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Surface and depth profiling techniques using XPS applied to the study of nickel-containing environmental particles
Applied Surface Science 68 (1993) 375-393 North-Holland
applied surface science
Surface and depth profiling techniques using XPS applied to the study of nickel-containing environmental particles J.T. R i c k m a n * Hoechst Celanese Corporation, Sunette Division, 25 Worlds Fair Dr., Somerset, NJ 08873, USA
and R.W. Linton Department of Chemistry, Unicersity of North Carolina, CB No. 3290, Chapel Hill, NC 27599, USA Received 11 November 1992; accepted for publication 21 January 1993
The objective of this study was to characterize the surface chemistry of micrometer-sized particles to determine their potential environmental impact. XPS (X-ray photoelectron spectroscopy) was employed to differentiate between the surface and bulk components of nickel-containing atmospheric pollutants from industrial sources. Three types of m e a s u r e m e n t s using XPS were used to access compositional information progressively deeper into the particle interiors. AR-XPS (angle-resolved XPS) was used to obtain a nondestructive depth profile of the outer surface ( ~ 10 rim). The composition was fairly homogeneous, including large concentrations of NiSO 4. Selective solvent leaching was then used to remove the NiSO 4 and access information at greater depths than available using A R - X P S of the native surfaces. Citrate leaching resulted in removal of most of the NiSO 4 and revealed other nickel-containing species present (e.g. sulfides). Ion beam sputtering also was used to remove the outer surface layers of the particles and obtain XPS information about the composition of particle interiors. After ion beam sputtering, XPS results were obtained in close agreement with bulk analysis results obtained using traditional wet chemical methods.
1. Introduction
In order to ascertain the biological and health effects of airborne particles, it is essential to acquire quantitative chemical speciation information, including data on the surface versus bulk composition. It is the surface which is the point of direct contact with the environment and thereby influences both toxicity and chemical reactivity. This research was part of an Environmental Protection Agency (EPA) cooperative project involving the speciation of particulate compounds, especially those containing nickel, emitted from industrial sources. The goal of this project was to * To whom correspondence should be addressed.
use surface analysis techniques to differentiate between surface versus bulk chemical components in environmental particles. Such information influences biological impact, but is not normally available in environmental analytical investigations. Information on the chemical forms of nickel m air is virtually nonexistent. Gilman [1] did determine in his study of certain types of nickel refinery samples that they contained 20% NiSO 4 • 7 H 2 0 , 59% Ni3S 2 and 6.5% NiO. There have been no studies on these compounds after they have been released into the air. It appears that the least soluble forms of nickel such as Ni3S 2 (derived from mining ore), Ni powder, NiO and Ni(OH) 2 are carcinogenic, while the most soluble such as NiS, NiC12 and NiSO 4 are not.
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
376
J.T. Rickman, R.W. Linton / Characterization o['nickel-containing em,ironmentalparticles
In order to distinguish the surface versus the bulk chemical components of the nickel-containing particles, XPS was used in three distinct modes. First, nondestructive depth profiles were acquired of the outer < 10 nm of the particles via angle-resolved XPS (AR-XPS). By changing 0 (angle of photoelectron ejection) from 15 ° (grazing angle and most surface-sensitive measure) in steps to 90 ° (most bulk-sensitive measure), quantitative elemental as well as chemical speciation information can be obtained with depth into the sample in a nondestructive manner. Young and McCaslin [2] have used AR-XPS in the study of compacted lead ion selective m e m b r a n e powders, They found that roughness had a negligible effect on the intensity ratios with a relative uncertainty of ~ 11% obtained using AR-XPS when comparing results obtained using experimental sensitivity factors to those derived theoretically. However, it is the degree of roughness which determines the extent of its effect. According to Fadley et al. [3], when the roughness is on the order of the attenuation length or larger of either the incident X-rays or the escaping electrons, shadowing occurs and the effects of roughness are seen. Fadley et al. report, in a review of the theoretical and experimental aspects of AR-XPS, that surface enhancement will be less at grazing angles for rough compared to flat surfaces, but still significant. The focus of this study is a relative comparison of semi-quantitative results obtained using three different modes of depth profiling. Quantitative XPS of rough surfaces has been studied by others and reviewed by Fulghum and Linton [4]. The second technique used to differentiate surface versus bulk components was selective solvent leaching coupled with XPS. This allowed the study of the surface predominance of NiSO 4 by obtaining XPS data before and after extracting the NiSO 4 from the particles. Other nickel species were thereby revealed whose presence were masked by the NiSO a surface layer. Thirdly, XPS was combined with ion bombardment in order to access information deeper into the particle interiors. This was done by collecting XPS data before and after sputtering the outer particle surface using an argon ion beam. Sputtering combined with XPS has been used previously
to study environmental particles, for example by Campbell et al. [5], Cabaniss and Linton [6] and Kaufherr et al. [7].
2. Materials and methods 2.1. Sample collection
The samples were collected from a process in the Fluidized Bed Roaster Building at one of the world's largest nickel refineries, INCO, Ltd. in Ontario, Canada. The refinery emissions contain a variety of detectable nickel species, including a suspected carcinogen, Ni3S 2 (nickel subsulfide). Mineral ores are first mechanically separated and the NiS (nickel sulfide) portion is introduced into a holding tank as a slurry. It is then dry filtered, and the residue is made into pellets and fed into the base of the roaster at 2200°F. Next, the particulates in the exhaust gas pass through a cyclone where the large dust particles are sent back to the base of the roaster. The small particle fraction passes on to an electrostatic precipitator where the product, consisting mainly of NiO, is recovered to be used in other processes. Anything not collected by the precipitator is sent to a common stack and vented to the atmosphere. The Fluidized Bed Roaster (FBR) Dust sample was collected from the area leading to the electrostatic precipitator. 2.2. General sample preparation and analysis conditions for X P S
Standards and samples analyzed as received were pressed in 0.25 mm thick indium foil (Alfa Products). The XPS instrument used in these studies was a Perkin-Elmer Physical Electronics Industries 5400 spectrometer. Typical XPS analysis conditions were: magnesium anode operated at 400 W power with an X-ray voltage of 15 kV and a pressure of 2 x 10 -9 Torr in the analysis chamber. Various apertures were used to get reasonable signal-to-noise: 1 x 3 mm, 1 × 1 ram, or a 600 /_~m diameter spot. The take-off angle between sample and spectrometer was 45 °. Survey scans 1000 eV wide were collected for 5 min
J.T. Rick•an, R.W. Linton / Characterization of nickel-containing eneironmental particles
with a resolution of 1 e V / s t e p , 25 m s / s t e p and a pass energy of 89.45 eV. Higher resolution windows for Ni 2p, O ls, C ls and S 2p were collected at 855, 532, 284 and 164 eV binding energies, respectively, with corresponding window widths of 50, 25, 25 and 25 eV. The time to collect a set of windows was ~ 20 min. The resolution was 0.1 e V / s t e p , 50 m s / step using a pass energy of 35.74 eV. Three methods of charge compensation were used, as appropriate: binding energy referencing to C ls at 284.6 eV; using low-energy electrons (1.0-2.4 eV) to flood sample; or binding energy referencing to In3ds/2 (mounting substrate) at 445.0 eV.
377
set of samples using the following procedure, as developed by Zatka [8] to speciate nickel compounds in industrial dusts. First, a 0.1M ammonium citrate solution was prepared by dissolving 8.5 g of diammonium citrate and 2.6 g of citric acid monohydrate in distilled water and diluting to 500 ml. The p H of the solution was then checked and adjusted as necessary to 4.4 using citric acid or ammonia. A citrate wash solution was prepared by diluting the ammonium citrate solution by a factor of 10. Next, approximately 10 mg of sample was placed on a moistened PVC m e m b r a n e in a Biichner funnel on a vacuum filter flask and wetted using a few drops of methanol. Then 5 ml of the ammonium citrate solution was added and gentle suction applied over a 5 - 1 0 min period until the leach solution had passed through the filter. The sample was next washed dropwise with no more than 3 ml of the citrate wash solution. The sample was then allowed to air-dry and pressed in indium foil for XPS analysis.
2.3. Specific sample preparation and analysis conditions for three modes of XPS 2.3.1. AR-XPS Five size fractions of NiS standard particles were obtained using a Sonic Sifter (Allen Bradley-Model 3LP): < 20, < 30, < 63, < 150 and > 150 /.Lm. XPS data was collected at eight different take-off angles: 15 °, 20 °, 25 °, 30 °, 35 °, 40 °, 70 ° and 90 ° .
2.3.3. Ion beam sputtering Standards and samples were sputtered using argon ions at 3 keV for 15 • i n with a 100% raster or alternatively for 12 min with a 50% raster with an ion beam current of ~ 5 - 6 /xA (standard sputter cleaning conditions).
2.3.2. Selective soh, ent leaching Selective solvent leaching to remove water soluble nickel salts was performed to give a second
NiS standard 1 O0 90 80
I
70 60 ._= E x
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NiS as received
50 < 20micron NiS
40 30 20 10 0 15
20
25
30
35
40
70
90
Take-off angle (degrees) Fig. 1. Plot of percent maximum photoelectron intensity versus take-off angle for sulfur in NiS as received and the < 20 p.m NiS.
