Evaluation of XPS for the quantitative determination of surface coverages: fluoride adsorption on hydrous ferric oxide particles

Journal of Electron Spectroscopy and Related Phenomena, 49 (1989) 101-118 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

101

EVALUATION OF XPS FOR THE QUANTITATIVE DETERMINATION OF SURFACE COVERAGES: FLUORIDE ADSORPTION ON HYDROUS FERRIC OXIDE PARTICLES

JULIA E. FULGHUM* and RICHARD W. LINTON** Department of Chemistry, University of North Carolina, Chapel Hill, NC 27599-3290 (U.S.A.) (Received 9 August 1988)

ABSTRACT Quantitative XPS approaches for homogeneous materials can be adapted for use with heterogeneous substrates having chemically modified surfaces. Several methods have been proposed for the XPS determination of adsorbate surface coverages on non-planar samples such as high surface area particles with overlayers. This paper evaluates various quantitative approaches through a study of the pH-dependent adsorption of fluoride on high surface area hydrous ferric oxide (HFO ) particles. A fluoride ion selective electrode (ISE) determines fluoride concentration remaining in solution, allowing for the estimation of fluoride surface coverages in the monolayer range on the HFO particles. Analysis of XPS measurements of fluoride adsorbed on the HFO surface are compared with the ISE results. This comparison shows that if the substrate and overlayer photopeaks are close together in binding energy, several different quantitative methods provide reasonable results. However, if there is a large binding energy difference between the photopeaks, only a threelayer exponential attenuation model taking adventitious carbon into account provides results that agree well with ISE-based calculations.

INTRODUCTION

XPS is frequently used in the analysis of non-planar samples with overlayers. However, most conventional XPS quantitation methods were developed for flat, homogeneous samples. Modified quantitation equations have been developed for the analysis of data from non-ideal samples, but a literature-based comparison of these approaches is difficult because of differences in instrumentation and sample type. Methods for the determination of coverages on rough surfaces have been recently reviewed [ 1 ] and are summarized in eqns. (l)-(6)in Table 1. Terminology is defined in Table 2. Equations have been put into a format for experimental evaluation with the specific spectrometer available (see Experimental section). *Present address: Chemistry Department, Kent State University, Kent, OH 44242, U.S.A. **Author to whom correspondence should be addressed.

0368-2048/89/$03.50

0 1989 Elsevier Science Publishers B.V.

102 TABLE 1 Summary of XPS quantitation

methods

(A) Surface sensitivity factors (Wagner [2] ) Correct bulk sensitivity

factors by AL to obtain surface sensitivity

S’ = s/n

factors (1)

(B) Layered catalyst models Angevine et al. [ 3 ] No attenuation of bulk or overlayer signal. Overlayer surface density given by d = NJ,, = n,/Ab I, 0,(1-l&/4) c=o&+PJ4)

E, d=Od J% NJb

(2)

Defosse [ 41 Sample consists of stacked alternating layers of bulk substrate and overlayer. Bulk substrate attenuates overlayer signal. Substrate consists of sheets or particles with dimensions determined by surface area of substrate. 10 I=Cdq b

q=1+2

(3a)

exp ( - t/A,) [ l-exp( --t/n,)

1

(3b) (3c)

(C) Exponential attenuation model (Dreiling [5] ) Substrate covered with one or two overlayers. Overlayer (s ) attenuate bulk substrate signal. Carbon contamination layer can be included. Two layer: Z,, Ka,x,&,cos~/E, ~=K~bxb&cosO/Eb Z,

-= Ill

[l-exp(-z/&,cos~)] exp ( -

zi2bcose)

S,x,[l-exp(-2/l,cose)] Sbxb exp ( - e/l2*c0se)

(da) (4b)

Three layer: Z,

Ko,x,l,cosB/E,X[1-exp(--z/l,cosB)]exp(--y/l,costI)

FTKcT~x~~Z~COS~/E~

Z, -=

45

exp[ - (2+y)/lbc0se]

