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TiOPc composites
Materials Science and Engineering B57 (1999) 87 – 91
Xerographic property, optical absorption, and X-Ray diffraction study of azo/TiOPc composites Ke-Jian Jiang a, Hong-Zheng Chen a,b, Mang Wang a,* a
Department of Polymer Science and Engineering, Zhejiang Uni6ersity, Hangzhou 310027, People’s Republic of China b State Key Lab of Silicon Materials, Hangzhou 310027, People’s Republic of China Received 17 January 1998
Abstract The photoconductivities of bilayer photoreceptor devices, containing azo pigments or titanium oxide phthalocyanine (TiOPc) or azo/TiOPc composites as the charge-generation materials (CGMs), and p-diethylaminobenzaldehyde-a-naphthalenyl-phenylhydrazone (DENPH) or p-dimethylaminobenzaldehyde diphenylhydrazone (DMDPH) as the charge-transportation materials (CTMs), have been studied. The results show that the photosensitivities of azo/TiOPc composites are higher than that of pure azo pigments or pure TiOPc. UV/Vis spectra indicate that the absorption spectra of the azo pigments can be broadened by blending with TiOPc. The X-ray diffraction patterns of the blended composites are also studied. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Azo pigment; Titanium oxide phthalocyanine; Photoconductivity; Composite
1. Introduction In recent years, organic photoconductive materials have been rapidly developed because of their non-toxicity, low cost, magnitude and variability of development, panchromaticity, mechanical and architectual flexibility [1]. Among the organic photogenerating pigments, such as azos, phthalocyanines, perylenes, and polycyclic quinones, azos and phthalocyanines have been extensively studied. Azo pigments primarily absorb and photorespond in the visible region (450 – 650 nm), and are particularly suitable for copiers where visible light sources such as tungsten, fluorescent, and xenon lamps are used [2]. However, they have low photosensitivity in the near infrared region. Metal-free phthalocyanine (aH2Pc) and metal phthalocyanines (such as TiOPc and VOPc) have optical absorptions in the near infrared region (550–850 nm) where GaAs diode lasers operate (750–850 nm) [3]. Therefore, phthalocyanines are suitable for diode printers. Unfortunately, they exhibit a
* Corresponding author. Tel.: +86-571-7951342; fax: + 86-5717951358.
low optical absorption in the 450–550 nm region, and cannot be used in white light imaging processes. Making a kind of photoreceptors which can be used for copiers (visible region: 450–650 nm) and diode laser printers (near infrared region: 750–850 nm) has been explored for some years. The possibility of using a single photogenerator which absorbs in the entire spectral range (400–850 nm) has been studied. A series of unsymmetrical squaraines and unsymmetrical azos, absorbing from visible to near infrared region, have been synthesized by Law et al. [4,5] In addition, incorporating two spectrally complementary photogenerators in a single CGL (charge-generation layer) or double CGLs has been reported. Horgan and his coworkers designed a photoreceptor which had two CGLs (a layer of trigonal selenium and a layer of vanadyl phthalocyanine) [6]. This kind of photoreceptors absorb and photorespond in the 400–900 nm spectral range. Loutfy et al. made a photoreceptor which had a single CGL (composed of a mixture of perylenes and phthalocyanines) [7], which also absorbs and photoresponds from visible to near infrared region. In the paper, we blended a series of azo pigments with titanium oxide phthalocyanine (TiOPc) to make a
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K.-J. Jiang et al. / Materials Science and Engineering B57 (1999) 87–91
Scheme 1. The structures of azo-1, azo-2, TiOPc, DENPH, and DMDPH.
single CGL, and studied the range of their spectral responses and their photoconductivities.
PH) were synthesized in our laboratory according to the reported methods [8–10]. Their structures are illustrated in Scheme 1. Other reagents were commercially available and in analytical purity grade.
