In situ total-electron-yield sulfur K-edge XAFS measurements during exposure of copper to an SO2-containing humid atmosphere

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Nuclear Instruments and Methods in Physics Research A 360 (1995) 634-641

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NUCLEAR INSTRUMENTS &METnoDS IN PHYSICS RESEARCH SectIon A

ELSEVIER

In situ total-electron-yield sulfur K-edge XAFS measurements during exposure of copper to an SOT-containing humid atmosphere Inho Song a,* , Brett Rickett a, Paul Janavicius

a, Joe H. Payer a, Mark R. Antonio b

a Department of Materials Science and Engineering, The Case School of Engineering, Case Western Reserve University, Cleoeland, OH 44106-7204, USA b Chemistry Diuision, Argonne National Laboratory, Argonne, IL 60439-4831, USA

Received 27 September 1994, revised form received 13 December 1994

Abstract A total-electron-yield (TEY) detector was designed and constructed measurements of the sulfur-containing species formed during exposure Using the detector, gas phase XAFS spectra were also collected for presents the experimental technique and examples of the sulfur K-edge

1. Introduction Understanding gas/solid reactions and the resulting products often requires the use of spectroscopic techniques that minimize the perturbation to the specimen and to the reaction environment during the measurement. This is particularly important for the study of atmospheric corrosion, as relative humidity plays an essential role in the corrosion process [1,2].Thus, when determining the structure and the valency of the reaction products, the use of techniques that involve vacuum or a charged particle beam has been a salient concern because they may cause inadvertent and significant perturbations. The objective of this research was to design, build and ‘demonstrate the use of an XAFS detector that is suitable for in situ measurement of the tarnish products formed during the sulfidation/oxidation of copper in a humid environment containing parts per billion (by volume) levels of SO,. Since the conversion-electron detection method of Mossbauer-effect spectroscopy was first applied to XAFS measurement [3], the technique of total electron yield (TEY) detection has become an invaluable tool for many types of XAFS measurements [4-61. However, the TEY detection mode is not always suitable for in situ XAFS measurements as it requires a special gaseous environment (e.g., hydrogen or helium), or vacuum for detection of the electron yield. Therefore, many previously reported in situ studies were conducted either in the fluorescence mode [7-91 or in the transmission mode [lo], which are the most

* Corresponding author. Tel. + 1 216 368 8935/8681, 216 368 3209, e-mail [email protected].

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0168-9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)00134-4

for in situ X-ray absorption fine structure (XAFS) of copper to a humid atmosphere containing SO,. both dry and humid SO, atmospheres. This work spectra collected during the study.

commonly used and direct methods of XAFS measurements. During the planning of the present work, several limitations inherent to the fluorescence detection technique became apparent. The species of interest contain sulfur, and it is known that the fluorescence yield is low for species of low atomic number (such as phosphorus, sulfur and chlorine) [ll]. Additionally, for some bulk or concentrated specimens, it is also known that fluorescence XAFS spectra can suffer from the effects of thickness [12] and/or self-absorption [ 131. These experimental artifacts alter the XAFS features such as edge inflection points and amplitudes, and thus quantitative comparisons between spectra become difficult as a result. In low energy fluorescence experiments, the material for the fluorescence detector window as well as the incident X-ray window (e.g., polycarbonate, polyimide, Mylar, aluminized Mylar, etc.) becomes a serious concern because of the attenuation of the incident and the Ka fluorescence radiations. In contrast, the TEY detection mode has several wellknown advantages over the fluorescence detection mode. In the TEY mode, experimental artifacts such as the thickness effect are absent [4,5], and for elements of low atomic number, the (nonradiative) electron yield is high, i.e., close to 1 [ll]. Because of the nature of the TEY detection method, windows are not used in the detector. Furthermore, the TEY mode provides near-surface sensitivity [4,5], which is an important feature for thin film studies. As in fluorescence XAFS measurements, Kim et al. [14] reported that Bragg diffraction may also interfere with TEY XAFS measurements. Based on all of these considerations, the TEY detection mode was selected for the present study, and the conventional TEY detector