J.T. Rickman, R.W. Linton / Characterization of nickel-containing environmental particles
378
Table 1 XPS data for NiS before and after leaching Sample
Ni 2P3/2
O ls
S 2p
BE ~)
%ac b)
FWHM c)
BE
%ac
FWHM
BE
%ac
FWHM
NiS
857.4
31.8
~ 6
533.6 531.5
22.0 33.2
2.50 2.31
Leached NiS
855.9 853.0
35.8 3.7
531.2
36.9
2.22
170.4 168.4 163.7 162.3 161.0 167.8 163.3 162.1 161.0
4.2 2.6 3.4 1.6 1.1 2.9 6.3 7.9 6.5
2.90 2.48 3.16 1.79 1.38 3.80 d) 2.80 1.46 1.10
3.40 1.59
a~ BE = binding energy in eV (referenced to C ls at 284.6 eV). b~ %ac = percent atomic concentration. o F W H M = full-width at h a l f - m a x i m u m in eV. d) Broad peak of Low intensity not suitable for curve-fitting.
3. Results and discussion: nickel-containing standards 3.1. AR-XPS
The NiS standard and two size fractions of NiS ( < 20 and > 150 txm) were analyzed by XPS. A plot of sulfur (nickel is similar) photoelectron intensity versus take-off angle (fig. 1) for all sizes
combined and the < 20/xm size range (the > 150 /xm size range is similar) summarizes the results. The plot in fig. 1 shows a relatively uniform spectrometer response at intermediate angles (25°-40°). The angular response, in general, is not expected to be completely flat (even for a flat surface), because of nonuniformity in X-ray illumination of, and angular acceptance of photoelectrons from, the analysis area over the range
Table 2 Binding energies (in eV) f r o m literature a~ for nickel, sulfur a n d / o r oxygen-containing standards Standard
Ni 2P3/2
O ls
Ni(OH) 2 SO 3 NiSO 4 SO: MSO 3 S
856.6, 855.8, 855.6
531.8, 531.7, 531.4
856.8
532.1
S 2p 171 169.2 168.1 167 b)
164.2
b)
163.9 ct
NiO~
Ni20 3 NiO NiO~,l~ NiS Ni~S z NiS~ Ni metal H 20 "~ bl o d)
857.3, 855.8 854.5, 854.3, 853.5 853.5 855.1,853.2, 852.8 854.1 d) ( A I K ~ )
531.8 529.9, 529.6 533.5 162.2 c) 162.1 dl
852.8, 852.7, 852.2 533.2
Values are from the Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics, u n l e s s o t h e r w i s e specified. From Craig et al. [9]. From Coyle [10]. From Ng and Hercules [11].
J. 22 Rickman, R. W. Linton / Characterization of nickel-containing ent,ironmental particles
of angles. For the NiS standard particles, this effect is evidenced by a decreased intensity at the lower and higher angles in both plots. This effect is lessened at the lower angles and enhanced at the higher angles when looking at a narrower size range. The greater decrease at higher angles for the sized fractions is probably due to the fact that the X-ray gun was positioned much closer to the sample when analyzing the two sized fractions (there were much higher intensities in general) than when the original standard was analyzed. Having the gun closer exaggerated the effects of stage tilt. Fortunately, sulfur and nickel behave similarly as a function of angle. Therefore, the percent atomic concentration, which normalizes the results, should reflect the concentration of elements in the sample with the same degree of accuracy regardless of angle.
3.2. Selective solvent leaching The NiS standard was analyzed before and after citrate leaching. The results are compiled in table 1. The NiS standard was chosen for this study in order to be consistent with the angular response study. The binding energies and F W H M (full-width at half-maximum intensity) values for the NiS standard can be compared to the literature values compiled in table 2. The binding energy and width of the nickel peak (857.4 eV, ~ 6 eV width), as well as the oxygen peak at 531.4 eV and the sulfur peak at 170.4 eV for the NiS standard, suggest that a NiSO 4 layer has grown on top of the NiS. This is not surprising since NiS is known to be unstable in air [11]. There appear to be two chemical forms of oxygen before leaching. One oxygen peak at 531.5 eV is possibly due to NiS(OH), which is known to be formed when NiS is exposed to air [12]. The other oxygen peak at 533.6 eV is probably from NiSO 4. Curve fitting of the peaks in the S2p window for the NiS standard results in five sulfur peaks. The sulfur peak at highest binding energy (170.4 eV) can be assigned to NiSO 4. The peak at 168.4 eV might indicate some NiS(OH), which would confirm the identification of the oxygen peak at 531.5 eV. The other three peaks represent three more re-
379
duced sulfur species (e.g. elemental S, NiS 2, NiS a n d / o r Ni3S2). Further confirmation of NiSO 4 is evidenced by shake-up peaks, which were present in the Ni2p window [13]. More detailed analysis of shake-up peaks for nickel-containing compounds (NiO, NiSO 4 and 68 other nickel compounds) is available in another source [14]. After leaching the NiS standard, there are two nickel peaks in the spectrum. The major one is at 855.9 eV, possibly due to a nickel/oxygencontaining compound; such compounds tend to have broader Ni2p peaks than do nickel/sulfurcontaining compounds. A second nickel peak at 853.0 eV is observed after leaching. Its binding energy is very close to the expected value for NiS (compare to literature values in table 2). Its narrow F W H M (1.59 eV) also indicates a nickel sulfide species. The oxygen peak at higher binding energy (previously assigned to NiSO 4) disappears as does the sulfur peak at 170.4 eV after leaching. This result is consistent with a significant loss of NiSO 4 species into the extracting solution. These results indicate that the citrate leach is successful. This is of interest since one of the goals of this project is assessing the reliability of wet chemical techniques routinely used to chemically speciate nickel compounds from industrial sources and proposed for use by the Environmental Protection Agency in environmental monitoring applications. Partial extraction of NiSO 4 would underestimate its concentration based only on wet chemical analysis results. Additionally, these results indicate that the surface composition of the standards (i.e. 99.99% NiS) differs markedly from bulk values.
3.3. Ion beam sputtering The nickel standards analyzed are listed in table 3. The results before and after sputtering are summarized in table 4. The only nickel standard that had reasonably close surface atomic concentrations and binding energies before sputtering compared to bulk speciation and stoichiometry was N i S O 4 . 7 H 2 0 . The nickel sulfide standards all have substantial oxygen surface contamination.
380
J.T. Rickman, R.W. Linton / Characterization of nickel-containing em~ironmental particles
Table 3 Source of nickel standards used in sputtering experiments Compound
Purity (%)
Source
Ni(OH) 2 NiSO4.7H20 a) Ni203 - H 2 0 NiO2 Ni(I|)O Ni(II)S Ni3S 2 NiS 2 Ni metal
97 99.999 0.9 H 2 0 _ b) 99.99 99.99 _ b~ 99.99 99.999
Aldrich Chemical Co., Inc. Aldrich Chemical Co., Inc. Alfa Products Alfa Products Aldrich Chemical Co., Inc. Alfa Products INCO, Ltd. Alfa Products Alfa Products
~" Sufficient vacuum for XPS analysis and subsequent ion sputtering could only be attained after drying the prepared NiSO4.7H20 standard in a vacuum oven at 60°C for 24 h. bl Not available.
Sputtering appears to affect the standards in one of the three following ways. The nickel peak is broadened (see table 4 for F W H M ) and a second oxygen peak appears (e.g. in the cases of Ni(OH) 2 and N i S O 4 - 7 H 2 0 ) . Prior studies by Christie et al. [15] indicate that metal sulfates might undergo a dissociation reaction upon ion beam bombardment. The XPS results indicate this as well: NiSO 4 -~ NiO(s) + SO2(g ) + ½02(g). In the second class of sputter-induced changes to the nickel standards, the width of the nickel peak either stays approximately the same or broadens, but no new oxygen peaks appear. There is, however, a decrease in concentration of oxygen at the higher binding energies (e.g. the oxides). The oxides appear to be least affected by sputtering. This is contrary to the results of Kim et al. [16] who found that oxides undergo reduction upon argon ion bombardment. It is possible that differences in ion beam dose in Kim's study versus this study explain the apparent discrepancies. The third way sputtering affects some nickel standards is that the nickel peak is narrowed and there is a loss of sulfur at the higher binding energy with a concomitant increase of sulfur concentration at the lower binding energies (e.g. in the case of the sulfides. Eventually this could result in total decomposition to elemental Ni and
S as Coyle et al. [10] found with transition-metal sulfides. A further discussion of the results from the sputtering of nickel standards is presented in another source with pertinent spectra included [13]. The important findings pertaining to this study are that the standards are inhomogeneous and sputtering causes changes which cannot be attributed simply to removal of contamination layers.
