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Hybrid Electrochromic Fluorescent Poly(DNTD)/CdSe@ZnSComposite FilmsHuige 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 atroom temperature and their electrochemical properties at different temperatureswere investigated by cyclic voltammetry (CV). At room temperature, the compositefilms exhibited asymmetry in the voltammograms arising from the heterogeneity inthe polymer chains and the morphology change caused by the QDs. A more positivecathodic 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 evaluatethe temperature effect on their electrochemical properties. The asymmetry in theoxidation/reduction process became more remarkable, and lower oxidationpotentials were observed. Pure poly(DNTD) thin film and its composite thin filmwith 0.05 wt % QD loading were prepared at 0 °C as well. Their CVs were per-formed at both room temperature and 0 °C and were compared with those of the films prepared at room temperature. Themorphology, 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 obtaineduniform thin films turned out to be made of nanoparticles under high resolution AFM observation. Fluorescent microscopicobservations 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 wasobserved to turn blue at an applied voltage of 1.4 V vs Ag/AgCl.

1. INTRODUCTIONSemiconducting or conducting polymers with spatially ex-tended p−p or p−n−p bonding systems have been intensivelyresearched since they can serve as active components in photo-voltaic cells, light emitting diodes, photodiodes, field effect tran-sistors, and other electronic devices.1,2 Among the conjugatedpolymers, poly(thiophene),3 poly(pyrrole),4,5 and poly(aniline)(PANI),6,7 and their derivatives are widely studied. Significantefforts have been devoted to manipulate the physicochemicalproperties of the conjugated polymers by tailoring the molec-ular structure of either the main chain or pendant groups8−10

or through the formation of the blends,11 laminates,12 orcomposites.13,14 One attractive area relating to conjugatedpolymers is their spectroelectrochemical properties, whichoccur upon the reduction (gain of electrons) or oxidation(loss of electrons), on the passage of an electrical current afterthe application of an appropriate electrode potential.15−17 Theelectrochromic properties of the conducting polymers havegained much popularity both from the academia and industryfor their easy processability, rapid response time, high opticalcontrast, and the ability to modify their structures to createmulticolor electrochromes.18−20 New electrochromic materials,

that is, thin films consisting of PANI and WO3 through molec-ular assembling synthesis21 and multilayer films of PANI andgrapheme22 have been reported.Semiconductor nanocrystals (NCs) have attracted much

interest due to their wide potential engineering applicationssuch as biology and medication, biological fluorescence label-ing and imaging.23−25 The primary focus has been on their opto-electronic properties, displaying a strong size-dependent quantumconfinement effect, which takes place when the quantum wellthickness becomes comparable to the de Broglie wavelength ofthe carriers (generally electrons and holes), leading to energylevels called “energy subbands”.26−28 Among the various kindsof NCs, cadmium selenide (CdSe) quantum dots (QDs) areundoubtedly the most widely studied owing to their tunableemission in the visible range and the availability of precursorsand the simplicity of crystallization.29−31 In order to increasethe photostability of the core and the quantum yield, core−shell structured QDs, for example, CdSe coated with ZnS as a

Received: December 7, 2011Revised: January 10, 2012Published: January 12, 2012

Article

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© 2012 American Chemical Society 4500 dx.doi.org/10.1021/jp2117906 | J. Phys. Chem. C 2012, 116, 4500−4510

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core−shell nanocomposite structure, have been achieved.32−35 Inaddition, the introduced ZnS shell can minimize the toxicity of theheavy metal cadmium.36

By virtue of the advantages of the conducting polymersand NCs, a strategy of combining these two components forapplications in various domains of electronics, for examples, asactive layers in light emitting diodes (LEDs), photodiodes andphotovoltaic cells, has been developed.37−39 It turns out thatthese new hybrid materials may lead to polymer nanocom-posites (PNCs) with properties unmatched by conventionalmaterials. For both of the conjugated polymers and the NCs,their properties can be tuned individually to adapt to eachother. This particularly applies to the positions of the highestoccupied molecular orbital (HOMO) and the lowest un-occupied molecular orbital (LUMO) energy levels, whichdetermine their electronic, redox, and luminescent properties.40

