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Facile synthesis and luminescence properties of TiO2:Eu3+ nanobelts
Optics & Laser Technology 49 (2013) 33–37
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Facile synthesis and luminescence properties of TiO2:Eu3 þ nanobelts Hongbo Li a, Keyan Zheng a, Ye Sheng a, Yanhua Song a, Hongguang Zhang a, Jing Huang a, Qisheng Huo b, Haifeng Zou a,n a b
College of Chemistry, Jilin University, Changchun 130012, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
abstract
Article history: Received 29 August 2012 Received in revised form 19 November 2012 Accepted 7 December 2012 Available online 7 January 2013
Uniform TiO2:Eu3 þ nanobelts have been successfully prepared by a simple hydrothermal method without any surfactant, catalyst or template. The as-synthesized products were characterized by X-ray diffraction, transmission electron microscopy and Fourier Transform Infrared Spectrophotometry. X-ray diffraction results demonstrate that all the diffraction peaks of the samples annealed at 500 1C can be well indexed to the pure anatase-phase TiO2. Under ultraviolet excitation, TiO2:Eu3 þ nanobelts exhibit red emission corresponding to the 5D0–7F2 transition of the Eu3 þ ions, and the photoluminescence intensity is the strongest when calcination temperature is 500 1C. It is expected that the TiO2:Eu3 þ nanobelts can be used as novel semiconductor luminescence materials. Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: TiO2:Eu3 þ nanobelts Calcination Luminescence properties
1. Introduction One-dimensional (1D) nano-materials, such as nanobelts, nanorods, and nanotubes, have become the focus of intensive research due to their physical and chemical properties and wide applications [1–4]. The properties of 1D nano-materials depend not only on their composition but also on their structure, morphology, phase, shape, size distribution, and spatial arrangement [5,6]. Nanobelts, which are a new morphology of 1D nanostructures with a rectangular cross section, could be an ideal system for fully understanding dimensionally confined transport phenomena and show promising applications in numerous areas, such as field-effect transistors, nanometer-sized ultrasensitive gas sensors, resonators and luminescent materials [6–8]. However, simple, rapid, inexpensive, environment friendly, and effective synthesis of such 1D nanostructure on a large scale still remains a challenge for researchers. Much research attention has been paid to the field of rareearth materials, since they have many potential applications based on their novel electronic, optical, and magnetic properties resulting from their 4f electrons [9–11]. As their host lattices are suitable for rare earths doping, rare earth doped TiO2 nanomaterials have been widely used in photocatalysis, luminescent materials, and other fields [12–14]. Among the lanthanide ions, Eu3 þ ion is a very important luminescent center and has been regarded as an attractive dopant for use as visible luminescent materials n
Corresponding author. Tel./fax: þ 86 431 85155275. E-mail address: [email protected] (H. Zou).
because of its strong red emissions. Eu3 þ -doped TiO2 luminescent nanocrystals were prepared by Ji-Guang Li, and bright red emissions were observed by exciting the TiO2 host with UV light or by directly exciting Eu3 þ at a wavelength beyond the absorption edge of TiO2 [15]. Eu3 þ -activated titania hollow spheres were synthesized using carbon spheres as hard templates and effective nonradiative energy transfer from the TiO2 hollow spheres host matrix to Eu3 þ ions was observed. This is due to changes of crystalline field in the environment around Eu3 þ ions occupying Ti4 þ sites [16]. Co-doped TiO2 nanostructures such as nanorods, nanowires, and nanotubes were synthesized through the solvothermal method by Kajari Das et al., and the Co was incorporated into the TiO2 lattice as Co2 þ and oxygen vacancies were created due to the substitution of the Ti4 þ ions by Co2 þ ions [17]. Ilaria Cacciotti et al. have synthesized rare earth-doped titania nanofibers by the electrospinning technique and the luminescence spectra of Eu- and Er-doped samples clearly testified the presence of lanthanide ions in the TiO2 host lattice [18]. However, there were few reports on the Gd3 þ ions and Eu3 þ ions co-doped TiO2 with a uniform belt-like shape. So it is significant to develop more facile, efficient, and low-cost techniques to fabricate large-scale and well-crystallized rare earth-doped TiO2 nanobelts. In this paper, uniform and well-dispersed belt-like TiO2 nanobelts doped with Eu3 þ have been successfully prepared through a simple hydrothermal approach and subsequent annealing treatment process. The anatase TiO2:Eu3 þ nanobelts, which inherit their parents’ morphology, were obtained after the calcination process, and the luminescence properties were investigated.
