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Porous ZnO nanobelts evolved from layered basic zinc acetate nanobelts
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Applied Surface Science 254 (2008) 3517–3521 www.elsevier.com/locate/apsusc
Porous ZnO nanobelts evolved from layered basic zinc acetate nanobelts Qingyue Cui, Ke Yu *, Ning Zhang, Ziqiang Zhu * Department of Electronic Engineering, East China Normal University, Shanghai 200241, PR China Received 13 September 2007; received in revised form 23 November 2007; accepted 23 November 2007 Available online 3 December 2007
Abstract Novel porous ZnO nanobelts were successfully synthesized by heating layered basic zinc acetate (LBZA) nanobelts in the air. The precursor of LBZA nanobelts consisted of a lamellar structure with two interlayer distances of 1.325 and 0.99 nm were prepared using a low-temperature, solution-based method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy are used to characterize the as-products. PL measurements show that the porous ZnO nanobelts have strong ultraviolet emission properties at 380 nm, while no defect-related visible emission is detected. The good performance for photoluminescence emission makes the porous ZnO nanobelts promising candidates for photonic and electronic device applications. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05.Dz; 81.05.Rm; 78.55.Et Keywords: ZnO nanostructures; Porous morphology; PL spectra
1. Introduction The development of novel nanostructured materials with controlled shapes, size and morphology has initiated great research interest for seeking novel properties and tailorable functions. In particular, zinc oxide, a wide-bandgap semiconductor [1], has proven valuable as an ideal functional component for devices and materials in catalysts [2], photonic crystals [3], gas sensors [4], light-emitting diodes [5], solar cells [6], lasers [7] and so on. ZnO nanostructures with various morphologies, including nanorings [8], nanobelts [9], nanorod arrays [10], nanocombs [11], have been fabricated by different methods. Among these, porous nanostructures are the best candidates for nanosieve filters [12], masks [13], and gas sensors [14], owing to the high surface-to-volume ratio as well as other excellent inherent properties. Recently, Shen et al. [15] reported the large-scale ZnO nanopore arrays, taking advantage of charged surfaces and controllable sizes in hexagonal morphologies of both ZnO and self-organized porous alumina
membranes (PAMs). PAMs were used as template assisted by electrochemical deposition. Compared with the method mentioned above, here our approach is simple, product orientated, and low cost. As we know, Hydroxyl double salts (HDS) deserve a lot of attention, because of special features based on their layered crystal structure. When there is only one kind of metal cation, the HDS compound is called a layered basic metal salt. An important example is layered basic zinc salt (LBZS). Crystal growth of layered basic zinc acetate (LBZA) in the solutions is twodimensional and the resultant particles have plate-like or sheetlike shapes [16]. In this paper, large-scale porous ZnO nanobelts were fabricated from a precursor of synthetic LBZA nanobelts. Although ZnO nanostructures have been extensively investigated for applications in luminescence [17–19], here room temperature photoluminescence measurements reveal a strong band edge emission at 380 nm, while no defect-related visible emission is detected. The novel ZnO porous nanobelts are expected to have great potential for sensing, catalysis and optical emission. 2. Experimental procedure
* Co-corresponding author. Tel.: +86 21 62233223; fax: +86 21 62232517. E-mail addresses: [email protected] (K. Yu), [email protected] (Z. Zhu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.11.044
We prepare the porous ZnO nanobelts though two steps. First, 1.0 g Zn(CH3COO)22H2O (analytical grade) was
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dissolved in deionized water (20 mL). An appropriate amount of ammonia (25 wt.%) was slowly added under magnetic stirring to keep the pH at 7.2. Then the milky solution was sealed in beaker (50 mL), heated to 40 8C for 10 h. When it cooled down, the colloid liquid was filtrated. After that, the precipitate was dried at 60 8C in air for 6 h. A white, thin membrane subsequently was found on the filter paper. Second, by heating the membrane in ambient circumstance at 700 8C for 2 h in a crucible, the yellowy samples were observed. The morphology and structure of the products were characterized by scanning electron microscopy (SEM, JEOLJSM-6700F), X-ray diffraction (XRD, D/MAX 2550V), and transmission electron microscopy (TEM, Philips Tecnai 20UTWIN). Fourier transform infrared (FTIR) spectra were recorded on pellets of the samples mixed with KBr on a Bruker EQUINOX FT-IR spectrometer in the range of 400– 4000 cm–1 at a resolution of 2 cm–1. Photoluminescence (PL) measurements were performed at room temperature, using a He–Cd laser line of 325 nm as an excitation source. 3. Results and discussion Fig. 1a shows SEM image of self-assembly LBZA nanobelts, demonstrating fairly uniform morphology. The yield of the nanobelts is 100%. The nanobelts are typically 300–4 mm wide and up to 40 mm long. From the side view, we estimate they are 50 nm thick in average. An enlarged view of typical structures is shown in the Fig. 1b. Interestingly, No
Fig. 1. (a, b) SEM images of self-assembly LBZA nanobelts obtained at 40 8C.
