DOI: 10.1021/cg900648q
Mesoporous NiCo2O4 Spinel: Influence of Calcination Temperature over Phase Purity and Thermal Stability
2009, Vol. 9 4814–4821
Moises Cabo,† Eva Pellicer,*,† Emma Rossinyol,†,‡ Onofre Castell,‡ Santiago Suri~ nach,† and Maria Dolors Bar o†,‡ †
Departament de Fı´sica and ‡Servei de Microsc opia, Facultat de Ci encies, Universitat Aut onoma de Barcelona, E-08193 Bellaterra, Spain Received June 10, 2009; Revised Manuscript Received August 26, 2009
ABSTRACT: Mesoporous NiCo2O4 spinel has been synthesized by nanocasting, using SBA-15 and KIT-6 silica as hard templates. Two temperatures of calcination were applied for the conversion of the spinel precursors (metal nitrates) into NiCo2O4. At 375 C the pure spinel was obtained, whereas at 550 C NiO impurities were detected. The mesoporous powders obtained were characterized by transmission electron microscopy, X-ray diffraction, Brunauer-Emmett-Teller analysis, and magnetic measurements. In addition, their thermal stability was assessed by post heat-treating the materials at 550 C: the mesostructure of the pure spinel (calcined at 375 C) collapsed, leading to the disruption of the porous network, while the mesoporosity of the powders calcined at 550 C was preserved. In all cases, the post-thermal treatment induced the segregation of NiO; the final NiO amount was found to be in the 7-10 wt % range.
1. Introduction One of the most well-known strategies for fabricating mesoporous metal and metal oxides is the nanocasting method by using mesostructured silica as a “hard” template.1-3 Several mesoporous silicas with various pore geometries (MCM-41, SBA-15, SBA-16, KIT-6...) can be used as templates to render a negative replica of a non-siliceous material.4 The synthesis of about 10 mesoporous transition metal oxides (Co3O4, CeO2, WO3, In2O3...) has already been reported, as triggered by their potential applications in catalysis, gassensing, photonic and electronic devices, and drug delivery to name a few.5 Basically, inorganic precursors are incorporated into the channels of the silica matrix, which serves as a host, and form an inorganic framework by further treatments, usually by thermal treatment. The first step comprises the impregnation of the silica matrix by the precursor, which can be done using different approaches (surface modification method, dual-solvent method, evaporation method, and solid-liquid method). Once the matrix pores are filled with the precursor, a thermal treatment is applied to decompose it and form the related oxide. The last step is the silica removal by using either HF or NaOH solution. Compared to mesoporous simple oxides, the synthesis of mesoporous binary or mixed metal oxides via a hard template route has been much less explored.6,7 NiCo2O4 is an interesting binary oxide with applications in several fields: electrocatalysis, flat panel displays, ferrofluid technology, drug delivery and local hyperthermia, optical limiters, chemical sensors, etc., which have been pursued by virtue of its low overpotential for oxygen evolution reaction (OER),8 high transparency in the infrared region,9 and magnetic10 and gas-sensing properties.11 This compound shows a spinel structure and its instability above usually 400-500 C has constrained the effectiveness of the synthetic pathways to those operating at low temperatures.12,13 Actually, the structure of NiCo2O4 has been a matter of controversy for many *To whom correspondence should be addressed. E-mail: eva.pellicer.