S,n,[l-exp(-2/l,cose)]exp(--y/l,c0se) Sbxbexp[ -

(z+y)/2bcose]

(4c) (4a

103 TABLE 1 (continued) (D) Subshell twin method (EbeE [6] ) Ratio two photoelectron peaks of the same element. Replace absolute imfp values with relative values based on energy dependence. (5) (E) Photopeak distortion

(Tougaard [ 71)

Transition metal peak shape distorted by overlayers. Peak shape distortion is related to photoelectron path length by a universal, empirical equation. D(R,&)

= (425+&)R

-1.3+1.4x10-‘Ek

TABLE 2 Terminology A: b: d: D(E): DVW,): E,,: Ek: G(E): I: J: L: N: 0: P: i: s: S’: t: w X: Y: 2: ;; I: 8:

surface area bulk substrate surface density (AU) detector efficiency ratio of peak area to the increase in background signal 30 eV below the peak energy binding energy kinetic energy analyzer transmission function background-subtracted photoelectron intensity X-ray line flux intensity angular asymmetry of photoemission atomic density overlayer density particle radius photoelectron path length (total distance travelled) bulk sensitivity factor surface sensitivity factor sheet thickness atomic weight atomic volume concentration surface layer thickness of carbon-containing contaminant layer monolayer thickness photoelectron cross-section angular asymmetry parameter electron attenuation length (AL) photoemission angle with respect to surface normal

(6)

104

In order to evaluate and compare quantitative XPS determinations of surface coverages, samples which can be characterized by an independent technique are required. In this study, pH-dependent F- adsorption on hydrous ferric oxide (HFO ) particles is used to evaluate surface coverages based on XPS measurements. Ion selective electrode (ISE) measurements of the solution phase allow for the independent determination of F- coverages for comparative purposes. EXPERIMENTAL

Sample preparation All chemicals used were reagent grade or better. F- experiments were conducted using plastic (polymethylpentene) beakers and pipets to avoid adsorption of F- onto the container walls. HFO was prepared by dropwise addition of CO,-free O.lM NaOH to O.lM Fe ( NOs)s until the pH exceeded 8.0. The precipitate was allowed to equilibrate for 20 h before use. The solution was continuously stirred during this period. This procedure yields an X-ray amorphous iron oxide which has been characterized by Dousma and de Bruyn [ 81 and by Harvey and Linton [ 91. For the adsorption experiments, 1 x 10W4MNaF was added to pH-adjusted 1 x 10W3MHFO in O.lM NaN03. The pH was readjusted after F- addition and the solution allowed to equilibrate for 4 h, with stirring. After the equilibration period, the pH was measured. The solution was then filtered through 0.4 ,um Nuclepore filters and the HFO precipitate allowed to air dry before XPS analysis. The filtrate was collected for ISE measurement of F- in solution. Instrumentation and analysis A F- ISE, Orion model # 94-09-00, and a Fisher Accumet Selective Ion Analyzer Model 750 digital meter were used to measure solution F- concentration. The electrode was calibrated using a nine point calibration curve over the region 1 x 10m6to 1 x 10m3M F- using solutions made from a Fisher standard O.lM F- solution. Linear calibration curves were obtained (slope= -55.45 mV decade-‘, r2 ~0.996). A Perkin-Elmer Physical Electronics Model 548 electron spectrometer was used for XPS analyses. This instrument has a Mg Ka! anode operated at 10 kV, 40 mA and a double-pass cylindrical mirror analyzer with an electron multiplier detector. Samples required 1 h to pump down in the introduction chamber. Typical chamber pressure during analysis was 8x lo-‘torr. Samples were pressed into In foil (Alfa Products, 99.9995% pure) for analysis. Low resolution (100 eV pass energy) survey spectra were acquired over

