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The electrode properties of polycrystalline SnO2 containing up to 10% Sb or Ru oxides
THE ELECTRODE PROPERTIES OF POLYCRYSTALLINE SnO, CONTAINING UP TO 10% Sb OR Ru OXIDES W. BADAWY,*
K. D~BLHOFER,+
Fritz-Haber-Institut
I. EISELT,
H. GERISCHER,
der Max-Planck-Gesellschaft, (Receioed
S. KRAUSE and J. MELSHEIMER
Faradayweg
7 March
4-6, D-1000 Berlin 33, F.R.C.
1984)
Abstrmct-Thin tin dioxide films containing up to cn. 10% Sb or Ru oxides were prepared by the hydrolysis of SnCI,/SbC15 or SnC1,/RuOa vapour mixtures at 42O”C, on conducting substrates. The distribution of electronic states in the band gap of the semiconducting materials was determined by UPS (ultraviolet photoelectron spectroscopy). The tilms were then used as electrodes and the electrode kinetics of the redoxsystems Fe’ + /a + and Ce” + j* + were determined. By correlating the UPS and the electrochemical results, a model for the charge-transfer mechanisms is suggested in which electrons can pass thenarrow semiconductor space charge barrier by mediation of the deep donor states in the band gap being introduced by the dopants.
INTRODUCTION SnO, is an attractive eIectrode materia1 for spectroscopic studies and can serve as a protective window for reactive substrates. It has a high transmittivity in the visible- ir range, its resistanceagainst chemical attack is high and thin film electrodes can be easily prepared, ag by chemical vapour deposition (CVD). SnOz has a band gap of % 3.7 eV. The SnO, employed as thin film electrode material usually contains oxygen vacancies in the lattice. In addition, it may be prepared to contain such donors as Cl and Sb. Thus the n-type carrier concentration can be raised to values above lo*’ cnm3, and correspondingly, the conductivity can be well above lo2 ohm-‘cm-‘. Still, the electrodes retain their semiconducting properties: depending on the standard potential of the redox-couple the exchange currents tend to be low and the Tafel plots are unsymmetric[ll~5]. SnOZ electrodes containing Ru (as a catalyst for the chlorine evolution reaction) have received some attention[4, 51. Electrochemical reactions were found to proceed on these electrodes quite effectively, also in the anodic direction. It was supposed that this behaviour was due to electronic states in the SnO, band gap[6]. They will affect the interfacial potential distribution and the charge-transfer barrier. In addition, these electronic states may effectively mediate electron transfer through the barrier, depending on their energetic position with respect to the redox-system in the electrolyte, and on their accessibility to CB electrons and depolarizerC7, S]. It was, therefore, considered important to determine the distribution of such states in the band gap and correlate it with the electrode properties. A particularly attractive method for such a study is ultraviolet photoelectron spectroscopy (UPS). With highly doped material many more electronic states in the band gap are occupied and thus accessible to this method. The sampling depth of the method is * Permanent address: Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt. t Author to whom correspondence should be addressed.
less than 1 nm, ie only the states at or near the surface, which are the most effective ones in electrode processes, are analysed.
EXPERIMENTAL SnO, films of thickness on glass or glassy carbon SnCl,
42o”c_
SnO,
+4HCl.
A 0.5 M SnCl,/ethylacetate solution was employed as a spray. It evaporated in front of the heated substrate, the SnO, formation proceeded thus from the vapour phase. This technique permitted the convenient incorporation of Sb or Ru into the SnOz during formation by adding the compounds SbC15 or Ru04 to the spraying solutions. More details on the preparation of the SnOl have been reported previously[9]. The conductivities of the SnOL samples were determined by a four-point probe to vary between 100-200 Scm- ‘. The n-type carrier density was determined by capacitance measurement in non-aqueous solvents to be (7 + 2) x 1Ol9 cme3. Adding up to 10 y0 Sb had little effect on the film conductivity. With higher Sb concentrations the films appeared milky and the conductivity dropped sharply; such films behaved as poor electrodes. The electrochemical results obtained with such films were therefore not included in the following discussion. With increasing ruthenium concentration in the Snot the conductivity decreased. Ru (note that in It was cu. 0.1 Scm- t with 6 atom-% the following we shall use the symbol 0d to indicate atom- y0 values). Standard potentiostatic techniques were employed for the electrochemical experiments. The SnO, was deposited on polished glassy carbon discs, diameter 8 mm, height 3 mm. They were mounted in a stainless steel holder used for the rotating disc system, then insulated with Teflon tape so that only the Sn&coated circular front face was exposed to the electrolyte. 1617
EA 29:12-p.
+ 4H,O
10&l 50 nm were prepared by the hydrolysis reaction:
W.
1618
BADAWY,
K.
DOBLHOFER, I. EISELT, H. GERISCHER,
The ultraviolet photoelectron spectra (UPS) were obtained with a Vacuum Generators combined ESCA/iJPS spectrometer. Usually, at least three spectra were taken with each sample: the first one without any pretreatment. The second was taken after 10 mm Ar* bombardment (500 V, 3 PA) which was expected to remove contaminations but not to alter significantly the SnOa surface; these spectra are report&in this work. Further sputtering increased the photoelectron emission in the energy range of the band gap. Apparently, more oxygen deficiencies were introduced. Even prolonged sputtering did not lead, howl ever, to a detectable signal at the Fermi energy, indicating that a metallic phase was not formed. With the instrument in the ESCA mode the concentrations of Ru and Sb in the SnOp samples were determined. RESULTS (a) Electrode
kinetics
of Fez+13+
Two typical Tafel plots are represented in Fig. 1. The exchange current densities (at q = 0) were found to have values between 2-20 @ cmw2. These include the samples containing up to 10 o/oSb and up to 6 0A Ru. The reproducibility of results obtained with electrodes prepared under identically adjusted preparation parameters was not much better than the above range. Thus, it is concluded that the exchange currents for the Fe2+‘3* reaction for all samples did not differ considerably. The cathodic Tafel slopes, t,, were similar for all the investigated samples. They ranged between 130 -=zI, < 190 mV decade-‘, corresponding to B z 0.35, where /I is the cathodic charge transfer coefficient. At anodic over-potentials remarkably linear Tafel lines (of slope ra) were obtained with pure SnOa electrodes. Regularly, t, > 500 mV decadee’ was
S.