4. Results and discussion: FBR Dust
4.1. A R - X P S
Examples of raw spectra from the angle-resolved XPS studies are shown in fig. 2. The spectra look very similar at all eight angles. The semi-quantitative results for the FBR Dust at the eight different angles are compiled in table 5. The measured binding energies for nickel, sulfur and oxygen do not change significantly with take-off angle with the exception of oxygen at 0 = 90 °. That is, there is no appreciable chemical change with depth. The binding energies for nickel, sulfur and oxygen correspond to NiSO 4. However, at 0 = 90 °, the appearance of a small, second oxygen peak is observed at a lower binding energy. At this depth into the sample, it appears that more of an underlying oxide component in the particles is being sampled. A plot of C, O and Ni concentration versus 0 in fig. 3, indicates a greater concentration of carbon at the surface (more grazing angles) with a corresponding depletion of nickel, thus indicating a carbon contamination layer on top of the nickel. The concentration of oxygen at the higher binding energy (sulfate oxygen) is slightly decreasing with depth which suggests sulfate enrichment at the surface. An additional plot of concentration versus 0 (fig. 4) for Ni and two types of sulfur (sulfate and other sulfur species), indicates a decrease in the sulfate sulfur signal at 0 = 90 °, consistent with the decreasing sulfate oxygen signal. The other sulfur signal appears to be depleted at the surface (more grazing angles) and slightly higher at 0 = 90 °.
Table 4 X P S d a t a o n nickel s t a n d a r d s b e f o r e a n d a f t e r s p u t t e r i n g Standard
O ls
N i 2 P 3 / 2 ~)
c.a)
B E b)
%ac
859.4 859.0
37.5 56.9
2.765 4.785
858.1 ~ 856.9
16.1 47.2
3.864 ~ 5
NiO 2
855.2
39.7
4.327
NiO 2 *
854.5
54.6
4.735
Ni20 3 .xH20
853.9
51.9
4.263
854.5
68.6
NiO
854.1
52.3
NiO *
854.9
65.3
NiS 2
853.6
19.4
5.101
NiS 2 *
852.9
55.9
2.433
NiS
857.4
31.8
NiS *
852.7
65.0
Ni(OH) 2 Ni(OH) 2 * NiSO 4 . 7H20 NiSO 4 . 7H20
NieO 3 .xH20
*
*
FWHM
~ 7.5 4.387 ~ 7
~ 6
1.849
S 2p
BE
%ac
FWHM
534.5 535.7 534.0 532.9 532.4 530.5 530.8 528.9 531.2 529.4 530.9 529.0 531.5 529.8 531.1 529.4 531.5 530.0 531.8
52.2 14.3 28.9 64.0 22.9 23.3 30.1 13.0 17.9 23.0 16.5 25.4 6.6 24.0 21.2 26.5 9.7 24.9 27.8
1.702 1.73 1.78 2.406 2.15 2.28 2.13 1.31 1.87 1.62 1.85 1.24 1.68 1.46 2.43 1.28 2.14 1.45 2.316
531.3 529.3
5.3 0.8
1.95 1.05
533.6 531.5
22.0 33.2
2.50 2.31
530.7
4.7
-
-
22.1 1.8 2.7 1.6 2.8 6.0 12.9 7.5 -
2.62 5.06 0 2.19 1.24 1.49 5.49 f~ 2.20 1.55 -
48.1
2.629
32.4
5.021
533.5 531.7
23.8 34.9
2.30 2.28
Ni3S 2 *
852.7
61.6
1.957
531.1
11.9
2.40
Ni m e t a l
856.1
45.4
5.762
531.6
43.9
2.38
2.5
529.6 529.9
10.7 11.2
1.35 1.626
.
-
162.? 168.7 164.7 163.6 162.3 165.2 163.2 162.1 -
857.8
.
-
-
2.0 1.26 6.2 f~
Ni3S 2
.
-
2.725 2.30 2.90 -
14.9 8.0 8.0
2.777
.
-
-
162.4 161.3 167.0
17.9
.
14.0 2.0 4.7 -
FWHM
-
531.5
S
-
2.86 1.71 1.00 5.60 0 1.93 1.30 1.15 2.90 2.48 3.16 1.79 1.38 6.09 f~
5.247
88.8
169.2 169.0 167.1 -
%ac
6.7 16.1 11.2 6.1 8.1 13.3 10.5 4.2 2.6 3.4 1.6 1.1 7.4
52.0
852.8
-
168.8 163.1 162.0 166.5 163.3 162.1 161.0 170.4 168.4 163.7 162.3 161.0 165.2
853.4
Ni m e t a l *
BE
161.5
-
* Sputtered. a) N o c u r v e fitting w a s u s e d f o r t h e Ni p e a k b e c a u s e o f (1) the t r a d e - o f f in t i m e v e r s u s u s e f u l n e s s o f i n f o r m a t i o n o b t a i n e d a n d (2) t h e difficulty in d e t e r m i n i n g t h e c o r r e c t n u m b e r o f p e a k s t h a t m a k e u p t h e Ni 2 p c u r v e w i t h o u t m a k i n g a s s u m p t i o n s a b o u t t h e Ni s p e c i e s p r e s e n t . M a t i e n z o et. al. [14] f o u n d t h a t N i S O 4 h a s o n e s h a k e - u p satellite f o r t h e 2P3/2 a n d t w o satellites f o r t h e 2Pl/2. N i O h a s o n e s h a k e - u p satellite f o r b o t h t h e 2 P 3 / 2 a n d 2 P l / 2 p e a k s . A m i x t u r e o f t h e s e t w o c o m p o u n d s w o u l d b e i m p o s s i b l e to deconvolute. b~ c) d~ e~ 0
N i 2 P 3 / 2 : o - = _+0.5 eV; O l s : o - = + 0 . 2 eV; S 2 p : o - = _+0.1 eV. Sensitivity f a c t o r s u s e d in c a l c u l a t i n g % a c : 3.653 f o r N i 2 p , 0.711 f o r O is, 0.57 f o r S 2 p a n d 0.296 f o r C ls. R e m a i n i n g % a c d u e to a d v e n t i t i o u s c a r b o n . U s e d e l e c t r o n f l o o d g u n to c o m p e n s a t e f o r s a m p l e c h a r g i n g . B r o a d p e a k o f low intensity. N o t s u i t a b l e f o r c u r v e fitting.
Z T. Rickman, R. IV.. Linton / Characterization of nickel-containing environmental particles
382
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Figs. 2d-2f. Raw spectra from the AR-XPS experiment on F B R Dust (take-off angle 40°): (d) In 3d window, (e) C Is window, (f) S 2p window.
J. T. Rickman, R. W. Linton / Characterization of nickel-containing environmental particles
384
Table 5 AR-XPS results ") on FBR Dust Angle
Ni 2P3/2
(deg)
BE b)
%ac
FWHM
BE
0 ls %ac
FWHM
S 2p BE
%ac
F WH M
15
856.4
12.6
3.929
531.5
46.7
2.736
20
856.5
10.4
3.626
531.9
47.4
2.674
25
856.4
13.9
4.021
531.6
46.5
2.574
30
856.8
15.5
4.1 ! l
532.0
46.4
2.562
856.1
16.7
4.349
531.4
46.1
2.616
40
856.4
16.3
4.226
532.0
45.9
2.506
70
857.0
16.0
3.982
532.0
44.8
2.651
90
856.3
16.8
3.580
531.5 527.9
41.8 4.3
2.882 -
6.5 1.1 6.3 1.4 6.8 1.6 6.4 1.7 6.7 1.2 6.4 1.5 6.6 1.9 5.4 1.5
2.78 2.68 2.82 2.64
35
168.5 Other c) 168.9 Other 168.7 Other 169.0 O t he r 168.4 O t he r 168.8 O t he r 168.9 Other 168.5 Other
2.65 2.68 2.69 2.87
~) These results do not include approximately 3% Cu and < 1% Ca detected in other XPS analysis of the FBR Dust. R e m a i n d e r of %ac due to adventitious C. b) Binding energy referencing to C l s at 284.6 eV. c) Other signifies a broad S component of low intensity whose binding energy would be difficult to assign.
photoelectrons on depth of analysis. The attenuation lengths for Ni2P3/2, O l s and S2p photoelectrons escaping from a bulk NiO matrix can be calculated using the following equation [17]:
This result gives further support to speculation that the FBR Dust particles consist of a surface enriched in NiSO4, with other nickel-containing species being surface depleted. The limitations of angle-resolved XPS include potential topographic effects on photoemission and the restraint imposed by the escape depth of
Am =
2170E -2 +
0.72(aE) 1/2,
where Am is the attenuation length (monolayers),
Fluidized Bed Roaster Dust (FBR Dust) 50 45
~ 40 ~
35
•
c
~ 30 E ~ 25
O
-~ 20 E
• _ _ _ _
~ ~s
N
Ni
--4
10 5 I
0 15
20
-
i
2,5
30
I
i
i
35
40
70
90
Take-off angle (degrees)
Fig. 3. Plot of C, O and Ni concentrations versus 0 for AR-XPS of F B R Dust.
J.T. Rickman, R.W. Linton / Characterization of nickel-containing environmental particles
385
Fluidized Bed Roaster Dust (FBR Dust) 18
fjll~
16 tO fc O U
14
~
•
12 10
.o
8
E
6
o
jr
+ --*
Ni S-sulfate S-other
4 2 0
5
20
25
30
35
40
70
90
Take-off angle (degrees) Fig. 4. Plot of atomic concentration versus 0 for Ni and two types of sulfur.
