Core–shell PEDOT:PSS—PVP nanofibers containing PbS nanoparticles through coaxial electrospinning

Synthetic Metals 220 (2016) 255–262

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Core–shell PEDOT:PSS—PVP nanofibers containing PbS nanoparticles through coaxial electrospinning I.E. Moreno-Corteza,b , A. Alvarado-Castañedaa,b , D.F. Garcia-Gutierreza,b , N.A. Garcia-Gomezc, S. Sepulveda-Guzmana,b , D.I. Garcia-Gutierreza,b,* a Universidad Autónoma de Nuevo León, UANL, Facultad de Ingeniería Mecánica y Eléctrica, FIME, Av. Universidad S/N, Cd. Universitaria, San Nicolás de los Garza, Nuevo León, C.P. 66450, Mexico b Universidad Autónoma de Nuevo León, Centro de Innovación, Investigación y Desarrollo en Ingeniería y Tecnología CIIDIT, Apodaca, Nuevo León, Mexico c Universidad Autónoma de Nuevo León, Facultad de Ciencias Químicas, FCQ, San Nicolás de los Garza, Nuevo León, C.P. 66450, Mexico

A R T I C L E I N F O

Article history: Received 31 March 2016 Received in revised form 27 May 2016 Accepted 24 June 2016 Available online xxx Keywords: PEDOT:PSS nanofibers Coaxial electrospinning PbS nanoparticles Core–shell nanofibers

A B S T R A C T

Coaxial electrospinning was used to fabricate core–shell nanofibers of PEDOT:PSS, located in the core, with PbS nanoparticles homogeneously distributed within it, and a PVP shell. This morphology was confirmed by HAADF-STEM results, while the composition and nature of the different phases was corroborated by EDXS and FTIR studies. Moreover, optoelectronic properties of the composite material based on the synthesized core–shell nanofibres were studied, showing an increment in the conductivity of the material by the addition of the PbS nanoparticles and also the appearance of photoresponse, effect not observed in either of the polymeric materials, but displayed by the PbS nanoparticles. ã 2016 Elsevier B.V. All rights reserved.

1. Introduction Intrinsically conductive polymers (ICP) have found applications in different areas such as antistatic layers, touch screens, organic light-emitting diodes (OLEDs), capacitors and organic solar cells [1]. Among them, poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is an ICP that posses outstanding properties such as excellent environmental stability in air, good thermal stability, tunable conductivity, the ability to form stable aqueous dispersions at low pH values and high transparency when it is synthesized in thin films. The commercial samples of this polymer exhibit conductivities as high as 1000 S cm
1 and work function values between 4.8 and 5.8 eV [2]. These electrical properties make PEDOT:PSS one of the most versatile and utilized conductive polymers, being used in a vast kind of commercial and industrial applications, such as gas sensors, tissue engineering, bionic devices coatings, among others [3]. Taking advantage of the water solubility provided by the polyelectrolyte, PEDOT:PSS has been used mostly in the form of films with a relative low

* Correspondence to: Km. 10 de la nueva carretera al Aeropuerto Internacional de Monterrey, PIIT Monterrey, C.P. 66600, Apodaca, Nuevo León, Mexico. E-mail addresses: [email protected], [email protected] (D.I. Garcia-Gutierrez). http://dx.doi.org/10.1016/j.synthmet.2016.06.019 0379-6779/ã 2016 Elsevier B.V. All rights reserved.

conductivity due to the formation of a PSS shell that hinders the contact between the polymer chains once the films are formed [4]. This conductivity could be enhanced through several methods such as thermal and light treatment, the contact with organic solvents, surfactants, salts and several types of acids. In particular, with the use of H2SO4 post-treatment, the reported conductivity has reached a maximum value of 4380 S cm
1 [5]. However, the specific mechanism in which these treatments work to enhance the conductivity of PEDOT:PSS is unclear, with conformational changes in the molecular chain as the most widely accepted explanation [6]. In general, most ordered and crystalline PEDOT films are formed using alternative techniques such as chemical vapor deposition (CVD) and vapor-phase polymerization (VPP) [7,8]. The morphology of the PEDOT:PSS films depends strongly on the synthesis method used for its fabrication. The final morphology has been reported as nanofibrillar in electrochemically deposited PEDOT:PSS films and as uniform and smooth thin films when the VPP technique is used [3,9]. In this regard, is desirable to increase the surface area as much as possible in order to reduce the impedance in the interaction between the conductive polymer and the surrounding polyelectrolyte. Following this subject, it has been suggested that nanostructures of this conductive polymer could show an improvement in conductivity, compared to their bulk

