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Electrochemical cells for in situ EXAFS
Nuclear Instruments and Methods in Physics Research 222 (1984) 347-350 North-Holland, Amsterdam
347
E L E C T R O C H E M I C A L CELLS F O R IN S I T U EXAFS M.E. K O R D E S C H a n d R.W. H O F F M A N Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA
The structural information obtainable from EXAFS can make an important contribution to traditional electrochemical studies of wet electrode surfaces. It is critical that the technique be made sufficiently near-surface sensitive while maintaining a properly controlled electrochemical environment. We report our experience with several electrochemical cells with both X-ray fluorescence and electron detection that were used specifically for the study of iron passivation in buffered borate. The cells may easily be applied to other materials and even corrosive environments.
1. Introduction Typical in situ electrochemical measurements using voltammetry or coulometry do not readily yield direct surface or interface structural information; electron spectroscopy and electron or photon diffraction are often employed in conjunction with "wet electrochemistry" methods. However, they are performed either in vacuum or ex situ environments. There is considerable controversy over the validity of such measurements. Hence, non-invasive, in situ techniques are essential to the understanding of wet electrode surfaces and processes. EXAFS is an ideal spectroscopy for electrochemical systems, since the photon probe can penetrate electrolyte layers and cell windows, as well as provide information about the electrolyte structure in addition to the electrode surface. EXAFS is not inherently surface sensitive, so that selective enhancement of the signal from the electrode under study relative to the remaining cell constituents is necessary. In corrosion, electrodeposition and battery electrode studies, the layers of interest may only be a few nm thick and represent only a small fraction of the total cell or sample dimension. Under these conditions, conventional transmission EXAFS is unable to give useful information. X-ray fluorescence and other detection methods, as well as special cell designs, are required [1]. For an ideal experiment, the interface or layer to be studied would be the sole contributor to the EXAFS signal. A practical approach to in situ electrochemical cells for thin layer studies is based on reducing all but the desired signal, while retaining as much of the in situ character as possible. In addition to the difficulty of obtaining optimal thin film samples, electrolyte contamination by dissolved electrode materials can contribute background signals that are difficult to subtract from the EXAFS data and absorption by the electrolyte 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
can severely reduce the signal strength. For the purposes of this paper we shall assume the present state of the art of fluorescence geometry [2]. The plane of the specimen is at 45 ° with respect to the incident and scattered direction. K# filtering of #x = ~ 5 appears optimal and chevron baffles reduce the effect of scattered radiation. For fluorescence, ion chambers are commonly used as detectors, with nitrogen for the incident and argon for the fluorescent ion chamber. The signals are ratioed to provide the data in the form of #x as a function of photon energy. Although the sensitivity of such a technique in fluorescence consists of submonolayer quantities, we are specifically concerned with the electrochemically grown layer on the clean surface of the same element. As a result, the specimen thickness or detection scheme must be adjusted such that the EXAFS signal from the overlayer is either larger or spectrally distinguishable from the underlying substrate, so that deconvolution is reliable. The sample structure for the passivation experiments used an Fe film of controlled thickness between 2-4 nm vacuum deposited onto a 20 nm thick conducting gold electrode which was itself evaporated onto a polyester (Melinex 505) [3] substrate, thus providing a sample and electrode structure which is transparent to the X-rays and is also sufficiently conducting to ensure uniform polarization of the electrode. Such a specimen was used in several cell geometries. A summary of the experimental procedure and the results of earlier in situ EXAFS and MES measurements of passive films are given in the review by Hoffman [4]. Each cell was sealed from the atmosphere and contained a reference and counter electrode for potentiostatic control of the Fe film working electrode. In addition, the electrolyte was deaerated prior to introduction into the cell. The cell could be flushed with VI. SPECIAL TECHNIQUES
M.E. Kordesch, R. IF. Hoffman / Electrochemical cells for in situ EXAFS
348
clean electrolyte if necessary, during the experiment. Except for the emersed electrode cell, to be described later, an electrolyte contamination in the ppm range could contribute an observable background signal. o
2. Cell design and performance o
2.1. Thin electrolyte cells
