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Structural, photoconductivity, and dielectric studies of polythiophene-tin oxide nanocomposites
Accepted Manuscript Title: Structural, photoconductivity, and dielectric studies of polythiophene-tin oxide nanocomposites Author: S. Murugavel M. Malathi PII: DOI: Reference:
S0025-5408(16)30199-4 http://dx.doi.org/doi:10.1016/j.materresbull.2016.05.004 MRB 8770
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Please cite this article as: S.Murugavel, M.Malathi, Structural, photoconductivity, and dielectric studies of polythiophene-tin oxide nanocomposites, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.05.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural, photoconductivity, and dielectric studies of polythiophene-tin oxide nanocomposites S.Murugavel1, M.Malathi* Condensed Matter Research Laboratory Materials Physics Division School of Advanced Sciences VIT University Fax: +91416 2243092 Tel: +91416 2202453 E-mail address: [email protected] (M.Malathi*) [email protected] (S. Murugavel) *Corresponding author
Graphical abstract
Highlights
Synthesis of polythiophene-tin oxide nanocomposites confirmed by FTIR and EDAX
SEM shows SnO2 nanoparticles embedded within polythiophene matrix
Stability and isoelectric point suggest nanoparticle–matrix interaction
High dielectric constant due to high Maxwell–Wagner interfacial polarization
Abstract Polythiophene-tinoxide (PT-SnO2) nanocomposites were prepared by in situ chemical oxidative polymerization, in the presence of various concentrations of SnO2 nanoparticles. Samples were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and Zeta potential measurements. Morphologies and elemental compositions were investigated by transmission electron microscopy, field-emission scanning electron microscopy and energy-dispersive Xray spectroscopy. The photoconductivity of the nanocomposites was studied by fielddependent dark and photo conductivity measurements. Their dielectric properties were
investigated using dielectric spectroscopy, in the frequency range of 1kHz‒1MHz. The results indicated that the SnO2nanoparticlesin the PT-SnO2nanocomposite were responsible for its enhanced dielectric performance.
Keywords: A. composites, D. dielectric properties, C. photoelectron spectroscopy, D. electrical properties, C. electron microscopy
1. Introduction Conducting polymers and their nanocomposites have been an important research area since their discovery in the mid-1970s. Conducting polymer-metal oxide nanocomposites have been extensively investigated, because of their interesting physiochemical properties and potential application in nanodevices [1-5]. Polymer nanocomposites have been synthesized by various physical and chemical methods, including insitu chemical oxidation, electrochemical polymerization, and melt processing [6-8]. Those with high dielectric constants are used in charge-storage devices, telecommunications, electromagnetic interference shielding, integral capacitor technology, and electromechanical applications [912]. Polythiophene (PT) is a promising conducting material, because of its high and controllable electrical conductivity[13-15]. Its low solubility and poor processability limit its application, so PT has been incorporated with metal oxide nanoparticles to overcome these problems. Incorporating metal oxide nanoparticles can enhance the electrical properties of conjugated polymers. PT composites containing Fe3O4, Al2O3, and TiO2 have all been thoroughly studied [16-18]. Nanoscale metal oxide particles are of particular interest, because of their size-dependent physical and chemical properties. SnO2 is a wide band gap (3‒6 eV) n-type semiconductor used in many applications, including electrode materials for Libatteries, gas sensors and antistatic coatings [1921].Optical charge generation and transport in photoconducting PT derivatives is receiving much current interest[22]. Photoconductivity is the enhancement of a material’s electrical conductivity by absorbing photons of a suitable energy. Polyfluorene, its copolymers including poly(para-phenylene vinylene), and substituted PT derivatives are all widely used in this field [23]. Photoconducting substituted PTs have applications in electronic devices including electro-optic modulators, optical signal processors, Photoreceptors, solarcells, and optical frequency doublers [24]. Photoconducting devices based on polymer composites with
internal donor/acceptor heterojunctions have also been investigated [25]. However, there are no reported studies investigating the photoconductivity of PT-SnO2 nanocomposites. PT and SnO2 are both widely applied in technology. Thus, PT-SnO2 nanocomposites were thought likely to possess interesting properties, which could potentially be useful when designing and developing polymer electronic devices. While there are some reported syntheses and morphological studies of PT-metal oxide nanocomposites [26-28], to the best of our knowledge none have probed the electric behavior of the PT-SnO2 nanocomposites. In addition to requiring good photoconductivity and dielectric performance, other properties of the material are also important when developing novel polymer electronic materials. These include structural order (i.e., morphology yielding optimum efficient charge transport), thermal stability, and surface charge. In the current study, we report the synthesis, structural characterization, zetapotential and photoconductivity measurements of PT-SnO2 nanocomposites. The dielectric behavior of PT-SnO2 nanocomposites containing 10, 20, 30, and 40 wt.