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counterparts [6,10]. Considering the different options available in polymeric nanomaterials, nanofibers produced by the electrospinning method are one of the most commonly reported nanostructures for polymeric materials; this is due to their unique characteristics, such as facile production, large area to volume ratio, high porosity, high degree of interconnection and the possibility to be retrieved from the synthesis medium once the reaction has concluded [11]. In the case of polymers with attractive electronic properties, nanofibers offer advantages like efficient electron transport and optical excitations [12,13]. As previously mentioned, the electrospinning is the technique par excellence to produce nanofibres because of its simplicity, versatility and high production of nanofibers; however, nanofibers have been produced through other techniques, such as: self-assembly, phase separation, drawing and template synthesis. Nonetheless, the preparation of ICP nanofibres by electrospinning still represents an important challenge, mainly due to the rigid backbone presented by this type of polymers that induces a high crystallinity and poor solubility in common solvents; characteristics that make them incompatible with the electrospinning process [9,14]. In the particular case of PEDOT:PSS, several routes have been explored to synthesize nanofibres; among these routes the use of “templates” based in nanoporous membranes can be highlighted. In this approach, the PEDOT is forced to take the shape of the porous membrane during the electropolimerization of EDOT monomers immersed in the same media as the template [15]. To overcome the problem of processing ICPs by the electrospinning technique, in one approach PEDOT:PSS is mixed with a carrier polymer that is easily processed by electrospinning. Most commonly used carrier polymers include polyethyleneoxide, polyvinylalcohol and polyvinylpirrolidone (PVP). Recent studies have reported to add other solvents to PEDOT:PSS solutions with the carrier polymers to increase the conductivity of the resulting nanofibers, these solvents include ethylene glycol, dimethylformamide and dimethylsulfoxide [10,12,16–18]. In a different approach, the electrospinning of precursor solutions composed by a carrier polymer and EDOT were carried out and, after the subsequently polymerization process, a coating of PEDOT over the surface of the carrier polymer nanofibers was formed [14,19,20]. The electrospinning technique can also be used to produce nanofibers with a core–shell morphology, through methods such as coaxial electrospinning, emulsion and suspension electrospinning [21–24]. This kind of morphology is sought-after in biosensors, catalyst and drug delivery applications. In this work we report on the use of coaxial electrospinning technique to fabricate nanofibers with core–shell morphology, ICP PEDOT:PSS was used as the core material, while the carrier polymer, PVP, was used as the shell material. In order to improve the optoelectronic characteristics of the core–shell nanofibers, lead sulfide nanoparticles (PbS NPs) were dispersed within the PEDOT: PSS core. Several reports have demonstrated that nanostructures involving PEDOT:PSS and PbS NPs exhibit improved optoelectronic properties such as, increments in electrical conductivity, wide optical absorption and photocurrent phenomena, many of them suitable for applications in electronic logic gates [25], solar cells [26] and extremely thin absorber layer (eta)—cells [27]. Generally, these nanostructures are processed as thin films by techniques that require several steps in order to increase the nanoparticles concentration and distribution such as spin coating and casting [28,29]. However, it is possible to enhance the optoelectronic performance of these nanostructures by forming a composite material of PEDOT:PSS and PbS nanoparticles through the design of 1D nanostructures of the conductive polymer, which would display improved features such as high surface to volume ratio, large porosity and tunable surface functionality; along with the incorporation of the PbS NPs inside these 1D nanostructures core

