Enhancement of Electroluminescence from n-Type Porous Silicon and Its Photoelectrochemical Behavior

Enhancement of Electroluminescence from n-Type Porous Silicon and Its Photoelectrochemical Behavior

Kako Ogasawara, Toshiyuki Momma, and Tetsuya Osaka* Department o/Applied Chemistry, School o/Science and Engineering; Kagami Memorial Laboratory/or Materials Science

and Technology, Waseda University, Shinjuku-ku, Tokyo 169, Japan

ABSTRACT

In a study of the electroluminescence (EL) of porous silicon on an n-type Si wafer using an $20~- electrolyte solution, the addition of C2H~OH to the solution was found to enhance the intensity of EL from the porous silicon. The porous silicon structure was classified into two types which were prepared based on whether the anodizing current density for forming the porous n-Si was above or below the saturated photocurrent. A single layer of fine pores was formed galvanostatically with i l lumination at a current density below the saturated photocurrent density, and a double layer of fine and rough pores was formed under the same conditions but at a current density above the saturated photocurrent density. The electrochemical and enhanced electroluminescent properties of the two types of porous silicon were studied.

Introduction Since the visible photoluminescence (PL) of porous sili-

con was reported by Canham in 1990,1 this material has been studied to clarify luminescence mechanism and to determine its possible use as a new material for optical device fabrication. Although the quantum-size effects of silicon ~ and siloxene formation on the pore surface, 2 and other mechanisms have been proposed as the origin of luminescence from porous silicon, no consensus has been reached. Despite the disagreement on the mechanism, the direct ex- aminat ion of the EL of porous silicon continues to be at- tractive because of the possibility of development of a new EL device based on the Si wafer. All-solid EL systems using porous silicon have been reported, ~-5 but their efficiency is low. The EL emission using electrolyte solutions also has been reported 6'7 which has the advantage of a large contact area for injecting the carriers.

In an electrolyte solution containing a strong oxidizing agent such as peroxodisulfate ion ($20~-), stable and strong EL can be produced from cathodieally biased porous silicon formed on n-type St. 7 The carrier injection mechanism for the EL in this system is considered to be the same as for the typical n-type semiconductor as shown in Fig. 1, involving the following equations a

S20~- + ec -~ SO~.- + SO;.- [1]

SO4- + ev --> SO~- [2]

The first step reaction (Eq. 1) is the reduction of $20~- ion to form the SO~- radical anion. In this step, the electron is extracted from the conduction band. The second step reaction (Eq. 2) is the reduction of the SO~- radical anion to SO~ . In this step, the hole is generated by extracting the

* Electrochemical Society Active Member.

EF

:iiii:ii!!i ' iii!ii!i!iiil i i!i!! ilili n-type semiconductor

�9 E / e V

~ $2082- + ec- _~ $Q2- _~ S04"-

e -

SO4"- + e v- ~ SO42-

E)ectrolyte solution

Fig. i. Schematic energy diagram for EL on an n-type semiconductor under cathodic bias in electrolyte solution containing $20~-.

1874

electron from the valence band. Light is emitted with the recombination of the generated hole and the electron in the conduction band. Even though an electrolyte solution is used, the penetrat ion of the aqueous electrolyte solution into the porous layer is difficult in the case of porous silicon. 9 Thus, the efficiency of EL emission from porous silicon is still not as high as that expected from its large surface area. The difficulty of electrolyte penetrat ion is thought to be caused by the fact that porous silicon was extremely hydrop.hobic. 9"1~ The EL spectra of porous silicon were reported to shift drastically toward shorter wavelengths as the applied potential was made more negative. 11-13

Many researchers have pointed out that the luminescence properties depend strongly on the conditions for preparation of the porous silicon sample) The formation of porous silicon was first reported in 1956.1~ The importance of the formation mechanism of the silicon porous layer recently has attracted increasing attention, 14-16 but the nature of the formation mechanism is still unclean Although the correlation between PL properties and the conditions of porous silicon preparation has been reported and discussed, 1'171" there are many factors, e.g., the doping type and level of the Si wafer, the temperature, the concentration of HF, and the conditions of electrolysis. The n-St wafer system, in partic- ular, has been further complicated by the necessity of light irradiation for dissolution. Thus, it is also important to un- derstand the relationship between the luminescence properties and the preparation conditions.

