Nanoceria prepared by electron beam evaporation

October 19 - 21, 2022 Brno, Czech Republic, EU NANOCERIA PREPARED BY ELECTRON BEAM EVAPORATION Naděžda PIZÚROVÁ, 2Antonín HLAVÁČEK, 1,3Zuzana KAVČIAKOVÁ, 1,3Pavla ROUPCOVÁ, 1 Ivo KUBĚNA, 1Jiří BURŠÍK, 4,5Sergey Yu. SOKOVNIN 1 1Institute 2Department 3CEITEC of Physics of Materials, Czech Academy of Sciences, Brno, Czech Republic, EU, [email protected], [email protected], [email protected], [email protected] of Bioanalytical Instrumentation, Institute of Analytical Chemistry, Czech Academy of Sciences, Brno, Czech Republic, EU, [email protected] - Central European Institute of Technology, Brno University of Technology, Brno, Czech Republic; EU, [email protected] 4Institute 5Ural of Electrophysics Ural Branch RAS, Yekaterinburg, Russia Federal University, Yekaterinburg, Russia; [email protected] https://doi.org/10.37904/nanocon.2022.4585 Abstract 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 Ce3+ and Ce4+ valency state ratio. In the nanoparticle form, cerium oxides contain a mixture of Ce3+ and Ce4+ 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 Ce4+ and Ce3+ 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. We used nanoparticles prepared by electron beam evaporation. This unique physical method includes nanoparticle creation through the fast cooling process followed by breaking radiation damaging nanoparticle surfaces to create surface off-stoichiometry. We prepared a sample containing clusters of a mixture of ultrasmall nanoparticles and approximately 100 nm particles. X-ray diffraction confirmed the CeO2 phase in both components. To extract the finest component, we used centrifugal size fractionation. We received 200 nm clusters of 2-10 nm nanoparticles. Nanoparticle shapes and facet types were analyzed using transmission electron microscopy methods. We found out most nanoparticles were formed with truncated octahedrons containing {1,1,1} and {1,0,0} facet types and truncated cuboctahedrons containing {1,1,1}, {1,0,0}, and additional {1,1,0} facets. No octahedron (without truncation) containing only {1,1,1} facets was observed. Nanoparticle shapes containing {1,1,0} and {1,0,0} are suitable for redox activity. Some amount of irregular shapes, beneficial for redox activity, was also observed. Spectroscopy methods confirmed Ce 3+ content. Keywords: Nanoceria, Ce3+ and Ce4+, electron beam evaporation, facet types, HRTEM 1. INTRODUCTION Cerium in compounds exists in two valency states, Ce3+ (Ce2O3) and Ce4+ (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 (Ce3+). The formation of oxygen vacancies on the CeO2 surfaces leads to the reduction of the Ce4+ October 19 - 21, 2022 Brno, Czech Republic, EU ions to Ce3+ and vice versa. Such oxygen off-stoichiometry correlates with the redox activity of cerium oxide nanoparticles [1-3]. Cerium oxides formed with the CeO2 fluorite lattice (Figure 1a) contain Ce4+. In the nanoparticle form, CeO2 contain also Ce3+ ions on the surfaces. With a nanoparticle size decrease, the number of surface oxygen vacancies and Ce3+ increases [4,5]. Therefore, the Ce3+/Ce4+ 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. Table 1 Surface energies for {1,1,1} and {1,0,0} according to the terminal atomic layer [7] facet {1,1,1} Ce 0.823 eVÅ-2 O 0.084 eVÅ-2 facet {1,0,0} Ce 0.777 eVÅ-2 O 0.455 eVÅ-2 Figure 1 a) CeO2 lattice, b) octahedron, the most typical CeO2 nanoparticle morphology containing only {1,1,1} facet types, c) cuboctahedron containing {1,1,1} and {1,0,0} facet types 2. EXPERIMENTAL 2.1. 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 CeO 2 synthesis, was published previously [8]. 2.2. 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 October 19 - 21, 2022 Brno, Czech Republic, EU 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. 2.3. Chemicals Centrifugal size fractionation: Methyl alcohol (p.a.), isopropyl alcohol (p.a.), and cyclohexane (p.a.) were from Ing. Petr Švec - PENTA s.r.o. (www.pentachemicals.eu). Sodium oleate (82 %), oleylamine (70 %, technical grade), octadec-1-ene (90 %, technical grade) were from Sigma-Aldrich (www.sigmaaldrich.com). 2.4. 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). 3. RESULTS AND DISCUSSION 3.1. 