more expectable manner. The peak is seen to become narrower as the rate increases or decreases. I t eventually approaches the same wave form in the extremely high and low regions. Although the magnitudes of S and E in these regions seem to be a little too high, they tend to cancel each other out when T is small. Thus, the upper portion of the profile becomes symmetrical and approaches the gaussian form as its limit. On the wings of the profile where T is large, more terms in the Gram-Charlier series must be considered. Figures 3 and 4 give the dependence of ml, mz, S , and E on kl and k-1, respectively. The rate dependence of S and E is seen to be more involved than that of the simple moments in all cases. The same is observed for the k and k' dependences. Although S and E may be more conveniently measured experimentally, the simple moments are seen to be more useful in the determination of on-column reaction rates. Since the log-log plot of the moment vs. the constant is very close to a parabolic function, a second-order equation with coefficients containing all other rate constants can be established for each moment. With the use of a set of these equations, one can estimate the on-column reaction rates in a real measurement. In principle, only the first four moments, Le., ml, m2, m3, and m4, or ml, m ~m, 3 , andm4, are required to calculate the four rate constants, h, k', hl, and k-1.
ACKNOWLEDGMENT We wish to thank S. H. Lin for many helpful discussions.
LITERATURE CITED (1) S. Dal Nogare and R. S. Juvet, Jr., "Gas-Liquid Chromatography", Interscience, New York, 1962. (2) G. S. G. Phillips, in "Gas Chromatography, 1970", R. Stock and S. G. Perry, Ed., Elsevier Publishing Co., New York. 1971. (3) C. S. G. Phillips, A. J. Hart-Davis, R. G. L. Saul, and J. Wormald, J. Gas Cbromatogr., 5, 424 (1967). (4) J. C. Giddings. "Dynamics of Chromatography, Part I: Principles of Theory", Marcel Dekker. New York, 1965. (5)J. C. Giddings in "Gas Chromatography, 1964", A. Goldup, Ed., Elsevier Publishing Co.. Amsterdam, 1964. (6) H. Purnell, "Gas Chromatography", John Wiley & Sons, New York. 1962. (7) J. Kallen and E. Heilbronner, Helv. Cbim. Acta, 43, 489 (1960). (8) R. A. Keller and J. C. Giddings, J. Cbromatogr., 3, 205 (1960). (9) D. W. Bassett and H. W. Habgood, J. Pbys. Cbem., 64, 769 (1960). (10) K. P. Li, D. L. Duewer, and R. S. Juvet, Jr., Anal. Chem., 46, 1209 (1974). (11) J. C. Giddings and H. Eyring, J. Pbys. Cbem., 59, 416 (1955). (12) E. Kucera, J. Cbromatogr., 19, 273 (1965). (13) 0. Grubner, Adv. Cbromatogr., 6, 173 (1968). (14) 0. Grubner and E. Kucera, "Gas Chromatographic 1965". Vortrage des 5. Symposium uber Gas Chromatographie, Akademie Veiag, Berlin, 1965. (15) 0. Grubner. Anal. Chem., 43, 1934 (1971). (16) E. Grushka, M. N. Myers, and J. C. Giddings, Anal. Chem., 42, 21 (1970). (17) E. Grushka, M. N. Myers, P. D. Schettler, and J. C. Giddings, Anal. Cbem., 41, 889 (1969). (18) H. Margenan and G. M. Murphy, "The Mathematics of Physics and Chemistry", D. Van Nostrand Company, Princeton, N.J., 2nd ed., 1964.
RECEIVEDfor review September 25, 1975. Accepted January 16, 1976. Financial support from the University Computing Support, University of Florida is gratefully acknowledged.
Electrochemical and Surface Characteristics of Tin Oxide and Indium Oxide Electrodes Neal R. Armstrong,' Albert W. C. Lin, Masamichi Fujihira,2 and Theodore Kuwana* Department of Chemistry, Ohio State University, Columbus, Ohio 432 10
The electrochemical characteristics of heavily doped tin oxide and indium oxide thin film electrodes have been correlated with results of surface analyses by x-ray photoelectron (ESCA) and Auger spectroscopy. From measurement of current, capacitance, and surface conductance as a function of the applied electrode potential, reglons of potential where surface reactions were possibly occurring could be delineated. ESCA/Auger analyses of these electrodes, which were poised in various potential regions, confirmed the changes in the stoichiometry of the metal oxides at the surface. Similar analyses were performed on tin oxide surfaces which had been modified by derivatization of the surface. Such modifications resulted in the lowering of the effective carrier density of the yrface. Depth and extent of coverage of these modified electrodes could be inferred from the Auger analysis coupled with argon ion sputtering of the surface.
During the last decade, thin conductive films on various substrates have been increasingly employed as electrodes. Present address, D e p a r t m e n t of Chemistry, M i c h i g a n State University, East Lansing, M i c h . 48824. Present address, T o h o k u University, Pharmaceutical Institute, Aobayama, Sendai, Japan.
