Electron beam induced transformation of MoO3 to MoO2 and a new phase MoO

Article abstract of DOI:10.1002/zaac.200400052

The transformation of MoO₃ induced by electron beam irradiation was studied by electron energy-loss spectroscopy (EELS) in combination with electron diffraction and high-resolution transmission electron microscopy (HRTEM) techniques. The routes of structure transformation were dependent on the applied electron current density. In case of low current density, MoO ₂ was obtained. In case of high current density, MoO with a rock-salt structure is suggested to be the final phase. The change in oxidation states of the Mo oxides was deduced from the features in energy-loss near edge structure (ELNES) of the O K-edge. Quantitative analysis was successfully employed on Mo M₃-edge and O K-edge to obtain the O/Mo ratio of the reduced phases. The mechanisms of different structure transformation behaviors were suggested in the frame of radiolysis enhanced diffusion.

Full text of DOI:10.1002/zaac.200400052

Electron Beam Induced Transformation of MoO3 to MoO2 and a
New Phase MoO
D. Wang*, D. S. Su, and R. Schlögl
Berlin/Germany, Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society
Received February 25th, 2004.
Abstract. The transformation of MoO3 induced by electron beam
irradiation was studied by electron energy-loss spectroscopy
(EELS) in combination with electron diffraction and high-resolu-
tion transmission electron microscopy (HRTEM) techniques. The
routes of structure transformation were dependent on the applied
electron current density. In case of low current density, MoO2 was
obtained. In case of high current density, MoO with a rock-salt
structure is suggested to be the final phase. The change in oxidation
states of the Mo oxides was deduced from the features in energy-
loss near edge structure (ELNES) of the O K-edge. Quantitative
analysis was successfully employed on Mo M3-edge and O K-edge
to obtain the O/Mo ratio of the reduced phases. The mechanisms
of different structure transformation behaviors were suggested in
the frame of radiolysis enhanced diffusion.
Keywords: Electron diffraction; High-resolution electron mi-
croscopy; Electron energy-loss spectroscopy; Molybdenum oxide;
Electron irradiation
Elektronenstrahl induzierte Transformation von MoO3 nach MoO2 und eine neue
MoO-Phase
Inhaltsübersicht. Die Elektronenstrahl induzierte Transformation
von MoO3 im Transmissions-Elektronenmikroskop (TEM) wurde
mit Hilfe von Elektronen-Energieverlust Spektrometrie (EELS) in
Kombination mit Elektronenbeugung und hochauflösender Trans-
missions-Elektronenmikroskopie (HRTEM) untersucht. Der Re-
duktionspfad hängt dabei von der Elektronendosis ab: Im Falle
einer kleinen Stromdichte endet die Transformation bei MoO2, wo-
hingegen die Transformation bei hoher Stromdichte bei MoO in
NaCl-Struktur zu enden scheint. Änderungen im Oxidationszu-
stand entlang der Transformation wurden anhand der Änderungen
in der Feinstruktur im Nahkantenbereich der Sauerstoff K-Ionisa-
tionskante im Elektronen-Energieverlust-Spektrum abgeleitet.
Eine quantitative Analyse anhand der Molybdän M3- und der Sau-
erstoff K-Ionisationskante erlaubte die Bestimmung des O/Mo-Ver-
hältnisses der reduzierten Phasen. Schließlich wurden mögliche
Mechanismen für die unterschiedlichen strukturellen Transforma-
tionen im Rahmen einer strahlungsinduzierten Diffusion vorge-
schlagen.
1 Introduction
tion generates localized electronic excitations with sufficient
energy for time much longer than atomic vibration, so that
the potential energy inherent in electronic excitations can
be converted to the momentum of a departing atom or ion
via different mechanisms [1].
