PERGAMON
Solid State Communications 116 (2000) 631±636
www.elsevier.com/locate/ssc
XPS comparison between nanocrystalline g-alumina and a new
high pressure polymorph
C.E. Mof®tt*, B. Chen, D.M. Wieliczka, M.B. Kruger
Department of Physics, University of Missouri-Kansas City, 5110 Rockhill Road, Kansas City, MO 64110, USA
Received 12 April 2000; received in revised form 25 August 2000; accepted 29 August 2000 by F.J. Di Salvo
Abstract
High surface area, nano-phase materials exhibit many interesting properties quite different from their bulk counterparts, and
are the focus of much technical investigation and engineering development based on these properties. Earlier experiments using
X-ray diffraction have indicated that a new phase of alumina exists upon non-hydrostatic compression of 67 nm particles of galumina to above 35 GPa and quasi-hydrostatic compression to pressures over 50 GPa. This phase is quenchable on decompression to 0 GPa, allowing for additional analysis that are unmanageable in the diamond anvil cell (DAC). For the present
paper, imaging X-ray photoelectron spectroscopy (XPS), which is ideally suited for surface chemistry measurements of the
small samples required in the DAC, is used. The collection of Bremsstrahlung excited Auger electrons in combination with
photoelectrons allows for a better comparison of chemical shifts, through the Auger parameter. No dramatic change was
observed, as is the case with many alumina polymorphs, but slight differences between the gamma phase and the new highpressure phase are noted, supporting the earlier X-ray results. Additionally, an extremely low adventitious carbon level was
observed on a g-phase sample and is discussed. q 2000 Elsevier Science Ltd. All rights reserved.
Keywords: D. Phase transitions; E. Photoelectron spectroscopies
PACS: 81.05.Ys; 79.60.-I; 07.35.1k; 81.05.Je
1. Introduction
There is currently avid interest in nanocrystalline (particle
size less than roughly 100 nm) materials due in part to some
of their extreme properties, as well as the ability to tailor
speci®c characteristics by selection of particle sizes. Surface
effects can be signi®cant due to the large surface area to
volume ratios inherent to nano-particles, and can result in
dramatic differences in material properties. Physical properties such as hardness and strength may be signi®cantly
enhanced due to the fact that the nanometer range is the
largest size that perfect crystals with no defects can be
grown. Electronic changes associated with the dominance
of quantum effects at small particle sizes are also being
explored. The ability to regulate physical and electronic
properties, through control of particle size, will be increas-
* Corresponding author. Tel.: 11-816-235-5443; fax: 11-816235-5221.
E-mail address: mof®[email protected] (C.E. Mof®tt).
ingly exploited in the future. Thus, a deeper understanding
of fundamental aspects of nanoparticle materials is essential.
Recent high pressure studies on a-alumina have demonstrated that under high pressures and temperatures it transforms into the Rh2O3 (II) structure [1]. More recent high
pressure, room temperature X-ray diffraction experiments
on nanocrystalline g-alumina have shown that it transforms
into another high-pressure phase at pressures greater than
,35 GPa. This phase is quenchable and can therefore be
studied under ambient conditions [2].
The interest in pressure-induced structural changes
extends across many ®elds, including geology, chemistry
and materials science. The ability to extract phase-change
information and determine potential new pathways for
materials development merits the dif®cult preparation and
analysis of the small samples, required for diamond anvil
cell (DAC) pressure experiments. As surface properties are
inherently important in catalytic reactions, so they are in the
general context regarding the properties of large surface
area materials.
Until recently, surface analytical techniques that can
0038-1098/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0038-109 8(00)00367-7
632
C.E. Mof®tt et al. / Solid State Communications 116 (2000) 631±636
extract chemical information, such as X-ray photoelectron
spectroscopy (XPS or ESCA), were restricted to use on
samples of roughly one centimeter in size or larger.
Advances in imaging and small area measurement now
push the limit beyond the 1 mm region, allowing chemical
state information to be recorded from relatively small
samples using monochromatic laboratory X-ray sources.
The higher energy resolution afforded by standard Al-Ka
monochromatic sources, as compared to ¯ood type sources,
has one inherent drawback in the analysis of aluminum
compounds. The photon energy cannot excite the 1s level,
which is required to stimulate the Al KLL Auger transition.
The Bremsstrahlung emitted by laboratory ¯ood sources
does, however, excite this particular transition. These spectra have become quite important when analyzing aluminum
compounds, since the development of the Auger parameter,
for chemical state identi®cation [3,4].
