- Email: [email protected]
Quantitative surface measurements of metal oxide powders by X-ray photoelectron spectroscopy (XPS)
Surface Science 71 (19781 231-246 0 North-Holland Publishing Company
QUANTITATIVE
SURFACE MEASUREMENTS OF METAL OXIDE POWDERS
BY X-RAY PHOTOELECTRON
SPECTROSCOPY (XPS)
Received 8 July 1977; manuscript received in final form 6 September 1977
A method is described and tested for estimating the concentration of elemental species on metal oxide powder surfaces using X-ray photoelectron spectroscopy. Intensity expressions are derived for a model of the surface and solved for the surface atomic densities and surface layer thickness. In tests of the method, a one-for-one carbon and oxygen increase was measured as methanol was adsorbed on ZnO powder and the surface of fluorided gamma alumina was shown to contain a fluorine-containing second phase.
1. Introduction The measurement of surface atomic concentrations is an important potential application of X-ray photoelectron spectroscopy (XPS). The XPS peak intensities are proportional, in the first approximation, to the surface atomic concentrations and the photoionization cross sections. There have been experimental measurements of photoionization cross sections using XPS peak intensities from reference materials [l-S]. These studies have shown that the potential for surface quantitative measurement exists but is limited by difficulties in accurate peak intensity measurement ].5], surface contamination and surface stoichiometry uncertainties. Theoretical calculations of photoionization cross sections based on a HartreeSlater model have also been made [2,6,7]. Recently, the cross sections calculated by Scofield [6] have been found to be substantially in agreement with experiment (51 when uncontaminated surfaces are used for reference. The use of these cross sections in more detailed mathematical modelling of experimental surfaces seems to be justified now. Chang [8] has developed a formalism for quantitative Auger electron spectroscopy (AES) that can be used to obtain approximate surface atomic concentrations on a variety of samples. In that work, a layered structure was used to model the surface segregation of elements. An exponential escape probability was used to derive expected Auger currents for the model. The surface atomic concentrations were obtained by solving the resulting intensity equations. In the present work, a similar model is used with appropriate modi~cations to derive expressions for X-ray photo231
232
M.J. Dreiling / Quantitative surface measurements
of metal oxide powders
electron spectroscopic peak intensities. A technique is given to solve the intensity equations for the relative atomic concentrations of surface species. The accuracy of the method given here is dependent upon several experimental limitations. Those limitations are discussed in the development of the method. In the following sections, the technique will be described for a general model of several surface layers having different stoichiometries. A specific model is then constructed that is appropriate for the experimental systems of fluorided alumina and methanol adsorbed on zinc oxide.
2. Experimental The X-ray photoelectron spectra were recorded using either a Physical Electronics Industries Inc. (PHI) XPS system or a Varian (IEE-15) photoelectron spectrometer. In the PHI system the X-ray source is a Mg anode and was operated at 40 mA and l- kV. The double pass cylindrical mirror analyzer (CMA) was operated at a 50 eV pass energy in the retarding grid mode (1 .O eV resolution). The pressure in the chamber was 1.3 X 1OF7 Pa during the measurement. The system was interfaced to a Hewlett-Packard 21MX minicomputer which controlled the
.’ .
PRINCIPAL
.
’
PEAK . . .
.
ENERGY
LOSS PEAK
ELECTRON
KINETIC
Fig. 1. A typical X-ray photoelectron spectrum included in the total peak integrated intensity.
ENERGY
Ei -_*
elemental
region.
The energy
loss peak
is
M.J. Dreiling /Quantitative surface measurements of metal oxide powders
233
CMA retarding grid voltage, and set a gate time on a Fluke 1953A programmable digital counter. The total count for the set CMA retarding grid voltage and gate time was recorded by the computer and stored on a magnetic disc file. FORTRAN programming was used. The spectra were collected in about four hours using sequential signal averaging so that any long term X-ray flux variations would be averaged over all elemental regions. Each elemental spectral region included the most intense photoelectron peak plus the energy loss peak as shown in fig. 1. In the IEE-I 5 system, the Mg X-ray source was operated at 50 mA and 9 kV. The pressure in the chamber was 1.3 X lOA Pa. The data were collected in the sequential signal averaging mode by the system computer (Varian 620L) and transferred via paper punched tape to the HP 2 1MX computer system for processing. In the data analysis, the spectral regions were recalled from the computer magnetic disc and displayed on a Tektronix interactive display terminal. A linear background, shown in fig. 1, was substracted and the remaining areas were calculated by numerical integration. The energy loss peak intensity was included in the total peak intensity in order to account for as many as possible of the emitted photoelectrons. This addition to the principal peak intensity amounted to about 0.2 to 0.3 of the total intensity. Inelastically scattered electrons that were not included in the discrete energy loss peak were ignored since it was not obvious how they should be measured [S]. The calculated areas were scaled for the scan time, number of scans and energy increment. Calculations of surface composition from the intensities were performed on the HP 21MX computer.
3. Theoretical A calculation of the surface composition from X-ray photoeiectron intensities requires a set of equations for the intensities that reflects the true surface layering. In the following, the expressions for the XPS intensities from a general system of several homogeneous layers are developed. The equations for the XPS intensities are derived assuming an exponential attenuation of electron intensity during passage through the solid. The photoelectron intensity from an incremental layer at depth z into the surface is given by tit = loi exp(-z/g&)
(1)
dz
where loi is the number of photoelectrons generated per unit volume, Xi is the mean free path of electrons of kinetic energy Ei, and g is a specimen-spectrometer geometrical factor that takes into account the angle of escape of the electron with respect to the surface norma [9]. The number of electrons generated per unit volume, Ioi, is given by
where K. is a constant
instrumental
factor including
X-ray flux, Xi is the atomic
volume concentration of element i and ui is the photoionization cross section for the photoelectron observed for element i. The transmission of the CMA varies as l/Ej [IO] in the retarding grid operational mode. The detected incremental intensity is written as dli = (Koixi/Ei)
exp(-z/g&)
dz ,
(3)
where the constant Know includes instrumental factors such as the analyzer acceptance angle. Upon integration. the total intensity, i!, for a bulk substrate covered by a surface layer of thickness z and having X: atoms of the element k per unit volume is I,! = Vok&WEk) The total intensity to be
(4)
exp(-z/&J
observed from the surface layer of element i is similarly found
ZF = (Ka~~~g~i/~i) [ 1 --
eXp(--Z/ghi)]
.
(5)
In the general case, eqs. (4) and (5) are written for each element and layer of the model system. The ~nfornlation that can be obtained from this system of equations depends upon the specific knowledge of the stoichiometry of the overlayer and bulk, surface layer thickness and the resolvability of surface and bulk intensity components. In the following, a specific model that is appropriate to the experimental data and specimens reported here will be treated in detail. Alternate models that are appropriate to other systems can be treated in similar ways.
4. Model for ZnO Eqs. (4) and (5) can be used to develop expressions for the photoelectroI1 intensities that would be observed from zinc oxide covered by a layer of methanol. The model for this system is shown in fig. 2. It consists of a bulk substrate ZnO that contains x: oxygen ions/run3 and an equal atomic density of zinc ions, x&. The bulk is immediately covered by an intermediate layery mn thick of carbon, hydroxyl ions and zinc ions that are not included in the bulk stoichiometry. The total atomic density (xt> +x& +xi,) of the intermediate layer is assumed to be equal to that of the bulk. This layer represents surface species found on a specimen of ZnO before further adsorption takes place. Finally, the intermediate layer is covered by the adsorbate or surface layer. z nm thick, of methanol containing x8 oxygen atoms/nm3, _x: carbon atoms/nm3 and xg hydrogen atoms~nm3~ where the sum xs + xz + x5 is equal to the atomic density of methanol. Having defined the physical structure and parameters of the system, the expressions for the photoelectron intensities from each layer can be derived. Eqs. (4) and (5) are the basis for the series of equations given in table 1 for the model shown in fig. 2. Each element in each of the three regions of the zinc oxide system is respons-
M.J. Dreiling /Quantitative
surface measurements
of metal oxide powders
235
VACUUM
SURFACE
LAYER
x:, x’, 4
CONTAINING
ATOMSlnd
OXYGEN
ATOMSlnd
CARBON
ATOMSlnmS
HYDROGEN
t S”lll
+
INTERMEDIATE
I 7 7 xZn
LAYER
CONTAINING
ATOMSlnm3
OXYGEN
ATOMSlnm3
CARBON
ATOMSinm3
ZINC
t Y “In
c
BULK
SUBSTRATE
CONTAINING
B X0
ATOMS/nmz
OXYGEN
xBZn
ATOMS/nm3
ZINC
Fig. 2. A schematic diagram for the model of the zinc oxide contaminants and is further covered by z nm of methanol.