It is well known that fast electrons interact with both
atomic nuclei and electrons, which introduce irradiation
damages to many materials when exposed to an electron
beam [1]. The mechanisms responsible for the damages can
be summarized as so called “knock-on displacement” and
“radiolysis process”. The former is the elastic interaction
between the incident electrons with atomic nuclei, which di-
rectly displaces the atoms from the equilibrium sites and
creates point defects (vacancy and interstitial as Frenkel
pair). When the incident electron energy exceeds a thresh-
old, permanent change in the material occurs. The latter is
the process in which fast electron-atomic electron interac-
Some transition metal oxides, such as V2O5, TiO2 and
MoO3, etc., are known to transform to suboxides by elec-
tron irradiation in an electron microscope [2]. Although the
possible damage increases the difficulty in characterizing
catalysts based on these transition metal oxides by trans-
mission electron microscopy (TEM), electron beam induced
structure change gives the opportunity for investigation of
reduction behavior, phase transition routes and phase sta-
bility of many transition metal oxides. The knowledge ob-
tained helps the understanding of reduction mechanisms of
catalysts in more complicated chemical environments.
Moreover, such pre-knowledge is important to distinguish
the structure change induced by chemical treatment from
that induced by electron irradiation during in-situ TEM
characterization of the materials under real reaction con-
ditions.
* Dr. Di Wang
Fritz-Haber-Institut der MPG
Faradayweg 4Ϫ6
D-14195 Berlin
Tel.: ϩ49-30-8413-4482
Fax: ϩ49-30-8413-4405
E-mail address: wangdi@fhi-berlin.mpg.de
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D. Wang, D. S. Su, R. Schlögl
The phase transition in maximal valence transition metal
oxides induced by electron irradiation has been intensively
studied [2Ϫ6] and some results are summarized in reference
[2]. For V2O5, WO3, TiO2 and Nb2O5, corresponding mon-
oxides based on a rock-salt structure were detected to be
the final products [4, 7]. Epitaxial relationships were ob-
served to exist between the reduced surface phase and the
parent oxides [8]. The Knotek-Feibelman mechanism [9]
was proposed to be responsible for the oxygen desorption
from the surface, which involves an interatomic Auger de-
cay process. Most of the investigations on the structure
change of MoO3 in an electron microscope concluded that
MoO2 was the product of reduction [10Ϫ12]. In the detailed
work of Bursill [10], a superlattice of oxygen vacancies was
observed in the electron diffraction pattern for the domain
structure in MoO3, which was exposed to electron beam in
an electron microscope. The oxygen vacancies were pro-
posed to be consumed in the formation of crystallographic
shear (CS) planes with prolonged electron irradiation. At
higher electron beam currents, MoO2 in form of tiny crys-
tals was detected as the product of MoO3 reduction. Topot-
actic mechanisms of phase transformation from MoO3 to
MoO2 were suggested in [12, 13]. Recently, Su reported an
unknown phase of molybdenum oxide having an oxidation
state lower than Mo4ϩ [6].
Thermal decomposition and reduction in various re-
ductive atmospheres for MoO3 were also extensively studied
[14Ϫ16]. Temperature programmed reduction (TPR) of
MoO3 with hydrogen (5-100 %) in the temperature range
from 573 to 823 K was studied by in-situ X-ray diffraction
(XRD) and X-ray absorption spectroscopy (XAS). At reac-
tion temperature below 698 K, the reduction of MoO3 to
MoO2 is a one-step process without formation of (crystal-
line) intermediates. At temperatures above 723 K and H2
concentrations higher than 10 vol%, Mo metal is the final
product. At temperatures above 698 K, Mo4O11 can be ob-
tained and its formation was explained as the product of a
parallel reaction of MoO3 and MoO2 [15]. Recently, in-situ
XAS of MoO3 in 10 vol% propene TPR from 300 K to
773 K at a heating rate of 5 K/min showed the formation
of a short range order structure which was suggested to be
the “Mo18O52” type crystallographic shear [16].
Fig. 1 The [100], [010] and [001] projected structures of MoO3
levels of electron current density are also studied. The
characterization and the detailed analysis on the results give
more information about the reduction mechanism of MoO3
under electron irradiation.
