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Synthesis, morphology, and luminescence of ZnNb2O6 nanocrystals by hydrothermal method
Nano-Structures & Nano-Objects (
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Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso
Synthesis, morphology, and luminescence of ZnNb2 O6 nanocrystals by hydrothermal method Masanori Hirano ∗ , Takaaki Okamoto Department of Applied Chemistry, Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota, 470-0392, Japan
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Article history: Received 29 May 2015 Accepted 3 November 2015 Available online xxxx Keywords: ZnNb2 O6 Photoluminescence Hydrothermal synthesis Nanocrystal
abstract The effect of hydrothermal treatment conditions on the formation, morphology, crystal growth, and photoluminescence of columbite-type ZnNb2 O6 nanocrystals having rose-like morphology that were directly formed from aqueous precursor solutions of ZnSO4 and NbCl5 in the presence of aqueous ammonia was investigated. The crystallization of ZnNb2 O6 nanocrystals was observed at 210 °C, but the hydrothermal treatment at 240 °C for 5 h under weakly basic condition around pH = 8.4 was necessary for the sufficient crystallite growth of columbite-type ZnNb2 O6 phase. On the other hand, the solid product formed at 240 °C from the precursor solution at pH = 9.0 was almost amorphous. The observation by means of the transmission electron microscopy, selected area diffraction, and energy dispersive X-ray spectrometry showed that the ZnNb2 O6 particles with the morphology like flower or rose consisted of arrayed nanosized-sheets having crystallite size of 11 nm. The ZnNb2 O6 particles grew from about 1 to 4 µm accompanying crystal growth with morphological change into rose-like shape via the solution and precipitation mechanism as hydrothermal holding time at 240 °C increased from 5 to 72 h. The asprepared ZnNb2 O6 nanocrystals showed a broad-band emission in the UV-blue region centered at 420 nm (and peaked at 360 nm) under excitation at 276 nm. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
∗
Corresponding author. Tel.: +81 565 48 8121; fax: +81 565 48 0076. E-mail address: [email protected] (M. Hirano).
http://dx.doi.org/10.1016/j.nanoso.2015.11.001 2352-507X/© 2015 Elsevier B.V. All rights reserved.
In recent years, great attention has been devoted to wet chemical routes to synthesize nanometer-sized particles of inorganic materials because their structures, crystalline phases, characteristics, and performances can be variously designed by controlling
2
M. Hirano, T. Okamoto / Nano-Structures & Nano-Objects (
their compositions and preparation conditions [1–3]. From a view point of green processing, the aqueous solution route including hydrothermal synthesis method is effective to form new compounds [4], solid solutions [5], and metastable phases [6] of complex oxides and oxide ceramics as nano-sized particles with peculiar morphological structures. Zinc niobate (ZnNb2 O6 ) that crystallizes in the columbite structure is one of representative members of orthorhombic columbite group, M2+ Nb2 O6 where M2+ = calcium, magnesium, or transition metal elements. Zinc niobate has been of great interest in the field of microwave dielectric ceramics [7–9], photocatalyst materials [10–12], and sensor materials [13]. Zinc niobate is one of materials satisfying the demand for low-cost but nonetheless highperformance dielectric ceramics as dielectric resonators because it has a resonant frequency (fr ) in the microwave region and has high quality factor (Q ) [7]. It has also been studied for the development of photocatalyst for water splitting with the aim at photon energy conversion [11]. On the other hand, ZnNb2 O6 has interesting luminescence properties and shows self-activated luminescence under excitation with ultraviolet (UV) light. Thus, it works as one of host crystals for doping activator ions [14–17]. Many investigations on the preparation of zinc niobate and its application have been carried out using several synthesis techniques that include solid-state reaction [18,19], sol–gel [20], coprecipitation [21], molten salt [22], thermal decomposition [23], hydrothermal [24], glycothermal [25], citrate complex [12], Pechini [26], combustion [17], and mechanochemical [27]. However, there have been only a few studies on the direct synthesis of ZnNb2 O6 nanocrystals through hydrothermal route. We have noticed that hydrothermally formed ZnNb2 O6 nanocrystals possess characteristic morphology like rose and their formation, structures and properties fairly depend on the hydrothermal treatment conditions. In this study, the effect of hydrothermal treatment conditions on the formation, morphology, crystal growth, and photoluminescence of ZnNb2 O6 nanocrystals that are formed directly from aqueous precursor solutions of ZnSO4 and NbCl5 has been investigated. As a result, the behavior of change in phase, structure, and morphology of ZnNb2 O6 nanocrystals depending on the treatment conditions has been clarified. 2. Experimental 2.1. Sample preparation Reagent-grade ZnSO4 · 7H2 O and NbCl5 were used as starting materials. A mixture of an aqueous solution of ZnSO4 · 7H2 O and ethanol solution of NbCl5 in the ratio of Zn/Nb = 0.50 was prepared in a Teflon container. The solution mixture was controlled by the addition of aqueous ammonia to have a weakly basic condition in the end stage of hydrothermal treatment. This solution mixture with cation concentration of 0.20 mol/dm3 (: Zn = 0.20 mol/dm3 , Nb = 0.40 mol/dm3 ) in the Teflon container was then placed in a stainless-steel vessel. The vessel was tightly sealed and it was heated at 180 ∼ 240 °C for 5 ∼ 72 h under rotation at 1.5 rpm. After hydrothermal treatment, the precipitates were washed with distilled water until the pH value of the rinsed water became 7.0, separated from the solution by centrifugation, and dried in an oven at 60 °C. 2.2. Characterization The powder X-ray diffraction (XRD) measurements were performed at room temperature for the as-prepared powders using CuKα radiation (XRD; model RINT-2000, Rigaku, Tokyo, Japan). The morphology of the samples was observed using transmission
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Fig. 1. XRD patterns of precipitates obtained from the precursor solutions with various pH under hydrothermal conditions at 240 °C for 5 h.
