Photoelectrochemical properties of p-type gallium phosphide electrodes coated with tetraphenylporphyrin

J. Electroanal. Chem., 159 (1983) 361-372

361

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

PHOTOELECTROCHEMICAL PROPERTIES OF p-TYPE GALLIUM PHOSPHIDE ELECTRODES COATED WITH TETRAPHENYLPORPHYRIN

HIROSHI YONEYAMA, HISASHI SHIOTA and HIDEO TAMURA

Department of Applied Chemistry, Faculty of Engineerin~ Osaka University, 2-1 Yamada-oka, Suita, Osaka 565 (Japan) (Received 22nd November 1983; in revised form 13th June 1983)

ABSTRACT Photoelectrochemical properties of p-type GaP coated with a thin tetraphenyl porphyrin (TPP) film were studied in phosphate buffer solutions containing a variety of oxidizing agents with focusing effects of photoexcitation of the semiconductor substrate. In situ reflection spectra have revealed that reduction of the TPP film occurs in the course of photoelectrode reactions and the coated TPP serve as an electron mediator for charge transport from the semiconductor substrate to electroactive species in solution. It will be shown that the coatings with porphyrins are useful for the modification of apparent electrocatalytlc activities of the semiconductor electrodes.

INTRODUCTION

The coating of a n-type semiconductor electrode surface with a photosensitive organic film has recently been studied using phthalocyanines with the objectives of extending spectral response of the electrodes [1,2] and of stabilizing the electrodes in aqueous solutions [3]. As for the former case, it has been shown that besides anodic photosensitization of the dye usually expected for n-type semiconductor electrodes [4,5], photoexcitation of the coated dye film caused cathodic photocurrents which were attributable to p-type behaviour of the film itself. Illumination of semiconductor electrodes coated with a very thin dye film will usually excite not only the dye film but also the semiconductor substrate. However, the effects of photoexcitation of the semiconductor substrate on photoelectrochemical properties of the coated semiconductor electrodes have not yet been studied in detail. Since the dye should have intrinsic energy levels (redox levels), the coated dye film must have some role in controlling electron transfer from the photoexcited semiconductor substrate to electroactive species in solution. In this paper, the photoelectrochemical behavior of p-type GaP electrodes coated with a tetraphenylporphyrin (TPP) film is reported with focussing effects of photoexcitation of the semiconductor substrates. A TPP film exhibits a behavior like a p-type semiconductor electrode [6-11], and thus how much the behavior is modified by photoexcitation of the p-type GaP substrate will be of interest for understanding 0022-0728/83/$03.00

© 1983 Elsevier Sequoia S.A.

362 the role of the coated dye layer in the appearance of the overall photoelectrochemical properties of the electrodes. From comparison of photocurrent-potential behavior of the TPP-coated p-type G a P electrodes with that of TPP-coated Pt electrodes and of the bare p-type G a P electrodes, it will be shown that the TPP film on the p-type G a P acts as an electron mediator for cathodic reactions on the coated electrodes. EXPERIMENTAL The (111) face of p-type G a P single crystals was used in the present study. It was polished with 1 /~m alumina, and then etched in hot aqua regia [12]. The coating with TPP was made by employing the vacuum deposition technique. The thickness of the deposited film was determined by measuring the absorbance of the film which was prepared on a glass plate at the same time as when the coating of the p-type G a P wafer was made. The amount of coating was ca. 1016 molecules cm -2. Electroactive species used were methylviologen (MV2+), p-benzoquinone (p-Q), F e ( E D T A ) - , Fe(bpy)33 +, Fe(CN) 3- a n d O 2 which were dissolved in a phosphate buffer of p H 6. The concentration of these species was 10 -3 M except for O 2 which was used by being saturated in the solution. A 500 W xenon lamp was used as a light source, and illumination of the electrode surfaces was made by focusing light by using a quartz lens.-Monochromatic light used for measurements of action spectra of photocurrents was obtained by using a grating monochromator (JASCO, Model CT-25). The number of photons incident on the electrode surface was measured by a calibrated Eppley thermopile. Action spectra reported in this paper are those corrected for the intensity variation of the monochromator output. Absorption spectra were obtained by using a Shimadzu MPS 5000 spectrophotometer. Reflection spectra of the TPP-coated p-type G a P electrodes were obtained in situ