KRAUSE AND J. MELSHEIMER
found, corresponding to a < 0.1, where 01is the anodic charge-transfer coefficient. Ruthenium incorporated in the SnO, caused a dramatic rise of t,. It was Ru in the SnO, and t, 2 350 mV decade- ’ at 0.5 0/O reached 150mV decade- I, at 1.5% Ru. When the ruthenium concentration in the SnOz was further raised, I! and~+,-remained unchanged at values correspondmg to c( x p = 0.38 f 0.05. However, the exchange current rose, reaching values of ea. 30 PA cm-’ at 10% Ru. No significant effect of up to 10 % Sb incorporation S@, was detectable within the experimental reproduce llity of the results. A concentration of in-the--%w ca. 5 o/0Sb produced films of particularly high stability. (b) Electrode kinetics of Ce’ + ‘4 + Results obtained with Ce 3+/4+ in H2S04 are summarized in Fig. 2. The currents were corrected for the background currents measured with pure HZS04. Again, the addition of Sb had little effect on the electron transfer rates. With increasing Ru concentration, on the other hand, the exchange current density rose to values as obtained on Pt eleetrodes[ lo] and the anodic branch became steeper than the cathodic one. Note that the current-voltage relations were logarithmic; however, the anodic and cathodic branches usually did not extrapolate to the same exchange currents. Even with the highest concentration of Ru, the sum of the anodic and cathodic CT coefficients a +/3 was (0.5.
THE UPS SPECTRA Figure 3 gives a UPS spectrum as obtained for the pure SnOx films. The secondary electron threshold was taken as the true sample threshold and thus as the zero
Fig. 1. Tafel plots obtained with the redox system Fe’ l ‘3+ (each 0.0025 M) in HZS04 (0.1 M). Electrodes rotated at 3300 min - I*. (# SnO,, within the limits of reproducibility identical with Sn03 + 5 % Sb; (0) Sn02 + 1.8% Ru.
SnO, containing up lo 10% Sb or Ru oxides
1619
0
\.
l ,
.\
o \
0
=A.
\ ‘A 0-111%5b~.A .
‘0
\
0
.
n.
0
\
‘A
.
.
.
. .
\
.
.
Ce%+. IM H,SO, 1 -
0. 5
I
I
-0.6
-0.3
- 0.2
\
I
-01
Fig. 2. Tafel plots obtained with the redox system Ce” + 14+ (each 0.05 M) in I M H,SO,. Electrodes rotated
at 3300 min- ‘: SnO&e.ctrodes containing the indicated atom- y0 of Sb or Ru.
N(E)1 counts
UV - Photoelectron
spectrum: SnO, 5
16000 -
12000 -
8000 -
4000 -
OE, leV Fig. 3. UPS spectrum obtained with SnO,. Fg is the literature value of the Sn02 band gap, used to obtain the conduction
band edge, EcB, from the derwxl valence band edge, EVB_ Q is the SnO, work function.
point of the electron energy, Fe, scale because the sample was biased to - 10 V during the measurement. The peak at about 12 eV corresponds to the Sn02 valence band[l l]. Extrapolation of the electron density function towards high energies (small binding energies) should lead to the valence band edge, E,, With the known value of the band gap, Es = 3.7eV[12-141, one can calculate the energy of electrons emitted from the conduction band edge, E,, = 17.5 eV. Note that with the highly doped semiconductor the Fermi level, E,, is probably not more than 0.1 eV below the conduction band edge[15].
Thus, the work-function of the samples can be determined, because Eva corresponds to the photon energy (21.21 eV) diminished by the sum of workfunction plus gap energy. Assuming E, = 17.4 eV on the energy scale of Fig. 3, a work-function Cp= 3.8 eV is obtained. This value is in excellent agreement with the results of Volta-potential measurements between SnOz and Cu (110) by the Kelvin probe, where Q, = 3.8 f 0.1 eV was obtained for SnO, samples prepared similarly to ours[16]. UPS spectra from SnO, samples containing various concentrations of Sb and Ru are given in Fig. 4. The
I620
W. BADAWY, K. DOBLHOFER,I. ELSELT,H. GERISCHER.S. KRAUSEAND J. MELSHEIMER
r UV -Photoelectron
0
AE,leV
1
Spectra
2
3
Fig. 4. UPS spectra of electrode materials used in this work. The symbol y0 indicates atom-% values, determined by ESCA. For comparison the UPS spectrum of pure RuO, is included.The energiesof electrons from the samples’ VB-edges (Fig. 3) were taken as zero of the AE, scale.
