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Journal of Colloid and Interface Science213,270–272 (1999)Article ID jcis.1999.6132, available online at http://www.idealibrary.com on

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LETTER TO THE EDITOR

Preparation and Characterization of Nanoscopic OrganicSemiconductor of Oxovanadium Phthalocyanine

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Nanoscopic particles of oxovanadium phthalocyanine (VOPc,hase-II) with different particle sizes are prepared by ag-regation of VOPc molecules in the presence of surfactants forhe first time. A size effect in the region of nanoscale wasbserved in the UV–visible spectra. The photoconductivity ofhe nanoscopic VOPc is much higher than that of bulkOPc. © 1999 Academic Press

Key Words: phthalocyanine; organic semiconductor colloid;anoscopic photoconductor.

A great deal of attention has been paid to the preparation and cherization of nanostructure systems during the past two decades (1–mportant reason for this is that nanoscopic materials may display oplectronic, catalytic, and structural properties which are not preseither isolated molecules or macroscopic solids (4 –7). In previous wttention is devoted to the nanoscopic materials of metals and s

norganic semiconductors (1–10), whereas reports on the preparatioroperties of nanoscopic organic semiconductors have so far been sfew works about the size effect on the absorption properties and c

eneration efficiency of metal-free phthalocyanine particles (averageicle size larger than 100 nm) prepared by the traditional milling meave been reported (11, 12). It has been concluded that smalleigment particles exhibit higher carrier generation efficiency withinxamined particle size region. Thus, it is reasonable to expect that fecrease in the particle size of phthalocyanine may greatly improvehotoconductivity. However, it seems impossible to prepare supeanoscopic particles with a narrow size distribution by using the traditilling method. On the other hand, because the change of crystalccurs during the milling process which may cause the shift of theisible spectra peaks (13), the observed size effect on the absoroperty mentioned above (11) can hardly be ascribed to different paizes only.In this work, we proposed a new method for preparing nanosc

rganic semiconductors of oxovanadium phthalocyanine (VOPc),orming a colloidal solution of VOPc by dispersing a concentrated H2SO4

olution of VOPc into water containing surfactant. This method charazed by the aggregation of VOPc molecules in the presence of surfacifferent from the traditional milling process which produces VOPc ples with large particle size from bulk VOPc. The method proposed hexpected to be of universal significance for preparing many kindanoscopic phthalocyanine materials, such as H2Pc, TiOPc, CuPc, annPc. Nanoscopic organic semiconductors (photoconductor) of V

nano-VOPc) with small particle size and narrow size distribution w

repared for the first time and the size effect on the absorption property a

270021-9797/99 $30.00opyright © 1999 by Academic Pressll rights of reproduction in any form reserved.

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hotoconductivity of the nanoscopic VOPc organic semiconductorhesame crystal form was studied. The VOPc particles were characty transmission electron microscopy (TEM), X-ray powder diffracXRD), and UV–visible spectra.

VOPc was synthesized and analyzed as previously (14). Bulk Vn phase-II form (B-VOPc) was obtained by acid-pasting the abroducts in concentrated H2SO4 into an ice–water mixture. In a typicxperiment for the preparation of VOPc colloids, a known quantity (0.6 mmol) of surfactant, hexadecyltrimethyl ammonium bromC16H33N(CH3)3Br)(CTAB), was added to 600 ml water and heated at 5nder stirring to get a transparent solution. VOPc (0.15 g, 0.26 missolved in 50 ml concentrated H2SO4 was added dropwise to the surfa

ant solution under vigorous stirring at 0 –5°C. A transparent blue collolution was obtained and washed to neutral with water by using anlter under stirring. A trace amount of H2SO4 was removed by anioxchange resin. By changing the surfactant and the concentration of

n concentrated H2SO4, nanoscopic VOPc with different particle sizes wrepared.Figure 1 shows the TEM pictures and particle size distributions o

anoscopic VOPc colloidal particles and the bulk VOPc. The colloOPc particles protected by CTAB (Fig. 1a) are mainly distributed wi

he range from 2 to 3.5 nm with an average diameter of 2.3 nm, wolloidal VOPc particles protected by polyoxyethylene(20) stearyl ePOSE) (Fig. 1b) have a wider distribution from 17 to 40 nm withverage diameter of 26 nm. Particles in the bulk VOPc have a veryistribution from 50 to 130 nm (Fig. 1c).In the solid state of VOPc, three polymorphs can be obtained an

