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Low temperature preparation of nanocrystalline SrTiO3 and BaTiO3 from alkaline earth nitrates and TiO2 nanocrystals
Powder Technology 212 (2011) 378–381
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Short Communication
Low temperature preparation of nanocrystalline SrTiO3 and BaTiO3 from alkaline earth nitrates and TiO2 nanocrystals Shuang Zhi Liu a,⁎, Tian Xi Wang b, Li Yun Yang a Chemistry and Engineering Department, Kaifeng University, Kaifeng 475004, Henan, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, Henan, China
20
(220)
SrTiO3 is a wide band gap semiconductor (Eg = 3.4 eV [1]) that can be used in many technical fields, such as photocatalysis [2,3], luminescence [4–6] and electroceramic industries [7–11], etc. Recently, great effort has been devoted to the synthesis of SrTiO3 nanopowders [5–7,12–16], driven by the excitement of understanding new science and the hope for better performances and novel applications. Among the many methods developed for the preparation of SrTiO3 powders, solid phase reaction synthesis is regarded as one of the most practical and economical ways [9–14]. However, in the conventional solid phase methods, SrTiO3 powders were usually produced via the reaction between the mechanically mixed SrCO3 or SrO and TiO2 powders above 1000 °C [9–11]. Unfortunately, the reactions at such high temperatures are beyond control and the resultant powders often have large particle size and wide size distribution, etc. Although the hydrothermal or microwave-assisted hydrothermal methods can prepare phase-pure SrTiO3 nanoparticles with uniform particle size at lower temperatures than those needed in the solid phase methods [15–20], many of them are not cost-effective in mass production of SrTiO3 nanopowders. Therefore, for industrial applications, it is still desirable to develop an alternative low temperature solid phase method for the preparation of nanocrystalline SrTiO3 powders at low cost. As a raw material for Sr, Sr(NO3)2 has a melting point of about 570 °C and a boiling point of about 645 °C [21]. Thus, when Sr(NO3) is used as a reactant, it will take on a liquid state at 600 °C, which can
enormously increase the contact area with TiO2 nanocrystals (just like TiO2 nanocrystals immersed in Sr(NO3)2 solution, and liquid–solid phase reaction may occur between them) and accelerate the reaction rate, leading to the lowered reaction temperature and even the improved properties of the resultant product. In addition, as TiO2 nanocrystals with high surface energy possess highly reactive activity, the utilization of them as a reactant can also lead to milder preparation conditions and even improved properties of the resultant products. So, in this communication, we report the low temperature (600 °C) synthesis of nanocrystalline SrTiO3 from low-melting-point Sr(NO3)2 and high-reactive-activity P25 TiO2 nanocrystals, as well as
(211)
1. Introduction
(210)
Keywords: Titanate Nanomaterials Chemical synthesis Characterization
Using low-melting-point Sr(NO3)2 (melting point: 570 °C) and high-reactive-activity TiO2 nanocrystals (Degussa P25 TiO2) as the raw materials, phase-pure SrTiO3 nanocrystals were prepared at 600 °C. X-ray diffraction and X-ray photoelectron spectroscopy revealed that the as-prepared products were cubic phase SrTiO3 with a surface composition of Sr0.97TiO2.92. Transmission electron microscopy image showed that the as-prepared SrTiO3 comprised nanocrystals with the size of about 24–44 nm. UV–vis absorption spectrum of the as-prepared SrTiO3 nanocrystals displayed a wide absorption peak centered at around 365 nm (3.4 eV), together with a tail at the lower energy side. This kind of low temperature and cost-effective method can also be extended to prepare BaTiO3 nanocrystals, simply by substituting Ba(NO3)2 for Sr(NO3)2. © 2011 Elsevier B.V. All rights reserved.
