Abstract
Surfactant-mediated chemical routes allow one to synthesize highly engineered shape- and size-controlled nanocrystals. However, the occurrence of capping agents on the surface of the nanocrystals is undesirable for selected applications. Here, a novel approach to the production of shape-controlled nanocrystals which exhibit high thermal stability is demonstrated. Ceria nanocubes obtained by surfactant-mediated synthesis are embedded inside a highly porous silica aerogel and thermally treated to remove the capping agent. Powder X-ray Diffraction and Scanning Transmission Electron Microscopy show the homogeneous dispersion of the nanocubes within the aerogel matrix. Remarkably, both the size and the shape of the ceria nanocubes are retained not only throughout the aerogel syntheses but also upon thermal treatments up to 900 °C, while avoiding their agglomeration. The reactivity of ceria is measured by in situ High-Energy Resolution Fluorescence Detected - X-ray Absorption Near Edge Spectroscopy at the Ce L3 edge, and shows the reversibility of redox cycles of ceria nanocubes when they are embedded in the aerogel. This demonstrates that the enhanced reactivity due to their prominent {100} crystal facets is preserved. In contrast, unsupported ceria nanocubes begin to agglomerate as soon as the capping agent decomposes, leading to a degradation of their reactivity already at 275 °C.
Highlights
Opment of synthetic routes for the preparation of crystalline nanoparticles (NPs) with well-defined morphology, where the best results have so far been achieved by surfactant-assisted high temperature thermal decomposition of inorganic precursors in solution [1,2,3]
This property is associated with the distinctive ability of cerium ions to undergo transition between Ce (III) and Ce(IV) oxidation states [36] which is quantified in terms of Oxygen Storage Capacity (OSC), defined as mmol of oxygen atoms released and stored per gram of ceria during a complete redox cycle [37]
High-Energy Resolution Fluorescence Detected (HERFD) - X-ray Absorption Near Edge Spectroscopy (XANES) data at the Ce L3 edge were collected on unsupported CeO2 NCs and on the NCs embedded in silica aerogel
Summary
Successful strategies to produce CeO2 NCs rely on the use of fatty acids such as oleic acid, which binds preferentially to the {1 0 0} crystal facets, promoting a shape-selective synthesis [45,46] This surfactant-assisted production makes it possible to control and prevent excessive growth of the ceria NCs, a key requirement to produce them with small sizes in order to enhance catalytic performance, as calculations show that the energy required to form an oxygen vacancy is strongly dependent on the size of the NPs [47,48]. We provide a significant advance in the production of surfactant-free morphology-controlled CeO2 NCs through their stabilization into a highly porous silica aerogel, a well-known solid with remarkable ultra-low apparent density, high porosity, large specific surface area, and low thermal conductivity, which can be obtained in different forms including monoliths [52,53].
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