Chemically Modified Surfaces in Catalysis and ... - ACS Publications

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Downloaded by MICHIGAN STATE UNIV on August 21, 2013 | http://pubs.acs.org Publication Date: July 2, 1982 | doi: 10.1021/bk-1982-0192.ch012

Improvements in Photoelectrochemical and Electrochromic Reactions at Chemically Modified Electrodes N. R. ARMSTRONG, T. MEZZA, C. L. LINKOUS, B. THACKER, T. KLOFTA, and R. CIESLINSKI University of Arizona, Department of Chemistry, Tucson, AZ 85721 Chemical and physical modification of semiconductor, metal and metal oxide electrodes has been carried out for the purpose of improvement of the v i s i b l e l i g h t response of these surfaces, or for the enhancement of the k i n e t i c s of the deposition of the n-heptyl viologen cation r a d i c a l . Phthalocyanine aggregates have been sublimed to the surfaces of either SnO2 or gold metallized-plastic f i l m electrodes and reactions observed which correspond to either photosensitization and energy conversion and/or photoelectrocatalysis -both with unusually high quantum e f f i c i e n c i e s of 2-9%. Metal oxide electrodes can be chemically or ion-beam modified to enhance the rate of nucleation of the n-heptylviologen cation r a d i c a l (n-HV++ + e- n-HV+.). The n-HV++ reduction follows an instantaneous nucleation mechanism and the nucleation s i t e density, No, can be increased through the addition of a silane layer to the surface, or after the bombardment with 1-10 molecular layers of 0.5-1.5 KeV, argon ions. As shown i n this symposium, interest i n chemical modification of electrode surfaces has been extended i n many directions, including the study of light-assisted redox reactions, and the use of modified electrodes i n electrochromic devices (1,2). Our own studies have centered on the study of metal and metal oxide electrodes modified with very thin films of phthalocyanines (PC) and on the electrochromic reaction of n-heptyl viologen on metal oxide electrodes, and on the effect on these reactions of changing substrate chemical and physical composition (4,5). In the case of the photoelectrochemical reactions, dyemodified electrodes may participate i n two types of photonstimulated reaction: a) photosensitization of the semiconductor substrate (leading to energy conversion) and b) a photoassisted c a t a l y t i c response (leading simply to an enhanced reaction rate 0097-6156/82/0192-0205 $6.00/0 © 1982 American Chemical Society In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


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CHEMICALLY MODIFIED SURFACES

(3). The rate of charge transfer at the PC-electrolyte i n t e r face and the rate of charge transfer at the substrate-PC i n t e r face are both photocurrent-determining processes. In the case of the n-heptyl viologen deposition, nucleation rates of the f i r s t molecular layers of t h i s molecule control the deposition rates of subsequent layers. The nucleation reaction follows the instantaneous nucleation model — and i s found to be highly sensitive to the chemical and physical nature of the electrode surface prior to deposition. RF-plasma of ion-beam etched surfaces generally show greatly enhanced nucleation and bulk deposition rates. Light-Assisted Electrochemical Reactions at Phthalocyanine Modified Surfaces. If solar energy conversion devices are the objective, i n the use of photoactive electrodes, three basic requirements must be met: a) The electrode must be receptive, and show i t s maximum e f f i c i e n c y , to l i g h t i n the r e d - v i s i b l e and nearinfrared regions of the spectrum, since most solar energy i s concentrated i n t h i s region. b) The power conversion efficiency of solar photons to electrochemical energy must be high ( i n excess of 10% i s desirable). This condition requires that the electrode material be o p t i c a l l y opaque, the electron-hole pair recombination events be minimized i n the s o l i d , and that the conduction and valence band edges of the electrode be favorably placed with respect to both the oxidation and reduction reaction e.m.f. of solution species so as to promote rapid rates of electron transfer. c) The electrode must be stable to the photoelectrochemic a l process. Unfortunately, many of the semiconductor materials which would s a t i s f y requirement (a) and (b) are not stable and undergo light-assisted corrosion instead of driving the desired redox reaction. Several important methods have been devised to chemically protect the surfaces of such materials as S i , CdS or CdSe, and the GaAs with the result however that the redox reactions that can be light-assisted are dictated by the redox e.m.f. of the surface-attached species (6). Modification of semiconductor electrode response with adsorbed or attached dye molecules i s an attractive alternative to other photoelectrochemical systems (7-13). Metal oxides which are stable or have very low corrosion rates but are transparent to v i s i b l e wavelength l i g h t can be used i n light-assisted electrochemical reactions when modified with monolayers and m u l t i layers of a wide variety of chromophores interposed between the electrode and electrolyte. With one exception, the i n i t i a l reports of energy conversion e f f i c i e n c i e s of electrodes with adsorbed dyes was disappointingly low. Recently however,

