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Synthesis, characterization and room temperature photoluminescence properties of briers-like ZnO nanoarchitectures
Materials Science and Engineering B 167 (2010) 177–181
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis, characterization and room temperature photoluminescence properties of briers-like ZnO nanoarchitectures Yude Wang a,∗ , Xiaodan Sun b , Hengde Li b a b
Department of Materials Science & Engineering, Yunnan University, Cuihu North Road 2#, 650091 Kunming, Yunnan Province, People’s Republic of China Department of Materials Science & Engineering, Tsinghua University, 100084 Beijing, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 16 October 2009 Received in revised form 30 January 2010 Accepted 1 February 2010 Keywords: Nanostructures Layered compound Chemical synthesis Zinc oxide Room temperature photoluminescence
a b s t r a c t Briers-like ZnO nanoarchitectures, which consisted of sword-like ZnO nanosheets, have been prepared by a facile organic CTAB (cetyltrimethylammonium bromide, CH3 (CH2 )15 N+ (CH3 )3 Br− ) inducing deposition process on the titanium substrate. The nanostructured composite has been characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoemission spectra (XPS). The XRD pattern indicates that the swardlike ZnO nanosheets are the high ordered nanolayered inorganic–organic composites. The ordered layered nanocomposite exhibits the room temperature photoluminescence (RTPL) characteristics. It is inferred that the RTPL of ZnO/CTAB ordered nanolayered composite might be induced by the interfacial effect between the ZnO and the surfactant CTAB. © 2010 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a promising luminescent material and used for various applications such as vacuum fluorescent displays due to its wide band-gap (3.37 eV), large exciton binding energy (60 meV), nonlinear optical property and room temperature ultraviolet emission [1]. As a wide bandgap semiconductor and luminescence material, nanostructured ZnO (nanoparticles, nanowires, nanobelts, and nanotubes) has been widely studied [2–5] and prepared with various routes, including solvothermal, hydrothermal, self-assembly and a template assisted sol–gel process at a relatively low concentration of zinc. The solution chemistry involved enables the synthesis of ZnO nanostructures with unique shapes such as in plate-like morphology and the formation of novel microstructures constructed from these nanoplates. The various ZnO nanostructures that has been produced, but the reports on the synthesis of plate-like or sheet-like ZnO nanostructures are rare. Thus, only a few papers have described the formation of ZnO superstructures produced by the assembly of nanoplates [6]. The designing the morphology of ZnO with the hierarchical nanostructures is now focus of current research for improving the physical and chemical performance in devices [7], for examination of the factors governing their growth and the discovery of new properties [6]. The effects of organic molecular on the morphology control of inorganic materials in the process of biomineralization have
∗ Corresponding author. Tel.: +86 871 6998372; fax: +86 871 5153832. E-mail address: [email protected] (Y. Wang). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.02.001
long been realized. Nowadays, these effects have been utilized to prepare inorganic materials with desired morphologies in different systems [8]. The flower-like single-crystal ZnO nanostructures prepared by the CTAB-assisted hydrothermal process at low temperature (120 ◦ C) were reported [9]. In this paper, we report a new method to obtain briers-like ZnO nanoarchitectures consisted of sword-like ZnO nanosheets on Ti substrates by the CTAB-assisted growth, which can be easily employed in the coating of complex geometries. The sword-like ZnO nanosheets were grown at room temperature. The structure, morphology, and composition of the materials were characterized by X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoemission spectra (XPS), respectively. 2. Experimental All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification. The titanium plates had the following composition (in mass percent): Fe 0.0015, Si 0.0010, C 0.0005, N 0.0003, H 0.00015, O 0.0015, with the balance being Ti. The plates were polished metallographically with SiC emery paper to remove the oxide surface layer. The final polishing was performed with No. 800 paper and the final thickness of the polished plates was about 1 mm. The plates were ultrasonically washed in acetone for about 10 min and rinsed in deionized water for 1 min. Subsequently, the plates were aged in deionized water at room temperature.
