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Uniform hollow TiO2:Sm3 + spheres: Solvothermal synthesis and luminescence properties
Powder Technology 239 (2013) 403–408
Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Uniform hollow TiO2:Sm 3 + spheres: Solvothermal synthesis and luminescence properties Hongguang Zhang a, Ye Sheng a, Yanhua Song a, Hongbo Li a, Jing Huang a, Keyan Zheng a, Qisheng Huo b, Xuechun Xu c, 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 Science, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 10 October 2012 Received in revised form 7 January 2013 Accepted 6 February 2013 Available online 14 February 2013 Keywords: TiO2:Sm3 + Hollow microspheres Luminescence
a b s t r a c t Sm-doped hollow titania microspheres were fabricated by a simple solvothermal method using carbon spheres as hard template. The samples were characterized by X-ray diffraction (XRD), fourier transform infrared spectrum (FT-IR), scanning electron microscope (SEM), transmission electron microscope (TEM) and photoluminescence spectrum. In TEM images, hollow spheres with uniform morphology can be clearly observed. XRD results confirm that the hollow TiO2 spheres are anatase phase. The photoluminescence (PL) spectra of Sm-doped hollow TiO2 microspheres are dominated by red-emission around 612 nm due to intra-atomic 4f → 4f (4G5/2 → 6H7/2) transition of Sm 3+. The effect of doping concentrations of Sm 3+ on the luminescence was investigated. The results show that the luminescence of TiO2:Sm 3+ hollow microspheres has the strongest emission intensity when the ratio of Sm3+ is 2 mol%. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction The development of nanocrystals with controllable size and morphology has attracted special interest, not only for their fundamental scientific interest, but also for their technological applications. Among all these nanocrystals, hollow colloidal particles have fascinating advantages, such as their low effective density, high specific surface area, and encapsulation ability. Based on these virtues, they have potential applications in various fields such as photonic devices [1], confined catalysis [2], biotechnology [3], electrochemical cells [4,5], and drug delivery [6]. For the preparation of hollow sphere materials, a variety of methods have been designed, including templating method [7], sol–gel reaction [8] and hydrothermal technology [9] etc. Among the reported synthesis techniques, the template-tailored route is a low-cost and high-yield approach and can realize the large-scale synthesis. Titania (TiO2) is suggested to be a promising host lattice for the luminescence of various optically active lanthanide ions because of its low cost, high transparency in the visible-light region, and good thermal, chemical, and mechanical properties [10–12]. Thereby, the various shapes of TiO2, such as nanorods [13], nanotubes [14], nanowires [15], nanorings [16] and nanoplates [17], have been synthesized via different methods. For example, P. Haro-González et al. [14] have prepared Eu 3 + doped TiO2 nanotubes via a hydrothermal method; Xuan Feng et al. [18] have reported the fabrication of TiO2 hollow spheres ⁎ Corresponding author. Tel./fax: +86 431 85155275. E-mail address: [email protected] (H. Zou).
doped with europium by using carbon spheres as template and subsequently removing the template by calcinating in air. Although the above methods have provided effective routes to prepare different morphologies of TiO2 doped with the rare earth ion. However, to the best of our knowledge, there have been few reports on the synthesis of uniform, well-dispersed submicron-scaled Sm-doped TiO2 hollow spheres and their corresponding luminescence. In this paper, we report the synthesis of monodisperse Sm-doped TiO2 hollow spheres by using carbon spheres as hard template, followed by a subsequent calcination process. In addition, the optical properties of the Sm-doped TiO2 hollow spheres are investigated in detail. It is important that the carbon spheres are green template and the hollow structure will achieve a reduction in the amount of the expensive rare earth raw materials. Additionally, because of the low density of TiO2 hollow spherical materials, the phosphor can be evenly dispersed, and give high packing densities of the coating. It is anticipated that these TiO2:Sm 3+ hollow spheres with intense PL could be applied in various photoelectric devices. 2. Experimental section 2.1. Materials Sm(NO3)3 aqueous solution was obtained by dissolving Sm2O3(99.99%) in dilute HNO3 solution under heating with ceaseless agitation. All other chemicals were analytical-grade reagents and were purchased from the Beijing Chemical Corporation and used
0032-5910/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.02.010
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without further purification. Water was purified with a Milli-Q system from Millipore (Bedford, MA, USA), and the ultrapure water's conductivity is about 0.054 μs/cm at 25 °C, also expressed as resistivity of 18.3 MΩ-cm. 2.2. Synthesis 2.2.1. Preparation of carbon template microspheres Monodisperse carbon spheres were synthesized by a facile hydrothermal method, as described in Li et al. [19]. In the preparation procedure, glucose (6.34 g) was dissolved in 64 ml of ultrapure water to form a clear solution. The solution was then sealed in a 100 ml teflon-lined stainless steel autoclave and maintained at 180 °C for 5.5 h. After the autoclave was cooled to room temperature naturally, the afforded dark-brown carbon spheres were collected by centrifugation subsequently washed with ultrapure water and ethanol for six times. Then the carbon spheres were oven-dried at 80 °C for 2 h. 2.2.2. Synthesis of TiO2:Sm 3+ hollow spheres In a typical synthesis of TiO2:Sm 3+ hollow spheres [20], first, the as-prepared carbon spheres template (0.1 g) was dispersed in 30 ml ethanol by ultrasonification, and then 1.2 g of polyvinyl pyrrolidone (Mw = 30,000) (PVP) and 1.5 ml of titanium tetrabutoxide (TBOT) were dissolved in 20 ml of ethanol and the solution was dropwise added to the above homogeneous mixture, finally, varying concentrations of samarium (1.0, 1.5, 2.0, 2.5 and 3.0 mol% compared to the concentration of titania) was rapidly added into the above mixture solution. The final solution was transferred to a 100 ml teflon-lined stainless steel autoclave and maintained at 125 °C for 72 h. Afterward, the reactor was cooled to room temperature naturally. The coated particles were centrifuged and redispersed into ethanol several times. Then they were collected and dried. The carbon cores were removed by calcinating in a furnace at 500 °C for 2 h in air. The final products obtained were white crystalline powders. 2.3. Characterization The crystalline structures of the samples were evaluated by X-ray diffraction (XRD) analyses, carried out on a XRD-6000 X-Ray diffractometer from Shimadzu with Cu Kα radiation (λ = 0.15405 nm). Fourier transform infrared (FT-IR) spectra were measured with the Nicolette 5PC FT-IR spectrophotometer using the KBr pellet technique. The size and morphology of the microstructures were analyzed using a field-emission scanning electron microscope (FESEM) with a Hitachi S-4800 FESEM. Transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) patterns were obtained by a FEI Tecnai G 2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. Photoluminescence (PL) excitation and emission spectra were recorded with a Jobin Yvon FluoroMax-4 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at room temperature.
