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Facile synthesis of TiO2:Eu3+ spindle shaped nanoparticles from titanate nanobelt precursors
Powder Technology 228 (2012) 277–283
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Facile synthesis of TiO2:Eu 3+ spindle shaped nanoparticles from titanate nanobelt precursors Hongbo Li a, Keyan Zheng a, Xuechun Xu c, Huan Zhao a, Yanhua Song a, Ye Sheng a, Qisheng Huo b, Haifeng Zou a,⁎ a b c
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 College of Earth Sciences, Jilin University, Changchun 130026, PR China
a r t i c l e
i n f o
Article history: Received 4 March 2012 Received in revised form 2 May 2012 Accepted 13 May 2012 Available online 19 May 2012 Keywords: Titanium oxide Eu3+ ions Doping Spindle-shaped nanoparticles Luminescent properties
a b s t r a c t Uniform TiO2:Eu3+ nanorods and spindle-shaped nanoparticles have been successfully prepared through simple calcination and two-step hydrothermal processes using titanate as the precursor. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectrophotometer, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence spectra (PL) were used to characterize these nanoparticles. On the basis of X-ray diffraction result, it can be assumed that the as-obtained precursors have the structure formula of titanate (H2Ti2O5·H2O). The as-formed nanotube precursors could transform to anatase TiO2:Eu3+ from nanotubes to nanorods and spindle-shaped nanoparticles with the calcination and hydrothermal processes. Under UV light excitation, both the TiO2:Eu3+ nanorods and spindle-shaped nanoparticles exhibit the strong red emission corresponding to the 5D0–7F2 transition of the Eu 3+ ions. In addition, the luminescence properties of the as-obtained samples are dependent on the defects on the surface, leading to stronger luminescence intensity. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Rare earth ion doped compounds have attracted a great amount of interest, due to their significant technological importance, and are used as high-performance luminescent devices, catalysts, time-resolved fluorescence labels for biological detection, and other functional materials based on their optical, electronic, and chemical characteristics [1–4]. If the compounds were fabricated by precisely controlling the size and morphologies, it is expected that they would have new physical and chemical properties [5]. Hexagonal monodisperse NaCeF4 and NaCeF4:Tb3+ spindle-shaped nanoparticles have been successfully synthesized by a polyol-mediated solvothermal route, and shape-dependent luminescence and energy transfer routes from Ce3+ to Tb3+ in NaCeF4:Tb3+ spindleshaped nanoparticles were observed by the modified local crystal field environment around rare earth ions [6]. SnO2 spindle-shaped nanoparticles and nanoparticles were obtained through hydrothermal and microwave irradiation methods, and the PL emission and decay time strongly depend on the shape of the nanocrystals [7]. Therefore, the controlled fabrication of rare-earth doped luminescent nanocrystals represents a great challenge and is a very important subject of research. Among the various nanostructures, one-dimensional (1D) nanostructures with their inherent anisotropy are the smallest dimension
⁎ Corresponding author. Tel./fax: + 86 431 85155275. E-mail address: [email protected] (H. Zou).
structures, including spindle-shaped nanoparticles, nanowires, nanotubes and nanoprisms, which have attracted extensive synthetic interest over the past years due to their numerous potential applications in the fabrication of electronic, optical, optoelectronic, and magnetic devices [8–12]. Y4O(OH)9NO3:Eu 3+ nanotubes and nanobundles were successfully synthesized from nanolamellar precursors in large quantities with a direct hydrothermal method, and the Y2O3:Eu 3+ nanotubes and nanobundles were obtained by subsequent calcination of the corresponding Y4O(OH)9NO3:Eu 3+ precursors [5]. Monodisperse and well-defined one-dimensional rare earth fluoride (β-NaREF4) (RE = Y, Sm, Eu, Gd, Tb, Dy and Ho) nanowires and spindle-shaped nanoparticles were successfully synthesized by in situ acid corrosion and an ion exchange approach using RE(OH)3 as precursors via a two-step hydrothermal route [13]. Particularly, rare earth doped TiO2 nanomaterials with electronic, optical, and catalytic properties have drawn particular attention because of their wide applications as high efficient luminescent materials, catalysts, and other functional materials [14–16]. Dramatic efforts have been dedicated to design and synthesis of controlled morphologies and size. Up to the present, RE doped TiO2 nanocrystals have been prepared through various chemical routes such as solvothermal method, sol–gel deposition, and electrospinning technique [17–19]. Among these methods, the hydrothermal process is a relatively simple synthetic method because it has some advantages, such as lower reaction temperature, lower energy consumption, easily controllable reaction conditions, size-selective growth, controllable morphology and
0032-5910/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.05.032