valence band edges derived by extrapolation were taken as the zero of the energy scale (AE,), in order to facilitate comparison of the density of states in the gap. The addition of Sb has no noticeable effect up to CLJ.lOa?& then the density of states in the energy gap decreases. Ruthenium, on the other hand, increases the density of electronic states in the band gap. For comparison, the UPS spectrum obtained with pure Ru02 is included; the sharp cut-off indicates metallic character and a work-function @ = 4.1 eV. DISCUSSION With n-type SnOz charge-transfer reactions with electroactive species in aqueous electrolytes are expected to proceed tiia conduction band electrons. The flat band potential in the 0.1 M H,SO, solution is cu. -+-0.1 V/&$10]. The redox systems considered have standard potentials, U O, at more positive values. This is not positive enough for the possiblity of hole injection into the valence band which is located at about + 3.8 V/&e. If the SnO,-electrode is polarized to the standard potentials,adepletion layer will be formed underneath the semiconductor surface which constitutes a barrier for electron transfer across the interface. The dark currents on semiconductor electrodes are normally determined by the rate at which electrons can pass over or across this space charge barrierC17, IS]. The straightforward model for the interfacial charge transfer processes would involve only the allowed conduction band states in the semiconductor. For lower values of donor density and dielectric constant (N, c lo=, E < 10) the dark current density (_j)/overvoltage (7) relation would be described[19] by j=j"[l-exp(-_%I]
(1)
where j, is the exchange current density. The obtained Tafel slopes would correspond to OL= 0 and /?= 1. with the donor density of ND = 7 Actually, x 10’9cm-3 a significant fraction of the applied q will appear across the Helmholtz double layer, reducing the effective space charge barrier. This leads to values of LZ> 0 and /I -Z 1. It should be noted, however, that the sum of D:+ B should remain unity[19, 203. Comparing the experimental results (Figs 1 and 2) it follows that this semiconductor electrode model does not apply to the investigated tin dioxide electrodes. The condition GC+~ = 1 is in no case fulfiIled. In addition, the Ce’+’ + reaction would have to proceed at a rate much smaller than the Fe’+“* couple, because the space charge barrier, qAU,,, were much higher due to the difference of standard potentials of CCL0.8 V. The situation is illustrated in the schematic diagram, Fig. 5. Another, much more reasonable possibility is the assumption that the electrons can pass the space charge barrier by hopping from the conduction band to surface states via electronic states of donors deeper in the band gap, which do not contribute to the bulk conductivity but can exchange an electron with the conduction band and the surface under the influence of the high electric field in the space charge layer. This model shall be further considered. The distribution of electronic states in the band gap can in principle be obtained from the UPS spectra. The measurements of Fig. 4 show a signal starting 1.1 eV below the conduction band edge with SnO, and SnO, (+ Sb) electrodes, and 0.5 eV below the band edge in presence of 6% Ru. Note that at these energies the density of states must be in the order of 10Z1 an-’ (eV)- ‘, because the donors near the conduction band edge are not detected with our instrument. Much lower densities of states can he effective, however, in electron transfer processes. Therefore, the relevant information available from capacitance meas-
SnO, containing up to lOoA Sb or Ru oxides E LqU)
Sn02
.
electrolyte
. 5
L
L
x I 51-10Zl I
3
2
1
x lelectrolytel
“In
Fig. 5. Schematic energy diagram of the interface SnOJaqueous redox electrolyte obtained by assuming absence of electronic states in the gap except of the ionized donors of constant density N, = 7 x 10’9cm-3, and a double layer capacitance, C, = 15 JLFCIK’. The potential distributions between the space charge and the Helmholtz layers (AU,,, AU,) are shown at the standard potentials (UC) of the considered
redox systems.
urements was included in the qualitative representetion of the distribution of states, given in Fig. 6. The low donor density at 0.2 < AU,, < 0.8 eV for the SnO, and SnO, (Sb) electrodes thus follows from the straight Mott-Schottky plots reported in the literature[l, lo]. At anodic bias where a depletion layer is formed, deeper lying donor states will be ionized and con-
Sn02
conductjon
band
+ Sb
5n02
+
sr,02 Sb density
of el. states
I
orb.
units
Fig. 6. Estimate of the distribution of electronic states in the
band gap of the indicated electrode materials, based on UPS
and capacitance
measurements. AU,, is the potential drop across the space charge layer.