In Chemically Modified Surfaces in Catalysis and Electrocatalysis; Miller, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

12.

ARMSTRONG ET AL.

Photoelectrochemical, Electrochromic Reactions

Tsubomura and coworkers have reported on high-surface area, ZnO electrodes with adsorbed rose bengal, which show an energy (power) conversion efficiency of 2.5% (14). Research i n our laboratory and by Osa and F u j i h i r a showed that i t i s possible to covalently attach monolayers of chromophores to metal-oxide semiconductor surfaces — with no compromise i n quantum efficiency to energy conversion compared with dyes adsorbed from solution (9-11). The quantum efficiency for these systems (ratio of photo-generated current to photons adsorbed i n the dye layer, n /n ) i s quite low, i n the range of 10~5 to 10~4 and argues against device applications of these simple modified electrodes without further improvements, such as l i n e a r , multielectrode stacks of dye-modified, semi-transparent electrodes (10). An electrode covered with several molecular layers of dye could be made to adsorb a l l of the v i s i b l e l i g h t , and obviate the need for the multielectrode stack. Very thick dye layers have tended not to be conductive or highly photoconductive so that their photoelectrochemical e f f i c i e n c i e s are no better and perhaps worse than those seen on electrodes modified with very thin dye films. Molecular disorder of the dye appears to be the dominant reason for lack of conductivity i n thick films of fluorescein-type, cyanine-type, and phthalocyanine-type dyes (12). I t has been shown however that ordered molecular systems (mainly conjugated, highly unsaturated hydrocarbons) have considerable potential as conductive media, and that these ordered systems may be used to chemically modify electrode surfaces (12, 15). Our attention has been directed to modifying Sn02 electrodes and l a t e r , metal electrodes with very thin films (10-100 molecular layers) of phthalocyanines which appear to aggregate when sublimed. The oriented phthalocyanine phase or phases sensitize the response of the Sn02 electrodes with e f f i c i e n c i e s many times greater than monomolecular layers of covalently attached chromophores or randomly oriented multilayer dye films (9). Our i n i t i a l studies have been conducted with phthalocyanines which we expected would orient i n a linear "pancake-stack," by virtue of the interaction between the central metal atoms — either a covalent bond or a strong electrostatic interaction. The synthesis, by Professor Malcolm Kenny at Case Western Reserve University, of a series of s i l i c o n phthalocyanine polymers (SiPc) (aggregates of the monomer of t h i s series, m-SiPc, are discussed here) and of a series of aluminum and gallium, fluoro and chloro phthalocyanines (AlPc-Cl, GaPc-Cl, AlPc-Cl, GaPc-F) allowed us to study several oriented systems (16). Some concepts of orientation possible with a l l Pc films are shown i n Figure 1. We now understand that these types of chromophore orientations are only some of several which are present on our electrode surfaces and which may lead to increased photoelectrochemical e f f i c i e n c i e s . Those phthalocyanines e


207

p


208



Figure la.

Silicon phthalocyanine polymer shown attached to SnO surface.


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ARMSTRONG ET AL.



12.