Fig. 1 shows a sagittal view of a variable thickness "bag cell" (the sample makes a 45 ° angle with the beam and detector) in the sample chamber of a standard fluorescence detector. The most significant features of this cell are the remotely controlled vise arrangement that allows expansion of the flexible cell and the retractable reference and counter electrodes that may be withdrawn above the beam after the electrode has been polarized. In the passivation experiments, the iron films were cathodically reduced at current densities of a few hundred # A / c m 2, with the counter electrode directly facing the sample. The electrolyte was then replaced with fresh solution, while the film was cathodically protected. The potential was then stepped into the passive region. The initial current surge is high and drops a few orders of magnitude in seconds. After the current stabilizes (typically 1 #A/cm2), the counter electrode is retracted above the vise and the bag squeezed between its jaws. The current flowing through the cell is monitored continually throughout the passivation; there is no significant variation in the current before the bag is squeezed compared to afterwards, although some noise is observed during the electrode retraction and squeezing. Fig. 2 shows fluorescence EXAFS data for an initially 4 nm thick iron film in the various stages of the
i
,
)<
D
j
J r h
I 69
7.1
I
I 75
73
ENERGY
in
c
I 77
I 79
k eV
Fig. 2. (a) X-ray fluorescence EXAFS of 4 nm iron film, dry; (b) cathodic protection; (c) passive; (d) electrolyte and background signal.
experiment. The spectrum 2a is of the dry film in the cell. In fig. 2b, the same film is shown under cathodic protection with the bag squeezed. The bag was expanded, the film passivated and again squeezed; the spectrum from the passivated film is shown in fig. 2c. A spectrum of the electrolyte with all the electrodes removed (including sample) is shown in fig. 2d. Although the bag-cell is electrochemically advantageous, the loss of signal when the electrolyte is added to the cell is unavoidable and larger than the loss of iron during cathodic reduction, but subsequently removed from the cell. The approximately 50% signal loss between wet and dry samples is further complicated by the background contribution (fig. 2d), even though the cell was flushed with clean borate. A 4 nm film is probably optimal for use in this experiment. Thinner (2 nm) films were also passivated in this cell; however, the background signal approaches that of the film, making data analysis difficult. We emphasize that this background
H I
G
I
F
I
.F
B
X
:=K
Fig. 1. (a) aluminum fluorescencechamber, sealed with Kapton tape windows; (b) vise assembly; (c) linear motion device; (d) polyethylene bag; (e) Pd-H reference electrode; (f) gold counter electrode; (g) iron film working electrode; (h) teflon bag closure; (i) evaporated iron film on gold film substrate, front view.
I
t
7
8 ENERGY
qn
a
9
keY
Fig. 3. (a) X-ray fluorescence EXAFS of 8 nm iron film taken through the substrate, liquid in cell; (b) without solution.
349
M.E. Kordesch, R. IV. Hoffman / Electrochemical cells for in situ EXAFS
may result from residual electrolyte contamination, as well as impurities in, or contamination of, the cell components. T h e S / N for fluorescence geometry may be improved by making the sample substrate and cell as X-ray transparent as possible, eliminating the undesirable scattering from these materials into the fluorescence detector. A simplification of the variable thickness cell was made using the sample structure as the entrance window of a fixed thickness lucite cell. The fluorescence EXAFS spectrum was acquired directly through the substrate, avoiding the electrolyte, the extra layer of bag and the necessity of withdrawing the reference and counter electrodes from their optimal placement. Fig. 3 shows two spectra of an 8 nm film with (a) and without (b) solution in this type of cell. These spectra show the effects of scattering from the solution below the absorption edge and far above the edge, but no significant difference in the edge step height. We have not yet used this type of cell in actual experiments. 2.2. Emersed cell construction
An additional improvement may be made by minimizing the electrolyte thickness. In suitably chosen systems, an electrode may be removed from the electrolyte while under potential control, without altering the electrode electrolyte interface [5]. Such an emersed electrode would be covered by an electrolyte layer less than 10 nm thlck [6,7]. When the electrode is maintained in a closed cell saturated with water vapor, there appears to be no change in the film electrochemistry [8]. A nearly in situ compromise short of complete emersion of the electrode may be accomplished by continually rotating a disk shaped electrode into the electrolyte, so that the bottom half is in the solution under potential control and the upper portion is analyzed. Such a geometry has
\
e:'½ ,'Z ,C
\
\ ~
.\
- -
\
Fig. 4. (a) aluminum fluorescence chamber, sealed with Kapton tape windows; (b) lucite cell; (c) electron detector, removed during X-ray fluorescence measurements; (d) Pd-H reference electrode; (e) gold foil counter electrode; (f) iron disk working electrode; (g) teflon bushing; (h) carbon brush assembly and brass shaft contact to working electrode; (i) phenolic insulator; (j) brass shaft to motor.
