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Synthesis and luminescence of high-brightness Gd2O2SO4:Tb3+ nanopieces and the enhanced luminescence by alkali metal ions co-doping
Journal of Luminescence 150 (2014) 50–54
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and luminescence of high-brightness Gd2O2SO4:Tb3 þ nanopieces and the enhanced luminescence by alkali metal ions co-doping Lixin Song a,b, Pingfan Du a,b, Qinxu Jiang a,b, Houbao Cao a,b, Jie Xiong a,b,n a
College of Materials and textile, Zhejiang Sci-Tech University, Hangzhou 310018, China Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 August 2013 Received in revised form 11 January 2014 Accepted 14 January 2014 Available online 22 January 2014
Gd2O2SO4:Tb3 þ nanopieces were synthesized by a combined approach of electrospinning and calcination at 1000 1C in mixed gas of sulfur dioxide and air. The nanopieces excited by a 230 nm light showed excellent green luminescence with the strongest emission peak at 545 nm due to the 5D4-7F5 transition of Tb3 þ . Interestingly, the intensity of emission peak at 545 nm of Gd2O2SO4:Tb3 þ nanopieces exhibited about two times stronger than that of the bulk Gd2O2SO4:Tb3 þ at the same doping concentrations of Tb3 þ . Besides, the effects of alkali metal ions doping on the luminescence of the nanopieces have been examined. The emission intensities were further enhanced by alkali metal ions doping, especially for Gd2O2SO4:Tb3 þ /Li þ . The optimal doping concentration of Li þ was 7%. & 2014 Elsevier B.V. All rights reserved.
Key words: Gd2O2SO4:Tb3 þ Alkali metal ions Nanopieces Electrospinning Photoluminescence
1. Introduction Rare earth doped phosphors have attracted much attention for their high luminescence efficiency, color purity, long emission lifetimes values, and potential applications [1–4]. The luminescent properties of rare earth phosphors greatly depended on their compositions, morphologies, sizes, crystallinity, etc. [5–7]. Host materials play an important role in the luminescence of phosphors. As it is well known, the effects of the sizes, dimensionality, and co-dopants on luminescence of rare earth doped phosphors depend on host materials. Rare earth oxysulfates (Ln2O2SO4) have been proved to be outstanding host materials [8–11]. Up to now, several methods were used to synthesize Ln2O2SO4 based phosphors. One is the thermal decomposition of hydrated lanthanide sulfates at high temperature under air or N2 [12–14]. But the process was very slow, and it costs 7 days for a single phase of Y2O2SO4 from decomposition of Y2(SO4)3. Shoji and Sakurai reported that a single phase Y2O2SO4:Eu3 þ was prepared by high-energy ball milling combined with carbon disulfide reducing the Y2O3 [9]. All the above Ln2O2SO4 samples were bulk. Generally, the emission intensities of nanoscale phosphors were
n Corresponding author at: College of Materials and textile, Zhejiang Sci-Tech University, Hangzhou 310018, China. Tel.: þ 86 751 86843603. E-mail address: [email protected] (J. Xiong).
0022-2313/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2014.01.043
stronger than those of the bulk phosphors [15,16]. To prepare Ln2O2SO4 nanostructures, the template-assisted synthesis and coprecipitation were employed, respectively [10,11,17]. The complicated post treatments need to separate and purify the samples during the process of both the methods. Recently, in our previous work, the Y2O2SO4:Eu3 þ nanopieces were obtained by electrospinning combined with calcination and exhibited abnormally stronger than those of electrospun Y2O3:Eu3 þ nanoribbons and the commercial phosphors [18]. Besides, simultaneous doping (co-doping) of other ions including rare earth ions [19,20] and alkali metal ions [21–23] in rare earth doped phosphors is an effect way to enhance its luminescent properties. Several groups reported that the intensities of phosphors were improved more evidently by Li þ doping than the Na þ and K þ doping [24,25]. Some researchers found that Na þ was the optimal co-dopant for SrB2O4:Tb3 þ among three alkali metal ions [23,26]. While Mari et al. [27] thought that the K þ doping can abnormally enhance the intensities of CaO:Eu3 þ comparison with the other two alkali metal ions. However, Chen et al. reported that the alkali carbonates (Li2CO3, Na2CO3, and K2CO3) doping in the BaY2ZnO5:Tb3 þ lowered its emission intensity [28]. The discrepancies in various works indicated that the co-dopants of alkali metal ions had different effects on luminescent properties of rare earth phosphors. However, to the best of our knowledge, there are no papers concerning Gd2O2SO4:Tb3 þ , much less about the difference in
L. Song et al. / Journal of Luminescence 150 (2014) 50–54
luminescence between 2D Ln2O2SO4 nanostructures and their bulk counterparts, and the effects of co-dopants on luminescence of Ln2O2SO4 phosphors. Thus, in this work, the Gd2O2SO4:Tb3 þ nanopieces were fabricated and a comparison of luminescent properties between the Gd2O2SO4:Tb3 þ nanopieces and the bulk Gd2O2SO4:Tb3 þ was conducted. Moreover, the effects of alkali metal ions co-doping and the co-doping concentration on the emission intensity of Gd2O2SO4:Tb3 þ nanopieces were investigated in detail.