2. Experimental part
2.2. UV/Vis spectra measurements 2.1. Materials Two azo pigments (azo-1 and azo-2), titanium oxide phthalocyanine (TiOPc), p-diethylaminobenzaldehydea-naphthalenylphenylhydrazone (DENPH), and p-dimethyl-aminobenzaldehyde diphenylhydrazone (DMD
UV/Vis absorption spectra of thin films (0.1 mm) of the pure azo pigments, TiOPc and azo-TiOPc composites (1:1, by weight) were obtained on a BECKMAN-DU50 UV-Visible recording spectrophotometer from 300 to 900 nm. The pigment dispersion systems
K.-J. Jiang et al. / Materials Science and Engineering B57 (1999) 87–91
89
Table 1 Photoconductivities of azo/TiOPc compositesa CTM
Composite system
TiOPc% (wt)
Vo (V)
Vr (V)
Rd (V s−1)
DV1 (%)
t1/2 (s)
E1/2 (lux · s)
DENPH
azo-1/TiOPc
0 20 40 60 80 100
841 723 639 592 597 812
46 39 42 19 32 43
18 23 29 32 30 39
70 84 89 94 87 78
0.57 0.32 0.28 0.23 0.29 0.44
17.1 9.6 8.4 6.9 8.7 13.2
DENPH
azo-2/TiOPc
0 20 40 60 80 100
893 835 784 692 677 812
44 37 46 32 38 43
15 23 36 41 35 39
65 70 74 83 78 78
0.65 0.42 0.37 0.30 0.36 0.44
19.5 12.6 11.1 9.0 10.8 13.2
DMDPH
azo-1/TiOPc
0 20 40 60 80 100
854 813 751 722 690 820
55 48 40 38 40 45
13 21 39 43 36 37
69 75 82 87 83 75
0.63 0.38 0.31 0.31 0.38 0.44
18.9 11.4 9.3 9.3 11.4 13.2
DMDPH
azo-2/TiOPc
0 20 40 60 80 100
869 834 785 703 672 820
93 51 45 39 35 45
15 27 34 31 38 37
63 70 79 88 86 75
0.69 0.56 0.44 0.31 0.31 0.44
20.7 16.8 13.2 9.3 9.3 13.2
a
(1) DENPH: p-diethylaminobenzaldehyde-a-naphthalenylphenylhydrazone; DMDPH: p-dimethylaminobenzaldehyde diphenylhydrazone. (2) Thicknesses of IFL, CGL, and CTL are about 1, 0.5, and 25 mm, respectively. (3) The exposure intensity (I) is 30 lux.
were processed in THF solvent by milling with sands, and were coated onto transparent quartz glasses by dip coating, which were then dried at 80°C for 30 min.
2.3. X-Ray diffraction pattern measurements X-ray diffraction patterns of the pure azo pigments, TiOPc and their blended composites(1:1, by wt), which were obtained by milling with sands in THF solvent, then separated and dried, were recorded on a Rigaku D/max3B X-ray diffraction instrument, using Cu Ka monochromatic radiation.
2.4. Photoreceptor de6ice fabrication and photoconducti6ity measurements [11] The double-layered photoreceptor device (P/R) was made by coating an interface layer (IFL, 1 mm) of polyamide (PA), then a charge generation layer (CGL, 0.5 mm) of pure azo pigments, or pure TiOPc, or azo/TiOPc composites, which were dispersed in polyvinylbutyral (PVB), and finally a charge transportation layer (CTL, 30 mm) of a mixture of DENPH or DMDPH and polycarbonate(PC) on an aluminium plate, in that order. A GDT-II model photoconductivity measuring device
was used with a 5 W, 24 V incandescence lamp, whose wavelength goes from visible to IR region. The light intensity of the exposure (I) is controlled at 30 lux. In the measurement, the surface of the P/R is negatively charged, and hole charge carriers are generated in CGL and injected into CTL under exposure, and the photoinduced discharge curve (PIDC) of the P/R is recorded, from which the following parameters could be obtained: Vo, Vr, Rd, DV1, t1/2, and E1/2. Here, Vo is the surface charged potential; Vr is the residual potential at 5 s after exposure; Rd is the rate of dark discharge; DV1 is the percentage of potential discharge after 1 s of exposure; t1/2 is the time of half photodischarge; E1/2 (equal to t1/2 × I) is the exposure energy for surface potential to half decay. E1/2 may be taken as an indication of photosensitivity. The lower the E1/2 value, the higher is the photosensitivity of the material.
