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The photoluminescence enhancement of electrospun poly(ethylene oxide) fibers with CdS and polyaniline inoculations
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Acta Materialia 56 (2008) 5775–5782 www.elsevier.com/locate/actamat
The photoluminescence enhancement of electrospun poly(ethylene oxide) fibers with CdS and polyaniline inoculations Guo Yu, Xiaohong Li *, Xiaojun Cai, Wenguo Cui, Shaobing Zhou, Jie Weng Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science & Engineering, Southwest Jiaotong University, Chengdu 610031, China Received 24 June 2008; received in revised form 30 July 2008; accepted 31 July 2008 Available online 30 August 2008
Abstract Blending electrospinning of cadmium sulfide (CdS) quantum dots (QD) with poly(ethylene oxide) (PEO) solution was employed to fabricate one-dimensional ultrafine fibers with an average diameter of 450 nm. This study focused on systematic investigations into the role of the matrix polymer and the optimal electrospinning parameters for enhancing the photoluminescence properties of fibrous composites. CdS QDs showed a homogeneous distribution within the composite fibers, and fluorescence spectra showed that PEO successfully passivated the interface defects and quenched the visible emission of CdS QDs. The QDs concentration and electrospinning voltage were found to play important roles in enhancing the passivation effect of PEO and adjusting the photoluminescence intensity of the composite fibers. Furthermore, the addition of polyaniline enhanced the photoluminescence intensity of the electrospun fibers, and an electron–hole mechanism was proposed. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Electrospinning; Quantum dots; Cadmium sulfide; Polyaniline; Photoluminescence
1. Introduction Quantum dots (QDs), especially with a size range of 1– 10 nm, strongly size-controlled spectral tunability and chemical flexibility, are of both fundamental and technological interest. Cadmium sulfide (CdS) is an example of such a material. Bulk CdS is characterized by a direct band gap of 2.4 eV and shifts of up to 1 eV, which can be achieved via variation of the QD size. Due to the tunable electronic band gaps, the optical properties of CdS nanocrystals have been widely investigated in such fields as optoelectronic, photocatalysis and biological labeling [1]. Comparing to bulk CdS, CdS nanoparticles usually have poor optical performance due to low crystallinity and vast surface trap. Therefore effective modification of the surface trap is essential to enhance the emission efficiency of QDs.
*
Corresponding author. Tel.: +86 28 87634023; fax: +86 28 87634649. E-mail addresses: [email protected] (X. Li), [email protected] (X. Li).
The capping of CdS nanocrystals with another larger band gap semiconductor such as ZnS is a well-established method for the enhancement of the emission efficiency [2]. Another strategy is to employ polymers, such as poly (vinyl alcohol) and poly(methyl methacrylate), to enhance the emission efficiency as well as to modify the surface trap [3,4]. This method provides a facile procedure to incorporate nanocrystals into solid substrates or to prepare thin films containing nanocrystals. The photoluminescence enhancement of CdS quantum dots was observed in polymer matrices, reflecting improved electronic passivation and suggesting an effective role of the polymers in blocking the surface state recombination. In order to further facilitate QDs application in diverse field such as light emitting diodes [5], solar cells [6], and photoelectric sensors [7], different kinds of assemblies of quantum dots were also employed. One-dimensional (1D) nanostructures such as nanorod and nanowire have been the subject of intensive research due to the large surface area to volume ratio and unique nanoscale architecture. In particular, polymer ultrafine
1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.07.056
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fibers have been of great interest recently owing to their potential uses in multifunctional membranes, biomedical structural elements, nanoscale electrical or photoelectronic devices [8,9]. So incorporating QDs into polymer fibers to assemble polymer-quantum dots composites are particularly challenging because they could sustain the advantages of mixing polymer fibers, such as long lengths, diversity in composition and uniformity in size, with the electrical or photoluminescence properties of quantum dots. Electrospinning is a very simple and versatile method of creating polymer-based high-functional and high-performance ultrafine fibers, and has gained much attention not only due to its versatility in spinning a wide variety of polymeric fibers but also due to its consistency in producing fibers in the submicron range [10]. Electrospun fibers, with the diameter between 50 nm and 5 lm, have shown many outstanding properties, such as large surface area, high length–diameter ratio, flexible surface functionalities, tunable surface morphologies, and superior mechanical performance. Attempts have been made on capping the quantum dots into polymer matrices and building 1D nanostructure polymeric fibers. Demir et al. prepared linear arrays of ZnSe quantum dots embedded in electrospun polylactide ultrafine fibers to investigate the effect of Anderson localization phenomenon [11]. Enhanced mutual orientation of fibers with dimensions ranging from 10 to several hundred nanometers may be fabricated into potential optical devices. Liu et al. dispersed CdSe/ZnS into polymeric waveguide ultrafine fiber by electrospinning and used the QDs photoluminescence as an internal light source [12]. Bashouti et al. embedded CdS quantum wires in poly(ethylene oxide) (PEO) matrix to demonstrate the emission spectra of the aligned quantum wires, showing a linear polarization [13]. All these works have concentrated on exploring applications of incorporating quantum dots into polymeric fibers. And systematic investigations into the role of the polymer and the optimal electrospinning process to enhance the photoluminescence properties would be essential to make wider applications of nanosized CdS in 1D ultrafine fibers. Interesting electrical and optical properties are expected to manifest when the conducting polymers interact with the semiconductor quantum dots. In particular, polyaniline (PANI), one of the most frequently investigated semiconducting polymers, is a p-type semiconductor, while CdS nanoparticles are n-type semiconductors. The hole-conducting ability of PANI combined with the electron-conducting inorganic CdS can increase the radiative electron–hole recombination rate so as to improve the optoelectronic performance of the inorganic semiconductors. The photoluminescence of CdS is enhanced in composites employing PANI and CdS. PANI/CdS solution [14], films [15] and microwires [16] have been fabricated and the optical properties were investigated via various methods of characterization. All the results concluded that the fluorescence radiation and emission efficiency can be unambiguously enhanced when CdS nanoparticles were
decorated with PANI. However, PANI itself is difficult to process into ultrafine fibers [17], and the stabilization and photoluminescence enhancement of PANI on CdS QDs in 1D polymeric fibers remain a challenge. In this study, CdS nanoparticles with size range below 6 nm were synthesized, and blending electrospinning of CdS nanoparticles with PEO solution was employed to fabricate ultrafine composite fibers with an average diameter of 450 nm. The photoluminescence properties of electrospun composites and the process parameters were optimized and the enhancement mechanism was discussed. In order to further realize the application of nanosized CdS in light emission material as well as to investigate the role of PANI to CdS in the electrospun fibers, the combination of PANI and CdS nanoparticles in PEO matrix was electrospun into 1D ultrafine fiber, in which the mechanism of photoluminescence enhancement by the addition of PANI was evaluated. 2. Experiment 2.1. Materials PEO (Mw = 900 kDa) and 1-octadecene (ODE) were supplied by Acros Co. (NJ, USA) and used without further purification. PANI (Mw = 100 kDa) and sulfur powder were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). All reagents were analytical grade or better, and purchased from Changzheng Regents Company (Chengdu, China), unless otherwise indicated. 2.2. Preparation of CdS QDs CdS QDs were prepared in ODE solution with oleic acid (OA) as a capping agent, following the procedures described in detail elsewhere [18] with some modifications. Briefly, CdO (0.051 g, 0.20 mmol) and OA (0.564 g, 2.00 mmol) were mixed in ODE and heated to 300 °C during 10 min under vigorous stirring and a flow of high-purity nitrogen. Sulfur powder (0.0032 g, 0.10 mmol) was dissolved in 2 ml ODE under ultrasonic treatment and then swiftly injected into the reaction mixture. The reaction temperature was kept at 260 °C for 20 min. After cooling to room temperature, mixture of chloroform and methanol (1/1.5, v/v) was added to extract the excess complex of cadmium and OA. After the extracting process was applied twice, the mud-like CdS nanocrystals were obtained by the addition of ethanol, and then were dispersed in chloroform for further use. 2.3. Characterization of CdS QDs The crystalline phase of CdS QDs was analyzed with X-ray powder diffraction (XRD, Philips X’Pert PRO, The Netherlands) over the 2h range from 10° to 60° with a scanning speed of 0.35 min 1, using Cu-Ka radiation ˚ ). CdS QDs were dispersed in toluene, and (k = 1.54060 A
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deposited onto copper grids. The size and dispersion behaviors were examined using a transmission electron microscope (TEM, Hitachi H-700H, Japan) operated at 15 kV. The absorption spectra were recorded with an ultraviolet spectrophotometer (Shimadzu 2550, Japan). A fluorescence spectrophotometer (Hitachi F-7000, Japan) was used to obtain the photoluminescence (PL) spectra.