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The plates were taken out after one day and rinsed in deionized water for 1 min and then dried at 60 ◦ C for one day. The synthesis method of ZnO nanostructures was based on the using of a cationic surfactant CTAB (cetyltrimethylammonium bromide, CH3 (CH2 )15 N+ (CH3 )3 Br− ), as structure directing agent, and the simple chemical materials (ZnNO3 and NH4 OH), as inorganic precursor and counterions respectively. Reaction was performed at room temperature. The synthetic procedure was as follows: (1) CTAB was mixed with the distilled deionized water with stirring until a homogenous solution was obtained; (2) NH4 OH (25 wt.% solution) was mixed with the distilled deionized water and then added into the CTAB solution with stirring; (3) when the mixing solution became homogenous, a Zn2+ solution of ZnNO3 diluted with distilled deionized water was introduced, producing a white slurry. The molar ratio of CTAB:ZnNO3 :NH4 OH:H2 O was 1.6:1.0:5.3:1347. Subsequently, one plate was placed in the mixed solution with white slurry at room temperature for up to 30 days to induce nanostructures formation. The Ti plates with ZnO nanostructures were dried at 50 ◦ C for 12 h after taken out from solution. Powder X-ray diffraction (XRD) data were carried out with a Rigaku D/max-RB diffractometer with Cu K␣ radiation ( = 1.5418 Å). Field emission scanning electron microscopy (FE-SEM) photograph was obtained by JSM-6301F equipped with EDX part for analysis. The samples for SEM were prepared by pasting the Ti plate with ZnO nanostructures in the conductive glue; this pasting was then sprayed with carbon. Fourier transformed infrared (FTIR) spectra, in the range of 4000–400 cm−1 , were recorded on PerkinElmer Spectrum GX infrared spectrophotometer. The samples for FTIR were prepared using the KBr technology, which were calibrated by polystyrene. The composition of the as-synthesized composite was determined by the X-ray photoemission spectra (XPS), which were recorded on a PerkinElmer PHI 5300 ESCA system with an Al K␣ X-ray beam and 250 W power. Photoluminescence (PL) measurements were carried out at room temperature using 310 nm wavelength as the excitation wavelength with a HITACHI 850 type visible–ultraviolet spectrometer with a Xe lamp as the excitation source.
3. Results and discussion The small angle X-ray diffraction pattern of the as-synthesized product is presented in Fig. 1(a). The pattern of zinc oxide assembled with surfactant contains only a series of low angle peaks. These equidistant diffraction peaks have been recognized to arise from an ordered nanolayered structure, similar to the lamellar silica [10,11] and aluminophosphate mesophase [12–14]. The peaks are attributed to the 0 0 1 rational reflections, characteristic of nanolayered structures. On the basis of the XRD results, the inter-layer distance of the mesolamellar structure is determined to be 16.86 Å. After calcinations, the small angle X-ray diffraction pattern disappear (Fig. 1(b)). Wide-angle X-ray diffraction patterns are presented for products as-synthesized and calcined at 500 ◦ C for 2 h as shown in Fig. 2. The peaks of as-synthesized sample clearly show the broad peaks that can be indexed according to their corresponding crystalline oxide phases. The materials exhibit reflections of comparable integral intensity in the region 2 20–80◦ that are characteristic of hexagonal wurtzite structure (JCPDS card 36-1451). The data indicate that the inorganic product may consist of crystalline oxide domains. The resulting material is highly crystalline after their calcinations because the diffraction peaks are higher and narrower. Comparing Fig. 2(a) with (b), the surfactant CTAB has been removed. The morphologies of the as-synthesized and calcined samples on titanium substrate were characterized by field emission electron microscopy (FE-SEM). Typical examples are shown in Fig. 3. Fig. 3(a) and (b) shows the FE-SEM images of briers-like ZnO/CTAB compos-
Fig. 1. XRD patterns at the small-angle of as-synthesized product (a) and calcined products at 500 ◦ C for 2 h (b).
ite and crystalline ZnO, respectively. The high magnification image reveals that the briers-like ZnO nanoarchitectures are made up of sword-like ZnO nanosheets (Fig. 3(c)). From the SEM observations, the ZnO nanocrystalline contains numerous dendrites (Fig. 3(d)), and almost all of them show same morphology. The oxidization of the ordered nanolayered structures in air produced wurtzite ZnO dendrites. In addition to carbon (from the spraying with carbon), the EDS result (Fig. 3(e)) demonstrates only elements Zn and O contained in the sample. Hence the calcined sample is hexagonal wurtzite ZnO.
Fig. 2. XRD patterns of the as-synthesized (a) and calcined products at 500 ◦ C for 2 h (b).