We recently found that the EL of porous silicon was enhanced by the addition of C~H5OH to the $20~- aqueous solution system) ~ Using this system, we try to discuss the EL properties of porous silicon in the electrolyte solution containing C2H5OH, and also try to determine the correlation between the EL properties and the preparation conditions.

Experimental All samples of porous silicon were prepared by anodizing

n-type Si (100) with a specific resistance of 2.4 - 3.0 tl cm in an aqueous solution of 10 weight percent (w/o) HF and 35 w/o C2H5OH under the i l lumination of a 60 W tungsten lamp from a distance of 30 cm. Anodization was performed galvanostatically at different current densities for 7 rain. The samples were rinsed, but their surfaces were not dried, and immersed in distilled water for 30 min. The samples were then dried in vacuum for 24 h. The sample morphol- ogy was studied using scanning electron microscopy (SEM). An electrochemical cell was formed from three elec- trodes consisting of a porous, silicon working, a Pt wire counter, and an Ag/AgC1 reference electrode. EL spectra were measured in an aqueous solution of 0.15 tool dm ~ (NH~)2S208, 0.20 tool dm -3 Na2SO4, and 3.5 tool dm -3 C2H5OH using a spectrophotometer (F-4500, Hitachi, Ltd.) with a photomultiplier tube (R928F, Hamamatsu Photon-

J. Electrochem. Soc., Vol. 142, No. 6, June 1995 �9 The Electrochemical Society, Inc. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 155.33.16.124Downloaded on 2014-10-28 to IP

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J. Electrochem. Soc., Vol. 142, No. 6, June 1995 �9 The Electrochemical Society, Inc. 1875

I

E o < 20 E

t'--

~- 0 0

f

I J I l L, I J J I t I I 0 0 .5

Potential / V vs. Ag/AgCI

Fig. 2. Voltammogram of n-Si in the aqueous solution of 10 w / o HF + 35 w / o C2HsOH. The current was measured at ,5 mV s -1 under an illumination with a 60 W tungsten lamp from a distance of 30 cm.

ics, K. K.). The scan rate of measuring wavelength was 4 nm s -1 from 400 to 800 nm, and it took 100 s for the measurement of each spectrum. The reduction potential of $20~- on porous silicon, at which the cathodic current was started, was determined by a potential sweep measurement in the electrolyte solution. The f latband potential of porous silicon was estimated using the Mott-Schottky plots obtained from impedance measurements at 20 kHz in an aqueous electrolyte solution of 0.20 mol dm -3 Na2SO4 and 3.5 mol dm -~ C2HsOH in the dark.

Results and Discussion Preparation and structure of porous n-Si.--Porous n-

type silicon is formed by anodizing n-type Si wafers under a reverse bias with irradiation to generate a photocurrent. Figure 2 shows the voltammogram of n-Si in an aqueous solution of 10 w/o HF and 35 w/o C2HsOH with the il lumi- nat ion of a tungsten lamp. A current density plateau at 7 to 8 m A c m -2 corresponds to the value of the saturated photocurrent. Arita and Sunahara 2~ suggested that porous layers with different morphologies can be grown by taking into account the correlation between the anodizing current density and the saturated photocurrent. We thus formed two types of porous silicon under a fixed condition of illu- minat ion at current densities below and above the saturated photocurrent. We used a constant anodizing period of 7 min for all samples, rather than a constant anodizing charge, because the value of saturated photocurrent density shown in Fig. 2 remained essentially unchanged during this period of time.

Figure 3 shows SEM images of the cross sections for porous silicon samples in two categories described below. Sample A, anodized at 2 mA cm -2, and sample B, anodized at 5 mA cm -2, were formed below the saturated photocurrent. The two samples have a single, very dense, thin top layer with a thickness of some hundreds of nanometers, namely~ nanoporous silicon. 2~ Samples C and D, anodized at a current density higher than the saturated photoeur- rent, dear ly have two layers, i.e., a very dense, thin top layer and a porous underlayer having pores with the ap- pearance of trees extending over 10 ~m deep, the latter being designated as a macroporous silicon layer. 2~ When the n-type silicon wafer is anodized at a current density above the saturated photocurrent, holes for the dissolution of silicon are provided by the breakdown phenomena at the interface of the pore tip, ~3 and thus pores are grown in the direction of the electric field, especially in the < 100> direction, as has already been made clear in Ref. 22. As a result, deep pores are formed along the bulk crystalline structure as shown in Fig. 3C and D.

