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Synthesis and luminescent properties of ZnNb2O6 nanocrystals for solar cell
Materials Letters 64 (2010) 2563–2565
Contents lists available at ScienceDirect
Materials Letters 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 / m a t l e t
Synthesis and luminescent properties of ZnNb2O6 nanocrystals for solar cell Yu-Jen Hsiao a, Te-Hua Fang b,⁎, Liang-Wen Ji c a b c
National Nano Device Laboratories, Tainan 741, Taiwan Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan
a r t i c l e
i n f o
Article history: Received 29 June 2010 Accepted 17 August 2010 Available online 23 August 2010 Keywords: Luminescence Sol–gel preparation Optical materials and properties
a b s t r a c t The phase formation, morphology and luminescent properties of ZnNb2O6 nanocrystals by the sol–gel method were investigated at a lower temperature than that of the traditional solid-state reaction method. The products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy (PL) and absorption spectra. The activation energy of ZnNb2O6 grain growth is obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of 20–40 nm. The PL spectra excited at 276 nm have a broad and strong blue emission band maximum at 450 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO6]7−. The optical absorption spectrum of the sample at a calcination temperature of 800 °C has a band gap energy of 3.68 eV. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The electro-optical properties of the metal niobates of ANb2O6 (A = Mg and Ca) have been studied extensively in recent years [1,2]. It is known that various compositions are possible in the Zn–Nb–O system. To date, three possible zinc niobium oxides have been identified: ZnNb2O6, Zn2 Nb34O87 and Zn3Nb2O8 [3–5]. Among these compounds, zinc niobate (ZnNb2O6) is one of the most well-known materials. ZnNb2O6 ceramics have excellent dielectric properties: Q × f = 87 300 GHz, or εr = 25 and τf = 56 ppm/°C [6]. It is investigated for the applications in microwave dielectric resonators and lowtemperature co-fired ceramics (LTCCs) [7]. ZnNb2O6 exhibits very strong blue luminescence with excitation by ultraviolet radiation of 375 nm at room temperature [8]. The absorption wavelength of the general solar cell is about 400–1000 nm, however, photoluminescence would be beneficial if the absorption wavelengths were shorter than 400 nm [9]. The photoluminescence of the ZnNb2O6 nanostructure is very promising for application to solar cells because of the absorption wavelengths of less than 400 nm. Currently, the nanostructures of zinc niobate (ZnNb2O6) have been synthesized by the rapid vibro-milling technique [4], the combustion synthesis method [8], and the molten salt route [10]. This study is to explore a sol–gel synthetic route for the preparation of single phase ZnNb2O6 oxides. Chemically synthesized ceramic powders often have better chemical homogeneity and a finer particle together with better control of particle morphology than those produced by the mixed oxide route
⁎ Corresponding author: Tel.: +886 7 381 4526x5336; fax: +886 7 3831373. E-mail address: [email protected] (T.-H. Fang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.08.053
[11]. To our knowledge, the works of the luminescence behavior of the niobate-based complex formed by the citric gel method are few. This fact motivates this work which discusses the phase formation, morphology and luminescent properties of ZnNb2O6 nanocrystals. 2. Experiments Pure ZnNb2O6 powders were prepared by the sol–gel method using zinc nitrate [Zn(NO3)2 6H2O], niobium chloride (NbCl5), ethylene glycol (EG) and citric acid anhydrous (CA). Their purities are over 99.9%. First, a stoichiometric amount of zinc nitrate, and niobium ethoxide were dissolved in distilled water. Niobium ethoxide, Nb(OC2H5)5, was synthesized from niobium chloride and ethanol, C2H5OH, according to the general reaction: NbCl5 ＋ 5C2 H5 OH→NbðOC2 H5 Þ5 ＋5HCl:
ð1Þ
A sufficient amount of citric acid was added to the former solution as a chelating agent to form a solution. Citric acid to the total metal ions in the molar ratio of 3:2 was used for this purpose. EG was also added to the above solution as a stabilizing agent. The precursor was dried in an oven at 120 °C for 10 h and then the powders were obtained after calcinations at 450–800 °C for 3 h in air. All of the above measurements were taken at room temperature. 3. Results and discussion XRD patterns of the precursor powders at heat-treatment temperatures of 450–800 °C for 3 h are shown in Fig. 1. All the peaks can be well-indexed to a pure orthorhombic phase of ZnNb2O6
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Y.-J. Hsiao et al. / Materials Letters 64 (2010) 2563–2565
Fig. 1. XRD of ZnNb2O6 precursor powders annealed at (a) 450, (b) 500, (c) 600, (d) 700 and (e) 800 °C for 3 h.