phase of the conductive polymer. To the best of our knowledge, there is no previous report of a core–shell nanostructure in PEDOT: PSS nanofibres with PbS NPs dispersed inside them, achieved exclusively using coaxial electrospinning. 2. Materials and methods 2.1. Materials All materials were purchased from Sigma-Aldrich except otherwise indicated. PbS NPs were synthesized by the one-pot rapid injection method reported by our group in reference [30], with reactions times of 10 min. Briefly, in a three neck round bottom flask 23 mL ODE, 0.45 g PbO and 3.6 mL of Oleic Acid (OA) were stirred, then this solution was heated until 150  C and maintained at this temperature for about an hour. For the sulfur precursor 2 mL of 1-octadecene, 0.2 mL of hexamethyldisilathiane (TMS) and 0.1 mL of dyphenilphosphine were added and stirred for 30 min in ambient temperature in a globe box filled with nitrogen. For the synthesis of the PbS nanoparticles, the sulfur precursor is quickly added with a syringe into the lead precursor, and the nanoparticles nucleaction and growth start immediately. After the reaction was finished, the nanoparticles were extracted from the reaction media, and any subproducts of the reaction, by means of centrifugation. The extraction was carried out using anhydrous methanol (twice) and then precipitating the nanoparticles with anhydrous acetone. The nanoparticles were isolated by centrifugation at 9500 RPM for 20 min. The precipitated nanoparticles were redispersed in chloroform. The extraction with anhydrous methanol was repeated twice. After this step, the nanoparticles were redispersed in anhydrous chloroform. 2.2. Synthesis of PEDOT:PSS 0.35 g of PSSNa and 0.06 g of EDOT were added in a vial with 20 mL of deionized water and stirred for 2 h. Then 1 mL of FeCl3 (20 mM) was added to the solution of PSSNa and EDOT and stirred for 24 h. The resultant dispersion was purified by dialysis with deionized water using a cellulose membrane for 48 h. The resultant dispersion of PEDOT:PSS was adjusted to a 3 pH value using a 10% v/v HCl solution. The PEDOT:PSS dispersion was freeze-dried and then redispersed in deionized water at a 1.3% w/w concentration. 2.3. PEDOT:PSS—PbS nanoparticles solution preparation In a 10 mL vial, 3 mL of a solution of 1.3% w/w of PEDOT:PSS in deionized water was added, then 150 mL of the Chloroform—PbS nanoparticles solution was added to the vial. Afterwards, this solution was stirred for 96 h at 700 rpm before the electrospinning process. 2.4. Coaxial electrospinning Once the solutions were prepared, the coaxial electrospinning process was carried out using a handcrafted coaxial syringe (Fig. S1), with the characteristics described in Table S1 in the supporting information. The PbS NPs with PEDOT:PSS dispersion was used as core material, and an 8% wt/wt Polyvinylpyrrolidone (PVP) solution in ethanol was used as shell material. The coaxial electrospinning process was carried out using a voltage of 25 kV and a distance between the syringe tip and the Al foil collector of 18 cm. Fig. S2 shows a schematic representation of the coaxial electrospinning process used. Two syringe pumps (Cole Parmer) were used in this process, named syringe pump 1 (core solution, 0.05 mL/h) and syringe pump 2 (shell, 0.8 mL/h). A power source