2. Experimental
Nanocrystalline g-Al2O3 was obtained (Nanophase Technologies Corporation, IL) with an average particle size of
67 nm. Compression of samples was performed non-hydrostatically in a Mao±Bell type DAC, at room temperature.
The sample chambers in the gaskets (spring steel and
rhenium) were ,180 mm in diameter, with the diamonds
having 350 mm culets. Pressure calibration of the cell was
made using ruby ¯uorescence, and ruby was included on one
side of certain test samples to ensure the transition pressure
had been reached. This transition pressure for the new phase
was determined from the previous X-ray diffraction work.
Originally, spring steel gaskets were used, but after preliminary XPS measurements indicated that the oxygen signal
from the potentially complex oxide on the gasket could not
be fully eliminated from the collected spectrum, a rhenium
gasket was used to minimize possible overlap and give a
second level for comparison.
Samples of the 67 nm g-Al2O3 starting material were
prepared and measured in three ways; in the cavity of a steel
gasket similar to the high pressure set-up, mounted on doublesided adhesive tape, and pressed into an indium coated sample
stub. The samples in the steel gasket were not as reliably ®xed
in the opening as the high-pressure phase, and after transfer
had regions with no material fully intact at the surface of the
cavity in the indentation. The additional samples on the tape
and indium allowed for much better signal to noise ratios at the
same settings, and also allowed for spectra to be taken that did
not incorporate additional oxygen signals that were present
from the sample in the steel gasket.
XPS measurements were performed on a Kratos Axis-HS
at a base pressure of , 2 £ 10210 Torr: A ¯ood type Mg-Ka
source was used for excitation, so that the Al KLL transition
could be studied along with the core levels. Calibration of the
spectrometer is made measuring the Ag 3d5/2 at 368.26 eV
binding energy and the Ag M4NN at 895.75 eV apparent bind-
ing energy from a silver standard, which assures calibration
over a broad energy range. Most spectra were collected with
the analyzer set at a pass-energy of 80 eV, which yields a
FWHM of just over 1.4 eV for the 3d 5/2 line from an Ag
standard. The reason for lower resolution operation stems
from the small area settings in the mode of collection. To
achieve higher spatial resolution, magnetic lensing coupled
with a selected area aperture in the lens stack are used,
which limit signal strength. The 1 mm aperture used for
these measurements yields the width of a sharp edge in the
range of 50±60 mm. Operating in this mode, the collection
time necessary to achieve good signal to noise resolution was
10 sweeps through core level regions at 0.1 eV steps and
300 ms dwell per step. For the Al KLL spectra, and when
valence band measurements were made, this required an
increase to 30 sweeps through the particular region, maintaining the same step size and dwell time.
The high-pressure phase was best measured in the physical
arrangement in which it was formed, which was compacted
alumina surrounded by a metal gasket. The indention of the
gasket material below the surface required that the sample be
slightly tilted, to ensure full illumination of the recessed
alumina sample with X-rays. Aluminum emissions were the
only features unique to the sample, since oxygen and carbon
were also evident on the gasket material, particularly so on
the spring steel gaskets. Maps based on the actual sample
emissions were used to set the selected area analysis positioning for collection of spectra from the center of the
alumina sample.
The magnetic mode of operation used in the imaging
tends to induce more sample charging in measurements on
insulators. A proprietary method of compensation, using an
electron ¯ood gun in the ®eld of the magnetic lens, allows
for the neutralization of the surface. Generally, referencing
to the adventitious carbon 1s energy position at 285.0 eV
can be used to further address sample charging issues [5].
Optimal compensation was somewhat dif®cult to assess on
most of the high-pressure phase samples, due to the unavoidable collection of some electrons emitted from carbon on the
grounded gaskets. Fortunately the use of the Auger parameter is independent of sample charging, and becomes an
invaluable aid in addressing samples such as these.
3. Results and discussion
As mentioned in the experimental detail section, some
problems associated with measurements of samples in
steel gaskets, due to the unavoidable detection of oxygen
and carbon signals emanating from the steel surface, were
experienced. Slight image shifting from the standard sample
position was also noted and is thought that this might be
related to changes in magnetic ®eld near the spring steel.
