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Article

Thermal Stability and Radiation Tolerance of Lanthanide-Doped Cerium Oxide Nanocubes

by
Kory Burns
1,2,
Paris C. Reuel
3,
Fernando Guerrero
3,
Eric Lang
1,
Ping Lu
1,
Assel Aitkaliyeva
2,
Khalid Hattar
1,* and
Timothy J. Boyle
3,*
1
Center for Integrated Nanotechnologies, Sandia National Laboratories, P.O. Box 8500, Albuquerque, NM 87185, USA
2
Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA
3
Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, NM 87106, USA
*
Authors to whom correspondence should be addressed.
Crystals 2021, 11(11), 1369; https://doi.org/10.3390/cryst11111369
Submission received: 1 October 2021 / Revised: 27 October 2021 / Accepted: 5 November 2021 / Published: 11 November 2021
(This article belongs to the Special Issue Optical and Spectroscopic Properties of Rare-Earth-Doped Crystals)
Browse Figures
Versions Notes

Abstract

:
The thermal and radiation stability of free-standing ceramic nanoparticles that are under consideration as potential fillers for the improved thermal and radiation stability of polymeric matrices were investigated by a set of transmission electron microscopy (TEM) studies. A series of lanthanide-doped ceria (Ln:CeOx; Ln = Nd, Er, Eu, Lu) nanocubes/nanoparticles was characterized as synthesized prior to inclusion into the polymers. The Ln:CeOx were synthesized from different solution precipitation (oleylamine (ON), hexamethylenetetramine (HMTA) and solvothermal (t-butylamine (TBA)) routes. The dopants were selected to explore the impact that the cation has on the final properties of the resultant nanoparticles. The baseline CeOx and the subsequent Ln:CeOx particles were isolated as: (i) ON-Ce (not applicable), Nd (34.2 nm), Er (27.8 nm), Eu (42.4 nm), and Lu (287.4 nm); (ii) HMTA-Ce (5.8 nm), Nd (6.6 nm), Er (370.0 nm), Eu (340.6 nm), and Lu (287.4 nm); and (iii) TBA-Ce (4.1 nm), Nd (5.0 nm), Er (3.8 nm), Eu (7.3 nm), and Lu (3.8 nm). The resulting Ln:CeOx nanomaterials were characterized using a variety of analytical tools, including: X-ray fluorescence (XRF), powder X-ray diffraction (pXRD), TEM with selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDS) for nanoscale elemental mapping. From these samples, the Eu:CeOx (ON, HMTA, and TBA) series were selected for stability studies due to the uniformity of the nanocubes. Through the focus on the nanoparticle properties, the thermal and radiation stability of these nanocubes were determined through in situ TEM heating and ex situ TEM irradiation. These results were coupled with data analysis to calculate the changes in size and aerial density. The particles were generally found to exhibit strong thermal stability but underwent amorphization as a result of heavy ion irradiation at high fluences.

Graphical Abstract

1. Introduction

Lanthanum oxide (LnOx) nanomaterials are of interest as potential fillers in polymeric coatings to protect internal electronic components and circuits from high temperature and ionization exposure (i.e., rad-hard). This is mainly due to their high Z number, which helps to prevent the ionization of energy and variable oxidation state, which can be accessed when exposed to ionizing radiation-reducing Compton and Auger effects [1]. Computational studies using WinXCom software indicate that g-photons (1 keV–100 GeV) interacting with LnOx (Ln–La, Ce, Pr, Nd, Eu, Dy, Sm, Er, Yb, Lu) are effective shielding agents [2,3]. A number of systems have demonstrated their utility, such as LnOx (La [4], Ce, Nd, Sm)-doped borate glasses, which were found to effect γ-ray shielding [5]. To overcome the brittle nature of the ceramic oxide materials, the development of composite coatings (polymers with LnOx nanofillers) allows for a flexible, stable layer that is also rad-hard.
While proving to be an effective shielding agent against electromagnetic radiation, ceria (CeO2) has been shown to be very sensitive to ion irradiation [6]. In ion–matter interaction theory, an impinging ion penetrates a target material and transfers its energy by both elastic and inelastic collisions with the target nuclei and target electrons, respectively. In previous reports, high doses of 2 MeV Au ion irradiation produced significant disorder on the Ce sub-lattice caused by inelastic collisions [6]. This is in direct contrast to 2.5 MeV electron beam irradiations, where elastic collisions produced isolated defects that were easily repaired by thermal annealing [7]. This suggests that dopant atoms within CeO2 may be warranted to enhance radiation stability and prevent significant lattice disorder upon interaction with external stimuli. There are several candidate Ln dopants that have shown promise when utilized in electronic components from high radiation environments, in part due to their high threshold energies [8]. The Ln cations discussed in this report were selected due to their availability, representation across the Ln series, diverse oxidation states available, and diverse applications: EuOx is used as an activation ion for color television phosphor, ErOx is used as a gate dielectric in CMOS logic circuits in space environments, NdOx is used as a ceramic capacitor, and LuOx is used as a laser crystal for solid-state lasers [3].
For this work, we were interested in exploiting ceria (CeOx) beyond the wide range of applications for which it is already used [9,10], including: catalysts [11,12], biomedical applications [1,12,13,14], actinide surrogates [15], pigments [16], sunscreen [12,13,17], and many other applications, as well as less frequently investigated rad-hard coatings. As mentioned above, studies have shown that doping Ln cations into the lattice of CeOx nanoparticles imparts changes to their final properties, including improved radiative recombination, reduced oxidation catalytic activity, enhanced UV-absorption capacity, and the promotion of intermediate electronic levels in the bandgap [12,14,17,18]. Herein, a study of lanthanide doped-CeOx (Ln:CeOx) nanomaterials was undertaken to evaluate the thermal and radiation stability of the nanoparticles through in situ Transmission Electron Microscopy (TEM) techniques [19].
In an effort to explore the properties of Ln:CeOx nanoparticles, a series of nanoparticles was generated using oleylamine (ON) [20], hexamethylenetetramine (HMTA) [21], and t-butylamine (TBA) [22] as surfactants. The lanthanide cations used for dopants included Nd, Eu, Er, and Lu, which were selected to represent the series and are well known to occupy alternative oxidation states. The doping was verified by Scanning Transmission Electron Microscopy (STEM)-based EDS elemental mapping. The structural evolution from annealing and irradiation was verified using the unique in situ ion irradiation TEM (I3TEM) available at Sandia National Laboratories. The results of the synthesis route, ligand, dopant, and resulting particulates were evaluated.

