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A facile preparation of potassium niobate nanorices with excellent optical response
Materials Letters 89 (2012) 324–326
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A facile preparation of potassium niobate nanorices with excellent optical response Xiangqing Li a, Shi-Zhao Kang a, Weijie Huang a, Guodong Li b, Jin Mu a,n a b
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 120 Caobao Road, Shanghai 200235, China State Key Laboratory of Inorganic Synthesis Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2012 Accepted 3 September 2012 Available online 10 September 2012
Potassium niobate nanoscrolls can be easily converted into rice-like nanostructures in the presence of water-soluble C60. The conversion mechanism of potassium niobate nanoscrolls is discussed according to the interaction between potassium niobate nanoscrolls and water-soluble C60. The rice-like potassium niobate nanoparticles show a stronger visible light absorption, suggesting an increment of surface electric charge in potassium niobate. Furthermore, the fluorescence of potassium niobate is quenched after complexing with water-soluble C60, which is ascribed to the reduced recombination of electron-hole pair and electron transmission between water-soluble C60 and the potassium niobate layers. & 2012 Elsevier B.V. All rights reserved.
Keywords: Potassium niobate Nanocomposite Microstructure Spectroscopy Solar energy materials
1. Introduction Potassium niobate has recently been a focus of researchers’ attention due to its distinctive photochemical and semiconductor properties [1], and good properties as precursors for the synthesis of novel composite nanostructures by the ionic exchange intercalation and exfoliation reactions [2]. Several investigations have elucidated its intercalation chemistry [3,4] and the structural details of its hydration [5]. Due to high layer charge density of the layered niobates, the direct intercalation of the species is hard to achieve. Based on the substitution of the interlayer cations by protons followed by an amine intercalation [6], the exfoliation of potassium niobate can produce a colloidal suspension of individual sheets or scroll structures. The sheets/scrolls of potassium niobate are superior host candidates for preparation of novel composite structure. Yao et al. have reported that scrolling of niobate nanosheets in the presence of magnetic nanoparticlechains can lead to peapod like structures [7]. Furthermore, with spectral analyses, Yao et al. found that the electron transfer from the interlayer molecule to the host layer is possible under illumination with visible light [8,9]. Both C60 molecule and their aggregates like clusters and micro-nanocrystals are now of much interest, because of their great potential applications in many research fields such as physics, chemistry, and electronics [10]. Recently, the solar cells consisting of C60 and organic or inorganic semiconductors have
n
Corresponding author. Tel.: þ86 21 60873061; fax: þ 86 21 64253317. E-mail address: [email protected] (J. Mu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.09.005
been widely investigated, because C60 possesses particular electronic structure and high electron affinity [11]. The results show that C60, as an excellent electron acceptor, can greatly increase the efficiency of solar cells [12]. In the limited reports of inorganic semiconductors/C60 composites, the C60 was mainly adsorbed through electrostatic forces onto the surface of inorganic semiconductors. As a result, the intimate contact between C60 and inorganic semiconductors matrix cannot be guaranteed. This problem may be overcome if C60 could be incorporated into inorganic semiconductors [13]. In the present article, without adding any other surfactants, only by a simple process, the potassium niobate/C60 composite with structurally well-defined morphology and good optical properties is prepared in aqueous solution. The influence of C60 on morphology and optical properties of potassium niobate are discussed in terms of the interaction between the C60 guest and the potassium niobate host layer.
2. Experimental Synthesis: Potassium niobate nanoscrolls [14] and watersoluble C60 [15] were prepared according to the methods reported in literatures, respectively. To fabricate the potassium niobate/C60 composite, initially, 200 mg of potassium niobate nanoscrolls were dispersed in 40 mL of C60 aqueous solution (0.075 mg mL 1). Then the mixture was stirred and heated at 60 1C for 8 h. Finally, the products were obtained by removing the solvent and drying.
X. Li et al. / Materials Letters 89 (2012) 324–326
Physical characterization: The morphological characterization of products was performed on a FEI Tecnai G2S-Twin transmission electron microscopy with a field emission gun operating at 200 kV (USA). A VARIAN Cary 500 UV–vis–NIR spectrophotometer was used to record the UV–vis diffuse reflectance absorption spectra (Japan). The fluorescence spectra of products were measured with a Shimadzu RF-5301PC spectrophotometer (Japan).