Table 1 The XI’S intensity
equations
for the model system
Surface
IS, = KOxS)
Layer
1: = K&
Intermediate
IL = Kd;b]
Layer
IL = KC&l
I;,
surface
shown in fig. 2
[ 1 - exp(-z/ghg)] [ 1 - exp(-z/ghC)] 1 - exp(-y/g%)1
ew-zIgho)
1 - exp(--Y/ghC)l exp(-z/ghC)
= Kz,,&
[ 1 - exp(-y/ghZn)l
Bulk
1: = K&$
Substrate
I!”
Where
Ki = KoightlEi
and the total measured
intensities
exp]-(a
= KZ”Gl
are
exp]-(a
+y)/ghol +Y)/gh7,nl
exp(-s/ghz,)
that
has y nm of surface
hLJ. Dreiling /Quantitative
236
Table 2 Surface concentration
-p-
equations
surface measurements
obtained
by inverting
of metal oxide powders
the intensity
equations
given in table 1
1 - exp(-z/g+)
where
and xsO + xz = estimated
surface
atomic
density
ible for a component of photoelectron intensity. In the case of XPS, the peaks usually overlap and it is difficult to obtain an accurate resolution into components that originate in the surface and bulk. The ZnO oxygen peak shows evidence of two oxygen species. The total intensity will be used in the following treatment. The total intensities as given in table I are used in the calculation of surface composition. The intensity equations in table 1 are normalized by dividing by the total intensity from Zn and inverting to obtain expressions for the atomic densities of oxygen and carbon in the surface layer. Table 2 gives the expressions for the surface oxygen (xz) and the surface carbon (x$) atomic densities. The system of equations in table 2 can be used to calculate the surface composition of zinc oxide given the XPS intensities, electron mean free paths, photoionization cross sections and the instrumental factor g. The electron mean free paths are estimated here from an empirical relation given by Chang [8], i.e., hi = 0.2 dJ!?i monolayers
,
(6)
where Ei is the kinetic energy of the electron. The definition of a monolayer thickness for the specimens treated here is the cube edge length of the average volume per atom in the layer or in the bulk. The photoionization cross sections used are those calculated by Scofield [6]. The instrumental geometrical factor used (0.75) is that calculated by Seah [9] for the CMA. The first step in applying the equations in table 2 to the methanol&ZnO system is to calculate the surface composition of the starting material (an electron beam cleaned ZnO). This ZnO contained carbon at the surface. A surface oxygen component was also indicated by an oxygen (Is) photoelectron peak at about 1.7 eV higher binding energy than the principal oxygen (1s) peak (fig. 3). The calculation
M.J. Dreiling 1 Quantitative surface measurements
of metal oxide powders
23-l
BULKOXYGEN COMPONENT
COMPONENT
ELECTRON
Fig. 3. The O(ls) photoelectron
KINETIC
ENERGY
+
peak for ZnO that has been cleaned by electron beam heating.
is performed by entering the known parameters (oi, Ei, g, hi, Zi, X&X&,) into the equation of table 2). An “intermediate” layer of Zn (xzn = 1 .O, y = 0.25 monolayers) is included in order to account for Zn that is involved in bonding in the surface layer. The surface oxygen and carbon is included in the “surface” layer. In the trial and error solution of the system of equations a starting value of z is used to calculate the total “surface” layer total atomic density (x”, +x8). The calculated surface atomic density is compared to the estimated value (equal to bulk ZnO, 83 atoms/nm3) and an adjustment made in z. For the model described here the calculated surface atomic density decreases monotonically with increasing z and one solution is found. The uniqueness of the solution was verified by calculating x5 + x8 for z = 0.0 to z = 20.0 in increments of 6z = 1.O. The results of the calculation are the individual surface atomic densities and the surface layer thickness. The surface atomic concentrations can be calculated from the individual atomic densities and the surface layer thickness, i.e., xfz atoms i/nm2. The object of the next calculation is to determine the atomic densities and the thickness of the layer of methanol after adsorption on the clean ZnO. The atomic densities (~8, x 8 , xi”) and total surface layer thickness (z +v) that were previously obtained for the ZnO are now used for x& .&x$~ and y in the calculation of methanol coverage. The system of equations is solved for x& x& and z as before
238
M.J. Dreiling / Quantitative surface measurements
except that the total of& x : 1staken to be one-third methanol, (89.25 atoms/nm3)/3, since no photoelectron
of metal oxide powders
the atomic density of liquid peak exists for hydrogen.
5. Results 5. I. Methanol on ZnO As part of a current ultraviolet photoelectron spectroscopic study in this laboratory [ 1 I], XPS peak intensities were measured for the C(ls), O(ls) and Zn(2p) photoelectrons for the system consisting of methanol adsorbed on ZnO powder. The XPS data complemented ultra-violet photoelectron spectroscopic (UPS) studies of methanol adsorbed. The system of methanol adsorbed on ZnO presents an opportunity to use the method described in the previous section to effectively separate the oxygen intensity into surface and bulk components and the carbon intensity into methanol and ZnO surface contamination components. The ZnO (Mallinkrodt Analytical Reagent) powder was pressed into pellets and examined in the PHI spectrometer. Exposure to a broad electron beam at 1000 V and 15 mA was found to clean sulfur, chlorine and most of the carbon contamination from the surface. After XPS measurements on the cleaned surface, the specimen was cooled to -180°C and methanol was adsorbed on the surface at a pressure of 1.3 X lo-’ Pa. After adsorption the system was pumped to 1.3 X 10e7 Pa while maintaining the -180°C specimen stage temperature. At the time of the measurement reported here, the ZnO pellet was not in sound contact with the cold stage and varying levels of methanol were obtained. The XPS measurements were used to quantitatively follow the concentration of methanol at the surface. Three adsorption experiments are described here. The intensities are listed in table 3. The first step in the analysis of the data was to characterize the cleaned ZnO surface. Observation of the oxygen (1s) peak indicated a significant surface oxygen component at about 1.7 eV higher binding energy than the major peak. A typical 0 (1 s) peak profile for a cleaned ZnO powder is shown in fig. 3. The resolution of the peak into two components shows that the minor component represents about 0.3 of the intensity of the major peak component. When the system equations are solved for the three cleaned specimens in table 2 without an intermediate layer (x6 =x& = xin = y = O.O), the ratios of the back calculated intensities &I!!? are between 0.027 and 0.13. Since the calculated values are significantly less than the observed value of 0.3 the above model is obviously incomplete. A new model was constructed where one-half the zinc ions in the ZnO surface monolayer were removed from the bulk stoichiometry and included in the intermediate layer, i.e., I = 1 0 . The solution of the system equations then gives an y = 0.25 monolayer, xzn increase in the surface oxygen content and the ratio of surface-to-bulk oxygen intensity Is//g is calculated to be between 0.16 and 0.52 for the three cleaned specimens, in better agreement with the observed value. This model for the cleaned
M.J. Dreiling / Quaatitativesurface measurements of metal oxide powders
239
Table 3 Surface carbon, oxygen and zinc intensities and the calculated surface coverage of carbon and oxygen for 0.25 monolayer of Zn in the intermediate layer for the system depicted in fig. 2 Specimen identification
Intensities
Zn (2~)
Surface concentrations C tls)
Cls)
X0
(O/nm2)
xc
z
(C/nm2)
tnm)
“Cleaned” ZnO (Ia) Methanol absorbed 15 min XPS scan time (lb) la: next 90 min XPS scan (lc) lb: next 15 min XPS scan
1.0 1.0
0.090
3.70
0.0182 1.09
7.7 35.8
27.1 36.4
0.420 2.25
1.0
3.85
1.01
38.0
33.6
2.25
1.0
4.01
0.960
40.0
31.8
2.25
“Cleaned” ZnO (2) After absorption of methanol 39 min XPS scan time “Cleaned” ZnO (3) After adsorption of methanol 94 min XPS scan time
1.0 1.0
0.0827 0.8411
0.0208 0.2352
4.52 23.9
30.6 22.5
0.426 1.45
1.0 1.0
0.0840 0.134
0.0198 0.0315
5.27 6.58
29.5 5.00
0.420 0.364
specimens, then, consists of the bulk ZnO, covered by an intermediate layer of 0.25 monolayer of Zn and topped by a layer consisting of carbon and surface oxygen. The surface layer thicknesses for the three cleaned specimens was found to be 1.83 to 1.86 lnonolayers consisting of 4.5 to 7.7 O/nm’ and 27 to 3 1 Q’nm*. Without the intermediate layer of Zn the results were 1.61 to 1.63 monolayers consisting of 1 .O to 4.0 0/nm2 and 2’7 to 30 CJnm’, A similar model was used to calculate the surface coverage for methanol except that, in the intermediate layer, the 0.25 monolayer of Zn was assumed to be homogeneously mixed with the carbon and oxygen, i.e., xkn = 4.76 Znfnm2, XL = 7.7 O/nm2 and XL = 27.1 C/nm’ for Specimen 1 in table 3. The results for the three methanol covered ZnO specimens are given in table 3 and plotted in fig. 4. Again, if the intermediate layer of Zn is eliminated the results for the methanol covered specimens are not strongly affected; for specimen la in table 3 there are found 35.0 of 0fnm2 and 35.6 C/nm2. A very nearly 1 : 1 ratio in atomic concentration oxygen to carbon is observed. The agreement with the 1 : I atomic ratio of oxygen to carbon in methanol gives confidence that the model is valid. It should be noted that elimination of the intermediate layer in the calculation of the surface concentrations of carbon and oxygen resulted in a slope of about 0.5 with an intercept at 20 C/rim*. That occurred because, in that model, the relatively dense layer of
240
M.J. Dreihg
i Quantitative surface measurements
ofmeta oxide powders
/
/
/
/
1
/
a f/
/
/ b.