electron microscopy (TEM; model JEM-2010, JEOL, Tokyo, Japan), selected area electron diffraction (SAED), and field emission scanning electron microscopy (FESEM; model JSM-6335FM, JEOL, Tokyo, Japan). Energy-dispersive X-ray spectrometer (EDS) was used to analyze the composition of samples. The crystallite size of ZnNb2 O6 phase was estimated from the line broadening of 131 diffraction peak, according to the Scherrer equation, DXRD = K λ/β cos θ , where θ is the Bragg angle of diffraction lines; K is a shape factor (K = 0.9 in this work); λ is the wavelength of incident X-rays, and β is the corrected half-width 2 given by β 2 = βm − βs2 , where βm is the measured half-width and βs is the half-width of a standard sample. The diffuse reflectance spectra measurements for powder samples have been made. The optical absorption of these prepared powders was measured using an ultraviolet–visible spectrophotometer (V-560, Nihon Bunko, Tokyo, Japan). The photoluminescence (PL) emissions of samples were measured using a spectrofluorometer (F-2700, Hitachi High-Tech, Japan) with Xe lamp. Powder samples were excited with 276 nm radiation from a 150 W xenon lamp. The emission wavelength was scanned from 300 to 700 nm at a scanning rate of 60 nm/min. 3. Results and discussion 3.1. Effect of pH of the precursor solution The effect of the pH of the precursor solution on the formation and crystallization of ZnNb2 O6 has been investigated. The XRD patterns of precipitates formed from the precursor solutions with various pH under hydrothermal conditions at 240 °C for 5 h are shown in Fig. 1. The crystalline phases appeared in the solid precipitates excluding the case formed from the precursor solution of pH = 9.0. Amorphous-like precipitate was obtained from the precursor solution of pH = 9.0, and Nb2 O5 phase appeared as the main phase at pH = 2.8. In the case of weakly acidic
M. Hirano, T. Okamoto / Nano-Structures & Nano-Objects (
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Table 1 Morphology, crystallite size, and optical band-gap of samples. Sample (Hydrothermal condition)
Morphology
Crystallite size (nm)
Optical band gap (eV)
240 °C for 5 h 240 °C for 24 h
Flower-like Rose-like
10.9 12.9
3.45 3.47
Fig. 3. XRD patterns of precipitates obtained from the weakly basic precursor solutions under hydrothermal conditions at various temperatures for 5 h.
Fig. 2. (a) The TEM image and SAED pattern of the area marked with circle in the TEM image of precipitates obtained from the precursor solution at pH = 8.4 under hydrothermal condition at 240 °C for 5 h. (b) The EDS spectrum of the area marked in the TEM image (a).
conditions at pH = 5.6 and 6.6, the coexistence of Nb2 O5 and columbite-type ZnNb2 O6 phase was observed though the amount of Nb2 O5 phase in the solid product increased with increased acidity of the precursor solution. Under weakly basic condition (pH = 8.4), a single phase of columbite-type ZnNb2 O6 with the highest crystallinity was obtained though almost a single phase of ZnNb2 O6 was also formed under neutral condition of pH = 7.3. In order to form a single phase of columbite-type ZnNb2 O6 , the strict pH control of the precursor solution is considered to be necessary due to the result that the solid product formed at 240 °C from the precursor solution at pH = 9.0 was almost amorphous. The TEM image of the columbite-type ZnNb2 O6 precipitates hydrothermally formed under weakly basic condition (pH = 8.4) at 240 °C for 5 h is given in Fig. 2(a). The ZnNb2 O6 precipitates have distinctive morphology like flower. It is observed that the crystals spread out radially from the center of the particle and that
those structures may be composed of arrayed nanosized-sheets. The crystallite size of the ZnNb2 O6 phase formed at 240 °C for 5 h was 10.9 nm as listed in Table 1. The selected area electron diffraction (SAED) pattern of the area that is marked with circle in Fig. 2(a), i.e., nanosized-sheet which is the part of the particles with the morphology like flower spreading out radially is also shown in Fig. 2(a). The SAED pattern shows that the area, nanosized-sheet making the morphology like flower has high crystallinity. Energydispersive X-ray spectrum analysis (EDS) was used to analyze the composition of the particles formed under hydrothermal conditions. The EDS spectrum of the area marked with circle in Fig. 2(a), nanosized-sheet making the morphology of flower is also shown in Fig. 2(b). The line of Cu detected in the EDS spectrum is the material used for the mesh in the holder of TEM. The EDS analysis proved that the nanosized-sheets making the structure like flower were composed of Zn, Nb, and O elements. These results suggest that the weakly basic hydrothermal condition around pH = 8.4 is necessary for the formation of nanocrystals with a single phase of columbite-type ZnNb2 O6 and morphology like flower. 3.2. Effect of hydrothermal treatment temperature The effect of hydrothermal treatment temperature on the formation of ZnNb2 O6 has been investigated to get the information on the hydrothermal crystallization temperature of the material. Hydrothermal treatment of the precursor solutions was conducted at various temperatures for 5 h under weakly basic condition. The XRD patterns of the precipitates formed at 180 ∼ 240 °C 5 h are shown in Fig. 3. The solid products obtained at temperatures
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Fig. 5. XRD patterns of precipitates obtained from the weakly basic precursor solutions under hydrothermal conditions at 240 °C for 5 and 24 h.
