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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.55 n.2 Concepción jun. 2010 

J. Chil. Chem. Soc, 55, N° 2 (2010), págs.: 261-265





1 Laboratorio de Fisicoquímica y Electroquímica de Sólidos. Departamento de Química de los Materiales, Facultad de Química y Biología, Universidad de Santiago de Chile, Av. L. B. O 'Higgins 3363, Santiago, Chile.
2 Departamento de Física, Facultad de Ciencia, Universidad de Santiago de Chile, Av. Ecuador 3493. Santiago, Chile.
3 Centro para la Investigación Interdisciplinaria Avanzada en Materiales, CIMAT, Av. Blanco Encalada 2008, Santiago, Chile.
4 Departamento de Química, Facultad de Ciencias Básicas, Universidad de Antofagasta, Campus Coloso, Antofagasta, Chile.


Two extreme compositions of the manganese-cobalt oxide system MnxCo3xO4 namely Co3O4 (x = 0) and MnCo2O4 (x = 1) were prepared in powder form at low temperature by thermal decomposition of nitrates in air. The synthesis was conducted at 150°C. The oxides obtained were characterized by different techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), surface area determination using the Brunauer-Emmett-Teller (BET) method, thermogravimetric analyses (TGA) and oxidation power (OP). XRD analysis revveals that the oxides crystalhze in a cubic spinel-type structure with a unit cell parameter (ao) that increases as cobalt is replaced by manganese. The MnCo2O4 showed 70 m2 g-1 and 13 nm of specific surface area and crystallite size respectively. The crystallite was bigger (20nm) when cobalt-manganese oxide is prepared as athin film. Both compounds exhibited nanometric size. The cationic distributions proposed for both oxides on the basis of the physicochemical characterization results showed an excellent agreement with those obtained from the XRD data refinement performed using the Rietveld method.

Keywords: Manganese cobalt oxide, Mixed oxides, Spinels, Solid Solution, Manganese-cobalt distribution


Spinel type oxides from transition metals, particularly the manganese-cobalt oxide family, are interesting systems because of their potential for a variety of applications in the conversion of energy 1,2. Many different synthetic routes have been attempted to prepare MnxCo3 xO4 oxides in powder form3. The commonest method for the preparation of these spinels has been the thermal decomposition of nitrate precursors at different temperatures. From a practical point of view, the search is oriented to obtain the highest specific area in addition to the highest intrinsic activity. Mixed valence oxides have rather low specific surface areas, mainly when they are prepared at high temperature by conventional ceramic methods or decomposition of dry salts routes. However, they may reach higher specific surface areas when they are obtained at lower temperatures by thermal decomposition of suitable precursors4.

In the thermal process of powder preparation, the higher specific areas are obtained at lower temperatures 5. On the other hand, when the powders are synthesized at low temperatures from aqueous solutions, water molecules and OH groups end up attached to the oxide structure4. Besides, thermally formed oxides are as a rule non-stoichiometric. A common feature is that non-stoichiometry decreases when the calcination temperature increases, thus paralleling the decrease in surface area 6. The solid-state properties of the two extreme compositions of a series of mixed oxides MnxCo3 xO4 (0 ≤ ≤ x £ 1), i.e, the spinel oxides of cobalt Co3O4 (x = 0) and of manganese an d cobalt MnCo2O4 (x = 1), have been more intensively investigated than the intermedíate compositions (with x varying from x = 0 to x = 1 and with Δx = 0.25).

The formula of a binary spinel oxide is A[B4]O4 in which A is a cation tetrahedrically coordinated with the oxygen while the B cation is octahedrically coordinated with the oxygen. Each cubic unit cell contains 8 formal units A[B4]O4, that is to say it corresponds to A8B16O32. Spinels are usually classified as normal, inverted or random2.

Owing to the múltiple oxidation states that the mixed oxides formed in the Mn-Co-O system can adopt, the determination of the oxidation states and cation distributions among both the tetrahedral and octahedral sublattices of the spinel structure are not simple tasks 7-9. There is overwhelming evidence in the literature that the distribution of the cationic oxidation states among the different crystallographic sites greatly depends on the preparation conditions and also determines the textural and morphological characteristics of the powder oxides 10.

