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Controlled synthesis from alginate gels of cobalt–manganese mixed oxide nanocrystals with peculiar magnetic properties
Catalysis Today 189 (2012) 49–54
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Controlled synthesis from alginate gels of cobalt–manganese mixed oxide nanocrystals with peculiar magnetic properties Pierre Agulhon a , Sandra Constant a , Bich Chiche a , Lenaïc Lartigue b , Joulia Larionova b , Francesco Di Renzo a , Franc¸oise Quignard a,∗ a Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UMII-ENSCM-UMI, Matériaux Avancés pour la Catalyse et la Santé, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France b Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UMII-ENSCM-UMI, Chimie Moléculaire et Organisation du Solide, Place Eugène Bataillon, CC1701, 34095 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 5 December 2011 Received in revised form 24 February 2012 Accepted 19 March 2012 Available online 21 April 2012 Keywords: Polysaccharide Aerogel Oxide
a b s t r a c t Tailored cobalt–manganese mixed oxide nanoparticles are obtained from ionotropic gels of alginate, a seaweed-derived carboxylic-functionalized polymer. (Co,Mn)3 O4 phases are formed by low-temperature calcination of the alginate aerogels. Cubic spinel or tetragonal hausmannite structures are formed if, respectively, cobalt or manganese is the main cation. Stable nano-sized mixed oxides are formed in the whole range of composition of the solid solution by calcination of the aerogel precursors at 450 ◦ C. The manganese-rich oxides are more dispersed than the cobalt-rich oxides, with surface areas higher than 80 m2 g−1 . The organization of the oxide nanocrystals in arrays reminiscent of the fibrillar structure of the parent gel prevents them from packing and confers them peculiar magnetic properties in terms of coercivity and magnetoresistance. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide materials at the nanoscale have been intensively investigated in the last 40 years due to the fundamental scientific interest in the understanding of new phenomena arising from the size reduction, but also for their technological applications. Due to the high surface-to-volume ratio, quantum size effect [1], and electrodynamic interactions, the nano-objects possess unique size-dependent physical and chemical properties that are strikingly different from those of the individual atoms as well as from their bulk counterparts. The chemical and physical properties of nanoparticles (NPs) are mainly governed by their size, shape, composition, crystallinity and structure. For this reason, an accurate control of these intrinsic parameters is the most important requirement for many future applications. On the other hand, the NPs behavior may be strongly influenced by their close environment. Thus, the properties of NPs-containing materials will also critically depend on the surface state of the NPs including interactions with ligands, matrixes or substrates. It is evident that all these phenomena can be differently controlled according to the synthetic methods adopted for the preparation and that the choice for the
∗ Corresponding author. Tel.: +33 4 67 16 34 60; fax: +33 4 67 16 34 70. E-mail address: [email protected] (F. Quignard). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.03.052
most suitable method will depend on the desired final applications envisaged for the material. Among various metal oxide nano-objects, transition metal mixed oxides such as Co–Mn, are well-know catalysts for, as instances, Fischer–Tropsch reaction [2,3], olefin formation from synthetic gas [4] or desulfurization [5]. Variations of particle size and composition of the (Co,Mn)3 O4 solid solutions significantly affected conversion and selectivity in insaturation and carbon number of Fischer–Tropsch catalysts [6,7]. For this reason, numerous mixed nanoparticles with various controlled size and shape and controlled surface state have been investigated. The mixed oxide nano-objects maybe classically produced by a co-precipitation of the corresponding metal salts or hydroxide precursors in the presence of surfactants [8], polymers [9] or micellar [10] stabilizing agents or via sol–gel methods in non-aqueous solvents [11,12]. Otherwise, the two-steps Pechini’s method [13] consisting on the synthesis of metal–citrate complexes included into organic matrix by polymerization with ethylene glycol and following calcination of these composites providing the nanostructured oxides thanks to the atomic dispersion of the metal into the network. Recently, alginates have been used as organic matrix in order to produce the monometallic oxide nanoparticles presenting much higher surface area in comparison with other methods [14]. Alginates are naturally carboxylate-functionalized polysaccharides abundantly produced by a brown algae. They are linear block-copolymers formed by two structurally isomeric
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P. Agulhon et al. / Catalysis Today 189 (2012) 49–54
Scheme 1. Alginate structural units.
carboxylated units: ␣-l-guluronate (G) and -d-mannuronate (M) (Scheme 1). Depending on the natural source, the structure of alginates is characterized by different M/G ratios and block lengths. The coordination of divalent cations (except Mg2+ ) on the carboxylate functions leads to the formation of an hydrogel, whose supercritical CO2 drying allows to form aerogels, good precursor for highly dispersed nanostructured oxides. M/G ratio and block distribution of a given alginate are expected to affect metal complexation and gelling behavior [15–18]. In this study we describe the synthesis and characterisations of tailored cobalt–manganese mixed oxide nano-particles by using ionotropic alginate gels. Six different alginates have been employed in order to investigate the importance of the natural variability of the resource on the formation of the gels and the derived oxides. The control of the phase and composition of these high-surface area magnetic oxides is a requisite for the design of tailored redox catalysts.
