Nanokristalliner Cobaltoxid-Spinell: Größenselektive Synthese, Defektchemie, Partikelwachstum
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Philipps-Universität Marburg
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Nanocrystalline (nc-) Co3O4 is a multifunctional material with a wide range of engineering applications. Currently, it is primary used in heterogeneous catalysis, energy storage and sensoric materials. Due to the fact that the physico-chemical properties are ultimately determined by the particle size, the atomic structure as well as the microstructure, systematic investigation and control of these parameters is essential for customized optimization of the properties of nc-Co3O4.
The present work focuses on the size-selective synthesis of average mean particle sizes of cobalt oxide spinel with a monomodal size distribution down to the ultrafine range using a cost-effective approach. A solution of cobalt acetate or cobalt nitrate is reacted with ammonia, air and varying proportions of water and ethanol to precipitate the product under simple reflux conditions. A size range of 35 to 2 nm is obtained by varying the water/ethanol and ammonia/water ratios. Starting with a 2% cobalt acetate solution with a given ammonia content (~2.3 wt%), a linear relationship is seen between the particle size and volume fraction of ethanol in the reaction mixture in the range from 11 nm (0vol% EtOH) to 2 nm (91vol% EtOH). Previous reasearch groups only accomplished this under hydrothermal conditions, for which the lower limit of the achievable particle size was only 3,5 nm. It was also found that the particle size is influenced not only by varying the ethanol/water ratio but also by increasing the ammonia concentration in the solution and by exchanging the counterion of the cobalt salt. Starting from the synthesis of an aqueous solution (no EtOH, 2.3 wt% NH3), a sigmoidal increase in particle size with increasing ammonia concentration was observed, which, at an ammonia concentration of 10 wt%, went into saturation at about 25 nm. A further increase in particle size up to 35 nm was achieved by performing the synthesis at an elevated concentration of ammonia with cobalt nitrate instead of cobalt acetate.
Isochronic grain growth (1h) in the temperature range from 250 to 500°C of particles with a different mean starting particle size, dS = 2 nm, 11 nm and 25 nm, differs strongly and decreased with increasing particle size until growth was completely blocking at approximately 25 nm.
Due to increased adsorption of foreign materials on nanocrystalline compounds as a result of a higher surface energy compared to their bulk counterparts, thermal analyzes of particle sizes in the range of 2 to 25 nm were performed. Gravimetric analysis of the excess mass, determined by thermal annealing up to 800°C and carried out both ex-situ and in-situ, by differential thermoanalyses, revealed an exponential relationship between mass loss and particle size with values between 4% for 25 nm particles and 25% for 2 nm particles.
The excess mass was attributed to physisorbed water, carbon dioxide and also acetamide, which was formed from ammonium acetate during the synthesis of cobalt oxide spinel.
Rietveld refinements showed an exponential increase in the lattice parameter with decreasing particle size between 808 pm for 12 nm particles and 813 pm for 2 nm particles. The exponential increase in the lattice parameter with decreasing particle size as well as the expansion of the cobalt oxide spinel lattice to about 813 pm (813.4 pm) has not been previously reported in the literature. More importantly, X-ray diffraction revealed that by performing the synthesis at different water/ethanol as well as ammonia/water ratios led to a particle size-dependent intensity variation of individual reflexes. This intensity modulation expressed itself in Rietveld refinements with reduced and different occupancy factors for Co2+ and Co3+, which thus implies a reduction of the X-ray density and formation of metal defects. An entropically favored, partial redistribution of cobalt atoms on additional, unoccupied octahedral and/or tetrahedral sites in the spinel structure - postulated by Tarascon as the sole cause of the intensity modulation - was excluded in the case of our samples by means of Rietveld refinements as well as pycnometric density determinations of nanocrystalline cobalt oxide spinel. In contrast to conventional cobalt oxide spinel, which has a density of 6.11 g/cm³, we observed X-ray densities of 5.5 g/cm³ for 25 nm particles down to 5.1 g/cm³ for 2 nm particles. The X-ray density gradually decreased with decreasing particle and then accelerated especially in the range between 10 to 2 nm. The decrease in density with decreasing particle size is detected not only by Rietveld refinements, but also by pycnometric measurements with helium as the analysis gas and by taking account of the excess masses of nanocrystalline materials. The pycnometrically determined densities in the 25 to 2 nm range confirm the dependency of the density on the particle size, although they differ with decreasing particle size to increasingly lower values compared to X-ray densities, with a maximum deviation of 25% for 2 nm sized particles. This means that the true density of ultrafine particles is even less than the density determined by Rietveld refinement.