Fig. 1. XRD patterns of Sm-doped TiO2 hollow spheres (a) before calcination, (b) after calcination at 500 °C and the standard data for anatase TiO2 (JCPDS 21-1272).
samples is below the detection limit of the XRD instrument, or the samarium species have been dispersed well on the surface of TiO2 hollow spheres. Fig. 1 shows that the calcination at 500 °C not only removed the template, but also made the formation of anatase phase. The average size of the titania nanocrystallites is approximately 12 nm, which was calculated from the full-width at half-maximum of the (101) reflection using the Scherrer equation: D = kλCu / βc cosθ, where k = 0.89, λCu = 0.15406 nm, βc is the full-width at half-maximum of the peak in radians corrected from instrumental broadening and θ is the peak angle. Fig. 2 shows the XRD patterns of the precursor heated in atmosphere at different temperatures for 2 h. It can be seen that there is no new peak that appeared when the calcination temperature is raised to 600 °C or 700 °C, but the widths of the diffraction peaks become smaller compared to those at 500 °C, which shows that the sizes of the titania nanocrystallites turn bigger at 600 °C and 700 °C. In addition, when the temperature reaches 800 °C, diffraction peaks of rutile begin to appear. However, when the calcination temperature is up to 900 °C, the anatase phase disappears and the rutile phase becomes dominant. 3.2. FT-IR spectra FT-IR spectra are used to identify the functional groups on the surfaces of carbon spheres, core–shell structures and TiO2:Sm 3+ hollow
3. Results and discussion 3.1. XRD analysis The phase structures of the as-synthesized samples before and after calcination were investigated by XRD method and the results are shown in Fig. 1. It can be seen that no obvious diffraction peak appears for the precursor (Fig. 1a), indicating that it is amorphous before calcination. After calcination at 500 °C for 2 h, all the diffraction peaks of the product (Fig. 1b) can be directly indexed to anatase-phase TiO2 (JCPDS No. 21-1172). No crystalline phase attributed to samarium oxide can be found. This can be ascribed to the reasons that the Sm content of our
Fig. 2. XRD patterns of TiO2: Sm3+ hollow spheres calcined in different temperatures: (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C, and (e) 900 °C, respectively. Colored sticks correspond to the diffraction peaks for anatase TiO2 (black), and rutile TiO2 (blue), corresponding to JCPDS file nos. 21-1272, and 65-0190, respectively.
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TiO2 [22]. The peaks at 3423 and 1635 cm −1 are the stretching vibration and the bending vibration of water [23], respectively. The peak at 1384 cm −1 could be attributed to symmetric stretching vibrations of COO − groups [19]. Besides, most of the characteristic peaks from the carbon spheres have disappeared in Fig. 3b, which reflected that the precursor has been coated on the surface of the carbon spheres completely. The typical spectrum of TiO2: Sm 3+ hollow spheres (Fig. 3c) is similar to that of the core–shell composites. However, after calcination at 500 °C, the intensities of peaks at 3423 and 1635 cm −1 decrease significantly, indicating that the physically absorbed or chemically absorbed water in the interlayer is released. What's more, it can be seen that the absorption peaks from the carbon spheres were almost absent, indicating that the carbon cores were completely removed. 3.3. SEM and TEM analysis Fig. 3. FT-IR spectra of the samples obtained by solvothermal treatment. (a) Carbon spheres, (b) core–shell structures, (c) TiO2:Sm3+ hollow spheres after calcination at 500 °C.
spheres. The spectrum of the carbon spheres sample is shown in Fig. 3a. The bands at 1703 cm −1 and 1620 cm −1 are assigned to C_O and C_C double bonds stretching modes [19], respectively. The peak at 2927 cm −1 is assigned to stretching vibrations of C\H bonds, and the bands at 1000–1400 cm −1 wavenumber ranges are due to C\OH stretching and OH bending vibrations [21]. Fig. 3b shows the spectrum of the core–shell structures. The intensity of the absorption band in the region of 400–800 cm −1 can be assigned to Ti\O stretching and Ti\O\Ti bridging stretching modes of the
SEM image of the carbon spheres sample is shown in Fig. 4a. The carbon microspheres have a diameter ranging from 100 to 160 nm and the overall morphology is uniform. The diameter of carbon microspheres can be tuned by altering the reaction temperature, reaction time and concentration of starting materials [19]. Fig. 4b shows the SEM image of the core–shell structures. It can be seen that the precursor particles still keep the spherical morphology of the as-prepared carbon spheres except for a slightly larger particle size, which should be clearly due to the coated TiO2 shell (this conclusion can be drawn from the FT-IR spectra). Additionally, it reveals that the particle surface is relatively rough and apparently built from the aggregation of anatase titania nanocrystals. The SEM image in Fig. 4c shows the TiO2:Sm3+ hollow
Fig. 4. SEM images of (a) carbon spheres, (b) core–shell structures, (c) TiO2:Sm3+ hollow spheres after calcination at 500 °C.
