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Low temperature solvothermal synthesis, optical and electric properties of tetragonal phase BaTiO3 nanocrystals using BaCO3 powder
Materials Letters 100 (2013) 1–3
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Low temperature solvothermal synthesis, optical and electric properties of tetragonal phase BaTiO3 nanocrystals using BaCO3 powder Haixin Bai, Xiaohua Liu n College of Sciences, Henan Agricultural University, Zhengzhou, Henan 450002, China
a r t i c l e i n f o
abstract
Article history: Received 26 December 2012 Accepted 27 February 2013 Available online 7 March 2013
A simple low temperature solvothermal method, which was based on the reactions of BaCO3, tetrabutyl titanate and NaOH in ethanol in an autoclave at 180 1C for 12 h, was proposed for the synthesis of BaTiO3. The characterization results from X-ray diffraction, Raman spectroscopy and transmission electron microscopy revealed the preparation of pure tetragonal phase BaTiO3 nanocrystals. The UV–vis diffuse reflectance spectrum indicated that the as-synthesized BaTiO3 nanocrystals had a direct band gap of about 3.42 eV. The room temperature photoluminescence spectrum of the as-synthesized BaTiO3 nanocrystals upon laser excitation at 325 nm exhibited a strong and broad emission in the wavelength range of about 480–625 nm, which may be originated from the surface states and other crystal defects. The impedance spectroscopy disclosed that the electric resistance and electric capacity of the assynthesized BaTiO3 nanocrystals were 1.4 104 O and 5.7 10 7 F, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Nanomaterials Optical materials and properties Powder technology
1. Introduction BaTiO3 is of great importance in many technical applications owing to its outstanding chemical and physical properties [1–8]. Nevertheless, the practical performance of BaTiO3 is related to its phase, size, crystal defects, etc., which ultimately depended on their preparation methods and preparation conditions [1–8]. Hence, much effort has been made to design novel routes for the synthesis of BaTiO3 nanomaterials with different characteristics, aiming at improving the properties of BaTiO3 powders for their intended purposes [1–8]. The solvothermal method is a versatile wet chemical process that has been widely used to synthesize inorganic nanomaterials [9–12]. The main advantage lies in its capability of obtaining highly crystalline, uniform-sized products at considerably low temperatures [9–12]. In addition, it also can induce the formation of metastable phase (e.g., obtaining high temperature stable phase at a much lower temperature [12]). Besides, the synthesis in nonaqueous solvents can avoid the incorporation of lattice hydroxyl groups, which can cause the formation of undesired porosity during the sintering of multilayer ceramic capacitors [5]. Herein, we report a low temperature (180 1C) and low cost solvothermal route for the synthesis of tetragonal phase and hydroxyl-free BaTiO3 nanocrystals, using BaCO3, tetrabutyl titanate and NaOH as the source materials and ethanol as the solvent.
n
Corresponding author. Tel.: þ86 371 63554844. E-mail addresses: [email protected], [email protected] (X. Liu). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.02.106
The structure, optical and electric properties of the as-synthesized product are characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, UV–vis diffuse reflectance spectrum, room temperature photoluminescence spectrum and impedance spectroscopy. The present work has at least two contributions. First, it for the first time adopts BaCO3 as the Ba precursor. As BaCO3 is one of main existing forms of Ba in natural ores, it is plentiful and cheap. Second, it is among the very limited studies on the optical properties of BaTiO3 nanocrystals.
2. Material and methods All the reagents were of analytical grade and used directly after purchase from Sinopharm Chemical Reagent Co. Ltd. The powders of 3 mmol BaCO3 and 80 mmol NaOH were weighed into a 50 ml Teflon-lined stainless steel autoclave, and 40 ml of ethanol was added and stirred for 10 min. Then, 1.0 ml of tetrabutyl titanate was added and stirred for 30 min. The autoclave was sealed and heated in an electric oven at 180 1C for 12 h, then cooled to ambient temperature naturally. The resultant precipitate was filtered, washed with deionized water, and dried in vacuum at 100 1C for 4 h. The obtained product was characterized by X-ray diffraction (XRD, Bruker AXS D8 ADVANCE X-Ray Diffractometer), Raman and room temperature photoluminescence spectra (Renishaw Invia Raman spectrometer), transmission electron microscopy (TEM, Philips Tecnai-12 transmission electron microscopy), UV– vis diffuse reflectance spectrum (Varian Cary 5000 UV–vis–NIR
2
H. Bai, X. Liu / Materials Letters 100 (2013) 1–3
spectrophotometer) and impedance spectroscopy (Autolab PGSTAT 30 electrochemical workstation).
