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Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3+, Yb3+ with silica shell
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CERAMICS INTERNATIONAL
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Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell Yangke Cun, Zhengwen Yangn, Jun Li, Bo Shao, Jianzhi Yang, Yida Wang, Jianbei Qiu, Zhiguo Song College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China Received 25 April 2015; received in revised form 19 May 2015; accepted 26 May 2015
Abstract In this paper, a simple effective approach has been developed to enhance upconversion emission of three-dimensionally ordered macroporous materials, which is attributed to formation of silica shell around surface of three-dimensionally ordered macroporous materials. This approach involves the preparation of Bi2Ti2O7:Er3 þ , Yb3 þ three-dimensionally ordered macroporous materials and the formation of amorphous SiO2 shell on the macroporous surfaces of three-dimensionally ordered macroporous materials. The influence of silica shell on the upconversion emission of Er3 þ ions was investigated, and the enhanced upconversion emission was observed in the Bi2Ti2O7:Er3 þ , Yb3 þ three-dimensionally ordered macroporous materials owing to the suppression of surface quenching effect caused by the formation of silica shells. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: SiO2 shells; Three-dimensionally ordered macroporous materials; Upconversion emission; Bi2Ti2O7:Er3 þ , Yb3 þ
1. Introduction Rare earth ions doped three-dimensionally ordered macroporous (3DOM) materials consists of a nano-sized skeleton surrounding uniform close-packed macrospores interconnected through windows, which have attracted extensive attention during the past decades due to their wide potential applications in solid-state lasers, lighting and displays and biosensor and so on [1–8]. In particular, in contrast to other luminescent materials such as organic dyes and semiconductors, the rare earth ions doped 3DOM materials have narrow emission bandwidth, high photostability, low toxicity, good chemical stability and biocompatibility, which make them more suitable applications in the medicine, biological fluorescence imaging and detections. However, for practical applications of rare earth doped 3DOM materials, one main challenge is lower luminescence efficiency, which is caused by the following two reasons. One is that much structure defects occurred in a nanon
Corresponding author. E-mail address: [email protected] (Z. Yang).
sized skeleton of the 3DOM materials, leading to the surface quenching of luminescence. Another is that the small absorption cross-section from the rare earth ions limits the emission efficiency of the 3DOM materials. Based on the above discussion, two main approaches can be used to improve the emission efficiency of the rare earth doped 3DOM materials. One is enhancement of excitation efficiency realized by using surface plasmon absorption of noble metal nanostructures or energy transfer between rare earth ions [9–11]. Another effective convenient strategy is to grow a utilization shell around the 3DOM materials surface, resulting in the formation of so-called core–shell structure [12,13]. Previous investigations have demonstrated that amorphous SiO2 shell formation around nanoparticles could enhance their optical properties due to the suppression of surface quenching [14–16]. However, there have been no experiment reports on the luminescence enhancement of the 3DOM materials by coating amorphous SiO2. Cubic pyrochlore Bi2Ti2O7 with relatively high dielectric constant, low dielectric loss and a high refractive index (n=2.1 at 550 nm) is one of the important functional materials [17,18], which could be used as UC
http://dx.doi.org/10.1016/j.ceramint.2015.05.144 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
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luminescent host material due to a relatively low phonon energy [19]. In this paper, a simple approach for upconversion (UC) emission enhancement of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials has been developed, which is attributed to the suppression of surface quenching effect caused by the formation of silica shell around surface of the 3DOM materials.