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). Figure 2 XRD patterns confirmed the CeO2 phase and a high content of the nanocrystalline component (blue) 3.2. 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 October 19 - 21, 2022 Brno, Czech Republic, EU 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). Figure 3 a) HRTEM image shows 5 nm truncated octahedron CeO2 nanoparticle in crystallographic direction Z= [1,0,1]; b) truncated cuboctahedron in the direction Z= [1,1,2] containing additional {1,1,0} facet types, and c) irregular nanoparticle in the direction Z = [1,1,4]. Evaluated Fast Fourier transform patterns and Inverse filtered Fast Fourier patterns are included in the corresponding images 3.3. 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 2nd satellite of Ce4+ d3/2, respectively (Figure 4b). These energies indicated the CeO2 phase content [9,10]. The identification of the Ce3+ 3d5/2 and Ce3+ d3/2 peaks was unclear. However, satellites at the energies of 884.26 eV and 902.69 eV confirmed the presence of Ce3+. The O 1s spectra in more detail are visible in Figure 4c. These results suggested Ce4+ in majority compare to the Ce3+ compounds. Figure 4 a) XPS wide spectra; b) Ce 3d; c) O1s spectra October 19 - 21, 2022 Brno, Czech Republic, EU 3.4. 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 Ce3+ oxidation state was found as often as Ce4+ 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. Figure 5 EELS measurements detect two oxidation states of cerium (Ce3+ and Ce4+) 4. 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 Ce3+ 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. ACKNOWLEDGEMENTS The research has been supported by the Czech Science Foundation (Project 21-31852J) and by Russian Foundation for Basic Research (Project 20-58-26002). We acknowledge CzechNanoLab Research Infrastructure supported by MEYS CR (LM2018110). REFERENCES [1] DHALL, A., SELF, W. Cerium oxide nanoparticles: A Brief Review of Their Synthesis Methods and Biomedical Applications. Antioxidants (Basel). 2018, vol. 7, no. 8, pp. 97. [2] ULITKO, M.V., NAUMOVA, A.S., SULTANOVA, T.R., VAZIROV, R.A., AGDANTSEVA, E., OLSHVANG, O. Yu., SOKOVNIN, S. Yu. Investigation of the effect of cerium dioxide nanoparticles on the radiosensitivity of various cell types. AIP Conference Proceedings. 2020, vol. 2313, 080031. [3] XU, C., QU, X. Cerium oxide nanoparticle: a remarkably versatile rare earth nanomaterial for biological applications. NPG Asia Mater. 2014, vol. 6, no. e90. October 19 - 21, 2022 Brno, Czech Republic, EU [4] SAIFI, M.A., SEAL, S., GODUGU, Ch. Nanoceria, the versatile nanoparticles: Promising biomedical applications. Journal of Controlled Release. 2021, vol. 338, pp.164-189. [5] DESHPANDE, S., PATIL, S., KUCHIBHATLA, S. VNT, SEAL, S. Size dependency variation in lattice parameter and valency states in nanocrystalline cerium oxide. Appl. Phys. Lett. 2005, vol. 87, 133113. [6] PÉREZ-BAILAC, P., LUSTEMBERG, P.G., GANDUGLIA-PIROVANO, M.V., Facet - dependent stability of nearsurface oxygen vacancies and excess charge localization at CeO2 surfaces. Journal of Physics: Condensed Matter. 2021, vol. 33, no. 50, 504003. [7] DAN, X., WANG Chao, XU Xi, LIU Ya, CHENG Xiaowei, FRONZI, M., BI Lei, ZHAO, X. S. Improving the sinterability of CeO2 by using plane-selective nanocubes. Journal of the European Ceramic Society. 2019, vol. 39, pp. 4429-4434. [8] IL’VES, V.G., SOKOVNIN, S.Yu.. Production and studies of properties of naanopowders on the basis of CeO2. 2012. Nanotechnol. Russ. 2012, vol. 7, pp. 213 - 226. [9] ELOIRDI, R., CAKIR, P., HUBER, F., SEIBERT, A., KONINGS, R., GOUDER, T. X-ray photoelectron spectroscopy study of the reduction and oxidation of uranium and cerium single oxide compared to (U-Ce) mixed oxide films. Applied Surface Science. 2018, vol. 457, pp. 566-571. [10] MASLAKOV, K.I., TETERIN, Yu.A., POPEL, A.J., TETERIN, A.Yu., IVANOV, K.E., KALMYKOV, N., PETROV, V.G., PETROV, V.G., PETROV, P.K., FARNAN, I. XPS study of ion irradiated and unirradiated CeO2 bulk and thin film samples. Applied Surface Science. 2018, vol. 448, pp. 154-162. [11] GARVIE, L.A.J., BUSECK, P.R. Determination of Ce4+/Ce3+ in electron beam damaged CeO2 by electron energyloss spectroscopy. Journal of Physics and Chemistry of Solids. 1999, vol. 60, pp. 1943-1947.