3. Results and discussion Fig. 1 shows the XRD pattern of the as-synthesized product. All its diffraction peaks can be readily indexed to tetragonal phase BaTiO3 (JCPDS card no. 01-089-1428). Thus, the as-synthesized BaTiO3 is free of the possible impurities such as TiO2, BaCO3, etc. Besides, the sharp and intense XRD peaks suggest that the assynthesized BaTiO3 is well crystallized. Raman spectroscopy is a very sensitive tool to identify the tetragonal or cubic phase structure of BaTiO3 [1–4]. For instance, cubic BaTiO3 inherently has no Raman active modes, whereas eight Raman active modes are expected for the noncentrosymmetric tetragonal structure, 3A1gþB1gþ4Eg [1–4]. Fig. 2 shows the Raman spectrum of the as-synthesized BaTiO3. The peaks at around 185 and 513 cm 1 can be assigned to the fundamental transverse component of the optical (TO) mode of A1 symmetry, and the peak at about 722 cm 1 is related to the highest frequency longitudinal optical mode (LO) of A1 symmetry [1–5]. The peak at 306 cm 1, which can be attributed to the B1 mode and
20
30
40
50
60
70
F(R )
(311) (113)
(310) (301) (103)
(300) (212)
(220) (202)
(211) (112)
(210) (201) (102)
(200) (002)
(111)
(100) (001)
Intensity (a. u.)
(110) (101)
Fig. 3. TEM image of the as-synthesized BaTiO3.
80
2θ (deg.)
300
700
(F(R )hν)2
(722)
(513) 200
600
(306)
Intensity (a. u.)
(185)
Fig. 1. XRD pattern of the as-synthesized BaTiO3.
400 500 Wavelength (nm)
400
600 800 Raman shift (cm-1)
1000
Fig. 2. Raman spectrum of the as-synthesized BaTiO3.
1200
2.0
2.5
3.0
3.5 hν (eV)
4.0
4.5
5.0
Fig. 4. (a) UV–vis diffuse reflectance spectrum of the as-synthesized BaTiO3, and (b) Plot of (F(RN)hn)2 vs (hn) for estimating the optical band gap of this sample.
Intensity (a. u.)
H. Bai, X. Liu / Materials Letters 100 (2013) 1–3
400
450
500 550 600 Wavelength (nm)
650
700
Fig. 5. RTPL spectrum of the as-synthesized BaTiO3 nanocrystals upon laser excitation at 325 nm.
