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Enhanced ultraviolet emission from highly dispersed ZnO quantum dots embedded in poly(vinyl pyrrolidone) electrospun nanofibers
Journal of Colloid and Interface Science 347 (2010) 215–220
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Enhanced ultraviolet emission from highly dispersed ZnO quantum dots embedded in poly(vinyl pyrrolidone) electrospun nanofibers Zhenyi Zhang, Changlu Shao *, Fei Gao, Xinghua Li, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China
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Article history: Received 11 February 2010 Accepted 24 March 2010 Available online 27 March 2010 Keywords: Electrospinning Poly(vinyl pyrrolidone) ZnO quantum dots Photoluminescence
a b s t r a c t Highly dispersed ZnO quantum dots (QDs) in poly(vinyl pyrrolidone) (PVP) nanofibers have been successfully prepared by electrospinning technique. The structure and optical properties were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), resonant Raman spectra, Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric and differential thermal analysis (TG–DTA), ultraviolet (UV)–vis absorption spectra, and photoluminescence (PL) spectra. In the PVP/ZnO QDs composite nanofibers, PVP molecules could effectively prevent the aggregation of ZnO QDs and passivate the surface defects of ZnO QDs. Thus, by comparing ZnO QDs, the composite nanofibers exhibited a blue-shifted band gap and enhanced ultraviolet (UV) emission. Furthermore, the composite nanofibers prepared at higher voltage showed more intense UV emission than which obtained at lower voltage, suggesting that the UV emission intensity of the composite nanofibers could be controlled by adjusting the electrospinning voltage. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction As an important semiconducting oxide with a direct wide band gap (3.37 eV) and a large exciton binding energy (about 60 meV) at room temperature, zinc oxide (ZnO) has attracted much interest due to its novel physical properties and excellent chemical nature such as near-ultraviolet emission, conductivity, piezoelectricity, photocatalysis, sensitivity to gas, and so forth [1–4]. It is well known that low dimensional nanostructural materials provide a fundamental importance in bridging the gap between bulk material and molecular species. Moreover, the low dimensional semiconductor nanocrystals may have superior optical properties to bulk crystals because of quantum confinement effects. As a result, low dimensional ZnO nanocrystals, such as nanorods [5], nanowires [6], and nanorings [7], have been reported comprehensively. Comparing to bulk ZnO materials, however, nanostructural ZnO usually exhibits stronger visible luminescence related to surface defects [8]. It is great challenge to enhance the ultraviolet (UV) emission of nanostructural ZnO materials. Thus, in recent years, the optical properties of ZnO nanocrystals embedded in organic polymers, such as poly(vinyl alcohol) (PVA), poly(methyl methacrylate) (PMMA) and poly(vinyl pyrrolidone) (PVP) [9–11], and inorganic materials, such as CaF2, MgO, SiO2 and BaF2 [12–15], have been widely studied. Especially, polymer/ZnO nanocrystals
* Corresponding author. E-mail address: [email protected] (C. Shao). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.03.052
composites with one dimensional (1D) nanostructure have been of immense interest because of their large length-to-diameter ratio, special optical and conductivity properties, and so forth, which might be very useful to fabrication of nanoscale electronic and optoelectronic devices. Notably, electrospinning, a remarkably simple and versatile technique, has been exploited for nearly one century to process polymers and related materials into 1D structural fibers for a variety of applications [16]. By using this facile method, many kinds of composite nanofibers with optoelectronic properties have been prepared, such as poly(vinyl pyrrolidone) (PVP)/poly(p-phenylene vinylene) (PPV) [17], poly[2,5-(20 -ethylhexyloxy)]-1,4-phenylenevinylene(BEHPPV)/1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)-C61 (PCBM) [18], polymer/rare-earth complex [19–21], poly(ethylene oxide) (PEO)/teraamino-phthalocyanine copper (II) [(NH2)4PcCu] [22], PVP/Ln3+ doped NaYF4 [23], PVP/PbS [24], PVP/CdS [25], and so forth. Furthermore, our group has studied the photoluminescence (PL) properties of PVA/ZnO quantum dots (QDs), and PEO/ZnO QDs composite nanofibers [26,27]. The investigation indicated that ZnO QDs capped with different polymers could exhibit different PL properties due to the interactions between polymers and ZnO QDs. Thus, it is expect that highly dispersed ZnO QDs in PVP electrospun nanofibers maybe possess a novel PL properties. In the past, Guo et al. found a markedly enhanced near-band-edge UV photoluminescence and significantly reduced defect-related green emission from highly monodisperse PVP-capped ZnO nanoparticles [28]. And, Yang et al. reported an enhanced UV emission from PVP surface modified ZnO