2. Experimental 2.1. Materials and synthesis The Gd2O2SO4:Tb3þ nanopieces and Gd2O2SO4:Tb3 þ /M þ (M¼Li, Na, and K) nanopieces were prepared by electrospinning and calcination in mixed gas of sulfur dioxide and air according to our previous work [18]. Typically, 0.71 g Poly(vinyl pyrrolidone) (PVP, Mw¼ 1,300,000), 0.57 g the acetate of gadolinium acetate tetrahydrate and terbium acetate tetrahydrate at a required mole ratio [viz., Tb: Gd¼0.05:1], were dissolved in 4 mL dimethyl formamide (DMF) to form Gd2O2SO4:Tb3 þ precursor electrospinning solution. Then the solution was electrospun into the composite nanofibers (NFs) with a voltage of 15 kV, a distance between the tip of the needle and the collector being fixed at 12 cm, and a feeding rate of 0.5 mL/h. To obtain Gd2O2SO4:Tb3 þ /M þ (M¼Li, Na, and K) samples, a little of M2CO3 with a required molar ratio M:Gd of 0.06:1 and the molar ratio Li:Gd varying from 0 to 0.10, were respectively added into the electrospinning solutions, and then all the solutions were electrospun into NFs. Each electrospun NFs were blended with excess amount of sulfur (the mass as 15 times as NFs) in a crucible. The crucible along with the samples was covered and was calcined in a muffle furnace at 1000 1C in air to obtain the nanopieces. Furthermore, to prepare the bulk Gd2O2SO4:Tb3 þ , the excess amount of sulfur powder was directly added in the electrospinning solution and then they were calcined at 1000 1C in air. 2.2. Characterization The X-ray diffraction (XRD) patterns of the samples were carried out on a Thermo ARL-X’TRA using a Ni-filtered Cu Kα radiation (λ ¼ 1.5418 Å, Thermo ARL-X’TRA), scans were made from 151 to 851 (2θ). The field emission scanning electron microscope (FESEM, ULTRA 55, ZEISS, Germany) was used to observe the morphologies of the samples. The transmission electron (TEM) images and high-resolution transmission electron (HRTEM) images were performed using a JEM2100 transmission microscope. The Hitachi F4600 fluorescence-spectrophotometer
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equipped with a 150 W xenon lamp was employed to characterize photoluminescence (PL) of the samples.
3. Results and discussion The XRD patterns of the samples are exhibited in Fig. 1. A broad band in a range from 151 to 401 is attributed to PVP and no Ln (CH3COO)3 (Ln ¼Gd, Tb) phases are observed in the as-spun NFs, suggesting that non-crystalline Ln(CH3COO)3 4H2O is dispersed homogeneously in the hybrid NFs. After the samples were calcined as shown in Fig. 1(A) (b–e), the broad band disappeared and some well-defined diffraction peaks appeared, indicating the removal of PVP. Almost all of these peaks can be indexed to Gd2O2SO4 according to JCPDS Card nos. 29-0613 and 41-0683 [17]. No additional peak for other phases is found, implying that both Tb3 þ ions and M þ ions have been effectively dissolved into Gd2O2SO4 lattice without obvious change in the host structure. A careful comparison of the three strongest diffraction peaks of Gd2O2SO4:Tb3 þ /M þ is depicted in Fig. 1(B). With Li þ as additives, the diffraction peaks shift to a larger angle, while both Na þ and K þ dopings in Gd2O2SO4:Tb3 þ lead to the diffraction peaks slightly shifting to a smaller angle. These shifts of the peaks may be due to the substitution of the Gd3 þ by alkali metal ions in host lattice. In view of the Bragg Equation 2dsinθ ¼ λ (λ is the wavelength of the X-ray) and their radius [r (Gd3 þ )¼0.94 nm, r (Li þ ) ¼ 0.68 nm, r (Na þ )¼0.97 nm, and r (K þ )¼ 1.33 nm], the Li þ has been dissolved into host lattice by replacing Gd3 þ , resulting in the crystal plane spacing decreasing, while the increase of the crystal plane spacing is due to the substitution of the Na þ and K þ into Gd3 þ sites. The FESEM