J. Phys. Chem. 1992,96, 6367-6371
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anthracene molecules are adsorbed randomly, and with increasing T, they are rearranged to a crystalline state, where the molecules are oriented, their short axis parallel to the substrate and their long axis almost perpendicular to it. It should be noted that the angular distributionmeasurements of luminescence are useful for the study of the orientation of photophysical products such as excimers or exciplexes on a metal substrate. Registry No. Ag, 7440-22-4; anthracene, 120-12-7.
(4) Ishibashi, Y.; Ohshima, S.;Kajiwara, T. Surf.Sci. 1988, 201, 311. (5) Sano, H.; Mizutani, G.; Ushioda, S.Surf.Sci. 1989, 223, 621. (6) Maruyama, Y.; Takamiya-Ichikawa,K. Int. J. Quantum Chem. 1980, 18, 587. (7) Ferguson, J.; Mau, A. W.-H. Mod. Phys. 1974, 27, 377. (8) Hofmann, J.; Seefeld, K. P.; Hofberger, W.; Bgssler, H. Mol. Phys. 1979. 37. 973. (9) Seki, H.; Itoh, U. J . Chem. Phys. 1980, 72, 2166. (10) Greenler, R. G. Surf Sci. 1977, 69, 647. (11) Chambers, R. W.; Kajiwara, T.; Kearns, D. R. J. Phys. Chem. 1974,
References and Notes (1) Avouris, P.; Persson, B. N. J. J . Phys. Chem. 1984, 88, 837. (2) Waldeck, D. H.; Alivistos, A. P.; Harris, C. B. Surf Sci. 1985, 158,
(12) Seki, K.; Inokuchi, H. Chem. Phys. Lerr. 1982, 89, 268. (13) Johnson, P.; Christy, R. Phys. Reu. 1972, 86, 4370. (14) Michl, J.; Thulstrup, E. J. Spectroscopy with Polarized Light; VCH: New York, 1986; p 405. (1 5) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; p 327. (16) Harada, Y.; Ozaki, Y. Jpn. J. Appl. Phys. 1987,26, 1201.
103.
( 3 ) Tro, N. J.; Nishimura, A. M.; George, S.M. J . Phys. Chem. 1989, 93,
3276.
Photochemical Reaction of H,FeOs,( CO)
Adsorbed on the Surface of Silica
Sadaaki Yamamoto,* Central Research Institute, Mitsui Toatsu Chemicals, Znc., 1 190, Kasama, Sakae, Yokohama, Japan
Yasushi Miyamoto, Robert M. Lewis, Mitsuo Koizumi, Kuroda Solid Surface Project, Research Development Corporation of Japan, Tsukuba Research Consortium, 5-9-4 Tokodai, Tsukuba, Zbaraki 300-26, Japan
Yoshiyuki Morioka, Department of Chemistry, Faculty of Science, Tohoku University, Aoba- ku, Aramaki, Sendai, Japan
Kiyotaka Asakura, and Haruo Kurodat Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo, Japan (Received: September 3, 1991)
The photochemical reaction of silica-supported H,F~OS~(CO),~ induced by irradiation with visible light was investigated by using FT-IRand UV-visible reflectance spectroscopies. The structure of the photoproduct was studied by laser Raman and EXAFS spectroscopies. H , F ~ O S ~ ( C O molecules ) ~ ~ that are adsorbed in a molecularly dispersed state on the surface of silica were found to lose one CO ligand when irradiated with light in the region of the first electronic transition of the carbonyl cluster. The reaction yielded a coordinatively unsaturated species, H,F~OS~(CO),~. It was concluded that this unstable photoproduct is stabilized on the surface of silica and does not undergo secondary reactions because of an interaction with surface hydroxyl groups.
