Steady-State Spectroscopic and Photovoltage Studies of Hemin and

Particularly, the photovoltaic response value at the region of 400 nm which corresponds to the Soret ... A. Mazzaglia , M.T. Sciortino , N. Kandoth , ...
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Langmuir 1999, 15, 6969-6974

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Steady-State Spectroscopic and Photovoltage Studies of Hemin and Hemin/n-Octadecylamine Langmuir-Blodgett Films Jian Jin,† Lin Song Li,†,§ Xiaoqing Wang,‡ Yi Li,† Yan Jie Zhang,† Xia Chen,† Yuan-zong Li,‡ and Tie Jin Li*,† Center for Intelligent Materials United Research (CIMUR), Department of Chemistry, Jilin University, Changchun, 130023, People’s Republic of China, and Department of Chemistry, Peking University, Beijing 100871, People’s Republic of China Received February 4, 1999. In Final Form: April 27, 1999 Hemin (iron(III) protoporphyrin IX chloride) and mixed hemin/n-octadecylamine (ODA) with the molar ratio of 1:2 were used to investigate the Langmuir monolayer behavior and to prepare the LangmuirBlodgett (LB) films. It showed that well-defined Langmuir monolayers were formed at the air-water interface and could be successfully transferred onto solid substrates. UV-vis spectra of hemin indicated the formation of H-aggregate in a LB matrix for the blue shift of the Soret band from 400 nm in solution to 370 nm in LB film. The inlay of ODA molecules partly reduced the aggregation of hemin molecules. Both the small-angle X-ray diffraction (XRD) pattern of hemin and mixed hemin/ODA LB multilayers exhibited an ordered layer structure in an LB matrix with the d-value of bilayer hemin equal to 4.8 nm. Surface photovoltage spectroscopy (SPS) was used to study the photoinduced interfacial charge-transfer process between a monolayer or multilayer of hemin and mixed hemin/ODA LB films on n- and p-type Si substrates. The results showed that the hemin LB monolayer film had a photosensitization effect on n-type Si. Particularly, the photovoltaic response value at the region of 400 nm which corresponds to the Soret band absorption of hemin molecules was increased 6-fold. The mixed hemin/ODA LB monolayer and multilayer films increased the photovoltaic conversion efficiency by separating the hemin molecules into individual ones with about a 15-fold and 7-fold increment of response value at the Soret band, respectively. But the SPS response value of the hemin LB multilayer film was decreased almost 20-fold and the photovoltaic conversion efficiency was largely reduced.

Introduction Porphyrin and its analogous exist in nature and creature’s bodies extensively. As they have well-defined structures, they are more important in many life processes, such as the transfer, the storage and the activity of dioxygen.1 The investigation of porphyrins have been performed since they were discovered. Their spectral and structural properties have been fully understood in recent years. Now, many works have been aimed on the evolution of these artificial biological systems to gain access to the other chemical or physical processes by organized monolayer assemblies.2 The Langmuir-Blodgett (LB) technique as an imaginative approach for the study of novel properties of organic semiconductor thin films which are controlled on the order of the molecular level has attracted much attention.3-5 The first study on the monomolecular layer of a variety of porphyrins on the surface of water was reported by * To whom correspondence should be addressed. Fax: 86-4318923907. E-mail: [email protected]. † Jilin University. ‡ Peking University. § Present address: Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. (1) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol 3. (2) (a) Ulman, A. An Introduction of Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Petty, M. C. Langmuir-Blodgett film: an introduction, Cambridge University Press: Cambridge, 1996. (3) Liu, C. Y.; Pan, H. L.; Fox, M. A.; Bard, A. J. Science 1993, 261, 897. (4) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.; Schwartz, D. K. Science 1994, 263, 1726. (5) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607.