2. Experimental

Nanoparticle preparations were conducted on the bench-top under ambient atmospheric conditions using chemicals obtained and used as received from Aldrich Chemical Company, Inc. (Milwaukee, WI, USA). The lanthanide chloride hydrates ([LnCl3•6H2O]) and nitrates ([Ln(NO3)3•6H2O]) were synthesized in-house using the appropriate metal and concentrated acid ((aq) HCl or (aq) HNO3. All of the Ln precursors structures were verified by single-crystal X-ray experiments. Three routes to nanocubes or nanorods were evaluated, with modifications noted below: (i) oleylamine (ON) [17], (ii) hexamethylenetetramine (HMTA) [18], or (iii) t-butylamine (TBA) [19]. All the generated nanoparticle samples (ON, HMTA, TBA) were characterized by a variety of analytical methods: X-ray Fluorescence (XRF), Powder X-ray Diffraction (pXRD), Fourier Transformed InfraRed Spectroscopy (FTIR), and Dynamic Light Scattering (DLS). Full details of the instrumentation and product characterization are available in the Supplementary Materials. General descriptions of the three different routes to CeO2 and Ln:CeOx, where Ln = Eu, Er, and Lu are presented below. The yields were not determined due to the presence of excess surfactants. A summary of the analytical characterization can be found in Table 1.
X-ray Fluorescence (XRF). A ThermoFisher ARL (West Palm Beach, FL, USA) Quant’X EDXRF, DLS Spectrometer utilizing UniQuant software was used for all the analyses. The system uses a Fundamental Parameters approach based on the Sherman equation for the direct measurement of elemental concentrations based on integrated fluorescent peak intensities. In air, using a medium count rate, a single--repetition, multi-scan excitation (C Thin (5 kV, 60 s); Al (12 kV, 100 s), Pd Thick (28 kV, 100 s); Cu Thick (50 kV, 100 s)) was used to evaluate each sample.
Powder X-ray Diffraction (pXRD). The powders were collected on a Bruker D8 Avance diffractometer employing Cu Kα radiation (1.5406 Å) and a RTMS X’Celerator detector. The data were collected over a 2θ range of 5–70° at a scan rate of 0.083°/s and a zero-background holder was employed. The XRD patterns were analyzed using Bruker EVA software and indexed using the Powder Diffraction File PDF-4 + 2013.
Fourier Transformed InfraRed Spectroscopy (FTIR). The FTIR spectral data were collected on a Bruker Vector 22 MIR Spectrometer in an atmosphere of flowing nitrogen using an ATR powder attachment.
Dynamic Light Scattering (DLS). The DLS data were collected on a Malvern Instruments Zetasizer Nanoseries (NanoZS). All the samples were dispersed by sonication for 10 min at 50–60 Hz in their respective solvents (ON–methanol, Parr–tol, HMTA–water). Following the sonication, the samples were diluted and loaded into 1 cm glass cuvettes. A total of 10 sets of scans was performed for each sample to generate an average size distribution by intensity.
  • Oleylamine (ON) [17]. In a round-bottomed flask, (CeCl3•6H2O) and any dopant (LnCl3•6H2O) where Ln = Nd, Eu, Er, and Lu were added to ON. After heating the reaction to 90 °C and stirring for 1 h, the reaction was then warmed to 265 °C. The reaction turned from pale brown to black. After cooling to room temperature, the solution was collected by centrifugation, washed with ethanol and hexanes and used without further manipulations.
  • Hexamethylenetetramine (HMTA) [18]. In a round-bottomed flask, (Ce(NO3)3•6H2O) and any dopant (Ln(NO3)3•6H2O) where Ln = Nd, Eu, Er, and Lu were dissolved in water. An equal volume of concentrated HMTA was added at room temperature and the reaction was allowed to stir (24 h). The precipitate was collected by centrifugation, washed with hexanes and used without further manipulations.
  • t-butylamine (TBA) [19]. In an Ehrlyenmeyer flask, (Ce(NO3)3•6H2O) and any dopant (Ln(NO3)3•6H2O) where Ln = Nd, Eu, Er, and Lu were dissolved in water and poured into a TeflonTM-lined Parr-bombTM. To this mixture, toluene, oleic acid, and t-butylamine were added; the sample was sealed and heated at 180 °C for 12 h. The precipitate was collected by centrifugation, washed with hexanes, and used without further manipulations.
Transmission Electron Microscopy (TEM). The TEM samples were prepared by making a slurry of the powders isolated (vide supra) in methanol, sonicating them for 10 min at 50–60 Hz, and then dropping one drop of solution onto a lacey carbon TEM grid and allow for the solution to volatilize.
The TEM micrographs and selected area electron diffraction (SAED) patterns were collected using a highly modified JEOL 2100 TEM operating at an accelerating voltage of 200 keV. A FEI TitanTM G2 80–200 STEM with a Cs probe corrector and ChemiSTEMTM technology (X-FEGTM and SuperXTM EDS with four windowless silicon drift detectors) operated at 200 kV was used for compositional and structural analysis using EDS spectral imaging and high-angle annular dark-field (HAADF) imaging. The EDS spectral imaging was acquired as a series of frames, where the same region was scanned multiple times with a total acquisition time of over 30 min. An electron probe of size of about 0.13 nm, with a convergence angle of 18.1 mrad, and a current of ~75 pA was used for the EDS acquisition. The Ce L lines and Eu L lines were used for constructing the EDS maps of Ce and Eu, respectively. Since the Ce L lines overlap with the Eu L lines significantly, the EDS spectra were deconvoluted pixel-by-pixel using pure spectra of Ce and Eu as references before map construction. The HAADF images were recorded under similar optical conditions using an annular detector with a collection range of 60–160 mrad. The thermal anneals were performed utilizing the Gatan Heating stage in a JEOL 2100 up to 500 °C.
Heavy Ion Irradiation. The ex situ ion irradiations were performed on previously prepared TEM samples for the powders listed from the preparation routes noted above. The specimens were subjected to 15 MeV Au4+ ion irradiation at room temperature using a 6 MV HVEE EN tandem accelerator. The ion fluence range included 1 × 1013 and 1 × 1015 cm−2.