E the kinetic energy of the photoelectron (eV), and a the monolayer thickness (nm). The monolayer thickness can be calculated as shown below:
Table 6 Attenuation lengths for Ni, O and S in NiO Element
Peak
E (eV)
Am (ML)
A.(nm)
a 3 = Z/(pnN)
Ni O S
2P3/2 ls 2p
398.6 722.6 1088.6
6.611 8.887 10.905
1.393 1.872 2.297
1024,
where A is the molecular weight (g/mol), p the bulk density (kg/m3), n the number of atoms in a
Table 7 XPS data on FBR Dust Ni2P3/2
O ls
BE
%ac ")
FBR Dust h)
856.4
16.3
Leached FBR Dust b)
855.7
19.8
Sputtered FBR Dust ¢)
856.0
51.7
FWHM
S 2p
BE
%ac
FWHM
BE
%ac
FWHM
4.226
532.0
45.9
2.506
4.277
533.7 532.1
18.8 26.1
2.41 2.39
531.5 529.8
17.3 24.9
2.25 2.14
168.8 Other 170.6 166.6 163.7 161.5 169.3 167.2 165.4 162.5 160.9
6.4 1.5 1.5 0.6 1.0 0.6 0.5 1.8 0.7 1.6 1.6
2.68 2.85 2.79 2.40 2.15 2.29 2.51 1.86 2.20 2.00
~ 7
") Remaining %ac due to adventitious C. b) Binding energy referencing to C ls at 284.6 eV. c) Charge compensation using low-energy electrons to flood sample.
386
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J.T. Rickman, R.W. Linton / Characterization of nickel-containing environmental particles
molecule of the bulk material, and N Avogadro's number. To get the attenuation length in nanometers (An), the following calculation is used:
4.2. Selective soluent leaching Raw spectra for the FBR Dust are shown in fig. 2. Raw spectra and curve fits for the leached F B R Dust can be found in fig. 5. The binding energies, percent atomic concentrations and F W H M before and after leaching are compiled in table 7. Citrate leaching resulted in removal of 75% of the NiSO 4 present as evidenced by the decrease in intensity of the sulfur peak at 168.8 eV. Other sulfur species were revealed as well at 166.6,
A n = a A m.
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v
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388
J.T. Rickman, R.W. Linton / Characterization of nickel-containing enuironmentalparticles
citrate leach solution is maintained at a proper pH using ammonia. It is also hard to quantify the efficiency of the leaching process within the particles. Ion beam sputtering represents an additional alternative to avoid some of these potential problems with selective solvent leaching and to access information deeper into the particle interior than is possible using angle-resolved XPS.
163.7 and 161.5 eV. Possible species assignment would be NiSO (nickel sulfite) at 166.6 eV, S (elemental sulfur) at 163.7 eV, and a sulfide at 161.5 eV. The leach for NiSO 4 was not complete possibly because the NiSO 4 was re-deposited on the surface of the particles upon drying. It is difficult to assess the chemical/physical effects of the leach on the particles such as how selective the citrate leach is for NiSO 4 and the accessibility and solubility of other species in the extracting solution. The fact that Ni(OH) 2 is known to be soluble in ammonia forming [Ni(NH3)6] 2+ [12] would be a problem if Ni(OH) 2 were in the sample since the I
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Raw spectra for the as-received FBR Dust are shown in fig. 2. Raw spectra and curve fits for the sputtered F B R Dust can be found in fig. 6. The [
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390
J.T. Rickman, R.W. Linton / Characterization of nickel-containing enuironmental particles
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J. T. Rickman, R. W. Linton / Characterization of nickel-containing environmental particles
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Table 8 Atomic percentages obtained from XPS data collected at 40 ° tilt angle on FBR Dust
Ni O S-total S-sulfate S-sulfite S-elemental S-sulfide S-other C
As received
Leached
Sputtered
16.3 45.9 7.9 6.4 < 0.5 < 0.5 < 0.5 1.5 29.9
19.8 44.9 3.7 1.5 0.6 1.0 0.6 < 0.5 31.6
51.7 42.2 6.1 0.5 1.8 0.7 3.2 < 0,5 < 0.9
binding energies, percent atomic concentrations and F W H M before and after sputtering are compiled in table 7. Sputtering results in a change in the proportions of surface sulfur species present. The peak at 167.2 eV corresponds to NiSO 3 (nickel sulfite) which may have been reduced from NiSO 4 (169.3 eV), as observed in the NiSO4 standard after sputtering. The peak at 165.4 eV is probably elemental sulfur. The sulfur peaks at 162.5 and 160.9 eV can be assigned to the sulfides. They are more likely reflective of nickel sulfide species
392
J.T. Rickman, R.W. Linton / Characterization of nickel-containing ent'ironmental particles
inside the particles rather than ion beam reduction of sulfate to sulfide species since the NiSO 4 • 7 H 2 0 standard did not show ion beam reduction to sulfide under the sputter conditions used. The new oxygen peak is probably NiO that has been produced through dissociation of NiSO 4 by the ion beam as appeared to happen with the standard or it could be NiO within the particle. The loss of NiSO 4 is much less than the growth of the NiO peak. Thus, it is likely that much of the NiO reflects species inherent to the particle interiors. This is also consistent with bulk analyses (wet chemical techniques and X-ray powder diffraction) indicating the major component is oxide. 4.4. Summary o f results and discussion on FBR Dust A summary of the results from all three techniques is shown in table 8. Using AR-XPS (results under as received in table 8), a contaminant carbon layer on top of NiSO 4 was observed. At the greatest analysis depth (0 = 90°), a hint of a decrease in NiSO 4 and a new oxygen-containing species is seen. Because of the limited depth of analysis, selective solvent leaching was used to remove the NiSO 4 and facilitate detection of other nickel species present. This experiment was only partially successful and was limited by the uncertainties of selectivity, solubility and surface re-deposition in using the citrate leach. Finally, ion beam sputtering was used to access information deeper into the particle interior. The carbon overlayer was removed by sputtering and a depth was reached in the sample where one-half of the sulfur is sulfide and the other half is NiSO 3, elemental sulfur (probably produced by the reduction of NiSO 4 by the ion beam) and a little NiSO 4 that was not reduced. Jach and Powell [18] in a similar study of environmental particles using XPS also detected sulfate on unsputtered particles and sulfide after sputtering. The XPS results can be compared to bulk ICP (inductively coupled plasma) results by normalizing to nickel (table 9). Sputtering appears to probe most deeply into the bulk since the results compare very favorably with those obtained by ICP bulk analysis. With AR-XPS and leaching,
Table 9 Atomic concentrations normalized to Ni for FBR Dust XPS
O/Ni S/Ni C/Ni
ICP
As received
Leached
Sputtered
2.82 0.48 1.83
2.27 0.19 1.60
0.82 0.12 -
0.86 0.13
sulfur and oxygen ratios greater than those obtained using ICP are observed, indicating that the surface enriched NiSO 4 layer is being sampled. Through our study of standards and correlation of techniques, better evaluation of the chemical species present in the surface versus the bulk components of these particles is possible. From a biological perspective, the presence of a surface layer enriched in soluble NiSO 4 suggests that environmental impact assessments based on particle standards assumed to be homogeneous may be seriously in error. The surface properties of both standard and environmental particle samples must be considered in more detail.
Acknowledgements The authors gratefully acknowledge the support of F.E. Butler and the US Environmental Protection Agency which funded this project under cooperative agreement CR-812908-01-1.
References [1] J.P.W. Gilman, Cancer Res. 22 (1962) 158. [2] V. Young and P.C. McCaslin, Anal. Chem. 57 (1985) 485. [3] C.S. Fadley, R.J. Baird, W. Siekhaus, T. Novakov and S.A.L Bergstrom, J. Electron Spectrosc. Rel. Phen. 4 (1974) 93. [4] J.E. Fulghum and R.W. Linton, Surf. Interf. Anal. 13 (1988) 186. [5] J.A. Campbell, R.D. Smith and L.E. Davis, Appl. Spectrosc. 32 (1978) 316. [6] G.E. Cabaniss and R.W. Linton, Environ. Sci. Technol. 18 (1984) 271. [7] N. Kaufherr, M. Shenasa and D. Lichtman, Environ. Sci. Technol. 19 (1985) 609. [8] V.J. Zatka, Chemical Speciation of Nickel Phases in Industrial Dusts (INCO, Ontario, Canada, 1987).
J.T. Rickman, R.W. Linton / Characterization of nickel-containing environmental particles [9] N.L. Craig, A.B. Harker and T. Novakov, Atmos. Environ. 8 (1974) 15. [10] G.J. Coyle, T. Tsang and I. Adler, J. Electron Spectrosc. 20 (1980) 169. [11] K.T. Ng and D.M. Hercules, J. Phys. Chem. 80 (1976) 2094. [12] A.G. Sharpe, Inorganic Chemistry (Longman, New York, 1981) p. 596. [13] J.T. Rickman, PhD Dissertation, University of North Carolina at Chapel Hill (1989) p. 105.
