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copper phthalocyanine composites with improved compatibility
Materials Science and Engineering B 117 (2005) 296–301
Fabrication of carbon nanotubes/copper phthalocyanine composites with improved compatibility Yong Wang, Hong-Zheng Chen∗ , Han-Ying Li, Mang Wang∗ Department of Polymer Science and Engineering, State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 27 September 2004; received in revised form 7 November 2004; accepted 18 December 2004
Abstract Modified multiwalled carbon nanotubes (MWCNTs) bonded dodecyl chain and copper-tetra(4-dodecoxy-carboxyl)phthalocyanine (CuPcdc) were prepared respectively, resulting in improved solubility in organic solvents, good compatibility between these two compounds and film-forming characteristic. The novel composite of the modified MWCNTs/CuPcdc was prepared by the solution-blending method. Transmission electron microscope (TEM) showed the good compatibility between the modified MWCNTs and CuPcdc due to a stronger interaction between these two compounds, which is demonstrated by X-ray photoelectron spectroscopy (XPS) of the composite and by comparing the UV–vis absorption spectrum of the composite with that of modified MWCNTs/copper phthalocyanine (CuPc)/polyvinylbutyral (PVB). The stronger interaction is favoring to the photoinduced charge transfer as demonstrated via the study of the photoconductivity of modified MWCNTs/CuPcdc composites. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Copper phthalocyanine; Compatibility; Electronic interaction; Photoconductivity; Charge transfer
1. Introduction Recently, the composites of metal phthalocyanines (MPc)/carbon nanotubes (CNTs) have inspired considerable research interest because of their high quantum efficiency facilitated by charge transfer between them and the complementary properties of the composites, and have been regarded as promising candidates for the fabrication of donor (MPc)–acceptor (CNTs) heterojunction diodes and photovoltaic devices [1–3]. Our previous work demonstrated that the photoconductivity of oxotitanium phthalocyanine (TiOPc)/multiwalled carbon nanotubes (MWCNTs) composite was improved due to the photoinduced charge transfer from the excited TiOPc to MWCNTs. [1] It is well known that an effective approach getting high efficiency of charge transfer is to control the interface properties between the donor and acceptor, which has been demonstrated by Yu et al. [4] in an interpenetrating bicontinuous network. However, both CNTs ∗
Corresponding authors. Tel.: +86 571 87952557; fax: +86 571 87951635. E-mail addresses: [email protected] (H.-Z. Chen), [email protected] (M. Wang). 0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.007
and MPc have poor solubility in organic solvents and consequently inferior compatibility between them, so that the uniform interface between them cannot be formed easily, which limited the efficiency of charge transfer. Therefore, fabricating CNTs-based composites with the desired compatibility remains a great challenge. In this paper, MWCNTs and copper phthalocyanine (CuPc) were attached with long dodecyl chain via covalent bond, respectively (as shown in Fig. 1), aiming at improving their solubility in organic solvents and the compatibility between them. Another objective is to obtain the CNTs/CuPc film with improved compatibility by simple solution-blending method. The electronic interactions between modified MWCNTs and modified CuPc were investigated as well.
2. Experimental 2.1. Materials and equipment Materials. MWCNTs was produced by catalytic pyrolysis of ethylene and purified as reported by Smalley and cowork-
Y. Wang et al. / Materials Science and Engineering B 117 (2005) 296–301
297
Fig. 1. Chemical structures of: (a) CuPcdc and (b) modified MWCNTs.
ers [5]. CuPc (copper-tetra(4-carboxyl)phthalocyanine), CuPcdc, tetra-n-octylammonium bromide (TOAB), dodecyl bromide(DBr), dodecyl alcohol (DOH), polymethyl methacrylate (PMMA), polycarbonate (PC), and N,N -diethyl-4-aminobenzaldehyde-1-pheny-(␣-naphthyl)hydrazone (BAH) were commercially available and used without further purification. All solvents used were in analytical grade and were distilled before use. FTIR measurement was carried out on a VECTOR 22 Fourier transform infrared spectrometer. 1 H NMR spectrum was measured on an Avance 500 MHz spectrometer. The morphology of the MWCNTs and the microstructure of the composites were observed by transmission electron microscope (TEM) JEM 200CX. UV–vis absorption was recorded by a CARY Bio100 spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on an AXIS ULTRA X-ray photoelectron spectrometer. Film thickness was measured on an Elektko-Physik minitest 2000 thin film measuring apparatus. 2.2. Synthesis of modified MWCNTs bonded dodecyl chains and copper-tetra(4-dodecoxy-carboxyl) phthalocyanine (CuPcdc) Modified MWCNTs with dodecyl chain were prepared by the phase-transfer reaction as reported by Guo and coworkers [6]. MWCNTs were shortened and converted into the carboxyl-terminated MWCNTs via sonication in 1:3 in volume ratio concentrated nitric acid–sulfuric acid at ∼50 ◦ C. The carboxyl-terminated MWCNTs was sonicated in aque-
