Trivalent cerium ions in CeO_2 are the key active species in a wide range of catalytic and electro-catalytic reactions. We employed ambient pressure X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy to quantify simultaneously the concentration of the reactive Ce^3+ species on the surface and in the bulk of Sm-doped CeO_2(100) in hundreds of millitorr of H2–H2O gas mixtures. Under relatively oxidizing conditions, when the bulk cerium is almost entirely in the 4+ oxidation state, the surface concentration of the reduced Ce^3+ species can be over 180 times the bulk concentration. Furthermore, in stark contrast to the bulk, the surface’s 3+ oxidation state is also highly stable, with concentration almost independent of temperature and oxygen partial pressure. Our thermodynamic measurements reveal that the difference between the bulk and surface partial molar entropies plays a key role in this stabilization. The high concentration and stability of reactive surface Ce^3+ over wide ranges of temperature and oxygen partial pressure may be responsible for the high activity of doped ceria in many pollution-control and energy-conversion reactions, under conditions at which Ce^3+ is not abundant in the bulk

Bluhm, Hendrik

Chueh, William C.

El Gabaly, Farid

Grass, Michael E.

Haile, Sossina M.

Hao, Yong

Jabeen, Naila

Liu, Zhi

McCarty, Kevin F.

McDaniel, Anthony H.

English

Caltech Authors - Main

 1
Highly Enhanced Concentration and Stability of 
Reactive Ce3+ on Doped CeO2 Surface Revealed  
In operando 
William C. Chueh1,2,*, Anthony H. McDaniel1, Michael E. Grass3, Yong Hao2, Naila Jabeen3,4, 
Zhi Liu3, Sossina M. Haile2, Kevin F. McCarty1, Hendrik Bluhm5, Farid El Gabaly1 
1Sandia National Laboratories, Livermore, California 94551, United States 
2Materials Science, California Institute of Technology, Pasadena, California 91125, United States 
3Advanced Light Source, Lawrence Berkley National Laboratory, Berkeley, California 94720 United 
States 
4Nanosciences & Catalysis Division, National Centre for Physics, Islamabad 44000 Pakistan 
5Chemical Sciences Division, Lawrence Berkley National Laboratory, Berkeley, California 94720 
United States 
Supplementary Information 
 
Photolithography Procedure 
Metal patterns were fabricated by metal liftoff photolithography. Positive photoresist (Shipley 1813) 
was spin-coated onto YSZ substrates at 2,000 to 3,000 rpm and baked at 115 °C for 150 s. The sample 
was then aligned with the glass plate mask using a Karl Suss MJB 3 contact aligner and exposed to UV 
 2
light for 45 to 75 s. Next, the photoresist was developed by immersing the sample in Microposit MF-
319 solution for ~ 30 to 60 s. After rinsing by deionized water and drying, the sample was cleaned using 
an oxygen plasma to remove residual organics and then transferred to a DC magnetron sputtering 
system (AJA International) where Pt (99.99 % purity) was deposited in 3 milltorr Ar. The growth rate 
was approximately 10 nm min-1 and the total thickness approximately 200 nm. Metal liftoff was 
performed by immersing the samples in acetone at room temperature. The above procedure was 
repeated for the other side of the substrate to make a symmetric cell.  
 
Additional Physical Characterization  
Epitaxy of the film was confirmed by X-ray diffraction (Figure S1). In-plane diffraction pattern shows 
the expected four-fold rotation symmetry of the (220) peak when the (200) oriented sample was titled by 
45 ° and rotated about the sample normal axis. Furthermore, the SDC thin film shows excellent registry 
alignment with the YSZ substrate. Out-of-plane diffraction pattern only contained the (200) family 
peaks, again confirming the eptixay. The rocking curve yielded a full-width half-max of 0.30°. A cross-
sectional scanning electron micrograph of a typical sample, Figure S2, shows the two morphological 
regions of SDC: columnar-type growth over Pt and epitaxial growth over YSZ.  
 