2. Experimental The ordered template was self-assembled by 380 nm polystyrene (PS) microspheres according to vertical deposition process reported by our works [19,20]. The 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials were fabricated by a template assisted route. The Bi2Ti2O7:3 mol% Er3 þ , 3 mol% Yb3 þ precursor solution was prepared by using tetrabutyl titanate ( C16H36O4Ti), Bi(NO3)3.5H2O, Yb2O3 and Er2O3 as raw materials without further purification. The Yb(NO3)3 and Er(NO3)3 were prepared by dissolving Yb2O3 and Er2O3 in hot nitric acid, respectively. In the preparation of Bi2Ti2O7: 3 mol% Er3 þ , 3 mol% Yb3 þ sol, appropriate amounts of Yb(NO3)3 and Er(NO3)3 and C16H36O4Ti were dissolved in ethanol, respectively. The Bi (NO3)3.5 H2O was dissolved in a mixture of glycol and acetic acid. Then the above solutions were mixed and stirred for 2 h to form a homogeneous solution. The prepared precursor solutions were infiltrated into the voids of the ordered PS templates. The 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials were obtained after the removal of PS microspheres by heat treatment at 670 1C. The x mol/L SiO2 (x=0.1, 0.2, 0.3) precursor solution was prepared by dissolving different amounts of tetraethyl orthosilicate [Si(CH3CH2O)4, TEOS] in the glycol. The x mol/L SiO2 (x=0.1, 0.2, 0.3) precursor solution was added into the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials. The Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials with silica shell were prepared after sintering at 450 1C. For comparison, the Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials without infiltration of silica sol were second sintered at 450 1C. Pore structure features of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials with or without SiO2 shells were determined by a scanning electron microscope (SEM) after the samples were sputtered with a thin layer of gold. X-ray diffraction (XRD) patterns of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials were recorded via the X-ray diffraction (D8 ADVANCE). The UC emission spectra of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials were measured by a F-7000 (HITACHIU Company, Japan) Fluorescence spectrophotometer under a 980 nm infrared laser excitation. The transmittance spectra of the 3DOM were determined using a HITACHIU-4100 instrument. The microstructures of the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials were observed by the optical microscope. The transmission electron microscope (TEM) images of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials were taken on a JEOL 2100 transmission electron microscope.
determine their crystalline structures. Fig. 1 shows the XRD pattern of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials before and after the formation of the SiO2 shells. The broad pick range from 171 to 781 was attributed to the diffraction of glass substrate. All the 3DOM Bi2Ti2O7 samples exhibit X-ray diffraction peaks well in accordance with the corresponding standard card (No.32-0118) of cubic phase Bi2Ti2O7, indicating that the crystalline structure of Bi2Ti2O7 is not altered by doping Yb3 þ and Er3 þ and the formation of amorphous SiO2 shells. Two step processes were used to prepare the Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials with silica shells, which involve the preparation of the Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials and then immersion in a SiO2 sol to allow the formation of SiO2 shell on the pore surfaces of 3DOM materials. Fig. 2 shows the typical SEM images of the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials before and after the formation of the SiO2 shells. The Bi2Ti2O7: Er3 þ , Yb3 þ samples before the addition of SiO2 sol exhibit three-dimensionally ordered inter-connected macrospores structures. Inside each air pores there are dark holes corresponding to the air spheres in sub-layer. It can be seen from the Fig. 2 that the infiltration of the SiO2 sol has significant influence on the morphology of materials. The long-range ordered hexagonal arrangement was retained after addition of SiO2 sol with low concentration ranging from 0.1 to 0.2 M. When the concentration of SiO2 sol increased to the 0.3 M, the ordered structure of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material was destroyed, as shown in Fig. 2(d). This ordered structure is illustrated by the transmittance spectra of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials with and without the SiO2 shells formed by 0.1, 0.2 and 0.3 M SiO2 sol, as shown in Fig. 3. The transmittance spectrum of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material without the SiO2 shells showed a broad dip at around 510 nm, which is contributed to the Bragg diffraction of visible light from the ordered porous structure.
3. Results and discussion The XRD spectra of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials before and after the formation SiO2 shells are measured to
Fig. 1. XRD patterns of 3DOM Bi2Ti2O7:Er3 þ ,Yb3 þ materials without the SiO2 shells (a) and with the SiO2 shells formed by 0.1 M (b), 0.2 M (c), 0.3 M (d) SiO2 sol.
Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
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Fig. 2. SEM images of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials without SiO2 shells (a) and with SiO2 shells prepared by 0.1 M (b), 0.2 M (c), 0.3 M (d) SiO2 sol.
Fig. 3. The transmittance spectra of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials without SiO2 shells sintering at 450 1C (a) and with SiO2 shells formed by 0.1 M (b), 0.2 M (c) and 0.3 M (d) SiO2 sol. Transmittance spectrum of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials without sintering at 450 1C (e).
