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Langmuir 2006, 22, 344-348
Solvent-Stabilized Oxovanadium Phthalocyanine Nanoparticles and Their Application in Xerographic Photoreceptors Xinran Zhang, Yongfeng Wang, Yan Ma, Yingchun Ye, Yuan Wang,* and Kai Wu* State Key Lab for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China ReceiVed August 10, 2005. In Final Form: October 16, 2005 A stable organic sol of solvent-stabilized oxovanadium phthalocynine (VOPc) nanoparticles with excellent photoconductivity was successfully prepared by ultrasonificating a prepared nanoscopic VOPc powder in1,2dichloroethane (C2H4Cl2) without any additive. These solvent-stabilized VOPc nanoparticles have a size distribution from 2 to 20 nm with an average diameter of 4.6 nm. The VOPc concentration of these organic sols could be as high as 100 g/L. The nanoscopic VOPc particles were well-dispersed in an insulating polycarbonate (PC) resin, resulting in single-layered photoreceptors with high surface charge durability in the dark and excellent photoconductivity. Based on the light-assisted scanning tunneling microscopy (STM) measurements, the charge transport mechanism of these photoreceptors was ascribed to light-induced enhancement of electron tunneling through the VOPc-nanoparticle/ insulator junctions.
1. Introduction Miniaturization of electronic devices and pursuit of electronic, optical, or photoelectric devices with high resolutions have been the key driving force in searching for new types of “bottom-up” architectures that make use of nanoscopic building blocks or functional molecules. Great efforts have been made in the exploration of potentially applicable inorganic semiconductor nanoparticles.1-9 On the other hand, organic compounds are much more diversiform than inorganic ones and have great potential in fabricating soft devices. Recently, studies on organic nanoparticles, especially those of aromatic and functional dye molecules, are gradually emerging.10-21 Different from the * Corresponding authors. E-mail:
[email protected] (Y.W.);
[email protected] (K.W.). Phone: +86-10-62757497 (Y.W.); +86-106275-4005 (K.W.). Fax: +86-10-6276-5769 (Y.W.); +86-10-6275-4005 (K.W.). (1) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706-8715. (2) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699-701. (3) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013-2016. (4) Ridley, B. A.; Nivi, B.; Jacobson, J. M. Science 1999, 286, 746-749. (5) Zhang, J. Z. J. Phys. Chem. B 2000, 104, 7239-7253. (6) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 183-184. (7) Schmelz, O.; Mews, A.; Basche´, T.; Herrmann, A.; Mu¨llen, K. Langmuir 2001, 17, 2861-2865. (8) Tang, Z.; Wang, Y.; Kotov, N. A. Langmuir 2002, 18, 7035-7040. (9) Cushing, B. L.; Kolesnichenko, V. L.; O’Connor, C. J. Chem. ReV. 2004, 104, 3893-3946. (10) Fu, H.; Yao, J. J. Am. Chem. Soc. 2001, 123, 1434-1439. (11) Gong, X. C.; Milic, T.; Xu, C.; Batteas, J. D.; Drain, C. M. J. Am. Chem. Soc. 2002, 124, 14290-14291. (12) An, B.-K.; Kwon, S.-K.; Jung, S.-D.; Park, S. Y. J. Am. Chem. Soc. 2002, 124, 14410-14415. (13) Kim, H. Y.; Bjorklund, T. G.; Lim, S.-H.; Bardeen, C. J. Langmuir 2003, 19, 3941-3946. (14) Debuigne, F.; Jeunieau, L.; Wiame, M.; B.Nagy, J. Langmuir 2000, 16, 7605-7611. (15) Jenekhe, S. A.; Yi, S. J. AdV. Mater. 2000, 12, 1274-1278. (16) Ye, J.; Chen, H.; Wang, M. J. Mater. Sci. 2003, 38, 4021-4025. (17) Tamaki, Y.; Asahi, T.; Masuhara, H. J. Phys. Chem. A 2002, 106, 21352139. (18) Li, B.; Kawakami, T.; Hiramatsu, M. Appl. Surf. Sci. 2003, 210, 171176. (19) Nitschke, C.; O’Flaherty, S. M.; Kro¨ll, M.; Blau, W. J. J. Phys. Chem. B 2004, 108, 1287-1295. (20) Wang, Y.; Deng, K.; Gui, L.; Tang, Y.; Zhou, J.; Cai, L.; Qiu, J.; Ren, D.; Wang, Y. J. Colloid Interface Sci. 1999, 213, 270-272.