Introduction
Metal carbonyl compounds are often used to prepare highlydispersed supported metal catalysts.' Usually, a thermal treatment followed by reduction at an elevated temperature in a hydrogen atmosphere is used to remove the CO ligands or to produce small metal clusters on the support. Decarbonylation of metal carbonyl compounds can also be done by using light irradiation. Because photochemical processes are often more selective than thermal processes, the use of a photochemical process to make unstable surface bound chemical species that cannot be obtained by conventional thermal p r o c a w appears reasonable. Relatively little is known, however, about the photochemical behavior of metal carbonyl compounds on solid surfa~es,~-'~ although the photochemistry of metal carbonyl compounds in solution has been extensively studied.'* We previously found that the surface bound hydride anion cluster HFe3(CO),,- can be formed when Fe3(C0)', adsorbed 'Also at Kurcda Solid Surface Project.
on the surface of silica is irradiated with visible light.' This species cannot be obtained by thermal treatment of Fe3(CO),2supported on silica. With the success of the photochemical approach in this system, it was important to explore the limits of such a photochemical approach. Another challenging carbonyl system is that of bimetallic carbonyl clusters. These carbonyl clusters often decompose when thermally treated. In the present paper, we report the photochemical reaction of the bimetallic carbonyl cluster compound H2FeO~3(C0)13 supported on silica. Experimental Section
H2FeOs3(C0)13 was prepared according to the method reported in the 1iterat~re.l~ n-Hexane used in the sample preparations was dried over CaC12and then distilled from Na or LiA1H4prior to use. We used two types of silica as the support, Aerosil 380 (Nippon Aerosil), which has surface OH groups, and RY200 (Nippon Aerosil), in which all surface silanol groups have been replaced by methyl groups. We refer to the first type of silica as 'hydroxy silica" and the second type as "methyl silica". Before
0022-365419212096-6367$03.00/00 1992 American Chemical Society
Yamamoto et al.
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400
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Figure 1. Light-induced UV-visible diffuse reflectance spectral change of H , F ~ O S ~ ( C O physisorbed ),~ on silica versus time (min): (-) 0, (---) 11, and (a-) 36. The transmittance curve of the filter, Toshiba L42, is also shown (---).
use, the silica powders were well dried by heating them at an elevated temperature in vacuo (1 X lo4 Torr) for 12 h. The hydroxy silica was heated to 723 K and the methyl silica was heated to 473 K. H2FeOs3(CO),3was impregnated onto the silicas according to the following procedure. Dried silica powder was put into an n-hexane solution of H2FeOs3(CO)13 and stirred for 30 min under an oxygen- and water-free atmosphere in a drybox (Vacuum Atmospheres Co.). After the mixing process was completed, the suspension was transferred to a vessel connected to a glass vacuum line and the n-hexane was removed by trap-to-trap distillation. The nominal metal loading was 4 wt 9% in each sample used in the present study. Hereafter we denote the samples as H2FeOs3(C0)13/Si02(hydroxy) when H2FeOs3(CO)13 was adsorbed on the hydroxy silica and H2FeOs3(C0),,/SiO2(methyl) when the methyl silica was used as the support. The irradiation with visible light was done on the free-flowing powder form of the sample contained in a reaction cell having quartz windows. The samples were irradiated by using a deep UV Xe lamp (Ushio UXM-SOMD, 500W) as the light source, a water filter to remove infrared radiation, and a Toshiba L42 filter to remove light with a wavelength below 400 nm. The gas evolved from the sample during the irradiation with light was analyzed by using a gas analysis system equipped with a quadrupole mass analyzer. UV-visible diffuse-reflectance spectra were measured with a Shimazu UV-265FS spectrophotometer equipped for diffuse reflectance measurement. FT-IR spectra were obtained with a Nicolet 170SX FT-IR spectrometer. The FT-IR spectral measurements were done in situ with the sample contained in an evacuable photochemical reaction chamber equipped with two opposing quartz windows for the light irradiation. These windows were oriented normal to two KBr (or CaF2) windows used during the measurement of the IR spectra. The sample was aligned with the appropriate windows during irradiation or spectral measurement. Raman spectra were recorded in 90' scattering geometry by a multichannel spectrometer equipped with an intensified diode array detector (Tracor-Nothern, TN-6133).20 The 514.5-nm line of an argon ion laser and 647 nm of krypton ion laser were used as the stimulating light. To avoid thermal decomposition of the clusters, the laser was operated at a low power (ca. 25 mw) with the beam slightly defocussed. EXAFS data were obtained by use of the EXAFS spectrometers at lines BL-6B and BL-7C in the Photon Factory at the National Laboratory for High-Energy Physics in Japan (PF-KEK). ResUJtS
Figure 1 shows the UV-visible diffuse reflectance spectrum of
H2FeOs3(CO)13/Si02(hydroxy) and the change caused by light irradiation. As can be seen from the transmittance curve of the cutoff filter used, Toshiba L42 shown in the same figure, we are irradiating the sample with light in the region of the first absorption band of H2FeOs3(CO),,. The diffuse reflectance spec-
00
1800
2000 Wave nu mber (cm-')
Figure 2. Light-induced FT-IR spectral change of HzFeOs,(CO)13 physisorbed on silica versus time (min): (1) 0, (2) 2, (3) 4, (4) 6, (5) 8, (6) 10,(7) 15, and (8) 60.