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Fig. 3. SEM images of sample: (a) as-synthesized briers-like ZnO nanoarchitectured inorganic–organic composite; (b), (c) and (d) wurtzite ZnO briers-like dentrites obtained after oxidization of as-synthesized inorganic–organic composites at 500 ◦ C for 2 h in air. (e) Energy dispersive spectroscopy (EDS) of calcined sample. (f) SEM image of as-synthesized product in solution.
After the old oxide surface layer was removed through polishing, a fresh oxide layer would quickly form on the Ti plate. When the oxidized titanium plates are aged in the water, the surface reactions can take place. According to the potential-pH (i.e., E-pH diagram) for the Ti–H2 O corrosion system, the passive TiO2 layer is stable in pure water [15]. Therefore, the titanium plate surface oxidation will continue to thicken the oxide layer, and at the same time the oxide layer will induce dissociative decomposition of water molecules, resulting in a covering of hydroxyl groups, including half acidic (doubly coordinated) and half mainly basic (singly coordinated) hydroxyls, termed TiOH and Ti–OH groups, respectively [16,17]. The existence of hydroxyl groups changes the ligands of Zn2+ ions in growth unit [18]. The addition of CTAB caused the formation of briers-like shapes. Remarkably a hierarchical organisation of the surface morphology was built up in this self-assembly process. For the product in solution, the morphology is shown in Fig. 3(f). It is different that of the material lied on Ti. It is possible that the organization of the materials is due to the fact that the material was placed on the Ti plate and consequently the preferentially orientation to the substrate. Further work will be carried out to give a more detailed explanation on the growth mechanism.
Fig. 4 shows the FTIR spectra in the range 4000–400 cm−1 for the CTAB and as-synthesized nanocomposite. Some bands are observed in the region 2800–3000 cm−1 and attributed to CTAB surfactants [19,20]. CTAB FTIR spectrum (Fig. 4(a)) shows two intense bands, assigned to asymmetric (2921 cm−1 ) and symmetric (2853 cm−1 ) stretching vibrations of C–CH2 in the methylene chains. The CH2 stretching vibrations are considered to be related with the physical state (monomer, micelle or solid) of surfactant [20]. It indicates that the surfactant is present in the as-synthesized composite as micelles. The sharp bands in the region of 1350–1500 cm−1 are attributed to the deformation of –CH2 – and –CH3 [20] of the incorporated surfactants. The broad band between 3200 and 3600 cm−1 and the band centered at 1631 cm−1 found on all samples are assigned to O–H stretching and deformation vibrations of weakbound water [21]. It is because the measured sample adsorbed the water from the KBr. The broad bands between 400 and 850 cm−1 (Fig. 4(b)) are attributed to the framework vibrations of zinc oxide [22]. The XPS spectrum (Fig. 5(a)) shows two peaks of 2p3/2 and 2p1/2 at 1022.8 and 1045.7 eV with a better symmetry, and they are assigned to the lattice zinc in zinc oxide. The peak separation
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Fig. 4. FTIR spectra for CTAB (a) and as-synthesized briers-like ZnO nanoarchitectured inorganic–organic composite (b).
between these two peaks is 22.9 eV. The values correspond to a 2p3 binding energy of Zn(II) ion (indexed Standard ESCA Spectra of the Elements and Line Energy Information, Co., USA). The small shift in the 2p3/2 peak position from the crystalline ZnO to the nanolayered structure ZnO/CTAB composite (from 1021.7 (indexed Hand of Xray photoelectron spectroscopy) to 1022.8 eV) indicates a change of microenvironments for zinc. This shift (1.1 eV) confirms that the largest and the majority of Zn atoms remain in Zn-CTAB composites, in the same formal valence state of Zn2+ within an oxygen deficient ZnO1−x matrix [23,24]. This Zn enrichment or oxygen defects is due to the strong interaction between the CTAB and ZnO and to the weakly crystalline nature of the hydrous zinc oxide composite.
Fig. 6. Room temperature photoluminescence spectrum of as-synthesized brierslike ZnO nanoarchitectured inorganic–organic composite in the excitation wavelength 310 nm.
