Langmuir 1999, 15, 6921-6924
6921
Formation and Photoelectric Properties of C60EDTA-Mn+ Monolayer Films Wen Zhang,‡ Yaru Shi,‡ Liangbing Gan,*,‡ Nianzu Wu,† Chunhui Huang,*,‡ and Dengguo Wu‡ State Key Laboratory of Rare Earth Materials Chemistry and Applications, and Institute of Physical Chemistry, Peking University, Beijing 100871, People’s Republic of China Received December 14, 1998. In Final Form: May 20, 1999 A novel amphiphilic C60EDTA derivative forms a stable monolayer on water subphases containing metal cations such as Ca2+, Cd2+, Cu2+, Nd3+, or Eu3+. The monolayer films can be effectively transferred to ITO or normal glass substrates. XPS spectra show that the metal cations are incorporated into the LangmuirBlodgett films, which exhibit photoinduced electric current. The resemblance of the action and absorption spectra of the modified ITO electrodes suggests that the C60EDTA-Mn+ films are responsible for the generation of the photocurrent. The photocurrent flow direction of these C60EDTA-Mn+ films (anodic) is different from that for C60EDTA films (cathodic). Films containing rare earth elements exhibit a relatively large photocurrent compared with that for films containing other metals. Some factors that may influence the photocurrent generation have been studied, and the mechanism of the photocurrent is proposed.
1. Introduction The photochemical and photophysical properties of C60 and its functional derivatives have gained extensive attention, as they exhibit a variety of interesting excitedstate properties.1-5 The application of fullerene photoactivity is mainly concerned with its strong electronaccepting capacity. Electron transfer from various electron donors to photoexcited C60 has been reported.2 The photovoltaic responses of C60 thin films and polymer-C60 heterojunction devices have been studied in recent years.6 However, studies on fullerene derivatives are still rare. The introduction of hydrophilic groups into a highly hydrophobic C60 molecule made it possible to form stable LB film.7 We have recently reported the formation of stable monolayer and multilayer films from amphiphilic C60 derivatives. Efficient photocurrent generation from these LB-film-modified semiconductor electrodes has been observed.8 In this paper we describe the metal ion binding properties of C60EDTA Langmuir films and the investigation of the effect of a series of metal cations on the photoelectric properties of the C60EDTA film. 2. Experimental Section 2.1. Materials. C60EDTA tetramethyl ester was prepared by the photochemical reaction between C60 and EDTA tetramethyl †
Institute of Physical Chemistry. State Key Laboratory of Rare Earth Materials Chemistry and Applications. ‡
(1) Guldi, D. M.; Asmus, K. D. J. J. Phys. Chem. A 1997, 101, 1472. (2) (a) Arbogast, J. W.; Foote, C. S.; Kao, M. J. Am. Chem. Soc. 1992, 114, 2277. (b) Guldi, D. M.; Huie, R. E.; Neta, P.; Hungerbuhler, H.; Asmus, K.-D. Chem. Phys. Lett. 1994, 223, 511. (3) Kamat, P. V. J. Am. Chem. Soc. 1991, 113, 9705. (4) Kuciauskas, D.; Lin, S.; Seely, G. R.; Moore, A. L.; Gust, D. J. Phys. Chem. 1996, 100, 15926. (5) Maggini, M.; Dono, A.; Scorrano, G.; Prato, M. J. Chem. Soc., Chem. Commun. 1995, 843. (6) Srdanov, V. I.; Lee, C. H.; Sariciftci, N. S. Thin Solid Films 1995, 257, 223. (7) (a) Maliszewskyj, N. C.; Heiney, P. A.; Jones, D. R.; Strongin, R. M.; Cichy, M. A.; Smith, A. B., III. Langmuir 1993, 9, 1439. (b) Ravaine, S.; Le Pecq, F.; Mingotaud, C.; Delhaes, P.; Wudl, F.; Patterson, L. K. J. Phys. Chem. 1995, 99, 9551. (8) (a) Luo, C. P.; Huang, C. H.; Gan, L. B.; Zhou, D. J.; Xia, W. S.; Zhuang, Q. K.; Zhao, Y. L.; Huang, Y. Y. J. Phys. Chem. 1996, 100, 16685. (b) Zhang, W.; Shi, Y. R.; Gan, L. B.; Huang, C. H.; Luo, H. X.; Wu, D. G.; Li, N. Q. J. Phys. Chem. B 1999, 103, 675.
