Replicated by Mesoporous Carbon. Microporous Mesoporous Mater. 2008, 116, 461−468. (39) Liang, C.; Li, Z.; Dai, S. Mesoporous Carbon Materials: Synthesis and Modification. Angew. Chem., Int. Ed. 2008, 47, 3696− 3717. (40) Xia, Y.; Yang, Z.; Mokaya, R. Templated Nanoscale Porous Carbons. Nanoscale 2010, 2, 639−659. (41) Yen, H.; Seo, Y.; Guillet-Nicolas, R.; Kaliaguine, S.; Kleitz, F. One-step Impregnation Hard Templating Synthesis of High Surfacearea Nanostructured Mixed Metal Oxides (NiFe2O4, CuFe2O4 and Cu/CeO2). Chem. Commun. 2011, 47, 10473−10475. (42) Yen, H.; Seo, Y.; Kaliaguine, S.; Kleitz, F. Tailored Mesostructured Copper/Ceria Catalysts with Enhanced Performance for Preferential Oxidation of CO at Low Temperature. Angew. Chem., Int. Ed. 2012, 51, 12032−12035. (43) Smått, J.-H.; Weidenthaler, C.; Rosenholm, J. B.; Lindén, M. Hierarchically Porous Metal Oxide Monoliths Prepared by the Nanocasting Route. Chem. Mater. 2006, 18, 1443−1450. (44) Smått, J.-H.; Lindén, M.; Wagner, T.; Kohl, C.-D.; Tiemann, M. Micrometer-sized Nanoporous Tin Dioxide Spheres for Gas Sensing. Sens. Actuators, B 2011, 155, 483−488. (45) Smått, J.-H.; Schüwer, N.; Järn, M.; Lindner, W.; Lindén, M. Synthesis of Micrometer Sized Mesoporous Metal Oxide Spheres by Nanocasting. Microporous Mesoporous Mater. 2008, 112, 308−318. (46) Lepoutre, S.; Julián-López, B.; Sanchez, C.; Amenitsch, H.; Lindén, M.; Grosso, D. Nanocasted Mesoporous Nanocrystalline ZnO Thin Films. J. Mater. Chem. 2010, 20, 537−542. (47) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Synthesis of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120, 6024− 6036. (48) Kleitz, F.; Choi, S. H.; Ryoo, R. Cubic Ia3d Large Mesoporous Silica: Synthesis and Replication to Platinum Nanowires, Carbon Nanorods and Carbon Nanotubes. Chem. Commun. 2003, 2136−2137. (49) Park, T.-J.; Papaefthymiou, G. C.; Viescas, A. J.; Moodenbaugh, A. R.; Wong, S. S. Size-dependent Magnetic Properties of Singlecrystalline Multiferroic BiFeO3 Nanoparticles. Nano Lett. 2007, 7, 766−772. (50) Ichiyanagi, Y.; Yamada, S. The Size-dependent Magnetic Properties of Co3O4 Nanoparticles. Polyhedron 2005, 24, 2813−2816. (51) Makhlouf, S. A. Magnetic Properties of Co3O4 Nanoparticles. J. Magn. Magn. Mater. 2002, 246, 184−190. (52) Wang, Y.; Yang, C. M.; Schmidt, W.; Spliethoff, B.; Bill, B.; Schü th, F. Weakly Ferromagnetic Ordered Mesoporous Co3O4 Synthesized by Nanocasting from Vinyl-Functionalized Cubic Ia3d Mesoporous Silica. Adv. Mater. 2005, 17, 53−56. 24478


Nanostructure-Related Magnetic Properties of Various Mesoporous

Recommend Documents