Interface Energies of Nanocrystalline Doped Ceria: Effects of

Article pubs.acs.org/JPCC

Interface Energies of Nanocrystalline Doped Ceria: Effects of Manganese Segregation Longjia Wu,† Jeffery A. Aguiar,‡ Pratik P. Dholabhai,§ Terry Holesinger,∥ Toshihiro Aoki,⊥ Blas P. Uberuaga,§ and Ricardo H. R. Castro*,† †

Department of Chemical Engineering and Materials Science and NEAT ORU, University of California, Davis, Davis, California 95616, United States ‡ Microscopy and Imaging Group, National Renewable Energy Laboratory, Golden, Colorado 80401, United States § Materials Science and Technology Division and ∥Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ⊥ LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe, Arizona 85287, United States ABSTRACT: The thermodynamics of nanoparticles is strongly dependent on their surface energy as it accounts for a large fraction of the total atomic volume. Grain boundary energies are equally important as the formation of this solid−solid interface is inevitable during synthesis, processing, and application via agglomeration or sintering. The objective of this work is to apply microcalorimetric techniques and atomistic modeling to understand the role of manganese as a dopant and its impact on the interface energies of ceria nanoparticles. Based on the collection of microcalorimetric data, manganese decreases both grain boundary and surface energies with a particularly remarkable effect on the grain boundary energy (0.87 J m−2 for CeO2 and 0.30 J m−2 for 10 mol % Mn). This was attributed to segregation of Mn to both grain boundaries and surfaces, as evidenced by electron microscopy and atomistic modeling examining the segregation of Mn to the (111) surface and to grain boundaries (GB) in CeO2. Noteworthy, the segregation was generally greater to grain boundaries than to surfaces, consistently with the larger energy decrease, which suggest that doped nanoparticles have stronger driving force for aggregation.

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temperatures (1350 °C) and was attributed to the grain boundary segregation.11 A similar phenomenon has been described for Gd3+-doped ceria where the grain size was reported to be greatly decreased with increasing Gd 3+ content.12 Segregated dopants are expected to affect the kinetics of coarsening by pinning boundary movement, a phenomenon attributed to the difficulty in moving boundaries containing a dopant, also called solute drag effect. That is, an increase in the activation energy occurs since not only the dopant but also a defect cloud surrounding it, formed as a result of charge compensation and lattice mismatches between dopant and matrix, have to move. For example, Chen et al. had investigated the effect of multiple dopants,13 such as Mg2+, Ca2+, and Y3+, on the grain boundary mobility of CeO2, and they had found out that at high dopant concentration even diffusion-enhancing dopants, such as Mg2+ and Ca2+, would show a strong solute drag effect to suppress grain boundary mobility. Further, from a

INTRODUCTION Ceria is a fluorite structure material that can easily form oxygen vacancies1,2 due to reduction of Ce4+ to Ce3+. This character of ceria makes it a very important material, since the catalytically active surfaces and high ionic conductivity can be applied in many different applications, such as oxidative catalysis,3 sensors,4 and solid oxide fuel cell.5 For example, ceria has been recently successfully applied in three-way catalytic converters6,7 because of its ability to shift between reduced and oxidized state induced by the oxygen concentration change. Additionally, ceria is also a good additive to zirconia-based solid electrolytes that leads to higher ionic conductivity at lower temperature which can be used for solid oxide fuel cells.8 While ceria particles with nanometer sizes show enhanced performance in such applications due to increased surface area, the instability due to interface energy excesses may cause the nanostructure to collapse under operation conditions, especially at moderate to high temperature. Designed dopant segregation at interfaces is an effective way to improve upon the stability of nanoparticles.9,10 For instance, Ca2+ was used as a dopant in ceria and the grain growth behavior was studied by Rahaman and Zhou.9 Suppression of grain growth was observed at high © 2015 American Chemical Society

Received: September 22, 2015 Revised: November 12, 2015 Published: November 16, 2015 27855

DOI: 10.1021/acs.jpcc.5b09255 J. Phys. Chem. C 2015, 119, 27855−27864

The Journal of Physical Chemistry C

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thermodynamics perspective, dopant enrichment can decrease interfacial energies, which is the driving force for coarsening, as predicted by the Gibbs adsorption isotherm equation for two components under dilute conditions:14,15 dγ = −RT Γ2,1 d ln x 2

EXPERIMENTAL PROCEDURE

Sample Synthesis and Characterization. Mn-doped ceria and pure ceria nanoparticles were synthesized by the coprecipitation method. Cerium nitrate hexahydrate [Ce(NO3)3·6H2O, 99.99%, Alfa Aesar] and manganese carbonate [MnCO3 , 99.9%, Alfa Aesar] were used as precursor compounds, and ammonia solution was used as the precipitant. During the synthesis, cerium nitrate hexahydrate, appropriate manganese carbonate (to achieve desired composition), and a small amount of nitrate acid were dissolved into distilled water to form 100 mL of the stock solutions with 0.15 M of overall cations concentration. Then the solutions were dripped into an equal volume of ammonia solution (1.5 M) at room temperature. After homogenizing, the hydroxide suspension was centrifuged and washed repeatedly with distilled water and ethanol, dried at 90 °C for 24 h, and calcined at 600 °C for 8 h under oxygen flow to obtain pure ceria nanoparticles as well as the Mn-doped ceria nanoparticles. X-ray powder diffraction (XRD) patterns were taken using a Bruker D8 Advance diffractometer operated at an accelerating voltage of 40 kV and an emission current of 40 mA with Cu Kα radiation (λ = 1.5406 Å) and a spinning sample holder. Data were acquired over a 2θ range of 20°−90°, with a 0.017° step size and 0.7 s dwell time. Crystallite sizes were refined from diffraction peak broadening, by using WPF (Whole Pattern Fitting) refinement in the JADE software (version 6.11, 2002, Materials Data Inc., Livermore, CA). The surface area of the Mn-doped ceria samples were measured using a Micromeritics ASAP 2020 instrument, based on the Brunauer−Emmett−Teller (BET) method. Before analysis, all samples were degassed under vacuum at 400 °C for 12 h to get an anhydrous surface (as determined by thermal analysis at the condition where no more water loss is observed) and then oxidized under oxygen (P = 700 mmHg) at 400 °C also for 12 h to allow oxidation of the surface reduced by the vacuum. During the analysis, five-point adsorption isotherms of nitrogen were acquired at the relative pressure range from 0.05 to 0.30 at −196 °C. Each sample was measured three times to get an average value of the surface area. To study the Mn interface segregation, analytical transmission electron microscopy was performed on the probecorrected JEOL ARM 200F located at the LeRoy Center for Solid State Science at Arizona State University. The JEOL ARM is equipped with a field-emission gun that was operated in STEM (scanning transmission electron microscopy) mode at 200 kV, a Gatan Enfinium electron energy loss image filter, and a high solid angle 50 mm2 X-ray detector. Electron energy loss (EEL) X-ray spectral chemical imaging was utilized to acquire the O-K, Ce-M, and Mn-L edges with the best achievable spatial and energy resolution for the microscope. Given the heightened sensitivity to beam damage, where cubic CeO2 easily transforms to the fluorite-derivative bixbyite structure, Ce2O3, structural and spectral imaging was performed under reduced beam current conditions and subsecond exposures. Core loss EELS was performed using beyond a 15 mrad collection half-angles providing an energy resolution defined by the full width at half-maximum of the zero-loss peak of 0.98 eV. The acquisition time to resolve EELS near edge fine structures was performed over a series of consecutive and compensated subsecond exposures. All collected EELS spectra were aligned based on their peak maxima, individually dark count subtracted, and summed to produce the results shown here. The core-loss

