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A facile synthesis to Zn2SiO4:Mn2+ phosphor with controllable size and morphology at low temperature
Journal of Colloid and Interface Science 314 (2007) 510–513 www.elsevier.com/locate/jcis
A facile synthesis to Zn2SiO4 :Mn2+ phosphor with controllable size and morphology at low temperature Tian Jun Lou b,∗ , Jing Hui Zeng a,b,∗ , Xiang Dong Lou c , Hai Li Fu a , Ye Feng Wang a , Rui Li Ma a , Lin Jian Tong a , Ya Li Chen b a School of Chemistry and Materials Science, Shaanxi Normal University, 199 South Chang’an Road, Xi’an 710062, China b College of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China c College of Chemistry & Environmental Science, Henan Normal University, Xinxiang 453007, China
Received 23 February 2007; accepted 4 June 2007 Available online 21 June 2007
Abstract Sphere- and rod-shaped Zn2 SiO4 :Mn2+ phosphor nanocrystals were synthesized at 230 ◦ C. The process consists of tuning the surfactant concentration in the oil/surfactant/ethanol system. Powder X-ray (XRD) and transmission electron microscopy (TEM) were used to characterize the phase purity, size and morphology. Photoluminescent (PL) spectra were collected and analyzed. Fourier transform infrared (FT-IR) spectra of the samples indicate the removal of surfactant in the phosphor nanoparticles. As a result, the sphere-shaped phosphor nanoparticles of 100 nm in size can be redispersed in ethanol ultrasonically. The suspension maintain stable for over 48 h. © 2007 Elsevier Inc. All rights reserved. Keywords: Nanocrystal; Photoluminescence; Solvothermal; Controlled synthesis
1. Introduction Nanometer sized inorganic phosphors have aroused much attention in the fields such biolabeling, detection, imaging, and disease treatments recently [1]. These applications require nanoparticles to be monodispersed and “water-soluble.” Additional requests are the comparable nanoparticle size to biologic targets. Up to now, these researches are concentrated on the II– VI semiconductors and gold nanoparticles. Gold nanoparticles are high affinity to biomolecules. Although the II–VI semiconductor phosphors have many advantages over traditional organic biolabeling reagents, such as narrow and symmetric emission peaks which can be excited using one excitation wavelength and can be used as multicolor labeling, low photodegradation rates, long fluorescence lifetimes, they show latent toxicity to biomolecules compared with their organic counterparts. So, before their biological applications, reducing the toxicity * Corresponding authors.
E-mail addresses: [email protected] (T.J. Lou), [email protected] (J.H. Zeng). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.026
with silica coating is one of the routine processes [1a,2]. Another reason for the silica capping is to increase their water solubility. Core/shell fluorescent silica nanoparticles for chemical sensing was also reported recently on organic phosphors [3]. Amorphous silica appears to be biocompatible [4] and nontoxic material [5]. Dye-doped silica nanoparticles have been used in the biologic applications [6]. Silicate with less latent toxicity and can be easily coated with amorphous silica, in this case, can be a promising candidate for the biologic labeling application. The emissions can be tuned by different doping ions. For example, Mn2+ or Tb3+ doping can make green emission. Eu3+ doping emits red light. Ce3+ doping can make blue phosphors [7]. Among these phosphors, Mn-doped Zn2 SiO4 is a well-known green phosphor for its high luminescent efficiency and chemical stability. Zn2 SiO4 :Mn2+ phosphor is widely used in cathode ray tubes, plasma display panels, and lamps as a green phosphor [8]. The films of Zn2 SiO4 :Mn2+ are applied in electroluminescent devices due to its better chemical stability than the traditional materials [9]. It is also used in medical imaging technology, and found to be appropriate for low-voltage radiography and fluoroscopy,
T.J. Lou et al. / Journal of Colloid and Interface Science 314 (2007) 510–513
especially in mammographic applications [10]. All these applications benefit from its long emission lifetime, which is 25 ms and is suitable for minimum flicker in displays [11]. Zn2 SiO4 :Mn2+ was synthesized traditionally at elevated temperatures from 950–1150 ◦ C [8a]. It is difficult to control the distribution of the doped metal ions and the morphology of the particles. The aggregation of the willemite particles cannot be avoided [12]. There are many new methods reported recently for the synthesis of Zn2 SiO4 :Mn2+ phosphors, including sol– gel process [13], polymer precursor method [14], spray pyrolysis method [15], chemical vapor synthesis from organometallic single-source precursor [16], and hydrothermal methods [17– 19]. Among them, the control