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design was modified to accommodate the use of corrosive gas and to increase the sampling area for improved signal. The utilization of the TEY detection mode for the present experiment was made possible by the fact that the nitrogen normally present in “air” does not play a role in the corrosion process. Therefore, the nitrogen, which would be otherwise present in the in situ exposure environment was replaced by helium, which is also nonreactive. Importantly, helium now served as a detector gas for the TEY detection mode. A He(SO%)/O,(2O%o) mixed gas was used as the carrier gas for the corrosive in situ environment containing 200 ppb SO,. Following design and fabrication, the detector performance was verified through XAFS experiments on solid and gas phase model compounds. In this report, the design of the TEY detector for in situ tarnishing experiments and examples of the XAFS data obtained from the TEY detector are presented.

(A)

Detector Gas In

Detector Gas Out

Fluorescence Detector x-ray Beam TEY i in-srtu Cell

/I

I I RH Sensor

Exposure Atmosphere

In

E&sure Atmosphere

Out

2. Detector design and experimental procedure Two features were incorporated into the design of the in situ TEY detector: 1) a large sampling area (approximately 75 mm*) by using a grazing incidence geometry (the projected area would be l-2 mm’ when using the common geometry, either a normal or a 45 degree incident X-ray beam) under focused beam (e.g., 1 mm X 1 mm) conditions, and 2) a sulfidation- and oxidation-resistant electron collection grid. In order to have a large sampling area, the detector was designed to work in a grazing incidence geometry, either with or without a goniometer as shown in Fig. 1. Although is well-understood that a long beam path length associated with the grazing incidence geometry can degrade the signal-to-noise ratio, the use of the glancing angle geometry was necessary to allow the sampling of a wide area along the whole specimen length. This is because in laboratory testing of atmospheric corrosion the formation and the distribution of tarnish products depend on the flow pattern of the reactive gas in the in situ cell. If the X-ray beam probes the specimen region where the flow pattern restricts the free access of pollutant gas to the surface of specimen, the result may erroneously indicate a slower corrosion kinetics than it would be during the in situ studies. Therefore, despite the possible degradation of signal-to-noise, the in situ TEY detector was designed and operated in the glancing angle geometry. Also, the grazing incidence geometry was necessary in order to keep the TEY collection electrode out of the path of the X-ray beam. This geometry prevents the spurious spectral contamination by the species, which have been adsorbed or reacted on the surface of the electron collection electrode, but which are not related to the reaction product on the actual specimen. Additionally, the grazing incidence geometry facilitates the use of a small pre-monochromator vertical slit, which determines the beam divergence and hence the experimental energy resolution. To prevent sulfi-

TEYI / Cell Base

Specimk (2500 A Cu / Glass Slide)

/

Fig. 1. Total-electron-yield (TEY) detector/cell assembly. (A) In situ TEY detector/cell. A fluorescence ion-chamber detector is shown stacked on top of the TEY detector/cell. (B) Cross section of the detector assembly. The details of the fluorescence ionchamber detector are not shown for clarity. The volume of the TEY detector/cell is approximately 130 ems. The cell was used in a grazing geometry to increase the footprint of the beam on the specimen.

dation or oxidation of the electron collection electrode, gold was chosen as the material for this electrode. Fig. 1A shows the external view of the TEY detector, and Fig. 1B shows a cross section of the detector assembly placed in the grazing incidence geometry. Fig. 2 shows an exploded schematic of the TEY detector. The detector consists of four main components - cell housing, incident X-ray window, TEY collection grid, and specimen stage. The cell housing was made of polycarbonate, the incident X-ray window was polycarbonate (5 pm thick, Spex), the TEY collection grid was a gold mesh (90% transparent, 30 lines/in., Buckbee Mears), and the specimen stage was made of polycarbonate. On the specimen stage, two copper clips were provided to hold the specimen in place and to make an electrical connection between the specimen and the electronics. Copper was chosen as the material for the clips in order to prevent the formation of a galvanic couple between the clips and the copper specimen in the corrosive in situ environment. For other in situ studies, it is recommended that the material of these clips be changed to the same material as the specimen. As shown in Figs. 1 and 2, the TEY detector has a provision for an optional placement of a fluorescence