105

the binding energy range O-1000 eV for all samples. No In was detected in the survey scans. High resolution windows were acquired at 50 eV pass energy and a scan speed of 1 eV see-‘. Typical binding energy windows (eV) for analysis were Fe2p 745-700, Fe3p 70-45, Cls 305-280, Fls 700-675, Nls 420-395, and Nals 1086-1061. Binding energies were charge-corrected by referencing to adventitious carbon at 284.6 eV. Peak areas were measured using a straight line background. Sensitivity factors are from Wagner and coworkers [ 2,101. RESULTS AND DISCUSSION

Adsorption isotherms F- adsorption was studied for comparison with oxyanion adsorption on HFO [ 111. F- adsorption on HFO is pH-dependent, as shown in the adsorption

isotherms in Fig. 1. Figure la plots adsorbed F- (from ISE measurements) vs. pH; Fig. lb shows Fls:Fe2p photoelectron intensity ratios vs. pH. The two isotherms are qualitatively similar. The goal of this evaluation was to determine which XPS data analysis method(s) result in surface coverages which agree best with ISE-based coverages. Linear regression analysis was used to compare the surface coverages determined from XPS and ISE data. Since the adsorption isotherms are similar, a good correlation coefficient ( r2) is expected from the linear regression. The most successful model is thus the one that comes closest to a unity slope and a zero intercept, the ideal one-to-one correlation between XPS and ISE results. The plots do result in a near-zero intercept for XPS surface coverages based on either Fls : Fe2p or Fls: Fe3p ratios, which indicates that the assumptions made below about surface area and molecular weight are reasonable. The comparison of slopes is a relative one. If the same assumptions are made about surface area and molecular weight, the same surface coverages should result from the two techniques. Calculation of surface coverages from ISE data The available surface area for adsorption can be calculated from the surface area and molecular weight of HFO. The values for surface area and molecular weight have a significant effect on the calculations. Values reported for HFO surface area vary from 120 m2 g-l to 800 m2 g-’ [12]. Measurement of the surface area by Brunauer, Emmett and Teller analysis gave on the order of 200 m2 g-’ for air-dried HFO. As both Davis [ 121 and Benjamin [ 131 report, surface area measurements tend to give areas of between 100-300 m2 g-l while calculated estimates can be as high as 800 m2 g-l. Taking the structure of HFO as Fe (OH ) 3 gives a molecular weight of 107 g

O.OW+O

1

5

4

6

7

0

PH

007

0.01

0.05

.s

.

0.04

m.

2

2 x 9

0.03

.

iL

.

n

0.02 . .

n

.

.

.

0.01

:

0.00 4

5

6

7

8

PH

Fig. 1. Adsorption of 1 X 10e4M NaF on 1 X 10e3M HFO: (a) adsorbed F- vs. pH as determined by F- ISE; (b) uncorrected Fls: Fe2p photopeak intensities vs. pH.

107

mol-’ while Harvey and Linton [9] have proposed the formula Fez0,*3.26H,0 based on thermal analysis results. This gives a molecular weight of 219 g mol-l. However, this molecular weight is expected to be too high, since dehydration will occur in the vacuum chamber. Hirokawa and Danzaki [ 141 have reported that inorganic compounds which are dehydrated at 400’ C under normal pressure will lose water in a vacuum of lo-* torr. Harvey and Linton [ 91 observed loss of water beginning at 150°C in the thermal analysis of HFO. Thus a molecular weight of between 107 and 219 g mol-’ is expected owing to water loss. The HFO surface area available for adsorption is the product of the surface area, the molecular weight of HFO and the amount of HFO in solution. Using values of 200 m2 g-l, 107 g mol-l and 4~ 10m4moles respectively gives 8.6 m2. As the product of the molecular weight and the surface area decreases, the surface available for adsorption decreases, increasing the calculated surface coverage values. Thus, 107 g mol-l and 200 m2 g-’ give the maximum surface coverages for the range of molecular weight and surface area values considered to be reasonable. The surface coverage is given by the product of the number of F- ions per unit area of the HFO surface and the HFO surface area per F-. The number of F- ions is determined from ISE measurements and the HFO surface area is determined as explained above. A value for the surface area occupied by adsorbed F- of 0.2 nm2/F- was taken from Hingston [ 151. These numbers were used to establish reference values for comparison with XPS. Submonolayer F- coverages result from these calculations, indicating that, for the XPS measurements, no significant Fe attenuation by the F- will occur. The correction term for F- attenuation of the Fe photopeak is thus expected to play a minor role in the models to be discussed. Calculation of surface coverages from XPS data The models summarized in Table 1 have been used to determine F- surface coverages from the XPS data. Two different types of information result. The methods of Wagner [ 21 and Defosse [ 41 give atomic ratios of F: Fe or atoms F : g Fe. Surface coverages can be calculated, based on the available surface area for adsorption and the size of the adsorbing F- ion. Surface layer thicknesses result from the methods of Dreiling [ 51, Ebel [ 61 and Tougaard [ 71. Surface coverages can be determined from the Dreiling exponential attenuation model based on a calculated theoretical monolayer thickness [ 51. In all cases, surface coverages of less than 1 monolayer result, consistent with the ISE data. Application of the models in Table 1 requires both terms related to photoemission (a, j?, A) and those which describe the bulk substrate (surface area, density). Photoelectron cross-sections, a, were taken from Scofield [ 161. The theoretical values show good agreement with experimental values for elements with atomic number < 50 [ 17,181. Asymmetry parameters are from Reilman

