5 Integrated Chemical Systems: n-Silicon/Silicide/Catalyst Systems
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ALLEN J. BARD, FU-REN F. FAN, G. A. HOPE, and R. G. KEIL The University of Texas, Department of Chemistry, Austin, TX 78712
The paper by Wrighton describes semiconductor systems which incorporate other components, such as polymer layers, to produce useful electrode structures. The development of such multicomponent, multiphase systems, which we call "integrated chemical systems" by analogy to the integrated circuits used in semicon ductor devices, clearly represents an important new trend in chemistry. The design of useful semiconductor electrodes and powders will require surface modification to passivate surface states to improve efficiency, to protect the surface from photodecomposition, to catalyze desired reactions and to provide sen sitizers. For example the system p-GaAs/viologen polymer/Pt can be used for the photodriven evolution of hydrogen. The develop ment of such systems will probably require the application of techniques very different from those normally used in chemical synthesis, e.g., molecular beam epitaxy, sputtering, ion implan tation, spin coating, and other methods borrowed from solid state physics and semiconductor technology. These integrated chemical systems will have properties different and, we hope, more useful, than that of the individual components. Such syner gistic effects are well-known in biological systems where the overall behavior of the complex structure is usually more than the simple sum of the parts. I would l i k e to describe b r i e f l y an i n t e g r a t e d chemical sys tem r e c e n t l y under i n v e s t i g a t i o n i n our l a b o r a t o r y based on ntype s i l i c o n which i l l u s t r a t e s some of the above features (1,2). As Wrighton p o i n t s out i n h i s paper, η-Si e l e c t r o d e s are u s u a l l y unstable i n aqueous s o l u t i o n s , because they tend to form a p a s s i v a t i n g oxide f i l m under i r r a d i a t i o n . We have found that by form ing a platinum s u i c i d e l a y e r on the surface of the S i e l e c t r o d e (by f l a s h evaporation of Pt on the p r e t r e a t e d S i surface followed by annealing) the e l e c t r o d e w i l l show s t a b l e operation i n aqueous photoelectrochemical (PEC) c e l l s . The current-voltage curves f o r an η-Si e l e c t r o d e (coated with 40 angstroms of Pt and annealed a t 400 C i n vacuum f o r 10 minutes) i n a s o l u t i o n of 1 M FeCl2, 0.1 M 0097-6156/83/0211-0093$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Photovoltage, V
Figure 1. Photocurrent-photovoltage characteristics of the cell η-Si (Pt suicide coated)/1.0 M FeCl , 0.1 M FeCl , 1 M HCl/Pt at 65 nW/cm illumination. Pt thickness deposited ~ 40 À and annealing temperature 400 °C at ~ 10' torr for 10 min. Key: a, before long-term stability test; and b, after long-term stability test. 2
2
s
6
Figure 2. Voltammetric curves of Pt (curve a) and n-Si(Ir)/Ru0 (curve b) electrode in 11 M LiCl at pH = 7. Light intensity 65 nW/cm , and scan rate 100 mV/s. (Reproduced with permission from Réf. 1. Copyright 1982, The Electrochemical Society, Inc.) 2
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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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BARD E T A L .
Integrated Chemical
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z
F e C l ^ , and 1 M HC1 under i l l u m i n a t i o n of 65 mW/cm (tungstenhalogen lamp) are shown i n F i g u r e 1. T h i s c e l l has operated f o r more than 20 days with no a p p r e c i a b l e change i n performance or decomposition at a maximum power conversion e f f i c i e n c y of about 6%. To s t a b i l i z e such an e l e c t r o d e to oxidants stronger than i r o n ( I I I ) a c a t a l y s t such as Ru0£ must be added to the s u r f a c e to allow the r a p i d t r a n s f e r of photogenerated holes to a s o l u t i o n species before S i r e a c t i o n s occur. Under these c o n d i t i o n s even c h l o r i n e and bromine e v o l u t i o n are p o s s i b l e on S i . For example an i r i d i u m s i l i c i d e coated η-Si e l e c t r o d e with Ru02 c a t a l y s t i n 11 M L i C l s o l u t i o n w i l l evolve c h l o r i n e at p o t e n t i a l s about 0.4V l e s s p o s i t i v e than the r e v e r s i b l e c h l o r i d e / c h l o r i n e p o t e n t i a l (Figure 2) and show no a p p r e c i a b l e d e t e r i o r a t i o n a f t e r seven days of continuous i r r a d i a t i o n . In looking towards the 21st Century, I p r e d i c t that i n t e r f a c i a l photochemical and e l e c t r o c h e m i c a l processes at designed and i n t e g r a t e d chemical systems w i l l play an important r o l e i n the development of energy conversion and other devices. LITERATURE CITED
1. Fan, F. R., Hope, G. Α., and Bard, A. J. J. Electrochem. Soc., in press. 2. Fan, F. R., Keil, R. G., and Bard, A. J., J. Am. Chem. Soc., submitted. RECEIVED
December 30,
1982
In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