3. Results and discussion
3.1. Photoconducti6ities of azo/TiOPc composites Photoreceptors were prepared with various blended composites containing different proportions of the azo-1/ TiOPc or azo-2/TiOPc as CGL, p-diethylaminoben-
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K.-J. Jiang et al. / Materials Science and Engineering B57 (1999) 87–91
zaldehyde-a-naphthalenylphenylhydrazone (DENPH) as CTL. Their photoconductivities were measured and listed in Table 1. From Table 1, we can see that, to azo-1/TiOPc system, E1/2 values of pure azo-1 and TiOPc are 17.1 and 13.2 lux s, respectively, with the increase of the content of TiOPc in the azo-1/TiOPc composite system, E1/2 value decreases and reaches the minimum (6.9 lux s) at azo-1/ TiOPc= 4/6(by wt), then, with the increase of the content of TiOPc again, E1/2 value increases slightly. Similarly, to azo-2/TiOPc system, with the increase of the content of TiOPc in the composite system, E1/2 value also decreases and reaches the minimum (9.0 lux s) at the ratio of azo-2/TiOPc = 4/6 (by wt). These observations indicate that photoconductivities of the two blended composite systems are better than that of the pure azo pigments or TiOPc. When p-dimethylaminobenzaldehyde diphenylhydrazone (DMDPH) was used as CTM, the same conclusion can be drawn (see Table 1). The t1/2 and E1/2 values decrease and DV1 value increases with the increase of the content of TiOPc in the two azo/TiOPc composite systems, and the photosensitivity also reaches the highest at the ratio of azo/TiOPc =4/6 (by wt), indicating better photoconductivity than that of the pure azo and TiOPc. In addition, from Table 1, we find that, as the content of TiOPc increases in the two blended composite systems, the surface charge potential (Vo) decreases and the rate of dark discharge (Rd) increases, which suggests that during charging, the dark currents enhance as the content of TiOPc increases in the blended composites [12]. It was reported that TiOPc belong to n-type pigment (zeta potential is about 2.8 mV), and azo pigments belong to p-type (zeta potential B 0). When they are blended, they would be held together by attractive forces of the opposite charge of their partical surface, and form exciplexes which might partly decomposed to free carriers assisted by the electric field [12]. There is no doubt that the number of the exciplexes generated during charging should reach the maximum when azo and TiOPc at equal mole numbers. The formation of the exciplexes might not only lead to much bigger dark current of the photoreceptor, but also be benefit to photo current of the photoreceptor, hence raise the photoconductivities of the blended composites.
Fig. 1. UV/Vis absorption spectra of azo-1 (a), TiOPc (b), and azo-1/TiOPc (1:1, by wt) composite (c) in thin film.
nm (the Q-bands), the other in the near-UV at 300–400 nm (the Soret band). When the two azo pigments are blended with TiOPc, respectively, the absorption spectra of the two blended composite systems are almost overlapped each other. That is to say, the absorption spectra of the two composites are broadened from the visible to the near infrared region.
3.3. Analyses of X-ray diffraction patterns Azo pigments are generally less crystalline than phthalocyanine compounds [13]. From Figs. 3 and 4, we
3.2. UV/Vis absorption The UV/Vis absorption spectra of azo-1, azo-2, TiOPc and the two azo/TiOPc blended composites (1:1, by wt) were studied in the thin films (see Figs. 1 and 2). The absorption spectra of azo-1 and azo-2 range from 450 to 650 nm. Both the two azo pigments show a main absorption peak (lmax of azo-1 and azo-2 is 551 and 555 nm, respectively) and a shoulder peak. TiOPc have two absorption bands, one in the visible region at 600–850
Fig. 2. UV/Vis absorption spectra of azo-2 (a), TiOPc (b), and azo-2/TiOPc (1:1, by wt) composite (c) in thin film.