108

et al. [ 191,as recommended by Seah [ 171. The modification of p suggested by Ebel and coworkers [ 201 has little effect on values for these samples. Attenuation length (AL) values were calculated from the empirical equations developed by Seah and Dench [ 211. Equation (7) is the Seah and Dench formulation, while eqn. (8) is a modification recommended by Seah [ 171 for inorganic compounds other than alkali halides. Equations (7) and (8) give the AL in monolayers, where a is the atom size and is as defined by Seah and Dench [ 211. Ek is the kinetic energy of the photoelectron. (7) 2, =o.55(UEk)“2

(8)

Values calculated from eqns. (7) and (8) were used for comparative purposes. AL values from eqn. (8) are in good agreement with the values recently calculated by Tanuma et al. [ 221. A in nanometers can be calculated using eqn. (9). 2, =all,

(9)

For Fe2p, this gives 1.50 nm using eqn. (7)) and 1.06 nm using eqn. (8). Table 3 contains AL values utilized for the photoelectron transitions of interest. Since the Fls and Fe2p photoelectron peaks have similar kinetic energies at 569.6 and 542.6 eV, respectively, the ALs are very similar. To examine the effect of a substantial kinetic energy difference, the Fe3p photoelectron peak at 1194 eV was also used, Each of the quantitative models requires slightly different information about the sample. Atomic volume concentrations are required for the methods of Defosse and Angevine et al. (eqns. (2 ) and (3a) ) . A molecular weight of 107 g mol-’ gives a volume concentration of 20.2 atoms Fe nmm3,while 219 g mol-’ gives 9.9 atoms Fe nmY3. The exponential attenuation model (eqns. (4a)(4~) ) requires the surface atomic density. In this case the total number of atoms nrnm3is determined, to give 140 atoms nmm3based on a molecular weight of 107 g mol-‘. This calculation of surface coverage also requires a theoretical TABLE 3 In values Photoelectron

Ew. (7) (nm)

Eqn. (8) (nm)