Therefore, conjugated polymer/nanocrystal hybrids are muchmore superior to their corresponding individuals where thedesign flexibility is concerned.N,N′-Di[p-phenylamino(phenyl)]-1,4,5,8-naphthalene tetra-

carboxylic diimide (DNTD), a bis(diphenylamine)naphthalenediimide, belongs to the class of conjugated conducting polymerswith a chemical structural motif of R−X−R, where R representsthe diphenylamine (DPA) group and X represents naphthalenetetracarboxylic diimide (NTCDI) group as the electroactivecenter. Materials made from this monomer exhibit low dielectric,thermal/oxidative stability41 and increased bulk conductivitythrough π-stacking interactions.42 Though the electrochemicaland optical properties of the pure poly(DNTD) polymer thinfilms have been widely investigated and well elucidated by cyclicvoltammetry, UV−visible spectroelectrochemistry,43 and electro-chemical quartz crystal microbalance (EQCM),44 the combinationof the poly(DNTD) polymer with NCs in the nanocomposite thinfilms for using as electrochromic and fluorescent materials havebeen rarely explored.In this study, hybrid electrochromic nanocomposite thin

films consisting of poly(DNTD) and core@shell CdSe@ZnSquantum dots (QDs) were fabricated by the cyclic voltammetrymethod. The physicochemical properties of the nanocompositethin films are studied and compared with those of pure poly-(DNTD) thin films. The electrochemical properties, optical, andspectroelectrochemistry (SEC) properties were studied usingtwo different working electrodes, that is, platinum (Pt) diskwith an area of 0.018 cm2 and indium tin oxide (ITO) coatedglass slides. The effects of temperature on the film growth andelectrochemical properties of the polymer and its nanocompositethin films were also investigated. The composite films showedpromising potential in the electrochromic device applications,while the fluorescent properties were introduced successfully.

2. EXPERIMENTAL SECTION

2.1. Materials. Methylene chloride (CH2Cl2, 99.9%) andtetrabutylammonium hexafluorophosphate (TBAPF6, 98%) werepurchased from Alfar-Aesar. CdSe@ZnS quantum dots withoctadecylamine ligand were supplied by Ocean NanoTech LLC(Springdale, Arkansas USA). The monomer DNTD was syn-thesized following the procedures in the literature,42 and Scheme 1shows the chemical structure of the DNTD. The microscopeglass slides and indium tin oxide (ITO) coated glass slides werepurchased from Fisher and Delta technologies, Limited, respec-tively. ITO coated glass slides were degreased by a mixed aqueoussolution containing 1 mL of 28.86 wt % ammonium hydroxide,

1 mL of 30.0 wt % hydrogen peroxide (both from Fisher), and10 mL of deionized water before the usage.

2.2. Electrochemical Synthesis of Pure Polymer andNanocomposite Thin Films. Pure poly(DNTD) and its QDnanocomposite thin films were prepared by oxidative electro-polymerization from CH2Cl2 solutions containing monomer(0.5 mM), QDs and electrolyte TBAPF6 (0.1 M) with multiplecycles of cyclic voltammetry (CV) at room temperature.Figure 1a−d shows the different colors of the CH2Cl2 solutions

containing only monomer DNTD and QDs/DNTD with dif-ferent QD loadings. CV was performed on an electrochemicalworking station VersaSTAT 4 potentiostat (Princeton AppliedResearch). A typical electrochemical cell consisting of a refer-ence electrode, a working electrode, and a counter electrodewas employed. An Ag/AgCl electrode saturated with KCl servedas the reference electrode and a platinum (Pt) wire as the counterelectrode. ITO coated glass slide and a platinum (Pt) disk with anarea of 0.018 cm2 were used as the working electrode for opticalcharacterizations and electrochemical characterizations, respec-tively. A long path length homemade spectroelectrochemical cellwith Teflon cell body with front and rear windows clapped withtwo steel plates was employed when the ITO glass slide was usedas the working electrode. A typical electrochemical polymer-ization was performed 10 cycles scanned back and forth from0 to 1.8 V vs Ag/AgCl at a scan rate of 200 mV/s.42 Toinvestigate the temperature effect on the performance of the thinfilms, pure poly(DNTD) and its nanocomposite thin films with0.05 wt % of QDs at 0 °C were also prepared for comparison.

2.3. Characterizations. The electrochemical properties ofpure poly(DNTD) and its nanocomposite thin films wereinvestigated by CV scanned from −1.2 to 1.5 V vs Ag/AgClusing Pt disk as the working electrode. The three-electrode cellwas bubbled with ultrahigh purity nitrogen (Airgas) for at least15 min before the scan. The monomer-free solution is 0.1 MTBAPF6 in CH2Cl2. The CVs of all the thin films producedwere 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 micro-scopy (AFM, Agilent 5600 AFM system with multipurpose90 μm scanner). Imaging was done in acoustic ac mode (AAC)using a silicon tip with a force constant of 2.8 N/m and aresonance frequency of 70 kHz.

