Hybridization of thiol-functionalized poly(phenylacetylene) with cadmium sulfide nanorods: Improved miscibility and enhanced photoconductivity

Article abstract of DOI:10.1039/b617595c

Molecules of a thiol-functionalized phenylacetylene derivative were assembled on the CdS nanorod surface and copolymerized with phenylacetylene, affording an inorganic semiconductor-conjugated polymer hybrid with excellent solubility and high photoconduct

Full text of DOI:10.1039/b617595c

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Hybridization of thiol-functionalized poly(phenylacetylene) with
cadmium sulfide nanorods: improved miscibility and enhanced
photoconductivity{
Hai-Peng Xu,a Bo-Yu Xie,a Wang-Zhang Yuan,a Jing-Zhi Sun,*a Feng Yang,a Yong-Qiang Dong,ab
Anjun Qin,ab Shuang Zhang,a Mang Wanga and Ben Zhong Tang*ab
Received (in Cambridge, UK) 4th December 2006, Accepted 14th February 2007
First published as an Advance Article on the web 23rd February 2007
DOI: 10.1039/b617595c
accomplished the hybridization of CdS nanorods with poly-
(phenylacetylene) (PPA) chains (Scheme 1). Nanorods, rather than
nanoparticles, were used in this study because the former naturally
provide a directed path for charge transport.1 PPA is chosen as the
Molecules of a thiol-functionalized phenylacetylene derivative
were assembled on the CdS nanorod surface and copolymer-
ized with phenylacetylene, affording an inorganic semiconduc-
tor–conjugated polymer hybrid with excellent solubility and
high photoconductivity.
parent form of polymer because it is
a well-investigated
photoconductive polyacetylene derivative.5 It was envisaged that
the nanohybrid of CdS and PPA would exhibit high photo-
conductivity. In this Communication, we prove this is indeed the
case.
Nanoscopic hybridization of inorganic semiconductors with
organic polymers offers the possibility to create new hybrids with
combined advantageous attributes from the two constituents (e.g.,
high charge mobility of inorganics and ready processability of
organics), which may find technological applications as active
materials in the construction of advanced photoelectronic devices
such as photovoltaic cells (PVCs) or solar cells for the generation
of ‘‘green electricity’’ by harvesting light from the sun.1 Promising
results have been obtained along this line of research endeavor.
CdSe nanorods, for example, were hybridized nanodimensionally
with a soluble polythiophene derivative, whose PVCs exhibited
high external quantum efficiencies.1a Nanocomposites of ZnO
nanocrystals and a poly(phenylenevinylene) derivative were made
and their PVCs showed impressive performance.1b Polyacetylenes
are the best-known conjugated polymers,2 but their hybridization
with inorganic semiconductors remains an almost unexplored area
of research.3
The CdS nanorods used in this study were prepared by a
solvothermal reaction according to a published procedure (ESI{).6
A phenylacetylene (PA) monomer carrying a thiol end group (M1)
was synthesized via the multi-step reaction route shown in Scheme
S1 (ESI{). The intermediate obtained in each reaction step and the
final product (M1) were characterized by standard spectroscopic
methods, from which satisfactory analysis data were obtained.
Inherently, inorganic semiconductors and organic polymers are
mutually immiscible. Their mixtures tend to be nonuniform and
inhomogeneous due to the detrimental processes involved such as
self-aggregation and phase separation. In our previous work, we
tackled this problem by taking advantage of the electrostatic
interactions between metallic and ammonium ions and succeeded
in the preparation of homogeneous films of perovskite–poly(1-
phenyl-1-alkyne) hybrids.3 In this work, we tried to develop a new
approach. By utilizing the bonding interactions of a transition
metal semiconductor with
a
thiol functional group,4 we
aDepartment of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, China. E-mail: sunjz@zju.edu.cn;
Fax: +86-571-8795-3734; Tel: +86-571-8795-3797
bDepartment of Chemistry, The Hong Kong University of Science &
Technology, Clear Water Bay, Kowloon, Hong Kong, China.