Fe2p Fe3p Fls

1.50 2.06 1.42

1.06 1.57 1.09

109

monolayer thickness. The monolayer thickness is defined as the cube edge length of the average volume per atom [ 51. Based on an atomic volume concentration of 140 atoms nrnm3for HFO, a theoretical monolayer thickness of 0.2 nm is obtained. Monolayer thicknesses calculated from eqns. (4a)-(4d) in Table 1 are divided by the theoretical monolayer thickness to get surface coverage values in monolayers. Comparison of models Results from eqns. (l)-(4) in Table 1 are summarized in Tables 4 and 5. Table 4 contains results from XPS models using Fls : Fe2p ratios. Two different attenuation length values (Table 3) and atomic concentration values ( xFe) were tested with appropriate changes in the ISE-based calculations. The models were further evaluated using Fls and Fe3p photoelectron peaks, which differ in kinetic energy by approximately 600 eV. Using peaks with a large difference in kinetic energy gives substantially different results, as summarized in Table 5. Surface sensitivity factors The Fls bulk and surface sensitivity factors are both 1, since F was chosen as the reference element. The Fls and Fe2p ALs are very similar, so the bulk and surface sensitivity factors reported by Wagner for Fe2p are the same [ 21. The surface sensitivity factor approach results in a slope of 1.00 from the linear regression analysis. The surface sensitivity factor for Fe3p is 0.17, compared with a bulk sensitivity factor of 0.26 [ 21. Surface coverages based on Fe3p are lower than those for Fe2p by a factor of approximately two, and the high slope indicates results do not correlate as well as for Fe2p. ISE and XPS results are compared in Fig. 2. Results based on Fe2p and Fe3p are clearly different. Note that in both cases, the maximum coverage is well below 1 monolayer. Layered catalyst models The Angevine et al. and Defosse layer models replace the product of the AL of the overlayer and the atomic density with a surface density [ 3,4]. The Defosse model assumes a multilayer structure which describes the bulk substrate as stacked non-porous “sheets” or “particles” [ 41. Separate equations are developed for these two cases (layer and sphere models, Table 1) . These models are not very sensitive to changes in the atomic concentration values (2cre),but significant variation results from changing the AL value. Slopes are lower for J”reZp= 1.50 nm than for a value of 1.06 nm. A slope less than one indicates a surface coverage which is too high relative to coverages calculated from ISE data. For the Angevine et al. model [ 31, an AL of 1.06 nm gives reasonable results, but an AL of 1.50 nm gives a slope which is too low by about 25%. Both AL values work with the Defosse layer model [ 41, giving slopes which approach

110 TABLE 4 Comparison of models using Fe2p Model and important parameters

Intercept

r2

Slope

Fe2p (107)

0.84

1.00+0.12

&= 1.06, xFe=20.2 ,&= 1.06, x,+= 9.9 &e=1.50,x,,=20.2 A&= 1.50, xFe= 9.9

0.85 0.86 0.85 0.84

1.11 f0.13 1.16f0.13 0.78 f 0.09 0.77 f 0.09

0.00 -0.02 0.01 0.00

+,=20.2 .%,?e= 9.9 z&=20.2 xFe= 9.9

0.85 0.84 0.85 0.83

1.26 rt0.14 1.18kO.14 0.88f0.10 0.88f0.11

0.01 0.01 0.01 0.00

AFe=1.06,XF,=20.2 AFe= 1.06, xFe= 9.9 1,=1.50, xF.=20.2 A,+= 1.50, X,?e= 9.9

0.85 0.83 0.84 0.84

2.20 f. 0.25 1.98kO.24 1.55f0.18 1.55f0.18

0.00 0.01 0.01 0.00

1.09

0.83

0.99 If:0.13

0.02

2.02

0.83

0.78f0.10

0.02

lw= 1.06, J.F= 1.09 xFe= 140.6, ,+ = 73.4

0.83

0.55 k 0.07

0.02

1.09

0.82

1.26 f 0.16

0.02

2.02

0.81

0.70 f 0.09

0.02

1,=1.06, AF= 1.09 xFe= 140.6, %F= 73.4

0.81

0.68 f 0.09

0.02

Wagner” 1

0.00

Angevine” 1 2 3 4

Defosse layer model’ 1 2 3 4

&,=1.06, &= 1.06, 1,,=1.50, &,=1.50,

Defosse sphere modeld 1 2 3 4

Dreiling 2-layer FP model’ 1

AFe= 1.06, &= XFe =xF

2

;i&= 1.39, &= XFe = XF

3

Dreiling S-layer SF model’ 1

dFe= 1.06, &= %Fe = ZF

2

d&=1.39,&= xFe = XF

3

111 TABLE 4 (continued)

Dreiling 34ayer modelg (y=O.2 nm) are= 1.06, IF= 1.09 rFe= r,?, SF ,IFe= 1.06, IF= 1.09 XFe= rr, FP (y= 1.0 nm) ire= 1.06, Ar= 1.09 XFe= rr, SF AFe=1.06, dF= 1.09 &=rr, FP