and TEM were applied to characterize the samples. As shown in Fig. 2(a), the diameter of as-spun NFs is about 200– 800 nm. After calcination, the typical irregular nanopieces with thicknesses of 50–90 nm and in-plane sizes of 120–600 nm are obtained in Fig. 2(b and c). Fig. 2(d) exhibits that the Ln2O2SO4 is a typical crystalline phase with well-resolved lattice fringes. The distinct lattice spacing of 0.302 nm and 0.293 nm correspond to (103) crystal plane and (110) crystal plane, respectively, according to JCPDS Card no. 29-0613. Based on the results of XRD, FESEM and TEM, and our previous work [18,29], the formation courses of the Gd2O2SO4:Tb3 þ and Gd2O2SO4:Tb3 þ /M þ nanopieces from the as-spun NFs are as follows: the mixed gas is formed by reaction between the sulfur and excess air as shown in Eq. (1). Then the Ln2O3 (Ln ¼Gd and Tb) is obtained from Ln(CH3COO)3 4H2O according to Eq. (2) [30,31]. During this process, the as-spun NFs act as a structure directing template leading to mass transfer occurring in limited range. With the further increase in temperature, the PVP is completely
Fig. 1. The XRD patterns (A) of all the samples and (B) magnified view of 2θ from 29.51 to 32.51.
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Fig. 2. FESEM images of the samples: (a) the as-spun NFs and (b) the NFs after calcination, and TEM images of Gd2O2SO4:Tb3 þ nanopieces (c) and its HRTEM (d).
decomposed, and the Ln2O3 NFs are formed. Subsequently, the Ln2O2SO4 is generated by the mixed gas of SO2 and O2 reducing the Ln2O3 NFs according to Eq. (3). The Ln2O2SO4 consists of the alternative layers of the sulfated groups (SO24 ) and (LnO)22 þ cations, and each sulfate oxygen is coordinated with two rare earth atoms [30,32]. In other words, the (LnO) layers can be described as 2D framework of the centered tetrahedral OLn4 linked by four of their edges, leading to the 2D nucleation of Ln2O2SO4. Because the mass transfer is limited by Ln2O3 NFs, the 2D Ln2O2SO4 crystal nuclei grow in several hundreds nanometers scale, resulting in the formation of irregular nanopieces. Furthermore, M2CO3 (M ¼Li, Na, and K) is decomposed and the M þ is respectively dissolved into the lattice of Ln2O2SO4 during the growth of Ln2O2SO4 [24,33,34]. S þ O2 -SO2
ð1Þ
Ln ðCH3 COOÞ3 4H2 O- Ln ðCH3 COOÞ3 -Ln2 ðCO3 Þ3 -Ln2 O2 CO3 -Ln2 O3
ð2Þ Ln2 O3 þ SO2 þ O2 -Ln2 O2 SO4
ð3Þ
Fig. 3 depicts the PL excitation and emission spectra of Gd2O2SO4:Tb3 þ nanopieces and bulk Gd2O2SO4:Tb3 þ . In Fig. 3(a and b), the excitation spectra of the two samples (monitored at 545 nm) are similar and exhibit a sharp strong band of 230 nm due to the parity- and spin-allowed transition 4f8-4f75d1 of Tb3 þ and some broad bands around 303 nm owing to the 8S7/2-6IJ transitions of Gd3 þ and the f-f transitions of Tb3 þ [35–37]. The emission spectra of both the bulk Gd2O2SO4:Tb3 þ (Fig. 3c) and the Gd2O2SO4:Tb3 þ nanopieces (Fig. 3d), recorded at 230 nm excitation, consist of several emission peaks due to the 5D3-7FJ (J ¼4, 5) and 5D4-7FJ (J¼ 3–6) transitions of Tb3 þ . The peak with the maximum intensity locates at 545 nm in green region arising of 5D4-7F5 transition. The other emission peaks center at 417 nm (5D3-7F5), 438 nm (5D3-7F4), 487 nm (5D4-7F3), 587 nm (5D4-7F4), and 621 nm (5D4-7F6). Additionally, the emission intensities of the Gd2O2SO4:Tb3 þ nanopieces are strikingly stronger than those of the bulk counterparts. This may be attributed to the exchange interaction between the Tb3 þ ions. The exchange interaction would restrain the luminescent centers of emission by non-radiative relaxation [16,38]. The interaction was hindered to
Fig. 3. PL excitation (a and b) and emission spectra (c and d) of the bulk Gd2O2SO4: Tb3 þ and Gd2O2SO4:Tb3 þ nanopieces, respectively.
