Photoluminescence properties of the rare-earth ions in the TiO2 host nanofibers prepared via electrospinning

Photoluminescence properties of the rare-earth ions in the TiO2 host nanofibers prepared via electrospinning

Materials Research Bulletin 44 (2009) 408–414 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.elsevier.c...

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Materials Research Bulletin 44 (2009) 408–414

Contents lists available at ScienceDirect

Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Photoluminescence properties of the rare-earth ions in the TiO2 host nanofibers prepared via electrospinning Haiying Wang, Yu Wang, Yang Yang, Xiang Li, Ce Wang * Alan G. MacDiarmid Institute, Jilin University, Changchun 130012, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 15 November 2007 Received in revised form 25 March 2008 Accepted 1 May 2008 Available online 4 May 2008

Luminescent rare-earth (RE) ions doped TiO2 nanofibers have been prepared by electrospinning of a mixture solution of rare-earth acetylacetone (RE(C5H7O2)3)/titanium tetraisopropoxide (Ti (OiPr)4)/ poly(vinyl pyrrolidone) (PVP) (RE = Eu, Er, Ce, Pr), followed by calcination at high temperature. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) analyses demonstrated the morphology and the structure of the rare-earth doped TiO2 nanofibers. Exciting the nanofibers results in an energy transfer from surface states of TiO2 to that of the rare-earth ions and the photoluminescence is observed from the crystal field states of the rare-earth ions. ß 2008 Elsevier Ltd. All rights reserved.

Keywords: A. Composites A. Nanostructures D. Luminescent

1. Introduction The design and preparation of one-dimensional nanostructural materials, such as nanorods, nanowires, or nanofibers have attracted great attention because of their potential unique properties and applications [1–3]. The generation of these materials using laser ablation [4], vapor-phase approach [5], template [6], and other methods [7–9] has been introduced in some works. Additionally, electrospinning technique as a simple and low-cost method for making nanofibers has been utilized in the preparation of many one-dimensional nanostructural materials as well [10–12]. In recent years, there has been a growing interest in the fabrication of semiconductor oxides nanofibers such as WO3, ZnO, and TiO2 nanofibers by this technique [13–15]. Rare-earth ions have been chosen for study as dopants because they are extremely useful in optical applications due to their sharp, near-monochromatic emission lines. The research on the luminescent properties of the rare-earth elements hosted in several crystalline matrices, such as fluoride glasses [16–18], metal oxides [19,20], phosphors [21], metal organic complexes [22,23], and a variety of semiconductor materials [24–26], is strongly motivated due to their technological applications in optoelectronics devices and flat panel displays [27,28]. As a host material, TiO2 is considered as a promising semiconductor with outstanding optical and thermal properties [29–31]. Synthesis of nanoparticulate rare-

* Corresponding author. Tel.: +86 431 85168292; fax: +86 431 85168292. E-mail address: [email protected] (C. Wang). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.05.001

earth doped TiO2 films and nanocrystals can be obtained by a sol– gel method [32–35]. However, no work has been reported to prepare the TiO2 nanofibers doped with rare-earth ions. Here, we report the rare-earth doped TiO2 nanofibers obtained by electrospinning. The high specific surface of the nanofibers results from small diameters will be beneficial in variety of applications. These nanofibers exhibit a good luminescent property, which is probably applicable for making of optoelectronic nanodevices. 2. Experimental procedures 2.1. Materials Europium acetylacetone (Eu (C5H7O2)3), erbium acetylacetone (Er (C5H7O2)3), cerium acetylacetone (Ce (C5H7O2)3), and praseodymium acetylacetone (Pr (C5H7O2)3) were prepared by reacting europium oxide (Eu2O3) (Aldrich), erbium oxide (Er2O3) (Aldrich), cerium oxide (Ce2O3) (Aldrich), praseodymium oxide (Pr2O3) (Aldrich) with acetylacetone (C5H8O2) (Tianjin Guangfu Fine Chemical Research Institute, China). Poly(vinyl pyrrolidone) (PVP, MW = 1,300,000, Aldrich) and titanium tetraisopropoxide (Ti (OiPr)4) (Tianjin Guangfu Fine Chemical Research Institute, China) were used without further purification. Ethanol (G.R.) was used as solvent. 2.2. Instruments The scanning electron microscopy (SEM) images were recorded on a SHIMADZU SSX-550 microscope. The transmission electron microscopy (TEM) was measured through a JEM-2000EX electron