ous NaOH (5 mM) for 2 min and converted into the sodium salt form. To this black homogeneous suspension were added TOAB and DBr, and the mixture was refluxed under vigorous stirring. After 4 h, the suspension became clear and colorless, and black precipitation was observed. The precipitation was collected and dissolved in CHCl3 . After filtration, the solution was washed with a 15% NaCl aqueous solution and dried with anhydrous CaSO4 . The resulting CHCl3 solution was concentrated and then precipitated into ethanol. The black solid of MWCNTs bonded dodecyl chains was collected and dried in a vacuum at 80 ◦ C. FTIR (KBr pellet, cm−1 ): 1206 (C O stretch of the ester), 1578 (C C stretch of MWCNTs backbones), 1732 (C O stretch of the ester), 2846 and 2917 (C H stretch of the alkyl chains). CuPcdc was synthesized and purified as reported by Jin et al. [7]. CuPc(COOH)4 (3.0 g) was refluxed in excess SOCl2 (50 ml) mixed with several drops of dimethyl formamide (DMF) at 70 ◦ C for 24 h. The excess SOCl2 was removed by distillation and the remaining blue solid was dried in the vacuum. The solid (1.0 g) reacted with DOH (1.7 g) in 100 ml benzene mixed with several drops of pyridine at 100 ◦ C for 40 h. After filtration, the resulting benzene solution was concentrated and then precipitated into acetone, which was repeated three times. The blue solid was collected and dried in a vacuum at 80 ◦ C for 24 h. FTIR (KBr pellet, cm−1 ): 670, 730, and 750 (C H stretch of the macro-ring of CuPc); 1720 (C O stretch of the ester); 2850 and 2920 (C H stretch of the alkyl chains). Anal. Calc. for CuPcdc (C89 H131 CuN8 O8 ): C, 70.63%; H, 7.86%; N, 7.94%. Found: C, 70.56%; H, 7.62%; N, 7.64%.
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2.3. Preparation of modified MWCNTs/CuPcdc composites The modified MWCNTs/CuPcdc composite was obtained by mixing the solutions containing the two components, respectively. The composite film was prepared by castcoating method. For a comparison, the modified MWCNTs/CuPc/PVB composite was fabricated as follows: CuPc was firstly dispersed in PVB homogeneously by ball milling in CHCl3 for 8 h, then the slurry was blended with the CHCl3 solution of modified MWCNTs and sonicated for 2 h. The composite film was prepared by dip coating method. 2.4. Fabrication of photoreceptor and measurement of photoconductivity The photoconductivity of modified MWCNTs/CuPcdc composites was studied in dual-layer photoreceptors. The photoreceptor was made of the interface layer (IFL), the charge generation layer (CGL), and the charge transport layer (CTL) in turn on an aluminum substrate as reported by Chen et al. [8]. The IFL contained PMMA with 1 m thickness. The CGL was formed with 1 m thickness from the CHCl3 solution containing CuPcdc or modified MWCNTs/CuPcdc. The CTL consisted of 50 wt.% N,N -diethyl-4-aminobenzaldehyde-1-pheny-(␣-naphthyl)-hydrazone (BAH) in polycarbonate (PC) matrix, and the thickness was 25 m. Photoconductivity measurements were carried out on a GDT-II model photoconductivity-measuring device by the photoinduced xerographic discharge technique [8], which included charging, dark decaying, and illuminating steps. A halogen lamp (5 W, 24 V) was used as light source. In the measurement, the surface of the dual-layer photoreceptor was negatively charged in the dark with initial surface potential (V0 ). As soon as the lamp was lit, charge carriers were generated in the CGL and injected into the CTL. The surface voltage decreased because of photoinduced carriers recombining with the surface charge. The photoinduced discharge curve was output by a computer, from which we can obtain V0 , residual potential (Vr ), dark decaying rate (Rd ), and time from original surface potential to half under illumination (t1/2 ). A desired photoreceptor should have a large V0 and small Vr , Rd , and t1/2 . The smaller the t1/2 is, the higher the photosensitivity of the photoreceptor is.
Fig. 2. TGA curves of: (a) carboxyl-terminated MWCNTs and (b) modified MWCNTs.
Thermogravimetric analysis (TGA) experiments were performed to analyze the modified MWCNTs in the nitrogen atmosphere with a heating rate of 10 ◦ C/min. As shown in Fig. 2, there is one inflection at ∼470 ◦ C in the TGA curve of carboxyl-terminated MWCNTs, indicating that the tubes begin to decompose at this temperature [9]. However, the TGA curve of the modified MWCNTs with dodecyl chains shows obvious weight loss at ∼280 ◦ C, which should be attributed to the loss of the alkyl chains [10]. Therefore, it is believed that MWCNTs bonded by dodecyl groups are indeed obtained. The weight loss between ∼280 and ∼470 ◦ C can be used to estimate the weight percentage of alkyl chains attached to the tubes, although at ∼470 ◦ C the alkyl chains may not be completely removed. From Fig. 2, the weight percentage of attached alkyl chains in the modified MWCNTs is determined to be ∼12%, corresponding to the tube weight percentage ∼86. 3.2. Compatibility and electronic interaction The result of TEM investigation is shown in Fig. 3. The unmodified MWCNTs were nearly endless bundles (Fig. 3a).