-180 0 180
YSZ 
(220)
SDC
(220)
 / 
In
te
ns
ity
 / 
ar
b.
 u
ni
t
20 40 60 80 100 120
SD
C
 (2
00
)
SD
C
 (6
00
)
SD
C
 (4
00
)
YS
Z 
(4
00
)
2 / 
In
te
ns
ity
 / 
ar
b.
 u
ni
t
YS
Z 
(2
00
)
15 16 17 18
SDC (200)
In
te
ns
ity
 / 
ar
b.
 u
ni
t
 / 
FWMH = 
0.30 
 
Figure S1. (a) In-plane, (b) out-of-plane, and (c) rocking curve X-ray diffraction pattern for SDC(100) 
without the burried Pt electrical contacts. Cu kα X-ray source (45 kV, 40 mA) was used. 
B CA 
 3
SDC on YSZ SDC on Pt
YSZ
Pt
1 µm
 
Figure S2. Cross-sectional scanning electron micrograph of SDC(100) with buried electrical contacts. 
(a) (b)
 
Figure S3. In-situ low-energy electron diffraction patterns for SDC collected at (a) 1.8 × 10-6 Torr O2 , 
740 °C (oxidizing condition) and (b) 1.0 × 10-7 Torr H2, 600 °C (reducing condition) using 19 eV 
electrons. The patterns are consistent with a first-order diffraction from SDC(100). These diffuse 
features are due to secondary electrons.  
 
Electrochemical Impedance Spectroscopy 
A one-dimensional Poisson-Nernst-Planck transport model has been proposed and validated for the 
thin film solid-state electrochemical cell utilized here.1,2 This transport model, discussed briefly below, 
is used to determine the bulk oxidation state, 3+[Ce ]bulk , from the impedance response. We consider 
mixed oxygen vacancy and electron conducting thin films grown on both sides of a pure ion-conducting 
 4
substrate. Because of the aspect ratio of the thin films used here, the rate of oxygen incorporation and 
release upon perturbation in SDC is limited by surface reactions rather than by bulk transport. By 
further supposing that oxygen ion transfer across the ceria-gas external interface is the rate-limiting step, 
the model predicts a single half-circle arc in the Nyquist plot described by the following equivalent 
circuit:  
 12 extchem intC C
2 extionR 2
IC
ionR
 
where ICionR  is the resistance of the YSZ ion-conducting substrate, chemC  is the chemical capacitance, and 
ext
ionC  and 
ext
ionR  are the surface capacitance and reaction resistance (for the external ceria-gas interface), 
respectively. The factors of 2 and ½ account for the fact that the electrochemical cell is symmetric. The 
chemical capacitance, in particular, is the variation in oxygen stoichiometry upon a variation in the 
chemical potential: 
 
1
2
2 2
1 1ion eon
chem
ion ion ion eon
d dC e V
z dc z dc
      
 (S1) 
where V  is the thin film volume, e  is the electron charge, z  is the formal charge,   is the chemical 
potential, and c  is the defect concentration. Subscripts “ion” and “eon” denote the ionic defect (oxygen 
vacancy in this case) and the electronic defect (polarons), respectively. In heavily-doped ceria such as 
the SDC, the bulk defect chemistry is well-known.3 Under the dilute-solution limit, the chemical 
potential for both defects can be expressed as 0 0lni i B i ik T c c   . Finally, we recognize that under 
moderately reducing conditions, the oxygen vacancy concentration is fixed by the dopant concentration 
( 12ion dopc c ). Combining these expressions with Eq. (S1), we obtain the following relationship between 
the chemical capacitance and the electron concentration:  
 5
 