E-mail: tangbenz@ust.hk; Fax: +852-2358-1594; Tel: +852-2358-7375
{ Electronic supplementary information (ESI) available: Preparation and
characterization data of M1, C1, H1, B1, B2 and CdS; 1H NMR spectra
of M1 and H1; MS spectrum of M1; absorption spectra of CdS, M1 and
C1; (HR)TEM images and EDX data of C1; TGA thermograms of PPA,
CdS and H1; SEM and TEM images of B1; SEM images of B2; TEM
image of H1; determination of CdS content in H1 by elemental analysis;
experimental procedures for the photoreceptor fabrication and photo-
conductivity measurement. See DOI: 10.1039/b617595c
Scheme 1 Assembly of the molecules of monomer M1 on the surface of
CdS nanorod gives composite C1, whose copolymerization with
phenylacetylene yields CdS–poly(phenylacetylene) hybrid H1.
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We initially planned to prepare the hybrid by incorporating the
CdS nanorods into preformed polymer of M1, as we did in our
previous work on the perovskite system.3 Monomer M1 could,
however, not be polymerized by the Mo-, W-, Ta- and Nb-based
‘‘classic’’ catalysts for acetylene polymerization2 because of the
‘‘toxic’’ effect of its mercapto hydrogen on the transition-metal
catalysts. While the monomer could be polymerized by rhodium-
based catalysts such as [Rh(cod)Cl]2 (cod 5 1,5-cyclooctadiene),
the polymerization product was completely insoluble in common
solvents, possibly due to the ‘‘physical cross-linking’’ between the
‘‘free’’ thiol pendant groups.
To verify the polymeric nature of H1, its molecular weight was
estimated by gel permeation chromatography (GPC). To prevent
the GPC system from potential damage by the inorganic
nanorods, the hybrid was washed with concentrated hydrochloric
acid to remove the inorganic component. After the acid
pretreatment, the organic portion was subjected to GPC analysis,
which gave a weight-average molecular weight of 72620 and a
polydispersity index of 2.94, indicating that the acetylene
polymerization reaction had proceeded as expected.
Upon excitation with UV light of 330 nm, the hybrid exhibits a
fluorescence spectrum with emission maxima at 398, 458, 496 and
515 nm (Fig. 2). From the comparison with the spectra of its CdS
and PPA parents, the photoluminescence of H1 is clearly
dominated by the attributes from its CdS component.7 This is
easy to understand, because PPA and its derivatives are known to
be weak light emitters.2
We thus considered an alternative strategy: to allow molecules
of M1 to assemble on the CdS nanorod surface, for it should
consume the active mercapto hydrogen of the monomer
(cf. Scheme 1). The nanorods and the monomer were thus mixed
in dichloromethane (DCM) and gently stirred for 24 h. The
resulting nanocomposite (C1) was collected by precipitation and
was then washed with DCM several times. The indelible binding of
the M1 molecules on the surfaces of the CdS nanorods and the
complete removal of the unbound M1 molecules were confirmed
by chromatographic and spectroscopic analyses (ESI{).
Composite C1 was soluble in organic solvents and could thus be
used as a nanodimensional monomer to copolymerize with PA in
the presence of a Rh catalyst of [Rh(cod)Cl]2 (ESI{). The lack of
an active mercapto hydrogen in C1 enabled its copolymerization
mixture to remain homogeneous during the whole reaction
process. Moreover, the isolated hybrid (H1) was soluble in organic
solvents such as chloroform, DCM and tetrahydrofuran. Thin
films could be readily prepared by simply casting the hybrid
solutions onto solid substrates.
The morphology of a thin film cast from an H1 solution on an
aluminum plate is characterized by a continuous surface decorated
by discrete rod-like structures (Fig. 3A). According to the principle
of electron microscopy, and by comparison with the morphology
of the original nanorods used in the fabrication of H1 (cf. Fig. 3D),
the rod-like structures with high contrast can be assigned to the
CdS nanorod component in the hybrid. The nanorods are
dispersively distributed in the film: some of them lie parallel to
the film surface, while others tilt from the surface normal. As can
be seen from a deliberately generated fissure, the CdS nanorods are
interspersed well throughout the film (Fig. 3B). In sharp contrast,
the morphology of the film prepared by casting a solution of a
blend of CdS and PPA (denoted as B1) features very large
aggregates of the CdS nanorods, as manifested by its fluctuated,
uneven surface (Fig. 3C). Evidently, the bonding interactions
between the thiol groups in the polymer and the cadmium species
in the nanorods have helped to enhance the miscibility of the two
components, making it possible to uniformly disperse the CdS
nanorods in the PPA matrix.