3. Results and discussion 3.1. Chemical modification of MWCNTs The reactions for modified MWCNTs with dodecyl chain were carried out in the following way: MWCNTs (COOH)n DBr/H2 O
NaOH/H2 O
−→
sonicate
MWCNTs (COO− Na+ )n
−→ MWCNTs (COOC12 H25 )n
TOAB
Fig. 3. TEM images of: (a) MWCNTs before modification; (b) MWCNTs after modification; (c) modified MWCNTs/CuPc/PVB composite (1:10:10 by wt.); (d) modified MWCNTs/CuPcdc composite (1:10 by wt.).
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After being chemically modified, MWCNTs bundles were dispersed uniformly (Fig. 3b). Fig. 3c shows the microstructure of modified MWCNTs/CuPc/PVB composite. The modified MWCNTs were dispersed uniformly in PVB matrix, indicating good miscibility between modified MWCNTs and PVB. However, the good interfaces were not formed between the microcrystals of CuPc and the modified MWCNTs. Fig. 3d shows the modified MWCNTs in the MWCNTs/CuPcdc composite. Each threadlike modified CNT was dispersed uniformly in CuPcdc film, suggesting its good compatibility with CuPcdc. Therefore, the dodecyl-functionalized MWCNTs show enhanced miscibility with organic substance. Meanwhile, CuPcdc is readily soluble in CHCl3 and has good filmforming characteristic. As a result, the uniform composite film based on these two compounds is obtained. The good compatibility between them might be due to the – interaction of conjugated rings between the CNT backbones and the macro ring of CuPc besides the dodecyl long chain interactions between the modified CNTs and CuPcdc. Fig. 4 shows UV–vis absorption spectra of CuPcdc, modified MWCNTs and CuPcdc/modified MWCNTs in CHCl3 solutions and their thin films on quartz. In CHCl3 solution, besides a peak of Soret band at 335 nm, CuPcdc shows two peaks of Q band at 616 and 681 nm, representing the characteristic bands of the dimer and the monomer of CuPcdc, respectively [11]. Modified MWCNTs exhibit a very broad and
Fig. 4. UV–vis absorption spectra of: (a) CuPcdc; (b) modified MWCNTs; (c) modified MWCNTs/CuPcdc (1:10 by wt.) in CHCl3 solutions (c = 15.8 mg/l) and their thin films on quartz.
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Table 1 Binding energy (Eb , eV) of CuPcdc and modified MWCNTs/CuPcdc compositea
CuPcdc MWCNTs/CuPcdc
N 1s
Cu 2p3/2
Cu 2p1/2
398.7 399.1
935.4 935.6
955.2 955.6
a
In the composite, all peaks are induced by the binding energy of atoms in CuPcdc.
featureless spectrum. The absorption bands of the solution containing modified MWCNTs and CuPcdc are the superposition of the absorption of these two compounds. However, the absorption spectrum of the modified MWCNTs/CuPcdc (1:10 by wt.) composite in film on quartz changes greatly when compared to that of pure CuPcdc film. Three main peaks of the composite are red-shifted by 2, 10, and 14 nm, respectively, and show broader absorption in the longer wavelength, which have not been observed in the pure CuPcdc film. These changes might stem from the interaction between the modified MWCNTs and CuPcdc. The existence of the interaction between the donor and the acceptor was further proved via XPS analysis. Table 1 lists the binding energy (Eb ) of various atoms in CuPcdc before and after doping with modified MWCNTs measured from the XPS. The binding energy was corrected by using Eb of C 1s (284.6 eV) in benzene as calibration. From Table 1, we can find that the binding energy of N and Cu atoms in CuPcdc changed after doping with MWCNTs. Eb of N 1s increased by 0.4 eV, and Eb of Cu 2p3/2 and Cu 2p1/2 increased by 0.2 and 0.4 eV, respectively. As we know, the binding energy is correlated with the electronic density around nuclear. The higher the electronic density is, the lower the binding energy is. Therefore, to CuPcdc doped with modified MWCNTs, the electronic density around N and Cu atoms decreases, indicating that intermolecular charge transfer occurs from CuPcdc to MWCNTs in the composite. Fig. 5 shows UV–vis absorption spectra of the composite based on CuPc and modified MWCNTs on quartz. It is
Fig. 5. UV–vis absorption spectra of: (a) CuPc/PVB composite (1:1 by wt.); (b) modified MWCNTs/CuPc/PVB composite (1:10:10 by wt.); (c) modified MWCNTs films on quartz.
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Table 2 Photoconductive properties of CuPcdc and modified CuPcdc/MWCNT composites
CuPcdc MWCNT/CuPcdc (100/2 by wt.) MWCNT/CuPcdc (100/5 by wt.) MWCNT/CuPcdc (100/8 by wt.) MWCNT/CuPcdc (100/10 by wt.) MWCNT/CuPcdc (100/20 by wt.)