2. Materials and methods Alginate samples were supplied by FMC Biopolymer (Norway) and Sigma–Aldrich. They were characterized by 1 H NMR according to Grasdalen [19,20] Alginates, as block copolymers, are described by the sequence of their building blocks. More specifically, by the mannuronate (FM ) and guluronate (FG ) fractions, the fractions of diad and triad sequences, an parameter which describes the alternation of M and G groups (0 < < 1: block copolymer, = 1 statistical polymer, 1 < < 2: alternated polymer), and the average number of G units in G-blocks (NG ) and in G-blocks of more than one G unit (NG>1 ). These properties are reported in Table 1 for the alginates used in this study. Hydrogel beads were obtained by dropwise addition of 2% alginate solution in gelling solution containing Co2+ and/or Mn2+ with a total concentration of 0.1 mol L−1 . Those beads were progressively ethanol-exchanged in water–ethanol baths of increasing concentration (10-30-50-70-90-100%). Finally alcohol was replaced by liquid CO2 (Polaron 3100 autoclave) which was eliminated under supercritical conditions (typically 40 ◦ C and 85 bar). Oxide samples were obtained by calcination of the aerogels under air flow at 450 ◦ C (8 h, 3 ◦ C/min heating rate). Nitrogen adsorption–desorption isotherms at −196 ◦ C were recorded on a Micromeritics Tristar apparatus. Prior to the analysis, the aerogel samples were outgassed at 50 ◦ C and oxide samples at 250 ◦ C. The volume of the adsorbed monolayer was evaluated by the BET equation and the surface area was calculated by assuming a N2 molecule to cover 0.162 nm2 . Powder X-ray diffraction (XRD) patterns were recorded on a Brucker diffractometer with Cu K␣ radiation. Crystallographic identification and lattice parameters calculation were made using the FPM (Full Pattern Matching) module contained within the DIFFRACplus BASIC (Bruker AXS) program package. Crystallite size was
measured from the broadening of the diffraction lines with Scherrer equation. Scanning electron microscopy (SEM) pictures were recorded on a Hitachi S-4800 microscope. The elemental analyses were obtained on calcinated aerogels (450 ◦ C for 8 h) by energy dispersive X-ray spectroscopy on a Hitachi S-4500 microscope without any preliminary metallization. The Co/Mn ratio of the oxides corresponds to the metal ratio in the parent gels, as calcination does not change this metal ratio. Magnetic measurements in DC mode were carried out on a Quantum Design MPMS-XL SQUID magnetometer. The thermal evolution of magnetization was recorded in the temperature range from 2 to 300 K. The sample was cooled from room temperature down to 2 K in the absence of magnetic field and was heated up while the magnetization was recorded under a magnetic field of 50 Oe. Hysteresis loop were measured at 2.5 K under a maximum applied field of 5 T. All magnetic measurements were performed on dry powders and corrected for the sample holder. 3. Results and discussion 3.1. Heterometallic ionotropic alginate aerogels The relative affinity of cobalt and manganese cations for different alginates can be determined by the comparison of the Mn/(Mn + Co) ratios in the gel and the parent solution. This relation is plotted in Fig. 1 in the complete range of composition. All experimental results are on the same curve, showing that the final composition of the heterometallic gels does not depend on the alginate type. The field of composition of the alginate used is wide enough to show that neither the global M/G ratio nor the block distribution affects the relative affinity for cobalt and manganese
Fig. 1. Manganese fraction in the gel vs. manganese fraction in gelling solution for various alginates.
P. Agulhon et al. / Catalysis Today 189 (2012) 49–54
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Table 1 Characteristics of the various alginate samples used in this study; FX represent the fraction of X pattern, NZ is the average number of Z units in Z-blocks and characterizes the block structure alginate chains.
LG1 LG2 LG3 LG4 HG1 HG2
FM
FG
FMM
FMG FGM
FGG
FGGG
FGGM FMGG
FMGM
NM
NG
NG>1
0.67 0.63 0.61 0.60 0.37 0.34
0.33 0.37 0.39 0.40 0.63 0.66
0.54 0.46 0.40 0.43 0.26 0.25
0.13 0.17 0.21 0.17 0.10 0.09
0.21 0.20 0.19 0.23 0.53 0.58
0.18 0.15 0.16 0.18 0.49 0.53
0.02 0.03 0.01 0.01 0.05 0.03
0.12 0.11 0.20 0.16 0.09 0.08
0.57 0.71 0.86 0.70 0.45 0.38
5 4 3 4 4 4
3 2 2 2 6 8
9 8 14 24 11 17
cations. Since the composition of the gels does not depend on the alginate type, other properties will be discussed for two representative guluronate-rich and mannuronate-rich alginates, respectively HG1 and LG1.
3.2. Aerogel textural properties The surface areas of two series of aerogel samples are reported in Fig. 2 as a function of the manganese fraction. All data are in the range 70–330 m2 g−1 . The surface area strongly depends on the alginate type: the high-G alginate provides aerogels with a surface area more than twice the surface area of aerogels from the low-G alginate, whatever the cation ratio. The metal ratio also influences the surface area, which decreases with manganese content. The surface area of pure manganese aerogels is about the half of the surface area of pure cobalt aerogels.
3.3. Oxide phases The oxide phases formed by calcination of the aerogels depend on the fraction of each metal. Cubic spinel or tetragonal hausmannite structures are formed depending if cobalt or manganese is the main metal. In the transition zone where the Mn/(Mn + Co) ratio is close to 0.5, the two phases co-exist. The volume of the asymmetric unit of the phases formed is reported in Fig. 3 as a function of the manganese fraction. In the case of spinel, insertion of manganese in the lattice slightly increases the unit volume since manganese radius is higher than cobalt one. For the same reason, the adjunction of cobalt in the hausmannite structure decreases the unit volume.
Fig. 2. Surface area of aerogel precursors obtained from alginate HG1 (circles) and LG1 (squares).