% SnO2 was investigated over the frequency range of 1kHz‒1MHz. 2. Experimental 2.1. Materials All chemicals used were of analytic reagent (AR) grade. PT was prepared from freshly double distilled thiophene (Aldrich). Anhydrous ferric chloride (FeCl3) was purchased from Fluka. Stannouschloride dihydrate(SnCl2·2H2O), chloroform and ammonia solution were purchased from Merck. Deionized water was used in all reactions. 2.2. Synthesis of SnO2 nanoparticles SnO2 nanoparticles were synthesized by coprecipitation, using SnCl2 as a Sn source. 4g (0.2mol) of SnCl2·2H2O was dissolved in 200ml of water. 6 ml of ammonia solution was added, and the mixture was stirred vigorously at room temperature for 1.5 h to form a
precipitate. The white gel precipitate was allowed to settle for 14 h, collected by filtration and thoroughly rinsed with distilled water. The precipitate was heated at 110°C until dry, and then calcined at 500°C for 5h to yield SnO2 nanoparticles. 2.3. Preparation of PT-SnO2nanocomposite The PT-SnO2 nanocomposite was prepared by in situ chemical oxidative polymerization of thiophene monomer, in the presence of SnO2 nanoparticles. In a typical synthesis, 0.2g of SnO2 nanoparticles and 0.05 mol of thiophene were dispersed in 100 ml of chloroform and stirred for several minutes. 0.25 mol of anhydrous FeCl3 in chloroform was added under vigorous stirring. The reaction mixture was stirred with a magnetic stirrer bar at room temperature for 4h, during which its color changed from grey to black. The precipitate was collected by filtration, and washed thoroughly with methanol and doubly distilled water to remove unreacted oxidants and monomer. The resulting powder was dried under vacuum at 70°C for 8h. PT-SnO2 nanocomposites were synthesized containing 10, 20, 30, and 40 wt.% of SnO2 nanoparticles. PT was prepared similarly, but in the absence of SnO2 nanoparticles. 2.4. Characterization X-ray diffraction (XRD) patterns were collected at 2θ of 10‒80° using a diffractometer (XRD-Smart Lab, Rigaku, Japan). XRD patterns were analyzed by indexing observed peaks with standard JCPDS values. Fourier transform infrared (FTIR) spectra were obtained at 4000‒400cm−1 using a FTIR spectrometer (Spectrum RX1, Perkin Elmer, MA, USA). Sample morphologies were observed by transmission electron microscopy (TEM; JEOL 3010, Japan) and field-emission scanning electron microscopy (FESEM; SUPRA 55, Carl Zeiss, Germany). Energy-dispersive X-ray analysis (EDAX) was used to determine the elemental compositions of PT and the PT-SnO2 nanocomposites. Thermal stabilities of the
samples were investigated by thermo gravimetric analysis (TGA: TG/DTA 6200), at temperatures from 30 to 900°C at a heating rate of 10°C/min in a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) (AXIS ULTRA from AXIS 165) was used to obtain information about the chemical states of samples. Zeta potential measurements (Horiba) were used to determine the surface charge and isoelectric points of samples in water. Photoconductivity measurements were conducted using a Keithley Picoammeter 6485. PT and PT-SnO2 powders were finely ground with an agate mortar. Pellets of~10mm in diameter and 2mm in thickness were prepared by hydraulically pressing powdersat5.5t. Silver paste was then coated on each face to create contact with two electrodes. The pellet was then placed into sample holder. Dielectric measurements were carried out using a LCR HiTester apparatus (HIOKI 3532-50LCRHiTester, Japan), over the frequency range 1kHz‒1MHz. 3. Results and Discussion 3.1. XRD Fig. 1a shows the XRD pattern of PT, which containeda peak at 2θ of 21.5°. Fig. 1b shows the XRD pattern of SnO2 nanoparticles, in which all peaks were well assigned to the tetragonal rutile structure of SnO2 with high crystallinity. No diffraction peaks for impurities were detected. The average crystallite size (D) was calculated to be 30 nm, using the DebyeScherrer formula (D=0.9λ/βcosθ, where λ is the X-ray wavelength, β is the full width at half maximum intensity, and θ is the diffraction angle). Fig. 1c shows the XRD pattern of the PTSnO2 nanocomposite containing 10 wt.% SnO2, which confirmed the presence of SnO2. All major XRD peaks for SnO2 were observed in the pattern of the PT-SnO2 nanocomposite, implying that PT was deposited on the SnO2 nanoparticle surface. 3.2. FTIR spectroscopy Fig. 2 shows the FTIR spectra of PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2. The characteristic absorption peaks of PT at 2782, 1643, 1042, 797, 630and
