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12640 Phys. Chem. Chem. Phys., 2011, 13, 12640–12645 This journal is c the Owner Societies 2011
Cite this: Phys. Chem. Chem. Phys., 2011, 13, 12640–12645
XPS revelation of tungsten edges as a potential donor-type catalyst
Yanguang Nie,aXi Zhang,
aShouzhi Ma,
aYan Wang,
bJisheng Pan
cand
Chang Q. Sun*ade
Received 27th January 2011, Accepted 13th May 2011
DOI: 10.1039/c1cp21421g
We report an efficient yet simple technology of photoelectron spectroscopic purification for
identifying the capability of, and direction of charge flow in, a catalyst in a reaction, which has
enabled the finding, for the first time, of the similarity of the valence band of tungsten edges to
that of Rh adatoms and Ag/Pd alloy and hence suggested that W undercoordinated atoms could
be a suitable candidate for replacing the costly Rh adatoms and Ag/Pd alloy as a cheaper, richer,
and efficient donor-type catalyst for CO and NO oxidation applications. The new technology and
new findings will be stimulating to the community for new catalyst design and identification and
provide a better understanding of the electronic process of a catalytic reaction associated with
undercoordinated atoms.
1. Introduction
Atomic undercoordination associated with vacancies, defects,
terrace edges, and nanostructures of various shapes demon-
strate excellent properties, such as the extremely high catalytic
ability that cannot be seen even from a flat surface of the same
specimen, such as Au, Rh, and Pt.1 The altered local structural
and electronic environment modifies the bond length, bond
energy, potential trap depth, and hence the Hamiltonian, work
function, electroaffinity, and the atomic cohesive energy that
locally determine the performance of a material, such as the
catalytic, electronic, dielectric, optic, magnetic, mechanical,
and thermal properties.2 Thus, understanding the bonding and
the energetic behavior of electrons localized in atomic-scaled
zones surrounding undercoordinated atoms is the key for one
to harness the process of catalytic reaction.
X-Ray photoelectron spectroscopy (XPS) is a powerful tool
for detecting the energetic behavior of electrons in the valance
band and below, showing the fingerprints of the crystal
potential change with the local chemical and coordination
environment and its consequences on the electronic energy and
structure in the deeper core bands.3 Generally, XPS data can
be decomposed into several components corresponding to
contributions from bulk (B) and surfaces (Si, i = 1,2. . .) of
different atomic layers in sequence. Three kinds of surface core
level shift (SCLS), i.e., positive, negative, and mixed shift are
generally assumed for the component assignment. Fig. 1a
illustrates the positive SCLS, which means that the B and Si(i = 1,2,. . .) are arranged in the sequence of S1, S2,. . ., and B
from lower (larger absolute value of energy) to higher binding
energy in the XPS profile.4 A represents the entrapped
components due to the undercoordinated adatoms or edge
atoms and P represents the screened polarization states.
The SCLS is often attributed to the ‘‘initial–final’’ states5 or
the ‘‘surface bond contraction’’6 effects. The latter has been
confirmed experimentally from Ta,7 Nb,8 and Mo9 surfaces
and Au10 and Cu11 atomic clusters using XPS and low-energy
electron diffraction. The first interlayer spacing of a W(320)
surface has been found to contract by up to 25%.12 However,
decomposing the XPS spectra with derivatives of quantitative
information of bonds and electronic energy has long been
problematic because of the lack of constraints for the XPS
profile deconvolution: (i) the number of components in one
XPS spectrum; (ii) the energy separation between the components
and their correlations; (iii) the reference point from which the
core level shifts; and (iv) the direction of the energy shift
upon surface and edge formation. Establishment of these
constraints is highly needed, in addition to a consistent under-
standing of the nature and the physical origin of the SCLS and
their indications in catalyst design.
The SCLS ofW(100), (110), and (111) surfaces13 and their (320)
and (540) vicinals14 have been well measured for more than 30
years using different techniques, such as the synchrotron and
XPS, unfortunately with discrepancies in the assignment of the
direction of the energy shifts and the energy of the bulk
component.13b,d,14b,15 A negative shift was assigned with the bulk
component at 31.4 eV and the surface component at 31.1 eV.
a School of Electrical and Electronic Engineering,Nanyang Technological University, Singapore 639798.E-mail: [email protected]
b School of Information and Electronic Engineering,Hunan University of Science and Technology, Xiangtan 411201,China
c Institute of Materials Research and Engineering, Agency for Science,Technology and Research (A*STAR), Singapore 117602
d School of Materials Science, Jilin University, Changchun 130012,China
e Faculty of Materials, Photoelectronics and Physics,Xiangtan University, Changsha 400073, China
PCCP Dynamic Article Links
www.rsc.org/pccp PAPER
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Much more information regarding the interatomic binding
energy and the effect of undercoordination is supposed to be
given by the sophisticated measurements. For the past 30 years,
progress towards such information has been limited because of
the lack of suitable theories and decoding technologies.
