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Synthesis, characterization and optical activity of La-doped ZnWO4 nanorods by hydrothermal method
Superlattices and Microstructures 67 (2014) 197–206
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Synthesis, characterization and optical activity of La-doped ZnWO4 nanorods by hydrothermal method Jirapong Arin a, Phattranit Dumrongrojthanath a, Oranuch Yayapao b, Anukorn Phuruangrat c,⇑, Somchai Thongtem b, Titipun Thongtem a,⇑ a
Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand c Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand b
a r t i c l e
i n f o
Article history: Received 15 December 2013 Accepted 26 December 2013 Available online 2 January 2014 Keywords: La doped ZnWO4 Hydrothermal Nanorods
a b s t r a c t One-dimensional La doped ZnWO4 nanostructures have been synthesized by hydrothermal method. The effect of various La contents doped ZnWO4 samples on phase, morphologies and optical properties was investigated and characterized by X-ray diffraction (XRD), field scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Raman and UV–visible spectroscopy, respectively. The results of XRD, SEM and SAED revealed that all products were one-dimensional monoclinic ZnWO4 nanorods while doped La3+ constants into ZnWO4 lattice. The observation of TEM and HRTEM showed that the obtained La3+ doped ZnWO4 with different La constants were homogenous nanorods shapes with grow along [0 2 1] direction. The optical property result showed that La3+ doped ZnWO4 is a red shifted absorption when comparing to pure ZnWO4. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In the past decade, ZnWO4 has been wildly researched because of its interesting in chemical and physical properties such as laser hosts, magnetic, scintillated, luminescent and catalytic properties and potential applications such as optical fibers, scintillator materials, magnetic properties, and ⇑ Corresponding authors. Tel.: +66 (0)74 288 374; fax: +66 (0)74 288 395. E-mail address: [email protected] (A. Phuruangrat). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.12.024
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heterogeneous catalysis [1–3]. ZnWO4 is a materials in wolframite structure and belongs monoclinic with C2h point group symmetry and P2/c space group. There are two formula units per primitive cell, having the lattice parameters a = 4.69263 Å, b = 5.72129 Å, c = 4.92805 Å and b = 90.6321°. The presence of two non-equivalent oxygen atoms is responsible for three pairs of Zn–O and W–O bonds, having different lengths. Thus, both Zn and W ions are surrounded by six oxygen atoms coordination, forming distorted ZnO6 and WO6 octahedral coordination which either edge-sharing ZnO6 octahedron or edge-sharing WO6 octahedra are corner-linked by zigzag chains in parallel to [0 0 1] [1,2,4]. To increase the luminescence efficiency of phosphor, researchers have tried various approaches including modify phosphor particle size and surface morphology, adding conductive materials, mixing flux materials and doping a sensitizer to enhance excitation energy absorption and energy transfer to the activator [5]. Doping method is a suitable synthesis method to increase the optical efficiency of oxide phosphor, rare-earth (RE) with their particular 4f–5d and 4f–4f electronic transition was used due to their unique optical transitions happen between the spin orbit levels split under the different manifolds weak crystal field and promising applications in optoelectronic devices [6–9]. By doping with RE3+ ions, it improves the luminescence efficiency by energy transfer processes [10]. For example, Anandan et al. report a high photocatalytic activity in the degradation of organic contaminants of Ladoped TiO2 due to the suppression of electron–hole recombination, large content of oxygen vacancies, and strong absorption of OH ions on the surface of the catalyst [11]. Jia et al. shows a strong emission peak centering at around 405 nm of La-doped ZnO which exhibited a red shift compared with the near band emission of pure ZnO because the incorporation of La3+ ions [12]. Here, we report one-dimensional La doped ZnWO4 with differences La contents nanostructures by hydrothermal method. The effects of La doping on the structural and optical properties of ZnWO4 were also investigated and discussed in detail. 