A Novel Routine for the Fabrication of Y-Type Oxotitanium

Jun 2, 2016 - The high-gravity technology, carried out in a rotating packed bed (RPB) ... So far, as we know, no attempts have been done to study the ...
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A Novel Routine for the Fabrication of Y‑Type Oxotitanium Phthalocyanine Nanocrystals in High-Gravity Rotating Packed Beds Kai Wu,† Miao-Ling Xie,† Jian-Feng Chen,†,‡ and Yuan Le*,† †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: In this work, external circulation rotating packed bed (EX-RPB) was combined with internal circulation rotating packed bed (IN-RPB) for the preparation of Y-type oxotitanium phthalocyanine nanocrystals (Y-TiOPc NCs) for the first time. The operating conditions in EX-RPB and in IN-RPB were investigated in detail. X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet−visible (UV-vis) absorption spectroscopy analysis, and elemental analysis were performed, and xerographic properties were analyzed, to characterize the products. The experimental results indicated Y-TiOPc NCs with the average size of 6 nm were successfully prepared. The Q-band absorption peak of Y-TiOPc NCs centered at 782 nm, implying their potential application in the xerographic photoreceptors of laser printers. The photoreceptor fabricated with Y-TiOPc NCs exhibited an initial surface potential of 703.56 V, a photosensitivity of 0.166 μJ/cm2, a residual potential of 26.48 V, and a dark decay rate of 10.64 V/s. This work made a significant improvement to control the crystal form of nanoparticles at a large scale.

1. INTRODUCTION The fast development of novel nanostructured photoelectric devices demands higher-performance semiconductor materials. As one of the most important organic semiconductor materials, phthalocyanines (Pcs) have attracted much attention, because of their good photoelectrical properties associated with their chemical and thermal stability.1−6 Oxotitanium phthalocyanine (TiOPc) has been regarded as one of the best Pcs photoconductors, since it exhibits remarkable absorption and photosensitivity properties in the near-infrared wavelength region.7 TiOPc displays polymorphism and TiOPc in different crystal types exhibits very different photoconductive properties. Among them, Y-TiOPc has the highest photosensitivity to nearinfrared light, with a charge-carrier photogeneration quantum efficiency of >90% under high electric field, so it is widely used as charge-generation material in the xerographic photoreceptors of laser printers.8 Y-type oxotitanium phthalocyanine (Y-TiOPc) was conventionally prepared by transforming other crystal forms via organic solvents as crystal form regulators.7,8 The photosensitivity of Y-TiOPc is influenced by environmental factors, as well as the particle size. Small particles could afford high resolution in photoelectric devices (such as xerographic photoreceptors or image sensors) and would enhance the efficiency of photoinduced charge separation, because of the high surface-to-volume ratio.9 So far, many researchers have developed different methods to prepare the Y-TiOPc nanocrystals (NCs). Wang et al.10 proposed a colloidal-solutionmediated phase transition process to prepare Y-TiOPc NCs © 2016 American Chemical Society

with an average size of 3.4 nm, which were used in positively charged single-layered photoreceptors. Li et al.11 developed a microemulsion phase transfer method to prepare Y-TiOPc NCs with an average size of 2.3 ± 0.8 nm, which afforded an impressive photosensitivity of E1/2 = 0.064 μJ/cm2. However, the above methods were carried out on a small scale, which restricted their wider application and further commercialization. Therefore, it is necessary to develop an efficient process for scale-up production. The high-gravity technology, carried out in a rotating packed bed (RPB), is regarded as an effective process intensification technology, which can generate an acceleration of 1−3 orders of magnitude larger than the gravitational acceleration on Earth. In the high-gravity field, the liquids going through the packing are spread or split into very fine droplets, threads, and thin films in the porous packing. Thus, mass transfer and micromixing can be intensified greatly in the RPB.12,13 Until now, the high gravity technology, implemented by external circulation rotating packed bed (EX-RPB), was successfully applied in the fast reaction processes, such as absorption,14 distillation, polymer devolatilization,15 reactive crystallization, and so on.16−21 Because of the continuous operation and structure of EX-RPB, it is not appropriate for the slow reaction processes. In order to overcome this shortage, our group has designed a Received: Revised: Accepted: Published: 6753

January 28, 2016 June 1, 2016 June 2, 2016 June 2, 2016 DOI: 10.1021/acs.iecr.6b00397 Ind. Eng. Chem. Res. 2016, 55, 6753−6759