3. Results and discussion As seen from the TEM images shown in Fig. 1a and b, the exfoliation of acid-exchanged potassium niobate with tetra (n-butyl) ammonium hydroxide in water produces the colloidal suspension with morphologies both high aspect ratio scrolls and
Fig. 1. TEM images of potassium niobate: in the absence of water-soluble C60 (a,b) and in the presence of water-soluble C60 (c,d).
325
sheets. The sheets are partially rolled at the edges, with a flat unrolled central region. The scrolls are 100–200 nm in length, and most have outer diameters ranging from 15 to 25 nm. Fig. 1c and d presents TEM images of potassium niobate/C60 composite. It is found that the scrolls/sheets structures are converted into ricelike nanoparticles after the introduction of C60. The particles are with uniform grain diameter and good dispersity. Obviously, the water-soluble C60 is essential in the formation of these rice-like nanostructures. A possible mechanism for the formation of the rice-like nanostructures is proposed (Fig. 2): initially, in the presence of water-soluble C60, the scrolls are opened and changed into sheets because the forces controlling rolling and unrolling of potassium niobate sheets are closely balanced; then, sheets of potassium niobate are rescrolled by means of the hydroxyl interaction of sheets of potassium niobate and water-soluble C60; and finally, a novel rice-like nanostructure can achieved. Obviously, the [NbO6] octahedron units and sheets of potassium niobate are not changed in the process. Fig. 2 illustrates this proposed mechanism of the formation of rice-like nanostructured potassium niobate. Further, as shown in Fig. 3, the characteristic peaks corresponding to C60 [11] cannot be observed besides the peaks of pure potassium niobate nanoscrolls. The similar XRD patterns of pure potassium niobate nanoscrolls and potassium niobate/C60 composite indicate that C60 could be encapsulated by the potassium niobate layer and the layer structure of nanoscrolls changes little after the introduction of C60, which is coincident with the result of Fig. 2.
Fig. 3. XRD patterns of the pure potassium niobate nanoscrolls (a) and potassium niobate/C60 composite (b).
Fig. 2. Proposed mechanism for the formation of the nanorice: (a) potassium niobate nanoscroll, (b) the scroll is opened and changed into sheet in the presence of watersoluble C60, (c) potassium niobate sheet reconvolutes, and (d) rice-like nanoparticle formed.
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X. Li et al. / Materials Letters 89 (2012) 324–326
water-soluble C60. It is suggested that C60 is very vital to the morphological conversion of potassium niobate from scrolls to rice-like nanoparticles. Importantly, the rice-like potassium niobate nanoparticles have a stronger visible-light absorbance, increased surface electric charges and higher separation efficiency of electron-hole pairs. The composite particles containing both inorganic and organic components are potentially useful in the synthesis of photonic band gap materials, nanoscale photoelectron-devices, and solar energy materials.
Fig. 4. (a) Solid diffuse reflectance UV–vis spectra and (b) fluorescence spectra of the pure potassium niobate nanoscrolls (A) and potassium niobate/C60 composite (B), lex ¼270 nm.
In addition, it should be noted that this is not the only plausible mechanism for the formation of rice-like potassium niobate nanoparticles. Structures could certainly be formed by the insert of C60 into rolled potassium niobate, which leads to reconvolution of potassium niobate layers. The potassium niobate in the presence of water-soluble C60 has a better response in the visible light range as seen from its UV–vis spectrum (Fig. 4a). It shows a much stronger absorption in the whole range of 350–800 nm, which indicates an increment of surface electric charge in potassium niobate due to the introduction of C60 [16]. As a result, it can be deduced that there exist some electronic interactions between C60 and potassium niobate sheets. It is well known that fluorescence spectrum is often employed to characterize surface structure and excited states and to monitor surface processes of the products [17]. As shown in Fig. 4b, the fluorescence spectrum of potassium niobate scrolls is characterized by two peaks at around 361 and 467 nm. Rice-like potassium niobate/C60 composite shows a similar fluorescence spectrum while the fluorescence intensity decreases. This phenomenon may be ascribed to reduced recombination of electron-hole pairs due to introduction of C60. There exist some electron transmission between potassium niobate sheets and the C60 [18]. 4. Conclusions By a simple process, potassium niobate nanoparticles with uniform rice-like nanostructure were achieved in the presence of
Acknowledgments Authors acknowledge the financial support by National Nature Science Foundation of China (Grant no. 20933007).