3. Results and Discussion

The interest in the addition of Ln:CeOx nanomaterial into polymers as a means through which to improve their properties (thermal and radiation tolerance) led us to prepare and characterize the particles prior to more complex studies. The dopants (none, Nd, Eu, Er, Lu) were selected to explore the size and electronic impact on the final behavior of the resulting Ln:CeOx. Three selected paths focused on synthesis routes that could produce morphologically varied particles, involving different processing parameters, amenability processing for scale up, and different amine surfactants. Details concerning the: Section 3.1. Synthesis and Characterization of the nanoparticles, Section 3.2. Dopant Mapping for Eu, Section 3.3. Thermal Stability, and Section 3.4. Radiation Stability are presented sequentially in the sections below.

3.1. Synthesis and Characterization

The synthesis of nanoparticles following three routes (ON [17], HMTA [18], and TBA [19]) were undertaken as described in the Section 2, with full details available in the Supplementary Materials. The results of the syntheses are presented, followed by in situ and ex situ TEM thermal and ion irradiation measurements, respectively.
Oleylamine (ON). For the ON preparation, the resulting precipitates were independently collected by centrifugation. The green (unprocessed), undoped Ce particles were found to be CeO2 by pXRD patterns. For the dopants, initial efforts focused on whether the dopant had precipitated with, or within, the CeOx matrix. The XRF analysis confirmed the presence of each of the different cations (Nd, Eu, Er, and Lu). However, there was also a trace amount of Cl that was associated with the starting precursor ligands. Noted in the Nd:CeOx(ON) sample was the presence of Zr, which was attributed to a mis-assignment or external contamination. The pXRD patterns of the doped species were in agreement with several different phases of CeOx along with numerous, minor unidentified peaks which were attributed to the organic surfactants. This assignment proved valid when the samples were thermally treated and the lower ‘organic’ peaks were lost and phase pure CeO2 was identified. The FTIR analyses of all these samples verified that ON was present on each sample. Further analysis of the particulates available in solution were determined by the DLS measurements. For these samples, the particles were found to range from 257–518 nm in size. The larger size noted for the Eu dopant was attributed to the clustering of the smaller particles in solution. In order to assess the size of the crystallites formed, a TEM analysis was undertaken. For the undoped species, a polymeric matrix was found to surround the particles. The source of this is not known at this time, but its presence made high-resolution identification (bright field imaging and SAED) of the final particles difficult. Nonetheless, fine particles in the order of ~1–2 nm in diameter were observed. By contrast, the doped-CeOx specimens were found to have formed nanocubes that were ~30 nm in dimension. Further, these particles were found to be highly crystalline, as determined by the SAED analyses.
Hexamethylenetetramine (HMTA). Similarly, the XRF of HMTA showed each powder had the proper Ln cations present. The pXRD patterns of the green undoped Ce as well as the doped Nd matched the phase pure CeO2. For the rest of the Ln:CeOx(HMTA) samples, again, a number of different phases of cerium oxide with minor HMTA species were present. Similarly, heat treatment produced phase pure CeO2 and the loss of the ‘organic’ peaks. The FTIR clearly showed HMTA present on each of the samples. The DLS analyses showed a wide range of particles available from 33–463 nm. Again, Eu displayed two different sizes of particulate. The TEM analysis confirmed the small (~2–6 nm) octahedral-shaped particles for the undoped species, while the doped particles featured similarly shaped species, with sizes ranging from 80–200 nm.
t-butylamine (TBA). The substantially smaller amount of material made available by the Parr bomb approach using TBA made characterization much more difficult. However, for each sample, XRF showed the proper cations were present and that most phases were CeO2 with Lu having an additional carbonate phase. For each, similar FTIR data were compiled and were consistent with the TBA and the other species that were used in the reduction of the nitrate precursor. The DLS data were very consistent in comparison, ranging only from 23–68 nm in size, with both Nd and Er demonstrating some cluster formation (222 and 424 nm, respectively). TEM analysis showed crystallite samples that were in the order of ~2–7 nm in size for the doped and undoped species.