Recently, we incorporated the bond order–length–strength
(BOLS) correlation mechanism into the tight-binding theory16
(BOLS-TB) to examine the CLS of undercoordinated systems
from the perspective of Hamiltonian perturbation. The BOLS-TB
algorithm has enabled quantitative information about the
bonds and electronic energy from the SCLS of Ru,17 Be,18
Rh and Pd surfaces,19 carbon allotropes,16,17 Pt and Rh
adatoms,19 and Cu/Pd and Ag/Pd nanoalloys.20 It has been
consistently confirmed that the shorter and stronger bonds
between undercoordinated atoms derive globally positive
CLS because of the undercoordination-induced quantum
entrapment. The occupancy of the polarized states at the
upper edge of the valence band dictates the direction of charge
flow between the catalyst and the reactant in the process of
catalytic reaction.19,20
The objective of this work is to show that an incorporation
of the BOLS-TB algorithm and the recently-developed atomistic
photoelectron distillation spectroscopy (APDS) purification
method into the well-measured XPS data 13d,14b,21 has enabled
the discrimination of the edge states from those of the surface
and the bulk, which suggests that the W edge could perform
the same as Rh adatoms and AgPd alloy as a donor-type
catalyst with quantitative information about the 4f level
energy of an isolated W atom and its bulk shift, as well as
the local bond length, binding energy density, and the atomic
cohesive energy.
2. Principles
According to the BOLS-TB algorithm,22 a specific nth energy
level of a specimen and its shift upon interatomic interaction
being involved follow the relations,18
DEvðzÞ ¼ EvðzÞ �Evð0Þ ¼ az þ zbz ðCore level shiftÞEvð0Þ ¼ �hn; ijVatomðrÞjn; ii ðAtomic core levelÞ
az ¼ �hn; ijVcrystðrÞð1þDHÞjn; ii / Ez ðExchange integralÞbz ¼ �hn; ijVcrystðrÞð1þDHÞjn; ji / Ez ðOverlap integralÞ
8>><>>:
ð1Þ
where |n,ii is the specifically localized Bloch wave function at a
specific ith atomic site. DH is the perturbation to the Hamiltonian
due to the effect of bond-order loss. The intraatomic potential,
Vatom(r), intrinsically defines the core level of an isolated atom,
Ev(0); the interatomic potential, Vcryst(r), determines the shift
of the core level, DEv (z = 12), when the bulk is formed.
The z is the effective coordination number (CN or z) of the
considered atom. For a bcc bulk, the effective z value is
12 rather than 8 for normalization purposes. The Ev(0) and
DEv (z = 12) are intrinsic constants that do not change with
the environment. The Ev(12) moves deeper or positively with
respect to the Ev(0) because of the additional Vcryst(r). Most
importantly, the energy shift of DEv(z) is proportional to the
cohesive energy per bond at equilibrium, Ez. Any perturbation
to the interatomic potential or the bond energy will lead to the
energy shift.
According to the BOLS scheme, bonds between under-
coordinated atoms are shorter and stronger. Local quantum
entrapment and densification of bonding charge and binding
energy will happen, which perturbs the interatomic potential.
The core level shift of the undercoordinated atoms is therefore
deeper than that of the bulk. On the other hand, polarization
of the electrons at the upper-edge of the valence band by the
locally, densely, and entrapped core electrons also provide
perturbation to the Hamiltonian through screening and splitting
the crystal potential, and thus,
Fig. 1 (a) Illustration of the XPS spectral components with positive
CLS caused by atomic undercoordination. The P, B, Si, and A
components represent, respectively, contributions from the polarization,
bulk, surfaces, and adatoms. (b) The surface atomic structures
of the (110) vicinal (540) and (320) surfaces with edge atoms
density. The edge density of (320) is 0.28 monolayer (ML) and the
(540) is 0.16 ML.