References [1] Suzuki S, Teshima K, Yamaguchi A, Yubuta K, Shishidoc T, Oishi S. Cryst Eng Comm 2012;14:987–92. [2] Saupe GB, Waraksa CC, Kim HN, Han YJ, Kaschak DM, Skimmer DM, et al. Chem Mater 2000;12:1556–62. [3] Lagaly G, Beneke K. J Inorg Nucl Chem 1976;38:1513–8. [4] Kinomura N, Kumada N, Muto F. J Chem Soc Dalton Trans 1985;11:2349–51. [5] Gasperin M, Le Bihan MT. J Solid State Chem 1982;43:346–53. [6] Bizeto MA, Constantino VRL. Mater Res Bull 2004;39:1729–36. [7] Yao Y, Chaubey GS, Wiley JB. J Am Chem Soc 2012;134:2450–2. [8] Yao K, Nishimura S, Imai Y, Wang H, Ma T, Abe E, et al. Langmuir 2003; 19:321–5. [9] Furube A, Shiozawa T, Ishikawa A, Wada A, Domen K, Hirose C. J Phys Chem B 2002;106:3065–72. [10] Hsin YL, Lin CF, Liang YC, Hwang KC, Horng JC, Ho JA, et al. Adv Funct Mater 2008;18:2048–56. [11] Masuhara A, Tan Z, Kasai H, Nakanishi H, Oikawa H. Mol Cryst Liq Cryst 2008;492:262–7. ¨ [12] Cremer J, Bauerle P, Wienk MM, Janssen RAJ. Chem Mater 2006;18:5832–4. [13] Long YZ, Lu Y, Huang Y, Peng YC, Kang SZ, Mu J. J Phys Chem C 2009;113:13899–905. [14] Sarahan MC, Carroll EC, Allen M, Larsen DS, Browning ND, Osterloh FE. J Solid State Chem 2008;181:1678–83. [15] Ko W, Heo J, Nam J, Lee K. Ultrasonics 2004;41:727–30. [16] Wang W, Philippe S, Philippe K, Joaquim LF. Appl Catal B Environ 2005;56: 305–12. [17] Yu Y, Yu JC, Chan CY, Che YK, Zhao JC, Ding L, et al. Appl Catal B 2005;61: 1–11. [18] Kan F, Shinobu I, Teruhisa O, Michio M. J Photochem Photobiol 2000;132: 99–104.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A facile preparation of potassium niobate nanorices with excellent optical response Xiangqing Li a, Shi-Zhao Kang a, Weijie Huang a, Guodong Li b, Jin Mu a,n a b
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, 120 Caobao Road, Shanghai 200235, China State Key Laboratory of Inorganic Synthesis Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2012 Accepted 3 September 2012 Available online 10 September 2012
Potassium niobate nanoscrolls can be easily converted into rice-like nanostructures in the presence of water-soluble C60. The conversion mechanism of potassium niobate nanoscrolls is discussed according to the interaction between potassium niobate nanoscrolls and water-soluble C60. The rice-like potassium niobate nanoparticles show a stronger visible light absorption, suggesting an increment of surface electric charge in potassium niobate. Furthermore, the fluorescence of potassium niobate is quenched after complexing with water-soluble C60, which is ascribed to the reduced recombination of electron-hole pair and electron transmission between water-soluble C60 and the potassium niobate layers. & 2012 Elsevier B.V. All rights reserved.
Keywords: Potassium niobate Nanocomposite Microstructure Spectroscopy Solar energy materials
1. Introduction Potassium niobate has recently been a focus of researchers’ attention due to its distinctive photochemical and semiconductor properties [1], and good properties as precursors for the synthesis of novel composite nanostructures by the ionic exchange intercalation and exfoliation reactions [2]. Several investigations have elucidated its intercalation chemistry [3,4] and the structural details of its hydration [5]. Due to high layer charge density of the layered niobates, the direct intercalation of the species is hard to achieve. Based on the substitution of the interlayer cations by protons followed by an amine intercalation [6], the exfoliation of potassium niobate can produce a colloidal suspension of individual sheets or scroll structures. The sheets/scrolls of potassium niobate are superior host candidates for preparation of novel composite structure. Yao et al. have reported that scrolling of niobate nanosheets in the presence of magnetic nanoparticlechains can lead to peapod like structures [7]. Furthermore, with spectral analyses, Yao et al. found that the electron transfer from the interlayer molecule to the host layer is possible under illumination with visible light [8,9]. Both C60 molecule and their aggregates like clusters and micro-nanocrystals are now of much interest, because of their great potential applications in many research fields such as physics, chemistry, and electronics [10]. Recently, the solar cells consisting of C60 and organic or inorganic semiconductors have
n
Corresponding author. Tel.: þ86 21 60873061; fax: þ 86 21 64253317. E-mail address: [email protected] (J. Mu).