0030-3992/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.12.007
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H. Li et al. / Optics & Laser Technology 49 (2013) 33–37
2. Experimental section 2.1. Materials Commercial TiO2 powder, absolute ethanol, concentrated hydrochloric acid, nitric acid, sodium hydroxide, and Eu2O3 (99.9%) were purchased from Beijing Chemical Co. All chemicals were analytical grade reagents and used directly without further purification. Eu(NO3)3 aqueous solution was obtained by dissolving Eu2O3 (99.99%) in dilute HNO3 solution under heating with ceaseless agitation. 2.2. Preparation In the present synthesis process, 0.5 g TiO2 powder was mixed with 10 M NaOH aqueous solution and absolute ethanol (the volume ratio of NaOH aqueous solution to ethanol was 1:1). Different molar ratios Eu(NO3)3 (0.1 M) solution was added into the above mixture. The solution was transfered into a teflon container enclosed in a stainless steel autoclave, and aged at 180 1C for 24 h. After naturally cooling to room temperature, the precursors were separated by filtration, washed with 0.1 M dilute HCl and deionized water several times until the pH was adjusted to about 7.0, and then dried at 60 1C for 10 h in air. The final products were obtained through a heat treatment at desired temperatures (300–900 1C) for 4 h with a heating rate of 1 1C min 1.
Fig. 1. XRD patterns of TiO2:Eu3 þ nanobelts precursor (a) and the annealed products prepared at 300 1C (b), 500 1C (c), 700 1C (d), and 900 1C (e), along with the standard data for anatase-phase TiO2 (JCPDS card no. 21-1272) as a reference.
nanobelts samples are very sharp and strong at 500 1C (Fig. 1c), indicating that products with high crystallinity can be synthesized by this method. 3.2. Morphology
2.3. Characterization The structure of TiO2:Eu3 þ nanobelts was examined by standard X-ray diffraction (XRD; Rigaku D/max-B II (Cu Ka ray)). The morphologies of the samples were inspected by transmission electron microscopy (TEM; JEM-2000EX). Fourier Transform Infrared (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. Excitation and emission spectra were recorded using a Jobin Yvon FluoroMax-4 equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at room temperature.
3. Results and discussion 3.1. Structures Fig. 1 shows the XRD patterns of the as-prepared precursor sample (a) and those annealed from 300 1C to 900 1C (b–e), as well as the data of anatase-phase TiO2 (JCPDS no. 21-1272). All the diffraction peaks of the precursor can be readily indexed to H2Ti2O5 H2O [19], except that the pattern has one weak peak marked with ‘‘n’’. The peaks may be caused by a small amount of unreacted precursors (Fig. 1a). All samples heated above 300 1C show the X-ray diffraction of the anatase crystal phase (JCPDS 211272) at 2y ¼25.31; these peaks become sharper as the temperature is increased, and the precursor becomes anatase-phase TiO2 absolutely when the temperature reaches at 500 1C (Fig. 1c). Above 700 1C the transformation of anatase to rutile is detected (Fig. 1d) and after calcining the nanobelts at 900 1C, the anatase phase transforms to rutile phase absolutely (Fig. 1e). The introduction of different lanthanide ions has little effect on the phase structures of the products [20]. The XRD patterns of TiO2:Eu3 þ nanobelts annealed at 500 1C illustrate that all of the diffraction peaks can be indexed to the anatase-phase TiO2, indicating that the Eu3 þ ions have been effectively doped into the TiO2 host lattice. It can also be seen that the diffraction peaks of TiO2:Eu3 þ
The TEM images of the precursor prepared from hydrothermal reaction and the TiO2 nanobelts after annealing at different temperatures are shown in Fig. 2. Fig. 2a shows a panoramic TEM image of the precursor, which consists of belt-like nanostructures with a mean length of several micrometers. The highly magnified TEM image of the nanobelts shows that each nanobelt consists of many smaller nanobelts with width about 200–400 nm and length about several micrometers, with typical width-tothickness ratios of 5–10 (Fig. 2b). The belt-like morphology is maintained with calcination at 300 1C and 500 1C (Fig. 2c and d). The nanobelts obtained at 300 1C had no obvious changes in the length, but only reduction in width (Fig. 2c). As the annealing temperature was raised to 500 1C, the width of TiO2:Eu3 þ nanobelts changed greatly, being about 50 nm (Fig. 2d). The SAED pattern shows a set of broad diffuse rings instead of spots due to the random orientation of the crystallites, corresponding to diffraction from different planes of the nanocrystallites. The SAED patterns are consistent with an anatase phase structure of TiO2 with strong ring patterns due to (101), (004), (200), (211), (204) and (200) planes, in good agreement with XRD patterns and demonstrating its polycrystalline nature. However, after annealing at 700 1C, some of the nanobelts could not maintain their belt-like morphology, which are ready to crumble into a powder form (Fig. 2e). At 900 1C, the belt-like morphology appears to comprise aggregates of nanoparticles, 50–200 nm in diameter (Fig. 2f). 3.3. FT-IR spectra analysis Fig. 3 shows the FT-IR spectra of the TiO2:Eu3 þ nanobelts precursor (a) and the annealed products (b)–(e). The peaks located at 400–1000 cm 1 are characteristics of the formation of O–Ti–O network [21]. It is interesting to note that the peaks at 400–1000 cm 1 changed with the calcination temperature, and achieved the strongest stretching when the temperature reached 500 1C. The bands at 561, 503 cm 1 and 559, 501 cm 1 are attributed to Eu–O stretching modes [22]. The peaks at about
H. Li et al. / Optics & Laser Technology 49 (2013) 33–37
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Fig. 2. (a) Low- and (b) high-magnification TEM images of TiO2:Eu3 þ nanobelts precursor, and the final products annealed at 300 1C (c), 500 1C (d), 700 1C (e) and 900 1C (f).