some extent due to the boundary of nanopiece. Furthermore, the emission efficiency of the nanopieces may improve because the electron transfer in the nanopiece from excitation to energy level 5 D4 takes place directly without going through intermediate level due to the confinement of Tb3 þ imposed by the host boundaries of the nanopieces comparing with that in the bulk [39,40]. The PL emission spectra of Gd2O2SO4:Tb3 þ nanopieces and Gd2O2SO4:Tb3 þ /M þ (M ¼Li, Na, K) nanopieces are exhibited in Fig. 4. It is observed that the intensity of the peak arising of 5 D4-7F5 transition in Gd2O2SO4:Tb3 þ nanopieces increases by about 34%, 27%, and 18% with the alkali metal ions (Li þ , Na þ , and K þ ) co-doping, respectively. The incorporation of Li þ , Na þ , and K þ in the host lattice would eventually create a substantial number of oxygen vacancies on the surface of the phosphor [23,24]. These oxygen vacancies might act as sensitizers for the energy transfer from host to Tb3 þ due to the strong mixing of charge transfer states, and enhance the emission intensity [40,41]. Moreover, the substitution of Li þ , Na þ , and K þ into Gd3 þ sites is associated with the mismatches in sizes and electro negativities between co-dopants and host cations [24]. With the radii of codoping ions deceasing (from K þ to Li þ ), the electro negativities increase and the emission intensities of Gd2O2SO4:Tb3 þ are enhanced drastically by the co-doping.
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nanopieces are expected to have potential applications in field emission display devices (FEDs), light emitting diodes (LEDs) and other optically functional devices and the electrospinning technique described in this work could also be used to fabricate other material nanopieces.
Acknowledgment
Fig. 4. The PL emission spectra of Gd2O2SO4:Tb3 þ nanopieces and Gd2O2SO4:Tb3 þ / M þ (M¼ Li, Na, and K) nanopieces with molar ratio of M/Gd of 0.06.
The financial support of this work was provided by Top Priority Discipline of Textile Science and Engineering of Zhejiang Province for Cultivation of Outstanding Graduate Thesis (2013YBPY06), Special Program for International S&T Cooperation Projects of Zhejiang Province (2012C24012) and the Program for Zhejiang Leading Team of Science and Technology Innovation (2011R50003). References
Fig. 5. Relationship between PL emission intensity of 5D4-7F5 transition and the concentration of Li þ in Gd2O2SO4:Tb3 þ /xLi þ (x ¼0–10%) nanopieces.
Fig. 5 depicts the PL emission intensities of 5D4-7F5 transition varying with the doping concentration (x) of Li þ ions in the Gd2O2SO4:Tb3 þ /xLi þ nanopieces (where x¼ 0, 0.02, 0.04, 0.06, 0.07, 0.08, 0.09, and 0.10) with the Tb3 þ concentration maintaining 5%. The emission intensity increases with the increasing x, reaches a maximum value of x ¼0.7, and afterwards decreases gradually. These may be due to the following: the oxygen vacancies as sensitizers or inter-mediators increase with the Li þ ions increasing, benefiting to energy transfer from the host to Tb3 þ ions and leading to the increase in emission intensity. With the further increase in Li þ ions, the defects increase, associate, and may act as energy sinks as well as destroying the crystallinity, resulting in the emission intensity decreasing [40,42].