The transmittance spectra change of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material is also present in the Fig. 3 with the increasing of SiO2 sol concentration. It is noted that an obvious dip was observed when the SiO2 sol concentration is lower than 0.2 M. No obvious dip occurred when the SiO2 sol concentration is 0.3 M, which suggested that the ordered structure of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material was destroyed due to addition of high concentration SiO2 sol. The variety of ordered structures of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material was observed by the
optical microscope images, as shown in Fig. 4. The Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material with SiO2 sol concentration lower than 0.2 M exhibited the blue–green color attributed to the Bragg reflection of the periodic structures of 3DOM material, while no color was observed when the SiO2 sol concentration is 0.3 M. To further demonstrate the formation of core–shell structured 3DOM materials, TEM observations were performed. Fig. 5 shows the TEM micrographs of powder sample grinded from 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ filled with 0.3 M SiO2 sol. The core and shell domains of the 3DOM samples can be distinguished clearly due to their different electron penetrabilities. The cores are black and the shells have gray color, which suggested that the Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials with silica shells were prepared. Fig. 6 shows the UC luminescence spectra of the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ samples with or without SiO2 shells under the excitation of 980 nm. Lots of the UC emission bands from Er3 þ ions could be observed. All samples show three main peaks located at 525, 548 and 662 nm, which was attributed to the 2H11/ 4 4 4 4 4 2- I15/2 (525 nm), S3/2- I15/2 (548 nm), F9/2- I15/2 (662 nm) 3þ of Er , respectively. The UC emission mechanism was investigated based on the relation between excitation light power and the emission intensity [21,22]. As shown in Fig. 7(a), the three and two photons UC process are responsible for the green and red emission, respectively. Fig. 7(b) shows the possible mechanism of UC emission. In the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material, ground
Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
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Fig. 4. The optical microscope images of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials without SiO2 shells and with SiO2 shells (a) formed by 0.1 M (b), 0.2 M (c) and 0.3 M (d) SiO2 sol.(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 5. The TEM micrographs of powder sample grinded from 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ filled with 0.3 M SiO2 sol.
Fig. 6. The UC spectra of Bi2Ti2O7:Er3 þ ,Yb3 þ 3DOM materials without SiO2 shells and with SiO2 shells (a) formed by 0.1 M (b), 0.2 M (c) and 0.3 M (d) SiO2 sol. UC spectrum of Bi2Ti2O7:Er3 þ ,Yb3 þ 3DOM materials without sintering at 450 1C (e).(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
state Er3 þ was excited to the 4I11/2 state by the energy transfer from excited state Yb3 þ . Then the subsequent nonradiative relaxation populated to the 4F9/2 level, resulting in the red UC emission. The Er3 þ ions located at the 4F9/2 state was excited to 4 G11/2 state by the two steps energy transfer from excited state Yb3 þ . Then 4G11/2 state relaxed nonradiatively to the 2H11/2 and 4 S3/2 states, leading to the green UC emission. In addition, the 4F9/2
state population can be realized by nonradiative relaxation of the 2 H11/2 and 4S3/2 states, which generated the red UC emission. In contrast to the UC luminescence intensity of the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials without the SiO2 shells, it is obvious that the green and red UC emissions were significantly enhanced in the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ samples with the SiO2 shells. The 2.2, 3.0 and 5.5 enhancement factors were evidenced in the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ with the SiO2
Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
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Fig. 7. (a) Log–Log plots of pump power dependence of the UC emission of 3DOM Bi2Ti2O7:Er3 þ ,Yb3 þ coating with 0.3 M SiO2 sol, (b) the mechanism of UC emission.(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