The hybrid formation was confirmed by spectroscopic analyses
(ESI{). A set of IR spectra is shown in Fig. 1 as an example. In the
spectrum of M1, the MC–H stretching band is seen at 3293 cm21
.
This peak disappears in the spectrum of H1 (Fig. 1B), indicating
that the acetylenic triple bond has been completely consumed by
the copolymerization reaction. On the other hand, absorption
bands associated with the stretching of the methylene and carbonyl
groups are observed at 2926–2851 and y1675 cm21, respectively.
These bands are absent in the spectrum of the PPA parent
(Fig. 1C), confirming the integration of the C1 unit into the hybrid
structure.
Thermogravimetric analysis (TGA) has been commonly used to
estimate the weight fraction of a thermally stable component in a
composite material.3,8 The content of the CdS nanorods in the
hybrid was estimated by measuring the TGA thermograms of H1
and PPA (the thermogram of the nanorods was not taken because
the sample was obtained by sintering the as-prepared CdS at
Fig. 2 Photoluminescence spectra of thin films of H1 and CdS [blended
with poly(methyl methacrylate) or PMMA]. Data for a PPA film are
shown for comparison. Excitation wavelength: 330 nm.
Fig. 1 IR spectra of (A) monomer M1 and (B) hybrid H1. Data for PPA
are shown in part C for comparison.
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Chem. Commun., 2007, 1322–1324 | 1323

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to photoinduced charge transfers (PICT) between the donors
and the acceptors.9,11 As an n-type semiconductor, CdS works as
an electron acceptor, while PPA functions as an electron donor in
the hybrid. The PICT process takes place when the photoreceptor
is illuminated. The strong bonding of the CdS nanorods with
the PPA chains and the uniform distribution of the CdS nanorods
in H1 facilitate the PICT process and the successive charge
separation and transport, hence enhancing its photosensitivity.
In summary, in this work we have developed a facile strategy for
the hybridization of CdS nanorods with polyacetylene chains.
Taking an approach of assembly plus copolymerization, we have
successfully prepared a CdS–PPA nanohybrid that is fully soluble
in common organic solvents and that can form homogeneous films
when its solutions are cast on solid substrates. With the aid of the
bonding interactions between the thiol groups and the cadmium
atoms, the nanorods can be uniformly dispersed in the polymer
matrix. The uniform distribution of the CdS nanorods in the
hybrid film and the efficient PICT process between the n-type
nanorods and the p-type polymer chains have synergistically
boosted the photosensitivity of the hybrid photoreceptor. Noting
that the thiol group can bond with metallic species in many other
inorganic semiconductors, the process developed in this work may
prove to be a versatile approach towards the fabrication of a
wealth of inorganic–organic hybrids, using various combinations
of different nanostructured inorganic semiconductors and con-
jugated organic polymers.
Fig. 3 SEM images of (A) an as-fabricated H1 film, (B) an H1 film with
a deliberately torn fissure, and (C) a film of CdS–PPA blend (B1) that was
prepared by the polymerization of phenylacetylene in the presence of the
CdS nanorods. (D) TEM image of the CdS nanorods used in this study.
Table 1 Photosensitivity of the double-layered photoreceptor device
prepared by using H1, CdS, PPA, B1 or B2 as charge generation
material (CGM)a
CGM V0 (V) Rd (V s21
) )
Vr (V) t1/2 (s) S (1023 mm2 (mW s)21
H1
CdS
PPA
B1
1027
749
891
656
675
25
22
25
30
35
317
308
308
226
252
2.63
4.74
3.67
3.35
4.30
34.3
19.0
24.5
26.9
21.0
This work was partially supported by the National Science
Foundation of China (20634020, 50573065 and 50603021), the
Ministry of Science and Technology of China (2002CB613401),
and the Research Grants Council of Hong Kong (602706 and
603505). B. Z. T. acknowledges support from the Cao Guangbiao
Foundation of Zhejiang University.
B2
a
Exposed to a light with an intensity of 11 mW mm22 from a
halogen lamp. Abbreviations: V0
5 initial surface potential;
Rd 5 dark decay rate; Vr 5 residual surface potential; t1/2 5 half
discharge time under exposure; and S 5 photosensitivity. B2 denotes
a simple blend of CdS and preformed PPA.