of size and morphology became more and more important in recent reports. In the hydrothermal synthesis, the products are most acicular and/or spherical shape with the dimensions in the micron or submicron range [17,18]. Rod-like Zn2 SiO4 :Mn2+ nanoparticles were prepared hydrothermally with dimensions of 20–30 nm in diameter and ∼300 nm in length. Nevertheless, the hydrothermal synthesis of Zn2 SiO4 :Mn2+ nanoparticles from spheres to jujubes and rods in the nanometer range is rarely reported. In this paper, we report a facile synthesis to the size and morphology controllable Zn2 SiO4 :Mn2+ phosphors at 230 ◦ C. The nanoparticles are well dispersed with the diameter of about 100 nm. The nanorods are of ∼300 nm in length and 30 nm in diameter, while the dimensions of the jujubes lie in the middle of the nanoparticles and nanorods. The spherical phosphor nanoparticles can be suspended in ethanol over 48 h without observable deposition. 2. Experimental Zinc acetate, manganese chloride, and sodium oleate were weighed and put into Teflon liner. Ethanol, n-heptane, and ammonia hydroxide were added to the liner in sequence. Then tetraethoxysilane was drip into the solution dropwise under stirring. The different amounts of chemicals that added for the formation of Zn2 SiO4 :Mn2+ morphologies from nanobead, jujubes to nanorods were listed in Section 3. After the solution being stirred for half an hour, the liner was put into an autoclave, sealed and heated to 230 ◦ C for 10–12 h. The autoclaves were then allowed to cool to room temperature naturally. The products were collected by filtration, washing and drying. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max2550VB+/PC X-ray diffractometer. Transmission electron microscopy (TEM) images were taken on Hitachi H-600 TEM with an accelerating voltage of 100 kV. The photoluminescent spectra were recorded on a Perkin–Elmer PELS50B spectrometer using 250 nm excitation wavelength. Fourier transform infrared (FT-IR) spectra were collected on a Brucker EQUINX55 spectrometer. 3. Results and discussion Mn2+ doped zinc silicate nanocrystalline phosphors were synthesized using zinc acetate, ammonia hydroxide, and manganese chloride at 230 ◦ C. The phase purities of the products
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Fig. 1. XRD patterns of Zn2 SiO4 :Mn2+ with different morphologies. (a) Spheres, (b) jujubes, (c) nanorods, (d) aggregated nanoparticles. Bottom: standard JCPDS card No. 37-1845 for Zn2 SiO4 (willemite). Due to the small aspect ratio differences from jujubes, nanorods to spheres and aggregates, there in no obvious differences in the XRD patterns. Table 1 Experimental parameters to control the morphologies of Zn2 SiO4 :Mn2+ phosphor nanoparticles No.
Ethanol (ml)
n-Heptane (ml)
Sodium oleate (g)
Morphology
EP1 EP2 EP3 EP4
0.5 1 3.8 4.4
34 33 30 30
0.5 1.5 1.5 0.5
Spherical shape Jujube shape Nanorods Nanoparticle aggregates
are proved by the powder X-ray diffraction patterns (Fig. 1). In all the cases, the diffraction peaks can be well indexed to the willemite (JCPDS card No. 37-1845). Although there exists preferred growth direction in the jujubes and nanorods which is described in the following section, the preferred growth is not reflected in the XRD patterns, which may be related to the relative small aspect ratio of the jujubes and nanorods. Ethanol, n-heptane, and sodium oleate were used to tune the morphology of the products. The typical transmission electron microscopy (TEM) images are shown as Fig. 2. From the images, Zn2 SiO4 :Mn2+ with different sizes and morphologies can be well controlled. Nanoparticles with diameter about 100 nm were first received with experimental parameter EP1 (see Table 1, Fig. 2a). With the increasing amount of sodium oleate, the spherical shaped nanoparticles show the trends to jujubes (EP2, Fig. 2b). The jujube-like particles are almost 200 nm in length with a diameter of about 100 nm. This is an interim morphology from spheres to rods. Ethanol content plays an important role in the formation of nanorods. When the concentration of ethanol increases to 3.8 ml, nanorods dominate the products (EP3, Fig. 2c). Anyway, the oleate also plays a key role in the stabilizing of the morphology. If the concentration of sodium
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Fig. 3. FT-IR spectra of Zn2 SiO4 :Mn2+ with different morphologies. (a) Spheres, (b) jujubes, (c) nanorods, (d) aggregated nanoparticles. The absence of νC=O peak at 1711 cm−1 indicates the removing of oleate during the washing process. Fig. 2. TEM microimages of Zn2 SiO4 :Mn2+ synthesized at different experimental conditions. (a) Spherical nanocrystals of about 100 nm; (b) jujube-shaped nanocrystals; (c) nanorods with ∼30 nm in diameter and ∼300 nm in length; (d) nanoparticle aggregates.