shells formed by 0.1, 0.2 and 0.3 M SiO2 sol, respectively. There are two possible reasons for the enhancement of UC luminescence in the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials with the SiO2 shells. One is the second sintering at 450 1C in order to form the SiO2 shells around the surface of 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials. In order to investigate the influence of second sintering on UC emission, the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials without the SiO2 sol infiltration was sintered at 450 1C together with 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials with the SiO2 sol infiltration, and the corresponding UC emission spectrum is shown in Fig. 6. It is noted that no significant change was observed in the UC emission spectrum of 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials sintered at 450 1C without SiO2 sol infiltration, which indicated that the 450 1C sintering has no obvious influence on the UC emission of 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials. Another is suppression of surface quenching effect attributed to the formation of SiO2 shells. In the present work, selfassembly and sol–gel approach have been used to fabricate 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials. The main disadvantage of this method is the presence of a relatively large number of structural defects, which are inherent to the synthesis of the 3DOM materials. The UC emission intensity of the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials corresponds to a sum of optical emissions from doping Er3 þ ions at the surface and in the interior of the 3DOM materials. In contrast to the UC emission from interior Er3 þ ions, the UC emission from surface Er3 þ ions were quenched because of the excitation energy quenching caused by surface defects and impurities [23–25]. When the SiO2 shells were formed on the surface of the 3DOM materials, the surface quenching effect was suppressed, resulting in the UC emission enhancement of the 3DOM materials. 4. Conclusion We prepared the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials by the sol–gel method in combination with the PS template, then the surface of the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials were coated with the SiO2. The influence of SiO2 shells on the UC
emission properties of 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials were carefully studied. In contrast to the UC luminescence intensity of three dimensional ordered macroporous Bi2Ti2O7: Er3 þ , Yb3 þ without the SiO2 coating, the UC emission intensity of three dimensional ordered macroporous Bi2Ti2O7:Er3 þ , Yb3 þ with SiO2 shells is greatly enhanced due to an effective suppression of surface quenching effect caused by surface defects.
Acknowledgments This work was supported by Reserve Talents Project of Yunnan Province (2012HB068), Applied Basic Research Program of Yunnan Province (2014FB127) and Talent Youth Science Foundation of College of Materials Science and Technology, Kunming University of Science and Technology (20140205).
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Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
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Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell Yangke Cun, Zhengwen Yangn, Jun Li, Bo Shao, Jianzhi Yang, Yida Wang, Jianbei Qiu, Zhiguo Song College of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China Received 25 April 2015; received in revised form 19 May 2015; accepted 26 May 2015
Abstract In this paper, a simple effective approach has been developed to enhance upconversion emission of three-dimensionally ordered macroporous materials, which is attributed to formation of silica shell around surface of three-dimensionally ordered macroporous materials. This approach involves the preparation of Bi2Ti2O7:Er3 þ , Yb3 þ three-dimensionally ordered macroporous materials and the formation of amorphous SiO2 shell on the macroporous surfaces of three-dimensionally ordered macroporous materials. The influence of silica shell on the upconversion emission of Er3 þ ions was investigated, and the enhanced upconversion emission was observed in the Bi2Ti2O7:Er3 þ , Yb3 þ three-dimensionally ordered macroporous materials owing to the suppression of surface quenching effect caused by the formation of silica shells. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: SiO2 shells; Three-dimensionally ordered macroporous materials; Upconversion emission; Bi2Ti2O7:Er3 þ , Yb3 þ
1. Introduction Rare earth ions doped three-dimensionally ordered macroporous (3DOM) materials consists of a nano-sized skeleton surrounding uniform close-packed macrospores interconnected through windows, which have attracted extensive attention during the past decades due to their wide potential applications in solid-state lasers, lighting and displays and biosensor and so on [1–8]. In particular, in contrast to other luminescent materials such as organic dyes and semiconductors, the rare earth ions doped 3DOM materials have narrow emission bandwidth, high photostability, low toxicity, good chemical stability and biocompatibility, which make them more suitable applications in the medicine, biological fluorescence imaging and detections. However, for practical applications of rare earth doped 3DOM materials, one main challenge is lower luminescence efficiency, which is caused by the following two reasons. One is that much structure defects occurred in a nanon
Corresponding author. E-mail address: [email protected] (Z. Yang).