Early investigations by Wickham and Croft11 and Naka et al.12 have shown that a single stable phase of the cubic manganese-cobalt spinel exists at 1000°C only for the 0 ≤x ≤1.3 composition range. It is now widely accepted in the literature 13-19 that Co3O4 exhibits a normal cationic distribution, with low spin Co3+ in octahedral sites and high spin Co2+ in tetrahedral sites: Co2+[CoIII CoIII]. In contrast, the cationic distribution of MnCo2O4 is not well established. Several cationic distributions have been proposed in stoichiometric MnCo2O4 and non-stoichiometric MnxCo3-xO4+d oxides on the basis of (i) electrical conductivity 10: Co2+[Co2+ Mn4+]O4 or Co3+[Co3+ Mn2+]O4, ii) magnetism Co2+[Co3+2-x Mn3+x]O4 by Wickham and Croft11 and Co2+[Co2+ Mn4+]O4, by Blasse 20, and iii) neutron diffraction and magnetism measurements: Co2+[Co3+2-x Mn3+x]O4 by Boucher et al.21. MnxCo3- xO4 oxides are commonly described as being inverse spinels in which the manganese cations show preference for octahedral sites. Studies of the surface composition of MnCo2O4 by XPS have shown that the Co is present as Co2+ and Co3+ while Mn is mainly present as Mn3+ ions 22. Ravindranathan et al. 23 reported onthe synthesis of MnCo2O4 from its organic precursor decomposition below 250°C. Yamamoto et al. 24 used neutron diffraction technique to study the oxide MnCo2O4 synthesised by a wet procedure at 80°C, and have proposed Mn1-xCox [MnxCo2-x]O4 as the atomic distribution, where x is the inversion parameter. On the basis of chemical analysis, DRX, TGA, DTA and oxidation power measurements, Gautier, Fuentealba and Cabezas 25 proposed a global formulae and ionic stoichiometry of MnxCo3-xO4 (1 ≥ x ≥ 0.25) prepared at 400°C.

Depending on the preparation method and the calcination temperature, Gautier et al. have suggested the cationic distributions: Co2+[Co3+ Mn2+0.35Mn+3+0.29Mn4+0.36]O4 and Co2+[Co3+0.95 Mn2+0.015 Mn3+0.50 Mn4+0.485 0.05]O4,27. The occurrence of Mn4+ cations in octahedral sites has been again considered by Jabry, Rousset and Lagrange 2S from X-ray and electrical conductivity measurements: Co2+[Co2+x Co2+2(1-x) Mn4+x]O4 . Ríos et al.1 have prepared by thermal decomposition at 400°C of nitrates salts the entire series MnxCo3 xO4 (0 ≤ x ≤ 1) and proposed the corresponding cationic distributions. In the recent past we have undertaken the study of the title compound by spray pyrolysis at 150°C of the two extreme compositions Co3O4 and MnCo2O4, but were unable to obtain cationic distribution measurements due to material paucity 5. Co3O4 was prepared by a sol-gel process at 300°C 29 with large specific area (27 m2 g-1) while De Vidales et al.4 have prepared MnCo2O4 by a sol-gel route at 80°C with larger specific surface area. The temperature at which the Co3O4 spinel oxide is formed changes with the heating rate. Pope et al. 30 observed that the decomposition was already completed at 200°C, when the heating rate was lowered, while Garavaglia et al.31 reported that at sufficiently long times Co3O4 is formed at 150°C (24 h).

In this work, we report the successful preparation at low temperature, on the two extreme compositions of MnxCo3-xO4 material, Co3O4 (x = 0) and MnCo2O4(x = 1), using a thermal decomposition procedure with proper salt precursors. A series of physicochemical measurements have enabled us to propose the cationic distributions which have been confirmed by the application of the Rietveld structural analysis method. A further crystallographic study for the rest ofthe oxides family, namely for x = 0.25, 0.5 and 0.75, and a study ofthe effects of solid state properties of the entire series of the spinel-type MnxCo3- xO4 (0 ≤ x ≤ 1) compounds on their electrocatalytic properties, are currently in progress.


Preparation of MnCo3O4 oxides

The chemical synthesis ofthe polycrystalline compounds MnxCo3-xO4 (x = 0 and 1.0) were conducted by thermal decomposition of nitrate solutions.

Co3O4 synthesis

Co(NO3)2 • 6H2O (Fluka p.a. ref. 60833) was allowed to dissolve in its crystallization water by gradually heating it up to 100°C. The solution was then evaporated and the residual solid nitrate was further decomposed in air at 150°C.