Article

Industrial & Engineering Chemistry Research

TiOPc NCs. Precipitation is a fast reaction process, while crystal transformation is a slow reaction process. In this paper, we applied EX-RPB to control the particle size of TiOPc nanoparticles and applied IN-RPB to fabricate uniform YTiOPc NCs. The basic principle of RPB is to create a highgravity environment via the action of centrifugal force, which is reflected by the high-gravity factor. The high-gravity factor (β) of RPB is determined by eq 1:

new experimental setup (internal circulation rotating packed bed (IN-RPB)) as the batch reactor for the slow reaction processes.22 Except for the mode of operation, the liquid in the EX-RPB and IN-RPB flows onto the inside edge of the rotator through the distributor and lifter, respectively. IN-RPB could effectively afford the high-gravity environment for the nucleation process and crystal growth process in the slow reaction processes, and avoid the poor crystallinity of nanoparticles prepared in EX-RPB. So far, as we know, no attempts have been done to study the effect of IN-RPB on the crystal transformation and relative products. Our group’s previous work reported the synthesis of 30−50 nm Y-TiOPc using EX-RPB, in which crystal transformation was conducted by maintaining circulation between EX-RPB and solvent tank (with crystal regulator) via pumping.23 Because of the fact that the precipitation of raw TiOPc and crystal transformation were carried out in the EX-RPB at the same time, the effects of these two processes on the particle size were not investigated in detail. In fact, the parameters of these two processes have a significant effect on the particle size and crystal form. In this work, we first proposed the combination of EXRPB and IN-RPB for preparation of nanodispersed Y-TiOPc NCs. The effects of operating conditions on particle size and crystal form in EX-RPB and IN-RPB were discussed in detail. Furthermore, the xerographic properties of the photoreceptors contained the as-prepared Y-TiOPc NCs were examined.

β=

ω 2r g

(1)

Furthermore, the mean high-gravity factor (β̅) in RPB can be calculated using eq 2: r

β̅ =

∫r 2 2πrβ dr 1

r2

∫r 2πr dr

=

1

2ω 2(r12 + r1r2 + r2 2) 3(r1 + r2)g

(2)

According to our previous work, the larger β value of RPB could contribute to enhancing the micromixing and achieving a more homogeneous environment.24 Meanwhile, in the precipitation process, the temperature has a significant effect on the crystal growth rate, which can be expressed as eq 3: dl = K g(Ci − C*)b dt

(3)

The value of b is usually between 1 and 3 and decreases with the reduction of temperature.25 It means that the lower temperature is beneficial to slowing the rate of crystal growth. Therefore, the highest β values of EX-RPB and IN-RPB, and the lower temperature in precipitation, were adopted in the study. In a typical experiment, the concentrated sulfuric acid solution of crude TiOPc (TiOPc/H2SO4) was prepared by adding the raw TiOPc (150 mg) to sulfuric acid (50 mL, 98 wt %), followed by filtering. Brij 56 (375 mg) and PEG 1000 (375 mg) were dissolved at a 1:1 (w/w) ratio in water (500 mL) to form an aqueous surfactant solution. The TiOPc/ H2SO4, the surfactant solution, and the crystal transformation solvent (DCE, 55 mL) were added into storage containers 10, 11, and 12, respectively. A TiOPc slurry was obtained by pumping TiOPc/H2SO4 and the surfactant aqueous solution into EX-RPB at a temperature of 0 °C at β = 146, using pumps 8 and 9. In the precipitation process of TiOPc, TiOPc slurry flowed back to storage container 11, where the slurry was repumped into the EX-RPB continuously until TiOPc/H2SO4 was completely consumed. After the precipitation process in EX-RPB, TiOPc slurry and DCE was pumped into IN-RPB through pumps 18 and 7. The crystal transformation process was carried out in IN-RPB at β = 198. Finally, the as-prepared Y-TiOPc NCs was filtrated and washed with deionized (DI) water and methanol. As a control experiment, the TiOPc nanoparticles were obtained by adding TiOPc/H2SO4 and surfactant aqueous solution into the stirred tank reactor (STR), which was composed of a beaker and a stirrer, under the same operation conditions. Y-TiOPc NCs were also prepared by mixing TiOPc slurry obtained in EX-RPB and dichloroethene (DCE) in STR for 30 min under other same operating conditions. 2.3. Fabrication of Organic Photoreceptors. Y-TiOPc NCs/PVB nanodispersion was formed by dissolving 0.15 g of Y-TiOPc NCs and 0.15 g of PVB in 10 mL of CYC/2-butanone (CYC/MEK, v/v = 6:1) via ultrasonication. The dispersion