(1)

where subscripts “1” and “2” represent solvent and solute, respectively, γ is the interface energy, T is temperature, R is gas constant, Γ2,1 is the Gibbs excess at the interface, and x2 is the molar fraction of the solute (dopant) in the bulk. As an example, our previous work16 showed that ceria grain size decreases with increasing Mn3+ content due to the reduction of the surface energy caused by the Mn3+ surface segregation, as directly quantified by scanning transmission electron microscopy based electron energy loss spectroscopy (EELS). Thus, dopants are expected to alter both the thermodynamics and the kinetics associated with coarsening and grain growth in a way that reduces the overall grain size. Although most experimental works on nanoparticle stability focus on surface effects, coalescence processes can be strongly dependent on the energetics of the formation of solid−solid interfaces, such as grain boundaries, as intermediates for net grain enlargement.17 For instance, during sintering, particles that touch each other form a so-called neck or a grain boundary with a distinct energy compared to the surface. The energetics associate with neck formation (sometimes referred to as aggregation) and eventually coarsening can be described by18 dGtotal = γS dAS + γGB dA GB

Article

(2)

where dGtotal is the variation of total free energy for the interface, dAS and dAGB are the surface area variation and grain boundary area variation, respectively, and γS and γGB are the surface and grain boundary energies, respectively. Manganese has been used as a dopant in ceria to increase nanostability and enhance catalytic activities. In a previous work,16 the effect of Mn on the surface energy of ceria was studied by water adsorption microcalorimetry, and a reduction of surface energy was observed with increasing Mn content and associated increase in nanostability at given temperature. However, the role of the dopant on the grain boundary stability was not considered. In this article, in order to fully describe the thermodynamics of Mn-doped CeO2 nanocrystals, we report on the effect of Mn on the grain boundary energies of ceria by using a combination of high-temperature oxide melt drop solution calorimetry with water adsorption microcalorimetry.19,20 A significant decrease in grain boundary energy was observed with increasing dopant concentration. This decrease was more remarkable than the effect of the dopant on the surface energy, consistent with a greater enthalpy of segregation. Interface segregation was confirmed by electron energy loss spectroscopy (EELS). The grain boundary and surface segregation profiles were calculated based on the results of interface energies using an analytical model. Experimental observations of segregation were consistent with atomistic modeling showing that segregation of both single Mn atoms and dopant-defect clusters containing Mn segregate much more strongly to GBs than to the (111) surface in CeO2. The results suggest that the overall higher effect of the dopant in decreasing the GB energy as compared to surface energy can result in an easier formation of aggregates while maintaining nanocrystalline sizes since effective growth is pinned. 27856


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(3)

Additionally, for each prepared nanopowder sample, a bulk sample with negligible surface/grain boundary area was also needed as a reference. Therefore, all of the Mn-doped ceria and pure ceria samples were heated at 1200 °C for 12 h to obtain the corresponding bulk reference. Prior to the measurement, all of the prepared samples went through the same degassing and oxidizing procedure as the samples for water adsorption calorimetry to maintain a consistent surface condition and surface chemistry. After degassing and oxidizing, the samples were stored in a room under constant humidity and temperature (50% humidity, 24 °C) for at least 3 days to equilibrate the water content on the samples. Then the water content was measured by thermogravimetry, and the samples were ready for drop solution calorimetry. During the experiment, about a 5 mg pellet of the sample was loosely pressed and dropped from room temperature into a molten sodium molybdate (3Na2O·4MoO3) solvent at 700 °C for dissolution. Meanwhile, the calorimetry setup was flushed with 40 mL/min oxygen flow to bring out the water vapor released from the sample, and oxygen gas was also bubbled through the solvent at 5 mL/min to aid dissolution of the sample as well as to keep the oxidation state of the dissolved metal cations. For each sample, at least eight pellets were dropped separately to get statistically reasonable results. Calibration of the instrument was done using the heat content of α-alumina. Atomistic Simulation of Segregation. Atomistic modeling was used to examine the segregation of Mn to the (111) surface, chosen as it has been shown to be the most energetically stable,26 and to grain boundaries in CeO2. The atomistic modeling used empirical potentials of the Buckingham form, supplemented by long-range Coulomb interactions summed using the Ewald method. Potential parameters utilized and the appropriate references are summarized in Table 1.