Fig. 4. (a) The TEM image and SAED pattern of the area marked with circle in the TEM image of the precipitates obtained from the weakly basic precursor solution under hydrothermal condition at 225 °C for 5 h. (b) The EDS spectrum of the area marked in the TEM image (a).
lower than 180 °C were amorphous. The slight crystallization of ZnNb2 O6 nanocrystals was observed at 210 °C, but the amorphous phase as a main phase coexisted with a small amount of crystalline ZnNb2 O6 phase in the precipitates formed at 210 ∼ 225 °C. The precipitates formed at 240 °C were detected as almost a single phase corresponding to the columbite-type ZnNb2 O6 structure, and no diffraction peaks due to another crystalline phases was detected. The TEM image and the SAED pattern of the area marked with circle in the TEM image of the ZnNb2 O6 precipitates that were hydrothermally formed under weakly basic condition at 225 °C for 5 h are given in Fig. 4(a). The EDS spectrum of the area marked with circle in the TEM image of the ZnNb2 O6 precipitates is also shown in Fig. 4(b). The EDS spectrum of the precipitates formed at 225 °C was almost similar to that of ZnNb2 O6 nanocrystals with sufficient crystallinity that was shown in Fig. 2(b). The result obtained from the EDS analysis of two samples shows that the precipitates formed at 225 °C have the same composition as the ZnNb2 O6 nanocrystals with sufficient crystallinity that were formed at 240 °C (Fig. 2(b)). Although the coexisting of a very slight amount of fibrous-like products (indicated by an arrow) was observed, the TEM image showed that the precipitates that
Fig. 6. TEM images and SAED patterns of precipitates obtained from the weakly basic precursor solutions under hydrothermal condition at 240 °C for (a) 5 h and (b) 24 h.
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Fig. 7. FESEM images of precipitates obtained from the weakly basic precursor solutions under hydrothermal condition at 240 °C for (a) 5 h (b) 24 h and (c) 72 h.
were formed at 225 °C consisted of very small particles. The SAED pattern (Fig. 4(a)) indicated that those fine particles consisted of almost amorphous and the crystallization was not sufficient, which accorded with the result from the XRD pattern of the solid product. As a result, the hydrothermal treatment at 240 °C was necessary for the sufficient crystallite growth of the columbite-type ZnNb2 O6 phase under the weakly basic condition. 3.3. Effect of hydrothermal treatment time The behavior of the change in the size, structure, and morphology of ZnNb2 O6 nanocrystals with increasing hydrothermal treatment time has been investigated. The XRD patterns of the precipitates formed from the weakly basic precursor solutions under hydrothermal conditions at 240 °C for 5 and 24 h are shown in Fig. 5. All the precipitates were detected as almost a single phase corresponding to the columbite-type ZnNb2 O6 structure, and no diffraction peaks due to another crystalline phases was detected. It is observed that the diffraction lines become sharper and the crystallinity of the product is improved as the hydrothermal treatment time is prolonged. The crystallite size of the samples is listed in Table 1. The crystallite size increased from 11 to 13 nm when the hydrothermal holding time was prolonged from 5 to 24 h. The TEM images and SAED patterns of the precipitates formed at 240 °C for 5 and 24 h are shown in Fig. 6(a) and (b), respectively. The increase in the size and crystal growth in the particles with the morphology like flower are clearly observed by comparing the TEM image for 5 h with that for 24 h. The morphology of the precipitates formed under hydrothermal condition for 24 h is rather like rose than
flower. According to the SAED analysis, it was proved that their precipitates possessed sufficient crystallinity. The FESEM images of the precipitates formed at 240 °C for various holding times are shown in Fig. 7. The change in the morphology of the particles from flower- to rose-like structure accompanying the increase in the particle size is clearly observed. The average particle size of ZnNb2 O6 is plotted in Fig. 8 against hydrothermal holding time. At 240 °C, as hydrothermal holding time increased from 5 to 72 h, the ZnNb2 O6 particles grew from about 1 to 4 µm accompanying the morphological change from flower- to rose-like shape. It was clarified that the columbite-type ZnNb2 O6 particles with the morphology like rose consisting of arrayed nanosized-sheets, which had crystallite size of 13 nm were formed after hydrothermal treatment for 24 h by means of TEM, FESEM, EDS, SAED, and XRD. The particles grew from about 1 to 4 µm as hydrothermal holding time at 240 °C increased from 5 to 72 h. It is considered that the observed crystal growth with morphological change from flower- into rose-like particle through the prolonged treatment time is promoted via the solution and precipitation mechanism under hydrothermal condition. The strictly controlled hydrothermal condition is found to be necessary to synthesize ZnNb2 O6 nanocrystals and to arrange their morphological structure. 3.4. Optical properties of as-prepared ZnNb2 O6 nanocrystals In Fig. 9, the diffuse reflectance spectra of the as-prepared samples are shown. A broad absorption band peak is appeared at the low wavelength UV region (250 ∼ 280 nm) in the each samples.
