Article
pubs.acs.org/JPCC
Hybrid Electrochromic Fluorescent Poly(DNTD)/CdSe@ZnS
Composite Films
Huige Wei,† Xingru Yan,† Yunfeng Li,†,‡ Shijie Wu,§ Andrew Wang,# Suying Wei,*,‡ and Zhanhu Guo*,†
†
Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont,
Texas 77710, United States
‡
Department of Chemistry and Biochemistry, Lamar University, Beaumont, Texas 77710, United States
§
Agilent Technologies, Inc, Chandler, Arizona 85226, United States
#
Ocean NanoTech, LLC, 2143 Worth Lane, Springdale, Arkansas 72764, United States
ABSTRACT: Hybrid electrochromic poly(DNTD)/CdSe@ZnS quantum dots
(QDs) composite films were prepared using an electropolymerization technique at
room temperature and their electrochemical properties at different temperatures
were investigated by cyclic voltammetry (CV). At room temperature, the composite
films exhibited asymmetry in the voltammograms arising from the heterogeneity in
the polymer chains and the morphology change caused by the QDs. A more positive
cathodic potential than that of the pure polymer films indicated that poly(DNTD)
became more difficult to be oxidized as a consequence of the incorporated QDs.
The CVs of these nanocomposite thin films were also carried out at 0 °C to evaluate
the temperature effect on their electrochemical properties. The asymmetry in the
oxidation/reduction process became more remarkable, and lower oxidation
potentials were observed. Pure poly(DNTD) thin film and its composite thin film
with 0.05 wt % QD loading were prepared at 0 °C as well. Their CVs were performed at both room temperature and 0 °C and were compared with those of the films prepared at room temperature. The
morphology, optical, and spectroelectrochemistry (SEC) properties of the thin films were studied using atomic force microscopy
(AFM), fluorescent microscopy, and a UV−visible spectrometer coupled with the potentiostat, respectively. The obtained
uniform thin films turned out to be made of nanoparticles under high resolution AFM observation. Fluorescent microscopic
observations revealed the fluorescent emission of the nanocomposites with the QDs embedded in the hosting polymer matrix.
The nanocomposite thin films exhibited varying absorbance bands when different potentials were applied, and the film was
observed to turn blue at an applied voltage of 1.4 V vs Ag/AgCl.
1. INTRODUCTION
Semiconducting or conducting polymers with spatially extended p−p or p−n−p bonding systems have been intensively
researched since they can serve as active components in photovoltaic cells, light emitting diodes, photodiodes, field effect transistors, and other electronic devices.1,2 Among the conjugated
polymers, poly(thiophene),3 poly(pyrrole),4,5 and poly(aniline)
(PANI),6,7 and their derivatives are widely studied. Significant
efforts have been devoted to manipulate the physicochemical
properties of the conjugated polymers by tailoring the molecular structure of either the main chain or pendant groups8−10
or through the formation of the blends, 11 laminates,12 or
composites.13,14 One attractive area relating to conjugated
polymers is their spectroelectrochemical properties, which
occur upon the reduction (gain of electrons) or oxidation
(loss of electrons), on the passage of an electrical current after
the application of an appropriate electrode potential.15−17 The
electrochromic properties of the conducting polymers have
gained much popularity both from the academia and industry
for their easy processability, rapid response time, high optical
contrast, and the ability to modify their structures to create
multicolor electrochromes.18−20 New electrochromic materials,
© 2012 American Chemical Society
that is, thin films consisting of PANI and WO3 through molecular assembling synthesis21 and multilayer films of PANI and
grapheme22 have been reported.