209


210

which lead to high photoelectrochemical e f f i c i e n c i e s share the a b i l i t y to form aggregates on the electrode surface which leads to a v i s i b l e spectrum which i s broadened and red-shifted from that of the monomeric form (9). Figure 2 shows the dark versus illuminated current/voltage response of two types of phthalocyanine-modified Sn02 (m-SiPcSn02 and GaPc-Cl-Sn02> electrodes, and the current/voltage response of a platinum electrode, a l l i n a pH - 4, 10" M hydroquinone (H2Q) solution. Several features of these curves deserve discussion. The oxidation of H2Q on platinum i s k i n e t i c a l l y slow i n pH • 4 aqueous media (17). On the illuminated Pc-Sn02 electrodes, however, the oxidation process i s considerably enhanced, the onset potential for the oxidation process i s actuall y negative of the potential observed on the Pt electrode. A closed-cycle photovoltaic c e l l i s possible using a Pt cathode (Q + 2e- + 2H+ t H2Q) and an illuminated Pc-Sn02 electrode ( 2Q Z Q + 2H* + 2e~). Preliminary experiments have shown that an open c i r c u i t photovoltage, V = -0.20 v o l t s vs. Pt i s obtained using the SiPc-Sn02 electrode system under the conditions described i n Figure 2. The shape of the photocurrent/voltage curves on the Pc-Sn02 electrodes suggests some strong s i m i l a r i t i e s and differences between the two systems. In both cases, the photoelectrochemical efficiency (n /n ) increases sharply i n the potential range near +0.2 volts vs. Ag/AgCl which i s near the EO for the quinone/hydroquinone redox couple (18). The maximum quantum efficiency i n creases up to 2% for both the GaPc-Cl and SiPc modified electrodes with increasing p o s i t i v e bias potential (9). The photoassisted reaction i s d e f i n i t e l y the oxidation of hydroquinone, and does not i r r e v e r s i b l y consume the phthalocyanine on the electrode surface. As with many semiconductor photoelectrolysis reactions, the oxidation process i s not mass transport controlled at bias potentials negative of +0.6 volts (where the dark current process begins). The raction rate on both electrodes i s l i n e a r l y controlled by photon flux up to.400-500 watts/cm^ at which point, continuous-wave (cw) laser l i g h t or pulsed-dye laser l i g h t begins to saturate the dye, resulting i n degradation of the dye layer. The difference between the two electrode materials i s seen i n that on the m-SiPc-Sn02 electrodes only the oxidation of H2Q Q i s enhanced, and on the GaPc-Cl-SnO^ electrodes, both the oxidation and the reduction, Q + H2Q, show enhanced rates upon illumination. This a b i l i t y of the GaPc-Cl modified electrodes to photoenhance both the oxidation and reduction redox processes has been further explored using gold, metallized p l a s t i c films (Au-MPOTE, Sierracin Corporation) modified with 10-100 molecular layer thicknesses of t h i s phthalocyanine. Figure 3 shows the l i g h t and dark i/V behavior of such an electrode modified with a nonporous f i l m of GaPc-Cl. The dark i/V behavior of an unmodified gold electrode i n the same solution i s shown for comparison.


3

H

Q # c >

e

p



2

-r

3 0

ua/cm

2

t

t

2

s

Key: GaPc-Cl-SnO electrode, dark vs. light, and a current! voltage trace for a platinum electrode of the same area; and b, SiPc-Cl-SnO electrode, dark vs. light. All solutions were 10' M^HtQ* pH 4, illumination with polychromatic light (470-900 nm), ca. Ï00 mW/cm . Potential scan rates were 10 mV/s. Reproduced with permission from Journal of Electroanalytical Chemistry.

Figure 2. Dark vs. illuminated current/voltage curves for two types of phthalocyanine-modified Sn0 electrodes.

ight




212

mV vs. Ag/AgCI Figure 3. Dark vs. illuminated current/voltage curves for thin-layer voltammetry on plain and GaPc-Cl-modified Au-MPOTE's. Conditions as for Figure 2, except the potential scan was carried out at 2 mV/s.