In Fig. 5(b), it can be seen that the O1s XPS is symmetric. The peak at about 531.8 eV is due to the ZnO crystal lattice oxygen [25]. The room temperature photoluminescence spectrum of the ZnO/CTAB inorganic–organic nanocomposite is shown in Fig. 6. The emission spectrum of the excitation at 310 nm gives three peaks at 429, 464, 537 nm, and a peak centered at 480 nm, which is the shoulder of the broad peak. It indicates that the emission band is made up of two overlapping emission band centered at 464 and 480 nm. The inner interfacial defects between the ZnO and surfactant are contribute to the relatively shorter wavelength of luminescence (less than 500 nm), while the outmost surfactant has the ability to additionally luminescence at long wavelengths (more than 500 nm) [26]. However, the emission spectrum has not been observed for the sample treated at 500 ◦ C. Absolutely no luminescence is observed for the CTAB alone. ZnO/CTAB inorganic–organic nanocomposite is very different the ZnO nanowires [27] and nanosheets [28]. In the past several decades, the green luminescence mechanisms of ZnO have been studied [29–33]. The green peak can be attributed to the deep-trap-mediated emission. The origin of the deep trap in ZnO is not yet clearly understood but is generally attributed to structural defects, single ionized vacancies, and impurities [27,34–36]. In our investigation, RTPL of Zn/CTAB nanocomposite is different from that of bulk and other ZnO nanostructures. The zinc oxide/CTAB ordered nanolayered composite shows the ordered zinc oxide/CTAB superlattices structure. This peculiar structure might have a profound effect on the chemical and physical properties of zinc oxide. We think that the interfacial effect of Zn-CTAB nanocomposites between zinc oxide and the organic surfactant CTAB might be similar to the nanoparticles coated with stearic acid reported by Zou et al. [37] and ZnO nanoparticles coated with SDS reported by Wang [26]. The similar results were obtained in TiO2 mesolamellar [38], meolamellar NiO/dodecylamine [39], and ZnO/dodecylamine inorganic–organic nanocomposite [22]. The quantum yield of photoluminescence and the emission life, as well as the intensity of luminescence of the zinc oxide/CTAB inorganic–organic nanocomposite are being investigated to get a definite understanding. 4. Conclusion
Fig. 5. XPS spectra of the as-synthesized briers-like ZnO nanoarchitectured inorganic–organic composite: (a) Zn2p and (b) O1s.
The briers-like ZnO nanoarchitectures, which consisted of sword-like ZnO nanosheets on the titanium plates can be generated by a simple organic template method. The results investigated by XRD, FE-SEM, and XPS indicated that these nanostructured ZnO have fine hexagonal wurtzite crystal structure. It is found that
Y. Wang et al. / Materials Science and Engineering B 167 (2010) 177–181
as-synthesized composite displays room temperature photoluminescence. The emission features confirm that the RTPL of zinc oxide/CTAB ordered layered nanocomposite is different from that of bulk and nanoparticles zinc oxides. The interface effect between zinc oxide and CTAB is the probably main cause of the room temperature photoluminescence. Further investigation of the formation mechanism, the properties and the applications of this kind of nanostructure are being performed. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 50662006), and the Natural Science Foundation of Yunnan Province, China (No. 2006E0013M). The authors thank Y.J. Yan for their help in the SEM experiments in Tsinghua University. References [1] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407–409. [2] G.D. Wei, W.P. Qin, W. Han, W.Y. Yang, F.M. Gao, G.Z. Jing, R.J. Kim, D.S. Zhang, K.Z. Zheng, L.L. Wang, L. Liu, J. Phys. Chem. C 113 (2009) 19432–19438. [3] Y.W. Wang, L.D. Zhang, G.Z. Wang, X.S. Peng, Z.Q. Chu, C.H. Liang, J. Cryst. Growth 234 (2002) 171–175. [4] J. Zhang, L.D. Sun, C.S. Liao, C.H. Yan, Chem. Commun. (2002) 262–263. [5] Y. He, W.B. Sang, J.A. Wang, R.F. Wu, J.H. Min, J. Nanopart. Res. 7 (2005) 307–311. [6] C.L. Kuo, T.J. Kuo, M.H. Huang, J. Phys. Chem. B 109 (2005) 20115–20121. [7] E. Hosono, S. Fujihara, I. Honna, H.S. Zhou, Adv. Mater. 17 (2005) 2091–2094. [8] X.D. Sun, X.D. Kong, Y.D. Wang, C.L. Ma, F.Z. Cui, H.D. Li, Mater. Sci. Engine C 26 (2006) 653–656. [9] H. Zhang, D.R. Yang, Y.J. Ji, X.Y. Ma, J. Xu, D.L. Que, J. Phys. Chem. B 108 (2004) 3955–3958. [10] J.S. Beck, J.C. Vartuli, W.U. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834–10843.