393
[14] LJ. Matienzo, L.O. Yin, S.O. Grim and W.E. Swartz, Inorg. Chem. 12 (1973) 2764. [15] A.B. Christie, J. Lee, I. Sutherland and J.M. Walls, Appl. Surf. Sci. 15 (1983) 224. [16] K.S. Kim, J.W. Amy Baitinger and N. Winograd, J. Electron Spectrosc. Rel. Phen. 5 (1974) 351. [17] M.P. Seah and W.A. Dench, Surf. Interf. Anal. 1 (1979) 2. [18] T. Jach and C.J. Powell, Environ. Sci. Technol. 18 (1984) 58.
applied surface science
Surface and depth profiling techniques using XPS applied to the study of nickel-containing environmental particles J.T. R i c k m a n * Hoechst Celanese Corporation, Sunette Division, 25 Worlds Fair Dr., Somerset, NJ 08873, USA
and R.W. Linton Department of Chemistry, Unicersity of North Carolina, CB No. 3290, Chapel Hill, NC 27599, USA Received 11 November 1992; accepted for publication 21 January 1993
The objective of this study was to characterize the surface chemistry of micrometer-sized particles to determine their potential environmental impact. XPS (X-ray photoelectron spectroscopy) was employed to differentiate between the surface and bulk components of nickel-containing atmospheric pollutants from industrial sources. Three types of m e a s u r e m e n t s using XPS were used to access compositional information progressively deeper into the particle interiors. AR-XPS (angle-resolved XPS) was used to obtain a nondestructive depth profile of the outer surface ( ~ 10 rim). The composition was fairly homogeneous, including large concentrations of NiSO 4. Selective solvent leaching was then used to remove the NiSO 4 and access information at greater depths than available using A R - X P S of the native surfaces. Citrate leaching resulted in removal of most of the NiSO 4 and revealed other nickel-containing species present (e.g. sulfides). Ion beam sputtering also was used to remove the outer surface layers of the particles and obtain XPS information about the composition of particle interiors. After ion beam sputtering, XPS results were obtained in close agreement with bulk analysis results obtained using traditional wet chemical methods.
1. Introduction
In order to ascertain the biological and health effects of airborne particles, it is essential to acquire quantitative chemical speciation information, including data on the surface versus bulk composition. It is the surface which is the point of direct contact with the environment and thereby influences both toxicity and chemical reactivity. This research was part of an Environmental Protection Agency (EPA) cooperative project involving the speciation of particulate compounds, especially those containing nickel, emitted from industrial sources. The goal of this project was to * To whom correspondence should be addressed.
use surface analysis techniques to differentiate between surface versus bulk chemical components in environmental particles. Such information influences biological impact, but is not normally available in environmental analytical investigations. Information on the chemical forms of nickel m air is virtually nonexistent. Gilman [1] did determine in his study of certain types of nickel refinery samples that they contained 20% NiSO 4 • 7 H 2 0 , 59% Ni3S 2 and 6.5% NiO. There have been no studies on these compounds after they have been released into the air. It appears that the least soluble forms of nickel such as Ni3S 2 (derived from mining ore), Ni powder, NiO and Ni(OH) 2 are carcinogenic, while the most soluble such as NiS, NiC12 and NiSO 4 are not.
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
376
J.T. Rickman, R.W. Linton / Characterization o['nickel-containing em,ironmentalparticles
In order to distinguish the surface versus the bulk chemical components of the nickel-containing particles, XPS was used in three distinct modes. First, nondestructive depth profiles were acquired of the outer < 10 nm of the particles via angle-resolved XPS (AR-XPS). By changing 0 (angle of photoelectron ejection) from 15 ° (grazing angle and most surface-sensitive measure) in steps to 90 ° (most bulk-sensitive measure), quantitative elemental as well as chemical speciation information can be obtained with depth into the sample in a nondestructive manner. Young and McCaslin [2] have used AR-XPS in the study of compacted lead ion selective m e m b r a n e powders, They found that roughness had a negligible effect on the intensity ratios with a relative uncertainty of ~ 11% obtained using AR-XPS when comparing results obtained using experimental sensitivity factors to those derived theoretically. However, it is the degree of roughness which determines the extent of its effect. According to Fadley et al. [3], when the roughness is on the order of the attenuation length or larger of either the incident X-rays or the escaping electrons, shadowing occurs and the effects of roughness are seen. Fadley et al. report, in a review of the theoretical and experimental aspects of AR-XPS, that surface enhancement will be less at grazing angles for rough compared to flat surfaces, but still significant. The focus of this study is a relative comparison of semi-quantitative results obtained using three different modes of depth profiling. Quantitative XPS of rough surfaces has been studied by others and reviewed by Fulghum and Linton [4]. The second technique used to differentiate surface versus bulk components was selective solvent leaching coupled with XPS. This allowed the study of the surface predominance of NiSO 4 by obtaining XPS data before and after extracting the NiSO 4 from the particles. Other nickel species were thereby revealed whose presence were masked by the NiSO a surface layer. Thirdly, XPS was combined with ion bombardment in order to access information deeper into the particle interiors. This was done by collecting XPS data before and after sputtering the outer particle surface using an argon ion beam. Sputtering combined with XPS has been used previously
to study environmental particles, for example by Campbell et al. [5], Cabaniss and Linton [6] and Kaufherr et al. [7].
2. Materials and methods 2.1. Sample collection
The samples were collected from a process in the Fluidized Bed Roaster Building at one of the world's largest nickel refineries, INCO, Ltd. in Ontario, Canada. The refinery emissions contain a variety of detectable nickel species, including a suspected carcinogen, Ni3S 2 (nickel subsulfide). Mineral ores are first mechanically separated and the NiS (nickel sulfide) portion is introduced into a holding tank as a slurry. It is then dry filtered, and the residue is made into pellets and fed into the base of the roaster at 2200°F. Next, the particulates in the exhaust gas pass through a cyclone where the large dust particles are sent back to the base of the roaster. The small particle fraction passes on to an electrostatic precipitator where the product, consisting mainly of NiO, is recovered to be used in other processes. Anything not collected by the precipitator is sent to a common stack and vented to the atmosphere. The Fluidized Bed Roaster (FBR) Dust sample was collected from the area leading to the electrostatic precipitator. 2.2. General sample preparation and analysis conditions for X P S
Standards and samples analyzed as received were pressed in 0.25 mm thick indium foil (Alfa Products). The XPS instrument used in these studies was a Perkin-Elmer Physical Electronics Industries 5400 spectrometer. Typical XPS analysis conditions were: magnesium anode operated at 400 W power with an X-ray voltage of 15 kV and a pressure of 2 x 10 -9 Torr in the analysis chamber. Various apertures were used to get reasonable signal-to-noise: 1 x 3 mm, 1 × 1 ram, or a 600 /_~m diameter spot. The take-off angle between sample and spectrometer was 45 °. Survey scans 1000 eV wide were collected for 5 min
J.T. Rick•an, R.W. Linton / Characterization of nickel-containing eneironmental particles
with a resolution of 1 e V / s t e p , 25 m s / s t e p and a pass energy of 89.45 eV. Higher resolution windows for Ni 2p, O ls, C ls and S 2p were collected at 855, 532, 284 and 164 eV binding energies, respectively, with corresponding window widths of 50, 25, 25 and 25 eV. The time to collect a set of windows was ~ 20 min. The resolution was 0.1 e V / s t e p , 50 m s / step using a pass energy of 35.74 eV. Three methods of charge compensation were used, as appropriate: binding energy referencing to C ls at 284.6 eV; using low-energy electrons (1.0-2.4 eV) to flood sample; or binding energy referencing to In3ds/2 (mounting substrate) at 445.0 eV.
377
set of samples using the following procedure, as developed by Zatka [8] to speciate nickel compounds in industrial dusts. First, a 0.1M ammonium citrate solution was prepared by dissolving 8.5 g of diammonium citrate and 2.6 g of citric acid monohydrate in distilled water and diluting to 500 ml. The p H of the solution was then checked and adjusted as necessary to 4.4 using citric acid or ammonia. A citrate wash solution was prepared by diluting the ammonium citrate solution by a factor of 10. Next, approximately 10 mg of sample was placed on a moistened PVC m e m b r a n e in a Biichner funnel on a vacuum filter flask and wetted using a few drops of methanol. Then 5 ml of the ammonium citrate solution was added and gentle suction applied over a 5 - 1 0 min period until the leach solution had passed through the filter. The sample was next washed dropwise with no more than 3 ml of the citrate wash solution. The sample was then allowed to air-dry and pressed in indium foil for XPS analysis.
2.3. Specific sample preparation and analysis conditions for three modes of XPS 2.3.1. AR-XPS Five size fractions of NiS standard particles were obtained using a Sonic Sifter (Allen Bradley-Model 3LP): < 20, < 30, < 63, < 150 and > 150 /.Lm. XPS data was collected at eight different take-off angles: 15 °, 20 °, 25 °, 30 °, 35 °, 40 °, 70 ° and 90 ° .