Scheme 1. Chemical Structure of the Monomer DNTD

Figure 1. Images of DNTD (a) and QDs/DNTD dissolved in CH2Cl2with QD loading of (b) 0.05 wt %, (c) 0.1 wt %, and (d) 0.2 wt %,respectively.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp2117906 | J. Phys. Chem. C 2012, 116, 4500−45104501

UV−vis spectra of the pure DNTD and DNTD/QDsCH2Cl2 solutions with the electrolyte were recorded with a50 bio ultraviolet visible (UV−vis) spectrophotometer. Theirphotoluminescence (PL) spectra were recorded by a VarianCARY Eclipse fluorescence spectrophotometer. The solutionswere put into a standard 10 mm path length quartz cuvette fortesting. The solution of QDs was diluted to 1% of its originalconcentration with CH2Cl2 before measurement. The excita-tion wavelength is 350 nm. The films grown on ITO coated glassslide were further examined by an Olympus BX51 fluorescencemicroscope. Images were obtained using Olympus DP72 cameraaccompanied with Olympus CellSens software.The spectroelectrochemistry measurements were performed

on a Varian Cary 50 Version 3 UV−visible spectrometer coupledwith the potentiostat for applying electrochemical potentials.Solutions in the spectroelectrochemistry (SEC) cell were treatedby the same manner bubbled with ultrahigh purity nitrogen for atleast 15 min. Each spectrum was taken when the electrochemicalcell current decayed to zero and the data was obtained in themode of medium in Varian Cary WinUV Bio software. At eachapplied potential, a minimum of three spectra were taken toverify consistency.

3. RESULTS AND DISCUSSION3.1. Synthesis of Poly(DNTD) and Poly(DNTD)/QD

Nanocomposite Films. Figure 2a,b shows the UV−vis ab-sorption (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 aflat curve without any absorption peaks. The QDs show two ab-sorption peaks at 562 and 595 nm, corresponding to the ab-sorption 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 isPlanck’s constant, 4.135 × 10−15 eV·s) characteristic of anarrow-size distribution of CdSe@ZnS QDs.45 Cyclic voltam-metry has been used widely to determine the HOMO andLUMO levels of QDs.46,47 The onsets of the first oxidationpeak and reduction peak in the CV are assumed to correspondto the withdraw of an electron from the bonding HOMO andan addition of an electron to the antibonding LUMO, respec-tively.48 The onset potential is determined from the intersec-tion of the two tangents drawn at the rising oxidation or reductioncurrent and background current in the CV.49,50 Figure 3a,bshows the reduction voltammetry of the bare Pt (without QDs)(a) and 0.2 wt % QDs (b) in CH2Cl2 solution. The onsetpotential of −0.55 V vs Ag/AgCl, Figure 3b, could be obtainedemploying the graphical method. Therefore, the LUMO energywith respect to the vacuum value of QDs is calculated to be−3.82 eV on the basis of the assumption that the energy valueof Ag/AgCl is −4.37 eV in a vacuum.51−53 The bandgap (energydifference between HOMO and LUMO) of the QDs is calculatedto be 2.10 eV at 595 nm in the UV−vis spectra, Figure 2a, for theQDs in CH2Cl2 solutions. Correspondingly, the HOMO energyof the QDs with respect to the vacuum value is calculated to be−5.92 eV. Similar HOMO and LUMO energy values have beenobtained by others.54 The measured bandgap value shows a blueshift 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 ofthe low concentration of the QDs, and the absorption positionsshift to longer wavelength, indicating an interaction between QDsand the monomer. The fluorescence spectra, Figure 2b, depict the

emission peak of the QDs solution at 612 nm at an excitationwavelength of 350 nm. However, the QD/DNTD solutions showa shifted emission peak at 616 nm, consistent with the result ofUV−vis analysis, that some interaction between QDs and themonomer does exist.Figure 4a−d shows the cyclic voltammetric growth of pure

poly(DNTD) films and its nanocomposite films with a QDloading of 0.05, 0.1, and 0.2 wt %, respectively, on a Pt elec-trode (0.018 cm2), scanned in CH2Cl2 solution containing 0.1 MTBAPF6 supporting electrolyte at room temperature. The scan

Figure 2. (a) UV−vis absorption and (b) fluorescence spectra of QDs,TBAPF6, QDs/TBAPF6, and QDs/DNTD with different loadings ofQDs in CH2Cl2. The excitation wavelength for fluorescence spectra is350 nm.

Figure 3. Reduction voltammetry recorded on Pt electrode in the (a)absence and (b) presence of QDs (0.2 wt %) in CH2Cl2 with TBAPF6as the supporting electrolyte. The scan rate is 200 mV/s.