between the hydroxyl groups and TiO2 nanobelts through hydrogen bond. However, the peaks become weaker with the rise of temperature. Increasing the treatment temperature results in a structural transformation from titanate to anatase-phase and rutile-phase TiO2, which is in accord with the results of XRD analysis (Fig. 1). 3.4. Luminescence properties
Fig. 3. FT-IR spectra of TiO2:Eu3 þ nanobelts precursor (a) and the final products annealed at 300 1C (b), 500 1C (c), 700 1C (d) and 900 1C (e).
3400 cm 1 correspond to stretching vibrations of hydroxyl groups; the presence of surface water is also confirmed by its bending mode at 1630 cm 1, indicating a strong interaction
Fig. 4 shows the excitation and emission spectra of the TiO2:Eu3 þ nanobelts prepared at 180 1C for 24 h and then annealed at 500 1C. The as-obtained sample exhibits a strong red emission under short ultraviolet irradiation, which can be confirmed by the emission spectrum of the TiO2:Eu3 þ sample excitation at 393 nm. The excitation spectrum (Fig. 4a) consists of a weak absorption band at 308 nm attributed to the Eu3 þ –O2 transitions originating from a charge transfer band (CTB) transition [23]. The products synthesized under this method show the characteristic emission of Eu3 þ ions, which originated from the transitions between the excited state 5D0 to the ground state 7FJ (J ¼0–4) of the Eu3 þ ions (Fig. 4b). The intensity of different 5 D0–7FJ transitions of these emission peaks depends on the local symmetry of the crystal field of Eu3 þ ions. If the Eu3 þ ions occupy an inversion symmetry site in the crystal lattice, the orange–red
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H. Li et al. / Optics & Laser Technology 49 (2013) 33–37
Fig. 5. Emission intensity of Eu3 þ ions at 614 nm as a function of its doping concentration.
Fig. 4. Excitation (a) and emission (b) spectra of Eu3 þ -doped TiO2 nanobelts.
emission, magnetic dipole transition 5D0–7F1 (around 590 nm) is the dominant transition. On the contrary, the electric dipole transition 5D0–7F2 (around 614 nm) is the dominant transition [24,25]. Therefore, the strongest emission line at 614 nm originates from the electric dipole transition 5D0–7F2. Next, we present the results of room-temperature (RT) photoluminescence measurements for the samples with Eu3 þ doping levels in the range of 2–12 mol% (Fig. 5). Under the excitation of 393 nm, the emission intensities at 614 nm are different, and the best doping concentration is 8 mol%. 3.5. Effect of calcination temperature The effect of calcination temperature on emission intensity has been investigated, and the results are presented in Fig. 6. It can be seen that annealing temperature affects luminescence of the nanobelts significantly. In Fig. 6, the emission spectra were obtained under the excited wavelength of 393 nm. They all show the characteristic emission of Eu3 þ ions at 614 nm which is ascribed to electric dipole 5D0–7F2 transition of Eu3 þ ions. However, emission intensity increased along with the annealing temperature below 500 1C. The crystallization of the products improved with the increase of the temperature, and the defects
Fig. 6. Emission spectra of TiO2:Eu3 þ nanobelts precursor and after annealing at 300 1C, 500 1C, 700 1C and 900 1C.
decreased accordingly [26]. The annealing temperature causes an important increasing of the luminescence because of the wellformed crystal structure of TiO2:Eu3 þ nanobelts. Furthermore, the relative PL intensity of TiO2:Eu3 þ nanobelts was found to be higher than that of titanate (H2Ti2O5 H2O):Eu3 þ nanobelts which may be due to the quenching effect of H2O in the titanate crystal lattice [27]. When the annealing temperature increased from 700 1C to 900 1C, the pure rutile phase TiO2 was formed. Comparing the emission intensity of anatase phase TiO2 when the annealing temperature is 500 1C, the luminescence intensity decreased; this may be due to phase transition from anatase to rutile phase of TiO2 [28].
4. Conclusion In summary, we have successfully prepared TiO2:Eu3 þ nanobelts using the hydrothermal method. The XRD results show that anatase-phase TiO2 can be formed at temperatures above 300 1C, and the optimal calcination temperature is 500 1C. Under UV excitation, TiO2:Eu3 þ nanobelts exhibit red emission.