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QDs [29]. However, to the best of our knowledge, there was no report on the PL properties of highly dispersed ZnO quantum dots in PVP electrospun nanofibers. In our present work, we reported a successfully attempt to the fabrication of PVP/ZnO QDs composite nanofibers by electrospinning method. By comparing the ZnO QDs, the PVP/ZnO composite nanofibers exhibited enhanced UV emission, because PVP molecules effectively passivated the surface defects of ZnO QDs. Notably, the composite nanofibers showed more intensive UV emission at higher electrospinning voltages. 2. Materials and methods In our experiments, the preparing process consisted of two steps. At first, for the preparation of the precursor, 1 g of poly(vinyl pyrrolidone) (PVP) powder (Mn = 900,000) was dissolved in 10 ml of ethanol. And, then the ZnO QDs obtained by using the method of literature [30] was mixed with the above PVP and ethanol solution, for which the ZnO QDs concentration was 0.1 M. Subsequently, the above PVP/ZnO composite precursor solution was drawn into a hypodermic syringe for electrospinning under different voltage (10, 12, 14, and 16 kV). The obtained PVP/ZnO QDs composite nanofibers were collected at a distance about 12 cm to the syringe tip for the following characterizations. Scanning electron microscopy (SEM; XL-30 ESEM FEG, Micro FEI Philips) and transmission electron microscopy (TEM; high resolution TEM [HRTEM], JEM-3010) were used to characterize the morphologies of the products. X-ray diffraction (XRD) measurement was carried out using a D/max 2500 XRD spectrometer (Rigaku) with Cu Ka line of 0.1541 nm. Fourier transform infrared (FT-IR) spectra were obtained on Magna 560 FT-IR spectrometer with a resolution of 1 cm1. Thermal gravimetric and differential thermal analysis (TG–DTA) analysis was carried out on a NETZSCH STA 449C thermoanalyzer in N2 atmosphere. The UV–Vis absorption spectra were measured at room temperature with a Lambda 900 UV–vis spectrophotometer (Perkin–Elmer). Photoluminescence (PL) and resonant Raman spectra were collected with a Jobin–Yvon HR800 micro-Raman spectrometer using the 325 nm line of a He– Cd laser as the excitation source. 3. Results and discussion Fig. 1a and b showed a typical SEM images of the PVP/ZnO QDs composite nanofibers prepared at the electrospinning voltage of 10 kV. From Fig. 1a, it could be observed that the composite nanofibers aligned in random orientation because of the bending instability associated with the spinning jet. Fig. 1b displayed the corresponding SEM image with higher magnification. It was showed that the diameters of above composite nanofibers ranged from 100 nm to 250 nm. The morphologies of the composite nanofibers obtained at the electrospinning voltages of 12, 14, 16 kV (not shown) were similar with that attained at the electrospinning voltage of 10 kV. Afterward, the PVP/ZnO QDs composite nanofibers prepared at different voltages (from 10 to 16 kV) was examined by HRTEM in Fig. 1c–f, which further confirmed that ZnO QDs have been successfully embedded in the PVP nanofibers. Besides, with the increase of the electrospinning voltages, more ZnO QDs in the composite nanofibers were obvious more dispersible. It was implied that the electrospinning voltage might play a crucial role to direct the dispersion of ZnO QDs in the composite nanofibers during the electrospinning process. The HRTEM image of a single ZnO QDs in the composite nanofibers in Fig. 1g given evidence of that ZnO QDs were spherical with the diameter about 4 nm. From the XRD curve of the PVP/ZnO QDs composite nanofibers in Fig. 1h, a broad peak around 22° appeared, corresponding to PVP semicrystalline in the composite nanofibers. But, the signals of ZnO QDs
were not clearly observed, which might be owing to its small size, low content and high dispersion in PVP nanofibers. The FT-IR spectra of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers obtained at the electrospinning voltages of 10 kV were shown in Fig. 2. From the Fig. 2A, it was observed that the PVP nanofibers exhibited strong peaks at around 1466 and 1285 cm1, both attributed to the C@C and CAN stretching vibration, respectively. Another characteristic peak of PVP was the very strong peak at 1661 cm1, which was attributed to C@O stretching vibration. However, in the FT-IR spectrum of PVP/ZnO QDs composite nanofibers, besides the characteristic vibration bands of PVP, a new intense broadband between 400 and 750 cm1 assigned to the ZnAO vibration of ZnO was appeared, indicating that the composite nanofibers were composed of PVP and ZnO QDs [31]. Notably, by comparing the FT-IR spectrum of the pure PVP nanofibers, the strong peak of the carbonyl group stretching was shifted to 1650 cm1 in the composite nanofibers (Fig. 2B). It was reported that the type of interaction between the inorganic nanoparticles and the carbonyl group in PVP might cause a shift in FT-IR frequency because the metal atoms accepted an electron pair of the carbonyl oxygen [24,25,32]. In our present work, there might exist the oxygen vacancy defects on the surface of ZnO QDs, resulting in the weak UV emission of as-prepared ZnO QDs. However, the composite nanofibers exhibited an enhanced UV emission (as shown in Fig. 6). Those meant that the electron pair of the carbonyl oxygen of PVP molecule could make up for the oxygen vacancy of ZnO QDs and thus passivated the surface defects of ZnO QDs. Furthermore, Fig. 