traditional milling methods that result in nanoparticles with large sizes (usually >100 nm) and wide size distributions, several effective ways have been developed to prepare small organic nanoparticles, including reprecipitation method,10-13 microemulsion method,14 complexation-mediated solubilization,15,16 laser ablation,17,18 and microwave method.19 Phthalocyanines (Pcs) are one of the most important organic semiconductors. Their strong Q-band absorption (600-800 nm) and excellent photoinduced charge generation properties facilitate their application in the xerographic photoreceptors of copiers and laser printers.22-24 Also, they are very promising materials for image sensors,25 photodetectors,15 optical limiters,19 etc. Because of the insolubility of unsubstituted Pcs in almost all kinds of common solvents, people have to make colloidal solutions of them for their applications. Compared to large particles of Pcs, the advantages of small nanoparticles derive at least from the following two aspects: (1) they could afford high resolutions in photoelectric devices (such as xerographic photoreceptors or image sensors) made from them; (2) the high surface-to-volume ratio of small photoconductive nanoparticles would enhance the efficiency of photoinduced charge separation since it was believed that only excitons generated near the surface of the particles significantly contribute to charge separation, whereas lightinduced excitons near the center of the large particles are most probably deactivated before reaching the surface.26 Usually, additives such as surfactants or amphiphilic polymers are necessary for the preparation of colloidal solutions of small nanoparticles of Pcs. However, such additives may have adverse impact on the performance of photoelectric devices containing Pcs since the electrical properties of Pcs are very sensitive to additives.27 Thus, stable organic sols of solvent-stabilized Pcs (21) Liu, W.; Wang, Y.; Gui, L.; Tang, Y. Langmuir 1999, 15, 2130-2133. (22) Loutfy, R. O.; Hor, A. M.; Hsiao, C. K.; Baranyi, G.; Kazmaier, P. Pure Appl. Chem. 1988, 60, 1047-1054. (23) Law, K. Y. Chem. ReV. 1993, 93, 449-486. (24) Borsenberger, P. M.; Weiss, D. S. Organic Photoreceptors for Xerography; Marcel Dekker Inc.: New York, 1998; Chapter 6. (25) Street, R. A.; Graham, J.; Popovic, Z. D.; Hor, A.; Ready, S.; Ho, J. J. Non-Cryst. Solids 2002, 299-302, 1240-1244. (26) Niimi, T.; Umeda, M. J. Phys. Chem. B 2002, 106, 3657-3661. (27) Dini, D.; Hanack, M. In The Porphyrin Handbook, Vol. 17/Phthalocyanines: Properties and Materials; Kardish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2003; p 19.
10.1021/la0521746 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/19/2005
VOPc Nanoparticles in Xerographic Photoreceptors Chart 1. Molecular Structure of VOPc
nanoparticles with good photoconductivity should be very attractive, and overcoming the obstacle in the preparaton of them is of great importance. Previously, we reported the preparation of aqueous sols of surfactant-protected small nanoparticles of oxovanadium phthalocynine (VOPc, Chart 1) and other phthalocyanines.20,21 In this work, a modified procedure was adopted to prepare a nanoscopic VOPc powder (N-VOPc) that could be well-dispersed in organic solvents such as 1,2-dichloroethane (C2H4Cl2) in the absence of any additive, resulting in stable organic colloidal solutions of small VOPc nanoparticles. These solvent-stabilized nanoparticles of VOPc were also well-dispersed in an insulating polycarbonate (PC) resin to produce single-layered photoreceptors with excellent xerographic properties. On the basis of lightassisted STM study, we proposed that the light-induced enhancement of electron tunneling through the VOPc-nanoparticle/insulator junctions is an important reason for bringing on the effective charge transportation in the single-layered photoreceptors containing the VOPc nanoparticles. 2. Experimental Section Chemicals. Starting material of VOPc powder (B-VOPc), in phaseII crystal form,28 was kindly supplied by Professor Liming Yang at the Institute of Chemistry, Chinese Academy of Sciences. Elemental analyses showed that the composition of this powder was in accord with that of theoretical values. Polycarbonate resin (PC, ca. M.W. 64000) and polyoxyethylene (23) lauryl ether (POLE) were purchased from Acros and used as-received. Water was purified through an Ultra-pure Water System (Beijing Epoch Company). 