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M. Hirano, T. Okamoto / Nano-Structures & Nano-Objects (
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Fig. 8. The average particle size of ZnNb2 O6 plotted as a function of hydrothermal holding time.
Fig. 10. Emission spectra of as-prepared ZnNb2 O6 nanocrystals formed at 240 °C for 5–72 h. (λex = 276 nm).
be partly due to an insufficiency of crystallinity in as-prepared samples.
Fig. 9. Diffuse reflectance spectra of as-prepared ZnNb2 O6 nanocrystals formed at 180–240 °C for 5 h.
The optical band-gap of samples was evaluated from the diffuse reflectance spectra based on the onset of absorption. The optical band gap values of the as-prepared ZnNb2 O6 nanocrystals were 3.5 eV as listed in Table 1. The photoluminescence spectra of the as-prepared ZnNb2 O6 nanocrystals measured under excitation at 276 nm at room temperature, are shown in Fig. 10. The nanocrystalline ZnNb2 O6 showed a broad band emission in the UV-blue region peaked at around 360 nm and centered at 420 nm under excitation at 276 nm. The samples formed at 240 °C for 10 and 24 h showed rather high intensity, but their emission intensities of the as-prepared ZnNb2 O6 nanocrystals were not so high, which is considered to
Summary Columbite-type ZnNb2 O6 nanocrystals with the particle size in the range of 1–4 µm having rose-like morphology, which consisted of arrayed nanosized-sheets having crystallite size of 11 ∼ 13 nm, were directly formed from aqueous precursor solutions of ZnSO4 and NbCl5 under weakly basic hydrothermal conditions at 240 °C in the presence of aqueous ammonia. The structure, composition, and crystal growth of the ZnNb2 O6 nanocrystals accompanying morphological change were investigated using TEM, SAED, EDS, FESEM, and XRD. The hydrothermal treatment conditions, e.g., temperature, pH of the precursor solution, and holding time had effect on the crystalline phase, crystal growth, crystallite size, particle size, morphology, photoluminescence of ZnNb2 O6 nanocrystals. The particle size and morphology of the ZnNb2 O6 nanocrystals changed from about 1 to 4 µm and from flower- to rose-like shape accompanying crystal growth via the solution and precipitation mechanism depending on prolonged hydrothermal treatment time. A broad-band emission in the UVblue region centered at 420 nm (and peaked at 360 nm) under excitation at 276 nm was observed in the as-prepared ZnNb2 O6 nanocrystals. References [1] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Recent advances in the liquid-phase syntheses of inorganic nanoparticles, Chem. Rev. 104 (2004) 3893–3946. [2] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shape, Chem. Rev. 105 (2005) 1025–1102. [3] S. Feng, R. Xu, New materials in hydrothermal synthesis, Acc. Chem. Res. 34 (2001) 239–247. [4] M. Hirano, H. Morikawa, M. Inagaki, M. Toyoda, Direct synthesis of new zircontype ZrGeO4 and Zr(Ge, Si)O4 solid solutions, J. Am. Ceram. Soc. 85 (2002) 1915–1920. [5] M. Hirano, E. Kato, Hydrothermal synthesis and sintering of fine powders in CeO2 –ZrO2 system, J. Ceram. Soc. Japan 104 (1996) 958–962. [6] M. Hirano, K. Ota, Direct formation and photocatalytic performance of anatase (TiO2 )/silica (SiO2 ) composite nanoparticles, J. Am. Ceram. Soc. 87 (2004) 1567–1570.
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[17] Y. Zhou, Z. Qiu, M. Lü, Q. Ma, A. Zhang, G. Zhou, H. Zhang, Z. Yang, Photoluminescence characteristics of pure and Dy-doped ZnNb2 O6 nanoparticles prepared by a combustion method, J. Phys. Chem. C. 111 (2007) 10190–10193. [18] A. Ngamjarurojana, O. Khamman, R. Yimnirun, S. Ananta, Effect of calcination condition on phase formation and particle size of zinc niobate powders synthesized by solid-state reaction, Mater. Lett. 60 (2006) 2867–2872. [19] A. Belous, O. Ovchar, B. Jancar, J. Bezjak, The effect of non-stoichometry on the microstructure and microwave dielectric properties of the columbites A2+ Nb2 O6 , J. Eur. Ceram. Soc. 27 (2007) 2933–2936. [20] C.-H. Hsu, P.-C. Yang, H.-Wen Yang, S.-F. Yan, H.-H. Tung, Study and fabrication of ZnNb2 O6 thin films by sol–gel method, Thin Solid Films 519 (2011) 5030–5032. [21] V.V. Deshpande, M.M. Patil, S.C. Navale, V. Ravi, A coprecipitation technique to prepare ZnNb2 O6 powders, Bull. Mater. Sci. 28 (2005) 205–207. [22] L. Guo, J. Dai, J. Tian, Z. Zhu, T. He, Molten salt synthesis of ZnNb2 O6 powder, Mater. Res. Bull. 42 (2007) 2013–2016. [23] O.A. Shlyakhtin, Y.-J. Oh, Low temperature sintering of Zn3 Nb2 O8 ceramics from fine powders, J. Am. Ceram. Soc. 89 (2006) 3366–3372. [24] Y.-Y. Zhuang, Y.-P. Pu, D. Liu, X. Shi, Preparation of ZnNb2 O6 powders by hydrothermal method, J. Synth. Cryst. 40 (2011) 668–671. [25] H. Kominami, M. Inoue, T. Inui, Formation of niobium double oxides by the glycothermal method, Catal. Today 16 (1993) 309–317. [26] Y. Zhang, S. Wang, X. Zhou, B. Fu, Z. Yue, Preparation of ZnNb2 O6 nano-powders by pechini metod, Key Eng. Mater. 434–435 (2010) 221–223. [27] L.B. Kong, J. Ma, H. Huang, R.F. Zhang, T.S. Zhang, Zinc niobate derived from mechanochemically activated oxides, J. Alloys Compd. 347 (2002) 308–313.