For each of these routes, the nanoparticles were isolated, with only a few featuring residual starting ligands. The pXRD patterns revealed that organic moieties that were consistent with the various surfactants (ON, HMTA, TBA) for the different processes. The heat treatment of the samples led to the formation of phase pure CeO2. The DLS showed much larger particles were present, but this was attributed to clustering. The crystal size was confirmed to be nanoparticles by the TEM analysis for each sample, as shown in Figure 1: (i) ON-Ce (not applicable), Nd (34.2 nm), Er (27.8 nm), Eu (42.4 nm), Lu (287.4 nm); (ii) HMTA-Ce (5.8 nm), Nd (6.6 nm), Er (370.0 nm), Eu (340.6 nm), Lu (287.4 nm); and (iii) TBA-Ce (4.1 nm), Nd (5.0 nm), Er (3.8 nm), Eu (7.3 nm), Lu (3.8 nm). The ON all appeared to be below 50 nm in size and formed cubes, except for the Lu species, which were an order of magnitude larger in size. In comparison, the HMTA particles were similar for Ce and Nd (~5–6 nm) but the other doped species were again an order of magnitude larger. Further, these particles were irregular round species. Finally, the TBA nanocubes all proved to be <8 nm.
The samples were analyzed by XRF, which confirmed that the various dopants and Ce were present. In some instances, minor residual cations were observed, but in general, the cations introduced were isolated in the precipitate. The pXRD patterns confirmed for all the samples that CeO2 was formed, with no indication of minor dopant oxide phases. This implies that the Ln dopants were successfully incorporated into the CeO2 lattice. TEM elemental mapping was used to verify this concept (vide infra). The FTIR data revealed the presence of the various surfactants (HDA, ON, and TBA) on the isolated particles. It is difficult to distinguish between bound and free ligands, but it is believed that after extensive washing, the remaining bends and shifts are associated with the bound surfactants. DLS data revealed that sub-micron species were present in solution, which is consistent with the TEM data collected (vide infra). See Table 1 for a summary of the various results obtained on each sample and Supplementary Materials for the raw data.

3.2. Dopant Mapping for Eu

Based on the TEM images of the dopants across the different preparation routes, the variability in their oxidation states and their more centric sizes, the Eu:CeOx samples were selected for further study by HAADF imaging and EDS mapping (ON, HMTA, and TBA in Figure 2, Figure 3 and Figure 4, respectively). While the above data (see Table 1) confirm the presence of the desired Eu dopant, its size, and its initial phase assignments, it was critical to verify the that Eu dopant was within the CeOx matrix; thus, elemental mapping was undertaken.
For the ON sample, the CeOx(ON) exhibited a much finer particle size (<2 nm; Figure 2a) than the doped Eu:CeOx(ON) samples (20–50 nm; Figure 2b). The EDS mapping for the Eu:CeOx(ON) species verified that the Eu-dopant was present within the CeOx matrix (Figure 2c). Notably, several minor contaminates were identified that were not involved in the synthesis, such as a Si-based polymeric matrix. Additionally, Cl particles consistent with the XRF studies were found within the particles that most likely originated from the starting precursors. As shown in Figure 3, the CeOx(HMTA) particles were found to feature an average size of ~4 nm, whereas the Eu:CeOx(HMTA) were found to be considerably larger (~300 nm). The EDS mapping showed the uniform distribution of Eu within the large Eu:CeOx(HMTA) particles (Figure 3c). As in the Eu:CeOx(ON), Cl contamination was observed, along with other smaller impurities (Fe and Si), the presence of which is unaccounted for as they were not used in the part of the synthesis; however, as these elements were not identified by the XRF studies, it is believed an external contamination source may have accounted for their presence. The CeOx(TBA) and Eu:CeOx(TBA) samples were determined to be cubes 2–5 nm in size (Figure 4a,b) and the distribution of Eu within the Eu:CeOx was verified by the EDS maps (Figure 4c).