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mat was heated from 25 to 150 °C with a heating rate of 10 °C min 1. The PL spectra were recorded with the fluorescence spectrophotometer as mentioned above while the emissive light irradiated the sample at 45°, and the excitation wavelength for all the samples were fixed at 360 nm. Fluorescence microscopy (Leica DMR, Germany) was used to collect the fibers images under UV illumination, and the real object images were taken with a digital camera.
2.4. Preparation of PEO/CdS ultrafine fibers 3. Results and discussion PEO was dissolved in chloroform and acetone (2/1, v/v) at a concentration of 3.0 wt.%, followed by the addition of CdS under vigorous stirring to give a solution for electrospinning. In order to further enhance the photoluminescence of CdS nanoparticles, PANI was introduced to enwrap CdS nanoparticles. PANI was dissolved in chloroform, and added into the electrospinning solution described above. The electrospinning process was performed as described elsewhere [19]. Briefly, the electrospinning apparatus was equipped with a high-voltage station (Tianjing High Voltage Power Supply Co., Tianjing, China) of maximal voltage of 50 kV. The polymer solution was added in a 2 ml syringe attached to a circular-shaped metal needle as the nozzle. An oblong counter electrode was located about 20 cm from the capillary tip. The flow rate of the polymer solution was controlled by a precision pump (Zhejiang University Medical Instrument Co., Hangzhou, China) to maintain a steady flow from the capillary outlet. As a control to electrospun fibrous mats, 0.5 ml of the composite solution was dropped onto a 2 2 cm2 clean silicon sheet and left to dry under ambient conditions to form the thin film. 2.5. Characterization of CdS loaded electrospun fibers The electrospun fibrous mats, mounted on metal stubs by using conductive double-sided tape, were sputter-coated with gold for a period up to 120 s. The morphology was observed using scanning electron microscopy (SEM, FEI Quanta 200, The Netherlands) operated at an accelerating voltage of 20 kV. The fiber diameter was measured from SEM images, and five images were used for each fiber sample. From each image, at least 20 different fibers and 100 different segments were randomly selected and their diameters were measured to generate an average by using Photoshop 8.0 edition. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR, Thermos Nicolet 5700, Madison, WI) was performed to analyze the composite fibers, and the spectra were collected over the range of 4000–400 cm 1. The dispersion of QDs on the fibers was detected by TEM as described above after directly depositing the fibrous mat onto the copper grid. Selected area electron diffraction patterns (SAED) were taken with a camera length of 79 cm. Differential scanning calorimetry (DSC) was performed in perforated and covered aluminum pans under nitrogen purge (Netzsch STA 449C, Bavaria, Germany). Approximately 1 mg of polymer or fibrous
3.1. Characterization of CdS QDs Fig. 1A shows the UV–vis absorption and PL spectra of CdS QDs. The band gap energy for bulk CdS was 2.49 eV, which corresponded to a wavelength 512 nm, and absorption of shorter wavelengths of below 420 nm was observed for CdS nanoparticles. As the particle size decreases, the band gap increases while the wavelength of absorbance onset shifts to shorter wavelengths, and the mean size of CdS QDs can be determined from the onset wavelength [20]. As shown in Fig. 1A, the sizes of 3.0, 3.4 and 4.0 nm were obtained for QDs with the absorption wavelength of 380, 390 and 420 nm, respectively. The PL spectra indicated a strong and narrow band-edge emission at 410, 426 and 443 nm for QDs with the sizes of 3.0, 3.4 and 4.0 nm, respectively. As shown in Fig. 1A, the full width at half maximum (FWHM) of the three QDs was around 20 nm, indicating a narrow size distribution in solution. The low intensity and wide band in the range of 500–700 nm can be ascribed to defect states at the surface of nanocrystals [21]. Fig. 1B presents a TEM image of 4.0 nm CdS nanocrystals, indicating a well conform of the observed particle size with the calculations from the absorption spectra. The SAED pattern of the CdS nanocrystals revealed three strong diffraction rings patterns, indicating that the CdS grew as a multiple crystal. The distance between the CdS wafer (d) was obtained according to the formula d = Lk/R, where R is radius of diffraction ring, L a constant of the CCD camera, and k the wavelength of an electron. By assigning the elements to Cd and S in PCPDFWIN software, the h, k and l results were the (1 1 1), (2 2 0) and (3 1 1) planes, indicating that CdS nanoparticles existed as the zinc-blende cubic lattice. Fig. 1C shows XRD patterns of CdS QDs. The peaks at 26°, 44° and 52° fit well to the typical diffraction of the zinc-blende CdS, while the strong peaks at 19° related to the capping of organic molecules of OA at the nanocrystals surface [22]. The peaks width of XRD patterns varied inversely as the sizes of CdS nanocrystals, and the average size can be further calculated by the Scherrer equation: b1/2 = (Kk)/(D cos h) [23], where b1/2 is the FWHM in 2 h calculated by the XRD equipment software, K a constant set to 1, k the X-ray wavelength in Angstroms, D roughly the average crystallite size, and h the diffraction angle of the corresponding reflex. CdS QDs with the sizes of 2.7, 3.1 and 3.7 nm were
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Fig. 1. UV–vis absorption and PL spectra (A), TEM image and SAED pattern (B) and XRD profiles (C) of CdS QDs.
obtained, which was in good agreement with those found from TEM and UV–vis absorption spectra. 3.2. Characterization of PEO/CdS composite ultrafine fibers Fig. 2A shows the SEM image of electrospun PEO/CdS fibers, indicating smooth and uniform morphology with the size of 450 ± 50 nm. A uniform and separate distribution of QDs in the PEO matrix is also clearly revealed in the TEM image as shown in Fig. 2B. The CdS nanoparticles are roughly spherical in shape and separated from each other, each having a diameter of around 4 nm. The SAED pattern of the composite indicated that CdS nanoparticles existed in PEO ultrafine fibers as the zinc-blende cubic lattice, which was in agreement with that before electrospinning. Fig. 2C shows the fluorescence images of PEO/CdS composite ultrafine fibers under UV excitation. Strong and continuous fluorescence was observed throughout the whole PEO/CdS fibers, and the bright dots may ascribe to the cross-sections of the composite fibers. 3.3. The interactions of PEO and CdS in composite fibers The interactions between CdS and PEO and the structural changes of PEO/CdS composites are determined by ATR-FTIR. Fig. 3A shows the spectra of the electrospun PEO fibers, solvent-casting film and electrospun fibers of
PEO/CdS composites. For pure PEO fiber, the bands at about 1456 and 1350 cm 1 were attributed to the vibrations of CH2 group, and those at about 1112 and 962 cm 1 were assigned to the vibrations of CAO group. For PEO/CdS film and electrospun fibers, an additional weak peak at 405 cm 1 was due to the vibration absorption of the CdAS bond [24]. Meanwhile, a notable blue shift from 1112 to 1100 cm 1 was detected for the vibration of CAOAC bond of PEO after the introduction of CdS. The oxygen atoms of the backbone of PEO molecules formed CdAO band with excess cadmium atoms at the surface of the CdS nanocrystals, thus making the vibration of CAOAC bond shift to shorter wave numbers. As shown in Fig. 3A, the CH2 bending at 1280 and 1240 cm 1 appeared stronger for electrospun PEO and PEO/CdS fibers than those of PEO/CdS composite films. It was indicated that these two bands are very sensitive to the structure change of the alkyl chains [25]. During the electrospinning process, the high-voltage, high-speed swing and solvent evaporation may have effect on reconstructing chemical group distribution. Chemical groups with lower binding energy could be enriched on the fiber surface [26]. So the blue shifts may arise from the orientation of the PEO chains in the fibers during the electrospinning process. To further determine the interactions between PEO and CdS, DSC analysis of the electrospun PEO fibers, solventcasting film and electrospun fibers of PEO/CdS composites
Fig. 2. SEM (A), TEM and SAED patterns (B) and fluorescence microscope images (C) of electrospun PEO/CdS composite fibers.