MnCo2O4 synthesis

The aqueous precursor solutions were made up of cobalt and manganese nitrate salts (analytical grade) Co(NO3)2 • 6H2O (Fluka p.a. ref. 60833) and Mn(NO3)2 • 4H2O (Merck p.a. ref. 5942), whose concentrations were 0.100 and 0.0947 M, respectively. To avoid metal hydroxide precipitation, these solutions were slightly acidified with 1 mL of concentrated HNO3 (Merck p.a.) per liter. The expected stoichiometry was obtained by mixing the required amounts ofthe starting solutions to yield the desired concentration. The overall concentration ofthe mixed solutions was 0.08 M.

The above mentioned cobalt nitrate and the corresponding mixture were separately heated slowly on a sand bath to evaporate the water until a dry solid residue was obtained. Further decomposition in air at 150°C was conducted until complete cessation of the red-coloured nitrous vapours (NOx), coming from the nitrates decomposition. The residual powder was carefully ground, sieved (400 mesh) and then calcined inside a tubular furnace for 24 hours in an alumina crucible at 150°C in air. After cooling to room temperature (RT), the resulting powder was again submitted to prolonged cycles of grinding and reheated at the same temperature, to ensure the total decomposition of the oxides. Total calcination time was 5 days. All the final products were black in colour. Once the binary oxides formation was accomplished the samples were stored in a desiccator under vacuum.

Chemical analysis

The accurate cationic concentrations of Co and Mn both in the mother liquors and in the final products were controlled by Induced Coupled Plasma-Optical Emission (ICP-OE) Spectroscopy, using an Óptima 2000 DV Perkin-Elmer apparatus, not only for checking their actual rate, but also because there is always some doubt about the composition of manganese nitrate due to the hygroscopic character of the starting salt. For these classical analyses, the samples were dissolved in acidic media and then the metallic cation ratios were determined using Mn and Co standards from Perkin-Elmer (1 mg mL-1). The cationic Co/Mn ratios present in solution were satisfactorily retained in the oxide powders. Within experimental error (±2%), the composition of all the compounds were in good agreement with the expected ones.

X-ray diffraction

Crystallographic structure and purity of the samples were carried out by X-ray powder diffraction (XRD), using a Siemens D500 diffractometer at RT employing a Cu anode Kα1 (λ = 0.154060 nm) and Kα2 (λ = 0.1540431 nm) in Bragg-Brentano geometry. The voltage and intensity current were set to 40 kV and 30 mA, respectively. The data were collected in steps of 0.02° (29) with a fixed-time counting of 5 s. Profiles were measured in the range 10° to 110° (29) since no detectable peaks were observed beyond these values. The refinement of X-ray diffraction results was performed using the Rietveld's profile analysis method with the Fullprof Suite program32.

SEM analysis

In order to estimate the average aggregate size and crystallinity degree ofthe oxides, the microstructures ofthe powders were examined by scanning electron microscopy (SEM) using a JEOL apparatus (JSM 5410). The details of surface morphology were studied at magnifications of up to 5000X by scanning electron microscopy.

Thermogravimetric analysis

The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves were obtained using a TA Instruments SDT 2960 under a dry static air atmosphere. About a 40 mg sample was used in each run, and α-Al2O3 was the inert reference material. The material was heated from RT up to 800°C at a constant rate of 20°C min-1.

Specific surface areas

The specific surface areas of the oxides were measured by the Brunauer-Emmet and Teller (BET) method. Nitrogen physisorption measurements ofthe fresh samples were performed in a standard Sorptometer Micromeritics ASAP 2010.

Oxidation power q(x).

The oxidation power q(x) of the oxide can be defined as the degree of oxidation power of the spinel with regard to a soft reduction agent 33. This parameter (eq mol-1) was determined by chemical reduction of the oxide cations Mn+ with n ≥ 3 until oxidation state 2 was reached using a mild reducing agent, such as vanadyl sulphate sulphuric solution (1 N VOSO4) according to the reaction:

This measurement reflects the quantity of electrons per mole of (MnxCo3 x)8+ with q(x) >2 necessary to level all oxidation states to 2. The oxide samples (50 mg approximately) were dissolved in a known excess amount (50 mL) of VOSO4 (Merck p.a.) in sulfuric acid solution (to avoid cationic hydrolysis). Because ofthe extended periods of digestion required to dissolve the samples, the mixture was heated at 90°C for 48 hours. After complete dissolution of the oxide, the unreacted VOSO4 was potentiometrically titrated with 1 N KMnO4 solution, which had been previously standardized with oxalate solution according to McBride's method 34. The oxidation power was calculated according to the equation:

where VVO and NVO and VK and NK are the volume and concentration of the VOSO, solution and the KMnO4, solution, respectively, Mox and mox corresponding to the molar mass and weight of the oxide. Each determination was made at least three times. The overall standard deviation was ± 0.02.