1621
tribute to the space charge. Therefore, the determination of the donor concentration from capacitance measurements can give a larger number than that from conductivity measurements. This is observed with the introduction ofruthenium, in which case the measured capacitance rises while the conductivity decreases. For thecharge transfer reaction the existence of such deeper donor levels has two consequences: (1) the space charge layer is thinner than calculated with a model assuming constant donor density, since the density of positively charged states increases from the bulk to the surface. Tunnelling will therefore be enhanced and (2) the donor states in the space charge layer, being close enough to each other and to the surface, can mediate electron transfer by a hopping mechanism. The latter effect will offer locally different passes for electron exchange between the surface and the conduction band since the spatial distribution of donor states will be quite inhomogeneous, more so in view of the polycrystalline structure of the material. Grain boundaries might be particularly preferred pathways for electron transfer. Taking into account these assumptions we have designed in Fig. 7 a model for the space charge layer which appears much more realistic than that of Fig. 5 and permits electron passage through the space charge barrier by electron exchange between occupied and vacant donor states. If there are enough electronic states on the surface which can exchange electrons with the redox system the rate-determining step will be the electron passage through the barrier and the occupation of surface states will be in equilibrium with the redox system. This will mean a variation of the density of occupied and vacant surface states with applied bias since part of it will charge the voltage drop in the Helmholtz double layer. However, as long as the passage of electrons through the space charge barrier remains rate determining, this will not affect the measurable reaction rate. It will only reduce the effect of the bias variation since only the change of the potential drop in the space charge layer can influence the passing electron current. This explains that a + B -z I and gives also some idea for the reason of the very small charge transfer coefficients. Figure 8 illustrates the situation which is proposed to prevail during an anodic and a cathodic electrode reaction. If the electrons on their way through the barrier have to pass several donor states in single steps, only the shift of these states relative to each other with a change of the voltage will affect the electron transfer rate between two states in the space charge barrier or these donor states and the surface or the conduction band respectively. This means a division of a single charge transfer step between the semiconductor and the bulk into several smaller steps and a shift of the passage of electrons to different donor and surface states with a variation of the bias. Depending on the slowest steps in this chain, the apparent charge transfer coefficients will vary considerably with the material and the potential range in which the redox reaction occurs. In any case, smaller values of CLand j.3 than obtained for metal electrodes ari to be expected[7,8]. This picture applies well to the Ce3+ ‘* l reaction on all the considered electrodes and to the Fe2 + k3+ reaction on SnO, + Ru. Figure 6 suggests that at the respective standard potentials electronic states are
W. BADAWY,K.DOBLHOFER,
LEISELT,
H. GERISCHER,
S. KRAUSEAND
J. MELSHEIMER
conduction
(b)
(al
Fig. 7. Semiconductor electrode containing electronic states deep in the band gap, at (a) the Rat band potential, U,,, and (b) at the standard potential of a redox system [U’ = &(redox)/qJ. Electrons can penetrate the space charge barrier by a hopping mechanism. The distribution functions for electron energy states of the redox components (Ox’, Red) in the electrolyte are included schematically (W(E)).
at Ua Iequilibrium)
-7
(Ox + e--Red)
Fig. 8. Model of electrochemical charge-transfer
+q (Red +
Ox+e-1
involving surface states in equilibrium with a redox system.
An applied overvbltage, 4, is divided up between the space charge and Helmholtz layers. The equilibrium situation corresponds
available in the band gap. Their charging/discharging leads to AU, changes with U, as required for the mechanism symbolized in Fig. 8. These states are also available for a hopping type electron transfer across the space charge barrier. The inhomogeneity of the material has not yet been taken into consideration in this scheme. Its influence goes in the same direction, namely reducing the effect of a bias variation on the reaction rate by opening and closing different paths at different bias. At present we cannot give more than this rather qualitative descrip-
to Fig. 7(b).
tion. Better defined model materials are needed for a more accurate interpretation of such electrodes. Acknowle~~emencs-Theauthorsacknowledge thoroughdiscussions with Dr K. Jacobi on the interpretation of the UPS spectra. Valuable comments by Prof. J. Ulstrup (The Technical University of Denmark, Lyngby) concerning the mechanism of interfacial electron transfer, are gratefully acknowledged. The results from capacitance measurements in non-aqueous solvents were made available to us by Dr R. McIntyre.
SnO,
containing
up to 10% Sb or Ru oxides
REFERENCES 1. F. Mijller and R. Memming, Ber. Bunsenges. Phys. Chem. 76, 469 (1972). 2. H. A. Laitinen, Dsnki Kagaku 44, 626 (1976). 3. N. R. Armstrong, A. W. C. Lin, M. Fujihira and T. Kuwana, Attalyr. Chem. 48, 741 (1976). 4. T. A. Chertykovtseva, Z. D. Skuridina, D. M. Shub and V. A. Veselovskii, Elektrokhimiya 14, 1412 (1978). 5. C. Iwakura, M. Inai, T. Wemura and H. Tamura, Electrochim. Acta 26, 579 (1981). 6. P. Triggs, F. L&y and F. E. Wagner, Mat. Res. Bull. in press; R. Kijtz and K. Miiller, BBC Forschungszentrum, Baden, private communication; J. B. Goodenough and A. Hammett, Lecture I11.3, 34th JSE Meeting, Erlangen, i&23 September 1983. 7. W. Schmickler, Ber. Bunsenges. Phys. Chem. 82, 477 (1976). 8. J. Ulstrup, Charge Transfer Processes in Condensed Media, Lecture Notes in Chemistry 10. Springer, Berlin (1979). 9. W. Badawy, F. Decker and K. Doblhofer, Solar Energy Materials 8, 363 (1983).
1623
10. D. Elliott, D. L. Zellmer and H. A. Laitinen, J. elscrrochem. sac. 117, 1343 (1970). 11. Z. M. Jarzebski and J. P. Marton, J. efectrochem. Sot. 123, 299C (1976). 12. T. Arai, J. Phys. Sac. Jap. 15, 916 (1960). 13. H. Kaneko and K. Miyake, J. a&. Phys. 53,3629 (1982). 14. H. Kim and H. A. Laitinen, J. electrochem. Sot. 122, 53 ( 1975). 15. G. Mierdel, Ekktrophysik, Chapter 3.4. VEB Verlag Technik, Berlin (1972). 16. K. Bange and J. Sass, private communication; cf K. Bange, dissertation, TU Berlin (1983). 17. H. Gerisoher, Semiconductor electrode reactions, in Advances in Electrochemistry and Electrochemical Engineering (Edited by P. Delahay and C. W. Tobias), Vol. 1, p. 139. Interscience, New York (1961). 18. S. R. Morrison, Electrochemistry at Semicondnctor nnd Oxidized Metal Electrodes, Chapter 2. Plenum Press, New York (1980). 19. H. Geriseher, 2. phys. Chem. 21, 48 (1961). 20. K. Uosski and H. Kita, J. electrochem. Sot. 130, 985 (1983); Discussion by H. Gerischer, discussion section of J. elecrrochem. Sot. December 1983.