Some overlap of the oxygen and carbon spectral contributions from the steel limited the use of these core level spectra for charge referencing. The aluminum spectra, however,
C.E. Mof®tt et al. / Solid State Communications 116 (2000) 631±636
Fig. 1. Al 2p photoelectron map zooming in on sample region. The
X labeled as p1 marks the center of the analysis region where core
spectra were collected.
were only assumed to emanate from the sample itself, and
the Auger (modi®ed) parameter can be used to deduce
chemical state information without knowing the exact
charge corrected positions of the peaks [3,4,6]. To minimize
any magnetic interactions and the spectral contribution from
the oxide on the gasket, a sample made in a rhenium gasket
was also investigated.
The process for analyzing the samples is discussed and
illustrated for the high-pressure phase sample in the rhenium
gasket. Initially, a photoelectron map was formed using the
intensity distribution of electrons with a 40.5 eV binding
energy, the observed position of the Re 4f 7/2 transition.
Taking a core level Al 2p spectrum from the center of the
low intensity region revealed some signal. A corresponding
map was then generated based on collected electron intensity from the maximum of the Al 2p peak at 73.1 eV binding
Fig. 2. Carbon 1s spectra taken using selected area aperture with
center of collection region at p1 in Fig. 1, showing charging shift of
components on alumina sample and adjacent grounded Re gasket
material.
633
energy. The step size was then reduced to 16 mm and a new
map centered on the high Al concentration was formed,
while still maintaining the same 50 mm selected area
aperture setting (Fig. 1). All of the maps required the collection and subtraction of a background map taken at a binding
energy just lower than the respective core level. Finally,
core levels from the aluminum sample were collected,
with the center of the collection marked by the X, labeled
p1, in Fig. 1. The images are not shown, but a similar
process was used to locate and map the samples prepared
in steel gaskets, using maps based on Fe 2p, O 1s, and Al 2p
energies. Core level spectra were obtained from the center
of the region of high aluminum photoelectron intensity, as
was the case for the Re gasket sample.
For one sample, the use of a smaller selected area aperture
(,25 mm edge measurement) was attempted, but the signal
to noise ratio of the spectra was extremely poor for a reasonable collection time, particularly for the aluminum Auger
spectrum. The photoelectron and Auger electron spectra
used for analysis were collected with the same 50 mm
selected area aperture used in the mapping. This aperture
setting would ideally only allow for collection of electrons
emanating from the samples, which were roughly 150 mm
across, but it was observed that both Fe and Re signals, as
well as additional oxygen signals were collected in all
instances. It was thought that the lower oxygen level on
rhenium, as opposed to that on the spring steel, would
give an indication of which oxygen signal was from the
alumina and which was from the gasket.
The primary assumption in the analysis is that the
collected carbon and oxygen signals originate from both
the alumina sample and the gasket material, while the
aluminum signals only originate from the sample region.
As mentioned in the introduction, the carbon 1s position is
Fig. 3. Rhenium 4f spectra taken with both charge neutralization
settings and after sputtering, showing the constancy of the peak
positions.
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C.E. Mof®tt et al. / Solid State Communications 116 (2000) 631±636
Table 1
Position of peak components from Re gasket sample with and without the charge neutralizer; core level photoelectron peaks are given
in binding energy and Auger peaks in kinetic energy. The bracketed
designation of Y or N indicates whether or not the particular peak is
associated with the alumina sample
Peak
Position (eV)
Without charge neutralization
C 1s-1 [Y]
287.60
C 1s-2 [N]
285.31
O 1s-1 [Y]
533.79
O 1s-2 [N]
532.38
O 1s-3 [N]
535.30
Al 2p [Y]
77.18
Al KLL [Y]
1384.53
With charge neutralization
C 1s-1 [Y]
283.61
C 1s-2 [N]
284.93
C 1s-3 [N]
287.62
O 1s-1 [Y]
529.80
O 1s-2 [N]
531.63
Al 2p [Y]
73.34
Al KLL [Y]
1388.39
Charge corrected position
for alumina related peaks
(eV)
285.0
531.19
74.58
1387.13
a ˆ 1461.71 eV
285.0
531.19
74.73
1387.0
a ˆ 1461.73 eV
generally used as the charging reference to determine actual
photoelectron binding energies and Auger kinetic energies
of peaks from insulating samples. To properly use this
method when analyzing regions of different possible potential, the contribution from the particular region of interest
must be identi®ed. The enclosure of an insulating sample by
a grounded metallic gasket made this distinctly feasible. Fig.