/
/
/
/
/ 3
/
/
/
/
/
/
/
/ /
/ /
l
/ / I
I
I
I
10
20
30
40
OXYGEN
4. The surface
SURFACE
CONCENTRATION
culated from XPS
concentrations of carbon and oxygen intensities. Specimen 1 was measured
exchange
for carbon
Fig.
of oxygen
IATOMShm2I
on methanol covered iMI as Calat three successive times and an
was observed.
carbon at the surface of the cleaned ZnO was erroneousfy assumed to be distributed I~oInogeneously in the ~l~ethanol layer. As the XPS measurement was carried out there appeared to be an increase in the surface oxygen to surface carbon ratio for the methanol-covered specimens. This is illustrated in fig. 4 for the specimen no. 1. This specimen had been measured by XPS in three successive scans of 1.5,90 and 15 min duration at 1.33 X 1O’w7Pa and -180°C. An exchange of oxygen and carbon atoms appears to have taken place, possibly as an exchange of Ha0 from the vacuum chamber with the surface methanol. Water was subsequently observed in the vacuum system using a quadrupole mass spectrometer. The water was removed in later experiments by employment of a liquid nitrogen cryopanel to adsorb Ha0 before cooling the ZnO sample stage. The surface concentrations listed for specimens 2 and 3 also appear to be slightly high in oxygen with respect to carbon in the methanol layer. The run times were 39
241
M.J. Dreiling / Quantitativesurface measurements of metal oxide powders
min for specimen 2 and 94 min for specimen 3 so, considering for specimen 1, some excess oxygen is not unexpected.
the effect observed
5.2. Comparison of several ZnO peak intensities Zinc oxide powder was examined in the Varian IEE-15 photoelectron spectrometer in order to compare the intensities of the Zn photoelectron peaks. Since it is not well known how peaks should be measured to obtain accurate intensities, it is worthwhile to compare the results of surface composition calculations that involve several of the Zn peaks. If substantial agreement is found for the surface composition for several combinations of peaks, it becomes reasonable to use just one of them in future calculations. The method of peak area measurement of the model itself must be reexamined to determine the cause of the discrepancy if agreement is not found. Table 4 lists the intensities of peaks that can be measured by the Varian IEE-15 spectrometer using MgKa radiation. All intensities were measured in the way shown in fig. 1 except for the Zn (3d) peak. The secondary electron peak that is associated with the Zn (3d) peak is not well defined so its contribution was estimated to be 0.3 that of the principal peak. Experimental relative photoionization cross sections were calculated assuming a homogeneous mixture of atoms by the following equation derived from eq. (4)
uzn=
~dco~o~z,,Ezn = 0.0112
Zzn(Ezn)1’2 )
xzJ&o~zn
Table 4 Surface carbon and oxygen concentrations on ZnO powder calculated from XPS intensities; the spectrum was taken on a Varian IEE-1.5 X-ray photoelectron spectrometer ____~_ Relative Kinetic Cross-sections Photoelecx0 X0 (ions/rim*) (ions/rim*) intensities energy tron peak Theory b Exp ’ (eV) Zn Zn Zn Zn
(3d) (3p) (3s) (2P112)
2.52 a 8.19 2.34 57.4
(2PN2) 0 (1s) C (1s)
9.46 4.60
a The secondary
electron
1242 1162 1112 -220 721 968
0.897 2.58 0.873 27.3
0.995 3.13 0.874 9.52
1.8 -1.9 4.1 10
54 49 56 62
2.85 1.0 ____
peak was not measured.
sity .
b Values given by Scofield [ 61. c Related to O(ls) assuming stoichiometric
ZnO.
This value is 1.3 times the elastic peak inten-
242
M.J. Dreiling / Quantitative surface measurements
of metal oxide powders
where the theoretical cross section for the oxygen (Is) peak (uO = 2.85), empirical mean free path from eq. (6) and stoichiometry of ZnO (1 : 1 Zn : 0) are used. The experimental cross sections calculated from eq. (7) are listed in table 4. Fairly good agreement is obtained for those Zn peaks at the higher kinetic energies but the value calculated for the low kinetic energy Zn (2~) combination of peaks is about one-third the theoretical value. It appears that the low energy Zn (2~) electrons are much attenuated with respect to the other Zn peaks resulting in a low calculated value. The carbon intensity is sighificant with respect to the other intensities indicating that contamination of the surface exists, and may be sufficient to cause selective filtering of low energy electrons. In order to account for the overlayer in the intensity calculations, the model developed for the cleaned ZnO is solved for the surface atom concentrations. The surface layer was assumed to contain 0.5 monolayer of Zn and have an atom density equal to that of the bulk ZnO. The resultant surface ion concentration is given in the last column of table 4. The values of the oxygen surface layer show a value of about 5 * 5 0 atoms/nm* and the carbon concentration was found to be 5.5 + 5 C atoms/nm* in a layer three monolayers thick, where a monolayer is defined as 20 atoms/nm* at the density of the bulk ZnO. The agreement between the calculated surface carbon concentrations for different Zn XPS peaks is much better than that found for that between the experimental cross sections. The difference in the calculated surface oxygen concentrations is larger because of their greater sensitivity for the measured Zn peak intensity. This exercise shows that the model gives a systematic accounting for surface carbon and allows one to use any set of elemental intensities to calculate surface compositions.