K.-J. Jiang et al. / Materials Science and Engineering B57 (1999) 87–91
Fig. 3. X-Ray diffraction patterns of azo-1/TiOPc (1:1, by wt) composite (a), TiOPc (b), and azo-1 (c).
indeed found that TiOPc had some strong diffraction peaks which meant much crystalline, while azo-1 and azo-2 had much wider peaks, suggesting little crystalline. In Fig. 3,we found that the peaks of azo-1/TiOPc blended composite (1:1, by wt) are at 2u: 7.7, 10.4, 12.7, 13.2, 15.1, 22.7, 24.4, 25.4, and 28.7°, while pure azo-1 has peaks at 5.0, 7.6, 9.0, 12.0, 13.2, 14.5, 23.8, 26.1, and 27.6°, and pure TiOPc has peaks at 7.6, 10.5, 12.9, 16.4, 18.3, 22.6, 24.4, 25.4, and 28.9°. It is clear that after blending, some peaks were shifted, some peaks disappeared, and some new peaks appeared. For example, the azo-1/TiOPc composite showed sharper peaks than both pure TiOPc and azo-1 did, especially at 2u =7.7°, where the stronger intensity peak was also observed. The same phenomenon was observed in the azo-2/TiOPc composite system (see Fig. 4). For instance, the wide peak of azo-2 at 2u= 5.1° was almost disappeared in the azo-2/TiOPc composite, and the peak of TiOPc at 2u =7.6° was shifted to 7.4° and became sharper in the composite. It suggests that the X-ray diffraction patterns of the blended composites are not simply overlapped by TiOPc and the two azo pigments, respectively, and that the blending leads to changes of the crystals of TiOPc or azo pigments, which would result in the form of the exciplexes from n-type TiOPc and p-type azo pigments, and increase the photoconductivities of the blended composites. .
4. Conclusions (1) The photoconductivities of azo-1/TiOPc and azo-2/TiOPc two composite systems are better than that of the pure azo pigments (azo-1 and azo-2) or TiOPc, and reach the best at azo/TiOPc = 4/6 (by wt). (2) Absorption spectra have been broadened from
91
Fig. 4. X-Ray diffraction patterns of azo-2/TiOPc (1:1, by wt) composite (a), TiOPc (b), and azo-2 (c).
visible to near-infrared region by blending azo-1 or azo-2 with TiOPc. (3) Exciplexes might form by attractive forces of the opposite charges of n-type TiOPc particle surface and p-type azo pigment particle surface. (4)The form of exciplexes would be benefit to increase the photoconductivities of the azo-1/TiOPc and azo-2/TiOPc two composite systems.
Acknowledgements The work was financed by the National Natural Science Foundation of China (Grant No. 69890230 and 59603003).
