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Fabrication of size controllable YVO4 nanoparticles via microemulsion-mediated synthetic process
Solid State Communications 124 (2002) 35–38 www.elsevier.com/locate/ssc
Fabrication of size controllable YVO4 nanoparticles via microemulsion-mediated synthetic process Lingdong Suna, Yingxin Zhangb, Jun Zhanga, Chunhua Yana,*, Chunsheng Liaoa, Yiqiang Lub a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Lab in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry, Peking University, Beijing 100871, People’s Republic of China b Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China Received 18 July 2002; accepted 10 August 2002 by Z. Gan
Abstract Narrow size distributed YVO4 nanoparticles with controllable size were successfully fabricated by direct reaction of Na3VO4 and Y(NO3)3 in CTAB microemulsion systems. By the confinement of the microemulsion droplets, with manipulating the pH value of microemulsion media to 7, 8, 9, 10, the size of YVO4 nanoparticles can be accurately controlled as 8.9, 11.1, 13.9 and 47.2 nm, respectively. X-ray diffraction (XRD) and transmission electron microscope (TEM) were utilized to characterize the structure, size and shape of the nanoparticles, which indicated tetragonal phase YVO4 nanoparticles displaying ellipsoidal or lozenge-like shape. As pH value is an important factor in our experiments and W value (W ¼ [H2O]/[CTAB]) is the key parameter in the formation of microemulsion droplets, their influences on the YVO4 nanoparticles formation were studied. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 81.07.Bc; 81.16.Be; 82.33.Nq Keywords: A. Nanostructures; B. Chemical synthesis; E. Microemulsion-mediated
1. Introduction Yttrium orthovanadate (YVO4) has been known to be a very attractive host lattice for several metal ions. For example, when doped with trivalent Eu3þ ions, it can be utilized as a red phosphor in cathode ray tube (CRT) [1], and scintillator in medical image detectors [2]; while adulterated with certain rare earth oxide, it becomes the optical maser materials [3]. Moreover, it is also a promising polarizer [4] and laser host material in single crystal form [5]. Since Goldschmidt and Haraldsen fabricated YVO4 firstly in 1928 [6], many synthetic methods, such as solid state reaction, solution combustion process [7], hydrolyzed colloid reaction (HCR) technique [8], and urea precipitation followed by calcinations [9], etc. have been developed to * Corresponding author. Tel./fax: þ 86-10-6275-4179. E-mail address: [email protected] (C. Yan).
prepare YVO4 material in view of its important utilization in optoelectronic fields. Solid-state reaction is one of the most common routes to obtain YVO4 materials. However, because of the high reaction temperature and long crystallization process in nature, it is difficult to prepare the nanocrystals free from agglomeration and oxygen deficiencies, which will contribute to the non-radiative recombination paths and therefore be harmful to the optical properties of YVO4. To this extent, the preparation of nanosized YVO4 is necessary to meet the requirement of the host of the phosphors with decreasing size and narrow size distribution, demanded by recent development of displays towards the high quality, efficiency and miniaturization. Meanwhile, the reduction of the particle size will also lead to the decrease of the optimum screening weight, which results in the abatement of the overall thickness of the coating and elimination of pinholes and mottling of the screen [9]. And, the application of nanosized phosphors has
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 4 4 9 - 0
36
L. Sun et al. / Solid State Communications 124 (2002) 35–38
Fig. 1. XRD patterns of YVO4 prepared under different pH (W ¼ 16).
attracted considerable attentions since the report of increased quantum efficiency with decreasing particle size [10]. It has been proved that microemulsion method is not only an effective process to obtain size controllable nanoparticles with narrow distribution by the confinement of microemulsion droplets, but also a simple and convenient route with low costing. Although it has been utilized to synthesize many nanomaterials with controllable size, there is no report on the preparation of YVO4 nanoparticles yet. In this paper, microemulsion-mediated process was employed to synthesize size controllable YVO4 nanoparticles. The results show that the size and size distribution of the nanoparticles can be controlled accurately by modulating the pH and W value (W ¼ [H2O]/[CTAB]) of microemulsion mediated environment in a relatively wide range.