1þ DH ¼C�mz ¼ Ez=E0 ðTrap depressionÞ
p ¼ ðEvðpÞ � Evð0ÞÞ=ðEvð12Þ � Evð0ÞÞ ðPolarizationÞ
(
Cz ¼ dz=d0 ¼ 2=f1þ exp½ð12� zÞ=ð8zÞ�g ðBond contraction coefficientÞ
ð2Þ
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12642 Phys. Chem. Chem. Phys., 2011, 13, 12640–12645 This journal is c the Owner Societies 2011
En(p) represents the peak energy of the polarization component in
the XPS. Cz is the Goldschmidt–Pauling coefficient of bond
contraction. E0 and d0 represent, respectively, the bond energy
and bond length in the ideal bulk. The bond nature indicator
m has been optimized to be unity for metals. The p is the
coefficient of polarization to be determined from the XPS
measurement. Thus, correlation between the XPS components
follows the criterion,
EvðxÞ � Evð0ÞEvð12Þ � Evð0Þ
¼ C�1z ðEntrapmentÞp ðPolarizationÞ
�ð3Þ
The x represents z or p. If the polarization-entrapment
coupling effect is apparent, the term C�1z is then replaced by
pC�1z , the trapped states will be moved up from the otherwise
low-z position to energy close to the bulk component. For
situations without apparent polarization, the relation evolves,
EvðzÞ � Evð0ÞEvðz0Þ � Evð0Þ
¼ Cz0
Cz; or; Evð0Þ ¼
Cz0Evðz0Þ � CzEvðzÞCz0 � Cz
ð4Þ
This relation allows us to determine the Ev(0) for an isolated
atom and the bulk shift, which have been a long pursuit of the
community. The accuracy of the determined Ev(0) depends on
the database size collected from the same materials. If there
are a total of n components of B and Si components for
various surfaces of the specimen, there will be a combination
of C2n = n!/(2!(n � 2)!) possible Ev(0) values. In the present
case, n = 7 and C2n = 21, as given in Table 1. By taking the
average of the Ev(0) and the standard deviation, s, we have theexpression for the z-resolved CLS for the XPS components:
E4f(z) = hE4f(0)i � s + [E4f(12) � E4f(0)]/Cz (5)
Besides, with the derived z values for various surface and
subsurface layers of differ crystal orientations, we are able to
elucidate the z-resolved local strain, binding energy density,
and the cohesive energy per discrete atom in the surface
skins.18 Such information is particularly fundamentally important
for one to understand and control processes at sites surrounding
undercoordinated atoms.
One can imagine what will happen if we subtract the XPS
spectrum collected from the flat surface from that collected
from the edged surface under the same measurement
conditions, upon background correction and spectral area
normalization of both. The residual spectrum keeps only the
features due to the edge atoms within zones of only a one or
two atomic layer size. Such an APDS process enables the
purification of the edge states as the APDS filters out the
artifact background and bulk information, such as the back-
ground uncertainty and the ‘‘initial–final states’’ effect that
exists throughout the course of measurements.23
3. Results and discussion
3.1 The APDS of edge atoms
Fig. 1b shows the atomic arrangement of the (320) and (540)
surfaces with a considerable fraction of undercoordinated
atoms compared with the flat (110) surface. From the original
W 4f spectra shown in Fig. 2a for the (110), (320) and (540)
Table 1 BOLS elucidated information regarding the atomic-layer (S1, S2) and crystal-orientation resolved effective CN (z), local strain (Cz � 1),the relative binding energy density (C�4z ) and atomic cohesive energy (zibC
�1z ) from the measured XPS W 4f SCLS. The zib = zi/zb is the relative
coordination number. The spectral deconvolution using the BOLS-TB algorithm derives the energy level of an isolated W atom as E4f(0) = 28.910 �0.006 eV and its bulk shift DE4f(12) = 2.173 eV with the z-resolved CLS: E4f(z) = 28.910 � 0.006 + 2.173C�1z for surface and edge atoms
i E4f(i) (eV) z Cz � 1 (%) E-density [C�4z ] DEC(i)/EC(B) [zibC�1z � 1](%)
W B 31.083 12 0 1 0W(100)21 S2 31.283 5.161 �8.26 1.41 �53.12
S1 31.398 3.970 �12.57 1.71 �62.16W(110)14b S2 31.240 5.829 �6.61 1.31 �47.99
S1 31.402 3.942 �12.71 1.72 �62.36W(111)13d S2 31.275 5.270 �7.96 1.39 �52.28
S1 31.370 4.195 �11.58 1.64 �60.46
Fig. 2 From the normalized the (110), (540), and (320) XPS 4f7/2
spectra24 (a) one can hardly discriminate the contribution of the edge
atoms from those of the un-edged (110) surface; but (b) the APDS, or
subtraction of the un-edged from the edged W(540) and (320) surfaces,
can resolve the edge states unambiguously with the P and P + T extra
states and the B and the additional valley at the bottom edge.