i
I
i
I /
/
-
o x
6.9
7.1
D , 7.5 ENERGY
I
I 7.5
in
I k
I 7.7
I
7.9
eV
Fig. 5. (a) X-ray fluorescence EXAFS of 4 nm iron film in emersion cell, dry; (b) cathodic protection; (c) passive; (d) blank electrode in cell.
two advantages: the extremely thin electrolyte layer in the beam would allow only a grossly contaminated electrolyte to contribute to the EXAFS signal and only negligible absorption of the fluorescent radiation takes place in the electrolyte remaining on the electrode surface. The emersed specimen concept allows the use of fluorescent X-ray detection, or detection of high energy electrons. Fig 4 shows the emersed electrode cell with the electron detector in place in front of the sample. For X-ray fluorescence detection, the electron counter is removed and the complete cell assembly turned 45 ° . This design provides good electrochemical control of the sample electrode while it is immersed; however, some loss of the in situ character may result during emersion. A 4 nm iron film passivated in this cell with the electrode rotating is shown in fig. 5; dry (a), cathodically protected (b) and passive (c). Although the polarization of the electrode was accomplished with the sample rotating (0.2 rpm), the sample was held stationary during the data acquisition. The current behavior in the emersed cell is similar to that in the bag cell. There is a periodic variation in the current due to the rotation and shape of the electrode; however, the current density at passive potentials is comparable to that of the bag cell. The nonuniformity of the evaporated iron films and the particular rotation speed chosen introduced undesirable background signals into the EXAFS spectra. An approximately 10% variation in sample thickness caused a corresponding variation in the EXAFS signal. Since the disk made about 4 rotations during one 20 min scan, "humps" in the background with this 5 rain period were observed. In principle, rotation of the sample does not interfere with the EXAFS spectra. "Spinners" rotating at several hundred rpm have been used to average over sample inhomogeneities. In this case, either a more VI. SPECIAL TECHNIQUES
350
M.E. Kordesch, R.W. Hoffman / Electrochemical cells for in situ EXAFS
electron Mossbauer spectra of wet electrodes have been successfully acquired [8]. The near surface sensitivity of electron E X A F S and the non-vacuum, non-thin film character of these measurements make them suited to corrosion and surface modification studies.
5.0
4O c 30 o == o 20
3. Summa~
×
O0 60
70 ENERGY
80 in
90
keV
Fig. 6. (a) electron yield EXAFS of 1 mm thick iron plate; (b) transmission EXAFS of 5 pm thick iron foil; (c) same as (a) with tape on surface of plate to block electrons.
uniform sample or a faster rotation speed would overcome these difficulties. We have successfully made several scans at short sampling times (0.2 s / p t compared to 2 s / p t in fig. 4 or 4 s / p t in fig. 2) and added these spectra. The advantage of the emersed electrode fluorescence detection mode is the absence of a thick electrolyte layer in front of the electrode. As is clear from fig. 5, there is no significant loss of signal between wet and dry spectra. (The small variation between the edge height in (a)-(c) is due to the rotation of the sample and its thickness variation.) A blank electrode (gold, no iron) rotated through iron containing electrolyte gave no observable edge and is qualitatively the same as fig. 4d. The actual thickness of the emersed electrolyte layer is not known; however, it may be possible to measure the thickness using a known Fe concentration in solution. Another advantage of emersion is the possibility of performing electron yield E X A F S on the wet electrode surface. Secondary electrons are emitted after the absorption process. Normally electron detection is restricted to UHV, but high energy Auger electrons may be detected using a helium gas flow detector operated as an ion chamber [9]. The detector assembly, shown in fig. 4, has been used to acquire electron yield E X A F S from a 1 mm thick iron plate; this spectrum is shown in fig. 6a with an iron foil transmission spectrum (b) and a spectrum of the plate with tape over the surface to block the electrons (c). Although the electron detector has not yet been tested in wet environments, similar conversion
The cell designs described above provide a means for surface layer E X A F S in wet, electrochemically controlled environments. Varying degrees of in situ measurements are possible; ranging from an electrode surrounded by bulk electrolyte to a multiply or continually emersed film. Iron layers as thin as 1 nm are easily analyzed with X-ray fluorescence E X A F S with good S / N in as little as 2 rain; however, the sensitivity of these techniques allows for substantial improvement. In all of the cells described, the reduction of background signals in the sample-substrate structure and contamination free cell materials are necessary. We would like to thank J.A. Mann, J. Rusek, J. Wainright, D. Sandstrom and J. Eldridge for their contribution to this work. This work was supported by the Office of Naval Research Grant N00014-79C-0795. SSRL is supported by the National Science Foundation and the US Department of Energy.