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better particle size distribution [20]. It has been proven to be an effectively way and convenient process in preparation of high-quality anisotropic nanostructures due to its great chemical flexibility and synthetic tenability. Co-doped TiO2 nanostructures such as nanowires and nanotubes were synthesized through solvothermal method by Kajari Das et al., the result indicates that the Co was incorporated into the TiO2 lattice as Co2+ and oxygen vacancies were created [21]. Ilaria Cacciotti et al. have synthesized rare earth-doped titania nanofibers by electrospinning technique and the luminescence spectra of Eu- and Er-doped samples clearly testified the presence of lanthanide ions in the TiO2 host lattice [22]. Among the lanthanide ions, as a most important luminescent center, Eu 3+ ion has been regarded as an attractive dopant for visible luminescent materials because of their strong red emissions. Eu3+-activated titania hollow spheres were synthesized using carbon spheres as hard templates, the effective nonradiative energy transfer from the TiO2 hollow spheres host matrix to Eu3+ ions was observed [23]. However, there is no report on the synthesis of rare earth doped TiO2 spindle-shaped nanoparticles through the two-step hydrothermal process. Herein, we report the preparation of the uniform titanate nanobelt precursors through a simple hydrothermal process. The as-obtained precursor has been successfully utilized to fabricate highly uniform TiO2:Eu 3+ spindle-shaped nanoparticles. On the basis of X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results, it can be assumed that the precursors have the structure formula of titanate (H2Ti2O5·H2O). However, the nanobelts could be transformed to spindle-shaped TiO2 after the two-step hydrothermal process. Furthermore, the formation process of the as-synthesized TiO2:Eu 3+ spindle-shaped nanoparticles has been investigated in detail. 2. Experimental section
were then filtered, and dried in an oven at 80 °C for 12 h to give the final products. 2.3. Characterization The structure of the TiO2 1D nanostructures was examined by standard X-ray diffraction (XRD) (Rigaku D/max-B II) under Cu Kα (λ = 1.5405 Å) radiation at a scanning rate of 6°/min with 2θ ranging from 10° to 70°. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Scanning electron microscopy (SEM) measurements were conducted on a Hitachi S-4800 scanning electron microscope with an accelerating voltage of 15 kV. Energydispersive X-ray (EDX) spectroscopy analysis was performed with an H JEOL JXA-840 EDX system attached to the SEM microscope. The samples were prepared by sonicating powdered samples in ethanol, evaporating one drop of suspension on a Si slide, and then evaporating a thin Pt film on the slide. Low- and high-resolution transmission electron microscopies (TEM) were performed by using an FEI Tecnai G2 S-Twin instrument with a field emission gun operating at 200 kV. The powders were dispersed in absolute ethanol and sonicated before TEM characterization. Fourier transform infrared (FT-IR) spectroscopy was used to examine, by means of powderpressed KBr pellets, in the wave number range from 4000 to 500 cm −1, at a resolution of 4 cm −1, with the KBr/powder weight ratio about 100–200, using a Perkin-Elmer 580B infrared spectrophotometer at room temperature. The luminescence properties of the TiO2:Eu 3+ spindle-shaped nanoparticles were investigated by excitation and fluorescence emission spectra, which were recorded at room temperature with a Jobin Yvon Fluoro Max-4 equipped with a 150 W xenon lamp as the excitation source. The slits for both excitation and emission were set as 0.5 nm, the scanning speed was fixed at 240 nm/ min, the resolution is 1 nm and the scanning range was from 250 to 750 nm. All the measurements were performed at room temperature.
2.1. Materials 3. Results and discussion The anatase-type titania powder (32 nm in diameter, 3 m 2/g in specific surface area), absolute ethanol, concentrated hydrochloric acid, sodium hydroxide, and rare earth oxide (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.
3.1. Phase identification, morphology of Eu 3+-doped titanate nanobelts Fig. 1 shows the XRD patterns of the Eu 3+-doped titanate nanobelt precursor. Most of the diffraction peaks are readily indexed to be H2Ti2O5·H2O, with lattice constants of a = 18.030, b = 3.784 and c = 2.998 (JCPDS file no. 47-0124). The except peak marked with “*” may be caused by a small amount of unreacted anatase titania raw
2.2. Preparation 2.2.1. Hydrothermal synthesis of the Eu 3+ doped titanate nanobelt precursor Titanate nanobelts were prepared using a chemical process similar to that described by Kasuga et al. [24]. In a typical synthetic process, 0.5 g of TiO2 powder was mixed with 10 ml 10 M NaOH aqueous solution, 10 ml absolute ethanol and 0.1 M Eu(NO3)3 solutions. The solution was placed in Teflon container enclosed in a stainless steel autoclave, aged at 180 °C for 24 h. The samples were washed with 0.1 M HCl aqueous solution and deionized water several times until the pH value of the washing solution was about 7.0, and dried at 80 °C for 12 h in air. 2.2.2. Hydrothermal synthesis of the TiO2:Eu 3+ spindle-shaped nanoparticles The TiO2:Eu 3+ spindle-shaped nanoparticles were prepared by the hydrothermal conversion from the precursor. 2 g of the nanobelt precursor was mixed with 80 ml of deionized water and the suspensions were obtained. The suspensions were subjected to hydrothermal post-treatment in a 100 ml Teflon-lined autoclave at 200 °C for 72 h. After the hydrothermal post-treatment, the resulting slurries
Fig. 1. XRD patterns of the as-synthesized titanate nanobelt precursor, along with the standard data for titanate (JCPDS card no. 47‐0124) and anatase titania (JCPDS card no. 21‐1272) as references.