3.4. Textural properties of the oxides The crystallite size of the oxides formed by calcination at 450 ◦ C, as evaluated from the XRD patterns, are reported in Fig. 4a. Cobalt pure oxide presents spinel crystallites with size close to 30 nm. Doping with manganese immediately decreases the crystallite size, which reaches values below 20 nm from 5% doping. For manganese-richer compositions, all samples show nanocrystallite size 12 ± 5 nm, whatever the phase structure. The surface area of the oxides, reported in Fig. 4b, increases with the decrease of crystallite size. Cobalt pure oxides develop the lowest surface areas, while manganese-richer oxides are more dispersed and reach 60 m2 g−1 for samples from alginate HG1 and up to 87 m2 g−1 for samples from alginate LG1. The calcination temperature was chosen according to thermogravimetric studies. At 450 ◦ C all the organic part of aerogel is removed. When calcination is performed at 600 ◦ C, the crystalline phase is the same as it was at 450 ◦ C but bigger crystallites are obtained and the mean diameter increases, for example, from 12 nm to 22 nm for Co–Mn oxide (70 Mn%). Along with this phenomenon, the surface area decreases from 59 m2 g−1 to 20 m2 g−1 . SEM images of cross-sections of the same sample, reported in Fig. 5, show thread-like structures of crystallites, reminiscent of the fibrils of the parent aerogel [21]. Crystallite size is clearly larger when the sample has been calcined at higher temperature. The TEM imaging of the sample calcined at 600 ◦ C, presented as an example in Fig. 5, shows an inter-reticular distance of 0.48 nm, in good agreement with the hausmannite structure (d101 = 0.482 nm).
Fig. 3. M3 O4 unit volume in Co–Mn oxides.
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P. Agulhon et al. / Catalysis Today 189 (2012) 49–54
Fig. 4. Crystallite size (a) and surface areas (b) of Co–Mn oxides depending composition.
Fig. 5. SEM images of Co–Mn oxide (70 Mn%) for two different calcination temperatures TEM images of 600 ◦ C calcinated Co–Mn sample.
3.5. Magnetic properties The magnetic measurements have been performed on Co3 O4 , Mn3 O4 and some mixed oxides by using dc (direct current) mode by using SQUID magnetometer working in 1.8–350 K temperature range. Figs. 6 and 7 show the temperature dependence of the magnetization performed following zero field-cooled (ZFC)/fieldcooled (FC) magnetization procedure. In the ZFC experiment, the
sample was cooled in the absence of a static magnetic field and the magnetization was then recorded as a function of the temperature under a 50 Oe field. The FC magnetization data were collected after cooling the sample with the same field of 50 Oe. In its bulk form, Co3 O4 is an antiferromagnetic material having a normal cubic spinel structure AB2 O4 . The Co3+ ions have no moment at the octahedral B sites, while the Co2+ ions have a permanent moment of 3.25 B at the tetrahedral A sites [22]. Its Néel temperature has
Fig. 6. Magnetic measurement for Co3 O4 sample: (a) ZFC/FC curve with an applied field of 5 mT; (b) hysteresis loop at 2.5 K.
P. Agulhon et al. / Catalysis Today 189 (2012) 49–54
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Fig. 7. Magnetic measurement of Mn3 O4 : (a) ZFC/FC curve with an applied field of 5 mT; (b) hysteresis loop at 2.5 K.
been reported between 30 K and 40 K [23]. The magnetic behavior of Co3 O4 at nano-scale shows a size dependence of blocking (or freezing temperature), hysteresis effect and various conflicting claims have been made to determine the spin-glass or superparamagnetic behavior of these particles [24]. The ZFC curve of Co3 O4 (Fig. 6a) exhibits a maximum at Tmax = 38 K that corresponds to the de-freezing temperature or blocking temperature for the nanoparticles. The shape of this curve and the value of the maximum temperature are similar to what is observed for the antiferromagnetic Co3 O4 nanoparticles with diameter around 30 nm previously reported by Zhu [25]. This size corresponds to what is estimated from the XRD measurements. The FC curve follows the ZFC curve at high temperature and separates from the ZFC after Tmax . The decrease of the FC curve after the ZFC peak suggests the presence of strong interparticles dipolar interactions. The magnetization as a function of the applied field was measured at 2.5 K with the field swept from ±50 Oe. Fig. 6b shows the hysteretic behavior of the Co3 O4 . The ZFC hysteresis loop is symmetrical about the origin with a coercive field (HC ) of 200 Oe, and is coherent with value report in the literature [24]. The ZFC curve of Mn3 O4 (Fig. 7a) shows a maximum at 41 K corresponding to the blocking (or freezing) temperature for these particles. The FC curve follows initially the ZFC curve, separates from the ZFC curve, then increases rapidly and tends to saturation at low temperature. Both curves are in agreement with what was previously observed for spherical Mn3 O4 nanoparticles or nanoobjects larger than 10 nm (Tmax = 41 K) [26]. The field dependence of the magnetization performed for Mn3 O4 samples sample at 2.5 K displays a hysteresis with a high coercive field of 12,800 Oe (Fig. 7b,
Table 2 Blocking temperature, coercive field and magnetization value of the oxides.
Co3 O4 20% Mn 70% Mn Mn3 O4 a
TB (K)
HC (kOe)
MS a (emu/g)
38 33 89 43
0.2 3.2 1.5 12.8
33 10 4 19
Magnetization value observed for 50 kOe applied magnetic fields.