Chart 1. Chemical Structure of C60EDTA
ester as reported.9 C60EDTA (Chart 1) was prepared by the hydrolysis of its tetramethyl ester by a procedure similar to Hirsch’s.10 Under nitrogen atmosphere, 70 mL of toluene was added to a reaction flask containing 100 mg of C60EDTA tetramethyl ester and 160 mg of NaH. The mixture was stirred for 3.5 h at 60 °C and then 3 mL of methanol was added. As soon as the methanol was added, vigorous gas evolution occurred along with the formation of a precipitate. After 10 min of stirring, the precipitate was separated by centrifugation. It was then treated with 20 mL of 2 M H2SO4, washed with water, and dried under vacuum. Yield: ∼90%. IR spectrum: 1735, 1631, 1224, 1113, 1081, 1058, 593, 563, 554, 527, 466. MALDI-TOF-MS: 720 (C60+, 100%), 1010 (M+, 30%). Elemental analysis. Found (Calcd) for C60C10H14N2O8‚2H2SO4‚5H2O: C, 64.85 (64.71); H, 1.94 (2.16); N, 2.10 (2.16). Hydroquinone (H2Q) was reagent grade and recrystallized before use. Chloroform and DMSO were purified by distillation. Deionized water purified by passing through an EASY pure RF compact ultrapure water system (Barnstead Co.) was used in all experiments. 2.2. LB Film Preparation. A monolayer of C60EDTA was obtained on a NIMA 622 computer-controlled Langmuir trough (UK). The subphase was a 1 × 10-3 M metal salt solution such as CaCl2, CdBr2, CuCl2, NdCl3, or Eu(ClO4)3. A spreading solution of C60EDTA was prepared by dissolving it in DMSO and then diluting with chloroform to a concentration of 1.98 × 10-5 M. Five milliliters ((1%) of the solution was carefully deposited on the clean subphase in about 1 h. After the evaporation of the solvent for 30 min, the floating film was compressed at a rate of 40 cm2/min and the surface pressure-area (π-A) isotherm was recorded. The monolayer was deposited onto the hydrophilic, pretreated, transparent indium-tin oxide (ITO) glass substrate or glass plate at a rate of 5 mm/min (vertical dipping) under a constant surface pressure of 15 mN/m. A typical transfer ratio was 0.95 ( 0.05. (9) Gan, L. B.; Jiang, J. F.; Zhang, W.; Su, Y.; Shi, Y. R.; Huang, C. H.; Pan, J. Q.; Lu, M. J.; Wu, Y. J. Org. Chem. 1998, 63 (13), 4240.
10.1021/la981706y CCC: $15.00 © 1999 American Chemical Society Published on Web 07/14/1999
6922
Langmuir, Vol. 15, No. 20, 1999
Zhang et al.
Table 1. Influence of Cations in the Subphase on Film Formation and the Binding Energy of the Metals in LB Films sample
metal ionic radii (Å)
limiting molecular area (Å2)
collapse pressure (mN/m)
subphase pH
binding energy (eV)
C60EDTA-Ca
0.99
106
18
5.6
C60EDTA-Cd
0.97
115
17
5.6
C60EDTA-Cu C60EDTA-Nd C60EDTA-Eu C60EDTA
0.72 0.99 0.95
102 113 112 102
18 24 21 20
5.2 4.5 4.5 5.6
346.9 (Ca 2p3/2) 350.5 (Ca 2p1/2) 405.6 (Cd 3d5/2) 412.3 (Cd 3d3/2) 933.9 (Cu 2p3/2) 984.1 (Nd 3d5/2) 1135.4 (Eu 3d5/2)
Figure 1. Surface pressure-area isotherms of C60EDTA on different subphases (1 × 10-3 M metal salt solutions and pure water), 293 ( 1 K.