*
These films have been made from vapor deposited or evaporated noble metals or from thermally decomposed and deposited metal oxides. In addition to their use for fundamental studies of charge-transfer processes, such films deposited on transparent substrates have been widely employed for optically coupled electrochemical experiments (1-10). One prime example is transparent electrodes made from tin oxide (SnOz) films on glass or quartz which were introduced some ten years ago ( I ) . The electrochemical and surface characteristics of films made from noble metals, Le., Pt and Au, are fairly well understood because of their similarities to the extensively studied bulk metal electrodes. On the other hand, metal oxide films are less well understood. Part of the problem arises from the fact that the film properties are quite dependent on the various fabrication methods. For tin oxide, a surface layer of 0.5- to 1-p thick film is produced by thermal decomposition of an acidic solution of SnC14 sprayed onto a heated glass or quartz substrate ( I I 14). The pure tin oxide films are polycrystalline in nature, and the conductivity is believed to be due to a defect structure with oxygen deficiencies and/or interstitial tin, as well as the incorporation of chlorine impurities in the crystalline lattice of the cassiterite structure (12, 14). The high conductivity of these films has stimulated many studies of their electrical and optical properties (14-20). More reANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
* 741
cently, various rf and ion sputtering techniques have been used to produce high quality semiconductor films (SnO2 and Inz03)from pressed targets of metal oxides (21-23). Doped tin oxide is most commonly made by the addition of antimony. With replacement of the Sn(1V) in the lattice sites of tin oxide by Sb(III), the carrier density (n-type) can be increased to greater than lozo carriers/cm3. Surface resistances of less than about 15 R/sq. can be achieved. These SnO2 films have served as fairly inert and chemically stable surfaces for electrochemical studies. The fundamental electrochemical properties of SnOz electrodes have been studied by Memming and Mollers (10, 24), and Laitinen and coworkers (11, 12, 25). The space charge capacitance in the anodic region followed the MottSchottky relationship, where 1/C2 was linear with the applied potential. The extrapolated intercept of the 1/C2 plot gave a flat band potential which was linearly dependent on p H (60 mV/pH). This dependence was rationalized on the basis of a possible surface acid/base equilibrium: -SnOH e -SnOor
, -SnOH
-Sn+
+ HC
+ OH-
(1)
(2)
Kirkov (13) has speculated about the chemistry of the SnOz surface, particularly for changes caused by anodic or cathodic polarizations, from potential-time curves recorded during constant current perturbations followed by open circuit relaxation. His observations, coupled with others, suggested that potential dependent surface changes such as reduction of tin(1V) oxide to tin(I1) or tin(0) occurred a t negative potentials. However, there has been no direct euidence of such changes in the chemical composition or elemental valences by independent physical methods. The same holds true for the tin doped indium oxide films (on glass) which have been used as optically transparent electrodes (OTE) recently in our laboratory. In this paper, the electrochemical characteristics (current-potential, charge-potential, and surface conductancepotential) of doped tin oxide and indium oxide film electrodes will be discussed and compared to those of Pt and Au film electrodes. Also, the macroscopic surface structure, the elemental composition and distribution, and elemental valence states which have been determined by scanning electron microscopy (SEM), x-ray fluorescence spectroscopy, Auger spectroscopy and x-ray photoelectron spectroscopy (ESCA) will be presented. Recently, there has been a great deal of interest in the chemical modification of surfaces (26, 27) as a means of producing selective and catalytic electrodes. Our objective in surface modification has been to directly bind coenzymes, enzymes, and mediators which participate in selective charge-transfer reactions. Initial efforts have been devoted to the study of binding methods and the development of analytical methods for characterizing these surfaces. Both Auger and ESCA have served admirably in identifying the surface elemental and compositional changes and also in providing semiquantitative information about the extent of the surface coverage. Preliminary analytical results on such modified oxide surfaces will be discussed.
EXPERIMENTAL The basic cell design used for the surface conductance and electrochemical studies has been described previously (28). This design allows sandwich-like positioning of the semiconductor film electrode to a Lucite cell body with cell volume of approximately 1 ml. For determination of capacitance, the electrode area was decreased (0.2 cm2 geometric area) to minimize problems due to fre742
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
2.0 I
1.5 I
1.0
0.5 I
0.0 I
-0.5
1
Volts vs, Ag/AgCl
Figure 1. Comparison of current voltage curves for Pt, Au, SnOn and ln203electrodes in 1N H2S04 media
Potential scan rate was 87 mV/s. All electrodes were I cm2 in geometric area. ( a )Platinum thin film, ( b ) gold thin film, (c)Sn02thin film, (d) ln203 thin film
Table I. Tabulation of Effective Carrier Density (no) Values for Some SnO, and In,O, Electrodes Value of no (carriers/cm3) Electrode
Method: Plasma frequency
Capacitancea
Auger
SnO,/Sb 1x lo2' 3.8 x l o z o 8 x lozo In, 0, /Sn 9 x lozo 1.2 x l o z 1 3-9 x loz1 a no values f o r o t h e r SnO,/Sb surfaces d e t e r m i n e d by capacitance a r e r e p o r t e d in ref. (11)and ( 2 4 ) .
quency dispersion. The solid state potentiostat was similar to those previously described. An amplifier/bridge circuit (28) was used to measure the dc conductance of the films. The conductances were tabulated as a conductance change (AC) divided by an initial surface conductance (C), which was obtained by balancing the bridge circuit while the electrode was held a t a given rest potential. When the electrode potential was scanned, the output from the bridge corresponded to the ratio (ACIC). Charge/potential data for the electrodes were obtained in a similar manner. An integrating, operational amplifier circuit was used, the output was zeroed while the electrode potential was held constant as before. The charge/potential curves were therefore referenced to this initial charge value. Capacitance measurements were made by the method of Gileadi and co-workers (29). A small amplitude triangular wave of between 100 Hz and 10 kHz was superimposed on a slowly changing, linear ramp potential. Calibration of the output current response was made by means of an external circuit. Compensation for the iR drop between the reference and the working electrode was made by the usual positive feedback to the control amplifier of the potentiostat. The antimony doped SnOz on 0.38-in. thick glass was obtained from Corning Glass Co. The tin doped In203 on quartz or glass (0.38-in. thick) was manufactured by Pittsburgh Plate Glass Co. The thickness of these films varied between 700-900 nm and 400500 nm for the SnOn and InzOs, respectively, as determined from