[email protected]. pubs.acs.org/crystal
Published on Web 09/10/2009
years. It can be described as an inverse spinel Ni1-xCoxOy where x = 0.67 and y = 4/3, x being the mole fraction of Co ions in the oxide (x = Co/(Co þ Ni)).14 In a typical inverse spinel, Co ions are equally distributed between the tetrahedral and octahedral sites, whereas Ni ions are only located at octahedral sites. Recently, Ogale and co-workers reported on the synthesis of nearly monodispersed NiCo2O4 nanoparticles by a combustion method using glycine as a fuel and cobalt/nickel nitrates as oxidizer.12 The synthesis of mesoporous NiCo2O4 nanoparticles in a single step by using nonionic surfactant Brij-35 as a structure-directing agent and a heterometallic single-source precursor as the inorganic source has already been reported.15 Though the nanoparticles displayed a unimodal pore distribution, the NiCo2O4 spinel phase was accompanied by undesirable phases such as NiO and Ni and Co in metallic form. The highest spinel phase content was 81 wt %. Herein we report the hard template route to synthesize mesoporous NiCo2O4 spinel from SBA-15 (space group P6mm) and KIT-6 (space group Ia3d) silica templates, paying special attention to the influence of the calcination temperature over the phase purity. With this aim, two temperatures of calcination, namely, 375 and 550 C, were used for the conversion of the inorganic precursors (nitrates) into the NiCo2O4 compound. These values were chosen to be below and above the temperature at which the spinel becomes unstable (400-500 C). A detailed structural characterization is also presented to evaluate the thermal stability of the obtained porous crystals. For that purpose, we explored the degree of spinel decomposition and the possible loss of mesoporosity after a thermal treatment at 550 C. 2. Experimental Section Mesoporous silica SBA-15 was synthesized by dissolving 6.0 g of Pluronic P123 copolymer (kindly supplied by BASF Corporation) in diluted HCl. 12.5 g of tetraethyl ortosilicate (TEOS, from SigmaAldrich), which served as the silicon source, was then added and the solution was stirred for 24 h at a constant temperature (about 37 C). The hydrothermal treatment was carried out at 90 C in a sealed r 2009 American Chemical Society
Article
Crystal Growth & Design, Vol. 9, No. 11, 2009
4815
Figure 1. TEM images of (a, b) SBA-15 silica, (c, d) SBA-15 templated NiCo2O4 (calcined at 550 C). The arrow in (c) points to a group of randomly oriented nanoparticles. The d spacings of 2.8, 2.4, and 2.0 A˚ correspond to (220), (311), and (222), respectively, of the cubic NiCo2O4. container and the solid obtained was filtered, copiously washed with water, and finally calcined at 550 C for 5 h to remove the organics. Mesoporous silica KIT-6 was synthesized under similar conditions except for the addition of 1-butanol after dissolution of the P123 surfactant. The evaporation method was used as an impregnation step. 0.150 g of mesoporous silica template was mixed with 0.192 g of Co(NO3)2 3 6H2O and 0.096 g of Ni(NO3)2 3 6H2O (both from SigmaAldrich, 99.999% purity) dissolved in ethanol. The Co/Ni molar ratio was 2:1. The mixture was stirred for 30 min in a crucible and left for ethanol evaporation overnight. During solvent evaporation the metal nitrates migrated into the silica nanochannels by capillary action. The crucible was then placed in a tubular furnace and the impregnated silica was calcined. The furnace temperature was increased to 375 or 550 C at a rate of 3 C/min and held at these temperatures for 5 h and 4 h, respectively, under atmospheric conditions. At the end of this process, the furnace was slowly cooled down to room temperature. The silica matrix was removed with 2 M NaOH solution at 70 C under stirring. The resulting mesoporous replicas were collected after centrifugation and decanting off the supernatant, copiously rinsed in absolute ethanol, and finally dried. To evaluate the thermal stability of the powders, these were placed in a crucible and put again in the furnace. The heat-treatment was carried out in air as follows: the furnace temperature was increased to 550 C at a rate of 3 C/min, held at the target value for 4 h, and finally cooled down to room temperature. The porous crystals obtained were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) analysis. TEM characterization was performed on a Jeol-JEM 2011 microscope operated at 200 kV. The powders were dispersed in ethanol and then placed dropwise onto a holey carbon supported grid. XRD patterns were collected on a Phillips X’Pert diffractometer in the 15-902θ range
(step size = 0.03, step time = 10 s) using Cu KR radiation, at a voltage of 50 kV and 40 mA of current. The microstructural parameters were evaluated by fitting the full XRD patterns using the materials analysis using diffraction (MAUD) Rietveld refinement software.16,17 The goodness of fit was checked by the weighted R-value, Rw (%), which was well below 10% in all fittings. BET analyses were carried out on a Micromeritics ASAP 2020 accelerated surface area and porosimetry analyzer. N2 adsorption/desorption isotherms were recorded at 77 K after degassing the powders at 175-200 C for 10 h. Magnetic measurements were carried out using a superconducting quantum interference device (SQUID). Hysteresis loops were recorded in the ( 70 kOe range at 10 K after zero-fieldcooling (ZFC) from room temperature.