2 Experimental
For TEM investigations, commercial powder of MoO3 was gently
crushed in ethanol and dispersed onto a copper mesh grid covered
with a holy carbon film. A Philips CM200 FEG electron micro-
scope, operating at 200 kV and equipped with a Gatan imaging
filter, GIF100, was used for TEM observation and EELS measure-
ments. The pressure of the specimen chamber was kept lower than
10Ϫ7 Torr. The electron irradiation was performed at a current den-
sity of about 1.3 A/cm2 and 57.6 A/cm2, respectively. Electron en-
ergy-loss spectra were measured in image mode with convergence
angle Ͱ ഠ 7 mrad and collection angle β ഠ 16 mrad.
3 Results
In the present paper, the transformations of MoO3 in-
duced by electron beam irradiation are further studied, es-
pecially by means of electron energy-loss spectroscopy
(EELS) in combination with electron diffraction and high-
resolution transmission electron microscopy (HRTEM)
techniques. The energy-loss near edge structure (ELNES)
on appropriate ionization edges of metal and oxygen atoms
can serve as “fingerprints” of the changes in oxidation state,
in chemical bonding and in the coordination environment
of the detected species for the oxides in different reduction
degree. In addition, the local concentration ratio of the con-
stituent elements can be derived by applying quantitative
EELS analysis, and thus helps the determination of the fi-
nal phase, especially for the unknown structures. In the pre-
sent experiment, transformation routes affected by different
Ͱ-MoO3 has the space group of Pbnm with cell parameters
˚
˚
˚
a ϭ 3.96 A, b ϭ 13.86 A and c ϭ 3.70 A. It consists of
double layers of linked distorted MoO6 octahedra parallel
to (010) planes. In each double layer MoO6 octahedra form
edge sharing zig-zag rows along the [001] direction and cor-
ner sharing rows along the [100] direction. The [100], [010]
and [001] projected structures of MoO3 are shown in Fig.
1. Successive layers are held together by weak van der
Waals forces only, with easy cleavage along (010) planes.
Therefore, most of the crystallites for TEM examination are
of platelet shape, oriented with [010] direction nearly paral-
lel to the incident electron beam. The crystallites were
quickly tilted to the [010] projection with diverged beam
illumination before they were subjected to the irradiation
treatment.
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Electron Beam Induced Transformation of MoO3
Fig. 3 HRTEM image showing the contrast arising from CS planes,
denoted in the enlarged part by triangles.
spots imply that the defect planes are periodically produced
in the crystal. According to the pattern in Fig. 2c, these
planes are parallel to the MoO3 c axis. The present obser-
vation is in agreement with the crystallographic shear (CS)
mechanism suggested by Bursill, for which a shear vector
of 1/2aϩ1/7b was assigned [10]. The HRTEM image in Fig.
3 also shows the contrast arising from these CS planes.
They can be seen extending into the bulk of the crystal,
though CS planes near the surface exhibit a different and
less distinct contrast due to the highly disordered surface
area.
The intermediate phase MoO3Ϫx with CS defects seems
fairly stable under the electron beam. With further ir-
radiation up to about two hours, there is only little change
in the geometry and intensities in the diffraction pattern.
After two hours, further change is observed to start in the
diffraction pattern as shown in Fig. 2d. Diffuse spots ap-
pear, overlapping with the MoO3 diffraction pattern (indi-
cated by solid arrows), and tails are seen for some MoO3
primary diffractions (indicated by hollow arrows). These
observations are probably produced by changes of the
MoO6 octahedra both in orientation and in interatomic dis-
tances. These features become more pronounced and MoO3
diffractions greatly diminish as seen in Fig. 2e (diffraction
pattern of the sample after 200 minutes of irradiation).
Both some discontinuous diffraction rings and distinct dif-
fraction spots can be seen, indicating that the newly devel-
oped structure has an epitaxial relationship with the MoO3
matrix, however, not uniformly oriented among different
grains. After about 360 minutes, a diffraction pattern as
shown in Fig. 2f is obtained and no more change takes
place with further irradiation. Calibrated by the MoO3
[010] diffraction pattern, all the diffraction spots in Fig. 2f
can be attributed to MoO2. One set of diffraction spots out-
lined by frame 1 in Fig. 2f coincides with the reciprocal
Fig. 2 The series of electron diffraction patterns of Mo-oxides sub-
jected to prolonged irradiation by electron beam of low current
density (about 1.3 A/cm2). The corresponding irradiation time is
(a) 0 minute, (b) 10 minutes, (c) 60 minutes, (d) 120 minutes, (e)
200 minutes and (f) 360 minutes, respectively.