oleate is too low, only nanoparticle aggregates can be observed in the final products (EP4, Fig. 2d). The size and morphology controlling of nanocrystals in the presence of oleic acid or oleic ammine is intensively reported in recent literatures [20]. A general technique for the synthesis of a series of inorganic nanomaterials was reported recently [20e]. NaYF4 nanorods with controlled uniform sizes were prepared using the modified technique in the presence of oleic acid as the surfactant [21]. In the present process, sodium oleate serves as the surfactant. The topology of the reaction vessels were tuned with the different ethanol/sodium oleate/n-heptane ratios. With the changing ratios, the topologies can be made from spherical to worm-like. As the result, the inorganic materials synthesized are confined by the topologies of the reaction media. The sphere, jujube and rod-shaped nanoparticles were thus synthesized. The detailed experimental parameters to control the sizes and morphologies of the products are listed in Table 1. Fourier transform infrared (FT-IR) spectra of the samples with different morphologies are shown in Fig. 3. The absorption bands of 952 cm−1 (νSi–OH ), 802 cm−1 (νSi–O–Si ) and 577 cm−1 (νZnO4 ) suggest the formation of Zn2 SiO4 [22], which was in good agreement of that concluded by X-ray diffraction (XRD) patterns. As we can see, νC=O band at 1711 cm−1 for C=O bond in oleate is absent for all the samples, which indicates that oleate was fully removed during the washing process. The removal of oleate favors the suspension of the as-prepared phosphor nanoparticles in polar solvents. EP1 samples can be suspended in ethanol for over 48 h without observable deposition. As a result, the reducing in the intensity of the photoluminescent (PL)
Fig. 4. PL spectra of (a) EP1, (b) EP2, (c) EP3, (d) EP4 and suspension of samples and photoluminescent pictures on ethanol suspensions for different period of suspension times. (e) EP1, immediately after ultrasonic dispersion, (f) EP1, 48 h after dispersion, (g) EP2, immediately after ultrasonic dispersion, (h) EP2, 4 h after dispersion. Insert of (e), (f), (g), and (h): photoluminescent image.