1
2 1
2eon chem B dop
e Vc
C k T c
     
 (S2) 
In a wide band gap semiconductor such as ceria ( gE   6 eV), electron-hole pair formation due to 
thermal excitation is negligible. Therefore, the polaron concentration can be directly correlated to the 
bulk oxidation state, with 3+ 0[Ce ] /bulk eon eonc c , where 0eonc  is the concentration of all Ce ions. This 
approach of correlating the thin film capacitance to the bulk oxidation state can be easily extended to 
non-dilute solutions by modifying the functional form of the chemical potential.  
A typical impedance spectrum is shown in Figure S4 in the Nyquist form. A small high-frequency 
feature is visible, which is not predicted by the impedance model. It has been shown elsewhere that this 
feature is related to the electronic current constriction near the perimeter of the buried metal current 
collector.4 We account for this effect by adding an additional resistor-capacitor network in series with 
equivalent circuit. Furthermore, we also observe that the main arc in the Nyquist plot takes a slightly 
depressed semi-circular shape, rather than a perfect half-circle. This non-ideal behavior can attributed to 
the dispersion in the characteristic frequency, for example, due to variation in the film thickness. To 
account for such non-ideality, we replace the ideal capacitor in the parallel resistor-capacitor network 
with a constant-phase element, with the impedance given as   1mCPEZ j Y     , where 1j    and   
is the angular frequency. It can be shown that the equivalent capacitance of the constant-phase element 
is 1 1 1m meqC Y R
 , where R  is the resistance. The modified equivalent circuit is fitted to the impedance 
spectrum, Figure S4. The surface capacitance ( extionC ) was reported elsewhere
1 to be approximately 
4 mF cm-2 and independent of temperature and pressure. Using Eq. (S2), the bulk Ce3+ fraction is 
calculated from the fitted chemical capacitance. 
 6
0 200 400 600 800 1000
0
100
200
300
400
500
 
-Im
(Z
 / 
)
Re(Z / )
0.019 Hz
T = 586 C, pO2 = 4.5  10
-25 atm
 
Figure S4. Typical measured (dots) and fitted (line) impedance spectrum in the Nyquist plot at zero-
bias. Data collected in-operando during an APXPS experiment.  
 
XPS Quantification Procedure 
The Ce3+ concentration was quantified using two independent methods based on (1) valence band 
(localized Ce 4f) and (2) core level (Ce 3d) photoelectron spectra. Upon removal of oxygen from ceria, 
the gap state located approximately 1.5 eV above the O 2p band edge appears and has been assigned to 
occupied, localized Ce 4f orbitals.5-9 Accordingly, the peak area can be used to determine directly the 
average Ce oxidation state. Here, the Ce 4f integrated area is normalized to the Ce 4d area, the latter of 
which originates from both Ce3+ and Ce4+. The fraction of Ce in the 3+ oxidation state is given by: 
 Ce 4 Ce 43+
Ce 4 Ce 4
[Ce ] f dsurf
d f
A
A


   (S3) 
where (Photon Flux) (Cross Section) (Angular Efficiency)i     is the photon-energy-dependent 
normalization factor and A  is the integrated peak area (the entire peak area is used when no final state 
is indicated in the subscript).  Photon flux was measured experimentally at the beamline using a 
calibrated photodiode whereas the theoretically-computed atomic subshell photoionization cross 
sections and asymmetry parameters were taken from reference 10. In this analysis, it is assumed that 4f 
occupancy is predominately in the gap state rather than in the O 2p-Ce 4f hybridized orbital, i.e., it is 
 7
fully localized. It was verified experimentally that the gas-phase X-ray attenuation was not a strong 
function of h  between 270 eV and 390 eV.  
Turning to the core-level analysis, the Ce3+ concentration was determined using a linear combination 
of two Ce 3d spectra: 
 
'''
'''
Ce 3 ,3+ Ce 3
Ce 3Ce 3 ,
[Ce ] 1
oxd
d u d
surf oxd
dd u
A A
A A
   (S4) 
where the superscript “oxd” indicates the reference spectra taken under a sufficiently oxidizing 
condition such that 3+[Ce ]surf  can be approximated as zero, and '''u  denotes the highest binding energy 
peak in the Ce 3d envelope.  Here, a photoelectron spectrum collected at 465 °C and 7 × 10-6 Torr O2 is 
taken as the oxidized reference spectrum (Figure 3b). Under this condition, an occupied Ce 4f state in 
the band gap is completely absent and confirms the oxidized nature of ceria surface. This approach 
assumes that the mixed valent Ce 3d spectra is a linear combination of the fully oxidized and fully 
reduced spectra, and that the '''Ce 3 ,d u  peak area scales linearly with 4+[Ce ]surf . These assumptions are 
generally valid for moderate values of 3+[Ce ]surf .
11  
Finally, the dopant (Sm) concentration was determined using: 
 Sm 3 Sm 3
Ce 4 Ce 4 Sm 3 Sm 3
/[Sm]
/ /
d d
surf
d d d d
A
A A