indicate the asymmetry of TiO6 octahedra, is a characteristic peak for tetragonal phase BaTiO3 [1–5]. It can be seen that the peak at 306 cm 1 in Fig. 2 is sharp and strong, confirming the formation of tetragonal phase BaTiO3. This was consistent with the aforementioned XRD result (Fig. 1). Furthermore, unlike the Raman spectra of the BaTiO3 products synthesized in aqueous solutions, there is no peak observable at around 805–808 cm 1 in Fig. 2, indicating that the as-synthesized BaTiO3 is free of hydroxyl lattice groups [1–4]. Fig. 3 shows the TEM image of the as-synthesized BaTiO3. As can be seen, this product comprised nanocrystals with the sizes of about 44–80 nm. The UV–vis diffuse reflectance spectrum of the as-synthesized BaTiO3 nanocrystals was measured and converted into the absorption mode (Fig. 4(a)) using the Kubelka–Munk Function [13] FðR1 Þ ¼ ð1R1 Þ2 =2R1 ¼ a=S R1 ¼ Rsample =RBaSO4 where F(RN), R, a and S are the Kubelka–Munk function, reflectance, absorption coefficient and scattering coefficient, respectively. The band gap (Eg) of this sample was determined based on the theory of optical absorption for direct band gap semiconductors [14,15]
ahn ¼ BðhnEg Þ1=2 where hn and B are discrete photon energy and a constant relative to the material, respectively. For the diffused reflectance spectra, F(RN) can be used instead of a for estimating the Eg value [13]. So, the curve of (F(RN)hn)2 vs (hn) for this sample is plotted in Fig. 4(b). By extrapolating the linear portion of the (F(RN)hn)2 vs (hn) curve to
3
F(RN)¼0, the Eg of the as-synthesized BaTiO3 nanocrystals was estimated to be 3.42 eV. Fig. 5 shows the RTPL spectrum of the as-synthesized BaTiO3 nanocrystals upon laser excitation at 325 nm. It exhibited a strong and broad emission in the wavelength range of about 480– 625 nm, which was obviously red-shifted as compared with its Eg (3.42 eV). The room temperature photoluminescence of this sample was probably originated from its ‘‘broken’’ surface or dangling bonds (The electrons in the surface dangling bonds of nanocrystalline materials are in the unpaired state and can be considered as a kind of defects [16].) and other crystal defects (such as nonstoichiometry) [16,17]. The electric properties of the as-synthesized BaTiO3 nanocrystals were measured by impedance spectroscopy. The obtained results revealed that the electric resistance and electric capacity of the as-synthesized BaTiO3 nanocrystals were 1.4 104 O and 5.7 10 7 F, respectively.
4. Conclusions Tetragonal phase BaTiO3 nanocrystals were synthesized via the solvothermal reactions of BaCO3, tetrabutyl titanate and NaOH in ethanol at 180 1C, which is much lower than the reaction temperatures (above 1000 1C) required by conventional solid phase methods. The as-synthesized BaTiO3 nanocrystals had a direct band gap of about 3.42 eV, and exhibited a defect-induced strong and broad emission in the wavelength range of about 480–625 nm upon laser excitation at 325 at room temperature. The electric resistance and electric capacity of the as-synthesized BaTiO3 nanocrystals were determined to be 1.4 104 O and 5.7 10 7 F, respectively, by impedance spectroscopy. References [1] Tian X, Li J, Chen K, Han J, Pan S. Cryst Growth Des 2010;10:3990–5. [2] Zhu YF, Zhang L, Natsuki T, Fu YQ, Ni QQ. ACS Appl Mater Interfaces 2012;4:2101–6. [3] Maxim F, Ferreira P, Vilarinho PM, Aimable A. Cryst Growth Des 2010;10: 3996–4004. ˇ ´ epanovic´ MJ, Dojcˇilovic´ J. Ceram Int 2011;37:2513–8. [4] Pavlovic´ VP, Krstic´ J, Sc [5] Zhang YC, Wang GL, Li KW, Zhang M, Hu XY. J Cryst Growth 2006;290:513–7. [6] Katsuki H, Furuta S, Komarneni S. Mater Lett 2012;83:8–10. ¨ zen M, Mertens M, Schroeven M, Snijkers F, Cool P. Mater Lett 2012;67: [7] O 154–7. [8] Singh KC, Nath AK. Mater Lett 2011;65:970–3. [9] Zhang YC, Wang GL. Mater Lett 2008;62:673–5. [10] Ji B, Chen D, Jiao X, Zhao Z, Jiao Y. Mater Lett 2010;64:1836–8. [11] Zhang YC, Qiao T, Hu XY. J Solid State Chem 2004;177:4093–7. [12] Zhang YC, Wang GY, Hu XY, Shi QF, Qiao T. J Cryst Growth 2005;284:554–60. [13] Zhang YC, Li J, Zhang M, Dionysiou DD. Environ Sci Technol 2011;45: 9324–31. [14] Wang P, Fan C, Wang Y, Ding G, Yuan P. Mater Res Bull 2013;48:869–77. [15] Ahadi K, Mahdavi SM, Nemati A, Tabesh M. Mater Lett 2012;72:107–9. [16] Li J, Zhang YC, Wang TX, Zhang M. Mater Lett 2011;65:1556–8. [17] Li H, Huang S, Zhang W, Pan W. J Alloy Compd 2013;551:131–5.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Low temperature solvothermal synthesis, optical and electric properties of tetragonal phase BaTiO3 nanocrystals using BaCO3 powder Haixin Bai, Xiaohua Liu n College of Sciences, Henan Agricultural University, Zhengzhou, Henan 450002, China