amed phase I, II, and III, respectively (15). X-ray power diffracatterns of the B-VOPc and nano-VOPc materials were measured. B

hem show diffraction peaks at Bragg angle (2u) of 7.5, 10.1, 12.6, 22.44.2, 25.3, and 28.6. This clearly indicates that both of them are in ph

orm. The diffraction peaks of nano-VOPc are broader than thos-VOPc. The average crystal grain size of the B-VOPc calculated fromcherrer equation is 28 nm, while that of the nano-VOPc prepared byOSE as the protective agent is 12 nm.Phthalocyanine and other functional dyes can form molecular aggre

n solution state or molecular crystals in solid states with close inteecular spacing and strong coupling interactions (15, 16). The counteraction can in turn result in the splitting of the ground and excnergy levels of molecules in the aggregates or solid. As a result oplitting, spectral shifts on the order of more than6100 nm of moleculaggregates containing two or a few dye molecules can be observed r

o the absorption peaks of monomer in solution (16). However, the ince of the size of nanoscopic organic semiconductors on the absoroperty has not been clearly revealed, although the quantum size efanoscopic metals and inorganic semiconductors has been intentudied.

ndFigure 2 shows the UV–visible spectra of B-VOPc and nano-VOPc of

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271LETTER TO THE EDITOR

nterests. It is obvious that the absorption bands of nano-VOPc shiftlue region relative to those of B-VOPc, and the blue shift in the UV reDlac 5 9 nm, Dlbc 5 6 nm, whereDlac represents the difference of tbsorption bands between spectrum a and c) is larger than thatear-IR region (Dlac 5 7 nm, Dlbc 5 4 nm). The surfactants themselvsed in this work do not cause any shift of the absorption peaks of Vs proved by UV–vis spectra examination and the nano-VOPc and B-

FIG. 1. Transmission electron micrographs of VOPc colloids: (a) CTrotected VOPc, (b) POSE-protected VOPc colloid, and (c) B-VOPc di

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re in the same crystal form; thus the observed blue shifts should be drom the change in particle size.

VOPc is an important organic photoconductor. To study the relahip between the particle size of VOPc and its photoconductivity, dayered photoreceptors using nano-VOPc and B-VOPc as the ceneration materials and 4-diethylaminobenzaldehyde diphenrazone as the transport material were prepared. Photoconductiv

he prepared photoreceptors is measured by an electrostatic paper aModel SP-428, Kawaguchi Electric Works Ltd.) under white lixposure (5 lux). The photoconductivity of nano-VOPc is much hi

han that of B-VOPc. The half-decay exposure (E1/ 2) and E1/5 of theual-layered photoreceptor of nano-VOPc (particle size: 26 nm)harge generation material are 0.75 and 1.7 luxz s, whereas those usi-VOPc as the charge generation material are 1.4 and 3.4 luxz s, respec

ively. We believe that the huge surface area of the nanoscopic Vhotoconductor plays an important role in the improvement of photouctivity, although there may exist other influence factors derived fromecrease in the particle size of VOPc photoconductor. We thinkanoscopic particles of phthalocyanine obtained here would provideossibility for the preparation of highly efficient organic photoconduc

n other photoelectric mutual conversion devices, such as electrolument devices, organic electrode, and image-forming devices, it is helpse nanoscopic particles of phthalocyanine to obtain uniform organiclms.

ACKNOWLEDGMENTS

The authors thank the TEM lab of Peking University for their helphe TEM measurements, and Miss. Chunmei Xu in the Institute of Cstry, Chinese Academy of Sciences, for help in the photoconductivityork.

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FIG. 2. UV–vis spectra of (a) CTAB-protected VOPc, (b) POSE-proteOPc colloid, and (c) B-VOPc dispersion.

therein]

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272 LETTER TO THE EDITOR

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Yuan Wang*,1

Kai Deng*Linlin Gui*Youchi Tang*Jinwei Zhou†Liangyuan Cai†Jiabai Qiu†Deyuan Ren†Yanqiao Wang†

Institute of Physical Chemistryeking Universityeijing 100871eople’s Republic of ChinaInstitute of Chemistryhinese Academy of Scienceseijing 100080eople’s Republic of China

Received October 6, 1998; accepted January 28, 1999

1 To whom correspondence should be addressed. E-mail: wan

pc.pku.edu.cn.