The decrease in density is probably due to a Co defect-related, partial substitution of O2- by OH- in the cobalt oxide spinel, which leads to an exponential increase in the lattice parameter. Furthermore, not only the particle size, but also the thermal treatment led to a variation in the density of nanocrystalline cobalt oxide spinel. Isochronic thermal treatment (1h) of precipitated cobalt oxide spinel at temperatures between 250-500°C led to a gradual increase in the X-ray density with increasing temperature. The increase in density of cobalt oxide spinel due to thermal treatment was also found in the pycnometric measurements.
The hitherto misunderstood relationships between particle size, chemical composition and structural parameters were studied by means of infrared spectroscopic and magnetic measurements, which indicate the strength of the intraparticle binding forces.
Infrared spectroscopy revealed a variation in the occupation of cobalt in the transverse mode at about 390 cm-1, which shifts by more than 15 wavenumbers (388 - 372 cm-1). We also observed a linear dependence of the most energetic transverse mode at about 650 cm-1 from the lattice parameter, starting from 657 cm-1 at 808 pm to 650 cm-1 at 813 pm.
The change in density due to thermal treatment of particles with blocked grain growth (25 nm) resulted in an increase in the antiferromagnetic ordering temperature, which so far is only attributed to a change in particle size. Raising the calcination temperature (1h) from 250°C to 425°C increased the Néel-temperature from 20 to 35 Kelvin.
Surface area measurements of nanocrystalline cobalt oxide spinel with particle sizes of 11 to 2 nm revealed abnormal behaviour: the highest specific surface area of 190 m²/g was not found for particles having the smallest particle size, but for 3 nm particles. This finding is attributed to the observable exponential increase in the excess mass for ultrafine particles. Moreover, the sorption measurements in the size range from 11 to 2 nm showed a linear function for both the pore volume and the average pore width of the particle size, with an absolute pore volume of 0.27 cm³/g for 11 nm particles down to 0.05 cm³/g for 2 nm particles, and an average pore width of 9.7 nm for 11 nm particles down to 2 nm for 4 nm particles. A strict correlation between the pore volume and the average pore size with particle size, obtained by varying the ethanol/water ratio, has not been previously observed.
In addition, the pore size distribution, and thus access of gas molecules to the solid surface of particles with blocked grain growth was significantly influenced by simple thermal treatment. The pore size distribution after isochronous thermal treatment (1h) at temperatures of 250 and 425°C widened as the prevailing temperature increased.
Also investigated was the reason for the different grain growth dynamics in dependence on the starting particle size as a result of isochronic thermal treatment (1h) at temperatures between 250 and 500°C, as well as the reason for the different grain growth behaviour even for an interim period with an identical particle size. Thus, complete blocking of grain growth for 25 nm particles was observed, whereas smaller particles showed a successively higher degree of grain growth. At a temperature of 500°C (1h), starting particles with an initial size of 2.7 nm were able to reach 40 nm, thus easily exceeding a particle size of 25 nm.
To elucidate the reason for the different grain growth behaviour in dependence on the starting particle size, cobalt oxide spinel compounds with different mean starting particle sizes dS = 12 nm, 9.7 nm, 6.9 nm, 6.3 nm, 4 nm und 2.5 nm were prepared from solution. They were grown at a temperature of 425°C and different growth times to produce an
identical mean particle size of 15 nm. Their particle size distribution, structural aspects and surface-specific parameters were then characterized by means of transmission electron microscopy, X-ray diffractometry and sorption measurements. Since the structural parameters derived from Rietveld refinements showed similar values, a significant contribution to subsequent grain growth from the structural parameters was excluded. Each sample showed a log-normal particle size distribution as well as a narrow pore size distribution. The results revealed that the smaller the initially starting particle size is, the smaller is the interparticle pore volume for an identical mean particle size. Subsequent isochronic thermal treatments at 600°C (2h) showed that the particle growth correlated with the interparticle volume: the lower the interparticle volume, the stronger the particle growth. According to these findings, the reason for the dependency of the grain growth dynamics on the starting particle size is attributed to the different pore volume of the initial starting particle sizes, which due to their initial differences in pore volume continues to differ with respect to this parameter even when the identical particle size is exceeded.
Finally, it should be noted that the present study raises the fundamental question of whether the generally observed nanocrystalline effects are actually a primary influence of the particle size or rather an influence of particle size and thermal treatment associated variation of the chemical composition.
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Dates
Created: 2013Issued: 2014-03-18Updated: 2014-03-18
Faculty
Fachbereich Chemie
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
Keywords
PorenspinelOberflächeCo3O4Cobaltnanocrystallinegrain growthdefectMikrostruktur
DFG-subjects
NanotechnologieSpinellTeilchengrößeCobaltWachstumDefekt
DDC-Numbers
540
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Jehl, Pierre (105197559X): Nanokristalliner Cobaltoxid-Spinell: Größenselektive Synthese, Defektchemie, Partikelwachstum. : Philipps-Universität Marburg 2014-03-18. DOI: https://doi.org/10.17192/z2014.0121.
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This item has been published with the following license: In Copyright