1,2-Dichloroethane (C2H4Cl2) was purchased from Beijing Chemical Corporation and redistilled before use. Preparation of the Organic Sols of Solvent-Stabilized VOPc Nanoparticles. At first, a powder of small VOPc nanoparticles (NVOPc) was prepared according to the previously reported method20 but with some modifications. In a typical experiment, a concentrated H2SO4 solution (50 mL) containing 0.15 g (0.26 mmol) of VOPc was added dropwise into an aqueous solution (500 mL) containing polyoxyethylene (23) lauryl ether (POLE) (1.5 g) under vigorous stirring at room temperature. Ultimately produced was a stable, blue, and transparent aqueous colloidal solution of VOPc nanoparticles. The obtained colloidal solution was then washed in an ultrafilter to pH ∼ 7 with pure water (18 MΩ). Then, the VOPc nanoparticles were filtered, washed alternatively with water and acetone to remove residual POLE, and dried in a vacuum at room temperature. Finally, in a typical experiment for the preparation of the organic sol of C2H4Cl2-stabilized VOPc nanoparticles, 100 mg of N-VOPc was added to 1 mL of C2H4Cl2 and treated by ultrasonication for 30 min, (28) Griffiths, C. H.; Walker, M. S.; Goldstein, P. Mol. Cryst. Liq. Cryst. 1976, 33, 149-170.
Langmuir, Vol. 22, No. 1, 2006 345 resulting in a stable organic sol of VOPc nanoparticles. This organic sol could be diluted to any concentration below 100 g/L. Preparation of Single-Layered Photoreceptors of VOPc Nanoparticles. A C2H4Cl2 solution of the PC resin (0.25 mL, 200 g/L) was added to the prepared VOPc colloidal solution formed by dispersion of 2-25 mg of N-VOPc in 0.25 mL of C2H4Cl2. The mixture was treated by ultrasonication for 5 min, followed by stirring in the dark for 3 h, and then was blade-coated onto an aluminum plate to form a film of ca. 20 µm in thickness after solvent evaporation. Finally, the film was heated at 60 °C for 3 h to remove residual C2H4Cl2. The weight ratios of VOPc to PC were 1:10 to 1:2 in the final photoreceptor films. Characterization. Transmission electron microscopy (TEM) photographs were taken on a JEM-2000FX transmission electron microscope operated at 160 kV. The TEM samples were prepared as follows. A drop of C2H4Cl2 colloidal solution containing VOPc particles and PC resin was dripped onto the surface of water. The solution spread quickly with the solvent evaporating during this course, and a thin film on water soon formed. The film was attached to a blank copper grid and then dried in a vacuum at room temperature. Elemental analyses were performed on a Vario EL elemental analyzer (Elementar Analysensysteme GmbH). UV-vis spectra were measured using a Lambda45 UV-vis spectrometer (Perkin-Elmer Instruments). X-ray diffraction (XRD) patterns were recorded by a Rigaku-2500Pc diffractometer. Photoinduced discharging curves were measured on an EPA-8200 electrostatic paper analyzer (Kawaguchi Co.). From these curves one could obtain the values of initial surface potential (V0, charged by high-voltage corona), decay rate of surface potential in the dark (Rd), exposure energy required to discharge to half of V0 (E1/2), exposure energy required to discharge to one-fifth of V0 (E1/5), and residual potential (VR). Structures and scanning tunneling spectroscopy (STS) data of the VOPc nanoparticles were measured with an Omicron variable temperature STM in ultrahigh vacuum (UHV) with a base pressure of ∼5 × 10-11 Torr. Tungsten tips were used to take the measurements. In imaging, constant current mode was employed. A sample for STM measurements was simply prepared by dumping a drop of a diluted N-VOPc/ C2H4Cl2 colloidal solution on an HOPG substrate and dried in a vacuum. In light-assisted STS measurements, the current feedback loop was broken after the tip was positioned above a specific VOPc particle. A linear bias voltage scan (essentially equivalent to a linear time scan) was conducted in a voltage range to avoid obvious change in the current caused by variation in voltage. All measurements were taken at room temperature. A photodiode laser MDL-300 (650 nm) triggered with a DG535 digital delay/pulse generator was adopted in the light-assisted STS measurements.
