photoelectronic properties of Q-CdS/polyaniline (PANI) nanocomposites
Fan Jun, Ji Xin, Zhang Weiguang, Yan Yunhui
Received Mar. 4, 2004.
(Dept. of Chem., South China Normal University, Guangzhou 510631)
Abstract A kind of Q-CdS nanoparticle
with narrow size distribution was successfully prepared by using n-octyl mercaptan as
ligand and its size was determined by UV-Vis spectra to be about 2.4 nm. Then, adulterated
polyaniline (PANI) was synthesized by emulsion polymerization using aniline and
n-dodecylbenzene sulfonic acid (DBSA). And then, a series of Q-CdS/PANI nanocomposites
with different CdS or PANI content were prepared by mixing Q-CdS with PANI. Finally, these
samples were characterized by UV-Vis spectra, conductance measurement and fluorescent
spectra. Some results obtained were listed as follows: first, obvious size quantization
effect was shown in their UV-Vis spectra; secondly, their conductivity increased
continuously with the content of Q-CdS in the nanocomposites; the band-gap emission
intensities (- 440 nm) from Q-CdS increased greatly and these peaks changed to sharpness,
however, the intensities of the surface state emission (-550 nm) slightly decreased, which
indicated that the surface was modified with the long chain of PANI and the defect was
reduced when Q-CdS nanoparticles were embedded in PANI.
Keyword Q－CdS, polyanilines (PANI),
nanocomposites, photoelectronic properties
In recent years, the studies of
synthesis, characterization and application of Q-CdS nanoparticles with narrow size
distribution have attracted much more attention owing to their size-dependent
photophysical, photochemical and non-linear optical properties, etc [1-4]. In
contrast to the bulk solids, Q-CdS nanoparticles exhibit different absorption and emission
in UV-vis spectra and fluorescent spectra etc, which varies with particle size. However,
these particles have a strong tendency to coalescence for their large surface-to-volume
ratio . In order to solve this problem, some strategies including
encapsulation in sol-gels and in polymer matrixes etc, have been investigated .
Polyaniline compounds (PANIs) are being considered as a kind of conducting polymer
substance and have wide potential application in conducting materials, light emitting
diodes (LEDs), optical devices, etc [7, 8] and they show many advantages in
recombining nanomaterials compared to other polymers matrixes. Some studies on the optical
and electronic properties of Q-CdS/PANI nanocomposites have been reported in recent years [6,
have been generated by many methods including molecular beam epitaxy, MOCVD,
microlithography, and direct chemical preparation, etc . In this paper,
preparation and photoelectronic properties of Q-CdS particles, PANI and the Q-CdS/PANI
nanocomposites have been reported. A kind of freshly Q-CdS nanoparticles with narrow
distribution were successfully prepared by using n-octyl mercaptan as ligand and
its dimension was determined by UV-Vis spectra to be about 2.4 nm. Then, the nanoparticles
were dispersed into freshly PANIs synthesized by emulsion polymerization and a series of
Q-CdS/PANI nanocomposites with different ratio were successfully obtained. Furthermore,
their photoelectronic properties have been characterized by UV-Vis spectra, conductance
measurement and fluorescent spectra, respectively.
2.1 Measurement and chemicals
n-Octyl mercaptan have been obtained from
J&K Chemical Ltd. Aniline was redistilled prior to use. Unless noted, other reagents
such as CdSO4, n-hexadecyltrimethylammonium bromide, dodecylbenzene sulfonic
acid (DBSA), N-methyl-pyrrolidone(NMP), (NH4)2S2O8,
xylene, toluene, acetone etc were of analytical grade and without further purification.
Saturated H2S solution was prepared in the laboratory.
UV-Vis spectra were measured on UV-Vis 8500 spectrophotometer
(Shanghai) in NMP solution. Fluorescent spectra were recorded with Hitachi F-2500
spectrophotofluorometer. Conductivity measurements were carried out in DDS-11A
conductometer using DJS-1 conductivity electrode in NMP solutions at room temperature.