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microscopy (Japan) with an accelerating voltage of 200 kV. X-ray diffraction (XRD) patterns were obtained with a Siemens D5005 diffractometer by Cu Ka radiation. FT-IR spectra of KBr powderpressed pellets were recorded on a BRUKER VECTOR 22 spectrometer. Elemental analysis was done using SEM (Shimadzu, SSX550) coupled to an X-ray detector for EDX analysis. UV–vis absorption spectra were recorded on a UV-3101 PC Spectrometer

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(SHIMADZU). Fluorescence spectra were investigated on a fluorometer of SPEXF212 model (SPEX). 2.3. Preparation of rare-earth doped TiO2 nanofibers In a typical procedure, an alcohol solution of PVP (13 wt.%) was prepared by dissolving PVP in alcohol under magnetic stirring for

Fig. 1. SEM images of TiO2:Eu nanofibers, TiO2:Er nanofibers, TiO2:Ce nanofibers, and TiO2:Pr nanofibers before calcinations (a, b, c, d) and after calcinations (e, f, g, h).

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1 h. 3.45 g solution was dropped slowly into a mixture solution of 1.5 g Ti (OiPr)4, 2 g acetic acid, and 1 g alcohol under vigorous stirring. Then, RE (C5H7O2)3 (RE = Eu, Er, Ce, Pr) acetic acid solutions were doped into the mixed solutions and stirred vigorously for 6 h to obtain the solutions for electrospinning, in which as the weight of acetic acid was kept at 1 g, Eu (C5H7O2)3, Er (C5H7O2)3, Ce (C5H7O2)3, and Pr (C5H7O2)3 were 0.099 g, 0.102 g, 0.096 g, and 0.097 g, respectively. The solutions were held separately in a spinning nozzle with a tip diameter of 1 mm. A copper pin connected to the anode of a high-voltage generator was placed in the solutions. A voltage of 15 kV was applied to the solutions and dense nanofiber webs were collected on an aluminum foil, which was at a distance of 12 cm from the tip of the nozzle. Finally, the film attached on the aluminum foil was separated and calcined at 500 8C for 6 h to prepare the rare-earth doped TiO2 nanofibers. 3. Results and discussion Fig. 1 shows the SEM images of Eu(C5H7O2)3/Ti(OiPr)4/PVP (Fig. 1a), Er (C5H7O2)3/Ti(OiPr)4/PVP (1b), Ce (C5H7O2)3/Ti(OiPr)4/ PVP (Fig. 1c), and Pr (C5H7O2)3/Ti (OiPr)4/PVP (Fig. 1d) nanofibers, respectively. It can be seen that the nanofibers are smooth and uniform. They are longer than several millimeters, with a diameter of approximate 200 nm. After calcinations at 500 8C (Fig. 1e–h), the

nanofibers remain intact but their average diameter was reduced to about 150 nm due to the removal of PVP from the nanofibers, resulting a rough surface occurring and the surface area increasing obviously. TEM images of the nanofibers after calcinations are shown in Fig. 2. It is seen that the fibers are composed of TiO2 (Fig. 2a) and TiO2:RE (RE = Eu, Er, Ce, Pr) nanocrystals (Fig. 2b–e), respectively. For the pure TiO2 nanofibers, the crystals are about 30 nm in diameter, whereas for the nanofibers containing the rare-earth ions, the size of crystal is reduced to about 10 nm. It can be suggested that the size reduction are resulted from the RE ions that suppressed the growth of TiO2 crystals. The diffraction rings in ED pattern (inset) for the pure TiO2 fibers could be indexed to (1 0 1) (2 0 0) (1 0 5) of anatase phase with (1 1 0) of rutile phase (Fig. 2a). However, the ED patterns (Fig. 2b–e) show anatase phase after doping RE ions. It indicates that the surrounding RE ions may inhibit a transition from the anatase to rutile TiO2 through a formation of Ti–O–RE bond [36]. The result is in good agreement with the calculation values from the XRD patterns, which are shown in Fig. 3. The XRD patterns of the undoped TiO2 calcined at 500 8C show a mixture of anatase and rutile (Fig. 3a), while Fig. 3b– e shows that the RE doped TiO2 calcinied at 500 8C are pure anatase phase. For the RE doped TiO2 calcinied at 700 8C (Fig. 3f–i), the major crystal phase is anatase accompanied by a rutile phase. Compared to the undoped TiO2, the phase transformation