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Fig. 2. (a) SEM image of Eu3+ doped titanate nanobelts. (b) Low-magnification TEM images of Eu3+ doped titanate nanobelts. (c) Cross-sectional TEM images of Eu3+-doped titanate nanobelts. (d) High‐magnification TEM images of Eu3+ doped titanate nanobelts, and inset is the SAED pattern of the Eu3+ doped titanate nanobelts.
materials (JCPDS file no. 21‐1272). In the present study, the anatase TiO2 is converted to sodium titanate through hydrothermal process. Then, after an ion exchange process, the Na + ions have been substituted by H + ions during the washing procedure by using diluted hydrochloric acid [25]. The morphology of the Eu 3+ doped titanate nanobelts was investigated with SEM, TEM and HRTEM. Fig. 2a shows the SEM image of the as-prepared nanobelt sample, and it can be clearly seen that the sample is composed of a large scale of nanobelts. To further study the fine structure of the above nanobelt product, TEM images were performed. The low- and high-magnification TEM images of the nanobelts are shown in Fig. 2b and d, clearly showing that the products are entirely composed of nanobelts with width of about 50–200 nm and lengths of several micrometers, which are consistent with the values shown in the SEM image (Fig. 2a). Cross-sectional TEM observation (Fig. 2c) shows that the cross sections of the nanobelts are rectangle‐like with typical width-to-thickness ratios of ~5 to 10. A typical SAED pattern taken from an area containing a large amount of nanobelts is shown in Fig. 2d. The SAED pattern takes with the electron beam covering the area encircled by the broken line. The round shape of the diffraction pattern indicates that a preferred orientation exists in the indicated area. In addition, the bright dot pattern highlighting the round rings indicates that the material within the encircled area contains larger crystals beside the polycrystalline material producing the rings. Fig. 3a and b shows the FT-IR spectra of the Eu 3+-doped titanate nanobelt precursor and the TiO2:Eu 3+ spindle-shaped nanoparticles, respectively. The peaks in the range of 3000–3700 cm − 1 can be attributed to symmetrical and asymmetrical stretching vibrations of O–H groups in the precursor, indicating that the structure of precursors contains O–H groups [26]. The sharp peak at 1639 cm − 1 is associated with the deformation of physisorbed H\O\H bonds [27]. The peaks located at 400–1000 cm − 1 (460 cm − 1, 486 cm − 1, 802 cm − 1, 905 cm − 1) are characteristics of the formation of O–Ti–O network
[28,29]. The peaks at 1419 cm − 1 and 1566 cm − 1 can be assigned as surface carbonate species by comparison with spectra of adsorbed carbonates on TiO2, and these are formed by the adsorption of atmospheric CO2 [30]. In Fig. 3b, it is interesting to note that the characteristic bands at 3000–3700 cm − 1 are changed, and shifted to higher wavenumbers. It has been revealed that there are two types of O–H groups occurring on the sample, which are terminal hydroxyl residing at an acidic site (bounding to one Ti 4+ cation) and bridging hydroxyl
Fig. 3. FT-IR spectra of nanobelt precursors (a) and the TiO2:Eu3+ spindle-shaped nanoparticles (b).
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Fig. 4. XRD pattern of the TiO2:Eu3+ spindle-shaped nanoparticle product.
residing at a two-coordinate or bridging O 2 − site (bounding to two Ti 4+ cations), respectively [31]. After post hydrothermal treatment, TiO2 exhibited the changes in wavenumber and decrease in the intensity of the 3424 cm − 1 band. This indicates that the terminal hydroxyl is removed from the surface of titanate. These provide additional evidence that the precursors have completely converted into crystalline TiO2 during the hydrothermal process, which is consistent with the XRD results of the precursors and spindle-shaped nanoparticle samples (Figs. 1 and 4). 3.2. The transformation of the TiO2:Eu 3+ spindle-shaped nanoparticles from titanate nanobelt precursor Fig. 4 shows the XRD pattern of the TiO2:Eu 3+ spindle-shaped nanoparticles. The pure phase, and all diffraction peaks can be
indexed easily as the anatase TiO2, which is in good agreement with the standard card no. 21-1272 (lattice constants of a = 3.785, b = 3.785 and c = 9.514). No peaks from impurities were observed, indicating that titanate transforms completely into the anatase phase of TiO2. The strong diffraction peaks indicate that the product has higher crystallinity. Fig. 5 shows SEM, TEM and HRTEM images of the TiO2:Eu 3+ spindle-shaped nanoparticles. From the SEM image (Fig. 5a), it can be seen that the sample is composed of uniform spindle-shaped structure. Fig. 5b shows typical TEM image of the TiO2:Eu 3+ spindle-shaped nanoparticles with diameters of 50–200 nm and lengths of 200–1000 nm, respectively. Further analysis of the crystalline structure of the spindle-shaped nanoparticles with HRTEM is depicted in Fig. 5c. The visible well-crystallized structure with lattice fringes about 0.37 nm was observed, which agreed with the (101) plane anatase phase of the typical anatase TiO2 nanoparticles [32]. These results further confirm the presence of highly crystalline anatase TiO2:Eu 3+ spindle-shaped nanoparticles, which agreed well with the XRD results. To confirm their composition, energy dispersive X-ray (EDX) spectrum was further used to investigate the TiO2:Eu 3+ spindle-shaped nanoparticles (Fig. 5d). It can be seen from Fig. 5d that the final product is mainly composed of the three elements (Ti, Eu, and O). The extra peaks in the spectrum come from the Pt in coating and Si in substrate for SEM measurement. The results effectively support the XRD results of spindle-shaped nanoparticle sample (Fig. 4).
3.3. Formation mechanism of the TiO2:Eu 3+ spindle-shaped nanoparticles Because of the topotactic transformation among the anatase structures, sodium titanate and titanate, there will be a definite relationship between the crystal-axis orientations of the three structures. To investigate the mechanism for the transformation, a sketch map was carried on the intermediate products in the transformation
Fig. 5. (a) SEM and (b) TEM images of the TiO2:Eu3+ spindle‐shaped nanoparticles. (c) HRTEM image of the TiO2:Eu3+ spindle‐shaped nanoparticles. (d) EDX spectrum of the TiO2: Eu3+ spindle‐shaped nanoparticles.