3. Results and Discussion First, TEM images were taken to confirm the formation of mesostructured powders by replication of the silica templates. No significant differences were found depending on the calcination temperature used for the synthesis (for simplicity, we have only shown here images corresponding to 550 C). One of the risks of the hard template route is the formation of bulk particles outside the silica host due to pore blocking, since such particles may not be nanostructured and grow even further during calcination. However, as this was not observed in any case, the choice of both the impregnation method and the temperatures of calcination were deemed appropriate. Figure 1 shows images of the SBA-15 silica template, displaying an expected structure as described in the literature, along with its corresponding replica.
4816
Crystal Growth & Design, Vol. 9, No. 11, 2009
Cabo et al.
Figure 2. TEM images of (a, b) KIT-6 silica, (c, d) KIT-6 templated NiCo2O4 (calcined at 550 C). The hexagonal ring is indicated in (d) along with the d spacings of 2.8 A˚ and 2.4 A˚ corresponding to (220) and (311) of the cubic NiCo2O4.
The replicated material is composed of nanowires of several hundreds of nanometers in length and 7-8 nm in diameter. The pore size of the parent SBA-15 silica is about 6-7 nm, thus confirming the growth of the material inside the mesochannels of the silica host, provided that we assume the uncertainty in the exact determination of these values by TEM.18 However, some isolated nanoparticles were also found, probably due to incomplete filling of the silica mesochannels (Figure 1c). When the load of the precursor is not enough to allow the formation of the small bridges which stack nanowires together, these small structures get free with the template removal and are not able to keep an organized structure but these random nanoparticles. Figure 1d shows a high-resolution TEM (HRTEM) image of the SBA-15 replica, in which the lattice fringes corresponding to (220), (311), and (222) planes are clearly seen, thus confirming that the framework walls are highly crystalline. The connecting bridge between two nanowires can also be observed. Representative TEM images of the KIT-6 silica template and its corresponding cubic mesoporous replica are displayed in Figure 2. Unlike the SBA-15 silica template, which shows a two-dimensional (2D) hexagonal symmetry, the KIT-6 silica is highly branched, displaying an interpenetrating bicontinuous network of channels (Figure 2a,b). These characteristics make the KIT-6 silica highly accessible to the metal sources. The corresponding replica predominantly features well-ordered three-dimensional (3D) large domains (Figure 2c). From each particle, the SAED patterns and HRTEM images indicate a
single-crystal feature. Figure 2d shows a detailed picture of the replicated material, in which the hexagonal rings typical of this mesophase can be observed. The diameter of these 3D large domains is much smaller than the KIT-6 silica particles, which can reach several micrometers in size. This suggests the simultaneous nucleation of several seeds inside the template. In the case of Cr2O3, it has been suggested that this may be due to mass transfer along different directions and disconnections of the particles during thermal decomposition and crystallization.19 In addition, it seems that uncoupled frameworks have been mostly formed. This may be due to a low interconnectivity within the gyroid structure of the silica template, thus allowing the precursor to fill one of the two enantiomeric subframeworks. Moreover, a single impregnation step was applied, which may also lead to the formation of uncoupled frameworks.20 The shape of the particles is different for both structures. While SBA-15 replica forms elongated structures, KIT-6 replica particles tend to be spherical in shape. This confirms a 3D growth resulted from the 3D pore system of both templates. For all replicas, energy dispersive X-ray microanalyses (EDS) confirmed the presence of both cobalt and nickel, together with the oxygen signal (not shown here). Silicon was not detected, thus proving the effectiveness of silica removal down to the trace level. The XRD patterns of the porous crystals are shown in Figure 3. The diffraction peaks are quite broad, reflecting the