K. D~BLHOFER,+
Fritz-Haber-Institut
I. EISELT,
H. GERISCHER,
der Max-Planck-Gesellschaft, (Receioed
S. KRAUSE and J. MELSHEIMER
Faradayweg
7 March
4-6, D-1000 Berlin 33, F.R.C.
1984)
Abstrmct-Thin tin dioxide films containing up to cn. 10% Sb or Ru oxides were prepared by the hydrolysis of SnCI,/SbC15 or SnC1,/RuOa vapour mixtures at 42O”C, on conducting substrates. The distribution of electronic states in the band gap of the semiconducting materials was determined by UPS (ultraviolet photoelectron spectroscopy). The tilms were then used as electrodes and the electrode kinetics of the redoxsystems Fe’ + /a + and Ce” + j* + were determined. By correlating the UPS and the electrochemical results, a model for the charge-transfer mechanisms is suggested in which electrons can pass thenarrow semiconductor space charge barrier by mediation of the deep donor states in the band gap being introduced by the dopants.
INTRODUCTION SnO, is an attractive eIectrode materia1 for spectroscopic studies and can serve as a protective window for reactive substrates. It has a high transmittivity in the visible- ir range, its resistanceagainst chemical attack is high and thin film electrodes can be easily prepared, ag by chemical vapour deposition (CVD). SnOz has a band gap of % 3.7 eV. The SnO, employed as thin film electrode material usually contains oxygen vacancies in the lattice. In addition, it may be prepared to contain such donors as Cl and Sb. Thus the n-type carrier concentration can be raised to values above lo*’ cnm3, and correspondingly, the conductivity can be well above lo2 ohm-‘cm-‘. Still, the electrodes retain their semiconducting properties: depending on the standard potential of the redox-couple the exchange currents tend to be low and the Tafel plots are unsymmetric[ll~5]. SnOZ electrodes containing Ru (as a catalyst for the chlorine evolution reaction) have received some attention[4, 51. Electrochemical reactions were found to proceed on these electrodes quite effectively, also in the anodic direction. It was supposed that this behaviour was due to electronic states in the SnO, band gap[6]. They will affect the interfacial potential distribution and the charge-transfer barrier. In addition, these electronic states may effectively mediate electron transfer through the barrier, depending on their energetic position with respect to the redox-system in the electrolyte, and on their accessibility to CB electrons and depolarizerC7, S]. It was, therefore, considered important to determine the distribution of such states in the band gap and correlate it with the electrode properties. A particularly attractive method for such a study is ultraviolet photoelectron spectroscopy (UPS). With highly doped material many more electronic states in the band gap are occupied and thus accessible to this method. The sampling depth of the method is * Permanent address: Chemistry Department, Faculty of Science, Cairo University, Cairo, Egypt. t Author to whom correspondence should be addressed.
less than 1 nm, ie only the states at or near the surface, which are the most effective ones in electrode processes, are analysed.
EXPERIMENTAL SnO, films of thickness on glass or glassy carbon SnCl,
42o”c_
SnO,
+4HCl.
A 0.5 M SnCl,/ethylacetate solution was employed as a spray. It evaporated in front of the heated substrate, the SnO, formation proceeded thus from the vapour phase. This technique permitted the convenient incorporation of Sb or Ru into the SnOz during formation by adding the compounds SbC15 or Ru04 to the spraying solutions. More details on the preparation of the SnOl have been reported previously[9]. The conductivities of the SnOL samples were determined by a four-point probe to vary between 100-200 Scm- ‘. The n-type carrier density was determined by capacitance measurement in non-aqueous solvents to be (7 + 2) x 1Ol9 cme3. Adding up to 10 y0 Sb had little effect on the film conductivity. With higher Sb concentrations the films appeared milky and the conductivity dropped sharply; such films behaved as poor electrodes. The electrochemical results obtained with such films were therefore not included in the following discussion. With increasing ruthenium concentration in the Snot the conductivity decreased. Ru (note that in It was cu. 0.1 Scm- t with 6 atom-% the following we shall use the symbol 0d to indicate atom- y0 values). Standard potentiostatic techniques were employed for the electrochemical experiments. The SnO, was deposited on polished glassy carbon discs, diameter 8 mm, height 3 mm. They were mounted in a stainless steel holder used for the rotating disc system, then insulated with Teflon tape so that only the Sn&coated circular front face was exposed to the electrolyte. 1617
EA 29:12-p.
+ 4H,O
10&l 50 nm were prepared by the hydrolysis reaction:
W.
1618
BADAWY,
K.
DOBLHOFER, I. EISELT, H. GERISCHER,
The ultraviolet photoelectron spectra (UPS) were obtained with a Vacuum Generators combined ESCA/iJPS spectrometer. Usually, at least three spectra were taken with each sample: the first one without any pretreatment. The second was taken after 10 mm Ar* bombardment (500 V, 3 PA) which was expected to remove contaminations but not to alter significantly the SnOa surface; these spectra are report&in this work. Further sputtering increased the photoelectron emission in the energy range of the band gap. Apparently, more oxygen deficiencies were introduced. Even prolonged sputtering did not lead, howl ever, to a detectable signal at the Fermi energy, indicating that a metallic phase was not formed. With the instrument in the ESCA mode the concentrations of Ru and Sb in the SnOp samples were determined. RESULTS (a) Electrode
kinetics
of Fez+13+
Two typical Tafel plots are represented in Fig. 1. The exchange current densities (at q = 0) were found to have values between 2-20 @ cmw2. These include the samples containing up to 10 o/oSb and up to 6 0A Ru. The reproducibility of results obtained with electrodes prepared under identically adjusted preparation parameters was not much better than the above range. Thus, it is concluded that the exchange currents for the Fe2+‘3* reaction for all samples did not differ considerably. The cathodic Tafel slopes, t,, were similar for all the investigated samples. They ranged between 130 -=zI, < 190 mV decade-‘, corresponding to B z 0.35, where /I is the cathodic charge transfer coefficient. At anodic over-potentials remarkably linear Tafel lines (of slope ra) were obtained with pure SnOa electrodes. Regularly, t, > 500 mV decadee’ was
S.