    (S5) 
Because of the proximity of the Sm 4d and Ce 4d binding energies, the same photon energy was used 
( h = 390 eV) and the electron attenuation length was assumed to be the same. The quantification result 
is shown in Figure S5. A linear background was used for Ce 3d, Ce 4d, and Sm 4d photoemission, 
whereas a Shirley background was used for the valence band spectra.  
To examine the consistency of the two quantification methods, 3+[Ce ]surf  determined from Ce 3d and 
Ce 4f analysis are plotted against each other in Figure S6. Overall, good linearity is obtained, with the 
slope close to 1. 
 8
10-32 10-30 10-28 10-26 10-24 10-22
0.0
0.1
0.2
0.3
0.4
0.5
Bulk 
 650 C
 586C 
 521C
 466 C
 
[S
m
] su
rf
 
pO2 / atm  
Figure S5. Surface Sm dopant fraction (relative to the total Sm and Ce concentration) as a function of 
temperature and oxygen activity. Dashed line indicates bulk dopant concentration quantified by electron 
probe microanalysis. Not plotted in is the Sm fraction at 465 °C and 7 × 10-6 Torr O2, [Sm]surf = 0.35.  
0.0 0.2 0.4 0.6 0.8 1.0
0.0
0.2
0.4
0.6
0.8
1.0
 650 C
 586C 
 521C
 466 C
 
[C
e3
+ ] s
ur
f f
ro
m
 C
e 
3d
[Ce3+]surf from Ce 4f  
Figure S6. Self-consistency check for surface Ce3+ fraction determined from Ce 4f and Ce 3d 
photoelectron peaks. 
 
 9
Effective Attenuation Length 
Calculation 
The effective attenuation length (EAL) was calculated using the NIST Standard Reference Database 
8212 based on the TPP-2M predictive equation for the electron inelastic mean free paths. The parameters 
used for the calculation are summarized in Table S1. Because the composition changes near the sample 
surface, it is important to examine how this affects the EALs. Specifically, the EALs were computed for 
two compositions: Sm0.2Ce0.8O1.9 (bulk) and Sm0.5Ce0.5O. The variation in EALs was less than 6 % in 
the kinetic energy range examined. Therefore, the scattering properties are taken to be uniform within 
the sample. 
 
 
 
Table S1. Parameters used for calculating the effective attenuation length of Ce 4f photoelectrons.  
*The six 5p electrons in Ce are included in the valance electron count as they contribute strongly to the 
energy-loss spectrum.13 ** Two values of band gap are reported in literature: the gap between valence 
band maximum and the localized Ce 4f (~ 3 eV) state, and the gap between the valence band maximum 
and the conduction band minimum (~ 6 eV). It is not immediately clear which value is more suitable in 
calculating the inelastic mean free path, though the calculated EALs differ by less than 3 %. 
Composition Sm0.2Ce0.8O1.9 
Density 7.2 g cm-3 
Number of valence electron* 20.4 
Asymmetry Parameter10 0.770 ( h  = 270 eV) 
Band gap** 6 eV 
X-ray incidence angle 
(with respect to surface normal) 
15 ° (Beamline 11.0.2) 
75 ° (Beamline 9.3.2) 
Electron emission angle 
(with respect to surface normal) 
-40 ° (Beamline 11.0.2) 
0 ° (Beamline 9.3.2) 
 10
Correction 
With the EALs calculated, it is possible to correct for the finite XPS penetration depth and estimate 
the composition on the topmost surface. The simplest approach is to approximate the sample as a bilayer 
structure, consisting of an overlayer (of thickness t ) with an unknown composition and a substrate with 
a known composition. By integrating the mass attenuation equation (with intensity originating from 
depth z  given by  0 exp ( cos )I I z EAL   , where   is the electron emission angle with respect to 
the surface normal), we obtain the following expression for the corrected composition of the surface 
overlayer: 
 