Article
Crystal Growth & Design, Vol. 9, No. 11, 2009
4817
Figure 3. XRD patterns of (A) KIT-6 and (B) SBA-15 replicas calcined at 375 C, and (C) KIT-6 and (D) SBA-15 replicas calcined at 550 C together with the curves generated from the full-pattern procedure and the corresponding difference between the experimental and the calculated profiles. A sketch of the NiCo2O4 spinel unit cell is shown as an inset. *NiO phase. Table 1. Structural Parameters Obtained after Rietveld Refinement of the XRD Patterns of KIT-6 and SBA-15 Templated NiCo2O4 (þ NiO) Calcined at the Indicated Temperatures (in C) NiO system: cubic, space group: Fm3m
NiCo2O4 system: cubic, space group: Fd3m
KIT-6 replica 375 SBA-15 replica 375 KIT-6 replica 550 SBA-15 replica 550
a ((5 10-4 A˚)
ÆDæ ((5 nm)
Æε2æ1/2 10-3 ((0.5)
wt %
a ((5 10-4 A˚)
ÆDæ ((5 nm)
Æε2æ1/2 10-3 ((0.5)
wt %
8.1304 8.1287 8.1264 8.1241
15 14 17 17
2.2 1.0 0.7 1.3
100 100 98 97
4.1750 4.1771
9 11
4.5 3.9
2 3
nanocrystalline nature of the replicas. The absence of a broad reflection at around 2θ = 25 related to amorphous silica corroborates the successful removal of the silica template. The diffractograms of the samples calcined at 375 C (Figure 3A,B) show the formation of a single phase, in good agreement with the computed pattern for cubic NiCo2O4 (PCPDF No. 73-1702) spinel. A picture of the spinel unit cell is shown as an inset in Figure 3A. Table 1 lists the cell parameter, a, crystallite size, ÆDæ, and microstrain, Æε2æ1/2, values extracted from the Rietveld refinement of the XRD data of the SBA-15 and KIT6 nanocast NiCo2O4. On the other hand, the XRD patterns of the powders calcined at 550 C show the presence of NiO (PCPDF No. 73-1519) together with the NiCo2O4 phase (Figure 3C,D). The reflections marked with an asterisk in the patterns correspond to the (200) and (220) reflections of NiO in the cubic phase, located at around 2θ = 43.4 and 2θ = 63.0, respectively. In fact, partial decomposition of the spinel into NiO is known to occur above 400-500 C.12,13 The corresponding structural parameters and the amount
Figure 4. N2-gas adsorption-desorption isotherms of KIT-6 (b) and SBA-15 (O) mesoporous replica calcined at 550 C. The KIT-6 replica isotherm has been shifted for clarity.
4818
Crystal Growth & Design, Vol. 9, No. 11, 2009
Cabo et al.
Figure 5. XRD patterns recorded (a) before and (b) after a thermal treatment at 550 C of (A) KIT-6 and (B) SBA-15 replicas calcined at 375 C, and (C) KIT-6 and (D) SBA-15 replicas calcined at 550 C. Patterns b are shown together with the curves generated from the full-pattern procedure and the corresponding difference between the experimental and the calculated profiles. *NiO phase. Table 2. Structural Parameters Obtained after Rietveld Refinement of the XRD Patterns of KIT-6 and SBA-15 Templated NiCo2O4 (þNiO)a NiO system: cubic, space group: Fm3m
NiCo2O4 system: cubic, space group: Fd3m
KIT-6 replica - 375f550 SBA-15 replica - 375f550 KIT-6 replica - 550f550 SBA-15 replica - 550f550 a
a ((5 10-4 A˚)
ÆDæ ((5 nm)
Æε2æ1/2 10-3 ((0.5)
wt%
a ((5 10-4 A˚)
ÆDæ ((5 nm)
Æε2æ1/2 10-3 ((0.5)
wt%
8.1132 8.1173 8.1168 8.1233
21 18 20 18
3.1 3.2 3.1 2.8
90 93 92 93
4.1787 4.1820 4.1798 4.1787
18 27 11 16
4.8 6.1 5.1 6.8
10 7 8 7
T1 f T2 means that powders were calcined at T1 (in C) and post heat-treated at T2 (in C).