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the titanate and anatase TiO2 have common structural features: both crystal lattices consist of the octahedra sharing four edges and the zigzag ribbons [33,34]. In the reaction with the deionized water, the titanate nanobelts dehydrated and the large structural units such as zigzag ribbons remain relatively unchanged, rearranging to form anatase lattice [33]. Consequently, the spindle-shaped nanoparticles of the anatase crystals obtained from the titanate nanobelts showing no morphological resemblance to the belts (Fig. 5). 3.4. Luminescence properties
Scheme 1. The phase transitions between anatase, sodium- and titanate by the twostep hydrothermal process.
process in detail, as shown in Scheme 1. The phase transitions in Scheme 1 reveal that both phases between anatase TiO2 and titanate can be converted into each other by the two-step reactions at appropriate reaction conditions. Scheme 2 shows the schematic drawing and typical TEM images of the oriented growth, overlap for the formation of nanobelt products, and spindle-shaped nanoparticles. In the formation process of nanobelts, there is a general agreement that the reaction proceeds through four stages: (1) slow dissolution of raw materials of TiO2 powder, (2) the nanosheets formed, accompanied by epitaxial growth of sodium titanate nanosheets along the axis and formed the smaller nanobelts, (3) the smaller nanobelts overlap each other, (4) exchanged of sodium ions by protons during washing and separation of nanobelts. In this case, if the crystallization rate was high enough, the growing of the nanosheets would go along the axis after exceeding a particular value. The nanosheets became too rigid to bend, and overlapped with each other, resulting in the formation of nanobelts. The treatment with dilute hydrochloric acid solution is actually an ion-exchange progress from the sodium titanate to titanate. The two-step hydrothermal process clearly transformed the titanate nanobelts to spindle-shaped nanoparticles. It is noted that
The PL intensity increases with increasing Eu 3+ ions concentration when the doping concentrations are 0.5 mol%, 1 mol%, 2 mol%, 3 mol%, 4 mol%, and the intensity reaches the maximum when the concentration of Eu 3+ ions is 2.0 mol%, then decreases with addition of Eu 3+ ions. This is the concentration quench effect induced by the cross-relaxation. Therefore, the best doping concentration is 2.0 mol%. The excitation and emission spectra for the TiO2:Eu 3+ spindleshaped nanoparticles are shown in Figs. 6 and 7. The excitation spectrum consists of a strong broad band between 250 nm and 350 nm, and the characteristic excitation lines of Eu 3+ within its 4f 6 configuration from 350 to 550 nm (Fig. 6). In general, the strong broad band between 250 nm and 350 nm is attributed to the charge transfer band (CTB) between the O 2 − and Eu 3+ ions [35,36]. The excitation lines peaking at 365, 384, 393, 419, 468, 525 and 528 nm assign to 7 F0– 5D4, 7F0– 5G2, 7F0– 5L6, 7F0– 5D3, 7F0– 5D2, 7F1– 5D1 and 7F0– 5D1 transitions, respectively. Excitation into the strongest 7F0– 5L6 transition of Eu 3+ at 393 nm yields the emission spectrum of the sample, which consists of the emission lines associated with the Eu 3+ transitions from the excited 5D0, 1 levels to the 7FJ (J = 0,1,2,3,4) levels, as shown in Fig. 7. There is no notable shift in the positions of the emission peaks compared to other Eu 3+ doped systems due to the shielding effect of 5s 25p 6 electrons [13]. It is well known that the relative intensity of the 5D0– 7F1 and 5D0– 7F2 transitions is determined by the symmetry of the crystal sites of the Eu 3+ ions. If Eu 3+ ion has a site with inversion symmetry, the 5D0– 7F1 transition dominates; while if Eu 3+ ion holds a site without inversion symmetry, the
Scheme 2. Schematic illustration for the formation process of the TiO2:Eu3+ spindle-shaped nanoparticles.
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Furthermore, this synthetic methodology appears to provide a gateway into the synthesis of one-dimensional nanomaterials.
Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 21171066), the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources, No: OSR05, 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 supported by the Graduate Innovation Fund of Jilin University.
References
Fig. 6. Excitation spectrum of the TiO2:Eu3+ spindle-shaped nanoparticles.
5
D0– 7F2 transition predominates [37]. In the case of the TiO2:Eu 3+ spindle-shaped nanoparticles, the most prominent red emission originates from the 5D0– 7F2 transitions, which are induced by the lack of inversion symmetry at the Eu 3+ ion sites and depend strongly on the site symmetry in a host crystal.
4. Conclusions Highly uniform TiO2:Eu 3+ spindle-shaped nanoparticles were synthesized by two-step hydrothermal processes. The width of the nanobelt precursor is about 50–200 nm and the lengths are about several micrometers. The diameters of the TiO2:Eu 3+ spindleshaped nanoparticles are about 50–200 nm and the lengths about 200–1000 nm. The phase transformation among the anatase structure, sodium titanate and titanate and the possible formation mechanism were also investigated in detail. The final products of the TiO2: Eu 3+ spindle-shaped nanoparticles show strong red emission intensity under ultraviolet excitation, which is due to the lack of inversion symmetry at the Eu3+ ions site and depends strongly on the site symmetry in a host crystal. It can be anticipated that the as-prepared product is a potential candidate as the novel semiconductor luminescent materials.