498cm−1corresponded to the C‒H aromatic stretching, C=C symmetric stretching, C‒H inplane deformation, C‒H out-of-plane stretching, C‒S thiophene ring stretching and C‒S‒C ring deformation, respectively. The FTIR spectrum of the PT-SnO2 nanocomposite contained these characteristic peaks of PT, but those at 2782, 1643, 1042, 797, 630 and 498 cm−1were shifted to 2350, 1699, 1110, 803, 613 and 486 cm−1, respectively, in the spectrum of the PTSnO2 nanocomposite. These shifts resulted from the incorporation of the SnO2 nanoparticles in PT, and suggested a significant interaction between SnO2 and PT. Interaction may have been via SnO2 and the PT sulfur moiety. Hydrogen bonding between SnO2 and PT may also have contributed to the shift in these characteristic absorption peaks. 3.3. TEM Understanding a sample’s particle size can be used to infer quantum size effects. The samples were investigated using TEM to determine their particle sizes. Fig.3a, b and c show TEM images of SnO2, PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2, respectively. The SnO2 particles had an average diameter of approximately 15nm, which was lower than the crystallite size of 30 nm estimated from the XRD results and Scherrer’s formula. The TEM image of the PT-SnO2 nanocomposite in Fig.3c indicates that SnO2 nanoparticles were dispersed within the PT matrix. 3.4. FESEM FESEM images of the samples are shown in Fig. 4. The image of the SnO2 nanoparticles in Fig. 4a shows a typical morphology distribution, in which the nanoparticles were predominantly spherical with diameters of 20‒80nm. A net-like structure was apparent in the image of PT in Fig.4b.SnO2 nanoparticles were uniformly distributed throughout the net-like matrix of PT, in the image of the nanocomposite in Fig. 4c. The morphology of the nanocomposite was highly micro-porous, which was responsible for the high interfacial area
between the two different phases. High interfacial areas led to strong interaction between the PT and SnO2 nanoparticles. 3.5. EDAX Table 1shows the elemental compositions of PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2, as obtained by EDAX. The results were consistent with the presence of SnO2 in the nanocomposite. 3.6. TGA The thermal stability of the samples was investigated by TGA under a N2 atmosphere. Fig.5 shows the TGA profile for PT and the nanocomposite containing 10 wt.% SnO2. PT was stable at up to 230°C, after which it largely decomposed, leaving a residual mass of 3% at 463°C.There was no further significant reduction in this residual mass at up to 900°C. The PT-SnO2 nanocomposite was stable up to 320°C,after which almost 60% of the samples’ mass decomposed by 750°C.The remaining mass was largely stable up to 900°C, with only 2% of further mass loss. Weight loss at <200°C arose from volatilization of small molecules such as H2O. The weight loss at 200‒750°C was caused by PT and some SnO2 with irregular structure. In this temperature range, PT gradually decomposed to H2S and C. The TGA results indicated that the PT-SnO2 nanocomposite decomposed at higher temperature than PT, and the PT-SnO2 nanocomposite had not completely decomposed at 900 °C. Thus, the thermal stability of PT was enhanced by the presence of the SnO2 nanoparticles. 3.7. XPS XPS was used to determine the chemical compositions of the sample surfaces, and to investigate the interaction between the SnO2 nanoparticles and PT. Fig.6 shows the XPS survey spectra of PT and the PT-SnO2nanocomposite containing 10 wt.% SnO2. XPS peaks corresponding to the C1s (287 eV), O1s (534 eV), S 2p1/2 (156eV), S 2p3/2 (166eV), Fe 2p3/2(715 eV) and Cl 2p (207 eV) states were observed in the spectrum of PT (Fig. 6a). Peaks
corresponding to the Sn 3d5/2(487 eV) and Sn 3d3/2(495 eV) states were also observed in the spectrum of the PT-SnO2 nanocomposite (Fig. 6b). The Fe 2p3/2and Cl 2pstates arosefrom the oxidation medium FeCl3. Another fine Fe 2p1/2(719.9eV) peak was observed because of spin orbital splitting. Two sulfur states were due to neutral and oxidized thiophene units in the polymer chain. The XPS results were consistent with the interaction of SnO2 with PT. 3.8. Zeta potential measurements Zeta potential is a measure of the average surface charge of particles in solution, andits identity can reveal the interaction between particles. Zeta potential was measured as a function of pH to identify the isoelectric point of the nanocomposite. Fig.7 shows the zeta potentials of PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2 at different pH values. The isoelectric point of PT was at pH 8.5. At this point,the zeta potential was zero because of equal amounts of protons and hydroxide ions. Adding 10 wt.% SnO2to the PT decreasedthe isoelectric point to pH 7. The surface charge of the nanocomposite at pH <7 was positive, and at pH >7 was negative. The shift in isoelectric point and change in PT surface charge when prepared in the presence of SnO2 also suggested a significant interaction between the PT and SnO2. 3.9. Photoconductivity The field-dependent dark and photoconductivity properties of pressed pellet samples were studied using a Keithley 485 picoammeter and a 1000-W Xe arc lamp light source. Dark current measurements were recorded by applying a bias to the sample. The results for PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2 are shown in Fig.8a and b, respectively. Low dark current values and a small increase in the photocurrent for PT were observed. A significantly enhanced photocurrent was observed for the PT-SnO2 nanocomposite (Fig. 8b). At 50V, the photocurrents of PT and the PT-SnO2 nanocomposite were 0.52 and 1.02µA, respectively. The enhanced photocurrent was attributed to light