3 showed the measured resonant Raman scattering spectra of the PVP/ZnO QDs composite nanofibers prepared at the electrospinning voltages of 10 kV. In our experiment, the resonant Raman scattering at room temperature was performed to investigate the vibrational properties of the composite nanofibers. The energy of the He–Cd laser line (325 nm) was 3.82 eV, which was higher than the band gap of ZnO (3.37 eV). In Fig. 3, a significant Raman band centered at 574, 1152, 1733 cm1 was observed, which was attributed to the 1–3 Raman longitudinal optical (LO) phonon mode of nanosized ZnO [33]. Moreover, two peaks at around 1360 and 1590 cm1 indicated by a rectangle were clearly observed, which originated from Raman scattering of disordered carbon [34]. To further confirm the interactions between PVP and ZnO QDS, the thermal behavior of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers were investigated in Fig. 4. As observed in the images of TG curve in Fig. 4, the pure PVP nanofibers appeared two major weight loss steps from 380 up to about 450 °C and from 450 up to 600 °C due to the decomposition of organic PVP. And, there was no residue above 600 °C, indicating that the organic PVP was decomposed absolutely. By comparing the TG curves of the as-spun PVP and PVP/ZnO QDs composite nanofibers, it could be deduced that the obvious weight losses appearing at 50–150, 380–450, and 450–550 °C in the TG curve of PVP/ZnO QDs composite nanofibers were ascribed to the evaporation of the absorbed water and the thermal decomposition of the organics. And, the clear plateau formed between 550 and 600 °C on the TG curve indirect indicated that the composite nanofibers might consist of PVP and ZnO QDs. Meanwhile, by comparing the DTA curve of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers in Fig. 4, it was found that three exothermic peaks were observed at around 410, 430, and 520 °C in the composite nanofibers, indicating that there might exist the weak interactions between the PVP and ZnO QDs. All above results, including the FT-IR, Raman and TG–DTA, demonstrated the existence of interactions between a PVP molecule and ZnO QDs via the metal atoms accepted an electron pair of the carbonyl oxygen. These interactions could be responsible for especial luminescent properties of the composite nanofibers. To give a study on the optical properties of the samples, the absorption and PL spectra for them were measured. Fig. 5 showed
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Fig. 1. SEM images of PVP/ZnO QDs composite nanofibers prepared at the electrospinning voltage of 10 kV: (a) low magnification, (b) high magnification; TEM images of the composite nanofibers obtained at the electrospinning voltages of: (c) 10, (d) 12, (e) 14, (f) 16 kV; (g) HRTEM image of a single ZnO quantum dot in the composite nanofibers; and (h) the XRD pattern of the composite nanofibers obtained at the electrospinning voltages of 10 kV.
the absorption spectra of ZnO QDs in ethanol, pure PVP nanofibers and PVP/ZnO QDs composite nanofbiers (the electrospinning voltage was 10 kV). From Fig. 5, it was well established that the size of ZnO QDs could be estimated from the excitonic absorption peak based on the effective mass approximation [15,35].
E ffi Ebulk þ g
2 p2 1 h 1 þ 0:248ERyd 2 2er me mh
ð1Þ
where E and Ebulk was the bandgap of quantum dots and bulk, r was g the particle radius, me was the effective mass of the electrons, mh was the effective mass of the holes, h was Planck constant divided by 2p, and e was the charge on the electron. In our experiment, tak¼ 3:37 eV, the electron and the hole ing, E = 3.41 eV and Ebulk g effective mass were me ¼ 0:24m0 and mh ¼ 0:45m0 , m0 was the free electron mass, and using the bulk exciton binding energy ERyd ¼ 60 meV. The ZnO QDs diameters in ethanol were calculated to be about 3.863 nm. These were consistent with the diameter of
ZnO QDs in composite nanofibers from above TEM analysis, indicating ZnO QDs was not further grow in the our experiment process. Furthermore, the characteristic peaks of exciton absorption of ZnO were observed for both of ZnO QDs and composite nanofibers. It was also conformed that ZnO QDs were successfully embedded in the PVP nanofibers. Furthermore, as the Bohr radius of bulk ZnO was 2.34 nm and the diameter of ZnO QDs in this work was about 4 nm from above results, the ratio of the QD diameter and the Bohr radius was nearly 2, which meant that a strong confinement occurred [36]. The exciton absorption peaks of ZnO QDs sol and PVP/ZnO QDs composite nanofibers (about 340 nm) were substantially blueshifted relative to that of the bulk ZnO (about 372 nm) due to the strong quantum confinement effects. However, the blueshifted for the ZnO QDs sol and PVP/ZnO QDs composite nanofibers might not only come from a blueshift from the confinement-induced shift of the electronic levels, but also come from a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius [37].
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Fig. 2. FT-IR spectra of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers obtained at the electrospinning voltages of 10 kV.
Fig. 3. The resonant Raman spectra of the PVP/ZnO QDs composite nanofibers obtained at the electrospinning voltages of 10 kV.