precipitate was observed when the pH was lower than 6, and ZnO precipitates were obtained if the pH was higher than 8, while keeping all other factors the same. So the pH plays a crucial role in the formation of LBZA nanobelts. In the case of the ZnO particle preparation, the ratio of Zn2+/ NH3 greatly affects the morphology of the obtained particles. If the ratio of Zn2+/NH3 is higher than 5.2/9, the ZnO nanoflower will be obtained. And we cannot obtained the nanobelts if the ratio is lower than 4/9. The crystal structure of LBZA nanobelts are similar to that of layered basic zinc acetate (LBZA) plate-like shapes, Zn5(OH)8(CH3COO)22H2O [17]. Fig. 2 shows XRD pattern of the as-prepared LBZA nanobelts synthesized under mild conditions. The diffraction patterns were different from that of LBZA [18]. The appearance of two groups of 0 0 l reflections indicates that a novel layered structure with two interlayer dspacings is formed. Three distinct diffraction peaks at 2u = 6.78, 13.388, 20.068, 33.88 are indexed as (0 0 1) I, (0 0 2) I, (0 0 3) I and (0 0 4) I. From the most intense peak at 2u = 6.78, the (0 0 1)I interlamellar distance of 1.325 nm was deduced. The other peak at 2u = 8.98, exhibits another interlayer distance was 0.99 nm. The wide-angle XRD pattern of the sample can be explained as h k 0 reflections. By heating LBZA nanobelts at different temperatures, porous ZnO nanobelts were obtained, without morphological deformation. Fig. 3 shows scanning electron microscopy (SEM) images of the ZnO materials that were made from LBZA nanobelts. Fig. 3(a) and (b) reveals that these LBZA belts are transformed into ZnO nanobelts with uniform pores and high pore density. The typical length of the ZnO is over 20 mm on average, consisting of abundant nanoparticles with a size of 80 nm. The morphology of nanobelts keeps very well under 500 8C, whilst morphology of nanobelts has been evolved into nanochains when the temperature up to 700 8C. The nanochains obtained at 700 8C by calcinations were shown in Fig. 3 (c) and (d). The formation of long nanoparticle chains in marginal sections of the nanobelts, due to the dense assembly of nanoparticles that attaches one particle to another, while the middle sections of the nanobelts are torn off into discrete nanoparticles, generating porous ZnO nanobelts with irregular pores or gaps between nanoparticles during the transformation.
Fig. 2. The XRD pattern of the as-prepared LBZA nanobelts.
Q. Cui et al. / Applied Surface Science 254 (2008) 3517–3521
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Fig. 3. SEM images of porous ZnO at different temperatures (a, b) 500 8C, (c, d) 700 8C.