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500
200
the photoproduct on silica, (B) HzFcOs3(C0)13physisorted on silica, and (C) solid HzFeOs3(CO)13.
trum of the nonirradiated sample of H2FeOs3(C0)13/Si02(hydroxy) shows poorly resolved bands with maxima at 360,400, and 465 nm. On irradiating the sample with visible light, a new absorption band grows at 330 nm. Clear isosbestic points are evident. Although the peak intensities decrease slightly, the product retains the bands described above that are associated with the FeOs, skeleton?' The FT-IR spectrum and its change upon light irradiation are shown in Figure 2. The spectrum of the nonirradiated H2FeOs3(CO),,/Si02(hydroxy) sample is composed of the bands associated with terminal CO groups, (2116 (w), 2089 (s), 2075 (s), and 2043 (s) cm-')as well as bands associated with bridging CO groups (1868 (w) and 1826 (w)). This FT-IR spectrum agrees well with the spectrum of H2FeOs3(C0)13in n-hexane solution. On the other hand, the FT-IR spectrum of H2FeOs3(CO)13/ Si02(methyl) was significantly different from these spectra and resembled the spectrum of poorly dispersed H2FeOs3(CO)13 powder in a KBr disk. The gas evolved from the sample during light irradiation was analyzed by mass spectroscopy and found to be CO. The total amount of the evolved CO was found to be 1 mol per mole of H2Fea3(CO)13* Figure 3 shows the laser Raman spectra, Observed with the 514.5-nm line from an argon ion laser, of H2FeOs3(CO)13/ Si02(hydroxy) before and after light irradiation. Included is the
Photochemical Reaction of HzFeOs3(CO)13
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The Journal of Physical Chemistry, Vol. 96, No. 15, 1992 6369
TABLE I: Curve-Fitting Results for Fe K-Edge EXAFS of H2FeOs3(CO)13Pbysisorbed on the Surface of Silica before a d after
; Pbotoirradiation
Fe-Qs
irradiated
nonirradiated solid H2FeOs3(CO),,
a
CN'
RIA
2fl 2fl 3
2.66 2.68 2.68
Coordination number.
I
1
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21 00 2000 1900 1800 Wave number (c m-'1 Figure 5. FT-IRabsorption spectra of the species extracted into n-hexane (-) and the photoproduct on silica (--).