4. Conclusions We have successfully prepared the Gd2O2SO4:Tb3 þ nanopieces by the combined method of electrospinning and calcination. The irregular nanopieces have thicknesses of 50–90 nm and in-plane sizes of 120–600 nm. Under UV excitation, the nanopieces exhibit excellent luminescent properties. The emission spectrum consists of 417 nm (5D3-7F5), 438 nm (5D3-7F4), 487 nm (5D4-7F3), 544 nm (5D4-7F5), 587 nm (5D4-7F4), and 621 nm (5D4-7F6). Besides, the emission intensities of the nanopieces are greatly stronger than those of the bulk counterparts. Besides, the emission intensities are further enhanced by alkali metal ions (Li þ , Na þ , and K þ ) doping, especially for Gd2O2SO4:Tb3 þ /Li þ nanopieces. The optimal doping concentration of Li þ is 7%. Furthermore, the
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Synthesis and luminescence of high-brightness Gd2O2SO4:Tb3 þ nanopieces and the enhanced luminescence by alkali metal ions co-doping Lixin Song a,b, Pingfan Du a,b, Qinxu Jiang a,b, Houbao Cao a,b, Jie Xiong a,b,n a
College of Materials and textile, Zhejiang Sci-Tech University, Hangzhou 310018, China Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, China
b
art ic l e i nf o
a b s t r a c t
Article history: Received 30 August 2013 Received in revised form 11 January 2014 Accepted 14 January 2014 Available online 22 January 2014
Gd2O2SO4:Tb3 þ nanopieces were synthesized by a combined approach of electrospinning and calcination at 1000 1C in mixed gas of sulfur dioxide and air. The nanopieces excited by a 230 nm light showed excellent green luminescence with the strongest emission peak at 545 nm due to the 5D4-7F5 transition of Tb3 þ . Interestingly, the intensity of emission peak at 545 nm of Gd2O2SO4:Tb3 þ nanopieces exhibited about two times stronger than that of the bulk Gd2O2SO4:Tb3 þ at the same doping concentrations of Tb3 þ . Besides, the effects of alkali metal ions doping on the luminescence of the nanopieces have been examined. The emission intensities were further enhanced by alkali metal ions doping, especially for Gd2O2SO4:Tb3 þ /Li þ . The optimal doping concentration of Li þ was 7%. & 2014 Elsevier B.V. All rights reserved.
Key words: Gd2O2SO4:Tb3 þ Alkali metal ions Nanopieces Electrospinning Photoluminescence
1. Introduction Rare earth doped phosphors have attracted much attention for their high luminescence efficiency, color purity, long emission lifetimes values, and potential applications [1–4]. The luminescent properties of rare earth phosphors greatly depended on their compositions, morphologies, sizes, crystallinity, etc. [5–7]. Host materials play an important role in the luminescence of phosphors. As it is well known, the effects of the sizes, dimensionality, and co-dopants on luminescence of rare earth doped phosphors depend on host materials. Rare earth oxysulfates (Ln2O2SO4) have been proved to be outstanding host materials [8–11]. Up to now, several methods were used to synthesize Ln2O2SO4 based phosphors. One is the thermal decomposition of hydrated lanthanide sulfates at high temperature under air or N2 [12–14]. But the process was very slow, and it costs 7 days for a single phase of Y2O2SO4 from decomposition of Y2(SO4)3. Shoji and Sakurai reported that a single phase Y2O2SO4:Eu3 þ was prepared by high-energy ball milling combined with carbon disulfide reducing the Y2O3 [9]. All the above Ln2O2SO4 samples were bulk. Generally, the emission intensities of nanoscale phosphors were
n Corresponding author at: College of Materials and textile, Zhejiang Sci-Tech University, Hangzhou 310018, China. Tel.: þ 86 751 86843603. E-mail address: [email protected] (J. Xiong).
0022-2313/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2014.01.043
stronger than those of the bulk phosphors [15,16]. To prepare Ln2O2SO4 nanostructures, the template-assisted synthesis and coprecipitation were employed, respectively [10,11,17]. The complicated post treatments need to separate and purify the samples during the process of both the methods. Recently, in our previous work, the Y2O2SO4:Eu3 þ nanopieces were obtained by electrospinning combined with calcination and exhibited abnormally stronger than those of electrospun Y2O3:Eu3 þ nanoribbons and the commercial phosphors [18]. Besides, simultaneous doping (co-doping) of other ions including rare earth ions [19,20] and alkali metal ions [21–23] in rare earth doped phosphors is an effect way to enhance its luminescent properties. Several groups reported that the intensities of phosphors were improved more evidently by Li þ doping than the Na þ and K þ doping [24,25]. Some researchers found that Na þ was the optimal co-dopant for SrB2O4:Tb3 þ among three alkali metal ions [23,26]. While Mari et al. [27] thought that the K þ doping can abnormally enhance the intensities of CaO:Eu3 þ comparison with the other two alkali metal ions. However, Chen et al. reported that the alkali carbonates (Li2CO3, Na2CO3, and K2CO3) doping in the BaY2ZnO5:Tb3 þ lowered its emission intensity [28]. The discrepancies in various works indicated that the co-dopants of alkali metal ions had different effects on luminescent properties of rare earth phosphors. However, to the best of our knowledge, there are no papers concerning Gd2O2SO4:Tb3 þ , much less about the difference in
L. Song et al. / Journal of Luminescence 150 (2014) 50–54
luminescence between 2D Ln2O2SO4 nanostructures and their bulk counterparts, and the effects of co-dopants on luminescence of Ln2O2SO4 phosphors. Thus, in this work, the Gd2O2SO4:Tb3 þ nanopieces were fabricated and a comparison of luminescent properties between the Gd2O2SO4:Tb3 þ nanopieces and the bulk Gd2O2SO4:Tb3 þ was conducted. Moreover, the effects of alkali metal ions co-doping and the co-doping concentration on the emission intensity of Gd2O2SO4:Tb3 þ nanopieces were investigated in detail.