the interference fringes measured in the visible region of the spectrum. The effective carrier density, no, was determined from the measured near infrared plasma frequency, A,, using the relationship (30):
CIA,= ( n o e 2 / m h ) ’ / 2
(3)
where c is the velocity of light, e is the elementary electron charge, and nzl is the effective electron mass. The no values are tabulated in Table I. The thin film oxide electrode were pretreated by cleaning in an ultrasonic bath, followed by successive washings with Haemsol detergent (30 min), isopropanol (30 min), and triply distilled water (30 min). The electrodes were then vacuum dried and stored in a controlled atmosphere glove box (Vacuum Atmospheres Corporation, HE-43-2) under purified, dried nitrogen. All manipulations of electrodes prior to and following electrolysis were done in the glove box. For surface analyses, the electrodes were rinsed thoroughly with triply distilled water and vacuum dried except in a few cases, where they were vacuum dried without pre-rinsing. Electrodes were then mounted onto appropriate holders for the surface analyses. SEM analysis was carried out using a Cambridge Model S4-10 instrument. Small amounts of carbon were vapor deposited on electrodes prior to analysis to prevent charging of the surface. Auger analyses were conducted on a Varian CMA-1 analyzer operated at approximately Torr. A primary beam energy of a 2 keV was used and modulation of the signal was 2 V, peak to peak. Depth profiling was performed using an argon ion gun with a voltage of 600 V and an argon pressure of ca. 9 X Torr, which resulted in a sputtering rate of about 5 8, per minute. This rate was determined from the removal of a thin film of known thickness. Several successive analysishputtering experiments were carried out for each electrode. The atomic ratios of each element were computed as a function of depth using the peak-to-peak amplitude of the dNldE Auger signal which had been corrected for the relative Auger sensitivity (from spectra recorded in reference 31) and ratioed to the same computation for another element. Pure standards of Sn, SnO, Sn02, In, and In203 were run for calibration purposes. For ESCA analyses, a DuPont Model 650 B instrument was used. Binding energies of the ESCA transitions were computed and corrected for charging effect by referencing to the 284.4 eV C(1s) peak (32). Deconvolution of the overlapping spectra was computer calculated with a Nova 800 (Data General Corp.) minicomputer using digital simulation methods. The ESCA transition for the pure species was first measured. Lorentzian contributions to the normally Gaussian line shape were estimated for use in the deconvolution. As a first approximation, ESCA line widths were assumed constant with binding energy. Relative atomic ratios for each species were also computed as in the Auger experiments. The magnitudes of each ESCA signal were corrected for differences in the x-ray cross section (33). R E S U L T S AND DISCUSSION General Electrochemical Behavior of SnOz a n d In&. For comparison purposes, cyclic voltammograms of P t , Au, SnOz, and In203 films on glass substrates obtained in 1 N solution are shown in Figure 1. Similar t o previous studies, the current-voltage (i-E) curves are separated for purpose of discussion, into three major regions. For both Au and Pt, these regions have been extensively studied and correspond to the “ideal” double layer region (I), the hydrogen region (11), and the oxide region (111). The potential widths of these regions vary with the scan rate of potential (34, 35). However, the most notable difference between the two metal film electrodes is the larger POtential width of region I and the absence of hydrogen adsorption on Au. With both electrodes, the films will be irreversibly destroyed if the potential is maintained in the hydrogen evolution region for any extended period of time. For SnOz and Inz03, the charge on the electrode changes monotonically throughout region I. Because the electrode is already an oxide, usually in the highest oxidation state of the metal, pronounced Faradaic currents associated with oxide formation are absent as the potential is increased in the positive direction. It is also quite a contrast to note that
the potential width of region I is much wider (ca. 1.5 V) than that of the metal films. Region I for these oxide films are bounded at positive potentials (region 111’) by oxygen evolution and at negative potentials (region 11’) by oxide reduction to a lower valence state of the metal, hydrogen evolution and/or reduction of surface to the metallic state (region 11”). Thus, similar to the metal electrodes, prolonged electrolysis at very negative potentials results in irreversible, detrimental changes to these metal oxide electrodes. A likely conclusion about SnOz and In203 electrodes as compared to Pt or Au, as seen in Figure 1, is the wide potential window apparenlty available for carrying out redox reactions. However, as pointed out by Memming and Mollers (24) for SnO2, carrier depletion occurs at potentials more positive than the flat band potential. Thus, at very positive potentials, the rate of any Faradaic reaction (e.g., cerous oxidation) may become limited by the charge transfer process of the semiconductor film. At low doping levels, tunneling appears to be the main mechanism. At high doping levels, other factors are important including composition of the surface ( 11, 12). The hydrogen region (11) on the metal films is replaced by regions 11’ and 11” on the semiconductors. The differentiation between regions I and 11’ is made to indicate the small Faradaic current which possibly involves the surface and appears to occur a t potentials in region 11’. The magnitude of the current was consistent with the partial reduction of the surface of the SnOz electrode as discussed by Kirkov (13). This current was observed only for anaerobic M 0 2 ) and was enhanced in alkasolutions (less than line solutions. The excursion of potential