2200
2 4 Distance(,& Figure 4. Fourier transforms of Fe K-edge EXAFS oscillations: (A) irradiated sample, (B) nonirradiated sample, and (C) solid H2FeOs3(C-
than that from the oxygen in CO ligands. We obtained a good fit as shown in Table I. The Fe-Os bond distance in 0)IO. H2FeOs3(C0)13/Si02(hydroxy) after the light irradiation (2.68 A) was found to be the same as those in H F ~ O S ~ ( C O ) ~ ~ / S ~ O ~ laser Raman spectrum of H2FeOs3(C0)13 powder. Unfortunately, and in the H2Fe(hydroxy) before light irradiation (2.68 it was not possible to obtain a spectrum with a high signal-tc-noise O S ~ ( C Opowder ) ~ ~ (2.68 A). This value is different from the ratio. This occurred because of the low concentration of the Fe-Fe distance (2.48 A) in Fe metal particles. Thus, we can rule supported metal carbonyl cluster and because the laser power had out the possibility that Fe metal particles were formed by light to be kept low to avoid thermal decomposition of the supported irradiation. The values obtained for the Fe-Os coordination metal carbonyl cluster. Nevertheless, we can still identify the three number was a little smaller in H2FeOs3(CO)13/Si02(hydroxy) peaks at 155, 192, and 220 cm-I in the spectrum of HzFeOs3compared to HzFeOs3(CO)13powder, but the difference is within (C0)13/Si02(hydroxy)before and after light irradiation. These the experimental error. are the same as those observed in the spectrum of the HzFeWe also studied the Os L3-edgeEXAFS. Since the second peak Os3(co)13 powder (Figure 3c). These peaks could not be observed of the Fourier transform of Os L3-edge EXAFS oscillation contains when the 647-nm line from a krypton ion laser was used as the contributions from three different atoms (Le., Os,Fe, and carbonyl stimulating light. In contrast, the 514.5-nm line falls in the region oxygen) it was not possible to derive precise values for the bond of the electronic absorption band; thus, peaks observed in the distances and coordination numbers by a curve-fitting analysis. spectra shown in Figure 3 are likely due to resonance Raman The results obtained, however, were in agreement with the exenhancement. The lack of peaks upon 647-nm excitation may pectation that the photoproduct has Os-Os bonding with a bond be due to the lower cross sections of this excitation wavelength. distance almost the same as that found for the original H2FeAs a result, the intensities may be less than the detection limit O S ~ ( C Opowder. )~~ of the instrument used. When the photoirradiated sample of H , F ~ O S ~ ( C O ) , ~ / S ~ O ~ To obtain more direct structural information, we studied the (hydroxy), which is yellow in color, was put into dry deaerated Fe K-edge EXAFS of HzFeOs3(C0),3/Si0z(hydroxy) before and n-hexane saturated with Ar, it was decolorized and the n-hexane after light irradiation. The Fourier transform of the Fe K-edge became yellow. The FT-IR spectrum of the extracted material EXAFS oscillation is shown in Figure 4 together with that of was found to change with time, indicating that the extracted H2FeOs3(C0)13powder. The first peak of the Fourier transform species is unstable. The FT-IR spectrum of the freshly extracted of H2FeOs3(CO)13powder is attributed to Fe-C and the second solution is shown in Figure 5 . This spectrum is essentially the peak to F e 4 s and Fe-O(C0). Note that the Fourier transform of the nonirradiated sample of H2FeOs3(CO)13/Si02(hydroxy) same as the spectrum of the photoirradiated sample, although the spectrum of the photoirradiated sample is much sharper. is very similar to that of H2FeOs3(C0)13powder, indicating that the H2FeOs3(C0)13 molecules are adsorbed on the surface of silica Discussion without significant change in its molecular structure. Although the Fourier transform of the light-irradiated sample is a little The bands in the UV-visible diffuse reflectance spectrum of different than that of the nonirradiated sample, the main features HzFeOs3(C0)13/SiOz(hydroxy) shown in Figure 1 agree well with remain unchanged. The first peak for all samples can be attributed the absorption bands of H2FeOs,(CO)13in solution.z1 These can to F d and the second one to Fe-Os and Fe-O(C0). be assigned to the electronic transitions associated with the F a 3 The possibility remains, however, that small Fe metal particles skeleton of the H2FeOs3(CO)13molecule. This indicates that the are formed in a manner similar to when the cluster on Si02 is FeOs, skeleton is still intact in the product. thermally treated. To eliminate this possibility, we did a curveThe sharp resolution of the peaks in the FT-IR spectra of fitting analysis to confirm that the second peak is due to Fe-Os H2FeOs3(C0)13/Si02(hydroxy) indicates that the carbonyl cluster molecules are in a molecularly dispersed state on the surface of bonding. The phase shift and amplitude functions for the F e O s bonding were derived from the second peak of the Fourier the hydroxy silica. In contrast, the FT-IR spectrum of transform of H2FeOs3(CO)13powder. This method of comparison HzFeOs3(CO),3/SiOz(methyl) shows broad peaks, indicating that is valid because the contribution from Os atoms is much larger the clusters are in an aggregated state.