2. Experimental 2.1. Materials and synthesis The Gd2O2SO4:Tb3þ nanopieces and Gd2O2SO4:Tb3 þ /M þ (M¼Li, Na, and K) nanopieces were prepared by electrospinning and calcination in mixed gas of sulfur dioxide and air according to our previous work [18]. Typically, 0.71 g Poly(vinyl pyrrolidone) (PVP, Mw¼ 1,300,000), 0.57 g the acetate of gadolinium acetate tetrahydrate and terbium acetate tetrahydrate at a required mole ratio [viz., Tb: Gd¼0.05:1], were dissolved in 4 mL dimethyl formamide (DMF) to form Gd2O2SO4:Tb3 þ precursor electrospinning solution. Then the solution was electrospun into the composite nanofibers (NFs) with a voltage of 15 kV, a distance between the tip of the needle and the collector being fixed at 12 cm, and a feeding rate of 0.5 mL/h. To obtain Gd2O2SO4:Tb3 þ /M þ (M¼Li, Na, and K) samples, a little of M2CO3 with a required molar ratio M:Gd of 0.06:1 and the molar ratio Li:Gd varying from 0 to 0.10, were respectively added into the electrospinning solutions, and then all the solutions were electrospun into NFs. Each electrospun NFs were blended with excess amount of sulfur (the mass as 15 times as NFs) in a crucible. The crucible along with the samples was covered and was calcined in a muffle furnace at 1000 1C in air to obtain the nanopieces. Furthermore, to prepare the bulk Gd2O2SO4:Tb3 þ , the excess amount of sulfur powder was directly added in the electrospinning solution and then they were calcined at 1000 1C in air. 2.2. Characterization The X-ray diffraction (XRD) patterns of the samples were carried out on a Thermo ARL-X’TRA using a Ni-filtered Cu Kα radiation (λ ¼ 1.5418 Å, Thermo ARL-X’TRA), scans were made from 151 to 851 (2θ). The field emission scanning electron microscope (FESEM, ULTRA 55, ZEISS, Germany) was used to observe the morphologies of the samples. The transmission electron (TEM) images and high-resolution transmission electron (HRTEM) images were performed using a JEM2100 transmission microscope. The Hitachi F4600 fluorescence-spectrophotometer
51
equipped with a 150 W xenon lamp was employed to characterize photoluminescence (PL) of the samples.
3. Results and discussion The XRD patterns of the samples are exhibited in Fig. 1. A broad band in a range from 151 to 401 is attributed to PVP and no Ln (CH3COO)3 (Ln ¼Gd, Tb) phases are observed in the as-spun NFs, suggesting that non-crystalline Ln(CH3COO)3 4H2O is dispersed homogeneously in the hybrid NFs. After the samples were calcined as shown in Fig. 1(A) (b–e), the broad band disappeared and some well-defined diffraction peaks appeared, indicating the removal of PVP. Almost all of these peaks can be indexed to Gd2O2SO4 according to JCPDS Card nos. 29-0613 and 41-0683 [17]. No additional peak for other phases is found, implying that both Tb3 þ ions and M þ ions have been effectively dissolved into Gd2O2SO4 lattice without obvious change in the host structure. A careful comparison of the three strongest diffraction peaks of Gd2O2SO4:Tb3 þ /M þ is depicted in Fig. 1(B). With Li þ as additives, the diffraction peaks shift to a larger angle, while both Na þ and K þ dopings in Gd2O2SO4:Tb3 þ lead to the diffraction peaks slightly shifting to a smaller angle. These shifts of the peaks may be due to the substitution of the Gd3 þ by alkali metal ions in host lattice. In view of the Bragg Equation 2dsinθ ¼ λ (λ is the wavelength of the X-ray) and their radius [r (Gd3 þ )¼0.94 nm, r (Li þ ) ¼ 0.68 nm, r (Na þ )¼0.97 nm, and r (K þ )¼ 1.33 nm], the Li þ has been dissolved into host lattice by replacing Gd3 þ , resulting in the crystal plane spacing decreasing, while the increase of the crystal plane spacing is due to the substitution of the Na þ and K þ into Gd3 þ sites. The FESEM and TEM were applied to