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Fig. 3. FTIR spectra (A) and DSC traces (B) of electrospun PEO fibers (a), solvent-casting film (b) and electrospun fibers of PEO/CdS composites (c); fluorescence spectra (C) and digital images (D) of the blend solution (a), solvent-casting film (b) and electrospun fibers of PEO/CdS composites (c) under UV light illumination.
were conducted, and the results are summarized in Fig. 3B. Compared with electrospun PEO fibers, the increase in the melting temperature (Tm) of PEO/CdS ultrafine fibers may occur both from the restriction of polymer chain end to CdS and from the interactions between CAOAC groups of the PEO with the Cd atom on the CdS surface. Comparison of PEO/CdS fibers and solution-casting films revealed that the Tm of electrospun fibers was higher than that measured in the casting film. It indicated that electrospinning may yield structures of a higher order compared to traditional solution-casting materials leading to improved thermal stabilities. Fig. 3C depicts the PL spectra of the blend solution, solvent-casting film and electrospun fibers of PEO/CdS composites. All samples exhibited the characteristic PL spectra of CdS QDs, and the positions of the emission bands in the electrospun fibrous mat and solvent-casting film were similar to that of the blend solution. However, compared with the composite solution, the intensity of trap-state emission (500–700 nm) decreased significantly. Besides, a narrower FWHM was detected in PL spectrum of electrospun PEO/CdS fibers, indicating more uniform and homogenous size distribution of the CdS QDs in fibers. Broad FWHM and slight red shift were observed in the emission peak of solvent-casting film, suggesting the existence of Fo¨rster resonance energy transfer (FRET) effect due to
the aggregation of CdS QDs [27]. The photoluminescence enhancement of electrospun fibers may attribute to two aspects. In the solvent-casting PEO/CdS film, the interaction between PEO and CdS was not strong enough to confine the aggregation of CdS QDs, thus there may exist large particles of CdS in the matrix of PEO. For electrospun composite fibers, however, under the effect of the electrostatic force, CdS nanoparticles with low aggregation were extracted and polymer chains were solidified instantly due to the fast evaporation rate of the solvent in the electrospinning process. This made homogeneous CdS QDs further confined to fiber matrix with no time to aggregate. On the other hand, due to the high surface volume ratio of nanoparticles, their properties were ultrasensitive to the surface state and environmental variations. The high electric field induced the polarization and orientation of CdS QDs, which led to well dispersion, embedment in and alignment along the PEO matrices. The existence of PEO made up for the sulfur vacancy and passivated the surface defects of CdS nanoparticles, so the interactions between CdS QDs and PEO molecules was enhanced as demonstrated by FTIR and DSC analyses, thereafter enhanced the band-edge emission and weakened the trapstate emission of CdS significantly. Real object image of the blend solution, solvent-casting film and electrospun fibers of PEO/CdS composites under
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UV light illumination were exhibited from left to right in Fig. 3D. Weak violet and strong white color in PEO/CdS blend solution was observed, indicating that there still existed large surface trap and sulfur vacancy on the surface of CdS nanoparticles. The surface of composite film was rather smooth and the aggregation structure of CdS QDs caused the color of film uneven and impurity. Homogeneous fibrous mats appeared strong and uniform violet in color due to the sufficient passivation of PEO, which is consistent with the PL spectra as shown in Fig. 3C. 3.4. Optimization of the PL properties of electrospun composite fibers The fluorescence density of QDs is dependent on their concentrations. To investigate the effect of CdS inoculations on the photoluminescence, composite fibers were electrospun from PEO/CdS blend solutions with different QDs concentrations, and Fig. 4A summarizes the PL spectra of the obtained fibers. As the QDs concentration increased from 0.25 to 2.0 wt.%, the emission peak of the electrospun ultrafine fibers accordingly shifted from 430 to 437 nm. This may be due to the stronger QDs aggregation with the increase of concentrations. And the emission peak of the QDs would red shift as a result of light readsorption or slight FRET occurred. Meanwhile, the band-edge emission intensity of PEO/CdS composite fibers appeared to increase with an increase in the CdS contents. However, this enhancement shows some decrease for fibers with the QDs content of 3.0%, indicating that higher CdS concentration may form congregation, which can otherwise result in the lower electron exchange on the interface and further reduce the emission intensity of PEO/CdS composite fibers. Fig. 4B shows the PL spectra of PEO/CdS fibers with the electrospinning voltage from 10 to 25 kV. With the increase in the electrospinning voltages, the PEO/CdS fibers showed more intensive band-edge emission and weaker trap-state emission. The inset of Fig. 4B shows the change of PL intensity ratios of the band-edge emission to the trap-state emissions (IB/IT) as a function of the elec-
trospinning voltage. The increase of IB/IT suggested that the passivation effect of PEO was strengthened by increasing the electrospinning voltage. By applying a high-voltage to the composite solution, PEO chains spread and the PEO/CdS clusters were embedded in PEO with smaller size under the extraction of fibers during the electrospinning process. The electrospinning voltage may induce the polarization and orientation of CdS nanoparticles, which were embedded in and aligned along the PEO matrix fibers. With an increase in the electrospinning voltages, more CdS nanoparticles with low aggregation were extracted [28]. Then the dispersion of the PEO/CdS clusters was narrower, and the passivation was more effective, resulting in larger IB/IT ratio of the fibers under higher electrospinning voltages. Fig. 4C presents the fluorescence spectra of PEO ultrafine fibers incorporated with CdS mixtures of equivalent amount of QDs with different sizes. Three emission peaks were observed for the blend solution at 410, 426 and 443 nm, corresponding to those of QDs with the sizes of 3.0, 3.4 and 4.0 nm, respectively. The red shift and overlap of emission peak was very distinct in the solvent-casting film due to the aggregation of CdS nanocrystals during the solvent evaporation. Both emission peaks and intensity ratios for the ultrafine fibers were in good accordance with those in solution, indicating the distribution of CdS QDs within the electrospun fibrous mat as good and uniform as that in the solution. 3.5. Characterization of PEO/CdS/PANI composite fibers In order to further enhance the photoluminescence of CdS nanoparticles, PANI was introduced to enwrap CdS nanoparticles, which was further electrospun into PEO fibers. The SEM image of the electrospun PEO fibrous mats containing 2.0% CdS and 2.0% PANI is illustrated in Fig. 5A, showing smooth and uniform morphology with the average fiber diameter of 800 nm. The addition of PANI did not appear to hinder the electrospinning process, but led to an increase in the fibers’ diameters compared
Fig. 4. (A) The band-edge emission spectra of electrospun PEO/CdS fibers with CdS inoculations of 0.25 (a), 0.5 (b), 1.0(c), 2.0 (d) and 3.0 wt.% (e); (B) PL spectra of PEO/CdS fibers under electrospinning voltages of 10 (a), 16 (b) and 25 (c) kV, and the inset shows the PL intensity ratios of the band-edge emission to the trap-state emissions (IB/IT) as a function of the electrospinning voltages; (C) PL spectra of the blend solution (a), solvent-casting film (b) and electrospun fibers (c) of PEO entrapped CdS mixtures of equivalent amounts of QDs with the size of 3.0, 3.4 and 4.0 nm.