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6310 The Journal of Physical Chemistry, Vol. 96, No. 15, 1992
As shown in Figure 2, the IR bands marked with a downward arrow, especially those due to bridging COS, decreased and new bands marked with an upward arrow increased as H2FeOs3(CO),,/Si02(hydroxy) was irradiated with visible light. Clear isosbestic points are visible in the spectra, indicating that only one chemical species is selectively formed. The observed change in the FT-IR spectrum indicates that the loss of a bridging CO is taking place during radiation. The same spectral change was not Prolonged irraobserved with H2FeOs3(CO),3/Si02(methyl). diation of the cluster on methyl silica led to an overall decrease in the absorption intensity, indicating that a slow decomposition reaction occurs on methyl silica. The gas evolution analysis provides some insight into the nature of the reaction that takes place. No indication could be found for the evolution of H2, whereas both CO and H2are evolved when H2FeOs3(CO)13/Si02(hydroxy) is treated thermally. Thermal treatment causes the FeOs, metal skeleton to break, resulting in Fe metal particles and the surface bound trinuclear Os species, Os3(CO) lo(H)(OSi=).22Thus, the gas evolution results indicate that a clean reaction involving loss of one carbonyl group is taking place. It is reported that metal-metal stretching frequencies in Raman spectra fall in the region 120-210 cm-l for a wide range of metals.,’ In fact, the Raman studies on tetranuclear hydride carbonyls have revealed that these carbonyl species show a set of Raman peaks in the frequency region 100-250 cm-’ that can be assigned to metal-metal stretching. For example, the isois known to also structural metal carbonyl cluster, H,Ru~(CO),~, give three peaks at 150, 174, and 213 cm-I, which have been attributed to the Ru-Ru stretching modes.24 Furthermore, the band at 192 cm-I in the Raman spectrum of H2FeOs3(CO),, supported on silica has been assigned to the Fe-Os stretching mode.22 Therefore, the three peaks observed for H,FeOs,(C0),,/SiO2 before and after light irradiation can safely be attributed to the metal-metal stretching vibrations of the FeOs, skeleton, indicating that it is still intact. The results described above indicate that the FeOs, skeleton of the photoproduct is intact and that the reaction involves the loss of one carbonyl group. Thus we can conclude that the photochemical process which takes place in the H2FeOs3(C0)i3/Si02(hydroxy) system is metal-CO bond dissociation yielding a product having a FeOs, skeleton whose structure is almost the same as that of the original metal carbonyl molecule. Next, we need to consider what type of reaction is taking place in this system. It was reported by Foley et al. that H2FeOs3(C0),,(PPh3) is formed when H2FeOs3(C0),, is irradiated in the presence of PPh, in a hydrocarbon solution and that H4FeOs3(CO),, is formed when the irradiation is done in the presence of H2.2’ Thus, one possible structure for the photoproduct could be H4FeOs3(C0)12 produced by the following photochemical reaction involving surface hydroxyl groups: H , F ~ O S ~ ( C O ) , 2HO--Si= ~ H,F~OS,(CO),~ + 2O-Sis + CO
+
-
Although the IR spectrum of H4FeOs3(C0),2has not been reported, it should be similar to that of H4FeRu3(C0),2since the IR spectrum of H2FeOs3(C0),, is similar to that of H2FeRu3(CO) Assuming this comparison is valid, H4FeOs3(C0),, cannot be the photoproduct in H2FeOs3(CO)13/Si02(hydroxy) because the IR spectrum of the light-irradiated H2FeOs3(CO)13/Si02(hydroxy)is sisnifcantly different than the spectrum of H4FeR~3(C0)12.25 Another possibility that must be considered is the formation of a surface bound hydride cluster, HFeOs,(CO),,-. HFeOs,(CO),,- is known to be formed thermally on basic oxides such as A120326or Mg02’ by a proton-transfer reaction. It is conceivable, therefore, that the same proton-transfer reaction is indued in the cluster on Si02(hydroxy)by visible light irradiation, resulting in the formation of HFeOs,(CO),,-. Fortunately, the IR s p e c ” of this product on AI2O3%and Mg02’ is known and it is different than that obtained for the photoproduct on Si02(hydroxy). In addition, the fact that 1 mol of CO is evolved per