KRAUSE AND J. MELSHEIMER
found, corresponding to a < 0.1, where 01is the anodic charge-transfer coefficient. Ruthenium incorporated in the SnO, caused a dramatic rise of t,. It was Ru in the SnO, and t, 2 350 mV decade- ’ at 0.5 0/O reached 150mV decade- I, at 1.5% Ru. When the ruthenium concentration in the SnOz was further raised, I! and~+,-remained unchanged at values correspondmg to c( x p = 0.38 f 0.05. However, the exchange current rose, reaching values of ea. 30 PA cm-’ at 10% Ru. No significant effect of up to 10 % Sb incorporation S@, was detectable within the experimental reproduce llity of the results. A concentration of in-the--%w ca. 5 o/0Sb produced films of particularly high stability. (b) Electrode kinetics of Ce’ + ‘4 + Results obtained with Ce 3+/4+ in H2S04 are summarized in Fig. 2. The currents were corrected for the background currents measured with pure HZS04. Again, the addition of Sb had little effect on the electron transfer rates. With increasing Ru concentration, on the other hand, the exchange current density rose to values as obtained on Pt eleetrodes[ lo] and the anodic branch became steeper than the cathodic one. Note that the current-voltage relations were logarithmic; however, the anodic and cathodic branches usually did not extrapolate to the same exchange currents. Even with the highest concentration of Ru, the sum of the anodic and cathodic CT coefficients a +/3 was (0.5.
THE UPS SPECTRA Figure 3 gives a UPS spectrum as obtained for the pure SnOx films. The secondary electron threshold was taken as the true sample threshold and thus as the zero
Fig. 1. Tafel plots obtained with the redox system Fe’ l ‘3+ (each 0.0025 M) in HZS04 (0.1 M). Electrodes rotated at 3300 min - I*. (# SnO,, within the limits of reproducibility identical with Sn03 + 5 % Sb; (0) Sn02 + 1.8% Ru.
SnO, containing up lo 10% Sb or Ru oxides
1619
0
\.
l ,
.\
o \
0
=A.
\ ‘A 0-111%5b~.A .
‘0
\
0
.
n.
0
\
‘A
.
.
.
. .
\
.
.
Ce%+. IM H,SO, 1 -
0. 5
I
I
-0.6
-0.3
- 0.2
\
I
-01
Fig. 2. Tafel plots obtained with the redox system Ce” + 14+ (each 0.05 M) in I M H,SO,. Electrodes rotated
at 3300 min- ‘: SnO&e.ctrodes containing the indicated atom- y0 of Sb or Ru.
N(E)1 counts
UV - Photoelectron
spectrum: SnO, 5
16000 -
12000 -
8000 -
4000 -
OE, leV Fig. 3. UPS spectrum obtained with SnO,. Fg is the literature value of the Sn02 band gap, used to obtain the conduction
band edge, EcB, from the derwxl valence band edge, EVB_ Q is the SnO, work function.
point of the electron energy, Fe, scale because the sample was biased to - 10 V during the measurement. The peak at about 12 eV corresponds to the Sn02 valence band[l l]. Extrapolation of the electron density function towards high energies (small binding energies) should lead to the valence band edge, E,, With the known value of the band gap, Es = 3.7eV[12-141, one can calculate the energy of electrons emitted from the conduction band edge, E,, = 17.5 eV. Note that with the highly doped semiconductor the Fermi level, E,, is probably not more than 0.1 eV below the conduction band edge[15].
Thus, the work-function of the samples can be determined, because Eva corresponds to the photon energy (21.21 eV) diminished by the sum of workfunction plus gap energy. Assuming E, = 17.4 eV on the energy scale of Fig. 3, a work-function Cp= 3.8 eV is obtained. This value is in excellent agreement with the results of Volta-potential measurements between SnOz and Cu (110) by the Kelvin probe, where Q, = 3.8 f 0.1 eV was obtained for SnO, samples prepared similarly to ours[16]. UPS spectra from SnO, samples containing various concentrations of Sb and Ru are given in Fig. 4. The
I620
W. BADAWY, K. DOBLHOFER,I. ELSELT,H. GERISCHER.S. KRAUSEAND J. MELSHEIMER
r UV -Photoelectron
0
AE,leV
1
Spectra
2
3
Fig. 4. UPS spectra of electrode materials used in this work. The symbol y0 indicates atom-% values, determined by ESCA. For comparison the UPS spectrum of pure RuO, is included.The energiesof electrons from the samples’ VB-edges (Fig. 3) were taken as zero of the AE, scale.