Fig. 2. TEM images of YVO4 nanoparticles prepared under different pH (W ¼ 16).
First, two types of quaternary microemulsions, designated as MA and MB were prepared separately. Both microemulsion contained surfactant CTAB (cetyltrimethylammonium bromide), cosurfactant n-hexanol and oil phase n-heptane in common, while, MA contained Y(NO3)3 aqueous solution, MB contained Na3VO4 and NaOH aqueous solution, respectively. Normally, the composition
of the four-component reverse micelles can be defined by three parameters: W (the molar ratio between water and CTAB), P (the molar ratio between n-hexanol and CTAB), and [CTAB] (the molar concentration of CTAB). In our case, the P, [CTAB] and the same initial molar concentrations of Na3VO4 and Y(NO3)3 were fixed as 5.27, 0.19 and 0.017 mol/l, respectively, and the pH was varied by modulating the concentration of NaOH in MB under certain W value (W ¼ 12, 16, 20, 24) to control the size of YVO4 nanoparticles. Then, MB was added dropwise to MA under constant stirring (vMB/vMA ¼ 1). Subsequently, the mixture was transferred into a 50 ml autoclave for hydrothermal treatment at 150 8C for 2 h to make the nanoparticles well crystallized. Naturally cooled to room temperature, the precipitates of YVO4 nanoparticles were separated from the reaction media by centrifugation and were washed with absolute ethanol and distilled water for several times. Finally, YVO4 nanosized product was dried under vacuum at 60 – 70 8C. The structure, size and morphology of YVO4 nanoparticles were characterized by X-ray diffraction (XRD, Rigaku, D/max-2000, Cu Ka radiation) and transmission electron microscope (TEM, Hitachi, H-800), respectively.
Table 1 Size of YVO4 nanoparticles prepared under different pH (W ¼ 16)
3. Results and discussion
2. Experimental
pH
2u (8)
FWHM (8)
D (nm)
7 8 9 10
25.030 24.937 24.980 24.904
0.9955 0.8103 0.6533 0.2516
8.9 11.1 13.9 47.2
The XRD patterns of YVO4 under different pH values were shown in Fig. 1, which can be indexed as the tetragonal structured YVO4 (JCPDS No. 17-0341). With the increase of the pH of microemulsion media, the diffraction peaks exhibited size-broadened reflections of YVO4 nanocrystallites. The particle size was calculated by fitting the full
L. Sun et al. / Solid State Communications 124 (2002) 35–38
Fig. 3. XRD patterns of YVO4 prepared under different W value (pH ¼ 8).
width at half maximum of (200) peaks with Scherrer Formula (Table 1). It can be seen that when adjusting the pH of microemulsion from 7, 8, 9 to 10 successively, the mean particle size of YVO4 varied from 8.9, 11.1, 13.9 to 47.2 nm accordingly, which are consistent with the results deduced from the TEM images. Fig. 2 shows the TEM images of YVO4 particles prepared under different pH (pH ¼ 7, 8, 9, 10). It is clear that both the size and the shape of particles changed with increasing the pH value. When pH was fixed at 7 or 8, the YVO4 particles exhibited an ellipsoidal shape, while the particle displayed a lozenge-like shape as pH value was increased to 9 or 10. In the present case, microemulsion was used as the reaction media to prepare YVO4 particles. Normally, the
Fig. 4. TEM images of YVO4 nanoparticles prepared under different W value (pH ¼ 8).
