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Journal of the American Chemical Society
Ser. 6, 29, 489 (1975). (23) See, for example, C. P. Andrieux, C. Blocman, J. M. Saveant, and J. M. Dumas-Bouchiat. J. Am. Chem. Soc., 101,3431 (1979). (24) J. F. Evans, T. Kuwana, M. T. Henne, and G. P. Royer, J. E k m m I . Chem., 80, 409 (1977); D. C-S. Tse and T. Kuwana, Anal. Chem., 50, 1315 (1978). (25) Unpublished work of M. Dautartls and J. Evans, University of Minnesota. ‘26) H. Lund and E. Holboth, Acta Chem. Sand., Ser. 6, 30, 895 (1976).
/ 102:lO / May 7, 1980
(27) C. P. Andrieux and J. M. Saveant, J. Electroanel. Chem., 93, 163 (1978). (28) The reproducibilityof volQ”0gams for allyl and benzyl hallde reductions on platinum has recently been studled and effects due to adsorbed halide were reported. A. J. Bard and A. Merz, J. Am. Chem. Sa.,101, 2959 (1979). (29) R. E.Buckles, W. E. Steinmetz, and N.G. Wheeler, J. Am. Chem. Soc., 72, 2496 (1950).
Use of Chemically Derivatized n-Type Silicon Photoelectrodes in Aqueous Media. Photooxidation of Iodide, Hexacyanoiron( 11), and Hexaammineruthenium( 11) at Ferrocene-Derivatized Photoanodes Andrew B. Bocarsly, Erick C. Walton, and Mark S. Wrighton* Contributionfrom the Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. Receioed August 20, 1979
Abstract: n-Type Si can be derivatized using ( 1 ,I’-ferrocenediy1)dichlorosilaneyielding a photoanode that can be used in aqueous electrolyte solutions under conditions where the naked (nonderivatized) n-type Si undergoes photoanodic corrosion yielding an insulating SiO, surface layer. Derivatized electrodes in aqueous electrolyte solution exhibit chemically reversible oxidation of the surface-confined, ferrocene-centered redox reagent when the electrode is illuminated. Oxidation is not detectable in the dark, consistent with the fact that n-type semiconductors are blocking to oxidation in the dark. At sufficiently high light
intensity, the surface-confined material can be oxidized at more negative potentials than when the same material is confined to the surface of a reversible electrode material such as Pt or Au. Light intensity of -40 mW/cm2 at 632.8 nm is typically suffi10-9 mol/cm2 cient to effect oxidation 200-400 mV thermodynamically uphill for electrodes exhibiting coverages of of electrochemically active material. Such electrodes can be used to effect the persistent oxidation of I-, [Fe(CN)6I4-, and [ R U ( N H ~ ) ~The ] ~ +current . efficiency for 31- 13- and [Fe(CN)6I4- -[Fe(CN)6I3- is measured to be >90% and these oxidations can be effected thermodynamically uphill, but with low efficiency. The formal potential of [Ru(NH3)6l3+/ [ R u ( N H ~ ) ~ is] ~sufficiently + negative, -0.2 V vs. SCE, that oxidation cannot be effected thermodynamically uphill with ntype Si. Oxidation of H20 using derivatized n-type Si does not occur under any conditions used. The oxidation of the three reductants used occurs by (1) photogeneration of electron-hole pairs by absorption of band-gap irradiation by n-type Si (band gap = 1. I eV); (2) oxidation of the surface-confined ferrocene-centered redox reagent by the photogenerated holes; ( 3 ) oxidation of the solution reductant (I-, [Fe(CN)6I4-, [RU(NH&]~+)by the surface-confined ferricenium oxidizing reagent. This mechanism is established by cyclic voltammetry. Current-time plots show that ferrocene-ferricenium turnover numbers exceed IO5 and constant (within 10’70) photocurrents can be obtained for > 5 h, whereas naked electrodes give photocurrents which decay by >90% within 5 min. Energy conversion efficiency for [Fe(CN)6I4- oxidation or 1- oxidation at 632.8 nm is of the order of 1%.