into region 11” resulted in the irreversible reduction of the semiconductor similar to previous results (11, 13). The In203electrode behaved similarly to SnOz. S u r f a c e Conductance. The surface conductance behavior of the SnO2 and hs03films is different from the behavior of the Pt and Au thin films. Conductance data for platinum in acidic media have been previously reported (28). Changes in conductance, LCIC, were observed as a function of applied potential and correlated to the change in electrode charge, Aq/q. These two quantities were assumed to be linearly related: S I C = cY(Aq/q) (4) -where (Y was found to be ca. 1.5 and 1.0 in the double layer region (I) for Pt ( 2 8 ) and Au (36), respectively. In the hydrogen region (11) for Pt, the value of (Y decreased to 0.65, consistent with the removal of conduction electrons by adsorbed hydrogen. In the study of both the platinum and gold films, pronounced hysteresis was observed in the AC/C vs. potential plots when the electrode potential was scanned across all three potential regions. This hysteresis was believed to be caused by the Faradaic processes involving the surface of the electrode, e.g., oxide formation. If the potential scan was limited to region (111) or region (I), no hysteresis was observed. The cyclic voltammetric i-E curve, the relative charge vs. potential, Aqlq-E, and the relative conductance vs. potential, LCIC-E curves for an InnO3electrode in a solution containing 1 M NazSO4 at p H 7 (phosphate buffer) are shown in Figure 2a, b, and c . For both SnOs and In203, there is no hysteresis in the AC/C and Aq/q curves if the potential scan is kept to about 0.1 V inside of the potential boundaries of either regions I or 11’. When the potential scan included both regions, hysteresis in the ACIC-E and Aqlq-E curves occurred. T h e hysteresis reflects the faradaic surface process of region 11’ and is similar to that reported previously ( 3 6 )for the tin oxide. ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
743
110
I I . l l l l / , , , . . . . I 0 -05 -10
to5
V
VI
Ag/AgCi
Figure 2. Current-potential. charge-potential, and conductance (AC/C)-potential relationships for an ln203 electrode in pH 7.0, phosphate buffer, 1 M Na2S04 (a1 Current-potential curves taken at a potentia scan rate of 87 mV/s. (b] Charge-potential C U N ~ S taken at above scan rate. (c)Conductance-potential curves taken at above scan rate. Dashed lines indicate behavior abSeNed when potential scan was confined to a patticular region
The proportionality, a,was 1.1 f 0.2 for both SnOz and In203 eleotrodes in region I. This value increased hy a factor of 2 or more in region 11' hut the day-to-day reproducibility was poor, particularly for the Sn02 electrode. The larger a value in region 11' probably reflects not only the increase in the population of the conduction electrons but also, the faradaically related surface changes. Capacitance Results. Capacitance measurements of semiconductor electrodes yielded information concerning the distribution of energy levels in the material and the presence of surface energy states (12,241, whereas on metal films, capacitance reflected charge due to the solutionmetal interface. For SnO2 and In& in region (I),the measured capacitance appeared to be determined solely by the space charge. Thus, the capacitance could be described by the Mott-Schottky relationship, namely:
(-)
2 1/C2 = ( E - EFB- k T / e ) ceoena
(5)
where C is the space charge capacitance, e is the dielectric constant for SnOz (e = 12.7, Ref. 12 and 24), eo is the permittivity in vacuum, no is the effective carrier density, E is the electrode potential measured with respect to the reference electrode, EFBis the flat hand potential (measured vs. the same reference electrode), and k , T,and e have their usual meanings. From the slopes of the 1/C2 vs. E plots, values of n, can he calculated. In Table I, the no values calculated from capacitance data for particular samples of S n 0 2 and In203 were compared to those determined from both Auger analysis and the measured plasma frequency, A,. The n, values for most electrodes in our laboratory are equal to or greater than lozo carrier/cm3. Good agreement was found for the no values determined by capacitance measurements and those calculated from A, and Auger results. The Mott-Schottky relationship predicts that the extrapolation of the linear 1/C2 vs. E plots should intersect the potential axis and give a value of the flat hand potential, E F B (with correction of k T l e ) . Experimentally, the 1/C2 vs. E plot deviates seriously from linearity as the value of 1/C2 decreases a t negative potentials (approaching EFB).Deviation is believed to he due to a surface Faradaic process which adds a Faradaic current component to the measured capacitance. The EFBvalues reported are therefore those obtained from extrapolating the linear portions 744
ANALYTiCAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
Figure 3. SEM photomicrographs of two samples of SnOP electrodes (a1 Surfaceresistance of 15 ohmstsq.. (b)surface resistance of 5 ohrndsq.
of the 1/C2 vs. E plots (for example, see Figure 9). Returning the electrode potential to region I appeared to reverse the behavior to simple space charge capacitance, as long as extreme cathodic potentials (less than -1.2 volts vs. Ag/ AgCl reference electrode, pH 7.0 phosphate buffer) have not been applied. The flat band potential of the SnO2 and In203 electrodes are indirectly influenced by the surface states of the semiconductors and by solution conditions. Because of the surface ionizable groups a t the surface (reactions 1 or 2), the flat hand potential was observed to shift cathodically with increasing pH. Similar observations have been made with earlier characterized SnOz and other semiconductors (12, 24). For example, EFBflat hand potentials of SnOz electrodes were -0.30 and -0.90 V (vs. AgIAgC1) in solutions of pH 0 and pH 7, respectively, where the shift is more than the predicted 60 mV/pH unit. However, the predicted pH dependence was observed in the restricted p H range of 4-8. Within this p H range, analysis was considered valid in solution containing similar buffers or counter ions, etc. The flat band potentials are in agreement with those reported for SnOz by Laitinen and coworkers (12),but disagree with those reported by Memming and Mollers (24). Those electrodes studied here and by Laitinen and coworkers are of extremely high carrier densities (2 X 1020-1021/~m3). Memming and Mollers investigated those with carrier densities less than 1OZ0/cm3. Semiconductors with higher carrier densities apparently will not show the same flat hand po-
Figure 5.
