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In the present study, the pore space of a mesoporous cerium oxide material is investigated, which forms by the self-assembly of primary particles into a spherical secondary structure possessing a disordered mesopore space. The material under study exhibits quite stable mesoporosity upon aging at high temperatures (800 °C) and is, thus, of potential interest in high-temperature catalysis. Here, different characterization techniques were applied to elucidate the structural evolution taking place between heat treatment at 400 °C and aging at 800 °C, i.e., in a water-containing atmosphere, which is usually detrimental to nanoscaled porosity. The changes in the mesoporosity were monitored by advanced physisorption experiments, including hysteresis scanning, and electron tomography analysis coupled with a 3D reconstruction of the mesopore space. These methods indicate that the 3D spatial arrangement of the primary particles during the synthesis under hydrothermal conditions via thermal hydrolysis is related to the thermal stability of the hierarchical mesopore structure. The assembly of the primary CeO2 particles (â¼4 nm in size) results in an interparticulate space constituting an open 3D mesopore network, as revealed by skeleton analysis of tomography data, being in conformity with hysteresis scanning. At elevated temperatures (800 °C), sinter processes occur resulting in the growth of the primary particles, but the 3D mesopore network and the spherical secondary structure are preserved.
RESUMEN
The formation of CeO2 colloidal particles upon heating an aqueous solution of (NH4)2Ce(NO3)6 to 100 °C was investigated by time-resolved in situ SAXS analysis using synchrotron radiation, providing absolute intensity data. In particular, the experiments were performed by applying different temperatures between room temperature and 100 °C as well as under variation of the ionic strength and concentration. Using validated SAXS evaluation tools (SASfit and McSAS software), the analyses revealed the presence of two types of particle populations possessing average dimensions of ca. 2 nm and 5-15 nm, with the latter being agglomerates of the 2 nm particles rather than single crystallites. The analysis revealed not only the changes in the size, but also the relative volume fractions of these two CeO2 particle populations as a function of the aforementioned parameters. Increasing the temperature increases the number of the 5-15 nm agglomerates on one hand by the enhanced nucleation rate of the primary particles. On the other hand, especially at high temperatures (90 and 100 °C) the larger agglomerate particles precipitate, resulting in interesting trends in the fractions of the two populations as a function of time, temperature, ionic strength, and precursor concentration. The experimental studies are complemented by calculating colloidal interaction energies based on classical DLVO theory. Thereby, this study provides detailed insight into the nucleation, growth, and agglomeration of CeO2 nanoparticles. The primary objective of this study is to provide a better understanding of the nucleation and growth of particles by the hydrolysis of the tetravalent cerium ion in aqueous solutions.
RESUMEN
[This corrects the article DOI: 10.3762/bjnano.11.23.].
RESUMEN
This study is dedicated to link the nanoscale pore space of carbon materials, prepared by hard-templating of meso-macroporous SiO2 monoliths, to the corresponding nanoscale polyaromatic microstructure using two different carbon precursors wthat generally exhibit markedly different carbonization properties, i.e., a graphitizable pitch and a non-graphitizable resin. The micro- and mesoporosity of these monolithic carbon materials was studied by the sorption behavior of a relatively large organic molecule (p-xylene) in comparison to typical gas adsorbates (Ar). In addition, to obtain a detailed view on the nanopore space small-angle neutron scattering (SANS) combined with in situ physisorption was applied, using deuterated p-xylene (DPX) as a contrast-matching agent in the neutron scattering process. The impact of the carbon precursor on the structural order on an atomic scale in terms of size and disorder of the carbon microstructure, on the nanopore structure, and on the template process is analyzed by special evaluation approaches for SANS and wide-angle X-ray scattering (WAXS). The WAXS analysis shows that the pitch-based monolithic material exhibits a more ordered microstructure consisting of larger graphene stacks and similar graphene layer sizes compared to the monolithic resin. Another major finding is the discrepancy in the accessible micro/mesoporosity between Ar and deuterated p-xylene that found for the two different carbon precursors, pitch and resin, which can be regarded as representative carbon precursors in general. These differences essentially indicate that physisorption using probe gases such as Ar or N2 can provide misleading parameters if to be used to appraise the accessibility of the nanoscale pore space.
RESUMEN
Lithium titanate Li4Ti5O12 (LTO) is regarded as a promising alternative to carbon-based anodes in lithium-ion batteries. Despite its stable structural framework, LTO exhibits disadvantages, such as the sluggish lithium-ion diffusion and poor electronic conductivity. To modify the performance of LTO as an anode material, nanosizing constitutes a promising approach and the impact is studied here by a systematical experimental approach. Phase-pure polycrystalline LTO nanoparticles (NPs) with high crystallinity and crystallite sizes ranging from 4 to 12 nm are prepared by an optimized solvothermal protocol and characterized by several state-of-the-art technologies, including high-resolution transmission electron microscopy, X-ray diffraction (XRD), pair distribution function (PDF) analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy. Through a wide array of electrochemical analyses, including charge/discharge profiles, cyclic voltammetry, and electrochemical impedance spectroscopy, a crystallite size of approx. 7 nm is identified as the optimum particle size. Such NPs exhibit as good reversible capacity as the ones with larger crystallite sizes but with a more pronounced interfacial charge storage. By decreasing the crystallite size to about 4 nm, the interfacial charge storage increases remarkably, however resulting in a loss of reversible capacity. An in-depth structural characterization using the PDF obtained from synchrotron XRD data indicates an enrichment in Ti for NPs with the small crystallite sizes, and this Ti-rich structure enables a higher Li storage. The electrochemical characterization confirms this result and furthermore points to a plausible reason as to why a higher Li storage in very small nanoparticles (4 nm) results in a loss in the reversible capacity.