References [1] R.O. Loutfy, A.M. Hor, P. Kazmaler, M. Tam, J. Imag. Sci. 33 (5) (1989) 151. [2] G. Dipaola-Baranyi, C.K. Hsiao, A.M. Hor, J. Imag. Sci. 34 (1990) 224. [3] Y. Fujimaki, H. Tadokoro, Y. Oda, H. Yoshioka, T. Homma, H. Moriguchi, K. Watanabe, A. Konishita, N. Hirose, A. Itami, S. Ikeuchi, J. Imag. Tech. 17 (1991) 202. [4] K.Y. Law, I.W. Tarawskyj, J. Imag. Sci. Tech. 39 (1995) 1. [5] K.Y. Law, Chem. Mater. 4 (1992) 605. [6] A.M. Horgan, US Patent 4,415,639 (1983). [7] R.O. Loutfy, A.M. Hor, G. Liebermann, A.J. Toth, C.K. Hsiao, K.M. Carmichael, E.G. Tokoli, US Patent 4,882,254 (1989). [8] R.B. Champ, M.D. Shattuck, US Patent 3,898,084 (1975). [9] T. Enokida, R. Hirohashi, T. Nakamura, J. Imag. Sci. 34 (6) (1990) 234. [10] T. Arita, M. Mabuchi, S. Umehara, K. Sakai, DE. 3,331,259 (1983). [11] H.Z. Chen, M. Wang, L.X. Feng, S.L. Yang, J. Polym. Sci. A: Chem. 31 (1993) 1165. [12] T. Nakazawa, A. Kawahara, Y. Wafanabe, Y. Mizuta, J. Imag. Sci. Tech. 38 (5) (1994) 421. [13] K.Y. Law, Chem.Rev. 93 (1) (1993) 449.
Xerographic property, optical absorption, and X-Ray diffraction study of azo/TiOPc composites Ke-Jian Jiang a, Hong-Zheng Chen a,b, Mang Wang a,* a
Department of Polymer Science and Engineering, Zhejiang Uni6ersity, Hangzhou 310027, People’s Republic of China b State Key Lab of Silicon Materials, Hangzhou 310027, People’s Republic of China Received 17 January 1998
Abstract The photoconductivities of bilayer photoreceptor devices, containing azo pigments or titanium oxide phthalocyanine (TiOPc) or azo/TiOPc composites as the charge-generation materials (CGMs), and p-diethylaminobenzaldehyde-a-naphthalenyl-phenylhydrazone (DENPH) or p-dimethylaminobenzaldehyde diphenylhydrazone (DMDPH) as the charge-transportation materials (CTMs), have been studied. The results show that the photosensitivities of azo/TiOPc composites are higher than that of pure azo pigments or pure TiOPc. UV/Vis spectra indicate that the absorption spectra of the azo pigments can be broadened by blending with TiOPc. The X-ray diffraction patterns of the blended composites are also studied. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Azo pigment; Titanium oxide phthalocyanine; Photoconductivity; Composite
1. Introduction In recent years, organic photoconductive materials have been rapidly developed because of their non-toxicity, low cost, magnitude and variability of development, panchromaticity, mechanical and architectual flexibility [1]. Among the organic photogenerating pigments, such as azos, phthalocyanines, perylenes, and polycyclic quinones, azos and phthalocyanines have been extensively studied. Azo pigments primarily absorb and photorespond in the visible region (450 – 650 nm), and are particularly suitable for copiers where visible light sources such as tungsten, fluorescent, and xenon lamps are used [2]. However, they have low photosensitivity in the near infrared region. Metal-free phthalocyanine (aH2Pc) and metal phthalocyanines (such as TiOPc and VOPc) have optical absorptions in the near infrared region (550–850 nm) where GaAs diode lasers operate (750–850 nm) [3]. Therefore, phthalocyanines are suitable for diode printers. Unfortunately, they exhibit a
* Corresponding author. Tel.: +86-571-7951342; fax: + 86-5717951358.
low optical absorption in the 450–550 nm region, and cannot be used in white light imaging processes. Making a kind of photoreceptors which can be used for copiers (visible region: 450–650 nm) and diode laser printers (near infrared region: 750–850 nm) has been explored for some years. The possibility of using a single photogenerator which absorbs in the entire spectral range (400–850 nm) has been studied. A series of unsymmetrical squaraines and unsymmetrical azos, absorbing from visible to near infrared region, have been synthesized by Law et al. [4,5] In addition, incorporating two spectrally complementary photogenerators in a single CGL (charge-generation layer) or double CGLs has been reported. Horgan and his coworkers designed a photoreceptor which had two CGLs (a layer of trigonal selenium and a layer of vanadyl phthalocyanine) [6]. This kind of photoreceptors absorb and photorespond in the 400–900 nm spectral range. Loutfy et al. made a photoreceptor which had a single CGL (composed of a mixture of perylenes and phthalocyanines) [7], which also absorbs and photoresponds from visible to near infrared region. In the paper, we blended a series of azo pigments with titanium oxide phthalocyanine (TiOPc) to make a
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 3 0 7 - 9
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K.-J. Jiang et al. / Materials Science and Engineering B57 (1999) 87–91
Scheme 1. The structures of azo-1, azo-2, TiOPc, DENPH, and DMDPH.
single CGL, and studied the range of their spectral responses and their photoconductivities.