(4)

Table 1. Short-Range Pair Potential Parameters for MnDoped Ceria

EELS spectra were furthermore processed to reduce effects of plural scattering events using Fourier-log deconvolution. Given the proximity of O-K and Mn-L core-loss near-edge onsets, we applied a triple window background subtraction that utilized the known partial cross-section profiles for each of these transitions. Integrated windows were then applied at these edges, as well as the intense Ce-M lines, and processed into elemental maps as discussed elsewhere. The point resolved MnL and Ce-M lines were further processed to refine the fine structure associated with each of these elements, where the results were scrutinized for the presence of Ce3+.21,22 To measure the enthalpy of water adsorption and hence implement the water corrections for the results of the drop solution calorimetry, water adsorption microcalorimetry, a combination of a Micromeritics ASAP 2020 instrument and a Setaram Sensys Calvet microcalorimeter, was performed. Prior to analysis, the sample was degassed and oxidized with the same procedure of the surface area measurement. During the experiment, the sample was kept at 25 °C, and about 2 μmol water per dose was kept pumping into the system. An empty tube run was performed to measure the amount of water adsorbed to the tube and used for later correction. Details of this experimental setup and methodology can be found in our previous publication.23 As a result of the calcination procedures, partial agglomeration (sintering) of nanoparticles is inevitable given the high reactivity of nanocrystals. This is a critical parameter that needs quantification for reliable calorimetric assessments. While surface areas and crystallite sizes can be directly measured as described above, grain boundary area quantification requires geometrical assumptions. In order to calculate grain boundary area (AGB), the following equations were used:19,20

AI =

6000 ρGSXRD

A GB =

AI − AS 2

where AI is the overall interface area (assuming all particles are isolated), ρ is the theoretical density of the sample, GSXRD is the grain size refined from X-ray diffraction peak (also from TEM), and AS is the surface area measured by the BET method. Equation 3 has the assumption that all the nanoparticles are spherical and show narrow size distribution (a reasonable assumption for ceria powders prepared by coprecipitation according to TEM analyses16). For the unlikely case of nanoparticles with negligible agglomeration, AI and AS should be equal to each other. The division by 2 in eq 4 is because the grain boundary is formed by two surfaces during agglomeration. High-Temperature Oxide Melt Drop Solution Calorimetry for Grain Boundary Energy Measurement. Hightemperature oxide melt drop solution calorimetry was performed in a custom-build Tian-Calvet twin microcalorimeter.24,25 Pure ceria samples and Mn-doped ceria samples were prepared in different procedures for the drop solution calorimetry. In order to obtain different surface/grain boundary areas for accurate interface energy measurement, pure ceria samples were treated at 900 °C for different times (1 and 5 min). However, for the doped samples, different temperatures and times of the heat treatment may vary the distribution of the Mn dopant in the samples, causing the change of the interface energy, so that all of the doped samples were calcined at the same temperature (600 °C) for the same period of time (8 h).

interaction

A (eV)

ρ (Å)

C (eV Å−6)

ref

Ce −O O2−−O2− Mn3+−O2−

1809.68 9547.96 922.83

0.3547 0.2192 0.3389

20.40 32.00 0.00

53 53 54

4+

2−

Atomistic calculations were performed within the framework of LAMMPS.27 To calculate the segregation energy of Mn near GBs, we examined three grain boundary structuresΣ3 (111)/ [110] symmetric tilt, Σ5 (310)/[001] symmetric tilt, and Σ5 (001) θ = 36.87 symmetric twist grain boundariesused in our previous work on both CeO228 and UO2.29 Optimized structures of stoichiometric undoped ceria grain boundaries were obtained by energy minimization along all three directions, and the forces on all atoms were allowed to relax. These structures were subsequently annealed with hightemperature molecular dynamics to ensure their stability. Application of PBC in all dimensions resulted in two GBs existing within each simulation cell. This is essential to avoid any electrostatic dipole originating from surface termination. While all three GBs are high symmetry/low sigma boundaries, they represent a range of local atomic arrangements that provide insight into how GB atomic structure influences Mn segregation. In each case, we substituted each Ce ion within the structure, one by one, with Mn3+. For the Σ3 tilt, Σ5 tilt, and 27857


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The Journal of Physical Chemistry C Σ5 twist GB, the sizes of the respective GB models are 7.31 × 2.36 × 3.38 nm3 (4416 atoms), 6.79 × 3.36 × 4.28 nm3 (7488 atoms), and 8.61 × 3.38 × 3.38 nm3 (7680 atoms). For the surface calculations, the size of the slab is 2.86 × 7.81 × 2.86 nm3 (2808 atoms), wherein a 4 nm vacuum is added along the Y-axis. Two configurations of Mn were investigated for all of these systems: (a) MnCe3+ and (b) 2MnCe3+ + VO2−, though, in the case of the GBs, the results for the clustered defect were reported previously.28 For the calculations for doped ceria, the cell volume was held constant at the undoped ceria cell volume.

in Table 2. The data are consistent with grain sizes measured from TEM images. Surface areas measured by BET are also shown in Table 2. Comparing the surface area with the total interface area calculated by assuming isolated crystals and using the sizes from XRD refinement, one may note a significant discrepancy. This indicates a non-negligible level of agglomeration. The grain boundary areas for all nanosamples were calculated and are also listed in Table 2. Here we want to emphasize that although the pKs of the cations might be different during precipitation, a large excess of ammonia solutions (5 times the necessary amount) has been used to make sure all cations would precipitate at the same kinetics. The chemical compositions of as-synthesized powders have been determined by microprobe analysis, and the results agreed with the nominal compositions within experimental errors. That is for the chemical compositions of as-synthesized Mn-doped CeO2 powders (Ce1−XMnXO2−(X/2)), X = 0.018 ± 0.001 for 2 mol % Mn dopant concentration, X = 0.048 ± 0.003 for 5 mol % Mn dopant concentration, and X = 0.095 ± 0.003 for 10 mol % Mn dopant concentration. Grain Boundary Energy for Mn-Doped Ceria Samples. High-temperature oxide melt drop solution calorimetry was used to measure the grain boundary energy of Mn-doped ceria nanoparticles. The samples listed in Table 2 were dissolved in a molten sodium molybdate (3Na2O·4MoO3) solvent, and the heat of dissolution correlated with the microstructure of each sample throughout a detailed thermochemical cycle describing the process:

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RESULTS AND DISCUSSION Structure Characterization of Mn-Doped Ceria Samples. Figure 1 shows the X-ray diffraction patterns of pure ceria

Ce1 − xMnxO2 − (x /2)nH 2O(nano,25 °C) Figure 1. X-ray diffraction pattern for pure ceria and Mn-doped ceria samples annealed at different temperature for different times.