0.82

1.27f0.18

0.02

0.82

1.00+0.14

0.02

0.83

1.36f0.19

0.01

0.82

1.03f0.17

0.02

“Equation ( 1) , molecular weight of 107 g mol- ’ assumed for HFO. bEquation (2 ) , A in nanometers, x in atoms Fe nms3. “Equations (3a) and (3b), 1 in nanometers, x:in atoms Fe nmm3. dEquations (3a) and (3c ) ,I. in nanometers, x in atoms Fe nme3. “Equation (4a), FP = first principles, 1 in nanometers, x in total atoms nme3. Equation (4b), SF = sensitivity factors, 1 in nanometers, x in total atoms nme3. gEquations (4c ) and (4d), FP = first principles, SF = sensitivity factors, 1 in nanometers.

1.0 within the standard deviation of the linear regression analysis. The Defosse sphere model shows the same trend with changes in the AL, but the slopes are too high by a factor of 1.5-2.0. The choice of HFO surface area values is not significant for any of these models since it cancels in the conversion to surface coverage. Results based on Fls and Fe3p are worse than those for Fls and Fe2p. Slopes from the linear regression analysis are too high by factors of 1.5 to 3. Exponential attenuation model The Dreiling two-layer model (eqns. (4a) and (4b) ) assumes that the overlayer attenuates the signal from the bulk substrate [ 51. As shown in Table 4, changing the values of the AL or the atomic concentration changes the correlation. This model is not very sensitive to the AL for the overlayer, but variation in the bulk substrate AL has a significant effect. Since the overlayer thickness is significantly less than the Fls AL, changing 2, by a factor of two results in less than a 10% change in calculated layer thicknesses. A 200% change results for the same variation in the substrate AL. Increasing .ItFdecreases the calculated surface coverage, while increasing xi?, increases it. A change of 5075% results from a factor of two change in x. Calculating xi+ based on Fe (OH )3 and xr based on NaF gives a slope of 0.55. The best results are obtained using a first principles approach (eqn. (4a) ) with IF&p= 1.06 nm, AFls= I.09 nm and xFe= xi&Results using tabulated sensitivity factors are not as good (eqn. (4b ) ) , although the same combination of 1 and x values gives the best results. It is

112 TABLE 5 Comparison of models using Fe3p Intercept

r2

Slope

Fe3p

0.76

1.68f0.27

1 &aP= 1.57, x,=20.2 1 &sP= 1.57, x,=9.9

0.76 0.77

1.45 k 0.22 1.55 + 0.24

0.79 0.79

2.12f0.31 2.16f0.31

0.00 0.00

0.75 0.66

2.95 f 0.48 2.66 + 0.53

0.01 0.01

0.78

1.36kO.20

0.01

0.78

1.45f0.21

0.01

0.78

1.31 k 0.21

0.01

0.76

1.35 k 0.23

0.01

0.75

0.97F0.17

0.01

0.76

0.98 f0.17

0.01

Model and important parameters Wagner” 1

0.01

Angevine” 1 2

0.00 -0.01

Defosse layer model’ 1 2

1 &..+= 1.57, x&=20.2 J?F&,= 1.57, X&=9.9

Defosse sphere modeld 1 2

1 F&+1.57, x,=20.2 1 ,,=1.57, x&=9.9

Dreiling Z-layer model’ 1 2

1 F&,=1.57,&=1.09 xFe= %F,SF I F&,=1.57,&=1.09 xFe= XF, FP

Dreiling 3-layer model’ (~~0.2 nm) AF+,= 1.57, &?=l.O9 xFe= +, SF I F@=l.57,&.=1.09 xFe= %F,FP (y=l.Onm) ii F&,=1.57,&=1.09 xFe= XF,SF 1 F&$,=1.57, IF= 1.09 nFe= %F,FP

“Equation ( 1) bEquation (2 ) , A in nanometers, x in atoms Fe nmd3. “Equations (3a) and (3b), I in nanometers, x in atoms Fe nmd3. dEquations (3a) and (3~)) L in nanometers, x in atoms Fe nme3. ‘Equations (4a) and (4b), SF=sensitivity factors, FP =fint principles, I in nanometers, z in total atoms nmm3. %quations (4~) and (4d), FP = first principles, SF = sensitivity factors, 1 in nanometers.

