R. MEMMING AND B. TRIBUTSCH
562
Electrochemical Investigations on the Spectral Sensitization Phosphide Electrodes
Philips Zentrallaboratorium GmbH, Laboratorium Hamburg, Hamburg, Germany
and
N. Tributsch
Phy:r.ikalisch Chemisches Institut, Technische Hochschule, Munich, Germany
(Received June 9, 1970)
Publication costs assisted bg Philips Zentrallaboratorium
The spectral sensitization of gallium phosphide electrodes in contact with an electrolyte was studied using cryptocyanine,pseudoisocyanine, methylene blue, rhodamine E, rose bengal, and crystal violet as seiisitiziiig dyes. A sensit,ized cathodic photocurrent was observed with n- and p-type electrodes. It is caused by an elecixon transfer from the valence band of GaP to the ground level of the dye. In certain caws (rhodamin €3, rose bengal, crystal violet) it is postulated that, besides charge transfer, energy transfer also takes place. The influence of 0 2 and H?O2 on the sensitization is discussed and the results compared with Lhose obtained with ZnO electrodes.
Introduction In most serniconductors the conductivity can be increased by light absorption. The energy of the incident light must be sufficiently large for electrons to be excited from the yalence into the conduction band. Photoconduction was also observed at lower energies if light a a s absorbed by a dye being in contact with the semi~onduator.’-~ Similar effects were also studied at scmiconductor/electrolyte interfaces. The rate of certain electrochemical reactions can be influenced by varying the conceniration of electrons and holes in the semiconductoi~electrode, e.g., by light absorption within the crystal. As sl own for ZnO, electrode processes may also be wnsitized by light absorption in the dye which is adsorbed 011 ZnQ electrode^.^,^ I n all papem on sensitization effects a t surfaceb, the problem is discussed whether the sensitization is caused by energy or charge transfer. The mechanism is still not completely understood. I t is rather difficult to solve this problem nith semiconductors being in contact with a gas atmosphere because only the space charge at the semiconductor surfacr is varied by a transfer of electrons bctneen dye and crystal as shown by photopotential and photoconductivity measurement^.'-^ Nore information can be ob1ained with semiconductor/ electrolyte pnterfaces since charge carriers transferred from the dye to the crystal or vice versa can be removed from the surface by .ionic conduclancr in the electrolyte. As shown for the ZnO/dye/electrolyte interface, electrons excited 111 the dye from its ground to a higher energy stale are transferred into the conduction band of ZnO.G Investigations on the supersensitization by reducing agents support the charge-transfer mechanism7 The Journal of Physical C’ilemistry, Vol. 76, No.
4, 1971
I n order to get inore information about transfrr mechanisms we studied the spectral sensitization of GaP electrodes. The band gap of GaP (2.25 eV) is sufficiently large to permit sensitization experiments with a variety of dyes. A further advantage of GaP is the fact that n- and p-type crystals are available. The electrochemical behavior of Gap8 and several reaction mechanisms of redox ~ y s t e m shave ~ , ~ ~previously been studied in detail. Experimental Section The investigations were performed with single crystals of GaP as electrode materials. We used n- and $-type material doped with T e and Zn, respectively. The crystals showed a conductivity of about 1 ohm-cm . were glued in (carrier density IO1’ ~ m - ~ ) They Araldite (Ciba) sockets and connected with a Teflon rod. For the measurements a Teflon cell having a glass windom- on the bottom was uaed. The Teflon (1) R. C. Nelson, J . Opt. Sac. Amer., 46, 13, 1016 (1956); 5 1 , 1182, 1186 (1961). (2) A. Terinin and I. Akimov, Z. Phys. Chem. (Leipzig), 217, 307 (1961); J. Phys. Chem., 69, 732 (1965); Dokl. Akad. Nauk BSSR, 172, 23 (1967). (3) H. Nleier, “Die Photochemie der organischen Farbstoffe,” Springer, Berlin, 1963. (4) I. Akimov, “Collection: Elementary Photoprocesses in Molecules,” Xauka, 397 (1966). (5) E. Michel-Beyerle, H. Gerischer, F. Rebentrost, and 13. Tributsch, Electrochim. Acta, 13, 1509 (1968). (6) H. Gerischer and H. Tributsch, B w .Bunaenges. P h w . Chem., 72, 437 (1968). (7) E. Tributsch and H. Gerischer, ibid., 73, 251 (1969). (8) R. Memrning and G . Schwandt, Electrochim Acta, 13, 1299 (1968). (9) K. H. Beckmann and R. Mcmming, J . Electrorhsm. Sac., 116,
368 (1969). (10) R. Memming, ibid., 116, 785 (1969).