-
Substantial improvement in n-type semiconductor-based cells for light to electricity energy conversion has been possible through judicious choice of aqueous and nonaqueous electrolyte/redox W e have recently adopted a strategy involving chemical derivatization of the surface of n-type semiconductor photoanodes in order to design photosensitive interfaces for use in a large number of thermodynamically uphill oxidation processes. The essence of our strategy is to bind redox reagents to the surface of the n-type semiconductor. The bound reagents can be oxidized in a thermodynamically uphill fashion by irradiation of the semiconductor and the oxidized form can then oxidize solution species. The point is that the surface-confined material serves two functions: ( I ) photogenerated holes are transferred to the attached redox reagent to preclude semiconductor decomposition and (2) the photooxidized, surface-confined material oxidizes solution species that may or may not be directly competitively oxidized a t the naked electrode. Just what species in solution can be oxidized does not depend on the kinetics of hole transfer from semiconductor to the solution species, but rather on the nature of the surface-confined material. Thus, electrodes can be “designed’’ to oxidize certain species selectively. In energy conversion applications what can be oxidized, overall efficiency, and durability are parameters of central importance. Our 0002-7863/80/1502-3390$0 1.OO/O
earlier articles have been concerned with the preliminary aspects of derivatized n-type Si? Ge,Io and GaAs’I characterized in nonaqueous media by electrochemical techniques. In this article we wish to amplify our resultsgc on the use of derivatized n-type Si in aqueous electrolyte solutions. These are important measurements inasmuch as naked Si is useless in aqueous electrolytes and yet has a band gap, E B G ,of 1.1 eVi2 that is nearly optimal from the point of view of solar energy conversion. Our new results show that the uphill oxidation of I- to 13- can be sustained using an n-type Si photoelectrode derivatized with (1 ,1’-ferrocenediyl)dichlorosilane.
Results and Discussion A. Characterization of Naked and Derivatized n-Type Si Electrodes in Aqueous Electrodes. Freshly HF-etched, single-crystal, n-type Si photoanodes ,in aqueous 0.1 M NaC104 (pH 0-6) are not durable, and we have been unable to observe any sustained photoanodic reaction other than that represented by the equation xHzO
+ Si
hu
SO,
+ 2xe- + 2xHt
Note that the surfaces of Si used here are not pure Si; the surfaces invariably have a thin air oxide.I3 Irradiation of the 0 1980 American Chemical Society
Bocarsly, Walton, Wrighton
/ Chemically Deriuatized
n- Type Silicon Photoelectrodes
3391
Table I. Passivation of Naked n-Type Si by Illumination in
ni
Aqueous Media” passivation potential, V vs. S C E b
electrode
coulombs passed, X lo4 cm-*
fraction of remaining photocurrentd
A A A A A
2.0 6.0 15.0 20.0 27.5
0.96 0.9 1 0.84 0.67 0.33
+o. 1 +o. 1 +o. 1 +o. 1
A A
10.0 30.0
A
55.0
A
57.6
0.89 0.80 0.36 0.33
c0.2 +0.2 f0.2
A
30.0 70.0
+OS
B
30.0
0.29
+0.5
B B B
30.0 40.0 45.0
0.16 0.06 0.02
B B B
7.0 30.0 50.5
0.76
0.0 0.0 0.0
0.0 0.0
+0.5 +OS
+0.5 +0.5 +0.5
A A
100
0.91 0.65 0.39
I
I
I
-0.6 -0.4 -0.2
I
I
0
+0.2
,
-0.6
-0.4
-0.2
0
t0.2
0.05 0.01
Electrolyte consists of 0.1 M NaC104(,,) adjusted to pH 1.3 with concentrated HCIO4. Illumination is from a 632.8-nm He-Ne laser beam expanded to illuminate total crystal face (0.1 cm2). Input power is 40 mW/cm2. Electrode is potentiostated at given potential under illumination. Data indicates number of coulombs passed at the passivation potential. Note that the time necessary to pass a given amount of electrons is a function of the potential. Photocurrent is measured at short-circuit in an EtOH solution of ferrocene/ferricenium with 0.1 M [n-Bu4N]C104 as the supporting electrolyte