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Fig. 5. TEM images of (a) core–shell structures, (b) TiO2:Sm3+ hollow spheres after calcination at 500 °C, (c) HRTEM image of TiO2:Sm3+ hollow spheres after calcination at 500 °C (inset: HRTEM image at higher magnification), (d) SAED pattern after calcination at 500 °C.
spheres generated by calcinating the C@TiO2:Sm 3+ composite particles. From the SEM image, it can be confirmed that the sample consists of relatively uniform microspheres with a diameter of 75–100 nm, suggesting that the shape and size of the final hollow phosphors are related to the carbon spheres template. The shrinkage of the particle size should be due to dehydration of the cross-linked structures on the carbon spheres and the further densification of the loose TiO2 precursor layer converting into closely compact oxides. To provide further insight into the TiO2:Sm3+ hollow microstructures, a TEM investigation was also performed. The high contrast of the shell and the core demonstrated in Fig. 5a clearly denotes that the core–shell structures have been formed. As expected, these spheres are well monodisperse and nearly uniform in dimension with particle diameters of 120–180 nm, which is consistent with the result shown in the SEM image (Fig. 4b). Fig. 5b shows a typical TEM image of TiO2: Sm 3+ hollow spheres, from which we can see the strong contrast between the dark edge and bright center, which indicates that the Sm-doped hollow structure of titania spheres has been formed. The uniform TiO2:Sm3+ hollow spheres have outer diameters of 75–100 nm and inner diameters of 55–75 nm. It also reveals that the utilization of carbon spheres as the template has resulted in the formation of spherical shells with a relatively dense arrangement of thin titania nanolayers. Fig. 5c presents a typical HRTEM lattice image of the shells of TiO2:Sm 3+ hollow spheres. The magnified HRTEM image (inset in Fig. 5c) shows clear lattice fringes, and the interplanar distance calculated from the lattice fringes of the HRTEM is 0.353 nm, which matches well with the crystallographic spacings of the (101) crystal plane of anatase phase. This confirms the single anatase phase structure of TiO2:Sm3+ hollow spheres. The typical SAED pattern (Fig. 5d) indicates that the shells
consist of polycrystalline nature of anatase phase of TiO2:Sm 3+ hollow spheres. It is further confirmed that the shell is indeed made up of highly crystalline anatase TiO2 nanoparticles.
Fig. 6. Schematic illustration of the formation process of TiO2:Sm3+ hollow spheres.
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Fig. 7. Photoluminescence excitation (a) and emission (b) spectra of TiO2 hollow spheres doped with 2 mol% of samarium.
3.4. Possible formation mechanism of TiO2:Sm 3+ hollow microspheres Fig. 6 shows the possible formation mechanism of the evolution from glucose to carbon spheres and the final hollow TiO2:Sm 3+ spheres. The formation of hollow TiO2:Sm 3+ spheres may experience three stages. First, hydrothermal carbonization of glucose solution synthesized of monodisperse carbon sphere template. The surface of the carbon spheres is hydrophilic and functionalized with OH and C_O groups, which provide an available reactive site for further titania deposition. Then, the hydrolyzed Sm-doped amorphous TiO2 species could homogeneously coated over the carbon spheres template via the catalytic self-condensation between the hydroxyl groups among titania species. As shown in Fig. 4b, after the solvothermal treatment without any other precipitating agent introduced, TBOT hydrolyzed and deposited uniformly on the surface of carbon spheres, formed the C@TiO2:Sm 3+ core–shell structures. In addition, PVP was utilized as dispersing agent. Finally, Sm-doped hollow titania spheres were obtained when carbon spheres were removed by calcinating at 500 °C for 2 h in air. 3.5. Photoluminescence properties The excitation spectrum of TiO2:Sm 3+ hollow spheres after calcination at 500 °C is shown in Fig. 7a, which is obtained by monitoring the emission of the 4f → 4f ( 4G5/2 → 6H7/2) transition of Sm 3+ at 612 nm. It can be seen that the excitation spectrum consists of a strong and broad band centered around 362 nm, which corresponds
to the anatase titania host absorption, confirming the effective energy transfer from TiO2 host to Sm 3+ ions [24]. The remaining relative weak excitation bands at about 475 nm can be attributed to the direct excitation of Sm 3+ [25]. As seen in Fig. 7a, the intensity of the transition at 362 nm is found to be the highest in all the spectra. Fig. 7b shows the emission spectrum of TiO2:Sm 3+ hollow spheres in the wavelength range of 550–750 nm under 362 nm excitation. There are four main sharp emission peaks at near 580, 612, 660 and 726 nm, among which the intensity of the 612 nm peak is the highest. It is concluded that the emissions are caused by the f–f forbidden transitions of the 4f electrons of Sm3+, corresponding to 4G5/2 → 6H5/2 (580 nm), 6H7/2 (612 nm), 6H9/2 (660 nm) and 6H11/2 (726 nm), respectively. The fine structure can be explained by taking into account that the 6HJ levels are split into J + 1/2 sublevels in the crystal field [26].