2.2 Preparation of Q-CdS nanoparticles
The particles were prepared according to the
literature method . n-Hexadecyl trimethylammonium bromide (5 mmol) was
slowly added to the mixture of CdSO4 (10 mmol) and n-octyl mercaptan (20
mmol) under vigorous stirring. The particle size was controlled by the quantity of
saturated H2S solution. After 6 hrs, the crude mixture was extracted repeatedly
by using a mixture of toluene and acetone, and then post-precipitation techniques were
applied to separate Q-CdS particles with different sizes.
2.3 Preparation of conductive Polyaniline(PANI)
Synthetic route of PANI was shown in Scheme 1. A solution of (NH4)2S2O8
(0.02 mol) in 10 mL water was added dropwise to the mixture of redistilled aniline
(0.05 mol) and DBSA (0.075 mol) in xylene/H2O solution. Reactive temperature
was steadily controlled in the range of 0 - 5°C. After stirring for 12 hrs, the
precipitate of polyaniline-dodecylbenzene sulfonate was formed by adding acetone into the
emulsion. After filtrating and washing, the blackish green solid was obtained and dried in
vacuum for 48 hrs.
Scheme 1 Synthetic Route of the conductive PANI-DBSA adulterants
2.4 Preparation of Q-CdS/PANI nanocompositesRESULTS AND DISCUSSION
The mixture of Q-CdS nanoparitcles and PANIs with different ratio were dissolved in
N-methyl-pyrrolidone (NMP) and stirred vigorously at room temperature for 2 hrs.
3.1 UV-Vis spectra of the samples
Absorption peaks in different position would be observed in UV-Vis spectra for Q-CdS
particles with different size. So, UV-Vis spectrum is a kind of the basic technique of
measuring the particle size from the absorption peaks and also estimating the size
distribution from the sharpness of peaks [9, 13]. In general, the Brus equation
 was used to calculate the particle size from the absorption peak,
Where, ER* is the
energies of excited state, ER* = hc / l . With respect to CdS, the
value of the electron effective mass (me) is 0.19 electron masses; the value of
the hole mass (mh) is 0.8; the value of dielectric constant (e ) is 5.7 and 2R
is the particle size. Finally, Eg must be identified with bulk band gap of CdS,
UV-Vis spectra of Q-CdS nanoparticles, PANI and Q-CdS/PANI
nanocomposites are shown in Fig.1, respectively. Absorption peak in 292 nm is assigned for
the characteristic absorption band of the Q-CdS nanoparticles and the size is determined
to about 2.4 nm according to the above-mentioned Brus equation.
Fig.1 UV-Vis spectra of
Q-CdS nanoparticles, PANI and Q-CdS/PANI nanocomposites
spectrum of conductive PANI shows two strong peaks in Fig. 1. The peak at about 320 nm is
assigned to the p-p *
transition of benzene rings, and the other at around 630 nm represent the transition of
the quinoid rings in long PANI chains, that is to say that characteristic
PANI were successfully prepared by emulsion polymerization.
Table 1 Conductivity of
Q-CdS, PANI and Q-CdS/PANIs nanocomposite in NMP solution
The absorption spectrum turns much greater changes when the Q-CdS
nanoparticles were dispersed in PANI matrix. Compared to Q-CdS and PANI, the spectrum of
Q-CdS/PANI nanocomposite shows two strong peaks with one weak shoulder. Very strong and
sharp absorption in about 292 nm can be attributed to the characteristic absorption of
Q-CdS particles, however, the absorption peak at about 320 nm that be assigned to aryl p-p* transition
becomes much weaker than that of single PANI sample. It can be explained that surface
deficiencies of the Q-CdS samples decreased and their stabilities increased greatly when
the CdS particles were embedded with PANI. The transition from the quinoid rings of the
PANI (about 630 nm) becomes weaker than before.
3.2 Conductivity measurement
Conductivity of Q-CdS, PANI and Q-CdS/PANI nanocomposite in NMP solution are listed in
Table 1. Q-CdS nanoparticle shows weak conductivity before mixed. In comparison,
Q-CdS/PANI nanocomposites show stronger conductivity than that of the pure samples.