Fig. 2. TEM and ED patterns of TiO2 nanofibers (a), TiO2:Eu nanofibers (b), TiO2:Er nanofibers (c), TiO2:Ce nanofibers (d), and TiO2:Pr nanofibers (e).

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Fig. 3. XRD patterns of TiO2 nanofibers (a), TiO2:Eu nanofibers (b), TiO2:Er nanofibers (c), TiO2:Ce nanofibers (d), TiO2:Pr nanofibers (e), TiO2:Eu nanofibers (calcined at 500 8C) (f), TiO2:Er nanofibers (g), TiO2:Ce nanofibers (h), and TiO2:Pr nanofibers (calcined at 700 8C) (i).

temperature from anatase to rutile for the RE doped TiO2 is obviously increased. The average crystal size of the RE doped samples is decreased to about 10 nm, which can be determined from the peak width of (1 0 1) using scherrer’s equation, D = Kl/ b cos u, where K = 0.9, l is the X-ray wavelength at 0.15405 nm. It is seen that due to the mismatch of the ionic sizes of rare-earth ion and Ti4+, rare-earth ions can hardly enter into the TiO2 lattice and probably substitute Ti4+ on the crystallite surface of interstitials of TiO2 nanocrystals to form Ti–O–RE bonds, which would prevent from the nucleation that is necessary for the transformation from anatase to rutile, resulting in a remarkable increase in phase transformation temperature. Furthermore, the formation of Ti–O– RE bonds would retard the interaction of the crystals as well as the transfer and rearrangement of Ti and O atoms in the crystals, leading to a decrease in diameter of the RE doped TiO2 nanocrystals. No rare-earth oxide phase is found in the XRD pattern, indicating that the doping degree is lower than a solid solution limit [37]. The formation of TiO2:RE (RE = Eu, Er, Ce, Pr) nanofibers is evidenced by FT-IR spectra. In the case of the composite fibers before calcinations (Fig. 4a–d), the band at about 1650 cm1 results from the vibration of C O groups of PVP. However, the characteristic peaks of PVP disappear after heating 500 8C for 6 h, which indicates that the polymer was degraded (Fig. 4e–h). Instead, there are new peaks below 450 cm1 assigning to TiO2, indicating the formation of pure inorganic fibers. Some residual

OH groups are still present after calcinations, which are attributed to the H2O absorbed on the surface of KBr pellets. The TGA curve of the Ce (C5H7O2)3/Ti(OiPr)4/PVP (Fig. 5) shows three steps and a total weight loss of ca. 78%. The first step of ca. 7% from 50 8C to 230 8C could be attributed to the desorption of the water, and the second significant weight loss of ca. 53%

Fig. 4. FT-IR spectra of TiO2:Eu nanofibers, TiO2:Er nanofibers, TiO2:Ce nanofibers, and TiO2:Pr nanofibers before calcinations (a, b, c, d) and after calcinations (e, f, g, h).