V0 (V)
Vr (V)
Rd (V/s)
t1/2 (s)
554 517 526 496 487 364
217 207 199 174 178 158
12 9 11 11 14 24
2.1 1.8 1.5 1.4 1.5 2.1
V0 , initial surface potential; Vr , residual potential; Rd , dark decaying rate; t1/2 , half-decaying time.
clear that the absorption spectrum of CuPc/PVB composite before and after doping modified MWCNTs changes slightly, indicating the absence of significant electronic interaction between MWCNTs and CuPc. The above results from UV–vis spectra show that the interaction between the donor and the acceptor in the modified MWCNTs/CuPcdc composite is stronger than that in the modified MWCNTs/CuPc/PVB composite, which is due to the uniform interface formed in the composite. 3.3. Photoconductivity of modified MWCNTs/CuPcdc composites Table 2 presents the photoconductive properties of CuPcdc and modified MWCNTs/CuPcdc composites. The initial surface potential (V0 ) and half-discharge time (t1/2 ) of modified MWCNTs/CuPcdc composites with various CNT-doping content is given in Fig. 6. We can find that, under the same exposure condition, the photoreceptor of pure CuPcdc shows a half-discharge time (t1/2 ) of 2.1 s and a residual potential (Vr ) of 217 V. When doped with a small amount of MWCNTs of 2 wt.%, both Vr and t1/2 decrease to 207 V and 1.8 s, respectively. The t1/2 decreases firstly with increasing the content of MWCNTs in the composites and reaches the minimum
in the composite doped with 8 wt.% MWCNTs. It can be concluded that the photosensitivity of CuPcdc doped with suitable amount of MWCNTs is higher than that of undoped CuPcdc. The V0 decreases consistently with increasing the content of MWCNTs in the composites, which represents the fall of the ability of retaining charge. It is explained that the good electric conductivity of MWCNTs induces the lower charged V0 and that poor photoconductivity of the photoreceptor made from CuPcdc doped with over 8 wt.% MWCNTs might be induced by the whole process predominated by the discharge of MWCNTs. For a comparison, photoreceptors with CuPc and the modified MWCNTs/CuPc composites as CGM are also studied. The photosensitivity is much higher than that of CuPcdc and the modified MWCNTs/CuPcdc because of poor crystallizability of CuPcdc. The enhanced photosensitivity of modified MWCNTs/CuPcdc might result from the interaction between MWCNTs and CuPcdc as demonstrated above. It was reported that photoinduced charge transfer might occur between the excited poly(p-phenylene vinylene) and MWCNTs, which can enhance the efficiency of charge carrier separation, hence leading to a better photoconductivity [12]. In our modified MWCNTs/CuPcdc composite case, the interfaces between MWCNTs and CuPcdc act as the dissociation centers, where the bound photoinduced excitons split into free charge carriers. Electrons generated by CuPcdc excitons transfer to MWCNTs (acceptor), leaving holes on CuPcdc (donor), hence resulting in higher photosensitivity in the modified MWCNTs/CuPcdc composite. MWCNTs can sufficiently be conductive with high mobility for electron because of the one-dimensional nature of the conduction electron states, which allows removing the photogenerated electrons out of the recombination ranges. So, early time recombination of carriers is inhibited by the spatial separation of electron (on MWCNTs) and hole (on CuPcdc), which enhances the carrier lifetime. In other words, it is the photoinduced charge transfer from CuPcdc to MWCNTs in the composite that contributes to higher photosensitivity. 4. Conclusions
Fig. 6. V0 and t1/2 of modified MWCNTs/CuPcdc composites with different MWCNTs-doping contents.
Both MWCNTs and CuPc were attached with long alkyl chain by covalent bonds, respectively. The modified MWCNTs and CuPcdc showed enhanced solubility in organic solvents, good compatibility between these two compounds and the film-forming characteristic. The modified MWCNTs/CuPcdc composites showed a stronger interaction because of improved compatibility between them. Enhanced photosensitivity was observed in the photoreceptor made from this composite due to the photoinduced charge transfer from CuPcdc to MWCNTs. The photosensitivity reached the maximum in the composite doped with 8 wt.% modified MWCNTs. The result is a benefit for us to design photoconductive or photovoltaic devices with high efficiency of charge carrier generation.
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Acknowledgment This work was financed by the National Natural Science Foundation of China (Nos. 90201009, 50225312 and 50433020). References [1] L. Cao, H.Z. Chen, M. Wang, J.Z. Sun, J. Phys. Chem. B 106 (2002) 8971. [2] L. Cao, H.Z. Chen, H.B. Zhan, L. Zhu, J.Z. Sun, M. Wang, Adv. Mater. 15 (2003) 909. [3] Z.L. Yang, H.Z. Chen, L. Cao, H.Y. Li, M. Wang, Chin. Chem. Lett. 15 (2004) 717.
