MS&T 2014_Longjia Wu
-
Upload
longjia-wu -
Category
Documents
-
view
179 -
download
0
Transcript of MS&T 2014_Longjia Wu
![Page 1: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/1.jpg)
Surface Segregation on Manganese doped Ceria
Nanoparticles and Relationship with Nanostability
Longjia WuUniversity of California, Davis
(Advisor: Ricardo Castro)
![Page 2: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/2.jpg)
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.
![Page 3: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/3.jpg)
• Surface segregation could improve nanostability.
22 1 2 2,1 2
2
d dln dln1xRT n n x RT x
A x
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.
![Page 4: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/4.jpg)
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.
![Page 5: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/5.jpg)
Synthesis: Co-precipitation methodCe and Mn
Precursor
Dripping into
ammonia
Mixture of
hydroxide
Calcination at 600C
Mn-CeO2 NPs
![Page 6: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/6.jpg)
Mn segregation study X-ray diffraction pattern
20 30 40 50 60 70 80 900.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Inte
nsity
(a.u
.)
Two Theta (degrees)
0%Mn
2%Mn
5%Mn
10%Mn
SampleLattice
Parameter, Å
Crystallite size,
nm (XRD)CeO2 5.41295±0.00030 10.8±0.4
2%Mn CeO2 5.41021±0.00037 9.6±0.3
5%Mn CeO2 5.40695±0.00045 8.5±0.310%Mn CeO2
5.40505±0.00049 7.3±0.3
![Page 7: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/7.jpg)
Electron Energy Loss Spectroscopy (EELS)
•a
•Line 1
•Line 2
•b1 2 3 4 5 6
0
2
4
6
8
10
12
EELS
inten
sity (
coun
ts*10
3 )
Relative distance (pixels)
•c •Line 1
1 2 3 4 5 60
2
4
6
8
10
12
14
16
18
EELS
inten
sity (
coun
ts*10
3 )
Relative Distance (pixels)
•d •Line 2
10 %Mn-CeO2
Mn EELS intensity
![Page 8: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/8.jpg)
segregation effect on surface energy
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.70
5
10
15
20
25
30
35
40
Wat
er C
over
age
(H2O
/nm
2)Relative Pressure (P/Po)
CeO2
-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
Diff
eren
tial H
eat o
f Ads
orpt
ion
(kJ/
mol
)
Water adsorption microcalorimetryWater 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.
![Page 9: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/9.jpg)
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
-180
-160
-140
-120
-100
-80
-60
-40
-200 2 4 6 8 10 12 14 16 18
0%Mn 2%Mn 5%Mn 10%Mn -44KJ/mol
Diff
eren
tial H
eat o
f Ads
oprp
tion
(kJ/
mol
)
Water Coverage (H2O/nm2)
B
More work
1. Castro, R. H, Quach, D. V.,The Journal of Physical Chemistry C. 2012, 116, 24726-24733.
![Page 10: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/10.jpg)
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
0 2 4 6 8 10
0.94
0.96
0.98
1.00
1.02
1.04
1.06
1.08
Sur
face
ene
rgy
(J/m
2)
Dopant Concentration (mol%)
Surface energy (J/m2)
70
72
74
76
78
80
Surface area (m2/g)
Sur
face
are
a (m
2/g)
![Page 11: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/11.jpg)
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
RTH
xx
xx sseg
b
b
s
s,
Mn
Mn
Mn
Mn exp11
Mnsb
ss 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.
![Page 12: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/12.jpg)
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
0 2 4 6 8 10
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Mol
e Fr
actio
n of
Mn
in B
ulk
or S
urfa
ce
Dopant Concentration (mol%)
Segregated on surface Dissolved in bulk
2%Mn 5%Mn 10%Mn
XMnb 0.0016 0.0041 0.0086
XMns 0.0879 0.1951 0.3395
![Page 13: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/13.jpg)
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.
![Page 14: MS&T 2014_Longjia Wu](https://reader035.fdocument.pub/reader035/viewer/2022062310/58a8d48b1a28ab5a368b509d/html5/thumbnails/14.jpg)
Thanks for your attention