H. Li et al. / Optics & Laser Technology 49 (2013) 33–37
The calcination temperature has a large influence not only on the crystal structure and morphology, but also on the luminescence properties. It is expected that TiO2:Eu3 þ nanobelts can be a potential candidate as novel semiconductor fluorescence materials.
Acknowledgment This work is financially supported by the National Natural Science Foundation of China (Grant no. 21171066), the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2010-05) and Project 20111129 supported by Graduate Innovation Fund of Jilin University. References [1] Gates B, Yin YD, Xia YN. A solution-phase approach to the synthesis of uniform nanowires of crystalline selenium with lateral dimensions in the range of 10–30 nm. Journal of the American Chemical Society 2000;122:12582–3. [2] Lin SS, Hong JI, Song JH, Zhu Y, He HP, Xu Z, et al. Phosphorus doped Zn1 xMgxO nanowire arrays. Nano Letters 2009;9:3877–82. [3] Xia YN, Yang PD, Sun YG, Wu YY, Mayers B, Gates B, et al. One-dimensional nanostructures: synthesis, characterization, and applications. Advanced Materials 2003;15:353–89. [4] Song YH, You HP, Yang M, Zheng YH, Liu K, Jia G, et al. Facile synthesis and luminescence of Sr–5(PO4)(3)CI:Eu2 þ nanorod bundles via a hydrothermal route. Inorganic Chemistry 2010;49:1674–8. [5] Wang ZL, Song JH. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006;312:242–6. [6] Liu K, You HP, Zheng YH, Jia G, Song YH, Huang YJ, et al. Facile and rapid fabrication of metal–organic framework nanobelts and color-tunable photoluminescence properties. Journal of Materials Chemistry 2010;20:3272–9. [7] Lipsitz RS, Tjandra N. Carbonyl CSA restraints from solution NMR for protein structure refinement. Journal of the American Chemical Society 2001;123:11065–6. [8] Yang RS, Wang ZL. Springs, rings, and spirals of rutile-structured tin oxide nanobelts. Journal of the American Chemical Society 2006;128:1466–7. [9] Zhuang JL, Liang LF, Sung HHY, Yang XF, Wu MM, Williams ID, et al. Controlled hydrothermal growth and up-conversion emission of NaLnF(4) (Ln¼ Y, Dy–Yb). Inorganic Chemistry 2007;46:5404–10. [10] Liang LF, Xu HF, Su Q, Konishi H, Jiang YB, Wu MM, et al. Hydrothermal synthesis of prismatic NaHoF4 microtubes and NaSmF4 nanotubes. Inorganic Chemistry 2004;43:1594–6. [11] Palmer MS, Neurock M, Olken MM. Periodic density functional theory study of methane activation over La2O3: activity of O2 , O , O2(2 ), oxygen point defect, and Sr2 þ -doped surface sites. Journal of the American Chemical Society 2002;124:8452–61.
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[12] He ZQ, Xu X, Song S, Xie L, Tu JJ, Chen JM, et al. A visible light-driven titanium dioxide photocatalyst codoped with lanthanum and iodine: an application in the degradation of oxalic acid. Journal of Physical Chemistry C 2008;112:16431–7. [13] Cacciotti I, Bianco A, Pezzotti G, Gusmano G. Terbium and ytterbium-doped titania luminescent nanofibers by means of electrospinning technique. Materials Chemistry and Physics 2011;126:532–41. [14] Mohamed RM, Mkhalid IA. The effect of rare earth dopants on the structure, surface texture and photocatalytic properties of TiO2–SiO2 prepared by sol– gel method. Journal of Alloys and Compounds 2010;501:143–7. [15] Li JG, Wang XH, Watanabe K, Ishigaki T. Phase structure and luminescence properties of Eu3 þ -doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists. Journal of Physical Chemistry B 2006;110:1121–7. [16] Feng XA, Yang L, Zhang NC, Liu YL. A facile one-pot hydrothermal method to prepare europium-doped titania hollow phosphors and their sensitized luminescence properties. Journal of Alloys and Compounds 2010;506:728–33. [17] De SK, Das K, Sharma SN, Kumar M. Morphology dependent luminescence properties of co doped TiO(2) nanostructures. Journal of Physical Chemistry C 2009;113:14783–92. [18] Cacciotti I, Bianco A, Pezzotti G, Gusmano G. Synthesis, thermal behaviour and luminescence properties of rare earth-doped titania nanofibers. Chemical Engineering Journal 2011;166:751–64. [19] Nakahira A, Kato W, Tamai M, Isshiki T, Nishio K, Aritani H. Synthesis of nanotube from a layered H2Ti4O9 center dot H2O in a hydrothermal treatment using various titania sources. Journal of Materials Science 2004;39:4239–45. [20] Guo N, You HP, Song YH, Yang M, Liu K, Zheng YH, et al. White-light emission from a single-emitting-component Ca9Gd(PO4)(7):Eu2 þ ,Mn2 þ phosphor with tunable luminescent properties for near-UV light-emitting