Semiconductor nanocrystals (NCs) have attracted much
interest due to their wide potential engineering applications
such as biology and medication, biological fluorescence labeling and imaging.23−25 The primary focus has been on their optoelectronic properties, displaying a strong size-dependent quantum
confinement effect, which takes place when the quantum well
thickness becomes comparable to the de Broglie wavelength of
the carriers (generally electrons and holes), leading to energy
levels called “energy subbands”.26−28 Among the various kinds
of NCs, cadmium selenide (CdSe) quantum dots (QDs) are
undoubtedly the most widely studied owing to their tunable
emission in the visible range and the availability of precursors
and the simplicity of crystallization.29−31 In order to increase
the photostability of the core and the quantum yield, core−
shell structured QDs, for example, CdSe coated with ZnS as a
Received: December 7, 2011
Revised: January 10, 2012
Published: January 12, 2012
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core−shell nanocomposite structure, have been achieved.32−35 In
addition, the introduced ZnS shell can minimize the toxicity of the
heavy metal cadmium.36
By virtue of the advantages of the conducting polymers
and NCs, a strategy of combining these two components for
applications in various domains of electronics, for examples, as
active layers in light emitting diodes (LEDs), photodiodes and
photovoltaic cells, has been developed.37−39 It turns out that
these new hybrid materials may lead to polymer nanocomposites (PNCs) with properties unmatched by conventional
materials. For both of the conjugated polymers and the NCs,
their properties can be tuned individually to adapt to each
other. This particularly applies to the positions of the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, which
determine their electronic, redox, and luminescent properties.40
Therefore, conjugated polymer/nanocrystal hybrids are much
more superior to their corresponding individuals where the
design flexibility is concerned.
N,N′-Di[p-phenylamino(phenyl)]-1,4,5,8-naphthalene tetracarboxylic diimide (DNTD), a bis(diphenylamine)naphthalene
diimide, belongs to the class of conjugated conducting polymers
with a chemical structural motif of R−X−R, where R represents
the diphenylamine (DPA) group and X represents naphthalene
tetracarboxylic diimide (NTCDI) group as the electroactive
center. Materials made from this monomer exhibit low dielectric,
thermal/oxidative stability41 and increased bulk conductivity
through π-stacking interactions.42 Though the electrochemical
and optical properties of the pure poly(DNTD) polymer thin
films have been widely investigated and well elucidated by cyclic
voltammetry, UV−visible spectroelectrochemistry,43 and electrochemical quartz crystal microbalance (EQCM),44 the combination
of the poly(DNTD) polymer with NCs in the nanocomposite thin
films for using as electrochromic and fluorescent materials have
been rarely explored.
In this study, hybrid electrochromic nanocomposite thin
films consisting of poly(DNTD) and core@shell CdSe@ZnS
quantum dots (QDs) were fabricated by the cyclic voltammetry
method. The physicochemical properties of the nanocomposite
thin films are studied and compared with those of pure poly(DNTD) thin films. The electrochemical properties, optical, and
spectroelectrochemistry (SEC) properties were studied using
two different working electrodes, that is, platinum (Pt) disk
with an area of 0.018 cm2 and indium tin oxide (ITO) coated
glass slides. The effects of temperature on the film growth and
electrochemical properties of the polymer and its nanocomposite
thin films were also investigated. The composite films showed
promising potential in the electrochromic device applications,
while the fluorescent properties were introduced successfully.
Scheme 1. Chemical Structure of the Monomer DNTD
1 mL of 30.0 wt % hydrogen peroxide (both from Fisher), and
10 mL of deionized water before the usage.
2.2. Electrochemical Synthesis of Pure Polymer and
Nanocomposite Thin Films. Pure poly(DNTD) and its QD
nanocomposite thin films were prepared by oxidative electropolymerization from CH2Cl2 solutions containing monomer
(0.5 mM), QDs and electrolyte TBAPF6 (0.1 M) with multiple
cycles of cyclic voltammetry (CV) at room temperature.