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Fig. 5. SEM (A) and TEM (B) images of PEO/CdS/PANI composite fibers.
with electrospun PEO/CdS fibers. Although electrospun fibers were distributed randomly in the non-woven mat, the surface of composite fibers is rather rough and uneven. Fig. 5B shows the TEM image of electrospun PEO/CdS/ PANI composite fibers. It was indicated that CdS nanocrystals enwrapped by PANI dispersed loosely and equably throughout the ultrafine fibers. Fig. 6A shows UV–vis spectra of PANI, electrospun PEO/CdS and PEO/CdS/PANI composite fibers. The spectrum of PANI showed two strong peaks. The peak at about 320 nm was assigned to the p–p* transition of benzene rings, and the other at around 630 nm represented the transition of the quinoid rings in long PANI chains [29]. Absorption peak at 420 nm was assigned for the characteristic absorption band of the CdS nanoparticles in PEO/ CdS composite fibers. The absorption spectra were changed greatly when the CdS nanoparticles were enwrapped by PANI. The spectrum of PEO/CdS/PANI ultrafine fibers showed two strong peaks with one weak shoulder. Very strong and sharp absorption at about 420 nm can be attributed to the characteristic absorption of CdS particles; however, the absorption peak at about 320 nm assigned to aryl p–p* transition became much weaker and broader than that of the single PANI sample. It can be explained that the absorption of CdS overlapped with that of PANI at 320 nm and their stabilities increased greatly when the
CdS particles were enwrapped by PANI. The transition from the quinoid rings of the PANI (about 630 nm) became weaker than before. Fig. 6B outlines the PL spectra of electrospun PEO/CdS fibers containing different ratios of PANI. Similar spectra were observed for all samples despite of different CdS/ PANI ratios. As can be seen in the Fig. 6B, the introduction of PANI increased the PL intensity remarkably. The PL intensity of PEO/CdS fibers increased from 1847 in the absence of PANI to 4242 for fibers with the ratio of PANI/CdS of 0.5, and increased successively to 5765 for those with the ratio of 1.0. However, along with the ratio of PANI/CdS increased to 2.0, the PL intensity of composite fibers decreased to 4450, revealing that the PL intensity of PEO/CdS fibers can be enhanced dominantly by the addition of PANI, but was not proportionally linear to its amount. The phenomenon may be explained by the boundary effect, which derived from the change of the refractive index of CdS particle surface owing to the modification of PANI, and would play crucial roles for the fluorescent intensity of CdS/PANI [30]. A blue shift from 437 to 429 nm in PL spectra was also observed after PANI inoculation. This can be understood by the relationship between the energy level (DE) and wavelength (k) according to the formula DE = hc/k, where h stands for the Planck constant, and c the light velocity [31]. With the increase
Fig. 6. (A) UV–vis spectra of PANI (a), electrospun PEO/CdS (b) and PEO/CdS/PANI composite fibers (c). (B) PL spectra of PEO/CdS/PANI composite fibers containing 2.0 wt.% CdS (a) and additional 1.0 (b), 2.0 (c) and 4.0 wt.% of PANI (d).
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in the amount of PANI, CdS nanoparticles were enwrapped by PANI so perfectly that the distance between individual CdS nanoparticles became large, resulting in an increase in the emissive energy (DE). Thus, the decrease in the wavelength and the blue shift in the PL spectra were observed as shown in Fig. 6B. The enhanced photoluminescence of semiconductor nanostructures may have no relation with quantum confine effect because PANI is not a fluorescence polymer and there is no obvious quantum confinement for 4.0 nm CdS in fibers with the dimension of 800 nm. And obviously the enhanced photoluminescence did not derive from surface passivation because PEO has successfully made up for the sulfur vacancy and passivated the surface defects of CdS nanoparticles. It seems that the main contribution of the PL enhancement in the PEO/CdS /PANI systems came from mechanisms other than the passivation. The enhanced photoluminescence of semiconductor nanostructures maybe was a process of separating the photo-generated electron and the photo-generated hole. The excitation light used for stimulate the band transition of CdS (with the threshold of 2.52 eV) can easily generate the separation of holes and electrons in PANI. With CdS, it was energetically favorable for the photo-generated electron transferred from the PANI to nanocrystal. Alternatively, if the exciton was created on the nanocrystal, or transferred onto the nanoparticles by Forster transfer, the hole can subsequently transfer to PANI, again producing a charge separated state with an electron on the nanoparticle and a hole on PANI [32]. The electron–hole mechanism was demonstrated for this photoluminescence enhancement of electrospun PEO/CdS/PANI composite fibers. 4. Conclusions Blending electrospinning of CdS nanoparticles with PEO solution was employed to fabricate ultrafine composite fibers with the average diameter of 450 nm. QDs have homogeneous distribution in the PEO/CdS composite fibers, and the emission efficiency of electrospun fibers increased and the trap-state emission decreased significantly. The QDs concentration and electrospinning voltage were found to play important roles on enhancing the passivation effect of PEO and adjusting the PL intensity of the composite fibers. The combinations of PANI and CdS nanoparticles were electrospun into 1D ultrafine PEO fiber. The PL intensity was observed to be significantly enhanced by the introduction of PANI, and the electron–hole mechanism was proposed for the luminescence enhancement. This kind of materials is expected to be applicable to the preparation of electroluminescent devices and nano-optoelectronic devices.