Figure 2. Successive compression and expansion cycles with a monolayer of C60EDTA on a Eu(ClO4)3 subphase, 293 ( 1 K.
3. Results and Discussion 3.1. Langmuir-Blodgett Film. A Langmuir film of C60EDTA was formed on the surface of aqueous CaCl2, CdCl2, CuCl2, NdCl3, or Eu(ClO4)3 solutions. Figure 1 shows the π-A isotherms of C60EDTA on different subphases. From the linear part of the curves, the limiting area per molecule, ranging from 102 to 115 Å2, is extrapolated (Table 1). These values are slightly larger than that on pure water (102 Å2), suggesting the interaction of metal cations with the carboxyls of C60EDTA. The limiting area per molecule has been reported at around 100 Å2 for several other monolayer films of C60 derivatives.11 The π-A isotherms exhibit well-defined liquid phases and a distinct transition to the solid phase. The collapse pressures of the monolayer films on these metal cationic subphases are all below 25 mN/m. The monolayers of C60-
EDTA on NdCl3 and Eu(ClO4)3 subphases collapsed at a little higher surface pressure (Table 1), in agreement with the relatively higher positive charge of the rare earth metals (+3) and the thus stronger interaction with the EDTA moiety compared to that for M2+ ions. After the collapse pressure, further compression results in a distinct phase change and a decrease of the limiting area, suggesting the formation of multilayer or partial multilayer films. Figure 2 shows the π-A isotherms of the successive compression and expansion cycles for a C60EDTA-Eu3+ film. After an initial compression to π ) 20 mN/m, the expansion process followed a curve very close to that for the compression. Recompression yielded a curve very close to the original one, with only a slight decrease of the measured molecular area. This is one of a few examples with little hysteresis for fullerene monolayers.12 A K+ aqueous subphase has been reported to assist the monolayer formation of C60 crown ether derivatives.13 A possible arrangement of C60EDTA on metal cationic subphase is shown in Chart 2. The C60EDTA Langmuir film is transferred onto a solid surface by compressing the monolayer film to 15 mN/m and then upwardly withdrawing a hydrophilic pretreated substrate at 5 mm/min. The pH of the subphase is a little different from metal to metal. The experimental pH values are listed in Table 1. Although the addition of metal salts to the water subphase affects the molecular area at the air-water interface, the metal cations do not alter the UV-vis spectra of the C60EDTA-Mn+ film. This suggests that the metal cations are not close enough to the C60 moiety to affect its electronic structure upon binding.
(10) Lamparth, L.; Hirsch, A. J. Chem. Soc., Chem. Commun. 1994, 1727. (11) (a) Vaknin, D.; Wang, J. Y.; Uphaus, R. A. Langmuir 1995, 11, 1435. (b) Ravaine, S.; Le Pecq, F.; Mingotaud, C.; Delhaes, P.; Hummelen, J. C.; Wudl, F.; Patterson, L. K. J. Phys. Chem. 1995, 99, 9551.
(12) Cardullo, F.; Diederich, F.; Echegoyen, L.; Habicher, T.; Jayaraman, N.; Leblance, R. M.; Stoddart, J. F.; Wang, S. P. Langmuir 1998, 14, 1955. (13) Wang, S. P.; Leblance, R. M.; Arias, F.; Echegoyen, L. Langmuir 1997, 13, 1672.
2.3. Photoelectrochemical Measurement. The photocurrent measurements were carried out on a model 600 voltammetric analyzer (CH Instruments Inc.) and a 500 W xenon lamp (Ushio Electric, Japan). A series of filters (Toshiba, Japan) with a certain band-pass were used to obtain different wavelengths of incident light. The intensity of incident light was measured with a power and energy meter (Scientech 372, Boulder, CO). The IR light was filtered throughout the experiment with a Toshiba IRA-25s filter. A three-electrode cell having a flat window for illumination of the working electrode was used. The counter and reference electrodes were Pt wire and saturated calomel, respectively. KCl solution (0.1 M) was used as the electrolyte solution. All experiments were carried out under nitrogen atmosphere.