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(Spellman CZE1000R) was utilized to supply the necessary voltage to carry out the electrospinning process. 2.5. Characterization The synthesized core–shell PEDOT:PSS-PVP nanofibers containing PbS nanoparticles were characterized by means of transmission electron microscopy (TEM) using a FEI TEM Titan G2 80-300 operated at 300 kV. The TEM is equipped with Scanning-Transmission Electron Microscopy (STEM) capabilities, a High Angle Annular Dark Field (HAADF) detector from Fischione; a Bright Field (BF) STEM detector from Gatan; an Annular Dark Field (ADF) STEM detector from Gatan and an EDAX energy dispersive X-ray spectroscopy (EDXS) detector. FTIR absorption spectra were acquired using a Thermo Fisher Nicolet FT-IR spectrometer, with a detection range between 400 cm
1 and 4000 cm
1 and a resolution of 4 cm
1. The samples were analyzed using the ATR system available in the equipment. Finally, a Keithley 6487 picoammeter was used to perform the conductivity and photoresponse tests. 3. Results and discussion 3.1. PbS nanoparticles characteristics Fig. 1 shows the main characteristics of the PbS nanoparticles obtained after the synthesis and used in the fabrication of the core–shell composite nanofibers. The nanoparticles presented an optical absorption peak approximately at 1250 nm (Fig. 1a). This absorption peak value was displayed by PbS nanoparticles with an average size of 5.6 nm and a standard deviation of 1.1 nm, both numbers were calculated after measuring 100 nanoparticles from High Resolution TEM (HRTEM) images, as the one observed in Fig. 1b. EDXS (Fig. 1c) analyses confirmed their composition to be

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Pb and S; and Selected Area Electron Diffraction (SAED) patterns (Fig. 1d) confirmed their crystal structure to be FCC, as the diffraction pattern showed the presence of crystal planes associated to PbS, according to the JCPDS card 77-0244. 3.2. Nanofibers morphology The morphology of the PEDOT:PSS-PbS NPs—PVP nanofibers was analyzed using TEM. As it can be observed in Fig. 2, low magnification HAADF-STEM images suggest random oriented and non-woven deposited nanofibers. Furthermore, these images also suggest the feasibility to electrospun PEDOT:PSS-PbS NPs—PVP in a coaxial system without the presence of bead defects in the morphology of the electrospun nanofibers. In Fig. 3 higher magnification HAADF images are shown, at this magnification is possible to observe the distribution of the PbS NPs along the nanofibers structure (white arrows in Fig. 3a); in Fig. 3b we can observe a different area of the sample at a higher magnification, where the distribution of the nanoparticles along the core of the nanofibers can be distinguish more clearly (white arrows). The nanofibres average diameter was approximately 240 nm, ranging from 50 to 500 nm. Fig. 4 shows HAADF images of an isolated fiber produced through coaxial electrospinning. The core–shell morphology is evident by comparing the contrast through the diameter of the nanofiber, where a brighter core is covered by a less bright shell. Moreover, it is observed that the PbS NPs are distributed in the brighter inner material (Fig. 4b). Also, we can observe that PbS NPs are well dispersed, without the presence of any agglomerates (Fig. 4c–d). Hence, we can mention that the nanoparticles conserve their nanometric scale after the coaxial electrospinning process, with an average diameter of approximately 5.9 nm, which is very close to the value measured in the nanoparticles before they were introduced in the PEDOT:PSS (Fig. 4d). This behavior is in

Fig. 1. a) UV–vis-NIR absorbance spectra for the synthesized PbS nanoparticles. b) HRTEM image showing their size, morphology and size distribution. c) EDXS spectra showing their composition. d) SAED pattern indicating the crystal planes associated to the FCC structure.

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Fig. 2. HAADF-STEM images showing low magnification pictures depicting randomly oriented and non-woven deposited nanofibres.

Fig. 3. HAADF-STEM images showing a) and b) the distribution of PbS nanoparticles within the core–shell nanofibres.

Fig. 4. a) and b) HAADF-STEM images of an isolated nanofiber where the core–shell morphology is evident, along with the presence of PbS nanoparticles distributed along the nanofibre. c) and d) HAADF-STEM images at higher magnifications where the distribution and characteristics of the PbS nanoparticles are clearly observed.