3.3. Thermal Stability

In situ thermal annealing experiments were carried out on all the samples (ON, HMTA, TBA), as shown in Figure 5, Figure 6 and Figure 7, respectively. Each specimen was heated from room temperature to 500 °C at an average heating rate of 0.5 °C/s. For the ON specimens (CeOx(ON); Figure 5), the samples proved to be unstable under the electron beam, which was attributed to a high impurity concentration and the formation of complex oxides during the preparation route. This was emphasized further during annealing, and it was not possible to perform a statistical analysis. Further, the Eu:CeOx(ON) sample displayed an initial increase in areal density, followed by a decrease from 100–200 °C. In this temperature regime, sputtering dominated the response of the material, which was likely due to an increase in momentum exchange between the atoms in the solid, which was accelerated by an increase in temperature, and thus an increase in mobility. The CeOx(HMTA) samples were found to be mainly glommed together over the grid; nonetheless, well separated nanoparticles were observed in several areas, and these sections were used for thermal investigation. Upon heating, the nanoparticles that were in the highly concentrated regions were found to ‘de-cluster’, which was attributed to the thermal-induced mobility. It was also noted that the overall size of the nanoparticle material increased through the heating experiment. This was not due to sintering but, instead, to the emergence of larger nanoparticles from the room temperature cluster areas.
The sizes of the Eu:CeOx(HMTA) were unchanged over the temperature range investigated; however, the distance between the particles decreased as the organic ligands were thermally decomposed, allowing in situ clustering. Finally, the CeOx(TBA) particles, analyzed in the same fashion, displayed the highest areal density of any particle studied coupled with the smallest average size. These nanoparticles showed a relatively low mobility under annealing conditions but still agglomerated at higher temperatures. For the Eu:CeOx(TBA) samples, the particles appeared to destabilize at 200–300 °C, as suggested by the reduced areal density in comparison to the room temperature value. Table 2 provides a summary of the results from the thermal annealing experiments. In order to compare the various changes induced by the thermal study, the sizes of the samples were normalized and a plot of these versus the temperature was generated. As can be seen in Table 2, the Eu:CeOx nanocubes appeared to be more resistant to deformation compared to the CeOx samples. This implies that Ln dopants may be modulating the thermal resistance of nanocubes.

3.4. Irradiation Stability

The resistance of these nanoparticles to irradiation was evaluated by exposing the samples to different ion fluences. The TEM images of the ON, HDA, and TBA nanoparticles are shown in Figure 8, Figure 9 and Figure 10, respectively.
For the CeOx(ON) nanoparticles (see Figure 8i(a–c)), a decrease in crystallinity with increasing fluence was observed. This is consistent with the high concentration of defects that altered and/or destroyed the translational periodicity of the material. It was noted that at higher fluences, the ion irradiation also appeared to deplete the SiOx layer beneath the nanoparticles, while simultaneously forcing the aggregation of the particle. By contrast, in the Eu-doped analog of Figure 8ii(a–c), remarkable radiation resistance was observed in the nanoparticles, with diffraction patterns (DP) revealing high crystallinity through the highest fluence studied. What appear to be ablation pits formed from sputtering events on the nanoparticle surface are easily visible. These surface defects are highlighted with arrows in Figure 8ii(c).
Figure 9i(a–c) shows the CeOx (HMTA) preparation route. As can be observed, the polycrystalline green materials migrated towards a single crystalline/amorphous phase during exposure to various levels of ion irradiation. This process was likely akin to a recrystallization in the nanocubes with the simultaneous reduction in crystallinity at higher fluences. In Figure 9ii(a–c), the Eu-doped analog, it appears that the particles began to be sputtered at higher fluences, causing an overall reduction in the size of the nanoparticles. Additionally, the DP was amorphous where this process dominated the response of the material.
The CeOx(TBA) nanoparticles, under the same ion exposure as that noted above, are shown in Figure 10i(a–c). As can be observed, the samples agglomerated as a result of destabilization from the ion interaction. This was further verified by the DP’S transition from a polycrystalline to an amorphous state. Lastly, in Figure 10ii(a–c), the Eu-doped particles remained separated at higher fluences and displayed a small amount of crystalline behavior at the highest fluence studied. These doped nanoparticle retained their structural integrity better than CeOx(TBA) through ion irradiation.
The high thermal stability of the particles was expected of these systems, given the high melting temperature (2400 °C) of the CeOx, which resulted in homologous temperatures of 0.21. Although well above the operating temperature of many applications, this temperature regime shows that no significant nanoscale-enhanced sintering is observed in these systems. The poor irradiation stability observed in this work is similar to that seen in crystalline tungstate nanoparticles irradiated with heavy ions [23,24]. This in stark contrast to other amorphous ceramic nanoparticles and core@shell nanoparticles that have been explored, which suggests that the amorphous phase may be extremely important for radiation stability [25,26]. As such, future work will explore the role of both phase and doping on the stability of ceramic nanoparticles for both free-standing nanoparticles, as shown in this study, as well as those embedded in composite matrixes.