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Fig. 2. TEY detector/cell. A: Fluorescence detector; B: Polycarbonate window; C: Copper retaining ring (100 mm, diameter); D: Gold TEY collection grid; E: BNC connector to battery/amplifier; F: O-ring (95 mm, diameter); G: Copper clips; H: Specimen (2500 ,& copper on glass, 25 mmX75 mm); I: Electrical contact point; J: TEY/Cell base (115 mmX 115 mm); K: Retaining screws; L: Aluminum shielding plate.

detector. During the sole TEY mode operations, the socalled window at the top of the TEY detector serves merely as means of containing the in situ atmosphere. This window (polycarbonate) is attached to the TEY detector with double-sided tapes, or with polycarbonate adhesives. When a fluorescence detector needs to be mounted on the TEY detector, the window on the TEY detector can be removed, because most fluorescence detectors have their own windows, and the presence of an additional window will merely degrade the useful signal. However, the window on the TEY detector should stay if the fluorescence detector has a metallized (e.g., aluminized) window. This is because either the corrosive gas will ruin the metallized window or the metallized window may alter the composition of the corrosive in situ environment. In the sulfur K-edge studies, the use of gold in preference to other noble metals (e.g., platinum, etc.) as a TEY collection electrode could be beneficial for a reason other than the corrosion resistance. The Mm edge of gold (2743 eV) lies much farther away from the K-edge of sulfur (2472 eV) than the Mm edge of other noble metals (2457 eV for osmium, 2551 eV for iridium, and 2645 eV for platinum). This shifted, if not eliminated, the potential spectral contamination from the sulfur K-edge X-ray absorption near-edge structure (XANES) region. There may be a concern regarding possible spectral contamination of

the sulfur K-edge XANES by the extended X-ray absorption fine structure (EXAFS) of the gold Mt, edge (2291 eV). For the purpose of the present study this is considered noncritical because the sulfur K-edge XANES is more than 150 eV higher than the gold M,, edge, and the smooth EXAFS oscillations due to the gold M,, edge will not alter the sulfur K-edge XANES features that appear generally as well-defined peaks. Furthermore, in the present grazing incidence geometry of the TEY detector design, the gold electrode will not be in the path of the incident and specularly reflected X-ray beam. For the TEY collection electrode, gold in the form of a mesh was used instead of a foil to allow the transmission of fluorescence X-rays through the collection electrode to an optional fluorescence detector placed on top of the TEY detector/cell. The details of the fluorescence detector are not shown here, but the fluorescence detector can be used simultaneously with the TEY detector. All electrical connections were made with insulated BNC-type connectors through coax cables (Type RG-58C/U, Belden). Copper specimens (2500 A thick, 99.9%) were prepared by a combination of magnetron sputtering and evaporation onto optically flat glass substrates (Coming Glass Works, Part No. 2947: 76 mm X 25 mm X 1 mm). Prior to the sputtering of copper, the glass substrates were ultrasonically cleaned with methanol, acetone, and isopropanol for 10 min in each cleaning medium. The copper adhesion layer was initially formed by sputtering onto the substrate at 240 V pallet voltage and 1000 W forward power for 5 min at 15 mTorr chamber pressure. Then, the copper layer was etched with oxygen at 1050 V pallet voltage for 2 min at 8 mTorr chamber pressure. On this oxidized adhesion layer a metallic copper layer was formed at 240 V pallet voltage and 1000 W forward power for 10 s at 15 mTorr chamber pressure. The distance between the copper target and the substrate during sputtering was set to 7.5 cm. Following this sputtering operation, the actual copper specimen layer was formed by the evaporation method. The evaporation was done in 2 p,Torr for 4 min, at a distance of 5 cm from the soyrce. This yielded an optically flat copper film of 2500 A total thickness on the glass substrate. The thickness was monitored by a calibrated quartz crystal microbalance during the sputtering and evaporation processes. An X-ray photoelectron spectroscopy (XPS) survey scan confirmed that the film consisted of copper, oxygen (from surface oxide) and carbon (from adventitious hydrocarbon). XAPS data were collected at Beamline X-6B, National _Synchrotron Light Source, Brookhaven National Laboratory. The X-ray storage ring was operated at 2.5 GeV, 110-200 mA. The beamline was equipped with a double crystal Si(ll1) monochromator, a post-monochromator rhodium focusing mirror ( r,,sittal = 80 mm, rmeridional= 2.5 km), and a pre-monochromator vertical slit (0.3 mm> [15]. Measured from the source, the distance was 8.5 m to the vertical slit, 9.8 m to the monochromator, and 24 m to the