 
 
3+ 3+
3+ [Ce ] [Ce ] exp ( cos )[Ce ]
1 exp ( cos )
surf bulk
ol
t EAL
t EAL


     (S6) 
To carry out this correction, the overlayer thickness needs to be specified. While this quantity is not 
available experimentally, a lower bound can be readily estimated. By recognizing that 3+[Ce ]ol  cannot 
exceed one and assuming that the layer thickness does not change with experimental conditions, one 
obtains: 
 
 
 
3+ 3+[Ce ] [Ce ] exp ( cos )
1
1 exp ( cos )
surf bulk t EAL
t EAL


     (S7) 
Because the EAL depends on the overlayer thickness, we solve the equation iteratively. Dopant 
concentration in the overlayer is estimated in the same manner.  
Using Eq. (S7), the upper-bound limit for the 3+[Ce ] at the outermost surface is, for all temperature and 
pressure conditions, a factor of 2.5 - 2.6 greater than shown in Figure 2, with the lowest and highest 
values being 0.64 and 1.0, respectively. The variation of oxidation state with oxygen activity (i.e., the 
slope in Figure 2) and the chemical potentials are essentially unchanged. Assuming the same 
segregation length scale for Ce3+ and Sm, the upper-bound value for [Sm]surf  in the outermost surface 
 11
ranges from 0.33 to 0.43, again, slightly exceeding the directly computed values. The lower bound 
thickness (~ 2 Å) of the outermost layer corresponds to approximately half the unit cell thickness in 
ceria (ao = 5.4 Å) and suggests that approximately two atomic planes in the (100) orientation are 
disrupted by the surface termination. 
   
Surface OH Concentrations 
Surface OH concentration was qualitatively assessed by comparing O 1s photoelectron spectra 
collected under humidified and dry conditions. We first measured the O1s spectra under H2-H2O 
atmospheres at 650 °C. Subsequently, we evacuated the chamber to <10-7 Torr at ~460 °C, backfilled 
with 7 x 10-6 Torr O2, and collected an O 1s spectrum after equilibration. As evident in Figure S7, the 
spectra taken under dry and wet environments are qualitatively similar to each other and do not show a 
significant change in the high binding energy shoulder (OH is expected at ~2.5 eV above the lattice 
oxygen peak14). Based on temperature-programmed desorption results, OH on CeO2 begins to 
decompose at ~230 °C.14 Given our much higher sample temperature, we expect that any surface OH 
species would have desorbed from the surface at 460 °C under < 10-7 Torr. This data suggests that 
surface OH coverage is very low at 650 °C, particularly when compared to that of the Ce3+ 
concentration. We did not attempt to analyze O1s spectra further due to the lack of reference spectra 
under high temperature, ambient pressure conditions. We speculate that the high binding energy 
shoulder could be due to surface lattice oxygen, surface oxygen vacancy defects, or impurities. 
 12
536 534 532 530 528
 
 
In
te
ns
ity
 / 
ar
b
Binding Energy / eV
 10 mTorr H2O, 10 mTorr H2, 650 C
 300 mTorr H2O, 10 mTorr H2, 650 C  
 7 x 10-6 Torr O2, 466 C
 
Figure S7: O 1s spectra at various conditions at 650 °C. For clarity, the binding energy of the lattice 
oxygen peak is aligned to that in the spectrum taken under oxidizing condition (slight energy shifts are 
observed due change in the electron chemical potential in ceria upon interaction with the gas). Intensity 
is also normalized to the lattice oxygen peak. Measured at a photon energy of 800 eV. 
 13
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Highly Enhanced Concentration and Stability of Reactive Ce^3+ on Doped CeO_2 Surface Revealed In Operando

Interface Anal.

NIST Electron Effective-Attenuation-Length Database Version 1.3,

Solid State Ionics

https://authors.library.caltech.edu/31927/2/cm300574v_si_001.pdf

Highly Enhanced Concentration and Stability of Reactive Ce^3+ on Doped CeO_2 Surface Revealed In Operando

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