→ Ce1 − xMnxO2 − (x /2)(dissolved,700 °C) + nH 2O(gas,700 °C); ΔH1 = ΔHds,nano

and Mn-doped ceria samples prepared at different temperature and different calcination times used for high-temperature oxide melt drop solution calorimetry. Only the cubic fluorite structure was detected in all samples, with the absence of second phases. While the patterns for the sample calcined at 600 °C showed broad peaks consistent with nanosized grains, all compositions annealed at 1200 °C had sharp peaks, suggesting an increase of the grain size as a result of coarsening/grain growth. Grain sizes of all samples refined from diffraction peak (GSXRD) are shown

(5)

nH 2O(gas,700 °C) → nH 2O(gas,25 °C); ΔH2 = n( −25.0 ± 0.1) kJ/mol

(6)

Ce1 − xMnxO2 − (x /2)(nano,25 °C) + nH 2O(gas,25 °C) → Ce1 − xMnxO2 − (x /2)nH 2O(nano,25 °C);

ΔH3 = nΔHads (7)

Table 2. Characteristic of Grain Size, Interface Area, Water Content, and Calorimetric Information Acquired from Pure Ceria and Mn-Doped Ceria Samples Annealed at Different Temperatures for Different Times grain size (nm)

interface area (103 m2/mol)

ΔH (kJ/mol)

water content

samples

GSXRD

GSTEM

AS

AI

AGB

n (mol)

θ (H2O/nm2)

ΔHads

ΔHds

ΔHds,anhydrous

CeO2: 600 °C/8 h + 900 °C/1 min CeO2: 600 °C/8 h + 900 °C/5 min bulk CeO2:1200 °C/12 h 2MDC: 600 °C/8 h bulk 2MDC: 1200/12h 5MDC: 600 °C/8 h bulk 5MDC: 1200 °C/ 12 h 10MDC: 600 °C/8 h bulk 10MDC: 1200 °C/ 12 h

12.1 ± 0.4

13.0 ± 2.9

8.09 ± 0.02

11.82

1.87

0.1915

14.26

−56.51

80.49 ± 2.28

64.88 ± 2.28

21.3 ± 0.4

21.7 ± 6.0

2.73 ± 0.03

6.72

2.00

0.0751

16.57

−55.09

77.25 ± 1.64

71.23 ± 1.64

10.9 ± 3.1

9.27 ± 0.02

14.91

2.82

0.2269

14.73

−56.66

9.4 ± 1.9

12.82 ± 0.03

16.84

2.01

0.2858

13.42

−56.16

7.9 ± 2.4

12.92 ± 0.02

19.61

3.35

0.2509

11.69

−56.91

>100 9.6 ± 0.3 >100 8.5 ± 0.3 >100 7.3 ± 0.3 >100

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75.70 82.13 75.02 83.37 73.56

± ± ± ± ±

3.12 2.70 2.16 3.18 1.24

79.34 ± 1.90 72.01 ± 2.74

75.70 63.60 75.02 60.18 73.56

± ± ± ± ±

3.12 2.70 2.16 3.18 1.24

58.79 ± 1.90 72.01 ± 2.74


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and/or a lowering in grain boundary energy. On the other hand, according to Dey et al.,32 the mole fractions of dopant dissolved in bulk and segregated on the interface are directly dependent on the grain size of the sample. For the bulk samples, the grain sizes are much larger, and so the interface areas are smaller, so it is expected that Mn is highly concentrated at the limited interface areas. Although the enhanced segregation effect would lead to a much smaller interfacial energies for the bulk samples, because of the negligible interface area, the drop solution enthalpies for all of the bulk samples would be similar to each other, very consistent with our calorimetry results considering the error bar. Combining the microstructural characterization and the calorimetric data, eq 11 is reduced to two variables (the surface and grain boundary energies). While one could theoretically use a least-squares method to find proper values for both energies, as proposed in the literature,20 surface energies have been reported previously using accurate calorimetric procedure.16 This allows us to assess the grain boundary energies with high accuracy. The results are listed in Table 3.

Ce1 − xMnxO2 − (x /2)(nano,25 °C) → Ce1 − xMnxO2 − (x /2)(dissolved,700 °C); ΔH4 = ΔHds,nano,anhydrous

(8)

ΔH4 = ΔH1 + ΔH2 + ΔH3

(9)

Ce1 − xMnxO2 − (x /2)(bulk,25 °C) → Ce1 − xMnxO2 − (x /2)(dissolved,700 °C); ΔH5 = ΔHds,bulk

ΔHds,bulk − ΔHds,nano,anhydrous = γSAS + γGBA GB

(10) (11)

Here, ΔHds,nano is the drop solution enthalpy of nanosamples, ΔHads is the water adsorption enthalpy based on the results of water adsorption microcalorimetry, ΔHds,nano,anhydrous is the drop solution enthalpy of nanosamples after water correction, ΔHds,bulk is the drop solution enthalpy of the respective bulk samples (coarsened to negligible interface area), n is the amount of water adsorbed on one mole sample surface, and γS, γGB, AS, and AGB are the surface energy, grain boundary energy, surface area, and grain boundary area, respectively. According to the thermochemical cycle described above, the measured heat effect from the drop solution calorimetry includes the enthalpy related to water adsorbed on the sample surface, the heat of dissolution of the anhydrous nanosamples and bulk samples, and the excess enthalpy related to the surface and grain boundary. To be able to assess the excess enthalpy corresponding to the anhydrous interfaces only, the energetic contribution of water needs to be quantified. Therefore, the water content (n) of each sample was measured by TG-DSC and converted to water coverage (θ) based on the specific surface area of each sample. In addition to heat capacity, the actual energy involved in desorbing water molecules from the surface into a gas phase needs to be taken into account. Because this is a reversible process, one can evaluate it by the adsorption process instead.23 Therefore, water adsorption enthalpy (ΔHads) data for water coverage from zero (anhydrous) to the total amount of water in the sample was used (shown in Table 2). The data are available in a previous paper.16 With this, water-corrected drop solution enthalpy of anhydrous nanosamples (ΔHds,nano,anhydrous) can be calculated using eq 9. To obtain excess enthalpy related to the surface and grain boundary only, according to eq 11, the drop solution enthalpy of anhydrous nanosample needs to be subtracted from the drop solution enthalpy of corresponding bulk sample (ΔHds,bulk). As shown in Table 2, the measured drop solution enthalpy of bulk ceria is 75.70 ± 3.12 kJ/mol, which is in a good agreement with literature reports also measured by drop solution calorimetry.30,31 Corrected anhydrous drop solution enthalpies for the nanosamples are compiled in Table 2. For pure ceria, the values are lower in the nanosamples as compared to bulk due to the contributions of the surface energy and grain boundary energy. The values consistently increase with grain size from 64.88 kJ/mol for 12.1 nm to 71.23 kJ/mol for 21.3 nm. Likewise, for Mn-doped ceria samples the nanosamples show consistently lower heats of dissolution as compared to the respective bulk. By comparing the various concentrations of manganese, though, it is observe that with increasing Mn content, the drop solution enthalpy of nanosample continuously decreases from 63.60 kJ/mol for 2 mol % Mn down to 58.79 kJ/mol for 10 mol % Mn. This is consistent with the decrease in the grain size with increasing manganese content