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Article history: Received 16 December 2010 Received in revised form 25 April 2011 Accepted 11 June 2011 Available online 7 July 2011
a b s t r a c t
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i n f o
(110)
a r t i c l e
(100)
b
Intensity (a. u.)
a
30
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2θ (deg.) ⁎ Corresponding author. Tel.: + 86 15039010920. E-mail address: [email protected] (S.Z. Liu). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.06.010
Fig. 1. XRD pattern of the product obtained via heating the mixed powders of Sr(NO3)2 and P25 TiO2 in air at 600 °C for 10 h, combined with a subsequent washing process.
S.Z. Liu et al. / Powder Technology 212 (2011) 378–381
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2. Materials and methods The TiO2 nanocrystals used in our experiments are Degussa P25 TiO2 with a size of about 15–40 nm. Our choice of Degussa P25 TiO2 as a precursor for Ti is due to its availability and low cost. The powders of Sr(NO3)2 and Ba(NO3)2 (melting point: 590 °C) are of analytical grade and bought from Sinopharm Chemical Reagent Co., Ltd. The powders of 4 mmol Sr(NO3)2 or Ba(NO3)2 and 3.4 mmol P25 TiO2 were mixed and ground homogenously in a carnelian mortar, then transferred to a corundum crucible. The crucible was heated in a Muffle furnace at 600 °C for 10 h, then allowed to cool to ambient temperature naturally. The residue solid was washed with 1 mol/l HNO3 aqueous solution and deionized water to remove the impurity (such as SrCO3 or BaCO3), and dried in air at 80 °C. X-ray diffraction (XRD) patterns of the obtained products were recorded on a German Bruker AXS D8 ADVANCE X-ray diffractometer at room temperature. Transmission electron microscopy (TEM) images were taken on a Holland Philips Tecnai-12 transmission electron microscopy. XPS measurements were performed on an American Thermo-VG Scientific ESCALAB 250 XPS system with Al Kα radiation as the exciting source, where the binding energies were calibrated by referencing the C 1s peak (284.6 eV) to reduce the sample charge effect. UV–vis absorption spectra were recorded on a Japan Shimadzu UV-2550 spectrophotometer, with the samples being ultrasonically dispersed in distilled water.
Fig. 2. TEM image of the as-prepared SrTiO3.
Fig. 1 shows the XRD pattern of the product obtained via heating the mixed powders of Sr(NO3)2 and P25 TiO2 in air at 600 °C for 10 h, combined with a subsequent washing process. All its XRD peaks can
Ti 2p3/2
Intensity
Ti 2p1/2
Sr 4s Sr 4p
Sr 3p
Sr 3s
Sr 3d
C 1s
Ti 2p
Ti 2p
O(A)
Ti 2s
Survey
Intensity
3. Results and discussion
O 1s
the characterization of the resultant product by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–vis absorption spectra. Furthermore, it is found that a similar procedure can also be used to prepare BaTiO3 nanocrystals, simply by replacing Sr(NO3)2 with Ba(NO3)2.
1000
800
600
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200
0
470
468
466
Binding energy (eV) Sr 3d5/2
Sr 3d
464
462
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Binding energy (eV)
O1s
O1s O1s in SrTiO3 O1s in CO2
Sr 3d3/2
140
Intensity
Intensity
backgroud
138
136
134
132
130
128
540
538
536
Binding energy (eV)
534
532
530
528
Binding energy (eV) Fig. 3. XPS spectra of the as-prepared SrTiO3.
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S.Z. Liu et al. / Powder Technology 212 (2011) 378–381
Absorbance
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Wavelength (nm) Fig. 4. UV–vis absorption spectrum of the as-prepared SrTiO3.