References [1] P.A. Lee, P.H. Citrin, P. Eisenberger and B.M. Kincaid, Rev. Mod. Phys. 54 (1981) 769. [2] E.A. Stern and S.M. Heald, Rev. Sci. Instr. 50 (1979) 1579. [3] M.E. Kordesch and R.W. Hoffman, Thin Solid Films 107 (1983) 365. [4] R.W. Hoffman, 5th Int. Conf. on Passivity, Bordeaux, France, 1983. (Elsevier, Amsterdam, 1983) p. 147. [5] W.N. Hansen and D.M. Kolb, J. Electroanal. Chem. 110 (1980) 369. [6] F.T. Wanger and P.N. Ross, J. Electrochem. Soc. 130 (8) (1983) 1789. [7] J.G. Gordon I1, Extended abstract 82-1 (658), Electrochemical Society, 1982. [8] M.E. Kordesch, J. Eldridge, D. Scherson and R.W. Hoffman, Extended abstract 83-1 (55), Electrochemical Society, 1983. [9] M.E. Kordesch and R.W. Hoffman, Phys. Rev. B29 (1984) 491.
347
E L E C T R O C H E M I C A L CELLS F O R IN S I T U EXAFS M.E. K O R D E S C H a n d R.W. H O F F M A N Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA
The structural information obtainable from EXAFS can make an important contribution to traditional electrochemical studies of wet electrode surfaces. It is critical that the technique be made sufficiently near-surface sensitive while maintaining a properly controlled electrochemical environment. We report our experience with several electrochemical cells with both X-ray fluorescence and electron detection that were used specifically for the study of iron passivation in buffered borate. The cells may easily be applied to other materials and even corrosive environments.
1. Introduction Typical in situ electrochemical measurements using voltammetry or coulometry do not readily yield direct surface or interface structural information; electron spectroscopy and electron or photon diffraction are often employed in conjunction with "wet electrochemistry" methods. However, they are performed either in vacuum or ex situ environments. There is considerable controversy over the validity of such measurements. Hence, non-invasive, in situ techniques are essential to the understanding of wet electrode surfaces and processes. EXAFS is an ideal spectroscopy for electrochemical systems, since the photon probe can penetrate electrolyte layers and cell windows, as well as provide information about the electrolyte structure in addition to the electrode surface. EXAFS is not inherently surface sensitive, so that selective enhancement of the signal from the electrode under study relative to the remaining cell constituents is necessary. In corrosion, electrodeposition and battery electrode studies, the layers of interest may only be a few nm thick and represent only a small fraction of the total cell or sample dimension. Under these conditions, conventional transmission EXAFS is unable to give useful information. X-ray fluorescence and other detection methods, as well as special cell designs, are required [1]. For an ideal experiment, the interface or layer to be studied would be the sole contributor to the EXAFS signal. A practical approach to in situ electrochemical cells for thin layer studies is based on reducing all but the desired signal, while retaining as much of the in situ character as possible. In addition to the difficulty of obtaining optimal thin film samples, electrolyte contamination by dissolved electrode materials can contribute background signals that are difficult to subtract from the EXAFS data and absorption by the electrolyte 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
can severely reduce the signal strength. For the purposes of this paper we shall assume the present state of the art of fluorescence geometry [2]. The plane of the specimen is at 45 ° with respect to the incident and scattered direction. K# filtering of #x = ~ 5 appears optimal and chevron baffles reduce the effect of scattered radiation. For fluorescence, ion chambers are commonly used as detectors, with nitrogen for the incident and argon for the fluorescent ion chamber. The signals are ratioed to provide the data in the form of #x as a function of photon energy. Although the sensitivity of such a technique in fluorescence consists of submonolayer quantities, we are specifically concerned with the electrochemically grown layer on the clean surface of the same element. As a result, the specimen thickness or detection scheme must be adjusted such that the EXAFS signal from