absorption by the SnO2 nanoparticles and PT. SnO2is an n-type semiconductor. When combined with PT, intermolecular donor-acceptor or charge-transfer complexes can be formed [25], and the absorption of photons in PT produces excitons. At electrodes, these excitons may dissociate to form charge carriers or polarons. High polaron mobility in the nanocomposite was responsible for the increased photocurrent. 3.10.AC conductivity and dielectric properties The AC conductivity and dielectric properties of PT and the PT-SnO2 nanocomposites were compared to investigate the effect of the SnO2 nanoparticles on PT.Fig.9 shows the effect of various SnO2 nanoparticle concentrations on the AC conductivity of PT as a function of frequency, at room temperature. AC conductivity increased with increasing frequency. At any given frequency, the AC conductivities of all PT-SnO2 nanocomposites were higher than that of PT.AC conductivity was also higher for nanocomposites containing higher n-type SnO2 nanoparticle concentrations, indicating the formation of excess charge carriers. Fig. 10a and b shows the dielectric constant and dielectric loss as a function of frequency, for PT and its nanocomposites at room temperature. For all samples, the dielectric constant and dielectric loss decreased with increasing frequency. Increasing the SnO2content of the nanocomposite also increased the dielectric constant and loss. The nanocomposites were heterogeneous, so the dielectric constant arose due to interfacial polarization. At low frequency, the dipoles easily followed the applied electric field, and polarization was at a maximum, so a high dielectric constant was attained. Incorporating 20wt.% of SnO2 nanoparticles enhanced the dielectric constant of the PT-SnO2 nanocomposite to 18.4 at 1kHz, which was about twice that recorded for PT (8.2) under the same conditions. A similar situation was observed for dielectric loss. Incorporating SnO2nanoparticles increased the dielectric loss of PT. The results indicated a higher dielectric constant and dielectric loss at
higher SnO2 nanoparticle concentrations, which may have been attributed to an increase in the crystallinity of the nanocomposites [29,30]. Adding SnO2nanoparticles resulted in more efficient storage and charge transport for the PT network, which resulted in a higher dielectric constant. This increase in dielectric constant could be explained by applying the Maxwell–Wagner double-layered model [30,31] to the interface between SnO2 and PT. According to this model, the conductivity and permittivity of the two layers are responsible for the dielectric behavior of the overall nanocomposite. The dielectric permittivity mainly depends on the grain boundary capacitance. The grain boundary capacitance increases with increasing SnO2 nanoparticles concentration. This creates a crystalline network contributing to the maximum net polarization, and thus increases the dielectric constant of the PT-SnO2 nanocomposite. Fig.11a and b shows the temperature dependence of the dielectric constant and dielectric loss, respectively, for the PT-SnO2 nanocomposite containing 10 wt.% SnO2. The dielectric constant increased with temperature but decreased with frequency. Increasing temperature freed up the dipoles, which could then move toward the interfaces of the heterogeneous nanocomposite. Interfacial polarization and therefore the dielectric constant were increased [32,33]. 4. Conclusion SnO2 nanoparticles were prepared via coprecipitation, and PT-SnO2 nanocomposites were prepared by in situ chemical oxidative polymerization, using different SnO2 nanoparticle concentrations. The size and size distribution of SnO2 nanoparticles were investigated by XRD and TEM. Their presence in the nanocomposites was confirmed by FTIR and EDAX. Shifts in FTIR absorption peaks indicated a significant interaction existed between the PT and SnO2 nanoparticles. FESEM indicated that SnO2 nanoparticles were embedded within the PT matrix. Further evidence of the interaction between the PT and SnO2
nanoparticles was given by the high thermal stability and shift in isoelectric point of the PTSnO2 nanocomposite. The chemical states of PT and the nanocomposite were studied by XPS. Potentialdependent photocurrent measurements of the PT-SnO2 nanocomposite showed a significant increase in photocurrent, when the nanocomposite was illuminated by visible light. The PTSnO2 nanocomposites exhibited higher AC conductivity than PT, due to their more semicrystalline nature. The PT-SnO2 nanocomposites exhibited improved dielectric behavior when compared with PT, which may have been attributed to high Maxwell–Wagner polarization at the interfaces between the SnO2 nanoparticles and PT matrix. These results suggest that the PT-SnO2 nanocomposite may have potential application in nanoscale dielectrics.
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Fig. 1. XRD patterns of (a) PT, (b) SnO2 and (C) the PT-SnO2 nanocomposite containing 10wt.% SnO2.
Fig. 2. FTIR spectra of (a) PT and (b) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 3. TEM images of (a) SnO2, PT and (c) the PT-SnO2 nanocomposite containing 10 wt.% SnO2
Fig. 4. FESEM images of (a) SnO2, (b) PT and (c) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 5. TGA profiles of (a) PT and (b) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 6. XPS spectra of (a) PT and (b) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 7. Zeta potential measurements of PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 8. Field-dependent dark current and photoconductivity of (a) PT and (b) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 9. AC conductivity as a function of frequency for PT and PT-SnO2 nanocomposites at room temperature.
Fig. 10. Frequency dependence of (a) dielectric constant and (b) dielectric loss for PT and PT-SnO2 nanocomposites at room temperature.