20
(101) (110) 30
Fig. 6. TEM image of the as-prepared BaTiO3.
cores reveals that the surface composition of this product is Sr0.97TiO2.92, which is slightly deficient in Sr and O. Fig. 4 shows the UV–vis absorption spectrum of the as-prepared SrTiO3 nanocrystals. As can be seen, this product displays a wide absorption peak centered at around 343 nm (3.6 eV), which is slightly blue-shifted compared with the band gap of bulk SrTiO3[1]. Besides, there is an absorption tail at the lower energy side, which may be due to the presence of surface states and defect levels in the nanocrystalline product [24,25]. A similar procedure has been further used to prepare BaTiO3 powders. Fig. 5 shows the XRD pattern of the product prepared via heating the mixed powders of Ba(NO3)2 and P25 TiO2 in air at 600 °C for 10 h, combined with a subsequent washing process. It displays only the characteristic XRD peaks of tetragonal phase BaTiO3 (JCPDS card no. 00-005-0626). From the TEM image in Fig. 6, it can be seen that the as-prepared BaTiO3 consists of nanocrystals with a size of about 38–158 nm. 4. Conclusions Phase-pure SrTiO3 and BaTiO3 nanocrystals were synthesized via the reactions between low-melting-point strontium or barium nitrate and high-reactive-activity P25 TiO2 nanocrystals at 600 °C. The as-prepared SrTiO3 nanocrystals exhibited a wide and strong UV–vis absorption, which may be used as a kind of photocatalyst or photoelectric material.
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(112) (211)
(002) (200)
(102) (201) (210)
References
(111)
(001) (100)
Intensity (a. u.)
be indexed to cubic perovskite structure SrTiO3 [JCPDS card no. 01-073-0661], and no XRD peaks arising from the possible impurities such as TiO2, SrCO3, SrO or Sr2TiO4, etc. are observed, suggesting the successful preparation of phase-pure SrTiO3. Fig. 2 shows the TEM image of the as-prepared SrTiO3 powders. As can be seen from Fig. 2, this product comprises mainly nanocrystals with a size of about 24–44 nm. However, the aggregation phenomenon occurs for this nanocrystalline product. This may be explained from the following two main reasons. First, nanocrystals generally possess large surface area and high surface energy, and have the tendency to aggregate into larger agglomeration to reduce their surface energy. Second, the current preparation was carried out at a relatively high temperature (600 °C), which may cause the sintering of the resultant product [22]. The surface composition of the as-prepared SrTiO3 nanocrystals was determined by XPS. The XPS survey spectrum in Fig. 3 reveals that this product is made of only Sr, Ti and O elements, except for C from the adsorbed CO2 molecules. From the high resolution XPS spectra of Sr 3d and Ti 2p (Fig. 3), the binding energies of Sr 3d5/2 and Ti 2p3/2 are observed at around 132.4 and 458.4 eV, respectively, which agreed with the literature values for SrTiO3[23]. Moreover, it is noted from the O 1s spectrum (Fig. 3) that there are only two kinds of O in this product: one is the O with a binding energy of 529.7 eV in SrTiO3[23], and the other with a binding energy of 532.4 eV is the O in the adsorbed CO2 molecules. The above XPS analysis further indicates that the as-prepared SrTiO3 is free of TiO2 and SrCO3 impurities, etc. Besides, quantification of the peak areas of the Sr 3d, Ti 2p and O 1s
60
70
2θ (deg.) Fig. 5. XRD pattern of the product obtained via heating the mixed powders of Ba(NO3)2 and P25 TiO2 in air at 600 °C for 10 h, combined with a subsequent washing process.