With the interactions between PVP molecules and ZnO QDs, the existence of PVP molecules might make up for the oxygen vacancy and passivated the surface defects of ZnO QDs, thereafter enhance the UV photoluminescence emission of ZnO QDs. Fig. 6A showed the PL spectra of PVP/ZnO QDs composites nanofibers obtained at the electrospinning voltage from 10 to 16 kV. The inset was the PL spectra of ZnO QDs. As observed in Fig. 6A, the narrow UV emission from PVP/ZnO QDs composite nanofibers at around 364.4 nm (3.40 eV) was consisted with the bandedge emission typically originated from the exciton combination of ZnO [33]. The broad of visible emission for above composite nanofibers at about 550 nm (2.25 eV) was related to the transition between the electron close to the conduction band and the hole at vacancy associated with the surface defects [38]. Besides, the position of the UV emission band did not shift in all composite nanofibers. However, the UV emission of all composite nanofibers showed two characteristics compared to that of ZnO QDs gel, namely, blueshifted peak energy and enhanced ultraviolet emission, which demonstrated that PVP molecules could effectively prevent the aggregations of ZnO QDs and passivate the defects on the surface of ZnO QDs. Thus, the inter-
Fig. 4. TG–DTA curves of thermal decomposition of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers obtained at the electrospinning voltage of 10 kV.
Z. Zhang et al. / Journal of Colloid and Interface Science 347 (2010) 215–220
Fig. 5. Absorption spectra of ZnO QDs in ethanol (ZnO QDs), pure PVP nanofibers, and the ZnO QDs in the PVP/ZnO QDs composite nanofibers. The electrospinning voltage was 10 kV.
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actions among the ZnO QDs could be decreased, however, which could not be neglected in ZnO QD gel. Furthermore, an interesting phenomenon observed in Fig. 6B, it was showed that the ratios of ultraviolet emission to visible emission (IUV/IVIS) of PVP/ZnO QDs composite nanofibers were significantly increased by increasing the electrospinning voltage, which meant that the passivation effect of PVP was strengthened by increasing electrospinning voltage. Based on above results, a possible mechanism was presented, as shown in Scheme 1. In the precursor solution, there might exist large amount of ZnO QDs clusters because of the high Gibbs’ surface free energy of the small sized ZnO nanocrystals [27]. As a result, the PVP molecules could only passivated the surface defects of the ZnO QDs cluster, but could hardly passivated the surface defects of every ZnO QDs. As we known, electrospinning was a drawing process based on two competitive forces: the electrostatic force and the surface tension [39]. When the applied electric voltage overcomed the surface tension of the precursor solution, one or several jets of the solution were ejected from the tip of the Taylor cone and fly towards the grounded plate. This electrified jet then undergone a stretching and whipping process, leading to the formation of a long and thin thread. In our experiment, during the electrospinning
Fig. 6. (A) PL spectra of the PVP/ZnO QDs composites nanofibers prepared at the electrospinning voltages of 10, 12, 14 and 16 kV at room temperature, respectively. Inset was the PL spectrum of ZnO QDs. (B) The integrate area ratios of ultraviolet emission to visible emission (IUV/IVIS) of above composite nanofibers versus the function of the electrospinning voltages.
Scheme 1. The scheme of the distribution of ZnO QDs in the PVP/ZnO QDs composites nanofibers under the electrospinning process.
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process, by applying the higher voltages to the precursor solution, the curved PVP chains could be spread, while the ZnO QDs clusters could be dispersed highly in PVP nanofibers due to the extraction of large electrostatic force. Thus, the higher electrospinning voltage might induce the polarization and orientation of ZnO QDs. ZnO QDs were embedded in PVP matrix fibers with high dispersion, which was showed in the TEM of Scheme 1, suggesting that the PVP molecules might nearly passivate every ZnO QDs in the composite nanofibers. However, by using the lower voltages, there might still exist a lot of ZnO QDs clusters in the composite nanofibers due to the extraction of weak electrostatic force, which meant that only the surface of ZnO QDs clusters could be passivated by PVP molecules. Furthermore, it was noticed that, being prepared before mixing with PVP and protected by PVP, ZnO QDs did not grow in the sequent process due to the position of the UV emission band of all the samples remained unchanged in the PL spectra. Those results showed that, with the increase of the voltages, more ZnO QDs got more dispersive in the composite nanofibers, while the passivation was more effective, resulting in larger IUV/IVIS ratio of the composite nanofibers. Namely, by adjusting the electrospinning voltage, the intensity of UV emission from the composite nanofibers could be controlled. 4. Conclusion In summary, the nanofibers of PVP/ZnO QDs composite were successfully prepared by electrospinning. By characterizing the structural and optical properties of above composite nanofibers, we found that PVP molecules effectively passivated the oxygen related defects on the surface of ZnO, which led to blueshifted peak energy and enhanced UV emission to the ZnO QDs. Moreover, the electrospinning voltage was found to play an important role on enhancing the dispersion of ZnO QDs in the composite nanofibers and the passivation effect of PVP, resulting in enhancing its UV emission. In particular, a striking phenomenon was observed in the TEM and PL spectra of the samples, that is, with increasing the electrospinning voltages, the ZnO QDs in the composite nanofibers got more dispersible, and the composite nanofibers showed larger IUV/IVIS. This method could control the PL of PVP/ZnO QDs composite nanofibers, and might also be applied to other composites materials. These kinds of 1D nanomaterials were expected to have potential applications in nano-optoelectronic devices due to their large length-to-diameter ratio and intense ultraviolet emission. Acknowledgments The present work is supported financially by the National Natural Science Foundation of China (No. 50572014, 50972027), and the Program for New Century Excellent Talents in University (NCET-05-0322).