as heat-treatment temperatures were greater than 500 °C. No second phase was detected for the pure samples. Note that the intensity of the diffraction peaks becomes sharper at higher temperatures, indicating that the crystallinity of ZnNb2O6 increases with the increase of the calcination temperature. The average grain sizes were determined from XRD according to the Scherrer's equation, The average grain sizes of powders calcined at 500, 600, 700, and 800 °C were about 17.8, 21.7, 28.4, and 39.5 nm, respectively. The activation energy of grain growth during powder sintering can be calculated by the Arrhenius equation. According to Coble's theory [12], the activation energy of grain growth during powder sintering can be calculated by the Arrhenius equation, d lnK Q = dT RT 2
ð2Þ
Fig. 2. (a) TEM images of as-synthesized nanocrystals at 700 °C, and (b) high-resolution TEM image of the nanocrystals and electron diffraction pattern.
where K is the specific reaction rate constant, Q is the activation energy, T is the absolute temperature, and R is the ideal gas constant. Bolen and co-workers showed that the value of K is related with grain size directly [13]. Thus the integral of Eq. (2) becomes
diffraction (SAED) pattern of the sample was consistent with the high crystallization of ZnNb2O6. Fig. 3 presents the excitation and emission spectra of the pure ZnNb2O6 samples at temperatures of 600–800 °C. The sample exhibits great absorption band from 250 to 300 nm by monitoring fluorescence at a wavelength of 450 nm. Blasse [14] indicated that the niobate complexes had two kinds of absorbing groups [NbO6]7− and [NbO4]3−, respectively. Only one peak was observed at wavelengths of 276 nm Therefore, the peaks of excitation, at about 276 nm, were associated with charge transfer bands of [NbO6]7− in the ZnNb2O6 system [8]. A strong blue emission band centered at 450 nm can be observed for pure samples. Here, the edge-shared NbO6 groups are efficient luminescent centers for the blue emission, which may be ascribed to self-trapped exciton recombination [15]. This luminescence effect depends on the Nb–O–Nb bonding that the conduction band is composed of Nb5+ 4d orbitals, and the valence band of O2− 2p orbitals between the corner-sharing octahedra [16]. In addition, the sample that was heat-treated at 800 °C yielded the most intense emission spectra (λex = 276 nm), associated with the higher absorbing intensity of the [NbO6]7− group. The UV–Vis absorption spectra of the ZnNb2O6 powders annealed at 800 °C for 3 h are shown in Fig. 4. The broad bands, peaking at 250 and 320 nm are attributed to the charge transfer from the oxygen ligands to the central niobium atom inside the NbO6 groups. The inset is the pure ZnNb2O6 behavior of optical absorption as the function of
log D =
Q 1 +A 2:303R T
ð3Þ
where D is the grain size and A is the intercept. From Eq. (3), by making a plot of log D versus the reciprocal of absolute temperature (1/T), a straight-line is obtained as shown in Fig. 2. The slope of the resulting Arrhenius plot is − Q/(2.303R) and the activation energy of grain growth can be obtained and the value of Q is about 18.4 kJ/mol. The particle size increased as the sintering temperature was increased. It is believed that a higher temperature enhanced higher atomic mobility and caused faster grain growth. Fig. 2(a) shows the low-magnification TEM image, and the morphology is clearly observed at 700 °C. The diameters of the nanocrystals are in the range of 20–40 nm. The high-magnification TEM image and the selected area electronic diffraction (SAED) pattern of the nanocrystals are shown in Fig. 2(b). It is conjectured that the assemble effect arising from nanocrystals, are responsible for the decreasing of surface energy. The interplanar distances of the crystal fringes is 0.51 nm, which is consistent with the (200) plane of ZnNb2O6. As shown in the inset Fig. 2(b), the selected area electronic
Y.-J. Hsiao et al. / Materials Letters 64 (2010) 2563–2565
2565
Fig. 3. The room-temperature excitation (λem = 450 nm) and emission (λex = 276 nm) spectra of pure ZnNb2O6.