2.3.3. Ion beam sputtering Standards and samples were sputtered using argon ions at 3 keV for 15 • i n with a 100% raster or alternatively for 12 min with a 50% raster with an ion beam current of ~ 5 - 6 /xA (standard sputter cleaning conditions).
2.3.2. Selective soh, ent leaching Selective solvent leaching to remove water soluble nickel salts was performed to give a second
NiS standard 1 O0 90 80
I
70 60 ._= E x
/
,/ /
//
\
"El\,~
• •
NiS as received
50 < 20micron NiS
40 30 20 10 0 15
20
25
30
35
40
70
90
Take-off angle (degrees) Fig. 1. Plot of percent maximum photoelectron intensity versus take-off angle for sulfur in NiS as received and the < 20 p.m NiS.
J.T. Rickman, R.W. Linton / Characterization of nickel-containing environmental particles
378
Table 1 XPS data for NiS before and after leaching Sample
Ni 2P3/2
O ls
S 2p
BE ~)
%ac b)
FWHM c)
BE
%ac
FWHM
BE
%ac
FWHM
NiS
857.4
31.8
~ 6
533.6 531.5
22.0 33.2
2.50 2.31
Leached NiS
855.9 853.0
35.8 3.7
531.2
36.9
2.22
170.4 168.4 163.7 162.3 161.0 167.8 163.3 162.1 161.0
4.2 2.6 3.4 1.6 1.1 2.9 6.3 7.9 6.5
2.90 2.48 3.16 1.79 1.38 3.80 d) 2.80 1.46 1.10
3.40 1.59
a~ BE = binding energy in eV (referenced to C ls at 284.6 eV). b~ %ac = percent atomic concentration. o F W H M = full-width at h a l f - m a x i m u m in eV. d) Broad peak of Low intensity not suitable for curve-fitting.
3. Results and discussion: nickel-containing standards 3.1. AR-XPS
The NiS standard and two size fractions of NiS ( < 20 and > 150 txm) were analyzed by XPS. A plot of sulfur (nickel is similar) photoelectron intensity versus take-off angle (fig. 1) for all sizes
combined and the < 20/xm size range (the > 150 /xm size range is similar) summarizes the results. The plot in fig. 1 shows a relatively uniform spectrometer response at intermediate angles (25°-40°). The angular response, in general, is not expected to be completely flat (even for a flat surface), because of nonuniformity in X-ray illumination of, and angular acceptance of photoelectrons from, the analysis area over the range
Table 2 Binding energies (in eV) f r o m literature a~ for nickel, sulfur a n d / o r oxygen-containing standards Standard
Ni 2P3/2
O ls
Ni(OH) 2 SO 3 NiSO 4 SO: MSO 3 S
856.6, 855.8, 855.6
531.8, 531.7, 531.4
856.8
532.1
S 2p 171 169.2 168.1 167 b)
164.2
b)
163.9 ct
NiO~
Ni20 3 NiO NiO~,l~ NiS Ni~S z NiS~ Ni metal H 20 "~ bl o d)
857.3, 855.8 854.5, 854.3, 853.5 853.5 855.1,853.2, 852.8 854.1 d) ( A I K ~ )
531.8 529.9, 529.6 533.5 162.2 c) 162.1 dl
852.8, 852.7, 852.2 533.2
Values are from the Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics, u n l e s s o t h e r w i s e specified. From Craig et al. [9]. From Coyle [10]. From Ng and Hercules [11].
J. 22 Rickman, R. W. Linton / Characterization of nickel-containing ent,ironmental particles
of angles. For the NiS standard particles, this effect is evidenced by a decreased intensity at the lower and higher angles in both plots. This effect is lessened at the lower angles and enhanced at the higher angles when looking at a narrower size range. The greater decrease at higher angles for the sized fractions is probably due to the fact that the X-ray gun was positioned much closer to the sample when analyzing the two sized fractions (there were much higher intensities in general) than when the original standard was analyzed. Having the gun closer exaggerated the effects of stage tilt. Fortunately, sulfur and nickel behave similarly as a function of angle. Therefore, the percent atomic concentration, which normalizes the results, should reflect the concentration of elements in the sample with the same degree of accuracy regardless of angle.
3.2. Selective solvent leaching The NiS standard was analyzed before and after citrate leaching. The results are compiled in table 1. The NiS standard was chosen for this study in order to be consistent with the angular response study. The binding energies and F W H M (full-width at half-maximum intensity) values for the NiS standard can be compared to the literature values compiled in table 2. The binding energy and width of the nickel peak (857.4 eV, ~ 6 eV width), as well as the oxygen peak at 531.4 eV and the sulfur peak at 170.4 eV for the NiS standard, suggest that a NiSO 4 layer has grown on top of the NiS. This is not surprising since NiS is known to be unstable in air [11]. There appear to be two chemical forms of oxygen before leaching. One oxygen peak at 531.5 eV is possibly due to NiS(OH), which is known to be formed when NiS is exposed to air [12]. The other oxygen peak at 533.6 eV is probably from NiSO 4. Curve fitting of the peaks in the S2p window for the NiS standard results in five sulfur peaks. The sulfur peak at highest binding energy (170.4 eV) can be assigned to NiSO 4. The peak at 168.4 eV might indicate some NiS(OH), which would confirm the identification of the oxygen peak at 531.5 eV. The other three peaks represent three more re-
379
duced sulfur species (e.g. elemental S, NiS 2, NiS a n d / o r Ni3S2). Further confirmation of NiSO 4 is evidenced by shake-up peaks, which were present in the Ni2p window [13]. More detailed analysis of shake-up peaks for nickel-containing compounds (NiO, NiSO 4 and 68 other nickel compounds) is available in another source [14]. After leaching the NiS standard, there are two nickel peaks in the spectrum. The major one is at 855.9 eV, possibly due to a nickel/oxygencontaining compound; such compounds tend to have broader Ni2p peaks than do nickel/sulfurcontaining compounds. A second nickel peak at 853.0 eV is observed after leaching. Its binding energy is very close to the expected value for NiS (compare to literature values in table 2). Its narrow F W H M (1.59 eV) also indicates a nickel sulfide species. The oxygen peak at higher binding energy (previously assigned to NiSO 4) disappears as does the sulfur peak at 170.4 eV after leaching. This result is consistent with a significant loss of NiSO 4 species into the extracting solution. These results indicate that the citrate leach is successful. This is of interest since one of the goals of this project is assessing the reliability of wet chemical techniques routinely used to chemically speciate nickel compounds from industrial sources and proposed for use by the Environmental Protection Agency in environmental monitoring applications. Partial extraction of NiSO 4 would underestimate its concentration based only on wet chemical analysis results. Additionally, these results indicate that the surface composition of the standards (i.e. 99.99% NiS) differs markedly from bulk values.
3.3. Ion beam sputtering The nickel standards analyzed are listed in table 3. The results before and after sputtering are summarized in table 4. The only nickel standard that had reasonably close surface atomic concentrations and binding energies before sputtering compared to bulk speciation and stoichiometry was N i S O 4 . 7 H 2 0 . The nickel sulfide standards all have substantial oxygen surface contamination.
380
J.T. Rickman, R.W. Linton / Characterization of nickel-containing em~ironmental particles
Table 3 Source of nickel standards used in sputtering experiments Compound
Purity (%)
Source
Ni(OH) 2 NiSO4.7H20 a) Ni203 - H 2 0 NiO2 Ni(I|)O Ni(II)S Ni3S 2 NiS 2 Ni metal
97 99.999 0.9 H 2 0 _ b) 99.99 99.99 _ b~ 99.99 99.999
Aldrich Chemical Co., Inc. Aldrich Chemical Co., Inc. Alfa Products Alfa Products Aldrich Chemical Co., Inc. Alfa Products INCO, Ltd. Alfa Products Alfa Products
~" Sufficient vacuum for XPS analysis and subsequent ion sputtering could only be attained after drying the prepared NiSO4.7H20 standard in a vacuum oven at 60°C for 24 h. bl Not available.
Sputtering appears to affect the standards in one of the three following ways. The nickel peak is broadened (see table 4 for F W H M ) and a second oxygen peak appears (e.g. in the cases of Ni(OH) 2 and N i S O 4 - 7 H 2 0 ) . Prior studies by Christie et al. [15] indicate that metal sulfates might undergo a dissociation reaction upon ion beam bombardment. The XPS results indicate this as well: NiSO 4 -~ NiO(s) + SO2(g ) + ½02(g). In the second class of sputter-induced changes to the nickel standards, the width of the nickel peak either stays approximately the same or broadens, but no new oxygen peaks appear. There is, however, a decrease in concentration of oxygen at the higher binding energies (e.g. the oxides). The oxides appear to be least affected by sputtering. This is contrary to the results of Kim et al. [16] who found that oxides undergo reduction upon argon ion bombardment. It is possible that differences in ion beam dose in Kim's study versus this study explain the apparent discrepancies. The third way sputtering affects some nickel standards is that the nickel peak is narrowed and there is a loss of sulfur at the higher binding energy with a concomitant increase of sulfur concentration at the lower binding energies (e.g. in the case of the sulfides. Eventually this could result in total decomposition to elemental Ni and
S as Coyle et al. [10] found with transition-metal sulfides. A further discussion of the results from the sputtering of nickel standards is presented in another source with pertinent spectra included [13]. The important findings pertaining to this study are that the standards are inhomogeneous and sputtering causes changes which cannot be attributed simply to removal of contamination layers.