is also not obviously after 48 h (Figs. 4e, 4f and insert images). Although the other samples (EP2–EP4) can be dispersed ultrasonically in ethanol, they deposited continuously, which may be related to the relatively large sizes and dimensions of the samples. Comparison images were taken using sample EP2 as an example (Figs. 4g, 4h). Strong PL can be observed immediately after ultrasonic dispersion (insert of Fig. 4g). After 4 h, the PL of the suspension is almost undetectable (insert of Fig. 4h). When excited by a 250 nm irradiation, all the samples show a broad emission peak centered at 522 nm (Figs. 4a– 4d), which corresponds to the 4 T1 → 4 A1 energy transfer in the Mn ions [23]. Mn2+ occupies part of the Zn2+ sites, which is coordinated by four oxygen atoms. The weak crystal field
T.J. Lou et al. / Journal of Colloid and Interface Science 314 (2007) 510–513
around Mn2+ results in the low splitting width of its 3d energy levels. As a result, an emission at high energy (green) is observed. 4. Conclusion Zn2 SiO4 :Mn2+ phosphor nanoparticles with controlled size and morphologies were prepared at 230 ◦ C. The morphologies can be turned from spheres to jujubes and rod-like. Intense green emissions were observed for all the samples. The 100 nm spherical phosphor nanoparticles can be ultrasonically dispersed in ethanol and maintain stable for over 48 h. The size and dimension of the spherical phosphor nanoparticles and its solubility along with its stability in polar solvent suggest that it can be used in the biologic labeling processes. Acknowledgment This work is supported by Shaanxi Provincial Natural Science Foundation, Grant No. 2006B06. References [1] (a) M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013; (b) W.C.W. Chen, S. Nie, Science 281 (1998) 2016; (c) M.Y. Han, X.H. Gao, J.Z. Su, S. Nie, Nat. Biotechnol. 19 (2001) 631; (d) X.H. Gao, Y.Y. Cui, R.M. Levenson, L.W.K. Chung, S. Nie, Nat. Biotechnol. 22 (2004) 969; (e) W.J. Parak, T. Pellegrino, C. Plank, Nanotechnology 16 (2005) R9; (f) N.L. Rosi, D.A. Giljohann, C.S. Thaxton, A.K.R. Lytton-Jean, M.S. Han, C.A. Mirkin, Science 312 (2006) 1027. [2] (a) W.J. Parak, D. Gerion, D. Zanchet, A.S. Woerz, T. Pellegrino, C. Micheel, S.C. Williams, M. Seitz, R.E. Bruehl, Z. Bryant, C. Bustamante, C.R. Bertozzi, A.P. Alivisatos, Chem. Mater. 14 (2002) 2113; (b) A. Schroedter, H. Weller, R. Eritja, W.E. Ford, J.M. Wessels, Nano Lett. 2 (2002) 1363. [3] (a) H. Ow, D.R. Larson, M. Srivastava, B.A. Baird, W. Webb, U. Wiesner, Nano Lett. 5 (2005) 113; (b) A. Burns, P. Sengupta, T. Zedayko, B. Baird, U. Wiesner, Small 2 (2006) 723. [4] N.L. Rosi, C.A. Mirkin, Chem. Rev. 105 (2005) 1547.
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[5] P.W. Smith, J. Cell Biochem. (Suppl.) 39 (2002) 54. [6] X.X. He, J.H. Duan, K.M. Wang, W.H. Tan, X. Lin, C.M. He, J. Nanosci. Nanotechnol. 4 (2004) 585. [7] N. Taghavinia, G. Lerondel, H. Makino, T. Yao, Thin Solid Films 503 (2006) 190. [8] (a) R. Morimo, R. Mochinage, K. Nakamura, Mater. Res. Bull. 29 (1994) 751; (b) J. Koike, Extended Abstract of the 3rd Int. Conf. on the Science and Technology of Display Phosphors, SID, Huntington Beach, CA, USA, 1997, p. 13; (c) A. Morell, N. El Khiati, J. Electrochem. Soc. 140 (1993) 2019. [9] T. Xiao, G. Liu, M. Adams, A.H. Kitai, Can. J. Phys. 74 (1996) 132. [10] I. Kandarakis, D. Cavouras, P. Prassopoulos, E. Kanelpoulos, C.D. Nomicos, G.S. Panayiotakis, Appl. Phys. A 67 (1998) 521. [11] G. Blasse, B.C. Grambier, Luminescent Materials, Springer, Berlin, 1994. [12] I.F. Chang, J.W. Brownlow, T.L. Sun, J.S. Wilson, J. Electrochem. Soc. 136 (1989) 3532. [13] D.Y. Kong, M. Yu, C.K. Lin, X.M. Liu, J. Lin, J. Fang, J. Electrochem. Soc. 152 (2005) H146. [14] K. Su, T.D. Tilley, J. Sailor, J. Am. Chem. Soc. 118 (1996) 