Fig. 7. Emission spectrum of the TiO2:Eu3+ spindle-shaped nanoparticles.
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Facile synthesis of TiO2:Eu 3+ spindle shaped nanoparticles from titanate nanobelt precursors Hongbo Li a, Keyan Zheng a, Xuechun Xu c, Huan Zhao a, Yanhua Song a, Ye Sheng a, Qisheng Huo b, Haifeng Zou a,⁎ a b c
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 College of Earth Sciences, Jilin University, Changchun 130026, PR China
a r t i c l e
i n f o
Article history: Received 4 March 2012 Received in revised form 2 May 2012 Accepted 13 May 2012 Available online 19 May 2012 Keywords: Titanium oxide Eu3+ ions Doping Spindle-shaped nanoparticles Luminescent properties
a b s t r a c t Uniform TiO2:Eu3+ nanorods and spindle-shaped nanoparticles have been successfully prepared through simple calcination and two-step hydrothermal processes using titanate as the precursor. X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectrophotometer, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence spectra (PL) were used to characterize these nanoparticles. On the basis of X-ray diffraction result, it can be assumed that the as-obtained precursors have the structure formula of titanate (H2Ti2O5·H2O). The as-formed nanotube precursors could transform to anatase TiO2:Eu3+ from nanotubes to nanorods and spindle-shaped nanoparticles with the calcination and hydrothermal processes. Under UV light excitation, both the TiO2:Eu3+ nanorods and spindle-shaped nanoparticles exhibit the strong red emission corresponding to the 5D0–7F2 transition of the Eu 3+ ions. In addition, the luminescence properties of the as-obtained samples are dependent on the defects on the surface, leading to stronger luminescence intensity. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Rare earth ion doped compounds have attracted a great amount of interest, due to their significant technological importance, and are used as high-performance luminescent devices, catalysts, time-resolved fluorescence labels for biological detection, and other functional materials based on their optical, electronic, and chemical characteristics [1–4]. If the compounds were fabricated by precisely controlling the size and morphologies, it is expected that they would have new physical and chemical properties [5]. Hexagonal monodisperse NaCeF4 and NaCeF4:Tb3+ spindle-shaped nanoparticles have been successfully synthesized by a polyol-mediated solvothermal route, and shape-dependent luminescence and energy transfer routes from Ce3+ to Tb3+ in NaCeF4:Tb3+ spindleshaped nanoparticles were observed by the modified local crystal field environment around rare earth ions [6]. SnO2 spindle-shaped nanoparticles and nanoparticles were obtained through hydrothermal and microwave irradiation methods, and the PL emission and decay time strongly depend on the shape of the nanocrystals [7]. Therefore, the controlled fabrication of rare-earth doped luminescent nanocrystals represents a great challenge and is a very important subject of research. Among the various nanostructures, one-dimensional (1D) nanostructures with their inherent anisotropy are the smallest dimension
⁎ Corresponding author. Tel./fax: + 86 431 85155275. E-mail address: [email protected] (H. Zou).
structures, including spindle-shaped nanoparticles, nanowires, nanotubes and nanoprisms, which have attracted extensive synthetic interest over the past years due to their numerous potential applications in the fabrication of electronic, optical, optoelectronic, and magnetic devices [8–12]. Y4O(OH)9NO3:Eu 3+ nanotubes and nanobundles were successfully synthesized from nanolamellar precursors in large quantities with a direct hydrothermal method, and the Y2O3:Eu 3+ nanotubes and nanobundles were obtained by subsequent calcination of the corresponding Y4O(OH)9NO3:Eu 3+ precursors [5]. Monodisperse and well-defined one-dimensional rare earth fluoride (β-NaREF4) (RE = Y, Sm, Eu, Gd, Tb, Dy and Ho) nanowires and spindle-shaped nanoparticles were successfully synthesized by in situ acid corrosion and an ion exchange approach using RE(OH)3 as precursors via a two-step hydrothermal route [13]. Particularly, rare earth doped TiO2 nanomaterials with electronic, optical, and catalytic properties have drawn particular attention because of their wide applications as high efficient luminescent materials, catalysts, and other functional materials [14–16]. Dramatic efforts have been dedicated to design and synthesis of controlled morphologies and size. Up to the present, RE doped TiO2 nanocrystals have been prepared through various chemical routes such as solvothermal method, sol–gel deposition, and electrospinning technique [17–19]. Among these methods, the hydrothermal process is a relatively simple synthetic method because it has some advantages, such as lower reaction temperature, lower energy consumption, easily controllable reaction conditions, size-selective growth, controllable morphology and
0032-5910/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2012.05.032