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Fig. 5. a) HAADF-STEM image indicating the line analyzed in the line scan experiment. b) EDXS line scan profiles for the different signals detected, N Ka, O Ka, C Ka and S Ka. c) HAADF-STEM image showing PbS nanoparticles close to the core region, the red rectangle indicates the region analyzed for the EDXS analysis. d) EDXS spectrum showing the presence of the Pb and S signals related to the PbS nanoparticles within the nanofibers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

accordance with other reports, in which nanoparticles dispersed in polymeric solutions are electrospun, and their characteristics, such as size, morphology and crystallinity are conserved within the nanofibers after the electrospinning process, this also corroborate the effectiveness of the dispersion method of PbS nanoparticles in the PEDOT:PSS [31–33]. Afterwards, EDXS line scan studies were performed on the core–shell nanofiber to elucidate the difference in composition between the brighter core phase and the less bright shell phase. Fig. 5 shows the results for this study. In order to detect the PEDOT: PSS phase we would expect an increment in the S signal due to the presence of sulfur in the PEDOT:PSS molecular structure and its absence in the PVP. As expected, in Fig. 5b the S signal appears only at the core region of the fiber; coincidentally, the intensity of the HAADF-STEM is higher in this central region (Fig. 5a), since the signal intensity is proportional to the atomic number (Z2) in this technique. In contrast, the C, N and O compositional signals remain constant along the nanofiber width, as they all are present in the PVP. In Fig. 5c we can see the nanoparticles well aligned along the core phase in the center of the nanofiber (red rectangle). The composition of the nanoparticles in this region was confirmed through an EDXS analysis that clearly shows the Pb and S signals (Fig. 5d). All these data confirm the core–shell morphology of the electrospun nanofibers, where the PEDOT:PSS is located in the core, with the PbS nanoparticles homogeneously distributed within it, and the PVP is located in the shell. In addition, the chemical structure of coaxial nanofibers with PEDOT:PSS-PbS NPs as core phase and PVP as shell phase was further studied by FTIR and the resulting spectrum is shown in Fig. 6, along with the spectra for pure PVP nanofibers, PEDOT:PSS in powder and the as-synthesized PbS nanoparticles in solution in tetrahydrofuran (THF). The absorption bands at 966 cm
1, 847 cm
1 and 684 cm
1 have been assigned to the stretching of the C-S bond in the thiophene ring of the PEDOT [23,34,35]; these three bands can be observed in the PEDOT and PEDOT:PSS-PbS
PVP core–shell nanofibers spectra, being the band close to

966 cm
1 the most evident in the composite core–shell nanofibers. The absorption bands observed at 1168 cm
1 and 1224 cm
1 are associated with the C

O

C bond stretching in the ethylene dioxy (alkylenedioxy) group in the PEDOT [20,36,37]. For the PVP nanofibers spectrum, several characteristic bands of the molecular structure of the PVP polymer can be clearly observed; one at 1657 cm
1 corresponding to the C¼O stretching vibration; three bands at 1420 cm
1, 1443 cm
1 and 1464 cm
1 are related to the CH2 bending vibration; and the band at 1289 cm
1 is related to the C

N stretching vibration; all of these bands can be clearly observed in both spectra, for the PVP only nanofibers and for the PEDOT:PSS-PbS—PVP core–shell nanofibers. In addition, according to the literature, the band at 1657 cm
1 shifts to higher wave numbers when PVP interacts with PbS nanoparticles, which has been attributed to the stabilization of PbS nanoparticles through the electrostatic interaction of the oxygen atoms of the polypyrrolidone units of PVP [38] and the Pb2+ on the nanoparticles’ surface. The fact that this band remains unchanged at 1657 cm
1 in our composite core–shell nanofibers, strongly suggest that PVP polymer has no interactions with PbS nanoparticles, which agrees with the PbS NPs dispersed only in the PEDOT:PSS core phase of the coaxial electrospun nanofibers. The FTIR spectrum of the PbS nanoparticles displayed the bands at 2929 cm
1 and 2879 cm
1 associated to asymmetric and symmetric stretching vibrations of C

H, and at 2852 cm
1 related to the symmetrical stretching vibration of methylene group; also a band at 1884 cm
1 related to the symmetric stretching of C

H can be observed. Other band can be observed associated to the stretching vibration of C¼O functional groups at 1765 cm
1; and associated to the vibration of COO- functional group in ionized carboxyl at 1572 cm
1, all of these bands supporting the idea of lead oleate in the surface of the PbS nanoparticles acting as capping layer [39]. Nevertheless, none of these signals could be observed in the spectra from the composite core–shell nanofibres, most likely due to the low concentration of nanoparticles found in the composite material. The strong signals observed in the region below 1200 cm
1 of the

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Fig. 6. FTIR spectra of the PEDOT:PSS-PbS NPs—PVP core–shell nanofibres, PVP nanofibers, PEDOT:PSS and oleate capped PbS nanoparticles.