The resultant APDS almost satisfies the criterion of spectral area
conservation.
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surfaces collected under the same experimental conditions,24
one can hardly discriminate the spectral features from one
another; the APDS process, however, makes a great difference,
as shown in Fig. 2b, proceeded by subtracting the spectrum
collected from the un-edged (110) surface from the edged ones
of the (540) and (320) surface after the standard process of
spectral normalization and background correction using the
Shirley method. The correctness of the APDS outcome is
justified by the criterion of spectral area conservation, which
means that the spectral loss and gain should be identical. The
normalization of the spectra is to minimize the artifacts due to
scattering of the X-rays by the edge atoms. Unexpectedly, two
extra components centered at 30.945 and 31.310 eV are present
and two valleys centered at 31.083 and 31.454 eV are generated.
The emergence of these spectral features indicate that the
electronic structure for the edge atoms is indeed different from
those at the bulk interior or at the flat surface, but at this
moment one can hardly tell why.
3.2 SCLS analysis of the W(110), (100) and (111) surfaces
In order to calibrate and understand the APDS features
in Fig. 2b, we need to decompose the XPS spectra from the
well-faceted surfaces with respect to the reported best fits19
using three components, the bulk, B, and the second and the
first surface layers, S2 and S1. Experimental conditions, such as
the incident beam energy and the emission angle, may change
the spectral appearance because of the X-rays’ penetration
depth. A spectrum collected at larger emission angles or with
lower incident beam energy collects information dominated by
the shallow surface, otherwise more bulk information is
collected. Varying experimental conditions can never change
the intrinsic properties of the surfaces, such as the atomic
coordination numbers of the surface and sub-surfaces, that are
the key factors used herewith. The order of the B, S2, and S1components and the separation between them follows the
constraint given in eqn (4). The decomposed z-resolved
components of the W(100), W(110), and W(111) 4f7/2 spectra
in Fig. 3 show that the atomic CN reduction leads to the
positive CLS. The CNs of the S1 and the S2 components across
the three W surfaces vary slightly because of the anisotropy of
crystal structure and atomic density.13,14 Deconvolution of the
three surfaces aims to enhance the accuracy in determining
the E4f7/2(0) and the DE4f7/2
(12) by minimizing the standard
deviation value, s. From the BOLS-TB enabled deconvolution,
we derived the following information: (i) the W 4f energy level of
E4f7/2(0) = 28.910 � 0.006 eV for an isolated W atom and its bulk
shift of DE4f7/2(12) = 2.173 eV; (ii) an analytical expression for
the z-resolved CLS: E4f7/2(z) = 28.910 + 2.173C�1z for the under-
coordinated edge and adatoms; (iii) the effective atomic CN of the
sublayers of different orientations and their derivatives on the local
bond strain, local bond energy, the ratio of binding energy density
and the atomic cohesive energy to the respective bulk values, as
summarized in Table 1.
3.3 Edge CLS purification
Based on the derived CN-dependent SCLS, E4f7/2(z) = 28.910 �
0.006 + 2.173C�1z , we can incorporate the z values into the
APDS spectra in Fig. 2b and hence clarify the origin of the
extra spectral features:
(i) The B valley at 31.083 eV = 28.910 + 2.173 eV is
unambiguously the bulk component that was defined in the
APDS without needing intuitive assignment. This finding
clarifies the long confusion13c regarding bulk component that
was always assumed at the bottom edge of the 4f band.
(ii) The additional T states below the bulk component are
the entrapped states because of the undercoordination-induced
bond strain and bond strength gain. The locally entrapped T
states polarize the valence electrons that cannot be directly
Fig. 3 Deconvolution of the XPS 4f7/2 spectra of (a) W(100),21
(b) (110),14b and (c) (111)13d surfaces using the B, S2 and S1 components
with the derived 4f level of an isolated W atom as E4f(0) = 28.910 �0.006 eV and its shift upon bulk formation, DE4f(12) = 2.173 eV, both
of which are intrinsic constants changing with neither the experimental
conditions nor the crystal orientations. The component energies
follows: DE4f(z) :DE4f(12) = C�1z with Cz being the bond contraction
coefficient. Deconvolution also confirms the positive SCLS and leads
to the quantitative information, as listed in Table 1. The baselines are
the standard spectral background correction.