5.3. Fluorided
gamma-alumina
The replacement of OH- at the surface of gamma-alumina by F- has been measured by chemical methods by Gerberich et al. [ 1 I]. It was reported that Freplaced OH- at the rate of about 2 : 1 until a surface concentration of about 4 F-/nm* was reached. Beyond that concentration about 8-10 F- per OH- was required. The conclusion, also based on catalytic activities of the samples, was made that the surface became more like that of AlF3 as the fluorine concentration increased. Based on the evidence reported in that work, the fluorided alumina system was chosen for a test of the method of interpreting XPS intensity data that was described in the previous section. The surface atomic concentrations x% and $-, obtained by XPS, should reflect the ratios reported by Gerberich et al. Specimens were prepared by treating gamma alumina in ammonium bifluoride solutions to provide a series of fluorided aluminas containing 0.0% to 5.0 wt% F. The specimens were then heated in air at 484°C for two hours. The surface fluorine concentrations were calculated from BET surface areas and the known weight fraction of fluorine. X-ray photoelectron spectra were collected for the O(ls), F(ls),
M.J. Dreiling / Quantitative surface measurements
Table 5 The XPS peak areas and binding
wt% F
0.0 0.5 1.0 2.0 2.8 4.0 5.0
energies
for fluorided
243
gamma-aluminas Binding
Intensities
of metal oxide powders
IF/IA1
IO/IA1
I&AI
F
0.00 0.206 0.398 0.704 0.960 1.32 1.67
9.15 8.37 8.31 8.28 8.10 7.98 7.94
0.43 0.30 0.35 0.31 0.43 0.41 0.42
684.7 684.7 685.0 685.1 685.1 685.2
_
energies
(eV)
0
C
Al
530.9 531.0 530.9 530.9 530.9 530.9 530.9
283.5 283.9 284.2 284.1 283.9 283.9 283.9
74.0 74.0 74.0 74.0 74.0 74.0 74.0
C (1 s) and the Al (2~) energy regions. The intensity data are listed in table 5. The intensities were normalized to that of the Al (2~) peak and the binding energies were referenced to that of the Al (2~) peak set at 74.0 eV. A linear increase in fluorine intensity, IF/IA,, and a corresponding decrease in oxygen intensity, Zo/ZAr, was observed in the series. The carbon intensity remained relatively constant. A 0.5 eV increase in the F (1s) binding energy is an indication of the development of a second fluorine compound, presumably AlF,, at the higher fluorine concentrations. The shift is in the proper direction for the compound AlF,, however, the lack of resolvable F (1s) or Al (2~) intensity components for the new compound prevents a less ambiguous identification. Fluorided surfaces of aluminum oxides were studied by Pitton et al. [ 131 using XPS. They observed an increase in the energy separation between the Al (2~) and F (1s) peak of about 0.7 eV when going from a fluorided alpha-alumina surface to AlFs. A system of intensity equations like that given for zinc oxide in tables 1 and 2 was derived for the fluorided alumina surface. Following the schematic diagram in fig. 2, a model consisting of xz +x8 +x$ atoms/nm3 in the surface layer, x$ atoms/nm3 in the intermediate layer and x2, +x8 atoms/nm3 in the bulk substrate was constructed. The atom density of both the surface and the intermediate layers was taken to be equal to that of the bulk gamma alumina, about 109 atoms/nm3. The instrumental geometrical factor g of 0.75 was used and the aluminum concentration in the intermediate layer was taken to be 33 Al/nm3, corresponding to about 0.3 monolayer. Experimental values for the mean free path of low energy electrons in Al,O, have been measured [14] and are shown in fig. 5 compared to values calculated from eq. (6) where a monolayer thickness in gamma alumina is taken to be 0.209 nm ((109 at/nm3)-‘Z3). The agreement between the two sets of mean free paths is satisfactory considering the experimental error in the mean free path measurement and the uncertainty in the densities of the Al203 in that experiment and the present one. These values calculated from eq. (6) were used in the calculation of the surface composition of the fluorided aluminas.
M.J. Dreiling /Quantitative
244
surface measurements
of metal oxide powders
+
Al (2~)
+ CIISI
i
+ !
+O(ISj
FitSI
l.DEXPERIMENTAL”AL”ES BATTYE.
efal. 1141
+ + + VALUES
OBTAINED
FROM
0.51 0
’
’
’
’
1
’
’
’
’
500
ELECTRON
Fig. 5. The experimental and the values calculated
1
’
1
1000 KINETK
ENERGY
1
Eq. 6.
1
1
I
1
15oca E, (ev)
values for the mean free path of low energy electrons from h = 0.2 E1/2 (2.09) A used in the present work.
in Al203
[ 141
Table 6 gives the surface fluorine concentrations calculated from the known surface area and fluorine weight fraction as compared to the surface fluorine concentrations obtained from the XPS measurement. Good agreement was obtained for the lowest levels of fluorine while, above 1% F the agreement is poor. At the higher levels of fluorine, the XPS values for the surface fluorine concentration are too low, indicating the development of a second phase. It may be concluded that the
Table 6 Surface oxygen wt% F
0.0 0.5 1.0 2.0 2.8 4.0 5.0
and fluorine
concentrations;
comparison
of calculated
and XPS measured
X@ (XPS)
xFz (talc)
x()2 (XPS)
xgz
(F/nm2)
(F/nm2)
(O/nm2)
(O/nm2)
0.0 0.693 1.34 2.31 3.21 4.44 5.56
0.0 0.737 1.53 3.17 4.55 6.78 8.88
8.9 2.96 2.90 2.76 1.92 1.21 1.09
3.0 2.6 2.3 1.7 1.4 1.2 1.0
(ref.
values
[ 121)
M.J. Dreiling / Quantitative surface measurements
of metal oxide powders
245
fluorine is taken into a second phase, possibly AlFs, at concentrations above 1 F/ nm*. The decrease in surface OH- concentration with increasing F concentration is in approximate agreement with the values reported by Gerberich et al. [ 121. The agreement may be fortuitous considering the development of the two phase system and the probable loss of some bulk oxygen in the development of the second phase. In spite of the uncertainty about the interpretation of the XPS values for the surface oxygen coverage, it is clear that the total surface oxygen is decreased and that the total surface fluorine does not increase linearly. This more detailed treatment of the XPS intensities can, therefore, be used for test models for the surface and to eliminate some of them, such as one that presumes uniform distribution of fluorine.
6. Conclusions Interpretation of XPS intensities measured for metal oxides powders can be greatly facilitated through the use of models of the surface layering. For a system such as methanol adsorbed on ZnO powder, it is necessary to take into account the bulk and surface zinc, the bulk, surface, and methanol oxygen, and the surface and methanol carbon when calculating intensities. In that way, equal increases in carbon and oxygen surface concentrations are found as methanol is adsorbed. Although important parameters such as the electron mean free path and the surface atomic density are not always accurately known, some important information can be obtained from measurements made on a well-chosen series of specimens. The formation of a second phase in fluorided alumina was inferred through the failure of the simple layer model to describe the XPS intensities at the higher fluorine concentrations in the series of specimens. In the application of this method, one must be careful to include all elements of the system, construct a reasonable model and use good estimates for the bulk and surface atomic densities. The procedure for measuring peak intensities must be consistent for all elements and carbon buildup in oil pumped vacuum systems should be monitored. Considerably more complex systems can be solved in the same manner as those described here. The mathematical model can be simply expanded to include additional elements or provision can be made for additional layers.
Acknowledgements The author is indebted to Dr. G.D. Parks and C.W. Krueger for their technical assistance in the XPS studies and to J.W. Myers for helpful discussions and preparation of the fluorided aluminas.
246
M.J. Dreiling /Quantitative
surface measurements
of metal oxide powders
References [l]
C.D. Wagner,
[ 21 V.I. Nefedov, [3] [4] [5] [6] [7] [8] [Y] [lo] [ 1 l] [ 121 [ 131 [ 141
Anal. Chem. 44 (1972) 1050. N.P. Sergushin, J.hl. Band and M.B. Trzhaskovskaya,
J. Electron Spcctrosc. 2 (1973) 383. H. Berthou and C.K. Jorgenson, Anal. Chem. 47 (1973) 482. W.J. Carter, G.K. Schweitzer and T. Carlson, J. Electron Spectrosc. 5 (1975) 827. L.J. Brillson and G.P. Ceasar, Surface Sci. 58 (1976) 457. J.H. Scofield, J. Electron Spectrosc. 8 (1976) 129. J.T. Huang and F.O. Ellison, Chem. Phys. Letters 25 (1974) 43. C.C. Chang, Surface Sci. 48 (1975) 9. M.P. Seah, Surface Sci. 32 (1972) 703. P.W. Palmberg, J. Vacuum Sci. Technol. 12 (1975) 379. M.J. Dreiling, G.D. Parks and C.W. Krueger, to be published. H.R. Gerberich, FE. Lutkinski and W.K. Hall, J. Catalysis 6 (1966) 209. 0. Pitton, C.K. Jorgenson and H. Berthou, Chem. Phys. Letters 40 (1976) 357. F.L. Battye, J.G. Jenkin, J. Liesegang and R.C.G. Leckey, Phys. Rev. BY (1974) 2887.