a r t i c l e i n f o
abstract
Article history: Received 26 December 2012 Accepted 27 February 2013 Available online 7 March 2013
A simple low temperature solvothermal method, which was based on the reactions of BaCO3, tetrabutyl titanate and NaOH in ethanol in an autoclave at 180 1C for 12 h, was proposed for the synthesis of BaTiO3. The characterization results from X-ray diffraction, Raman spectroscopy and transmission electron microscopy revealed the preparation of pure tetragonal phase BaTiO3 nanocrystals. The UV–vis diffuse reflectance spectrum indicated that the as-synthesized BaTiO3 nanocrystals had a direct band gap of about 3.42 eV. The room temperature photoluminescence spectrum of the as-synthesized BaTiO3 nanocrystals upon laser excitation at 325 nm exhibited a strong and broad emission in the wavelength range of about 480–625 nm, which may be originated from the surface states and other crystal defects. The impedance spectroscopy disclosed that the electric resistance and electric capacity of the assynthesized BaTiO3 nanocrystals were 1.4 104 O and 5.7 10 7 F, respectively. & 2013 Elsevier B.V. All rights reserved.
Keywords: Semiconductors Nanomaterials Optical materials and properties Powder technology
1. Introduction BaTiO3 is of great importance in many technical applications owing to its outstanding chemical and physical properties [1–8]. Nevertheless, the practical performance of BaTiO3 is related to its phase, size, crystal defects, etc., which ultimately depended on their preparation methods and preparation conditions [1–8]. Hence, much effort has been made to design novel routes for the synthesis of BaTiO3 nanomaterials with different characteristics, aiming at improving the properties of BaTiO3 powders for their intended purposes [1–8]. The solvothermal method is a versatile wet chemical process that has been widely used to synthesize inorganic nanomaterials [9–12]. The main advantage lies in its capability of obtaining highly crystalline, uniform-sized products at considerably low temperatures [9–12]. In addition, it also can induce the formation of metastable phase (e.g., obtaining high temperature stable phase at a much lower temperature [12]). Besides, the synthesis in nonaqueous solvents can avoid the incorporation of lattice hydroxyl groups, which can cause the formation of undesired porosity during the sintering of multilayer ceramic capacitors [5]. Herein, we report a low temperature (180 1C) and low cost solvothermal route for the synthesis of tetragonal phase and hydroxyl-free BaTiO3 nanocrystals, using BaCO3, tetrabutyl titanate and NaOH as the source materials and ethanol as the solvent.
n
Corresponding author. Tel.: þ86 371 63554844. E-mail addresses: [email protected], [email protected] (X. Liu). 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.02.106
The structure, optical and electric properties of the as-synthesized product are characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, UV–vis diffuse reflectance spectrum, room temperature photoluminescence spectrum and impedance spectroscopy. The present work has at least two contributions. First, it for the first time adopts BaCO3 as the Ba precursor. As BaCO3 is one of main existing forms of Ba in natural ores, it is plentiful and cheap. Second, it is among the very limited studies on the optical properties of BaTiO3 nanocrystals.
2. Material and methods All the reagents were of analytical grade and used directly after purchase from Sinopharm Chemical Reagent Co. Ltd. The powders of 3 mmol BaCO3 and 80 mmol NaOH were weighed into a 50 ml Teflon-lined stainless steel autoclave, and 40 ml of ethanol was added and stirred for 10 min. Then, 1.0 ml of tetrabutyl titanate was added and stirred for 30 min. The autoclave was sealed and heated in an electric oven at 180 1C for 12 h, then cooled to ambient temperature naturally. The resultant precipitate was filtered, washed with deionized water, and dried in vacuum at 100 1C for 4 h. The obtained product was characterized by X-ray diffraction (XRD, Bruker AXS D8 ADVANCE X-Ray Diffractometer), Raman and room temperature photoluminescence spectra (Renishaw Invia Raman spectrometer), transmission electron microscopy (TEM, Philips Tecnai-12 transmission electron microscopy), UV– vis diffuse reflectance spectrum (Varian Cary 5000 UV–vis–NIR
2
H. Bai, X. Liu / Materials Letters 100 (2013) 1–3
spectrophotometer) and impedance spectroscopy (Autolab PGSTAT 30 electrochemical workstation).