2. EXPERIMENTAL SECTION 2.1. Materials and Equipment. The crude TiOPc was kindly supplied by Tianjin University. Polyethylene glycol monocetyl ether (Brij 56) and poly(vinyl butyral) (PVB) (moelcular weight of MW = 90 000−120 000) were purchased from Sinopharm Chemical Regent Co., Ltd. and used without further purification. Polyethylene glycol (PEG) (MW = 10 000) and cyclohexanone (CYC) were commercially available from Xilong Chemical Co., Ltd. and used as received. Polycarbonate (PC) (MW = 45 000) was purchased from Sigma−Aldrich (St. Louis, MO, USA). Water was purified through an ultrapure water system (Beijing Zhongyangyongkang Environmental Protection Technology Co., Ltd.). Other chemicals of analytical grade were commercially available and used without further purification. The experimental setup for preparation of YTiOPc NCs is shown in Figure 1. The characteristics of RPBs are listed in Table 1. 2.2. Preparation of Y-TiOPc NCs. The preparation of YTiOPc NCs includes two steps: liquid precipitation, to prepare TiOPc nanoparticles, and crystal transformation, to obtain Y-

Figure 1. Schematic diagram of reaction apparatus. [Legend: 1, 14: casing; 2, 16: packed rotator; 3, 20: motor; 4: liquid distributors; 5, 15: seal ring; 6, 17: outlet; 7, 8, 9, 18: pump; 10: solution storage container; 11: antisolvent storage container; 12: crystal transformation regulator storage container; 13: product storage container; 19: lifter.] 6754

DOI: 10.1021/acs.iecr.6b00397 Ind. Eng. Chem. Res. 2016, 55, 6753−6759

Article

Industrial & Engineering Chemistry Research Table 1. Characteristics of Rotating Packed Beds (RPBs) Used in This Study Rotor

Packing

Operation Parameter

equipment

inner diameter (mm)

out diameter (mm)

axial height (mm)

material

porosity (%)

rotating speed (rpm)

EX-RPB IN-RPB

40 34

84 100

15 34

wire mesh wire mesh

95 95

0−2800 0−2800

then was coated onto the aluminum plate (10 cm × 10 cm) with an undercoat layer (polyamide, 0.5 μm) to form a charge generation layer (CGL) ∼1 μm thick, followed by drying at 333 K for 30 min. The charge-transport layer (CTL, ∼23.2 μm) was fabricated with PC blending N,N′-diphenyl-N,N′-di(3-tolyl)-4benzidine with a weight ratio of 1:1 by coating on the top of CGL, then dried at 353 K for 60 min. 2.4. Characterization. 2.4.1. X-ray Diffraction (XRD) Analysis. The crystal type of TiOPc was performed using a XRD-6000 diffractometer (Shimadzu, Japan). The measuring unit consisted of a rotating anode in transmission mode, with a specification that Cu Kα radiation was generated at an accelerating voltage of 30 mA and voltage of 40 kV. The samples were scanned from 5° to 50° with a scan speed of 5°/ min. The XRD samples were prepared by coating the Y-TiOPc NCs/CYC/MEK nanodispersion onto glass sheets. Finally, the glass sheets were dried under vacuum at room temperature. 2.4.2. Transmission Electron Microscopy (TEM) Analysis. TEM photographs were using a TEM system (Model JEM2100, JEOL, Japan). The samples were made by depositing the fresh Y-TiOPc NCs/CYC/MEK nanodispersion on copper grid dropwise and then drying the sample under vacuum at room temperature. 2.4.3. Ultraviolet−Visible Absorption Spectroscopy (UVvis) Analysis. UV-vis spectra were recorded between 300 and 900 nm on a Lambda 950 UV-vis spectrophotometer (PerkinElmer, USA). The samples were prepared by diluting the Y-TiOPc NCs/CYC/MEK nanodispersion to a certain concentration. 2.4.4. Elemental Analysis. Elemental analysis of product power was measured on an Vario EL elemental analyzer (Elementar Analysensyteme GmbH, Germany). 2.4.5. Photoconductivity Measurement. Photoconductivity measurements were made on a photoconductive printer test system (Model QEA PDT2000-LTM, Quality Engineering Associates, Inc., USA). In order to satisfy with the demand of the laser printer, the exposure wavelength is 780 nm, which is obtained through a 150 W halogen lamp in combination with a narrow-band filter of 780 nm. From the decay rate of surface potential in the dark of the organic photoconductor (OPC) and photoinduced discharging curves, the following parameters could be obtained: the values of initial surface potential (V0), the dark decay rate (Rd), exposure energy required to discharge to half of V0 (E1/2), and the residual potential (VR), which is the lowest photodischarged surface potential achievable for a particular OPC.