3.1 Irradiation with electron current density of about
1.3 A/cm2
3.1.1 Electron diffraction and HRTEM
Figs. 2aϪ2f show the series of electron diffraction patterns
corresponding to Mo-oxides after prolonged irradiation on
MoO3 from 0 to 360 minutes. Fig. 2a shows the diffraction
pattern from the sample at the very beginning of ir-
radiation, on which only diffractions from [010] projected
MoO3 can be seen. After several minutes, in diffraction pat-
tern as shown in Fig. 2b, the (100) diffraction becomes dif-
fused along the a* direction in reciprocal space. This indi-
cates the formation of some planar defects by agglomer-
ation of oxygen vacancies due to the loss of weakly
bounded oxygen. After about 30 minutes irradiation, satel-
lite spots appear in addition to the primary strong diffrac-
tions of MoO3 (see e.g., Fig. 2c, diffraction pattern with 60
minutes of irradiation). The sharp intensities of the satellite
¯
lattice projected on the MoO2 [111] zone axis. The sym-
metrically equivalent orientation can be taken such that an-
other set of diffractions outlined by frame 2 is also formed.
Obviously, the arrowed intensities cannot be included into
¯
the diffractions on [111] or its equivalent zone axes. Frame
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D. Wang, D. S. Su, R. Schlögl
3 is outlined to show the set of diffractions arising from
¯
MoO2 on [122] or its equivalent zone axes. Therefore, by
electron irradiation with low current density, MoO2 is the
final product. Epitaxial relationships may exist between the
formed MoO2 and the MoO3 matrix, but it is not unique.
3.1.2 EELS
The electron irradiation with low current density was ap-
plied to another MoO3 crystallite. A series of ELNES spec-
tra of the O K-edge characterized by oxygen 1s Ǟ 2p tran-
sition were taken during this process, as shown in Fig. 4.
The spectra were exposed with the same electron current
density as used for irradiation experiment. In these spectra,
features marked as A and B can be distinguished just above
the threshold, which reflect the transition to the O 2p or-
bitals hybridized with the Mo 4d orbitals. For the octa-
hedral environment of the molybdenum, the crystal field
splittings lead to separations of the low part of the conduc-
tion band into t2g and eg symmetry bands, corresponding
to the feature A at 530.2 eV and the feature B at 533.4 eV,
respectively. The features (denoted as C in Fig. 4) above
540 eV can be attributed to the O 2p and Mo 5sp hybridiz-
ation and keep almost unchanged throughout the ir-
radiation process. The first spectrum in Fig. 4 shows the
ELNES of the O K-edge of MoO3. Feature B is present as
a shoulder at the high energy-loss side of feature A. With
electron irradiation up to 120 minutes, the spectrum only
shows a small increase in feature B. After 180 minutes, the
intensity of feature A (compared with that of feature C)
decreases considerably while that of feature B increases so
that they have comparable intensities. This trend of change
in the intensity ratio between the two features continues un-
til after 300 minutes of irradiation. In the corresponding
spectrum of the reduced Mo oxide, feature A is still present,
but much weaker than feature B. No change in the spec-
trum can be observed with further electron irradiation. An
O K-edge ELNES spectrum was also taken from a reference
MoO2 powder sample under the similar experimental con-
ditions (the last spectrum in Fig. 4). It is in good agreement
with the spectrum from the final reduced Mo oxide. This
result also confirms the electron diffraction observations.