(in weight percent) of NiCo2O4 and NiO phases extracted from Rietveld refinement are shown in Table 1. Note that the amount of NiO formed is relatively low (2-3 wt %) even though the calcination temperature was well above 400 C. In all cases, the cell parameter of the NiCo2O4 phase is slightly larger than the charted value (a = 8.114 A˚). The difference is about 0.15% and can be ascribed to the nanocrystalline nature of the particles formed.21 Moreover, the formation of the NiO phase renders a Ni-deficient spinel, resulting in a lattice distortion from the ideal spinel structure. The Rietveld refinement also yielded a larger microstrain for the NiO phase, probably due to the hindered crystallization imposed by the silica mesochannels. BET analyses were carried out to determine the surface area of the synthesized replicas. N2 adsorption/desorption curves shown in Figure 4 can be classified as type IV isotherm according to the IUPAC.22 The capillary condensation step is not very pronounced, which constitutes
an indication of the relatively small size of ordered domains.18 BET surface areas are rather high, that is, 80.6 m2/g for the SBA-15 and 93.7 m2/g for the KIT-6 replica. Carreon et al. reported specific surface areas in the range of 35.0-83.8 m2/g for mesoporous nanocrystalline NiCo2O4 prepared by onestep nanocasting.15 In that work, the sample calcined at 450 C appeared to be nonmesoporous, showing a specific surface area as low as 12 m2/g. Hence, the hard template route would overcome the problems of structural instability encountered by other approaches since the material has been successfully replicated even at 550 C calcination temperature. In order to assess the thermal stability of the synthesized spinels, the materials were heat-treated under atmospheric conditions and the XRD patterns were further recorded. The heat-treatment induced the segregation of NiO from the spinel cell (Figure 5, pattern b), as the applied temperature exceeded 400 C, that is, the temperature at which the decomposition
Article
Crystal Growth & Design, Vol. 9, No. 11, 2009
4819
Figure 6. TEM images of NiCo2O4 powders before the thermal treatment at 550 C of (a) SBA-15 replica calcined at 375 C, (b) KIT-6 replica calcined at 375 C. TEM images after the thermal treatment at 550 C of (c) SBA-15 replica calcined at 375 C, (d) KIT-6 replica calcined at 375 C, (e) SBA-15 replica calcined at 550 C, and (f) KIT-6 replica calcined at 550 C. (Images of both replicas calcined at 550 C before the thermal treatment are already shown in Figures 1 and 2.)
begins.12,13 Co3O4 and/or Co and Ni metals were not detected, which means that if formed, their amounts are below the detection limit of the technique. The formation of NiO as the main secondary phase is in agreement with other works.23 Notice that in all cases the final NiO amount is slightly higher for the KIT-6 replica. As seen by BET, the KIT-6 replica has a higher surface area, thereby providing a greater free volume through which NiO can segregate. A more in-depth analysis of the microstructural parameters allows drawing some trends regarding the cell parameter and the crystallite size. In all cases, the heat-treatment makes the cell parameter of the NiCo2O4 phase decrease. Indeed, the final cell parameter is almost the same as the charted value, except for the SBA-15 replica calcined at 550 C, for which it is slightly larger (8.1233 A˚). It is worth noticing that the slight deviation of the cell parameter from the charted value can be partly attributed to the formation of a Co-rich off-stoichiometric NiCo2O4 spinel due to NiO segregation. Conversely, the cell parameter of the NiO phase increases, the KIT-6 replica showing the largest increase from 4.1750 to 4.1798 A˚ (compare Tables 1 and 2). Moreover, the microstrain associated with the NiO phase is higher than that of the spinel phase. As expected, the post heat-treatment made the crystallite size of the NiCo2O4 phase increase, reaching the same value irrespective of the calcination temperature. The crystal size of the NiO phase also increased for both replicas after annealing of the powders formerly calcined at 550 C.
The heat-treated powders were also imaged by TEM, revealing that the mesoporous frameworks of the spinel calcined at 375 C and heat-treated at 550 C collapsed, leading to the disruption of the 3D porous network (Figure 6a-d). Indexed SAED patterns of pure and heattreated NiCo2O4 spinel are shown in Figure S1, Supporting Information. For the latter, the diffraction spots related to NiCo2O4 appear together with the ones of the NiO phase, in agreement with XRD analyses. Henceforth, the spinel synthesized at 375 C is not thermally stable under these conditions. Probably, the collapse of the mesoporous structure does not occur suddenly but gradually with the increase of the treatment temperature starting from 400 C. In the case of the SBA-15 replica, an insufficient bridging between the nanowires due to the relatively low precursor loading used for the synthesis may explain the collapse of the mesostructure. Interestingly, the mesoporous structure of the porous crystals calcined at 550 C and heattreated at the same temperature was undamaged (Figure 6e,f). Therefore, though the spinel is accompanied by NiO as impurity, the porous network is thermally stable. Thus, the control over the structure is better. This means that we cannot solely advocate to the precursor loading effect to explain the collapse of the mesostructure of the spinel synthesized at 375 C since the amount of metal source was the same in all cases. In light of these results, further investigations are open, such as analyzing the influence of the heating time over the spinel decomposition process or the
4820
Crystal Growth & Design, Vol. 9, No. 11, 2009
Figure 7. Hysteresis loops for SBA-15 mesoporous powders synthesized at 375 C (pure spinel, Ο) and synthesized at 550 C followed by post heat-treatment at the same temperature (NiCo2O4 þ NiO, Δ). The inset shows a zoomed detail in the low field range.