Fig. 11. Frequency and temperature dependence of (a) dielectric constant and (b) dielectric loss for the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Table1 Elemental compositions as determined by EDAX. Element (wt.%) Sample
C
O
S
Fe
H
Sn
PT
62.09
6.81
28.64
0.23
1.67
_
containing 50.50
8.12
31.1
1.52
1.15
7.41
PT-SnO2
10 wt.% SnO2
S0025-5408(16)30199-4 http://dx.doi.org/doi:10.1016/j.materresbull.2016.05.004 MRB 8770
To appear in:
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Received date: Revised date: Accepted date:
5-11-2015 7-4-2016 2-5-2016
Please cite this article as: S.Murugavel, M.Malathi, Structural, photoconductivity, and dielectric studies of polythiophene-tin oxide nanocomposites, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.05.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural, photoconductivity, and dielectric studies of polythiophene-tin oxide nanocomposites S.Murugavel1, M.Malathi* Condensed Matter Research Laboratory Materials Physics Division School of Advanced Sciences VIT University Fax: +91416 2243092 Tel: +91416 2202453 E-mail address: [email protected] (M.Malathi*) [email protected] (S. Murugavel) *Corresponding author
Graphical abstract
Highlights
Synthesis of polythiophene-tin oxide nanocomposites confirmed by FTIR and EDAX
SEM shows SnO2 nanoparticles embedded within polythiophene matrix
Stability and isoelectric point suggest nanoparticle–matrix interaction
High dielectric constant due to high Maxwell–Wagner interfacial polarization
Abstract Polythiophene-tinoxide (PT-SnO2) nanocomposites were prepared by in situ chemical oxidative polymerization, in the presence of various concentrations of SnO2 nanoparticles. Samples were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy and Zeta potential measurements. Morphologies and elemental compositions were investigated by transmission electron microscopy, field-emission scanning electron microscopy and energy-dispersive Xray spectroscopy. The photoconductivity of the nanocomposites was studied by fielddependent dark and photo conductivity measurements. Their dielectric properties were
investigated using dielectric spectroscopy, in the frequency range of 1kHz‒1MHz. The results indicated that the SnO2nanoparticlesin the PT-SnO2nanocomposite were responsible for its enhanced dielectric performance.
Keywords: A. composites, D. dielectric properties, C. photoelectron spectroscopy, D. electrical properties, C. electron microscopy
1. Introduction Conducting polymers and their nanocomposites have been an important research area since their discovery in the mid-1970s. Conducting polymer-metal oxide nanocomposites have been extensively investigated, because of their interesting physiochemical properties and potential application in nanodevices [1-5]. Polymer nanocomposites have been synthesized by various physical and chemical methods, including insitu chemical oxidation, electrochemical polymerization, and melt processing [6-8]. Those with high dielectric constants are used in charge-storage devices, telecommunications, electromagnetic interference shielding, integral capacitor technology, and electromechanical applications [912]. Polythiophene (PT) is a promising conducting material, because of its high and controllable electrical conductivity[13-15]. Its low solubility and poor processability limit its application, so PT has been incorporated with metal oxide nanoparticles to overcome these problems. Incorporating metal oxide nanoparticles can enhance the electrical properties of conjugated polymers. PT composites containing Fe3O4, Al2O3, and TiO2 have all been thoroughly studied [16-18]. Nanoscale metal oxide particles are of particular interest, because of their size-dependent physical and chemical properties. SnO2 is a wide band gap (3‒6 eV) n-type semiconductor used in many applications, including electrode materials for Libatteries, gas sensors and antistatic coatings [1921].Optical charge generation and transport in photoconducting PT derivatives is receiving much current interest[22]. Photoconductivity is the enhancement of a material’s electrical conductivity by absorbing photons of a suitable energy. Polyfluorene, its copolymers including poly(para-phenylene vinylene), and substituted PT derivatives are all widely used in this field [23]. Photoconducting substituted PTs have applications in electronic devices including electro-optic modulators, optical signal processors, Photoreceptors, solarcells, and optical frequency doublers [24]. Photoconducting devices based on polymer composites with
internal donor/acceptor heterojunctions have also been investigated [25]. However, there are no reported studies investigating the photoconductivity of PT-SnO2 nanocomposites. PT and SnO2 are both widely applied in technology. Thus, PT-SnO2 nanocomposites were thought likely to possess interesting properties, which could potentially be useful when designing and developing polymer electronic devices. While there are some reported syntheses and morphological studies of PT-metal oxide nanocomposites [26-28], to the best of our knowledge none have probed the electric behavior of the PT-SnO2 nanocomposites. In addition to requiring good photoconductivity and dielectric performance, other properties of the material are also important when developing novel polymer electronic materials. These include structural order (i.e., morphology yielding optimum efficient charge transport), thermal stability, and surface charge. In the current study, we report the synthesis, structural characterization, zetapotential and photoconductivity measurements of PT-SnO2 nanocomposites. The dielectric behavior of PT-SnO2 nanocomposites containing 10, 20, 30, and 40 wt.