3.6. Concentration quenching of Sm 3+ in TiO2:Sm 3+ Fig. 8 shows the emission intensity of TiO2:Sm 3+ hollow spheres as a function of varying concentrations of samarium. The integrated intensity of the strongest emission peak at 612 nm in PL spectrum is used as the standard. As shown in Fig. 8, with the increasing of the Sm 3+ concentration from 1 to 3 mol%, the PL intensity of the 4 G5/2 → 6H7/2 transition increases first, reaching a maximum value at the concentration of 2 mol%, and then decreases with the increasing of Sm 3+ content due to the concentration quenching effect [27]. Thus, the optimum concentration of Sm 3+ is 2 mol% in TiO2:Sm 3+ hollow spheres.
4. Conclusions
Fig. 8. Emission intensity of TiO2:Sm3+ hollow spheres as a function of Sm3+ concentration.
In summary, Sm-doped titania hollow spheres were prepared through a simple and economical method using carbon spheres as template. The size of Sm-doped titania hollow spheres and thickness of the shell are very uniform. The inner diameter and the shell thickness of hollow titania spheres is 55–75 and 10–14 nm, respectively. The XRD result shows that the obtained Sm-doped titania hollow spheres sample is anatase phase after calcination at 500 °C for 2 h. Under the excitation of UV light, the as-synthesized Sm-doped titania hollow spheres exhibit bright red and orange emission, and it is found that the PL intensity of Sm-doped titania hollow spheres phosphor is relevant to the Sm ion concentration and the optimal Sm-doped concentration is 2 mol%. The uniform size, submicron scale, and luminescent property make the Sm-doped titania hollow spheres be a potential phosphor for lighting, displays and so on.
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Acknowledgments Financial support of this research is from the National Natural Science Foundation of China (Grant No. 21171066), the China Postdoctoral Science foundation (Grant No. 2012M510869), the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2013-27), and the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources, No. OSR-05. References [1] X.L. Xu, S.A. Asher, Synthesis and utilization of monodisperse hollow polymeric particles in photonic crystals, Journal of the American Chemical Society 126 (25) (Jun 30 2004) 7940–7945. [2] L. Shang, B.J. Li, W.J. Dong, B.Y. Chen, C.R. Li, W.H. Tang, et al., Heteronanostructure of Ag particle on titanate nanowire membrane with enhanced photocatalytic properties and bactericidal activities, Journal of Hazardous Materials 178 (1–3) (Jun 15 2010) 1109–1114. [3] H.X. Zhao, W.J. Dong, Y.Y. Zheng, A.P. Liu, J.M. Yao, C.R. Li, et al., The structural and biological properties of hydroxyapatite-modified titanate nanowire scaffolds, Biomaterials 32 (25) (Sep 2011) 5837–5846. [4] W.J. Dong, B.J. Li, Y. Li, X.B. Wang, L.N. An, C.R. Li, et al., General approach to well-defined perovskite MTiO3 (M = Ba, Sr, Ca, and Mg) nanostructures, Journal of Physical Chemistry C 115 (10) (Mar 17 2011) 3918–3925. [5] X.W. Lou, L.A. Archer, A general route to nonspherical anatase TiO2 hollow colloids and magnetic multifunctional particles, Advanced Materials 20 (10) (May 19 2008) 1853–1858. [6] W.R. Zhao, H.R. Chen, Y.S. Li, L. Li, M.D. Lang, J.L. Shi, Uniform rattle-type hollow magnetic mesoporous spheres as drug delivery carriers and their sustained-release property, Advanced Functional Materials 18 (18) (Sep 23 2008) 2780–2788. [7] J.G. Yu, W. Liu, H.G. Yu, A one-pot approach to hierarchically nanoporous titania hollow microspheres with high photocatalytic activity, Crystal Growth and Design 8 (3) (Mar 2008) 930–934. [8] W.S. Yang, Z.G. Teng, Y.D. Han, J. Li, F. Yan, Preparation of hollow mesoporous silica spheres by a sol–gel/emulsion approach, Microporous and Mesoporous Materials 127 (1–2) (Jan 2010) 67–72. [9] Qian Zhang, W.L., Shouxin Liu, Controlled fabrication of nanosized TiO2 hollow sphere particles via acid catalytic hydrolysis/hydrothermal treatment, Powder Technology 212 (2011) 145–150. [10] A. Conde-Gallardo, M. Garcia-Rocha, I. Hernandez-Calderon, R. Palomino-Merino, Photoluminescence properties of the Eu3+ activator ion in the TiO2 host matrix, Applied Physics Letters 78 (22) (May 28 2001) 3436–3438. [11] B. Julian, R. Corberan, E. Cordoncillo, P. Escribano, B. Viana, C. Sanchez, One-pot synthesis and optical properties of Eu3+-doped nanocrystalline TiO2 and ZrO2, Nanotechnology 16 (11) (Nov 2005) 2707–2713. [12] J.G. Li, X.H. Wang, K. Watanabe, T. Ishigaki, Phase structure and luminescence properties of Eu3+-doped TiO2 nanocrystals synthesized by Ar/O2− radio frequency
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Contents lists available at SciVerse ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Uniform hollow TiO2:Sm 3 + spheres: Solvothermal synthesis and luminescence properties Hongguang Zhang a, Ye Sheng a, Yanhua Song a, Hongbo Li a, Jing Huang a, Keyan Zheng a, Qisheng Huo b, Xuechun Xu c, 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 Science, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 10 October 2012 Received in revised form 7 January 2013 Accepted 6 February 2013 Available online 14 February 2013 Keywords: TiO2:Sm3 + Hollow microspheres Luminescence