[1] F.M. Pontes, E. Longo, E.R. Leite, E.J.H. Lee, J.A. Varela, P.S. Pizani, Photoluminescence at room temperature in amorphous SrTiO3 thin films obtained by chemical solution deposition, Mater. Chem. Phys. 77 (2002) 598–602. [2] U. Sulaeman, S. Yin, T. Sato, Visible light photocatalytic activity induced by the carboxyl group chemically bonded on the surface of SrTiO3, Appl. Catal. B: Environ. 102 (2011) 286–290. [3] N. Wang, D. Kong, H. He, Solvothermal synthesis of strontium titanate nanocrystallines from metatitanic acid and photocatalytic activities, Powder Technol. 207 (2011) 470–473. [4] F. Fujishiro, T. Arakawa, T. Hashimoto, Substitution site and photoluminescence spectra of Eu3+-substituted SrTiO3 prepared by Pechini method, Mater. Lett. 65 (2011) 1819–1821. [5] J. Meng, Y. Huang, W. Zhang, Z. Du, Z. Zhu, G. Zou, Photoluminescence in nanocrystalline BaTiO3 and SrTiO3, Phys. Lett. A 205 (1995) 72–76. [6] L.F. da Silva, L.J.Q. Maia, M.I.B. Bernardi, J.A. Andrés, V.R. Mastelaro, An improved method for preparation of SrTiO3 nanoparticles, Mater. Chem. Phys. 125 (2011) 168–173. [7] C.N. George, J.K. Thomas, R. Jose, H.P. Kumar, M.K. Suresh, V.R. Kumar, Synthesis and characterization of nanocrystalline strontium titanate through a modified
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combustion method and its sintering and dielectric properties, J. Alloy. Comp. 486 (2009) 711–715. S. Chao, F. Dogan, BaTiO3–SrTiO3 layered dielectrics for energy storage, Mater. Lett. 65 (2011) 978–981. A. Ioachim, M.I. Toacsan, M.G. Banciu, L. Nedelcu, F. Vasiliu, H.V. Alexandru, C. Berbecaru, G. Stoica, Barium strontium titanate-based perovskite materials for microwave applications, Prog. Solid State Chem. 35 (2007) 513–520. M.B. Park, N.H. Cho, The effect of the sintering atmosphere on the electrical and chemical characteristics of grain boundaries in SrTiO3 ceramics prepared from semiconducting powders, Solid State Ionics 154–155 (2002) 175–181. J. Li, S. Luo, M.A. Alim, The role of TiO2 powder on the SrTiO3-based synthesized varistor materials, Mater. Lett. 60 (2006) 720–724. Q.A. Zhu, J.G. Xu, S. Xiang, L.X. Chen, Z.G. Tan, Preparation of SrTiO3 nanoparticles by the combination of solid phase grinding and low temperature calcining, Mater. Lett. 65 (2011) 873–875. T.X. Wang, S.Z. Liu, J. Chen, Molten salt synthesis of SrTiO3 nanocrystals using nanocrystalline TiO2 as a precursor, Powder Technol. 205 (2011) 289–291. H.L. Li, Z.N. Du, G.L. Wang, Y.C. Zhang, Low temperature molten salt synthesis of SrTiO3 submicron crystallites and nanocrystals in the eutectic NaCl–KCl, Mater Lett 64 (2010) 431–434. A.Z. Simões, F. Moura, T.B. Onofre, M.A. Ramirez, J.A. Varela, E. Longo, Microwavehydrothermal synthesis of barium strontium titanate nanoparticles, J. Alloy. Comp. 508 (2010) 620–624. M. Makarova, A. Dejneka, J. Franc, J. Drahokoupil, L. Jastrabik, V. Trepakov, Soft chemistry preparation methods and properties of strontium titanate nanoparticles, Opt. Mater. 32 (2010) 803–806.
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[17] Y.M. Rangel-Hernandez, J.C. Rendón-Angeles, Z. Matamoros-Veloza, M.I. PechCanul, S. Diaz-de la Torre, K. Yanagisawa, One-step synthesis of fine SrTiO3 particles using SrSO4 ore under alkaline hydrothermal conditions, Chem. Eng. J. 155 (2009) 483–492. [18] L. Yang, Y. Wang, Y. Wang, X. Wang, X. Guo, G. Han, Synthesis of single-crystal Ba1 − xSrxTiO3 (x = 0–1) dendrites via a simple hydrothermal method, J. Alloy. Comp. 500 (2010) L1–L5. [19] K.A. Razak, A. Asadov, J. Yoo, E. Haemmerle, W. Gao, Structural and dielectric properties of barium strontium titanate produced by high temperature hydrothermal method, J. Alloy. Comp. 449 (2008) 19–23. [20] S.B. Deshpande, Y.B. Khollam, S.V. Bhoraskar, S.K. Date, S.R. Sainkar, H.S. Potdar, Synthesis and characterization of microwave-hydrothermally derived Ba1 − xSrxTiO3 powders, Mater. Lett. 59 (2005) 293–296. [21] P.W. Shen, J.T. Wang, Dictionary of Compounds, Shanghai Lexicographical Publishing House, Shanghai, 2002. [22] Li Jing, Zhang Yong Cai, Wang Tian Xi, Zhang Ming, Low temperature synthesis and optical properties of CaTiO3 nanoparticles from Ca(NO3)2·4H2O and TiO2 nanocrystals, Mater. Lett. 65 (2011) 1556–1558. [23] C.D. Wagner, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Minnesota, 1979. [24] C.D. Pinheiro, E. Longo, E.R. Leite, F.M. Pontes, R. Magnani, J.A. Varela, The role of defect states in the creation of photoluminescence in SrTiO3, Appl. Phys. A 77 (2003) 81–85. [25] W.F. Zhang, Z. Yin, M.S. Zhang, Study of photoluminescence and electronic states in nanophase strontium titanate, Appl. Phys. A 70 (2000) 93–96.