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specimen. The focused beam size at normal incidence was approximately 1 mm X 1 mm [16]. The source size was between 102 and 143 pm [17]. In this configuration, the total energy bandwidth of the monochromator, A&,,,, was calculated to be approximately 0.54 eV (AEmtal: a Gaussian convolution of the energy divergence of the synchrotron beamline, approximately 0.27 eV with a source size of 120 pm; and the rocking curve width of the Si(ll1) monochromator, approximately 0.33 eV at 2500 eV). Providing that the energy bandwidth (0.54 eV) and the natural width of the K-level (r,ota,, 0.59 eV) [18] add in quadrature, the overall estimated effective spectral resolution was approximately 0.8 eV. This results in an inevitable broadening (approximately 1.4 times the natural width) of the true spectrum. The sulfur K-edge absorption data were collected at room temperature with a step size of 1 eV in th,e pre-edge region, 0.1 eV in the XANES regioa, and 0.05 A-’ in the EXAFS region (approximately 3-9 A- ‘>. The data for the EXAFS region were collected in order to model the postedge absorption for normalization of the XAFS data. Integration times ranged from 1 to 3 s/point (I s for pre-edge, 2 s for XANES, and 3 s for post-edge). The absolute energy calibration was set by use of Na,S,O,. 5HZ0 (99.5%. Aldrich) in powder form. The first sulfur K-edge peak (Is-a;) of Na,S,O, 5Hz0 was set to 2469.2 eV _ _ [191. The raw X-ray absorption data were treated as follows: 1) The pre-edge data were modeled with a two-term linear function, which was fitted below the onset of the absorption edge and extrapolated above the absorption edge. 2) The post-edge data were modeled with a three-term quadratic function, which was fitted above the edge and extrapolated below the edge. 3) Normalization was performed by subtracting the pre-edge absorption approximation from the raw data and then dividing the difference by the post-edge absorption approximation. This normalization procedure facilitates a direct quantitative comparison of the shapes and intensities of features found in XANES. The first and second derivatives of the normalized XAFS data were obtained by a Fourier transform technique [20]. The in situ TEY detector performance was verified by measuring the XANES, particularly the first sulfur K-absorption peak (Is-a; 1, of Na,S,O, .5H,O. The Na,S,O, 5Hz0 powder was pressed onto the adhesive of aluminized Mylar tape (Type 850, 3M) with a nickel mesh (80% transparent, 30 lines/in., Buckbee Mears) overlayer. This was then mounted on the specimen stage. The measurement was done in a dry helium environment at a flow rate of approximately 50 ml/min. The gas phase experiment was performed without solid substances (e.g., collection grid, blank substrate, etc.) other than the polycarbonate cell windows in the beam path. A gold counter electrode evaporated onto glass was placed on the specimen stage below the beam path. This geometry aIlowed the sampling only of the gaseous species in the in