rograph and x-ray fluorescence of unused (a) DCM pnoramicrograpn. tb) x-ray fluorescence yield vs. energy. "outl' is spectrum taken Outside the surface defect. "in" is spectrum taken inside the surface defect
tential as those with a lower carrier density (1016-1020/cm3) (37). One possible explanation is that the potential across the Helmholtz region may itself change and cause a significant cathodic shift in the value determined for the flat band potential (37):
EFB= Ei,t - hT/e + e,cen,/2C~~
(6)
where Ei,t is the measured intercept potential of the capacitance plots and CH is the Helmholtz capacitance. Assuming an effective carrier concentration, no, of 1021/cm3and a Helmholtz capacitance of 10jcF/cm2,the magnitude of the correction must be -0.071 V. If the magnitude of the Helmholtz capacitance is lowered to 1wF/cm2, this correction becomes -7.10 V. Consideration of the Helmholtz capacitance may he very important in studies of these highly doped semiconductors. Variations in measured flat band potentials between various solvent media may be explained by the Helmholtz correction alone. Further definitive capacitance studies should he conducted to confirm this effect. Scanning Electron Microscopy (SEM). SEM was used to study the surface topography of the oxide electrodes and to determine the elemental composition by the simultaneous monitoring of the x-ray fluorescence. In Figures 3a and 3b, SEM photomicrographs are shown for two different samples (manufacturer A and B) of Sb doped tin oxide. Sample A has a rougher surface than B. Although the carrier densities of these two samples did not differ markedly, the surface resistance of sample A was ca. 15 Wsq., com-
Auger spectra of various Sn02 and In2O3 materials
(e) "Clean" unused sno, surface, ( b ) 50% mixture of KBr and powdered SnOz standard. ( c ) "Clean" unused ln20s surface
pared to 5 Wsq. for sample B. Possible correlation between surface roughness and electrical resistance has been suggested (8,28,30). Examination of the total surface under lower magnification revealed the presence of several large surface defects. The shape and depth of these defects appeared similar to craters which suggested that they were formed during the heating-vaporization process of film formation. The example shown in Figure 4 has a width of ca. 50 jc and a depth comparable to that of the S n 0 2 film thickness. X-ray fluorescence analysis indicated considerable depletion of tin inside the crater, as expected. The elements of silicon and aluminum were also found. These elements were a t much higher relative concentration inside the crater than outside. Because the penetration depth (about 1 j c ) of the x-ray fluorescence beam is greater than the film thickness, .the elements in the substrate, such as AI and Si, appear on all of the analyses. The x-ray fluorescence method has provided, however, considerable analytical information about the relative concentrations of tin and indium on these oxide electrodes which have been subjected to various electrolysis conditions (38). Auger Spectroscopy. An Auger spectrum for a sample of an unused, Sh doped SnOz electrode is shown in Figure 5a. The Sn Auger lines of the MNN transition appear a t 316, 324, 367,430, and 437 eV. The last two lines are most intense and were used mainly in the analysis. Carbon always appeared as a contaminant, usually a t quite low levels. The Sb Auger lines for the MNN transition a t energies of 454 and 462 eV are not well resolved from the additional tin lines a t 458 and 466 eV. Because of this overlap, the intensities of the Sh lines required correction by suhtraction of contribution from the Sn lines. Such correction is made ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
745
by knowing the relative intensities of Sn a t the 458-466 eV region with respect to those of Sn a t 430-437 eV. Thus, the relative Sb/Sn ratio can be computed from the intensity of the Sn lines with respect to the corrected Sb lines. Assuming a density of 6.95 g/cm3 for the casserite structure of tin oxide, a value of 8 X 1020atoms/cm3 was estimated for the S b concentration. This value agrees well with those calculated from the capacitance measurements (Table I). Apparent atomic ratios of oxygen to tin were computed next for the SnO2 by comparison of the oxygen 510 eV, and the tin 430,437 eV lines. Corrections were made for the relative Auger sensitivities of each element by comparison with tabulated spectra (31). These ratios were also computed for standard SnO2 substrates (Figure 5 b ) which were prepared by pressing pellets of 5050 mixtures of KBr and SnOz powder. Depth profiles (Figure 6) were obtained by taking an Auger spectrum after each successive argon ion sputtering of the surface. These depth profiles reflected changes in the O/Sn ratio as a function of average sputtering depth. This average depth is based on the following assumptions (as a first approximation): All Auger electrons have equal mean free path lengths, the sputtering rates were equal for all elements, and the surface was homogeneous and flat. As shown in Figure 6, the unused electrodes also exhibited greater than stoichiometric concentrations of oxygen a t the surface. Within about 20-30 8, of the surface, these ratios approached values reflecting stoichiometric concentrations of tin and oxygen and then remained essentially independent of depth. Depth profiles were also determined for SnO2 electrodes which had been subjected t o prior electrolysis. These profiles (Figure 6b, c, and d ) reflect the elemental composition as a function of depth and their dependence on the applied potentials. These potentials correspond to the previously discussed regions (e.g., Figure 6d-Region I, Figure 6b-Region 11’, Figure 6c-Region 11”). All profiles exhibited O/Sn ratios that were higher a t the surface. The higher oxygen content with respect to the metal may possibly be due to adsorbed surface contaminants including hydrated water. Little evidence was seen for damage, Le., reduction of SnO2, caused by the electron beam in these analyses. In addition, the SnO2 system did not appear to be susceptible to reduction by the argon ion beam (39). Electrodes that were poised a t anodic potentials (I) (e.g., +1.0 V vs. Ag/AgCl) and rinsed with distilled water before analysis showed abnormally high O/Sn ratios a t the surface (Figure 6d). The high oxygen content persisted to much greater sputtering depths. Also, the Auger lines for potassium and phosphorus were evident and persisted a t trace levels for a fair depth. These residual contaminants are undoubtedly from the phosphate buffer used in the solution. The level of carbon a t the surface was also high. The relatively high concentrations of oxygen may result from adsorbed (oxygen-containing) species on the electrode surface or a real depletion of tin from the surface. Experiments are under way to clarify this effect and to determine whether small amounts of Sn may be dissolving from the surface. For an electrode poised a t negative potentials, e.g., -0.45 V vs. Ag/AgCl, (region 11’) substantially lower O/Sn ratios were observed (Figure 6b). At -1.0 V vs. Ag/AgCl (region II”), the O/Sn ratio decreases even further to some substoichiometric oxide state (see the depth profile in Figure 6c). The SnO2 surface can be reduced to elemental tin when subjected to very negative potentials (11, 13). An Auger spectrum for a clean, Sn doped, In203 electrode is shown in Figure 5c. The major 404, 410 eV in doublet and the 430,437 Sn doublet are well resolved. Calculation of the Sn/In atomic ratio from the intensity of the Auger signals and from knowing the density of In203 (7.179 746
