Using the ScatterX78 vacuum chamber, enabling very fast SAXS and WAXS data collection, the hydrothermal synthesis of nanoceria was monitored in real time. Particle and crystallite sizes as a function of synthesis time were accurately documented over the 36-hour combined SAXS-WAXS experiment.
We performed an extensive in situ study of hydrothermal synthesis of nanoceria by combination of X-ray diffraction and scattering techniques. Here we present the results of time-resolved SAXS-WAXS characterization of the nanoceria synthesis process at room temperature. From the experimental data the crystallite and particle sizes as a function of the synthesis time were obtained.
Due to its high oxygen storage capability and remarkable redox properties, nanoceria found numerous applications including the use as a catalyst for petroleum refinement, as a polishing agent and as an electrolyte material for intermediate temperature solid-oxide fuel cells. The catalytic activity of nanoceria potentially can be utilized for the thermochemical water splitting for hydrogen production, and the treatment of some human diseases. Useful properties of nanoceria are directly influenced by size and morphology of the particles. In order to fine-tune the properties of nanoceria and to upscale the production it is essential to understand the synthesis process. We performed an extensive in situ study of hydrothermal synthesis of nanoceria by combination of X-ray diffraction and scattering techniques. Results of the time- resolved small- and wide-angle X-ray scattering (SAXS-WAXS) and the time-resolved total X-ray scattering experiments (for pair distribution function (PDF) analysis) can be found elsewhere[1,2]. Here we present the results of the in situ SAXS characterization of the nanoceria synthesis process in the temperature range from 5 to 90 °C.
A series of time-resolved small-angle X-ray scattering (SAXS) measurements at variable temperatures were performed using the Empyrean X-ray diffraction platform configured with a Cu tube, an X-ray focusing mirror, capillary spinner stage, PIXcel3D detector and Oxford Cryostream Plus compact. As a precursor for the synthesis reaction, (NH4)2Ce(NO3)6 (Sigma Aldrich, > 98%) was used. For each experimental series, a fresh 5 wt% distilled water dispersion of (NH4)2Ce(NO3)6 was prepared and placed in a glass capillary immediately prior to the measurement series, so that the synthesis reaction could be monitored in real time.
Figure 1. Nano-structured CeO2 is routinely used for cleaning and polishing of silicon wafers for ultra modern chip systems and solar cells.
The set of the background corrected WAXS patterns from the sample, measured during a 36 hour combined SAXS-WAXS experiment is shown in Figure 2a. All observed bragg peaks belong to fcc-CeO2. Using automatic sequential Rietveld refinement the crystallite size as a function of synthesis time was extracted (Figure 2b). Progressive growth of crystallite domains slows down after the first 15 h, signifying the deceleration of the synthesis reaction.
In the end of the synthesis (30-36 h) the average crystallite size reaches 4.2(2)nm. In the very beginning of the synthesis (first 5-10 min the sample SAXS curve shows only one 'broad' feature centered at about 2 degree 2 theta (Fig. 3a). After 15 minutes an additional feature, shifted towards lower values at 20 emerges. The new feature becomes increasingly pronounced with time and shifts further towards lower 20, signifying an increase in particle size. This feature is most likely related to the growing CeO2 particles as the synthesis process progress. However, the origin of the 'broad' feature is not conclusive. Similar results were reported by Allen et al (2008) where it was suggested that the 'broad' feature might be related to the formulation of a shell around the primary CeO2 nanoparticle, the shell consisting of fine clusters formed by precursor material and Ce3+ or Ce4+ ions . Another plausible explanation could be nano-sized precursor residue.
Figure 2. Isoline plot of 130 WAXS patterns measured over a 36 hour SAXS-WAXS experiement
For a conclusive answer with respect to the origin of the 'broad' feature, further investigations e.g. high resolution microscopy are needed. In order to extract the size as a function of time for both observed features the SAXS curves were fitted with the assumption of a bimodal distribution of hard sphere particles. As seen from figure 3b. the rapid particle growth is finalized within the first 15-17 h which is in good agreement with the WAXS and PDF data at the end of the synthesis process the particle diameter reaches 8.5 nm, which is higher than the observed average crystallite size. Thus on the average CeO2 particles consist of several crystallites.
Figure 3. Representatives SAXS curves measured over a 36 hour SAXS-WAXS experiment estimation of the volume weighted average dimensions of the primary nano-CeO2 particles and the 'braod' features as a function of time, obtained using EasySAXS software
The Empyrean diffractometer equipped with the ScatterX78 vacuum chamber and the PIXcel3D detector enables fast SAXS-WAXS data collection. Using this setup the hydrothermal synthesis of nanoceria was studied in situ. The kinetics of the synthesis process, as well as the crystallite and particle growth as a function of the synthesis time, were characterized.