the overlayer is either larger or spectrally distinguishable from the underlying substrate, so that deconvolution is reliable. The sample structure for the passivation experiments used an Fe film of controlled thickness between 2-4 nm vacuum deposited onto a 20 nm thick conducting gold electrode which was itself evaporated onto a polyester (Melinex 505) [3] substrate, thus providing a sample and electrode structure which is transparent to the X-rays and is also sufficiently conducting to ensure uniform polarization of the electrode. Such a specimen was used in several cell geometries. A summary of the experimental procedure and the results of earlier in situ EXAFS and MES measurements of passive films are given in the review by Hoffman [4]. Each cell was sealed from the atmosphere and contained a reference and counter electrode for potentiostatic control of the Fe film working electrode. In addition, the electrolyte was deaerated prior to introduction into the cell. The cell could be flushed with VI. SPECIAL TECHNIQUES
M.E. Kordesch, R. IF. Hoffman / Electrochemical cells for in situ EXAFS
348
clean electrolyte if necessary, during the experiment. Except for the emersed electrode cell, to be described later, an electrolyte contamination in the ppm range could contribute an observable background signal. o
2. Cell design and performance o
2.1. Thin electrolyte cells
Fig. 1 shows a sagittal view of a variable thickness "bag cell" (the sample makes a 45 ° angle with the beam and detector) in the sample chamber of a standard fluorescence detector. The most significant features of this cell are the remotely controlled vise arrangement that allows expansion of the flexible cell and the retractable reference and counter electrodes that may be withdrawn above the beam after the electrode has been polarized. In the passivation experiments, the iron films were cathodically reduced at current densities of a few hundred # A / c m 2, with the counter electrode directly facing the sample. The electrolyte was then replaced with fresh solution, while the film was cathodically protected. The potential was then stepped into the passive region. The initial current surge is high and drops a few orders of magnitude in seconds. After the current stabilizes (typically 1 #A/cm2), the counter electrode is retracted above the vise and the bag squeezed between its jaws. The current flowing through the cell is monitored continually throughout the passivation; there is no significant variation in the current before the bag is squeezed compared to afterwards, although some noise is observed during the electrode retraction and squeezing. Fig. 2 shows fluorescence EXAFS data for an initially 4 nm thick iron film in the various stages of the
i
,
)<
D
j
J r h
I 69
7.1
I
I 75
73
ENERGY
in
c
I 77
I 79
k eV
Fig. 2. (a) X-ray fluorescence EXAFS of 4 nm iron film, dry; (b) cathodic protection; (c) passive; (d) electrolyte and background signal.
experiment. The spectrum 2a is of the dry film in the cell. In fig. 2b, the same film is shown under cathodic protection with the bag squeezed. The bag was expanded, the film passivated and again squeezed; the spectrum from the passivated film is shown in fig. 2c. A spectrum of the electrolyte with all the electrodes removed (including sample) is shown in fig. 2d. Although the bag-cell is electrochemically advantageous, the loss of signal when the electrolyte is added to the cell is unavoidable and larger than the loss of iron during cathodic reduction, but subsequently removed from the cell. The approximately 50% signal loss between wet and dry samples is further complicated by the background contribution (fig. 2d), even though the cell was flushed with clean borate. A 4 nm film is probably optimal for use in this experiment. Thinner (2 nm) films were also passivated in this cell; however, the background signal approaches that of the film, making data analysis difficult. We emphasize that this background
H I
G
I
F
I
.F
B
X
:=K
Fig. 1. (a) aluminum fluorescencechamber, sealed with Kapton tape windows; (b) vise assembly; (c) linear motion device; (d) polyethylene bag; (e) Pd-H reference electrode; (f) gold counter electrode; (g) iron film working electrode; (h) teflon bag closure; (i) evaporated iron film on gold film substrate, front view.