TiOPc precipitation without the stabilizer are shown in Figure 2. The TiOPc nanoparticles ranged from 10 nm to 90 nm, and

Figure 2. (A) TEM photograph of TiOPc particles precipitated in EXRPB without stabilizer; (B) the corresponding particle size distributions of TiOPc particles.

the average size of TiOPc nanoparticles was 45 nm. Meanwhile, the particles were preformed on an irregular thin sheet with severe agglomeration. In comparison, the TiOPc particles stabilized by the combination of PEG 1000 and Brij 56 exhibited the regular morphology and average size decreased to 4 nm (Figure 3). The influence of surfactants on the particle

Figure 3. (A) TEM photograph of TiOPc particles precipitated in EXRPB with stabilizer; (B) the corresponding particle size distributions of TiOPc particles.

size of TiOPc nanoparticles during the liquid precipitation process could be explained by these two points: (1) retardation in the growth of particles by the absorption of stabilizer on the surface of particles; and (2) protection from particle aggregation by the steric hindrance.26 The precipitation process in EX-RPB is influenced by the solvent/antisolvent (S/AS) ratio, since the driving force of liquid precipitation is the supersaturation of a solution induced by mixing of an antisolvent. To research the influence of S/AS ratio on the mean size of TiOPc nanoparticles, the flow rate of surfactant solution was fixed at 400 mL/min, while the flow rate of TiOPc/H2SO4 was regulated to achieve various S/AS ratios. Figure 4 shows TEM images of TiOPc nanoparticles prepared at different S/AS ratios. As shown in Figure 4, it could be clearly observed that the TiOPc nanoparticle size decreased from 98 nm to 4 nm with the flow rate of TiOPc/H2SO4 decreased from 4 mL/min to 2 mL/min. However, when TiOPc/H2SO4 flow rate was further decreased, only a slight change in the mean size was observed. Such a complex

3. RESULTS AND DISCUSSION 3.1. Liquid Precipitation in the EX-RPB. Liquid precipitation method is able to take effective control of the particle size of nanoparticles. Commonly, the ultrafine particles are prepared in the presence of surfactants, individually or in combination, as the stabilizer.24 To investigate the effect of stabilizer on the mean size of TiOPc nanoparticles, the raw TiOPc was precipitated in EX-RPB using stabilizer or not, respectively. The TEM image and particle size distributions of 6755

DOI: 10.1021/acs.iecr.6b00397 Ind. Eng. Chem. Res. 2016, 55, 6753−6759

Article

Industrial & Engineering Chemistry Research

TiOPc is observed at 7.46° and 26.16° (2θ), respectively.29 Enokida reported that the X-ray pattern of Y-TiOPc has three obvious characteristic peaks located at 9.6°, 24.2°, and 27.3°, in which the most intense diffraction peak is observed at 27.3°.30 Generally, these three peaks have been used to judge the YTiOPc in many published papers.10,11,31,32 Figure 6 illustrates

Figure 4. TEM images of TiOPc nanoparticles prepared at different S/ AS ratios: (A) S/AS = 4/400, (B) S/AS = 3/400, (C) S/AS = 2/400, and (D) S/AS = 1.5/400.

phenomenon could be attributed to the fact that a decrease of S/AS ratio resulted in a higher supersaturation level. Accordingly, a higher level of supersaturation led to a faster nucleation rate and a smaller critical nucleus size. On the other hand, the decreased S/AS ratio virtually decreased the solute concentration on the formed TiOPc particle surface. Therefore, according to eq 3, the decreased value of Ci − C* resulted in a slower particle growth rate, thereby led to a smaller ultimate mean particle size. With the S/AS ratio decreased, the value of Ci − C* achieved a relatively constant level, and, hence, no obvious further size decrease was observed.27,28 Under the same conditions, TiOPc particles precipitated with the stabilizer in the STR. As shown in Figure 5, the particle size

Figure 6. XRD patterns of TiOPc prepared in IN-RPB at different temperatures.