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situ detector. Flowing at a rate of 20 ml/min, the SO, concentration was 107 ppm in helium (Certified Grade, Matheson Gas Products Inc.). The humidity was adjusted by passing a portion of the gas over a water bath. The relative humidity of the humidified gas at 24” C was 88%, whereas the gas taken directly from the cylinder was much lower than 1% relative humidity. The relative humidity was monitored by means of a capacitive polymer film relative humidity sensor (Type MC-2, Panametrics) positioned in the wall of the detector/cell. In order to minimize the loss of the SO,, all tubing, fittings, and components of the gas delivery system were made of either Teflon or glass. Flow rates were controlled by electronic mass flow controllers (Model 825, Edwards High Vacuum Products) calibrated versus an electronic bubble flow meter (Cole-Parmer lnstrument Co.). A gas phase experiment was also conducted at 200 ppb SO, to evaluate the strength of the gas phase SO, XAFS signal at this concentration. During the in situ exposure of copper to SO,, the incident X-ray beam will inevitably sample the sulfur-containing species in the gaseous environment in addition to the condensed phase sulfur-containing species formed on the surface of the copper specimen. If the interference by the gas phase SO, XAFS is not negligible, the collected in situ data must be corrected by subtracting this gas phase data obtained at 200 ppb SO,. The in situ exposure of the evaporated copper film was performed in a helium/oxygen environment (80% He/20% 02) containing 200 ppb (_+20 ppb) SO, at 90% relative humidity ( i4% RH) and 24” C ( + 1”C). Gas concentrations were monitored by a gas analyzer (Fluorescence SO, Analyzer, Model 8850, Monitor Labs) calibrated versus a NIST certified permeation tube of SO,. The flow rate was controlled at 1 I/min through the cell, equivalent to approximately eight volume exchanges per minute. The specimen was exposed to the corrosive environment for a total of 13 h. Sulfur K-edge XAFS experiments were also conducted on several sulfur-containing model compounds - Na,SO, (99.3%. Fisher). Na,SO, (99.8%. Fisher), Na,S20, 5H,O (99.5%. Aldrich), NaHSO, (58.8% assay as SO,, Fisher), Cu,S (99.5%, Aesar), CuS (99.99%, ultra dry, Aesar), CuSCN (99%, Aldrich). The model experiments were done in the TEY detection mode using a commercially available detector (EXAFS Co.).

3. Results and discussion Fig. 3 taken at a step size of 0.05 eV shows the first sulfur K-edge absorption peak (Is-a;) of Na,S,O, .5H,O at 2469.2 eV. The normalized XANES of Na,S,O,. 5HZ0, measured at 0.1 eV/step, is shown in the inset of Fig. 3. The peak assignments in Fig. 3 are after Sekiyama et al. [19]. The FWHM of the first absorption peak (Is-a;) is approximately 1.4 eV, which is larger than the estimated

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effective spectral resolution. This measured width may reflect more than just experimental broadening, as a narrower FWHM (approximately 0.8 eV) was observed for the sulfur K-absorption peak in the XANES of gas phase SO,. Thus, the origin of the broadening seen for the Na,S,O,. 5HzO XANES may not be caused entirely by experimental factors. Instead, the broadening may be attributable to solid state effects, i.e., the observed Is-a; resonance for Na,S,O, .5H,O reflects a transition to a band of af states rather than to a single a; orbital, as would be the case for an isolated gas phase molecule of SO,. Also, for solid state species, there may be variations in the local environment of sulfur due to disorder, multiple sites, hydration, etc., which are not germane to the gaseous state. These structural variations produce an inhomogeneous broadening of edge resonances in solid state XANES [21]. In summary, the observed peak widths are a convolution of the K-level width, the experimental bandwidth, and the final state bandwidth. The ultimate (i.e., lowest) experimental energy bandwidth can be clearly observed in the XANES data of the gas phase SO, experiments. Fig. 4 shows the sulfur K-edge XANES of dry SO, (approximately 107 ppm mixed in helium). The XANES features are well-resolved (FWHM of the first peak = 0.8

0

2460

2480

25ooi

f

L 2465

2466

2467

J

2468

2469

2470

2471

2472

2473

Energy (eV) Fig. 4. Sulfur K-edge absorption peak (Is-n ‘(3b,)) of dry gaseous SO, measured at 0.1 eV step size. The FWHM of the first peak is approximately 0.8 eV. The XANES of SO, is shown in the inset. The peak assignments are after Bodeur and Esteva

La.