I
t
7
8 ENERGY
qn
a
9
keY
Fig. 3. (a) X-ray fluorescence EXAFS of 8 nm iron film taken through the substrate, liquid in cell; (b) without solution.
349
M.E. Kordesch, R. IV. Hoffman / Electrochemical cells for in situ EXAFS
may result from residual electrolyte contamination, as well as impurities in, or contamination of, the cell components. T h e S / N for fluorescence geometry may be improved by making the sample substrate and cell as X-ray transparent as possible, eliminating the undesirable scattering from these materials into the fluorescence detector. A simplification of the variable thickness cell was made using the sample structure as the entrance window of a fixed thickness lucite cell. The fluorescence EXAFS spectrum was acquired directly through the substrate, avoiding the electrolyte, the extra layer of bag and the necessity of withdrawing the reference and counter electrodes from their optimal placement. Fig. 3 shows two spectra of an 8 nm film with (a) and without (b) solution in this type of cell. These spectra show the effects of scattering from the solution below the absorption edge and far above the edge, but no significant difference in the edge step height. We have not yet used this type of cell in actual experiments. 2.2. Emersed cell construction
An additional improvement may be made by minimizing the electrolyte thickness. In suitably chosen systems, an electrode may be removed from the electrolyte while under potential control, without altering the electrode electrolyte interface [5]. Such an emersed electrode would be covered by an electrolyte layer less than 10 nm thlck [6,7]. When the electrode is maintained in a closed cell saturated with water vapor, there appears to be no change in the film electrochemistry [8]. A nearly in situ compromise short of complete emersion of the electrode may be accomplished by continually rotating a disk shaped electrode into the electrolyte, so that the bottom half is in the solution under potential control and the upper portion is analyzed. Such a geometry has
\
e:'½ ,'Z ,C
\
\ ~
.\
- -
\
Fig. 4. (a) aluminum fluorescence chamber, sealed with Kapton tape windows; (b) lucite cell; (c) electron detector, removed during X-ray fluorescence measurements; (d) Pd-H reference electrode; (e) gold foil counter electrode; (f) iron disk working electrode; (g) teflon bushing; (h) carbon brush assembly and brass shaft contact to working electrode; (i) phenolic insulator; (j) brass shaft to motor.
i
I
i
I /
/
-
o x
6.9
7.1
D , 7.5 ENERGY
I
I 7.5
in
I k
I 7.7
I
7.9
eV
Fig. 5. (a) X-ray fluorescence EXAFS of 4 nm iron film in emersion cell, dry; (b) cathodic protection; (c) passive; (d) blank electrode in cell.
two advantages: the extremely thin electrolyte layer in the beam would allow only a grossly contaminated electrolyte to contribute to the EXAFS signal and only negligible absorption of the fluorescent radiation takes place in the electrolyte remaining on the electrode surface. The emersed specimen concept allows the use of fluorescent X-ray detection, or detection of high energy electrons. Fig 4 shows the emersed electrode cell with the electron detector in place in front of the sample. For X-ray fluorescence detection, the electron counter is removed and the complete cell assembly turned 45 ° . This design provides good electrochemical control of the sample electrode while it is immersed; however, some loss of the in situ character may result during emersion. A 4 nm iron film passivated in this cell with the electrode rotating is shown in fig. 5; dry (a), cathodically protected (b) and passive (c). Although the polarization of the electrode was accomplished with the sample rotating (0.2 rpm), the sample was held stationary during the data acquisition. The current behavior in the emersed cell is similar to that in the bag cell. There is a periodic variation in the current due to the rotation and shape of the electrode; however, the current density at passive potentials is comparable to that of the bag cell. The nonuniformity of the evaporated iron films and the particular rotation speed chosen introduced undesirable background signals into the EXAFS spectra. An approximately 10% variation in sample thickness caused a corresponding variation in the EXAFS signal. Since the disk made about 4 rotations during one 20 min scan, "humps" in the background with this 5 rain period were observed. In principle, rotation of the sample does not interfere with the EXAFS spectra. "Spinners" rotating at several hundred rpm have been used to average over sample inhomogeneities. In this case, either a more VI. SPECIAL TECHNIQUES
350
M.E. Kordesch, R.W. Hoffman / Electrochemical cells for in situ EXAFS
electron Mossbauer spectra of wet electrodes have been successfully acquired [8]. The near surface sensitivity of electron E X A F S and the non-vacuum, non-thin film character of these measurements make them suited to corrosion and surface modification studies.