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better particle size distribution [20]. It has been proven to be an effectively way and convenient process in preparation of high-quality anisotropic nanostructures due to its great chemical flexibility and synthetic tenability. Co-doped TiO2 nanostructures such as nanowires and nanotubes were synthesized through solvothermal method by Kajari Das et al., the result indicates that the Co was incorporated into the TiO2 lattice as Co2+ and oxygen vacancies were created [21]. Ilaria Cacciotti et al. have synthesized rare earth-doped titania nanofibers by electrospinning technique and the luminescence spectra of Eu- and Er-doped samples clearly testified the presence of lanthanide ions in the TiO2 host lattice [22]. Among the lanthanide ions, as a most important luminescent center, Eu 3+ ion has been regarded as an attractive dopant for visible luminescent materials because of their strong red emissions. Eu3+-activated titania hollow spheres were synthesized using carbon spheres as hard templates, the effective nonradiative energy transfer from the TiO2 hollow spheres host matrix to Eu3+ ions was observed [23]. However, there is no report on the synthesis of rare earth doped TiO2 spindle-shaped nanoparticles through the two-step hydrothermal process. Herein, we report the preparation of the uniform titanate nanobelt precursors through a simple hydrothermal process. The as-obtained precursor has been successfully utilized to fabricate highly uniform TiO2:Eu 3+ spindle-shaped nanoparticles. On the basis of X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results, it can be assumed that the precursors have the structure formula of titanate (H2Ti2O5·H2O). However, the nanobelts could be transformed to spindle-shaped TiO2 after the two-step hydrothermal process. Furthermore, the formation process of the as-synthesized TiO2:Eu 3+ spindle-shaped nanoparticles has been investigated in detail. 2. Experimental section
were then filtered, and dried in an oven at 80 °C for 12 h to give the final products. 2.3. Characterization The structure of the TiO2 1D nanostructures was examined by standard X-ray diffraction (XRD) (Rigaku D/max-B II) under Cu Kα (λ = 1.5405 Å) radiation at a scanning rate of 6°/min with 2θ ranging from 10° to 70°. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Scanning electron microscopy (SEM) measurements were conducted on a Hitachi S-4800 scanning electron microscope with an accelerating voltage of 15 kV. Energydispersive X-ray (EDX) spectroscopy analysis was performed with an H JEOL JXA-840 EDX system attached to the SEM microscope. The samples were prepared by sonicating powdered samples in ethanol, evaporating one drop of suspension on a Si slide, and then evaporating a thin Pt film on the slide. Low- and high-resolution transmission electron microscopies (TEM) were performed by using an FEI Tecnai G2 S-Twin instrument with a field emission gun operating at 200 kV. The powders were dispersed in absolute ethanol and sonicated before TEM characterization. Fourier transform infrared (FT-IR) spectroscopy was used to examine, by means of powderpressed KBr pellets, in the wave number range from 4000 to 500 cm −1, at a resolution of 4 cm −1, with the KBr/powder weight ratio about 100–200, using a Perkin-Elmer 580B infrared spectrophotometer at room temperature. The luminescence properties of the TiO2:Eu 3+ spindle-shaped nanoparticles were investigated by excitation and fluorescence emission spectra, which were recorded at room temperature with a Jobin Yvon Fluoro Max-4 equipped with a 150 W xenon lamp as the excitation source. The slits for both excitation and emission were set as 0.5 nm, the scanning speed was fixed at 240 nm/ min, the resolution is 1 nm and the scanning range was from 250 to 750 nm. All the measurements were performed at room temperature.
2.1. Materials 3. Results and discussion The anatase-type titania powder (32 nm in diameter, 3 m 2/g in specific surface area), absolute ethanol, concentrated hydrochloric acid, sodium hydroxide, and rare earth oxide (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.
3.1. Phase identification, morphology of Eu 3+-doped titanate nanobelts Fig. 1 shows the XRD patterns of the Eu 3+-doped titanate nanobelt precursor. Most of the diffraction peaks are readily indexed to be H2Ti2O5·H2O, with lattice constants of a = 18.030, b = 3.784 and c = 2.998 (JCPDS file no. 47-0124). The except peak marked with “*” may be caused by a small amount of unreacted anatase titania raw
2.2. Preparation 2.2.1. Hydrothermal synthesis of the Eu 3+ doped titanate nanobelt precursor Titanate nanobelts were prepared using a chemical process similar to that described by Kasuga et al. [24]. In a typical synthetic process, 0.5 g of TiO2 powder was mixed with 10 ml 10 M NaOH aqueous solution, 10 ml absolute ethanol and 0.1 M Eu(NO3)3 solutions. The solution was placed in Teflon container enclosed in a stainless steel autoclave, aged at 180 °C for 24 h. The samples were washed with 0.1 M HCl aqueous solution and deionized water several times until the pH value of the washing solution was about 7.0, and dried at 80 °C for 12 h in air. 2.2.2. Hydrothermal synthesis of the TiO2:Eu 3+ spindle-shaped nanoparticles The TiO2:Eu 3+ spindle-shaped nanoparticles were prepared by the hydrothermal conversion from the precursor. 2 g of the nanobelt precursor was mixed with 80 ml of deionized water and the suspensions were obtained. The suspensions were subjected to hydrothermal post-treatment in a 100 ml Teflon-lined autoclave at 200 °C for 72 h. After the hydrothermal post-treatment, the resulting slurries
Fig. 1. XRD patterns of the as-synthesized titanate nanobelt precursor, along with the standard data for titanate (JCPDS card no. 47‐0124) and anatase titania (JCPDS card no. 21‐1272) as references.