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 CVshapes. During the first positive scan, an irreversible peak appeared,which is attributed to the irreversible oxidation of diphenyl-amine.55−57 For the first negative scan and the second positivescan, two sets of quasi-reversible peaks are observed, consistentwith the formation of diphenylbenzidene (DPB) unit. Thesetwo peaks correspond to the oxidation of DPB to form cation(DPB+) and dication (DPB2+), respectively.42 The possiblestructures of DPB+ and DPB2+ are shown in Scheme 2.58 Thecurrent increased with increasing cycles, reflecting the growthof pure polymer and nanocomposite thin films. The polymer-ization occurs via the coupling of the para carbon on theterminal phenyl group, followed by proton loss and additionalelectron transfer reactions.43 Scheme 3 illustrates the proposedpolymerization mechanism (a) and possible reactions betweenthe dimer/oligmers and the ligands on the QD surface (b). The

ligands on the QD surface were oxidized to form radical cationsand served as the terminating radicals to react with the initialradical cation of the diphenylamine group in the dimer/oligmers.There are also chances that QDs ligands formed dications tocouple with or even more two dimer/oligmers.However, some differences between the pure poly(DNTD)

polymer and the nanocomposite thin films can be observed

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 roomtemperature, 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 CH2Cl2solution 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



upon closer observation. First, the irreversible anodic potentialfor 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 aQD loading of 0.05, 0.1, and 0.2 wt %, respectively. Figure 5a,bshows the oxidation voltammetry of bare Pt (a) and 0.2 wt %QDs (b) in CH2Cl2 solution. In the positive sweep, an obviousanodic peak at around +1.3 V vs Ag/AgCl was observed forQDs, corresponding to the oxidation of the QDs octadecyl-amine ligands. Therefore, these positive potentials for compositefilms are supposed to be caused by the combined irreversibleoxidation of the QDs octadecylamine ligands with thediphenylamine in the monomer as shown in Scheme 3b. Second,for the growth of pure poly(DNTD) polymer thin film, the peakat 1.0 V vs Ag/AgCl corresponding to the 1 e− oxidation of DBPbecame less distinct upon repeated CV scans. However, for thenanocomposite thin films, the peak at 1.0 V vs Ag/AgCl appearedmore pronounced and did not undergo appreciable changes inintensity with increasing the scans. This indicates that QDsfavor the one-electron transfer process and stabilize the state ofradical DPB+.At 0 °C, Figure 4e,f, DNTD monomers could still be electro-

polymerized. The thin films exhibit similar CV shapes comparedto those at room temperature except the anodic current becamecomparatively lower, indicating a lowed polymerization rate.59

However, both the anodic and cathodic current potentials exhibitnegligible differences between the CVs at two different temper-atures. This phenomenon suggests that a similar electropolymeriza-tion mechanism occurred for the thin films growing at room tem-perature and 0 °C even though the reaction rate was different.

Scheme 3. (a) Polymerization Mechanism of Pure Polymer and (b) Possible Reactions between Oxidized QDs and Dimer/Oligomers

Figure 5. Oxidation voltammetry recorded on Pt electrode in the (a)absence and (b) presence of QDs (0.2 wt %) in CH2Cl2 with TBAPF6as the supporting electrolyte. The scan rate is 200 mV/s.



3.2. Electrochemical Properties. After the electro-polymerization, the electrode with a thin film on the surfaceof Pt disk was rinsed with fresh CH2Cl2 and then was placedin a degassed CH2Cl2 solution containing only 0.1 M TBAPF6supporting electrolyte, and was cycled again. Figure 6a−dshows the CV of poly(DNTD) and poly(DNTD)/QDs nano-composite thin films, respectively, on a Pt electrode. The scanrate is 200 mV/s and scan range is from −1.2 to +1.5 V vsAg/AgCl. The CV data of pure polymer and nanocompositefilms is summarized in Table 1. The half wave potentials of Eox

and Ered are obtained by taking the average of their anodic andcathodic peaks, i.e., Eox = (Epa

ox+ Epcox)/2 and Ered = (Epa

red+Epc

ox)/2.48 For the pure poly(DNTD) thin films, Figure 6a, ineither positive (potential larger than 0 V vs Ag/AgCl) ornegative (potential lower than 0 V vs Ag/AgCl) scan, two pairsof well-defined redox peaks are observed. Therefore, poly-(DNTD) can be oxidized and reduced, indicating its p-nbipolar properties. In the positive scan, the two sets of theredox peaks, one with an Eox of about 0.94 V vs Ag/AgCl andthe other one of about 1.15 V vs Ag/AgCl, are associatedwith the oxidation of poly(DNTD) into DPB+ cation andDPB2+ dication.42 The pure polymer thin films also show twoseparated cathodic peaks, one at about −0.39 V vs Ag/AgCl

and the other at −0.82 V vs Ag/AgCl in the negative scan.These two reduction peaks are attributed to the formation ofthe naphthalene diimide anion and naphthalene diimide dianion,respectively.42 The possible chemical structures of those anionsand dianions are shown in Scheme 4. For poly(DNTD)/QDs