solution, ,Fredox = 0.27 V vs. SCE. Illumination is provided by a 5.2-mW He-Ne laser at 632.8 nm with a beam diameter of 1.0 mm. Fractions given are ratio of photocurrent before and after passivation. photoelectrode a t given potentials leads to rapid growth of the oxide to thicknesses that preclude the flow of current. Figure 1 compares the result of repetitive cyclic voltammograms for irradiated n-type Si in 0.1 M NaC104 and in 0.1 M NaC104 plus 5 m M K4[Fe(CN)6]; additionally plots of photocurrent against time a t a fixed potential in the two solutions are in accord with the cyclic voltammetry (vide infra). These data are representative; naked n-type Si in any aqueous electrolyte solution that we have investigated is not stable under illumination. The presence of redox-active material such as I-, [ Fe(CN)sI4-, or [ R U ( N H & ] ~ +gives little, if any, qualitative improvement in the constancy of the photocurrent at potentials where efficient photocurrents can initially be observed. The lack of constant photocurrents in the presence of the reductants evidences the inability of irradiated n-type Si to effect the efficient oxidation of the solution species. The photoanodic decomposition represented by eq l dominates the oxidation processes. The amount of oxide needed to suppress the flow of photocurrent can be measured from integration of current-time plots in aqueous media. However, it is well known that the nature of the oxide is very dependent on the rate a t which it is grown and the solution (or other medium) from which it is grown.13 Table I lists some quantitative data, under specified conditions, relating the number of coulombs corresponding to SiO, growth in H20/0.1 M NaC104 and the suppression of photocurrent for that electrode when used in a standard EtOH electrolyte solution of ferricenium/ferrocene. The experiment for a given electrode would be as follows: ( 1 ) the freshly HF-etched electrode is examined in the EtOH/ferricenium/ferrocene solution, E r e d o x +0.3 V vs. SCE, and the steady-state pho-
then put in the H20/0.1 M NaC104 solutibn and illuminated a t a given potential while current is integrated; (3) the electrode is then reexamined (after rinsing and drying with EtOH) in the EtOH/ferricenium/ferrocene solution to determine the extent to which the photocurrent was attenuated. The procedure was repeated until the photocurrent in the EtOH/ferricenium/ferrocene solution was significantly attenuated. Several electrodes were studied and different potentials were used in the HzO/O.l M NaC104 solution. Approximately 40 mW/cm2 of 632.8-nm irradiation was used in all cases. The essence of the results can be summarized by the observation that -lo-’ mol of electrons/cm2 is more than sufficient to form an SiO, layer thick enough to significantly attenuate photocurrent under the conditions routinely employed in this experimentation. n-Type Si can be derivatized with ( 1,l’-ferrocenediy1)dichlorosilane according to the procedure detailed in the Experimental Section.9-” The resulting surface can be characterized by cyclic voltammetry in an aqueous electrolyte solution. Figure 2 shows data for derivatized Si like that shown in Figure 1 for the naked Si. The derivatized n-type Si exhibits a persistent oxidation wave and a reduction wave of equal area in the cyclic voltammogram when the electrode is illuminated with ~ E B light. G In the dark there is no anodic current, but the reduction of the photogenerated, surface-confined oxidation product can be effected in the dark as evidenced by the cathodic wave when scanning from the anodic limit with the light off. The oxidation/reduction waves correspond to the chemistry represented by the equation light, potential
surface-ferrocene
7 surface-ferricenium
(2)
light or dark, potential
