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Photoelectric Conversion Property of Covalent-Attached Multilayer Self-Assembled Films Fabricated from Diazoresin and Fullerol Tingbing Cao,† Shuming Yang,‡ Yanlian Yang,† Chunhui Huang,‡ and Weixiao Cao*,† College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China, and State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, China Received February 22, 2001. In Final Form: July 2, 2001 A novel self-assembly multilayer film was fabricated from fullerenol (C60(ONa)n) and diazoresin (DR)sa photosensitive polymer on indium-tin-oxide substrate. The film shows good stability after UV irradiation and has photoelectric conversion property, which can be measured in a conventional photoelectrochemical cell. The cathodal photocurrent was measured to be about 800 nA/cm2 for a nine-layer film at 100 mW/cm2 white light. The photocurrent spectroscopy responses coincide with the absorption spectrum of the selfassembly film, which means that the C60(ONa)n/DR film is responsible for the photocurrent generation.
Fullerene (C60) has attracted much attention due to its unique chemical and physical properties.1 The photochemical and photophysical properties of fullerene and its functional derivatives have been studied extensively.2-6 Polymer-bound fullerenes are particularly interesting because they blend the polymer’s high solubility and processability and the unique properties of fullerene. Various kinds of C60-containing polymers have been synthesized and studied for understanding their optical, electrical, and solubility properties.7-10 It is known that fullerenes are strong electron acceptors. The electron transfer taking place from various electron donors to photoexcited C60 has been reported.3,4 Photoinduced electron transfer from conjugated polymers to C60 and enhancement of photoconductivity of the polymers have also been reported and were used to design the C60 heterojunction device.11 In the past decade a large number of fullerene derivatives have been synthesized to use as active components for specific electronic devices.12 A selfassembly technique is a favored approach to fabricate † College of Chemistry and Molecular Engineering, Peking University. ‡ State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University.
(1) (a) Talor, R.; Walton, D. R. M. Nature 1993, 363, 685. (b) Andreoni, W. The physics of fullerene-based and fullerene-related materials; Kluwer Academic Publishers: Boston, 2000. (2) Guldi, D. M.; Asmus, K. D. J. J. Phys. Chem. A 1997, 101, 1472. (3) Arbogast, J. W.; Foote, C. S.; Kao, M. J. Am. Chem. Soc. 1992, 114, 2277. (4) Guildi, D.; Huie, R. E.; Neta, P.; Hungerbuhler, H.; Asmus, K. D. Chem. Phys. Lett. 1994, 223, 511. (5) Kuciauskas, D.; Lin. S.; Seely, G. R.; Moore, A. L.; Gust, D. J. Phys. Chem. 1996, 100, 15926. (6) Maggini, M.; Dono, A.; Scorrano, G.; Prato, M. J. Chem. Soc., Chem. Commun. 1995, 843. (7) Okamura, H.; Minoda, M.; Komatsu, K.; Miyamoto, T. Macromol. Chem. Phys. 1997, 198, 777. (8) Weis, C.; Friedrich, C.; Mulhaupt, R.; Frey, H. Macromolecules 1995, 28, 403. (9) Sun, Y. P.; Ma, B.; Bunker, C. E.; Liu, B. J. Am. Chem. Soc. 1995, 117, 12705. (10) (a) Okamura, H.; Terauchi, T.; Minoda, M.; Fukuda, T.; Komatsu, K. Macromolecules 1997, 30, 5279. (b) Ederle, Y.; Mathis, C. Macromolecules 1997, 30, 2546. (11) Sacriciftci, S. N.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (12) Hirsch, A. The Chemistry of the Fullerenes; G. Thieme Verlag: Stuttgart, Germany, 1994.
ultrathin films,13 and some fullerene-containing films have been fabricated,14 but the practical properties of the fullerene-containing self-assembled multilayer (SAM) films have seldom been reported. In our previous work, we reported a novel C60-containing ultrathin film from a photosensitive polymersdiazoresin through a self-assembly technique.15 In this Letter, the photoelectric conversion property of the covalent attached film was investigated. Fullerol (C60(ONa)n)15 and diazoresin (DR)16 were synthesized in our lab and were deposited alternately on indium-tin-oxide (ITO) substrate as follows: DR was dissolved in deionized water (1 mg/1 mL) and C60(OH)n was dissolved in weak alkali aqueous solution (pH ≈8) with the concentration of 0.5 mg/1 mL. ITO glass was treated by ultrasonic cleaner and washed with deionized water before use. The treated substrate was immersed first in C60(ONa)n solution for 5 min, then thoroughly rinsed with water, and dried by a flow of air, followed by immersion in DR solution for 5 min, rinsing, and drying to complete a fabrication cycle. In each cycle, a layer of C60(ONa)n/DR was deposited on both sides of the substrate. Figure 1 shows the UV-vis spectra (on Shimadzu 2100) of multilayer films from the fabrication process. The characteristic band with λmax at 380 nm belongs to the diazonium group of diazoresin; the inset plot shows a good linear relationship between absorbance at 380 nm and the layer number, which indicates a smooth step-by-step fabrication has taken place on the ITO substrate. The fullerol and diazoresin were linked by electrostatic interaction from a cationic diazonium group (-N2+) and the anionic C60(O-)n to form a layer-by-layer ultrathin film. The diazoresin is a photosensitive polymer and decomposes easily under UV irradiation or heat; the ionic bond between -N2+ and O- can be easily transformed into covalent bond under UV light (as shown in Scheme 1), and the film gets very stable toward polar solvents. (13) Decher, G. Science, 1997, 277, 1232. (14) (a) Zheng, L. A.; Lairson; B. M.; Barrera, E. V. Appl. Phys. Lett. 2000, 77, 3242. (b) Feng, W. J.; Miller, B. Langmuir 1999, 15, 3152. (c) Liu, Y. J.; Wang, Y. X.; Lu, H. X. J. Phys. Chem. B 1999, 103, 2035. (15) Cao, T. B.; Gu, Z. N.; Zhou, X. H.; Cao, W. X. Chem. Lett. 2000, 12, 1370. (16) Cao, S. G.; Zhao, C.; Cao, W. X. Polym. Int. 1998, 45, 142.