QUANTITATIVE
SURFACE MEASUREMENTS OF METAL OXIDE POWDERS
BY X-RAY PHOTOELECTRON
SPECTROSCOPY (XPS)
Received 8 July 1977; manuscript received in final form 6 September 1977
A method is described and tested for estimating the concentration of elemental species on metal oxide powder surfaces using X-ray photoelectron spectroscopy. Intensity expressions are derived for a model of the surface and solved for the surface atomic densities and surface layer thickness. In tests of the method, a one-for-one carbon and oxygen increase was measured as methanol was adsorbed on ZnO powder and the surface of fluorided gamma alumina was shown to contain a fluorine-containing second phase.
1. Introduction The measurement of surface atomic concentrations is an important potential application of X-ray photoelectron spectroscopy (XPS). The XPS peak intensities are proportional, in the first approximation, to the surface atomic concentrations and the photoionization cross sections. There have been experimental measurements of photoionization cross sections using XPS peak intensities from reference materials [l-S]. These studies have shown that the potential for surface quantitative measurement exists but is limited by difficulties in accurate peak intensity measurement ].5], surface contamination and surface stoichiometry uncertainties. Theoretical calculations of photoionization cross sections based on a HartreeSlater model have also been made [2,6,7]. Recently, the cross sections calculated by Scofield [6] have been found to be substantially in agreement with experiment (51 when uncontaminated surfaces are used for reference. The use of these cross sections in more detailed mathematical modelling of experimental surfaces seems to be justified now. Chang [8] has developed a formalism for quantitative Auger electron spectroscopy (AES) that can be used to obtain approximate surface atomic concentrations on a variety of samples. In that work, a layered structure was used to model the surface segregation of elements. An exponential escape probability was used to derive expected Auger currents for the model. The surface atomic concentrations were obtained by solving the resulting intensity equations. In the present work, a similar model is used with appropriate modi~cations to derive expressions for X-ray photo231
232
M.J. Dreiling / Quantitative surface measurements
of metal oxide powders
electron spectroscopic peak intensities. A technique is given to solve the intensity equations for the relative atomic concentrations of surface species. The accuracy of the method given here is dependent upon several experimental limitations. Those limitations are discussed in the development of the method. In the following sections, the technique will be described for a general model of several surface layers having different stoichiometries. A specific model is then constructed that is appropriate for the experimental systems of fluorided alumina and methanol adsorbed on zinc oxide.
2. Experimental The X-ray photoelectron spectra were recorded using either a Physical Electronics Industries Inc. (PHI) XPS system or a Varian (IEE-15) photoelectron spectrometer. In the PHI system the X-ray source is a Mg anode and was operated at 40 mA and l- kV. The double pass cylindrical mirror analyzer (CMA) was operated at a 50 eV pass energy in the retarding grid mode (1 .O eV resolution). The pressure in the chamber was 1.3 X 1OF7 Pa during the measurement. The system was interfaced to a Hewlett-Packard 21MX minicomputer which controlled the
.’ .
PRINCIPAL
.
’
PEAK . . .
.
ENERGY
LOSS PEAK
ELECTRON
KINETIC
Fig. 1. A typical X-ray photoelectron spectrum included in the total peak integrated intensity.
ENERGY
Ei -_*
elemental
region.
The energy
loss peak
is
M.J. Dreiling /Quantitative surface measurements of metal oxide powders
233
CMA retarding grid voltage, and set a gate time on a Fluke 1953A programmable digital counter. The total count for the set CMA retarding grid voltage and gate time was recorded by the computer and stored on a magnetic disc file. FORTRAN programming was used. The spectra were collected in about four hours using sequential signal averaging so that any long term X-ray flux variations would be averaged over all elemental regions. Each elemental spectral region included the most intense photoelectron peak plus the energy loss peak as shown in fig. 1. In the IEE-I 5 system, the Mg X-ray source was operated at 50 mA and 9 kV. The pressure in the chamber was 1.3 X lOA Pa. The data were collected in the sequential signal averaging mode by the system computer (Varian 620L) and transferred via paper punched tape to the HP 2 1MX computer system for processing. In the data analysis, the spectral regions were recalled from the computer magnetic disc and displayed on a Tektronix interactive display terminal. A linear background, shown in fig. 1, was substracted and the remaining areas were calculated by numerical integration. The energy loss peak intensity was included in the total peak intensity in order to account for as many as possible of the emitted photoelectrons. This addition to the principal peak intensity amounted to about 0.2 to 0.3 of the total intensity. Inelastically scattered electrons that were not included in the discrete energy loss peak were ignored since it was not obvious how they should be measured [S]. The calculated areas were scaled for the scan time, number of scans and energy increment. Calculations of surface composition from the intensities were performed on the HP 21MX computer.
3. Theoretical A calculation of the surface composition from X-ray photoeiectron intensities requires a set of equations for the intensities that reflects the true surface layering. In the following, the expressions for the XPS intensities from a general system of several homogeneous layers are developed. The equations for the XPS intensities are derived assuming an exponential attenuation of electron intensity during passage through the solid. The photoelectron intensity from an incremental layer at depth z into the surface is given by tit = loi exp(-z/g&)
(1)
dz
where loi is the number of photoelectrons generated per unit volume, Xi is the mean free path of electrons of kinetic energy Ei, and g is a specimen-spectrometer geometrical factor that takes into account the angle of escape of the electron with respect to the surface norma [9]. The number of electrons generated per unit volume, Ioi, is given by
where K. is a constant
instrumental
factor including
X-ray flux, Xi is the atomic
volume concentration of element i and ui is the photoionization cross section for the photoelectron observed for element i. The transmission of the CMA varies as l/Ej [IO] in the retarding grid operational mode. The detected incremental intensity is written as dli = (Koixi/Ei)
exp(-z/g&)
dz ,
(3)
where the constant Know includes instrumental factors such as the analyzer acceptance angle. Upon integration. the total intensity, i!, for a bulk substrate covered by a surface layer of thickness z and having X: atoms of the element k per unit volume is I,! = Vok&WEk) The total intensity to be
(4)
exp(-z/&J
observed from the surface layer of element i is similarly found
ZF = (Ka~~~g~i/~i) [ 1 --
eXp(--Z/ghi)]
.
(5)
In the general case, eqs. (4) and (5) are written for each element and layer of the model system. The ~nfornlation that can be obtained from this system of equations depends upon the specific knowledge of the stoichiometry of the overlayer and bulk, surface layer thickness and the resolvability of surface and bulk intensity components. In the following, a specific model that is appropriate to the experimental data and specimens reported here will be treated in detail. Alternate models that are appropriate to other systems can be treated in similar ways.
4. Model for ZnO Eqs. (4) and (5) can be used to develop expressions for the photoelectroI1 intensities that would be observed from zinc oxide covered by a layer of methanol. The model for this system is shown in fig. 2. It consists of a bulk substrate ZnO that contains x: oxygen ions/run3 and an equal atomic density of zinc ions, x&. The bulk is immediately covered by an intermediate layery mn thick of carbon, hydroxyl ions and zinc ions that are not included in the bulk stoichiometry. The total atomic density (xt> +x& +xi,) of the intermediate layer is assumed to be equal to that of the bulk. This layer represents surface species found on a specimen of ZnO before further adsorption takes place. Finally, the intermediate layer is covered by the adsorbate or surface layer. z nm thick, of methanol containing x8 oxygen atoms/nm3, _x: carbon atoms/nm3 and xg hydrogen atoms~nm3~ where the sum xs + xz + x5 is equal to the atomic density of methanol. Having defined the physical structure and parameters of the system, the expressions for the photoelectron intensities from each layer can be derived. Eqs. (4) and (5) are the basis for the series of equations given in table 1 for the model shown in fig. 2. Each element in each of the three regions of the zinc oxide system is respons-
M.J. Dreiling /Quantitative
surface measurements
of metal oxide powders
235
VACUUM
SURFACE
LAYER
x:, x’, 4
CONTAINING
ATOMSlnd
OXYGEN
ATOMSlnd
CARBON
ATOMSlnmS
HYDROGEN
t S”lll
+
INTERMEDIATE
I 7 7 xZn
LAYER
CONTAINING
ATOMSlnm3
OXYGEN
ATOMSlnm3
CARBON
ATOMSinm3
ZINC
t Y “In
c
BULK
SUBSTRATE
CONTAINING
B X0
ATOMS/nmz
OXYGEN
xBZn
ATOMS/nm3
ZINC
Fig. 2. A schematic diagram for the model of the zinc oxide contaminants and is further covered by z nm of methanol.
