NANOCERIA PREPARED BY ELECTRON BEAM EVAPORATION

Cerium oxide nanoparticles (nanoceria) are currently one of the most investigated nanomaterials because of their attractive properties used in biomedical applications, catalysis, fuel cells, and many others. These attractive properties are connected with the Ce 3+ and Ce 4+ valency state ratio. In the nanoparticle form, cerium oxides contain a mixture of Ce 3+ and Ce 4+ on the nanoparticle surfaces. Switching between these two states requires oxygen vacancies. Therefore, nanoceria's inherent ability to act as an antioxidant in an environmentally-dependent manner and a “redox switch” to confer auto-regenerating capabilities by automatically shifting between Ce 4+ and Ce 3+ oxidation states is significantly affected by surface morphology. Regarding this demanded behavior, we aimed to characterize synthesized nanoparticle surface quality and its influence on the cerium oxidation states. The received results were used to evaluate the synthesis method's suitability for suggested utilization.


INTRODUCTION
Cerium in compounds exists in two valency states, Ce 3+ (Ce2O3) and Ce 4+ (CeO2). These valency types are associated with different properties usable for catalysis and biomedical applications, especially for radioprotection of healthy tissues during radiation therapy (Ce 4+ ) or for supporting radio-oncological treatment toxic effect (Ce 3+ ). The formation of oxygen vacancies on the CeO2 surfaces leads to the reduction of the Ce 4+ ions to Ce 3+ and vice versa. Such oxygen off-stoichiometry correlates with the redox activity of cerium oxide nanoparticles [1][2][3].
Cerium oxides formed with the CeO2 fluorite lattice (Figure 1a) contain Ce 4+ . In the nanoparticle form, CeO2 contain also Ce 3+ ions on the surfaces. With a nanoparticle size decrease, the number of surface oxygen vacancies and Ce 3+ increases [4,5]. Therefore, the Ce 3+ /Ce 4+ ratio depends on the nanoparticle size. Concurrently, nanoparticle shapes determined with surface facet types can also contribute to this surface effect. Pérez-Bailac et al. [6] demonstrated {1,1,0} facets are the most beneficial. The stable nanoparticle shapes are formed with the surface facets of the lowest surface energy. The lowest surface energies for the CeO2 phase were found for {1,1,1} and {1,0,0} facets depending on the terminal atomic layer. Table 1 containing the calculated surface energy values shows favorable O-terminated facets in both cases [7]. Considering these values, octahedrons are preferred (Figure 1b). This shape is formed with only {1,1,1} facet types, unsuitable for vacancy creation. The second lowest energy was found for {1,0,0}. Therefore, the shapes formed with {1,1,1} and {1,0,0} are assumed to be the second most favorable nanoparticle forms (Figure1c).
The consideration of the stable morphology, especially nanoparticle shapes and their surface crystallography, is required for redox efficiency and its time stability assessment. For this reason, we analyzed a sample prepared using electron beam evaporation and aged for two years. The high-resolution transmission electron microscopy (HRTEM) analyses, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and Electron energy-loss spectroscopy (EELS) were employed for morphological, phase, and oxidation state analyses. TEM samples were purified with centrifugal size fractionation to decrease nanoparticle clusters, receive better nanoparticle distribution, and make more facets visible.

Synthesis -Electron Beam Evaporation
The sample was produced through the evaporation of a solid target (CeO2 micropowder) by a pulsed electron beam in a H2 atmosphere using a NANOBEAM 2 device. This method, including the setup for CeO2 synthesis, was published previously [8].

Centrifugal Size Fractionation
The sample of nanoparticles (25 mg) was dispersed in a mixture of sodium oleate (1 g), oleylamine (1 g), and octadec-1-ene (5 g). The mixture was supplemented with 5 mL of methanol and slowly heated to 300 °C under a protective N2 atmosphere (methanol was evaporated at a lower temperature). The temperature of 300 °C was kept for 5 min. After cooling to laboratory temperature (~25 °C), the nanoparticles were precipitated by adding 10 mL of propanol and centrifuged (1500 g, 5 min). The pellet was dispersed in 5 mL of methanol and shortly sonicated in an ultrasonic bath (~20 s). After centrifugation (1500 g, 5 min), the pellet was dispersed in cyclohexane (1 mL) and shortly sonicated (~20 s). The cyclohexane dispersion was centrifuged (1500 g, 5 min), and the supernatant containing the CeO2 nanoparticles was applied on a TEM grid.