37
characteristic dimensions of microemulsion droplets are in nanometer region, and the microemulsion droplets may afford a confined environment for the growth of nanoparticles. The surfactant and cosurfactant adhered to the surface of nanoparticles served as a protective layer to prevent from collision and amalgamation of the droplets. Therefore, microemulsion media play an important role in modulating and confining the particle size of YVO4. Since that the form of vanadium ions are extremely sensitive to the pH of the solution, as reported by Ropp and Carroll [11], the vanadium ions existed as VO2þ ions when the solution was in strong acidity. When the pH rises to 2, vanadium ions were in the form of V10O62 28 principally; while the pH further increased to 5, vanadium ions were liable to the form of V3O32 9 . In our case, since the pH of the microemulsion ranges from 7 to 10, the following reactions might occur 32 þ 3OH2 þ V3 O32 9 $ 3VO4 þ 3H
ð1Þ
3þ ! YVO4 VO32 4 þY
ð2Þ
Based on the above-mentioned mechanisms, given that microemulsion droplets have been confined in nanometer region, the size of YVO4 nanocrystals can be controlled by adjusting the pH of microemulsion media. With increasing the pH of microemulsion media, the reaction favors the formation of VO32 4 in Eq. (1), thus leading to a favorable product of YVO4 nanoparticles according to Eq. (2). Therefore, the higher the pH value is, the faster the YVO4 nanoparticles form, which benefits the formation of larger particles. It is well known that the W value is a key parameter in determining the dimensions of microemulsion droplets. By adjusting W (W ¼ 12, 16, 20, 24) and keeping pH at 8, the influence of W on the particle size of YVO4 was investigated. Fig. 3 revealed the relationship between XRD patterns and W. It is anticipated that the size of droplets increased with the W value [12], thus, finally leading to an obvious increase of the particle size. However, in our experiments, the particle size deduced from the XRD patterns (Fig. 3) was slightly changed with W. This result was also confirmed by TEM images in Fig. 4. As is well known, the behavior of microemulsion is dependent on the performance of the surfactant and the presence of cosurfactant. In CTAB system, cosurfactant has a distinct influence on the microstructure of the droplets, and its alkyl tail increases the interfacial curvature by increasing the surfactant packing parameter, which favors the formation of small droplets with low water content. When W value increases, the microemulsion tends to form more, small, droplets, until no more surfactant can afford to build up of new micelles and a phase separation occurs, and that is why the increase of W has little effect on the size of the droplets [13]. This is the case in our microemulsion system when W value varied from 12 to 24.
38
L. Sun et al. / Solid State Communications 124 (2002) 35–38
In conclusion, we have reported a simple and effective method to synthesize YVO4 nanoparticles with narrow size distribution. Under certain W value (W ¼ 16), the controllable nanosized YVO4 can be achieved by adjusting the pH of microemulsion, while in comparison, the W value has a slight effect on the size of particles. In view of the convenience and effectiveness of this method, it may also be applied to the synthesis of other substances such as YVO4:Eu and YPO4-based phosphors.
Acknowledgements This work was supported by the NSFC (Nos 20001002, 20023005, 29831010), the State Key Project of Basic Research of MOST (G19980613), the Foundation for University Key Teacher by MOE, and the Founder Foundation of Peking University.
References [1] A.S. Osvaldo, A.C. Simone, R.I. Renata, J. Alloys Compd 316 (2000) 313. [2] G. Panayiotakis, D. Cavouras, I. Kandarakis, C. Nomicos, Appl. Phys. A 62 (1996) 483. [3] J.J. Rubin, L.G. van Uitert, J. Appl. Phys. 37 (1966) 2920. [4] E.A. Maunders, L.G. Deshazer, J. Opt. Soc. 61 (1971) 684. [5] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [6] E. Broch, Z. Phys. Chem. B 20 (1933) 345. [7] S. Ekambaram, K.C. Patil, J. Alloys Compd 217 (1995) 104. [8] S. Erdei, J. Mater. Sci. 30 (1995) 4950. [9] A. Newport, J. Silver, A. Vecht, J. Electrochem. Sci. 147 (2000) 3944. [10] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [11] R.C. Ropp, B. Carroll, J. Inorg. Nucl. Chem. 39 (1977) 1303. [12] J. Zhang, L.D. Sun, C. Qian, C.S. Liao, C.H. Yan, Chin. Sci. Bull. 46 (2001) 1873. [13] M.L. Curri, A. Agostiano, L. Manna, M.D. Monica, M. Catalano, L. Chiavarone, V. Spagnolo, M. Lugara, J. Phys. Chem. B 104 (2000) 8391.