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[4] G. Yu, J. Gao, A.J. Heeger, Science 270 (1995) 1789. [5] J. Liu, A.G. Rinzler, R.E. Smalley, Science 280 (1998) 1253. [6] Y.J. Qin, J.H. Shi, W. Wu, X.H. Li, Z.X. Guo, D.B. Zhu, J. Phys. Chem. B 107 (2003) 12899. [7] S.D. Jin, J.W. Guan, Q. Shen, W.Q. Chen, J. Inorg. Chem. 9 (1993) 1. [8] H.Z. Chen, R.S. Xu, M. Wang, J. Appl. Polym. Sci. 69 (1998) 2609. [9] D. Bom, R. Andrews, D. Jacques, J. Anthony, B.L. Chen, M.S. Meier, J.P. Selegue, NanoLetters 2 (2002) 615. [10] R.K. Saini, I.W. Chiang, H. Peng, R.E. Smalley, W.E. Billups, R.H. Hauge, J.L. Margrave, J. Am. Chem. Soc. 125 (2003) 3617. [11] A.R. Monahan, J.A. Brado, A.F. Deluca, J. Phys. Chem. 76 (1972) 446. [12] H.S. Woo, R. Czerw, S. Webster, Appl. Phys. Lett. 77 (2000) 1393.
Fabrication of carbon nanotubes/copper phthalocyanine composites with improved compatibility Yong Wang, Hong-Zheng Chen∗ , Han-Ying Li, Mang Wang∗ Department of Polymer Science and Engineering, State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 27 September 2004; received in revised form 7 November 2004; accepted 18 December 2004
Abstract Modified multiwalled carbon nanotubes (MWCNTs) bonded dodecyl chain and copper-tetra(4-dodecoxy-carboxyl)phthalocyanine (CuPcdc) were prepared respectively, resulting in improved solubility in organic solvents, good compatibility between these two compounds and film-forming characteristic. The novel composite of the modified MWCNTs/CuPcdc was prepared by the solution-blending method. Transmission electron microscope (TEM) showed the good compatibility between the modified MWCNTs and CuPcdc due to a stronger interaction between these two compounds, which is demonstrated by X-ray photoelectron spectroscopy (XPS) of the composite and by comparing the UV–vis absorption spectrum of the composite with that of modified MWCNTs/copper phthalocyanine (CuPc)/polyvinylbutyral (PVB). The stronger interaction is favoring to the photoinduced charge transfer as demonstrated via the study of the photoconductivity of modified MWCNTs/CuPcdc composites. © 2004 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Copper phthalocyanine; Compatibility; Electronic interaction; Photoconductivity; Charge transfer
1. Introduction Recently, the composites of metal phthalocyanines (MPc)/carbon nanotubes (CNTs) have inspired considerable research interest because of their high quantum efficiency facilitated by charge transfer between them and the complementary properties of the composites, and have been regarded as promising candidates for the fabrication of donor (MPc)–acceptor (CNTs) heterojunction diodes and photovoltaic devices [1–3]. Our previous work demonstrated that the photoconductivity of oxotitanium phthalocyanine (TiOPc)/multiwalled carbon nanotubes (MWCNTs) composite was improved due to the photoinduced charge transfer from the excited TiOPc to MWCNTs. [1] It is well known that an effective approach getting high efficiency of charge transfer is to control the interface properties between the donor and acceptor, which has been demonstrated by Yu et al. [4] in an interpenetrating bicontinuous network. However, both CNTs ∗
Corresponding authors. Tel.: +86 571 87952557; fax: +86 571 87951635. E-mail addresses: [email protected] (H.-Z. Chen), [email protected] (M. Wang). 0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2004.12.007
and MPc have poor solubility in organic solvents and consequently inferior compatibility between them, so that the uniform interface between them cannot be formed easily, which limited the efficiency of charge transfer. Therefore, fabricating CNTs-based composites with the desired compatibility remains a great challenge. In this paper, MWCNTs and copper phthalocyanine (CuPc) were attached with long dodecyl chain via covalent bond, respectively (as shown in Fig. 1), aiming at improving their solubility in organic solvents and the compatibility between them. Another objective is to obtain the CNTs/CuPc film with improved compatibility by simple solution-blending method. The electronic interactions between modified MWCNTs and modified CuPc were investigated as well.
2. Experimental 2.1. Materials and equipment Materials. MWCNTs was produced by catalytic pyrolysis of ethylene and purified as reported by Smalley and cowork-
Y. Wang et al. / Materials Science and Engineering B 117 (2005) 296–301
297
Fig. 1. Chemical structures of: (a) CuPcdc and (b) modified MWCNTs.