Obviously, temperature is the key factor in the transformation of ZnO. The morphology and microstructure of the obtained ZnO product by heating were further measured by TEM, revealing that the product has a porous beltlike nanostructure; Fig. 4 shows a typical TEM image, confirming that each nanobelt has a porous structure assembled by nanoparticles. The nanoparticles are 80–150 nm, most of them exhibit quasihexagonal or ellipsoidal cross sections. The porous ZnO nanobelts, obtained from the LBZA nanobelts by heating, are constructed by the connection or by the partial overlap of neighboring ZnO nanoparticles in the joint regions. The selected area electron
diffraction (SAED) pattern in the inset of Fig. 4 proves the nanoparticles are single crystal. To investigate the thermal decomposition behavior of LBZA nanobelts is very important, because it may bring in a new approach to fabricate kinds of metal oxides. The temperature resolved X-ray Diffraction of products transformed from LBZA nanobelts heated in the air were demonstrated in Fig. 5. At all temperatures, the LBZA belts can be transformed into ZnO with the wurtzite structure. The increase of diffraction intensity and the decrease of peak width in the belts heated at 500 8C indicate the increased crystallinity and crystalline size. No peaks for Zn or other impurities were detected in the spectrum.
Fig. 4. A TEM image of the porous ZnO nanobelt obtained by heating. Inset is the SAED pattern of the nanoparticle in the porous ZnO nanobelt.
Fig. 5. XRD patterns of ZnO products at different temperatures.
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Fig. 6. IR spectra of the LBZA nanobelts: (a) formed at 40 8C; (b) heated at 150 8C; (c) heated at 200 8C; (d) heated at 300 8C.
Through the information recorded by XRD, we can believe that ZnO have been completely formed at 300 8C. In order to understand how the porous ZnO were formed, the IR spectrums of the LBZA nanobelts were illustrated in Fig. 6, heated at 150, 200, 300 8C, respectively. Infrared studies confirm the composition established by XRD investigations. The curve (a) in Figure 6 represents IR spectrum of the LBZA nanobelts prepared in the first step, which clearly show the presence of acetate, hydroxyl, and water species. Polyol infrared band appeared at 1023 cm 1 in the spectra without any shift or splitting compared to pure polyol bands. The two bands observed at1398 and 1552 cm 1 are assigned to the symmetric COO– stretching vibration (ys(COO–)) mode the antisymmetric COO– stretching vibration (ya(COO–)) bands [20]. These two bands are the fingerprint of the acetate group. Hydroxyl groups and water are evidenced by the broad bands observed at high frequency, at 3486 cm 1. The broadening of these bands is due to hydrogen bond formation [21]. The curve (b) is very different from curve (a), because of the presence of the characteristic stretching of Zn–O bonds. It indicates that dehydration and dehydroxylation of zinc hydroxide occurred under150 8C. Meanwhile the bands vs and va of C O in acetate group lie in 1425 and 1568 cm 1, respectively, with a little shift compared to those of unheated nanobelts. The band assigned to Hydroxyl groups and water remains between 3440 and 3446 cm 1. Their intensities become weaker and more diffuse, due to release of acetate group and water. After heating at higher temperature, these bands become more and more obscure, and disappeared in the end. This conclusion agrees well with that from XRD patterns. The detailed mechanism for the transformation of porous ZnO nanostructures under our experimental conditions is not well understood and requires further study. Poul et al. [20] have discussed the thermal decomposition behavior of these kinds of metal hydroxide compounds. This can also explain the formation of our different ZnO nanobelts and nanochains. The first step is the departure of water intercalated into or