FTIR spectra for PbS nanoparticles, come from the THF used to redisperse the PbS nanoparticles for their optical characterization. Finally, the optoelectronic properties of the core–shell nanofibres were evaluated. The I–V curves were acquired using the two-probe approach, and the results are shown in Fig. 7. Three different materials were evaluated in this study: pure PVP nanofibers, core–shell “PEDOT:PSS—PVP” nanofibers and core–shell “PEDOT:PSS-PbS NPs—PVP” nanofibers. It can be seen how the slope of the curve is almost zero for the material based in

Fig. 7. I vs. V graphs for the materials based in nanofibers of pristine PVP, PEDOT: PSS—PVP and (PEDOT:PSS—PbS NPs)
PVP; the last sample based on the core–shell nanofibers with PbS nanoparticles dispersed within the PEDOT:PSS showed the highest conductance.

pure PVP nanofibers, as expected, since PVP is an insulator; whereas the slope of the curve for the material based in “PEDOT: PSS—PVP” nanofibers increases considerably, compared to the pure PVP case, due to the presence of the conductive polymer. However, the slope of the curve for the material based on the “PEDOT:PSSPbS NPs—PVP” nanofibers exhibits the highest value among these three materials. Since the slope of the I–V curve represents the conductance of the analyzed material, it is clear that the material based on the core–shell “PEDOT:PSS-PbS NPs—PVP” nanofibers shows the highest conductance among the materials analyzed; it becomes evident that the addition of the PbS nanoparticles into the PEDOT:PSS phase clearly increases the conductivity of the core–shell nanofibers. Previous studies on conductive polymers—semiconductor nanoparticles composite materials have demonstrated an increment in the hole and electron mobilities of this type of composite materials, compared to the pristine conductive polymer. A. Watt et al. [40] demonstrated that the inclusion of PbS nanoparticles in poly[2-methoxy-5-(20 -ethylhexyloxy-p-phenylenevinylene)] (MEH-PPV) both balances and markedly increases the hole and electron mobilities, the hole mobility is increased by a factor of 105 and the electron mobility is increased by a factor of 107. In their work, they also proposed two conduction pathways for similar composite systems: a polymer-nanoparticle donor-acceptor pathway, and a purely nanoparticle percolation pathway arising from the similar electron and hole mobilities in PbS. In our system we believe both conduction mechanisms are equally important; the one related to the nanoparticles-polymer, donor-acceptor pathway, where most of the conduction takes place in the polymer matrix, by the majority charge carrier in the polymer matrix, which for PEDOT: PSS are the holes; and the contribution arising from the nanoparticle percolation conduction pathway, in this conduction mechanism the 1-dimensional morphology of the nanofiber and the well-dispersed nanoparticles within this 1-dimensional structure, would create a percolation conduction pathway that will allow the conduction of the minority charge carriers in the PEDOT:PSS matrix, in this case the electrons, that will translate in an increment in the conductivity. Moreover, the photoresponse of the core–shell PEDOT:PSS-PbS NPs—PVP nanofibers was evaluated. This experiment consists on applying a constant voltage to the material kept on darkness for 20 s, using a two-probe configuration to measure the current, then the material is exposed to an external light source for another 20 s, and finally the external light source is turned off, and the current is measured for another 20 s back in darkness condition. Fig. 8 shows the results of a PEDOT:PSS thin film (Fig. 8a) and PEDOT:PSS-PbS NPs—PVP nanofibers (Fig. 8b) for this experiment. As it can be observed that pure PEDOT:PSS does not show photoresponse, since the current value measured does not show any variation at the time when the material is illuminated by the external light source, or when it is eventually turned off. On the other hand, in Fig. 8b a clear increment in the measured current can be observed at 20 s, when the external light source is turned on, and a clear decrement in the measured current can be observed at 40 s, when the external light source is turned off. These observations clearly show that the material based on the core–shell PEDOT:PSS-PbS NPs—PVP nanofibers shows photoresponse. All these results show that the material based in the core–shell PEDOT:PSS-PbS NPs—PVP nanofibers shows a higher conductivity than the value displayed by the same material without the added PbS nanoparticles; moreover, this material shows a clear photoresponse, as photogenerated charge carriers are able to contribute to the measured electrical current when the material is exposed to an external light source, this property is not displayed by either of the polymeric materials forming the nanofibres, but it is shown by the PbS nanoparticles.