4. Summary and Conclusions

The synthesis of nanoparticles of CeOx and Ln:CeOx (Ln = Nd, Eu, Er, and Lu) was performed by using ON, HMTA, or TBA. The ON and TBA proved to be nanocubes whereas the HMTA produced larger rounded particulates. Elemental mapping and XRF studies confirmed the presence of the lanthanides, which were randomly distributed within a CeOx matrix. Thermal analyses of the undoped and Eu-doped CeOx samples for each preparative route revealed that the Ln-doped species appeared to be more resistant to thermal assaults in comparison to the undoped samples. In the ion stability studies, it was found that all of the samples displayed poor phase stability when exposed to 15 MeV Au ion irradiation. However, the Ln-doped species appeared to be slightly more stable than the homometallic nanoparticles. This variation suggests a potential difference in the active microstructural mechanisms during thermal annealing and ion irradiation. These results suggest that doping with alternative lanthanide cations in CeOx could serve as a good shielding agent against external stimuli.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/cryst11111369/s1. Figure S1: Analytical data for generated CeOx (a) XRF [(i) ON, (ii) HMTA, (iii) TBA], (b) pXRD RT [(i) ON, (ii) HMTA, (iii) TBA], (b*) pXRD 600 °C [(i) ON, (ii) HMTA, (iii) TBA], (c) FTIR [(i) ON, (ii) HMTA, (iii) TBA], (d) DLS [(i) ON, (ii) HMTA, (iii) TBA], (e) TGA/DSC [(i) ON, (ii) HMTA, (iii) TBA]; Figure S2: Analytical data for generated Nd:CeOx (a) XRF [(i) ON, (ii) HMTA, (iii) TBA], (b) pXRD RT [(i) ON, (ii) HMTA, (iii) TBA], (b*) pXRD 600 °C [(i) ON, (ii) HMTA, (iii) TBA], (c) FTIR [(i) ON, (ii) HMTA, (iii) TBA], (d) DLS [(i) ON, (ii) HMTA, (iii) TBA]; Figure S3: Analytical data for generated Eu:CeOx (a) XRF [(i) ON, (ii) HMTA, (iii) TBA], (b) pXRD RT [(i) ON, (ii) HMTA, (iii) TBA], (b*) pXRD 600 °C [(i) ON, (ii) HMTA, (iii) TBA], (c) FTIR [(i) ON, (ii) HMTA, (iii) TBA], (d) DLS [(i) ON, (ii) HMTA, (iii) TBA], (e) TGA/DSC [(i) ON, (ii) HMTA, (iii) TBA]; Figure S4: Analytical data for generated Er:CeOx (a) XRF [(i) ON, (ii) HMTA, (iii) TBA], (b) pXRD RT [(i) ON, (ii) HMTA, (iii) TBA], (b*) pXRD 600 °C [(i) ON, (ii) HMTA, (iii) TBA], (c) FTIR [(i) ON, (ii) HMTA, (iii) TBA], (d) DLS [(i) ON, (ii) HMTA, (iii) TBA]; Figure S5: Analytical data for generated Lu:CeOx (a) XRF [(i) ON, (ii) HMTA, (iii) TBA], (b) pXRD RT [(i) ON, (ii) HMTA, (iii) TBA], (b*) pXRD 600 °C [(i) ON, (ii) HMTA, (iii) TBA], (c) FTIR [(i) ON, (ii) HMTA, (iii) TBA], (d) DLS [(i) ON, (ii) HMTA, (iii) TBA].

Author Contributions

Conceptualization, T.J.B. and K.H.; methodology, T.J.B., K.H. and K.B.; software, K.B.; validation, T.J.B. and K.H.; formal analysis, T.J.B., K.H., K.B., P.C.R. and F.G.; investigation, T.J.B.; resources, T.J.B., K.H. and P.L.; data curation, K.B., P.C.R., F.G. and P.L.; writing—original draft preparation, T.J.B. and K.B.; writing—review and editing, T.J.B., K.H., K.B., P.C.R., F.G., E.L., P.L. and A.A.; visualization, K.B. and P.C.R.; supervision, T.J.B. and K.H.; project administration, T.J.B. and K.H.; funding acquisition, T.J.B. and K.H. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Sandia National Laboratories.

Data Availability Statement

Not applicable.