adsorbed onto LBZA. The second step is dehydration and dehydroxylation of zinc hydroxide and release of the acetate groups. The IR spectrum implied that the ZnO obtained by heating at 150 8C still contains a certain amount of acetates (see Fig. 6 (b)). In the LBZA nanobelts, the packing of zinc hydroxide units in the 2D ab-plane is dense, while that along the c-axis is loose, because of the coordinated acetate anions and intercalated water molecules, thus allowing the preservation of the belt morphology after calcination. During calcination, water and gases from the pyrolysis of acetate groups are generated and released, resulting in the formation of porous nanobelts. As for the presence of ZnO chains are due to more violent tearing and less dense nanoparticals. The photoluminescence (PL) emissions of the as-grown porous ZnO nanobelts were measured at room temperature following excitation with a continuous-wave He–Cd laser (l = 325 nm). The spectra are shown in Fig. 7. All these products exhibit a strong UV emission. The UV emission is the band-edge emission resulting from the recombination of excitonic centers [22–24]. It can be seen that the exciton emission in the UV region increases in intensity, shifts to high energy, and decreases in the full width at half maximum as temperature is increased. The ZnO nanobelts obtained after annealing at 150 8C exhibit a broad green–yellow emission around 584 nm, which is usually considered to be related to various intrinsic defects produced during ZnO preparation [25]. But the absence of oxygen-vacancy-related green luminescence band in the other three curves inhibits the higher quality of the porous ZnO nanobelts fabricated in higher temperature. For many photoluminescent applications, it is desirable to have the intensity of the visible emission as low as possible. Hence, IUV:IVS is usually employed as an important criterion to evaluate the quality of ZnO. It is observed that the IUV:IVS ratio increases with the increase of the temperature. Especially, a strong UV emission centered at 380 nm coming from ZnO heated at 700 8C has the highest IUV:IVS ratio (IUV:IVS28). Defect-related visible emission is nearly no detected. The narrow (15 nm) full width at half maximum PL peak is the indication of the
Fig. 7. Room-temperature photoluminescence spectra of the ZnO products.
Q. Cui et al. / Applied Surface Science 254 (2008) 3517–3521
uniformity of the ZnO nanobelts. It is lower than the values reported for other ZnO nanostructures [26,27]. From this point of view, the porous ZnO nanobelts prepared at higher temperature has superior crystal quality over the other products obtained at lower temperature. The large IUV:IVS ratio suggests that the material could, after simple and comparatively high-temperature treatment, be employed in photoluminescent devices. 4. Conclusion In summary, a simple approach to fabricate new porous ZnO nanobelts was demonstrated. We used XRD patterns and IR spectra to investigate the thermal decomposition behavior of LBZA nanobelt. The morphology and quality of porous ZnO nanobelts were mainly affected by thermal decomposition temperature. Annealing the nanobelts in the air, at higher (700 8C) temperature, causes substantial enhancement of the UV PL, while no defect-related visible emission is detected. Acknowledgments The authors acknowledge the financial support from the NSF of China (Grant No. 60476004), and the Chinese National Key Basic Research Special Found (Grant No. 2006CB921700 and 2007CB924902). References [1] [2] [3] [4]
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[5] N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, L. Sakaguchi, K. Koumoto, Adv. Mater. 14 (2002) 418. [6] K.L. Chopra, S.R. Das, Thin Film Solar Cells, Plenum, New York, 1983, p. 321. [7] M.H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu, H. Kind, E. Weber, R. Russo, P.D. Yang, Science 292 (2001) 1897. [8] X.Y. Kong, Y. Ding, R.S. Yang, Z.L. Wang, Science 303 (2004) 1384. [9] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [10] J.J. Wu, S.C. Liu, Adv. Mater. 14 (2002) 215. [11] P. Yang, H. Yan, S. Mao, R. Russo, J. Johnson, R. Saykally, N. Morris, J.R. Pham, R. He, H.J. Choj, Adv. Funct. Mater. 12 (2002) 323. [12] C.V. Cojocaru, C. Harnagea, F. Rosei, A. Pignolet, Appl. Phys. Lett. 86 (2005) 183107. [13] X. Wang, C. Lao, E. Graugnard, C.J. Summers, Z.L. Wang, Nano Lett. 5 (2005) 1784. [14] G. Sberveglieri, S. Groppelli, P. Nelli, A. Camanzi, J. Mater. Sci. Lett. 10 (1991) 602. [15] G.Q. Ding, W.Z. Shen, M.J. Zheng, D.H. Fan, Appl. Phys. Lett. 88 (2006) 103106. [16] E. Hosono, S. Fujihara, T. Kimura, H. Imai, J. Colloid Interface Sci. 272 (2004) 391. [17] J.J. Wu, S.C. Liu, J. Phys. Chem. B 106 (2002) 9546. [18] Y. Wu, H. Yan, M. Huang, B. Messer, J.H. Song, P. Yang, Chem. Eur. J. 8 (2002) 1260. [19] J.P. Liu, X.T. Huang, Y.Y. Li, X.X. Ji, Z.K. Li, X. He, F.L. Sun, J. Phys. Chem. C 111 (2007) 4990. [20] L. Poul, N. Jouini, F. Fie´vet, Chem. Mater. 