% SnO2 was investigated over the frequency range of 1kHz‒1MHz. 2. Experimental 2.1. Materials All chemicals used were of analytic reagent (AR) grade. PT was prepared from freshly double distilled thiophene (Aldrich). Anhydrous ferric chloride (FeCl3) was purchased from Fluka. Stannouschloride dihydrate(SnCl2·2H2O), chloroform and ammonia solution were purchased from Merck. Deionized water was used in all reactions. 2.2. Synthesis of SnO2 nanoparticles SnO2 nanoparticles were synthesized by coprecipitation, using SnCl2 as a Sn source. 4g (0.2mol) of SnCl2·2H2O was dissolved in 200ml of water. 6 ml of ammonia solution was added, and the mixture was stirred vigorously at room temperature for 1.5 h to form a
precipitate. The white gel precipitate was allowed to settle for 14 h, collected by filtration and thoroughly rinsed with distilled water. The precipitate was heated at 110°C until dry, and then calcined at 500°C for 5h to yield SnO2 nanoparticles. 2.3. Preparation of PT-SnO2nanocomposite The PT-SnO2 nanocomposite was prepared by in situ chemical oxidative polymerization of thiophene monomer, in the presence of SnO2 nanoparticles. In a typical synthesis, 0.2g of SnO2 nanoparticles and 0.05 mol of thiophene were dispersed in 100 ml of chloroform and stirred for several minutes. 0.25 mol of anhydrous FeCl3 in chloroform was added under vigorous stirring. The reaction mixture was stirred with a magnetic stirrer bar at room temperature for 4h, during which its color changed from grey to black. The precipitate was collected by filtration, and washed thoroughly with methanol and doubly distilled water to remove unreacted oxidants and monomer. The resulting powder was dried under vacuum at 70°C for 8h. PT-SnO2 nanocomposites were synthesized containing 10, 20, 30, and 40 wt.% of SnO2 nanoparticles. PT was prepared similarly, but in the absence of SnO2 nanoparticles. 2.4. Characterization X-ray diffraction (XRD) patterns were collected at 2θ of 10‒80° using a diffractometer (XRD-Smart Lab, Rigaku, Japan). XRD patterns were analyzed by indexing observed peaks with standard JCPDS values. Fourier transform infrared (FTIR) spectra were obtained at 4000‒400cm−1 using a FTIR spectrometer (Spectrum RX1, Perkin Elmer, MA, USA). Sample morphologies were observed by transmission electron microscopy (TEM; JEOL 3010, Japan) and field-emission scanning electron microscopy (FESEM; SUPRA 55, Carl Zeiss, Germany). Energy-dispersive X-ray analysis (EDAX) was used to determine the elemental compositions of PT and the PT-SnO2 nanocomposites. Thermal stabilities of the
samples were investigated by thermo gravimetric analysis (TGA: TG/DTA 6200), at temperatures from 30 to 900°C at a heating rate of 10°C/min in a nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) (AXIS ULTRA from AXIS 165) was used to obtain information about the chemical states of samples. Zeta potential measurements (Horiba) were used to determine the surface charge and isoelectric points of samples in water. Photoconductivity measurements were conducted using a Keithley Picoammeter 6485. PT and PT-SnO2 powders were finely ground with an agate mortar. Pellets of~10mm in diameter and 2mm in thickness were prepared by hydraulically pressing powdersat5.5t. Silver paste was then coated on each face to create contact with two electrodes. The pellet was then placed into sample holder. Dielectric measurements were carried out using a LCR HiTester apparatus (HIOKI 3532-50LCRHiTester, Japan), over the frequency range 1kHz‒1MHz. 3. Results and Discussion 3.1. XRD Fig. 1a shows the XRD pattern of PT, which containeda peak at 2θ of 21.5°. Fig. 1b shows the XRD pattern of SnO2 nanoparticles, in which all peaks were well assigned to the tetragonal rutile structure of SnO2 with high crystallinity. No diffraction peaks for impurities were detected. The average crystallite size (D) was calculated to be 30 nm, using the DebyeScherrer formula (D=0.9λ/βcosθ, where λ is the X-ray wavelength, β is the full width at half maximum intensity, and θ is the diffraction angle). Fig. 1c shows the XRD pattern of the PTSnO2 nanocomposite containing 10 wt.% SnO2, which confirmed the presence of SnO2. All major XRD peaks for SnO2 were observed in the pattern of the PT-SnO2 nanocomposite, implying that PT was deposited on the SnO2 nanoparticle surface. 3.2. FTIR spectroscopy Fig. 2 shows the FTIR spectra of PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2. The characteristic absorption peaks of PT at 2782, 1643, 1042, 797, 630and
498cm−1corresponded to the C‒H aromatic stretching, C=C symmetric stretching, C‒H inplane deformation, C‒H out-of-plane stretching, C‒S thiophene ring stretching and C‒S‒C ring deformation, respectively. The FTIR spectrum of the PT-SnO2 nanocomposite contained these characteristic peaks of PT, but those at 2782, 1643, 1042, 797, 630 and 498 cm−1were shifted to 2350, 1699, 1110, 803, 613 and 486 cm−1, respectively, in the spectrum of the PTSnO2 nanocomposite. These shifts resulted from the incorporation of the SnO2 nanoparticles in PT, and suggested a significant interaction between SnO2 and PT. Interaction may have been via SnO2 and the PT sulfur moiety. Hydrogen bonding between SnO2 and PT may also have contributed to the shift in these characteristic absorption peaks. 