the effect of the crystal transformation temperature on the crystal pattern of TiOPc, using DCE as a crystal form regulator in the IN-RPB. When TiOPc underwent crystal transformation at temperatures of 25, 35, and 45 °C, the product exhibited the peak at 7.70°, 7.74°, and 7.66°, respectively, which were consistent with the characteristic peaks of α-TiOPc. However, these three products also gave the diffraction peaks at characteristic peaks of Y-TiOPc, which suggested that the products were a mixture of α-TiOPc and Y-TiOPc. When the temperature increased to 50 °C, the characteristic peaks of αTiOPc disappeared and the TiOPc NCs exhibited a typical Ytype crystal featured with the diffraction peaks at 9.67°, 24.28°, and 27.49°. With the temperature increased to 55 °C, the TiOPc NCs exhibited at the characteristic peaks of Y-TiOPc (9.58°, 24.18°, 27.4°) as well as the characteristic peak of βTiOPc (26.32°), indicating that the TiOPc NCs were a mixture of Y-TiOPc and β-TiOPc. Therefore, it could be concluded that the crystal pattern of TiOPc NCs changed from α-TiOPc to YTiOPc and finally formed β-TiOPc as the temperature increased. 3.3. Effect of Transformation Time on Crystal Pattern in IN-RPB. Figure 7 shows the XRD patterns of TiOPc prepared for different times in the IN-RPB, using DCE as a crystal form regulator. We observed that the XRD pattern of TiOPc NCs prepared for 5 min exhibited the peak at 7.70°, which revealed the presence of α-TiOPc. The sharp XRD peaks of product prepared for 15 min at 9.54°, 24.17°, and 27.37° indicated that Y-TiOPc NCs were successfully prepared. The high-resolution transmission electron microscopy (HRTEM) image of Y-TiOPc NCs in Figure 8A showed an interplanar distance of 0.32 nm, which was in accordance with the lattice spacing between the neighboring planes of Y-TiOPc at the 27.3° diffraction peak (calculated by Bragg’s law).33 As the time increased to 30 min and 1 h, the products exhibited peaks at 9.56°, 24.19°, 27.36° and 9.58°, 24.20°, 27.38°, respectively, which indicated that TiOPc NCs were still in the type of Y.

Figure 5. (Left) TEM photograph of TiOPc particles precipitated in the stirred tank reactor (STR); (right) corresponding particle size distributions of TiOPc particles.

clearly increased from 4 nm to 16 nm, and particle size distributions became broader than that prepared in EX-RPB. Obviously, micromixing performance in EX-RPB is helpful to create the homogeneous environment, which is favorable to prepare nanoparticles with small size and narrow size distributions in the fast reaction process. 3.2. Effect of Transformation Temperature on Crystal Pattern in IN-RPB. TiOPc consists of relatively planar molecules that, when crystallized, have a tendency to produce polymorphs that have unique spectral characteristics. The most intense diffraction peak of the X-ray patterns of α-TiOPc and β6756

DOI: 10.1021/acs.iecr.6b00397 Ind. Eng. Chem. Res. 2016, 55, 6753−6759

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Industrial & Engineering Chemistry Research

crystal, nucleation of stable-crystal, and crystal growth.34 The nucleation process and the crystal growth process are driven by supersaturation.35 Therefore, homogeneous supersaturation is necessary to prepare smaller nanoparticles with narrow particle size distributions during the crystal transformation process. INRPB could extremely intensify mass transfer and micromixing, improved by 1−3 orders of magnitude than STR, to reach the region where the characteristic micromixing time is less than the nucleation induction time and induce higher surpersaturation. As a result, the nucleation rate was accelerated and particle growth was suppressed, thereby generating nanoparticles with smaller size and narrower particle size distributions. 3.5. Characterization of Y-TiOPc NCs. Elemental analysis of the Y-TiOPc NCs prepared by high-gravity technology was performed to investigate the purity of Y-TiOPc NCs, as shown in Table 2. The sulfur content in the product was 0.078%. The Table 2. Elemental Analysis Results of Y-TiOPc NCs Figure 7. XRD patterns of TiOPc prepared in IN-RPB at 50 °C at different times.

Composition (%) theory experiment

N

C

H

S

18.925 19.440

66.120 66.640

3.034 2.780

0.098 0.000

stabilizers (Brij 56 and PEG 10000) contain carbon, hydrogen, and oxygen; however, the carbon content of the product was 66.12% below the theoretical value. These results indicated that SO4−2 and the stabilizers were washed away. Because of the hydration water in the product, the hydrogen content was higher and the carbon content was lower than the theoretical value.36 We used elemental nitrogen as the reference, and we determined that the purity of Y-TiOPc was ∼97.6%. The UV-vis absorption spectra of a Y-TiOPc NCs/CYC/ MEK colloidal solution containing Y-TiOPc NCs with different average sizes are shown in Figure 9. UV-vis spectra measure-

Figure 8. TEM photographs of Y-TiOPc NCs prepared in different reactors: (A) IN-RPB and (B) STR. Particle size distributions of YTiOPc NCs prepared in different reactors: (C) IN-RPB and (D) STR.