Table 1 The XI’S intensity
equations
for the model system
Surface
IS, = KOxS)
Layer
1: = K&
Intermediate
IL = Kd;b]
Layer
IL = KC&l
I;,
surface
shown in fig. 2
[ 1 - exp(-z/ghg)] [ 1 - exp(-z/ghC)] 1 - exp(-y/g%)1
ew-zIgho)
1 - exp(--Y/ghC)l exp(-z/ghC)
= Kz,,&
[ 1 - exp(-y/ghZn)l
Bulk
1: = K&$
Substrate
I!”
Where
Ki = KoightlEi
and the total measured
intensities
exp]-(a
= KZ”Gl
are
exp]-(a
+y)/ghol +Y)/gh7,nl
exp(-s/ghz,)
that
has y nm of surface
hLJ. Dreiling /Quantitative
236
Table 2 Surface concentration
-p-
equations
surface measurements
obtained
by inverting
of metal oxide powders
the intensity
equations
given in table 1
1 - exp(-z/g+)
where
and xsO + xz = estimated
surface
atomic
density
ible for a component of photoelectron intensity. In the case of XPS, the peaks usually overlap and it is difficult to obtain an accurate resolution into components that originate in the surface and bulk. The ZnO oxygen peak shows evidence of two oxygen species. The total intensity will be used in the following treatment. The total intensities as given in table I are used in the calculation of surface composition. The intensity equations in table 1 are normalized by dividing by the total intensity from Zn and inverting to obtain expressions for the atomic densities of oxygen and carbon in the surface layer. Table 2 gives the expressions for the surface oxygen (xz) and the surface carbon (x$) atomic densities. The system of equations in table 2 can be used to calculate the surface composition of zinc oxide given the XPS intensities, electron mean free paths, photoionization cross sections and the instrumental factor g. The electron mean free paths are estimated here from an empirical relation given by Chang [8], i.e., hi = 0.2 dJ!?i monolayers
,
(6)
where Ei is the kinetic energy of the electron. The definition of a monolayer thickness for the specimens treated here is the cube edge length of the average volume per atom in the layer or in the bulk. The photoionization cross sections used are those calculated by Scofield [6]. The instrumental geometrical factor used (0.75) is that calculated by Seah [9] for the CMA. The first step in applying the equations in table 2 to the methanol&ZnO system is to calculate the surface composition of the starting material (an electron beam cleaned ZnO). This ZnO contained carbon at the surface. A surface oxygen component was also indicated by an oxygen (Is) photoelectron peak at about 1.7 eV higher binding energy than the principal oxygen (1s) peak (fig. 3). The calculation
M.J. Dreiling 1 Quantitative surface measurements
of metal oxide powders
23-l
BULKOXYGEN COMPONENT
COMPONENT
ELECTRON
Fig. 3. The O(ls) photoelectron
KINETIC
ENERGY
+
peak for ZnO that has been cleaned by electron beam heating.
is performed by entering the known parameters (oi, Ei, g, hi, Zi, X&X&,) into the equation of table 2). An “intermediate” layer of Zn (xzn = 1 .O, y = 0.25 monolayers) is included in order to account for Zn that is involved in bonding in the surface layer. The surface oxygen and carbon is included in the “surface” layer. In the trial and error solution of the system of equations a starting value of z is used to calculate the total “surface” layer total atomic density (x”, +x8). The calculated surface atomic density is compared to the estimated value (equal to bulk ZnO, 83 atoms/nm3) and an adjustment made in z. For the model described here the calculated surface atomic density decreases monotonically with increasing z and one solution is found. The uniqueness of the solution was verified by calculating x5 + x8 for z = 0.0 to z = 20.0 in increments of 6z = 1.O. The results of the calculation are the individual surface atomic densities and the surface layer thickness. The surface atomic concentrations can be calculated from the individual atomic densities and the surface layer thickness, i.e., xfz atoms i/nm2. The object of the next calculation is to determine the atomic densities and the thickness of the layer of methanol after adsorption on the clean ZnO. The atomic densities (~8, x 8 , xi”) and total surface layer thickness (z +v) that were previously obtained for the ZnO are now used for x& .&x$~ and y in the calculation of methanol coverage. The system of equations is solved for x& x& and z as before
238
M.J. Dreiling / Quantitative surface measurements
except that the total of& x : 1staken to be one-third methanol, (89.25 atoms/nm3)/3, since no photoelectron
of metal oxide powders
the atomic density of liquid peak exists for hydrogen.
5. Results 5. I. Methanol on ZnO As part of a current ultraviolet photoelectron spectroscopic study in this laboratory [ 1 I], XPS peak intensities were measured for the C(ls), O(ls) and Zn(2p) photoelectrons for the system consisting of methanol adsorbed on ZnO powder. The XPS data complemented ultra-violet photoelectron spectroscopic (UPS) studies of methanol adsorbed. The system of methanol adsorbed on ZnO presents an opportunity to use the method described in the previous section to effectively separate the oxygen intensity into surface and bulk components and the carbon intensity into methanol and ZnO surface contamination components. The ZnO (Mallinkrodt Analytical Reagent) powder was pressed into pellets and examined in the PHI spectrometer. Exposure to a broad electron beam at 1000 V and 15 mA was found to clean sulfur, chlorine and most of the carbon contamination from the surface. After XPS measurements on the cleaned surface, the specimen was cooled to -180°C and methanol was adsorbed on the surface at a pressure of 1.3 X lo-’ Pa. After adsorption the system was pumped to 1.3 X 10e7 Pa while maintaining the -180°C specimen stage temperature. At the time of the measurement reported here, the ZnO pellet was not in sound contact with the cold stage and varying levels of methanol were obtained. The XPS measurements were used to quantitatively follow the concentration of methanol at the surface. Three adsorption experiments are described here. The intensities are listed in table 3. The first step in the analysis of the data was to characterize the cleaned ZnO surface. Observation of the oxygen (1s) peak indicated a significant surface oxygen component at about 1.7 eV higher binding energy than the major peak. A typical 0 (1 s) peak profile for a cleaned ZnO powder is shown in fig. 3. The resolution of the peak into two components shows that the minor component represents about 0.3 of the intensity of the major peak component. When the system equations are solved for the three cleaned specimens in table 2 without an intermediate layer (x6 =x& = xin = y = O.O), the ratios of the back calculated intensities &I!!? are between 0.027 and 0.13. Since the calculated values are significantly less than the observed value of 0.3 the above model is obviously incomplete. A new model was constructed where one-half the zinc ions in the ZnO surface monolayer were removed from the bulk stoichiometry and included in the intermediate layer, i.e., I = 1 0 . The solution of the system equations then gives an y = 0.25 monolayer, xzn increase in the surface oxygen content and the ratio of surface-to-bulk oxygen intensity Is//g is calculated to be between 0.16 and 0.52 for the three cleaned specimens, in better agreement with the observed value. This model for the cleaned
M.J. Dreiling / Quaatitativesurface measurements of metal oxide powders
239
Table 3 Surface carbon, oxygen and zinc intensities and the calculated surface coverage of carbon and oxygen for 0.25 monolayer of Zn in the intermediate layer for the system depicted in fig. 2 Specimen identification
Intensities
Zn (2~)
Surface concentrations C tls)
Cls)
X0
(O/nm2)
xc
z
(C/nm2)
tnm)
“Cleaned” ZnO (Ia) Methanol absorbed 15 min XPS scan time (lb) la: next 90 min XPS scan (lc) lb: next 15 min XPS scan
1.0 1.0
0.090
3.70
0.0182 1.09
7.7 35.8
27.1 36.4
0.420 2.25
1.0
3.85
1.01
38.0
33.6
2.25
1.0
4.01
0.960
40.0
31.8
2.25
“Cleaned” ZnO (2) After absorption of methanol 39 min XPS scan time “Cleaned” ZnO (3) After adsorption of methanol 94 min XPS scan time
1.0 1.0
0.0827 0.8411
0.0208 0.2352
4.52 23.9
30.6 22.5
0.426 1.45
1.0 1.0
0.0840 0.134
0.0198 0.0315
5.27 6.58
29.5 5.00
0.420 0.364
specimens, then, consists of the bulk ZnO, covered by an intermediate layer of 0.25 monolayer of Zn and topped by a layer consisting of carbon and surface oxygen. The surface layer thicknesses for the three cleaned specimens was found to be 1.83 to 1.86 lnonolayers consisting of 4.5 to 7.7 O/nm’ and 27 to 3 1 Q’nm*. Without the intermediate layer of Zn the results were 1.61 to 1.63 monolayers consisting of 1 .O to 4.0 0/nm2 and 2’7 to 30 CJnm’, A similar model was used to calculate the surface coverage for methanol except that, in the intermediate layer, the 0.25 monolayer of Zn was assumed to be homogeneously mixed with the carbon and oxygen, i.e., xkn = 4.76 Znfnm2, XL = 7.7 O/nm2 and XL = 27.1 C/nm’ for Specimen 1 in table 3. The results for the three methanol covered ZnO specimens are given in table 3 and plotted in fig. 4. Again, if the intermediate layer of Zn is eliminated the results for the methanol covered specimens are not strongly affected; for specimen la in table 3 there are found 35.0 of 0fnm2 and 35.6 C/nm2. A very nearly 1 : 1 ratio in atomic concentration oxygen to carbon is observed. The agreement with the 1 : I atomic ratio of oxygen to carbon in methanol gives confidence that the model is valid. It should be noted that elimination of the intermediate layer in the calculation of the surface concentrations of carbon and oxygen resulted in a slope of about 0.5 with an intercept at 20 C/rim*. That occurred because, in that model, the relatively dense layer of
240
M.J. Dreihg
i Quantitative surface measurements
ofmeta oxide powders
/
/
/
/
1
/
a f/
/
/ b.