Acknowledgments

This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 11 01369 g001 550
Figure 1. TEM images of column (i) TBA, (ii) ON, and (iii) HMTA-generated nanoparticles of column (a) CeOx, (b) Nd:CeOx, (c) Eu:CeOx, (d) Er:CeOx, and (e) Lu:CeOx.
Figure 1. TEM images of column (i) TBA, (ii) ON, and (iii) HMTA-generated nanoparticles of column (a) CeOx, (b) Nd:CeOx, (c) Eu:CeOx, (d) Er:CeOx, and (e) Lu:CeOx.
Crystals 11 01369 g001
Crystals 11 01369 g002 550
Figure 2. STEM HAADF imaging and EDS mapping of nanoparticles generated by ON: (a) HAADF of CeOx and (b) HAADF of Eu:CeOx; (c) Ce and Eu L map of the Eu:CeOx NPs (Eu L—red, Ce–green).
Figure 2. STEM HAADF imaging and EDS mapping of nanoparticles generated by ON: (a) HAADF of CeOx and (b) HAADF of Eu:CeOx; (c) Ce and Eu L map of the Eu:CeOx NPs (Eu L—red, Ce–green).
Crystals 11 01369 g002
Crystals 11 01369 g003 550
Figure 3. STEM HAADF imaging and EDS mapping of nanoparticles generated by HMTA: (a) HAADF of CeOx and (b) HAADF of Eu:CeOx; (c) Ce and Eu L map of the Eu:CeOx NPs (Eu L—red, Ce L—green).
Figure 3. STEM HAADF imaging and EDS mapping of nanoparticles generated by HMTA: (a) HAADF of CeOx and (b) HAADF of Eu:CeOx; (c) Ce and Eu L map of the Eu:CeOx NPs (Eu L—red, Ce L—green).
Crystals 11 01369 g003
Crystals 11 01369 g004 550
Figure 4. STEM HAADF imaging and EDS mapping of nanoparticles generated by TBA: (a) HAADF of CeOx and (b) HAADF of Eu:CeOx; (c) Ce L and Eu L map of the Eu:CeOx NPs (Eu L—red, CeL—green).
Figure 4. STEM HAADF imaging and EDS mapping of nanoparticles generated by TBA: (a) HAADF of CeOx and (b) HAADF of Eu:CeOx; (c) Ce L and Eu L map of the Eu:CeOx NPs (Eu L—red, CeL—green).
Crystals 11 01369 g004
Crystals 11 01369 g005 550
Figure 5. TEM images of nanoparticles generated from ON preparation route during thermal annealing: (i) 25 °C and (ii) 500 °C where (a) CeOx (scale bar = 100 nm) and (b) Eu: CeOx (scale bar = 200 nm).
Figure 5. TEM images of nanoparticles generated from ON preparation route during thermal annealing: (i) 25 °C and (ii) 500 °C where (a) CeOx (scale bar = 100 nm) and (b) Eu: CeOx (scale bar = 200 nm).
Crystals 11 01369 g005
Crystals 11 01369 g006 550
Figure 6. TEM images of nanoparticles generated from HMTA preparation route during thermal annealing: (i) 25 °C and (ii) 500 °C where (a) CeOx (scale bar = 100 nm) and (b) Eu:CeOx (scale bar = 500 nm).
Figure 6. TEM images of nanoparticles generated from HMTA preparation route during thermal annealing: (i) 25 °C and (ii) 500 °C where (a) CeOx (scale bar = 100 nm) and (b) Eu:CeOx (scale bar = 500 nm).
Crystals 11 01369 g006
Crystals 11 01369 g007 550
Figure 7. TEM images of nanoparticles generated from TBA preparation route during thermal annealing: (i) 25 °C and (ii) 500 °C where (a) CeOx (scale bar = 100 nm) and (b) Eu:CeOx (scale bar = 100 nm).
Figure 7. TEM images of nanoparticles generated from TBA preparation route during thermal annealing: (i) 25 °C and (ii) 500 °C where (a) CeOx (scale bar = 100 nm) and (b) Eu:CeOx (scale bar = 100 nm).
Crystals 11 01369 g007
Crystals 11 01369 g008 550
Figure 8. TEM images and SAED patterns of (i) CeOx and (ii) Eu:CeOx nanoparticles generated from ON preparation route during ion irradiation: (a) green, (b) 1 × 1013 cm−2, and (c) 1 × 1015 cm−2. Respective SAED patterns are displayed in the inset of each micrograph. Scale bars are 100 nm in all micrographs.
Figure 8. TEM images and SAED patterns of (i) CeOx and (ii) Eu:CeOx nanoparticles generated from ON preparation route during ion irradiation: (a) green, (b) 1 × 1013 cm−2, and (c) 1 × 1015 cm−2. Respective SAED patterns are displayed in the inset of each micrograph. Scale bars are 100 nm in all micrographs.
Crystals 11 01369 g008
Crystals 11 01369 g009 550
Figure 9. TEM images and SAED patterns of (i) CeOx and (ii) Eu:CeOx nanoparticles generated from HMTA preparation route during ion irradiation: (a) green, (b) 1 × 1013 cm−2, and (c) 1 × 1015 cm−2. Respective SAED patterns are displayed in the inset of each micrograph. Scale bars are 100 nm in all micrographs.
Figure 9. TEM images and SAED patterns of (i) CeOx and (ii) Eu:CeOx nanoparticles generated from HMTA preparation route during ion irradiation: (a) green, (b) 1 × 1013 cm−2, and (c) 1 × 1015 cm−2. Respective SAED patterns are displayed in the inset of each micrograph. Scale bars are 100 nm in all micrographs.