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Enhanced ultraviolet emission from highly dispersed ZnO quantum dots embedded in poly(vinyl pyrrolidone) electrospun nanofibers Zhenyi Zhang, Changlu Shao *, Fei Gao, Xinghua Li, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, Key Laboratory of UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 11 February 2010 Accepted 24 March 2010 Available online 27 March 2010 Keywords: Electrospinning Poly(vinyl pyrrolidone) ZnO quantum dots Photoluminescence
a b s t r a c t Highly dispersed ZnO quantum dots (QDs) in poly(vinyl pyrrolidone) (PVP) nanofibers have been successfully prepared by electrospinning technique. The structure and optical properties were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), resonant Raman spectra, Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric and differential thermal analysis (TG–DTA), ultraviolet (UV)–vis absorption spectra, and photoluminescence (PL) spectra. In the PVP/ZnO QDs composite nanofibers, PVP molecules could effectively prevent the aggregation of ZnO QDs and passivate the surface defects of ZnO QDs. Thus, by comparing ZnO QDs, the composite nanofibers exhibited a blue-shifted band gap and enhanced ultraviolet (UV) emission. Furthermore, the composite nanofibers prepared at higher voltage showed more intense UV emission than which obtained at lower voltage, suggesting that the UV emission intensity of the composite nanofibers could be controlled by adjusting the electrospinning voltage. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction As an important semiconducting oxide with a direct wide band gap (3.37 eV) and a large exciton binding energy (about 60 meV) at room temperature, zinc oxide (ZnO) has attracted much interest due to its novel physical properties and excellent chemical nature such as near-ultraviolet emission, conductivity, piezoelectricity, photocatalysis, sensitivity to gas, and so forth [1–4]. It is well known that low dimensional nanostructural materials provide a fundamental importance in bridging the gap between bulk material and molecular species. Moreover, the low dimensional semiconductor nanocrystals may have superior optical properties to bulk crystals because of quantum confinement effects. As a result, low dimensional ZnO nanocrystals, such as nanorods [5], nanowires [6], and nanorings [7], have been reported comprehensively. Comparing to bulk ZnO materials, however, nanostructural ZnO usually exhibits stronger visible luminescence related to surface defects [8]. It is great challenge to enhance the ultraviolet (UV) emission of nanostructural ZnO materials. Thus, in recent years, the optical properties of ZnO nanocrystals embedded in organic polymers, such as poly(vinyl alcohol) (PVA), poly(methyl methacrylate) (PMMA) and poly(vinyl pyrrolidone) (PVP) [9–11], and inorganic materials, such as CaF2, MgO, SiO2 and BaF2 [12–15], have been widely studied. Especially, polymer/ZnO nanocrystals
* Corresponding author. E-mail address: [email protected] (C. Shao). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.03.052
composites with one dimensional (1D) nanostructure have been of immense interest because of their large length-to-diameter ratio, special optical and conductivity properties, and so forth, which might be very useful to fabrication of nanoscale electronic and optoelectronic devices. Notably, electrospinning, a remarkably simple and versatile technique, has been exploited for nearly one century to process polymers and related materials into 1D structural fibers for a variety of applications [16]. By using this facile method, many kinds of composite nanofibers with optoelectronic properties have been prepared, such as poly(vinyl pyrrolidone) (PVP)/poly(p-phenylene vinylene) (PPV) [17], poly[2,5-(20 -ethylhexyloxy)]-1,4-phenylenevinylene(BEHPPV)/1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)-C61 (PCBM) [18], polymer/rare-earth complex [19–21], poly(ethylene oxide) (PEO)/teraamino-phthalocyanine copper (II) [(NH2)4PcCu] [22], PVP/Ln3+ doped NaYF4 [23], PVP/PbS [24], PVP/CdS [25], and so forth. Furthermore, our group has studied the photoluminescence (PL) properties of PVA/ZnO quantum dots (QDs), and PEO/ZnO QDs composite nanofibers [26,27]. The investigation indicated that ZnO QDs capped with different polymers could exhibit different PL properties due to the interactions between polymers and ZnO QDs. Thus, it is expect that highly dispersed ZnO QDs in PVP electrospun nanofibers maybe possess a novel PL properties. In the past, Guo et al. found a markedly enhanced near-band-edge UV photoluminescence and significantly reduced defect-related green emission from highly monodisperse PVP-capped ZnO nanoparticles [28]. And, Yang et al. reported an enhanced UV emission from PVP surface modified ZnO