Fig. 4. Absorption spectra of ZnNb2O6 at 800 °C. The inset is the function of photon energy.
photon energy. We have measured and estimated the band gap from the absorption. For a direct band gap semiconductor, the absorbance in the vicinity of the onset due to the electronic transition is given by the following equation [17]:
visible light absorption edge of the sample calcined at 800 °C was corresponded to a band gap energy of 3.68 eV.
α=
1 = 2 C hν−Eg hν
References ð4Þ
where α is the absorption coefficient, C is the constant, hν is the photon energy and Eg is the band gap. The inset of Fig. 4 shows the relationship of (αhν)2 and hν. Extrapolation of the linear region gives a band gap of 3.68 eV. 4. Conclusion The well-crystallized orthorhombic ZnNb2O6 can be obtained by heat-treatment at 500 °C from XRD. The activation energy of the ZnNb2O6 nanocrystal grain growth can be obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of 20–40 nm. The excitation wavelengths at about 276 nm, were associated with the charge transfer bands of [NbO6]7−. The PL spectra under 276 nm excitation that showed a broad and strong blue emission peaks at about 450 nm, were originated from the niobate octahedra group. The
[1] Hsiao YJ, Liu CW, Dai BT, Chang YH. J Alloy Compd 2009;475:698. [2] Fang TH, Hsiao YJ, Chang YS, Ji LW, Kang SH. Curr Opin Solid State Mater Sci 2008;12:51. [3] Pollard AJ. J Am Ceram Soc 1961;44:630. [4] Prasatkhetragarn A, Yimnirun R, Ananta S. Mater Lett 2007;61:3873. [5] Prasatkhetragarn A, Ketsuwan P, Ananta S, Yimnirun R. Mater Lett 2010;61:1113. [6] Lee HJ, Kim IT, Hong KS. Jpn J Appl Phys 1997;36:1318. [7] Feng G, Jiaji L, Rongzi H, Zhen L, Changsheng T. Ceram Int 2009;35:2687. [8] Zhou Y, Qiu Z, Lü M, Ma Q, Zhang A, Zhou G, Zhang H, Yang Z. J Phys Chem C 2007;111:10190. [9] Rodríguez VD, Tikhomirov VK, Méndez-Ramos J, Yanes AC, Moshchalkov VV. Sol Energy Mater Sol Cells 2010;94:1612. [10] Guo L, Dai J, Tian J, Zhu Z, He T. Mater Res Bull 2007;42:2013. [11] Hsiao YJ, Chang YH, Chang YS. J Am Ceram Soc 2007;90:2287. [12] Coble RL. J Appl Phys 1961;32:793. [13] Jarcho M, Bolen CH, Doremus RH. J Mater Sci 1976;11:2027. [14] Blasse G. “The luminescence of closed-shell transition-metal complexes”. Struct Bond 1980;42:1. [15] Srivastava AM, Ackerman JF. J Solid State Chem 1997;134:187. [16] Hara K, Tominaga S, Takano A, Dantani K, Yoshino J, Kukimoto H. Jpn J Appl Phys 1992;31:L1661. [17] Zhang A, Lü M, Wang S, Zhou G, Wang S, Zhou Y. J Alloys Compd 2007;433:L7.