characterize the samples. As shown in Fig. 2(a), the diameter of as-spun NFs is about 200– 800 nm. After calcination, the typical irregular nanopieces with thicknesses of 50–90 nm and in-plane sizes of 120–600 nm are obtained in Fig. 2(b and c). Fig. 2(d) exhibits that the Ln2O2SO4 is a typical crystalline phase with well-resolved lattice fringes. The distinct lattice spacing of 0.302 nm and 0.293 nm correspond to (103) crystal plane and (110) crystal plane, respectively, according to JCPDS Card no. 29-0613. Based on the results of XRD, FESEM and TEM, and our previous work [18,29], the formation courses of the Gd2O2SO4:Tb3 þ and Gd2O2SO4:Tb3 þ /M þ nanopieces from the as-spun NFs are as follows: the mixed gas is formed by reaction between the sulfur and excess air as shown in Eq. (1). Then the Ln2O3 (Ln ¼Gd and Tb) is obtained from Ln(CH3COO)3 4H2O according to Eq. (2) [30,31]. During this process, the as-spun NFs act as a structure directing template leading to mass transfer occurring in limited range. With the further increase in temperature, the PVP is completely
Fig. 1. The XRD patterns (A) of all the samples and (B) magnified view of 2θ from 29.51 to 32.51.
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Fig. 2. FESEM images of the samples: (a) the as-spun NFs and (b) the NFs after calcination, and TEM images of Gd2O2SO4:Tb3 þ nanopieces (c) and its HRTEM (d).
decomposed, and the Ln2O3 NFs are formed. Subsequently, the Ln2O2SO4 is generated by the mixed gas of SO2 and O2 reducing the Ln2O3 NFs according to Eq. (3). The Ln2O2SO4 consists of the alternative layers of the sulfated groups (SO24 ) and (LnO)22 þ cations, and each sulfate oxygen is coordinated with two rare earth atoms [30,32]. In other words, the (LnO) layers can be described as 2D framework of the centered tetrahedral OLn4 linked by four of their edges, leading to the 2D nucleation of Ln2O2SO4. Because the mass transfer is limited by Ln2O3 NFs, the 2D Ln2O2SO4 crystal nuclei grow in several hundreds nanometers scale, resulting in the formation of irregular nanopieces. Furthermore, M2CO3 (M ¼Li, Na, and K) is decomposed and the M þ is respectively dissolved into the lattice of Ln2O2SO4 during the growth of Ln2O2SO4 [24,33,34]. S þ O2 -SO2
ð1Þ
Ln ðCH3 COOÞ3 4H2 O- Ln ðCH3 COOÞ3 -Ln2 ðCO3 Þ3 -Ln2 O2 CO3 -Ln2 O3
ð2Þ Ln2 O3 þ SO2 þ O2 -Ln2 O2 SO4
ð3Þ
Fig. 3 depicts the PL excitation and emission spectra of Gd2O2SO4:Tb3 þ nanopieces and bulk Gd2O2SO4:Tb3 þ . In Fig. 3(a and b), the excitation spectra of the two samples (monitored at 545 nm) are similar and exhibit a sharp strong band of 230 nm due to the parity- and spin-allowed transition 4f8-4f75d1 of Tb3 þ and some broad bands around 303 nm owing to the 8S7/2-6IJ transitions of Gd3 þ and the f-f transitions of Tb3 þ [35–37]. The emission spectra of both the bulk Gd2O2SO4:Tb3 þ (Fig. 3c) and the Gd2O2SO4:Tb3 þ nanopieces (Fig. 3d), recorded at 230 nm excitation, consist of several emission peaks due to the 5D3-7FJ (J ¼4, 5) and 5D4-7FJ (J¼ 3–6) transitions of Tb3 þ . The peak with the maximum intensity locates at 545 nm in green region arising of 5D4-7F5 transition. The other emission peaks center at 417 nm (5D3-7F5), 438 nm (5D3-7F4), 487 nm (5D4-7F3), 587 nm (5D4-7F4), and 621 nm (5D4-7F6). Additionally, the emission intensities of the Gd2O2SO4:Tb3 þ nanopieces are strikingly stronger than those of the bulk counterparts. This may be attributed to the exchange interaction between the Tb3 þ ions. The exchange interaction would restrain the luminescent centers of emission by non-radiative relaxation [16,38]. The interaction was hindered to
Fig. 3. PL excitation (a and b) and emission spectra (c and d) of the bulk Gd2O2SO4: Tb3 þ and Gd2O2SO4:Tb3 þ nanopieces, respectively.