diodes. Journal of Materials Chemistry 2010;20:9061–7. [21] Chen Y, Kang KS, Yoo KH, Jyoti N, Kim J. Cause of slow phase transformation of TiO2 nanorods. Journal of Physical Chemistry C 2009;113:19753–5. [22] Ferdov S, Ferreira RAS, Lin Z. Hydrothermal synthesis, structural investigation, photoluminescence features, and emission quantum yield of Eu and Eu– Gd silicates with apatite-type structure. Chemistry of Materials 2006;18:5958–64. [23] Wu CC, Chen KB, Lee CS, Chen TM, Cheng BM. Synthesis and VUV photoluminescence characterization of (Y,Gd)(V,P)O 4:Eu3 þ as a potential redemitting PDP phosphor. Chemistry of Materials 2007;19:3278–85. [24] Jayasankar CK, Ravi Kanth Kumar VV. Judd–Ofelt intensity analysis and spectral studies of Pr(III) ions in alkali zinc borosulphate glasses. Materials Chemistry and Physics 1996;46:84–91. [25] Lu LCZhouguang, Tang Yougen, Li Yadong. Preparation and luminescence properties of Eu3 þ -doped MSnO3 (M ¼ Ca, Sr and Ba) perovskite materials. Journal of Alloys and Compounds 2005;387:L1–4. [26] Qian XPXianhua, Zhang Dafeng, Li Lei, Li Meijing, Wu Shutong. Combustion synthesis and luminescence properties of NaY1 xEux(WO4)2 phosphors. Journal of Luminescence 2011;131:1692–5. [27] Buckner SW, Konold RL, Jelliss PA. Luminescence quenching in PbS nanoparticles. Chemical Physics Letters 2004;394:400–4. [28] Jing FL, Harako S, Komuro S, Zhao XW. Luminescence properties of Sm3 þ doped TiO2 thin films prepared by laser ablation. Journal of Physics D: Applied Physics 2009;42.
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Facile synthesis and luminescence properties of TiO2:Eu3 þ nanobelts Hongbo Li a, Keyan Zheng a, Ye Sheng a, Yanhua Song a, Hongguang Zhang a, Jing Huang a, Qisheng Huo b, Haifeng Zou a,n a b
College of Chemistry, Jilin University, Changchun 130012, PR China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
abstract
Article history: Received 29 August 2012 Received in revised form 19 November 2012 Accepted 7 December 2012 Available online 7 January 2013
Uniform TiO2:Eu3 þ nanobelts have been successfully prepared by a simple hydrothermal method without any surfactant, catalyst or template. The as-synthesized products were characterized by X-ray diffraction, transmission electron microscopy and Fourier Transform Infrared Spectrophotometry. X-ray diffraction results demonstrate that all the diffraction peaks of the samples annealed at 500 1C can be well indexed to the pure anatase-phase TiO2. Under ultraviolet excitation, TiO2:Eu3 þ nanobelts exhibit red emission corresponding to the 5D0–7F2 transition of the Eu3 þ ions, and the photoluminescence intensity is the strongest when calcination temperature is 500 1C. It is expected that the TiO2:Eu3 þ nanobelts can be used as novel semiconductor luminescence materials. Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved.
Keywords: TiO2:Eu3 þ nanobelts Calcination Luminescence properties
1. Introduction One-dimensional (1D) nano-materials, such as nanobelts, nanorods, and nanotubes, have become the focus of intensive research due to their physical and chemical properties and wide applications [1–4]. The properties of 1D nano-materials depend not only on their composition but also on their structure, morphology, phase, shape, size distribution, and spatial arrangement [5,6]. Nanobelts, which are a new morphology of 1D nanostructures with a rectangular cross section, could be an ideal system for fully understanding dimensionally confined transport phenomena and show promising applications in numerous areas, such as field-effect transistors, nanometer-sized ultrasensitive gas sensors, resonators and luminescent materials [6–8]. However, simple, rapid, inexpensive, environment friendly, and effective synthesis of such 1D nanostructure on a large scale still remains a challenge for researchers. Much research attention has been paid to the field of rareearth materials, since they have many potential applications based on their novel electronic, optical, and magnetic properties resulting from their 4f electrons [9–11]. As their host lattices are suitable for rare earths doping, rare earth doped TiO2 nanomaterials have been widely used in photocatalysis, luminescent materials, and other fields [12–14]. Among the lanthanide ions, Eu3 þ ion is a very important luminescent center and has been regarded as an attractive dopant for use as visible luminescent materials n
Corresponding author. Tel./fax: þ 86 431 85155275. E-mail address: [email protected] (H. Zou).