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Contents lists available at ScienceDirect
Nano-Structures & Nano-Objects journal homepage: www.elsevier.com/locate/nanoso
Synthesis, morphology, and luminescence of ZnNb2 O6 nanocrystals by hydrothermal method Masanori Hirano ∗ , Takaaki Okamoto Department of Applied Chemistry, Faculty of Engineering, Aichi Institute of Technology, Yakusa, Toyota, 470-0392, Japan
graphical
article
abstract
info
Article history: Received 29 May 2015 Accepted 3 November 2015 Available online xxxx Keywords: ZnNb2 O6 Photoluminescence Hydrothermal synthesis Nanocrystal
abstract The effect of hydrothermal treatment conditions on the formation, morphology, crystal growth, and photoluminescence of columbite-type ZnNb2 O6 nanocrystals having rose-like morphology that were directly formed from aqueous precursor solutions of ZnSO4 and NbCl5 in the presence of aqueous ammonia was investigated. The crystallization of ZnNb2 O6 nanocrystals was observed at 210 °C, but the hydrothermal treatment at 240 °C for 5 h under weakly basic condition around pH = 8.4 was necessary for the sufficient crystallite growth of columbite-type ZnNb2 O6 phase. On the other hand, the solid product formed at 240 °C from the precursor solution at pH = 9.0 was almost amorphous. The observation by means of the transmission electron microscopy, selected area diffraction, and energy dispersive X-ray spectrometry showed that the ZnNb2 O6 particles with the morphology like flower or rose consisted of arrayed nanosized-sheets having crystallite size of 11 nm. The ZnNb2 O6 particles grew from about 1 to 4 µm accompanying crystal growth with morphological change into rose-like shape via the solution and precipitation mechanism as hydrothermal holding time at 240 °C increased from 5 to 72 h. The asprepared ZnNb2 O6 nanocrystals showed a broad-band emission in the UV-blue region centered at 420 nm (and peaked at 360 nm) under excitation at 276 nm. © 2015 Elsevier B.V. All rights reserved.
1. Introduction
∗
Corresponding author. Tel.: +81 565 48 8121; fax: +81 565 48 0076. E-mail address: [email protected] (M. Hirano).
http://dx.doi.org/10.1016/j.nanoso.2015.11.001 2352-507X/© 2015 Elsevier B.V. All rights reserved.
In recent years, great attention has been devoted to wet chemical routes to synthesize nanometer-sized particles of inorganic materials because their structures, crystalline phases, characteristics, and performances can be variously designed by controlling
2
M. Hirano, T. Okamoto / Nano-Structures & Nano-Objects (
their compositions and preparation conditions [1–3]. From a view point of green processing, the aqueous solution route including hydrothermal synthesis method is effective to form new compounds [4], solid solutions [5], and metastable phases [6] of complex oxides and oxide ceramics as nano-sized particles with peculiar morphological structures. Zinc niobate (ZnNb2 O6 ) that crystallizes in the columbite structure is one of representative members of orthorhombic columbite group, M2+ Nb2 O6 where M2+ = calcium, magnesium, or transition metal elements. Zinc niobate has been of great interest in the field of microwave dielectric ceramics [7–9], photocatalyst materials [10–12], and sensor materials [13]. Zinc niobate is one of materials satisfying the demand for low-cost but nonetheless highperformance dielectric ceramics as dielectric resonators because it has a resonant frequency (fr ) in the microwave region and has high quality factor (Q ) [7]. It has also been studied for the development of photocatalyst for water splitting with the aim at photon energy conversion [11]. On the other hand, ZnNb2 O6 has interesting luminescence properties and shows self-activated luminescence under excitation with ultraviolet (UV) light. Thus, it works as one of host crystals for doping activator ions [14–17]. Many investigations on the preparation of zinc niobate and its application have been carried out using several synthesis techniques that include solid-state reaction [18,19], sol–gel [20], coprecipitation [21], molten salt [22], thermal decomposition [23], hydrothermal [24], glycothermal [25], citrate complex [12], Pechini [26], combustion [17], and mechanochemical [27]. However, there have been only a few studies on the direct synthesis of ZnNb2 O6 nanocrystals through hydrothermal route. We have noticed that hydrothermally formed ZnNb2 O6 nanocrystals possess characteristic morphology like rose and their formation, structures and properties fairly depend on the hydrothermal treatment conditions. In this study, the effect of hydrothermal treatment conditions on the formation, morphology, crystal growth, and photoluminescence of ZnNb2 O6 nanocrystals that are formed directly from aqueous precursor solutions of ZnSO4 and NbCl5 has been investigated. As a result, the behavior of change in phase, structure, and morphology of ZnNb2 O6 nanocrystals depending on the treatment conditions has been clarified. 