These characteristics are qualitatively the same as reported previously9 in nonaqueous media. The data in Figure 2 illustrate that the surfaee-confined material can be oxidized and reduced a large number of times without substantial deterioration. The position of the photoanodic peak for the derivatized electrode depends on light i n t e n ~ i t y At . ~ low light intensity there is not necessarily even a peak; the photocurrent corresponding to oxidation of the material on the surface is controlled by the hole generation rate. When a peak is observed, the photocurrent becomes limited by the amount of reduced
3392
Journal of the American Chemical Society (b)
(c)
/ 102:lO / May 7, 1980
200 mv/s I O 0 mv/s
scan I scan 1000 0
f
-
c
c
!03
0
u -
U
f
Light off
V
4
1
1
1
I
1
-0.8 -0.6 -0.4 -0.2
I
0
I
I
1
+0.2 -0.4 -0.2
I
1
0 +0.2 Potential, V.vs. S C E
I 1 I I -0.6 -0.4 -0.2
/
0
+0.2
Figure 2. Cyclic voltammetric characterization of three different derivatized n-type Si electrodes in 0.1 M NaC104, pH 1.3. Scans are initated at the negative limit. Illumination is from a He-Ne laser at 632.8 nm, -4.5 mW over the entire electrode surface (0.1 cm2). (a) Cyclic voltammogram (100 mV/s) in the dark ( - - -), under illumination (-), and an illuminated anodic scan followed by a dark cathodic return (-.-). (b) The electrode is scanned (200 mV/s) under illumination 1000 times between -0.5 and +0.3 V vs. SCE indicating durability of surface. (c) Scan rate dependence of an illuminated electrode.
material on the surface. The most negative photoanodic peak is observed at the highest light intensity. The most negative peak observed so far from n-type Si in aqueous solution is a t --0.05 V vs. SCE; more typically, we observe peaks in the range 0.1-0.2 V vs. S C E a t the light intensity routinely employed, -40 mW/cm2 a t 632.8 nm. By comparison to derivatized Pt or Au using (1,l'-ferrocenediyl)dichlorosilane as the derivatizing reagent,14 the derivatized n-type Si exhibits waves that are more negative a t sufficiently high light intensity. Derivatized Pt or Au exhibits cyclic voltammetric waves centered at -+OS V vs. SCE, which, as expected, is very close to the formal potential of the ferricenium/ferrocene system in s o l ~ t i o n . ' ~That . ' ~ waves on Si can be more negative indicates that the surface-confined ferrocene can be oxidized in an uphill manner by up to -0.5 V when using a high light intensity. Assuming that the photoanodic wave is symmetrical the peak potential represents the potential a t which the ferricenium/ ferrocene ratio is unity. W e take this potential to be the potential at which the derivatizing layer is at the formal potential of the ferricenium/ferrocene systemsgbFrom the results for Pt and Au we take Eo(ferricenium/ferrocene) = +0.45 V vs. S C E for aqueous electrolyte solutions contacting the surface-confined redox-active material. Thus, the position of the photoanodic peak defines what solution species are energetically capable of oxidation at that electrode potential, assuming that oxidation of solution species occurs exclusively by what we have termed mediated o ~ i d a t i o ni.e., , ~ the direct oxidation of solution reductant by the bound redox reagent. The data show that for electrode potentials of 0.0-0.2 V vs. S C E the oxidizing power of the surface can be -+0.45 V vs. S C E when the irradiation is sufficiently intense. The cyclic voltammograms generally are not taken to positive excursions that would bring the ferricenium/ferrocene ratio any higher than -10. Thus, if the potential of the derivatizing reagent layer obeys the Nernst equation, the oxidizing power does not significantly exceed +0.5 V vs. SCE. The formal potential for 02/H20 is 1.OO to +0.6 V vs. S C E for a p H range 0-7. Accordingly, the derivatized photoelectrode does not have the oxidizing power to evolve 0 2 from H20 when the 0 2 is a t unit activity and p H 9999/ 1 would be optimistic giving a -+0.7 V vs. SCE oxidizing power. But mediated oxidation