C60EDTA-Mn+ Monolayer Films
Langmuir, Vol. 15, No. 20, 1999 6923
Chart 2. Possible Arrangement of C60EDTA on a Metal Cationic Subphase
Table 2. Photocurrent Densities of C60EDTA-Mn+ LB Films and Quantum Yields photocurrent (nA/cm2) sample C60EDTA-Ca C60EDTA-Cd C60EDTA-Cu C60EDTA-Nd C60EDTA-Eu C60EDTA
0.1 M 0.1 M KCl, 0.1 M KCl, 0.2 V quantum KCl 20 mM H2Q bias, 20 mM H2Q, yielda (%) 13 15 15 62 70 -15
130 135 138 218 226 127
398 405 416 546 590
2.87 2.92 3.00 3.94 4.25
a Quantum yield is calculated using the photocurrent density measured at 0.2 V bias voltage in 0.1 M KCl containing 20 mM H2Q, λ ) 404 nm.
Figure 3. XPS spectra of C60EDTA-Cu2+(A) and C60EDTAEu3+ (B) LB films on glass plate.
Elemental analysis of LB films by X-ray photoelectron spectroscopy (XPS) reveals that the atoms present in the films are carbon, oxygen, nitrogen, and the incorporated metal. Figure 3 shows the XPS spectra of the C60EDTACu2+ and C60EDTA-Eu3+ LB films on glass plates. The presence of Cu2+ or Eu3+ in the LB film is clearly shown in the spectra. The binding energies of the metal in LB films are shown in Table 1. From the peak areas, which are integrated and corrected with atomic sensitivity factors, a 1:1 C60EDTA to metal ratio (with derivation (15%) can be obtained for all the C60EDTA-Mn+ films. 3.2. Photocurrent Generation from C60EDTA-Mn+ Modified Electrodes. Anodic photocurrents ranging from 13 to 70 nA/cm2 have been observed when the C60EDTA-Mn+ modified ITO electrodes are illuminated by 404 nm light (2.1 mW/cm2). These anodic photocurrents indicate that electrons flow from the electrolyte solution through the LB film to the electrode. Table 2 gives the values of photocurrents and photocurrent quantum yields for the C60EDTA-Mn+ LB films. The photocurrent of the C60EDTA monolayer film is also shown for comparison. In 0.1 M KCl solution the photoelectric responses of C60EDTA-Mn+ films are very different from that of a C60EDTA film from which a cathodic photocurrent was observed. In our previous study, the pH dependence of photocurrent for the C60EDTA film has been observed. When C60EDTA exists in acidic forms, cathodic photocurrent is observed, while anodic photocurrent is produced when it exists in basic forms.8b The introduction of metal cations onto the film of C60EDTA effectively replaces the
acidic protons on the EDTA substituent producing the same situation as the in basic forms. Therefore, the basic forms of C60EDTA and C60EDTA-Mn+ films have the same anodic photocurrent. When the electron donor H2Q was added to the electrolyte solution, the observed anodic photocurrents of C60EDTA-Mn+ films increase markedly with increasing H2Q concentration. In the presence of H2Q the magnitudes of photocurrents for C60EDTA-Cd2+, C60EDTA-Ca2+, and C60EDTA-Cu2+ films are close to that of C60EDTA (Table 2), indicating that these M2+ cations play a minor role in the electron-transfer efficiency under this condition. The C60EDTA film containing a rare earth metal (Eu3+, Nd3+), however, exhibited a marked increase of photocurrent. The photocurrent densities of these rare earth films are more than 4 times (0.1 M KCl) or approximately 1.7 times (0.1 M KCl, 20 mM H2Q) higher than those of the films containing other metal ions. This enhancement of the photocurrent cannot be caused by electron transfer from Eu(III) or Nd(III) to the C60 moiety because they are relatively redox inert. One possible explanation is the high positive charge of the rare earth cations, which may enhance the efficiency of electron transfer from the donor to the excited state of C60EDTA-Eu3+ or C60EDTA-Nd3+. 