2. Experimental procedure All the chemicals used in the experiment, including zinc nitrate hexahydrate (Zn(NO3)26H2O), lanthanum nitrate hexahydrate (La(NO3)36H2O), sodium tungstate dihydrate (Na2WO42H2O) were used as zinc, lanthanum, tungsten sources, respectively are of analytic grade reagents without further purification. In a typical procedure, the mixture containing 0.005 mol Zn(NO3)26H2O and different amount of La(NO3)36H2O (0.2, 0.4, 0.6, 1.0 and 2.0 wt%) was dissolved into 60 ml deionized water under rigorous stirring for 30 min. Then, 0.005 mol Na2WO42H2O in 20 ml deionized water was added and the mixture solution was transferred into a Teflon lined steel autoclave of 100 ml, and the autoclave was heated at 180 °C for 24 h. Afterward, the autoclave was cooled to room temperature gradually. The white precipitate was washed with ethanol and distilled water three times. Then, it was dried at 80 °C for 12 h. The products were characterized by X-ray diffraction (XRD) on a Philips, XPert MPD X-ray diffractometer equipped with Cu Ka radiation over a range from 10° to 60°. The scanning electron microscopy (SEM) images were carried out using a JEOL JSM-6335F field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) and high-resolution field emission transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) pattern were taken on a JEOL, JEM-2010 transmission electron microscopy with an acceleration voltage of 200 kV. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker Tensor 27 FT-IR spectrometer with a KBr disk. Raman spectra were recorded on HORIBA JOBIN YVON T64000 Raman spectrometer with 50 mW and 514.5 nm wavelength Ar laser. UV–visible absorption spectra were recorded on a PerkinElmer, Lambda 25 UV–visible spectroscopy in wavelength range 200–800 nm. 3. Results and discussion The phase structure of the products were examined by XRD. Fig. 1 shows XRD patterns of as-prepared pure ZnWO4 and La-doped ZnWO4 samples. For undoped ZnWO4, all diffraction peaks can be readily indexed to the pure monoclinic ZnWO4 structure with space group of P2/c and comprises
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Fig. 1. XRD pattern of as-prepared 0–2% La doped ZnWO4 samples synthesized by hydrothermal method.
infinite zigzag chains, running parallel to [0 0 1], of either edge-sharing ZnO6 octahedron or edge-sharing WO6 octahedra [1,2,14] which was in good agreement with JCPDS No. 15-0774 [13]. For La-doped ZnWO4, they are still indexed the ZnWO4 and no detected diffraction peaks of La2WO6, La2O3 or other impurity phases in La-doped ZnWO4 samples. It indicated that the existence of La3+ did not significantly influence the phase and crystallization of the products and suggested that La3+ ions would uniformly substitute into the Zn2+ sites or interstitial sites in ZnWO4 lattice. Fig. 1b of La-doped ZnWO4 with compared to the pure ZnWO4 in 2h = 17.5–40°, it can be seen that all peaks were slightly shifted
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Fig. 2. FTIR and Raman spectra of 0–2% La doped ZnWO4.
towards smaller diffraction angle. They suggested that the larger La3+ substitutes the smaller Zn2+ site partly so that the slightly shifting peak towards smaller diffraction angle. The ionic radius of La3+ (1.06 Å) is slightly larger than that of Zn2+ (0.74 Å), but much larger than that of W6+ (0.62 Å) in 6-fold coordination [15–17], so La3+ prefers to locating in Zn2+ sites without inversion center. As the amount of La3+ increases, it is observed that the corresponding XRD peaks move to lower degree according to previous reports which confirm that La3+ ions are inserted in ZnWO4 lattice cell [11,12,15–19]. FTIR spectra of the La-doped ZnWO4 samples with various concentration of La present in Fig. 2a. The products were mixed and diluted 10 times with potassium bromide (KBr). The background
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Fig. 3. SEM images of (a) 0%, (b) 0.4%, (c) 1% and (d) 2% La doped ZnWO4.