Notes and references
1 (a) W. U. Huynh, J. J. Dittme and A. P. Alivisatos, Science, 2002, 295,
2425; (b) W. J. E. Beek, M. M. Wienk and R. A. J. Janssen, Adv.
Mater., 2004, 16, 1009.
2 J. W. Y. Lam and B. Z. Tang, Acc. Chem. Res., 2005, 38, 745.
3 (a) H. Xu, J. Sun, A. Qin, J. Hua, Z. Li, Y. Q. Dong, H. Xu, W. Yuan,
Y. Ma, M. Wang and B. Z. Tang, J. Phys. Chem. B, 2006, 110, 21701;
(b) J. Hua, Z. Li, J. W. Y. Lam, H. Xu, J. Sun, Y. P. Dong, Y. Q. Dong,
A. Qin, W. Yuan, H. Chen, M. Wang and B. Z. Tang, Macromolecules,
2005, 38, 8127.
4 (a) J. C. Love, L. A. Estroff, J. K. Kriebel, R. G. Nuzzo and
G. M. Whitesides, Chem. Rev., 2005, 105, 1103; (b) T. T. Liang,
Y. Naitoh, M. Honkawa, T. Ishida and W. Mizutani, J. Am. Chem.
Soc., 2006, 128, 1372; (c) A. Shavel, N. Gaponik and A. Eychmuller,
J. Phys. Chem. B, 2006, 110, 19280; (d) A. C. Wisher, I. Bronstein and
V. Chechik, Chem. Commun., 2006, 1637; (e) C. L. McGuiness,
A. Shaporenk, C. K. Mars, S. Uppili, M. Zharrukov and D. L. Allara,
J. Am. Chem. Soc., 2006, 128, 5231.
5 B. Z. Tang, H. Chen, R. Xu, J. W. Y. Lam, K. K. L. Cheuk,
H. N. C. Wong and M. Wang, Chem. Mater., 2000, 12, 213.
6 Y. Li, H. Liao, Y. Ding, Y. Fan, Y. Zhang and Y. Qian, Inorg. Chem.,
1999, 38, 1382.
7 D. Xu, Z. Liu, J. Liang and Y. Qian, J. Phys. Chem. B, 2005, 109,
14344.
8 B. Z. Tang and H. Xu, Macromolecules, 1999, 32, 2569.
9 P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for
Imaging Systems, Marcel Dekker, New York, 1993.
10 H.-P. Xu, M.-M. Shi, H.-Z. Chen, M. Wang and B. Z. Tang,
Chin. J. Polym. Sci., 2005, 23, 675.
11 (a) L. Cao, H. Chen, M. Wang, J. Sun, X. Zhang and F. Kong, J. Phys.
Chem. B, 2002, 106, 8971; (b) J. Sun, J. Cao, H. Li, J. Hong and
M. Wang, Chem. J. Chin. Univ., 2006, 26, 1722.
y650 uC; ESI{). From the TGA data (ESI{, Fig. S6), the nanorod
content was found to be ca. 10.8 wt%. Elemental analyses of the
materials were also carried out (ESI{, Table S2), which gave a CdS
nanorod content of y9 wt%, very close to the value estimated
from the TGA analysis.
Thanks to its solubility and film-forming capability, H1 can be
used as an active material to construct photoelectronic devices. A
series of double-layered photoreceptors were prepared by a
solution-casting process and their photoconductivities were
evaluated by a photoinduced discharge technique (ESI{).3a,5,9
Upon exposure to illumination by a halogen lamp, the initial
surface potential of the photoreceptor using H1 as CGM dropped
quickly to its half value in as short a time as 2.63 s (Table 1,
entry 1). Its photosensitivity (S) is 34.3 6 1023 mm2 (mW s)21
,
higher than those of all the devices using its parent forms of PPA
and CdS as well as their blends (B1 and B2) as CGMs.
In our previous study on the photoconduction in the devices
prepared using polyacetylene derivatives as CGMs, we found that
the introduction of phthalimido groups into poly(1-phenyl-1-
alkyne)s as pendants can dramatically improve the performances
of the devices due to the formation of charge-transfer
complexes.3a,10 Photoreceptors prepared from donor–acceptor
nanocomposites are known to exhibit high photosensitivities owing
1324 | Chem. Commun., 2007, 1322–1324
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