12 (2000) 3123. [21] F.M. Labajos, V. Rives, M.A. Ulibarri, J. Mater. Sci. 27 (1992) 1546. [22] J.L. Yang, S.J. An, W.I. Park, G.C. Yi, W.Y. Choi, Adv. Mater. 16 (2004) 1661. [23] G.Z. Shen, Y. Bando, B.D. Liu, D. Golberg, C.J. Lee, Adv. Funct. Mater. 16 (2006) 410. [24] X.H. Han, G.Z. Wang, L. Zhou, J.G. Hou, Chem. Commun. 2 (2006) 212. [25] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230. [26] T. Mahalingam, K.M. Lee, K.H. Park, S. Lee, Y. Ahn, J.Y. Park, K.H. Koh, Nanotechnology 18 (2007), 035606. [27] L. Guo, Y. Ji, H. Xu, J. Am. Chem. Soc. 124 (2002) 14864.
Applied Surface Science 254 (2008) 3517–3521 www.elsevier.com/locate/apsusc
Porous ZnO nanobelts evolved from layered basic zinc acetate nanobelts Qingyue Cui, Ke Yu *, Ning Zhang, Ziqiang Zhu * Department of Electronic Engineering, East China Normal University, Shanghai 200241, PR China Received 13 September 2007; received in revised form 23 November 2007; accepted 23 November 2007 Available online 3 December 2007
Abstract Novel porous ZnO nanobelts were successfully synthesized by heating layered basic zinc acetate (LBZA) nanobelts in the air. The precursor of LBZA nanobelts consisted of a lamellar structure with two interlayer distances of 1.325 and 0.99 nm were prepared using a low-temperature, solution-based method. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy are used to characterize the as-products. PL measurements show that the porous ZnO nanobelts have strong ultraviolet emission properties at 380 nm, while no defect-related visible emission is detected. The good performance for photoluminescence emission makes the porous ZnO nanobelts promising candidates for photonic and electronic device applications. # 2007 Elsevier B.V. All rights reserved. PACS : 81.05.Dz; 81.05.Rm; 78.55.Et Keywords: ZnO nanostructures; Porous morphology; PL spectra
1. Introduction The development of novel nanostructured materials with controlled shapes, size and morphology has initiated great research interest for seeking novel properties and tailorable functions. In particular, zinc oxide, a wide-bandgap semiconductor [1], has proven valuable as an ideal functional component for devices and materials in catalysts [2], photonic crystals [3], gas sensors [4], light-emitting diodes [5], solar cells [6], lasers [7] and so on. ZnO nanostructures with various morphologies, including nanorings [8], nanobelts [9], nanorod arrays [10], nanocombs [11], have been fabricated by different methods. Among these, porous nanostructures are the best candidates for nanosieve filters [12], masks [13], and gas sensors [14], owing to the high surface-to-volume ratio as well as other excellent inherent properties. Recently, Shen et al. [15] reported the large-scale ZnO nanopore arrays, taking advantage of charged surfaces and controllable sizes in hexagonal morphologies of both ZnO and self-organized porous alumina
membranes (PAMs). PAMs were used as template assisted by electrochemical deposition. Compared with the method mentioned above, here our approach is simple, product orientated, and low cost. As we know, Hydroxyl double salts (HDS) deserve a lot of attention, because of special features based on their layered crystal structure. When there is only one kind of metal cation, the HDS compound is called a layered basic metal salt. An important example is layered basic zinc salt (LBZS). Crystal growth of layered basic zinc acetate (LBZA) in the solutions is twodimensional and the resultant particles have plate-like or sheetlike shapes [16]. In this paper, large-scale porous ZnO nanobelts were fabricated from a precursor of synthetic LBZA nanobelts. Although ZnO nanostructures have been extensively investigated for applications in luminescence [17–19], here room temperature photoluminescence measurements reveal a strong band edge emission at 380 nm, while no defect-related visible emission is detected. The novel ZnO porous nanobelts are expected to have great potential for sensing, catalysis and optical emission. 2. Experimental procedure