3459. [15] (a) Y.C. Kang, H.D. Park, Appl. Phys. A 77 (2003) 529; (b) C.H. Lee, Y.C. Kim, K.Y. Jung, J.G. Choi, Mater. Sci. Eng. B 117 (2005) 210. [16] A. Roy, S. Polarz, S. Rabe, B. Rellinghaus, H. Zähres, F.E. Kruis, M. Driess, Chem. Eur. J. 10 (2004) 1565. [17] T.S. Ahmadi, M. Haase, H. Weller, Mater. Res. Bull. 35 (2000) 1869. [18] (a) Q.H. Li, S. Komarneni, R. Roy, J. Mater. Sci. 30 (1995) 2358; (b) S.W. Lu, T. Copeland, B.I. Lee, W. Tong, B.K. Wanger, W. Park, F. Zhang, J. Phys. Chem. Solids 62 (2001) 177; (c) C. Yoon, S. Kang, J. Mater. Res. 16 (2001) 1210. [19] H.F. Wang, Y.Q. Ma, G.S. Yi, D.P. Chen, Mater. Chem. Phys. 82 (2003) 414. [20] (a) V.F. Puntes, D. Zanchet, C.K. Erdonmez, A.P. Alivisatos, J. Am. Chem. Soc. 124 (2002) 12874; (b) T. Hyeon, S.S. Lee, J. Park, Y. Chung, H.B. Na, J. Am. Chem. Soc. 123 (2001) 12798; (c) E.V. Shevchenko, D.V. Talapin, A.L. Rogach, A. Kornowski, M. Haanse, H. Weller, J. Am. Chem. Soc. 124 (2002) 11480; (d) C.H. Lee, M. Kim, T. Kim, A. Kim, J. Paek, J.W. Lee, S.-Y. Choi, Y. Kim, J.-B. Park, K. Lee, J. Am. Chem. Soc. 128 (2006) 9326; (e) X. Wang, J. Zhuang, Q. Peng, Y.D. Li, Nature 437 (2006) 121. [21] L.Y. Wang, Y.D. Li, Nano Lett. 6 (2006) 1654. [22] (a) Y. Chen, J.O. Iroh, Chem. Mater. 11 (1999) 1218; (b) A.M. Pires, M.R. Davolos, Chem. Mater. 13 (2001) 21. [23] P.D. Johnson, in: P. Goldberg (Ed.), Luminescence of Inorganic Solids, Academic, New York, 1966, p. 299, and references therein.
A facile synthesis to Zn2SiO4 :Mn2+ phosphor with controllable size and morphology at low temperature Tian Jun Lou b,∗ , Jing Hui Zeng a,b,∗ , Xiang Dong Lou c , Hai Li Fu a , Ye Feng Wang a , Rui Li Ma a , Lin Jian Tong a , Ya Li Chen b a School of Chemistry and Materials Science, Shaanxi Normal University, 199 South Chang’an Road, Xi’an 710062, China b College of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, China c College of Chemistry & Environmental Science, Henan Normal University, Xinxiang 453007, China
Received 23 February 2007; accepted 4 June 2007 Available online 21 June 2007
Abstract Sphere- and rod-shaped Zn2 SiO4 :Mn2+ phosphor nanocrystals were synthesized at 230 ◦ C. The process consists of tuning the surfactant concentration in the oil/surfactant/ethanol system. Powder X-ray (XRD) and transmission electron microscopy (TEM) were used to characterize the phase purity, size and morphology. Photoluminescent (PL) spectra were collected and analyzed. Fourier transform infrared (FT-IR) spectra of the samples indicate the removal of surfactant in the phosphor nanoparticles. As a result, the sphere-shaped phosphor nanoparticles of 100 nm in size can be redispersed in ethanol ultrasonically. The suspension maintain stable for over 48 h. © 2007 Elsevier Inc. All rights reserved. Keywords: Nanocrystal; Photoluminescence; Solvothermal; Controlled synthesis
1. Introduction Nanometer sized inorganic phosphors have aroused much attention in the fields such biolabeling, detection, imaging, and disease treatments recently [1]. These applications require nanoparticles to be monodispersed and “water-soluble.” Additional requests are the comparable nanoparticle size to biologic targets. Up to now, these researches are concentrated on the II– VI semiconductors and gold nanoparticles. Gold nanoparticles are high affinity to biomolecules. Although the II–VI semiconductor phosphors have many advantages over traditional organic biolabeling reagents, such as narrow and symmetric emission peaks which can be excited using one excitation wavelength and can be used as multicolor labeling, low photodegradation rates, long fluorescence lifetimes, they show latent toxicity to biomolecules compared with their organic counterparts. So, before their biological applications, reducing the toxicity * Corresponding authors.