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Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis, characterization and room temperature photoluminescence properties of briers-like ZnO nanoarchitectures Yude Wang a,∗ , Xiaodan Sun b , Hengde Li b a b
Department of Materials Science & Engineering, Yunnan University, Cuihu North Road 2#, 650091 Kunming, Yunnan Province, People’s Republic of China Department of Materials Science & Engineering, Tsinghua University, 100084 Beijing, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 16 October 2009 Received in revised form 30 January 2010 Accepted 1 February 2010 Keywords: Nanostructures Layered compound Chemical synthesis Zinc oxide Room temperature photoluminescence
a b s t r a c t Briers-like ZnO nanoarchitectures, which consisted of sword-like ZnO nanosheets, have been prepared by a facile organic CTAB (cetyltrimethylammonium bromide, CH3 (CH2 )15 N+ (CH3 )3 Br− ) inducing deposition process on the titanium substrate. The nanostructured composite has been characterized by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoemission spectra (XPS). The XRD pattern indicates that the swardlike ZnO nanosheets are the high ordered nanolayered inorganic–organic composites. The ordered layered nanocomposite exhibits the room temperature photoluminescence (RTPL) characteristics. It is inferred that the RTPL of ZnO/CTAB ordered nanolayered composite might be induced by the interfacial effect between the ZnO and the surfactant CTAB. © 2010 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a promising luminescent material and used for various applications such as vacuum fluorescent displays due to its wide band-gap (3.37 eV), large exciton binding energy (60 meV), nonlinear optical property and room temperature ultraviolet emission [1]. As a wide bandgap semiconductor and luminescence material, nanostructured ZnO (nanoparticles, nanowires, nanobelts, and nanotubes) has been widely studied [2–5] and prepared with various routes, including solvothermal, hydrothermal, self-assembly and a template assisted sol–gel process at a relatively low concentration of zinc. The solution chemistry involved enables the synthesis of ZnO nanostructures with unique shapes such as in plate-like morphology and the formation of novel microstructures constructed from these nanoplates. The various ZnO nanostructures that has been produced, but the reports on the synthesis of plate-like or sheet-like ZnO nanostructures are rare. Thus, only a few papers have described the formation of ZnO superstructures produced by the assembly of nanoplates [6]. The designing the morphology of ZnO with the hierarchical nanostructures is now focus of current research for improving the physical and chemical performance in devices [7], for examination of the factors governing their growth and the discovery of new properties [6]. The effects of organic molecular on the morphology control of inorganic materials in the process of biomineralization have
∗ Corresponding author. Tel.: +86 871 6998372; fax: +86 871 5153832. E-mail address: [email protected] (Y. Wang). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.02.001
long been realized. Nowadays, these effects have been utilized to prepare inorganic materials with desired morphologies in different systems [8]. The flower-like single-crystal ZnO nanostructures prepared by the CTAB-assisted hydrothermal process at low temperature (120 ◦ C) were reported [9]. In this paper, we report a new method to obtain briers-like ZnO nanoarchitectures consisted of sword-like ZnO nanosheets on Ti substrates by the CTAB-assisted growth, which can be easily employed in the coating of complex geometries. The sword-like ZnO nanosheets were grown at room temperature. The structure, morphology, and composition of the materials were characterized by X-ray diffraction (XRD) analysis, field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoemission spectra (XPS), respectively. 2. Experimental All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification. The titanium plates had the following composition (in mass percent): Fe 0.0015, Si 0.0010, C 0.0005, N 0.0003, H 0.00015, O 0.0015, with the balance being Ti. The plates were polished metallographically with SiC emery paper to remove the oxide surface layer. The final polishing was performed with No. 800 paper and the final thickness of the polished plates was about 1 mm. The plates were ultrasonically washed in acetone for about 10 min and rinsed in deionized water for 1 min. Subsequently, the plates were aged in deionized water at room temperature.
