1. The document summarizes research on manganese (Mn) doped ceria nanoparticles and the relationship between Mn doping and nanoparticle stability.
2. Experiments showed that Mn preferentially segregates to the surface of CeO2 nanoparticles, with more Mn accumulating at higher doping levels. Surface segregation lowers the surface energy and increases the surface area and stability of the nanoparticles.
3. Thermodynamic calculations determined the enthalpy of Mn surface segregation is strongly negative, indicating Mn has a strong tendency to segregate. The results demonstrate that Mn surface segregation enhances the thermodynamic stability of CeO2 nanoparticles.
1. Surface Segregation on
Manganese doped Ceria
Nanoparticles and Relationship
with Nanostability
Longjia Wu
University of California, Davis
(Advisor: Ricardo Castro)
2. Why surface segregation is important?
Nanostability Adding dopants
Second phase
Solid solution
Surface segregation
• Surface segregation will change the surface chemistry.
• Thermodynamic stability of nanoparticles (nanostability) is very important for
applications requiring high surface area.
3. • Surface segregation could improve nanostability.
2
2 1 2 2,1 2
2
d dln dln
1
xRT
n n x RT x
A x
γ
= − − = − Γ
−
Gibbs adsorption
(Dilute solution)
When surface segregation happens,
Surface energy decreases
Coarsening model
(Ostwald ripening)
Particle size decreases
1. Shaw, D. J.; Costello, B.; Butterworth-Heinemann: Oxford, U.K., 1991.
2. Kang, S.-J. L.; Butterworth-Heinemann: Oxford, U.K., 2004.
4. Our system: Mn doped CeO2
Mn doped CeO2 Nanoparticles
Mn dopant
CeO2
Nanoparticles
Possible driving forces
for Mn segregation
Formation of space charge layer
(segregation of oxygen vacancies )
Elastic strain energy
caused by size mismatch
(Mn3+
: 0.58Å, Ce4+
: 1.01Å)
• The goal of our research: achieving thermodynamically designed highly stable CeO2
nanoparticles by doping Mn.
1. Johnson, W. Metallurgical and Materials Transactions A. 1977, 8, 1413-1422.
2. Rahaman, M.; Zhou, Y. Journal of the European Ceramic Society. 1995, 15, 939-950.
8. segregation effect on surface energy
Water adsorption microcalorimetry
Water adsorption isotherm
and heat of adsorption as a function of Pr
1. Drazin, J. W.; Castro, R. H. R. Journal of Physical Chemistry C. 2014, 118, 10131-10142.
9. Coverage,
H2O.nm-2
Heat of water adsorption
0%Mn
CeO2
2%Mn
CeO2
5%Mn
CeO2
10%Mn
CeO2
1.66 -114.4 -117.5 -105.7 -100.8
3.32 -93.3 -97.9 -90.1 -85.7
6.64 -71.2 -74.5 -70.6 -68.23
When the second derivative of the isotherm curve is zero
(heat of adsorption go back to -44 KJ/mol)
Surface energy
for different Mn concentration
More work
1. Castro, R. H, Quach, D. V.,The Journal of Physical Chemistry C. 2012, 116, 24726-24733.
10. Surface energy and nanostability
• An increase in the overall stability of CeO2 nanoparticles happens with
decreasing surface energy, due to Mn surface segregation.
CeO2 2%Mn 5%Mn 10%Mn
Surface
energy (J/m2)
1.076 1.048 0.966 0.945
Surface area
(m2/g)
70.77 72.67 76.35 78.95
11. Enthalpy of Mn surface segregation
• Enthalpy of surface segregation can represent the ability of dopant
to segregate on the host particles’ surface.
ssegsss H ,0 ∆Γ+= γγ
∆
−
−
=
− RT
H
x
x
x
x sseg
b
b
s
s
,
Mn
Mn
Mn
Mn
exp
11
( ) Mns
b
s
s
xfxfx =−+ 1MnMn
Krill’s model
Langmuir isotherm
Molar conservation
ΔH seg, s = -29.66 KJ/mol
(A strong tendency for segregation)
1. Krill Iii, C.; Ehrhardt, H.; Birringer, R., Zeitschrift für Metallkunde. 2005, 96, 1134-1141.
2. Wynblatt, P.; Rohrer, G. S., Journal of the European Ceramic Society. 2003, 23, 2841-2848.
12. Amount of surface excess
• The results show that most of the Mn dopant will be segregated on the
surface and only a small part of Mn will dissolve in the bulk phase
2%Mn 5%Mn 10%Mn
XMn
b
0.0016 0.0041 0.0086
XMn
s
0.0879 0.1951 0.3395
13. Conclusion
• For Mn doped CeO2 nanoparticles, most of the Mn ion is
segregated on the CeO2 particles’ surface, and only small amount
of the Mn ion will form solid solution.