Table 2). These nanoparticles exhibit huge coercivity in comparison with the values previously reported for Mn3 O4 nanoparticles (for instance HC = 2500 Oe) [27] that may be explained by the presence of interparticles interactions. Fig. 8 shows the ZFC curves measured for samples with 20% and 70% of Mn as well as for two pure Co3 O4 , Mn3 O4 samples for comparison. The ZFC curve of Co–Mn oxide (20 Mn%) shows only one peak with Tmax equal to 33 K indicating the blocking (or freezing) temperature for these nanoparticles while Co–Mn oxide (70 Mn%) presents also one peak with Tmax = 70 K. The presence of one peak for both samples clearly indicates the presence of one mixed oxide phase in each nanoparticle and not the mixture of Co3 O4 and Mn3 O4 nanoparticles in agreement with the results of XRD analysis. In addition, the appearance of a blocking (freezing) temperature at 70 K also obviously designates the formation of a well-defined mixed Co–Mn oxide phase. The field dependence of the magnetization performed for these samples shows that all of them present a hysteretic behavior with coercive field of 3.2 and 1.5 kOe for 20 and 70% of Mn, respectively. The very low magnetic permeability of
Fig. 8. Magnetic measurement of Co3 O4 (), 20% Mn (䊉), 70% Mn (), Mn3 O4 (), (a) ZFC curve with an applied field of 5 mT (the y-axis is right for Mn3 O4 and left for the other samples); (b) hysteresis loop at 2.5 K.
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the mixed oxides is in agreement with the giant magnetoresistance postulated for these materials [28]. 4. Conclusion Co–Mn heterometallic ionotropic gel of alginate can be easily obtained in the whole range of composition. Supercritical CO2 drying affords to obtain aerogels of cation-linked polymers with surface areas ranging from 70 to 330 m2 g−1 with a perfect control of the composition in terms of cations content. They are good precursors for the synthesis of oxides, since the atomic dispersion of the cations in the polysaccharide and the low temperature of thermal degradation of the organics shift the balance between nucleation and growth toward the formation of a larger number of smaller oxide crystals. The Co–Mn mixed oxides formed present surface areas as high as 80 m2 g−1 . The natural variability of alginates has no influence on the composition of the oxides but controls their dispersion. The organization of the oxide nanocrystals in arrays reminiscent of the fibrillar structure of the parent gel confers them peculiar magnetic properties in term of coercivity and magnetoresistance and prevents them from packing, a property potentially useful for catalytic applications. Globally, the alginate route allows a fine control of composition and surface area of mixed Co–Mn oxides in the whole field of composition of the solid solutions. Its easy implementation recommends it as a preferred way for the preparation of oxidation catalysts for which the ratios of transition metal cations and the size of particles strongly affect conversion and selectivity. Acknowledgments We thank Dr. G. Mosser and S. Quignard (UPMC UMR 7574) for transmission electronic microscopy. References [1] (a) T. Hyeon, Chemical Communications (2003) 927; (b) K.J. Klabunde, in: K.J. Klabunde (Ed.), Nanoscale Materials in Chemistry, Wiley Intersciences, New York, 2001; (c) M.R. Diehl, J.-Y. Yu, J.R. Heath, G.A. Doyle, S. Sun, C.B. Murray, Journal of Physical Chemistry B 105 (2001) 7913; (d) D.L. Leslie-Pelescky, R.D. Rieke, Chemistry of Materials 8 (1996) 1770. [2] M.J. Keyser, R.C. Everson, R.L. Espinoza, Applied Catalysis A 171 (1998) 99–107. [3] T.E. Feltes, L. Espinosa-Alonso, E.d. Smit, L. D’Souza, R.J. Meyer, B.M. Weckhuysen, J.R. Regalbuto, Journal of Catalysis 270 (2010) 95–102.
[4] (a) R.G. Copperthwaite, G.J. Hutchings, M. van der Riet, J. Woodhouse, Industrial and Engineering Chemistry Research 26 (1987) 869–874; (b) S.E. Colley, R.G. Copperthwaite, G.J. Hutchings, M. van der Riet, Industrial and Engineering Chemistry Research 27 (8) (1988) 1339–1344. [5] A.A. Mirzaei, M. Faizi, R. Habibpour, Applied Catalysis A 306 (2006) 98–107. [6] J.T. Sampanthar, H. Xiao, J. Dou, T.Y. Nah, X. Rong, W.P. Kwan, Applied Catalysis B 63 (2006) 85–93. [7] Y.Q. Zhang, B. Zhong, Q. Wang, Journal of Natural Gas Chemistry 6 (1997) 275–283. [8] G. Vaidyanathan, S. Sendhilnathan, Physica B 403 (2008) 2157–2167. [9] P. Sivakumar, R. Ramesh, A. Ramanand, S. Ponnusamy, C. Muthamizhchelvan, Materials Letters 65 (2011) 483–485. [10] C.J. O’Connor, C.T. Seip, E.E. Carpenter, S. Li, V.T. John, Nanostructured Materials 12 (1999) 65–70. [11] Z.G. Wu, Y.X. Zhao, L.P. Xu, D.S. Liu, Journal of Non-Crystalline Solids 330 (2003) 274–277. [12] M.S. Tokumoto, S.H. Pulcinelli, C.V. Santilli, V. Briois, Journal of Physical Chemistry B 107 (2002) 568–574. [13] M.P. Pechini, US Patent No. 3,330,697 (1967). [14] R. Horga, F. Di Renzo, F. Quignard, Applied Catalysis A 325 (2007) 251–255. [15] O. Smidsrød, Faraday Discussions of the Chemical Society 57 (1974) 263. [16] K.I. Draget, G. Skjak-Braek, O. Smidsrød, Carbohydrate Polymers 25 (1994) 31–38. [17] N. Velings, M.M. Mestdagh, Polymer Gels and Networks 3 (1995) 311. [18] F. Quignard, R. Valentin, F. Di Renzo, New Journal of Chemistry 32 (2008) 1300–1310. [19] H. Grasdalen, B. Larsen, O. Smidsrød, Carbohydrate Research 68 (1979) 23–31. [20] H. Grasdalen, Carbohydrate Research 118 (1983) 255–260. [21] M. Robitzer, L. David, C. Rochas, F. Di Renzo, F. Quignard, Langmuir 24 (2008) 12547–12552. [22] W.L. Roth, Journal of Physics and Chemistry of Solids 25 (1964) 1–10. [23] (a) D.A. Resnick, K. Gilmore1, Y.U. Idzerda, M.T. Klem, M. Allen, T. Douglas, E. Arenholz, M. Young, Journal of Applied Physics 99 (2006) 08Q501; (b) M. Sato, S. Kohiki, Y. Hayakawa, Y. Sonoda, T. Babasaki, H. Deguchi, M. Mitome, Journal of Applied Physics 88 (5) (2000) 2771. [24] (a) V. Bisht, K.P. Rajeev, Solid State Communications 151 (2010) 1275–1279; (b) Y. Ichiyanagi, S. Yamada, Polyhedron 24 (2005) 2813–2816; (c) S. Takada, M. Fujii, S. Kohiki, Nano Letters 1 (2001) 379–382; (d) Y. Ichiyanagi, Y. Kimishima, S. Yamada, Journal of Magnetism and Magnetic Materials 272 (2004) E1245–E1246; (e) S.A. Makhlouf, Journal of Magnetism and Magnetic Materials 246 (2002) 184–190; (f) L. He, C. Chen, N. Wang, W. Chou, L. Guo, Journal of Applied Physics 102 (2007) 103911; (g) T. Mousavand, T. Naka, K. Sato, S. Ohara, M. Umetsu, S. Takami, T. Nakane, A. Matsushita, T. Adschiri, Physical Review B 79 (2009) 144411; (h) S. Li, H. Bi, B. Cui, F. Zhang, Y. Du, X. Jiang, C. Yang, Q. Yu, Y. Zhu, Journal of Applied Physics 95 (2004) 7420; (i) P. Dutta, M.S. Seehra, S. Thota, J. Kumar, Journal of Physics: Condensed Matter 20 (2008) 015218. [25] H.T. Zhu, J. Luo, J.K. Liang, G.H. Rao, J.B. Li, J.Y. Zhang, Z.M. Du, Physica B 403 (2008) 3141–3145. [26] L. He, Solid State Communications 150 (2010) 743–745. [27] B. Folch, J. Larionova, Y. Guari, Ch. Guérin, C. Rebeil, Journal of Solid State Chemistry 178 (2005) 2368–2375. [28] J. Philip, T.R.N. Kutty, Materials Letters 39 (1999) 311–317.
Contents lists available at SciVerse ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Controlled synthesis from alginate gels of cobalt–manganese mixed oxide nanocrystals with peculiar magnetic properties Pierre Agulhon a , Sandra Constant a , Bich Chiche a , Lenaïc Lartigue b , Joulia Larionova b , Francesco Di Renzo a , Franc¸oise Quignard a,∗ a Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UMII-ENSCM-UMI, Matériaux Avancés pour la Catalyse et la Santé, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France b Institut Charles Gerhardt Montpellier, UMR 5253, CNRS-UMII-ENSCM-UMI, Chimie Moléculaire et Organisation du Solide, Place Eugène Bataillon, CC1701, 34095 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 5 December 2011 Received in revised form 24 February 2012 Accepted 19 March 2012 Available online 21 April 2012 Keywords: Polysaccharide Aerogel Oxide
a b s t r a c t Tailored cobalt–manganese mixed oxide nanoparticles are obtained from ionotropic gels of alginate, a seaweed-derived carboxylic-functionalized polymer. (Co,Mn)3 O4 phases are formed by low-temperature calcination of the alginate aerogels. Cubic spinel or tetragonal hausmannite structures are formed if, respectively, cobalt or manganese is the main cation. Stable nano-sized mixed oxides are formed in the whole range of composition of the solid solution by calcination of the aerogel precursors at 450 ◦ C. The manganese-rich oxides are more dispersed than the cobalt-rich oxides, with surface areas higher than 80 m2 g−1 . The organization of the oxide nanocrystals in arrays reminiscent of the fibrillar structure of the parent gel prevents them from packing and confers them peculiar magnetic properties in terms of coercivity and magnetoresistance. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide materials at the nanoscale have been intensively investigated in the last 40 years due to the fundamental scientific interest in the understanding of new phenomena arising from the size reduction, but also for their technological applications. Due to the high surface-to-volume ratio, quantum size effect [1], and electrodynamic interactions, the nano-objects possess unique size-dependent physical and chemical properties that are strikingly different from those of the individual atoms as well as from their bulk counterparts. The chemical and physical properties of nanoparticles (NPs) are mainly governed by their size, shape, composition, crystallinity and structure. For this reason, an accurate control of these intrinsic parameters is the most important requirement for many future applications. On the other hand, the NPs behavior may be strongly influenced by their close environment. Thus, the properties of NPs-containing materials will also critically depend on the surface state of the NPs including interactions with ligands, matrixes or substrates. It is evident that all these phenomena can be differently controlled according to the synthetic methods adopted for the preparation and that the choice for the
∗ Corresponding author. Tel.: +33 4 67 16 34 60; fax: +33 4 67 16 34 70. E-mail address: [email protected] (F. Quignard). 0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.03.052
most suitable method will depend on the desired final applications envisaged for the material. Among various metal oxide nano-objects, transition metal mixed oxides such as Co–Mn, are well-know catalysts for, as instances, Fischer–Tropsch reaction [2,3], olefin formation from synthetic gas [4] or desulfurization [5]. Variations of particle size and composition of the (Co,Mn)3 O4 solid solutions significantly affected conversion and selectivity in insaturation and carbon number of Fischer–Tropsch catalysts [6,7]. For this reason, numerous mixed nanoparticles with various controlled size and shape and controlled surface state have been investigated. The mixed oxide nano-objects maybe classically produced by a co-precipitation of the corresponding metal salts or hydroxide precursors in the presence of surfactants [8], polymers [9] or micellar [10] stabilizing agents or via sol–gel methods in non-aqueous solvents [11,12]. Otherwise, the two-steps Pechini’s method [13] consisting on the synthesis of metal–citrate complexes included into organic matrix by polymerization with ethylene glycol and following calcination of these composites providing the nanostructured oxides thanks to the atomic dispersion of the metal into the network. Recently, alginates have been used as organic matrix in order to produce the monometallic oxide nanoparticles presenting much higher surface area in comparison with other methods [14]. Alginates are naturally carboxylate-functionalized polysaccharides abundantly produced by a brown algae. They are linear block-copolymers formed by two structurally isomeric
50
P. Agulhon et al. / Catalysis Today 189 (2012) 49–54
Scheme 1. Alginate structural units.