However, the small peak of TiOPc prepared for 2 h at 26.48° matched that of β-TiOPc. It meant that Y-TiOPc transformed to β-TiOPc. As the crystal transformation time increased, it exhibited a trend similar to that observed as the temperature increasedthe crystal pattern of TiOPc NCs transformed from α-TiOPc to Y-TiOPcand β-TiOPc was finally obtained. 3.4. Comparison of Y-TiOPc NCs Prepared under Different Reactors. The morphology of Y-TiOPc NCs prepared in the IN-RPB is illustrated in Figure 8A. We could observe that Y-TiOPc NCs with an average size of 6 nm were almost nanodispersed. In order to investigate the advantages of mass transfer performance and homogeneous environment in IN-RPB on the crystal transformation process, Y-TiOPc NCs were prepared in the STR using DCE as a crystal form regulator for 30 min at a temperature of 50 °C. The average size of Y-TiOPc NCs prepared in the STR was 100 nm. Furthermore, the particle size distributions of Y-TiOPc NCs prepared in the STR became broader. as shown in Figures 6C and 6D. The results could be attributed to intensified mass transfer and micromixing in the IN-RPB. Solvent-induced crystal transformation includes three steps: dissolution of meso-

Figure 9. UV-vis absorption spectra of Y-TiOPc NCs/CYC/MEK colloidal solution containing Y-TiOPc with different average sizes.

ments revealed that, in the Y-TiOPc NCs/CYC/MEK colloidal solution, an equilibrium between the solvent-stabilized YTiOPc NCs (characterized by the broad absorption band in the near-infrared (NIR) region) and a very small quantity of dissolved TiOPc molecules (characterized by the absorption peak centered at ca. 692 nm) was established. There was no difference in the absorption band at ∼692 nm, which was attributed to the dissolved TiOPc molecules; however, the absorption peaks of Y-TiOPc NCs were evidently blue-shifted from 360 and 816 nm to 356 and 782 nm as the size of Y6757

DOI: 10.1021/acs.iecr.6b00397 Ind. Eng. Chem. Res. 2016, 55, 6753−6759

Article

Industrial & Engineering Chemistry Research Notes

TiOPc decreased from 100 to 6 nm. Commonly, it is convincing that the interactions between organic molecules are so weak that only a narrow energy bandwidth can form. That is why the properties of organic solids do not change much with their particle size. However, there are strong interactions between two adjacent TiOPc molecules, so the energy gap will be broaden when the particle size decreases, resulting in the blue-shifts of the light-absorption bands. YTiOPc NCs prepared by the present methods were surfactantfree and were dispersed in the same solvent; therefore, the observed blue-shifts should be derived solely from the decrease in the size of the NCs. We confirmed that the Y-TiOPc, with an average size of 6 nm, was more suitable to be applied in the laser printers. The double-layered photoreceptors were fabricated with YTiOPc/PVB nanodispersion containing the Y-TiOPc with different average sizes. The characteristic parameters of the photoreceptors are listed in Table 3. A good photoreceptor that

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Key Basic Research Program of China (No. 2015CB932100), National Key Technology Support Program (No. 2014BAE12B01), and Beijing Municipal Science and Technology Project (No. Z151100003315005).

■

Table 3. Photoelectric Properties of Photoreceptors Based on Y-TiOPc NCs with Different Average Sizes Value

Greek Symbols

property

particle size = 6 nm

particle size = 100 nm

V0 (V) Rd (V/s) E50 (μJ/cm2) VR (V)

703.56 10.64 0.166 26.48

690.05 19.92 0.262 117.10

■

ω = rotor speed (rad/s) β = high-gravity level (dimensionless) β̅ = mean high-gravity level (dimensionless)

REFERENCES

(1) Martinez-Diaz, M. V.; de la Torre, G.; Torres, T. ChemInform abstract: lighting porphyrins and phthalocyanines for molecular photovoltaics. Chem. Commun. 2010, 46, 7090−7108. (2) Melville, O. A.; Lessard, B. H.; Bender, T. P. Phthalocyanine based organic thin-film transistors: A review of recent advances. ACS Appl. Mater. Interfaces 2015, 7, 13105−13118. (3) Minch, B. A.; Xia, W.; Donley, C. L.; Hernandez, R. M.; Carter, C.; Carducci, M. D.; Dawson, A.; O'Brien, D. F.; Armstrong, N. R.; et al. Octakis (2-benzyloxyethylsulfanyl) copper(II) phthalocyanine: A new liquid crystalline discotic material with benzyl-terminated, thioether-linked side chains. Chem. Mater. 2005, 17, 1618−1627. (4) Deng, Z. B.; Lü, Z. Y.; Chen, Y. L.; et al. Aluminum phthalocyanine chloride as a hole injection enhancer in organic light-emitting diodes. Solid-State Electron. 2013, 89, 22−25. (5) Jiang, Z.; Shao, J. W.; Yang, T. T.; Wang, J.; Jia, L. Pharmaceutical development, composition and quantitative analysis of phthalocyanine as the photosensitizer for cancer photodynamic therapy. J. Pharm. Biomed. Anal. 2014, 87, 98−104. (6) Zanjanchi, M. A.; Ebrahimian, A.; Arvand, M. Sulphonated cobalt phthalocyanine−MCM-41: An active photocatalyst for degradation of 2,4-dichlorophenol. J. Hazard. Mater. 2010, 175, 992−1000. (7) Weiss, D. S.; Abkowitz, M. Advances in organic photoconductor technology. Chem. Rev. 2010, 110, 479−526. (8) Law, K. Y. Organic photoconductive materials: Recent trends and developments. Chem. Rev. 1993, 93, 449−486. (9) Niimi, T.; Umeda, M. Electron transfer between a photoexcited azo pigment particle and an electron donor molecule in a solid system. J. Phys. Chem. B 2002, 106, 3657−3661. (10) Liang, D.; Peng, W.; Wang, Y. Solvent-stabilized Y-type oxotitanium phthalocyanine photoconductive nanoparticles: Preparation and application in single-layered photoreceptors. Adv. Mater. 2012, 24, 5249−5253. (11) Li, X. L.; Xiao, Y.; Wang, S. R.; Li, X. G. Ultra-photosensitive Ytype titanylphthalocyanine nanocrystals: Preparation and photoelectric properties. Dyes Pigm. 2016, 125, 44−53. (12) Chen, J. F.; Zheng, C.; Chen, G. A. Interaction of macro- and micromixing on particle size distribution in reactive precipitation. Chem. Eng. Sci. 1996, 51, 1957−1966.