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QDs [29]. However, to the best of our knowledge, there was no report on the PL properties of highly dispersed ZnO quantum dots in PVP electrospun nanofibers. In our present work, we reported a successfully attempt to the fabrication of PVP/ZnO QDs composite nanofibers by electrospinning method. By comparing the ZnO QDs, the PVP/ZnO composite nanofibers exhibited enhanced UV emission, because PVP molecules effectively passivated the surface defects of ZnO QDs. Notably, the composite nanofibers showed more intensive UV emission at higher electrospinning voltages. 2. Materials and methods In our experiments, the preparing process consisted of two steps. At first, for the preparation of the precursor, 1 g of poly(vinyl pyrrolidone) (PVP) powder (Mn = 900,000) was dissolved in 10 ml of ethanol. And, then the ZnO QDs obtained by using the method of literature [30] was mixed with the above PVP and ethanol solution, for which the ZnO QDs concentration was 0.1 M. Subsequently, the above PVP/ZnO composite precursor solution was drawn into a hypodermic syringe for electrospinning under different voltage (10, 12, 14, and 16 kV). The obtained PVP/ZnO QDs composite nanofibers were collected at a distance about 12 cm to the syringe tip for the following characterizations. Scanning electron microscopy (SEM; XL-30 ESEM FEG, Micro FEI Philips) and transmission electron microscopy (TEM; high resolution TEM [HRTEM], JEM-3010) were used to characterize the morphologies of the products. X-ray diffraction (XRD) measurement was carried out using a D/max 2500 XRD spectrometer (Rigaku) with Cu Ka line of 0.1541 nm. Fourier transform infrared (FT-IR) spectra were obtained on Magna 560 FT-IR spectrometer with a resolution of 1 cm1. Thermal gravimetric and differential thermal analysis (TG–DTA) analysis was carried out on a NETZSCH STA 449C thermoanalyzer in N2 atmosphere. The UV–Vis absorption spectra were measured at room temperature with a Lambda 900 UV–vis spectrophotometer (Perkin–Elmer). Photoluminescence (PL) and resonant Raman spectra were collected with a Jobin–Yvon HR800 micro-Raman spectrometer using the 325 nm line of a He– Cd laser as the excitation source. 3. Results and discussion Fig. 1a and b showed a typical SEM images of the PVP/ZnO QDs composite nanofibers prepared at the electrospinning voltage of 10 kV. From Fig. 1a, it could be observed that the composite nanofibers aligned in random orientation because of the bending instability associated with the spinning jet. Fig. 1b displayed the corresponding SEM image with higher magnification. It was showed that the diameters of above composite nanofibers ranged from 100 nm to 250 nm. The morphologies of the composite nanofibers obtained at the electrospinning voltages of 12, 14, 16 kV (not shown) were similar with that attained at the electrospinning voltage of 10 kV. Afterward, the PVP/ZnO QDs composite nanofibers prepared at different voltages (from 10 to 16 kV) was examined by HRTEM in Fig. 1c–f, which further confirmed that ZnO QDs have been successfully embedded in the PVP nanofibers. Besides, with the increase of the electrospinning voltages, more ZnO QDs in the composite nanofibers were obvious more dispersible. It was implied that the electrospinning voltage might play a crucial role to direct the dispersion of ZnO QDs in the composite nanofibers during the electrospinning process. The HRTEM image of a single ZnO QDs in the composite nanofibers in Fig. 1g given evidence of that ZnO QDs were spherical with the diameter about 4 nm. From the XRD curve of the PVP/ZnO QDs composite nanofibers in Fig. 1h, a broad peak around 22° appeared, corresponding to PVP semicrystalline in the composite nanofibers. But, the signals of ZnO QDs
were not clearly observed, which might be owing to its small size, low content and high dispersion in PVP nanofibers. The FT-IR spectra of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers obtained at the electrospinning voltages of 10 kV were shown in Fig. 2. From the Fig. 2A, it was observed that the PVP nanofibers exhibited strong peaks at around 1466 and 1285 cm1, both attributed to the C@C and CAN stretching vibration, respectively. Another characteristic peak of PVP was the very strong peak at 1661 cm1, which was attributed to C@O stretching vibration. However, in the FT-IR spectrum of PVP/ZnO QDs composite nanofibers, besides the characteristic vibration bands of PVP, a new intense broadband between 400 and 750 cm1 assigned to the ZnAO vibration of ZnO was appeared, indicating that the composite nanofibers were composed of PVP and ZnO QDs [31]. Notably, by comparing the FT-IR spectrum of the pure PVP nanofibers, the strong peak of the carbonyl group stretching was shifted to 1650 cm1 in the composite nanofibers (Fig. 2B). It was reported that the type of interaction between the inorganic nanoparticles and the carbonyl group in PVP might cause a shift in FT-IR frequency because the metal atoms accepted an electron pair of the carbonyl oxygen [24,25,32]. In our present work, there might exist the oxygen vacancy defects on the surface of ZnO QDs, resulting in the weak UV emission of as-prepared ZnO QDs. However, the composite nanofibers exhibited an enhanced UV emission (as shown in Fig. 6). Those meant that the electron pair of the carbonyl oxygen of PVP molecule could make up for the oxygen vacancy of ZnO QDs and thus passivated the surface defects of ZnO QDs. Furthermore, Fig. 3 showed the measured resonant Raman scattering spectra of the PVP/ZnO QDs composite