Contents lists available at ScienceDirect
Materials Letters 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 / m a t l e t
Synthesis and luminescent properties of ZnNb2O6 nanocrystals for solar cell Yu-Jen Hsiao a, Te-Hua Fang b,⁎, Liang-Wen Ji c a b c
National Nano Device Laboratories, Tainan 741, Taiwan Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan
a r t i c l e
i n f o
Article history: Received 29 June 2010 Accepted 17 August 2010 Available online 23 August 2010 Keywords: Luminescence Sol–gel preparation Optical materials and properties
a b s t r a c t The phase formation, morphology and luminescent properties of ZnNb2O6 nanocrystals by the sol–gel method were investigated at a lower temperature than that of the traditional solid-state reaction method. The products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), photoluminescence spectroscopy (PL) and absorption spectra. The activation energy of ZnNb2O6 grain growth is obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of 20–40 nm. The PL spectra excited at 276 nm have a broad and strong blue emission band maximum at 450 nm, corresponding to the self-activated luminescence of the niobate octahedra group [NbO6]7−. The optical absorption spectrum of the sample at a calcination temperature of 800 °C has a band gap energy of 3.68 eV. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The electro-optical properties of the metal niobates of ANb2O6 (A = Mg and Ca) have been studied extensively in recent years [1,2]. It is known that various compositions are possible in the Zn–Nb–O system. To date, three possible zinc niobium oxides have been identified: ZnNb2O6, Zn2 Nb34O87 and Zn3Nb2O8 [3–5]. Among these compounds, zinc niobate (ZnNb2O6) is one of the most well-known materials. ZnNb2O6 ceramics have excellent dielectric properties: Q × f = 87 300 GHz, or εr = 25 and τf = 56 ppm/°C [6]. It is investigated for the applications in microwave dielectric resonators and lowtemperature co-fired ceramics (LTCCs) [7]. ZnNb2O6 exhibits very strong blue luminescence with excitation by ultraviolet radiation of 375 nm at room temperature [8]. The absorption wavelength of the general solar cell is about 400–1000 nm, however, photoluminescence would be beneficial if the absorption wavelengths were shorter than 400 nm [9]. The photoluminescence of the ZnNb2O6 nanostructure is very promising for application to solar cells because of the absorption wavelengths of less than 400 nm. Currently, the nanostructures of zinc niobate (ZnNb2O6) have been synthesized by the rapid vibro-milling technique [4], the combustion synthesis method [8], and the molten salt route [10]. This study is to explore a sol–gel synthetic route for the preparation of single phase ZnNb2O6 oxides. Chemically synthesized ceramic powders often have better chemical homogeneity and a finer particle together with better control of particle morphology than those produced by the mixed oxide route
⁎ Corresponding author: Tel.: +886 7 381 4526x5336; fax: +886 7 3831373. E-mail address: [email protected] (T.-H. Fang). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.08.053
[11]. To our knowledge, the works of the luminescence behavior of the niobate-based complex formed by the citric gel method are few. This fact motivates this work which discusses the phase formation, morphology and luminescent properties of ZnNb2O6 nanocrystals. 2. Experiments Pure ZnNb2O6 powders were prepared by the sol–gel method using zinc nitrate [Zn(NO3)2 6H2O], niobium chloride (NbCl5), ethylene glycol (EG) and citric acid anhydrous (CA). Their purities are over 99.9%. First, a stoichiometric amount of zinc nitrate, and niobium ethoxide were dissolved in distilled water. Niobium ethoxide, Nb(OC2H5)5, was synthesized from niobium chloride and ethanol, C2H5OH, according to the general reaction: NbCl5 ＋ 5C2 H5 OH→NbðOC2 H5 Þ5 ＋5HCl:
ð1Þ
A sufficient amount of citric acid was added to the former solution as a chelating agent to form a solution. Citric acid to the total metal ions in the molar ratio of 3:2 was used for this purpose. EG was also added to the above solution as a stabilizing agent. The precursor was dried in an oven at 120 °C for 10 h and then the powders were obtained after calcinations at 450–800 °C for 3 h in air. All of the above measurements were taken at room temperature. 3. Results and discussion XRD patterns of the precursor powders at heat-treatment temperatures of 450–800 °C for 3 h are shown in Fig. 1. All the peaks can be well-indexed to a pure orthorhombic phase of ZnNb2O6
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Y.-J. Hsiao et al. / Materials Letters 64 (2010) 2563–2565
Fig. 1. XRD of ZnNb2O6 precursor powders annealed at (a) 450, (b) 500, (c) 600, (d) 700 and (e) 800 °C for 3 h.
