In situ monitoring of the hydrothermal synthesis of nanoceria. Time-resolved total X-ray scattering for pair distribution function analysis

Due to its high oxygen storage capability and remarkable redox properties, nanoceria finds numerous applications including its use in coatings, in production of photosensitive glass, as a catalyst for petroleum refinement, as a polishing agent and as an electrolyte material for intermediate temperature solid-oxide fuel cells. Catalytic activity of nanoceria potentially can be utilized for the thermochemical water splitting for hydrogen production, and the treatment of some human diseases. The 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 a combination of X-ray diffraction and scattering techniques. The results of time-resolved small- and wide-angle X-ray scattering (SAXS- WAXS) at variable temperatures [1,2] suggest that at room temperature the active nucleation and growth of particles is occurring within the first 24 hours. Thanks to the high efficiency of the new GaliPIX3D detector, pair distribution function (PDF) analysis [3] of the processes occurring over this time framework is now possible on a laboratory diffractometer. Here we present results of in situ time- resolved monitoring of hydrothermal synthesis of nanoceria using the PDF method.

Experimental Time-resolved total X-ray scattering measurements for PDF analysis were performed using the Empyrean X-ray diffraction platform configured with a Mo tube, an X-ray focusing mirror, capillary spinner stage and GaliPIX3D detector. A 5 wt% water dispersion of (NH4)2Ce(NO3)6 (Sigma Aldrich, > 98%) was prepared and placed in a glass capillary immediately prior to the experiment. A series of repeated 3-hour measurements monitoring the synthesis reaction in real time were performed over 24 hours.

Figure 1. Nano-structured CeO2 is routinely used for cleaning and polishing of silicon wafers for ultra modern chip systems and solar cells.

Results and discussion Representative experimental PDFs obtained during 24-hour monitoring of the hydrothermal synthesis of nanoceria from a water dispersion of (NH ) Ce(NO ) are shown in Figure 2. As seen from the figure, after 4 hours into the synthesis process only Ce-O and Ce-Ce distances within the first 5 Å can be clearly observed above the noise level. With the further growth of nanoceria particles the intensity of PDF peaks is increasing and ordering at the larger radial distances becomes apparent. Furthermore the gradual increase of the Ce-Ce distances is observed.

Experimental PDF results were fitted to a model of the cubic structure of CeO2 (Fm-3m). A representative example of a PDF fit is shown in Figure 3a, and the obtained time dependence of the cubic CeO2 lattice parameter is given in Figure 3b. As seen from the figure, in the beginning of the synthesis process the CeO2 lattice parameter continuously increases reaching the value of 5.431(3) Å in 15 hours. Afterwards the lattice parameter remains constant. This correlates with the results of the time-resolved SAXS-WAXS experiment[1], where the most active particle growth is observed within the first 15-17 hours of the hydrothermal synthesis of nanoceria at room temperature. From the results of the SAXS and PDF experiments direct influence of the particle size on CeO2 lattice parameter is apparent.

Figure 2. Representative experimental PDFs obtained during 24-hour monitoring of the nanoceria synthesis. PDFgetX3 software [4] was used for data normalization and calculation of the PDFs.

Figure 3. (a) A representative fit of the experimental PDF with the cubic CeO2 structure (Fm-3m). A good agreement between the experimental (blue circles) and calculated (red line) PDFs is obtained; data analysis was performed using PDFgui software package [5]. (b) Lattice parameter of CeO2 as a function of synthesis time.

Conclusion The excellent hard radiation performance of the new GaliPIX3D detector enables the time-resolved PDF study of the relatively slow (~24 h at room temperature) hydrothermal synthesis process of nanoceria. A continuous increase in the dimension of the CeO2 lattice parameter was observed within the first 15 hours of the synthesis.

In situ monitoring of the hydrothermal synthesis of nanoceria. Time-resolved total X-ray scattering for pair distribution function analysis

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