nanofibers prepared at the electrospinning voltages of 10 kV. In our experiment, the resonant Raman scattering at room temperature was performed to investigate the vibrational properties of the composite nanofibers. The energy of the He–Cd laser line (325 nm) was 3.82 eV, which was higher than the band gap of ZnO (3.37 eV). In Fig. 3, a significant Raman band centered at 574, 1152, 1733 cm1 was observed, which was attributed to the 1–3 Raman longitudinal optical (LO) phonon mode of nanosized ZnO [33]. Moreover, two peaks at around 1360 and 1590 cm1 indicated by a rectangle were clearly observed, which originated from Raman scattering of disordered carbon [34]. To further confirm the interactions between PVP and ZnO QDS, the thermal behavior of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers were investigated in Fig. 4. As observed in the images of TG curve in Fig. 4, the pure PVP nanofibers appeared two major weight loss steps from 380 up to about 450 °C and from 450 up to 600 °C due to the decomposition of organic PVP. And, there was no residue above 600 °C, indicating that the organic PVP was decomposed absolutely. By comparing the TG curves of the as-spun PVP and PVP/ZnO QDs composite nanofibers, it could be deduced that the obvious weight losses appearing at 50–150, 380–450, and 450–550 °C in the TG curve of PVP/ZnO QDs composite nanofibers were ascribed to the evaporation of the absorbed water and the thermal decomposition of the organics. And, the clear plateau formed between 550 and 600 °C on the TG curve indirect indicated that the composite nanofibers might consist of PVP and ZnO QDs. Meanwhile, by comparing the DTA curve of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers in Fig. 4, it was found that three exothermic peaks were observed at around 410, 430, and 520 °C in the composite nanofibers, indicating that there might exist the weak interactions between the PVP and ZnO QDs. All above results, including the FT-IR, Raman and TG–DTA, demonstrated the existence of interactions between a PVP molecule and ZnO QDs via the metal atoms accepted an electron pair of the carbonyl oxygen. These interactions could be responsible for especial luminescent properties of the composite nanofibers. To give a study on the optical properties of the samples, the absorption and PL spectra for them were measured. Fig. 5 showed
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Fig. 1. SEM images of PVP/ZnO QDs composite nanofibers prepared at the electrospinning voltage of 10 kV: (a) low magnification, (b) high magnification; TEM images of the composite nanofibers obtained at the electrospinning voltages of: (c) 10, (d) 12, (e) 14, (f) 16 kV; (g) HRTEM image of a single ZnO quantum dot in the composite nanofibers; and (h) the XRD pattern of the composite nanofibers obtained at the electrospinning voltages of 10 kV.
the absorption spectra of ZnO QDs in ethanol, pure PVP nanofibers and PVP/ZnO QDs composite nanofbiers (the electrospinning voltage was 10 kV). From Fig. 5, it was well established that the size of ZnO QDs could be estimated from the excitonic absorption peak based on the effective mass approximation [15,35].
E ffi Ebulk þ g
2 p2 1 h 1 þ 0:248ERyd 2 2er me mh
ð1Þ
where E and Ebulk was the bandgap of quantum dots and bulk, r was g the particle radius, me was the effective mass of the electrons, mh was the effective mass of the holes, h was Planck constant divided by 2p, and e was the charge on the electron. In our experiment, tak¼ 3:37 eV, the electron and the hole ing, E = 3.41 eV and Ebulk g effective mass were me ¼ 0:24m0 and mh ¼ 0:45m0 , m0 was the free electron mass, and using the bulk exciton binding energy ERyd ¼ 60 meV. The ZnO QDs diameters in ethanol were calculated to be about 3.863 nm. These were consistent with the diameter of
ZnO QDs in composite nanofibers from above TEM analysis, indicating ZnO QDs was not further grow in the our experiment process. Furthermore, the characteristic peaks of exciton absorption of ZnO were observed for both of ZnO QDs and composite nanofibers. It was also conformed that ZnO QDs were successfully embedded in the PVP nanofibers. Furthermore, as the Bohr radius of bulk ZnO was 2.34 nm and the diameter of ZnO QDs in this work was about 4 nm from above results, the ratio of the QD diameter and the Bohr radius was nearly 2, which meant that a strong confinement occurred [36]. The exciton absorption peaks of ZnO QDs sol and PVP/ZnO QDs composite nanofibers (about 340 nm) were substantially blueshifted relative to that of the bulk ZnO (about 372 nm) due to the strong quantum confinement effects. However, the blueshifted for the ZnO QDs sol and PVP/ZnO QDs composite nanofibers might not only come from a blueshift from the confinement-induced shift of the electronic levels, but also come from a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius [37].
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Fig. 2. FT-IR spectra of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers obtained at the electrospinning voltages of 10 kV.
Fig. 3. The resonant Raman spectra of the PVP/ZnO QDs composite nanofibers obtained at the electrospinning voltages of 10 kV.