nanocomposite thin films, Figure 6b−d, during the forwardnegative scan, the cathodic potential did not change too muchcompared to those of the pure poly(DNTD) thin films, one atabout −0.37 vs Ag/AgCl and the other at −0.83 V vs Ag/AgCl,Table 1. Therefore, the same redox species are produced. Thusthe QDs have negligible effect on the naphthalene diimidemoiety in the polymer. When the potential began to sweepbackward, asymmetry in the oxidation/reduction process wasobserved. The anodic peak at around 0.94 V vs Ag/AgCldisappeared and only one single prominent oxidation peakat 1.17 V vs Ag/AgCl appeared, and the oxidation potentialshifted toward more positive positions with the increase ofQD loading. Meanwhile, in the cathodic branch of the cyclicvoltammogram, two distinguishable reduction peaks instead ofone were observed. This asymmetric behavior might be attrib-uted to the heterogeneity in the polymer during the eletro-polymerization as shown in Scheme 3. The incorporated QDsgave rise to different polymers with varying conjugated chainlength. In addition, the introduction of QDs in the polymermay also affect the morphology or crystallinity of the polymer

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.

Table 1. Electrochemical Data of Polymer Films Synthesizedat Room Temperaturea

polymer filmsfirst

Eox (V)secondEox (V)

first Ered

(V)secondEred (V) Eg,EC (eV)b

pure polymer 0.94 1.15 −0.39 −0.82 1.330.05 wt % of QDs 1.17 −0.37 −0.83 1.540.1 wt % of QDs 1.23 −0.37 −0.83 1.600.2 wt % of QDs 1.24 −0.37 −0.83 1.61aPotentials vs Ag/AgCl; 0.1 M TBAPF6, CH2Cl2, scan rate: 200 mV/s.bEg, EC=1st E

ox − 1st Ered.

Scheme 4. Possible Anion and Dianion Chemical Structuresof DNTD



thin film, which also play a part in influencing the electro-chemical properties of the nanocomposite films.The first Eox and Ered provide some valuable information on

the HOMO and LUMO levels regarding the polymer and com-posite films.60 Given the higher first Eox for the nanocompositefilms, Table 1, it can be inferred that poly(DNTD) becamemore difficult to be oxidized. On the other side, the bandgapobtained by the electrochemical method, Eg,EC, of the polymer

increased with the increase of the loading of QDs. Eg,EC iscalculated to be 1.33, 1.54, 1.60, and 1.61 eV, Table 1, for thepure polymer and the nanocomposite films with a QD loadingof 0.05, 0.1, and 0.2 wt %, respectively. The phenomenon mightbe explained that the polymer became less planar and less con-jugated in the hybrid films, thus giving rise to a widenedband gap.48 Similar phenomena have been observed for thenanocomposites of the diaminopyrimidine functionalized

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.



poly(3-hexylthiophene) and thymine-capped CdSe nanocrystalsas a result of the formation of hydrogen bonding between thepolymer and the nanocrystals.50

Cyclic voltammograms of poly(DNTD) and poly(DNTD)/QDs nanocomposite films at 0 °C were also performed to in-vestigate the effect of temperature on the electrochemical prop-erties of these films prepared at room temperature. Figure 7a−dshows the CVs of pure poly(DNTD) and QDs/poly(DNTD)nanocomposite thin films with a QD loading of 0.05, 0.1, and

0.2 wt %, respectively, on a Pt electrode (0.018 cm2), scannedin CH2Cl2 solution containing 0.1 M TBAPF6 supporting elec-trolyte 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 interestingphenomenon is also observed for the CVs of the polymersprepared at 0 °C, Figure 8. The polymer films appear to havemore symmetrical CV shapes at 0 °C than at room tempera-ture. Many factors such as interactions among charged sites,61,62

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 % atroom temperature, and of (e) pure polymer at 0 °C, respectively, grown on ITO glass.



differences in the properties of the oxidized and neutral polymer,63

or heterogeneity of the polymer film64,65 have been proposedfor the asymmetry in the redox behavior of the polymers. Herethe asymmetry of the pure polymer might be caused by theconformational change of the polymer backbone induced bythe temperature difference between the film growth and CV

test. For example, the planar chain of the polymer film syn-thesized at room temperature is more subjected to be a twistedconformation when the CV test was conducted at 0 °C. For thenanocomposite thin films, the main reason might lie in thereduced conjugated length caused by the incorporation of theQDs in the polymer chains.