A cationic distribution derived from the X-ray diffraction pattern and the oxidation power were computed using the method of Poix35 based on the best fit with the equation: a = 2.0995a + [5.18182b2 - 1.4107α2]½ in which α = Σ xi(Mi - O)Td/1/åxi and ß = åxi(Mi - O)oh/åxi, with x. being the stoichiometric coefficient of the cation Mi and Mi - O the cation oxygen distances in the tetrahedral (Td) and octahedral (Oh) sites according to the Shannon and Prewitt data 36 and, considering the energetic preference of a cation for either site, according to Paul and Basu 37. The presence of the Mn3+ and Mn4+ ions in octahedral sites and the occupancy of tetrahedral sites by the Co2+ and Mn2+ ions for the cobalt-manganese spinel have been already established in the literature 38-40.


3.1.      Structural properties of Co3O4 and MnCo2O4 oxides.

Figures 1A and 1B show the X-ray diffraction patterns of the Co3O4 and MnCo2O4 phases respectively. Even if the crystalline state is not better defined due to the low temperature of synthesis 41, Rietveld analysis shows single phases corresponding to the expected compounds which crystallize in the face centered cubic system spinel type, spatial group oh7 (Fd3m). All the main peaks (indicated by their hkl Índices) could be indexed by comparison with the diffraction patterns of Co3O4 and MnCo2O4. Both the position and the relative intensities of the diffraction lines were found to be in good agreement with the ASTM X-ray powder data files JCPDS-ICDD for Co3O4 and MnCo2O4, thus showing that a single phase of each component was present. A remarkable common feature of the X-ray spectra of both spinel oxides prepared at such a low temperature was the good definition of the diffraction lines. The mayor peak in both XRD profiles was found to correspond to the (311) crystallographic plañe. The Co3O4 lattice constant (ao = 0.8079 nm) shows an acceptable good agreement with the 8-418 ASTM file value of 0.8084 nm for a product prepared at 850 °C. For MnCo2O4 a more pronounced discrepancy was observed (ao = 0.8089 nm) with the 23-1237 ASTM file value of 0.8269 nm, not only due to differences between the preparation method (ceramic procedure), but also due to the temperature of the preparation (720°C). This higher variation for MnCo2O4 suggests the presence of cations with a high oxidation state that must be distributed in a smaller size cell and a cationic stoichiometric deficiency should not be ruled out.

The diffractograms in Fig. 1 show a loss of crystallinity going from Co3O4to MnCo2O4. Broader diffraction peaks at the same positions were observed indicating that MnCo2O4 (13 nm) have a smaller crystallite size than Co3O4 (45nm) (see Table 1).

The fractional coordinates of the atoms in the cell as well as the starting refinement were established on the basis of the model by Knop et al. 18 for Co3O4 (ao = 0.80835 nm), which provides the best results for the fit. With this model the families of planes shown in Fig. 1 were indexed. The results of applying the structure refinement procedure to the experimentally determined intensities for the entire set of diffraction lines are shown in table I. This table includes the lattice parameter, the Bragg factor and the goodness of fit (GOF) that provides the refinement. Besides, it also shows the coherent diffraction size (crystallite) and the non-uniform micro deformation values obtained using the Wilson equation42 and full width at half máximum (FWHM) in the case of pseudo Voigt function of the peaks that are shown. The cubic cell parameter increases with an increase in manganese content, as expected. This result was interpreted as an effect of the substitution of Co-cations by larger Mn-cations43 as can seen in Table II. The decrease of the lattice parameter of the Co3O4with respect to the starting model can be attributed to the low temperature of the preparation.

The powders of the oxides present an average crystal size in the range of 13-45 nm, confirming that the oxide particles were actually nanometric. The low crystal size can be explained by the low temperature used in the preparation of the oxides.


Figures 2A and 2B show SEM micrographs of the samples Co3O4 and MnCo2O4. The magnified images show differences in the morphology between the products. From a morphological point of view the particles of Co3O4 are agglomerated while MnCo2O4 shows particles geometrically defined (nearly trapezoidal crystallites). The average grain size of both samples measuredfrom SEM micrographs lies in the range 1—8 mm.