a b s t r a c t Sm-doped hollow titania microspheres were fabricated by a simple solvothermal method using carbon spheres as hard template. The samples were characterized by X-ray diffraction (XRD), fourier transform infrared spectrum (FT-IR), scanning electron microscope (SEM), transmission electron microscope (TEM) and photoluminescence spectrum. In TEM images, hollow spheres with uniform morphology can be clearly observed. XRD results confirm that the hollow TiO2 spheres are anatase phase. The photoluminescence (PL) spectra of Sm-doped hollow TiO2 microspheres are dominated by red-emission around 612 nm due to intra-atomic 4f → 4f (4G5/2 → 6H7/2) transition of Sm 3+. The effect of doping concentrations of Sm 3+ on the luminescence was investigated. The results show that the luminescence of TiO2:Sm 3+ hollow microspheres has the strongest emission intensity when the ratio of Sm3+ is 2 mol%. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction The development of nanocrystals with controllable size and morphology has attracted special interest, not only for their fundamental scientific interest, but also for their technological applications. Among all these nanocrystals, hollow colloidal particles have fascinating advantages, such as their low effective density, high specific surface area, and encapsulation ability. Based on these virtues, they have potential applications in various fields such as photonic devices [1], confined catalysis [2], biotechnology [3], electrochemical cells [4,5], and drug delivery [6]. For the preparation of hollow sphere materials, a variety of methods have been designed, including templating method [7], sol–gel reaction [8] and hydrothermal technology [9] etc. Among the reported synthesis techniques, the template-tailored route is a low-cost and high-yield approach and can realize the large-scale synthesis. Titania (TiO2) is suggested to be a promising host lattice for the luminescence of various optically active lanthanide ions because of its low cost, high transparency in the visible-light region, and good thermal, chemical, and mechanical properties [10–12]. Thereby, the various shapes of TiO2, such as nanorods [13], nanotubes [14], nanowires [15], nanorings [16] and nanoplates [17], have been synthesized via different methods. For example, P. Haro-González et al. [14] have prepared Eu 3 + doped TiO2 nanotubes via a hydrothermal method; Xuan Feng et al. [18] have reported the fabrication of TiO2 hollow spheres ⁎ Corresponding author. Tel./fax: +86 431 85155275. E-mail address: [email protected] (H. Zou).
doped with europium by using carbon spheres as template and subsequently removing the template by calcinating in air. Although the above methods have provided effective routes to prepare different morphologies of TiO2 doped with the rare earth ion. However, to the best of our knowledge, there have been few reports on the synthesis of uniform, well-dispersed submicron-scaled Sm-doped TiO2 hollow spheres and their corresponding luminescence. In this paper, we report the synthesis of monodisperse Sm-doped TiO2 hollow spheres by using carbon spheres as hard template, followed by a subsequent calcination process. In addition, the optical properties of the Sm-doped TiO2 hollow spheres are investigated in detail. It is important that the carbon spheres are green template and the hollow structure will achieve a reduction in the amount of the expensive rare earth raw materials. Additionally, because of the low density of TiO2 hollow spherical materials, the phosphor can be evenly dispersed, and give high packing densities of the coating. It is anticipated that these TiO2:Sm 3+ hollow spheres with intense PL could be applied in various photoelectric devices. 2. Experimental section 2.1. Materials Sm(NO3)3 aqueous solution was obtained by dissolving Sm2O3(99.99%) in dilute HNO3 solution under heating with ceaseless agitation. All other chemicals were analytical-grade reagents and were purchased from the Beijing Chemical Corporation and used
0032-5910/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.powtec.2013.02.010
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without further purification. Water was purified with a Milli-Q system from Millipore (Bedford, MA, USA), and the ultrapure water's conductivity is about 0.054 μs/cm at 25 °C, also expressed as resistivity of 18.3 MΩ-cm. 2.2. Synthesis 2.2.1. Preparation of carbon template microspheres Monodisperse carbon spheres were synthesized by a facile hydrothermal method, as described in Li et al. [19]. In the preparation procedure, glucose (6.34 g) was dissolved in 64 ml of ultrapure water to form a clear solution. The solution was then sealed in a 100 ml teflon-lined stainless steel autoclave and maintained at 180 °C for 5.5 h. After the autoclave was cooled to room temperature naturally, the afforded dark-brown carbon spheres were collected by centrifugation subsequently washed with ultrapure water and ethanol for six times. Then the carbon spheres were oven-dried at 80 °C for 2 h. 2.2.2. Synthesis of TiO2:Sm 3+ hollow spheres In a typical synthesis of TiO2:Sm 3+ hollow spheres [20], first, the as-prepared carbon spheres template (0.1 g) was dispersed in 30 ml ethanol by ultrasonification, and then 1.2 g of polyvinyl pyrrolidone (Mw = 30,000) (PVP) and 1.5 ml of titanium tetrabutoxide (TBOT) were dissolved in 20 ml of ethanol and the solution was dropwise added to the above homogeneous mixture, finally, varying concentrations of samarium (1.0, 1.5, 2.0, 2.5 and 3.0 mol% compared to the concentration of titania) was rapidly added into the above mixture solution. The final solution was transferred to a 100 ml teflon-lined stainless steel autoclave and maintained at 125 °C for 72 h. Afterward, the reactor was cooled to room temperature naturally. The coated particles were centrifuged and redispersed into ethanol several times. Then they were collected and dried. The carbon cores were removed by calcinating in a furnace at 500 °C for 2 h in air. The final products obtained were white crystalline powders. 2.3. Characterization The crystalline structures of the samples were evaluated by X-ray diffraction (XRD) analyses, carried out on a XRD-6000 X-Ray diffractometer from Shimadzu with Cu Kα radiation (λ = 0.15405 nm). Fourier transform infrared (FT-IR) spectra were measured with the Nicolette 5PC FT-IR spectrophotometer using the KBr pellet technique. The size and morphology of the microstructures were analyzed using a field-emission scanning electron microscope (FESEM) with a Hitachi S-4800 FESEM. Transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) patterns were obtained by a FEI Tecnai G 2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. Photoluminescence (PL) excitation and emission spectra were recorded with a Jobin Yvon FluoroMax-4 fluorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source. All the measurements were performed at room temperature.