Moreover, the conductivity increases gradually with the content of Q-CdS particles in the
nanocomposites. It would be explained that when Q-CdS particles were dispersed in the
PANI, the surface of PANI would be modified to have high conductivity for Q-state
particles have much more activity surface. The properties that the conductivity would be
easily enhanced and altered by adding the different content of Q-CdS may play an important
role in assembling the nanocomposite electrodes and be attracted wide attention in many
Q-CdS/PANIs (mass ratio)
Conductivity (m s/cm)
3.3 Fluorescent spectra
3.3.1. Fluorescent spectra of Q-CdS, PANI and Q-CdS/PANI
Fluorescent spectra of Q-CdS, PANI and Q-CdS/PANI nanocomposites in NMP solution are
shown in Fig. 2 for an excitation wavelength of 365 nm. A sharp band-gap emission (about
440 nm) and surface state emission (approximate 550 nm) are observed in a Q-CdS sample
(see Fig.2). Solid line exhibits fluorescent spectra of the PANI, and two emission peaks
are detected in the range of about 400 nm to 450 nm. Emission spectra of Q-CdS
incorporated PANI exhibit two stronger and sharper bands in the range of about 400 nm to
450 nm than that of pure Q-CdS or PANI which is the characteristic exciton emission of
Q-CdS particles, however, the surface state emission about - 550 nm become much weaker
than that of pure Q-CdS. It may be explained that the imperfect surface of Q-CdS particles
was modified by the encapsulation of PANI and Q-state particles become more stable after
encapsulated with PANI. On the other hand, the intensity of Q-CdS/PANI nanocomposites
increased remarkably owing to the peak's superposition between Q-CdS and PANI.
Fig. 2 Fluorescent spectra of Q-CdS, PANI and Q-CdS/PANI nanocomposites in
3.3.2. Fluorescent spectra
of the Q-CdS/PANI nanocomposites with different contents of PANI
Fluorescent spectra of Q-CdS/PANI nanocomposites with different contents of PANI are
shown in Fig.3. Similar spectra are observed for all samples in despite of having
different ratios. In the first instance (from a to b), the emission intensities increased
obviously with increasing the content of PANI; then, the intensities yet decreased step by
step with the ratio of PANI (from b to d). When increasing to 8:1, the fluorescence of the
nanocomposite was almost quenched. The phenomenon may be explained that boundary effect,
which derived from the change of refractive index in Q-CdS particle surface owing to the
modification of PANI to Q-CdS, would play crucial role for the fluorescent intensity of
Fig.3 Fluorescent spectra of Q-CdS/PANI composites in different
ratios between PANI and Q-CdS. (a) 1:1, (b) 2:1, (c) 5:1, (d) 8:1
The Q-CdS nanoparticles with narrow
distribution were obtained by using post-precipitation techniques and the average size
determined from the band-edge emission by using Brus equation is about 2.4 nm. Then, the
Q-CdS nanoparticles were dispersed into freshly PANI under vigorously stirring and a
series of Q-CdS/PANI nanocomposites with different ratios were formed. Furthermore, their
photoelectronic properties have been investigated by UV-Vis spectra, conductance
measurement and fluorescent spectra. Conductivity of the composites turns stronger than
that of the Q-CdS particles and PANI. Fluorescent intensities of the samples changed
continuously with the ratios due to the recombination of excitons and the emission from
trap states. The band-gap emission (- 440 nm) intensities of all the nanocomposite samples
increased greatly and these peaks changed to sharpness, however, the intensities of the
surface state emission (- 550 nm) slightly decreased, which indicated that long chain of
PANI modified the surface of Q-CdS and reduced the surface defect when Q-CdS nanoparticles
were embedded in PANI.
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范军 纪欣 章伟光* 闫云辉
(华南师范大学化学系, 广州, 510631)
关键词 Q-CdS, 聚苯胺(PANI)，纳米杂化物，光电性能