PH) were synthesized in our laboratory according to the reported methods [8–10]. Their structures are illustrated in Scheme 1. Other reagents were commercially available and in analytical purity grade.
2. Experimental part
2.2. UV/Vis spectra measurements 2.1. Materials Two azo pigments (azo-1 and azo-2), titanium oxide phthalocyanine (TiOPc), p-diethylaminobenzaldehydea-naphthalenylphenylhydrazone (DENPH), and p-dimethyl-aminobenzaldehyde diphenylhydrazone (DMD
UV/Vis absorption spectra of thin films (0.1 mm) of the pure azo pigments, TiOPc and azo-TiOPc composites (1:1, by weight) were obtained on a BECKMAN-DU50 UV-Visible recording spectrophotometer from 300 to 900 nm. The pigment dispersion systems
K.-J. Jiang et al. / Materials Science and Engineering B57 (1999) 87–91
89
Table 1 Photoconductivities of azo/TiOPc compositesa CTM
Composite system
TiOPc% (wt)
Vo (V)
Vr (V)
Rd (V s−1)
DV1 (%)
t1/2 (s)
E1/2 (lux · s)
DENPH
azo-1/TiOPc
0 20 40 60 80 100
841 723 639 592 597 812
46 39 42 19 32 43
18 23 29 32 30 39
70 84 89 94 87 78
0.57 0.32 0.28 0.23 0.29 0.44
17.1 9.6 8.4 6.9 8.7 13.2
DENPH
azo-2/TiOPc
0 20 40 60 80 100
893 835 784 692 677 812
44 37 46 32 38 43
15 23 36 41 35 39
65 70 74 83 78 78
0.65 0.42 0.37 0.30 0.36 0.44
19.5 12.6 11.1 9.0 10.8 13.2
DMDPH
azo-1/TiOPc
0 20 40 60 80 100
854 813 751 722 690 820
55 48 40 38 40 45
13 21 39 43 36 37
69 75 82 87 83 75
0.63 0.38 0.31 0.31 0.38 0.44
18.9 11.4 9.3 9.3 11.4 13.2
DMDPH
azo-2/TiOPc
0 20 40 60 80 100
869 834 785 703 672 820
93 51 45 39 35 45
15 27 34 31 38 37
63 70 79 88 86 75
0.69 0.56 0.44 0.31 0.31 0.44
20.7 16.8 13.2 9.3 9.3 13.2
a
(1) DENPH: p-diethylaminobenzaldehyde-a-naphthalenylphenylhydrazone; DMDPH: p-dimethylaminobenzaldehyde diphenylhydrazone. (2) Thicknesses of IFL, CGL, and CTL are about 1, 0.5, and 25 mm, respectively. (3) The exposure intensity (I) is 30 lux.
were processed in THF solvent by milling with sands, and were coated onto transparent quartz glasses by dip coating, which were then dried at 80°C for 30 min.
2.3. X-Ray diffraction pattern measurements X-ray diffraction patterns of the pure azo pigments, TiOPc and their blended composites(1:1, by wt), which were obtained by milling with sands in THF solvent, then separated and dried, were recorded on a Rigaku D/max3B X-ray diffraction instrument, using Cu Ka monochromatic radiation.