Fig. 1. XRD patterns of Sm-doped TiO2 hollow spheres (a) before calcination, (b) after calcination at 500 °C and the standard data for anatase TiO2 (JCPDS 21-1272).
samples is below the detection limit of the XRD instrument, or the samarium species have been dispersed well on the surface of TiO2 hollow spheres. Fig. 1 shows that the calcination at 500 °C not only removed the template, but also made the formation of anatase phase. The average size of the titania nanocrystallites is approximately 12 nm, which was calculated from the full-width at half-maximum of the (101) reflection using the Scherrer equation: D = kλCu / βc cosθ, where k = 0.89, λCu = 0.15406 nm, βc is the full-width at half-maximum of the peak in radians corrected from instrumental broadening and θ is the peak angle. Fig. 2 shows the XRD patterns of the precursor heated in atmosphere at different temperatures for 2 h. It can be seen that there is no new peak that appeared when the calcination temperature is raised to 600 °C or 700 °C, but the widths of the diffraction peaks become smaller compared to those at 500 °C, which shows that the sizes of the titania nanocrystallites turn bigger at 600 °C and 700 °C. In addition, when the temperature reaches 800 °C, diffraction peaks of rutile begin to appear. However, when the calcination temperature is up to 900 °C, the anatase phase disappears and the rutile phase becomes dominant. 3.2. FT-IR spectra FT-IR spectra are used to identify the functional groups on the surfaces of carbon spheres, core–shell structures and TiO2:Sm 3+ hollow
3. Results and discussion 3.1. XRD analysis The phase structures of the as-synthesized samples before and after calcination were investigated by XRD method and the results are shown in Fig. 1. It can be seen that no obvious diffraction peak appears for the precursor (Fig. 1a), indicating that it is amorphous before calcination. After calcination at 500 °C for 2 h, all the diffraction peaks of the product (Fig. 1b) can be directly indexed to anatase-phase TiO2 (JCPDS No. 21-1172). No crystalline phase attributed to samarium oxide can be found. This can be ascribed to the reasons that the Sm content of our
Fig. 2. XRD patterns of TiO2: Sm3+ hollow spheres calcined in different temperatures: (a) 500 °C, (b) 600 °C, (c) 700 °C, (d) 800 °C, and (e) 900 °C, respectively. Colored sticks correspond to the diffraction peaks for anatase TiO2 (black), and rutile TiO2 (blue), corresponding to JCPDS file nos. 21-1272, and 65-0190, respectively.
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TiO2 [22]. The peaks at 3423 and 1635 cm −1 are the stretching vibration and the bending vibration of water [23], respectively. The peak at 1384 cm −1 could be attributed to symmetric stretching vibrations of COO − groups [19]. Besides, most of the characteristic peaks from the carbon spheres have disappeared in Fig. 3b, which reflected that the precursor has been coated on the surface of the carbon spheres completely. The typical spectrum of TiO2: Sm 3+ hollow spheres (Fig. 3c) is similar to that of the core–shell composites. However, after calcination at 500 °C, the intensities of peaks at 3423 and 1635 cm −1 decrease significantly, indicating that the physically absorbed or chemically absorbed water in the interlayer is released. What's more, it can be seen that the absorption peaks from the carbon spheres were almost absent, indicating that the carbon cores were completely removed. 3.3. SEM and TEM analysis Fig. 3. FT-IR spectra of the samples obtained by solvothermal treatment. (a) Carbon spheres, (b) core–shell structures, (c) TiO2:Sm3+ hollow spheres after calcination at 500 °C.
spheres. The spectrum of the carbon spheres sample is shown in Fig. 3a. The bands at 1703 cm −1 and 1620 cm −1 are assigned to C_O and C_C double bonds stretching modes [19], respectively. The peak at 2927 cm −1 is assigned to stretching vibrations of C\H bonds, and the bands at 1000–1400 cm −1 wavenumber ranges are due to C\OH stretching and OH bending vibrations [21]. Fig. 3b shows the spectrum of the core–shell structures. The intensity of the absorption band in the region of 400–800 cm −1 can be assigned to Ti\O stretching and Ti\O\Ti bridging stretching modes of the
SEM image of the carbon spheres sample is shown in Fig. 4a. The carbon microspheres have a diameter ranging from 100 to 160 nm and the overall morphology is uniform. The diameter of carbon microspheres can be tuned by altering the reaction temperature, reaction time and concentration of starting materials [19]. Fig. 4b shows the SEM image of the core–shell structures. It can be seen that the precursor particles still keep the spherical morphology of the as-prepared carbon spheres except for a slightly larger particle size, which should be clearly due to the coated TiO2 shell (this conclusion can be drawn from the FT-IR spectra). Additionally, it reveals that the particle surface is relatively rough and apparently built from the aggregation of anatase titania nanocrystals. The SEM image in Fig. 4c shows the TiO2:Sm3+ hollow
Fig. 4. SEM images of (a) carbon spheres, (b) core–shell structures, (c) TiO2:Sm3+ hollow spheres after calcination at 500 °C.