• Mn segregation could cause the decrease in surface energy,
which is measured by water adsorption calorimetry.
• The strong dependence of the thermodynamic metastability of
ceria nanoparticles on Mn surface segregation was confirmed by
showing the close relationship between Mn concentration, surface
area, and surface energy.
Driving forces.1, Surfaces in ionic crystals can assume a net charge layer due to the allocation of defects at the interface. We assume Mn ion is 3+ in the system. So, when we use Mn3+ to dope Ce4+, the generated oxygen vacancies tend to segregate at the surface to compensate the net charge layer and probably lead to the Mn segregation.2, the elastic strain energy caused by the big size mismatch between Mn and Ce ionic radius will also provide additional driving force for segregation
Enter speaker notes here.
XRD: 1, only ceo2 fluorite structure, no second phase.2 if form solid solution, due to the ionic radius difference, peaks position shifting are expected towards higher 2θ angles; which was not obviously observed. 3, another information we can get from the xrd results is the clear Peak broadening with the increase of Mn concentration, indicating the decrease of CS, which is in a good agreement with the results from WPF refinement. This CS decrease show the change of surface energy which is probably due to the Mn segregation.
To directly test the surface segregation hypothesis, EELS was performed on the samples. Here is a 10mol %Mn-ceo2 particle. The EELS spectrum imaging of the Mn L3 edges (signal) was carried out from the surface area of the particle. And Here is a spatial distribution map of the Mn EELS intensities. Two intensity line profiles across the part of the CeO2 particle to edge of the particle are plotted. Both EELS profiles reveal larger Mn concentrations at the surface when compared to the volume, showing the direct evidences of Mn segregation.
After we proof the Mn surface segregation, we want to study the segregation effect on surface energy. In order to achieve this goal, we used water adsoprtion microcalorimetry. We put our sample in one side of the tube and leave the other side to be the reference. Before the experiment, we dry our sample inside the tube to let it go back to the anhydrous state. And during the experiment, ASAP 2020 system will continuously dope water into the tube and the water will be adsorbed on the sample’s surface. Then the heat of adsorption will be measured by the microcalorimeter.
The results from water adsorption microcalorimetry are usually two curves. One is adsorption isotherm curve. And the other one is heat of adsorption as a function of relative pressure or water coverage. Both of the curves represent the water adsorption behavior on the particle surface, from a highly reactive state to a liquid-like surface.
This is heat of adsorption as a function of water coverage for pure ceo2 and mn-ceo2 samples. The results show that the Mn doped samples have the similar behavior as compared to pure ceria. While the pure and 2mol% Mn samples are practically overlapping, the curves for 5 and 10 mol% Mn show a systematic decrease. In order to see this trend clearly, the heat of adsorption for the same coverages has been picked up for pure ceria and doped ceria samples and the results are shown in the table. Consistently with the observation, while the heats of adsorption for the pure and 2mol% ceria samples are quite similar, it decrease for the higher manganese doping. This behavior is consistent with an increased change on the surface caused by the segregation behavior.
Another important information provided by the heats of adsorption is the water coverage value where the second derivative of the isotherm curve is zero 15. This is a critical coverage where the surface during water adsorption starts to behave like liquid water. So by using this critical coverage and the corresponding integral heat of adsorption, we can calculate the surface energy for different mn concentration.
This is the surface energy for different mn concentration. By increasing the mn concentration we can see that the surface energy systematically decrease. By using BET method, we also can find out that the surface area will systematically increase with a higher mn concentration. Higher surface area means higher nanostability. So …
After get the surface energy value for different mn dopant concentration, we can calculate the enthalpy of Mn surface segregation, which represent the ability of dopant to segregate on the host particles surface. By using Krill’s model and combining the XXX and XXX, we calculate the enthalpy of mn surface segregation to be -29.66KJ/mol, which reveals a strong tendency for surface segregation.
By using the same method, we also calculate the mole fraction of Mn in the bulk and on the surface, what gives a quantitative analysis of the segregation profile. By adding Mn, both the amount of Mn in the bulk phase and on the surface will increase simultaneously, but the increase on the surface will be much faster than that in the bulk phase, suggesting most of the Mn dopant will be segregated on the surface and only a small part of Mn will dissolve in the bulk phase to form limited solid solution.