Alexander and co-workers.6 Consequently, many porphyrin derivatives modified with various substituents had been investigated.7-15 Most studies of porphyrin and its derivatives by the LB technique have mainly aimed at the research of their structural characterizations.8-10 Their attentions have been paid to one aspect of the physical properties of porphyrin and its derivatives, that is, how to obtain stable rigid Langmuir monolayers and Langmuir-Blodgett films. In fact, porphyrins without any modification do not form high-quality LB films by themselves because of their strong tendency of aggregation and their ambiguous amphipathy.14 Thus, two main methods are used to reduce the aggregation of porphyrins and to enhance their amphipathy. One is the chemical modification method. That is, porphyrins are functionalized with longer alkyl chains as the hydrophobic end to obtain more stable amphiphilic derivatives.11,12,14 The other (6) Alexander, A. E. J. Chem. Soc. 1937, 1813. (7) Dick, H. A.; Bolton, J. R.; Picard, G.; Munger, G.; Leblanc, R. M. Langmuir 1988, 4, 133. (8) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L. G.; Crossley, M. J. J. Phys. Chem. 1993, 97, 12862. (9) Chou, H.; Chen, C. T.; Stork, K. F.; Bohn, P. W.; Suslick, K. S. J. Phys.. Chem. 1994, 98, 383. (10) Aramata, K.; Kamachi, M.; Takahashi, M.; Yamagishi, A. Langmuir 1997, 13, 5161. (11) Koon, J. M.; Sudholter, E. J. R.; Schenning, A. P. H. J.; Nolte, R. J. M. Langmuir 1995, 11, 214. (12) Grieve M. B.; Hudson, A. J.; Richardson, T.; Johnstone, R. A. W.; Sobral, A. J. F. N.; Rocha Gonsalves, A. M. d’A. Thin Solid Films 1994, 243, 581. (13) Anikin, M.; Tkachenko, N. V.; Lemmetyinen, H. Langmuir, 1997, 13, 3002. (14) Azumi, R.; Matsumoto, M.; Kuroda, S.-I.; King, L. G.; Crossley, M. J. Langmuir 1995, 11, 4056. (15) Gregory, B. W.; Vaknin, D.; Gray, J. D.; Ocko, B. M.; Stroeve, P.; Cotton, T. M.; Struve, W. S. J. Phys. Chem. B 1997, 101, 2006

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is to use the porphyrins in conjunction with amphiphilic compounds for preparing a mixed monolayer.13 All these results have revealed that the orientation in porphyrins is really controlled in a film. Now, many different techniques have been used to understand the fascinating physical properties of thin films of porphyrin and its derivatives. For their welldefined molecular structure and extended π-electron system, they have been chosen as a good candidate for organic semiconductors and have exhibited good photovoltaic and photoelectrochemical properties. For instance, surface photovoltaic properties and interface charge transfer of the porphyrin layer was reported by E. Moons and co-workers.16 Surface photovoltage spectroscopy (SPS) is a powerful tool to study the surface and interface electronic structure of semiconductor materials and is widely used. It has been proven to be an extensive source of information about photoinduced electronic transport at real surfaces and interfaces of various semiconductors.17-20 In this work, we combine the LB technique with the surface photovoltage technique to comparatively investigate the structural characterization and photovoltaic behavior of a kind of porphyrin-hemin and its mixture with n-octadecylamine(ODA) LB monolayer and multilayer films. Hemin, as one of the substances of biological importance, has also attracted chemists’ attention for its various properties in thin film form.21-23 Meanwhile, a few concerns about hemin have been made by the LB technique, mainly because it is not a typical amphiphilic molecule. Our results show that a hemin LB film can be produced by adjusting the density of water (appropriate CdCl2 is added). When mixed with ODA, the quality of the LB film can be enhanced. The result of SPS reveals that a hemin LB monolayer film shows high photovoltaic conversion efficiency when modified on n-type Si. Moreover, the mixing of ODA molecules effectively increases the photovoltaic conversion efficiency. Experimental Details Hemin was purchased from the Sigma Co., with no further purification. Octadecylamine was A. R. grade and was recrystallized before use. DMF solvent was purified by reduced pressure distillation and Na particles were used for drying. Chloroform solution was distilled twice prior to use. All film experiments were carried out on a RMC-2T multicompartmental round trough from Mayer-Fein technic (Germany). For the film preparation of hemin, the spreading solution was prepared by dissolving the hemin in DMF with the concentration of 1.24 × 10-5 M. The subphase was deionized water (18 MΩ cm) containing CdCl2 of 2 × 10-4 M (here, the addition of CdCl2 was used to increase the density of solution and enhance the interaction between the COOH- group and subphase) . For the preparation of the mixed film, the hemin in DMF and the ODA in chloroform with the molar ratio of hemin/ODA ) 1:2 (in this case, the ratio of -NH2/COOH ) 1:1) was spread onto the water subphase. For all the LB deposition, the monolayers were compressed at the speed of 13 cm2/min and transferred onto solid substrates by a vertical lifting method. The transfer speed was 18 cm/min and the surface (16) Moons, E.; Goossens, A.; Savenije, T. J. Phys. Chem. B 1997, 101, 8492. (17) Gatos, H. C.; Lagowski, J. J. Vac. Sci. Technol. 1973, 10, 130. (18) Kronik, L.; Leibovitch, M.; Fefer, E.; Korobov, V.; Shapira, Y. J. Electeon. Mater. 1995, 24, 893. (19) Fefer, E.; Shapira, Y.; Balberg, I. Appl. Phys. Lett. 1995, 67, 371. (20) Li, L. S.; Zhang, J.; Wang, L. J.; Chen, Y. M.; Hui, Z.; Chi, L. F.; Fuchs, H.; Li, T. J. J. Vac. Sci. Technol. B 1997, 15, 1618. (21) Snyder, S. R.; White, H. S. J. Phys. Chem. 1995, 99, 5626. (22) Tao, N. J.; Cardenas, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11, 4445. (23) Sagara, T.; Fukuda, M.; Nakashima, N. J. Phys. Chem. B 1998, 102, 521.