We confirmed that the anodization process at the low current densities (2 and 5 mA cm -2) did not clearly yield a

double-layer structure as shown in Fig. 3C and D even when an anodization charge larger than the charges corre- sponding to samples C and D was given. At first glance, our results seem incompatible with the results by L6vy- C16ment et aI. that the double-layer structure is formed at low current densities if a charge exceeding a certain amount was passed. 21 After the forming of the thick porous Iayer, if anodization of the silicon under this layer is at- tempted, the irradiated light must penetrate through the thick porous layer onto the unanodized silicon. Since the intensity of the irradiated light after the penetration becomes weak due to absorption and scattering in the thick porous layer, the effective value of the saturated photocurrent density becomes low. Thus, at the interface region just etched, the saturated photocurrent density is lowered and the anodizing current density which is lower than the ini- tial saturated photocurrent density becomes higher than the resulting saturated photocurrent density. Therefore, our results are consistent with L6vy-C16ment results that the macroporous layer under the nanoporous layer was formed even at low current densities below the saturated photocurrent density.

Enhancement of EL from porous n-Si in electrolyte solution containing $20~- with addition of C2HsOH.--The mechanism of the EL from the porous layer on n-type Si in an electrolyte solution containing $20~- is considered the same as for the usual n-type semiconductors, as was mentioned in the Introduction. Such EL accompanies an H2 gas evolution according to Eq. 3

2H § + 2e- --+ H2 [3]

The current efficiency for the reduction of $20~- is low due to the reduction of HL The evolution and adsorption of hydrogen gas also may disturb the detection of the EL for this system. We therefore tried to control the H2 evolution during the observation of the EL by adding C2H5OH to the electrolyte solution containing SzO~-. The result was the strong enhancement of EL for this system as shown in Fig. 4, which presents typical EL spectra of porous silicon anodized at 15 m A c m -2. Figures 4A and B correspond to the spectra of porous silicon in the electrolyte solution of 0.20 mol dm -~ Na2SO4 containing 0.15 mol dm -3 $20~- with and without C2HsOH at -1 .7 Y vs. Ag]AgC1. The intensity in the electrolyte solution containing C2H5OH was about 30 times stronger than that without C2H5OH. Further, with C2H5OH light emission was continuously recognized in daylight, rather than only momentarily. In the electrolyte solution with C2H5OH, the attachment and evolution of H2 bubbles on the sample surface were not visible when the cathodic bias was applied, although the H2 bubbles were clearly observable in the solution without C2H5OH. As we already described in an earlier Letter, 19 however, the potential of H* reduction on porous silicon was not changed by the addition of C2H5OH. As has been pointed out by Uhlir 1~ and Halimaoui, ~ the porous silicon layer was so hydropho- bic that the aqueous solution had difficulty in penetrating into the layer. The addition of C2H5OH could make the nature of the electrolyte solution slightly organophitic. There- fore, the main reason for the EL enhancement appears to be that the addition of C2H~OH makes the $20~- ion penetration into pores easier. As a result, the effective area for light emission becomes larger, which produces the enhanced EL intensit): Another reason may be the fact that the interme- diate radical anion SO4- has a longer lifetime with the addition of C2H5OH.

In the electrolyte solution system without C2H~OH, the EL intensity has been reported to increase by gradual re- placement of the HF solution by the electrolyte for EL measurement. 9 In our experiments, it is most important that the EL from porous silicon is observable in daylight for a long time period.