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
Short Communication
Low temperature preparation of nanocrystalline SrTiO3 and BaTiO3 from alkaline earth nitrates and TiO2 nanocrystals Shuang Zhi Liu a,⁎, Tian Xi Wang b, Li Yun Yang a Chemistry and Engineering Department, Kaifeng University, Kaifeng 475004, Henan, China School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang 453003, Henan, China
20
(220)
SrTiO3 is a wide band gap semiconductor (Eg = 3.4 eV [1]) that can be used in many technical fields, such as photocatalysis [2,3], luminescence [4–6] and electroceramic industries [7–11], etc. Recently, great effort has been devoted to the synthesis of SrTiO3 nanopowders [5–7,12–16], driven by the excitement of understanding new science and the hope for better performances and novel applications. Among the many methods developed for the preparation of SrTiO3 powders, solid phase reaction synthesis is regarded as one of the most practical and economical ways [9–14]. However, in the conventional solid phase methods, SrTiO3 powders were usually produced via the reaction between the mechanically mixed SrCO3 or SrO and TiO2 powders above 1000 °C [9–11]. Unfortunately, the reactions at such high temperatures are beyond control and the resultant powders often have large particle size and wide size distribution, etc. Although the hydrothermal or microwave-assisted hydrothermal methods can prepare phase-pure SrTiO3 nanoparticles with uniform particle size at lower temperatures than those needed in the solid phase methods [15–20], many of them are not cost-effective in mass production of SrTiO3 nanopowders. Therefore, for industrial applications, it is still desirable to develop an alternative low temperature solid phase method for the preparation of nanocrystalline SrTiO3 powders at low cost. As a raw material for Sr, Sr(NO3)2 has a melting point of about 570 °C and a boiling point of about 645 °C [21]. Thus, when Sr(NO3) is used as a reactant, it will take on a liquid state at 600 °C, which can
enormously increase the contact area with TiO2 nanocrystals (just like TiO2 nanocrystals immersed in Sr(NO3)2 solution, and liquid–solid phase reaction may occur between them) and accelerate the reaction rate, leading to the lowered reaction temperature and even the improved properties of the resultant product. In addition, as TiO2 nanocrystals with high surface energy possess highly reactive activity, the utilization of them as a reactant can also lead to milder preparation conditions and even improved properties of the resultant products. So, in this communication, we report the low temperature (600 °C) synthesis of nanocrystalline SrTiO3 from low-melting-point Sr(NO3)2 and high-reactive-activity P25 TiO2 nanocrystals, as well as
(211)
1. Introduction
(210)
Keywords: Titanate Nanomaterials Chemical synthesis Characterization
Using low-melting-point Sr(NO3)2 (melting point: 570 °C) and high-reactive-activity TiO2 nanocrystals (Degussa P25 TiO2) as the raw materials, phase-pure SrTiO3 nanocrystals were prepared at 600 °C. X-ray diffraction and X-ray photoelectron spectroscopy revealed that the as-prepared products were cubic phase SrTiO3 with a surface composition of Sr0.97TiO2.92. Transmission electron microscopy image showed that the as-prepared SrTiO3 comprised nanocrystals with the size of about 24–44 nm. UV–vis absorption spectrum of the as-prepared SrTiO3 nanocrystals displayed a wide absorption peak centered at around 365 nm (3.4 eV), together with a tail at the lower energy side. This kind of low temperature and cost-effective method can also be extended to prepare BaTiO3 nanocrystals, simply by substituting Ba(NO3)2 for Sr(NO3)2. © 2011 Elsevier B.V. All rights reserved.