Fig. 4 The series of ELNES spectra of the O K-edge of Mo-oxide
subjected to the prolonged irradiation by electron beam of low cur-
rent density (about 1.3 A/cm2).
rent density case (see Fig. 2c), satellite spots are observed
in Fig. 5a. It indicates that CS mechanisms may also play
an important role in this stage of MoO3 reduction. How-
ever, the CS structure disappears quickly. In addition, pri-
mary diffractions become more or less diffused and new
spots appear very close to the 002 diffraction (denoted by
arrow in Fig. 5a), which can be attributed to the expansion
of unit cell along the MoO3 [001] direction. After about 40
minutes of electron irradiation, a diffraction pattern as
shown in Fig. 5b is obtained. It can be assigned to a face
centered cubic (fcc) structure with lattice parameters a ϭ
˚
b ϭ c ϭ 4.08 A. Other diffraction patterns observed by tilt-
ing the sample confirm this suggestion. But none of the
known molybdenum oxides or Mo metal coincides with the
structure of the reduced phase. Part of the Mo oxide crys-
tallite without carbon film support was selected to study
the structure change repeatedly and the same result was ob-
tained. This excludes the possibility of formation of Mo
carbides. Furthermore, to test the stability of the reduced
phase, the sample was exposed to air and stored at room
temperature for as long as four weeks. When examining the
same crystallite in the microscope again, the fcc structure
remained.
Figs. 6aϪ6c are the HRTEM images showing the struc-
ture evolution of the system. At the early stage of electron
irradiation, plenty of defects are quickly formed near the
surface of MoO3 and some areas are highly disordered (see
Fig. 6a). Then fine lattice fringes with interplanar distances
3.2 Irradiation with electron current density of about
57.6 A/cm2
˚
of about 2 A appear in Fig. 6b (indicated by the circles),
corresponding to the (200) planes of the reduced Mo-oxide
with fcc structure. The new phase finally develops into the
bulk and is uniformly oriented, as seen in Fig. 6c. However,
considerable amounts of defects still exist.
3.2.1 Electron diffraction and HRTEM
Higher electron current density introduces different routes
of structure transformation. A diffraction pattern identical
to Fig. 2a acquired at the beginning of high electron ir-
radiation confirms the initial phase to be perfect [010] pro-
jected MoO3. Figs. 5a and 5b show the diffraction patterns
of the Mo-oxide after 10 minutes and after 40 minutes of
electron irradiation, respectively. Changes in the diffraction
pattern are readily seen after short time of electron ir-
radiation. Similar to the early stage in the low electron cur-
3.2.2 EELS and quantitative analysis
A series of ELNES spectra (see Fig. 7) of the O K-edge
were acquired during the electron irradiation of high cur-
rent density until the spectrum showed no further changes.
Similar to the case of low electron current density, feature
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Electron Beam Induced Transformation of MoO3
Fig. 5 Diffraction patterns of the Mo-oxides (a) after 10 minutes
and (b) after 40 minutes of irradiation by electron beam of high
current density (about 57.6 A/cm2).
C above 540 eV keeps almost unchanged throughout the
irradiation process. The first spectrum, with feature B ap-
pearing as a shoulder of the predominant feature A, indi-
cates the initial state of MoO3. With electron irradiation of
up to about 40 minutes, a continuous decrease in the inten-
sity of feature A and increase in that of feature B (both
compared relatively with the intensity of feature C) are ob-
served. In the ELNES of the O K-edge of the reduced final
phase, feature A is present as a less distinguishable shoulder
of the feature B (see the last spectrum in Fig. 7). This is
obviously different from the low electron current density
case, in which MoO2 is the final product (see also the last
two spectra in Fig. 4).
By using EELS quantitative analysis, the O/Mo relative
concentration ratio in the unknown phase with fcc structure
can be derived by [17]
Fig. 6 The series of HRTEM images showing the structure evolu-
tion of Mo oxide subjected to the prolonged irradiation by electron
beam of high current density (about 57.6 A/cm2).
NO
IO (β,∆1)
σMo (β,∆2)
σO (β,∆1)
ϭ
·
,
(1)
NMo
IMo (β,∆2)
where β is the collection angle, ∆1 and ∆2 the integration
energy ranges on O and Mo inner-shell edges, respectively.