3.3. TEM Understanding a sample’s particle size can be used to infer quantum size effects. The samples were investigated using TEM to determine their particle sizes. Fig.3a, b and c show TEM images of SnO2, PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2, respectively. The SnO2 particles had an average diameter of approximately 15nm, which was lower than the crystallite size of 30 nm estimated from the XRD results and Scherrer’s formula. The TEM image of the PT-SnO2 nanocomposite in Fig.3c indicates that SnO2 nanoparticles were dispersed within the PT matrix. 3.4. FESEM FESEM images of the samples are shown in Fig. 4. The image of the SnO2 nanoparticles in Fig. 4a shows a typical morphology distribution, in which the nanoparticles were predominantly spherical with diameters of 20‒80nm. A net-like structure was apparent in the image of PT in Fig.4b.SnO2 nanoparticles were uniformly distributed throughout the net-like matrix of PT, in the image of the nanocomposite in Fig. 4c. The morphology of the nanocomposite was highly micro-porous, which was responsible for the high interfacial area
between the two different phases. High interfacial areas led to strong interaction between the PT and SnO2 nanoparticles. 3.5. EDAX Table 1shows the elemental compositions of PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2, as obtained by EDAX. The results were consistent with the presence of SnO2 in the nanocomposite. 3.6. TGA The thermal stability of the samples was investigated by TGA under a N2 atmosphere. Fig.5 shows the TGA profile for PT and the nanocomposite containing 10 wt.% SnO2. PT was stable at up to 230°C, after which it largely decomposed, leaving a residual mass of 3% at 463°C.There was no further significant reduction in this residual mass at up to 900°C. The PT-SnO2 nanocomposite was stable up to 320°C,after which almost 60% of the samples’ mass decomposed by 750°C.The remaining mass was largely stable up to 900°C, with only 2% of further mass loss. Weight loss at <200°C arose from volatilization of small molecules such as H2O. The weight loss at 200‒750°C was caused by PT and some SnO2 with irregular structure. In this temperature range, PT gradually decomposed to H2S and C. The TGA results indicated that the PT-SnO2 nanocomposite decomposed at higher temperature than PT, and the PT-SnO2 nanocomposite had not completely decomposed at 900 °C. Thus, the thermal stability of PT was enhanced by the presence of the SnO2 nanoparticles. 3.7. XPS XPS was used to determine the chemical compositions of the sample surfaces, and to investigate the interaction between the SnO2 nanoparticles and PT. Fig.6 shows the XPS survey spectra of PT and the PT-SnO2nanocomposite containing 10 wt.% SnO2. XPS peaks corresponding to the C1s (287 eV), O1s (534 eV), S 2p1/2 (156eV), S 2p3/2 (166eV), Fe 2p3/2(715 eV) and Cl 2p (207 eV) states were observed in the spectrum of PT (Fig. 6a). Peaks
corresponding to the Sn 3d5/2(487 eV) and Sn 3d3/2(495 eV) states were also observed in the spectrum of the PT-SnO2 nanocomposite (Fig. 6b). The Fe 2p3/2and Cl 2pstates arosefrom the oxidation medium FeCl3. Another fine Fe 2p1/2(719.9eV) peak was observed because of spin orbital splitting. Two sulfur states were due to neutral and oxidized thiophene units in the polymer chain. The XPS results were consistent with the interaction of SnO2 with PT. 3.8. Zeta potential measurements Zeta potential is a measure of the average surface charge of particles in solution, andits identity can reveal the interaction between particles. Zeta potential was measured as a function of pH to identify the isoelectric point of the nanocomposite. Fig.7 shows the zeta potentials of PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2 at different pH values. The isoelectric point of PT was at pH 8.5. At this point,the zeta potential was zero because of equal amounts of protons and hydroxide ions. Adding 10 wt.% SnO2to the PT decreasedthe isoelectric point to pH 7. The surface charge of the nanocomposite at pH <7 was positive, and at pH >7 was negative. The shift in isoelectric point and change in PT surface charge when prepared in the presence of SnO2 also suggested a significant interaction between the PT and SnO2. 3.9. Photoconductivity The field-dependent dark and photoconductivity properties of pressed pellet samples were studied using a Keithley 485 picoammeter and a 1000-W Xe arc lamp light source. Dark current measurements were recorded by applying a bias to the sample. The results for PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2 are shown in Fig.8a and b, respectively. Low dark current values and a small increase in the photocurrent for PT were observed. A significantly enhanced photocurrent was observed for the PT-SnO2 nanocomposite (Fig. 8b). At 50V, the photocurrents of PT and the PT-SnO2 nanocomposite were 0.52 and 1.02µA, respectively. The enhanced photocurrent was attributed to light