sized skeleton of the 3DOM materials, leading to the surface quenching of luminescence. Another is that the small absorption cross-section from the rare earth ions limits the emission efficiency of the 3DOM materials. Based on the above discussion, two main approaches can be used to improve the emission efficiency of the rare earth doped 3DOM materials. One is enhancement of excitation efficiency realized by using surface plasmon absorption of noble metal nanostructures or energy transfer between rare earth ions [9–11]. Another effective convenient strategy is to grow a utilization shell around the 3DOM materials surface, resulting in the formation of so-called core–shell structure [12,13]. Previous investigations have demonstrated that amorphous SiO2 shell formation around nanoparticles could enhance their optical properties due to the suppression of surface quenching [14–16]. However, there have been no experiment reports on the luminescence enhancement of the 3DOM materials by coating amorphous SiO2. Cubic pyrochlore Bi2Ti2O7 with relatively high dielectric constant, low dielectric loss and a high refractive index (n=2.1 at 550 nm) is one of the important functional materials [17,18], which could be used as UC
http://dx.doi.org/10.1016/j.ceramint.2015.05.144 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
Y. Cun et al. / Ceramics International ] (]]]]) ]]]–]]]
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luminescent host material due to a relatively low phonon energy [19]. In this paper, a simple approach for upconversion (UC) emission enhancement of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials has been developed, which is attributed to the suppression of surface quenching effect caused by the formation of silica shell around surface of the 3DOM materials.
2. Experimental The ordered template was self-assembled by 380 nm polystyrene (PS) microspheres according to vertical deposition process reported by our works [19,20]. The 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials were fabricated by a template assisted route. The Bi2Ti2O7:3 mol% Er3 þ , 3 mol% Yb3 þ precursor solution was prepared by using tetrabutyl titanate ( C16H36O4Ti), Bi(NO3)3.5H2O, Yb2O3 and Er2O3 as raw materials without further purification. The Yb(NO3)3 and Er(NO3)3 were prepared by dissolving Yb2O3 and Er2O3 in hot nitric acid, respectively. In the preparation of Bi2Ti2O7: 3 mol% Er3 þ , 3 mol% Yb3 þ sol, appropriate amounts of Yb(NO3)3 and Er(NO3)3 and C16H36O4Ti were dissolved in ethanol, respectively. The Bi (NO3)3.5 H2O was dissolved in a mixture of glycol and acetic acid. Then the above solutions were mixed and stirred for 2 h to form a homogeneous solution. The prepared precursor solutions were infiltrated into the voids of the ordered PS templates. The 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials were obtained after the removal of PS microspheres by heat treatment at 670 1C. The x mol/L SiO2 (x=0.1, 0.2, 0.3) precursor solution was prepared by dissolving different amounts of tetraethyl orthosilicate [Si(CH3CH2O)4, TEOS] in the glycol. The x mol/L SiO2 (x=0.1, 0.2, 0.3) precursor solution was added into the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials. The Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials with silica shell were prepared after sintering at 450 1C. For comparison, the Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials without infiltration of silica sol were second sintered at 450 1C. Pore structure features of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials with or without SiO2 shells were determined by a scanning electron microscope (SEM) after the samples were sputtered with a thin layer of gold. X-ray diffraction (XRD) patterns of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials were recorded via the X-ray diffraction (D8 ADVANCE). The UC emission spectra of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials were measured by a F-7000 (HITACHIU Company, Japan) Fluorescence spectrophotometer under a 980 nm infrared laser excitation. The transmittance spectra of the 3DOM were determined using a HITACHIU-4100 instrument. The microstructures of the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials were observed by the optical microscope. The transmission electron microscope (TEM) images of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials were taken on a JEOL 2100 transmission electron microscope.