I is the integrated intensity over the corresponding energy
range after subtracting the background and removing the
plural scattering by deconvolution method. σ is the corre-
sponding partial cross section for specific inner-shell ioniz-
ation. For Mo-oxides, O has its K-edge with threshold at
about 526 eV and Mo the M3-edge at about 391 eV. By
choosing an appropriate energy dispersion per channel
(0.5 eV in the present case), it is possible to obtain the EEL
spectra containing both the Mo M3-edge and the O K-edge.
In addition, when using the same objective aperture for
EEL spectra acquired in image mode, the collection angle
β is fixed. In the present cases, the integration energy range
for O K-edge is set from 526 eV to 576 eV and for Mo M3-
edge is set from 391 eV to 406 eV (indicated in Fig. 8 and
Fig. 9). Therefore, the partial cross sections of the O K-edge
σMo (β,∆2)
and the Mo M3-edge are also fixed.
in equation
σO (β,∆1)
(1) can then be replaced by a constant factor [18] and equa-
tion (1) can be written as
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D. Wang, D. S. Su, R. Schlögl
Fig. 9 EEL spectra containing both the O K-edge and the Mo M3-
edge of (a) MoO3 and (b) MoO with rock-salt structure after elec-
tron irradiation in high current density case. The integrated energy
ranges for the O K-edge and the Mo M3-edge are indicated by pairs
of vertical lines.
Fig. 7 The series of ELNES spectra of the O K-edge of Mo oxide
subjected to the prolonged irradiation by electron beam of high
current density (about 57.6 A/cm2).
Fig. 10 Simulated [001] projected electron diffraction pattern of
MoO with rock-salt structure.
Fig. 8 EEL spectra containing both the O K-edge and the Mo M3-
edge of (a) MoO3 and (b) MoO2 after electron irradiation in low
current density case. The integrated energy ranges for the O K-edge
and the Mo M3-edge are indicated by pairs of vertical lines.
in Figs. 9a and 9b, respectively. The K factor calibrated by
the MoO3 spectrum is 0.38 and the atomic ratio of O/Mo
in the fcc phase is derived as 0.90. Despite the error induced
mainly by the background fitting, the O/Mo ratio close to
1:1 and the fcc structure strongly suggest that MoO with
rock-salt structure is the final oxide in case of high current
density of irradiation. The [001] projected electron diffrac-
tion pattern is simulated according to the MoO model with
NO
IO (β,∆1)
ϭ
· K .
(2)
is
NMo
IMo (β,∆2)
NO
For MoO3 at the beginning of electron irradiation,
˚
rock-salt structure (a ϭ b ϭ c ϭ 4.08 A) by EMS online
NMo
package [19] and is shown in Fig. 10. The coincidence be-
tween the diffraction pattern in Fig. 10 and that in Fig. 5b
confirms our structure determination.
known to be 3 and the K factor can be calculated by meas-
uring the integrated intensity of the O K-edge and the Mo
M3-edge. A spectrum of MoO3 is shown in Fig. 8a and the
K factor obtained from it is 0.35. The measured spectrum
of MoO2 after electron irradiation in the low current den-
sity case is shown in Fig. 8b and the atomic ratio of O/Mo
can be obtained as 2.1 by using equation (2), which also
verifies the validity of the quantitative method. For the high
electron current density case, the spectrum of the initial
MoO3 and the final Mo-oxides with fcc structure are shown
4 Discussion and Conclusions
We have shown two different routes of MoO3 reduction in
an electron microscope by electron irradiation depending
on the current density. Using electron diffraction and
HRTEM, MoO2 and MoO with fcc structure are deter-
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Electron Beam Induced Transformation of MoO3
mined as the final states in the cases of low and high elec-
tron current densities, respectively. Because of the layered
structure of MoO3 along [010], the oxygen atoms between
the octahedral doubles layers are weakly bounded and
therefore easily lost. The oxygen vacancies exist in the early
stage of the irradiation as plenty of point defects. With the
increase of these vacancies, they aggregate into planar de-
fects and are accommodated by forming CS planes, as ob-
served in both cases. The fact that different current densities
lead to different routes of transformation indicates that
knock-on displacement should not be the main reason re-
sponsible for the reduction, since for a given material, the
knock-on effect is only related with the accelerating voltage.