12. ARMSTRONG ET AL.

Photoelectrochemical, Electrochromic Reactions 213

Since these voltammograms were obtained i n a thin layer electrochemical c e l l , considerable displacement of the voltammetric peaks can be seen because of the uncompensatable solution r e s i s tance. I t i s nevertheless apparent that i n the dark both the H2Q ** Q and the Q e l e c t r o l y s i s are suppressed, while under illumination the rate of oxidation and reduction i s enhanced over that observed on the p l a i n gold substrate. In addition, the extent of this k i n e t i c enhancement i s controlled by the flux of photons delivered to the electrode surface! For t h i s type of modified gold substrate, no apparent photopotential i s observed for the quinone/hydroquinone couple. We have previously shown that i t i s possible to form a gold oxide layer on the Au-MPOTE, using an O2-RF plasma, and/or electrochemical oxidation (5). We undertook the study of GaPc-Cl modified Au-MPOTE's which had 1-5 molecular layers of oxide placed on their surfaces, p r i o r to Pc-modification. The resultant i/V curves under illumination are similar to those i n Figure 3, with the onset potential for oxidation pushed successively positive with increasing coverage of the non-conductive oxide layer. In contrast, when the RF-plasma i s used i n a substrate cleaning procedure, prior to Pc deposition, the photoelectrochemical efficiency i s seen to increase. m-SiPc-modified Sn02 electrodes as described i n Figure 2 show photoelectrochemic a l quantum e f f i c i e n c i e s which improve from ca. 1% to ca. 9% when the Sn02 surface i s cleaned i n an O2-RF plasma prior to Pc deposition. These experiments demonstrated the importance of good e l e c t r i c a l communication between the Pc and the underlying substrate. It i s clear that some d i s t i n c t i o n needs to be made between the photoelectrochemical reactions on the Pc-modified semiconductor substrate, where true energy conversion may be observed — and on the Pc-modified metal substrate where k i n e t i c enhancement may be the dominant photoelectrochemical process. Figure 4 summarizes the energetics of photosensitization of an n-type semiconductor electrode by monolayer or thin multilayer coverages of a phthalocyanine such as m-SiPc. Dark equilibrium i s obtained for both monolayer and a continuous multilayer f i l m by equalizing of the a) Fermi potential of the semiconductor, b) the ground state E° of the dye (monolayer) or the Fermi potential of the p-type dye layer (multilayer) and c) the e.m.f. of the solution redox species. The energetics of the equilibrium case shown i n Figure 3 are possible provided that the electrochemical potent i a l s of the Fermi-level of the Sn02 (EF(Sn02))» Fermi-level of the phthalocyanine (Ep(p )) and the E r/OX were arranged before contact according to: Ep(g 02) F(Pc) °R/Ox* ~ radiation of the modified electrode surface causes: 1): the monolayer dye to achieve an excited state with new redox l e v e l s capable of donating an electron to the conduction band of the semiconductor substrate or 2) the formation of electron/hole pairs i n the phthalocyanine layer which separate and react at t n e

c

i i

-0.408V i l I i i l

0

5

I i < i

10

I I i

15

Transmittance

Time (sec) Figure 6. Current vs. time, and transmittance-decrease vs. time plots for n-heptylviologen reduction obtained on a clean (top) and ion-beam modified (bottom) ITO, MPOTE. Current increases with (time)* in the nucleation region, and then decreases, while the absorbance changes linearly. Rates of nucleation are enhanced on the modified surface. [n-HV~] = 10 M, 0.1 M KHP. Potential steps are indicated with each plot. S



220


-430 mv vs Ag/AgCI

-J

100

1

I

»

300

'

t (msec)

I

500

L

700

1/2

Figure 7. Current vs. (timefk plots for the nucleation of n-HV*' at various electrode surfaces. Dimethyldiethoxysilane (DMDE) modified ITO, MPOTE surface (a); ion-beam modified ITO, MPOTE, 10 ions/cm (as in Figures 5 and 6) (b); and clean ITO, MPOTE surface (c). Conditions as in Figure 7. 16

2


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ARMSTRONG E T AL.


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(25). The substoichiometric oxide surface may provide better electron transfer rates to the solutions species, preferred adsorption sites for the cation r a d i c a l , or especially the anion (as i n the case of Br" adsorption on t i n metal (22)), both of these effects leading to increased nucleation rates. Further studies of the n-heptyl viologen reaction on modified surfaces are reported elsewhere (25). I t i s clear that the interaction of ion-beams or plasmas with the electrode surface can be a powerful modification t o o l , complementary to chemical modification procedures for application to either photoelectrochemical or electrochromic reactions. Acknowledgements This research has been supported by grants from the National Science Foundation, CHE80-17571 and from the Sierracin Corporation.