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Fig. 2. (a) SEM image of Eu3+ doped titanate nanobelts. (b) Low-magnification TEM images of Eu3+ doped titanate nanobelts. (c) Cross-sectional TEM images of Eu3+-doped titanate nanobelts. (d) High‐magnification TEM images of Eu3+ doped titanate nanobelts, and inset is the SAED pattern of the Eu3+ doped titanate nanobelts.
materials (JCPDS file no. 21‐1272). In the present study, the anatase TiO2 is converted to sodium titanate through hydrothermal process. Then, after an ion exchange process, the Na + ions have been substituted by H + ions during the washing procedure by using diluted hydrochloric acid [25]. The morphology of the Eu 3+ doped titanate nanobelts was investigated with SEM, TEM and HRTEM. Fig. 2a shows the SEM image of the as-prepared nanobelt sample, and it can be clearly seen that the sample is composed of a large scale of nanobelts. To further study the fine structure of the above nanobelt product, TEM images were performed. The low- and high-magnification TEM images of the nanobelts are shown in Fig. 2b and d, clearly showing that the products are entirely composed of nanobelts with width of about 50–200 nm and lengths of several micrometers, which are consistent with the values shown in the SEM image (Fig. 2a). Cross-sectional TEM observation (Fig. 2c) shows that the cross sections of the nanobelts are rectangle‐like with typical width-to-thickness ratios of ~5 to 10. A typical SAED pattern taken from an area containing a large amount of nanobelts is shown in Fig. 2d. The SAED pattern takes with the electron beam covering the area encircled by the broken line. The round shape of the diffraction pattern indicates that a preferred orientation exists in the indicated area. In addition, the bright dot pattern highlighting the round rings indicates that the material within the encircled area contains larger crystals beside the polycrystalline material producing the rings. Fig. 3a and b shows the FT-IR spectra of the Eu 3+-doped titanate nanobelt precursor and the TiO2:Eu 3+ spindle-shaped nanoparticles, respectively. The peaks in the range of 3000–3700 cm − 1 can be attributed to symmetrical and asymmetrical stretching vibrations of O–H groups in the precursor, indicating that the structure of precursors contains O–H groups [26]. The sharp peak at 1639 cm − 1 is associated with the deformation of physisorbed H\O\H bonds [27]. The peaks located at 400–1000 cm − 1 (460 cm − 1, 486 cm − 1, 802 cm − 1, 905 cm − 1) are characteristics of the formation of O–Ti–O network
[28,29]. The peaks at 1419 cm − 1 and 1566 cm − 1 can be assigned as surface carbonate species by comparison with spectra of adsorbed carbonates on TiO2, and these are formed by the adsorption of atmospheric CO2 [30]. In Fig. 3b, it is interesting to note that the characteristic bands at 3000–3700 cm − 1 are changed, and shifted to higher wavenumbers. It has been revealed that there are two types of O–H groups occurring on the sample, which are terminal hydroxyl residing at an acidic site (bounding to one Ti 4+ cation) and bridging hydroxyl
Fig. 3. FT-IR spectra of nanobelt precursors (a) and the TiO2:Eu3+ spindle-shaped nanoparticles (b).
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Fig. 4. XRD pattern of the TiO2:Eu3+ spindle-shaped nanoparticle product.
residing at a two-coordinate or bridging O 2 − site (bounding to two Ti 4+ cations), respectively [31]. After post hydrothermal treatment, TiO2 exhibited the changes in wavenumber and decrease in the intensity of the 3424 cm − 1 band. This indicates that the terminal hydroxyl is removed from the surface of titanate. These provide additional evidence that the precursors have completely converted into crystalline TiO2 during the hydrothermal process, which is consistent with the XRD results of the precursors and spindle-shaped nanoparticle samples (Figs. 1 and 4). 3.2. The transformation of the TiO2:Eu 3+ spindle-shaped nanoparticles from titanate nanobelt precursor Fig. 4 shows the XRD pattern of the TiO2:Eu 3+ spindle-shaped nanoparticles. The pure phase, and all diffraction peaks can be
indexed easily as the anatase TiO2, which is in good agreement with the standard card no. 21-1272 (lattice constants of a = 3.785, b = 3.785 and c = 9.514). No peaks from impurities were observed, indicating that titanate transforms completely into the anatase phase of TiO2. The strong diffraction peaks indicate that the product has higher crystallinity. Fig. 5 shows SEM, TEM and HRTEM images of the TiO2:Eu 3+ spindle-shaped nanoparticles. From the SEM image (Fig. 5a), it can be seen that the sample is composed of uniform spindle-shaped structure. Fig. 5b shows typical TEM image of the TiO2:Eu 3+ spindle-shaped nanoparticles with diameters of 50–200 nm and lengths of 200–1000 nm, respectively. Further analysis of the crystalline structure of the spindle-shaped nanoparticles with HRTEM is depicted in Fig. 5c. The visible well-crystallized structure with lattice fringes about 0.37 nm was observed, which agreed with the (101) plane anatase phase of the typical anatase TiO2 nanoparticles [32]. These results further confirm the presence of highly crystalline anatase TiO2:Eu 3+ spindle-shaped nanoparticles, which agreed well with the XRD results. To confirm their composition, energy dispersive X-ray (EDX) spectrum was further used to investigate the TiO2:Eu 3+ spindle-shaped nanoparticles (Fig. 5d). It can be seen from Fig. 5d that the final product is mainly composed of the three elements (Ti, Eu, and O). The extra peaks in the spectrum come from the Pt in coating and Si in substrate for SEM measurement. The results effectively support the XRD results of spindle-shaped nanoparticle sample (Fig. 4).
3.3. Formation mechanism of the TiO2:Eu 3+ spindle-shaped nanoparticles Because of the topotactic transformation among the anatase structures, sodium titanate and titanate, there will be a definite relationship between the crystal-axis orientations of the three structures. To investigate the mechanism for the transformation, a sketch map was carried on the intermediate products in the transformation
Fig. 5. (a) SEM and (b) TEM images of the TiO2:Eu3+ spindle‐shaped nanoparticles. (c) HRTEM image of the TiO2:Eu3+ spindle‐shaped nanoparticles. (d) EDX spectrum of the TiO2: Eu3+ spindle‐shaped nanoparticles.