precursor loading level over the thermal stability of the porous network. We finally show the hysteresis loops at 10K for the NiCo2O4 replica synthesized at 375 C (exhibiting pure spinel phase) and the post heat-treated material synthesized at 550 C (having the maximum amount of NiO) (Figure 7). In both cases, the loops are characteristic of a ferro- or ferrimagnetic material, although the magnetization does not saturate even at the maximum applied field (70 kOe) as expected from magnetic surface disorder effects often observed at reduced dimensions.24 Nevertheless, the loops resemble those reported for NiCo2O4 nanoparticles.12 In fact, bulk NiCo2O4 is a ferrimagnet with a Curie temperature (TC) of about 400 C.25 The coercive field (HC) of the pure spinel replica is 800 Oe, and it slightly increases up to 860 Oe for the post heattreated sample, which could be attributed to the magnetic pinning effects of the NiO phase.26 The slight decrease of magnetization (M) for the post heat-treated sample can be ascribed to the segregation of antiferromagnetic NiO phase and the concomitant formation of off-stoichiometric nickeldeficient NiCo2O4. Though a more exhaustive magnetic characterization would be required depending on the eventual application (ferrofluid technology, magnetic carriers for targeted drug delivery, contrast enhancement agents for magnetic resonance imaging, etc.), these results already demonstrate that the mesoporous powder synthesized at 550 C, with increased thermal stability, holds magnetic properties similar to the pure spinel replica. 4. Conclusions Mesoporous NiCo2O4 in SBA-15 and KIT-6 structures has been successfully synthesized by a hard template route. The pure NiCo2O4 spinel was obtained at a low calcination temperature (375 C), whereas NiCo2O4 and NiO phases were formed at a higher calcination temperature (550 C). The mesoporous powders obtained were post heat-treated at 550 C in air to evaluate their thermal stability. In all cases, NiCo2O4 decomposed to NiO, yielding impurity amounts of about 7-10 wt %. However, whereas the mesostructure of the pure spinel collapsed, it was kept intact for the SBA-15 and KIT-6 replicas synthesized at 550 C. From the application viewpoint, we envisage that the structural collapse would affect material performance in some way. One can notice that
Cabo et al.
the use of the mesoporous spinel calcined at 375 C should be restricted to those applications where the operation temperature does not exceed 400 C. For instance, its use as a gassensing element under experimental conditions that require heat-treating the sensor at temperatures above 400 C to enable gas detection may be seriously compromised (apart from the NiO segregation, the mesostructure would collapse). Below this temperature, the mesoporous pure spinel could be, in principle, safely used, though this hypothesis needs to be further assessed. For those applications where the phase purity is not a critical issue, our results clearly demonstrate that mesoporous SBA-15 and KIT-6 NiCo2O4 (with over 90 wt % spinel phase) can be synthesized at 550 C and used at this temperature or lower, ensuring that the mesoporosity is preserved. Moreover, the magnetic characterization indicates that the powder synthesized at this temperature holds magnetic properties similar to the pure spinel replica, overcoming thus the limitation in the temperature range for the different applications. Acknowledgment. The authors thank the Servei de Microsc opia of the Universitat Aut onoma