carboxylated units: ␣-l-guluronate (G) and -d-mannuronate (M) (Scheme 1). Depending on the natural source, the structure of alginates is characterized by different M/G ratios and block lengths. The coordination of divalent cations (except Mg2+ ) on the carboxylate functions leads to the formation of an hydrogel, whose supercritical CO2 drying allows to form aerogels, good precursor for highly dispersed nanostructured oxides. M/G ratio and block distribution of a given alginate are expected to affect metal complexation and gelling behavior [15–18]. In this study we describe the synthesis and characterisations of tailored cobalt–manganese mixed oxide nano-particles by using ionotropic alginate gels. Six different alginates have been employed in order to investigate the importance of the natural variability of the resource on the formation of the gels and the derived oxides. The control of the phase and composition of these high-surface area magnetic oxides is a requisite for the design of tailored redox catalysts.
2. Materials and methods Alginate samples were supplied by FMC Biopolymer (Norway) and Sigma–Aldrich. They were characterized by 1 H NMR according to Grasdalen [19,20] Alginates, as block copolymers, are described by the sequence of their building blocks. More specifically, by the mannuronate (FM ) and guluronate (FG ) fractions, the fractions of diad and triad sequences, an parameter which describes the alternation of M and G groups (0 < < 1: block copolymer, = 1 statistical polymer, 1 < < 2: alternated polymer), and the average number of G units in G-blocks (NG ) and in G-blocks of more than one G unit (NG>1 ). These properties are reported in Table 1 for the alginates used in this study. Hydrogel beads were obtained by dropwise addition of 2% alginate solution in gelling solution containing Co2+ and/or Mn2+ with a total concentration of 0.1 mol L−1 . Those beads were progressively ethanol-exchanged in water–ethanol baths of increasing concentration (10-30-50-70-90-100%). Finally alcohol was replaced by liquid CO2 (Polaron 3100 autoclave) which was eliminated under supercritical conditions (typically 40 ◦ C and 85 bar). Oxide samples were obtained by calcination of the aerogels under air flow at 450 ◦ C (8 h, 3 ◦ C/min heating rate). Nitrogen adsorption–desorption isotherms at −196 ◦ C were recorded on a Micromeritics Tristar apparatus. Prior to the analysis, the aerogel samples were outgassed at 50 ◦ C and oxide samples at 250 ◦ C. The volume of the adsorbed monolayer was evaluated by the BET equation and the surface area was calculated by assuming a N2 molecule to cover 0.162 nm2 . Powder X-ray diffraction (XRD) patterns were recorded on a Brucker diffractometer with Cu K␣ radiation. Crystallographic identification and lattice parameters calculation were made using the FPM (Full Pattern Matching) module contained within the DIFFRACplus BASIC (Bruker AXS) program package. Crystallite size was
measured from the broadening of the diffraction lines with Scherrer equation. Scanning electron microscopy (SEM) pictures were recorded on a Hitachi S-4800 microscope. The elemental analyses were obtained on calcinated aerogels (450 ◦ C for 8 h) by energy dispersive X-ray spectroscopy on a Hitachi S-4500 microscope without any preliminary metallization. The Co/Mn ratio of the oxides corresponds to the metal ratio in the parent gels, as calcination does not change this metal ratio. Magnetic measurements in DC mode were carried out on a Quantum Design MPMS-XL SQUID magnetometer. The thermal evolution of magnetization was recorded in the temperature range from 2 to 300 K. The sample was cooled from room temperature down to 2 K in the absence of magnetic field and was heated up while the magnetization was recorded under a magnetic field of 50 Oe. Hysteresis loop were measured at 2.5 K under a maximum applied field of 5 T. All magnetic measurements were performed on dry powders and corrected for the sample holder. 3. Results and discussion 3.1. Heterometallic ionotropic alginate aerogels The relative affinity of cobalt and manganese cations for different alginates can be determined by the comparison of the Mn/(Mn + Co) ratios in the gel and the parent solution. This relation is plotted in Fig. 1 in the complete range of composition. All experimental results are on the same curve, showing that the final composition of the heterometallic gels does not depend on the alginate type. The field of composition of the alginate used is wide enough to show that neither the global M/G ratio nor the block distribution affects the relative affinity for cobalt and manganese
Fig. 1. Manganese fraction in the gel vs. manganese fraction in gelling solution for various alginates.
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Table 1 Characteristics of the various alginate samples used in this study; FX represent the fraction of X pattern, NZ is the average number of Z units in Z-blocks and characterizes the block structure alginate chains.