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Fig. 5. TEM images of (a) core–shell structures, (b) TiO2:Sm3+ hollow spheres after calcination at 500 °C, (c) HRTEM image of TiO2:Sm3+ hollow spheres after calcination at 500 °C (inset: HRTEM image at higher magnification), (d) SAED pattern after calcination at 500 °C.
spheres generated by calcinating the C@TiO2:Sm 3+ composite particles. From the SEM image, it can be confirmed that the sample consists of relatively uniform microspheres with a diameter of 75–100 nm, suggesting that the shape and size of the final hollow phosphors are related to the carbon spheres template. The shrinkage of the particle size should be due to dehydration of the cross-linked structures on the carbon spheres and the further densification of the loose TiO2 precursor layer converting into closely compact oxides. To provide further insight into the TiO2:Sm3+ hollow microstructures, a TEM investigation was also performed. The high contrast of the shell and the core demonstrated in Fig. 5a clearly denotes that the core–shell structures have been formed. As expected, these spheres are well monodisperse and nearly uniform in dimension with particle diameters of 120–180 nm, which is consistent with the result shown in the SEM image (Fig. 4b). Fig. 5b shows a typical TEM image of TiO2: Sm 3+ hollow spheres, from which we can see the strong contrast between the dark edge and bright center, which indicates that the Sm-doped hollow structure of titania spheres has been formed. The uniform TiO2:Sm3+ hollow spheres have outer diameters of 75–100 nm and inner diameters of 55–75 nm. It also reveals that the utilization of carbon spheres as the template has resulted in the formation of spherical shells with a relatively dense arrangement of thin titania nanolayers. Fig. 5c presents a typical HRTEM lattice image of the shells of TiO2:Sm 3+ hollow spheres. The magnified HRTEM image (inset in Fig. 5c) shows clear lattice fringes, and the interplanar distance calculated from the lattice fringes of the HRTEM is 0.353 nm, which matches well with the crystallographic spacings of the (101) crystal plane of anatase phase. This confirms the single anatase phase structure of TiO2:Sm3+ hollow spheres. The typical SAED pattern (Fig. 5d) indicates that the shells
consist of polycrystalline nature of anatase phase of TiO2:Sm 3+ hollow spheres. It is further confirmed that the shell is indeed made up of highly crystalline anatase TiO2 nanoparticles.
Fig. 6. Schematic illustration of the formation process of TiO2:Sm3+ hollow spheres.
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Fig. 7. Photoluminescence excitation (a) and emission (b) spectra of TiO2 hollow spheres doped with 2 mol% of samarium.
3.4. Possible formation mechanism of TiO2:Sm 3+ hollow microspheres Fig. 6 shows the possible formation mechanism of the evolution from glucose to carbon spheres and the final hollow TiO2:Sm 3+ spheres. The formation of hollow TiO2:Sm 3+ spheres may experience three stages. First, hydrothermal carbonization of glucose solution synthesized of monodisperse carbon sphere template. The surface of the carbon spheres is hydrophilic and functionalized with OH and C_O groups, which provide an available reactive site for further titania deposition. Then, the hydrolyzed Sm-doped amorphous TiO2 species could homogeneously coated over the carbon spheres template via the catalytic self-condensation between the hydroxyl groups among titania species. As shown in Fig. 4b, after the solvothermal treatment without any other precipitating agent introduced, TBOT hydrolyzed and deposited uniformly on the surface of carbon spheres, formed the C@TiO2:Sm 3+ core–shell structures. In addition, PVP was utilized as dispersing agent. Finally, Sm-doped hollow titania spheres were obtained when carbon spheres were removed by calcinating at 500 °C for 2 h in air. 3.5. Photoluminescence properties The excitation spectrum of TiO2:Sm 3+ hollow spheres after calcination at 500 °C is shown in Fig. 7a, which is obtained by monitoring the emission of the 4f → 4f ( 4G5/2 → 6H7/2) transition of Sm 3+ at 612 nm. It can be seen that the excitation spectrum consists of a strong and broad band centered around 362 nm, which corresponds
to the anatase titania host absorption, confirming the effective energy transfer from TiO2 host to Sm 3+ ions [24]. The remaining relative weak excitation bands at about 475 nm can be attributed to the direct excitation of Sm 3+ [25]. As seen in Fig. 7a, the intensity of the transition at 362 nm is found to be the highest in all the spectra. Fig. 7b shows the emission spectrum of TiO2:Sm 3+ hollow spheres in the wavelength range of 550–750 nm under 362 nm excitation. There are four main sharp emission peaks at near 580, 612, 660 and 726 nm, among which the intensity of the 612 nm peak is the highest. It is concluded that the emissions are caused by the f–f forbidden transitions of the 4f electrons of Sm3+, corresponding to 4G5/2 → 6H5/2 (580 nm), 6H7/2 (612 nm), 6H9/2 (660 nm) and 6H11/2 (726 nm), respectively. The fine structure can be explained by taking into account that the 6HJ levels are split into J + 1/2 sublevels in the crystal field [26].