,““,““,““,““,““,“’

a

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2466 Energy

2469

2470

2471

2472

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Fig. 3. Sulfur K-edge absorption peak (Is-a; ) of Na,S,O, .5H,O taken at 0.05 eV/step. The FWHM of the first peak is approximately 1.4 eV. The normalized XANES of Na,S,O, .5H,O taken at 0.1 eV/step is shown in the inset. In the inset, A: terminal S

ls-13a; (terminal and central S 3po), B: onset of terminal S 1s ionization, C: central S ls-13a; (terminal and central S Jpcr), D: central S ls-9e; (central S 3~~1, E: central S ls-14a; (central S 3s and 3pa), F: shape resonance. The peak assignments are after Sekiyama et al. [19].

eV), and the overall structure is in agreement with other work [22,23]. In Fig. 4, particularly notable are the first three absorption features: 1) sulfur ls-r * (3b,) bound-state resonance at 2471.2 eV, 2) Is-y * (9a,) shape resonance at 2475.9 eV, and 3) Is-o *(6b,) shape resonance 2476.8 eV [22]. Additional peaks due to Rydberg states reported by Bodeur and Esteva [22] are observed near 2479 eV. In the present gas phase data, the FWHM (approximately 0.8 eV) of the first absorption peak (IS-T * (3b,)) is narrower than the FWHM (approximately 1.4 eV) of the Is-a; peak of Na,S,O, . SH,O. Furthermore, the width (0.8 eV) of the IS-T *(3b,) peak observed here is narrower than that for the transmission data (1.5 eV> reported by Bodeur and Esteva [22]. It is not clear, however, whether their transmission data were broadened by thickness effects or by the experimental conditions, such as the bandwidth of the Ge(ll1) monochromator, the slit width, etc. These observations suggest that the ultimate experimental energy resolution may be best determined by conducting a gas phase experiment with a TEY detector such as one described in the present work. Based on the FWHM of the Is-v * (3b,) peak of the gas phase SO, XANES data, the actual experimental resolution of the present study is estimated to be 0.53 eV. This agrees with the experimental resolution (0.54 eV) estimated in the previous section. This resolution is about the same as the natural width of the sulfur K-level, r tota,, 0.59 eV. As such, this results in a 40% broadening

I. Song et al. / Nucl. Instr. and Meth. in Phys. Rex A 360 (1995) 634-641

of the true spectrum. Using this new value for the experimental resolution, the bandwidth of the final state may be estimated. For Na,S,O, .5H,O, the width of the final state (a; ) is calculated to be approximately 1 eV. Fig. 5 (Curve B) shows the XANES data of humid SOa (107 ppm SO, mixed with helium, 88% relative humidity). Also shown in Fig. 5 are the data taken from dry SO, (Curve A, previously shown in Fig. 4) and the data taken from SO, in transition between humid and dry states (Curves C). The similarity between all three XANES spectra suggests that there are little differences in chemistry (IV + state) and structure (C,, symmetry) between the dry and the humid states of SO,. This indicates either that gaseous SO, and gaseous Hz0 do not form an acid as a result of gas phase mixing or that the amount of acid which forms is undetectably small under the present experimental conditions. It is known that gaseous SO, reacts with liquid Ha0 to form what is commonly referred to as sulfurous acid, i.e., when gaseous SO, is passed through water, it produces an acidic solution with a mild acidity attributed to the ionization of sulfurous acid (H,SO,). However, sulfurous acid has never been isolated as a pure compound, and there is no evidence that H,SO, exists in a molecular form at all [24]. In Fig. 5, it is interesting to note the extremely high pre-edge background for humid SO, (Curve B). This appears to be caused by the capacitive charging of the

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2450

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2470

2480

2490

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Energy (eV) Fig. 5. Sulfur K-edge XANES of “humid” (88% relative humidity) gaseous SO, measured in TEY mode. The sulfur K-edge XANBS of “dry” SO, and that of the transition state between humid and dry are also shown for comparison. (A) Dry SO, in dashed line, (B) Humid SO, (88% RH) in solid line, (C) During the transition from humid to dry state in dotted line.