3.3. Morphology and Optical Properties. The polymerand nanocomposite thin films grown on the ITO coated glassslides were rinsed with CH2Cl2 and dried for AFM character-ization. Figure 9 presents the morphology and thickness of thefilms grown at room temperature (a−e) and pure polymer filmgrown at 0 °C (f). At lower resolution, these films exhibitcompact and continuous film structures. However, under highresolution, evenly sized and uniformly distributed nanoparticlesare observed. For the films synthesized at room temperature,the thickness of the film increased with increasing the QDloading. The thickness is roughly estimated to be 17, 25, 30,and 35 nm for the pure polymer and nanocomposite filmswith a QD loading of 0.05, 0.1, and 0.2 wt %, respectively.Accordingly, the particle size increases with increasing thethickness of the film. This morphology change induced by theQDs may account for the changes in the electrochemical prop-erties of the nanocomposite films compared to that of the purepolymer. At lower temperature 0 °C, the pure poly(DNTD)polymer film exhibits more orientated and smaller particlesthan that synthesized at room temperature. This phenomenonis 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 examinedunder the fluorescent microscope. Figure 10 shows the fluo-rescent microscope images of the pure poly(DNTD) polymer

Figure 10. Bright field fluorescent microscopy images of (a) purepolymer, 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 indark field.

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.



and nanocomposite films. The pure polymer thin film shows nofluorescent emission signal as expected, Figure 10a. However,in the nanocomposite thin films with a QD loading of 0.05 and2.0 wt %, uniform red emission signals are observed, indicatingthat QDs were fairly uniformly distributed in the hostingpoly(DNTD) polymer matrix.3.4. Spectroelectrochemistry of Polymer and Compo-

site Films. Polymer and nanocomposite thin films can be bothoxidized and reduced as revealed from the results of CVs;therefore, it is expected that the visible spectra shall be changedwhen subjected to different potentials.67,68 Figure 11 shows thespectroelectrochemistry of pure poly(DNTD) and nano-composite thin films deposited on the ITO coated glass slidesat different potentials. The spectrum of the neutral films ofpoly(DNTD) and nanocomposites show a less absorption bandabove 400 nm. When the potential was increased to 0.9 V vsAg/AgCl, a peak appeared at 450 nm, which is assigned to bethe formation of the cation (DPB+).42 As the potential wasfurther increased to 1.4 V vs Ag/AgCl, the band at 450 nmdisappeared, and a broad absorption at 613 nm was observed,corresponding to the oxidation of the cation (DPB+) to thedication (DPB2+). Correspondingly, the thin film turns to blue.At negative potential, for example, −0.5 V vs Ag/AgCl, a peakat 480 nm was observed, which corresponds to the naphthalenediimide anion absorption. The films remain in the anion stateduring the reduced potential. As for the nanocomposite thinfilms, similar absorption spectra were observed when the appliedpotentials 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

1.4 V vs Ag/AgCl, respectively. At 0 V vs Ag/AgCl, thenanocomposite film appeared nearly transparent; when appliedto a potential of 1.4 V vs Ag/AgCl, the film turned to blue. Theunique electrochromic properties promise potential applicationsof these hybrid nanocomposite thin films in the electrochromicdevices.

4. CONCLUSIONHybrid electrochromic composite films composed of poly-(DNTD) and CdSe@ZnS quantum dots (QDs) have beensynthesized by electrodeposition and were electrochemicallyand spectroelectrochemically characterized. QDs are observedto be fairly uniformly dispersed in the polymer matrix. Thecomposite films showed bipolar properties and became lesseasily oxidized with a widened bandgap as a result of reducedconjugated length caused by the incorporation of QDs in thepolymer chain. The spectroelectrochemistry (SEC) propertiesof the films grown on ITO exhibit varied absorption bandswhen applied to different potentials and the color turned blueat high positive potential, for example, 1.4 V vs Ag/AgCl. Theeffect of temperature on the composite films growth and theirelectrochemical properties were also studied by CVs, showingless asymmetry in the redox process and lowered oxidationpotential at low temperature. These new hybrid nanocompositethin films exhibit both unique fluorescent properties and novelelectrochromic phenomena.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] (Z.G.); [email protected](S.W.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe financial support from the National Science Foundation-Nanoscale Interdisciplinary Research Team and Materials Process-ing and Manufacturing under Grant Number of CMMI 10-30755is kindly acknowledged.