3.6. Concentration quenching of Sm 3+ in TiO2:Sm 3+ Fig. 8 shows the emission intensity of TiO2:Sm 3+ hollow spheres as a function of varying concentrations of samarium. The integrated intensity of the strongest emission peak at 612 nm in PL spectrum is used as the standard. As shown in Fig. 8, with the increasing of the Sm 3+ concentration from 1 to 3 mol%, the PL intensity of the 4 G5/2 → 6H7/2 transition increases first, reaching a maximum value at the concentration of 2 mol%, and then decreases with the increasing of Sm 3+ content due to the concentration quenching effect [27]. Thus, the optimum concentration of Sm 3+ is 2 mol% in TiO2:Sm 3+ hollow spheres.
4. Conclusions
Fig. 8. Emission intensity of TiO2:Sm3+ hollow spheres as a function of Sm3+ concentration.
In summary, Sm-doped titania hollow spheres were prepared through a simple and economical method using carbon spheres as template. The size of Sm-doped titania hollow spheres and thickness of the shell are very uniform. The inner diameter and the shell thickness of hollow titania spheres is 55–75 and 10–14 nm, respectively. The XRD result shows that the obtained Sm-doped titania hollow spheres sample is anatase phase after calcination at 500 °C for 2 h. Under the excitation of UV light, the as-synthesized Sm-doped titania hollow spheres exhibit bright red and orange emission, and it is found that the PL intensity of Sm-doped titania hollow spheres phosphor is relevant to the Sm ion concentration and the optimal Sm-doped concentration is 2 mol%. The uniform size, submicron scale, and luminescent property make the Sm-doped titania hollow spheres be a potential phosphor for lighting, displays and so on.
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Acknowledgments Financial support of this research is from the National Natural Science Foundation of China (Grant No. 21171066), the China Postdoctoral Science foundation (Grant No. 2012M510869), the Opening Research Funds Projects of the State Key Laboratory of Inorganic Synthesis and Preparative Chemistry and College of Chemistry, Jilin University (2013-27), and the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources, No. OSR-05. References [1] X.L. Xu, S.A. Asher, Synthesis and utilization of monodisperse hollow polymeric particles in photonic crystals, Journal of the American Chemical Society 126 (25) (Jun 30 2004) 7940–7945. [2] L. Shang, B.J. Li, W.J. Dong, B.Y. Chen, C.R. Li, W.H. Tang, et al., Heteronanostructure of Ag particle on titanate nanowire membrane with enhanced photocatalytic properties and bactericidal activities, Journal of Hazardous Materials 178 (1–3) (Jun 15 2010) 1109–1114. [3] H.X. Zhao, W.J. Dong, Y.Y. Zheng, A.P. Liu, J.M. Yao, C.R. Li, et al., The structural and biological properties of hydroxyapatite-modified titanate nanowire scaffolds, Biomaterials 32 (25) (Sep 2011) 5837–5846. [4] W.J. Dong, B.J. Li, Y. Li, X.B. Wang, L.N. An, C.R. Li, et al., General approach to well-defined perovskite MTiO3 (M = Ba, Sr, Ca, and Mg) nanostructures, Journal of Physical Chemistry C 115 (10) (Mar 17 2011) 3918–3925. [5] X.W. Lou, L.A. Archer, A general route to nonspherical anatase TiO2 hollow colloids and magnetic multifunctional particles, Advanced Materials 20 (10) (May 19 2008) 1853–1858. [6] W.R. Zhao, H.R. Chen, Y.S. Li, L. Li, M.D. Lang, J.L. Shi, Uniform rattle-type hollow magnetic mesoporous spheres as drug delivery carriers and their sustained-release property, Advanced Functional Materials 18 (18) (Sep 23 2008) 2780–2788. [7] J.G. Yu, W. Liu, H.G. Yu, A one-pot approach to hierarchically nanoporous titania hollow microspheres with high photocatalytic activity, Crystal Growth and Design 8 (3) (Mar 2008) 930–934. [8] W.S. Yang, Z.G. Teng, Y.D. Han, J. Li, F. Yan, Preparation of hollow mesoporous silica spheres by a sol–gel/emulsion approach, Microporous and Mesoporous Materials 127 (1–2) (Jan 2010) 67–72. [9] Qian Zhang, W.L., Shouxin Liu, Controlled fabrication of nanosized TiO2 hollow sphere particles via acid catalytic hydrolysis/hydrothermal treatment, Powder Technology 212 (2011) 145–150. [10] A. Conde-Gallardo, M. Garcia-Rocha, I. Hernandez-Calderon, R. Palomino-Merino, Photoluminescence properties of the Eu3+ activator ion in the TiO2 host matrix, Applied Physics Letters 78 (22) (May 28 2001) 3436–3438. [11] B. Julian, R. Corberan, E. Cordoncillo, P. Escribano, B. Viana, C. Sanchez, One-pot synthesis and optical properties of Eu3+-doped nanocrystalline TiO2 and ZrO2, Nanotechnology 16 (11) (Nov 2005) 2707–2713. [12] J.G. Li, X.H. Wang, K. Watanabe, T. Ishigaki, Phase structure and luminescence properties of Eu3+-doped TiO2 nanocrystals synthesized by Ar/O2− radio frequency
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