LG1 LG2 LG3 LG4 HG1 HG2
FM
FG
FMM
FMG FGM
FGG
FGGG
FGGM FMGG
FMGM
NM
NG
NG>1
0.67 0.63 0.61 0.60 0.37 0.34
0.33 0.37 0.39 0.40 0.63 0.66
0.54 0.46 0.40 0.43 0.26 0.25
0.13 0.17 0.21 0.17 0.10 0.09
0.21 0.20 0.19 0.23 0.53 0.58
0.18 0.15 0.16 0.18 0.49 0.53
0.02 0.03 0.01 0.01 0.05 0.03
0.12 0.11 0.20 0.16 0.09 0.08
0.57 0.71 0.86 0.70 0.45 0.38
5 4 3 4 4 4
3 2 2 2 6 8
9 8 14 24 11 17
cations. Since the composition of the gels does not depend on the alginate type, other properties will be discussed for two representative guluronate-rich and mannuronate-rich alginates, respectively HG1 and LG1.
3.2. Aerogel textural properties The surface areas of two series of aerogel samples are reported in Fig. 2 as a function of the manganese fraction. All data are in the range 70–330 m2 g−1 . The surface area strongly depends on the alginate type: the high-G alginate provides aerogels with a surface area more than twice the surface area of aerogels from the low-G alginate, whatever the cation ratio. The metal ratio also influences the surface area, which decreases with manganese content. The surface area of pure manganese aerogels is about the half of the surface area of pure cobalt aerogels.
3.3. Oxide phases The oxide phases formed by calcination of the aerogels depend on the fraction of each metal. Cubic spinel or tetragonal hausmannite structures are formed depending if cobalt or manganese is the main metal. In the transition zone where the Mn/(Mn + Co) ratio is close to 0.5, the two phases co-exist. The volume of the asymmetric unit of the phases formed is reported in Fig. 3 as a function of the manganese fraction. In the case of spinel, insertion of manganese in the lattice slightly increases the unit volume since manganese radius is higher than cobalt one. For the same reason, the adjunction of cobalt in the hausmannite structure decreases the unit volume.
Fig. 2. Surface area of aerogel precursors obtained from alginate HG1 (circles) and LG1 (squares).
3.4. Textural properties of the oxides The crystallite size of the oxides formed by calcination at 450 ◦ C, as evaluated from the XRD patterns, are reported in Fig. 4a. Cobalt pure oxide presents spinel crystallites with size close to 30 nm. Doping with manganese immediately decreases the crystallite size, which reaches values below 20 nm from 5% doping. For manganese-richer compositions, all samples show nanocrystallite size 12 ± 5 nm, whatever the phase structure. The surface area of the oxides, reported in Fig. 4b, increases with the decrease of crystallite size. Cobalt pure oxides develop the lowest surface areas, while manganese-richer oxides are more dispersed and reach 60 m2 g−1 for samples from alginate HG1 and up to 87 m2 g−1 for samples from alginate LG1. The calcination temperature was chosen according to thermogravimetric studies. At 450 ◦ C all the organic part of aerogel is removed. When calcination is performed at 600 ◦ C, the crystalline phase is the same as it was at 450 ◦ C but bigger crystallites are obtained and the mean diameter increases, for example, from 12 nm to 22 nm for Co–Mn oxide (70 Mn%). Along with this phenomenon, the surface area decreases from 59 m2 g−1 to 20 m2 g−1 . SEM images of cross-sections of the same sample, reported in Fig. 5, show thread-like structures of crystallites, reminiscent of the fibrils of the parent aerogel [21]. Crystallite size is clearly larger when the sample has been calcined at higher temperature. The TEM imaging of the sample calcined at 600 ◦ C, presented as an example in Fig. 5, shows an inter-reticular distance of 0.48 nm, in good agreement with the hausmannite structure (d101 = 0.482 nm).
Fig. 3. M3 O4 unit volume in Co–Mn oxides.
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Fig. 4. Crystallite size (a) and surface areas (b) of Co–Mn oxides depending composition.
Fig. 5. SEM images of Co–Mn oxide (70 Mn%) for two different calcination temperatures TEM images of 600 ◦ C calcinated Co–Mn sample.
3.5. Magnetic properties The magnetic measurements have been performed on Co3 O4 , Mn3 O4 and some mixed oxides by using dc (direct current) mode by using SQUID magnetometer working in 1.8–350 K temperature range. Figs. 6 and 7 show the temperature dependence of the magnetization performed following zero field-cooled (ZFC)/fieldcooled (FC) magnetization procedure. In the ZFC experiment, the
sample was cooled in the absence of a static magnetic field and the magnetization was then recorded as a function of the temperature under a 50 Oe field. The FC magnetization data were collected after cooling the sample with the same field of 50 Oe. In its bulk form, Co3 O4 is an antiferromagnetic material having a normal cubic spinel structure AB2 O4 . The Co3+ ions have no moment at the octahedral B sites, while the Co2+ ions have a permanent moment of 3.25 B at the tetrahedral A sites [22]. Its Néel temperature has
Fig. 6. Magnetic measurement for Co3 O4 sample: (a) ZFC/FC curve with an applied field of 5 mT; (b) hysteresis loop at 2.5 K.