2 shows the carbon spectra from the region centered on the
point marked with an X and labeled p1 in Fig. 1, both with
and without the use of the charge neutralizer. The peak that
is shifted to the highest binding energy without the charge
neutralizer is assumed to originate from the alumina sample,
and allows for the correction of the other peaks to account
for charging effects. With the charge neutralizer on, the
surface of the alumina is over compensated and this peak
resides at lower binding energy than the adventitious carbon
on the gasket. The carbon associated with the rhenium
gasket also exhibits a shift with the different charge neutralization conditions, although the rhenium 4f peak shows no
shift, indicating that it is well grounded. The Re spectra are
shown in Fig. 3 in both charge neutralization states and after
sputtering. Sputtering appears to have deposited rhenium
across the entire sample (Fig. 3), and it is observed that
the binding energy after sputtering to exposed fresh metal
is unchanged.
Using the information from the manipulation of the
sample charging allows for the more precise assignment
of Al 2p binding energy and Al KL23L23 kinetic energy.
The carbon spectrum without charge neutralization (Fig.
2) is ®t with two Gaussian±Lorentzian functions with
30% Lorentzian composition on a Shirley background [6].
Neither the carbon nor the oxygen peaks could be ®t in the
charge neutralized state by simply letting the component
positions change and using the exact ®tting parameters
used for the spectra without charge neutralization. This
may be indicative of varied differential charging and
compensation [7,8], since peak widths can appear to
broaden with varying charge compensation within the
sampling volume.
The difference between the higher binding energy carbon
Table 2
Al 2p and Al KLL peak positions and the combined Auger parameter from the different low pressure (g) and quenched high-pressure (hp)
phases; the second number in parentheses indicates the charge corrected value to the carbon 1s at 285.0 eV binding energy, when it could be
applied due to the ability to distinguish the peak origins
Sample
Al 2p binding energy (eV)
Al KLL kinetic energy (eV)
Auger parameter a (eV)
g-Al2O3 on Tape
g-Al2O3 in Fe gasket
g-Al2O3 on In
74.49 (74.19)
72.93
74.01
1387.02 (1387.32)
1388.54
1387.39
1461.51
1461.47
1461.39
Average
1461.46
72.91
72.67
1388.76
1389.09
1461.67
1461.76
77.41
1384.26
1461.67
73.34 (74.73)
1388.39 (1387.0)
1461.73
77.18 (74.58)
1384.53 (1387.13)
1461.71
Average
1461.71
Hp-Al2O3 in Fe gasket #1
Hp-Al2O3 in Fe gasket #2
with charge neutralization
Hp-Al2O3 in Fe gasket #2
without charge neutralization
Hp-Al2O3 in Re gasket with
charge neutralization
Hp-Al2O3 in Re gasket
without charge neutralization
C.E. Mof®tt et al. / Solid State Communications 116 (2000) 631±636
peak position and 285.0 eV is then assumed to be the correction factor for the peaks of the alumina constituents. This is
fairly straightforward for the aluminum peaks, which only
appeared to have one component. The O 1s peak, however,
had large asymmetries that required a second ®tting peak.
The source of the required second peak was assumed to be
from the gasket. The asymmetry changes with charge
neutralization, in commensurate directions to the carbon
shifts, indicating that the dominant peak in the oxygen spectrum is from the alumina sample.
The positions of the dominant component peak, as
obtained in the curve ®tting results, are then shifted by the
same amount required for the C 1s peak from the sample
region to be moved to 285.0 eV. Using this requirement, the
addition of a third peak on the higher binding energy side
was necessary to obtain a good ®t. The contribution of this
peak was small but not negligible, and without its use the
dominant peak became unreasonably wide, but still did not
yield better ®tting results. The sum of the areas of the two
smaller peaks was restricted to contribute the same amount
to the combined spectrum as that of the non-dominant peak
from the charge-neutralized spectrum. The position of the
oxygen on the Re gasket, away from the sample, shifted with
changes in charge neutralization, similar to the position
changes of the gasket associated component of the
combined oxygen spectrum from the sample region, further
con®rming the assessment that it was associated with the
gasket material.
Table 1 gives the values obtained by curve ®tting the
spectra from the sample in the Re gasket, both with and
without charge neutralization and also shows the ®nal
charge corrected values. Table 2 then gives the Al 2p and
Al KLL positions (not charge corrected in most instances)
and the Auger parameters calculated from them for both of
the alumina phases.