Figure 1a−d shows the different colors of the CH2Cl2 solutions
Figure 1. Images of DNTD (a) and QDs/DNTD dissolved in CH2Cl2
with QD loading of (b) 0.05 wt %, (c) 0.1 wt %, and (d) 0.2 wt %,
respectively.
containing only monomer DNTD and QDs/DNTD with different QD loadings. CV was performed on an electrochemical
working station VersaSTAT 4 potentiostat (Princeton Applied
Research). A typical electrochemical cell consisting of a reference electrode, a working electrode, and a counter electrode
was employed. An Ag/AgCl electrode saturated with KCl served
as the reference electrode and a platinum (Pt) wire as the counter
electrode. ITO coated glass slide and a platinum (Pt) disk with an
area of 0.018 cm2 were used as the working electrode for optical
characterizations and electrochemical characterizations, respectively. A long path length homemade spectroelectrochemical cell
with Teflon cell body with front and rear windows clapped with
two steel plates was employed when the ITO glass slide was used
as the working electrode. A typical electrochemical polymerization was performed 10 cycles scanned back and forth from
0 to 1.8 V vs Ag/AgCl at a scan rate of 200 mV/s.42 To
investigate the temperature effect on the performance of the thin
films, pure poly(DNTD) and its nanocomposite thin films with
0.05 wt % of QDs at 0 °C were also prepared for comparison.
2.3. Characterizations. The electrochemical properties of
pure poly(DNTD) and its nanocomposite thin films were
investigated by CV scanned from −1.2 to 1.5 V vs Ag/AgCl
using Pt disk as the working electrode. The three-electrode cell
was bubbled with ultrahigh purity nitrogen (Airgas) for at least
15 min before the scan. The monomer-free solution is 0.1 M
TBAPF6 in CH2Cl2. The CVs of all the thin films produced
were performed at both room temperature and 0 °C.
The morphology and thickness of the thin films grown on
the ITO glass slides were characterized by atomic force microscopy (AFM, Agilent 5600 AFM system with multipurpose
90 μm scanner). Imaging was done in acoustic ac mode (AAC)
using a silicon tip with a force constant of 2.8 N/m and a
resonance frequency of 70 kHz.
2. EXPERIMENTAL SECTION
2.1. Materials. Methylene chloride (CH2Cl2, 99.9%) and
tetrabutylammonium hexafluorophosphate (TBAPF6, 98%) were
purchased from Alfar-Aesar. CdSe@ZnS quantum dots with
octadecylamine ligand were supplied by Ocean NanoTech LLC
(Springdale, Arkansas USA). The monomer DNTD was synthesized following the procedures in the literature,42 and Scheme 1
shows the chemical structure of the DNTD. The microscope
glass slides and indium tin oxide (ITO) coated glass slides were
purchased from Fisher and Delta technologies, Limited, respectively. ITO coated glass slides were degreased by a mixed aqueous
solution containing 1 mL of 28.86 wt % ammonium hydroxide,
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UV−vis spectra of the pure DNTD and DNTD/QDs
CH2Cl2 solutions with the electrolyte were recorded with a
50 bio ultraviolet visible (UV−vis) spectrophotometer. Their
photoluminescence (PL) spectra were recorded by a Varian
CARY Eclipse fluorescence spectrophotometer. The solutions
were put into a standard 10 mm path length quartz cuvette for
testing. The solution of QDs was diluted to 1% of its original
concentration with CH2Cl2 before measurement. The excitation wavelength is 350 nm. The films grown on ITO coated glass
slide were further examined by an Olympus BX51 fluorescence
microscope. Images were obtained using Olympus DP72 camera
accompanied with Olympus CellSens software.
The spectroelectrochemistry measurements were performed
on a Varian Cary 50 Version 3 UV−visible spectrometer coupled
with the potentiostat for applying electrochemical potentials.
Solutions in the spectroelectrochemistry (SEC) cell were treated
by the same manner bubbled with ultrahigh purity nitrogen for at
least 15 min. Each spectrum was taken when the electrochemical
cell current decayed to zero and the data was obtained in the
mode of medium in Varian Cary WinUV Bio software. At each
applied potential, a minimum of three spectra were taken to
verify consistency.