Fabrication of size controllable YVO4 nanoparticles via microemulsion-mediated synthetic process Lingdong Suna, Yingxin Zhangb, Jun Zhanga, Chunhua Yana,*, Chunsheng Liaoa, Yiqiang Lub a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Lab in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry, Peking University, Beijing 100871, People’s Republic of China b Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China Received 18 July 2002; accepted 10 August 2002 by Z. Gan
Abstract Narrow size distributed YVO4 nanoparticles with controllable size were successfully fabricated by direct reaction of Na3VO4 and Y(NO3)3 in CTAB microemulsion systems. By the confinement of the microemulsion droplets, with manipulating the pH value of microemulsion media to 7, 8, 9, 10, the size of YVO4 nanoparticles can be accurately controlled as 8.9, 11.1, 13.9 and 47.2 nm, respectively. X-ray diffraction (XRD) and transmission electron microscope (TEM) were utilized to characterize the structure, size and shape of the nanoparticles, which indicated tetragonal phase YVO4 nanoparticles displaying ellipsoidal or lozenge-like shape. As pH value is an important factor in our experiments and W value (W ¼ [H2O]/[CTAB]) is the key parameter in the formation of microemulsion droplets, their influences on the YVO4 nanoparticles formation were studied. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 81.07.Bc; 81.16.Be; 82.33.Nq Keywords: A. Nanostructures; B. Chemical synthesis; E. Microemulsion-mediated
1. Introduction Yttrium orthovanadate (YVO4) has been known to be a very attractive host lattice for several metal ions. For example, when doped with trivalent Eu3þ ions, it can be utilized as a red phosphor in cathode ray tube (CRT) [1], and scintillator in medical image detectors [2]; while adulterated with certain rare earth oxide, it becomes the optical maser materials [3]. Moreover, it is also a promising polarizer [4] and laser host material in single crystal form [5]. Since Goldschmidt and Haraldsen fabricated YVO4 firstly in 1928 [6], many synthetic methods, such as solid state reaction, solution combustion process [7], hydrolyzed colloid reaction (HCR) technique [8], and urea precipitation followed by calcinations [9], etc. have been developed to * Corresponding author. Tel./fax: þ 86-10-6275-4179. E-mail address: [email protected] (C. Yan).
prepare YVO4 material in view of its important utilization in optoelectronic fields. Solid-state reaction is one of the most common routes to obtain YVO4 materials. However, because of the high reaction temperature and long crystallization process in nature, it is difficult to prepare the nanocrystals free from agglomeration and oxygen deficiencies, which will contribute to the non-radiative recombination paths and therefore be harmful to the optical properties of YVO4. To this extent, the preparation of nanosized YVO4 is necessary to meet the requirement of the host of the phosphors with decreasing size and narrow size distribution, demanded by recent development of displays towards the high quality, efficiency and miniaturization. Meanwhile, the reduction of the particle size will also lead to the decrease of the optimum screening weight, which results in the abatement of the overall thickness of the coating and elimination of pinholes and mottling of the screen [9]. And, the application of nanosized phosphors has
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 4 4 9 - 0
36
L. Sun et al. / Solid State Communications 124 (2002) 35–38
Fig. 1. XRD patterns of YVO4 prepared under different pH (W ¼ 16).
attracted considerable attentions since the report of increased quantum efficiency with decreasing particle size [10]. It has been proved that microemulsion method is not only an effective process to obtain size controllable nanoparticles with narrow distribution by the confinement of microemulsion droplets, but also a simple and convenient route with low costing. Although it has been utilized to synthesize many nanomaterials with controllable size, there is no report on the preparation of YVO4 nanoparticles yet. In this paper, microemulsion-mediated process was employed to synthesize size controllable YVO4 nanoparticles. The results show that the size and size distribution of the nanoparticles can be controlled accurately by modulating the pH and W value (W ¼ [H2O]/[CTAB]) of microemulsion mediated environment in a relatively wide range.
Fig. 2. TEM images of YVO4 nanoparticles prepared under different pH (W ¼ 16).