The binding energy of the Al 2p peak for the g-alumina,
using carbon referencing on the tape substrate sample, is
somewhat higher, and the Al KL23L23 position somewhat
lower, than those reported in the literature [9]. The Auger
parameters for all of the g-alumina samples are, however,
near the reported value. Additional spectra were obtained
from this tape sample with higher energy resolution (20 eV
pass energy, which corresponds to the measurement of the
width of the Ag 3d5/2 line ,0.9 eV). The higher resolution
spectra indicated a distinct shoulder on the higher binding
(lower kinetic) energy side of the Al 2p (KLL) peaks which
produced a roughly 0.1 eV higher Auger parameter than the
main peak with an intensity of ,20% of the combined spectrum. The surface of aluminum oxides grown on metal
substrates is known to have some degree of hydroxylation
if exposed to atmosphere [10±12]. The third oxygen peak in
Table 1, required on the Re gasket sample without charge
neutralization, might represent a hydroxyl contribution to
the O 1s spectra [10,11]. This would be obscured by the
non-alumina associated peak with charge neutralization,
which closely coincides with an oxygen peak taken from a
635
point on the Re gasket far away from the sample. On the
other high-pressure samples it is dif®cult to determine
whether the origin of additional oxide peaks and asymmetries is the alumina sample or a region of the gasket
material. A second higher binding energy peak, with
lower intensity than the main peak, was also observed on
all the g-alumina samples. This tends to indicate that some
Fig. 4. Spectra from g-Al2O3 pressed on In coated sample stub,
which has no appreciable C 1s peak from adventitious carbon
though it had been exposed to atmosphere and never sputtered.
636
C.E. Mof®tt et al. / Solid State Communications 116 (2000) 631±636
degree of surface hydroxylation could be present on all the
samples, which could not be deconvolved from the Al 2p
and KLL spectra and contributes to the shifted energies.
This second oxygen peak contributes less than 25% to the
total O 1s peak collected in all instances, but could be
responsible for the shifted aluminum electron energies.
Argon sputtering of some samples was performed to
determine whether the possibly hydroxylated surface
could be removed. Apparent in Fig. 3 is the effect of deposition of gasket material across the sample surface. Preferential sputtering of one element in a compound and the
formation of intermediate compounds are both possible
consequences of ion sputtering. The sample in the rhenium
gasket developed a second Al KLL peak after sputtering. A
sample in a spring steel gasket appeared to maintain individual aluminum peaks with quite similar Auger parameters,
and the oxygen peak did not change appreciably, so differentiation between the gasket oxygen peak and that from the
alumina could not be conclusively made. The recessed
nature of the indentation in the gaskets may have limited
the ion rate through shadowing in this case. Sputtering, then,
did not appear to address the hydroxylation concerns.
A fairly unique phenomenon was observed in the XPS
analysis of the g-alumina pressed in indium, whose spectra,
using the charge neutralizer with no sputtering whatsoever,
are shown in Fig. 4. The absence of any resolvable C 1s peak
from a sample exposed to atmosphere is quite unusual. This
is after 10 sweeps through the region using the same settings
as the other spectral regions, which were also obtained with
10 sweeps, except for the valence band and that required 30
sweeps. These were obtained using the same settings used
on the small samples. The lack of an In 3d peak indicated
that the analysis was in a region with good alumina coverage. One seemingly plausible explanation for this is that, if
the pressed sample were jolted during transfer, some
rearrangement of the powder may have occurred exposing
a different surface to the vacuum. However, this sample was
formed from powder that had been exposed to atmosphere
itself and all the surfaces should have some level of carbon
on them, so this explanation appears incorrect. The other
possible explanation is that the high surface area of the
known catalytic phase has some unusual properties affecting
the adsorption of hydrocarbons from the atmosphere. This
behavior was not observed on the tape-mounted sample, but
this could be due to a different source of carbon contamination or some collection of electrons from the tape material
itself.
4. Conclusions
A new high-pressure polymorph of Al2O3 was analyzed
with imaging XPS and it was observed that the Auger parameter of this metastable phase was between 0.2 and 0.3 eV
higher than that of the nanocrystalline g-Al2O3 starting
material. Modi®cation of charge neutralizer settings allowed
for the difference in charging between the sample and the
gasket material to be resolved, thus allowing the assessment
of the sample peak positions. Some level of hydroxylation of
both alumina phases appeared likely from additional peak
components on all of the samples, but a clear relationship
could only be asserted on certain samples due to partial
collection of electrons originating from the gasket. The
use of a rhenium gasket facilitated the analysis due to its
different surface oxide level, contrasting that of an oxidized
spring steel gasket. An unusual lack of adventitious carbon
on one g-Al2O3 sample was unexpected and may have implications regarding high surface area materials, especially
known catalytic phases.
Acknowledgements
This work was partially supported by the National
Science Foundation and the ACS Petroleum Research Fund.
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