ers [5]. CuPc (copper-tetra(4-carboxyl)phthalocyanine), CuPcdc, tetra-n-octylammonium bromide (TOAB), dodecyl bromide(DBr), dodecyl alcohol (DOH), polymethyl methacrylate (PMMA), polycarbonate (PC), and N,N -diethyl-4-aminobenzaldehyde-1-pheny-(␣-naphthyl)hydrazone (BAH) were commercially available and used without further purification. All solvents used were in analytical grade and were distilled before use. FTIR measurement was carried out on a VECTOR 22 Fourier transform infrared spectrometer. 1 H NMR spectrum was measured on an Avance 500 MHz spectrometer. The morphology of the MWCNTs and the microstructure of the composites were observed by transmission electron microscope (TEM) JEM 200CX. UV–vis absorption was recorded by a CARY Bio100 spectrophotometer. X-ray photoelectron spectroscopy (XPS) analysis was performed on an AXIS ULTRA X-ray photoelectron spectrometer. Film thickness was measured on an Elektko-Physik minitest 2000 thin film measuring apparatus. 2.2. Synthesis of modified MWCNTs bonded dodecyl chains and copper-tetra(4-dodecoxy-carboxyl) phthalocyanine (CuPcdc) Modified MWCNTs with dodecyl chain were prepared by the phase-transfer reaction as reported by Guo and coworkers [6]. MWCNTs were shortened and converted into the carboxyl-terminated MWCNTs via sonication in 1:3 in volume ratio concentrated nitric acid–sulfuric acid at ∼50 ◦ C. The carboxyl-terminated MWCNTs was sonicated in aque-
ous NaOH (5 mM) for 2 min and converted into the sodium salt form. To this black homogeneous suspension were added TOAB and DBr, and the mixture was refluxed under vigorous stirring. After 4 h, the suspension became clear and colorless, and black precipitation was observed. The precipitation was collected and dissolved in CHCl3 . After filtration, the solution was washed with a 15% NaCl aqueous solution and dried with anhydrous CaSO4 . The resulting CHCl3 solution was concentrated and then precipitated into ethanol. The black solid of MWCNTs bonded dodecyl chains was collected and dried in a vacuum at 80 ◦ C. FTIR (KBr pellet, cm−1 ): 1206 (C O stretch of the ester), 1578 (C C stretch of MWCNTs backbones), 1732 (C O stretch of the ester), 2846 and 2917 (C H stretch of the alkyl chains). CuPcdc was synthesized and purified as reported by Jin et al. [7]. CuPc(COOH)4 (3.0 g) was refluxed in excess SOCl2 (50 ml) mixed with several drops of dimethyl formamide (DMF) at 70 ◦ C for 24 h. The excess SOCl2 was removed by distillation and the remaining blue solid was dried in the vacuum. The solid (1.0 g) reacted with DOH (1.7 g) in 100 ml benzene mixed with several drops of pyridine at 100 ◦ C for 40 h. After filtration, the resulting benzene solution was concentrated and then precipitated into acetone, which was repeated three times. The blue solid was collected and dried in a vacuum at 80 ◦ C for 24 h. FTIR (KBr pellet, cm−1 ): 670, 730, and 750 (C H stretch of the macro-ring of CuPc); 1720 (C O stretch of the ester); 2850 and 2920 (C H stretch of the alkyl chains). Anal. Calc. for CuPcdc (C89 H131 CuN8 O8 ): C, 70.63%; H, 7.86%; N, 7.94%. Found: C, 70.56%; H, 7.62%; N, 7.64%.
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2.3. Preparation of modified MWCNTs/CuPcdc composites The modified MWCNTs/CuPcdc composite was obtained by mixing the solutions containing the two components, respectively. The composite film was prepared by castcoating method. For a comparison, the modified MWCNTs/CuPc/PVB composite was fabricated as follows: CuPc was firstly dispersed in PVB homogeneously by ball milling in CHCl3 for 8 h, then the slurry was blended with the CHCl3 solution of modified MWCNTs and sonicated for 2 h. The composite film was prepared by dip coating method. 2.4. Fabrication of photoreceptor and measurement of photoconductivity The photoconductivity of modified MWCNTs/CuPcdc composites was studied in dual-layer photoreceptors. The photoreceptor was made of the interface layer (IFL), the charge generation layer (CGL), and the charge transport layer (CTL) in turn on an aluminum substrate as reported by Chen et al. [8]. The IFL contained PMMA with 1 m thickness. The CGL was formed with 1 m thickness from the CHCl3 solution containing CuPcdc or modified MWCNTs/CuPcdc. The CTL consisted of 50 wt.% N,N -diethyl-4-aminobenzaldehyde-1-pheny-(␣-naphthyl)-hydrazone (BAH) in polycarbonate (PC) matrix, and the thickness was 25 m. Photoconductivity measurements were carried out on a GDT-II model photoconductivity-measuring device by the photoinduced xerographic discharge technique [8], which included charging, dark decaying, and illuminating steps. A halogen lamp (5 W, 24 V) was used as light source. In the measurement, the surface of the dual-layer photoreceptor was negatively charged in the dark with initial surface potential (V0 ). As soon as the lamp was lit, charge carriers were generated in the CGL and injected into the CTL. The surface voltage decreased because of photoinduced carriers recombining with the surface charge. The photoinduced discharge curve was output by a computer, from which we can obtain V0 , residual potential (Vr ), dark decaying rate (Rd ), and time from original surface potential to half under illumination (t1/2 ). A desired photoreceptor should have a large V0 and small Vr , Rd , and t1/2 . The smaller the t1/2 is, the higher the photosensitivity of the photoreceptor is.
Fig. 2. TGA curves of: (a) carboxyl-terminated MWCNTs and (b) modified MWCNTs.