0.10 n

Fe2p

A

Fe 3p

0.40

XPS (Monolayers)

Fig. 2. Comparison of F- monolayer coverages on HFO as determined by ISE and by XPS surface sensitivity factors (Eqn. ( 1), Table 1) .

probable that using experimentally-derived sensitivity factors for the specific instrument used in this study might improve the correlation [ 231. Results of the Dreiling two-layer model based on Fls and Fe3p are better than for the models previously discussed. However, as with all previous models, they are not as good as the results for Fls and Fe2p. A slope of 1.36 (Fe3p) is obtained as compared with 0.99 (Fe2p). The exponential attenuation model can be expanded to include multiple layer samples. Since the adsorbed anions will be covered by a layer of hydrocarbon contamination, Dreiling’s three-layer

114

model (eqns. ( 4c ) or ( 4d ) ) may be especially applicable [ 5 1. This model treats the sample as a three-layer structure consisting of HFO, adsorbed F- and an adventitious carbon layer. For the evaluation of this model, the carbon layer was given thicknesses of 0.2 and 1 nm to approximate light and heavy carbon con~mination. Carbon layer thicknesses can be estimated from experimental data using the two-layer model on HFO samples containing no adsorbed F- (eqn. (4a) ). Results of the three-layer model using Fe2p or Fe3p are given in Tables 4 and 5. Adding a carbon overlayer decreases the surface coverage calculated using Fls and Fe2p, but changing the thickness of the carbon layer has little effect. Since the ALs are similar, the Fe2p and Fls photopeak intensities should be similarly attenuated by the C layers. A difference between first principles (eqn. (4~)) and sensitivity factor approaches (eqn. (4d) ) is apparent. Using sensitivity factors gives lower surface coverages; first principles results correlate better with ISE data. For example, if y is 1.0 nm, then first principles calculations using Fe2p results gives r2 =0.82 with a slope of 1.03. The corresponding sensitivity factor based calculations result in r 2=0.83, but a slope of 1.36. Similar trends are observed if X= 0.2 nm is used for the carbon contaminant layer thickness. Calculations using both first principles and sensitivity factor approaches for Fe3p and Fls result in increasing surface coverages as the C overlayer thickness increases. A C overlayer of 0.2 nm gives an r2 value of 0.79 and a slope of 1.31 while 1.0 nm gives an r2 value of 0.75 and a slope of 0.96. It is of interest to note that first principles and sensitivity factor approaches give comparable results for the Fe3p case. This three-layer exponential attenuation model is the only model which gives results which are most consistent with the surface coverages from ISE data. It is the only model which can provide slopes near unity for the Fe3p case.

Other models The subshell twin method (eqn. (5 ) ) and Tougaard methods (eqn. (6) ) are not useful for the samples under study. The subshell twin method gives an average overlayer thickness of approximately 2 nm. This is a reasonable value since it ineludes not only adsorbed F- but hydroxyl groups on the HFO surface, electrolyte adsorbed from solution and carbon. The method of Tougaard determines the Fe3p path length, which includes the average distance travelled through the HFO as well as the adsorbed layers. A path length of approximately 6 nm results from this calculation. These do not provide specific information about the F- coverage; rather they estimate total overlayer thickness including contaminant layers. However, they are useful in providing rough estimates of the total overlayer thickness without having to make extensive correlative measurements with other techniques.