3. RESULTS AND DISCUSSION
3.1. Synthesis of Poly(DNTD) and Poly(DNTD)/QD
Nanocomposite Films. Figure 2a,b shows the UV−vis absorption (a) and fluorescence spectra (b) of QDs and QDs/
DNTD in methylene chloride. In the UV−vis absorption,
TBAPF6 and DNTD/TBAPF6 in the CH2Cl2 solutions show a
flat curve without any absorption peaks. The QDs show two absorption peaks at 562 and 595 nm, corresponding to the absorption energy of 2.21 and 2.10 eV, respectively (ν = c/λ,
E = h × ν, ν is light wave frequency, s−1; c is light speed, 3 ×
108 m s−1; λ is wavelength, m; E is wave energy, eV; h is
Planck’s constant, 4.135 × 10−15 eV·s) characteristic of a
narrow-size distribution of CdSe@ZnS QDs.45 Cyclic voltammetry has been used widely to determine the HOMO and
LUMO levels of QDs.46,47 The onsets of the first oxidation
peak and reduction peak in the CV are assumed to correspond
to the withdraw of an electron from the bonding HOMO and
an addition of an electron to the antibonding LUMO, respectively.48 The onset potential is determined from the intersection of the two tangents drawn at the rising oxidation or reduction
current and background current in the CV.49,50 Figure 3a,b
shows the reduction voltammetry of the bare Pt (without QDs)
(a) and 0.2 wt % QDs (b) in CH2Cl2 solution. The onset
potential of −0.55 V vs Ag/AgCl, Figure 3b, could be obtained
employing the graphical method. Therefore, the LUMO energy
with respect to the vacuum value of QDs is calculated to be
−3.82 eV on the basis of the assumption that the energy value
of Ag/AgCl is −4.37 eV in a vacuum.51−53 The bandgap (energy
difference between HOMO and LUMO) of the QDs is calculated
to be 2.10 eV at 595 nm in the UV−vis spectra, Figure 2a, for the
QDs in CH2Cl2 solutions. Correspondingly, the HOMO energy
of the QDs with respect to the vacuum value is calculated to be
−5.92 eV. Similar HOMO and LUMO energy values have been
obtained by others.54 The measured bandgap value shows a blue
shift relative to the band gap of the bulk cubic CdSe (1.77 eV),45
which is attributed to the small size and the effect of ZnS shell.
The QDs/DNTD solutions show a weak absorption because of
the low concentration of the QDs, and the absorption positions
shift to longer wavelength, indicating an interaction between QDs
and the monomer. The fluorescence spectra, Figure 2b, depict the
Figure 2. (a) UV−vis absorption and (b) fluorescence spectra of QDs,
TBAPF6, QDs/TBAPF6, and QDs/DNTD with different loadings of
QDs in CH2Cl2. The excitation wavelength for fluorescence spectra is
350 nm.
Figure 3. Reduction voltammetry recorded on Pt electrode in the (a)
absence and (b) presence of QDs (0.2 wt %) in CH2Cl2 with TBAPF6
as the supporting electrolyte. The scan rate is 200 mV/s.
emission peak of the QDs solution at 612 nm at an excitation
wavelength of 350 nm. However, the QD/DNTD solutions show
a shifted emission peak at 616 nm, consistent with the result of
UV−vis analysis, that some interaction between QDs and the
monomer does exist.
Figure 4a−d shows the cyclic voltammetric growth of pure
poly(DNTD) films and its nanocomposite films with a QD
loading of 0.05, 0.1, and 0.2 wt %, respectively, on a Pt electrode (0.018 cm2), scanned in CH2Cl2 solution containing 0.1 M
TBAPF6 supporting electrolyte at room temperature. The scan
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Figure 4. Films growth of (a) pure DNTD, (b) 0.05 wt % QDs/DNTD, (c) 0.10 wt % QDs/DNTD, (d) 0.20 wt % QDs/DNTD at room
temperature, and of (e) pure DNTD, and (f) 0.05 wt % QDs/DNTD at 0 °C, respectively, on a Pt disk electrode (area = 0.018 cm2) in CH2Cl2
solution containing 0.1 M TBAPF6 and 0.1 mM DNTD monomer at room temperature. Scan rate is 200 mV/s.
Scheme 2. Possible Cation (DPB+) and Dication (DPB2+)
Chemical Structures
rate is 200 mV/s and scan range is from 0 to 1.5 V vs Ag/AgCl.