Yamamoto et al. mole of H2FeOs3(CO),3implies that each parent metal carbonyl lost one CO ligand upon visible light irradiation. Therefore, the product should have one less CO ligand. These experimental findings rule out the possibility that the product is the hydride anion HFeOs3(CO),). Another possibility that can be considered is the formation of an electronically deficient species such as H2FeOs3(C0),, formed by the loss of one CO ligand. Recently, we found that such an unsaturated species is formed when H2FeOs3(CO),, isolated in a polystyrene film or 3-methylpentane glass at 77 K is irradiated with visible light. This reaction involves the photochemical dissociation of one CO ligand. Furthermore, the isolated species re-forms H , F ~ O S ~ ( C Owhen ) , ~ warmed to 298 K by recombination of the product with CO remaining at nearby sites.28 The IR spectrum of the above photoproduct in 3-methylpentane glass or a polystyrene film is similar to that of the photoproduct in H2FeOs3(CO),,/Si02(hydroxy). From the resemblance of the IR spectrum,as well as other spectral data supporting the retention of the original metal skeleton, it is reasonable to conclude that the photoproduct is H,F~OS,(CO),~.Thus, the photochemical process which takes place in the H2FeOs3(CO),,/Si02(hydroxy) system is likely to be the following: H2FeOs3(CO),(ads) H,FeOs3(CO),,(ads) + CO(gas)
-
The finding that the complex could be extracted into n-hexane means that the photoproduct, presumably the coordinatively unsaturated H2FeOs3(CO)12, is interacting rather weakly with hydroxyl groups on the surface of silica. Hydroxyl groups are known to act as weak ligands? thus lending support to the scheme proposed above. The proposed scheme is further supported by the dependence of the H,F~OS~(CO)~, reactivity on the degree of dispenion. The initial photochemical process on SiO,(methyl) where the carbonyl clusters are in an aggregated state is probably CO dissociation as seen on SiO,(hydroxy). The recombination reaction between H2FeOs3(CO)12and CO may occur efficiently because of a “cage effect” inside of the aggregated particles, similar to that found in films and in solution. Thus, effective reaction does not occur. In contrast, CO can be efficiently removed from the cluster molecules on the outermost surface of aggregated particles. This H2FeOs3(C0),2,however, cannot be stabilized by nearby hydroxyl groups, and so decomposition results. Thus, the photoprocess is slow and eventually leads to decomposition of H2FeOs3(CO),, on Si02(methyl).
Conclusion The bimetallic carbonyl cluster H,F~OS,(CO),~ adsorbed on the surface of silica having surface hydroxyl groups undergoes a photoinduced CO dissociation reaction when irradiated with light that stimulates the first electronic absorption band of the cluster. This reaction yields selectively the coordinatively unsaturated H,F~OS,(CO),~ on the surface of silica. In solution, the same primary photochemical process involving loss of one CO ligand occurs,but the unstable photoproduct easily undergoes secondary reactions. On the other hand, with H2FeOs3(C0)13/Si02(hydroxy) the primary photoproduct is stabilized by an interaction with surface OH groups and the dissociated CO is lost from the surface. This interaction with surface hydroxyls prevents secondary reactions, thus explaining why the coordinatively unsaturated H2FeOs3(C0)12, which is normally unstable, is efficiently formed when H2Fd)s,(CO),3/Si02 is irradiated with visible light. The results of this study suggest that normally unstable species can be prepared and stabilized on the surface of silica when supported bimetallic carbonyl clusters are irradiated. Regisby No, H2FeOS,(CO),3, 12563-74-5;CO, 630-08-0; Aerosil380, 7631-86-9.