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363

by employing the experimental setup shown in Fig. 1 with a block diagram, which is different from that usually employed for measurements of reflection spectra. The purpose of the in situ measurements in the present study was to obtain information on changes in the composition of the TPP film caused by cathodic photocurrents the magnitude of which dependent on the illumination intensity, Employment of the standard technique [13], in which the electrode surface is usually illuminated with monochromatic light and the reflected light is detected, was unsuitable for the purpose of the present study because of low illumination intensities of the monochromatic light. Thus, the electrode surface was illuminated with the full light of the xenon lamp with an angle of incidence of ca. 45 ° , and the reflected light in the direction normal to the electrode surface was led to a photomultiplier through the monochromator. The intensity of reflected light was measured both under conditions of negative polarization large enough to cause appreciable photocurrents and under open-circuit conditions where no appreciable change in the surface composition was expected, and the ratio of the former to the latter was obtained as a function of wavelengths of reflected light. Since photoelectrochemical reduction of an electroactive species of interest at illuminated semiconductor electrodes usually competes with that of proton or water molecules in aqueous solutions, the competing ratio was measured by employing the ring-disk electrode technique. The ring-disk electrode was made of a p-type GaP or a TPP-coated p-type GaP disk electrode and a Au ring electrode. The collection efficiency of the ring electrode was assumed to be 0.17 which was obtained experimentally when the p-type GaP disk electrode was replaced with a Pt disk electrode of the same geometric area. Measurements were carried out under nitrogen atmosphere at the rotation rate of the electrode at 2000 rpm by polarizing the disk electrode at - 1 . 0 V vs. SCE and the ring at 0.4 V vs. SCE. RESULTS A N D DISCUSSION

Photosensitivity of TPP-coated p-type GaP Action spectra of cathodic photocurrents obtained at the bare p-type GaP and the TPP-coated p-type CaP electrodes are shown in Fig. 2, together with an absorption spectrum of a TPP film on a glass plate. The photocurrent of the TPP-coated p-type G a P electrode is smaller than that of the bare p-type GaP electrode in a region of wavelengths shorter than the intrinsic absorption threshold of GaP. Either one of the following two, or both, will be responsible for the occurrence of the photocurrent decrease by the TPP coating. One is that the TPP film absorbs a part of the fight quanta whose energy is sufficiently great to excite the p-type GaP substrate without any significant contribution to the cathodic photocurrents. The other is that by the coating, new recombination centers are formed at the surface of the semiconductor where a part of the photogenerated electrons are annihilated. According to the inset in Fig. 2, photon absorbed in the TPP film can contribute to the cathodic photocurrents with low quantum efficiencies, because the cathodic photocurrents can be seen

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in wavelengths beyond the intrinsic absorption threshold of GaP with the wavelength dependence of the absorption spectrum of the TPP film. It is evident from these results that the cathodic photocurrents mainly come from the photoexcitation of the GaP substrate. Thus, the coating with a thick TPP film such a s 1017 molecules cm-2 was not effective for producing cathodic photocurrents. A coating as thin as possible was desired, but that of 1015 molecules c m - 2 with the use of the present technique was insufficient to cover the entire surfaces of GaP, as found from observations by a scanning electron microscope. A coating with 1016 molecules cm -2 was then adopted in the present study, and the results given below concern those obtained at the electrodes coated with this amount. Photocurrent-potential curves

Cathodic photocurrent-potential curves of a variety of oxidizing agents at the TPP-coated p-type GaP and the bare p-type electrodes, obtained at 0.1 V s -1, are shown in Figs. 3 and 4, respectively, and those at the TPP-coated Pt electrodes are given in Fig. 5 which were obtained by steady-state measurements to eliminate the contribution of charging currents of capacitive components of the film which shadowed faradic currents under non-steady measurements. It is noticed in the results at the bare p-type GaP that there is tittle difference in the magnitude of cathodic photocurrents among the oxidizing agents used. The difference in tight absorption in the electrolytes is responsible for this. In order to minimize this light absorption, the electrode was set in an electrolytic cell as close as

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366

possible to its cell window, but discrepancies of photocurrents by as much as ca. 20% must be accepted according to the difference in the light absorption in the electrolytes. In cases of the TPP-coated p-type GaP and the TPP-coated Pt electrodes, different situations can be seen in the results. The magnitude of photocurrents at these electrodes seems to be influenced by the kind of oxidizing agent. It is especially noticeable that the photocurrent at the TPP-coated Pt electrode is very large in the cases of oxygen and p-benzoquinone. In order to confirm whether or not the redox potential of the electroactive species in solution plays a principal role in determining the magnitude of photocurrents, the reactivities of several redox species other than those used in the bare p-type GaP and the TPP-coated p-type GaP electrodes were investigated at - 0 . 2 V vs. SCE. The results are also included in Fig. 6c, from which it is suggested that the observed high reactivities of oxygen and p-benzoquinone are rather special. A photochemical reaction, like that forming an intermediate complex, may be involved in these cases, as postulated, for example, in ref. 14. In the case of the TPP-coated p-type GaP electrode, the high rea.ctivities of these species are shadowed by relatively large photocurrents originating from the p-type GaP substrate. However, another noticeable tendency appears in this electrode. The photocurrent at this electrode seems to be influenced by the redox potential in the potentials negative of ca. 0 V vs. SCE. The more negative the redox potential, the larger the photocurrent. Concerning this observation, it should be noted that the lower potential limit chosen for the TPP-coated Pt electrode was - 0 . 2 V vs. SCE, while that for the TPP-coated p-type GaP was - 1.0 V vs. SCE. The difference in the