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Figure 1. Surface pressure-area (π-A) isotherm of (a) hemin and (b) mixed hemin/ODA monolayers on the water subphase containing CdCl2 of 2 × 10-4 M. pressure was maintained at 20 mN/m for the hemin monolayer and 30 mN/m for the mixed hemin/ODA monolayer. All the monolayer experiments were performed at 20 ( 1 °C. At the range investigated, Y-type deposition was obtained with the transfer ratio of 0.7-0.8 for hemin and 1.0 ( 0.1 for mixed hemin/ ODA. The p-type Si (100) and n-type Si (100) substrates with hydrophilic surfaces were cleaned in turn by chloroform, ethanol, and deionized water by sonication for 10 min. The resistivity is 5-8 Ω m. CaF2 substrates were cleaned by a surfactant and sonicated in deionized water. The infrared spectra were recorded with a Nicolet 5DX spectrometer. An average of 200 scans were used to ensure a good signal-to-noise ratio. UV-vis spectra were obtained on a Shimadzu UV-3100 and UV-365 spectrophotometer (Japan). Small-angle X-ray diffraction patterns were recorded with the diffraction vector perpendicular to the plane of the films using a Rigaku D/max rA X-ray diffractometer. The surface photovoltage (SPV) measurements were carried out on a home-built apparatus with a solid junction photovoltaic cell using a light source-monochromator-lock-in detection technique. The principle and cell structure were described in detail elsewhere.24 The electrode was made of optical glass coated with indium and tin oxides (ITO). Monochromatic light was obtained by passing light from a 500-W xenon lamp through a double-prism monochromator (Hilger and Watts, D300). A lockin amplifier (Brookdeal, 9503-3C), synchronized with a light chopper, was employed to amplify the photovoltage signal. The spectra were normalized to unity at their maxim and the characteristic bands of the xenon lamp were subtracted by a computer.

Results and Discussion 1. Monolayers on the Subphase Surface and Their Transfer. Figure 1 shows the π-A isotherms of the hemin monolayer and mixed hemin/ODA monolayer formed on the subphase. For the isotherm of hemin, the larger slope indicates a relatively stronger interaction force between hemin molecules. The limiting area per hemin molecule, as estimated by extrapolating the linear region of the isotherm to zero pressure, is approximately 0.6 nm2. Further, in this isotherm a smaller molecular area at zero pressure is obtained than the molecular area which one porphyrin ring should be occupied when it lies flat on water (about 1.45 nm2).25 It comes from the molecule aggregation at the air-water interface. In general, there is a substantial attractive π-π interaction between porphyrin rings. This attraction is sufficiently strong that (24) Wang, D. J.; Zhang, J. J. Photochem. Photobiol. A 1996, 93, 21. (25) Bergeron, J. A.; Gaines, G. L., Jr.; Bellamy, W. D. J. Colloid Interface Sci. 1967, 25, 97.

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Figure 2. Infrared spectra of 15-layer (a) hemin and (b) mixed hemin/ODA LB films on CaF2 substrates.