115 CONCLUSIONS

All of the methods summarized in eqns. (1 )- (4)) Table 1, can provide useful estimates of surface coverages if the kinetic energies of the peaks are close together. For F- adsorbed on HFO, if Fe2p peak areas, a 3LFeof 1.06 nm, a surface area of 200 m2 g-’ and a molecular weight of either 107 or 219 g mol-’ are used, results from eqns. (1 )- (4) correlate similarly with ISE data as shown in Table 4. However, from Table 5 it is apparent that using an Fe peak further in energy (Fe3p) from the Fls decreases agreement with ISE data unless the three-layer exponential attenuation model (eqns. (4~) or (4d) ) is used. The slopes from the Fls: Fe2p and Fls : Fe3p examples should be equal to each other and close to 1.0. Figure 3a compares slopes resulting from Fls: Fe2p and Fls: Fe3p for all of the models. Slopes differ for the Fe2p and Fe3p cases except for the Dreiling three-layer model. This result is due to adventitious carbon effects on the relative attenuation of the Fe3p compared to the Fe2p photoelectrons. Since Fe2p and Fls have similar kinetic energies, and therefore similar attenuation lengths, the Fe2p and Fls photoelectron peaks will be similarly attenuated by hydrocarbon overlayers. However, Fe3p, with a larger attenuation length, will be less attenuated by the hydrocarbon overlayer than Fe2p, resulting in the calculation of lower F- surface coverages. This is observed for all the models that do not take a surface carbon layer into account. The best case results for each approach using Fe3p data are shown in Fig. 3b, indicating that although correlation coefficients are similar for all the models, only the Dreiling three-layer model yields a slope of 1.0. A weak point in the application of most of the models is the choice of AL. The models which give F : Fe ratios either do not require, or are not very sensitive to the F AL since the F- layer thickness is much less than hr. However, small changes in the AL for Fe can result in large changes in results. In most cases, an AL of 1.06 nm [ 171 appears to give results which correlate best with the ISE data. Fls and Fe2p are among the best possible cases for analysis, since the ALs are very close. As the elements of interest get further apart in energy, the error due to differential attenuation increases. The Dreiling three-layer model (eqn. (4~) or (4d) ) appears to be the best model to correct for this effect. Although a three-layer exponential attenuation model gives good quantitative results, there are future improvements that can be investigated. Background subtraction techniques should be given further consideration. For example, Hawn and De Koven [ 241 have corrected for inelastic energy losses in iron oxide spectra by a deconvolution and ratio method. This method is reported to be more reliable than a linear or sigmoidal background approximation. Tougaard [ 251 has suggested a correction for inelastic peaks using a “universal function”. Elastic scattering effects have been evaluated by Tofterup [ 261 andEbe1 et al. [ 271. Tofterup [ 261 has developed a deconvolution method

4

Slope from F Is/ Fe 3p

10

n

Ideal

l

Wagner[Z] 3p

x

Angevme [3]

A

Layer 141

q

Sphere 141

+

OredIng 2 [S]

l

+x

A

.

I

05

00-I 0

1

2

Fig. 3. Comparison of best case results for XPS-ISE correlations: (a) slope from Fls : Fe2p vs. Fls:Fe3p for each model; (b) best case from each quantitative XPS model using Fe3p and Fls photopeaks.

117

for removing inelastic and elastic scattering contributions. However, Ebel et al. [ 271 reported results of Monte Carlo calculations, showing that XPS analysis with no references requires no elastic scatter correction. Surface roughness effects, not taken into account here, have been considered in several different studies. Kuipers et al. [ 281 assumed that all photoemission angles are of equal probability for a random surface. A correction for the effect of surface roughness on photoelectron distributions was developed, although use of this approach has not been demonstrated. Ebel and Wernisch [ 291 modelled the angular dependence of photoelectron ejection by considering a surface composed of differently oriented cubes. They suggest roughness depth criteria within which angle-resolved methods may be applied to rough surfaces. Recent instruments have angular response functions which are more linear than that of the PHI 548, making the application of angle-resolved methods of particular interest.

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