because of its strong red emissions. Eu3 þ -doped TiO2 luminescent nanocrystals were prepared by Ji-Guang Li, and bright red emissions were observed by exciting the TiO2 host with UV light or by directly exciting Eu3 þ at a wavelength beyond the absorption edge of TiO2 [15]. Eu3 þ -activated titania hollow spheres were synthesized using carbon spheres as hard templates and effective nonradiative energy transfer from the TiO2 hollow spheres host matrix to Eu3 þ ions was observed. This is due to changes of crystalline field in the environment around Eu3 þ ions occupying Ti4 þ sites [16]. Co-doped TiO2 nanostructures such as nanorods, nanowires, and nanotubes were synthesized through the solvothermal method by Kajari Das et al., and the Co was incorporated into the TiO2 lattice as Co2 þ and oxygen vacancies were created due to the substitution of the Ti4 þ ions by Co2 þ ions [17]. Ilaria Cacciotti et al. have synthesized rare earth-doped titania nanofibers by the electrospinning technique and the luminescence spectra of Eu- and Er-doped samples clearly testified the presence of lanthanide ions in the TiO2 host lattice [18]. However, there were few reports on the Gd3 þ ions and Eu3 þ ions co-doped TiO2 with a uniform belt-like shape. So it is significant to develop more facile, efficient, and low-cost techniques to fabricate large-scale and well-crystallized rare earth-doped TiO2 nanobelts. In this paper, uniform and well-dispersed belt-like TiO2 nanobelts doped with Eu3 þ have been successfully prepared through a simple hydrothermal approach and subsequent annealing treatment process. The anatase TiO2:Eu3 þ nanobelts, which inherit their parents’ morphology, were obtained after the calcination process, and the luminescence properties were investigated.
0030-3992/$ - see front matter Crown Copyright & 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.12.007
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H. Li et al. / Optics & Laser Technology 49 (2013) 33–37
2. Experimental section 2.1. Materials Commercial TiO2 powder, absolute ethanol, concentrated hydrochloric acid, nitric acid, sodium hydroxide, and Eu2O3 (99.9%) were purchased from Beijing Chemical Co. All chemicals were analytical grade reagents and used directly without further purification. Eu(NO3)3 aqueous solution was obtained by dissolving Eu2O3 (99.99%) in dilute HNO3 solution under heating with ceaseless agitation. 2.2. Preparation In the present synthesis process, 0.5 g TiO2 powder was mixed with 10 M NaOH aqueous solution and absolute ethanol (the volume ratio of NaOH aqueous solution to ethanol was 1:1). Different molar ratios Eu(NO3)3 (0.1 M) solution was added into the above mixture. The solution was transfered into a teflon container enclosed in a stainless steel autoclave, and aged at 180 1C for 24 h. After naturally cooling to room temperature, the precursors were separated by filtration, washed with 0.1 M dilute HCl and deionized water several times until the pH was adjusted to about 7.0, and then dried at 60 1C for 10 h in air. The final products were obtained through a heat treatment at desired temperatures (300–900 1C) for 4 h with a heating rate of 1 1C min 1.
Fig. 1. XRD patterns of TiO2:Eu3 þ nanobelts precursor (a) and the annealed products prepared at 300 1C (b), 500 1C (c), 700 1C (d), and 900 1C (e), along with the standard data for anatase-phase TiO2 (JCPDS card no. 21-1272) as a reference.
nanobelts samples are very sharp and strong at 500 1C (Fig. 1c), indicating that products with high crystallinity can be synthesized by this method. 3.2. Morphology
2.3. Characterization The structure of TiO2:Eu3 þ nanobelts was examined by standard X-ray diffraction (XRD; Rigaku D/max-B II (Cu Ka ray)). The morphologies of the samples were inspected by transmission electron microscopy (TEM; JEM-2000EX). Fourier Transform Infrared (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. Excitation and emission spectra were recorded using a Jobin Yvon FluoroMax-4 equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at room temperature.