determine their crystalline structures. Fig. 1 shows the XRD pattern of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials before and after the formation of the SiO2 shells. The broad pick range from 171 to 781 was attributed to the diffraction of glass substrate. All the 3DOM Bi2Ti2O7 samples exhibit X-ray diffraction peaks well in accordance with the corresponding standard card (No.32-0118) of cubic phase Bi2Ti2O7, indicating that the crystalline structure of Bi2Ti2O7 is not altered by doping Yb3 þ and Er3 þ and the formation of amorphous SiO2 shells. Two step processes were used to prepare the Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials with silica shells, which involve the preparation of the Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials and then immersion in a SiO2 sol to allow the formation of SiO2 shell on the pore surfaces of 3DOM materials. Fig. 2 shows the typical SEM images of the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials before and after the formation of the SiO2 shells. The Bi2Ti2O7: Er3 þ , Yb3 þ samples before the addition of SiO2 sol exhibit three-dimensionally ordered inter-connected macrospores structures. Inside each air pores there are dark holes corresponding to the air spheres in sub-layer. It can be seen from the Fig. 2 that the infiltration of the SiO2 sol has significant influence on the morphology of materials. The long-range ordered hexagonal arrangement was retained after addition of SiO2 sol with low concentration ranging from 0.1 to 0.2 M. When the concentration of SiO2 sol increased to the 0.3 M, the ordered structure of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material was destroyed, as shown in Fig. 2(d). This ordered structure is illustrated by the transmittance spectra of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials with and without the SiO2 shells formed by 0.1, 0.2 and 0.3 M SiO2 sol, as shown in Fig. 3. The transmittance spectrum of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material without the SiO2 shells showed a broad dip at around 510 nm, which is contributed to the Bragg diffraction of visible light from the ordered porous structure.
3. Results and discussion The XRD spectra of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials before and after the formation SiO2 shells are measured to
Fig. 1. XRD patterns of 3DOM Bi2Ti2O7:Er3 þ ,Yb3 þ materials without the SiO2 shells (a) and with the SiO2 shells formed by 0.1 M (b), 0.2 M (c), 0.3 M (d) SiO2 sol.
Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
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Fig. 2. SEM images of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials without SiO2 shells (a) and with SiO2 shells prepared by 0.1 M (b), 0.2 M (c), 0.3 M (d) SiO2 sol.
Fig. 3. The transmittance spectra of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials without SiO2 shells sintering at 450 1C (a) and with SiO2 shells formed by 0.1 M (b), 0.2 M (c) and 0.3 M (d) SiO2 sol. Transmittance spectrum of Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials without sintering at 450 1C (e).
The transmittance spectra change of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material is also present in the Fig. 3 with the increasing of SiO2 sol concentration. It is noted that an obvious dip was observed when the SiO2 sol concentration is lower than 0.2 M. No obvious dip occurred when the SiO2 sol concentration is 0.3 M, which suggested that the ordered structure of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material was destroyed due to addition of high concentration SiO2 sol. The variety of ordered structures of Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material was observed by the
optical microscope images, as shown in Fig. 4. The Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material with SiO2 sol concentration lower than 0.2 M exhibited the blue–green color attributed to the Bragg reflection of the periodic structures of 3DOM material, while no color was observed when the SiO2 sol concentration is 0.3 M. To further demonstrate the formation of core–shell structured 3DOM materials, TEM observations were performed. Fig. 5 shows the TEM micrographs of powder sample grinded from 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ filled with 0.3 M SiO2 sol. The core and shell domains of the 3DOM samples can be distinguished clearly due to their different electron penetrabilities. The cores are black and the shells have gray color, which suggested that the Bi2Ti2O7:Er3 þ , Yb3 þ 3DOM materials with silica shells were prepared. Fig. 6 shows the UC luminescence spectra of the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ samples with or without SiO2 shells under the excitation of 980 nm. Lots of the UC emission bands from Er3 þ ions could be observed. All samples show three main peaks located at 525, 548 and 662 nm, which was attributed to the 2H11/ 4 4 4 4 4 2- I15/2 (525 nm), S3/2- I15/2 (548 nm), F9/2- I15/2 (662 nm) 3þ of Er , respectively. The UC emission mechanism was investigated based on the relation between excitation light power and the emission intensity [21,22]. As shown in Fig. 7(a), the three and two photons UC process are responsible for the green and red emission, respectively. Fig. 7(b) shows the possible mechanism of UC emission. In the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM material, ground
Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
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Fig. 4. The optical microscope images of the Bi2Ti2O7: Er3 þ , Yb3 þ 3DOM materials without SiO2 shells and with SiO2 shells (a) formed by 0.1 M (b), 0.2 M (c) and 0.3 M (d) SiO2 sol.(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 5. The TEM micrographs of powder sample grinded from 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ filled with 0.3 M SiO2 sol.