2.4. Photoreceptor de6ice fabrication and photoconducti6ity measurements [11] The double-layered photoreceptor device (P/R) was made by coating an interface layer (IFL, 1 mm) of polyamide (PA), then a charge generation layer (CGL, 0.5 mm) of pure azo pigments, or pure TiOPc, or azo/TiOPc composites, which were dispersed in polyvinylbutyral (PVB), and finally a charge transportation layer (CTL, 30 mm) of a mixture of DENPH or DMDPH and polycarbonate(PC) on an aluminium plate, in that order. A GDT-II model photoconductivity measuring device
was used with a 5 W, 24 V incandescence lamp, whose wavelength goes from visible to IR region. The light intensity of the exposure (I) is controlled at 30 lux. In the measurement, the surface of the P/R is negatively charged, and hole charge carriers are generated in CGL and injected into CTL under exposure, and the photoinduced discharge curve (PIDC) of the P/R is recorded, from which the following parameters could be obtained: Vo, Vr, Rd, DV1, t1/2, and E1/2. Here, Vo is the surface charged potential; Vr is the residual potential at 5 s after exposure; Rd is the rate of dark discharge; DV1 is the percentage of potential discharge after 1 s of exposure; t1/2 is the time of half photodischarge; E1/2 (equal to t1/2 × I) is the exposure energy for surface potential to half decay. E1/2 may be taken as an indication of photosensitivity. The lower the E1/2 value, the higher is the photosensitivity of the material.
3. Results and discussion
3.1. Photoconducti6ities of azo/TiOPc composites Photoreceptors were prepared with various blended composites containing different proportions of the azo-1/ TiOPc or azo-2/TiOPc as CGL, p-diethylaminoben-
90
K.-J. Jiang et al. / Materials Science and Engineering B57 (1999) 87–91
zaldehyde-a-naphthalenylphenylhydrazone (DENPH) as CTL. Their photoconductivities were measured and listed in Table 1. From Table 1, we can see that, to azo-1/TiOPc system, E1/2 values of pure azo-1 and TiOPc are 17.1 and 13.2 lux s, respectively, with the increase of the content of TiOPc in the azo-1/TiOPc composite system, E1/2 value decreases and reaches the minimum (6.9 lux s) at azo-1/ TiOPc= 4/6(by wt), then, with the increase of the content of TiOPc again, E1/2 value increases slightly. Similarly, to azo-2/TiOPc system, with the increase of the content of TiOPc in the composite system, E1/2 value also decreases and reaches the minimum (9.0 lux s) at the ratio of azo-2/TiOPc = 4/6 (by wt). These observations indicate that photoconductivities of the two blended composite systems are better than that of the pure azo pigments or TiOPc. When p-dimethylaminobenzaldehyde diphenylhydrazone (DMDPH) was used as CTM, the same conclusion can be drawn (see Table 1). The t1/2 and E1/2 values decrease and DV1 value increases with the increase of the content of TiOPc in the two azo/TiOPc composite systems, and the photosensitivity also reaches the highest at the ratio of azo/TiOPc =4/6 (by wt), indicating better photoconductivity than that of the pure azo and TiOPc. In addition, from Table 1, we find that, as the content of TiOPc increases in the two blended composite systems, the surface charge potential (Vo) decreases and the rate of dark discharge (Rd) increases, which suggests that during charging, the dark currents enhance as the content of TiOPc increases in the blended composites [12]. It was reported that TiOPc belong to n-type pigment (zeta potential is about 2.8 mV), and azo pigments belong to p-type (zeta potential B 0). When they are blended, they would be held together by attractive forces of the opposite charge of their partical surface, and form exciplexes which might partly decomposed to free carriers assisted by the electric field [12]. There is no doubt that the number of the exciplexes generated during charging should reach the maximum when azo and TiOPc at equal mole numbers. The formation of the exciplexes might not only lead to much bigger dark current of the photoreceptor, but also be benefit to photo current of the photoreceptor, hence raise the photoconductivities of the blended composites.