has high V0 (>500 V), low E1/2, low Rd, and low VR would be beneficial for xerographic use. This table shows that VR decreased from 117.10 V to 26.48 V and Rd gradually decreased from 19.92 V/s to 10.64 V/s as the average size of Y-TiOPc NCs in the Y-TiOPc NCs/PVB film decreased. The value of E1/2 changed from 0.262 μJ/cm2 to 0.166 μJ/cm2 when the average size of Y-TiOPc decreased from 100 to 6 nm. We affirmed that the particle size had great influence on the performance of the photoconductor. As a result, the highgravity method is a promising approach to prepare the Y-TiOPc NCs applied as the charge generation material in the photoreceptors.

4. CONCLUSIONS Y-type oxotitanium phthalocyanine nanocrystals (Y-TiOPc NCs) were successfully prepared by combining an external circulation rotating packed bed (EX-RPB) and an internal circulation rotating packed bed (IN-RPB), using dichloroethene (DCE) as a crystal form regulator. The TiOPc nanoparticles, with an average size of 4 nm, were precipitated in the EX-RPB, and the uniform Y-TiOPc NCs with an average size of 6 nm were successfully prepared in the IN-RPB. The Qband absorption (600−800 nm) of the as-prepared Y-TiOPc NCs were centered at 782 nm, and the photosensitivity was 0.166 μJ/cm2. Therefore, we significantly improved the rapid crystal transformation method for the mass production of YTiOPc NCs, applied in the organic photoconductor (OPC) and other manufacture of high-performance photoelectric devices.

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NOTATION r = radius of packing (m) r1 = inner radius of packing (m) r2 = outer radius of packing (m) g = acceleration of gravity (m/s2) b = growth order (dimensionless) l = crystal size (m) t = time (s) Kg = crystal growth rate constant (dependent on b) Ci = solute concentration on the crystal surface (mol/L) C* = saturation concentration (mol/L)

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*Tel.: +86-10-64447274. Fax: +86-10-64423474. E-mail: [email protected]. 6758

DOI: 10.1021/acs.iecr.6b00397 Ind. Eng. Chem. Res. 2016, 55, 6753−6759

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Industrial & Engineering Chemistry Research

(35) Chen, J. F.; Wang, Y. H.; Guo, F.; Wang, X. M.; Zheng, C. Synthesis of nanoparticles with novel technology: high gravity reactive precipitation. Ind. Eng. Chem. Res. 2000, 39, 948−954. (36) Oda, Y.; Homma, T.; Fujimaki, Y. Near-infrared sensitive photoreceptors incorporating a new polymorph of oxotitanium phthalocyanine. Denshi Shashin Gakkaishi 1990, 29, 250−258.