Table 3. Measured Surface and Grain Boundary Energies of Pure Ceria and Mn-Doped Ceria Samplesa CeO2 surface energy (J/m2) grain boundary energy (J/m2)

2% Mn−CeO2

5% Mn−CeO2

10% Mn−CeO2

1.08

1.05

0.97

0.95

0.87; 0.81;19 0.7733

0.61

0.49

0.30

a

Surface energy data are from our previous work, and literature data for grain boundary energy of ceria are reported for comparison.

The grain boundary energy of our ceria sample is 0.87 J/m2, which matches well with values reported in the literature. For example, Chen et al.33 studied the influence of grain boundary energy on the grain size evolution in nanocrystalline materials through modeling the gadolinium-doped ceria nanocrystalline system. By combining the generalized parabolic grain growth law with solute segregation equations, such as the Gibbs adsorption equation and the Mclean equation, the model had been established and used to analyze the grain growth mechanism with respect to grain boundary segregation and grain boundary energy. By fitting the experimental data of a series of grain sizes at different annealing temperatures into the model, the grain boundary energy of pure ceria had been calculated to be 0.77 J/m2, which is very close to the value reported here. In addition, Hayun et al.19 synthesized ceria nanoparticles by using the nonaqueous sol−gel method and measured the interface energy of the nanoparticles also by hightemperature oxide melt drop solution calorimetry. They reported that the measured grain boundary energy of nanoceria was 0.81 ± 0.14 J/m2. The grain boundary energy of pure ceria reported here lies within the error range of their value, showing good consistency. The results in Table 3 also reveal the influence of the Mn content on the grain boundary energy. As the Mn concentration increases, the grain boundary energy significantly decreases down to 0.30 J/m2 for 10 mol % Mn-doped ceria nanoparticles, only one-third of the grain boundary energy for pure ceria. The surface energy does not decrease as dramatically as the grain boundary energy, suggesting a smaller effect of manganese on that interface, which can be attributed to the 27859


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Figure 2. Relative segregation energies of Mn in the +3 charge states at three model grain boundaries in CeO2. All energies are measured relative to the lowest energy position for Mn in +3 charge state. A few high-energy configurations are not shown for clarity and because they are irrelevant for the overall behavior of the dopants near these boundaries.

amount of manganese segregated to each microstructural feature. Atomistic Simulations and Electron Microscopy. Based on the results described above, Mn dopants significantly decrease the grain boundary energy of ceria nanoparticles in addition to moderately decreasing the surface energy. This phenomenon is likely due to Mn segregation to those interfaces, as previously proposed.16 From a thermodynamic perspective, a decrease in interface energy can be related to the amount of dopant segregated to the interface. From a chemistry viewpoint, segregation can be induced by, among other things, ionic size mismatch and differences in the valence between dopant and matrix ions. According to the literature,34,35 the adopted annealing procedure (600 °C, 8 h) should induce Mn to be in the 3+ valence state in ceria. This causes a charge mismatch with Ce4+ which, in combination with a significant radius difference15 between Mn3+ (0.58 Å) and Ce4+ (1.01 Å),36 is presumably responsible for the segregation of Mn. Figure 2 summarizes the results of atomistic simulations for Mn3+ near the three grain boundaries considered. While the details differ, there are trends that are generic to all three boundaries, with a significant gain in energy as the dopant is placed at the GB plane in each case. Typically, charged dopants, upon incorporation into an ionic material, must be compensated by oppositely charged defects. Mn3+ should be compensated by oxygen vacancies, forming the defect complex 2MnCe3+ + VO2−. In the calculations reported in Figure 2, we did not account for these charge-compensating defects. Rather, those calculations should be viewed as limiting behavior in the dilute limit when dopants are not associated with defects. However, we recently examined the segregation of the bound 2MnCe3+ + VO2− complex to the same three grain boundaries.28 The segregation energies, as found from the atomistic calculations, are summarized in Table 4, in which a negative energy indicates the dopant is attracted to the boundary. Analogous to the results for the segregation energies of dopantdefect complexes, we found strong segregation tendencies for the isolated dopant at all the grain boundaries considered, but the magnitude of segregation is typically weaker for the isolated dopants. The difference in segregation strength between the isolated dopant and the dopant-defect cluster could be due to stronger segregation of oxygen vacancies and in-boundary binding of the dopant-defect complex. That said, there are differences in the trends between the isolated dopant and the clusters. For the dopant-defect clusters, segregation to the Σ5 twist boundary was found to be the strongest, whereas for the isolated dopants, segregation to the Σ3 tilt boundary was strongest, suggesting that certain boundaries may accommodate the complex structure of the dopant-defect cluster better than other boundaries. For both the cases, isolated dopants and

Table 4. Segregation Energies (Eseg) per Dopant Atom for the (111) Surface and the Three Grain Boundaries Considereda Eseg (eV) interface (111) surface Σ3 tilt GB Σ5 tilt GB Σ5 twist GB

MnCe

3+

−0.32 −3.42 −1.65 −2.5

2MnCe

3+

Eseg (kJ/mol) + VO

−1.04 −2.91 −2.05 −5.50

2−

MnCe

3+

−30.88 −329.98 −159.20 −241.20

2MnCe3+ + VO2− −100.34 −280.19 −197.31 −530.19

a

Segregation energies for the clustered (2MnCe3+ + VO2−) cases are taken from ref 28, wherein the energies correspond to the difference between the lowest energies in the bulk and at the grain boundaries. The energies given in ref 28 are for dopant-defect complexes containing two dopant ions. To facilitate comparison with isolated dopant energies, those numbers are converted to “per dopant” energies here.