4. Results and discussion: FBR Dust
4.1. A R - X P S
Examples of raw spectra from the angle-resolved XPS studies are shown in fig. 2. The spectra look very similar at all eight angles. The semi-quantitative results for the FBR Dust at the eight different angles are compiled in table 5. The measured binding energies for nickel, sulfur and oxygen do not change significantly with take-off angle with the exception of oxygen at 0 = 90 °. That is, there is no appreciable chemical change with depth. The binding energies for nickel, sulfur and oxygen correspond to NiSO 4. However, at 0 = 90 °, the appearance of a small, second oxygen peak is observed at a lower binding energy. At this depth into the sample, it appears that more of an underlying oxide component in the particles is being sampled. A plot of C, O and Ni concentration versus 0 in fig. 3, indicates a greater concentration of carbon at the surface (more grazing angles) with a corresponding depletion of nickel, thus indicating a carbon contamination layer on top of the nickel. The concentration of oxygen at the higher binding energy (sulfate oxygen) is slightly decreasing with depth which suggests sulfate enrichment at the surface. An additional plot of concentration versus 0 (fig. 4) for Ni and two types of sulfur (sulfate and other sulfur species), indicates a decrease in the sulfate sulfur signal at 0 = 90 °, consistent with the decreasing sulfate oxygen signal. The other sulfur signal appears to be depleted at the surface (more grazing angles) and slightly higher at 0 = 90 °.
Table 4 X P S d a t a o n nickel s t a n d a r d s b e f o r e a n d a f t e r s p u t t e r i n g Standard
O ls
N i 2 P 3 / 2 ~)
c.a)
B E b)
%ac
859.4 859.0
37.5 56.9
2.765 4.785
858.1 ~ 856.9
16.1 47.2
3.864 ~ 5
NiO 2
855.2
39.7
4.327
NiO 2 *
854.5
54.6
4.735
Ni20 3 .xH20
853.9
51.9
4.263
854.5
68.6
NiO
854.1
52.3
NiO *
854.9
65.3
NiS 2
853.6
19.4
5.101
NiS 2 *
852.9
55.9
2.433
NiS
857.4
31.8
NiS *
852.7
65.0
Ni(OH) 2 Ni(OH) 2 * NiSO 4 . 7H20 NiSO 4 . 7H20
NieO 3 .xH20
*
*
FWHM
~ 7.5 4.387 ~ 7
~ 6
1.849
S 2p
BE
%ac
FWHM
534.5 535.7 534.0 532.9 532.4 530.5 530.8 528.9 531.2 529.4 530.9 529.0 531.5 529.8 531.1 529.4 531.5 530.0 531.8
52.2 14.3 28.9 64.0 22.9 23.3 30.1 13.0 17.9 23.0 16.5 25.4 6.6 24.0 21.2 26.5 9.7 24.9 27.8
1.702 1.73 1.78 2.406 2.15 2.28 2.13 1.31 1.87 1.62 1.85 1.24 1.68 1.46 2.43 1.28 2.14 1.45 2.316
531.3 529.3
5.3 0.8
1.95 1.05
533.6 531.5
22.0 33.2
2.50 2.31
530.7
4.7
-
-
22.1 1.8 2.7 1.6 2.8 6.0 12.9 7.5 -
2.62 5.06 0 2.19 1.24 1.49 5.49 f~ 2.20 1.55 -
48.1
2.629
32.4
5.021
533.5 531.7
23.8 34.9
2.30 2.28
Ni3S 2 *
852.7
61.6
1.957
531.1
11.9
2.40
Ni m e t a l
856.1
45.4
5.762
531.6
43.9
2.38
2.5
529.6 529.9
10.7 11.2
1.35 1.626
.
-
162.? 168.7 164.7 163.6 162.3 165.2 163.2 162.1 -
857.8
.
-
-
2.0 1.26 6.2 f~
Ni3S 2
.
-
2.725 2.30 2.90 -
14.9 8.0 8.0
2.777
.
-
-
162.4 161.3 167.0
17.9
.
14.0 2.0 4.7 -
FWHM
-
531.5
S
-
2.86 1.71 1.00 5.60 0 1.93 1.30 1.15 2.90 2.48 3.16 1.79 1.38 6.09 f~
5.247
88.8
169.2 169.0 167.1 -
%ac
6.7 16.1 11.2 6.1 8.1 13.3 10.5 4.2 2.6 3.4 1.6 1.1 7.4
52.0
852.8
-
168.8 163.1 162.0 166.5 163.3 162.1 161.0 170.4 168.4 163.7 162.3 161.0 165.2
853.4
Ni m e t a l *
BE
161.5
-
* Sputtered. a) N o c u r v e fitting w a s u s e d f o r t h e Ni p e a k b e c a u s e o f (1) the t r a d e - o f f in t i m e v e r s u s u s e f u l n e s s o f i n f o r m a t i o n o b t a i n e d a n d (2) t h e difficulty in d e t e r m i n i n g t h e c o r r e c t n u m b e r o f p e a k s t h a t m a k e u p t h e Ni 2 p c u r v e w i t h o u t m a k i n g a s s u m p t i o n s a b o u t t h e Ni s p e c i e s p r e s e n t . M a t i e n z o et. al. [14] f o u n d t h a t N i S O 4 h a s o n e s h a k e - u p satellite f o r t h e 2P3/2 a n d t w o satellites f o r t h e 2Pl/2. N i O h a s o n e s h a k e - u p satellite f o r b o t h t h e 2 P 3 / 2 a n d 2 P l / 2 p e a k s . A m i x t u r e o f t h e s e t w o c o m p o u n d s w o u l d b e i m p o s s i b l e to deconvolute. b~ c) d~ e~ 0
N i 2 P 3 / 2 : o - = _+0.5 eV; O l s : o - = + 0 . 2 eV; S 2 p : o - = _+0.1 eV. Sensitivity f a c t o r s u s e d in c a l c u l a t i n g % a c : 3.653 f o r N i 2 p , 0.711 f o r O is, 0.57 f o r S 2 p a n d 0.296 f o r C ls. R e m a i n i n g % a c d u e to a d v e n t i t i o u s c a r b o n . U s e d e l e c t r o n f l o o d g u n to c o m p e n s a t e f o r s a m p l e c h a r g i n g . B r o a d p e a k o f low intensity. N o t s u i t a b l e f o r c u r v e fitting.
Z T. Rickman, R. IV.. Linton / Characterization of nickel-containing environmental particles
382
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J. T. Rickman, R. W. Linton / Characterization of nickel-containing environmental particles ]0
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J. T. Rickman, R. W. Linton / Characterization of nickel-containing environmental particles
384
Table 5 AR-XPS results ") on FBR Dust Angle
Ni 2P3/2
(deg)
BE b)
%ac
FWHM
BE
0 ls %ac
FWHM
S 2p BE
%ac
F WH M
15
856.4
12.6
3.929
531.5
46.7
2.736
20
856.5
10.4
3.626
531.9
47.4
2.674
25
856.4
13.9
4.021
531.6
46.5
2.574
30
856.8
15.5
4.1 ! l
532.0
46.4
2.562
856.1
16.7
4.349
531.4
46.1
2.616
40
856.4
16.3
4.226
532.0
45.9
2.506
70
857.0
16.0
3.982
532.0
44.8
2.651
90
856.3
16.8
3.580
531.5 527.9
41.8 4.3
2.882 -
6.5 1.1 6.3 1.4 6.8 1.6 6.4 1.7 6.7 1.2 6.4 1.5 6.6 1.9 5.4 1.5
2.78 2.68 2.82 2.64
35
168.5 Other c) 168.9 Other 168.7 Other 169.0 O t he r 168.4 O t he r 168.8 O t he r 168.9 Other 168.5 Other
2.65 2.68 2.69 2.87
~) These results do not include approximately 3% Cu and < 1% Ca detected in other XPS analysis of the FBR Dust. R e m a i n d e r of %ac due to adventitious C. b) Binding energy referencing to C l s at 284.6 eV. c) Other signifies a broad S component of low intensity whose binding energy would be difficult to assign.
photoelectrons on depth of analysis. The attenuation lengths for Ni2P3/2, O l s and S2p photoelectrons escaping from a bulk NiO matrix can be calculated using the following equation [17]:
This result gives further support to speculation that the FBR Dust particles consist of a surface enriched in NiSO4, with other nickel-containing species being surface depleted. The limitations of angle-resolved XPS include potential topographic effects on photoemission and the restraint imposed by the escape depth of
Am =
2170E -2 +
0.72(aE) 1/2,
where Am is the attenuation length (monolayers),
Fluidized Bed Roaster Dust (FBR Dust) 50 45
~ 40 ~
35
•
c
~ 30 E ~ 25
O
-~ 20 E
• _ _ _ _
~ ~s
N
Ni
--4
10 5 I
0 15
20
-
i
2,5
30
I
i
i
35
40
70
90
Take-off angle (degrees)
Fig. 3. Plot of C, O and Ni concentrations versus 0 for AR-XPS of F B R Dust.