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The plates were taken out after one day and rinsed in deionized water for 1 min and then dried at 60 ◦ C for one day. The synthesis method of ZnO nanostructures was based on the using of a cationic surfactant CTAB (cetyltrimethylammonium bromide, CH3 (CH2 )15 N+ (CH3 )3 Br− ), as structure directing agent, and the simple chemical materials (ZnNO3 and NH4 OH), as inorganic precursor and counterions respectively. Reaction was performed at room temperature. The synthetic procedure was as follows: (1) CTAB was mixed with the distilled deionized water with stirring until a homogenous solution was obtained; (2) NH4 OH (25 wt.% solution) was mixed with the distilled deionized water and then added into the CTAB solution with stirring; (3) when the mixing solution became homogenous, a Zn2+ solution of ZnNO3 diluted with distilled deionized water was introduced, producing a white slurry. The molar ratio of CTAB:ZnNO3 :NH4 OH:H2 O was 1.6:1.0:5.3:1347. Subsequently, one plate was placed in the mixed solution with white slurry at room temperature for up to 30 days to induce nanostructures formation. The Ti plates with ZnO nanostructures were dried at 50 ◦ C for 12 h after taken out from solution. Powder X-ray diffraction (XRD) data were carried out with a Rigaku D/max-RB diffractometer with Cu K␣ radiation ( = 1.5418 Å). Field emission scanning electron microscopy (FE-SEM) photograph was obtained by JSM-6301F equipped with EDX part for analysis. The samples for SEM were prepared by pasting the Ti plate with ZnO nanostructures in the conductive glue; this pasting was then sprayed with carbon. Fourier transformed infrared (FTIR) spectra, in the range of 4000–400 cm−1 , were recorded on PerkinElmer Spectrum GX infrared spectrophotometer. The samples for FTIR were prepared using the KBr technology, which were calibrated by polystyrene. The composition of the as-synthesized composite was determined by the X-ray photoemission spectra (XPS), which were recorded on a PerkinElmer PHI 5300 ESCA system with an Al K␣ X-ray beam and 250 W power. Photoluminescence (PL) measurements were carried out at room temperature using 310 nm wavelength as the excitation wavelength with a HITACHI 850 type visible–ultraviolet spectrometer with a Xe lamp as the excitation source.
3. Results and discussion The small angle X-ray diffraction pattern of the as-synthesized product is presented in Fig. 1(a). The pattern of zinc oxide assembled with surfactant contains only a series of low angle peaks. These equidistant diffraction peaks have been recognized to arise from an ordered nanolayered structure, similar to the lamellar silica [10,11] and aluminophosphate mesophase [12–14]. The peaks are attributed to the 0 0 1 rational reflections, characteristic of nanolayered structures. On the basis of the XRD results, the inter-layer distance of the mesolamellar structure is determined to be 16.86 Å. After calcinations, the small angle X-ray diffraction pattern disappear (Fig. 1(b)). Wide-angle X-ray diffraction patterns are presented for products as-synthesized and calcined at 500 ◦ C for 2 h as shown in Fig. 2. The peaks of as-synthesized sample clearly show the broad peaks that can be indexed according to their corresponding crystalline oxide phases. The materials exhibit reflections of comparable integral intensity in the region 2 20–80◦ that are characteristic of hexagonal wurtzite structure (JCPDS card 36-1451). The data indicate that the inorganic product may consist of crystalline oxide domains. The resulting material is highly crystalline after their calcinations because the diffraction peaks are higher and narrower. Comparing Fig. 2(a) with (b), the surfactant CTAB has been removed. The morphologies of the as-synthesized and calcined samples on titanium substrate were characterized by field emission electron microscopy (FE-SEM). Typical examples are shown in Fig. 3. Fig. 3(a) and (b) shows the FE-SEM images of briers-like ZnO/CTAB compos-
Fig. 1. XRD patterns at the small-angle of as-synthesized product (a) and calcined products at 500 ◦ C for 2 h (b).
ite and crystalline ZnO, respectively. The high magnification image reveals that the briers-like ZnO nanoarchitectures are made up of sword-like ZnO nanosheets (Fig. 3(c)). From the SEM observations, the ZnO nanocrystalline contains numerous dendrites (Fig. 3(d)), and almost all of them show same morphology. The oxidization of the ordered nanolayered structures in air produced wurtzite ZnO dendrites. In addition to carbon (from the spraying with carbon), the EDS result (Fig. 3(e)) demonstrates only elements Zn and O contained in the sample. Hence the calcined sample is hexagonal wurtzite ZnO.
Fig. 2. XRD patterns of the as-synthesized (a) and calcined products at 500 ◦ C for 2 h (b).
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Fig. 3. SEM images of sample: (a) as-synthesized briers-like ZnO nanoarchitectured inorganic–organic composite; (b), (c) and (d) wurtzite ZnO briers-like dentrites obtained after oxidization of as-synthesized inorganic–organic composites at 500 ◦ C for 2 h in air. (e) Energy dispersive spectroscopy (EDS) of calcined sample. (f) SEM image of as-synthesized product in solution.