* Co-corresponding author. Tel.: +86 21 62233223; fax: +86 21 62232517. E-mail addresses: [email protected] (K. Yu), [email protected] (Z. Zhu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.11.044
We prepare the porous ZnO nanobelts though two steps. First, 1.0 g Zn(CH3COO)22H2O (analytical grade) was
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Q. Cui et al. / Applied Surface Science 254 (2008) 3517–3521
dissolved in deionized water (20 mL). An appropriate amount of ammonia (25 wt.%) was slowly added under magnetic stirring to keep the pH at 7.2. Then the milky solution was sealed in beaker (50 mL), heated to 40 8C for 10 h. When it cooled down, the colloid liquid was filtrated. After that, the precipitate was dried at 60 8C in air for 6 h. A white, thin membrane subsequently was found on the filter paper. Second, by heating the membrane in ambient circumstance at 700 8C for 2 h in a crucible, the yellowy samples were observed. The morphology and structure of the products were characterized by scanning electron microscopy (SEM, JEOLJSM-6700F), X-ray diffraction (XRD, D/MAX 2550V), and transmission electron microscopy (TEM, Philips Tecnai 20UTWIN). Fourier transform infrared (FTIR) spectra were recorded on pellets of the samples mixed with KBr on a Bruker EQUINOX FT-IR spectrometer in the range of 400– 4000 cm–1 at a resolution of 2 cm–1. Photoluminescence (PL) measurements were performed at room temperature, using a He–Cd laser line of 325 nm as an excitation source. 3. Results and discussion Fig. 1a shows SEM image of self-assembly LBZA nanobelts, demonstrating fairly uniform morphology. The yield of the nanobelts is 100%. The nanobelts are typically 300–4 mm wide and up to 40 mm long. From the side view, we estimate they are 50 nm thick in average. An enlarged view of typical structures is shown in the Fig. 1b. Interestingly, No
Fig. 1. (a, b) SEM images of self-assembly LBZA nanobelts obtained at 40 8C.
precipitate was observed when the pH was lower than 6, and ZnO precipitates were obtained if the pH was higher than 8, while keeping all other factors the same. So the pH plays a crucial role in the formation of LBZA nanobelts. In the case of the ZnO particle preparation, the ratio of Zn2+/ NH3 greatly affects the morphology of the obtained particles. If the ratio of Zn2+/NH3 is higher than 5.2/9, the ZnO nanoflower will be obtained. And we cannot obtained the nanobelts if the ratio is lower than 4/9. The crystal structure of LBZA nanobelts are similar to that of layered basic zinc acetate (LBZA) plate-like shapes, Zn5(OH)8(CH3COO)22H2O [17]. Fig. 2 shows XRD pattern of the as-prepared LBZA nanobelts synthesized under mild conditions. The diffraction patterns were different from that of LBZA [18]. The appearance of two groups of 0 0 l reflections indicates that a novel layered structure with two interlayer dspacings is formed. Three distinct diffraction peaks at 2u = 6.78, 13.388, 20.068, 33.88 are indexed as (0 0 1) I, (0 0 2) I, (0 0 3) I and (0 0 4) I. From the most intense peak at 2u = 6.78, the (0 0 1)I interlamellar distance of 1.325 nm was deduced. The other peak at 2u = 8.98, exhibits another interlayer distance was 0.99 nm. The wide-angle XRD pattern of the sample can be explained as h k 0 reflections. By heating LBZA nanobelts at different temperatures, porous ZnO nanobelts were obtained, without morphological deformation. Fig. 3 shows scanning electron microscopy (SEM) images of the ZnO materials that were made from LBZA nanobelts. Fig. 3(a) and (b) reveals that these LBZA belts are transformed into ZnO nanobelts with uniform pores and high pore density. The typical length of the ZnO is over 20 mm on average, consisting of abundant nanoparticles with a size of 80 nm. The morphology of nanobelts keeps very well under 500 8C, whilst morphology of nanobelts has been evolved into nanochains when the temperature up to 700 8C. The nanochains obtained at 700 8C by calcinations were shown in Fig. 3 (c) and (d). The formation of long nanoparticle chains in marginal sections of the nanobelts, due to the dense assembly of nanoparticles that attaches one particle to another, while the middle sections of the nanobelts are torn off into discrete nanoparticles, generating porous ZnO nanobelts with irregular pores or gaps between nanoparticles during the transformation.