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detected using XPS, but it happens to the Rh and Ag/Pd alloy,
as confirmed using ultra-violet photoelectron spectroscopy,
showing the consistency of charge polarization direction in all
the bands of the same specimen.20,22
(iii) The P component at 30.945 eV results from the screening
and splitting of the crystal potential by the polarized valence
electrons. We can estimate the polarization coefficient with the
known energies of the P and the B components, p = [E4f7/2(p) �
E4f7/2(0)]/[E4f7/2
(12) � E4f7/2(0)] = (30.945 � 28.910)/2.173 =
93.6%, which means that the crystal potential has been
partially screened and elevated by 6.4% of the bulk value.
The otherwise T component turns out to be T + P with an
additional valley at the bottom edge of the core band because
of the coupling effect of entrapment and polarization. The T
component is supposed to add a component at energy corres-
ponding to z o 4, if the polarization is absent or it is
sufficiently weak. The state loss (second valley) at 31.454 eV
with an effective CN of 3.57 is supposed to be absent; the
strong interaction between edge atoms should enhance the
intensity of the states at lower-z positions instead, if no
polarization happens. However, as we discussed, the screening
effect also applies to the trapped states, and therefore, this
valley becomes present, and the T component becomes P + T.
The C�13 is replaced with pC�13 = C�13.75, which means that the
original edge states located at z = 3 shift up to energy being
equivalent to z = 3.75. The edge bond is strengthened by
[E4f7/2(T + P) � E4f7/2
(0)]/[E4f7/2(12) � E4f7/2
(0)] = (31.310 �28.910)/2.173 = 1.104, or 10.4%, because of the joint effect of
entrapment and polarization. It should be 1.104/p = 1.104/
0.936 = 1.18 instead, if no polarization occurs.
3.4 Potential catalytic behavior
The extra P and the P + T states in the APDS are due to the
edge atoms only, as the APDS has filtered out the background
and bulk information. It has been confirmed that the valence
and the core electrons of a specimen shift simultaneously in the
same direction because of the screening effect to the core
charge, such as the cases of AgPd and CuPd bimetallic alloy
catalysts,20 and the Pt and Rh adatoms.22 The APDS of W
edges share the same attribute to those of Rh adatoms and
AgPd alloy, as shown in Fig. 4. The latter have been identified
as donor-type catalysts compared to the Pt adatoms and CuPd
alloy that are the opposite, because of the respective
dominance of polarization and entrapment effects. From the
electronic structure, we can suggest that W edges should
perform the same as Rh and Ag/Pd as n-type catalysts, though
experimental confirmation is needed. Nevertheless, the APDS
can help us to search for new catalysts and identify the
catalytic nature of existing catalysts.
4. Conclusion
The BOLS-TB enabled XPS deconvolution of the W(100),
(110) ,and (111) surfaces has led to quantitative information
about the 4f energy level of an isolated W atom as 28.910 eV
and its bulk shift of 2.173 eV. The positive core-level shift
originates from the stronger and shorter bonds between
undercoordinated atoms, which follow the prediction of the
Goldschmidt–Pauling rule of bond contraction and the theory
of BOLS correlation. The deconvolution provides profound
information about the effective atomic CN, local bond strain,
bond energy, binding energy density and the atomic cohesive
energy in the surface skin of up to two atomic layers of
different orientations. Further APDS processing revealed
extra features of the bulk valley, polarization and the joint
effect of entrapment and polarization and their physical
indications, which confirm the positive core-level shift and
clarify the long confusion in the bulk component assignment.
Results show the BOLS expectation of the edge states as
resulting from the undercoordination-induced local bond
contraction and the associated quantum entrapment and
densification of core electrons, and the polarization of
the otherwise conducting electrons of W edge atoms by the
entrapped core charge. Most strikingly, being similar to the
spectral features of Rh adatoms and Ag/Pd alloy, the W edge
is suggested to serve as a donor-type catalyst. If it works, the
impact to catalytic industrial processes would be enormous.
As demonstrated, the APDS should provide a powerful tool
for one to purify information from atomic-scaled zones
surrounding undercoordinated atoms regarding the local bond
and energetic behavior of electrons, which is helpful for us to
search for new catalysts.
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