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the titanate and anatase TiO2 have common structural features: both crystal lattices consist of the octahedra sharing four edges and the zigzag ribbons [33,34]. In the reaction with the deionized water, the titanate nanobelts dehydrated and the large structural units such as zigzag ribbons remain relatively unchanged, rearranging to form anatase lattice [33]. Consequently, the spindle-shaped nanoparticles of the anatase crystals obtained from the titanate nanobelts showing no morphological resemblance to the belts (Fig. 5). 3.4. Luminescence properties
Scheme 1. The phase transitions between anatase, sodium- and titanate by the twostep hydrothermal process.
process in detail, as shown in Scheme 1. The phase transitions in Scheme 1 reveal that both phases between anatase TiO2 and titanate can be converted into each other by the two-step reactions at appropriate reaction conditions. Scheme 2 shows the schematic drawing and typical TEM images of the oriented growth, overlap for the formation of nanobelt products, and spindle-shaped nanoparticles. In the formation process of nanobelts, there is a general agreement that the reaction proceeds through four stages: (1) slow dissolution of raw materials of TiO2 powder, (2) the nanosheets formed, accompanied by epitaxial growth of sodium titanate nanosheets along the axis and formed the smaller nanobelts, (3) the smaller nanobelts overlap each other, (4) exchanged of sodium ions by protons during washing and separation of nanobelts. In this case, if the crystallization rate was high enough, the growing of the nanosheets would go along the axis after exceeding a particular value. The nanosheets became too rigid to bend, and overlapped with each other, resulting in the formation of nanobelts. The treatment with dilute hydrochloric acid solution is actually an ion-exchange progress from the sodium titanate to titanate. The two-step hydrothermal process clearly transformed the titanate nanobelts to spindle-shaped nanoparticles. It is noted that
The PL intensity increases with increasing Eu 3+ ions concentration when the doping concentrations are 0.5 mol%, 1 mol%, 2 mol%, 3 mol%, 4 mol%, and the intensity reaches the maximum when the concentration of Eu 3+ ions is 2.0 mol%, then decreases with addition of Eu 3+ ions. This is the concentration quench effect induced by the cross-relaxation. Therefore, the best doping concentration is 2.0 mol%. The excitation and emission spectra for the TiO2:Eu 3+ spindleshaped nanoparticles are shown in Figs. 6 and 7. The excitation spectrum consists of a strong broad band between 250 nm and 350 nm, and the characteristic excitation lines of Eu 3+ within its 4f 6 configuration from 350 to 550 nm (Fig. 6). In general, the strong broad band between 250 nm and 350 nm is attributed to the charge transfer band (CTB) between the O 2 − and Eu 3+ ions [35,36]. The excitation lines peaking at 365, 384, 393, 419, 468, 525 and 528 nm assign to 7 F0– 5D4, 7F0– 5G2, 7F0– 5L6, 7F0– 5D3, 7F0– 5D2, 7F1– 5D1 and 7F0– 5D1 transitions, respectively. Excitation into the strongest 7F0– 5L6 transition of Eu 3+ at 393 nm yields the emission spectrum of the sample, which consists of the emission lines associated with the Eu 3+ transitions from the excited 5D0, 1 levels to the 7FJ (J = 0,1,2,3,4) levels, as shown in Fig. 7. There is no notable shift in the positions of the emission peaks compared to other Eu 3+ doped systems due to the shielding effect of 5s 25p 6 electrons [13]. It is well known that the relative intensity of the 5D0– 7F1 and 5D0– 7F2 transitions is determined by the symmetry of the crystal sites of the Eu 3+ ions. If Eu 3+ ion has a site with inversion symmetry, the 5D0– 7F1 transition dominates; while if Eu 3+ ion holds a site without inversion symmetry, the
Scheme 2. Schematic illustration for the formation process of the TiO2:Eu3+ spindle-shaped nanoparticles.
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Furthermore, this synthetic methodology appears to provide a gateway into the synthesis of one-dimensional nanomaterials.
Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 21171066), the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources, No: OSR05, 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 supported by the Graduate Innovation Fund of Jilin University.
References
Fig. 6. Excitation spectrum of the TiO2:Eu3+ spindle-shaped nanoparticles.
5
D0– 7F2 transition predominates [37]. In the case of the TiO2:Eu 3+ spindle-shaped nanoparticles, the most prominent red emission originates from the 5D0– 7F2 transitions, which are induced by the lack of inversion symmetry at the Eu 3+ ion sites and depend strongly on the site symmetry in a host crystal.
4. Conclusions Highly uniform TiO2:Eu 3+ spindle-shaped nanoparticles were synthesized by two-step hydrothermal processes. The width of the nanobelt precursor is about 50–200 nm and the lengths are about several micrometers. The diameters of the TiO2:Eu 3+ spindleshaped nanoparticles are about 50–200 nm and the lengths about 200–1000 nm. The phase transformation among the anatase structure, sodium titanate and titanate and the possible formation mechanism were also investigated in detail. The final products of the TiO2: Eu 3+ spindle-shaped nanoparticles show strong red emission intensity under ultraviolet excitation, which is due to the lack of inversion symmetry at the Eu3+ ions site and depends strongly on the site symmetry in a host crystal. It can be anticipated that the as-prepared product is a potential candidate as the novel semiconductor luminescent materials.
Fig. 7. Emission spectrum of the TiO2:Eu3+ spindle-shaped nanoparticles.
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