First, two types of quaternary microemulsions, designated as MA and MB were prepared separately. Both microemulsion contained surfactant CTAB (cetyltrimethylammonium bromide), cosurfactant n-hexanol and oil phase n-heptane in common, while, MA contained Y(NO3)3 aqueous solution, MB contained Na3VO4 and NaOH aqueous solution, respectively. Normally, the composition
of the four-component reverse micelles can be defined by three parameters: W (the molar ratio between water and CTAB), P (the molar ratio between n-hexanol and CTAB), and [CTAB] (the molar concentration of CTAB). In our case, the P, [CTAB] and the same initial molar concentrations of Na3VO4 and Y(NO3)3 were fixed as 5.27, 0.19 and 0.017 mol/l, respectively, and the pH was varied by modulating the concentration of NaOH in MB under certain W value (W ¼ 12, 16, 20, 24) to control the size of YVO4 nanoparticles. Then, MB was added dropwise to MA under constant stirring (vMB/vMA ¼ 1). Subsequently, the mixture was transferred into a 50 ml autoclave for hydrothermal treatment at 150 8C for 2 h to make the nanoparticles well crystallized. Naturally cooled to room temperature, the precipitates of YVO4 nanoparticles were separated from the reaction media by centrifugation and were washed with absolute ethanol and distilled water for several times. Finally, YVO4 nanosized product was dried under vacuum at 60 – 70 8C. The structure, size and morphology of YVO4 nanoparticles were characterized by X-ray diffraction (XRD, Rigaku, D/max-2000, Cu Ka radiation) and transmission electron microscope (TEM, Hitachi, H-800), respectively.
Table 1 Size of YVO4 nanoparticles prepared under different pH (W ¼ 16)
3. Results and discussion
2. Experimental
pH
2u (8)
FWHM (8)
D (nm)
7 8 9 10
25.030 24.937 24.980 24.904
0.9955 0.8103 0.6533 0.2516
8.9 11.1 13.9 47.2
The XRD patterns of YVO4 under different pH values were shown in Fig. 1, which can be indexed as the tetragonal structured YVO4 (JCPDS No. 17-0341). With the increase of the pH of microemulsion media, the diffraction peaks exhibited size-broadened reflections of YVO4 nanocrystallites. The particle size was calculated by fitting the full
L. Sun et al. / Solid State Communications 124 (2002) 35–38
Fig. 3. XRD patterns of YVO4 prepared under different W value (pH ¼ 8).
width at half maximum of (200) peaks with Scherrer Formula (Table 1). It can be seen that when adjusting the pH of microemulsion from 7, 8, 9 to 10 successively, the mean particle size of YVO4 varied from 8.9, 11.1, 13.9 to 47.2 nm accordingly, which are consistent with the results deduced from the TEM images. Fig. 2 shows the TEM images of YVO4 particles prepared under different pH (pH ¼ 7, 8, 9, 10). It is clear that both the size and the shape of particles changed with increasing the pH value. When pH was fixed at 7 or 8, the YVO4 particles exhibited an ellipsoidal shape, while the particle displayed a lozenge-like shape as pH value was increased to 9 or 10. In the present case, microemulsion was used as the reaction media to prepare YVO4 particles. Normally, the
Fig. 4. TEM images of YVO4 nanoparticles prepared under different W value (pH ¼ 8).