spectrum recorded using KBr was subtracted from the products spectrum. The ZnWO4 samples show six main absorption bands at 430–1000 cm1. The absorption bands at 834 and 877 cm1 can be due to the stretching modes of W–O in WO6 octahedra. The bands at 610 and 532 cm1 were due to symmetrical vibrations of bridge oxygen atoms of the Zn–O–W groups. The absorption bands at 473 and 430 cm1 can be assigned to symmetric and asymmetric deformation modes of W–O bonds and Zn–O bonds in WO6 and ZnO6 octahedra, respectively [1,2,20,21]. These vibrations could be identified to the synthesized ZnWO4. Obviously, the board bands in the 3400 cm1 is assigned to the O–H vibrations of H2O molecule. It is also evident that the prepared samples contain a significant amount of surfaceadsorbed water and some structural water. Raman spectra of wolframite ZnWO4 has 36 lattice vibration modes as C = 8Ag + 10Bg + 8Au + 10Bu by group theory. Only 18 modes of 8Ag and 10Bg are Raman active while15 modes of 7Au and 8Bu are infrared (IR) active, and 3 modes of 1Au and 2Bu are acoustic vibrations. Raman vibration of monoclinic wolframite ZnWO4 was referred to the incompressibility of WO6 octahedrons with respect to the ZnO6 octahedrons [2,22,23]. Raman spectra of the samples in the range 100–1000 cm1 are shown in Fig. 2b. Their peaks were assigned to the stretches of the W–O bands. The peak at 900 cm1 was interpreted as an antisymmetric bridging mode associated with the tungstate chain. The bands about 300 cm1 were assigned to the modes of terminal WO2 group. The W–O bonds of intermediate length are characteristic of bridging W–O bonds and are assigned to Raman mode stretching wavenumbers in the 700– 1000 cm1 [2,23–25]. The general morphologies of the as-prepared La-doped ZnWO4 products are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observation. Fig. 3 shows the SEM images of La-doped ZnWO4 with difference La contents. As can be seen from Fig. 3a, it reveals that the as-synthesized pure ZnWO4 product consists of uniform nanorods and no detection of other shapes. It can be seen that these nanorods have a uniform diameter of about 20 nm and lengths up to 0.5–1 lm. The La-doped ZnWO4 samples can be found that the morphologies of sample (Fig. 3b– d) are almost different. They found that the length of nanorod was decreased when increased the content of La doping. Finally, the completely short nanorods were produced at 2% La-doped ZnWO4. In
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Fig. 4. TEM images of (a) 0%, (b) 0.4%, (c) 1% and (d) 2% La doped ZnWO4.
addition, the EDS spectra show the chemical composition of sample which revealed the presence elementary components of Zn, W and O in ZnWO4 and Zn, La, W and O in La-doped ZnWO4. The morphologies of the as-prepared La doped ZnWO4 samples were further studied by TEM. Fig. 4 shows the morphologies of the as-synthesized samples were strongly dependent on La content doping. The ZnWO4 without La doping as shown in Fig. 4a shows the uniform rod-shaped crystals and a majority of the crystals had exceeded 1500 nm in length. Fig. 4b–d shows the TEM image of La-doped ZnWO4 for La = 0.4%, 1% and 2% by weight. Increasing of La content in ZnWO4, the products are still one-dimensional ZnWO4 nanostructures but not uniform in length. The products are mixture of short and long nanorod-shaped crystals can be seen. For 2% La-doped ZnWO4 with two length ranges of 100–200 nm and 400–800 nm were observed. To understand the preferential orientation growth, ZnWO4 nanorods and 2% La-doped ZnWO4 nanorods have been further studied using high resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) techniques and the results were shown in Figs. 5 and 6. The pure ZnWO4 sample consists of unifrom nanorods with average width and length of 20 nm and >1500 nm (aspect ratio of >75). Each nanorod is straight and perfect over their entire lengths. The electron diffraction pattern of a single ZnWO4 nanorod (Fig. 5c) obtained by focusing the electron beam perpendicular to the longitudinal axis presented in Fig. 5b. The SAED pattern revealed the product is a single-crystalline nature of the nanorods which can be index to (0 2 1), (1 2 1) and (1 0 0) planes with [0 1–2] as zone axis. So the nanorods grew preferentially along the [0 2 1] direction, which was parallel to the (1 0 0) face. The HRTEM image showed only one periodic fringe spacing which
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Fig. 5. TEM, HRTEM images and SAED pattern of ZnWO4 sample.
confirmed that nanorod is a nanocrystalline structure. The lattice spacing of nanorod is along the longitudinal axis of the growth direction of nanorods which lattice spacing is 0.4674 nm which corresponded to the (1 0 0) plane of monoclinic ZnWO4, as marked by an arrow in Fig. 5d. It confirms that the single ZnWO4 nanorod is grow along the [0 2 1] direction. Fig. 6 presents TEM and HRTEM images and SAED pattern of as-synthesized 2% La-doped ZnWO4 sample. TEM image show that the product is rod-like in shape, which indicates the symmetrical growth of nanorods along a certain direction. It can be seen that the nanorods are about 10–25 nm in diameter and 60–600 nm in length with aspect ratio of 6–24. SAED pattern is essentially identified over the entire rod from individual ZnWO4 nanorod in Fig. 6b, indicating the brightness sport pattern. It presents that product is a single-crystalline nature. SAED pattern revealed the (0 2 1), (1 2 1) and (1 0 0) plans in zone axis of [0 1–2] in accordance with the XRD result. Fig. 6d reveals the HRTEM image of 2% La-doped ZnWO4. A lattice spacing of about 0.4673 nm can be observed, corresponding to the separation between the (1 0 0) planes of 2% La-doped ZnWO4. It indicates that the growing plane and the growing direction of the nanorods are in the [0 2 1] direction, as marked by an arrow in Fig. 6d. Thus, both ZnWO4 and 2% La-doped ZnWO4 have the same growth direction along [0 2 1] which were confirmed by SAED and HRTEM results. The optical absorption of the as-synthesized ZnWO4 and La-doped ZnWO4 nanorods was measured by UV–visible spectroscopy in wavelength of 200–800 nm in ethanol solution as shown in Fig. 7a. All samples show the spectra with two shoulder absorptions at around 240–260 nm and 280–290 nm
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Fig. 6. TEM, HRTEM images and SAED pattern of as-synthesized 2% La-doped ZnWO4 sample.