10.1021/la010283o CCC: $20.00 © 2001 American Chemical Society Published on Web 09/06/2001
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Figure 1. UV-vis spectra of C60(ONa)n/DR multilayer films with various layer numbers on ITO substrate. Layer number (bottom to top) are 2, 4, 6, 8, 10, and 12. Scheme 1. Schematic Illustration of the Structural Change for the C60(ONa)n/DR Multilayrs Film under UV Irradiation
Atomic force microscopy (AFM) (Nanoscope IIIA, Digital Instruments, Inc., tapping mode, air, ambient temperature) was utilized to visualize the surface morphology of the SAM film on the hydrophilic mica. Figure 2 shows the morphology of a five-layer film. The image indicated that the film surface is very flat with 0.3 nm average roughness over the 1 × 1 µm region. After UV irradiation, the morphology of the film shows no obvious change despite the bond nature between layers being changed from ionic into covalent. A conventional three-electrode cell was used to measure the photoelectrochemical properties of the SAM film. The ITO glass modified with SAM films (after UV irradiation) was used as a working electrode with an effective contact area of 1 cm2. A platinum wire was used as a counter electrode and the saturated calomel electrode as a reference electrode. A solution of 0.5 M KCl was selected as the supporting electrolyte in all measurements. The photocurrent measurements were carried out on a model 600 voltammetric analyzer (CH Instruments, USA), and a 500-W xenon lamp (Ushio Electric, Japan) was used as the light source. The intensity of incident light was measured with a power and energy meter (Scientech 372,
Figure 2. AFM image of the C60(ONa)n/DR film on mica in the 1 × 1 µm region.
Boulder, CO). The IR light was filtered throughout the experiment with a Toshiba IRA-25s. The ITO glass modified with 1-, 3-, 5-, 7-, 9-, 11-, and 13-layer SAM films, respectively, was utilized as the working electrode. Steady cathodal photocurrents were measured when each of the modified ITO electrodes was irradiated by a white light (100 mW cm-2). The relationship between photocurrent and layer number of SAM films is shown in Figure 3. It shows that the photocurrent is consecutive increases with increasing of the layer number when the film is thinner (until the ninth layer). The increase of the photocurrent responses indicates that the C60-containing multilayer film is surely an active species in the photoelectric conversion. When the film gets thicker, as shown in Figure 3, the photocurrent shows no steady increase and even minor decrease. This fact implies that at least two factors should be taken into account: one is the increasing probability of recombination and another is the cell resistance becomes larger with increasing film thickness.
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Figure 3. Relationship between photocurrent and layer number of SAM films.
Figure 4. Effect of bias potential on the photocurrent generation of the five-layer C60(ONa)n/DR film upon the irradiation of 100 mW/cm2 white light in 0.5 M KCl solution.
To determine the polarity of the current flow, the effect of bias voltage was investigated for the five-layer film as shown in Figure 4. From Figure 4, we found the cathodal photocurrent increases with increase of the negative bias
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Figure 5. Action (9) and absorption spectra of C60(ONa)n/DR self-assembled film on ITO substrate. The intensities of different wavelengths were all normalized.
potential, and anodic photocurrent increases as the positive bias potential increases. The figure indicates that the electron is transferred through the C60-containing film from ITO to the electrolyte, and the negative bias potential added to the ITO electrode can promote electron transfer through this routine. With a change of the excitation wavelengths within the range of 340-550 nm, a photocurrent action spectrum is obtained as shown in Figure 5, which is close to the absorption maximum in the range of investigated wavelength. The close match of the action and absorption spectra of the modified electrodes suggests that the C60(ONa)n/DR multilayer film is responsible for the generation of the observed photocurrent. In conclusion, covalent-attached self-assembled multilayer films containing polymer bound C60 were fabricated on ITO and mica substrates. The SAM films are very stable toward polar solvents after irradiated by UV light, and photoelectric conversion properties were observed for this SAM as well. Acknowledgment. The authors wish to thank National Nature Science Foundation of China (29874001) for the financial support of this work. LA010283O