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Fig. 8. Graphs showing the results for the photoresponse experiments. a) No response is observed in the sample based in pristine PEDOT:PSS. b) Photoresponse showed by the composite material based in the core–shell (PEDOT:PSS—PbS NPs)—PVP nanofibers.

4. Conclusions core–shell nanofibers of PEDOT:PSS-PbS NPs as inner material and PVP as outer material were successfully obtained by coaxial electrospinning technique. The core–shell and defect-free morphology, with a diameter in the nanometer scale of the nanofibers were confirmed by HAADF-STEM images. The distribution of PbS nanoparticles in PEDOT:PSS, as well as the core–shell ordering, was confirmed by EDXS line scan analyses. The core–shell nanofibers also exhibited a clear photoresponse and a higher conductivity than the one observed in the case for the material without the PbS nanoparticles. Finally, this work contributes to setting up the basis to use the coaxial electrospinning technique in the synthesis of conducting nanofibers and nanowires involving this important ICP with well dispersed semiconducting nanoparticles inside it. Acknowledgements This work was supported by CONACYT (Mexico) through project number 154303 and 236812; and UANL PAICYT program through project number IT721-11. DFGG thanks and acknowledge the financial support received from CONACYT Mexico. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2016.06.019. References [1] W. Lövenich, PEDOT—properties and applications, Polym. Sci. Ser. C 56 (1) (2014) 135–143. [2] K. Sun, S. Zhang, P. Li, Review on application of PEDOTs and PEDOT: PSS in energy conversion and storage devices, J. Mater. Sci: Mater. Electron. 26 (2015) 4438–4462. [3] D.C. Martin, J. Wu, C.M. Shaw, Z. King, S.A. Spanninga, S. Richardson-Burns, J. Hendricks, J. Yang, The morphology of poly(3,4-Ethylenedioxythiophene), Polym. Rev. 50 (3) (2010) 340–384. [4] N. Naujoks, J. Dual, B.U. Lang, E. Mu, Microscopical investigations of PEDOT: PSS thin films, Adv. Funct. Mater. 19 (2009) 1215–1220. [5] N. Kim, S. Kee, S.H. Lee, B.H. Lee, Y.H. Kahng, Y.-R. Jo, B.-J. Kim, K. Lee, Highly conductive PEDOT:PSS nanofibrils induced by solution-Processed crystallization, Adv. Mater. 26 (14) (2014) 2268–2272. [6] H. Shi, C. Liu, Q. Jiang, J. Xu, Effective approaches to improve the electrical conductivity of PEDOT: PSS: a review, Adv. Electron. Mater. 1 (4) (2015) 1–16. [7] A. Mohammadi, M.A. Hasan, B. Liedberg, I. Lundström, W.R. Salanek, Chemical vapour deposition (CVD) of conducting polymers: polypyrrole, Synth. Met. 14 (3) (2003) 189–197 (1986). [8] J. Kim, E. Kim, Y. Won, H. Lee, K. Suh, The preparation and characteristics of conductive poly(3,4-ethylenedioxythiophene) thin film by vapor-phase polymerization, Synth. Met. 139 (2) (2003) 485–489.

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