2. Experimental 2.1. Sample preparation Reagent-grade ZnSO4 · 7H2 O and NbCl5 were used as starting materials. A mixture of an aqueous solution of ZnSO4 · 7H2 O and ethanol solution of NbCl5 in the ratio of Zn/Nb = 0.50 was prepared in a Teflon container. The solution mixture was controlled by the addition of aqueous ammonia to have a weakly basic condition in the end stage of hydrothermal treatment. This solution mixture with cation concentration of 0.20 mol/dm3 (: Zn = 0.20 mol/dm3 , Nb = 0.40 mol/dm3 ) in the Teflon container was then placed in a stainless-steel vessel. The vessel was tightly sealed and it was heated at 180 ∼ 240 °C for 5 ∼ 72 h under rotation at 1.5 rpm. After hydrothermal treatment, the precipitates were washed with distilled water until the pH value of the rinsed water became 7.0, separated from the solution by centrifugation, and dried in an oven at 60 °C. 2.2. Characterization The powder X-ray diffraction (XRD) measurements were performed at room temperature for the as-prepared powders using CuKα radiation (XRD; model RINT-2000, Rigaku, Tokyo, Japan). The morphology of the samples was observed using transmission
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Fig. 1. XRD patterns of precipitates obtained from the precursor solutions with various pH under hydrothermal conditions at 240 °C for 5 h.
electron microscopy (TEM; model JEM-2010, JEOL, Tokyo, Japan), selected area electron diffraction (SAED), and field emission scanning electron microscopy (FESEM; model JSM-6335FM, JEOL, Tokyo, Japan). Energy-dispersive X-ray spectrometer (EDS) was used to analyze the composition of samples. The crystallite size of ZnNb2 O6 phase was estimated from the line broadening of 131 diffraction peak, according to the Scherrer equation, DXRD = K λ/β cos θ , where θ is the Bragg angle of diffraction lines; K is a shape factor (K = 0.9 in this work); λ is the wavelength of incident X-rays, and β is the corrected half-width 2 given by β 2 = βm − βs2 , where βm is the measured half-width and βs is the half-width of a standard sample. The diffuse reflectance spectra measurements for powder samples have been made. The optical absorption of these prepared powders was measured using an ultraviolet–visible spectrophotometer (V-560, Nihon Bunko, Tokyo, Japan). The photoluminescence (PL) emissions of samples were measured using a spectrofluorometer (F-2700, Hitachi High-Tech, Japan) with Xe lamp. Powder samples were excited with 276 nm radiation from a 150 W xenon lamp. The emission wavelength was scanned from 300 to 700 nm at a scanning rate of 60 nm/min. 3. Results and discussion 3.1. Effect of pH of the precursor solution The effect of the pH of the precursor solution on the formation and crystallization of ZnNb2 O6 has been investigated. The XRD patterns of precipitates formed from the precursor solutions with various pH under hydrothermal conditions at 240 °C for 5 h are shown in Fig. 1. The crystalline phases appeared in the solid precipitates excluding the case formed from the precursor solution of pH = 9.0. Amorphous-like precipitate was obtained from the precursor solution of pH = 9.0, and Nb2 O5 phase appeared as the main phase at pH = 2.8. In the case of weakly acidic
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Table 1 Morphology, crystallite size, and optical band-gap of samples. Sample (Hydrothermal condition)
Morphology
Crystallite size (nm)
Optical band gap (eV)
240 °C for 5 h 240 °C for 24 h
Flower-like Rose-like
10.9 12.9
3.45 3.47
Fig. 3. XRD patterns of precipitates obtained from the weakly basic precursor solutions under hydrothermal conditions at various temperatures for 5 h.
Fig. 2. (a) The TEM image and SAED pattern of the area marked with circle in the TEM image of precipitates obtained from the precursor solution at pH = 8.4 under hydrothermal condition at 240 °C for 5 h. (b) The EDS spectrum of the area marked in the TEM image (a).