which assigned to be the intrinsic band gap transition. The band gaps of the as-synthesized ZnWO4 nanorods were calculated by the below equation.
ahm ¼ hm Eg
n
ð1Þ
where a is the absorbance, h the Planck constant, m the photon frequency, Eg the energy gap, and n the pure numbers associated with the different types of electronic transitions [1,14,17,18]. For n = 1/2, 2, 3/2 and 3, the transitions are the direct allowed, indirect allowed, direct forbidden, and indirect forbidden, respectively. The value of n for ZnWO4 equals to 1/2 and its band gap was estimated to be 3.59, 3.53, 3.36, 3.24 and 3.11 eV for ZnWO4, 0.2% La-doped ZnWO4, 0.6% La-doped ZnWO4, 1.0% Ladoped ZnWO4 and 2.0% La-doped ZnWO4, respectively. It is worth to note that the La content in ZnWO4 plays the role in decreasing the energy band gap. 4. Conclusions In this research, 0–2% La doped ZnWO4 nanorods have been synthesized by hydrothermal method. XRD patterns of pure ZnWO4 and La doped ZnWO4 samples are a wolframite monoclinic ZnWO4 structure. Raman and FTIR spectra show a vibration of Zn–O, W–O and Zn–O–W in WO6 and ZnO6 octahedra. HRTEM image and SAED pattern confirmed that the nanorods grew preferentially along the [0 2 1]
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Fig. 7. (a) UV–visible spectra and (b) (ahm)2 vs hv curves of products.
direction. The UV–visible spectra show the great absorbance in UV region assigned to be the intrinsic band gap transition. Acknowledgements We wish to thank the National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), for providing financial support through the project P-10-11345, and the Thailand’s Office of the Higher Education Commission through the National Research University Project (NRU) for Chiang Mai University (CMU), and the Center for Innovation in Chemistry (PERCH-CIC), including the Graduate School of CMU through a general support.
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Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Synthesis, characterization and optical activity of La-doped ZnWO4 nanorods by hydrothermal method Jirapong Arin a, Phattranit Dumrongrojthanath a, Oranuch Yayapao b, Anukorn Phuruangrat c,⇑, Somchai Thongtem b, Titipun Thongtem a,⇑ a
Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand c Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai, Songkhla 90112, Thailand b
a r t i c l e
i n f o
Article history: Received 15 December 2013 Accepted 26 December 2013 Available online 2 January 2014 Keywords: La doped ZnWO4 Hydrothermal Nanorods
a b s t r a c t One-dimensional La doped ZnWO4 nanostructures have been synthesized by hydrothermal method. The effect of various La contents doped ZnWO4 samples on phase, morphologies and optical properties was investigated and characterized by X-ray diffraction (XRD), field scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), Raman and UV–visible spectroscopy, respectively. The results of XRD, SEM and SAED revealed that all products were one-dimensional monoclinic ZnWO4 nanorods while doped La3+ constants into ZnWO4 lattice. The observation of TEM and HRTEM showed that the obtained La3+ doped ZnWO4 with different La constants were homogenous nanorods shapes with grow along [0 2 1] direction. The optical property result showed that La3+ doped ZnWO4 is a red shifted absorption when comparing to pure ZnWO4. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In the past decade, ZnWO4 has been wildly researched because of its interesting in chemical and physical properties such as laser hosts, magnetic, scintillated, luminescent and catalytic properties and potential applications such as optical fibers, scintillator materials, magnetic properties, and ⇑ Corresponding authors. Tel.: +66 (0)74 288 374; fax: +66 (0)74 288 395. E-mail address: [email protected] (A. Phuruangrat). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.12.024