■ REFERENCES(1) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608−622.(2) Kim, H.; Abdala, A. A.; Macosko, C. W. Macromolecules 2010, 43,6515−6530.(3) Adachi, T.; Brazard, J.; Ono, R. J.; Hanson, B.; Traub, M. C.; Wu,Z. Q.; Li, Z.; Bolinger, J. C.; Ganesan, V.; Bielawski, C. W. J. Phys.Chem. Lett. 2011, 2, 1400−1404.(4) Zhu, J.; Wei, S.; Zhang, L.; Mao, Y.; Ryu, J.; Mavinakuli, P.; Karki,A. B.; Young, D. P.; Guo, Z. J. Phys. Chem. C 2010, 114, 16335−16342.(5) Ding, K.; Jia, H.; Wei, S.; Guo, Z. Ind. Eng. Chem. Res. 2011, 50,7077−7082.(6) Wang, C. C.; Song, J. F.; Bao, H. M.; Shen, Q. D.; Yang, C. Z.Adv. Funct. Mater. 2008, 18, 1299−1306.(7) Zhu, J.; Wei, S.; Zhang, L.; Mao, Y.; Ryu, J.; Haldolaarachchige,N.; Young, D. P.; Guo, Z. J. Mater. Chem. 2011, 21, 3952−3959.(8) Bidan, G.; Jarjayes, O.; Fruchart, J. M.; Hannecart, E. Adv. Mater.1994, 6, 152−155.(9) Li, M.; Xu, P.; Yang, J.; Ying, H.; Haubner, K.; Dunsch, L.; Yang,S. J. Phys. Chem. C 2011, 115, 4584−4593.(10) Lowe, J.; Holdcroft, S. Macromolecules 1995, 28, 4608−4616.(11) Meeker, D. L.; Mudigonda, D. S. K.; Osborn, J. M.; Loveday,D. C.; Ferraris, J. P. Macromolecules 1998, 31, 2943−2946.(12) Boehme, J. L.; Mudigonda, D. S. K.; Ferraris, J. P. Chem. Mater.2001, 13, 4469−4472.

Figure 12. Color change for composite film with 0.05 wt % loading ofQDs when applied to (a) 0 V, and (b) 1.4 V, respectively.



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(13) Mavinakuli, P.; Wei, S.; Wang, Q.; Karki, A. B.; Dhage, S.; Wang,Z.; Young, D. P.; Guo, Z. J. Phys. Chem. C 2010, 114, 3874−3882.(14) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem.Mater. 2003, 15, 1575−1586.(15) Rosseinsky, D. R.; Mortimer, R. J. Adv. Mater. 2001, 13, 783−793.(16) Mortimer, R. J. Chem. Soc. Rev. 1997, 26, 147−156.(17) Niklasson, G. A.; Granqvist, C. G. J. Mater. Chem. 2006, 17,127−156.(18) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Displays 2006, 27,2−18.(19) Argun, A. A.; Aubert, P. H.; Thompson, B. C.; Schwendeman, I.;Gaupp, C. L.; Hwang, J.; Pinto, N. J.; Tanner, D. B.; MacDiarmid,A. G.; Reynolds, J. R. Chem. Mater. 2004, 16, 4401−4412.(20) Zhu, J.; Wei, S.; Alexander, M. Jr.; Dang, T. D.; Ho, T. C.; Guo,Z. Adv. Funct. Mater. 2010, 20, 3076−3084.(21) Zhang, J.; Tu, J.; Zhang, D.; Qiao, Y.; Xia, X.; Wang, X.; Gu, C.J. Mater. Chem. 2011, 21, 17316−17324.(22) Sheng, H. B. K.; Sun, Y.; Li, C.; Shi, G. Polymer 2011, 52, 5567−5572.(23) Gerion, D.; Pinaud, F.; Williams, S. C.; Parak, W. J.; Zanchet,D.; Weiss, S.; Alivisatos, A. P. J. Phys. Chem. B 2001, 105, 8861−8871.(24) Gao, J.; Xu, B. Nano Today 2009, 4, 37−51.(25) Patra, M.; Manoth, M.; Singh, V.; Siddaramana Gowd, G.;Choudhry, V.; Vadera, S.; Kumar, N. J. Lumin. 2009, 129, 320−324.(26) Norris, D.; Sacra, A.; Murray, C.; Bawendi, M. Phys. Rev. Lett.1994, 72, 2612−2615.(27) Efros, A. L.; Rosen, M. Annu. Rev. Mater. Sci. 2000, 30, 475−521.(28) Nirmal, M.; Brus, L. Acc. Chem. Res. 1999, 32, 407−414.(29) Frasco, M. F.; Chaniotakis, N. Sensors 2009, 9, 7266−7286.(30) Wang, Y.; Tang, Z.; Correa-Duarte, M. A.; Pastoriza-Santos, I.;Giersig, M.; Kotov, N. A.; Liz-Marzan, L. M. J. Phys. Chem. B 2004,108, 15461−15469.(31) Suo, B.; Su, X.; Wu, J.; Chen, D.; Wang, A.; Guo, Z. Mater.Chem. Phys. 2010, 119, 237−242.(32) Weller, H. Angew. Chem., Int. Ed. 1993, 32, 41−53.(33) Murphy, C. J. Anal. Chem. 2002, 74, 520−526.(34) Dabbousi, B.; Rodriguez-Viejo, J.; Mikulec, F.; Heine, J.;Mattoussi, H.; Ober, R.; Jensen, K.; Bawendi, M. J. Phys. Chem. B1997, 101, 9463−9475.(35) Willard, D. M.; Carillo, L. L.; Jung, J.; Van Orden, A. Nano Lett.2001, 1, 469−474.(36) Wei, S.; Sampathi, J.; Guo, Z.; Anumandla, N.; Rutman, D.;Kucknoor, A.; James, L.; Wang, A. Polymer 2011, 52, 5817−5829.(37) Colvin, V.; Schlamp, M.; Alivisatos, A. Nature 1994, 370, 354−357.(38) Holder, E.; Tessler, N.; Rogach, A. L. J. Mater. Chem. 2008, 18,1064−1078.(39) Descalzo, A. B.; Martínez-Manez, R.; Sancenon, F.; Hoffmann,K.; Rurack, K. Angew. Chem., Int. Ed. 2006, 45, 5924−5948.(40) Reiss, P.; Couderc, E.; De Girolamo, J.; Pron, A. Nanoscale2010, 3, 446−489.(41) Sroog, C. Prog. Polym. Sci. 1991, 16, 561−694.(42) Wang, L.; Goodloe, G.; Stallman, B.; Cammarata, V. Chem.Mater. 1996, 8, 1175−1181.(43) Wang, L.; Wang, Q.; Cammarata, V. J. Electrochem. Soc. 1998,145, 2648−2654.(44) Cammarata, V.; Hao, N.; Metz, J.; Liang, J. ACS SymposiumSeries 832; American Chemical Society: Washington, DC, 2003; pp59−74.(45) Pan, A.; Yang, H.; Liu, R.; Yu, R.; Zou, B.; Wang, Z. J. Am.Chem. Soc. 2005, 127, 15692−15693.(46) Inamdar, S. N.; Ingole, P. P.; Haram, S. K. ChemPhysChem2008, 9, 2574−2579.(47) Haram, S. K.; Quinn, B. M.; Bard, A. J. J. Am. Chem. Soc. 2001,123, 8860−8861.(48) Johansson, T.; Mammo, W.; Svensson, M.; Andersson, M. R.;Inganas, O. J. Mater. Chem. 2003, 13, 1316−1323.