Basic electrochemical behavior of porous n-Si in electrolyte solution containing $20~- with addition of C2HsOH.--The enhanced EL mechanism in porous silicon in the system containing C2H5OH may be considered to be

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 155.33.16.124Downloaded on 2014-10-28 to IP


1876 J. Electrochem. Soc., Vol. 142, No. 6, June 1995 �9 The Electrochemical Society, Inc.

Fig. 3. SEM images for cross sections of different porous Si samples. Anodizing current densities are shown in the figure.

the same as that from the bulk semiconductor electrode. Therefore, it is important to investigate the electrochemical behavior of porous silicon as a semiconductor electrode. However, because of the possibility of visible light emission from porous silicon, the band structure of porous silicon must differ from that of bulk silicon. Therefore, it is important to determine the hand structure of porous silicon. First, we measured the flatband potentials of these samples anodized at different current densities. The measurement of the flatband potential of porous silicon in a normal aqueous solution was not stable under the anodic bias due to the dissolution of silicon. However, porous silicon was comparatively hard to dissolve under the anodic condition with the addition of C2HsOH. Figure 5A shows typical

(B) wi thou t C2HsOH 4000 - -

) wJth CzHaOH -~ 3000- lo

40O 6OO SO0 / \ .G 8 2ooo

8 =~ tooo

=, ,'- {B) without CzHsOH J

o| I I I 400 500 600 700 800

Wavelength /nm

Fig. 4. El. spectra of porous n-Si in the aqueous electrolyte solution of 0.20 mol elm -3 Na~O4 containing 0.15 mol dm -3 (NH4)2S208 {A)

3 with 3.5 mol dm- C2H5OH and (B) without C2H5OH. The porous Si 2 sample formed at 15 mA cm- was biased at - 1.7 V vs. Ag/AgCI in

the electrolyte solution.

Mott-Schottky plots for a porous silicon sample. Since the linearity is maintained, but in a narrow potential range in comparison with a plot for a normal semiconductor electrode, the measurement of the flatband potential was possible. Figure 5B shows the correlation between the anodizing current density and the flatband potential. The plot is roughly divided into two regions, and the boundary appears between 5 and 7 mAcm -2. In the region of lower anodizing current density, the flatband potential increased with the increase in anodizing current density. In the higher region, the flatband potential remained unchanged with relation to the anodizing current density. The two regions correspond to those of the porous silicon macrostructure produced below or above the saturated photoeurrenl density of 7 to 8 mA em -z. Therefore, the flatband potential of porous silicon samples and their macrostruetures may be correlated. However, porous silicon shows the penetration of the electrolyte solution from the surface to inner region, namely, to the space charge region, which has a gradient of potential. Thus, it is difficult to consider that the values obtained for these flatband potentials correspond to the normal flatband potential defined for the smooth semiconductor electrode, and the interpretation of these values re- quires further investigation.

Figure 6A shows voltammograms of typical porous silicon samples for the cathodic potential sweep in an electrolyte solution with (solid line) and without (dashed line) $20~-. In this figure, the solid line is the reduction current of SzO ~- and the dashed line is that for H § obtained from the potential sweep measurement using a sample anodized at 15 mAcm -2. The reduction of SzO~- occurs at a more positive potential than that of H +. In Fig. 6B the reduction potential for each sample is defined as the intersection of the lines of linear extrapolation of two segments of the current-potential curve obtained from the potential measurement in the electrolyte solution with $20~-, as indicated by




o 20 kHz

10 kHz

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- 0 .6

b

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0

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[ J n

0 0.2 0.4

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(A) o

l

0 . 6

(D cn

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o -~ ~.-

11

_

-0.2 -

- 0 . 4 -

0

(g)

P i z

! / t

/ 0 0 0 0

I I I I 10 20

Anodizing current density / mA cm -2 Fig. 5. Flatband potentials of porous silicon samples as a function

of anodizing current densily. The flatband potential was calculated from the Mott-Scholtky plots by an impedance measurement at 10 and 20 kHz in an aqueous electrolyte solution of 0.20 mol dm -~ Na2SO4 + 3.5 mol dm -3 C2HsOH. (A) Typical Mott-Scholtky plots for porous Si anodized at 10 mA cm -2. (B) Relationship between flatband potential which was estimated from a Mott-Schottky plot at 20 kHz and anodizing current density.

the arrows. In Fig. 6C, the reduction potential is plotted as a function of anodizing current density. It is seen from a comparison of Fig. 5B and Fig. 6C that the reduction potential is more negative than the f latband potential. However, in the reduction potential-anodizing current density plot (Fig. 6C), two regions can be distinguished with the same boundary of the anodizing current density of 7 to 8 mA cm -2 as observed in the f latband potential-current density plot (Fig. 5B). Since the reduction potential of the n-type semiconductor is considered to be determined by the flatband potential, porous silicon is considered to behave like a normal semiconductor electrode.