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heterogeneous catalysis [1–3]. ZnWO4 is a materials in wolframite structure and belongs monoclinic with C2h point group symmetry and P2/c space group. There are two formula units per primitive cell, having the lattice parameters a = 4.69263 Å, b = 5.72129 Å, c = 4.92805 Å and b = 90.6321°. The presence of two non-equivalent oxygen atoms is responsible for three pairs of Zn–O and W–O bonds, having different lengths. Thus, both Zn and W ions are surrounded by six oxygen atoms coordination, forming distorted ZnO6 and WO6 octahedral coordination which either edge-sharing ZnO6 octahedron or edge-sharing WO6 octahedra are corner-linked by zigzag chains in parallel to [0 0 1] [1,2,4]. To increase the luminescence efficiency of phosphor, researchers have tried various approaches including modify phosphor particle size and surface morphology, adding conductive materials, mixing flux materials and doping a sensitizer to enhance excitation energy absorption and energy transfer to the activator [5]. Doping method is a suitable synthesis method to increase the optical efficiency of oxide phosphor, rare-earth (RE) with their particular 4f–5d and 4f–4f electronic transition was used due to their unique optical transitions happen between the spin orbit levels split under the different manifolds weak crystal field and promising applications in optoelectronic devices [6–9]. By doping with RE3+ ions, it improves the luminescence efficiency by energy transfer processes [10]. For example, Anandan et al. report a high photocatalytic activity in the degradation of organic contaminants of Ladoped TiO2 due to the suppression of electron–hole recombination, large content of oxygen vacancies, and strong absorption of OH ions on the surface of the catalyst [11]. Jia et al. shows a strong emission peak centering at around 405 nm of La-doped ZnO which exhibited a red shift compared with the near band emission of pure ZnO because the incorporation of La3+ ions [12]. Here, we report one-dimensional La doped ZnWO4 with differences La contents nanostructures by hydrothermal method. The effects of La doping on the structural and optical properties of ZnWO4 were also investigated and discussed in detail. 2. Experimental procedure All the chemicals used in the experiment, including zinc nitrate hexahydrate (Zn(NO3)26H2O), lanthanum nitrate hexahydrate (La(NO3)36H2O), sodium tungstate dihydrate (Na2WO42H2O) were used as zinc, lanthanum, tungsten sources, respectively are of analytic grade reagents without further purification. In a typical procedure, the mixture containing 0.005 mol Zn(NO3)26H2O and different amount of La(NO3)36H2O (0.2, 0.4, 0.6, 1.0 and 2.0 wt%) was dissolved into 60 ml deionized water under rigorous stirring for 30 min. Then, 0.005 mol Na2WO42H2O in 20 ml deionized water was added and the mixture solution was transferred into a Teflon lined steel autoclave of 100 ml, and the autoclave was heated at 180 °C for 24 h. Afterward, the autoclave was cooled to room temperature gradually. The white precipitate was washed with ethanol and distilled water three times. Then, it was dried at 80 °C for 12 h. The products were characterized by X-ray diffraction (XRD) on a Philips, XPert MPD X-ray diffractometer equipped with Cu Ka radiation over a range from 10° to 60°. The scanning electron microscopy (SEM) images were carried out using a JEOL JSM-6335F field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) and high-resolution field emission transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) pattern were taken on a JEOL, JEM-2010 transmission electron microscopy with an acceleration voltage of 200 kV. Fourier transform infrared spectra (FT-IR) were recorded on a Bruker Tensor 27 FT-IR spectrometer with a KBr disk. Raman spectra were recorded on HORIBA JOBIN YVON T64000 Raman spectrometer with 50 mW and 514.5 nm wavelength Ar laser. UV–visible absorption spectra were recorded on a PerkinElmer, Lambda 25 UV–visible spectroscopy in wavelength range 200–800 nm. 3. Results and discussion The phase structure of the products were examined by XRD. Fig. 1 shows XRD patterns of as-prepared pure ZnWO4 and La-doped ZnWO4 samples. For undoped ZnWO4, all diffraction peaks can be readily indexed to the pure monoclinic ZnWO4 structure with space group of P2/c and comprises
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199
Fig. 1. XRD pattern of as-prepared 0–2% La doped ZnWO4 samples synthesized by hydrothermal method.
