Surface Segregation on Manganese doped Ceria

Nanoparticles and Relationship with Nanostability

Longjia WuUniversity of California, Davis

(Advisor: Ricardo Castro)

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.

• 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.

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.

Synthesis: Co-precipitation methodCe and Mn

Precursor

Dripping into

ammonia

Mixture of

hydroxide

Calcination at 600C

Mn-CeO2 NPs

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

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

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.

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.

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)

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.

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

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.

Thanks for your attention