With the interactions between PVP molecules and ZnO QDs, the existence of PVP molecules might make up for the oxygen vacancy and passivated the surface defects of ZnO QDs, thereafter enhance the UV photoluminescence emission of ZnO QDs. Fig. 6A showed the PL spectra of PVP/ZnO QDs composites nanofibers obtained at the electrospinning voltage from 10 to 16 kV. The inset was the PL spectra of ZnO QDs. As observed in Fig. 6A, the narrow UV emission from PVP/ZnO QDs composite nanofibers at around 364.4 nm (3.40 eV) was consisted with the bandedge emission typically originated from the exciton combination of ZnO [33]. The broad of visible emission for above composite nanofibers at about 550 nm (2.25 eV) was related to the transition between the electron close to the conduction band and the hole at vacancy associated with the surface defects [38]. Besides, the position of the UV emission band did not shift in all composite nanofibers. However, the UV emission of all composite nanofibers showed two characteristics compared to that of ZnO QDs gel, namely, blueshifted peak energy and enhanced ultraviolet emission, which demonstrated that PVP molecules could effectively prevent the aggregations of ZnO QDs and passivate the defects on the surface of ZnO QDs. Thus, the inter-
Fig. 4. TG–DTA curves of thermal decomposition of the pure PVP nanofibers and PVP/ZnO QDs composite nanofibers obtained at the electrospinning voltage of 10 kV.
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Fig. 5. Absorption spectra of ZnO QDs in ethanol (ZnO QDs), pure PVP nanofibers, and the ZnO QDs in the PVP/ZnO QDs composite nanofibers. The electrospinning voltage was 10 kV.
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actions among the ZnO QDs could be decreased, however, which could not be neglected in ZnO QD gel. Furthermore, an interesting phenomenon observed in Fig. 6B, it was showed that the ratios of ultraviolet emission to visible emission (IUV/IVIS) of PVP/ZnO QDs composite nanofibers were significantly increased by increasing the electrospinning voltage, which meant that the passivation effect of PVP was strengthened by increasing electrospinning voltage. Based on above results, a possible mechanism was presented, as shown in Scheme 1. In the precursor solution, there might exist large amount of ZnO QDs clusters because of the high Gibbs’ surface free energy of the small sized ZnO nanocrystals [27]. As a result, the PVP molecules could only passivated the surface defects of the ZnO QDs cluster, but could hardly passivated the surface defects of every ZnO QDs. As we known, electrospinning was a drawing process based on two competitive forces: the electrostatic force and the surface tension [39]. When the applied electric voltage overcomed the surface tension of the precursor solution, one or several jets of the solution were ejected from the tip of the Taylor cone and fly towards the grounded plate. This electrified jet then undergone a stretching and whipping process, leading to the formation of a long and thin thread. In our experiment, during the electrospinning
Fig. 6. (A) PL spectra of the PVP/ZnO QDs composites nanofibers prepared at the electrospinning voltages of 10, 12, 14 and 16 kV at room temperature, respectively. Inset was the PL spectrum of ZnO QDs. (B) The integrate area ratios of ultraviolet emission to visible emission (IUV/IVIS) of above composite nanofibers versus the function of the electrospinning voltages.
Scheme 1. The scheme of the distribution of ZnO QDs in the PVP/ZnO QDs composites nanofibers under the electrospinning process.
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process, by applying the higher voltages to the precursor solution, the curved PVP chains could be spread, while the ZnO QDs clusters could be dispersed highly in PVP nanofibers due to the extraction of large electrostatic force. Thus, the higher electrospinning voltage might induce the polarization and orientation of ZnO QDs. ZnO QDs were embedded in PVP matrix fibers with high dispersion, which was showed in the TEM of Scheme 1, suggesting that the PVP molecules might nearly passivate every ZnO QDs in the composite nanofibers. However, by using the lower voltages, there might still exist a lot of ZnO QDs clusters in the composite nanofibers due to the extraction of weak electrostatic force, which meant that only the surface of ZnO QDs clusters could be passivated by PVP molecules. Furthermore, it was noticed that, being prepared before mixing with PVP and protected by PVP, ZnO QDs did not grow in the sequent process due to the position of the UV emission band of all the samples remained unchanged in the PL spectra. Those results showed that, with the increase of the voltages, more ZnO QDs got more dispersive in the composite nanofibers, while the passivation was more effective, resulting in larger IUV/IVIS ratio of the composite nanofibers. Namely, by adjusting the electrospinning voltage, the intensity of UV emission from the composite nanofibers could be controlled. 4. Conclusion In summary, the nanofibers of PVP/ZnO QDs composite were successfully prepared by electrospinning. By characterizing the structural and optical properties of above composite nanofibers, we found that PVP molecules effectively passivated the oxygen related defects on the surface of ZnO, which led to blueshifted peak energy and enhanced UV emission to the ZnO QDs. Moreover, the electrospinning voltage was found to play an important role on enhancing the dispersion of ZnO QDs in the composite nanofibers and the passivation effect of PVP, resulting in enhancing its UV emission. In particular, a striking phenomenon was observed in the TEM and PL spectra of the samples, that is, with increasing the electrospinning voltages, the ZnO QDs in the composite nanofibers got more dispersible, and the composite nanofibers showed larger IUV/IVIS. This method could control the PL of PVP/ZnO QDs composite nanofibers, and might also be applied to other composites materials. These kinds of 1D nanomaterials were expected to have potential applications in nano-optoelectronic devices due to their large length-to-diameter ratio and intense ultraviolet emission. Acknowledgments The present work is supported financially by the National Natural Science Foundation of China (No. 50572014, 50972027), and the Program for New Century Excellent Talents in University (NCET-05-0322).
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