(13) Yang, H. J.; Chu, G. W.; Zhang, J. W.; Shen, Z. G.; Chen, J. F. Micromixing efficiency in a rotating packed bed: experiments and simulation. Ind. Eng. Chem. Res. 2005, 44, 7730−7737. (14) Yi, F.; Zou, H. K.; Chu, G. W.; Shao, L.; Chen, J. F. Modeling and experimental studies on absorption of CO2 by benfield solution in rotating packed bed. Chem. Eng. J. 2009, 145, 377−384. (15) Chen, J.-F.; Gao, H.; Zou, H.-K.; Chu, G.-W.; Zhang, L.; Shao, L.; Xiang, Y.; Wu, Y. X. Cationic polymerization in rotating packed bed reactor: experimental and modeling. AIChE J. 2010, 56, 1053−1062. (16) Chen, J. F.; Shao, L. Mass production of nanoparticles by high gravity reactive precipitation technology with low cost. China Particuol. 2003, 1, 64−69. (17) Yang, H. J.; Chu, G. W.; Zhang, J. W.; Shen, Z. G.; Chen, J. F. Micromixing efficiency in a rotating packed bed: experiments and simulation. Ind. Eng. Chem. Res. 2005, 44, 7730−7737. (18) Wang, D. G.; Guo, F.; Chen, J. F.; et al. Preparation of nano aluminium trihydroxide by high gravity reactive precipitation. Chem. Eng. J. 2006, 121, 109−114. (19) Zhao, R. H.; Li, C. P.; Guo, F.; et al. Scale-up preparation of organized mesoporous alumina in a rotating packed bed. Ind. Eng. Chem. Res. 2007, 46, 3317−3320. (20) Yang, Q.; Wang, J. X.; Guo, F.; et al. Preparation of hydroxyaptite nanoparticles by using high-gravity reactive precipitation combined with hydrothermal method. Ind. Eng. Chem. Res. 2010, 49, 9857−9863. (21) Chen, J.; Li, Y.; Wang, Y.; et al. Preparation and characterization of zinc sulfide nanoparticles under high-gravity environment. Mater. Res. Bull. 2004, 39, 185−194. (22) Chen, J. F.; Hu, T. T.; Zhao, H.; Chu, G. W.; et al. A high-gravity field device: Internal rotating packed bed. Chin. Patent CN201260790Y, 2009. (23) Shao, L.; Yu, Y. X.; Bian, S. G.; Chen, J. F.; Li, X. Synthesis of nanosized Y-type TiOPc by a high gravity method. J. Mater. Sci. 2005, 40, 4373−4374. (24) Lu, X. W.; Wu, W.; Chen, J. F.; et al. Preparation of polyaniline nanofibers by high gravity chemical oxidative polymerization. Ind. Eng. Chem. Res. 2011, 50, 5589−5595. (25) Yu, L.; Li, C. X.; Le, Y.; et al. Stabilized amorphous glibenclamide nanoparticles by high-gravity technique. Mater. Chem. Phys. 2011, 130, 361−366. (26) Dinunzio, J. C.; Miller, D. A.; Yang, W.; Mcginity, G. W.; Williams, R. O. Amorphous compositions using concentration enhancing polymers for improved bioavailability of itraconazole. Mol. Pharmaceutics 2008, 5, 968−980. (27) Zhang, Z. B.; Shen, Z. G.; Wang, J. X.; Zhao, H.; Chen, J. F.; Yun, J. Nanonization of megestrol acetate by liquid precipitation. Ind. Eng. Chem. Res. 2009, 48, 8493−8499. (28) Kuang, Y. Y.; Zhang, Z. B.; Xie, M. L.; Wang, J. X.; Le, Y.; Chen, J. F. Large-scale preparation of amorphous cefixime nanoparticles by antisolvent precipitation in a high-gravity rotating packed bed. Ind. Eng. Chem. Res. 2015, 54, 8157−8165. (29) Okada, O.; Klein, M. L. Phase transition and water molecules in titanylphthalocyanine phase Y crystal. Phys. Chem. Chem. Phys. 2001, 3, 1530−4. (30) Enokida, T.; Hirohashi, R.; Nakamura, T. Polymorphs of oxotitanium phthalocyanine and their applications for photoreceptors. J. Imaging Sci. Technol. 1990, 34, 234−242. (31) Chao, W.; Zhang, X.; Xiao, C.; et al. An excellent single-layered photoreceptor composed of oxotitanium phthalocyanine nanoparticles and an insulating resin. J. Colloid Interface Sci. 2008, 325, 198−202. (32) Wang, W. B.; Li, X. G.; Wang, S. R.; et al. The preparation of high photosensitive TiOPc. Dyes Pigm. 2007, 72, 38−41. (33) Mizuguchi, J.; Yamakami, H.; Kojima, Y.; Sasaki, C.; Osano, Y.; et al. Structural characterization of Y-like titanylphthalocyanine. J. Imaging. Sci. Technol. 2003, 47, 25−29. (34) Manoli, F.; Dalas, E. Spontaneous precipitation of calcium carbonate in the presence of ethanol, isopropanol and diethylene glycol. J. Cryst. Growth 2000, 218, 359−364. 6759

DOI: 10.1021/acs.iecr.6b00397 Ind. Eng. Chem. Res. 2016, 55, 6753−6759