Crystals 11 01369 g009
Crystals 11 01369 g010 550
Figure 10. TEM images of (i) CeOx and (ii) Eu:CeOx nanoparticles generated from TBA preparation route during ion irradiation: (a) green, (b) 1 × 1013 cm−2, and (c) 1 × 1015 cm−2. Respective SAED patterns are displayed in the inset of each micrograph. Scale bars are 100 nm in all micrographs.
Figure 10. TEM images of (i) CeOx and (ii) Eu:CeOx nanoparticles generated from TBA preparation route during ion irradiation: (a) green, (b) 1 × 1013 cm−2, and (c) 1 × 1015 cm−2. Respective SAED patterns are displayed in the inset of each micrograph. Scale bars are 100 nm in all micrographs.
Crystals 11 01369 g010
Table
Table 1. Summarized results of XRF, pXRD, ATR-FTIR, and DLS for CeO2, Nd:CeO2, Eu:CeO2, Er:CeO2, and Lu:CeO2. The ATR-FTIR spectrum bends and stretches are further noted with medium (m), strong (s), weak (w), broad (br). Graphical representations of the data shown can be found in the Supplementary Materials.
Table 1. Summarized results of XRF, pXRD, ATR-FTIR, and DLS for CeO2, Nd:CeO2, Eu:CeO2, Er:CeO2, and Lu:CeO2. The ATR-FTIR spectrum bends and stretches are further noted with medium (m), strong (s), weak (w), broad (br). Graphical representations of the data shown can be found in the Supplementary Materials.
CeO2Nd:CeOxEu:CeOxEr:CeOxLu:CeOx
XRFCe (major)
Eu (minor)
Cl(minor)		Ce (major)
Nd (minor)
Cl (minor)
Zr (minor)		Ce (major)
Eu (minor)
Cl (minor)		Ce (major)
Er (minor)		Ce (major)
Lu (minor)
pXRD(25 °C)
PDF
00-067-0123 CeO2 ceria|Cerium
Oxide		(600 °C)
PDF
00-067-0123 CeO2 ceria|Cerium
Oxide		(600 °C)
PDF
00-067-0122 CeO2 ceria|Cerium
Oxide		(600 °C)
PDF
00-067-0123 CeO2 ceria|Cerium
Oxide		(600 °C)
PDF
00-067-0123 CeO2 ceria|Cerium
Oxide
ATR-FTIR (cm−1)3317.93 (m)
2920.58 (s)
2851.09 (m)
2163.86 (w)
2037.01 (w)
1613.62 (m)
1460.77 (w)
1411.48 (m)
1064.97 (w)
965.24 (w)		3355.01 (m, br)
2923.12 (m)
2853.10 (m)
2583.18 (w)
2164.12 (w)
2079.88 (w)
1980.97 (w)
1467.17 (s)
1393.84 (s)
1260.87 (w)
1083.29 (w)
843.62 (m)
802.36 (m)
708.90 (m)		3340.37 (m)
3003.64 (w)
2920.74 (s)
2851.20 (m)
2162.81 (w)
2035.73 (w)
1494.59 (m)
1460.04 (s)
1404.08 (m)
1079.23 (w)
965.75 (w)
841.10 (w)
720.57 (m)		3323.03 (s)
3213.14 (s)
2920.59 (s)
2851.22 (m)
2214.27 (w)
1624.61 (s)
1462.78 (w)
1410.58 (m)
1049.92 (w)
966.31 (w)
719.16 (s)		3315.26 (s)
3210.01 (s)
2922.62 (m)
2852.34 (w)
2216.86 (w)
1618.16 (s)
1411.97 (m)
1117.11 (w)
966.47 (w)
699.03 (m)
DLS (nm)257 (100%)295 (100%)257 (54%)
901 (46%)		343 (100%)518 (100%)
Table
Table 2. Aerial density (μm−2) and size (nm) versus temperature (°C) for (a) ON, (b) HMTA, and (c) TBA preparation of CeO2 and Eu:CeO2.
Table 2. Aerial density (μm−2) and size (nm) versus temperature (°C) for (a) ON, (b) HMTA, and (c) TBA preparation of CeO2 and Eu:CeO2.
Prep
Route		Temp
(°C)		SampleAerial
Density
(µm−2)		Size
(nm)		SampleAerial
Density
(µm−2)		Size
(nm)
ON25CeO2N/AN/AEu:CeO2190.6342.4
100201.0642.34
200189.1442.42
300177.2342.42
400171.2742.43
500160.8542.45
HMTA25CeO229195.77Eu:CeO22.204340.57
10032165.752.204340.61
20037975.792.204340.6
30049325.842.204340.59
40046765.812.204340.6
50046495.812.204340.62
TBA25CeO215,8824.07Eu:CeO245937.3
10016,4454.0449317.27
20015,5194.0950267.23
30015,5014.0943427.31
40014,8184.139107.34
50014,0834.1238507.34
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Burns, K.; Reuel, P.C.; Guerrero, F.; Lang, E.; Lu, P.; Aitkaliyeva, A.; Hattar, K.; Boyle, T.J. Thermal Stability and Radiation Tolerance of Lanthanide-Doped Cerium Oxide Nanocubes. Crystals 2021, 11, 1369. https://doi.org/10.3390/cryst11111369

AMA Style

Burns K, Reuel PC, Guerrero F, Lang E, Lu P, Aitkaliyeva A, Hattar K, Boyle TJ. Thermal Stability and Radiation Tolerance of Lanthanide-Doped Cerium Oxide Nanocubes. Crystals. 2021; 11(11):1369. https://doi.org/10.3390/cryst11111369

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Burns, Kory, Paris C. Reuel, Fernando Guerrero, Eric Lang, Ping Lu, Assel Aitkaliyeva, Khalid Hattar, and Timothy J. Boyle. 2021. "Thermal Stability and Radiation Tolerance of Lanthanide-Doped Cerium Oxide Nanocubes" Crystals 11, no. 11: 1369. https://doi.org/10.3390/cryst11111369

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