rates would likely be very low for species whose formal potential is so positive. Further, there is no special reason to expect a ferricenium-centered reagent to be able to effect the multielectron oxidation of H20 without being irreversibly hydroxylated or otherwise destroyed. Finally, we find that significantly positive excursions with derivatized n-type Si appear to result in irreversible oxide growth undermining the derivatizing layer. Positive excursions to +0.6 V have been carried out without ill effects, but more routinely we avoid potentials which are >0.2 V more positive than the photoanodic peak. With respect to durability, we have encountered no difficulty with the cathodic excursion. In the p H range 0-7 the H2/H20 potential is -0.23 to -0.63 V vs. SCE. But in this potential range we observe no cathodic current; very negative potentials (-- 1.5 V vs. SCE) are needed to effect H2 evolution a t either the naked or derivatized n-type Si in aqueous solutions (pH 0-7). Derivatized electrodes have been routinely scanned to - 1 .O V vs. S C E without deleterious effects on electrochemical behavior of the confined material. The lack of efficient H2 evolution is not a particularly surprising result, since very few materials are good H2 electrodes. But we find additionally that naked (or derivatized) n-type Si is a poor cathode for either the reduction of [Fe(CN)6I3- or [Ru(NH&I3+; very negative potentials are needed to reduce either of these ions compared to what obtains for Pt or for the ferricenium ferrocene system on n-type Si, Figures 3 and 4. The extent to which the reduction peak is more negative seems to depend significantly on the oxide layer thickness. But in no case does the reduction of [Fe(cN)6l3-, [Ru(NH3)6I3+,or 13- occur with good kinetics. Cyclic voltammetry shows broad peaks with a more negative cathodic current peak at faster scan rates. Finally, we have been unable to establish any clear-cut, systematic effects
-
Bocarsly, Walton, Wrighton
/ Chemically Deriuatized n- Type Silicon Photoelectrodes
3393
200 pA
100 mv/s
150 mv/s -1.0 -0.8 -0.6 -0.4 -0.2
+ 0 2 C0.4
0
POTENTIAL, V vs SCE
Figure 3. Comparison of cyclic voltammetry of 1.O mM K,[Fe(CN)6] in 0.1 M NaC104, pH 5, aqueous solution at naked, n-type Si (nonilluminated) and at a Pt-foil electrode.
5 1
u
0
t-0
6s [L
-
LL
30 0 0 0
f
" -110
-68
-0:6 -d.4 -0.2
b
+0:2
-
POTENTIAL, V vs. SCE
-0.4
Figure 4. Cyclic voltammetry (50 mV/s) of illuminated, chemically derivatized n-type Si with and without [Ru(NH&]CI,. The reduction peak at -0.75 V vs. SCE is associated with the [Ru(NH3)613+ [Ru(NH3)6l2+ reduction. After the [Ru(NH3)6I2+is generated by the excursion to - 1.O V vs. SCE, there is a slightly larger photoanodic current associated with ferrocene ferricenium plus regeneration of [Ru(NH3)6I3+.The ferricenium ferrocene return peak is slightly attenuated when the [Ru(NH3)6I2+ is generated, evidencing a mediated oxidation mechanism.
-
--
on the cathodic behavior of n-type Si upon derivatization with the (1 ,1'-ferrocenediy1)dichlorosilane. One characteristic of the derivatized electrodes remains to be mentioned: coverage and kinetics. The oxidation of surface-confined material in the H20/0.1 M NaC104 occurs rapidly a t sufficiently high light intensity. Generally, the kinetics for the photooxidation are good as evidenced by the linear plot of peak photocurrent against scan rate, a t sufficiently high hole generation rates from appropriate light intensity. But the range of scan rates where linear plots are found does seem to depend on coverage. At coverages which are in the range of mol/cm2 of projected area the fastest kinetics obtain and linear plots a t scan rates up to 300 mV/s obtain. But oligomeric coverages, up to mol/cm2 of projected area, using (1 ,l'-ferrocenediyl)dichlorosilane can be obtained and plots of peak current vs. scan rate fall from linearity even at