We measured the potential at which the light emission starts (hereafter called luminescence potential) by scanning the potential in the negative direction from the rest potential. The results are shown in Fig. 7A. Figure 7B demonstrates the correlation between the luminescence

potential and the anodizing current density. Light emission starts at more positive potentials for samples anodized at higher current densities. The shape of the plot in Fig. 7B shows clearly the same tendency as that observed for the reduction potential shown in Fig. 6B. The similarity between the two plots suggests that the light emission arises from the reduction of $20~-, and the emission is EL for the type of carrier injection from the electrolyte solution.

The electrochemical properties clearly changed in the range of anodizing current density between 5 and 10 mA cm -2. The current density range corresponds to the saturated photocurrent density, and also to the range where the structural change is observed. Since there appears to be good agreement among the f latband potential, the reduction potential of $20~-, and the luminescence potential, this light emission from porous silicon is expected to be the EL from an n-type semiconductor.

Potential dependence of EL from porous n-St in electrolyte solution containing $20# with addition of C2HsOH.--In EL from porous silicon, the spectrum de- pended on the applied potential as already pointed out in Ref. 11, 12, and 23. Figure 8 shows the spectra at different potentials for different silicon samples anodized at different current densities. As mentioned above, however, the luminescence potential was different for each sample, and hence each sample is expected to show an EL with a peak at a different wavelength at the same applied potential.

As shown in Fig. 8, the blue-shift tendency is demonstrated with the increase in the cathodic bias for each sample. The intensity also increases with the increase in the cathodic bias. The samples anodized at higher current densities of 10 and 15 m A c m -~ and lower current densities of 2 and 5 m A c m -2 have different behaviors in peak shift tendency. Figure 9 shows the relationship between the peak wavelength and the potential of the cathodic bias. The peaks for low current density samples A and B show almost the same peak shift behavior. For higher current density samples C and D, the peak shifts toward longer wavelengths with the increase in anodizing current density. The EL emission in the longer wavelength region may be af- fected by the inner layer with pores, because the peaks f o r samples C and D are situated at longer wavelengths than those for samples A and B.

Bsiesy et al. reported a large spectral shift and tried to explain this phenomenon from the size distribution of silicon clusters in the porous layer. 23 Canham pointed out that the gradation of the luminescence property from the surface toward the bulk may originate in the distribution of the silicon crystal size. 24 Andsager et al. reported that the PL from porous silicon had different wavelengths in the direction of thickness, and they suggested h possibility of degradational change of luminescence propertyY On the basis of the potential dependence of the EL spectra, we also assume that a gradational change may occur in the porous layer. That is, the porous layer may be composed of minute silicon columns which produce the quantum-size effect, and the size of the column may change in the direction of the thickness of the porous layer. Considering that the EL spectra shift toward the shorter wavelength with the increase in the cathodic bias as shown in Fig. 8 and that the cathodic bias as the forward bias makes it easier for the electron in the conduction band to move to the region nearer the surface, we expect that the active area for the reduction of $20~ may move from the inner area to the nearby surface region. The distribution of the diameter of the silicon cluster in the direction of the thickness may be from smaller to larger values from the surface to the inner layer region. This assumption corresponds well with the potential dependence of the EL spectra, namely, the peaks of spectra shift from a longer to a shorter wavelength with the increase in cathodic potential. However, even if such size distribution exists in the porous layer, the difference in size of the c]usters is only a few nanometers by calculation. Therefore the direct evidence for the confirming such small difference of cluster size is difficult to obtain. There may be other factors for the change in the EL spectra, such as the




i E

-0.5 / / /

/

,/ -2.0 -1.5

I I -1.0 -0.5

Potential / V vs. Ag/AgCl

_A(A) 1

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- 0 . 6 <

> > m

c- ,~ - 0 . 8

. s

rr - 1 - I

0 5

O /

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Anodizing current density / mA cm -2