valence band edges derived by extrapolation were taken as the zero of the energy scale (AE,), in order to facilitate comparison of the density of states in the gap. The addition of Sb has no noticeable effect up to CLJ.lOa?& then the density of states in the energy gap decreases. Ruthenium, on the other hand, increases the density of electronic states in the band gap. For comparison, the UPS spectrum obtained with pure Ru02 is included; the sharp cut-off indicates metallic character and a work-function @ = 4.1 eV. DISCUSSION With n-type SnOz charge-transfer reactions with electroactive species in aqueous electrolytes are expected to proceed tiia conduction band electrons. The flat band potential in the 0.1 M H,SO, solution is cu. -+-0.1 V/&$10]. The redox systems considered have standard potentials, U O, at more positive values. This is not positive enough for the possiblity of hole injection into the valence band which is located at about + 3.8 V/&e. If the SnO,-electrode is polarized to the standard potentials,adepletion layer will be formed underneath the semiconductor surface which constitutes a barrier for electron transfer across the interface. The dark currents on semiconductor electrodes are normally determined by the rate at which electrons can pass over or across this space charge barrierC17, IS]. The straightforward model for the interfacial charge transfer processes would involve only the allowed conduction band states in the semiconductor. For lower values of donor density and dielectric constant (N, c lo=, E < 10) the dark current density (_j)/overvoltage (7) relation would be described[19] by j=j"[l-exp(-_%I]
(1)
where j, is the exchange current density. The obtained Tafel slopes would correspond to OL= 0 and /?= 1. with the donor density of ND = 7 Actually, x 10’9cm-3 a significant fraction of the applied q will appear across the Helmholtz double layer, reducing the effective space charge barrier. This leads to values of LZ> 0 and /I -Z 1. It should be noted, however, that the sum of D:+ B should remain unity[19, 203. Comparing the experimental results (Figs 1 and 2) it follows that this semiconductor electrode model does not apply to the investigated tin dioxide electrodes. The condition GC+~ = 1 is in no case fulfiIled. In addition, the Ce’+’ + reaction would have to proceed at a rate much smaller than the Fe’+“* couple, because the space charge barrier, qAU,,, were much higher due to the difference of standard potentials of CCL0.8 V. The situation is illustrated in the schematic diagram, Fig. 5. Another, much more reasonable possibility is the assumption that the electrons can pass the space charge barrier by hopping from the conduction band to surface states via electronic states of donors deeper in the band gap, which do not contribute to the bulk conductivity but can exchange an electron with the conduction band and the surface under the influence of the high electric field in the space charge layer. This model shall be further considered. The distribution of electronic states in the band gap can in principle be obtained from the UPS spectra. The measurements of Fig. 4 show a signal starting 1.1 eV below the conduction band edge with SnO, and SnO, (+ Sb) electrodes, and 0.5 eV below the band edge in presence of 6% Ru. Note that at these energies the density of states must be in the order of 10Z1 an-’ (eV)- ‘, because the donors near the conduction band edge are not detected with our instrument. Much lower densities of states can he effective, however, in electron transfer processes. Therefore, the relevant information available from capacitance meas-
SnO, containing up to lOoA Sb or Ru oxides E LqU)
Sn02
.
electrolyte
. 5
L
L
x I 51-10Zl I
3
2
1
x lelectrolytel
“In
Fig. 5. Schematic energy diagram of the interface SnOJaqueous redox electrolyte obtained by assuming absence of electronic states in the gap except of the ionized donors of constant density N, = 7 x 10’9cm-3, and a double layer capacitance, C, = 15 JLFCIK’. The potential distributions between the space charge and the Helmholtz layers (AU,,, AU,) are shown at the standard potentials (UC) of the considered
redox systems.
urements was included in the qualitative representetion of the distribution of states, given in Fig. 6. The low donor density at 0.2 < AU,, < 0.8 eV for the SnO, and SnO, (Sb) electrodes thus follows from the straight Mott-Schottky plots reported in the literature[l, lo]. At anodic bias where a depletion layer is formed, deeper lying donor states will be ionized and con-
Sn02
conductjon
band
+ Sb
5n02
+
sr,02 Sb density
of el. states
I
orb.
units
Fig. 6. Estimate of the distribution of electronic states in the
band gap of the indicated electrode materials, based on UPS
and capacitance
measurements. AU,, is the potential drop across the space charge layer.
1621
tribute to the space charge. Therefore, the determination of the donor concentration from capacitance measurements can give a larger number than that from conductivity measurements. This is observed with the introduction ofruthenium, in which case the measured capacitance rises while the conductivity decreases. For thecharge transfer reaction the existence of such deeper donor levels has two consequences: (1) the space charge layer is thinner than calculated with a model assuming constant donor density, since the density of positively charged states increases from the bulk to the surface. Tunnelling will therefore be enhanced and (2) the donor states in the space charge layer, being close enough to each other and to the surface, can mediate electron transfer by a hopping mechanism. The latter effect will offer locally different passes for electron exchange between the surface and the conduction band since the spatial distribution of donor states will be quite inhomogeneous, more so in view of the polycrystalline structure of the material. Grain boundaries might be particularly preferred pathways for electron transfer. Taking into account these assumptions we have designed in Fig. 7 a model for the space charge layer which appears much more realistic than that of Fig. 5 and permits electron passage through the space charge barrier by electron exchange between occupied and vacant donor states. If there are enough electronic states on the surface which can exchange electrons with the redox system the rate-determining step will be the electron passage through the barrier and the occupation of surface states will be in equilibrium with the redox system. This will mean a variation of the density of occupied and vacant surface states with applied bias since part of it will charge the voltage drop in the Helmholtz double layer. However, as long as the passage of electrons through the space charge barrier remains rate determining, this will not affect the measurable reaction rate. It will only reduce the effect of the bias variation since only the change of the potential drop in the space charge layer can influence the passing electron current. This explains that a + B -z I and gives also some idea for the reason of the very small charge transfer coefficients. Figure 8 illustrates the situation which is proposed to prevail during an anodic and a cathodic electrode reaction. If the electrons on their way through the barrier have to pass several donor states in single steps, only the shift of these states relative to each other with a change of the voltage will affect the electron transfer rate between two states in the space charge barrier or these donor states and the surface or the conduction band respectively. This means a division of a single charge transfer step between the semiconductor and the bulk into several smaller steps and a shift of the passage of electrons to different donor and surface states with a variation of the bias. Depending on the slowest steps in this chain, the apparent charge transfer coefficients will vary considerably with the material and the potential range in which the redox reaction occurs. In any case, smaller values of CLand j.3 than obtained for metal electrodes ari to be expected[7,8]. This picture applies well to the Ce3+ ‘* l reaction on all the considered electrodes and to the Fe2 + k3+ reaction on SnO, + Ru. Figure 6 suggests that at the respective standard potentials electronic states are
W. BADAWY,K.DOBLHOFER,
LEISELT,
H. GERISCHER,
S. KRAUSEAND
J. MELSHEIMER
conduction
(b)
(al
Fig. 7. Semiconductor electrode containing electronic states deep in the band gap, at (a) the Rat band potential, U,,, and (b) at the standard potential of a redox system [U’ = &(redox)/qJ. Electrons can penetrate the space charge barrier by a hopping mechanism. The distribution functions for electron energy states of the redox components (Ox’, Red) in the electrolyte are included schematically (W(E)).