de Barcelona for the equipment facilities and greatly acknowledge BASF Corporation for kindly supplying the P123 precursor. We acknowledge funding from the Spanish Ministerio de Ciencia e Innovaci on (MICINN) through MAT 2007-66309-C02-02 project. We thank MATGAS 2000 AIE for the provision of their facilities, Dr. Pau Solsona for technical assistance in the BET experiments, and Dr. M. Estrader and A. L opez-Ortega for the magnetic measurements. We also thank Prof. J. Nogues for critical reading of the manuscript and enlightening comments. E. Pellicer is indebted to the DURSI of the Generalitat de Catalunya for the postdoctoral Beatriu de Pin os fellowship. Supporting Information Available: SAED patterns of nanocast SBA-15 NiCo2O4 synthesized at 375 C before and after thermal treatment at 550 C. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Tiemann, M. Chem. Mater. 2008, 20, 961–971. (2) Guo, X.-J.; Yang, C.-M.; Liu, P.-H.; Cheng, M.-H.; Chao, K.-J. Cryst. Growth Des. 2005, 5, 33–36. (3) 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. J. Am. Chem. Soc. 2004, 126, 865–875. (4) Wan, Y.; Yang, H.; Zhao, D. Acc. Chem. Res. 2006, 39, 423–432. (5) Yue, W.; Zhou, W. Prog. Nat. Sci. 2008, 18, 1329–1338. (6) T€ uys€ uz, H.; Lehmann, C. W.; Bongard, H.; Tesche, B.; Schimdt, R.; Sch€ uth, F. J. Am. Chem. Soc. 2008, 130, 11510–11517. (7) Xia, Y.; Mokaya, R. J. Mater. Chem. 2005, 15, 3126–3131. (8) Cui, B.; Lin, H.; Li, J.-B.; Li, X.; Yang, J.; Tao, J. Adv. Funct. Mater. 2008, 18, 1440–1447. (9) Windisch, C. F., Jr.; Exarhos, G. J.; Ferris, K. F.; Engelhard, M. H.; Stewart, D. C. Thin Solid Fims 2001, 398-399, 45–52. (10) Marco, J. F.; Gancedo, J. R.; Gracia, M.; Gautier, J. L.; Rı´ os, E. I.; Palmer, H. M.; Greaves, C.; Berry, F. J. J. Mater. Chem. 2001, 11, 3087–3093. (11) Zhang, G. Y.; Guo, B.; Chen, J. Sens. Actuators, B 2006, 114, 402– 409. (12) Verma, S.; Joshi, H. M.; Jagadale, T.; Chawla, A.; Chandra, R.; Ogale, S. J. Phys. Chem. C 2008, 112, 15106–15112. (13) Kobayashi, Y.; Ke, X.; Hata, H.; Schiffer, P.; Mallouk, T. E. Chem. Mater. 2008, 20, 2374–2381. (14) Windisch, C. F., Jr.; Exarhos, G. J.; Owings, R. R. J. Appl. Phys. 2004, 95, 5435–5442. (15) Carreon, M. A.; Guliants, V. V.; Yuan, L.; Hughett, A. R.; Dozier, A.; Seisenbaeva, G. A.; Kessler, V. G. Eur. J. Inorg. Chem. 2006, 4983–4988.
Article (16) Lutterotti, L.; Ceccato, R.; Dal Maschio, R.; Pagani, E. Mater. Sci. Forum 1998, 278-281, 87–92. (17) http://www.ing.unitn.it/∼maud/ (18) Rumplecker, A.; Kleitz, F.; Salabas, E. L.; Sch€ uth, F. Chem. Mater. 2007, 19, 485–496. (19) Jiao, K.; Zhang, B.; Yue, B.; Ren, Y.; Liu, S.; Yan, S.; Dickinson, C.; Zhou, W.; He, H. Chem. Commun. 2005, 5618–5620. (20) Rossinyol, E.; Prim, A.; Pellicer, E.; Arbiol, J.; Hernandez-Ramı´ rez, F.; Peir o, F.; Cornet, A.; Morante, J. R.; Solovyov, L. A.; Tian, B.; Bo, T.; Zhao, D. Adv. Funct. Mater. 2007, 17, 1801–1806.
Crystal Growth & Design, Vol. 9, No. 11, 2009
4821
(21) Zhang, F.; Chan, S.-W.; Spanier, J. E.; Apak, E.; Jin, Q.; Robinson, R. D.; Herman, I. P. Appl. Phys. Lett. 2002, 80, 127–129. (22) Sing, K. S. W. Pure Appl. Chem. 1985, 57, 603–619. (23) Chi, B.; Li, J.; Han, Y.; Chen, Y. Int. J. Hydrogen Energy 2004, 29, 605–610. (24) Batlle, X.; Labarta, A. J. Phys. D: Appl. Phys. 2002, 35, R15–R42. (25) Kim, J.-G.; Pugmire, D. L.; Battaglia, D.; Langell, M. A. Appl. Surf. Sci. 2000, 165, 70–84. (26) Nogues, J.; Sort, J.; Langlais, V.; Skumryev, V.; Suri~ nach, S.; Mu~ noz, J. S.; Bar o, M. D. Phys. Rep. 2005, 422, 65–117.