Literature Cited 1. See papers by Van Duyne, Kaufman, Diaz, Kuwana, Meyer, Wrighton, Finklea, and Lundgren in this symposium. 2. A comprehensive review of this area up through 1980 is available by R. W. Murray, Accts. Chem. Res., 1980, 12, 135. 3. Kuwana, T., this symposium, paper #96. 4. Cieslinski, R.; Armstrong, N. J. Electrochem. Soc. 1980, 127, 2606. 5. Armstrong, N. R.; White, J . R. J . Electroanal. Chem. 1982, 131, 121. 6. See recent reviews by Bard, A. J. Science 1980, 207, 139 and J . Photochem. 1970, 10, 59; Wrighton, M. S. Accts. Chem. Res. 1970, 12, 303, and paper #98, this symposium. 7. Gerischer, H . , Topics in Applied Physics, Vol. 31, "Solar Energy Conversion," Seraphin, B. O., Ed.; Springer-Verlay: New York; 1979; pp. 115-169. 8. Hawn, D.; Armstrong, N. R. J. Phys. Chem. 1978, 82, 1288; Shepard, V. R.; Armstrong, N. R. J. Phys. Chem. 1989, 83, 1268. 9. Mezza, T.; Linkous, C.; Shepard, V. R.; Armstrong, N. R.; Nohr, R.; Kenney, M. J. Electroanal. Chem. 1981, 124, 311. 10. Armstrong, N. R.; Shepard, V. R. J . Electroanal. Chem. 1982, 131, 113. 11. Osa, T.; Fujihira, M. Nature, 1976, 264, 349. 12. Dähne, S. Photographic Sci. and Eng. 1979, 23, 219; Saunders, V. I; Lovell, S. P. Ibid, 1970, 24, 171, 176. 13. Iwasaki, T.; Sumi, S.; Fujishima, A . ; Honda, K. Ibid. 1979, 23, 17. 14. Matsumura, M.; Matsudaira, S.; Tsubomura, H.; Takata, M.; Yanagida, H. I and EC Prod. Res. and Dev. 1980, 19, 415. 15. Schoch, K. F . , J r . ; Kundulkar, B. R.; Marks, T. J . J . Amer. Chem. Soc. 1979, 101, 7071.


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17. 18. 19. 20.


21. 22. 23. 24. 25.


Janson, T. R.; Kane, A. R.; Sullivan, T. F . ; Knox, K.; Kenney, M. J. Amer. Chem. Soc. 1969, 91, 5210; Kusnesol, R. M.; Wynne, K.; Nohr, R.; and Kenney, M. J. Chem. Soc. Chem. Comm. 1980, 121. Vetter, K. J. Zeit. Elektrochimie, 1952, 56, 797. Oyama, N . ; Anson, F. Anal. Chem. 1980, 52, 1192. Collman, J. P.; Denisevich, P.; Konai, Y . ; Marrocco, M . ; Koval, K.; Anson, F. J . Amer. Chem. Soc. 1980, 102, 6027. Tse, D. C. S.; Kuwana, T. Anal. Chem. 1979, 51, 2257; Bettelheim, A . ; Chan, R. J . H.; Kuwana, T. J. Electroanal. Chem. 1979, 99, 39. Andrieus, C. P.; Dumas-Bouchiat, J . M.; Saveant, J . M. J . Electroanal. Chem., 1978, 87, 39; 1978, 93, 163,; 1980, 114, 159; and in press. Fletcher, S.; Duff, L . ; Barradas, R. G. J. Electroanal. Chem. 1979, 100, 759. Bruinink, J.; Kregting, C. G. A . ; Ponjee, J . J . J . Electrochem. Soc. 1977, 124, 1854; Jasinski, R. Ibid. 1978, 125, 1619; Ibid. 1979, 126, 167. Bunawardena, B . ; Hills, G.; Montenegro, I. Chem. Soc. Faraday Symp. 1977, #12, pp. 90-100. Cieslinski, R.; Armstrong, N. R. J. Electroanal. Chem. submitted for publication.

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Chemically Modified Surfaces in Catalysis and ... - ACS Publications

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