In addition, similar phenomena of phase changes from
MoO3 to MoO2 observed in a 100 kV electron microscope
[10] also support this point. Therefore radiolysis mecha-
nisms play an important role, which involves the displace-
ment of atoms by transferring the energy deposited in exci-
ton states [1]. Considering either MoO2 with a distorted
rutile structure or the MoO with fcc structure, the trans-
formation from layered MoO3 to either phase must involve
the reconstruction of a considerable number of MoO6 oc-
tahedra. In our experiment, the temperature increase aris-
ing from electron beam heating is estimated using the
method introduced in [1] and proves limited. It means that
the thermal diffusion is negligibly small. However, the
radiolysis process can enhance the diffusion by displacing
the already existing defects in the bulk of the material [7].
Obviously, this diffusion rate is proportional to the current
density of the incident electron beam. In case of low current
density, the process is slow and MoO2 develops from MoO3
following several epitaxial relationships by rearranging the
octahedra, while in case of much higher current density,
due to high diffusion rate, the random movement of defects
makes it more favorable for grains of a new fcc phase to
nucleate. The grains of this new phase also keep uniformly
epitaxial relationship with the matrix and grow rapidly into
the bulk of MoO3.
bitals and only two empty orbitals are available. This ex-
plains the feature A as a shoulder in the last spectrum in
Fig. 7. In addition, the relative weight of t2g and eg contri-
bution is influenced by the distortion of MoO6 octahedra
[22]. From MoO3 to the rutile-like MoO2 and MoO with
rock-salt structure, the octahedra are less distorted, thus in-
creasing the weight of σ (eg) bond and reducing that of π
(t2g) bond. The relative increase in the intensity of feature
B and the further decrease in feature A can be attributed
to the effect of less distorted octahedra.
Other transition metal oxides, such as V2O5 and TiO2,
etc., are reported to be reduced under electron irradiation
to the monoxides with rock-salt structure [3, 7]. Similar
mechanisms may drive this type of transformation. Though
MoO with rock-salt structure was not reported before, it
may exist as small domains within the MoO3 matrix, as
observed in the present experiment.
Our observations of electron beam induced structure
transformations of MoO3 in different routes are highly rel-
evant for the understanding of the phase transformations
in molybdenum oxide systems, in solid state chemistry and
catalysis. The CS mechanism at the beginning stage of re-
duction in both cases (low and high current densities) sup-
ports that this mechanism is generally applicable to dis-
cussions of bulk transition of molybdenum oxides. How-
ever, our results reveal also the complexity of solid state
chemistry of this system depending on chemical or physical
parameters. The reduction of MoO3 to MoO has not been
reported earlier by the current redox discussion of MoO3
in heterogeneous catalysis. MoO with rock-salt structure
could be unexpected since it may hinder the redox process
of MoO3 in catalytic processes. Our work shows that the
formation of MoO in extreme conditions is possible and
can be one of the deactive phases in MoO3 based catalysts.
References
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The ELNES of the O K-edge is very sensitive to the oxi-
dation states of the probed sample, especially for the fea-
tures at about 530 eV and 533.5 eV, which reflect the tran-
sition to the unoccupied t2g and eg anti bonding states of O
2pϪMo 4d hybridized orbitals in a Mo-O octahedral con-
figuration [20, 21]. For MoO3, the maximal valence oxide,
the six t2g and the four eg orbitals are empty and O 1s elec-
trons can be excited into both orbitals. Therefore, the corre-
sponding two features can be observed in the ELNES spec-
trum of MoO3. In MoO2, the t2g orbitals are filled with two
electrons and a decrease in feature A is observed in ELNES
spectrum. The oxide after electron irradiation of high cur-
rent density shows further decreased intensity in feature A,
which indicates a lower oxidation state than that of MoO2.
This is supported by the quantitative analysis on the Mo
M3-edge and the O K-edge, from which the O/Mo stoichi-
ometry is obtained to be close to 1. Therefore, MoO with
rock-salt structure could be the final phase. Assuming the
case of MoO, there are four electrons filled in the t2g or-
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