conditions at pH = 5.6 and 6.6, the coexistence of Nb2 O5 and columbite-type ZnNb2 O6 phase was observed though the amount of Nb2 O5 phase in the solid product increased with increased acidity of the precursor solution. Under weakly basic condition (pH = 8.4), a single phase of columbite-type ZnNb2 O6 with the highest crystallinity was obtained though almost a single phase of ZnNb2 O6 was also formed under neutral condition of pH = 7.3. In order to form a single phase of columbite-type ZnNb2 O6 , the strict pH control of the precursor solution is considered to be necessary due to the result that the solid product formed at 240 °C from the precursor solution at pH = 9.0 was almost amorphous. The TEM image of the columbite-type ZnNb2 O6 precipitates hydrothermally formed under weakly basic condition (pH = 8.4) at 240 °C for 5 h is given in Fig. 2(a). The ZnNb2 O6 precipitates have distinctive morphology like flower. It is observed that the crystals spread out radially from the center of the particle and that
those structures may be composed of arrayed nanosized-sheets. The crystallite size of the ZnNb2 O6 phase formed at 240 °C for 5 h was 10.9 nm as listed in Table 1. The selected area electron diffraction (SAED) pattern of the area that is marked with circle in Fig. 2(a), i.e., nanosized-sheet which is the part of the particles with the morphology like flower spreading out radially is also shown in Fig. 2(a). The SAED pattern shows that the area, nanosized-sheet making the morphology like flower has high crystallinity. Energydispersive X-ray spectrum analysis (EDS) was used to analyze the composition of the particles formed under hydrothermal conditions. The EDS spectrum of the area marked with circle in Fig. 2(a), nanosized-sheet making the morphology of flower is also shown in Fig. 2(b). The line of Cu detected in the EDS spectrum is the material used for the mesh in the holder of TEM. The EDS analysis proved that the nanosized-sheets making the structure like flower were composed of Zn, Nb, and O elements. These results suggest that the weakly basic hydrothermal condition around pH = 8.4 is necessary for the formation of nanocrystals with a single phase of columbite-type ZnNb2 O6 and morphology like flower. 3.2. Effect of hydrothermal treatment temperature The effect of hydrothermal treatment temperature on the formation of ZnNb2 O6 has been investigated to get the information on the hydrothermal crystallization temperature of the material. Hydrothermal treatment of the precursor solutions was conducted at various temperatures for 5 h under weakly basic condition. The XRD patterns of the precipitates formed at 180 ∼ 240 °C 5 h are shown in Fig. 3. The solid products obtained at temperatures
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Fig. 5. XRD patterns of precipitates obtained from the weakly basic precursor solutions under hydrothermal conditions at 240 °C for 5 and 24 h.
Fig. 4. (a) The TEM image and SAED pattern of the area marked with circle in the TEM image of the precipitates obtained from the weakly basic precursor solution under hydrothermal condition at 225 °C for 5 h. (b) The EDS spectrum of the area marked in the TEM image (a).
lower than 180 °C were amorphous. The slight crystallization of ZnNb2 O6 nanocrystals was observed at 210 °C, but the amorphous phase as a main phase coexisted with a small amount of crystalline ZnNb2 O6 phase in the precipitates formed at 210 ∼ 225 °C. The precipitates formed at 240 °C were detected as almost a single phase corresponding to the columbite-type ZnNb2 O6 structure, and no diffraction peaks due to another crystalline phases was detected. The TEM image and the SAED pattern of the area marked with circle in the TEM image of the ZnNb2 O6 precipitates that were hydrothermally formed under weakly basic condition at 225 °C for 5 h are given in Fig. 4(a). The EDS spectrum of the area marked with circle in the TEM image of the ZnNb2 O6 precipitates is also shown in Fig. 4(b). The EDS spectrum of the precipitates formed at 225 °C was almost similar to that of ZnNb2 O6 nanocrystals with sufficient crystallinity that was shown in Fig. 2(b). The result obtained from the EDS analysis of two samples shows that the precipitates formed at 225 °C have the same composition as the ZnNb2 O6 nanocrystals with sufficient crystallinity that were formed at 240 °C (Fig. 2(b)). Although the coexisting of a very slight amount of fibrous-like products (indicated by an arrow) was observed, the TEM image showed that the precipitates that
Fig. 6. TEM images and SAED patterns of precipitates obtained from the weakly basic precursor solutions under hydrothermal condition at 240 °C for (a) 5 h and (b) 24 h.
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Fig. 7. FESEM images of precipitates obtained from the weakly basic precursor solutions under hydrothermal condition at 240 °C for (a) 5 h (b) 24 h and (c) 72 h.