Acknowledgments This work was supported by National Natural Science Foundation of China (30570501), Program for New Century Excellent Talents in University (NECT-06-0801), Specialized Research Fund for the Doctoral Program of Higher Education (20050613025), Fok Ying Tong Education Foundation (104032), Ministry of Education of China. References [1] Wang Y, Tang ZY, Nicholas AK. Nanotoday 2005;20:20. [2] Steckel JS, Zimmer JP, Coe-Sullivan S, Stott NE, Bulovic V, Bawendi MG. Angew Chem Int Ed 2004;43:2154. [3] Tamborra M, Striccoli M, Comparelli R, Curri ML, Petrella A, Agostiano A. Nanotechnology 2004;15:240. [4] Wei CM, Hou SS. Colloid Polym Sci 2007;285:1343. [5] Tang AW, Teng F, Gao YH, Li D, Zhao SL, Liang CJ, et al. J Lumin 2007;650:649. [6] Choi SH, Song HJ, Park K, Yumb JH, Kimb SS, Sung YE. J Photochem Photobiol A 2006;179:135. [7] Carotenuto G, Longoa A, Repetto P, Perlo P, Ambrosio L. Sensor Actuat B – Chem 2007;125:202. [8] Frenot A, Chronakis IS. Curr Opin Colloid Interf Sci 2003;8:64. [9] Pinto NJ, Johnson TJ, MacDiarmid AG, Mueller CH, Theofylaktos N, Robinson DC, et al. Appl Phys Lett 2003;83:4244. [10] Subbiah T, Bhat GS, Tock RW, Parameswaran S, Ramkumar SS. J Appl Polym Sci 2005;96:557. [11] Schlecht S, Tan ST, Yosef M, Dersch R, Wendorff JH, Jia ZH, et al. Chem Mater 2005;17:809. [12] Liu HQ, Edel JB, Bellan LM, Craighead HG. Small 2006;2:495. [13] Bashouti M, Salalha W, Brumer M, Zussman E, Lifshitz E. Chemphyschem 2006;7:102. [14] Godovsky DY, Varfolomeev AE, Zaretsky DF, Chandrakanthi RLN, Ku¨ndig A, Weder C, et al. Mater Chem 2001;11:2465. [15] Khanna PK, Kulkarni MV, Singh N, Lonkar SP, Subbarao VVVS, Viswanath AK. Mater Chem Phys 2006;95:24. [16] Lu XF, Gao H, Chen JY, Chao DM, Zhang WJ, Wei Y. Nanotechnology 2005;16:113. [17] Norris ID, Shaker MM, Ko FK, MacDiarmid AG. Synthetic Met 2000;114:109. [18] Yu WW, Peng X. Angew Chem Int Ed 2002;41:2368. [19] Cui WG, Li XH, Zhou SB, Weng J. J Appl Polym Sci 2007;103:3105. [20] Yu WW, Qu L, Guo W, Peng X. Chem Mater 2003;15:2854. [21] Ramsden JJ, Webber SE, Gratzel M. J Phys Chem 1985;89:2740. [22] Wang CL, Zhang H, Zhang JH, Li MJ, Han K, Yang B. J Colloid Interface Sci 2006;294:104. [23] Liao S, Xu GF, Wang W, Watari F, Cui FZ, Ramakrishna S, et al. Acta Biomater 2007;3:669. [24] Martin TP, Schaber H. Spectrochim Acta 1982;38:655. [25] Bianco A, Bertarelli C, Frisk S, Rabolt JF, Gallazzi MC, Zerbi G. Synthetic Met 2007;157:276. [26] Cui WG, Li XH, Zhou SB, Weng J. Polym Degrad Stabil 2008;93:731. [27] Li MJ, Zhang JH, Zhang H, Liu YF, Wang CL, Xu X, et al. Adv Funct Mater 2007;17:3650. [28] Sui XM, Shao CL, Liu YC. Polymer 2007;48:1459. [29] Stilwell DE, Park SM. J Electrochem Soc 1989;136:427. [30] Fan J, Ji X, Zhang WG, Yan YH. Chem J Int 2004;6:45. [31] Wang SG, Li YX, Wang YZ, Yang QB, Wei Y. Mater Lett 2007;61:4674. [32] Chandrakanthi RLN, Careem MA. Thin Solid Films 2002;417:51.
Acta Materialia 56 (2008) 5775–5782 www.elsevier.com/locate/actamat
The photoluminescence enhancement of electrospun poly(ethylene oxide) fibers with CdS and polyaniline inoculations Guo Yu, Xiaohong Li *, Xiaojun Cai, Wenguo Cui, Shaobing Zhou, Jie Weng Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science & Engineering, Southwest Jiaotong University, Chengdu 610031, China Received 24 June 2008; received in revised form 30 July 2008; accepted 31 July 2008 Available online 30 August 2008
Abstract Blending electrospinning of cadmium sulfide (CdS) quantum dots (QD) with poly(ethylene oxide) (PEO) solution was employed to fabricate one-dimensional ultrafine fibers with an average diameter of 450 nm. This study focused on systematic investigations into the role of the matrix polymer and the optimal electrospinning parameters for enhancing the photoluminescence properties of fibrous composites. CdS QDs showed a homogeneous distribution within the composite fibers, and fluorescence spectra showed that PEO successfully passivated the interface defects and quenched the visible emission of CdS QDs. The QDs concentration and electrospinning voltage were found to play important roles in enhancing the passivation effect of PEO and adjusting the photoluminescence intensity of the composite fibers. Furthermore, the addition of polyaniline enhanced the photoluminescence intensity of the electrospun fibers, and an electron–hole mechanism was proposed. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Electrospinning; Quantum dots; Cadmium sulfide; Polyaniline; Photoluminescence
1. Introduction Quantum dots (QDs), especially with a size range of 1– 10 nm, strongly size-controlled spectral tunability and chemical flexibility, are of both fundamental and technological interest. Cadmium sulfide (CdS) is an example of such a material. Bulk CdS is characterized by a direct band gap of 2.4 eV and shifts of up to 1 eV, which can be achieved via variation of the QD size. Due to the tunable electronic band gaps, the optical properties of CdS nanocrystals have been widely investigated in such fields as optoelectronic, photocatalysis and biological labeling [1]. Comparing to bulk CdS, CdS nanoparticles usually have poor optical performance due to low crystallinity and vast surface trap. Therefore effective modification of the surface trap is essential to enhance the emission efficiency of QDs.
*
Corresponding author. Tel.: +86 28 87634023; fax: +86 28 87634649. E-mail addresses: [email protected] (X. Li), [email protected] (X. Li).
The capping of CdS nanocrystals with another larger band gap semiconductor such as ZnS is a well-established method for the enhancement of the emission efficiency [2]. Another strategy is to employ polymers, such as poly (vinyl alcohol) and poly(methyl methacrylate), to enhance the emission efficiency as well as to modify the surface trap [3,4]. This method provides a facile procedure to incorporate nanocrystals into solid substrates or to prepare thin films containing nanocrystals. The photoluminescence enhancement of CdS quantum dots was observed in polymer matrices, reflecting improved electronic passivation and suggesting an effective role of the polymers in blocking the surface state recombination. In order to further facilitate QDs application in diverse field such as light emitting diodes [5], solar cells [6], and photoelectric sensors [7], different kinds of assemblies of quantum dots were also employed. One-dimensional (1D) nanostructures such as nanorod and nanowire have been the subject of intensive research due to the large surface area to volume ratio and unique nanoscale architecture. In particular, polymer ultrafine
1359-6454/$34.00 Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2008.07.056
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fibers have been of great interest recently owing to their potential uses in multifunctional membranes, biomedical structural elements, nanoscale electrical or photoelectronic devices [8,9]. So incorporating QDs into polymer fibers to assemble polymer-quantum dots composites are particularly challenging because they could sustain the advantages of mixing polymer fibers, such as long lengths, diversity in composition and uniformity in size, with the electrical or photoluminescence properties of quantum dots. Electrospinning is a very simple and versatile method of creating polymer-based high-functional and high-performance ultrafine fibers, and has gained much attention not only due to its versatility in spinning a wide variety of polymeric fibers but also due to its consistency in producing fibers in the submicron range [10]. Electrospun fibers, with the diameter between 50 nm and 5 lm, have shown many outstanding properties, such as large surface area, high length–diameter ratio, flexible surface functionalities, tunable surface morphologies, and superior mechanical performance. Attempts have been made on capping the quantum dots into polymer matrices and building 1D nanostructure polymeric fibers. Demir et al. prepared linear arrays of ZnSe quantum dots embedded in electrospun polylactide ultrafine fibers to investigate the effect of Anderson localization phenomenon [11]. Enhanced mutual orientation of fibers with dimensions ranging from 10 to several hundred nanometers may be fabricated into potential optical devices. Liu et al. dispersed CdSe/ZnS into polymeric waveguide ultrafine fiber by electrospinning and used the QDs photoluminescence as an internal light source [12]. Bashouti et al. embedded CdS quantum wires in poly(ethylene oxide) (PEO) matrix to demonstrate the emission spectra of the aligned quantum wires, showing a linear polarization [13]. All these works have concentrated on exploring applications of incorporating quantum dots into polymeric fibers. And systematic investigations into the role of the polymer and the optimal electrospinning process to enhance the photoluminescence properties would be essential to make wider applications of nanosized CdS in 1D ultrafine fibers. Interesting electrical and optical properties are expected to manifest when the conducting polymers interact with the semiconductor quantum dots. In particular, polyaniline (PANI), one of the most frequently investigated semiconducting polymers, is a p-type semiconductor, while CdS nanoparticles are n-type semiconductors. The hole-conducting ability of PANI combined with the electron-conducting inorganic CdS can increase the radiative electron–hole recombination rate so as to improve the optoelectronic performance of the inorganic semiconductors. The photoluminescence of CdS is enhanced in composites employing PANI and CdS. PANI/CdS solution [14], films [15] and microwires [16] have been fabricated and the optical properties were investigated via various methods of characterization. All the results concluded that the fluorescence radiation and emission efficiency can be unambiguously enhanced when CdS nanoparticles were