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5 mA crn -2-1L-

I 50/JA cm -2

I I I I ~ I I I I , I 10 15 -1.o -0.5 o

Potent ial / V vs .Ag/AgCl

Fig. 6. Potential dependence of reduction current of $20~- on porous Si samples as a function of anodizing current density. (A) Voltam- mogroms of porous silicon samples, anodized at 15 mA cm -2, in an aqueous electrolyte solution of 0.20 mol dm -3 Na~SO4 and 3.5 mol dm 3 C2HsOH with (solid line) and without (dashed line) 0.15 mol dm -3 (NH4)2S208 at 50 mV s-L (B) Voltammograms in an aqueous elec~olyte solution of 0.20 mol dm-3 Na2SO4 containing 0.15 mol dm-3 (NH4)2S208 with 3.5 mol dm-3 C2HsOH at 100 mV s-1 in the dark. (C) Relationship between the reduction potential of $20~- and the anodizing current density.

>.,,

1,0 r -

E

O E

O

E

E . J

(A)

2 m A c m -2

5 mA c m -2

10 m A c m -2

15 m A c m -2

1"

<

< - 0 . 8

> >

[ . -

o

r-

E " I

. .J

(B)

P s"

O

- 1 . 2 i I i 0 5 10 15

Anodizing current density / mA cm -~

- 1 . 5 - 1 . 0 - 0 . 5 P o t e n t i a l / V vs. A g / A g C I

Fig. 7. Luminescence potential for porous Si samples with varying anodizing current density. (A) EL intensity-potential curves in a 0.15 mol dm ~ (NH4)2S208 aqueous electrolyte solution of 2.0 mol dm 3 Na2SO~ containing 3.5 mol dm-~C2H5OH. (B) Relationship between the estimated luminescence potential and the anodizing current density.




distr ibut ion of the amount of siloxene and the dis tr ibut ion of terminated components on the porous silicon surface. In

o9 r- Q) t--

r-

Go

r-

E

d

I I ,~1 I I 1,9 V

ii\ ili'

~ -1.9 V

f l ~ -1.8 v

k-2.0 V

r t

i tlb~ -1.9 V

i -1.8 V l i l ' k i 1' / ,/~,.Tv

# ~ -2.0 V

: I f , i' I

I

i ,4, -l.'v

I i 19~k -1.7 v

I I I I

(A)

(B)

[D) !

!

400 500 600 700 800

Wavelength / nm

Fig. 8. Dependence of EL spectra for porous n-Si on wavelength as a function of cathodic potential. The spectra of (A), (B), (C), and (D) were obtained from samples anodized at 2, 5, 10, and 15 mA cm -~, respectively. The measurements were performed from positive to negative potentials. The anodizing current densities for the samples and the applied potentials are shown in the figure.

700

~ 6 5 0 q)

600

�9 2 mA cm -2 (A)

A 5 mA crn -2 (B) / / t

i ' 10 mA cm -2 (C) i , ~ /

�9 ~5 r~A om-~ (D) I V "/" -

- 2 . 0 - 1 . 9 - 1 . 8 - 1 . 7 - 1 . 6 Potential / V vs. Ag/AgCI

Fig. 9. Peak wavelength dependence of EL spectra on cathodic bias potential for porous n-Si samples formed at different anodizing current densities. These plots are drawn from data in Fig. 8.

the future we will a t tempt to find addit ional experimental support for the above assumption.

Conclusion Two types of porous n-Si samples which had different

morphologies were prepared to compare their electrochemical and electroluminescent properties. An almost 30-fold enhancement in the EL was found with an addi t ion of C~H~OH to the aqueous electrolyte solution. We studied the electrochemical and photoelectrochemical properties, and were able to divide the dependence of the f la tband potential, and reduction potent ia l of $20~- anion, and the luminescence potent ial on anodizing current density into two separate regions. The two regions corresponded precisely to two different structures.

Manuscript submit ted Sept. 30, 1994; revised manuscr ipt received Jan. 9, 1995.

Waseda University assisted in meeting the publication costs of this article.