were formed at 225 °C consisted of very small particles. The SAED pattern (Fig. 4(a)) indicated that those fine particles consisted of almost amorphous and the crystallization was not sufficient, which accorded with the result from the XRD pattern of the solid product. As a result, the hydrothermal treatment at 240 °C was necessary for the sufficient crystallite growth of the columbite-type ZnNb2 O6 phase under the weakly basic condition. 3.3. Effect of hydrothermal treatment time The behavior of the change in the size, structure, and morphology of ZnNb2 O6 nanocrystals with increasing hydrothermal treatment time has been investigated. The XRD patterns of the precipitates formed from the weakly basic precursor solutions under hydrothermal conditions at 240 °C for 5 and 24 h are shown in Fig. 5. All the precipitates were detected as almost a single phase corresponding to the columbite-type ZnNb2 O6 structure, and no diffraction peaks due to another crystalline phases was detected. It is observed that the diffraction lines become sharper and the crystallinity of the product is improved as the hydrothermal treatment time is prolonged. The crystallite size of the samples is listed in Table 1. The crystallite size increased from 11 to 13 nm when the hydrothermal holding time was prolonged from 5 to 24 h. The TEM images and SAED patterns of the precipitates formed at 240 °C for 5 and 24 h are shown in Fig. 6(a) and (b), respectively. The increase in the size and crystal growth in the particles with the morphology like flower are clearly observed by comparing the TEM image for 5 h with that for 24 h. The morphology of the precipitates formed under hydrothermal condition for 24 h is rather like rose than
flower. According to the SAED analysis, it was proved that their precipitates possessed sufficient crystallinity. The FESEM images of the precipitates formed at 240 °C for various holding times are shown in Fig. 7. The change in the morphology of the particles from flower- to rose-like structure accompanying the increase in the particle size is clearly observed. The average particle size of ZnNb2 O6 is plotted in Fig. 8 against hydrothermal holding time. At 240 °C, as hydrothermal holding time increased from 5 to 72 h, the ZnNb2 O6 particles grew from about 1 to 4 µm accompanying the morphological change from flower- to rose-like shape. It was clarified that the columbite-type ZnNb2 O6 particles with the morphology like rose consisting of arrayed nanosized-sheets, which had crystallite size of 13 nm were formed after hydrothermal treatment for 24 h by means of TEM, FESEM, EDS, SAED, and XRD. The particles grew from about 1 to 4 µm as hydrothermal holding time at 240 °C increased from 5 to 72 h. It is considered that the observed crystal growth with morphological change from flower- into rose-like particle through the prolonged treatment time is promoted via the solution and precipitation mechanism under hydrothermal condition. The strictly controlled hydrothermal condition is found to be necessary to synthesize ZnNb2 O6 nanocrystals and to arrange their morphological structure. 3.4. Optical properties of as-prepared ZnNb2 O6 nanocrystals In Fig. 9, the diffuse reflectance spectra of the as-prepared samples are shown. A broad absorption band peak is appeared at the low wavelength UV region (250 ∼ 280 nm) in the each samples.
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Fig. 8. The average particle size of ZnNb2 O6 plotted as a function of hydrothermal holding time.
Fig. 10. Emission spectra of as-prepared ZnNb2 O6 nanocrystals formed at 240 °C for 5–72 h. (λex = 276 nm).
be partly due to an insufficiency of crystallinity in as-prepared samples.
Fig. 9. Diffuse reflectance spectra of as-prepared ZnNb2 O6 nanocrystals formed at 180–240 °C for 5 h.
The optical band-gap of samples was evaluated from the diffuse reflectance spectra based on the onset of absorption. The optical band gap values of the as-prepared ZnNb2 O6 nanocrystals were 3.5 eV as listed in Table 1. The photoluminescence spectra of the as-prepared ZnNb2 O6 nanocrystals measured under excitation at 276 nm at room temperature, are shown in Fig. 10. The nanocrystalline ZnNb2 O6 showed a broad band emission in the UV-blue region peaked at around 360 nm and centered at 420 nm under excitation at 276 nm. The samples formed at 240 °C for 10 and 24 h showed rather high intensity, but their emission intensities of the as-prepared ZnNb2 O6 nanocrystals were not so high, which is considered to
Summary Columbite-type ZnNb2 O6 nanocrystals with the particle size in the range of 1–4 µm having rose-like morphology, which consisted of arrayed nanosized-sheets having crystallite size of 11 ∼ 13 nm, were directly formed from aqueous precursor solutions of ZnSO4 and NbCl5 under weakly basic hydrothermal conditions at 240 °C in the presence of aqueous ammonia. The structure, composition, and crystal growth of the ZnNb2 O6 nanocrystals accompanying morphological change were investigated using TEM, SAED, EDS, FESEM, and XRD. The hydrothermal treatment conditions, e.g., temperature, pH of the precursor solution, and holding time had effect on the crystalline phase, crystal growth, crystallite size, particle size, morphology, photoluminescence of ZnNb2 O6 nanocrystals. The particle size and morphology of the ZnNb2 O6 nanocrystals changed from about 1 to 4 µm and from flower- to rose-like shape accompanying crystal growth via the solution and precipitation mechanism depending on prolonged hydrothermal treatment time. A broad-band emission in the UVblue region centered at 420 nm (and peaked at 360 nm) under excitation at 276 nm was observed in the as-prepared ZnNb2 O6 nanocrystals. References [1] B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Recent advances in the liquid-phase syntheses of inorganic nanoparticles, Chem. Rev. 104 (2004) 3893–3946. [2] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chemistry and properties of nanocrystals of different shape, Chem. Rev. 105 (2005) 1025–1102. [3] S. Feng, R. Xu, New materials in hydrothermal synthesis, Acc. Chem. Res. 34 (2001) 239–247. [4] M. Hirano, H. Morikawa, M. Inagaki, M. Toyoda, Direct synthesis of new zircontype ZrGeO4 and Zr(Ge, Si)O4 solid solutions, J. Am. Ceram. Soc. 85 (2002) 1915–1920. [5] M. Hirano, E. Kato, Hydrothermal synthesis and sintering of fine powders in CeO2 –ZrO2 system, J. Ceram. Soc. Japan 104 (1996) 958–962. [6] M. Hirano, K. Ota, Direct formation and photocatalytic performance of anatase (TiO2 )/silica (SiO2 ) composite nanoparticles, J. Am. Ceram. Soc. 87 (2004) 1567–1570.
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