5.0
4O c 30 o == o 20
3. Summa~
×
O0 60
70 ENERGY
80 in
90
keV
Fig. 6. (a) electron yield EXAFS of 1 mm thick iron plate; (b) transmission EXAFS of 5 pm thick iron foil; (c) same as (a) with tape on surface of plate to block electrons.
uniform sample or a faster rotation speed would overcome these difficulties. We have successfully made several scans at short sampling times (0.2 s / p t compared to 2 s / p t in fig. 4 or 4 s / p t in fig. 2) and added these spectra. The advantage of the emersed electrode fluorescence detection mode is the absence of a thick electrolyte layer in front of the electrode. As is clear from fig. 5, there is no significant loss of signal between wet and dry spectra. (The small variation between the edge height in (a)-(c) is due to the rotation of the sample and its thickness variation.) A blank electrode (gold, no iron) rotated through iron containing electrolyte gave no observable edge and is qualitatively the same as fig. 4d. The actual thickness of the emersed electrolyte layer is not known; however, it may be possible to measure the thickness using a known Fe concentration in solution. Another advantage of emersion is the possibility of performing electron yield E X A F S on the wet electrode surface. Secondary electrons are emitted after the absorption process. Normally electron detection is restricted to UHV, but high energy Auger electrons may be detected using a helium gas flow detector operated as an ion chamber [9]. The detector assembly, shown in fig. 4, has been used to acquire electron yield E X A F S from a 1 mm thick iron plate; this spectrum is shown in fig. 6a with an iron foil transmission spectrum (b) and a spectrum of the plate with tape over the surface to block the electrons (c). Although the electron detector has not yet been tested in wet environments, similar conversion
The cell designs described above provide a means for surface layer E X A F S in wet, electrochemically controlled environments. Varying degrees of in situ measurements are possible; ranging from an electrode surrounded by bulk electrolyte to a multiply or continually emersed film. Iron layers as thin as 1 nm are easily analyzed with X-ray fluorescence E X A F S with good S / N in as little as 2 rain; however, the sensitivity of these techniques allows for substantial improvement. In all of the cells described, the reduction of background signals in the sample-substrate structure and contamination free cell materials are necessary. We would like to thank J.A. Mann, J. Rusek, J. Wainright, D. Sandstrom and J. Eldridge for their contribution to this work. This work was supported by the Office of Naval Research Grant N00014-79C-0795. SSRL is supported by the National Science Foundation and the US Department of Energy.
References [1] P.A. Lee, P.H. Citrin, P. Eisenberger and B.M. Kincaid, Rev. Mod. Phys. 54 (1981) 769. [2] E.A. Stern and S.M. Heald, Rev. Sci. Instr. 50 (1979) 1579. [3] M.E. Kordesch and R.W. Hoffman, Thin Solid Films 107 (1983) 365. [4] R.W. Hoffman, 5th Int. Conf. on Passivity, Bordeaux, France, 1983. (Elsevier, Amsterdam, 1983) p. 147. [5] W.N. Hansen and D.M. Kolb, J. Electroanal. Chem. 110 (1980) 369. [6] F.T. Wanger and P.N. Ross, J. Electrochem. Soc. 130 (8) (1983) 1789. [7] J.G. Gordon I1, Extended abstract 82-1 (658), Electrochemical Society, 1982. [8] M.E. Kordesch, J. Eldridge, D. Scherson and R.W. Hoffman, Extended abstract 83-1 (55), Electrochemical Society, 1983. [9] M.E. Kordesch and R.W. Hoffman, Phys. Rev. B29 (1984) 491.