(200)
Article history: Received 16 December 2010 Received in revised form 25 April 2011 Accepted 11 June 2011 Available online 7 July 2011
a b s t r a c t
(111)
i n f o
(110)
a r t i c l e
(100)
b
Intensity (a. u.)
a
30
40
50
60
70
2θ (deg.) ⁎ Corresponding author. Tel.: + 86 15039010920. E-mail address: [email protected] (S.Z. Liu). 0032-5910/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2011.06.010
Fig. 1. XRD pattern of the product obtained via heating the mixed powders of Sr(NO3)2 and P25 TiO2 in air at 600 °C for 10 h, combined with a subsequent washing process.
S.Z. Liu et al. / Powder Technology 212 (2011) 378–381
379
2. Materials and methods The TiO2 nanocrystals used in our experiments are Degussa P25 TiO2 with a size of about 15–40 nm. Our choice of Degussa P25 TiO2 as a precursor for Ti is due to its availability and low cost. The powders of Sr(NO3)2 and Ba(NO3)2 (melting point: 590 °C) are of analytical grade and bought from Sinopharm Chemical Reagent Co., Ltd. The powders of 4 mmol Sr(NO3)2 or Ba(NO3)2 and 3.4 mmol P25 TiO2 were mixed and ground homogenously in a carnelian mortar, then transferred to a corundum crucible. The crucible was heated in a Muffle furnace at 600 °C for 10 h, then allowed to cool to ambient temperature naturally. The residue solid was washed with 1 mol/l HNO3 aqueous solution and deionized water to remove the impurity (such as SrCO3 or BaCO3), and dried in air at 80 °C. X-ray diffraction (XRD) patterns of the obtained products were recorded on a German Bruker AXS D8 ADVANCE X-ray diffractometer at room temperature. Transmission electron microscopy (TEM) images were taken on a Holland Philips Tecnai-12 transmission electron microscopy. XPS measurements were performed on an American Thermo-VG Scientific ESCALAB 250 XPS system with Al Kα radiation as the exciting source, where the binding energies were calibrated by referencing the C 1s peak (284.6 eV) to reduce the sample charge effect. UV–vis absorption spectra were recorded on a Japan Shimadzu UV-2550 spectrophotometer, with the samples being ultrasonically dispersed in distilled water.
Fig. 2. TEM image of the as-prepared SrTiO3.
Fig. 1 shows the XRD pattern of the product obtained via heating the mixed powders of Sr(NO3)2 and P25 TiO2 in air at 600 °C for 10 h, combined with a subsequent washing process. All its XRD peaks can
Ti 2p3/2
Intensity
Ti 2p1/2
Sr 4s Sr 4p
Sr 3p
Sr 3s
Sr 3d
C 1s
Ti 2p
Ti 2p
O(A)
Ti 2s
Survey
Intensity
3. Results and discussion
O 1s
the characterization of the resultant product by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV–vis absorption spectra. Furthermore, it is found that a similar procedure can also be used to prepare BaTiO3 nanocrystals, simply by replacing Sr(NO3)2 with Ba(NO3)2.
1000
800
600
400
200
0
470
468
466
Binding energy (eV) Sr 3d5/2
Sr 3d
464
462
460
458
456
454
Binding energy (eV)
O1s
O1s O1s in SrTiO3 O1s in CO2
Sr 3d3/2
140
Intensity
Intensity
backgroud
138
136
134
132
130
128
540
538
536
Binding energy (eV)
534
532
530
528
Binding energy (eV) Fig. 3. XPS spectra of the as-prepared SrTiO3.