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lower potential limit chosen is believed to be responsible for the different behavior observed b e t w e e n these two k i n d s of electrodes, as suggested from results given i n the following two sections. If the lower potential limit for the former electrode was extended to more negative potentials, n o steady behavior of cathodic p h o t o c u r r e n t s was observed. Studies using the r i n g - d i s k electrode suggest that the c u r r e n t efficiencies for r e d u c t i o n of oxidizing agents at the T P P - c o a t e d p-type G a P electrodes b e c o m e high with positive redox potentials (Table 1), although the results o b t a i n e d at the bare p - t y p e G a P electrodes were rather scattered. Results o b t a i n e d at the T P P - c o a t e d Pt

TABLE 1 Current efficiencies for reduction of oxidizing agents at the bare and the TPP-coated p-type GaP electrodes MV2+ Bare p-type GaP TPP-coated p-type GaP

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electrodes were unsuccessful, because the magnitude of photocurrents was too small to be measured with the ring-disk electrode (see Fig. 5). The results given in Fig. 6b and Table 1 show that the coating with TPP modifies the electrocatalytic activities of the semiconductor substrate. This is also true for the coating with other porphyrins. An example is given in Fig. 7 for the case of the CoTPP coating, although detailed studies have not yet been completed. It is clearly shown in this figure that the cathodic photocurrent in the presence of oxygen is much larger than thos.e obtained in other oxidizing agents. It has already been shown [15,16] that Co-porphyrins are good electrocatalysts for oxygen reduction.

Transient response of photocurrents at the TPP-coated p-type GaP Electrodes Figure 8 shows transient response behavior of cathodic photocurrents in three kinds of electrolytes, observed at the commencement and termination of illumination. The most marked transient behavior was observed in the absence of an oxidizing agent where H ÷ or water must be involved in the reduction reaction. In the case of Fe(CN)~-, the transient behaviors appeared but were less marked. When oxygen having the most positive redox potential was reduced, no appreciable transient behavior appeared. These results give the suggestion that the transient behavior of cathodic photocurrents is related to reduction of the TPP film the degree of which is influenced by the redox potential of electroactive species in solutions.

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In situ reflection spectra of the TPP-coated p-type GaP electrodes Reflection spectra of the electrodes were then obtained in situ in the presence and absence of dissolved oxygen. As already described in the l~xperimental section, the change of the reflection spectra caused by cathodic photocurrents was evaluated by obtaining the ratio of the intensity of light reflected from the electrode surface polarized at - 1 . 0 V vs. SCE to that obtained under open-circuit conditions. The results obtained are .shown in Fig. 9, which shows that new absorption peaks are created by the cathodic polarization in the absence of oxygen. In the case where oxygen is reduced, no peak appeared. The peaks seem to be assignable to phlorin by referencing them to published spectra of urophlorin [17] which are included in the figure. The absorption peak at 650 nm in the spectrum obtained shows a good matching in its peak position with the referenced spectra obtained for pH > 9, although the present experiments were conducted in a buffer solution of pH 6.0. This is not strange, however, because the solutions on the electrode surface must become alkaline under hydrogen evolving conditions. It should be noted here that it is desirable that the validity of the phlorin formation should be further checked by comparing the spectrum obtained with those of tetraphenylphlorin in aqueous solutions which, however, are not available at present to our knowledge. The phlorin formation is also supported by the results reported for the electrochemical reduction of free base porphyrins of water-soluble meso-Tetra(4-N-methylpyridyl)porphyrin [18] and of meso-porphyrindimethylester [19].

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Energetic correlations Cyclic voltammograms of a TPP-coated graphite electrode were obtained and are s h o w n i n Fig. 10. A s h o u l d e r c a n b e s e e n a t ca. - 0 . 8 V vs. S C E i n t h e c o u r s e o f t h e

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cathodic potential scan. This shoulder is believed to be connected to the phlorin formation. Based on these results, energetic correlations of the semiconductor substrate, phlorin and redox species are shown in Fig. 11. It is understood that phlorin can mediate the charge transport from the semiconductor substrate to all the electroactive species chosen in the present study. Judging from the results on the in situ reflection spectra, a redox reaction between oxygen and phlorin occurs too rapidly for phlorin to be detected under the electrochemical time-scale used here. Where electrolytes have less oxidizing power, such as Fe(CN)36- , slight retention of phlorin occurs on the electrode surface, as judged from the transient behaviors of cathodic photocurrents (Fig. 8). CONCLUSIONS

The results presented in this paper show that the coated porphyrin film serves as an electron mediator capable of modifying the electrocatalytic activities of the semiconductor substrate. This discovery will attach a new scientific implification to the dye coating of semiconductor electrodes. Since prophyrins are known to have a variety of interesting chemical and electrochemical properties, the attachment of such functions to the semiconductor electrodes will be promising, and research in this direction is currently in progress.

372 ACKNOWLEDGEMENT T h i s r e s e a r c h was s u p p o r t e d b y a G r a n t - i n - A i d for S c i e n t i f i c R e s e a r c h N o . 56550552 f r o m t h e M i n i s t r y o f E d u c a t i o n , S c i e n c e a n d C u l t u r e . REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

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