as heat-treatment temperatures were greater than 500 °C. No second phase was detected for the pure samples. Note that the intensity of the diffraction peaks becomes sharper at higher temperatures, indicating that the crystallinity of ZnNb2O6 increases with the increase of the calcination temperature. The average grain sizes were determined from XRD according to the Scherrer's equation, The average grain sizes of powders calcined at 500, 600, 700, and 800 °C were about 17.8, 21.7, 28.4, and 39.5 nm, respectively. The activation energy of grain growth during powder sintering can be calculated by the Arrhenius equation. According to Coble's theory [12], the activation energy of grain growth during powder sintering can be calculated by the Arrhenius equation, d lnK Q = dT RT 2
ð2Þ
Fig. 2. (a) TEM images of as-synthesized nanocrystals at 700 °C, and (b) high-resolution TEM image of the nanocrystals and electron diffraction pattern.
where K is the specific reaction rate constant, Q is the activation energy, T is the absolute temperature, and R is the ideal gas constant. Bolen and co-workers showed that the value of K is related with grain size directly [13]. Thus the integral of Eq. (2) becomes
diffraction (SAED) pattern of the sample was consistent with the high crystallization of ZnNb2O6. Fig. 3 presents the excitation and emission spectra of the pure ZnNb2O6 samples at temperatures of 600–800 °C. The sample exhibits great absorption band from 250 to 300 nm by monitoring fluorescence at a wavelength of 450 nm. Blasse [14] indicated that the niobate complexes had two kinds of absorbing groups [NbO6]7− and [NbO4]3−, respectively. Only one peak was observed at wavelengths of 276 nm Therefore, the peaks of excitation, at about 276 nm, were associated with charge transfer bands of [NbO6]7− in the ZnNb2O6 system [8]. A strong blue emission band centered at 450 nm can be observed for pure samples. Here, the edge-shared NbO6 groups are efficient luminescent centers for the blue emission, which may be ascribed to self-trapped exciton recombination [15]. This luminescence effect depends on the Nb–O–Nb bonding that the conduction band is composed of Nb5+ 4d orbitals, and the valence band of O2− 2p orbitals between the corner-sharing octahedra [16]. In addition, the sample that was heat-treated at 800 °C yielded the most intense emission spectra (λex = 276 nm), associated with the higher absorbing intensity of the [NbO6]7− group. The UV–Vis absorption spectra of the ZnNb2O6 powders annealed at 800 °C for 3 h are shown in Fig. 4. The broad bands, peaking at 250 and 320 nm are attributed to the charge transfer from the oxygen ligands to the central niobium atom inside the NbO6 groups. The inset is the pure ZnNb2O6 behavior of optical absorption as the function of
log D =
Q 1 +A 2:303R T
ð3Þ
where D is the grain size and A is the intercept. From Eq. (3), by making a plot of log D versus the reciprocal of absolute temperature (1/T), a straight-line is obtained as shown in Fig. 2. The slope of the resulting Arrhenius plot is − Q/(2.303R) and the activation energy of grain growth can be obtained and the value of Q is about 18.4 kJ/mol. The particle size increased as the sintering temperature was increased. It is believed that a higher temperature enhanced higher atomic mobility and caused faster grain growth. Fig. 2(a) shows the low-magnification TEM image, and the morphology is clearly observed at 700 °C. The diameters of the nanocrystals are in the range of 20–40 nm. The high-magnification TEM image and the selected area electronic diffraction (SAED) pattern of the nanocrystals are shown in Fig. 2(b). It is conjectured that the assemble effect arising from nanocrystals, are responsible for the decreasing of surface energy. The interplanar distances of the crystal fringes is 0.51 nm, which is consistent with the (200) plane of ZnNb2O6. As shown in the inset Fig. 2(b), the selected area electronic
Y.-J. Hsiao et al. / Materials Letters 64 (2010) 2563–2565
2565
Fig. 3. The room-temperature excitation (λem = 450 nm) and emission (λex = 276 nm) spectra of pure ZnNb2O6.