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S.Z. Liu et al. / Powder Technology 212 (2011) 378–381
Absorbance
380
200
300
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis absorption spectrum of the as-prepared SrTiO3.
20
(101) (110) 30
Fig. 6. TEM image of the as-prepared BaTiO3.
cores reveals that the surface composition of this product is Sr0.97TiO2.92, which is slightly deficient in Sr and O. Fig. 4 shows the UV–vis absorption spectrum of the as-prepared SrTiO3 nanocrystals. As can be seen, this product displays a wide absorption peak centered at around 343 nm (3.6 eV), which is slightly blue-shifted compared with the band gap of bulk SrTiO3[1]. Besides, there is an absorption tail at the lower energy side, which may be due to the presence of surface states and defect levels in the nanocrystalline product [24,25]. A similar procedure has been further used to prepare BaTiO3 powders. Fig. 5 shows the XRD pattern of the product prepared via heating the mixed powders of Ba(NO3)2 and P25 TiO2 in air at 600 °C for 10 h, combined with a subsequent washing process. It displays only the characteristic XRD peaks of tetragonal phase BaTiO3 (JCPDS card no. 00-005-0626). From the TEM image in Fig. 6, it can be seen that the as-prepared BaTiO3 consists of nanocrystals with a size of about 38–158 nm. 4. Conclusions Phase-pure SrTiO3 and BaTiO3 nanocrystals were synthesized via the reactions between low-melting-point strontium or barium nitrate and high-reactive-activity P25 TiO2 nanocrystals at 600 °C. The as-prepared SrTiO3 nanocrystals exhibited a wide and strong UV–vis absorption, which may be used as a kind of photocatalyst or photoelectric material.
40
50
(202) (220)
(112) (211)
(002) (200)
(102) (201) (210)
References
(111)
(001) (100)
Intensity (a. u.)
be indexed to cubic perovskite structure SrTiO3 [JCPDS card no. 01-073-0661], and no XRD peaks arising from the possible impurities such as TiO2, SrCO3, SrO or Sr2TiO4, etc. are observed, suggesting the successful preparation of phase-pure SrTiO3. Fig. 2 shows the TEM image of the as-prepared SrTiO3 powders. As can be seen from Fig. 2, this product comprises mainly nanocrystals with a size of about 24–44 nm. However, the aggregation phenomenon occurs for this nanocrystalline product. This may be explained from the following two main reasons. First, nanocrystals generally possess large surface area and high surface energy, and have the tendency to aggregate into larger agglomeration to reduce their surface energy. Second, the current preparation was carried out at a relatively high temperature (600 °C), which may cause the sintering of the resultant product [22]. The surface composition of the as-prepared SrTiO3 nanocrystals was determined by XPS. The XPS survey spectrum in Fig. 3 reveals that this product is made of only Sr, Ti and O elements, except for C from the adsorbed CO2 molecules. From the high resolution XPS spectra of Sr 3d and Ti 2p (Fig. 3), the binding energies of Sr 3d5/2 and Ti 2p3/2 are observed at around 132.4 and 458.4 eV, respectively, which agreed with the literature values for SrTiO3[23]. Moreover, it is noted from the O 1s spectrum (Fig. 3) that there are only two kinds of O in this product: one is the O with a binding energy of 529.7 eV in SrTiO3[23], and the other with a binding energy of 532.4 eV is the O in the adsorbed CO2 molecules. The above XPS analysis further indicates that the as-prepared SrTiO3 is free of TiO2 and SrCO3 impurities, etc. Besides, quantification of the peak areas of the Sr 3d, Ti 2p and O 1s
60
70
2θ (deg.) Fig. 5. XRD pattern of the product obtained via heating the mixed powders of Ba(NO3)2 and P25 TiO2 in air at 600 °C for 10 h, combined with a subsequent washing process.
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