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
I
30I
1.0
L
_ _ _ _ - _ _ _ A-.->
-_ 0.0
IO
10
30
Spvllering
40 Deplh (Angrfrornsi
50
60
Figure 6. Depth profiles for various SnOs electrodes constructed from Auger spectra ( a ) “Clean” unused Sn02 electrode, ( b ) SnOn electrode equilibrated at -0.45 V vs. Ag/AgCI, ( c ) SnOn electrode equilibrated at -1.0 V vs. Ag/ ASCI,, (0)SnO? electrode equilibrated at +1.0 V vs. Ag/AgCI
g/cm3), gave an average Sn dopant concentration of 6 X 1021/cm3.This value is in remarkable agreement with that calculated from capacitance measurements (Table I). However, the Sn/In Auger ratios were dependent on depth and were found to vary from 0.31 a t the surface to 0.14 a t depths of about 300 8, (38). Auger analysis of Ins03 electrodes, which have been electrolyzed in potential regions I, 11’, II”, gave results which paralleled those changes found for the SnO2 system. The sputtering depths listed should be viewed as approximate since the actual sputtering rate for these oxides are known to no better than f 3 8,/min. Some variation may also occur from electrode to electrode. However, it is clear that any changes produced in the surface by pre-electrolysis extends into the bulk to appreciable depths. Another uncertainty in determining the exact values of guantitatiue elemental composition is due to slight shifts in the energy of tin or indium lines as a function of the sample pretreatment. These shifts reflect, of course, the changing valence states and atomic environments. On the other hand, these shifts are reproducible and provide a rather unambiguous identification of the previous history of the oxide surfaces. Results of detailed analysis of the Auger lines will be discussed in a separate paper (38). ESCA. Initial analysis of ESCA results focused on the comparison of the total signal intensities of the oxygen and tin signals as a function of pre-electrolysis conditions. Results indicated that the total oxygen content on the surface remained reasonably invariant (f2096) while the tin concentration decreased as the potential became more positive (corresponds to region I, -1.0 V). This increase in O/Sn ratio is quite similar to those observed by Auger analysis. Again, high oxygen content on the surface may be caused by surface contamination (compounds containing oxygen).
I
8 kcps
Sn(3d)
I
c
i
C(1s)
Figure 7. ESCA spectra of various SnOn electrodes, all electrodes sputter-etched to remove approximately 10 A of material (a) “Clean” unused SnOz electrode, (b) Sn02electrode equilibrated at -1.0 V vs. Ag/AgCI, (c) Sn02 electrode equilibrated at +1.0 V vs. Ag/AgCI
However, the major increase of the O/Sn ratio a t positive potentials is believed to be a consequence of the pre-electrolysis. The elemental composition coincides with that obtained by Auger analysis. Of the S n ESCA and Auger lines appearing in the total ESCA spectra as seen in Figure 7, only the tin ESCA (3d) lines were used for analytical purposes. Both the oxygen and tin lines were corrected for their relative ESCA sensitivities (33).For the “clean” unused SnOa electrode which has been sputtered with an argon beam, the oxygen to tin ratios are near that expected for a stoichiometric surface ( O h = 2.0) and are comparable with standard SnOn material. For an electrode poised a t potentials in region I, the oxygen to tin ratio (O/Sn = 3.6) is somewhat higher than stoichiometric amounts (See Figure 7c). For a SnOz electrode poised a t potentials in region 11’, the oxygen to tin ratio (O/Sn = 2.1) was similar to that of the unused surface. This similarity was probably due to the type of detection system used. That is, the electrolyzed area of the SnOz was less than that of the analyzed area. Thus, the analysis includes some clean SnOz surface. High resolution spectra indicated changes in stoichiometry which were not evident in the low resolution ones. Figure 7a, b, and c show ESCA low resolution spectra for unused and pre-electrolyzed SnOz which has been subject to argon etch (ca. 10 8, removed) prior t o analysis. In these spectra, the oxygen to tin ratio changes only for the anodized electrode (electrolyzed a t E = +1.0 V, region I) which has a ratio of 3.6 compared to that of 2.0 and 2.1 for the unused and cathodically poised (region 11’)ones.
The high resolution spectra for Sn(3d) lines for a clean SnOz electrode and for one electrolyzed in potential region 11” are shown in Figure 8. The tin signals for the clean electrode are symmetrical with a FWHM of 1.7 eV. After correcting for charging of the samples (binding energies referenced to the 284.4 eV C 1s peak), the binding energies computed for these transitions were consistent with a fully oxidized tin valence state (Table 11). For the electrode poised in potential region 11” (Figure 8 b ) , a resolved tin species of lower valence state than the bulk material was observed. The binding energy of this species, depending on the particular sample and ESCA instrumentation, was found to be shifted to lower energy by 1.0 to 1.7 eV compared to the fully oxidized surface. Reduction of the SnOz electrode in potential region 11’’ (11, 13) was apparent. Additional resolved ESCA shifts for the Sn(3d) lines for electrodes poised a t other potentials have not been detected. I t is suspected that valence state changes associated with these potential regions may be short lived and reversible during the transition time between solution and high vacuum. Also in some experiments with oxidized tin metal films in a high resolution ESCA instrument (e.g., the HewlettPackard, FWHM of Sn4+ (3d) = 1.3 eV), we have found that the lower valence states of tin in the presence of fully oxidized (Sn4+)may be difficult to detect if the intermediate state consists of less than 10% of the total tin (38).The existence of an intermediate valence for SnOn poised in potential region 11’ (e.g., SnO, SnsOJ has not yet been shown, even though Auger O/Sn ratios are consistent with lower valence states. ANALYTICAL CHEMISTRY, VOL. 48, NO. 4 , APRIL 1976
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I
495
Figure 9. (Capacitance)-* vs. potential relationships for two selected SnOn electrodes Top: silanized Sn02 (see text); Bottom: "Clean" %On. Data taken in pH 7.0, Dhosphate buffer
Figure 8. High resolution ESCA Sn(3d) lines for two SnOn electrodes (a) "Clean" unused SnO2 electrode surface, ( b )Sn02 electrode equilibrated at -1.5 V vs. AgIAgCI, (c) Deconvolution of the spectrum in b
ESCA studies have been previously carried out on clean Ins03 films doped with Sn (22). These studies indicated that the binding energy for the tin was closer to SnsOd rather than SnO in the In203 matrix. No electrolysis studies were carried out on these films. No data were reported for the valence states of oxygen. Our preliminary analysis of the high resolution ESCA spectra indicates the presence of more than one type of oxygen on the electrode surface. Studies of Modified Surfaces. Through methods analogous to those described by Murray and coworkers (26) and others (27, 40), SnO2 electrodes have been subjected to modification by the bonding of various chemical functional groups through silane esters
.