Pure polymer and nanocomposite thin films exhibit similar CV
shapes. During the first positive scan, an irreversible peak appeared,
which is attributed to the irreversible oxidation of diphenylamine.55−57 For the first negative scan and the second positive
scan, two sets of quasi-reversible peaks are observed, consistent
with the formation of diphenylbenzidene (DPB) unit. These
two peaks correspond to the oxidation of DPB to form cation
(DPB+) and dication (DPB2+), respectively.42 The possible
structures of DPB+ and DPB2+ are shown in Scheme 2.58 The
current increased with increasing cycles, reflecting the growth
of pure polymer and nanocomposite thin films. The polymerization occurs via the coupling of the para carbon on the
terminal phenyl group, followed by proton loss and additional
electron transfer reactions.43 Scheme 3 illustrates the proposed
polymerization mechanism (a) and possible reactions between
the dimer/oligmers and the ligands on the QD surface (b). The
ligands on the QD surface were oxidized to form radical cations
and served as the terminating radicals to react with the initial
radical cation of the diphenylamine group in the dimer/oligmers.
There are also chances that QDs ligands formed dications to
couple with or even more two dimer/oligmers.
However, some differences between the pure poly(DNTD)
polymer and the nanocomposite thin films can be observed
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Scheme 3. (a) Polymerization Mechanism of Pure Polymer and (b) Possible Reactions between Oxidized QDs and Dimer/
Oligomers
upon closer observation. First, the irreversible anodic potential
for pure polymer is around 1.34 V and is postponed to be 1.41,
1.42, and 1.45 V vs Ag/AgCl for the composite thin films with a
QD loading of 0.05, 0.1, and 0.2 wt %, respectively. Figure 5a,b
shows the oxidation voltammetry of bare Pt (a) and 0.2 wt %
QDs (b) in CH2Cl2 solution. In the positive sweep, an obvious
anodic peak at around +1.3 V vs Ag/AgCl was observed for
QDs, corresponding to the oxidation of the QDs octadecylamine ligands. Therefore, these positive potentials for composite
films are supposed to be caused by the combined irreversible
oxidation of the QDs octadecylamine ligands with the
diphenylamine in the monomer as shown in Scheme 3b. Second,
for the growth of pure poly(DNTD) polymer thin film, the peak
at 1.0 V vs Ag/AgCl corresponding to the 1 e− oxidation of DBP
became less distinct upon repeated CV scans. However, for the
nanocomposite thin films, the peak at 1.0 V vs Ag/AgCl appeared
more pronounced and did not undergo appreciable changes in
intensity with increasing the scans. This indicates that QDs
favor the one-electron transfer process and stabilize the state of
radical DPB+.
At 0 °C, Figure 4e,f, DNTD monomers could still be electropolymerized. The thin films exhibit similar CV shapes compared
to those at room temperature except the anodic current became
comparatively lower, indicating a lowed polymerization rate.59
Figure 5. Oxidation voltammetry recorded on Pt electrode in the (a)
absence and (b) presence of QDs (0.2 wt %) in CH2Cl2 with TBAPF6
as the supporting electrolyte. The scan rate is 200 mV/s.
However, both the anodic and cathodic current potentials exhibit
negligible differences between the CVs at two different temperatures. This phenomenon suggests that a similar electropolymerization mechanism occurred for the thin films growing at room temperature and 0 °C even though the reaction rate was different.
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Figure 6. Cyclic voltammograms of (a) pure DNTD, (b) 0.05 wt % QDs/DNTD, (c) 0.10 wt % QDs/DNTD, and (d) 0.20 wt % QDs/DNTD,
respectively, on Pt scanned in 0.1 M TBAPF6 CH2Cl2 DNTD-free solution with a scan rate of 200 mV/s at room temperature.
and the other at −0.82 V vs Ag/AgCl in the negative scan.