After the old oxide surface layer was removed through polishing, a fresh oxide layer would quickly form on the Ti plate. When the oxidized titanium plates are aged in the water, the surface reactions can take place. According to the potential-pH (i.e., E-pH diagram) for the Ti–H2 O corrosion system, the passive TiO2 layer is stable in pure water [15]. Therefore, the titanium plate surface oxidation will continue to thicken the oxide layer, and at the same time the oxide layer will induce dissociative decomposition of water molecules, resulting in a covering of hydroxyl groups, including half acidic (doubly coordinated) and half mainly basic (singly coordinated) hydroxyls, termed TiOH and Ti–OH groups, respectively [16,17]. The existence of hydroxyl groups changes the ligands of Zn2+ ions in growth unit [18]. The addition of CTAB caused the formation of briers-like shapes. Remarkably a hierarchical organisation of the surface morphology was built up in this self-assembly process. For the product in solution, the morphology is shown in Fig. 3(f). It is different that of the material lied on Ti. It is possible that the organization of the materials is due to the fact that the material was placed on the Ti plate and consequently the preferentially orientation to the substrate. Further work will be carried out to give a more detailed explanation on the growth mechanism.
Fig. 4 shows the FTIR spectra in the range 4000–400 cm−1 for the CTAB and as-synthesized nanocomposite. Some bands are observed in the region 2800–3000 cm−1 and attributed to CTAB surfactants [19,20]. CTAB FTIR spectrum (Fig. 4(a)) shows two intense bands, assigned to asymmetric (2921 cm−1 ) and symmetric (2853 cm−1 ) stretching vibrations of C–CH2 in the methylene chains. The CH2 stretching vibrations are considered to be related with the physical state (monomer, micelle or solid) of surfactant [20]. It indicates that the surfactant is present in the as-synthesized composite as micelles. The sharp bands in the region of 1350–1500 cm−1 are attributed to the deformation of –CH2 – and –CH3 [20] of the incorporated surfactants. The broad band between 3200 and 3600 cm−1 and the band centered at 1631 cm−1 found on all samples are assigned to O–H stretching and deformation vibrations of weakbound water [21]. It is because the measured sample adsorbed the water from the KBr. The broad bands between 400 and 850 cm−1 (Fig. 4(b)) are attributed to the framework vibrations of zinc oxide [22]. The XPS spectrum (Fig. 5(a)) shows two peaks of 2p3/2 and 2p1/2 at 1022.8 and 1045.7 eV with a better symmetry, and they are assigned to the lattice zinc in zinc oxide. The peak separation
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Fig. 4. FTIR spectra for CTAB (a) and as-synthesized briers-like ZnO nanoarchitectured inorganic–organic composite (b).
between these two peaks is 22.9 eV. The values correspond to a 2p3 binding energy of Zn(II) ion (indexed Standard ESCA Spectra of the Elements and Line Energy Information, Co., USA). The small shift in the 2p3/2 peak position from the crystalline ZnO to the nanolayered structure ZnO/CTAB composite (from 1021.7 (indexed Hand of Xray photoelectron spectroscopy) to 1022.8 eV) indicates a change of microenvironments for zinc. This shift (1.1 eV) confirms that the largest and the majority of Zn atoms remain in Zn-CTAB composites, in the same formal valence state of Zn2+ within an oxygen deficient ZnO1−x matrix [23,24]. This Zn enrichment or oxygen defects is due to the strong interaction between the CTAB and ZnO and to the weakly crystalline nature of the hydrous zinc oxide composite.
Fig. 6. Room temperature photoluminescence spectrum of as-synthesized brierslike ZnO nanoarchitectured inorganic–organic composite in the excitation wavelength 310 nm.