some extent due to the boundary of nanopiece. Furthermore, the emission efficiency of the nanopieces may improve because the electron transfer in the nanopiece from excitation to energy level 5 D4 takes place directly without going through intermediate level due to the confinement of Tb3 þ imposed by the host boundaries of the nanopieces comparing with that in the bulk [39,40]. The PL emission spectra of Gd2O2SO4:Tb3 þ nanopieces and Gd2O2SO4:Tb3 þ /M þ (M ¼Li, Na, K) nanopieces are exhibited in Fig. 4. It is observed that the intensity of the peak arising of 5 D4-7F5 transition in Gd2O2SO4:Tb3 þ nanopieces increases by about 34%, 27%, and 18% with the alkali metal ions (Li þ , Na þ , and K þ ) co-doping, respectively. The incorporation of Li þ , Na þ , and K þ in the host lattice would eventually create a substantial number of oxygen vacancies on the surface of the phosphor [23,24]. These oxygen vacancies might act as sensitizers for the energy transfer from host to Tb3 þ due to the strong mixing of charge transfer states, and enhance the emission intensity [40,41]. Moreover, the substitution of Li þ , Na þ , and K þ into Gd3 þ sites is associated with the mismatches in sizes and electro negativities between co-dopants and host cations [24]. With the radii of codoping ions deceasing (from K þ to Li þ ), the electro negativities increase and the emission intensities of Gd2O2SO4:Tb3 þ are enhanced drastically by the co-doping.
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nanopieces are expected to have potential applications in field emission display devices (FEDs), light emitting diodes (LEDs) and other optically functional devices and the electrospinning technique described in this work could also be used to fabricate other material nanopieces.
Acknowledgment
Fig. 4. The PL emission spectra of Gd2O2SO4:Tb3 þ nanopieces and Gd2O2SO4:Tb3 þ / M þ (M¼ Li, Na, and K) nanopieces with molar ratio of M/Gd of 0.06.
The financial support of this work was provided by Top Priority Discipline of Textile Science and Engineering of Zhejiang Province for Cultivation of Outstanding Graduate Thesis (2013YBPY06), Special Program for International S&T Cooperation Projects of Zhejiang Province (2012C24012) and the Program for Zhejiang Leading Team of Science and Technology Innovation (2011R50003). References
Fig. 5. Relationship between PL emission intensity of 5D4-7F5 transition and the concentration of Li þ in Gd2O2SO4:Tb3 þ /xLi þ (x ¼0–10%) nanopieces.
Fig. 5 depicts the PL emission intensities of 5D4-7F5 transition varying with the doping concentration (x) of Li þ ions in the Gd2O2SO4:Tb3 þ /xLi þ nanopieces (where x¼ 0, 0.02, 0.04, 0.06, 0.07, 0.08, 0.09, and 0.10) with the Tb3 þ concentration maintaining 5%. The emission intensity increases with the increasing x, reaches a maximum value of x ¼0.7, and afterwards decreases gradually. These may be due to the following: the oxygen vacancies as sensitizers or inter-mediators increase with the Li þ ions increasing, benefiting to energy transfer from the host to Tb3 þ ions and leading to the increase in emission intensity. With the further increase in Li þ ions, the defects increase, associate, and may act as energy sinks as well as destroying the crystallinity, resulting in the emission intensity decreasing [40,42].
4. Conclusions We have successfully prepared the Gd2O2SO4:Tb3 þ nanopieces by the combined method of electrospinning and calcination. The irregular nanopieces have thicknesses of 50–90 nm and in-plane sizes of 120–600 nm. Under UV excitation, the nanopieces exhibit excellent luminescent properties. The emission spectrum consists of 417 nm (5D3-7F5), 438 nm (5D3-7F4), 487 nm (5D4-7F3), 544 nm (5D4-7F5), 587 nm (5D4-7F4), and 621 nm (5D4-7F6). Besides, the emission intensities of the nanopieces are greatly stronger than those of the bulk counterparts. Besides, the emission intensities are further enhanced by alkali metal ions (Li þ , Na þ , and K þ ) doping, especially for Gd2O2SO4:Tb3 þ /Li þ nanopieces. The optimal doping concentration of Li þ is 7%. Furthermore, the
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