These two reduction peaks are attributed to the formation of
the naphthalene diimide anion and naphthalene diimide dianion,
respectively.42 The possible chemical structures of those anions
and dianions are shown in Scheme 4. For poly(DNTD)/QDs
3.2. Electrochemical Properties. After the electropolymerization, the electrode with a thin film on the surface
of Pt disk was rinsed with fresh CH2Cl2 and then was placed
in a degassed CH2Cl2 solution containing only 0.1 M TBAPF6
supporting electrolyte, and was cycled again. Figure 6a−d
shows the CV of poly(DNTD) and poly(DNTD)/QDs nanocomposite thin films, respectively, on a Pt electrode. The scan
rate is 200 mV/s and scan range is from −1.2 to +1.5 V vs
Ag/AgCl. The CV data of pure polymer and nanocomposite
films is summarized in Table 1. The half wave potentials of Eox
Scheme 4. Possible Anion and Dianion Chemical Structures
of DNTD
Table 1. Electrochemical Data of Polymer Films Synthesized
at Room Temperaturea
polymer films
first
Eox (V)
second
Eox (V)
first Ered
(V)
second
Ered (V)
Eg,EC (eV)b
pure polymer
0.05 wt % of QDs
0.1 wt % of QDs
0.2 wt % of QDs
0.94
1.17
1.23
1.24
1.15
−0.39
−0.37
−0.37
−0.37
−0.82
−0.83
−0.83
−0.83
1.33
1.54
1.60
1.61
a
b
nanocomposite thin films, Figure 6b−d, during the forward
negative scan, the cathodic potential did not change too much
compared to those of the pure poly(DNTD) thin films, one at
about −0.37 vs Ag/AgCl and the other at −0.83 V vs Ag/AgCl,
Table 1. Therefore, the same redox species are produced. Thus
the QDs have negligible effect on the naphthalene diimide
moiety in the polymer. When the potential began to sweep
backward, asymmetry in the oxidation/reduction process was
observed. The anodic peak at around 0.94 V vs Ag/AgCl
disappeared and only one single prominent oxidation peak
at 1.17 V vs Ag/AgCl appeared, and the oxidation potential
shifted toward more positive positions with the increase of
QD loading. Meanwhile, in the cathodic branch of the cyclic
voltammogram, two distinguishable reduction peaks instead of
one were observed. This asymmetric behavior might be attributed to the heterogeneity in the polymer during the eletropolymerization as shown in Scheme 3. The incorporated QDs
gave rise to different polymers with varying conjugated chain
length. In addition, the introduction of QDs in the polymer
may also affect the morphology or crystallinity of the polymer
Potentials vs Ag/AgCl; 0.1 M TBAPF6, CH2Cl2, scan rate: 200 mV/s.
Eg, EC=1st Eox − 1st Ered.
and Ered are obtained by taking the average of their anodic and
cathodic peaks, i.e., Eox = (Epaox+ Epcox)/2 and Ered = (Epared+
Epcox)/2.48 For the pure poly(DNTD) thin films, Figure 6a, in
either positive (potential larger than 0 V vs Ag/AgCl) or
negative (potential lower than 0 V vs Ag/AgCl) scan, two pairs
of well-defined redox peaks are observed. Therefore, poly(DNTD) can be oxidized and reduced, indicating its p-n
bipolar properties. In the positive scan, the two sets of the
redox peaks, one with an Eox of about 0.94 V vs Ag/AgCl and
the other one of about 1.15 V vs Ag/AgCl, are associated
with the oxidation of poly(DNTD) into DPB+ cation and
DPB2+ dication.42 The pure polymer thin films also show two
separated cathodic peaks, one at about −0.39 V vs Ag/AgCl
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Figure 7. Cyclic voltammograms of (a) pure poly(DNTD), and QDs/poly(DNTD) nanocomposites with a QD loading of (b) 0.05, (c) 0.10, and
(d) 0.20 wt %, respectively, on Pt scanned in 0.1 M TBAPF6CH2Cl2 DNTD-free solution with a scan rate of 200 mV/s at 0 °C.