decorated with PANI. However, PANI itself is difficult to process into ultrafine fibers [17], and the stabilization and photoluminescence enhancement of PANI on CdS QDs in 1D polymeric fibers remain a challenge. In this study, CdS nanoparticles with size range below 6 nm were synthesized, and blending electrospinning of CdS nanoparticles with PEO solution was employed to fabricate ultrafine composite fibers with an average diameter of 450 nm. The photoluminescence properties of electrospun composites and the process parameters were optimized and the enhancement mechanism was discussed. In order to further realize the application of nanosized CdS in light emission material as well as to investigate the role of PANI to CdS in the electrospun fibers, the combination of PANI and CdS nanoparticles in PEO matrix was electrospun into 1D ultrafine fiber, in which the mechanism of photoluminescence enhancement by the addition of PANI was evaluated. 2. Experiment 2.1. Materials PEO (Mw = 900 kDa) and 1-octadecene (ODE) were supplied by Acros Co. (NJ, USA) and used without further purification. PANI (Mw = 100 kDa) and sulfur powder were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). All reagents were analytical grade or better, and purchased from Changzheng Regents Company (Chengdu, China), unless otherwise indicated. 2.2. Preparation of CdS QDs CdS QDs were prepared in ODE solution with oleic acid (OA) as a capping agent, following the procedures described in detail elsewhere [18] with some modifications. Briefly, CdO (0.051 g, 0.20 mmol) and OA (0.564 g, 2.00 mmol) were mixed in ODE and heated to 300 °C during 10 min under vigorous stirring and a flow of high-purity nitrogen. Sulfur powder (0.0032 g, 0.10 mmol) was dissolved in 2 ml ODE under ultrasonic treatment and then swiftly injected into the reaction mixture. The reaction temperature was kept at 260 °C for 20 min. After cooling to room temperature, mixture of chloroform and methanol (1/1.5, v/v) was added to extract the excess complex of cadmium and OA. After the extracting process was applied twice, the mud-like CdS nanocrystals were obtained by the addition of ethanol, and then were dispersed in chloroform for further use. 2.3. Characterization of CdS QDs The crystalline phase of CdS QDs was analyzed with X-ray powder diffraction (XRD, Philips X’Pert PRO, The Netherlands) over the 2h range from 10° to 60° with a scanning speed of 0.35 min 1, using Cu-Ka radiation ˚ ). CdS QDs were dispersed in toluene, and (k = 1.54060 A
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deposited onto copper grids. The size and dispersion behaviors were examined using a transmission electron microscope (TEM, Hitachi H-700H, Japan) operated at 15 kV. The absorption spectra were recorded with an ultraviolet spectrophotometer (Shimadzu 2550, Japan). A fluorescence spectrophotometer (Hitachi F-7000, Japan) was used to obtain the photoluminescence (PL) spectra.
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mat was heated from 25 to 150 °C with a heating rate of 10 °C min 1. The PL spectra were recorded with the fluorescence spectrophotometer as mentioned above while the emissive light irradiated the sample at 45°, and the excitation wavelength for all the samples were fixed at 360 nm. Fluorescence microscopy (Leica DMR, Germany) was used to collect the fibers images under UV illumination, and the real object images were taken with a digital camera.
2.4. Preparation of PEO/CdS ultrafine fibers 3. Results and discussion PEO was dissolved in chloroform and acetone (2/1, v/v) at a concentration of 3.0 wt.%, followed by the addition of CdS under vigorous stirring to give a solution for electrospinning. In order to further enhance the photoluminescence of CdS nanoparticles, PANI was introduced to enwrap CdS nanoparticles. PANI was dissolved in chloroform, and added into the electrospinning solution described above. The electrospinning process was performed as described elsewhere [19]. Briefly, the electrospinning apparatus was equipped with a high-voltage station (Tianjing High Voltage Power Supply Co., Tianjing, China) of maximal voltage of 50 kV. The polymer solution was added in a 2 ml syringe attached to a circular-shaped metal needle as the nozzle. An oblong counter electrode was located about 20 cm from the capillary tip. The flow rate of the polymer solution was controlled by a precision pump (Zhejiang University Medical Instrument Co., Hangzhou, China) to maintain a steady flow from the capillary outlet. As a control to electrospun fibrous mats, 0.5 ml of the composite solution was dropped onto a 2 2 cm2 clean silicon sheet and left to dry under ambient conditions to form the thin film. 2.5. Characterization of CdS loaded electrospun fibers The electrospun fibrous mats, mounted on metal stubs by using conductive double-sided tape, were sputter-coated with gold for a period up to 120 s. The morphology was observed using scanning electron microscopy (SEM, FEI Quanta 200, The Netherlands) operated at an accelerating voltage of 20 kV. The fiber diameter was measured from SEM images, and five images were used for each fiber sample. From each image, at least 20 different fibers and 100 different segments were randomly selected and their diameters were measured to generate an average by using Photoshop 8.0 edition. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR, Thermos Nicolet 5700, Madison, WI) was performed to analyze the composite fibers, and the spectra were collected over the range of 4000–400 cm 1. The dispersion of QDs on the fibers was detected by TEM as described above after directly depositing the fibrous mat onto the copper grid. Selected area electron diffraction patterns (SAED) were taken with a camera length of 79 cm. Differential scanning calorimetry (DSC) was performed in perforated and covered aluminum pans under nitrogen purge (Netzsch STA 449C, Bavaria, Germany). Approximately 1 mg of polymer or fibrous
3.1. Characterization of CdS QDs Fig. 1A shows the UV–vis absorption and PL spectra of CdS QDs. The band gap energy for bulk CdS was 2.49 eV, which corresponded to a wavelength 512 nm, and absorption of shorter wavelengths of below 420 nm was observed for CdS nanoparticles. As the particle size decreases, the band gap increases while the wavelength of absorbance onset shifts to shorter wavelengths, and the mean size of CdS QDs can be determined from the onset wavelength [20]. As shown in Fig. 1A, the sizes of 3.0, 3.4 and 4.0 nm were obtained for QDs with the absorption wavelength of 380, 390 and 420 nm, respectively. The PL spectra indicated a strong and narrow band-edge emission at 410, 426 and 443 nm for QDs with the sizes of 3.0, 3.4 and 4.0 nm, respectively. As shown in Fig. 1A, the full width at half maximum (FWHM) of the three QDs was around 20 nm, indicating a narrow size distribution in solution. The low intensity and wide band in the range of 500–700 nm can be ascribed to defect states at the surface of nanocrystals [21]. Fig. 1B presents a TEM image of 4.0 nm CdS nanocrystals, indicating a well conform of the observed particle size with the calculations from the absorption spectra. The SAED pattern of the CdS nanocrystals revealed three strong diffraction rings patterns, indicating that the CdS grew as a multiple crystal. The distance between the CdS wafer (d) was obtained according to the formula d = Lk/R, where R is radius of diffraction ring, L a constant of the CCD camera, and k the wavelength of an electron. By assigning the elements to Cd and S in PCPDFWIN software, the h, k and l results were the (1 1 1), (2 2 0) and (3 1 1) planes, indicating that CdS nanoparticles existed as the zinc-blende cubic lattice. Fig. 1C shows XRD patterns of CdS QDs. The peaks at 26°, 44° and 52° fit well to the typical diffraction of the zinc-blende CdS, while the strong peaks at 19° related to the capping of organic molecules of OA at the nanocrystals surface [22]. The peaks width of XRD patterns varied inversely as the sizes of CdS nanocrystals, and the average size can be further calculated by the Scherrer equation: b1/2 = (Kk)/(D cos h) [23], where b1/2 is the FWHM in 2 h calculated by the XRD equipment software, K a constant set to 1, k the X-ray wavelength in Angstroms, D roughly the average crystallite size, and h the diffraction angle of the corresponding reflex. CdS QDs with the sizes of 2.7, 3.1 and 3.7 nm were
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Fig. 1. UV–vis absorption and PL spectra (A), TEM image and SAED pattern (B) and XRD profiles (C) of CdS QDs.