3.3. Mechanism of Photocurrent Generation. To determine the polarity of the current flow, the effect of bias voltage was investigated. For the clarity of description, C60EDTA-Eu3+ is selected as an example to discuss below. Under the illumination of 404 nm light, the anodic photocurrent of C60EDTA-Eu3+ increases as the positive bias of the electrode increases. The photocurrent is anodic in the presence of H2Q under the potential range from -0.15 to 0.2 V versus SCE (Figure 4A). This implies that the polarity of the electrical field caused by the applied positive voltage is the same as the polarity of the inner electrical field. When the bias voltage is more negative than -0.15 V versus SCE, a switch from anodic to cathodic was observed. The results show that the electron flow in either direction is energetically possible. The electron flow for the photocurrent generation is consistent with the following pathway: the excited state of C60EDTA-Eu3+, *C60EDTA-Eu3+, is reduced to an anion by the electron donor (H2Q), and then the electrons transfer
6924
Langmuir, Vol. 15, No. 20, 1999
Figure 4. (A) Photocurrent (0.1 M KCl, 20 mM H2Q) versus bias voltage for the monolayer of C60EDTA-Eu3+ (λ ) 404 nm). (B) Photocurrent (0.1 M KCl, 20 mM H2Q) versus light intensity for the monolayer of C60EDTA-Eu3+ film on ITO under white light illumination.
from (C60EDTA-Eu3+)•- to the ITO electrode, thus generating the observed anodic photocurrent. Since the photoelectric responses of these C60EDTA-Mn+ films are all similar, other metal-containing films should adopt the same electron-transfer pathway as that of C60EDTA-Eu3+. The influence of the light intensity and the wavelength on the photocurrent and its quantum yield were also investigated to get more information about the generation and recombination of the charge carriers. Under white light illumination, a linear relationship between the measured photocurrent (iph) and the light intensity (I) was obtained when the light intensity changes from 1.3 to 195 mW/cm2 (Figure 4B). The equation for the line can be expressed as iph ) KIm. The values of K equal to 10.83 and m equal to 1 are obtained. This indicates the separated-charge loss process is an unimolecular recombination process.14 Upon changing the excitation wavelengths within the range 404-800 nm, a photocurrent action spectrum is obtained with a maximum at 404 nm (Figure 5A), which is close to the absorption maximum in the range of investigated wavelength. The close match of the action (14) Donoan, K. J.; Sudiwala, R. V.; Wilson, E. G. Mol. Cryst. Liq. Cryst. 1991, 194, 337.
Zhang et al.
Figure 5. (A) ([) Photocurrent action spectrum (0.1 M KCl, 20 mM H2Q, λ ) 404 nm) and (s) absorption spectrum of a monolayer of C60EDTA-Eu3+ on ITO. (B) Effect of illumination wavelength on the quantum yields of the photocurrents.
and absorption spectra of the modified electrodes suggests that the photosensitization mechanism is responsible for the generation of the observed photocurrent. The quantum yields also depend on the wavelength of excitation (Figure 5B). Upon decreasing the wavelength of the irradiation light, an increase of the quantum yields is observed. At 404 nm, the quantum yield is 1.63% under the shortcircuit condition. The charge carriers are most efficiently generated from 400 to 500 nm within the investigated range of 400-800 nm. 4. Summary C60EDTA has a strong binding ability to metal cations. A series of metal cations in the subphases have been incorporated into C60EDTA LB films. This provides an effective method for the construction of C60 and metal cation-containing organized films. All the metal-containing C60EDTA films adopt a similar photodriven electrontransfer pathway which is different from that for C60EDTA films. A marked enhancement of photocurrent was observed by the introduction of rare earth metal (Eu3+, Nd3+). Acknowledgment. The authors thank the Natural Science Foundation of China for financial support (Grant s29825102 and 2967001). LA981706Y