/
/
/
/
/ 3
/
/
/
/
/
/
/
/ /
/ /
l
/ / I
I
I
I
10
20
30
40
OXYGEN
4. The surface
SURFACE
CONCENTRATION
culated from XPS
concentrations of carbon and oxygen intensities. Specimen 1 was measured
exchange
for carbon
Fig.
of oxygen
IATOMShm2I
on methanol covered iMI as Calat three successive times and an
was observed.
carbon at the surface of the cleaned ZnO was erroneousfy assumed to be distributed I~oInogeneously in the ~l~ethanol layer. As the XPS measurement was carried out there appeared to be an increase in the surface oxygen to surface carbon ratio for the methanol-covered specimens. This is illustrated in fig. 4 for the specimen no. 1. This specimen had been measured by XPS in three successive scans of 1.5,90 and 15 min duration at 1.33 X 1O’w7Pa and -180°C. An exchange of oxygen and carbon atoms appears to have taken place, possibly as an exchange of Ha0 from the vacuum chamber with the surface methanol. Water was subsequently observed in the vacuum system using a quadrupole mass spectrometer. The water was removed in later experiments by employment of a liquid nitrogen cryopanel to adsorb Ha0 before cooling the ZnO sample stage. The surface concentrations listed for specimens 2 and 3 also appear to be slightly high in oxygen with respect to carbon in the methanol layer. The run times were 39
241
M.J. Dreiling / Quantitativesurface measurements of metal oxide powders
min for specimen 2 and 94 min for specimen 3 so, considering for specimen 1, some excess oxygen is not unexpected.
the effect observed
5.2. Comparison of several ZnO peak intensities Zinc oxide powder was examined in the Varian IEE-15 photoelectron spectrometer in order to compare the intensities of the Zn photoelectron peaks. Since it is not well known how peaks should be measured to obtain accurate intensities, it is worthwhile to compare the results of surface composition calculations that involve several of the Zn peaks. If substantial agreement is found for the surface composition for several combinations of peaks, it becomes reasonable to use just one of them in future calculations. The method of peak area measurement of the model itself must be reexamined to determine the cause of the discrepancy if agreement is not found. Table 4 lists the intensities of peaks that can be measured by the Varian IEE-15 spectrometer using MgKa radiation. All intensities were measured in the way shown in fig. 1 except for the Zn (3d) peak. The secondary electron peak that is associated with the Zn (3d) peak is not well defined so its contribution was estimated to be 0.3 that of the principal peak. Experimental relative photoionization cross sections were calculated assuming a homogeneous mixture of atoms by the following equation derived from eq. (4)
uzn=
~dco~o~z,,Ezn = 0.0112
Zzn(Ezn)1’2 )
xzJ&o~zn
Table 4 Surface carbon and oxygen concentrations on ZnO powder calculated from XPS intensities; the spectrum was taken on a Varian IEE-1.5 X-ray photoelectron spectrometer ____~_ Relative Kinetic Cross-sections Photoelecx0 X0 (ions/rim*) (ions/rim*) intensities energy tron peak Theory b Exp ’ (eV) Zn Zn Zn Zn
(3d) (3p) (3s) (2P112)
2.52 a 8.19 2.34 57.4
(2PN2) 0 (1s) C (1s)
9.46 4.60
a The secondary
electron
1242 1162 1112 -220 721 968
0.897 2.58 0.873 27.3
0.995 3.13 0.874 9.52
1.8 -1.9 4.1 10
54 49 56 62
2.85 1.0 ____
peak was not measured.
sity .
b Values given by Scofield [ 61. c Related to O(ls) assuming stoichiometric
ZnO.
This value is 1.3 times the elastic peak inten-
242
M.J. Dreiling / Quantitative surface measurements
of metal oxide powders
where the theoretical cross section for the oxygen (Is) peak (uO = 2.85), empirical mean free path from eq. (6) and stoichiometry of ZnO (1 : 1 Zn : 0) are used. The experimental cross sections calculated from eq. (7) are listed in table 4. Fairly good agreement is obtained for those Zn peaks at the higher kinetic energies but the value calculated for the low kinetic energy Zn (2~) combination of peaks is about one-third the theoretical value. It appears that the low energy Zn (2~) electrons are much attenuated with respect to the other Zn peaks resulting in a low calculated value. The carbon intensity is sighificant with respect to the other intensities indicating that contamination of the surface exists, and may be sufficient to cause selective filtering of low energy electrons. In order to account for the overlayer in the intensity calculations, the model developed for the cleaned ZnO is solved for the surface atom concentrations. The surface layer was assumed to contain 0.5 monolayer of Zn and have an atom density equal to that of the bulk ZnO. The resultant surface ion concentration is given in the last column of table 4. The values of the oxygen surface layer show a value of about 5 * 5 0 atoms/nm* and the carbon concentration was found to be 5.5 + 5 C atoms/nm* in a layer three monolayers thick, where a monolayer is defined as 20 atoms/nm* at the density of the bulk ZnO. The agreement between the calculated surface carbon concentrations for different Zn XPS peaks is much better than that found for that between the experimental cross sections. The difference in the calculated surface oxygen concentrations is larger because of their greater sensitivity for the measured Zn peak intensity. This exercise shows that the model gives a systematic accounting for surface carbon and allows one to use any set of elemental intensities to calculate surface compositions.