3. Results and discussion 3.1. Structures Fig. 1 shows the XRD patterns of the as-prepared precursor sample (a) and those annealed from 300 1C to 900 1C (b–e), as well as the data of anatase-phase TiO2 (JCPDS no. 21-1272). All the diffraction peaks of the precursor can be readily indexed to H2Ti2O5 H2O [19], except that the pattern has one weak peak marked with ‘‘n’’. The peaks may be caused by a small amount of unreacted precursors (Fig. 1a). All samples heated above 300 1C show the X-ray diffraction of the anatase crystal phase (JCPDS 211272) at 2y ¼25.31; these peaks become sharper as the temperature is increased, and the precursor becomes anatase-phase TiO2 absolutely when the temperature reaches at 500 1C (Fig. 1c). Above 700 1C the transformation of anatase to rutile is detected (Fig. 1d) and after calcining the nanobelts at 900 1C, the anatase phase transforms to rutile phase absolutely (Fig. 1e). The introduction of different lanthanide ions has little effect on the phase structures of the products [20]. The XRD patterns of TiO2:Eu3 þ nanobelts annealed at 500 1C illustrate that all of the diffraction peaks can be indexed to the anatase-phase TiO2, indicating that the Eu3 þ ions have been effectively doped into the TiO2 host lattice. It can also be seen that the diffraction peaks of TiO2:Eu3 þ
The TEM images of the precursor prepared from hydrothermal reaction and the TiO2 nanobelts after annealing at different temperatures are shown in Fig. 2. Fig. 2a shows a panoramic TEM image of the precursor, which consists of belt-like nanostructures with a mean length of several micrometers. The highly magnified TEM image of the nanobelts shows that each nanobelt consists of many smaller nanobelts with width about 200–400 nm and length about several micrometers, with typical width-tothickness ratios of 5–10 (Fig. 2b). The belt-like morphology is maintained with calcination at 300 1C and 500 1C (Fig. 2c and d). The nanobelts obtained at 300 1C had no obvious changes in the length, but only reduction in width (Fig. 2c). As the annealing temperature was raised to 500 1C, the width of TiO2:Eu3 þ nanobelts changed greatly, being about 50 nm (Fig. 2d). The SAED pattern shows a set of broad diffuse rings instead of spots due to the random orientation of the crystallites, corresponding to diffraction from different planes of the nanocrystallites. The SAED patterns are consistent with an anatase phase structure of TiO2 with strong ring patterns due to (101), (004), (200), (211), (204) and (200) planes, in good agreement with XRD patterns and demonstrating its polycrystalline nature. However, after annealing at 700 1C, some of the nanobelts could not maintain their belt-like morphology, which are ready to crumble into a powder form (Fig. 2e). At 900 1C, the belt-like morphology appears to comprise aggregates of nanoparticles, 50–200 nm in diameter (Fig. 2f). 3.3. FT-IR spectra analysis Fig. 3 shows the FT-IR spectra of the TiO2:Eu3 þ nanobelts precursor (a) and the annealed products (b)–(e). The peaks located at 400–1000 cm 1 are characteristics of the formation of O–Ti–O network [21]. It is interesting to note that the peaks at 400–1000 cm 1 changed with the calcination temperature, and achieved the strongest stretching when the temperature reached 500 1C. The bands at 561, 503 cm 1 and 559, 501 cm 1 are attributed to Eu–O stretching modes [22]. The peaks at about
H. Li et al. / Optics & Laser Technology 49 (2013) 33–37
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Fig. 2. (a) Low- and (b) high-magnification TEM images of TiO2:Eu3 þ nanobelts precursor, and the final products annealed at 300 1C (c), 500 1C (d), 700 1C (e) and 900 1C (f).
between the hydroxyl groups and TiO2 nanobelts through hydrogen bond. However, the peaks become weaker with the rise of temperature. Increasing the treatment temperature results in a structural transformation from titanate to anatase-phase and rutile-phase TiO2, which is in accord with the results of XRD analysis (Fig. 1). 3.4. Luminescence properties
Fig. 3. FT-IR spectra of TiO2:Eu3 þ nanobelts precursor (a) and the final products annealed at 300 1C (b), 500 1C (c), 700 1C (d) and 900 1C (e).
3400 cm 1 correspond to stretching vibrations of hydroxyl groups; the presence of surface water is also confirmed by its bending mode at 1630 cm 1, indicating a strong interaction
Fig. 4 shows the excitation and emission spectra of the TiO2:Eu3 þ nanobelts prepared at 180 1C for 24 h and then annealed at 500 1C. The as-obtained sample exhibits a strong red emission under short ultraviolet irradiation, which can be confirmed by the emission spectrum of the TiO2:Eu3 þ sample excitation at 393 nm. The excitation spectrum (Fig. 4a) consists of a weak absorption band at 308 nm attributed to the Eu3 þ –O2 transitions originating from a charge transfer band (CTB) transition [23]. The products synthesized under this method show the characteristic emission of Eu3 þ ions, which originated from the transitions between the excited state 5D0 to the ground state 7FJ (J ¼0–4) of the Eu3 þ ions (Fig. 4b). The intensity of different 5 D0–7FJ transitions of these emission peaks depends on the local symmetry of the crystal field of Eu3 þ ions. If the Eu3 þ ions occupy an inversion symmetry site in the crystal lattice, the orange–red
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Fig. 5. Emission intensity of Eu3 þ ions at 614 nm as a function of its doping concentration.