3. Results and discussion Fig. 1 shows the XRD pattern of the as-synthesized product. All its diffraction peaks can be readily indexed to tetragonal phase BaTiO3 (JCPDS card no. 01-089-1428). Thus, the as-synthesized BaTiO3 is free of the possible impurities such as TiO2, BaCO3, etc. Besides, the sharp and intense XRD peaks suggest that the assynthesized BaTiO3 is well crystallized. Raman spectroscopy is a very sensitive tool to identify the tetragonal or cubic phase structure of BaTiO3 [1–4]. For instance, cubic BaTiO3 inherently has no Raman active modes, whereas eight Raman active modes are expected for the noncentrosymmetric tetragonal structure, 3A1gþB1gþ4Eg [1–4]. Fig. 2 shows the Raman spectrum of the as-synthesized BaTiO3. The peaks at around 185 and 513 cm 1 can be assigned to the fundamental transverse component of the optical (TO) mode of A1 symmetry, and the peak at about 722 cm 1 is related to the highest frequency longitudinal optical mode (LO) of A1 symmetry [1–5]. The peak at 306 cm 1, which can be attributed to the B1 mode and
20
30
40
50
60
70
F(R )
(311) (113)
(310) (301) (103)
(300) (212)
(220) (202)
(211) (112)
(210) (201) (102)
(200) (002)
(111)
(100) (001)
Intensity (a. u.)
(110) (101)
Fig. 3. TEM image of the as-synthesized BaTiO3.
80
2θ (deg.)
300
700
(F(R )hν)2
(722)
(513) 200
600
(306)
Intensity (a. u.)
(185)
Fig. 1. XRD pattern of the as-synthesized BaTiO3.
400 500 Wavelength (nm)
400
600 800 Raman shift (cm-1)
1000
Fig. 2. Raman spectrum of the as-synthesized BaTiO3.
1200
2.0
2.5
3.0
3.5 hν (eV)
4.0
4.5
5.0
Fig. 4. (a) UV–vis diffuse reflectance spectrum of the as-synthesized BaTiO3, and (b) Plot of (F(RN)hn)2 vs (hn) for estimating the optical band gap of this sample.
Intensity (a. u.)
H. Bai, X. Liu / Materials Letters 100 (2013) 1–3
400
450
500 550 600 Wavelength (nm)
650
700
Fig. 5. RTPL spectrum of the as-synthesized BaTiO3 nanocrystals upon laser excitation at 325 nm.
indicate the asymmetry of TiO6 octahedra, is a characteristic peak for tetragonal phase BaTiO3 [1–5]. It can be seen that the peak at 306 cm 1 in Fig. 2 is sharp and strong, confirming the formation of tetragonal phase BaTiO3. This was consistent with the aforementioned XRD result (Fig. 1). Furthermore, unlike the Raman spectra of the BaTiO3 products synthesized in aqueous solutions, there is no peak observable at around 805–808 cm 1 in Fig. 2, indicating that the as-synthesized BaTiO3 is free of hydroxyl lattice groups [1–4]. Fig. 3 shows the TEM image of the as-synthesized BaTiO3. As can be seen, this product comprised nanocrystals with the sizes of about 44–80 nm. The UV–vis diffuse reflectance spectrum of the as-synthesized BaTiO3 nanocrystals was measured and converted into the absorption mode (Fig. 4(a)) using the Kubelka–Munk Function [13] FðR1 Þ ¼ ð1R1 Þ2 =2R1 ¼ a=S R1 ¼ Rsample =RBaSO4 where F(RN), R, a and S are the Kubelka–Munk function, reflectance, absorption coefficient and scattering coefficient, respectively. The band gap (Eg) of this sample was determined based on the theory of optical absorption for direct band gap semiconductors [14,15]