at Ua Iequilibrium)
-7
(Ox + e--Red)
Fig. 8. Model of electrochemical charge-transfer
+q (Red +
Ox+e-1
involving surface states in equilibrium with a redox system.
An applied overvbltage, 4, is divided up between the space charge and Helmholtz layers. The equilibrium situation corresponds
available in the band gap. Their charging/discharging leads to AU, changes with U, as required for the mechanism symbolized in Fig. 8. These states are also available for a hopping type electron transfer across the space charge barrier. The inhomogeneity of the material has not yet been taken into consideration in this scheme. Its influence goes in the same direction, namely reducing the effect of a bias variation on the reaction rate by opening and closing different paths at different bias. At present we cannot give more than this rather qualitative descrip-
to Fig. 7(b).
tion. Better defined model materials are needed for a more accurate interpretation of such electrodes. Acknowle~~emencs-Theauthorsacknowledge thoroughdiscussions with Dr K. Jacobi on the interpretation of the UPS spectra. Valuable comments by Prof. J. Ulstrup (The Technical University of Denmark, Lyngby) concerning the mechanism of interfacial electron transfer, are gratefully acknowledged. The results from capacitance measurements in non-aqueous solvents were made available to us by Dr R. McIntyre.
SnO,
containing
up to 10% Sb or Ru oxides
REFERENCES 1. F. Mijller and R. Memming, Ber. Bunsenges. Phys. Chem. 76, 469 (1972). 2. H. A. Laitinen, Dsnki Kagaku 44, 626 (1976). 3. N. R. Armstrong, A. W. C. Lin, M. Fujihira and T. Kuwana, Attalyr. Chem. 48, 741 (1976). 4. T. A. Chertykovtseva, Z. D. Skuridina, D. M. Shub and V. A. Veselovskii, Elektrokhimiya 14, 1412 (1978). 5. C. Iwakura, M. Inai, T. Wemura and H. Tamura, Electrochim. Acta 26, 579 (1981). 6. P. Triggs, F. L&y and F. E. Wagner, Mat. Res. Bull. in press; R. Kijtz and K. Miiller, BBC Forschungszentrum, Baden, private communication; J. B. Goodenough and A. Hammett, Lecture I11.3, 34th JSE Meeting, Erlangen, i&23 September 1983. 7. W. Schmickler, Ber. Bunsenges. Phys. Chem. 82, 477 (1976). 8. J. Ulstrup, Charge Transfer Processes in Condensed Media, Lecture Notes in Chemistry 10. Springer, Berlin (1979). 9. W. Badawy, F. Decker and K. Doblhofer, Solar Energy Materials 8, 363 (1983).
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10. D. Elliott, D. L. Zellmer and H. A. Laitinen, J. elscrrochem. sac. 117, 1343 (1970). 11. Z. M. Jarzebski and J. P. Marton, J. efectrochem. Sot. 123, 299C (1976). 12. T. Arai, J. Phys. Sac. Jap. 15, 916 (1960). 13. H. Kaneko and K. Miyake, J. a&. Phys. 53,3629 (1982). 14. H. Kim and H. A. Laitinen, J. electrochem. Sot. 122, 53 ( 1975). 15. G. Mierdel, Ekktrophysik, Chapter 3.4. VEB Verlag Technik, Berlin (1972). 16. K. Bange and J. Sass, private communication; cf K. Bange, dissertation, TU Berlin (1983). 17. H. Gerisoher, Semiconductor electrode reactions, in Advances in Electrochemistry and Electrochemical Engineering (Edited by P. Delahay and C. W. Tobias), Vol. 1, p. 139. Interscience, New York (1961). 18. S. R. Morrison, Electrochemistry at Semicondnctor nnd Oxidized Metal Electrodes, Chapter 2. Plenum Press, New York (1980). 19. H. Geriseher, 2. phys. Chem. 21, 48 (1961). 20. K. Uosski and H. Kita, J. electrochem. Sot. 130, 985 (1983); Discussion by H. Gerischer, discussion section of J. elecrrochem. Sot. December 1983.