Fig. 4. Excitation (a) and emission (b) spectra of Eu3 þ -doped TiO2 nanobelts.
emission, magnetic dipole transition 5D0–7F1 (around 590 nm) is the dominant transition. On the contrary, the electric dipole transition 5D0–7F2 (around 614 nm) is the dominant transition [24,25]. Therefore, the strongest emission line at 614 nm originates from the electric dipole transition 5D0–7F2. Next, we present the results of room-temperature (RT) photoluminescence measurements for the samples with Eu3 þ doping levels in the range of 2–12 mol% (Fig. 5). Under the excitation of 393 nm, the emission intensities at 614 nm are different, and the best doping concentration is 8 mol%. 3.5. Effect of calcination temperature The effect of calcination temperature on emission intensity has been investigated, and the results are presented in Fig. 6. It can be seen that annealing temperature affects luminescence of the nanobelts significantly. In Fig. 6, the emission spectra were obtained under the excited wavelength of 393 nm. They all show the characteristic emission of Eu3 þ ions at 614 nm which is ascribed to electric dipole 5D0–7F2 transition of Eu3 þ ions. However, emission intensity increased along with the annealing temperature below 500 1C. The crystallization of the products improved with the increase of the temperature, and the defects
Fig. 6. Emission spectra of TiO2:Eu3 þ nanobelts precursor and after annealing at 300 1C, 500 1C, 700 1C and 900 1C.
decreased accordingly [26]. The annealing temperature causes an important increasing of the luminescence because of the wellformed crystal structure of TiO2:Eu3 þ nanobelts. Furthermore, the relative PL intensity of TiO2:Eu3 þ nanobelts was found to be higher than that of titanate (H2Ti2O5 H2O):Eu3 þ nanobelts which may be due to the quenching effect of H2O in the titanate crystal lattice [27]. When the annealing temperature increased from 700 1C to 900 1C, the pure rutile phase TiO2 was formed. Comparing the emission intensity of anatase phase TiO2 when the annealing temperature is 500 1C, the luminescence intensity decreased; this may be due to phase transition from anatase to rutile phase of TiO2 [28].
4. Conclusion In summary, we have successfully prepared TiO2:Eu3 þ nanobelts using the hydrothermal method. The XRD results show that anatase-phase TiO2 can be formed at temperatures above 300 1C, and the optimal calcination temperature is 500 1C. Under UV excitation, TiO2:Eu3 þ nanobelts exhibit red emission.
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The calcination temperature has a large influence not only on the crystal structure and morphology, but also on the luminescence properties. It is expected that TiO2:Eu3 þ nanobelts can be a potential candidate as novel semiconductor fluorescence materials.
Acknowledgment This work is financially supported by the National Natural Science Foundation of China (Grant no. 21171066), the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2010-05) and Project 20111129 supported by Graduate Innovation Fund of Jilin University. References [1] Gates B, Yin YD, Xia YN. A solution-phase approach to the synthesis of uniform nanowires of crystalline selenium with lateral dimensions in the range of 10–30 nm. Journal of the American Chemical Society 2000;122:12582–3. [2] Lin SS, Hong JI, Song JH, Zhu Y, He HP, Xu Z, et al. Phosphorus doped Zn1 xMgxO nanowire arrays. Nano Letters 2009;9:3877–82. [3] Xia YN, Yang PD, Sun YG, Wu YY, Mayers B, Gates B, et al. One-dimensional nanostructures: synthesis, characterization, and applications. Advanced Materials 2003;15:353–89. [4] Song YH, You HP, Yang M, Zheng YH, Liu K, Jia G, et al. Facile synthesis and luminescence of Sr–5(PO4)(3)CI:Eu2 þ nanorod bundles via a hydrothermal route. Inorganic Chemistry 2010;49:1674–8. [5] Wang ZL, Song JH. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006;312:242–6. [6] Liu K, You HP, Zheng YH, Jia G, Song YH, Huang YJ, et al. Facile and rapid fabrication of metal–organic framework nanobelts and color-tunable photoluminescence properties. Journal of Materials Chemistry 2010;20:3272–9. [7] Lipsitz RS, Tjandra N. Carbonyl CSA restraints from solution NMR for protein structure refinement. Journal of the American Chemical Society 2001;123:11065–6. [8] Yang RS, Wang ZL. Springs, rings, and spirals of rutile-structured tin oxide nanobelts. Journal of the American Chemical Society 2006;128:1466–7. [9] Zhuang JL, Liang LF, Sung HHY, Yang XF, Wu MM, Williams ID, et al. Controlled hydrothermal growth and up-conversion emission of NaLnF(4) (Ln¼ Y, Dy–Yb). Inorganic Chemistry 2007;46:5404–10. [10] Liang LF, Xu HF, Su Q, Konishi H, Jiang YB, Wu MM, et al. Hydrothermal synthesis of prismatic NaHoF4 microtubes and NaSmF4 nanotubes. Inorganic Chemistry 2004;43:1594–6. [11] Palmer MS, Neurock M, Olken MM. Periodic density functional theory study of methane activation over La2O3: activity of O2 , O , O2(2 ), oxygen point defect, and Sr2 þ -doped surface sites. Journal of the American Chemical Society 2002;124:8452–61.
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