5.3. Fluorided
gamma-alumina
The replacement of OH- at the surface of gamma-alumina by F- has been measured by chemical methods by Gerberich et al. [ 1 I]. It was reported that Freplaced OH- at the rate of about 2 : 1 until a surface concentration of about 4 F-/nm* was reached. Beyond that concentration about 8-10 F- per OH- was required. The conclusion, also based on catalytic activities of the samples, was made that the surface became more like that of AlF3 as the fluorine concentration increased. Based on the evidence reported in that work, the fluorided alumina system was chosen for a test of the method of interpreting XPS intensity data that was described in the previous section. The surface atomic concentrations x% and $-, obtained by XPS, should reflect the ratios reported by Gerberich et al. Specimens were prepared by treating gamma alumina in ammonium bifluoride solutions to provide a series of fluorided aluminas containing 0.0% to 5.0 wt% F. The specimens were then heated in air at 484°C for two hours. The surface fluorine concentrations were calculated from BET surface areas and the known weight fraction of fluorine. X-ray photoelectron spectra were collected for the O(ls), F(ls),
M.J. Dreiling / Quantitative surface measurements
Table 5 The XPS peak areas and binding
wt% F
0.0 0.5 1.0 2.0 2.8 4.0 5.0
energies
for fluorided
243
gamma-aluminas Binding
Intensities
of metal oxide powders
IF/IA1
IO/IA1
I&AI
F
0.00 0.206 0.398 0.704 0.960 1.32 1.67
9.15 8.37 8.31 8.28 8.10 7.98 7.94
0.43 0.30 0.35 0.31 0.43 0.41 0.42
684.7 684.7 685.0 685.1 685.1 685.2
_
energies
(eV)
0
C
Al
530.9 531.0 530.9 530.9 530.9 530.9 530.9
283.5 283.9 284.2 284.1 283.9 283.9 283.9
74.0 74.0 74.0 74.0 74.0 74.0 74.0
C (1 s) and the Al (2~) energy regions. The intensity data are listed in table 5. The intensities were normalized to that of the Al (2~) peak and the binding energies were referenced to that of the Al (2~) peak set at 74.0 eV. A linear increase in fluorine intensity, IF/IA,, and a corresponding decrease in oxygen intensity, Zo/ZAr, was observed in the series. The carbon intensity remained relatively constant. A 0.5 eV increase in the F (1s) binding energy is an indication of the development of a second fluorine compound, presumably AlF,, at the higher fluorine concentrations. The shift is in the proper direction for the compound AlF,, however, the lack of resolvable F (1s) or Al (2~) intensity components for the new compound prevents a less ambiguous identification. Fluorided surfaces of aluminum oxides were studied by Pitton et al. [ 131 using XPS. They observed an increase in the energy separation between the Al (2~) and F (1s) peak of about 0.7 eV when going from a fluorided alpha-alumina surface to AlFs. A system of intensity equations like that given for zinc oxide in tables 1 and 2 was derived for the fluorided alumina surface. Following the schematic diagram in fig. 2, a model consisting of xz +x8 +x$ atoms/nm3 in the surface layer, x$ atoms/nm3 in the intermediate layer and x2, +x8 atoms/nm3 in the bulk substrate was constructed. The atom density of both the surface and the intermediate layers was taken to be equal to that of the bulk gamma alumina, about 109 atoms/nm3. The instrumental geometrical factor g of 0.75 was used and the aluminum concentration in the intermediate layer was taken to be 33 Al/nm3, corresponding to about 0.3 monolayer. Experimental values for the mean free path of low energy electrons in Al,O, have been measured [14] and are shown in fig. 5 compared to values calculated from eq. (6) where a monolayer thickness in gamma alumina is taken to be 0.209 nm ((109 at/nm3)-‘Z3). The agreement between the two sets of mean free paths is satisfactory considering the experimental error in the mean free path measurement and the uncertainty in the densities of the Al203 in that experiment and the present one. These values calculated from eq. (6) were used in the calculation of the surface composition of the fluorided aluminas.
M.J. Dreiling /Quantitative
244
surface measurements
of metal oxide powders
+
Al (2~)
+ CIISI
i
+ !
+O(ISj
FitSI
l.DEXPERIMENTAL”AL”ES BATTYE.
efal. 1141
+ + + VALUES
OBTAINED
FROM
0.51 0
’
’
’
’
1
’
’
’
’
500
ELECTRON
Fig. 5. The experimental and the values calculated
1
’
1
1000 KINETK
ENERGY
1
Eq. 6.
1
1
I
1
15oca E, (ev)
values for the mean free path of low energy electrons from h = 0.2 E1/2 (2.09) A used in the present work.
in Al203
[ 141
Table 6 gives the surface fluorine concentrations calculated from the known surface area and fluorine weight fraction as compared to the surface fluorine concentrations obtained from the XPS measurement. Good agreement was obtained for the lowest levels of fluorine while, above 1% F the agreement is poor. At the higher levels of fluorine, the XPS values for the surface fluorine concentration are too low, indicating the development of a second phase. It may be concluded that the
Table 6 Surface oxygen wt% F
0.0 0.5 1.0 2.0 2.8 4.0 5.0
and fluorine
concentrations;
comparison
of calculated
and XPS measured
X@ (XPS)
xFz (talc)
x()2 (XPS)
xgz
(F/nm2)
(F/nm2)
(O/nm2)
(O/nm2)
0.0 0.693 1.34 2.31 3.21 4.44 5.56
0.0 0.737 1.53 3.17 4.55 6.78 8.88
8.9 2.96 2.90 2.76 1.92 1.21 1.09
3.0 2.6 2.3 1.7 1.4 1.2 1.0
(ref.
values
[ 121)
M.J. Dreiling / Quantitative surface measurements
of metal oxide powders
245
fluorine is taken into a second phase, possibly AlFs, at concentrations above 1 F/ nm*. The decrease in surface OH- concentration with increasing F concentration is in approximate agreement with the values reported by Gerberich et al. [ 121. The agreement may be fortuitous considering the development of the two phase system and the probable loss of some bulk oxygen in the development of the second phase. In spite of the uncertainty about the interpretation of the XPS values for the surface oxygen coverage, it is clear that the total surface oxygen is decreased and that the total surface fluorine does not increase linearly. This more detailed treatment of the XPS intensities can, therefore, be used for test models for the surface and to eliminate some of them, such as one that presumes uniform distribution of fluorine.
6. Conclusions Interpretation of XPS intensities measured for metal oxides powders can be greatly facilitated through the use of models of the surface layering. For a system such as methanol adsorbed on ZnO powder, it is necessary to take into account the bulk and surface zinc, the bulk, surface, and methanol oxygen, and the surface and methanol carbon when calculating intensities. In that way, equal increases in carbon and oxygen surface concentrations are found as methanol is adsorbed. Although important parameters such as the electron mean free path and the surface atomic density are not always accurately known, some important information can be obtained from measurements made on a well-chosen series of specimens. The formation of a second phase in fluorided alumina was inferred through the failure of the simple layer model to describe the XPS intensities at the higher fluorine concentrations in the series of specimens. In the application of this method, one must be careful to include all elements of the system, construct a reasonable model and use good estimates for the bulk and surface atomic densities. The procedure for measuring peak intensities must be consistent for all elements and carbon buildup in oil pumped vacuum systems should be monitored. Considerably more complex systems can be solved in the same manner as those described here. The mathematical model can be simply expanded to include additional elements or provision can be made for additional layers.
Acknowledgements The author is indebted to Dr. G.D. Parks and C.W. Krueger for their technical assistance in the XPS studies and to J.W. Myers for helpful discussions and preparation of the fluorided aluminas.
246
M.J. Dreiling /Quantitative
surface measurements
of metal oxide powders
References [l]
C.D. Wagner,
[ 21 V.I. Nefedov, [3] [4] [5] [6] [7] [8] [Y] [lo] [ 1 l] [ 121 [ 131 [ 141
Anal. Chem. 44 (1972) 1050. N.P. Sergushin, J.hl. Band and M.B. Trzhaskovskaya,
J. Electron Spcctrosc. 2 (1973) 383. H. Berthou and C.K. Jorgenson, Anal. Chem. 47 (1973) 482. W.J. Carter, G.K. Schweitzer and T. Carlson, J. Electron Spectrosc. 5 (1975) 827. L.J. Brillson and G.P. Ceasar, Surface Sci. 58 (1976) 457. J.H. Scofield, J. Electron Spectrosc. 8 (1976) 129. J.T. Huang and F.O. Ellison, Chem. Phys. Letters 25 (1974) 43. C.C. Chang, Surface Sci. 48 (1975) 9. M.P. Seah, Surface Sci. 32 (1972) 703. P.W. Palmberg, J. Vacuum Sci. Technol. 12 (1975) 379. M.J. Dreiling, G.D. Parks and C.W. Krueger, to be published. H.R. Gerberich, FE. Lutkinski and W.K. Hall, J. Catalysis 6 (1966) 209. 0. Pitton, C.K. Jorgenson and H. Berthou, Chem. Phys. Letters 40 (1976) 357. F.L. Battye, J.G. Jenkin, J. Liesegang and R.C.G. Leckey, Phys. Rev. BY (1974) 2887.