3. Results and Discussion 3.1. Solvent-Stabilized VOPc Nanoparticles. It was found that particles of N-VOPc powder prepared in this work could be well-dispersed by ultrasonication in organic solvents such as chloroform or C2H4Cl2, especially the latter, and no protective agent was needed in this case. The obtained N-VOPc/C2H4Cl2 colloidal solutions were very stable and they could stand for at least 3 months without any precipitation. The VOPc concentration of these organic sols could be as high as 100 g/L. Although the starting material B-VOPc could also be dispersed in the same solvents by ultrasonication, precipitation would appear within several hours. Besides the outstanding stability of N-VOPc/ C2H4Cl2 colloidal solutions, the simplicity of these systems also offers the opportunities for investigating the intrinsic photoconductivity of VOPc nanoparticles. Figure 1 shows the typical TEM images of the VOPc particles obtained by dispersion of N-VOPc and B-VOPc in PC resin, respectively (for sample preparation, see the Experimental Section). The measured average diameter of small nanoparticles in the former case (N-particles) was 4.6 nm with a size distribution of 2-20 nm, while the average size of the large VOPc particles
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Figure 1. TEM images of (a) N-particles and (b) B-particles embedded in the PC resin. The size distribution of the nanoparticles in (a) is shown in (c).
Figure 3. UV-vis spectra of VOPc in the colloidal solutions and PC film. (a) N-VOPc/C2H4Cl2 colloidal solution, (b) diluting (a) to the concentration of ca. 1 mg/L, and (c) a PC film containing N-VOPc with a N-VOPc/PC weight ratio of 1:20.
Figure 2. XRD patterns of the films of B-VOPc (a) and N-VOPc (b) derived from evaporating the solvent of the C2H4Cl2 colloidal solutions. The peaks are indexed according to ref 28. Curve (a) is elevated by 3000 units.
in the latter case (B-particles) was about 105 nm. Apparently, the size of VOPc particles was reduced by more than 1 order of magnitude through the present process. Figure 2 shows that N-VOPc nanoparticles are in the crystal form of phase-II (curve b),28 the same as the starting material (curve a). Obviously, the diffraction peaks of N-VOPc are significantly broadened compared with those of B-VOPc, and the values of full-width at half-maximum (fwhm) of N-VOPc are 1.3-2.0 times those of B-VOPc, depending on different peaks (e.g., for the intense peak centered at 7.5°, the fwhm values of N-VOPc and B-VOPc are 0.465 and 0.245°, respectively.). With use of the Scherrer formula, the crystal grain sizes of N-VOPc and B-VOPc are estimated to be about 17 and 32 nm, respectively. These values are in agreement with the TEM measurement results. In the case of N-VOPc, although the majority of the VOPc particles have sizes less than 10 nm, their contibution to the XRD signals are smaller than that of larger particles. The crystal grain size of B-VOPc estimated from the XRD peaks implies that the large B-VOPc particles shown in the TEM image (Figure 1b) may be composed of several crystal grains. The stabilizing mechanism of N-particles in C2H4Cl2 is interesting. Elemental analyses of both N-VOPc and B-VOPc show that they were identical in composition, indicating that the surfactant molecules used to assist the preparation of VOPc nanoparticles in the aqueous solutions had been removed completely. So the good stability of N-particles in C2H4Cl2 could not be ascribed to the effect of protective agents. On the other hand, it was found that the prepared stable VOPc colloidal solution became unstable upon the addition of a small amount of organic electrolytes, such as tetraethylammonium perchlorate, tetrabutylammonium perchlorate, and tetrabutylammonium hexafluorophosphate. In such cases, N-particles agglomerated within several minutes and precipitate completely in several hours. This is the characteristic feature of colloidal particles that were stabilized by the repulsive electrostatic force between the particles due to the same kind of surface charges they carried. The generation of surface charges in an organic sol is possible
Figure 4. UV-vis spectra of (a) B-particles and (b) N-particles in C2H4Cl2.