Figure 3. Small-angle X-ray diffraction pattern of 15-layer (a) hemin and (b) mixed hemin/ODA LB films on Si substrates.

significant preaggregates can occur prior to the compression of the Langmuir monolayer.14 For instance, M. Anikin and co-workers reported that two kinds of porphyrin derivatives, TTP (5,10,15,20-tetraphenylporphyrin) and TBP (5,10,15,20-tetrakis(3,5-di-tert-butylphenyl) porphyrin, develop the small area per molecule about 0.1 and 0.25 nm2, respectively, even before lifting pressure. This also shows the strong tendency of aggregates to form between molecules.13 Being mixed with ODA, the isotherm of the mixed monolayer is quite different from that of hemin. A distinct liquid-condensed region is observed in curve b, much closer to the shape of the ODA isotherm. For the mixed monolayer, the molecular area of hemin can be obtained from the area of ODA which is about 0.2 nm2 and the molar ratio of hemin:ODA ) 1:2. The calculated result shows that the molecular area of hemin in the mixed monolayer is about 0.4 nm2, which is smaller than that of pure hemin. We think the smaller area per hemin molecule in the mixed monolayer arises from two possible reasons: one is due to the existence of ODA. The hemin molecules are separated effectively by ODA, which decreases the repulsive force between hemin molecules and causes them to pack more condensely. The second reason is that hemin molecules may be squeezed out from the ODA layer for the interaction between hemin and ODA molecules may not be strong enough.26 Depositions of the Y-type hemin multilayer and mixed hemin/ODA multilayer are all successful, only with the transfer ratio of the hemin multilayer being slightly smaller than that of mixed multilayer. This transfer property is also different from TBP and mixed TBP/ODA multilayers.13 It is reported that TBP molecules cannot be deposited onto a solid substrate for no real monomolecular layer form on the water surface and the mixed ODA/TBP film with the molar ratio of 1:2 cannot be obtained yet. We suggest that this different transfer property between hemin and TBP comes from the substituents attached on the porphyrin ring. The two carboxylic groups on the hemin molecule enhance its hydrophilicity and increase the interaction between hemin and subphase or hemin and the substrate. 2. Infrared Spectra of the Hemin and Mixed Hemin/ODA LB Multilayer. Figure 2 shows the infrared transmission spectra of the hemin and mixed hemin/ODA LB multilayer. It is observed that both the asymmetric and symmetric alkyl CH2 vibrations at about 2920 and 2850 cm-1 appear in the two spectra. But the intensity of

the mixed hemin/ODA multilayer is much higher than that of the hemin multilayer, indicating the existence of ODA molecules in the mixed system. In addition, there is a band at 1465 cm-1 which is attributed to the methylene scissoring vibration.27,28 It confirms the ordered arrangement of ODA molecules in LB film. Generally, the shift of the CdO band in the infrared spectrum can give us information about intramolecular interaction between molecules which contain the carboxylic group. As reported, the CdO stretching vibration is located at 1745 cm-1 in the free -COOH (no hydrogen bonding), at 1720-1730 cm-1 when the -COOH groups form a sideways dimer structure and at approximately 1700 cm-1 if the -COOH groups form ring dimer structures.29 In this report, the CdO stretching mode of hemin in curve a is at about 1727 cm-1, indicating the formation of a sideways dimer structure in the LB film. This is different from the IR result of hemin powder, in which the CdO stretching mode appears at 1700 cm-1 by forming the ring-hydrogen bonding dimmer.30 Additionally, there are two bands at 1542 and 1457 cm-1 in curve a, which correspond to the -COO- asymmetric and symmetric stretching vibrations. It indicates that some hemin molecules are protonated and partly form carboxylate with Cd2+.31 The relatively weak band at 1650 cm-1 is assigned to the CdC and CdN stretching vibration of the porphyrin macrocycle.32 When mixed with ODA molecules (curve b), a weak and broad band appears at the region of 1600 cm-1, which confirms the presence of νb(NH3+). It indicates there is partly interaction between ODA molecules and hemin molecules through the proton-transfer occurrence.33 3. X-ray Diffraction Pattern of Hemin and Mixed Hemin/ODA LB Multilayers. The small-angle XRD patterns of hemin and mixed hemin/ODA LB multilayers on Si substrates are shown in Figure 3. It shows both hemin and mixed hemin/ODA can form an ordered layer structure in LB films. For the diffraction pattern of hemin LB film, there is one signal observed at about 2θ ) 1.82°, corresponding to the layer spacing of 4.8 nm. The addition of CdCl2 into the water subphase converts the -COOH

(26) Aramata, K.; Kamachi, M.; Takahashi, M.; Yamagishi, A. Langmuir 1997, 13, 5161.