Fig. 4. Absorption spectra of ZnNb2O6 at 800 °C. The inset is the function of photon energy.
photon energy. We have measured and estimated the band gap from the absorption. For a direct band gap semiconductor, the absorbance in the vicinity of the onset due to the electronic transition is given by the following equation [17]:
visible light absorption edge of the sample calcined at 800 °C was corresponded to a band gap energy of 3.68 eV.
α=
1 = 2 C hν−Eg hν
References ð4Þ
where α is the absorption coefficient, C is the constant, hν is the photon energy and Eg is the band gap. The inset of Fig. 4 shows the relationship of (αhν)2 and hν. Extrapolation of the linear region gives a band gap of 3.68 eV. 4. Conclusion The well-crystallized orthorhombic ZnNb2O6 can be obtained by heat-treatment at 500 °C from XRD. The activation energy of the ZnNb2O6 nanocrystal grain growth can be obtained about 18.4 kJ/mol. The diameters of the nanocrystals are in the range of 20–40 nm. The excitation wavelengths at about 276 nm, were associated with the charge transfer bands of [NbO6]7−. The PL spectra under 276 nm excitation that showed a broad and strong blue emission peaks at about 450 nm, were originated from the niobate octahedra group. The
[1] Hsiao YJ, Liu CW, Dai BT, Chang YH. J Alloy Compd 2009;475:698. [2] Fang TH, Hsiao YJ, Chang YS, Ji LW, Kang SH. Curr Opin Solid State Mater Sci 2008;12:51. [3] Pollard AJ. J Am Ceram Soc 1961;44:630. [4] Prasatkhetragarn A, Yimnirun R, Ananta S. Mater Lett 2007;61:3873. [5] Prasatkhetragarn A, Ketsuwan P, Ananta S, Yimnirun R. Mater Lett 2010;61:1113. [6] Lee HJ, Kim IT, Hong KS. Jpn J Appl Phys 1997;36:1318. [7] Feng G, Jiaji L, Rongzi H, Zhen L, Changsheng T. Ceram Int 2009;35:2687. [8] Zhou Y, Qiu Z, Lü M, Ma Q, Zhang A, Zhou G, Zhang H, Yang Z. J Phys Chem C 2007;111:10190. [9] Rodríguez VD, Tikhomirov VK, Méndez-Ramos J, Yanes AC, Moshchalkov VV. Sol Energy Mater Sol Cells 2010;94:1612. [10] Guo L, Dai J, Tian J, Zhu Z, He T. Mater Res Bull 2007;42:2013. [11] Hsiao YJ, Chang YH, Chang YS. J Am Ceram Soc 2007;90:2287. [12] Coble RL. J Appl Phys 1961;32:793. [13] Jarcho M, Bolen CH, Doremus RH. J Mater Sci 1976;11:2027. [14] Blasse G. “The luminescence of closed-shell transition-metal complexes”. Struct Bond 1980;42:1. [15] Srivastava AM, Ackerman JF. J Solid State Chem 1997;134:187. [16] Hara K, Tominaga S, Takano A, Dantani K, Yoshino J, Kukimoto H. Jpn J Appl Phys 1992;31:L1661. [17] Zhang A, Lü M, Wang S, Zhou G, Wang S, Zhou Y. J Alloys Compd 2007;433:L7.