(49) Li, Y.; Cao, Y.; Gao, J.; Wang, D.; Yu, G.; Heeger, A. J. Synth.Met. 1999, 99, 243−248.(50) De Girolamo, J.; Reiss, P.; Zagorska, M.; De Bettignies, R.;Bailly, S.; Mevellec, J. Y.; Lefrant, S.; Travers, J. P.; Pron, A. Phys.Chem. Chem. Phys. 2008, 10, 4027−4035.(51) Polec, I.; Henckens, A.; Goris, L.; Nicolas, M.; Loi, M.;Adriaensens, P.; Lutsen, L.; Manca, J.; Vanderzande, D.; Sariciftci, N.J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1034−1045.(52) Liu, Y.; Liu, M.; Jen, A. K. Y. Acta Polym. 1999, 50, 105−108.(53) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bassler,H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7, 551−554.(54) Skompska, M. Synth. Met. 2010, 160, 1−15.(55) Yang, H.; Bard, A. J. J. Electroanal. Chem. Interfacial Electrochem.1991, 306, 87−109.(56) Yang, H.; Bard, A. J. J. Electroanal. Chem. 1992, 339, 423−449.(57) Yang, H.; Wipf, D. O.; Bard, A. J. J. Electroanal. Chem. 1992,331, 913−924.(58) Li, Y.; Patil, R.; Wei, S.; Guo, Z. J. Phys. Chem. C 2011, 115,22863−22869.(59) Wei, Y.; Sun, Y.; Tang, X. J. Phys. Chem. C 1989, 93, 4878−4881.(60) Yang, Y.; Arias, F.; Echegoyen, L.; Chibante, L. P. F; Flanagan,S.; Robertson, A.; Wilson, L. J. J. Am. Chem. Soc. 1995, 117, 7801−7804.(61) Roncali, J.; Marque, P.; Garreau, R.; Garnier, F.; Lemaire, M.Macromolecules 1990, 23, 1347−1352.(62) Zotti, G.; Schiavon, G. Synth. Met. 1989, 31, 347−357.(63) Chao, F.; Costa, M. Synth. Met. 1990, 39, 97−102.(64) Skompska, M. Electrochim. Acta 1998, 44, 357−362.(65) Kobayashi, S.; Nishikawa, T.; Takenobu, T.; Mori, S.; Shimoda,T.; Mitani, T.; Shimotani, H.; Yoshimoto, N.; Ogawa, S.; Iwasa, Y. Nat.Mater. 2004, 3, 317−322.(66) Painter, P. C.; Coleman, M. M. Fundamentals of Polymer Science;Technomic: Lancaster, PA, 1997.(67) Gale, R. J. Spectroelectrochemistry: Theory and Practice; Springer:Berlin, 1988.(68) Zoski, C. G. Handbook of Electrochemistry; Elsevier Science:Amsterdam, 2007.



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