(27) Koyama, Y.; Yanagishita, M.; Toda, S.; Matsuo, T. J. Colloid Interface Sci. 1977, 61, 438. (28) Dote, J. L.; Mowery, R. L. J. Phys. Chem. 1988, 92, 1571. (29) Peng, X.; Guan, S.; Chai, X.; Jiang, Y.; Li, T. J. J. Phys. Chem. 1992, 96, 3170. (30) Standard Infrared Grating Spectra, Sadtler Research Laboratories, Inc., 1979; Vol. 9-10, 8725. (31) Davies, G. H.; Yarwood, J. J. Spectrochim. Acta A 1987, 43, 1619. (32) Boucher, L. J.; Katz, J. J. J. Am. Chem. Soc. 1967, 89, 1340. (33) Jones, C. A.; Petty, M. C.; Roberts, G. G.; Davies, G.; Yarwood, J.; Ratcliffe, N. M.; Barton, J. W. Thin Solid Films 1987, 155, 187.

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Figure 4. UV-vis absorption spectra of (a) hemin in DMF solution with the concentration of 2 × 10-6 M, (b) hemin in alkaline solution with the same concentration, (c) hemin 15layer LB film on a CaF2 substrate, (d) mixed hemin/ODA in DMF/chloroform solution, and (e) mixed hemin/ODA 15-layer LB film on a CaF2 substrate.

group to the -COOCd group, so the existence of Cd2+ between two hemin layers increases the layer spacing. Taking into account the characterization of Y-type deposition, the d-value is equal to 2 molecular lengths of hemin. In the XRD pattern of the mixed hemin/ODA LB film, three distinct peaks are obtained at 2θ ) 1.54°, 1.84°, and 2.18°, corresponding to the layer spacing of 5.7, 4.8, and 4.0 nm. The appearance of different peak positions reveals the existence of phase separation in the mixed system, just like the structural characterization of the TBP/ODA mixture system.13 A Three type arrangement between two layers are obtained. The spacing value of 4.8 nm is equal to the d-value of two-layer hemin. According to K. Aramata’s report, porphyrin may be squeezed out of the fatty acid layer to take a parallel orientation in the mixed porphyrin/fatty acid Langmuir monolayer at the airwater interface under the pressure coming from the barrier.26 The reason is the interaction between the porphyrin and fatty acid is not strong enough. This effect will increase the layer spacing of the film. Moreover, some ODA molecules will also be changed to partial disorder and it will decrease the layer spacing. So three kinds of d-values have been obtained from the mixed hemin/ODA LB multilayer. 4. UV-Vis Spectra of a Hemin and Mixed Hemin/ ODA Solution and LB Films. Figure 4 shows the UVvis spectra of hemin and mixed hemin/ODA in solution and their LB multilayer on CaF2 substrates. All the spectra exhibit the characteristic Soret band at about 400 nm and weak Q-band between 500 and 600 nm of hemin. It has been reported that the presence of Cd2+ ions in the subphase have a strong influence on the absorption spectra