2450

639

2460

Energy (eV) Fig. 6. In situ sulfur K-edge KANBS of copper film (2500 w on glass) exposed to a humid SO, (200 ppb in He/O, (80%/20%)) environment, shown in solid line. Also shown are two consecutive ex situ measurements: the first measurement in dashed line, and the second one in dotted line.

detector system due to the increased dielectric constant of the gaseous medium between the specimen and the TEY collection grid caused by the introduction of moisture. An additional contribution to the high background may be the thin water layers formed on both electrodes under humid conditions [l], which will cause a change in the electrical response. Therefore, it is advisable to allow sufficient time to obtain the equilibrium background photocurrent after the X-ray photon shutter is opened. Based on the data shown in Fig. 5 and the energy scan rate, a 15-min equilibrium time will be sufficient for the present type of experiments. Fig. 6 shows the in situ sulfur K-edge XANES (indicated by a solid line) of copper exposed to a humid (90% RH), 200 ppb SO,, helium/oxygen (80%/20%) environment. Also shown are two consecutive XANES spectra (the first measurement denoted by a dashed line, and the second one by a dotted line) taken from the same specimen following a dry helium purge of the original in situ exposure environment. Under these high flow rate in situ conditions (1 l/min), the detector showed some degradation in signal/noise, probably because of the vibration of the TEY collection grid. A comparison of the data shown in Fig. 6 reveals obvious changes of the XANES upon introduction of the dry helium environment. It needs to be mentioned that there was no measurable contribution of gaseous SO, (200 ppb) to the measured sulfur K-edge

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XANES data. For these in situ data shown in Fig. 6, the incident photon flux determined from the ionization current through I, (incidence intensity) detector (flow type ionization chamber, He, 1 atm, 15 cm path length, 5 pm polycarbonate window) at 2465 eV (below the sulfur K-edge) with a detune by 80% was approximately 6 X 10” photons/s at approximately 170 rnA of X-ray storage ring current. The in situ total-electron-yield signal (i.e., current) measured at 2490 eV (above the sulfur K-edge) was on the order of 12 pA. The background current measured at 2465 eV (below the sulfur K-edge) was approximately 210 pA. The in situ TEY step height (Ir,,/I,) at the sulfur K-absorption edge was approximately 7 on top of the TEY background (ZTEy/Io) of 121. In order to facilitate the understanding of the detector performance and the tarnish process, the upper limit of sulfur that might have been sampled by the X-ray beam during the in situ measurement has been estimated. The estimation was based on the total consumption of available SO, (200 ppb SO,, 1 l/min for 13 h) by the copper specimen (76 mm X 25 mm). Since the X-ray beam (1 mm X 1 mm) saw only l/25 of the total area of the specimen in the present experimental setup, the estimated maximum amount of sulfur that might have been sampled by X-ray in the form of sulfur-containing corrosion product on the copper surface during the in situ measurement was 280 X 10e9 moles of molecules containing one sulfur atom per molecule. This is equivalent to 9 X 10e6 g of elemental sulfur. In Fig. 6 as the specimen dries out by the introduction of pure dry helium, the relative intensity of the first shoulder near 2466-2470 eV decreases, while the intensities of the second shoulder at approximately 2475 eV and the peak at approximately 2480 eV increase. The sulfur K-edge XANES data of model compounds shown in Fig. 7 suggest that in Fig. 6 the first shoulder near 2466-2470 eV corresponds to S(1 - , II - ), a sulfide; the second shoulder at approximately 2475 eV corresponds to S(IV + ), which includes species such as sulfite; and finally, the peak at approximately 2480 eV corresponds to S(VI + ), which includes sulfate-type species. It is noticed from the normalized XANES of the model compounds shown in Fig. 7 that the absorption intensities of sulfates are the highest and those of sulfides are the lowest. Roughly, the pure sulfates show 5 times the absorption intensities of the pure sulfides. In Fig. 7, it needs to be pointed out that sulfate (SO:-, VI + ) contamination is observed as a peak at approximately 2480 eV for all the sulfides except for the ultradry ones, and these sulfate peaks at 2480 eV are not a part of the characteristic features of sulfide XANES. For the exposed copper specimen, the intensities of the three sulfur-containing species are similar (2:l:l for S(I - , II ):S(IV + ):S(VI + )); however, as revealed by the model compound XANES data, the amount of S(I - , II - >, sulfide formed on copper may be an even larger contribution than it initially appears from the XANES data. Based on the present observations, it is reasonable to