E-mail addresses: [email protected] (T.J. Lou), [email protected] (J.H. Zeng). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.026
with silica coating is one of the routine processes [1a,2]. Another reason for the silica capping is to increase their water solubility. Core/shell fluorescent silica nanoparticles for chemical sensing was also reported recently on organic phosphors [3]. Amorphous silica appears to be biocompatible [4] and nontoxic material [5]. Dye-doped silica nanoparticles have been used in the biologic applications [6]. Silicate with less latent toxicity and can be easily coated with amorphous silica, in this case, can be a promising candidate for the biologic labeling application. The emissions can be tuned by different doping ions. For example, Mn2+ or Tb3+ doping can make green emission. Eu3+ doping emits red light. Ce3+ doping can make blue phosphors [7]. Among these phosphors, Mn-doped Zn2 SiO4 is a well-known green phosphor for its high luminescent efficiency and chemical stability. Zn2 SiO4 :Mn2+ phosphor is widely used in cathode ray tubes, plasma display panels, and lamps as a green phosphor [8]. The films of Zn2 SiO4 :Mn2+ are applied in electroluminescent devices due to its better chemical stability than the traditional materials [9]. It is also used in medical imaging technology, and found to be appropriate for low-voltage radiography and fluoroscopy,
T.J. Lou et al. / Journal of Colloid and Interface Science 314 (2007) 510–513
especially in mammographic applications [10]. All these applications benefit from its long emission lifetime, which is 25 ms and is suitable for minimum flicker in displays [11]. Zn2 SiO4 :Mn2+ was synthesized traditionally at elevated temperatures from 950–1150 ◦ C [8a]. It is difficult to control the distribution of the doped metal ions and the morphology of the particles. The aggregation of the willemite particles cannot be avoided [12]. There are many new methods reported recently for the synthesis of Zn2 SiO4 :Mn2+ phosphors, including sol– gel process [13], polymer precursor method [14], spray pyrolysis method [15], chemical vapor synthesis from organometallic single-source precursor [16], and hydrothermal methods [17– 19]. Among them, the control of size and morphology became more and more important in recent reports. In the hydrothermal synthesis, the products are most acicular and/or spherical shape with the dimensions in the micron or submicron range [17,18]. Rod-like Zn2 SiO4 :Mn2+ nanoparticles were prepared hydrothermally with dimensions of 20–30 nm in diameter and ∼300 nm in length. Nevertheless, the hydrothermal synthesis of Zn2 SiO4 :Mn2+ nanoparticles from spheres to jujubes and rods in the nanometer range is rarely reported. In this paper, we report a facile synthesis to the size and morphology controllable Zn2 SiO4 :Mn2+ phosphors at 230 ◦ C. The nanoparticles are well dispersed with the diameter of about 100 nm. The nanorods are of ∼300 nm in length and 30 nm in diameter, while the dimensions of the jujubes lie in the middle of the nanoparticles and nanorods. The spherical phosphor nanoparticles can be suspended in ethanol over 48 h without observable deposition. 2. Experimental Zinc acetate, manganese chloride, and sodium oleate were weighed and put into Teflon liner. Ethanol, n-heptane, and ammonia hydroxide were added to the liner in sequence. Then tetraethoxysilane was drip into the solution dropwise under stirring. The different amounts of chemicals that added for the formation of Zn2 SiO4 :Mn2+ morphologies from nanobead, jujubes to nanorods were listed in Section 3. After the solution being stirred for half an hour, the liner was put into an autoclave, sealed and heated to 230 ◦ C for 10–12 h. The autoclaves were then allowed to cool to room temperature naturally. The products were collected by filtration, washing and drying. Powder X-ray diffraction (XRD) patterns were collected on a Rigaku D/Max2550VB+/PC X-ray diffractometer. Transmission electron microscopy (TEM) images were taken on Hitachi H-600 TEM with an accelerating voltage of 100 kV. The photoluminescent spectra were recorded on a Perkin–Elmer PELS50B spectrometer using 250 nm excitation wavelength. Fourier transform infrared (FT-IR) spectra were collected on a Brucker EQUINX55 spectrometer. 3. Results and discussion Mn2+ doped zinc silicate nanocrystalline phosphors were synthesized using zinc acetate, ammonia hydroxide, and manganese chloride at 230 ◦ C. The phase purities of the products
511
Fig. 1. XRD patterns of Zn2 SiO4 :Mn2+ with different morphologies. (a) Spheres, (b) jujubes, (c) nanorods, (d) aggregated nanoparticles. Bottom: standard JCPDS card No. 37-1845 for Zn2 SiO4 (willemite). Due to the small aspect ratio differences from jujubes, nanorods to spheres and aggregates, there in no obvious differences in the XRD patterns. Table 1 Experimental parameters to control the morphologies of Zn2 SiO4 :Mn2+ phosphor nanoparticles No.