I
Y
n
n Z
I I
(--Si-(CH,),-R, where in present cases, R = aryl amine and 4-arylazo-lnaphtol. Details of the modification procedures are presented elsewhere ( 4 1 ) . Preliminary results of capacitance and surface analyses are presented to illustrate the characterization of chemically modified SnOz surfaces. The 1/C2 vs. potential plots for a clean SnO2 electrode and for an electrode which has been silanized (the silane ester is terminated with an aryl amine) are shown in Figure 9. Mott-Schottky behavior was observed for both electrodes in potential region I. Slight frequency dispersion was noted as the frequency of the modulating potential was varied. The silanized electrode behaved as a semiconductor with lower effective carrier density than for the clean SnOz electrode. The derivatization process may have possibly depleted the semiconductor of some Sb dopant, or may have effectively covered the tin oxide surface with a lower conductivity layer. These capacitance, as well as preliminary 748
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
Energy
(eV)
Figure 10. Auger spectra of a-napthol %On electrode (a) Spectrum of the surface, ( b ) spectrum taken with approximately 35-A sputter etched, (c) spectrum taken with approximately 100-A sputter etched
charge transfer rate measurements, are consistent with the results of Murray and coworkers (26) who observed lower charging currents and lower charge transfer rates on their derivatized SnO2 surface. Auger spectra of derivatized SnOz electrodes for \.arious sputtering depths are shown in Figure 10. The derivatization was by silinization with a-naphthol. These spectra should be compared to those for the clean SnO2 in Figure 5a. The 92-eV silicon signal was clearly evident. Also large
Table 11. ESCA Data for SnO, Electrodes Binding e n e r g y , e V a Electrode t y p e
"Clean" SnO, SnO, (+1.0 V y
Sn 3d5,,
0 1sb
Charge shift,c eV
Binding e n e r g y , eV c ~ s d
494.4 (494.1 )e 494.6 (494.0)
485.9 (485.7) 486.2 (485.6)
529.9 (529.6) 530.6 (530.1)
+4.4 (+3.7) +3.2 (+3.6)
288.8 (288.1) 287.6 (288.0)
(493.8)i 494.7 (494.9) 495.15 494
(485.4)i 486.2 (486.5) 486.6 485
(529.2) 530.4 (530.4) 530.7
('+2'I.)
(287.1) 287.1 (287.1) 287.4
S"
3*3,*
SnO, (-1.5 V)f SnO,/KBr pellet SnO film on glassg SnO 2
ooh
...
+2.7 (+2.7) +2.0
...
532
a All binding energies are corrected for charge shift. Precision is kO.3 eV. b Major component of O(1s) peak associated with SnO,. C Charge shift is obtained from the difference between 284.4 eV and the observed C ( 1 s ) binding energy. d Observed C(1s) binding energies. e Data in parentheses are ESCA results obtained after ca. 1 0 A of electrode surface removed by Ar+ sputtering. f Electrode equilibrated at these potentials in pH 7.0 phosphate buffer. g From ref. 22. From ref. 32. i Additional Sn(3d) lines observed at 4 9 1 . 8 and 4 8 3 . 4 eV.
i "1 I
0 ois. 0
N/Sn
A
Siih
Figure 11. Depth profile constructed from the Auger spectra of the surface modified Sn02 electrode from Figure 10
amounts of carbon, oxygen, and smaller amounts of nitrogen (381 eV) were found. Perhaps most striking was the low magnitude of a tin signal a t the surface. However, as the surface was subjected to argon ion sputtering, the relative magnitudes of the silicon, nitrogen, and carbon signals decreased while that of tin and oxygen increased (see Figures l o b and c). The argon sputtering effectively removed the silinized a-naphthol. The depth of the modified surface layer can be approximated from depth profiling experiments and the results are shown in Figure 11. All signals were corrected for their relative Auger sensitivity and ratioed t o the tin signal. An approximate value for the thickness was 80-100 A, based on the depth required to reach the stoichiometric Sn/O ratio of a "clean" tin oxide. This thickness value can be compared to a value determined by observing the tin signal level before and after the sputtering experiment. Assuming the escape depth of the tin Auger electron is 8 A, and that the yield of Auger electrons decays exponentially below the top surface layer, a calculated thickness of the derivatized layer of 24 8, was obtained. These ESCA-derivitization results have been duplicated with another modified surface containing silinized coenzyme substrate (41). Details of these experiments will be reported in another publication.
CONCLUSION
The heavily doped tin oxide and indium oxide film electrodes offer advantages over the usual Pt and Au films for Faradaic reactions because of the greater accessible potential range and lack of extensive surface reactions. There is, however, evidence for finite reduction of the oxide surface when the potential is scanned in the negative direction. Changes in the chemical composition of the surface as a function of the electrolysis potential were verified by combined ESCA and Auger analyses. Depth profiling by argon ion sputtering indicated that these changes penetrated the surface several tens of angstroms. The results of the surface analyses on tin oxide and indium oxide electrodes complement the expanding body of information on the surface composition and valence states of metal electrode surfaces (e.g., the metal, metal oxide states on Pt, Au, Pb, etc.). Few results have been reported to date on depth profiling studies of the valences of mixed metal oxide surfaces as was done in this study. The correlation between surface conductance and capacitance behavior and the indicated reduction of the surface as determined by Auger analysis in potential region 11' is particularly noteworthy. This reduction occurs a t potentials positive of the apparent flat band potential. I t will be interesting to evaluate the stoichiometry and valence states of a variety of mixed metal oxide surfaces when they are subjected to potentials where surface reduction might occur. The depth to which changes may take place is also a concern for future studies. Similar to a previous report on chemically modified electrode surfaces (26), ESCA or Auger analysis can provide verification of surface composition and some inferences to extent of coverage. Depth profiling by argon ion sputtering appears to be extremely useful for further characterization of these surfaces, particularly if meaningful experiments can be devised for examining these surfaces prior to and after electrochemical usage. Finally, several assumptions have been made in interpreting depth profiling results. Attempts will be made to test these assumptions, if possible, in future efforts. Nevertheless, the surface analysis methods are extremely promising and may be the only way in which elemental composition and valence states can be defined for characterizing mixed metal oxide and chemically modified surfaces. ACKNOWLEDGMENT We greatly appreciate the assistance of J. Lumsden, R. ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
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on modified tin oxide surfaces are hereby acknowledged.