Chemicals
Centrifugal

Characterization Methods
A Titan Themis 60-300 cubed transmission electron microscope was used for morphological analyses. The micrographs were processed with the TIA (ThermoFisher), the DigitalMicrograph, and the JEMS (by P. Stadelmann) softwares. An Empyrean diffractometer (PanAnalytical) with Co Kα1,2 was used for X-ray powder diffraction study. Electron energy-loss spectra were taken with a Gatan GIF with resolution of 0.9 eV. The Xray photoelectron spectroscopy (XPS) (Kratos AXIS Supra) was carried out with a monochromatic Al Kα (1486.7 eV) excitation source to determine the chemical composition and bonding environment of the sample. The CrystalMaker software was used for crystal structure and nanoparticle shape simulations (drawing, plots).

XRD
The synthesized sample was analyzed using X-ray diffraction (Figure 2) confirming only the CeO2 phase. The sample contained two morphological components. The first one contained  5 nm CeO2 nanoparticles (approximately 87 %, blue), the second one contained  150 nm crystallites (green).

TEM
We purified the finest morphological component with centrifugal size fractionation to prepare TEM samples.
We received approximately 100 nm nanoparticle clusters revealing individual nanoparticles at the cluster edges. TEM analysis revealed approximately 5 nm nanoparticles. We took 200 micrographs of 84 unduplicated nanoparticles at suitable orientations for facet type evaluation. The phase analyses using a Fast Fourier transform pattern evaluation confirmed the CeO2 phase. We observed octahedron shapes (Figure 3a) in most cases (approximately 80 %). All of these octahedrons were truncated, so {1,0,0} facet types were observed in all nanoparticles. Truncated cuboctahedrons typically contained additional (1,1,0) facets (Figure 3b). Some amounts of irregular nanoparticles were also observed (Figure 3c).

XPS
The XPS measurement proved the sample purity. Only the Ce and O were detected (Figure 4a). The visible peaks at the binding energies of 882.19 eV and 916.54 eV corresponded to Ce 4+ 3d5/2 and the 2 nd satellite of Ce 4+ d3/2, respectively (Figure 4b). These energies indicated the CeO2 phase content [9,10]. The identification of the Ce 3+ 3d5/2 and Ce 3+ d3/2 peaks was unclear. However, satellites at the energies of 884.26 eV and 902.69 eV confirmed the presence of Ce 3+ . The O 1s spectra in more detail are visible in Figure 4c. These results suggested Ce 4+ in majority compare to the Ce 3+ compounds.

EELS
EELS measurements were conducted in STEM mode and at an energy resolution of 0.9 eV. Data were taken from the thin areas of nanoparticle clusters near the surface. According to data received previously [11], we detected both oxidation states (Figure 5). The Ce 3+ oxidation state was found as often as Ce 4+ in the randomly chosen sites for analysis. These results indicate approximately equal representation of both states, contrary to XPS measurement results. This difference is intelligible because XPS measurements were conducted on the full sample surfaces, including large particles, in contrast to EELS data taken from the smallest nanoparticles at the cluster edge.

CONCLUSION
This work confirmed the possible long-period redox activity of the samples prepared using electron beam evaporation. The results proved that nanoparticles formed with truncated shapes can be active in the redox processes for a long time. Nanoparticles maintained their morphology even after thermal treatment in the purification process. The phase analyses of many nanoparticles using FFT pattern evaluation confirmed only the CeO2 phase. Therefore, the Ce 3+ state is assumed to be in the nanoparticle surface layer formed with the off-stoichiometry CeO2-x phase. The sample aging did not cause CeO2 -Ce2O3 phase transformation even in the ultra-small nanoparticles.