References and Notes ( 1 ) (a) Brenner, A. J. Chem. Soc., Chem. Commun. 1979, 251. (b) Ichikawa, M. J . Chem. Soc., Chem. Commun. 1976,26. (c) Anderson, J. R.; Elmes, P. S.; Howe, R. F.; Mainwaring, D. E. J. Caral. 1977, 50, 508. (d) Anderson, J. R.; Mainwaring, D. E. J . Caral. 1974, 35, 162. (2) Jackson, R. L.; Trusheim, M. R. J . Am. Chem. Soc. 1982,104,6590.
J. Phys. Chem. 1992, 96,6371-6374 (3) Tfusheim, M. R.; Jackson, R. L. J . Phys. Chem. 1983, 87, 1910. (4) Simon, R. C.; Gafney, H. D.; Morse, D. L. Inorg. Chem. 1983,22,573. ( 5 ) Darsillo, M.S.;Gafney, H. D.;Paquette, M. S . J. Am. Chem. Soc. 1987, 109, 3275. (6) Dieter, T.; Gafney, H. D.Inorg. Chem. 1988, 27, 1730. (7) Yamamoto, S.;Lewis, R. M.; Hotta, H.; Kurcda, H.Inorg. Chem. 1989, 28, 3091; Vacuum. 1990,41, 65. ( 8 ) Yamamoto, S.;Lewis, R. M.; Nabata, Y.; Hotta, H.; Kurcda, H. Inorg. Chem. 1990, 29, 4342. (9) Beck, A.; Dobos, S.; Guczi, L. Catal. Today 1989, 5, 149. (10) Gluck, N. S.;Ying, Z.; Bartosch, C. E.; Ho, W. J. Chem. Phys. 1987, 86, 4957. (11) Swanson, R.; Fried, C. M.; Chabal, Y. J. J . Chem. Phys. 1987,87, 5028. (12) Creighton, J. R. J . Appl. Phys. 1986, 59, 410. (13) Celii, F. G.; Whitmore, P. M.; Janda, K. C. J. Phys. Chem. 1988,92, 1604. (14) Foord, J. S.;Jackman, R. B. Chem. Phys. Lett. 1984, 112, 190. (15) Flynn, D. K.; Steinfeld, J. I.; Sethi, D. S.J . Appl. Phys. 1986, 59, 3914. (16) Germer, T. A.; Ho, W. J. Chem. Phys. 1988, 89, 562.
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(17) Wada, Y.;Nakaoka, C.; Morikawa, A. Chem. Loft. 1988, 25. (18) Geoffroy, G. L.; Wrighton, M. S."Organometallic Photochemistry"; Academic Press: New York, 1979. (19) Burkhardt, E. W.; Geoffroy, G. L. J . Organomer. Chem. 1980,198, 179. (20) Morioka, Y.; Wada, M.; Sawada, A. J. Phys. SOC.Jpn. 1988, 57, 3198. (21) Foley, H. C.; Geoffroy, G. L. J . Am. Chem. SOC.1981, 103, 7176. (22) Choplin, A.; Leconte, M.; Basset, J. M.; Shore, S.G.; Hsu,W. L. J . Mol. Catal. 1983, 21, 389. (23) Gager, H. M.; Lcwis, J.; Ware, M. J. J . Chem. Soc. Chem., Commun. 1966,616. (24) Kettle, S . F. A.; Stanghellini. Inorg. Chem. 1987, 26, 1626. (25) D o h , S.;hzoermenyi, I.; Mink, J.; Guczi, L. Inorg. Chim. Acta 1987, 134, 203. (26) Budge, J. R.; Lucke, B. F.; Gates, B. C.; Toran, J. J . Catal. 1985, 91, 1985. (27) Choplin, A.; Huang, L.; Basset, J. M.; Mathieu, R.; Siriwardane, U.; Shore, S . G. Organometallics 1986, 5, 1547. (28) Yamamoto, S.; Asakura, K.; Mochida, K.; Nitta, A.; Kuroda, H., to be published.