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Fig. 7. Sulfur K-edge XANES of various sulfur-containing model compounds. All spectra shown were normalized to the post-edge of unit absorption. These data were taken using a commercially available TEY detector (EXAFS Co.).

assume that all three species shown in Fig. 6 are condensed phases. The sulfide species are certainly solid state tarnish products, as supported by their low solubility products (K,, < 10-45) [25]. However, it is not certain whether the S(IV + ) and the S(VI + ) peaks in Fig. 6 are from solid state tarnish products or from the species of an aqueous electrolyte. The chemistry and structure of the electrolyte formed by SO, and liquid state H,O needs further investigation. The interpretation of the spectra in detail and the proposal of a mechanism for the sulfidation/oxidation reaction are outside the scope of the present paper. Nevertheless, the results shown in Fig. 6 clearly demonstrate the importance of conducting studies under in situ conditions.

4. Conclusion

A total-electron-yield (TEY) detector was designed and constructed for in situ XAFS measurements of the sulfurcontaining species formed during exposure of copper to a humid atmosphere containing SO,. Using the detector, gas phase XAFS were also measured for both dry and humid SO, atmospheres. In the solid state XANES data for Na2S,0, .5H,O, a spectral broadening by approximately 140% was indicated by the FWHM of the first sulfur absorption peak (Is-a; ).

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This is interpreted as the effect of the final state bandwidth of the solid state specimen in conjunction with the experimental broadening. In comparison, the first sulfur absorption peak (Is-a*(3b,)) in the XANES of gaseous SO, was resolved with a total spectral broadening of approximately 40%. The XANES data of gas phase SO, showed that the structure (C,,) and the chemistry (IV + ) of dry SO, did not change with the introduction of gaseous H,O (88% relative humidity) at room temperature. It was possible to record a sulfur K-edge TEY signal from the in situ condition, namely 20% O2 in helium at 90% relative humidity. The in situ XANES data taken from the surface of copper exposed to a corrosive environment (200 ppb SO, in Oz/He (20%/80%) at 90% RH) showed the presence of three different kinds of sulfur (I - /II - ; IV + ; and VI + ). The relative intensity (i.e., amount) of these species changed with time when the system was flushed with dry helium for ex situ measurements. In summary, the results of the present study highlight the importance of conducting in situ measurements particularly for the investigation of atmospheric corrosion in humid environments.

Acknowledgements The authors acknowledge the support of IBM-SUR (Grant No. 13141, NSF (Grant No. DMR-9015475), and B.A. MacDonald of the Division of Materials Research at the National Science Foundation. B.I. Rickett acknowledges the support of a National Science Foundation Graduate Fellowship. Technical assistance of Dr. Kegang Huang (MSD, Argonne National Laboratory) at X-6B, NSLS, is greatly appreciated. Finally, the general user beamtime (93-X-596) and the faculty-student research support program of NSLS, which is supported by US DOE, Division of Materials Science and Division of Chemical Science, are greatly acknowledged.

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