dopant-defect clusters, the Σ5 tilt boundary is found to exhibit the weakest segregation. In addition, because of the complex structure of the dopant-defect complex, there was greater variability in site energies at these boundaries than there are for the isolated dopant. Nonetheless, the overall trends indicating that the grain boundaries in doped-ceria induce strong segregation of Mn3+ dopants irrespective of their detailed nature are clear from the present calculations. We have also considered Mn segregation to the (111) surface of CeO2. Table 4 provides the segregation energy, per dopant atom, of Mn3+ in both isolated and clustered form. These results follow the same basic trend as described above for the grain boundaries. When Mn3+ associates with another Mn3+ and an oxygen vacancy, the segregation energy of that complex, per Mn atom, is stronger. However, the segregation energetics is significantly weaker at the surface than at the GBs, consistent with the calorimetric data. To experimentally demonstrate grain boundary segregation, analytical aberration corrected microscopy was used to reveal the details of grain boundary composition. Figure 3a is an atomic contrast STEM micrograph from 2 mol % Mn-doped ceria taken over a region containing several grain boundaries. Figure 3b, a higher magnification image of the same material, highlights the grain size of the materials is on the order of ∼16 nm. For a specific region of interest (Figure 3c), we have applied STEM-based EELS to resolve composite images of Mn (Figure 3d). There is significantly more Mn content present at the GBs than in the grain interiors, which is consistent with the energetics of segregation to boundaries (and surfaces) measured with calorimetry and atomistically simulated. Although Mn segregation is observed to all boundaries, the distribution is not even among all orientations. This is 27860


Article

The Journal of Physical Chemistry C γs = γs0 + ΓΔH s seg,s

(12)

γGB = γGB0 + ΓGBΔHseg,GB

(13)

where γs0, γGB0, γs, and γGB are the surface energy of pure ceria, grain boundary energy of pure ceria, surface energy of Mn doped ceria, and grain boundary energy of Mn-doped ceria, respectively; ΔHseg,s and ΔHseg,GB are the enthalpy of surface segregation and enthalpy of grain boundary segregation, respectively; and Γs and ΓGB are the excess Mn at the surface and the grain boundary. Here, based on the definition of excess interfacial Mn, Γs and ΓGB can be written as (xsMnns)/As and s GB (xGB MnnGB)/AGB, where xMn and xMn denote the mole fractions of Mn at the surface and at the grain boundary, ns and nGB represent the total mole numbers of molecules at the surface and at the grain boundary, and As and AGB are the total surface and grain boundary area. A/n is the molar area of the molecules at interface, approximately equal to Ω2/3Navg/m, where Ω is the average volume per molecule, Navg is Avogadro’s number, and m is the number of layers within the interface. To calculate the interface segregation enthalpies of Mn on SnO2 nanoparticles, Chang et al.45 made an assumption that surface had monolayer thickness (ms = 1) and the grain boundary had a thickness of three layers (mGB = 3). This is a reasonable assumption for Mndoped SnO2 system due to the occurrence of solute saturation in the grain boundary, indicated by the plateau of grain boundary energy when the dopant concentration is higher than 2.5 mol %. However, for the Mn-doped ceria system, there is no evidence of Mn saturation in the grain boundary from the grain boundary energy measurement (Table 3), and furthermore some low-angle grain boundaries, as suggested in Figure 3f, even show the Mn dopant is only segregated to specific positions without filling the entire boundary layer. Therefore, for this specific system, we assume both surface and grain boundary have monolayer thickness (ms = mGB = 1). For the fluorite crystal structure, Ω = a3/4 due to the four “molecules” in each unit cell, where a is the lattice constant. Therefore, the interface excess of Mn can be written as

Figure 3. (a, b) To image the clustering of Mn in nanostructured ceria, we imaged slightly sintered material codoped with Mn with aberration corrected high-angle annular dark field STEM. The bright areas are ceria and the dark areas are pores. (c) Following imaging we composed quantitative, (d) Mn-L spectral images. (e) For a lowconcentration Mn-doped boundary, we were further able to collect atomic column by column electron energy loss chemical images of (f) individual Ce and Mn columns segregating to a high-angle asymmetric GB. Specifically the cube-on-cube orientation relationship based on the (inset) fast Fourier transforms is in-plane (042)||(311) and out-ofplane [200]||[111].

attributed to differences in the energy of the boundaries as proposed in a recent work for this same system37,38 and as revealed by the atomistic calculations described above. That is, high-energy boundaries are typically better sinks for dopants because a higher net decrease in energy is expected. In a lowangle boundary, one may expect a lower concentration of dopants and also specific spatial distribution due to structural features. To check this possibility, a low-angle boundary was imaged and Mn mapped using atomically resolved STEM-EELS spectral imaging, as shown in Figure 3e. The specific orientation relationship is in-plane (042)||(311) and out-ofplane [200]||[111]. A composite chemical image of Mn shown in Figure 3f shows site-specific locales where the Mn is accumulated within the interface atomic structure. The boundary shows a kink shape, where the terminating atomic plane is jagged. Kinks are typically high-energy positions due to the stress accumulation and unsatisfied coordination numbers; the fact that Mn is segregating particularly in those positions reinforces the hypothesis that Mn is responsible for local energy reduction. Energy reduction is attributed to stronger (shorter) bonds (less excess energy), which we suspect is promoted by the Mn dopants. Similar behavior has also been shown in detail previously in the literature for ceria and other oxides, including strontium titanate and zinc oxide, where minimization of interface energies and site specificity associated with different dopant atoms occur.39−43 Interface Excess Dopant Quantification. The correlation between interfacial energies and excess dopant concentration has been previously established,44 such that one may calculate the amount of dopants segregated at the boundaries from energy measurements. In nanoparticles, the dopants can segregate to either the grain boundary or the surfaces. By studying isolated particles, solely considering surface segregation, Wu et al.16 have calculated the enthalpy of Mn surface segregation. By using a similar but extended approach, considering both surface and grain boundary segregation, one can also find segregation enthalpies for both interfaces, as qualitatively demonstrated by the atomistic simulations and microscopy studies. Specifically, the relationship between the enthalpy of interface segregation, ΔHseg, and the interface energy change, Δγ, due to solute segregation has been demonstrated by Krill et al.44 using the following equations:

Γs ≈

s msxMn

Ω2/3Navg

ΓGB ≈

(14)

GB mGBxMn

Ω2/3Navg

(15)

By using these two equations, interface excess of Mn is directly related to the mole fractions of Mn at the interface. Additionally, according to the Langmuir isotherm equation,10 the enthalpy of interface segregation can also be directly related to the mole fractions of Mn at the interface (xsMn, xGB Mn) and in the bulk (xbMn), shown as s b ⎛ ΔHseg,s ⎞ xMn xMn exp⎜ − = ⎟ s b 1 − xMn RT ⎠ ⎝ 1 − xMn GB xMn

1−

GB xMn

=

b xMn

1−

b xMn

⎛ ΔHseg,GB ⎞ exp⎜ − ⎟ RT ⎠ ⎝

(16)

(17)

Moreover, considering molar conservation, the following b relationship must be true for xsMn, xGB Mn, and xMn: s GB b xMn fs + xMn fGB + xMn (1 − fs − fGB ) = xMn

27861

(18)


Article

The Journal of Physical Chemistry C where xMn is the total Mn concentration; fs and f GB are the surface and grain boundary site fractions, which can be calculated from dividing the interface volume (Vs or VGB) by the total volume of the particles V. If we assume all the particles are spherical, fs and f GB can be written as fs =

4π (G /2)2 δs

As 6δsA s = G(A s + 2A GB) 4π (G /2) /3 (A s + 2A GB)

fGB =

that the number of available sites at the boundaries might be even smaller than at the surface (or mGB < ms), which would further increase the estimated disparity in segregation enthalpies at the surface and the boundaries. Using the same equations described above, the mole fraction of Mn at the grain boundary, at the surface, and in the bulk of ceria nanoparticles can also be calculated to show a quantitative segregation profile. As depicted in Figure 4, both the amount of

3

(19)

4π (G /2)2 δGB

2AGB 12δGBAGB = G(As + 2AGB) 4π (G /2) /3 (As + 2AGB) 3

(20)

where δs and δGB denote the thickness of surface and grain boundary, which can be expressed by msΩ1/3 and mGBΩ1/3, respectively, and G is the average diameter of the nanoparticles. Combining eqs 12−20 and using least-squares fitting, the enthalpies of Mn surface segregation as well as grain boundary segregation have been calculated. The results are ΔHseg,s = −30.8 kJ/mol and ΔH seg,GB = −37.2 kJ/mol. Surface segregation enthalpy is in good agreement with the value calculated by Wu et al. (−29.7 kJ/mol) for the same system16 and confirms the tendency for segregation. The value for the grain boundary is also exothermic and consistent with values reported in the literature for other dopants, such as Gd in ceria over a Gd concentration range 0−25% which shows an enthalpy of segregation at the GBs of −21.2 kJ/mol, and the average segregation enthalpy of Ta in ceria over the concentration range 0−2%, −53.1 kJ/mol.46 Note that although the absolute value of the grain boundary energy is usually smaller than that of the surface energy for ceramic materials, the grain boundary energy change was much larger than the surface energy change in Mn-doped CeO2 samples according to our calorimetry results, and the heat of segregation is directly proportional to the interfacial energy change, resulting in a larger heat of segregation to the grain boundary, as shown in our calculation. The energy change is not necessarily proportional to the absolute energy but on how defects are arranged at the interface and the number and quality of satisfied bonds. Our observations suggest that likely grain boundaries are better vacancy sinks than surfaces as a result of lower activation energies for the formation of charged defects on the grain boundaries.47,48 Therefore, more aliovalent cation dopants (Mn3+ in our case) would segregate to grain boundaries to compensate the opposite sign charged defects, indicating a larger enthalpy of grain boundary segregation. The similar phenomenon has been observed in Mn-doped SnO2 nanoparticles.45 The remarkable values of the Mn segregation enthalpies for both interfaces suggest a tendency for split segregation to both ceria surfaces and grain boundaries. Since the enthalpy of Mn grain boundary segregation is larger than the enthalpy of Mn surface segregation, which is consistent with the atomistic simulation results described above, the excess at the grain boundary would be greater than at the surface, leading to the greater decrease in the grain boundary energy as proved by the calorimetric results. The enthalpy of segregation estimated from this analysis for the grain boundaries is smaller than that determined from the atomistic calculations, but it is well established that while the types of potentials used here (with fixed full formal charges) provide physically reasonable trends, they often overestimate energetics. Further, both the EELS (Figure 3f) and the atomistic calculations (Figure 2) suggest

Figure 4. Mole fractions of Mn segregated on the grain boundary, on the surface, and dissolved in the bulk phase as a function of Mn dopant concentration.

Mn at the grain boundary and the surface will significantly increase with increasing Mn concentration, but the amount of Mn in the bulk phase barely changes, which is consistent with pronounced Mn segregation at interfaces as observed by STEM-based EELS. Additionally, the amount of Mn at the grain boundary is always higher than the amount of Mn at the surface, consistent with the decrease in the grain boundary energy trend. Note that there might be an argument about the increased oxygen vacancy concentration due to Mn3+ dopant, which may enhance ion transport and lead to severe grain growth behavior. However, although the dopants will create more oxygen vacancies, they would also combine with some oxygen vacancies to form dopant−oxygen vacancy association, which could limit the maximum attainable oxygen diffusivity, especially when dopant segregation happens.49−52 As Aidhy et al. showed in their recent work,49 despite the reduced oxygen migration barriers and increased oxygen vacancy concentrations at the grain boundaries, the segregated dopants attracted even more oxygen vacancies to form dopant−oxygen vacancy association, due to the additional potential gradients, at the grain boundaries in doped CeO2 system. They found out that the oxygen diffusivity in pure CeO2 system is much higher than the CeO2 system with segregated dopants. Therefore, according to their results, dopant segregation could reduce the oxygen diffusivity in nanocrystalline CeO2 and potentially impede the grain growth behavior from a kinetic aspect.