0167-577X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2012.09.005
been widely investigated, because C60 possesses particular electronic structure and high electron affinity [11]. The results show that C60, as an excellent electron acceptor, can greatly increase the efficiency of solar cells [12]. In the limited reports of inorganic semiconductors/C60 composites, the C60 was mainly adsorbed through electrostatic forces onto the surface of inorganic semiconductors. As a result, the intimate contact between C60 and inorganic semiconductors matrix cannot be guaranteed. This problem may be overcome if C60 could be incorporated into inorganic semiconductors [13]. In the present article, without adding any other surfactants, only by a simple process, the potassium niobate/C60 composite with structurally well-defined morphology and good optical properties is prepared in aqueous solution. The influence of C60 on morphology and optical properties of potassium niobate are discussed in terms of the interaction between the C60 guest and the potassium niobate host layer.
2. Experimental Synthesis: Potassium niobate nanoscrolls [14] and watersoluble C60 [15] were prepared according to the methods reported in literatures, respectively. To fabricate the potassium niobate/C60 composite, initially, 200 mg of potassium niobate nanoscrolls were dispersed in 40 mL of C60 aqueous solution (0.075 mg mL 1). Then the mixture was stirred and heated at 60 1C for 8 h. Finally, the products were obtained by removing the solvent and drying.
X. Li et al. / Materials Letters 89 (2012) 324–326
Physical characterization: The morphological characterization of products was performed on a FEI Tecnai G2S-Twin transmission electron microscopy with a field emission gun operating at 200 kV (USA). A VARIAN Cary 500 UV–vis–NIR spectrophotometer was used to record the UV–vis diffuse reflectance absorption spectra (Japan). The fluorescence spectra of products were measured with a Shimadzu RF-5301PC spectrophotometer (Japan).
3. Results and discussion As seen from the TEM images shown in Fig. 1a and b, the exfoliation of acid-exchanged potassium niobate with tetra (n-butyl) ammonium hydroxide in water produces the colloidal suspension with morphologies both high aspect ratio scrolls and
Fig. 1. TEM images of potassium niobate: in the absence of water-soluble C60 (a,b) and in the presence of water-soluble C60 (c,d).
325
sheets. The sheets are partially rolled at the edges, with a flat unrolled central region. The scrolls are 100–200 nm in length, and most have outer diameters ranging from 15 to 25 nm. Fig. 1c and d presents TEM images of potassium niobate/C60 composite. It is found that the scrolls/sheets structures are converted into ricelike nanoparticles after the introduction of C60. The particles are with uniform grain diameter and good dispersity. Obviously, the water-soluble C60 is essential in the formation of these rice-like nanostructures. A possible mechanism for the formation of the rice-like nanostructures is proposed (Fig. 2): initially, in the presence of water-soluble C60, the scrolls are opened and changed into sheets because the forces controlling rolling and unrolling of potassium niobate sheets are closely balanced; then, sheets of potassium niobate are rescrolled by means of the hydroxyl interaction of sheets of potassium niobate and water-soluble C60; and finally, a novel rice-like nanostructure can achieved. Obviously, the [NbO6] octahedron units and sheets of potassium niobate are not changed in the process. Fig. 2 illustrates this proposed mechanism of the formation of rice-like nanostructured potassium niobate. Further, as shown in Fig. 3, the characteristic peaks corresponding to C60 [11] cannot be observed besides the peaks of pure potassium niobate nanoscrolls. The similar XRD patterns of pure potassium niobate nanoscrolls and potassium niobate/C60 composite indicate that C60 could be encapsulated by the potassium niobate layer and the layer structure of nanoscrolls changes little after the introduction of C60, which is coincident with the result of Fig. 2.
Fig. 3. XRD patterns of the pure potassium niobate nanoscrolls (a) and potassium niobate/C60 composite (b).
Fig. 2. Proposed mechanism for the formation of the nanorice: (a) potassium niobate nanoscroll, (b) the scroll is opened and changed into sheet in the presence of watersoluble C60, (c) potassium niobate sheet reconvolutes, and (d) rice-like nanoparticle formed.