Fig. 2. The XRD pattern of the as-prepared LBZA nanobelts.
Q. Cui et al. / Applied Surface Science 254 (2008) 3517–3521
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Fig. 3. SEM images of porous ZnO at different temperatures (a, b) 500 8C, (c, d) 700 8C.
Obviously, temperature is the key factor in the transformation of ZnO. The morphology and microstructure of the obtained ZnO product by heating were further measured by TEM, revealing that the product has a porous beltlike nanostructure; Fig. 4 shows a typical TEM image, confirming that each nanobelt has a porous structure assembled by nanoparticles. The nanoparticles are 80–150 nm, most of them exhibit quasihexagonal or ellipsoidal cross sections. The porous ZnO nanobelts, obtained from the LBZA nanobelts by heating, are constructed by the connection or by the partial overlap of neighboring ZnO nanoparticles in the joint regions. The selected area electron
diffraction (SAED) pattern in the inset of Fig. 4 proves the nanoparticles are single crystal. To investigate the thermal decomposition behavior of LBZA nanobelts is very important, because it may bring in a new approach to fabricate kinds of metal oxides. The temperature resolved X-ray Diffraction of products transformed from LBZA nanobelts heated in the air were demonstrated in Fig. 5. At all temperatures, the LBZA belts can be transformed into ZnO with the wurtzite structure. The increase of diffraction intensity and the decrease of peak width in the belts heated at 500 8C indicate the increased crystallinity and crystalline size. No peaks for Zn or other impurities were detected in the spectrum.
Fig. 4. A TEM image of the porous ZnO nanobelt obtained by heating. Inset is the SAED pattern of the nanoparticle in the porous ZnO nanobelt.
Fig. 5. XRD patterns of ZnO products at different temperatures.
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Fig. 6. IR spectra of the LBZA nanobelts: (a) formed at 40 8C; (b) heated at 150 8C; (c) heated at 200 8C; (d) heated at 300 8C.