Fig. 6. The UC spectra of Bi2Ti2O7:Er3 þ ,Yb3 þ 3DOM materials without SiO2 shells and with SiO2 shells (a) formed by 0.1 M (b), 0.2 M (c) and 0.3 M (d) SiO2 sol. UC spectrum of Bi2Ti2O7:Er3 þ ,Yb3 þ 3DOM materials without sintering at 450 1C (e).(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
state Er3 þ was excited to the 4I11/2 state by the energy transfer from excited state Yb3 þ . Then the subsequent nonradiative relaxation populated to the 4F9/2 level, resulting in the red UC emission. The Er3 þ ions located at the 4F9/2 state was excited to 4 G11/2 state by the two steps energy transfer from excited state Yb3 þ . Then 4G11/2 state relaxed nonradiatively to the 2H11/2 and 4 S3/2 states, leading to the green UC emission. In addition, the 4F9/2
state population can be realized by nonradiative relaxation of the 2 H11/2 and 4S3/2 states, which generated the red UC emission. In contrast to the UC luminescence intensity of the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials without the SiO2 shells, it is obvious that the green and red UC emissions were significantly enhanced in the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ samples with the SiO2 shells. The 2.2, 3.0 and 5.5 enhancement factors were evidenced in the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ with the SiO2
Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144
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Fig. 7. (a) Log–Log plots of pump power dependence of the UC emission of 3DOM Bi2Ti2O7:Er3 þ ,Yb3 þ coating with 0.3 M SiO2 sol, (b) the mechanism of UC emission.(For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
shells formed by 0.1, 0.2 and 0.3 M SiO2 sol, respectively. There are two possible reasons for the enhancement of UC luminescence in the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials with the SiO2 shells. One is the second sintering at 450 1C in order to form the SiO2 shells around the surface of 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials. In order to investigate the influence of second sintering on UC emission, the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials without the SiO2 sol infiltration was sintered at 450 1C together with 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials with the SiO2 sol infiltration, and the corresponding UC emission spectrum is shown in Fig. 6. It is noted that no significant change was observed in the UC emission spectrum of 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials sintered at 450 1C without SiO2 sol infiltration, which indicated that the 450 1C sintering has no obvious influence on the UC emission of 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials. Another is suppression of surface quenching effect attributed to the formation of SiO2 shells. In the present work, selfassembly and sol–gel approach have been used to fabricate 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials. The main disadvantage of this method is the presence of a relatively large number of structural defects, which are inherent to the synthesis of the 3DOM materials. The UC emission intensity of the 3DOM Bi2Ti2O7: Er3 þ , Yb3 þ materials corresponds to a sum of optical emissions from doping Er3 þ ions at the surface and in the interior of the 3DOM materials. In contrast to the UC emission from interior Er3 þ ions, the UC emission from surface Er3 þ ions were quenched because of the excitation energy quenching caused by surface defects and impurities [23–25]. When the SiO2 shells were formed on the surface of the 3DOM materials, the surface quenching effect was suppressed, resulting in the UC emission enhancement of the 3DOM materials. 4. Conclusion We prepared the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials by the sol–gel method in combination with the PS template, then the surface of the 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials were coated with the SiO2. The influence of SiO2 shells on the UC
emission properties of 3DOM Bi2Ti2O7:Er3 þ , Yb3 þ materials were carefully studied. In contrast to the UC luminescence intensity of three dimensional ordered macroporous Bi2Ti2O7: Er3 þ , Yb3 þ without the SiO2 coating, the UC emission intensity of three dimensional ordered macroporous Bi2Ti2O7:Er3 þ , Yb3 þ with SiO2 shells is greatly enhanced due to an effective suppression of surface quenching effect caused by surface defects.
Acknowledgments This work was supported by Reserve Talents Project of Yunnan Province (2012HB068), Applied Basic Research Program of Yunnan Province (2014FB127) and Talent Youth Science Foundation of College of Materials Science and Technology, Kunming University of Science and Technology (20140205).
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Please cite this article as: Y. Cun, et al., Enhanced upconversion emission of three dimensionally ordered macroporous films Bi2Ti2O7:Er3 þ , Yb3 þ with silica shell, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.144