obtained, which was in good agreement with those found from TEM and UV–vis absorption spectra. 3.2. Characterization of PEO/CdS composite ultrafine fibers Fig. 2A shows the SEM image of electrospun PEO/CdS fibers, indicating smooth and uniform morphology with the size of 450 ± 50 nm. A uniform and separate distribution of QDs in the PEO matrix is also clearly revealed in the TEM image as shown in Fig. 2B. The CdS nanoparticles are roughly spherical in shape and separated from each other, each having a diameter of around 4 nm. The SAED pattern of the composite indicated that CdS nanoparticles existed in PEO ultrafine fibers as the zinc-blende cubic lattice, which was in agreement with that before electrospinning. Fig. 2C shows the fluorescence images of PEO/CdS composite ultrafine fibers under UV excitation. Strong and continuous fluorescence was observed throughout the whole PEO/CdS fibers, and the bright dots may ascribe to the cross-sections of the composite fibers. 3.3. The interactions of PEO and CdS in composite fibers The interactions between CdS and PEO and the structural changes of PEO/CdS composites are determined by ATR-FTIR. Fig. 3A shows the spectra of the electrospun PEO fibers, solvent-casting film and electrospun fibers of
PEO/CdS composites. For pure PEO fiber, the bands at about 1456 and 1350 cm 1 were attributed to the vibrations of CH2 group, and those at about 1112 and 962 cm 1 were assigned to the vibrations of CAO group. For PEO/CdS film and electrospun fibers, an additional weak peak at 405 cm 1 was due to the vibration absorption of the CdAS bond [24]. Meanwhile, a notable blue shift from 1112 to 1100 cm 1 was detected for the vibration of CAOAC bond of PEO after the introduction of CdS. The oxygen atoms of the backbone of PEO molecules formed CdAO band with excess cadmium atoms at the surface of the CdS nanocrystals, thus making the vibration of CAOAC bond shift to shorter wave numbers. As shown in Fig. 3A, the CH2 bending at 1280 and 1240 cm 1 appeared stronger for electrospun PEO and PEO/CdS fibers than those of PEO/CdS composite films. It was indicated that these two bands are very sensitive to the structure change of the alkyl chains [25]. During the electrospinning process, the high-voltage, high-speed swing and solvent evaporation may have effect on reconstructing chemical group distribution. Chemical groups with lower binding energy could be enriched on the fiber surface [26]. So the blue shifts may arise from the orientation of the PEO chains in the fibers during the electrospinning process. To further determine the interactions between PEO and CdS, DSC analysis of the electrospun PEO fibers, solventcasting film and electrospun fibers of PEO/CdS composites
Fig. 2. SEM (A), TEM and SAED patterns (B) and fluorescence microscope images (C) of electrospun PEO/CdS composite fibers.
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Fig. 3. FTIR spectra (A) and DSC traces (B) of electrospun PEO fibers (a), solvent-casting film (b) and electrospun fibers of PEO/CdS composites (c); fluorescence spectra (C) and digital images (D) of the blend solution (a), solvent-casting film (b) and electrospun fibers of PEO/CdS composites (c) under UV light illumination.
were conducted, and the results are summarized in Fig. 3B. Compared with electrospun PEO fibers, the increase in the melting temperature (Tm) of PEO/CdS ultrafine fibers may occur both from the restriction of polymer chain end to CdS and from the interactions between CAOAC groups of the PEO with the Cd atom on the CdS surface. Comparison of PEO/CdS fibers and solution-casting films revealed that the Tm of electrospun fibers was higher than that measured in the casting film. It indicated that electrospinning may yield structures of a higher order compared to traditional solution-casting materials leading to improved thermal stabilities. Fig. 3C depicts the PL spectra of the blend solution, solvent-casting film and electrospun fibers of PEO/CdS composites. All samples exhibited the characteristic PL spectra of CdS QDs, and the positions of the emission bands in the electrospun fibrous mat and solvent-casting film were similar to that of the blend solution. However, compared with the composite solution, the intensity of trap-state emission (500–700 nm) decreased significantly. Besides, a narrower FWHM was detected in PL spectrum of electrospun PEO/CdS fibers, indicating more uniform and homogenous size distribution of the CdS QDs in fibers. Broad FWHM and slight red shift were observed in the emission peak of solvent-casting film, suggesting the existence of Fo¨rster resonance energy transfer (FRET) effect due to
the aggregation of CdS QDs [27]. The photoluminescence enhancement of electrospun fibers may attribute to two aspects. In the solvent-casting PEO/CdS film, the interaction between PEO and CdS was not strong enough to confine the aggregation of CdS QDs, thus there may exist large particles of CdS in the matrix of PEO. For electrospun composite fibers, however, under the effect of the electrostatic force, CdS nanoparticles with low aggregation were extracted and polymer chains were solidified instantly due to the fast evaporation rate of the solvent in the electrospinning process. This made homogeneous CdS QDs further confined to fiber matrix with no time to aggregate. On the other hand, due to the high surface volume ratio of nanoparticles, their properties were ultrasensitive to the surface state and environmental variations. The high electric field induced the polarization and orientation of CdS QDs, which led to well dispersion, embedment in and alignment along the PEO matrices. The existence of PEO made up for the sulfur vacancy and passivated the surface defects of CdS nanoparticles, so the interactions between CdS QDs and PEO molecules was enhanced as demonstrated by FTIR and DSC analyses, thereafter enhanced the band-edge emission and weakened the trapstate emission of CdS significantly. Real object image of the blend solution, solvent-casting film and electrospun fibers of PEO/CdS composites under
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UV light illumination were exhibited from left to right in Fig. 3D. Weak violet and strong white color in PEO/CdS blend solution was observed, indicating that there still existed large surface trap and sulfur vacancy on the surface of CdS nanoparticles. The surface of composite film was rather smooth and the aggregation structure of CdS QDs caused the color of film uneven and impurity. Homogeneous fibrous mats appeared strong and uniform violet in color due to the sufficient passivation of PEO, which is consistent with the PL spectra as shown in Fig. 3C. 3.4. Optimization of the PL properties of electrospun composite fibers The fluorescence density of QDs is dependent on their concentrations. To investigate the effect of CdS inoculations on the photoluminescence, composite fibers were electrospun from PEO/CdS blend solutions with different QDs concentrations, and Fig. 4A summarizes the PL spectra of the obtained fibers. As the QDs concentration increased from 0.25 to 2.0 wt.%, the emission peak of the electrospun ultrafine fibers accordingly shifted from 430 to 437 nm. This may be due to the stronger QDs aggregation with the increase of concentrations. And the emission peak of the QDs would red shift as a result of light readsorption or slight FRET occurred. Meanwhile, the band-edge emission intensity of PEO/CdS composite fibers appeared to increase with an increase in the CdS contents. However, this enhancement shows some decrease for fibers with the QDs content of 3.0%, indicating that higher CdS concentration may form congregation, which can otherwise result in the lower electron exchange on the interface and further reduce the emission intensity of PEO/CdS composite fibers. Fig. 4B shows the PL spectra of PEO/CdS fibers with the electrospinning voltage from 10 to 25 kV. With the increase in the electrospinning voltages, the PEO/CdS fibers showed more intensive band-edge emission and weaker trap-state emission. The inset of Fig. 4B shows the change of PL intensity ratios of the band-edge emission to the trap-state emissions (IB/IT) as a function of the elec-