Thermogravimetric analysis (TGA) experiments were performed to analyze the modified MWCNTs in the nitrogen atmosphere with a heating rate of 10 ◦ C/min. As shown in Fig. 2, there is one inflection at ∼470 ◦ C in the TGA curve of carboxyl-terminated MWCNTs, indicating that the tubes begin to decompose at this temperature [9]. However, the TGA curve of the modified MWCNTs with dodecyl chains shows obvious weight loss at ∼280 ◦ C, which should be attributed to the loss of the alkyl chains [10]. Therefore, it is believed that MWCNTs bonded by dodecyl groups are indeed obtained. The weight loss between ∼280 and ∼470 ◦ C can be used to estimate the weight percentage of alkyl chains attached to the tubes, although at ∼470 ◦ C the alkyl chains may not be completely removed. From Fig. 2, the weight percentage of attached alkyl chains in the modified MWCNTs is determined to be ∼12%, corresponding to the tube weight percentage ∼86. 3.2. Compatibility and electronic interaction The result of TEM investigation is shown in Fig. 3. The unmodified MWCNTs were nearly endless bundles (Fig. 3a).
3. Results and discussion 3.1. Chemical modification of MWCNTs The reactions for modified MWCNTs with dodecyl chain were carried out in the following way: MWCNTs (COOH)n DBr/H2 O
NaOH/H2 O
−→
sonicate
MWCNTs (COO− Na+ )n
−→ MWCNTs (COOC12 H25 )n
TOAB
Fig. 3. TEM images of: (a) MWCNTs before modification; (b) MWCNTs after modification; (c) modified MWCNTs/CuPc/PVB composite (1:10:10 by wt.); (d) modified MWCNTs/CuPcdc composite (1:10 by wt.).
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After being chemically modified, MWCNTs bundles were dispersed uniformly (Fig. 3b). Fig. 3c shows the microstructure of modified MWCNTs/CuPc/PVB composite. The modified MWCNTs were dispersed uniformly in PVB matrix, indicating good miscibility between modified MWCNTs and PVB. However, the good interfaces were not formed between the microcrystals of CuPc and the modified MWCNTs. Fig. 3d shows the modified MWCNTs in the MWCNTs/CuPcdc composite. Each threadlike modified CNT was dispersed uniformly in CuPcdc film, suggesting its good compatibility with CuPcdc. Therefore, the dodecyl-functionalized MWCNTs show enhanced miscibility with organic substance. Meanwhile, CuPcdc is readily soluble in CHCl3 and has good filmforming characteristic. As a result, the uniform composite film based on these two compounds is obtained. The good compatibility between them might be due to the – interaction of conjugated rings between the CNT backbones and the macro ring of CuPc besides the dodecyl long chain interactions between the modified CNTs and CuPcdc. Fig. 4 shows UV–vis absorption spectra of CuPcdc, modified MWCNTs and CuPcdc/modified MWCNTs in CHCl3 solutions and their thin films on quartz. In CHCl3 solution, besides a peak of Soret band at 335 nm, CuPcdc shows two peaks of Q band at 616 and 681 nm, representing the characteristic bands of the dimer and the monomer of CuPcdc, respectively [11]. Modified MWCNTs exhibit a very broad and
Fig. 4. UV–vis absorption spectra of: (a) CuPcdc; (b) modified MWCNTs; (c) modified MWCNTs/CuPcdc (1:10 by wt.) in CHCl3 solutions (c = 15.8 mg/l) and their thin films on quartz.
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Table 1 Binding energy (Eb , eV) of CuPcdc and modified MWCNTs/CuPcdc compositea
CuPcdc MWCNTs/CuPcdc
N 1s
Cu 2p3/2
Cu 2p1/2
398.7 399.1
935.4 935.6
955.2 955.6
a
In the composite, all peaks are induced by the binding energy of atoms in CuPcdc.
featureless spectrum. The absorption bands of the solution containing modified MWCNTs and CuPcdc are the superposition of the absorption of these two compounds. However, the absorption spectrum of the modified MWCNTs/CuPcdc (1:10 by wt.) composite in film on quartz changes greatly when compared to that of pure CuPcdc film. Three main peaks of the composite are red-shifted by 2, 10, and 14 nm, respectively, and show broader absorption in the longer wavelength, which have not been observed in the pure CuPcdc film. These changes might stem from the interaction between the modified MWCNTs and CuPcdc. The existence of the interaction between the donor and the acceptor was further proved via XPS analysis. Table 1 lists the binding energy (Eb ) of various atoms in CuPcdc before and after doping with modified MWCNTs measured from the XPS. The binding energy was corrected by using Eb of C 1s (284.6 eV) in benzene as calibration. From Table 1, we can find that the binding energy of N and Cu atoms in CuPcdc changed after doping with MWCNTs. Eb of N 1s increased by 0.4 eV, and Eb of Cu 2p3/2 and Cu 2p1/2 increased by 0.2 and 0.4 eV, respectively. As we know, the binding energy is correlated with the electronic density around nuclear. The higher the electronic density is, the lower the binding energy is. Therefore, to CuPcdc doped with modified MWCNTs, the electronic density around N and Cu atoms decreases, indicating that intermolecular charge transfer occurs from CuPcdc to MWCNTs in the composite. Fig. 5 shows UV–vis absorption spectra of the composite based on CuPc and modified MWCNTs on quartz. It is
Fig. 5. UV–vis absorption spectra of: (a) CuPc/PVB composite (1:1 by wt.); (b) modified MWCNTs/CuPc/PVB composite (1:10:10 by wt.); (c) modified MWCNTs films on quartz.