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On the Lack of Activity of Substitutional Sn Atoms toward the Electro-oxidation of CO on Pt Anodes

Molecular Orbital Theory

Alfred B. Anderson and E. Grantscharova a Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078

Paul ShiUer Packard Electric DELPHI Systems, Warren, Ohio 44486

ABSTRACT

An atom superposition and electron delocalization molecular orbital (ASED-MO) study shows that t in atoms alloyed substi tutionally into p la t inum electrode surfaces are inactive in generating OH(ads) for the electro-oxidation of the CO(ads) poison generated during the operation of methanol fuel ceils. Such t in atoms, though they donate to Pt, are not good acceptors for HH20 lone-pair donation bond formation because of the way in which their 5p orbitals mix with the Pt valence band. Thus substi tutional Sn atoms in the Pt surface do not attract or activate H20. OH is also found to adsorb weakly to substi tutional Sn atoms compared to surface Pt atoms, the opposite of the diatomic SnO and Pro bond strengths. This is because the OH is essentially reduced by neighboring Pt atoms and not the Sn atom to which it is bound. When bound to substi tutional Sn atoms, OH is calculated to be relatively active in oxidizing CO(ads) on adjacent Pt atoms, but the inabil i ty of the surface to generate such OH implies a different mechanism must be responsible for the electrocatalysis, perhaps involving adsorbed Sn atoms or Sn complexes.

Inboduction The promoting effect of the addition of t in to pla t inum

for the electro-oxidation of water-soluble organic fuels was brought to the open literature by Cathro about 25 years ago. I Organic residues were known to plague the operation of fuel cells, causing an overpotential of ~0.6 V (vs. stan- dard hydrogen electrode (SHE)] above the thermodynamic potential of - 0 V. 2 Tin added to the surface reduced the overpotential by several hundred millivolts, depending on the fuels. The belief at the time was that an oxidized form of t in on the pla t inum surface reacted with the residues that otherwise poison the surface by blocking it, generating carbon dioxide, along with protons which entered the electrolyte, and electrons which entered the anode. The Sn(OH)2/Sn(OH)4 couple was mentioned as being possibly relevant by Cathro. There was a generally held belief at this time that OH radicals were the residue-removing oxidants and that their generation was paramount in improving fuel cell efficiency? 5 In association with metal atoms, OH radicals become OH- bound to oxidized metal atoms. Although HCO ~ and COH 7 have been proposed as the significant poisoning species, as more work has gone on, evidence has accumulated during the past decade that CO is the rate- controlling poison. T M

Uncertainty about the structure of the activating t in and the mechanism of its activation has persisted. Cathro's sug- gestion about an Sn(II)/Sn(IV) redox couple was not specific. Janssen and Moolhuysen proposed that the active t in was metallically alloyed into the Pt surface where it was assumed to weaken the absorption bonding of the poisoning residue and facilitate the adsorption of water molecules. 15 ASED-MO calculations by Shiller and Ander-

a On leave from Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria 1113.

son 16 demonstrated that such a ligand effect existed, with a 0.13 eV weakening of the CO adsorption for 0.25 monolayer t in coverage, in fair agreement with an experimental esti- mate of a 0.2 eV weakening for 0.33 monolayer coverage on Pt(111). However, as was noted in Ref. 16, weakening the CO adsorption energy does not prove a mechanism, and it must be considered whether t in could have an additional or alternative role of providing active oxygen. High-vacuum surface-science studies of CO adsorption on substi tutionally alloyed Pt surfaces have continued in the literature ~v'l~ but have not answered the electrochemical questions.

In the meantime, an ASED-MO study, again by Shiller and Anderson, z9 suggested the possibility that under proper circumstances H~O may react directly with adsorbed CO, producing CO2, 2H § and 2 e-. There is as yet no confirmation of this reaction. Recently, a series of experimental electrochemical studies by Gasteiger and co-work- ers 2~ and theoretical ASED-MO studies by Anderson and Grantscharova 23'24 have focused on OH(ads) as the oxidizing species on Pt and Ru/Pt surfaces. Ru/Pt alloy surfaces are the most effective methanol oxidation catalysts known to date. The accumulated experimental and theoretical evidence is that a major pathway for CO(ads) removal on Pt is

CO(ads) + OH(ads) ---> CO2 + H + + e- [1]

and that OH(ads) forms according to

H20 --> OH(ads) + H + + e- [2]

which is the potential-dependent rate-l imiting step responsible for much of the overpotential. A substi tutional Ru atom in a Pt surface was calculated to bind H20 prefer- entially and to cause reaction 2 to go at much lower potential, and the resulting OH, bound to substi tutional Ru in the Pt(111) surface, could easily oxidize CO according to reac-



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