Through the information recorded by XRD, we can believe that ZnO have been completely formed at 300 8C. In order to understand how the porous ZnO were formed, the IR spectrums of the LBZA nanobelts were illustrated in Fig. 6, heated at 150, 200, 300 8C, respectively. Infrared studies confirm the composition established by XRD investigations. The curve (a) in Figure 6 represents IR spectrum of the LBZA nanobelts prepared in the first step, which clearly show the presence of acetate, hydroxyl, and water species. Polyol infrared band appeared at 1023 cm 1 in the spectra without any shift or splitting compared to pure polyol bands. The two bands observed at1398 and 1552 cm 1 are assigned to the symmetric COO– stretching vibration (ys(COO–)) mode the antisymmetric COO– stretching vibration (ya(COO–)) bands [20]. These two bands are the fingerprint of the acetate group. Hydroxyl groups and water are evidenced by the broad bands observed at high frequency, at 3486 cm 1. The broadening of these bands is due to hydrogen bond formation [21]. The curve (b) is very different from curve (a), because of the presence of the characteristic stretching of Zn–O bonds. It indicates that dehydration and dehydroxylation of zinc hydroxide occurred under150 8C. Meanwhile the bands vs and va of C O in acetate group lie in 1425 and 1568 cm 1, respectively, with a little shift compared to those of unheated nanobelts. The band assigned to Hydroxyl groups and water remains between 3440 and 3446 cm 1. Their intensities become weaker and more diffuse, due to release of acetate group and water. After heating at higher temperature, these bands become more and more obscure, and disappeared in the end. This conclusion agrees well with that from XRD patterns. The detailed mechanism for the transformation of porous ZnO nanostructures under our experimental conditions is not well understood and requires further study. Poul et al. [20] have discussed the thermal decomposition behavior of these kinds of metal hydroxide compounds. This can also explain the formation of our different ZnO nanobelts and nanochains. The first step is the departure of water intercalated into or
adsorbed onto LBZA. The second step is dehydration and dehydroxylation of zinc hydroxide and release of the acetate groups. The IR spectrum implied that the ZnO obtained by heating at 150 8C still contains a certain amount of acetates (see Fig. 6 (b)). In the LBZA nanobelts, the packing of zinc hydroxide units in the 2D ab-plane is dense, while that along the c-axis is loose, because of the coordinated acetate anions and intercalated water molecules, thus allowing the preservation of the belt morphology after calcination. During calcination, water and gases from the pyrolysis of acetate groups are generated and released, resulting in the formation of porous nanobelts. As for the presence of ZnO chains are due to more violent tearing and less dense nanoparticals. The photoluminescence (PL) emissions of the as-grown porous ZnO nanobelts were measured at room temperature following excitation with a continuous-wave He–Cd laser (l = 325 nm). The spectra are shown in Fig. 7. All these products exhibit a strong UV emission. The UV emission is the band-edge emission resulting from the recombination of excitonic centers [22–24]. It can be seen that the exciton emission in the UV region increases in intensity, shifts to high energy, and decreases in the full width at half maximum as temperature is increased. The ZnO nanobelts obtained after annealing at 150 8C exhibit a broad green–yellow emission around 584 nm, which is usually considered to be related to various intrinsic defects produced during ZnO preparation [25]. But the absence of oxygen-vacancy-related green luminescence band in the other three curves inhibits the higher quality of the porous ZnO nanobelts fabricated in higher temperature. For many photoluminescent applications, it is desirable to have the intensity of the visible emission as low as possible. Hence, IUV:IVS is usually employed as an important criterion to evaluate the quality of ZnO. It is observed that the IUV:IVS ratio increases with the increase of the temperature. Especially, a strong UV emission centered at 380 nm coming from ZnO heated at 700 8C has the highest IUV:IVS ratio (IUV:IVS28). Defect-related visible emission is nearly no detected. The narrow (15 nm) full width at half maximum PL peak is the indication of the
Fig. 7. Room-temperature photoluminescence spectra of the ZnO products.
Q. Cui et al. / Applied Surface Science 254 (2008) 3517–3521
uniformity of the ZnO nanobelts. It is lower than the values reported for other ZnO nanostructures [26,27]. From this point of view, the porous ZnO nanobelts prepared at higher temperature has superior crystal quality over the other products obtained at lower temperature. The large IUV:IVS ratio suggests that the material could, after simple and comparatively high-temperature treatment, be employed in photoluminescent devices. 4. Conclusion In summary, a simple approach to fabricate new porous ZnO nanobelts was demonstrated. We used XRD patterns and IR spectra to investigate the thermal decomposition behavior of LBZA nanobelt. The morphology and quality of porous ZnO nanobelts were mainly affected by thermal decomposition temperature. Annealing the nanobelts in the air, at higher (700 8C) temperature, causes substantial enhancement of the UV PL, while no defect-related visible emission is detected. Acknowledgments The authors acknowledge the financial support from the NSF of China (Grant No. 60476004), and the Chinese National Key Basic Research Special Found (Grant No. 2006CB921700 and 2007CB924902). References [1] [2] [3] [4]
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