according to Coehn’s empirical rule.29 That is, when two dielectrics are in intimate contact, the electrostatic charge separation may occur. The substance having the higher dielectric constant will receive the positive charge, while the other one will receive the negative charge. From this viewpoint, the instability of the N-VOPc/C2H4Cl2 colloidal solution caused by the addition of electrolytes was quite reasonable since the foreign ions could neutralize the surface charges carried by the VOPc nanoparticles. It seems that the electrostatically repulsive force between VOPc particles was not very strong, as it could effectively stabilize N-particles in the prepared colloidal solutions, but it cannot prevent B-particles from aggregating. UV-vis spectra measurements (Figure 3) revealed that in the N-VOPc/C2H4Cl2 colloidal solution (spectrum a in Figure 3), there established an equilibrium between the solvent-stabilized VOPc nanoparticles (characterized by the broad absorption band in near-IR region) and a very small quantity of dissolved VOPc molecules (characterized by the absorption peak centered at ca. 690 nm). When the colloidal solution was diluted to a concentration of about 1 mg/L, the majority of VOPc molecules were dissolved in C2H4Cl2, and the UV-vis spectrum of this solution (spectrum b in Figure 3) was quite similar to that of VOPc molecules dissolved in pyridine.28 However, when embedded in PC resin, VOPc monomers seemed to no longer exist, as the peak at ca. 690 nm could not be observed (spectrum c in Figure 3). UV-vis spectra of both N-particles and B-particles in C2H4Cl2 are shown in Figure 4. Obviously, the absorption bands of the former are narrower than those of the latter. Furthermore, except for the peak at 692 nm, the other three absorption peaks of VOPc (29) Gement, A. Liquid Dielectrics; Chapman and Hall, Ltd.: London, 1933; p108.
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Table 1. Photoconductive Properties of N-VOPc Based Single-Layered Photoreceptorsa weight ratio of N-VOPc to PC
V0 (V)
Rd (V/s)
E1/2 (µJ/cm2)
E1/5 (µJ/cm2)
VR (V)
1:25 2:25 4:25 3:10 2:5 1:2
812 764 686 576 454 323
6 10 25 33 43 52
9.84 3.54 1.73 0.79 0.70 0.53
b 4.12 1.98 0.91 0.81 0.63
231 40 32 10 8 2
a All these data listed in the table were measured with a corona voltage of 5.6 kV and a monochromatic light of 1.0 µW (780 nm). b The surface potential of this photoreceptor could not be discharged to one-fifth of its V0 under the same conditions with other photoreceptors.
particles were all blue-shifted, from 355, 643, and 826 nm to 346, 630, and 820 nm, respectively, with a decrease in the particle size. For nanoparticles of inorganic semiconductors, it is generally accepted that reducing the particle size to a certain extent will enlarge the energy gap between the valence band (VB) and the conduction band (CB), and thus make peaks in the UV-vis spectra shift to shorter wavelength. Here, we have provided creditable proof of the size effect in an organic semiconductor because the VOPc particles with different sizes have the same crystal structure and are surrounded by the same solvent without any additives. 3.2. Xerographic Properties of the Single-Layered Photoreceptors Containing VOPc Nanoparticles. Pcs are widely used as charge generation materials (CGMs) in xerographic photoreceptors of copiers and laser printers.21-23 Usually these CGMs are assisted by charge transport materials (CTMs) to achieve fine photoconductivity of either single-30 or doublelayered15,20 photoreceptors. Obviously, the single-layered photoreceptor, composed of only one layer of photoconductive materials on a conductive substrate, is more convenient for developing new photoelectric devices and could decrease the cost of production to a great extent. And that the positively charged mode makes the photoreceptor emit ca. 90% less ozone than the usually applied negatively charged mode is also important in prolonging the device’s life and improving the air quality around the operators.31 To show the potential application of the prepared organic sol of C2H4Cl2-stabilized VOPc nanoparticles in photoelectric devices, single-layered photoreceptors were fabricated simply by coating the colloidal solutions containing the C2H4Cl2stabilized VOPc nanoparticles and a PC resin onto aluminum plates. Photoinduced discharging curves of these photoreceptors containing different amounts of N-VOPc particles were recorded in the positively charged mode. The characteristic parameters (see the Experimental Section) of the photoreceptors are listed in Table 1. For xerographic use, a photoreceptor is expected to have high V0 (>500 V) and low E1/2 (