326
X. Li et al. / Materials Letters 89 (2012) 324–326
water-soluble C60. It is suggested that C60 is very vital to the morphological conversion of potassium niobate from scrolls to rice-like nanoparticles. Importantly, the rice-like potassium niobate nanoparticles have a stronger visible-light absorbance, increased surface electric charges and higher separation efficiency of electron-hole pairs. The composite particles containing both inorganic and organic components are potentially useful in the synthesis of photonic band gap materials, nanoscale photoelectron-devices, and solar energy materials.
Fig. 4. (a) Solid diffuse reflectance UV–vis spectra and (b) fluorescence spectra of the pure potassium niobate nanoscrolls (A) and potassium niobate/C60 composite (B), lex ¼270 nm.
In addition, it should be noted that this is not the only plausible mechanism for the formation of rice-like potassium niobate nanoparticles. Structures could certainly be formed by the insert of C60 into rolled potassium niobate, which leads to reconvolution of potassium niobate layers. The potassium niobate in the presence of water-soluble C60 has a better response in the visible light range as seen from its UV–vis spectrum (Fig. 4a). It shows a much stronger absorption in the whole range of 350–800 nm, which indicates an increment of surface electric charge in potassium niobate due to the introduction of C60 [16]. As a result, it can be deduced that there exist some electronic interactions between C60 and potassium niobate sheets. It is well known that fluorescence spectrum is often employed to characterize surface structure and excited states and to monitor surface processes of the products [17]. As shown in Fig. 4b, the fluorescence spectrum of potassium niobate scrolls is characterized by two peaks at around 361 and 467 nm. Rice-like potassium niobate/C60 composite shows a similar fluorescence spectrum while the fluorescence intensity decreases. This phenomenon may be ascribed to reduced recombination of electron-hole pairs due to introduction of C60. There exist some electron transmission between potassium niobate sheets and the C60 [18]. 4. Conclusions By a simple process, potassium niobate nanoparticles with uniform rice-like nanostructure were achieved in the presence of
Acknowledgments Authors acknowledge the financial support by National Nature Science Foundation of China (Grant no. 20933007).
References [1] Suzuki S, Teshima K, Yamaguchi A, Yubuta K, Shishidoc T, Oishi S. Cryst Eng Comm 2012;14:987–92. [2] Saupe GB, Waraksa CC, Kim HN, Han YJ, Kaschak DM, Skimmer DM, et al. Chem Mater 2000;12:1556–62. [3] Lagaly G, Beneke K. J Inorg Nucl Chem 1976;38:1513–8. [4] Kinomura N, Kumada N, Muto F. J Chem Soc Dalton Trans 1985;11:2349–51. [5] Gasperin M, Le Bihan MT. J Solid State Chem 1982;43:346–53. [6] Bizeto MA, Constantino VRL. Mater Res Bull 2004;39:1729–36. [7] Yao Y, Chaubey GS, Wiley JB. J Am Chem Soc 2012;134:2450–2. [8] Yao K, Nishimura S, Imai Y, Wang H, Ma T, Abe E, et al. Langmuir 2003; 19:321–5. [9] Furube A, Shiozawa T, Ishikawa A, Wada A, Domen K, Hirose C. J Phys Chem B 2002;106:3065–72. [10] Hsin YL, Lin CF, Liang YC, Hwang KC, Horng JC, Ho JA, et al. Adv Funct Mater 2008;18:2048–56. [11] Masuhara A, Tan Z, Kasai H, Nakanishi H, Oikawa H. Mol Cryst Liq Cryst 2008;492:262–7. ¨ [12] Cremer J, Bauerle P, Wienk MM, Janssen RAJ. Chem Mater 2006;18:5832–4. [13] Long YZ, Lu Y, Huang Y, Peng YC, Kang SZ, Mu J. J Phys Chem C 2009;113:13899–905. [14] Sarahan MC, Carroll EC, Allen M, Larsen DS, Browning ND, Osterloh FE. J Solid State Chem 2008;181:1678–83. [15] Ko W, Heo J, Nam J, Lee K. Ultrasonics 2004;41:727–30. [16] Wang W, Philippe S, Philippe K, Joaquim LF. Appl Catal B Environ 2005;56: 305–12. [17] Yu Y, Yu JC, Chan CY, Che YK, Zhao JC, Ding L, et al. Appl Catal B 2005;61: 1–11. [18] Kan F, Shinobu I, Teruhisa O, Michio M. J Photochem Photobiol 2000;132: 99–104.