Fig. 1. UV/Vis absorption spectra of azo-1 (a), TiOPc (b), and azo-1/TiOPc (1:1, by wt) composite (c) in thin film.
nm (the Q-bands), the other in the near-UV at 300–400 nm (the Soret band). When the two azo pigments are blended with TiOPc, respectively, the absorption spectra of the two blended composite systems are almost overlapped each other. That is to say, the absorption spectra of the two composites are broadened from the visible to the near infrared region.
3.3. Analyses of X-ray diffraction patterns Azo pigments are generally less crystalline than phthalocyanine compounds [13]. From Figs. 3 and 4, we
3.2. UV/Vis absorption The UV/Vis absorption spectra of azo-1, azo-2, TiOPc and the two azo/TiOPc blended composites (1:1, by wt) were studied in the thin films (see Figs. 1 and 2). The absorption spectra of azo-1 and azo-2 range from 450 to 650 nm. Both the two azo pigments show a main absorption peak (lmax of azo-1 and azo-2 is 551 and 555 nm, respectively) and a shoulder peak. TiOPc have two absorption bands, one in the visible region at 600–850
Fig. 2. UV/Vis absorption spectra of azo-2 (a), TiOPc (b), and azo-2/TiOPc (1:1, by wt) composite (c) in thin film.
K.-J. Jiang et al. / Materials Science and Engineering B57 (1999) 87–91
Fig. 3. X-Ray diffraction patterns of azo-1/TiOPc (1:1, by wt) composite (a), TiOPc (b), and azo-1 (c).
indeed found that TiOPc had some strong diffraction peaks which meant much crystalline, while azo-1 and azo-2 had much wider peaks, suggesting little crystalline. In Fig. 3,we found that the peaks of azo-1/TiOPc blended composite (1:1, by wt) are at 2u: 7.7, 10.4, 12.7, 13.2, 15.1, 22.7, 24.4, 25.4, and 28.7°, while pure azo-1 has peaks at 5.0, 7.6, 9.0, 12.0, 13.2, 14.5, 23.8, 26.1, and 27.6°, and pure TiOPc has peaks at 7.6, 10.5, 12.9, 16.4, 18.3, 22.6, 24.4, 25.4, and 28.9°. It is clear that after blending, some peaks were shifted, some peaks disappeared, and some new peaks appeared. For example, the azo-1/TiOPc composite showed sharper peaks than both pure TiOPc and azo-1 did, especially at 2u =7.7°, where the stronger intensity peak was also observed. The same phenomenon was observed in the azo-2/TiOPc composite system (see Fig. 4). For instance, the wide peak of azo-2 at 2u= 5.1° was almost disappeared in the azo-2/TiOPc composite, and the peak of TiOPc at 2u =7.6° was shifted to 7.4° and became sharper in the composite. It suggests that the X-ray diffraction patterns of the blended composites are not simply overlapped by TiOPc and the two azo pigments, respectively, and that the blending leads to changes of the crystals of TiOPc or azo pigments, which would result in the form of the exciplexes from n-type TiOPc and p-type azo pigments, and increase the photoconductivities of the blended composites. .
4. Conclusions (1) The photoconductivities of azo-1/TiOPc and azo-2/TiOPc two composite systems are better than that of the pure azo pigments (azo-1 and azo-2) or TiOPc, and reach the best at azo/TiOPc = 4/6 (by wt). (2) Absorption spectra have been broadened from
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Fig. 4. X-Ray diffraction patterns of azo-2/TiOPc (1:1, by wt) composite (a), TiOPc (b), and azo-2 (c).
visible to near-infrared region by blending azo-1 or azo-2 with TiOPc. (3) Exciplexes might form by attractive forces of the opposite charges of n-type TiOPc particle surface and p-type azo pigment particle surface. (4)The form of exciplexes would be benefit to increase the photoconductivities of the azo-1/TiOPc and azo-2/TiOPc two composite systems.
Acknowledgements The work was financed by the National Natural Science Foundation of China (Grant No. 69890230 and 59603003).
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