■

CONCLUSION A combined experimental and atomistic modeling investigation of the chemistry and thermodynamics of polycrystalline Mndoped ceria was reported. The goal was to understand the relationship between segregation, atomic position, and thermodynamics of doped nanocrystalline ceria. Our results 27862


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Surface Excess on the SnO2 Nanoparticles and Relationship with Nanostability and Growth. Appl. Surf. Sci. 2011, 257, 4219−4226. (11) Kang, S.-J. L. Sintering: Densification, Grain Growth and Microstructure; Butterworth-Heinemann: Oxford, UK, 2004. (12) Hirano, M.; Inagaki, M. Preparation of Monodispersed Cerium (IV) Oxide Particles by Thermal Hydrolysis: Influence of the Presence of Urea and Gd Doping on Their Morphology and Growth. J. Mater. Chem. 2000, 10, 473−477. (13) Chen, P. L.; Chen, I. W. Grain Growth in CeO2: Dopant Effects, Defect Mechanism, and Solute Drag. J. Am. Ceram. Soc. 1996, 79, 1793−1800. (14) Shaw, D. J.; Costello, B. Introduction to Colloid and Surface Chemistry; Butterworth-Heinemann: Oxford, UK, 1993. (15) McLean, D. Grain Boundary in Metals; Oxford University Press: London, UK, 1957. (16) Wu, L.; Dey, S.; Gong, M.; Liu, F.; Castro, R. H. R. Surface Segregation on Manganese Doped Ceria Nanoparticles and Relationship with Nanostability. J. Phys. Chem. C 2014, 118, 30187−30196. (17) Song, X. Y.; Zhang, J. X.; Li, L. M.; Yang, K. Y.; Liu, G. Q. Correlation of Thermodynamics and Grain Growth Kinetics in Nanocrystalline Metals. Acta Mater. 2006, 54, 5541−5550. (18) Castro, R. H. R.; Torres, R. B.; Pereira, G. J.; Gouvea, D. Interface Energy Measurement of MgO and ZnO: Understanding the Thermodynamic Stability of Nanoparticles. Chem. Mater. 2010, 22, 2502−2509. (19) Hayun, S.; Shvareva, T. Y.; Navrotsky, A. Nanoceria−Energetics of Surfaces, Interfaces and Water Adsorption. J. Am. Ceram. Soc. 2011, 94, 3992−3999. (20) Costa, G. C.; Ushakov, S. V.; Castro, R. H. R.; Navrotsky, A.; Muccillo, R. Calorimetric Measurement of Surface and Interface Enthalpies of Yttria-Stabilized Zirconia (YSZ). Chem. Mater. 2010, 22, 2937−2945. (21) Aguiar, J. A.; Dholabhai, P. P.; Bi, Z.; Jia, Q.; Fu, E. G.; Wang, Y.; Aoki, T.; Zhu, J.; Misra, A.; Uberuaga, B. P. Linking Interfacial Step Structure and Chemistry with Locally Enhanced Radiation-Induced Amorphization at Oxide Heterointerfaces. Adv. Mater. Interfaces 2014, 1, 1300142−1300149. (22) Egerton, R. F. Electron Energy-Loss Spectroscopy in the Electron Microscope; Springer Science & Business Media: Berlin, Germany, 2011. (23) Castro, R. H.; Quach, D. V. Analysis of Anhydrous and Hydrated Surface Energies of Gamma-Al2O3 by Water Adsorption Microcalorimetry. J. Phys. Chem. C 2012, 116, 24726−24733. (24) Navrotsky, A. Progress and New Directions in High Temperature Calorimetry Revisited. Phys. Chem. Miner. 1997, 24, 222−241. (25) Navrotsky, A. Progress and New Directions in High Temperature Calorimetry. Phys. Chem. Miner. 1977, 2, 89−104. (26) Stanek, C. R.; Tan, A. H.; Owens, S. L.; Grimes, R. W. Atomistic Simulation of CeO2 Surface Hydroxylation: Implications for Glass Polishing. J. Mater. Sci. 2008, 43, 4157−4162. (27) Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1−19. (28) Dholabhai, P. P.; Aguiar, J. A.; Wu, L.; Holesinger, T. G.; Aoki, T.; Castro, R. H. R.; Uberuaga, B. P. Structure and Segregation of Dopant−Defect Complexes at Grain Boundaries in Nanocrystalline Doped Ceria. Phys. Chem. Chem. Phys. 2015, 17, 15375−15385. (29) Nerikar, P. V.; Rudman, K.; Desai, T. G.; Byler, D.; Unal, C.; McClellan, K. J.; Phillpot, S. R.; Sinnott, S. B.; Peralta, P.; Uberuaga, B. P. Grain Boundaries in Uranium Dioxide: Scanning Electron Microscopy Experiments and Atomistic Simulations. J. Am. Ceram. Soc. 2011, 94, 1893−1900. (30) Chen, W. Q.; Lee, T. A.; Navrotsky, A. Enthalpy of Formation of Yttria-Doped Ceria. J. Mater. Res. 2005, 20, 144−150. (31) Ushakov, S. V.; Helean, K. B.; Navrotsky, A.; Boatner, L. A. Thermochemistry of Rare-Earth Orthophosphates. J. Mater. Res. 2001, 16, 2623−2633.

show that Mn-doped ceria nanoparticles synthesized by coprecipitation exhibit Mn segregation to both surfaces and grain boundaries. The tendency for segregation of Mn3+ to both interfaces was determined by using atomistic simulations and confirmed by using an analytical model to describe the change in the experimentally observed, by microcalorimetry, interface energy changes. The segregation was further demonstrated by using high-resolution electron transmission microscopy and EELS mapping. The results imply that the concomitant effect of segregation on both surface and grain boundary must be considered when addressing properties of doped nanoparticles. Effective concentration of surface dopants, commonly of interest in catalysts and sensors, will be affected by the splitting of dopant with grain boundaries. The splitting is greatly affected by the enthalpy of segregation, which can be well predicted by using atomistic simulations. Microstructural evolution is also expected to be affected by the modified interfacial driving forces, and studies of coarsening behavior and nanostability taking into account these energetic effects should be of interest.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (R.H.R.C.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was also partially supported by UC Lab Fees Research Program 12-LF-239032. B.P.U. acknowledges support by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. DOE under contract DE-AC5206NA25396.

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Interface Energies of Nanocrystalline Doped Ceria: Effects of

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