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Fig. 7. Magnetic measurement of Mn3 O4 : (a) ZFC/FC curve with an applied field of 5 mT; (b) hysteresis loop at 2.5 K.
been reported between 30 K and 40 K [23]. The magnetic behavior of Co3 O4 at nano-scale shows a size dependence of blocking (or freezing temperature), hysteresis effect and various conflicting claims have been made to determine the spin-glass or superparamagnetic behavior of these particles [24]. The ZFC curve of Co3 O4 (Fig. 6a) exhibits a maximum at Tmax = 38 K that corresponds to the de-freezing temperature or blocking temperature for the nanoparticles. The shape of this curve and the value of the maximum temperature are similar to what is observed for the antiferromagnetic Co3 O4 nanoparticles with diameter around 30 nm previously reported by Zhu [25]. This size corresponds to what is estimated from the XRD measurements. The FC curve follows the ZFC curve at high temperature and separates from the ZFC after Tmax . The decrease of the FC curve after the ZFC peak suggests the presence of strong interparticles dipolar interactions. The magnetization as a function of the applied field was measured at 2.5 K with the field swept from ±50 Oe. Fig. 6b shows the hysteretic behavior of the Co3 O4 . The ZFC hysteresis loop is symmetrical about the origin with a coercive field (HC ) of 200 Oe, and is coherent with value report in the literature [24]. The ZFC curve of Mn3 O4 (Fig. 7a) shows a maximum at 41 K corresponding to the blocking (or freezing) temperature for these particles. The FC curve follows initially the ZFC curve, separates from the ZFC curve, then increases rapidly and tends to saturation at low temperature. Both curves are in agreement with what was previously observed for spherical Mn3 O4 nanoparticles or nanoobjects larger than 10 nm (Tmax = 41 K) [26]. The field dependence of the magnetization performed for Mn3 O4 samples sample at 2.5 K displays a hysteresis with a high coercive field of 12,800 Oe (Fig. 7b,
Table 2 Blocking temperature, coercive field and magnetization value of the oxides.
Co3 O4 20% Mn 70% Mn Mn3 O4 a
TB (K)
HC (kOe)
MS a (emu/g)
38 33 89 43
0.2 3.2 1.5 12.8
33 10 4 19
Magnetization value observed for 50 kOe applied magnetic fields.
Table 2). These nanoparticles exhibit huge coercivity in comparison with the values previously reported for Mn3 O4 nanoparticles (for instance HC = 2500 Oe) [27] that may be explained by the presence of interparticles interactions. Fig. 8 shows the ZFC curves measured for samples with 20% and 70% of Mn as well as for two pure Co3 O4 , Mn3 O4 samples for comparison. The ZFC curve of Co–Mn oxide (20 Mn%) shows only one peak with Tmax equal to 33 K indicating the blocking (or freezing) temperature for these nanoparticles while Co–Mn oxide (70 Mn%) presents also one peak with Tmax = 70 K. The presence of one peak for both samples clearly indicates the presence of one mixed oxide phase in each nanoparticle and not the mixture of Co3 O4 and Mn3 O4 nanoparticles in agreement with the results of XRD analysis. In addition, the appearance of a blocking (freezing) temperature at 70 K also obviously designates the formation of a well-defined mixed Co–Mn oxide phase. The field dependence of the magnetization performed for these samples shows that all of them present a hysteretic behavior with coercive field of 3.2 and 1.5 kOe for 20 and 70% of Mn, respectively. The very low magnetic permeability of
Fig. 8. Magnetic measurement of Co3 O4 (), 20% Mn (䊉), 70% Mn (), Mn3 O4 (), (a) ZFC curve with an applied field of 5 mT (the y-axis is right for Mn3 O4 and left for the other samples); (b) hysteresis loop at 2.5 K.
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the mixed oxides is in agreement with the giant magnetoresistance postulated for these materials [28]. 4. Conclusion Co–Mn heterometallic ionotropic gel of alginate can be easily obtained in the whole range of composition. Supercritical CO2 drying affords to obtain aerogels of cation-linked polymers with surface areas ranging from 70 to 330 m2 g−1 with a perfect control of the composition in terms of cations content. They are good precursors for the synthesis of oxides, since the atomic dispersion of the cations in the polysaccharide and the low temperature of thermal degradation of the organics shift the balance between nucleation and growth toward the formation of a larger number of smaller oxide crystals. The Co–Mn mixed oxides formed present surface areas as high as 80 m2 g−1 . The natural variability of alginates has no influence on the composition of the oxides but controls their dispersion. The organization of the oxide nanocrystals in arrays reminiscent of the fibrillar structure of the parent gel confers them peculiar magnetic properties in term of coercivity and magnetoresistance and prevents them from packing, a property potentially useful for catalytic applications. Globally, the alginate route allows a fine control of composition and surface area of mixed Co–Mn oxides in the whole field of composition of the solid solutions. Its easy implementation recommends it as a preferred way for the preparation of oxidation catalysts for which the ratios of transition metal cations and the size of particles strongly affect conversion and selectivity. Acknowledgments We thank Dr. G. Mosser and S. Quignard (UPMC UMR 7574) for transmission electronic microscopy. References [1] (a) T. Hyeon, Chemical Communications (2003) 927; (b) K.J. Klabunde, in: K.J. Klabunde (Ed.), Nanoscale Materials in Chemistry, Wiley Intersciences, New York, 2001; (c) M.R. Diehl, J.-Y. Yu, J.R. Heath, G.A. Doyle, S. Sun, C.B. Murray, Journal of Physical Chemistry B 105 (2001) 7913; (d) D.L. Leslie-Pelescky, R.D. Rieke, Chemistry of Materials 8 (1996) 1770. [2] M.J. Keyser, R.C. Everson, R.L. Espinoza, Applied Catalysis A 171 (1998) 99–107. [3] T.E. Feltes, L. Espinosa-Alonso, E.d. Smit, L. D’Souza, R.J. Meyer, B.M. Weckhuysen, J.R. Regalbuto, Journal of Catalysis 270 (2010) 95–102.
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