Figure 8. Cyclic voltammograms of (a) pure DNTD, (b) 0.05 wt % QDs/DNTD at room temperature, and (c) pure DNTD, (d) 0.05 wt % QDs/
DNTD at 0 °C.
increased with the increase of the loading of QDs. Eg,EC is
calculated to be 1.33, 1.54, 1.60, and 1.61 eV, Table 1, for the
pure polymer and the nanocomposite films with a QD loading
of 0.05, 0.1, and 0.2 wt %, respectively. The phenomenon might
be explained that the polymer became less planar and less conjugated in the hybrid films, thus giving rise to a widened
band gap.48 Similar phenomena have been observed for the
nanocomposites of the diaminopyrimidine functionalized
thin film, which also play a part in influencing the electrochemical properties of the nanocomposite films.
The first Eox and Ered provide some valuable information on
the HOMO and LUMO levels regarding the polymer and composite films.60 Given the higher first Eox for the nanocomposite
films, Table 1, it can be inferred that poly(DNTD) became
more difficult to be oxidized. On the other side, the bandgap
obtained by the electrochemical method, Eg,EC, of the polymer
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Figure 9. AFM images and thicknesses of (a) pure polymer and composite films with QDs loading of (b) 0.05 wt %, (c) 1.0 wt %, (d) 0.05 wt % at
room temperature, and of (e) pure polymer at 0 °C, respectively, grown on ITO glass.
0.2 wt %, respectively, on a Pt electrode (0.018 cm2), scanned
in CH2Cl2 solution containing 0.1 M TBAPF6 supporting electrolyte at 0 °C. Compared to those CVs at room temperature,
these films showed lower oxidation peak potentials. Besides,
the peaks of polymer became less symmetrical. This interesting
phenomenon is also observed for the CVs of the polymers
prepared at 0 °C, Figure 8. The polymer films appear to have
more symmetrical CV shapes at 0 °C than at room temperature. Many factors such as interactions among charged sites,61,62
poly(3-hexylthiophene) and thymine-capped CdSe nanocrystals
as a result of the formation of hydrogen bonding between the
polymer and the nanocrystals.50
Cyclic voltammograms of poly(DNTD) and poly(DNTD)/
QDs nanocomposite films at 0 °C were also performed to investigate the effect of temperature on the electrochemical properties of these films prepared at room temperature. Figure 7a−d
shows the CVs of pure poly(DNTD) and QDs/poly(DNTD)
nanocomposite thin films with a QD loading of 0.05, 0.1, and
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test. For example, the planar chain of the polymer film synthesized at room temperature is more subjected to be a twisted
conformation when the CV test was conducted at 0 °C. For the
nanocomposite thin films, the main reason might lie in the
reduced conjugated length caused by the incorporation of the
QDs in the polymer chains.
3.3. Morphology and Optical Properties. The polymer
and nanocomposite thin films grown on the ITO coated glass
slides were rinsed with CH2Cl2 and dried for AFM characterization. Figure 9 presents the morphology and thickness of the
films grown at room temperature (a−e) and pure polymer film
grown at 0 °C (f). At lower resolution, these films exhibit
compact and continuous film structures. However, under high
resolution, evenly sized and uniformly distributed nanoparticles
are observed. For the films synthesized at room temperature,
the thickness of the film increased with increasing the QD
loading. The thickness is roughly estimated to be 17, 25, 30,
and 35 nm for the pure polymer and nanocomposite films
with a QD loading of 0.05, 0.1, and 0.2 wt %, respectively.
Accordingly, the particle size increases with increasing the
thickness of the film. This morphology change induced by the
QDs may account for the changes in the electrochemical properties of the nanocomposite films compared to that of the pure
polymer. At lower temperature 0 °C, the pure poly(DNTD)
polymer film exhibits more orientated and smaller particles
than that synthesized at room temperature. This phenomenon
is reasonable for that at high temperature, the crystals grow fast,
yielding bigger-sized particles and less-orderly structures.66
The films grown on ITO glass slides were further examined
under the fluorescent microscope. Figure 10 shows the fluorescent microscope images of the pure poly(DNTD) polymer
Figure 10. Bright field fluorescent microscopy images of (a) pure
polymer, and composite films with QDs loading of (b) 0.05 wt %, and
(c) 2.0 wt % grown on ITO glass slides. a*−c* correspond to a−c in
dark field.