Ethanol (ml)
n-Heptane (ml)
Sodium oleate (g)
Morphology
EP1 EP2 EP3 EP4
0.5 1 3.8 4.4
34 33 30 30
0.5 1.5 1.5 0.5
Spherical shape Jujube shape Nanorods Nanoparticle aggregates
are proved by the powder X-ray diffraction patterns (Fig. 1). In all the cases, the diffraction peaks can be well indexed to the willemite (JCPDS card No. 37-1845). Although there exists preferred growth direction in the jujubes and nanorods which is described in the following section, the preferred growth is not reflected in the XRD patterns, which may be related to the relative small aspect ratio of the jujubes and nanorods. Ethanol, n-heptane, and sodium oleate were used to tune the morphology of the products. The typical transmission electron microscopy (TEM) images are shown as Fig. 2. From the images, Zn2 SiO4 :Mn2+ with different sizes and morphologies can be well controlled. Nanoparticles with diameter about 100 nm were first received with experimental parameter EP1 (see Table 1, Fig. 2a). With the increasing amount of sodium oleate, the spherical shaped nanoparticles show the trends to jujubes (EP2, Fig. 2b). The jujube-like particles are almost 200 nm in length with a diameter of about 100 nm. This is an interim morphology from spheres to rods. Ethanol content plays an important role in the formation of nanorods. When the concentration of ethanol increases to 3.8 ml, nanorods dominate the products (EP3, Fig. 2c). Anyway, the oleate also plays a key role in the stabilizing of the morphology. If the concentration of sodium
512
T.J. Lou et al. / Journal of Colloid and Interface Science 314 (2007) 510–513
Fig. 3. FT-IR spectra of Zn2 SiO4 :Mn2+ with different morphologies. (a) Spheres, (b) jujubes, (c) nanorods, (d) aggregated nanoparticles. The absence of νC=O peak at 1711 cm−1 indicates the removing of oleate during the washing process. Fig. 2. TEM microimages of Zn2 SiO4 :Mn2+ synthesized at different experimental conditions. (a) Spherical nanocrystals of about 100 nm; (b) jujube-shaped nanocrystals; (c) nanorods with ∼30 nm in diameter and ∼300 nm in length; (d) nanoparticle aggregates.
oleate is too low, only nanoparticle aggregates can be observed in the final products (EP4, Fig. 2d). The size and morphology controlling of nanocrystals in the presence of oleic acid or oleic ammine is intensively reported in recent literatures [20]. A general technique for the synthesis of a series of inorganic nanomaterials was reported recently [20e]. NaYF4 nanorods with controlled uniform sizes were prepared using the modified technique in the presence of oleic acid as the surfactant [21]. In the present process, sodium oleate serves as the surfactant. The topology of the reaction vessels were tuned with the different ethanol/sodium oleate/n-heptane ratios. With the changing ratios, the topologies can be made from spherical to worm-like. As the result, the inorganic materials synthesized are confined by the topologies of the reaction media. The sphere, jujube and rod-shaped nanoparticles were thus synthesized. The detailed experimental parameters to control the sizes and morphologies of the products are listed in Table 1. Fourier transform infrared (FT-IR) spectra of the samples with different morphologies are shown in Fig. 3. The absorption bands of 952 cm−1 (νSi–OH ), 802 cm−1 (νSi–O–Si ) and 577 cm−1 (νZnO4 ) suggest the formation of Zn2 SiO4 [22], which was in good agreement of that concluded by X-ray diffraction (XRD) patterns. As we can see, νC=O band at 1711 cm−1 for C=O bond in oleate is absent for all the samples, which indicates that oleate was fully removed during the washing process. The removal of oleate favors the suspension of the as-prepared phosphor nanoparticles in polar solvents. EP1 samples can be suspended in ethanol for over 48 h without observable deposition. As a result, the reducing in the intensity of the photoluminescent (PL)
Fig. 4. PL spectra of (a) EP1, (b) EP2, (c) EP3, (d) EP4 and suspension of samples and photoluminescent pictures on ethanol suspensions for different period of suspension times. (e) EP1, immediately after ultrasonic dispersion, (f) EP1, 48 h after dispersion, (g) EP2, immediately after ultrasonic dispersion, (h) EP2, 4 h after dispersion. Insert of (e), (f), (g), and (h): photoluminescent image.