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Fig. 5. TG curve of Ce (C5H7O2)3/Ti(OiPr)4/PVP nanofibers.

between 230 8C and 328 8C was attributed to the decomposition of PVP. From 328 8C to 491 8C, there was a weight loss of ca. 18%, which was assigned to the removal of water molecules from the hydroxyls on titanium atoms and a small amount of residual organic material. These results show that the organic template and PVP were removable from the fibers upon calcination in air at 491 8C. Above 490 8C, there was no further weight loss, which indicated that the organic component had been entirely decomposed. The existence of RE elements was indicated by EDX spectra of TiO2:RE fibers shown in Fig. 6b–e. The presence of O, Ti, Eu, Er, Ce, and Pr corresponds to TiO2:Eu, TiO2:Er, TiO2:Ce, TiO2:Pr, respectively, compared to the O, Ti elements in pure TiO2 (Fig. 4a) and no other impurity peaks occurred. As shown in Fig. 7, the UV–vis (diffuses reflectance) spectra of all TiO2:RE nanofibers were similar. The peaks at about 322– 445 nm in absorption spectra are corresponding to the titania semiconductor band gap. A blue shift occurs in the absorption edge, compared to that of pure TiO2 nanofibers due to a decrease in

Fig. 6. EDX spectra of TiO2 nanofibers (a), TiO2:Eu nanofibers (b), TiO2:Er nanofibers (c), TiO2:Ce nanofibers (d), and TiO2:Pr nanofibers (e).

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Fig. 7. UV–vis absorption spectra of TiO2 nanofibers (a), TiO2:Eu nanofibers (b), TiO2:Er nanofibers (c), TiO2:Ce nanofibers (d), and TiO2:Pr nanofibers (e).

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in strong emissions. Moreover, when the fibers are excited below the titania band gap, no emission from rare-earth ions are present. There have been literature precedents that the surface states of TiO2 crystals have been reported and assigned to the coordinatively unsaturated titanium ions on the crystalline surface [38]. Furthermore, as mentioned previously, the radii of the rare-earth ions are too large to allow them to replace titania in an anatase crystal, and the unresolved PL spectra are consistent with rareearths in a glass-like environment. These facts indicate that the rare-earth ions were located at the edge of the TiO2 nanocrystallites, or in close proximity to the surface states of TiO2 crystals. So, an energy transfer process is that the band gap of titania absorbed UV light, then relaxed it to the surface states, followed by the energy transfer to the crystal field states of the rare-earth ions. Because of the small size and the large number of nanocrystallites in these materials, there are a great number of the surface states available to transfer the energy to the crystal field states of the rare-earth ions. 4. Conclusion

crystallite size. None of the absorption peaks for any of the rareearth ions can be observed in the spectra due to the weak absorption cross-section of the parity forbidden nature of the f–f transitions in all of the rare earth in combination with the small diameter of the nanofibers. Fig. 8 shows the excitation and the emission spectra of the TiO2:RE (RE = Eu, Er, Ce, Pr) nanofibers. The most intense transitions are 5D0 ! 7F4 of TiO2:Eu (Fig. 8a), 4F9/2 ! 4I15/2 of TiO2:Er (Fig. 8b), 5 D1 ! 2F7/2 of TiO2:Ce (Fig. 8c), and 1S0 ! 1I6 of TiO2:Pr (Fig. 8d) nanofibers. The excitation spectra of these intense emissions clearly reflect the band gap of the titania as the main contribution to the excitation. Additionally, excitation of these fibers above the measured titania nanocrystallites band gap result

We have successfully prepared the rare-earth ions doped TiO2 nanofibers by an electrospinning technique. The nanofibers were composed of TiO2:RE nanocrystals with an average diameter of 10 nm, which were smaller than that of the TiO2 nanocrystals in the undoped TiO2 nanofibers. The energy transfer from the surface traps of the TiO2 nanocrystals to the crystal field states of the rareearth ion induced remarkable luminescent properties, which is probably applicable for the making of optoelectronic nanodevices. Furthermore, a high specific surface area of the nanofibers resulting from its rough surface and small diameters will be useful in a variety of applications. This method is general and can be expanded to the large-scale synthesis of other rare-earth doped semiconductor nanofibers.

Fig. 8. Excitation and photoluminescence spectra of TiO2:Eu nanofibers (a), TiO2:Er nanofibers (b), TiO2:Ce nanofibers (c), and TiO2:Pr nanofibers (d).

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