trospinning voltage. The increase of IB/IT suggested that the passivation effect of PEO was strengthened by increasing the electrospinning voltage. By applying a high-voltage to the composite solution, PEO chains spread and the PEO/CdS clusters were embedded in PEO with smaller size under the extraction of fibers during the electrospinning process. The electrospinning voltage may induce the polarization and orientation of CdS nanoparticles, which were embedded in and aligned along the PEO matrix fibers. With an increase in the electrospinning voltages, more CdS nanoparticles with low aggregation were extracted [28]. Then the dispersion of the PEO/CdS clusters was narrower, and the passivation was more effective, resulting in larger IB/IT ratio of the fibers under higher electrospinning voltages. Fig. 4C presents the fluorescence spectra of PEO ultrafine fibers incorporated with CdS mixtures of equivalent amount of QDs with different sizes. Three emission peaks were observed for the blend solution at 410, 426 and 443 nm, corresponding to those of QDs with the sizes of 3.0, 3.4 and 4.0 nm, respectively. The red shift and overlap of emission peak was very distinct in the solvent-casting film due to the aggregation of CdS nanocrystals during the solvent evaporation. Both emission peaks and intensity ratios for the ultrafine fibers were in good accordance with those in solution, indicating the distribution of CdS QDs within the electrospun fibrous mat as good and uniform as that in the solution. 3.5. Characterization of PEO/CdS/PANI composite fibers In order to further enhance the photoluminescence of CdS nanoparticles, PANI was introduced to enwrap CdS nanoparticles, which was further electrospun into PEO fibers. The SEM image of the electrospun PEO fibrous mats containing 2.0% CdS and 2.0% PANI is illustrated in Fig. 5A, showing smooth and uniform morphology with the average fiber diameter of 800 nm. The addition of PANI did not appear to hinder the electrospinning process, but led to an increase in the fibers’ diameters compared
Fig. 4. (A) The band-edge emission spectra of electrospun PEO/CdS fibers with CdS inoculations of 0.25 (a), 0.5 (b), 1.0(c), 2.0 (d) and 3.0 wt.% (e); (B) PL spectra of PEO/CdS fibers under electrospinning voltages of 10 (a), 16 (b) and 25 (c) kV, and the inset shows the PL intensity ratios of the band-edge emission to the trap-state emissions (IB/IT) as a function of the electrospinning voltages; (C) PL spectra of the blend solution (a), solvent-casting film (b) and electrospun fibers (c) of PEO entrapped CdS mixtures of equivalent amounts of QDs with the size of 3.0, 3.4 and 4.0 nm.
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Fig. 5. SEM (A) and TEM (B) images of PEO/CdS/PANI composite fibers.
with electrospun PEO/CdS fibers. Although electrospun fibers were distributed randomly in the non-woven mat, the surface of composite fibers is rather rough and uneven. Fig. 5B shows the TEM image of electrospun PEO/CdS/ PANI composite fibers. It was indicated that CdS nanocrystals enwrapped by PANI dispersed loosely and equably throughout the ultrafine fibers. Fig. 6A shows UV–vis spectra of PANI, electrospun PEO/CdS and PEO/CdS/PANI composite fibers. The spectrum of PANI showed two strong peaks. The peak at about 320 nm was assigned to the p–p* transition of benzene rings, and the other at around 630 nm represented the transition of the quinoid rings in long PANI chains [29]. Absorption peak at 420 nm was assigned for the characteristic absorption band of the CdS nanoparticles in PEO/ CdS composite fibers. The absorption spectra were changed greatly when the CdS nanoparticles were enwrapped by PANI. The spectrum of PEO/CdS/PANI ultrafine fibers showed two strong peaks with one weak shoulder. Very strong and sharp absorption at about 420 nm can be attributed to the characteristic absorption of CdS particles; however, the absorption peak at about 320 nm assigned to aryl p–p* transition became much weaker and broader than that of the single PANI sample. It can be explained that the absorption of CdS overlapped with that of PANI at 320 nm and their stabilities increased greatly when the
CdS particles were enwrapped by PANI. The transition from the quinoid rings of the PANI (about 630 nm) became weaker than before. Fig. 6B outlines the PL spectra of electrospun PEO/CdS fibers containing different ratios of PANI. Similar spectra were observed for all samples despite of different CdS/ PANI ratios. As can be seen in the Fig. 6B, the introduction of PANI increased the PL intensity remarkably. The PL intensity of PEO/CdS fibers increased from 1847 in the absence of PANI to 4242 for fibers with the ratio of PANI/CdS of 0.5, and increased successively to 5765 for those with the ratio of 1.0. However, along with the ratio of PANI/CdS increased to 2.0, the PL intensity of composite fibers decreased to 4450, revealing that the PL intensity of PEO/CdS fibers can be enhanced dominantly by the addition of PANI, but was not proportionally linear to its amount. The phenomenon may be explained by the boundary effect, which derived from the change of the refractive index of CdS particle surface owing to the modification of PANI, and would play crucial roles for the fluorescent intensity of CdS/PANI [30]. A blue shift from 437 to 429 nm in PL spectra was also observed after PANI inoculation. This can be understood by the relationship between the energy level (DE) and wavelength (k) according to the formula DE = hc/k, where h stands for the Planck constant, and c the light velocity [31]. With the increase
Fig. 6. (A) UV–vis spectra of PANI (a), electrospun PEO/CdS (b) and PEO/CdS/PANI composite fibers (c). (B) PL spectra of PEO/CdS/PANI composite fibers containing 2.0 wt.% CdS (a) and additional 1.0 (b), 2.0 (c) and 4.0 wt.% of PANI (d).
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in the amount of PANI, CdS nanoparticles were enwrapped by PANI so perfectly that the distance between individual CdS nanoparticles became large, resulting in an increase in the emissive energy (DE). Thus, the decrease in the wavelength and the blue shift in the PL spectra were observed as shown in Fig. 6B. The enhanced photoluminescence of semiconductor nanostructures may have no relation with quantum confine effect because PANI is not a fluorescence polymer and there is no obvious quantum confinement for 4.0 nm CdS in fibers with the dimension of 800 nm. And obviously the enhanced photoluminescence did not derive from surface passivation because PEO has successfully made up for the sulfur vacancy and passivated the surface defects of CdS nanoparticles. It seems that the main contribution of the PL enhancement in the PEO/CdS /PANI systems came from mechanisms other than the passivation. The enhanced photoluminescence of semiconductor nanostructures maybe was a process of separating the photo-generated electron and the photo-generated hole. The excitation light used for stimulate the band transition of CdS (with the threshold of 2.52 eV) can easily generate the separation of holes and electrons in PANI. With CdS, it was energetically favorable for the photo-generated electron transferred from the PANI to nanocrystal. Alternatively, if the exciton was created on the nanocrystal, or transferred onto the nanoparticles by Forster transfer, the hole can subsequently transfer to PANI, again producing a charge separated state with an electron on the nanoparticle and a hole on PANI [32]. The electron–hole mechanism was demonstrated for this photoluminescence enhancement of electrospun PEO/CdS/PANI composite fibers. 4. Conclusions Blending electrospinning of CdS nanoparticles with PEO solution was employed to fabricate ultrafine composite fibers with the average diameter of 450 nm. QDs have homogeneous distribution in the PEO/CdS composite fibers, and the emission efficiency of electrospun fibers increased and the trap-state emission decreased significantly. The QDs concentration and electrospinning voltage were found to play important roles on enhancing the passivation effect of PEO and adjusting the PL intensity of the composite fibers. The combinations of PANI and CdS nanoparticles were electrospun into 1D ultrafine PEO fiber. The PL intensity was observed to be significantly enhanced by the introduction of PANI, and the electron–hole mechanism was proposed for the luminescence enhancement. This kind of materials is expected to be applicable to the preparation of electroluminescent devices and nano-optoelectronic devices.
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