LITERATURE CITED (1) T. Kuwana, R. K. Darlington, and D. W. Leedy, Anal. Chem., 36, 2023 (1964). N. Winograd and T. Kuwana, "Spectroelectrochernistry at Optically Transparent Electrodes," in "Electroanalytical Chemistry," Vol. 7, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1974, and references therein. Papers in "Optical Studies of Adsorbed Layers at Interfaces," in Symposia of the Faraday Society, No. 4, 1970. T. Kuwana, Ber. Bunsenges, Phys. Chem., 77, 858 (1973) and references therein. Several papers in "Intermediates in Electrochemical Reactions," in Faraday Discussions of the Chemical Society, No. 56 (1973). Chapters in "Advances in Electrochemistry and Electrochemical Engineering", Vol. 9, R. H. Muller, Ed., John Wiley. New York, N.Y. 1973. Wm. Heineman and T. Kuwana, Bioelectrochem. Bioenergetics, 1, 389 (1974). R. K. Quinn, N. R. Armstronq, N. E. Vanderborah. J. Vac. Sci. Techno/.. 12, 160 (1975). C. Li and G. S. Wilson, Anal. Chem., 45, 2370 (1973), and references therein. F. Mollers and R . Memming, Ber. Bensenges Phys. Chem., 77, 879 11973). H. A.'Laitinen, C. A. Vincent, and T. M. Bednarski. J. Electrochem. Soc.. 115. 1024 (1968). D. Eiliot. 6. L. Zelmer: and H. A. Laitinen, J. Electrochem. Soc., 117, 1343 (1970). P. Kirkov. Electrochim. Acta, 7, 519, 533 (1972). A. Rohatgi, T. R. Viverito, and L. H. Slack, J. Am. Ceram. Soc., 57, 278 (1974). C. A. Vincent, J. Electrochem. Soc., 119, 515 (1972). C. A. Vincent and D. G. C. Weston, J. Electrochem. Soc., 119, 51% (1972). L. D. Loch, J. Electrochem. SOC.,110, 1081 (1963). A. Lerner. Photogr. Sci. Eng., 13, 103 (1969). M. Nagasawa and S. Shionoya, J. Appl. Phys. (Jpn), I O , 472 (1971); ibld, I O , 727 (1971). T. Arai, J. Phys. SOC.Jpn., 15, 916 (1960).
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
(26) (27) (28) (29) (30) (31) (32)
(33)
(34) (35) (36) (37) (38) (39) (40) (41)
P. R. Moses, L. Wier. and R. W. Murray, Anal. Chem.. 47, 1882 (1975). L. Miller, Private communication. M. Fujihira and T. Kuwana, Electrochim. Acta., 20, 565 (1975). M. Babai, N. Tshernikovski, and E. Gileadi, J. Electrochem. Soc., 119, 1018 (1972). K. L. Chopra, "Thin Film Phenomena," McGraw-Hill. New York, N.Y. 1969. "Handbook of Auger Electron Spectroscopy," Physical Electronics Industries, Inc., Edina, Minn., 1972. K. Siegbahn. C. Vordling, A. Fahlman, R. Nordberg. K. Hamrin, J. Hedman, G. Johansson. T. Bergmark, S. E. Karlsson, I. Lindgren, and B. Lindberg, "ESCA-Atomic. Molecular, and Solid State Structure Studied by Means of Electron Spectroscopy", Almquist and Wiksells, Uppsala, Sweden, 1967. J. H. Scofield, Lawrence Livermore Laboratory, Report No. UCRL51326, Jan. 1973, received courtesy of Bulletin 650-1007, Instrument Products Division, E.I. du Pont de Nemours and Co.. Inc., Monrovia, Calif., Sept. 1974. H. Angerstein-Kozlowska, B. E. Conway, and W. B. A. Sharp, J. Electroanal. Chem., 43, 9 (1973). B. E. Conway and S. Gottesfeld, J. Chem. Soc., Faraday, Trans., 1090 (1973). W. J. Anderson and W. N. Hansen, J. Electrochem. Soc., 121, 1570 (1974); J. Elecfroanal. Chem., 43, 329 (1973). R. De Gryse, W. P. Gomes, F. Cardon, and J. Vennik, J. Electrochem. Soc., 122, 711 (1975). N. R. Armstrong, A. W. C. Lin, M. Fujhara, and T. Kuwana, to be published. K. S. Kim and N. Winograd, Surf. Sci., 43, 625 (1974). B. Carriere, P. Legare, and G. Maire, J. Chlm. Phys., 71, 17 (1974). N. R. Armstrong, Mary E. Henne, T. Kuwana, and G. Royer. "Covalent Attachment of Nicotinamide-Adenine-Dinucleotide to Metal Oxide Electrodes," Abstract of November 1975, ACS meeting, Mexico City.
RECEIVEDfor review October 8, 1975. Accepted January 5 , 1976. The financial support provided by NSF grant MPS 7304882 and NIH-PHS grant No. GM 19181 is gratefully acknowledged.