SPECTRAL SENSITXZATION OF GaP ELECTRODES rod with the Gap-electrode could be shifted ton-ards the cell windo\\ so that any distance between the two could be adjusted. I n order to avoid iight absorption within the electrolyte this distance was kept very small ( excited singlet state with the conduction band of ZnO. Super- and Desensitization. Experiments with ZnU have shown that reducing agents increase the anodic sensitization current.' This result was interpreted as an electron transfer from the reducing agent to the ground state of the excited dye. Since the ground state was then occupied the excited electron could not return from the upper singlet level to the ground level. Consequently, the probability of an electron transfer to the conduction band was much higher. According to this interpretation one should expect €01-GaP LL desensitization after addition of a reducing agent. As illustrated in Figure 13 the ground level of the excited dye may trap an electron from the reducing agent, so that it is blocked for an electron transfer from the valence band of Gap. Whether a reducing agent works efficiently as a desensitizer depends on the tJwo rate constants kv and k~ (as defined in Figure 13). Since most reducing agents which enhanced the sensitization current with ZnO electrodes did not show any effect with GnP it must be concluded that the coupling between the valence band of GaP and the ground level of the dye is quite strong (Icv >> k ~ ) . a desensitization with HzOz. Since the spectral distribution was not changed by HzOzwe have to conclude that H202only influences the charge trandeer. Obviously, HzOz acts as a reducing agent i n this case. This result was surprising. It was checked that no reactions between HzOz and dye occurred already in the dark. We expected an enhancement of the sensitization by RzOzbecause an oxidizing agent could capture an electron from the excited singlet level of the dye, as also indicated in Figure 13. Obviously, the ground state of the dye seems t o fit much better to the energy level of the redox system Hn02-02(UE -- +0.9 V) than its excited level t o that of H202-OW- ( U E = 1.77 V). As mentioned above, an oxidizing agent should enhance the sensitization with GaP electrodes. Such an effect was observed with molecular oxygen (Figure
+
(14) F. Lohmann, Ber. Bunsenges. Phys. Chem., 70, 428 (1966).
The Journal of Physical Chemistry,Vol. 76,No. 4, 1971
570 9). In this case the spectral distribution was independent of the amlount of oxygen dissolved in the electrolyte, 2 . 9 . ) we only observed a supersensitization in a wavelength range where charge transfer is postulated. The cutoff at the short wavelength side of the spectra as found with rhodamine B, rose bengal, and crystal violet 1s iiot affected by oxygen. This is reasonable since we postulated a fast energy transfer from higher vibrational levels of the excited molecules in the “cutoff region.” I n photochemical processes, oxygen, however, plays a special role since there are several possibilities for energy or charge transfer. Various mechanisms are discussed in the literature as follows. a“ The simplest interpretation is the assumption that oxygen i s directly reduced by the excited dye.15 According to recent investigations, however, this mechanism can be omitted.16 b. Kaiitsky17 suggested an energy transfer from an exefted dye to oxygen by which oxygen should be transferred from i t s triplet ground state to its excited singlet state, With certain sensitizers, e.g., with a variety of fluorescein dyes, this mechanism could be proved.’? In OUT case we had to assume that holes are injected from the excited oxygen (02*) into the valence band of G a P ~ Since we observed a supersensitization not only with fluorescein dyes but also with triphenylmethane dye^ wibh which energy transfer to O2 never -cvas found. the Kautsky mechanism seems to be rather unlikely in our case. c The best possibility explaining the oxygen effect is afiforded by the assumption of a charge-transfer com-
The Journal of Physical Chemistry, Vol. 76, No. 4, lB7l
R. MEMMING AND plex between sensitizer (D) and oxygen (02),Kearns and E(hanl8 have calculated the energies of various states of the system [D.. . 0 2 ] complex as a function of intermolecular distance and have shown that a minimum of the potential energy exists for a charge-transfer complex [D”. . .O;-]. Since the negative charge is shifted relatively towards oxygen an electron transfer from the valence band (V.B.) to the ground state of the dye is facilitated, as
The charge transfer is followed by a splitting of the complex and the 0 2 - radical reacts further with the solution.
Acknowledgments. The authors are indebted to Professor Gerischer (T.H. Munich), Dr. Beckmann and Dr. Mollers (both Philips Hamburg) for many valuable discussions. Thanks are also due to Ir. Peters (Philips Nat. Lab. Eindhoven) for providing us with single crystals and to Ing. Kursten for performing the experiments. (15) J. Weiss, Naturwisselzschaften, 2 3 , 610 (1 935).
(16) K. Gollnick, Advan. Photochem., 6 , 1 (1968). (17) H. Kautsky and A. Hirsch, Chem. Ber., 64,2677 (1931). (18) A. K. Kahn and D. R. Kearns, J. Chem. Phys., 48, 3272 (1968).