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of the hemin LB film.34 It was argued that the Q-band between 500 and 600 nm of hemin was due to an intermolecular proton bridge between a nitrogen of one porphyrin and the carboxylic group of a neighboring group.35 The interaction is drastically reduced if Cd2+ binds to the carboxylic groups. So the very weak Q-band obtained here indicates the Cd2+ binding to the carboxylic groups in the LB film. Generally, for porphyrins, the solvent medium is quite important in determining the extent of aggregation. The ionic strength and dielectric constant both can have a profound effect upon the aggregation.36 Because of the lower dielectric constant, there seems to be no trend of aggregation of porphyrins in nonaqueous solvents at the concentration accessible to UV-vis spectroscopy.37 Meanwhile many water-soluble metalloporphyrins are prone to aggregation in solution. In our work, the concentrations of hemin in solution are as low as possible. So there should be no aggregate formed in DMF solution. The wavelength of the Soret band maxima at 390 and 400 nm in water (pH 8) and in DMF solution, respectively, are in agreement with the absorption band of free hemin at 386 nm.38 But in the absorption spectrum of the alkaline solution, there is a new band at 350 nm. According to reports,39 this band should be ascribed to the weak absorption of the hemin dimer; in this case, hemin molecules aggregate in the form of µ-oxo bihemin because of the effect of OH- in water.40 When the hemin monolayer at the air-water interface is transferred onto the CaF2 substrate, the blue shift of the Soret band and Q-band compared with that of the solution spectra is observed, which suggests that hemin molecules form faceto-face aggregates (i.e., H-aggregate). This shift to higher energy has been attributed to exciton coupling and through-space π-π interactions.41 As reported, the frequency shift of the aggregate’s absorption peak relative to the monomer’s absorption peak is inversely proportional to the cube of the distance between the individual molecules.9 So the larger blue shift of hemin in an LB film implies that the close packing of hemin molecules causes a larger overlap between adjacent porphyrin rings and the distance between the hemin molecules is shortened. For the mixed hemin/ODA system, the absorption spectrum of the mixed LB film exhibits a relatively broadened band around 400 nm compared with that of solution. We think it is the result of different types of structures of hemin in the mixed system. As we have described in the XRD results, the hemin molecules in the mixed LB system is not a well-ordered arrangement. The hemin may be squeezed out from ODA molecules. So the broadened absorption band is obtained. 5. SPS of Hemin and Mixed Hemin/ODA LB Monolayers and Multilayers. Hemin, like other porphyrin molecules, can act as a kind of typical photoinduced charge-transfer sensitizer as a result of its high density of π-electrons and good chemical and thermal stability. (34) Miller, A.; Knoll, W.; Mowald, H. Thin Solid Films 1985, 133, 83. (35) Ruaudel-Teixier, A.; Barraud, A.; Belbeoch, B.; Roulliay, M. Thin Solid Films 1983, 99, 33. (36) Gallagher, W. A.; Elliott, W. B. Ann. N. Y. Acad. Sci. 1973, 206, 463. (37) Yanagi, Y.; Sekuzu, I.; Orii, Y.; Okunuk, K. J. Biochem (Tokyo) 1972, 71, 47. (38) Sagara, T.; Takeuchi, S.; Kumazaki, K.; Nakashima, N. J. Electroanal. Chem. 1995, 396, 525. (39) Simplicio, J.; Schwenzer, K. Biochemistry 1973, 12, 1923. (40) Medhi, O.; Silver, J. Inorg. Chim. Acta 1988, 153, 133. (41) van Willigen, H.; Chandrashekar, T. K.; Das, U.; Ebersde, M. H. Porphyrins: Excited States and Dynamics; Gouterman, M.; Rentzepis, P. M., Straub, K. D., Eds.; ACS Symposium Series 321; American Chemical Society: Washington, DC, 1986; pp 140-153.

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Figure 5. Surface photovoltage spectra of (a) a hemin monolayer, (b) mixed hemin/ODA-monolayer-modified n-type Si, and (c) blank n-type Si.

Meanwhile, no research reports on the photoinduced interfacial charge-transfer process between hemin and semiconductors by the LB technique. Here, we use the SPS technique to study this process by transferring the hemin and mixed hemin/ODA monolayer and multilayer onto n- and p-type Si substrates. Figure 5 shows the SPS of hemin and mixed hemin/ ODA LB monolayer films on n-type Si. As a comparison, the SPS of the blank n-type Si is also shown in Figure 5. Distinguishing from those of blank n-type Si, the photovoltaic response values of hemin-modified and mixed hemin/ODA-modified n-type Si are increased about 6- and 15-fold, respectively. It indicates that hemin and mixed hemin/ODA monolayers have the photosensitization effect on n-type Si. In addition, both of the two SPS of the monolayer-modified n-type Si exhibit a new band from 400 to 460 nm, which correspond to the Soret band absorption of hemin molecules. It is caused by an electron transition from the a1u(π) to eg(π*) orbital of the hemin molecule under illumination.1 Then, the photoinduced electrons inject from the eg(π*) orbital of the hemin molecule into the conduction band of the n-type Si. The SPS of mixed hemin/ODA-modified n-type Si shows a larger response value than that of the hemin modified n-type Si, particularly in the Soret absorption region. It indicates that the inlay of ODA in the mixed LB monolayer enhances the photovoltaic conversion efficiency of hemin. In the mixed hemin/ODA LB film, ODA molecules can separate hemin partly. Thus, hemin molecules do not form a large aggregate. So the band gap between the HOMO and LUMO orbital of hemin is relatively reduced compared with that of the pure hemin LB film in which hemin forms H-aggregate. As a result, more electrons in hemin are excited from HOMO to LUMO under UV light irradiation and more excited electrons inject to the conduction band of n-type Si. Of course, the SPS response is increased. The SPS of hemin and mixed hemin/ODA multilayer LB films on n-type Si substrates are shown in Figure 6. It can be seen that the response value of the heminmultilayer-modified n-type Si is reduced about 20-fold compared with that of blank n-type Si. Meanwhile the response value of the mixed hemin/ODA-multilayermodified n-type Si still shows a larger response value than that of blank n-type Si, though a slightly smaller response than that of its monolayer-modified n-type Si is obtained. This result suggests that only the monolayer which is adjacent to the surface of n-type Si plays a key role in the process of photoinduced interfacial charge transfer. As for the multilayer of hemin, the photovoltaic response