absorption by the SnO2 nanoparticles and PT. SnO2is an n-type semiconductor. When combined with PT, intermolecular donor-acceptor or charge-transfer complexes can be formed [25], and the absorption of photons in PT produces excitons. At electrodes, these excitons may dissociate to form charge carriers or polarons. High polaron mobility in the nanocomposite was responsible for the increased photocurrent. 3.10.AC conductivity and dielectric properties The AC conductivity and dielectric properties of PT and the PT-SnO2 nanocomposites were compared to investigate the effect of the SnO2 nanoparticles on PT.Fig.9 shows the effect of various SnO2 nanoparticle concentrations on the AC conductivity of PT as a function of frequency, at room temperature. AC conductivity increased with increasing frequency. At any given frequency, the AC conductivities of all PT-SnO2 nanocomposites were higher than that of PT.AC conductivity was also higher for nanocomposites containing higher n-type SnO2 nanoparticle concentrations, indicating the formation of excess charge carriers. Fig. 10a and b shows the dielectric constant and dielectric loss as a function of frequency, for PT and its nanocomposites at room temperature. For all samples, the dielectric constant and dielectric loss decreased with increasing frequency. Increasing the SnO2content of the nanocomposite also increased the dielectric constant and loss. The nanocomposites were heterogeneous, so the dielectric constant arose due to interfacial polarization. At low frequency, the dipoles easily followed the applied electric field, and polarization was at a maximum, so a high dielectric constant was attained. Incorporating 20wt.% of SnO2 nanoparticles enhanced the dielectric constant of the PT-SnO2 nanocomposite to 18.4 at 1kHz, which was about twice that recorded for PT (8.2) under the same conditions. A similar situation was observed for dielectric loss. Incorporating SnO2nanoparticles increased the dielectric loss of PT. The results indicated a higher dielectric constant and dielectric loss at
higher SnO2 nanoparticle concentrations, which may have been attributed to an increase in the crystallinity of the nanocomposites [29,30]. Adding SnO2nanoparticles resulted in more efficient storage and charge transport for the PT network, which resulted in a higher dielectric constant. This increase in dielectric constant could be explained by applying the Maxwell–Wagner double-layered model [30,31] to the interface between SnO2 and PT. According to this model, the conductivity and permittivity of the two layers are responsible for the dielectric behavior of the overall nanocomposite. The dielectric permittivity mainly depends on the grain boundary capacitance. The grain boundary capacitance increases with increasing SnO2 nanoparticles concentration. This creates a crystalline network contributing to the maximum net polarization, and thus increases the dielectric constant of the PT-SnO2 nanocomposite. Fig.11a and b shows the temperature dependence of the dielectric constant and dielectric loss, respectively, for the PT-SnO2 nanocomposite containing 10 wt.% SnO2. The dielectric constant increased with temperature but decreased with frequency. Increasing temperature freed up the dipoles, which could then move toward the interfaces of the heterogeneous nanocomposite. Interfacial polarization and therefore the dielectric constant were increased [32,33]. 4. Conclusion SnO2 nanoparticles were prepared via coprecipitation, and PT-SnO2 nanocomposites were prepared by in situ chemical oxidative polymerization, using different SnO2 nanoparticle concentrations. The size and size distribution of SnO2 nanoparticles were investigated by XRD and TEM. Their presence in the nanocomposites was confirmed by FTIR and EDAX. Shifts in FTIR absorption peaks indicated a significant interaction existed between the PT and SnO2 nanoparticles. FESEM indicated that SnO2 nanoparticles were embedded within the PT matrix. Further evidence of the interaction between the PT and SnO2
nanoparticles was given by the high thermal stability and shift in isoelectric point of the PTSnO2 nanocomposite. The chemical states of PT and the nanocomposite were studied by XPS. Potentialdependent photocurrent measurements of the PT-SnO2 nanocomposite showed a significant increase in photocurrent, when the nanocomposite was illuminated by visible light. The PTSnO2 nanocomposites exhibited higher AC conductivity than PT, due to their more semicrystalline nature. The PT-SnO2 nanocomposites exhibited improved dielectric behavior when compared with PT, which may have been attributed to high Maxwell–Wagner polarization at the interfaces between the SnO2 nanoparticles and PT matrix. These results suggest that the PT-SnO2 nanocomposite may have potential application in nanoscale dielectrics.
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Fig. 1. XRD patterns of (a) PT, (b) SnO2 and (C) the PT-SnO2 nanocomposite containing 10wt.% SnO2.
Fig. 2. FTIR spectra of (a) PT and (b) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 3. TEM images of (a) SnO2, PT and (c) the PT-SnO2 nanocomposite containing 10 wt.% SnO2
Fig. 4. FESEM images of (a) SnO2, (b) PT and (c) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 5. TGA profiles of (a) PT and (b) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 6. XPS spectra of (a) PT and (b) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 7. Zeta potential measurements of PT and the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 8. Field-dependent dark current and photoconductivity of (a) PT and (b) the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Fig. 9. AC conductivity as a function of frequency for PT and PT-SnO2 nanocomposites at room temperature.
Fig. 10. Frequency dependence of (a) dielectric constant and (b) dielectric loss for PT and PT-SnO2 nanocomposites at room temperature.
Fig. 11. Frequency and temperature dependence of (a) dielectric constant and (b) dielectric loss for the PT-SnO2 nanocomposite containing 10 wt.% SnO2.
Table1 Elemental compositions as determined by EDAX. Element (wt.%) Sample
C
O
S
Fe
H
Sn
PT
62.09
6.81
28.64
0.23
1.67
_
containing 50.50
8.12
31.1
1.52
1.15
7.41
PT-SnO2
10 wt.% SnO2