is also not obviously after 48 h (Figs. 4e, 4f and insert images). Although the other samples (EP2–EP4) can be dispersed ultrasonically in ethanol, they deposited continuously, which may be related to the relatively large sizes and dimensions of the samples. Comparison images were taken using sample EP2 as an example (Figs. 4g, 4h). Strong PL can be observed immediately after ultrasonic dispersion (insert of Fig. 4g). After 4 h, the PL of the suspension is almost undetectable (insert of Fig. 4h). When excited by a 250 nm irradiation, all the samples show a broad emission peak centered at 522 nm (Figs. 4a– 4d), which corresponds to the 4 T1 → 4 A1 energy transfer in the Mn ions [23]. Mn2+ occupies part of the Zn2+ sites, which is coordinated by four oxygen atoms. The weak crystal field
T.J. Lou et al. / Journal of Colloid and Interface Science 314 (2007) 510–513
around Mn2+ results in the low splitting width of its 3d energy levels. As a result, an emission at high energy (green) is observed. 4. Conclusion Zn2 SiO4 :Mn2+ phosphor nanoparticles with controlled size and morphologies were prepared at 230 ◦ C. The morphologies can be turned from spheres to jujubes and rod-like. Intense green emissions were observed for all the samples. The 100 nm spherical phosphor nanoparticles can be ultrasonically dispersed in ethanol and maintain stable for over 48 h. The size and dimension of the spherical phosphor nanoparticles and its solubility along with its stability in polar solvent suggest that it can be used in the biologic labeling processes. Acknowledgment This work is supported by Shaanxi Provincial Natural Science Foundation, Grant No. 2006B06. References [1] (a) M. Bruchez Jr., M. Moronne, P. Gin, S. Weiss, A.P. Alivisatos, Science 281 (1998) 2013; (b) W.C.W. Chen, S. Nie, Science 281 (1998) 2016; (c) M.Y. Han, X.H. Gao, J.Z. Su, S. Nie, Nat. Biotechnol. 19 (2001) 631; (d) X.H. Gao, Y.Y. Cui, R.M. Levenson, L.W.K. Chung, S. Nie, Nat. Biotechnol. 22 (2004) 969; (e) W.J. Parak, T. Pellegrino, C. Plank, Nanotechnology 16 (2005) R9; (f) N.L. Rosi, D.A. Giljohann, C.S. Thaxton, A.K.R. Lytton-Jean, M.S. Han, C.A. Mirkin, Science 312 (2006) 1027. [2] (a) W.J. Parak, D. Gerion, D. Zanchet, A.S. Woerz, T. Pellegrino, C. Micheel, S.C. Williams, M. Seitz, R.E. Bruehl, Z. Bryant, C. Bustamante, C.R. Bertozzi, A.P. Alivisatos, Chem. Mater. 14 (2002) 2113; (b) A. Schroedter, H. Weller, R. Eritja, W.E. Ford, J.M. Wessels, Nano Lett. 2 (2002) 1363. [3] (a) H. Ow, D.R. Larson, M. Srivastava, B.A. Baird, W. Webb, U. Wiesner, Nano Lett. 5 (2005) 113; (b) A. Burns, P. Sengupta, T. Zedayko, B. Baird, U. Wiesner, Small 2 (2006) 723. [4] N.L. Rosi, C.A. Mirkin, Chem. Rev. 105 (2005) 1547.
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