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Figure 6. Surface photovoltage spectra of 13-layer (a) hemin, (b) mixed hemin/ODA-modified n-type Si, and (c) blank n-type Si.

Figure 7. Surface photovoltage spectra of 13-layer (a) hemin, (b) mixed hemin/ODA-modified p-type Si, and (c) blank p-type Si.

value is largely decreased, which is probably caused by the larger overlap of closely packed hemin molecules. With the increasing of hemin layer, the energy barrier between n-type Si and ITO is increased correspondingly, which hinders the transport of photoinduced charge carriers. When mixed with ODA molecules, the aggregation degree of hemin molecules is largely reduced. As mentioned above, the existence of ODA molecules in every hemin layer improves the electron transfer from the HOMO to LUMO orbital and the number of electrons which inject from hemin to n-type Si is also increased. Moreover, A. Haran has reported the order: the all-trans monolayer has a higher charge-transfer efficiency than a disorder monolayer.42 At this case, the “all-trans effect” leads to stronger electronic coupling through the chain. In our work, the ODA molecules in the mixed LB film take an ordered, all-trans arrangement which can be concluded from the position and intensity of CH2 vibration in Figure 2. These ordered ODA chains can be a channel for long-range electron transfer between adjacent layers to ensure the transport of electrons. So the photovoltaic conversion efficiency is not hindered so much, and the photovoltaic response value still can be kept larger than blank n-type Si. Figure 7 shows the SPS of hemin and mixed hemin/ ODA multilayer LB films on p-type Si substrates. It is observed that both the response value of hemin-modified p-type Si and mixed hemin/ODA modified p-type Si are almost reduced 20-fold compared with that of blank p-type (42) Haran, A.; Waldeck, D. H.; Naaman, R.; Moons, E.; Cahen, D. Science 1994, 263, 948.

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Si. In the range below 400 nm, the SPS response value of modified p-type Si is reduced so much that it is reversed. In this case, the band bending is increased rather than decreased. It indicates that the high-density photoinduced electrons inject from the modified layer into the conduction band of p-type Si. For the SPS of hemin and mixed hemin/ ODA-monolayer-modified p-type Si, the response value is also greatly reduced compared with that of blank p-type Si (data not shown here). Conclusion In this work, we investigate the behavior of the monolayer and its LB films of hemin which is not modified with any substituent groups and mixed hemin/ODA. Their properties are measured by π-A isotherm, infrared spectra, UV-vis spectra, and small-angle XRD pattern. The results show that the hemin molecules form H-

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aggregates in an LB matrix and a well-ordered layer structure can be obtained. The mixing of ODA molecules can reduce the aggregation partly. Studies on the surface photovoltaic properties of hemin and mixed hemin/ODA monolayers and multilayers modified on n-type Si and p-type Si are performed by SPS. The photoinduced interfacial charge-transfer process between hemin and the Si substrate is studied. The introduction of ODA molecules can effectively increase the photovoltaic response value by partly preventing the formation of hemin aggregates. Acknowledgment. The authors acknowledge the National Climbing Program and the National Natural Science Foundation of China (NNSFC) for the provision of financial support. LA990116C