differences in the properties of the oxidized and neutral polymer,63
or heterogeneity of the polymer film64,65 have been proposed
for the asymmetry in the redox behavior of the polymers. Here
the asymmetry of the pure polymer might be caused by the
conformational change of the polymer backbone induced by
the temperature difference between the film growth and CV
Figure 11. UV−vis spectra of (a) pure polymer, (b) 0.05 wt % QDs/DNTD, (c) 1.0 wt % QDs/DNTD, and (d) 2.0 wt % QDs/DNTD films,
respectively, grown on ITO glass slide.
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1.4 V vs Ag/AgCl, respectively. At 0 V vs Ag/AgCl, the
nanocomposite film appeared nearly transparent; when applied
to a potential of 1.4 V vs Ag/AgCl, the film turned to blue. The
unique electrochromic properties promise potential applications
of these hybrid nanocomposite thin films in the electrochromic
devices.
and nanocomposite films. The pure polymer thin film shows no
fluorescent emission signal as expected, Figure 10a. However,
in the nanocomposite thin films with a QD loading of 0.05 and
2.0 wt %, uniform red emission signals are observed, indicating
that QDs were fairly uniformly distributed in the hosting
poly(DNTD) polymer matrix.
3.4. Spectroelectrochemistry of Polymer and Composite Films. Polymer and nanocomposite thin films can be both
oxidized and reduced as revealed from the results of CVs;
therefore, it is expected that the visible spectra shall be changed
when subjected to different potentials.67,68 Figure 11 shows the
spectroelectrochemistry of pure poly(DNTD) and nanocomposite thin films deposited on the ITO coated glass slides
at different potentials. The spectrum of the neutral films of
poly(DNTD) and nanocomposites show a less absorption band
above 400 nm. When the potential was increased to 0.9 V vs
Ag/AgCl, a peak appeared at 450 nm, which is assigned to be
the formation of the cation (DPB+).42 As the potential was
further increased to 1.4 V vs Ag/AgCl, the band at 450 nm
disappeared, and a broad absorption at 613 nm was observed,
corresponding to the oxidation of the cation (DPB+) to the
dication (DPB2+). Correspondingly, the thin film turns to blue.
At negative potential, for example, −0.5 V vs Ag/AgCl, a peak
at 480 nm was observed, which corresponds to the naphthalene
diimide anion absorption. The films remain in the anion state
during the reduced potential. As for the nanocomposite thin
films, similar absorption spectra were observed when the applied
potentials were varied.
Figure 12a,b shows the color change for the composite
film with 0.05 wt % loading of QDs when subjected to 0 and
4. CONCLUSION
Hybrid electrochromic composite films composed of poly(DNTD) and CdSe@ZnS quantum dots (QDs) have been
synthesized by electrodeposition and were electrochemically
and spectroelectrochemically characterized. QDs are observed
to be fairly uniformly dispersed in the polymer matrix. The
composite films showed bipolar properties and became less
easily oxidized with a widened bandgap as a result of reduced
conjugated length caused by the incorporation of QDs in the
polymer chain. The spectroelectrochemistry (SEC) properties
of the films grown on ITO exhibit varied absorption bands
when applied to different potentials and the color turned blue
at high positive potential, for example, 1.4 V vs Ag/AgCl. The
effect of temperature on the composite films growth and their
electrochemical properties were also studied by CVs, showing
less asymmetry in the redox process and lowered oxidation
potential at low temperature. These new hybrid nanocomposite
thin films exhibit both unique fluorescent properties and novel
electrochromic phenomena.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected] (Z.G.); [email protected]
(S.W.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The financial support from the National Science FoundationNanoscale Interdisciplinary Research Team and Materials Processing and Manufacturing under Grant Number of CMMI 10-30755
is kindly acknowledged.
■
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