J.T. Rickman, R.W. Linton / Characterization of nickel-containing environmental particles
385
Fluidized Bed Roaster Dust (FBR Dust) 18
fjll~
16 tO fc O U
14
~
•
12 10
.o
8
E
6
o
jr
+ --*
Ni S-sulfate S-other
4 2 0
5
20
25
30
35
40
70
90
Take-off angle (degrees) Fig. 4. Plot of atomic concentration versus 0 for Ni and two types of sulfur.
E the kinetic energy of the photoelectron (eV), and a the monolayer thickness (nm). The monolayer thickness can be calculated as shown below:
Table 6 Attenuation lengths for Ni, O and S in NiO Element
Peak
E (eV)
Am (ML)
A.(nm)
a 3 = Z/(pnN)
Ni O S
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1.393 1.872 2.297
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Table 7 XPS data on FBR Dust Ni2P3/2
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51.7
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BE
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4.226
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2.506
4.277
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2.41 2.39
531.5 529.8
17.3 24.9
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6.4 1.5 1.5 0.6 1.0 0.6 0.5 1.8 0.7 1.6 1.6
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~ 7
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J.T. Rickman, R.W. Linton / Characterization of nickel-containing environmental particles
molecule of the bulk material, and N Avogadro's number. To get the attenuation length in nanometers (An), the following calculation is used:
4.2. Selective soluent leaching Raw spectra for the FBR Dust are shown in fig. 2. Raw spectra and curve fits for the leached F B R Dust can be found in fig. 5. The binding energies, percent atomic concentrations and F W H M before and after leaching are compiled in table 7. Citrate leaching resulted in removal of 75% of the NiSO 4 present as evidenced by the decrease in intensity of the sulfur peak at 168.8 eV. Other sulfur species were revealed as well at 166.6,
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388
J.T. Rickman, R.W. Linton / Characterization of nickel-containing enuironmentalparticles
citrate leach solution is maintained at a proper pH using ammonia. It is also hard to quantify the efficiency of the leaching process within the particles. Ion beam sputtering represents an additional alternative to avoid some of these potential problems with selective solvent leaching and to access information deeper into the particle interior than is possible using angle-resolved XPS.
163.7 and 161.5 eV. Possible species assignment would be NiSO (nickel sulfite) at 166.6 eV, S (elemental sulfur) at 163.7 eV, and a sulfide at 161.5 eV. The leach for NiSO 4 was not complete possibly because the NiSO 4 was re-deposited on the surface of the particles upon drying. It is difficult to assess the chemical/physical effects of the leach on the particles such as how selective the citrate leach is for NiSO 4 and the accessibility and solubility of other species in the extracting solution. The fact that Ni(OH) 2 is known to be soluble in ammonia forming [Ni(NH3)6] 2+ [12] would be a problem if Ni(OH) 2 were in the sample since the I
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J. T. Rickman, R. W. Linton / Characterization of nickel-containing environmental particles
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Table 8 Atomic percentages obtained from XPS data collected at 40 ° tilt angle on FBR Dust
Ni O S-total S-sulfate S-sulfite S-elemental S-sulfide S-other C
As received
Leached
Sputtered
16.3 45.9 7.9 6.4 < 0.5 < 0.5 < 0.5 1.5 29.9
19.8 44.9 3.7 1.5 0.6 1.0 0.6 < 0.5 31.6
51.7 42.2 6.1 0.5 1.8 0.7 3.2 < 0,5 < 0.9
binding energies, percent atomic concentrations and F W H M before and after sputtering are compiled in table 7. Sputtering results in a change in the proportions of surface sulfur species present. The peak at 167.2 eV corresponds to NiSO 3 (nickel sulfite) which may have been reduced from NiSO 4 (169.3 eV), as observed in the NiSO4 standard after sputtering. The peak at 165.4 eV is probably elemental sulfur. The sulfur peaks at 162.5 and 160.9 eV can be assigned to the sulfides. They are more likely reflective of nickel sulfide species
392
J.T. Rickman, R.W. Linton / Characterization of nickel-containing ent'ironmental particles
inside the particles rather than ion beam reduction of sulfate to sulfide species since the NiSO 4 • 7 H 2 0 standard did not show ion beam reduction to sulfide under the sputter conditions used. The new oxygen peak is probably NiO that has been produced through dissociation of NiSO 4 by the ion beam as appeared to happen with the standard or it could be NiO within the particle. The loss of NiSO 4 is much less than the growth of the NiO peak. Thus, it is likely that much of the NiO reflects species inherent to the particle interiors. This is also consistent with bulk analyses (wet chemical techniques and X-ray powder diffraction) indicating the major component is oxide. 4.4. Summary o f results and discussion on FBR Dust A summary of the results from all three techniques is shown in table 8. Using AR-XPS (results under as received in table 8), a contaminant carbon layer on top of NiSO 4 was observed. At the greatest analysis depth (0 = 90°), a hint of a decrease in NiSO 4 and a new oxygen-containing species is seen. Because of the limited depth of analysis, selective solvent leaching was used to remove the NiSO 4 and facilitate detection of other nickel species present. This experiment was only partially successful and was limited by the uncertainties of selectivity, solubility and surface re-deposition in using the citrate leach. Finally, ion beam sputtering was used to access information deeper into the particle interior. The carbon overlayer was removed by sputtering and a depth was reached in the sample where one-half of the sulfur is sulfide and the other half is NiSO 3, elemental sulfur (probably produced by the reduction of NiSO 4 by the ion beam) and a little NiSO 4 that was not reduced. Jach and Powell [18] in a similar study of environmental particles using XPS also detected sulfate on unsputtered particles and sulfide after sputtering. The XPS results can be compared to bulk ICP (inductively coupled plasma) results by normalizing to nickel (table 9). Sputtering appears to probe most deeply into the bulk since the results compare very favorably with those obtained by ICP bulk analysis. With AR-XPS and leaching,
Table 9 Atomic concentrations normalized to Ni for FBR Dust XPS
O/Ni S/Ni C/Ni
ICP
As received
Leached
Sputtered
2.82 0.48 1.83
2.27 0.19 1.60
0.82 0.12 -
0.86 0.13
sulfur and oxygen ratios greater than those obtained using ICP are observed, indicating that the surface enriched NiSO 4 layer is being sampled. Through our study of standards and correlation of techniques, better evaluation of the chemical species present in the surface versus the bulk components of these particles is possible. From a biological perspective, the presence of a surface layer enriched in soluble NiSO 4 suggests that environmental impact assessments based on particle standards assumed to be homogeneous may be seriously in error. The surface properties of both standard and environmental particle samples must be considered in more detail.
Acknowledgements The authors gratefully acknowledge the support of F.E. Butler and the US Environmental Protection Agency which funded this project under cooperative agreement CR-812908-01-1.
References [1] J.P.W. Gilman, Cancer Res. 22 (1962) 158. [2] V. Young and P.C. McCaslin, Anal. Chem. 57 (1985) 485. [3] C.S. Fadley, R.J. Baird, W. Siekhaus, T. Novakov and S.A.L Bergstrom, J. Electron Spectrosc. Rel. Phen. 4 (1974) 93. [4] J.E. Fulghum and R.W. Linton, Surf. Interf. Anal. 13 (1988) 186. [5] J.A. Campbell, R.D. Smith and L.E. Davis, Appl. Spectrosc. 32 (1978) 316. [6] G.E. Cabaniss and R.W. Linton, Environ. Sci. Technol. 18 (1984) 271. [7] N. Kaufherr, M. Shenasa and D. Lichtman, Environ. Sci. Technol. 19 (1985) 609. [8] V.J. Zatka, Chemical Speciation of Nickel Phases in Industrial Dusts (INCO, Ontario, Canada, 1987).
J.T. Rickman, R.W. Linton / Characterization of nickel-containing environmental particles [9] N.L. Craig, A.B. Harker and T. Novakov, Atmos. Environ. 8 (1974) 15. [10] G.J. Coyle, T. Tsang and I. Adler, J. Electron Spectrosc. 20 (1980) 169. [11] K.T. Ng and D.M. Hercules, J. Phys. Chem. 80 (1976) 2094. [12] A.G. Sharpe, Inorganic Chemistry (Longman, New York, 1981) p. 596. [13] J.T. Rickman, PhD Dissertation, University of North Carolina at Chapel Hill (1989) p. 105.
393
[14] LJ. Matienzo, L.O. Yin, S.O. Grim and W.E. Swartz, Inorg. Chem. 12 (1973) 2764. [15] A.B. Christie, J. Lee, I. Sutherland and J.M. Walls, Appl. Surf. Sci. 15 (1983) 224. [16] K.S. Kim, J.W. Amy Baitinger and N. Winograd, J. Electron Spectrosc. Rel. Phen. 5 (1974) 351. [17] M.P. Seah and W.A. Dench, Surf. Interf. Anal. 1 (1979) 2. [18] T. Jach and C.J. Powell, Environ. Sci. Technol. 18 (1984) 58.