Power Dependent Effects in the Luminescence Decay of GaAs/Eiectrolyte Contacts at the Flat Band Potential J. F. Kauffman; B. A. Balko, and G. L. Richmond* Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, Oregon 97403 (Received: January 24, 1992; In Final Form: April 9, 1992)
Saturation of surface traps has been observed in the GaAs/NaS photoelectrochemical system under modest excitation conditions. Saturation is shown to result in a surface minority trapping velocity that is dependent on time as well as laser excitation power. These saturation effects are observed by studying the luminescence decays of GaAs as a function of excitation pulse power under potentiostatic control at the flat band potential. The decays also indicate that surface minority carrier trapping is fast compared with processes which remove minority carriers from trap states. These results suggest that time-resolved experiments under high injection open circuit conditions may underestimate the surface minority trapping rate under typical solar conditions.
Introduction Semiconductor liquid junction solar cells are capable of extremely efficient conversion of solar radiation to electricity.' However, their widespread use is hampered by photocorrosion which limits their useful lifetime to an unacceptably short period. To overcome this corrosion problem researchers have focused on the design of electrode/electrolyte systems which have chargetransfer rates far in excess of the rate of photocorrosion.2 For this purpose, a detailed understanding of the charge-transfer kinetics is desirable. However, due to the mechanistic complexity of the charge-transfer process, formulating a diffinitive kinetic scheme and measuring the rate of each reactive pathway presents a challenge to the experimentalist. Recently picosecond laser technology has been applied to this problem. Several groups3have used luminescence decays to determine the effect of electrolyte concentration on the effective surface minority trapping velocity (STV)! Miller et al. have used a transient grating method to directly measure interfacial chargetransfer rates from Ti02 and GaAs following high energy pulsed i l l u ~ ~ h t i o nBoth . ~ *of ~ these studies were. carried out under high injection conditions in order to obviate effects due to the space charge field. We have recently performed a series of experiments in which picosecond excitation has been used to excite a semiconductor electrode. The experiments were performed under potentiostatic control, with careful attention paid to the power 'Permanent addrcss: Department of Chemistry, University of Missouri, Columbia, MO 65211.
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of the incident radiation. By controlling the minority camer flux and depletion layer width in this manner, we have been able to elucidate detailed information regarding kinetics of interfacial charge transfer as well as the quality of the electrode surface with respect to surface traps. Here we present the results of a portion of these studies in which the effect of laser power on the luminescence decay of GaAs is examined while the electrode is held at the flat band potential. The results show that surface traps can be substantially saturated with picosecond pulses of even moderate power. This appears as a reduction in the effective STV. We discuss the implications of this result for the interpretation of other experiments performed under high injection.
Experimental Section All experiments were performed on Si-doped n-GaAs (100) (Crystal Specialties) with a carrier concentration of 2 x 10'' mr3. Low resistance contacts were made by rubbing InGa eutectic on the back surface of the electrode after etching with 3:l:l H2SO4/H2O2/H2O.A copper wire was attached to the contact region with silver epoxy, and the electrode was mounted on a glass tube with Apiezon W wax. (The results presented here have been reproduced using samples which had contacts made from 700-A vapor deposited Au:Ge films annealed at 450 K in forming gas.) The front surface of the electrodes were etched with a 0.05% Br2/MeOH solution for 15 s, rinsed with MeOH, and then dried. The BrJMeOH treatment was followed by a 30-s KOH wash, water rinse, and drying. This cycle was repeated at least 3 times before the electrode was placed in an airtight, optically accessible 0 1992 American Chemical Society