37
characteristic dimensions of microemulsion droplets are in nanometer region, and the microemulsion droplets may afford a confined environment for the growth of nanoparticles. The surfactant and cosurfactant adhered to the surface of nanoparticles served as a protective layer to prevent from collision and amalgamation of the droplets. Therefore, microemulsion media play an important role in modulating and confining the particle size of YVO4. Since that the form of vanadium ions are extremely sensitive to the pH of the solution, as reported by Ropp and Carroll [11], the vanadium ions existed as VO2þ ions when the solution was in strong acidity. When the pH rises to 2, vanadium ions were in the form of V10O62 28 principally; while the pH further increased to 5, vanadium ions were liable to the form of V3O32 9 . In our case, since the pH of the microemulsion ranges from 7 to 10, the following reactions might occur 32 þ 3OH2 þ V3 O32 9 $ 3VO4 þ 3H
ð1Þ
3þ ! YVO4 VO32 4 þY
ð2Þ
Based on the above-mentioned mechanisms, given that microemulsion droplets have been confined in nanometer region, the size of YVO4 nanocrystals can be controlled by adjusting the pH of microemulsion media. With increasing the pH of microemulsion media, the reaction favors the formation of VO32 4 in Eq. (1), thus leading to a favorable product of YVO4 nanoparticles according to Eq. (2). Therefore, the higher the pH value is, the faster the YVO4 nanoparticles form, which benefits the formation of larger particles. It is well known that the W value is a key parameter in determining the dimensions of microemulsion droplets. By adjusting W (W ¼ 12, 16, 20, 24) and keeping pH at 8, the influence of W on the particle size of YVO4 was investigated. Fig. 3 revealed the relationship between XRD patterns and W. It is anticipated that the size of droplets increased with the W value [12], thus, finally leading to an obvious increase of the particle size. However, in our experiments, the particle size deduced from the XRD patterns (Fig. 3) was slightly changed with W. This result was also confirmed by TEM images in Fig. 4. As is well known, the behavior of microemulsion is dependent on the performance of the surfactant and the presence of cosurfactant. In CTAB system, cosurfactant has a distinct influence on the microstructure of the droplets, and its alkyl tail increases the interfacial curvature by increasing the surfactant packing parameter, which favors the formation of small droplets with low water content. When W value increases, the microemulsion tends to form more, small, droplets, until no more surfactant can afford to build up of new micelles and a phase separation occurs, and that is why the increase of W has little effect on the size of the droplets [13]. This is the case in our microemulsion system when W value varied from 12 to 24.
38
L. Sun et al. / Solid State Communications 124 (2002) 35–38
In conclusion, we have reported a simple and effective method to synthesize YVO4 nanoparticles with narrow size distribution. Under certain W value (W ¼ 16), the controllable nanosized YVO4 can be achieved by adjusting the pH of microemulsion, while in comparison, the W value has a slight effect on the size of particles. In view of the convenience and effectiveness of this method, it may also be applied to the synthesis of other substances such as YVO4:Eu and YPO4-based phosphors.
Acknowledgements This work was supported by the NSFC (Nos 20001002, 20023005, 29831010), the State Key Project of Basic Research of MOST (G19980613), the Foundation for University Key Teacher by MOE, and the Founder Foundation of Peking University.
References [1] A.S. Osvaldo, A.C. Simone, R.I. Renata, J. Alloys Compd 316 (2000) 313. [2] G. Panayiotakis, D. Cavouras, I. Kandarakis, C. Nomicos, Appl. Phys. A 62 (1996) 483. [3] J.J. Rubin, L.G. van Uitert, J. Appl. Phys. 37 (1966) 2920. [4] E.A. Maunders, L.G. Deshazer, J. Opt. Soc. 61 (1971) 684. [5] J.R. O’Connor, Appl. Phys. Lett. 9 (1966) 407. [6] E. Broch, Z. Phys. Chem. B 20 (1933) 345. [7] S. Ekambaram, K.C. Patil, J. Alloys Compd 217 (1995) 104. [8] S. Erdei, J. Mater. Sci. 30 (1995) 4950. [9] A. Newport, J. Silver, A. Vecht, J. Electrochem. Sci. 147 (2000) 3944. [10] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416. [11] R.C. Ropp, B. Carroll, J. Inorg. Nucl. Chem. 39 (1977) 1303. [12] J. Zhang, L.D. Sun, C. Qian, C.S. Liao, C.H. Yan, Chin. Sci. Bull. 46 (2001) 1873. [13] M.L. Curri, A. Agostiano, L. Manna, M.D. Monica, M. Catalano, L. Chiavarone, V. Spagnolo, M. Lugara, J. Phys. Chem. B 104 (2000) 8391.