Thermogravimetric studies

The thermogravimetric analysis (TGA) in air of dried Co3O4 and MnCo2O4powders are presented in Figure 3 and 4 respectively. Both TG curves show a thermal decomposition process involving several steps that begin at RT and finish near 600°C. On the curves, mainly three regions of mass loss can be distinguished. The processes occurring during the course of heating should include: physisorbed water evolution, water from OH groups adsorbed onto the surface of the oxide particles44 and a progressive loss of the excess oxygen in the non-stoichiometric oxide 31, namely Co3O4+d and MnCo2O4+d. Gyrdasova et al.45 have prepared MnCo2O4 from manganese cobalt oxalate. They reported the formation of a cubic spinel phase at 200°C. According to TG data the difference between the calculated and experimentally determined values of mass loss points to a higher oxygen eontent and eonsequently cationic deficieney. Further heating of MnCo2O4 at 700°C make lose oxygen and converts to the stoichiometric spinel MnCo2O4. At this temperature its lattice parameter is a = 0.8226 nm. The increase in a is likely due to the reduetion of manganese or cobalt.

Specific surface areas

The BET analysis performed on MnCo2O4 gave an average specific surface area of 70 m2 g-1. This value is higher than the one reported in the literature for MnCo2O4 prepared by a sol-gel process at 80°C with a surface area of 34 m2 g-1; the area increased to 44 m2 g-1 when annealing at 200°C4.

Oxidation power

The amount of equivalent per oxide mole, q(x), is higher for MnCo2O4. This suggests that the replacement of Co ions by Mn ions at the B octahedral sites would generate Mn ions with higher oxidation states.

Cationic distributions

A major challenge associated to the Mn-Co-O spinel-type systems is the determination of the metal oxidation states and cation distributions among the tetrahedral (labelled as A sites) and octahedral (B sites) sublattices of the spinel-related structure46. To put forward a tentative cation distribution for the compounds, the method of characteristic distances (invariants) developed by Poix 35 has been used along with literature data. In Mn-Co spinels the most likely cationic distributions reported in the literature indicates predominantly the presence of Co2+ and Mn2+ ions at A sites and Co3+, Mn3+ and Mn4+ ions at B sites leading to an ionic configuration47. Table III gathers the experimental values of the lattice constant, a , and the global oxidizing power, q(x), from which best fitted cationic distributions displayed in Table IV were established. The values for the calculated parameters a and q(x) on the basis of these cationic distributions are also shown in Table III.

We are now able to propose the most probable cationic distribution of the compounds as a result of a coherent study that includes:

        -  Con+-O and Mnn+-O distances, where n can be 2, 3 or 4 35, considering cation lattice site preferences.

        -  similarities between the calculated global oxidizing power qcalc values and the experimental ones qexp.

        - the excellent fit between XDR results and the results obtained from the Rietveld refinement.


In earlier work10, the preparation of the title compounds by spray pyrolysis at the same temperature (150°C) as film electrodes was successfully undertaken by us, under rigourous preparation conditions with ao = 0.8068 and q(x) = 1.6 for Co3O4, and ao = 0.8197 and q(x) = 1.9 for MtiCo2O4. Following the present results on the studied oxides we have now successfully accomplished the preparation of low temperature Co3O4 and MnCo2O4 materials by thermal decomposition of nitrate precursors. The syntheses of the compounds require a thermal treatment for several days to achieve thermal equilibrium. The formation of a cubic spinel-type phase for Co3O4 and MnCo2O4 has been achieved. The spinel oxides thus prepared have a large specific surface area (70 m2 g-1) and both types are nanometric materials. The cobalt oxide showed 45 nm of crystallite size while manganese-cobalt oxide only 13 nm. However when MnCo2O4 was prepared as a thin film using the spray pyrolysis technique at the same temperature, the crystallite was bigger5 (20 nm) thus showing the effect of the preparation method.

The cationic distribution proposed on the basis of a series of physicochemical measurements show an excellent agreement with the cationic distribution obtained from the Rietveld structural analysis method of XRD data.

The preparation and the crystallographic study of the rest of the oxides family (x=0.25 and 0.75) is currently in progress. A further electrochemical characterization study will complete this work.


The authors acknowledge financial support from the DICYT-USACH (project 05-0742EVHC) and JLG thanks also CONICYT (Fondecyt 1050178).



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(Received: November 17, 2009 - Accepted: January 26, 2010)

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