ahn ¼ BðhnEg Þ1=2 where hn and B are discrete photon energy and a constant relative to the material, respectively. For the diffused reflectance spectra, F(RN) can be used instead of a for estimating the Eg value [13]. So, the curve of (F(RN)hn)2 vs (hn) for this sample is plotted in Fig. 4(b). By extrapolating the linear portion of the (F(RN)hn)2 vs (hn) curve to
3
F(RN)¼0, the Eg of the as-synthesized BaTiO3 nanocrystals was estimated to be 3.42 eV. Fig. 5 shows the RTPL spectrum of the as-synthesized BaTiO3 nanocrystals upon laser excitation at 325 nm. It exhibited a strong and broad emission in the wavelength range of about 480– 625 nm, which was obviously red-shifted as compared with its Eg (3.42 eV). The room temperature photoluminescence of this sample was probably originated from its ‘‘broken’’ surface or dangling bonds (The electrons in the surface dangling bonds of nanocrystalline materials are in the unpaired state and can be considered as a kind of defects [16].) and other crystal defects (such as nonstoichiometry) [16,17]. The electric properties of the as-synthesized BaTiO3 nanocrystals were measured by impedance spectroscopy. The obtained results revealed that the electric resistance and electric capacity of the as-synthesized BaTiO3 nanocrystals were 1.4 104 O and 5.7 10 7 F, respectively.
4. Conclusions Tetragonal phase BaTiO3 nanocrystals were synthesized via the solvothermal reactions of BaCO3, tetrabutyl titanate and NaOH in ethanol at 180 1C, which is much lower than the reaction temperatures (above 1000 1C) required by conventional solid phase methods. The as-synthesized BaTiO3 nanocrystals had a direct band gap of about 3.42 eV, and exhibited a defect-induced strong and broad emission in the wavelength range of about 480–625 nm upon laser excitation at 325 at room temperature. The electric resistance and electric capacity of the as-synthesized BaTiO3 nanocrystals were determined to be 1.4 104 O and 5.7 10 7 F, respectively, by impedance spectroscopy. References [1] Tian X, Li J, Chen K, Han J, Pan S. Cryst Growth Des 2010;10:3990–5. [2] Zhu YF, Zhang L, Natsuki T, Fu YQ, Ni QQ. ACS Appl Mater Interfaces 2012;4:2101–6. [3] Maxim F, Ferreira P, Vilarinho PM, Aimable A. Cryst Growth Des 2010;10: 3996–4004. ˇ ´ epanovic´ MJ, Dojcˇilovic´ J. Ceram Int 2011;37:2513–8. [4] Pavlovic´ VP, Krstic´ J, Sc [5] Zhang YC, Wang GL, Li KW, Zhang M, Hu XY. J Cryst Growth 2006;290:513–7. [6] Katsuki H, Furuta S, Komarneni S. Mater Lett 2012;83:8–10. ¨ zen M, Mertens M, Schroeven M, Snijkers F, Cool P. Mater Lett 2012;67: [7] O 154–7. [8] Singh KC, Nath AK. Mater Lett 2011;65:970–3. [9] Zhang YC, Wang GL. Mater Lett 2008;62:673–5. [10] Ji B, Chen D, Jiao X, Zhao Z, Jiao Y. Mater Lett 2010;64:1836–8. [11] Zhang YC, Qiao T, Hu XY. J Solid State Chem 2004;177:4093–7. [12] Zhang YC, Wang GY, Hu XY, Shi QF, Qiao T. J Cryst Growth 2005;284:554–60. [13] Zhang YC, Li J, Zhang M, Dionysiou DD. Environ Sci Technol 2011;45: 9324–31. [14] Wang P, Fan C, Wang Y, Ding G, Yuan P. Mater Res Bull 2013;48:869–77. [15] Ahadi K, Mahdavi SM, Nemati A, Tabesh M. Mater Lett 2012;72:107–9. [16] Li J, Zhang YC, Wang TX, Zhang M. Mater Lett 2011;65:1556–8. [17] Li H, Huang S, Zhang W, Pan W. J Alloy Compd 2013;551:131–5.