In Fig. 5(b), it can be seen that the O1s XPS is symmetric. The peak at about 531.8 eV is due to the ZnO crystal lattice oxygen [25]. The room temperature photoluminescence spectrum of the ZnO/CTAB inorganic–organic nanocomposite is shown in Fig. 6. The emission spectrum of the excitation at 310 nm gives three peaks at 429, 464, 537 nm, and a peak centered at 480 nm, which is the shoulder of the broad peak. It indicates that the emission band is made up of two overlapping emission band centered at 464 and 480 nm. The inner interfacial defects between the ZnO and surfactant are contribute to the relatively shorter wavelength of luminescence (less than 500 nm), while the outmost surfactant has the ability to additionally luminescence at long wavelengths (more than 500 nm) [26]. However, the emission spectrum has not been observed for the sample treated at 500 ◦ C. Absolutely no luminescence is observed for the CTAB alone. ZnO/CTAB inorganic–organic nanocomposite is very different the ZnO nanowires [27] and nanosheets [28]. In the past several decades, the green luminescence mechanisms of ZnO have been studied [29–33]. The green peak can be attributed to the deep-trap-mediated emission. The origin of the deep trap in ZnO is not yet clearly understood but is generally attributed to structural defects, single ionized vacancies, and impurities [27,34–36]. In our investigation, RTPL of Zn/CTAB nanocomposite is different from that of bulk and other ZnO nanostructures. The zinc oxide/CTAB ordered nanolayered composite shows the ordered zinc oxide/CTAB superlattices structure. This peculiar structure might have a profound effect on the chemical and physical properties of zinc oxide. We think that the interfacial effect of Zn-CTAB nanocomposites between zinc oxide and the organic surfactant CTAB might be similar to the nanoparticles coated with stearic acid reported by Zou et al. [37] and ZnO nanoparticles coated with SDS reported by Wang [26]. The similar results were obtained in TiO2 mesolamellar [38], meolamellar NiO/dodecylamine [39], and ZnO/dodecylamine inorganic–organic nanocomposite [22]. The quantum yield of photoluminescence and the emission life, as well as the intensity of luminescence of the zinc oxide/CTAB inorganic–organic nanocomposite are being investigated to get a definite understanding. 4. Conclusion
Fig. 5. XPS spectra of the as-synthesized briers-like ZnO nanoarchitectured inorganic–organic composite: (a) Zn2p and (b) O1s.
The briers-like ZnO nanoarchitectures, which consisted of sword-like ZnO nanosheets on the titanium plates can be generated by a simple organic template method. The results investigated by XRD, FE-SEM, and XPS indicated that these nanostructured ZnO have fine hexagonal wurtzite crystal structure. It is found that
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as-synthesized composite displays room temperature photoluminescence. The emission features confirm that the RTPL of zinc oxide/CTAB ordered layered nanocomposite is different from that of bulk and nanoparticles zinc oxides. The interface effect between zinc oxide and CTAB is the probably main cause of the room temperature photoluminescence. Further investigation of the formation mechanism, the properties and the applications of this kind of nanostructure are being performed. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 50662006), and the Natural Science Foundation of Yunnan Province, China (No. 2006E0013M). The authors thank Y.J. Yan for their help in the SEM experiments in Tsinghua University. References [1] Y.C. Kong, D.P. Yu, B. Zhang, W. Fang, S.Q. Feng, Appl. Phys. Lett. 78 (2001) 407–409. [2] G.D. Wei, W.P. Qin, W. Han, W.Y. Yang, F.M. Gao, G.Z. Jing, R.J. Kim, D.S. Zhang, K.Z. Zheng, L.L. Wang, L. Liu, J. Phys. Chem. C 113 (2009) 19432–19438. [3] Y.W. Wang, L.D. Zhang, G.Z. Wang, X.S. Peng, Z.Q. Chu, C.H. Liang, J. Cryst. Growth 234 (2002) 171–175. [4] J. Zhang, L.D. Sun, C.S. Liao, C.H. Yan, Chem. Commun. (2002) 262–263. [5] Y. He, W.B. Sang, J.A. Wang, R.F. Wu, J.H. Min, J. Nanopart. Res. 7 (2005) 307–311. [6] C.L. Kuo, T.J. Kuo, M.H. Huang, J. Phys. Chem. B 109 (2005) 20115–20121. [7] E. Hosono, S. Fujihara, I. Honna, H.S. Zhou, Adv. Mater. 17 (2005) 2091–2094. [8] X.D. Sun, X.D. Kong, Y.D. Wang, C.L. Ma, F.Z. Cui, H.D. Li, Mater. Sci. Engine C 26 (2006) 653–656. [9] H. Zhang, D.R. Yang, Y.J. Ji, X.Y. Ma, J. Xu, D.L. Que, J. Phys. Chem. B 108 (2004) 3955–3958. [10] J.S. Beck, J.C. Vartuli, W.U. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834–10843.
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