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Table 2 Photoconductive properties of CuPcdc and modified CuPcdc/MWCNT composites
CuPcdc MWCNT/CuPcdc (100/2 by wt.) MWCNT/CuPcdc (100/5 by wt.) MWCNT/CuPcdc (100/8 by wt.) MWCNT/CuPcdc (100/10 by wt.) MWCNT/CuPcdc (100/20 by wt.)
V0 (V)
Vr (V)
Rd (V/s)
t1/2 (s)
554 517 526 496 487 364
217 207 199 174 178 158
12 9 11 11 14 24
2.1 1.8 1.5 1.4 1.5 2.1
V0 , initial surface potential; Vr , residual potential; Rd , dark decaying rate; t1/2 , half-decaying time.
clear that the absorption spectrum of CuPc/PVB composite before and after doping modified MWCNTs changes slightly, indicating the absence of significant electronic interaction between MWCNTs and CuPc. The above results from UV–vis spectra show that the interaction between the donor and the acceptor in the modified MWCNTs/CuPcdc composite is stronger than that in the modified MWCNTs/CuPc/PVB composite, which is due to the uniform interface formed in the composite. 3.3. Photoconductivity of modified MWCNTs/CuPcdc composites Table 2 presents the photoconductive properties of CuPcdc and modified MWCNTs/CuPcdc composites. The initial surface potential (V0 ) and half-discharge time (t1/2 ) of modified MWCNTs/CuPcdc composites with various CNT-doping content is given in Fig. 6. We can find that, under the same exposure condition, the photoreceptor of pure CuPcdc shows a half-discharge time (t1/2 ) of 2.1 s and a residual potential (Vr ) of 217 V. When doped with a small amount of MWCNTs of 2 wt.%, both Vr and t1/2 decrease to 207 V and 1.8 s, respectively. The t1/2 decreases firstly with increasing the content of MWCNTs in the composites and reaches the minimum
in the composite doped with 8 wt.% MWCNTs. It can be concluded that the photosensitivity of CuPcdc doped with suitable amount of MWCNTs is higher than that of undoped CuPcdc. The V0 decreases consistently with increasing the content of MWCNTs in the composites, which represents the fall of the ability of retaining charge. It is explained that the good electric conductivity of MWCNTs induces the lower charged V0 and that poor photoconductivity of the photoreceptor made from CuPcdc doped with over 8 wt.% MWCNTs might be induced by the whole process predominated by the discharge of MWCNTs. For a comparison, photoreceptors with CuPc and the modified MWCNTs/CuPc composites as CGM are also studied. The photosensitivity is much higher than that of CuPcdc and the modified MWCNTs/CuPcdc because of poor crystallizability of CuPcdc. The enhanced photosensitivity of modified MWCNTs/CuPcdc might result from the interaction between MWCNTs and CuPcdc as demonstrated above. It was reported that photoinduced charge transfer might occur between the excited poly(p-phenylene vinylene) and MWCNTs, which can enhance the efficiency of charge carrier separation, hence leading to a better photoconductivity [12]. In our modified MWCNTs/CuPcdc composite case, the interfaces between MWCNTs and CuPcdc act as the dissociation centers, where the bound photoinduced excitons split into free charge carriers. Electrons generated by CuPcdc excitons transfer to MWCNTs (acceptor), leaving holes on CuPcdc (donor), hence resulting in higher photosensitivity in the modified MWCNTs/CuPcdc composite. MWCNTs can sufficiently be conductive with high mobility for electron because of the one-dimensional nature of the conduction electron states, which allows removing the photogenerated electrons out of the recombination ranges. So, early time recombination of carriers is inhibited by the spatial separation of electron (on MWCNTs) and hole (on CuPcdc), which enhances the carrier lifetime. In other words, it is the photoinduced charge transfer from CuPcdc to MWCNTs in the composite that contributes to higher photosensitivity. 4. Conclusions
Fig. 6. V0 and t1/2 of modified MWCNTs/CuPcdc composites with different MWCNTs-doping contents.
Both MWCNTs and CuPc were attached with long alkyl chain by covalent bonds, respectively. The modified MWCNTs and CuPcdc showed enhanced solubility in organic solvents, good compatibility between these two compounds and the film-forming characteristic. The modified MWCNTs/CuPcdc composites showed a stronger interaction because of improved compatibility between them. Enhanced photosensitivity was observed in the photoreceptor made from this composite due to the photoinduced charge transfer from CuPcdc to MWCNTs. The photosensitivity reached the maximum in the composite doped with 8 wt.% modified MWCNTs. The result is a benefit for us to design photoconductive or photovoltaic devices with high efficiency of charge carrier generation.
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