infinite zigzag chains, running parallel to [0 0 1], of either edge-sharing ZnO6 octahedron or edge-sharing WO6 octahedra [1,2,14] which was in good agreement with JCPDS No. 15-0774 [13]. For La-doped ZnWO4, they are still indexed the ZnWO4 and no detected diffraction peaks of La2WO6, La2O3 or other impurity phases in La-doped ZnWO4 samples. It indicated that the existence of La3+ did not significantly influence the phase and crystallization of the products and suggested that La3+ ions would uniformly substitute into the Zn2+ sites or interstitial sites in ZnWO4 lattice. Fig. 1b of La-doped ZnWO4 with compared to the pure ZnWO4 in 2h = 17.5–40°, it can be seen that all peaks were slightly shifted
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Fig. 2. FTIR and Raman spectra of 0–2% La doped ZnWO4.
towards smaller diffraction angle. They suggested that the larger La3+ substitutes the smaller Zn2+ site partly so that the slightly shifting peak towards smaller diffraction angle. The ionic radius of La3+ (1.06 Å) is slightly larger than that of Zn2+ (0.74 Å), but much larger than that of W6+ (0.62 Å) in 6-fold coordination [15–17], so La3+ prefers to locating in Zn2+ sites without inversion center. As the amount of La3+ increases, it is observed that the corresponding XRD peaks move to lower degree according to previous reports which confirm that La3+ ions are inserted in ZnWO4 lattice cell [11,12,15–19]. FTIR spectra of the La-doped ZnWO4 samples with various concentration of La present in Fig. 2a. The products were mixed and diluted 10 times with potassium bromide (KBr). The background
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Fig. 3. SEM images of (a) 0%, (b) 0.4%, (c) 1% and (d) 2% La doped ZnWO4.
spectrum recorded using KBr was subtracted from the products spectrum. The ZnWO4 samples show six main absorption bands at 430–1000 cm1. The absorption bands at 834 and 877 cm1 can be due to the stretching modes of W–O in WO6 octahedra. The bands at 610 and 532 cm1 were due to symmetrical vibrations of bridge oxygen atoms of the Zn–O–W groups. The absorption bands at 473 and 430 cm1 can be assigned to symmetric and asymmetric deformation modes of W–O bonds and Zn–O bonds in WO6 and ZnO6 octahedra, respectively [1,2,20,21]. These vibrations could be identified to the synthesized ZnWO4. Obviously, the board bands in the 3400 cm1 is assigned to the O–H vibrations of H2O molecule. It is also evident that the prepared samples contain a significant amount of surfaceadsorbed water and some structural water. Raman spectra of wolframite ZnWO4 has 36 lattice vibration modes as C = 8Ag + 10Bg + 8Au + 10Bu by group theory. Only 18 modes of 8Ag and 10Bg are Raman active while15 modes of 7Au and 8Bu are infrared (IR) active, and 3 modes of 1Au and 2Bu are acoustic vibrations. Raman vibration of monoclinic wolframite ZnWO4 was referred to the incompressibility of WO6 octahedrons with respect to the ZnO6 octahedrons [2,22,23]. Raman spectra of the samples in the range 100–1000 cm1 are shown in Fig. 2b. Their peaks were assigned to the stretches of the W–O bands. The peak at 900 cm1 was interpreted as an antisymmetric bridging mode associated with the tungstate chain. The bands about 300 cm1 were assigned to the modes of terminal WO2 group. The W–O bonds of intermediate length are characteristic of bridging W–O bonds and are assigned to Raman mode stretching wavenumbers in the 700– 1000 cm1 [2,23–25]. The general morphologies of the as-prepared La-doped ZnWO4 products are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observation. Fig. 3 shows the SEM images of La-doped ZnWO4 with difference La contents. As can be seen from Fig. 3a, it reveals that the as-synthesized pure ZnWO4 product consists of uniform nanorods and no detection of other shapes. It can be seen that these nanorods have a uniform diameter of about 20 nm and lengths up to 0.5–1 lm. The La-doped ZnWO4 samples can be found that the morphologies of sample (Fig. 3b– d) are almost different. They found that the length of nanorod was decreased when increased the content of La doping. Finally, the completely short nanorods were produced at 2% La-doped ZnWO4. In
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Fig. 4. TEM images of (a) 0%, (b) 0.4%, (c) 1% and (d) 2% La doped ZnWO4.
addition, the EDS spectra show the chemical composition of sample which revealed the presence elementary components of Zn, W and O in ZnWO4 and Zn, La, W and O in La-doped ZnWO4. The morphologies of the as-prepared La doped ZnWO4 samples were further studied by TEM. Fig. 4 shows the morphologies of the as-synthesized samples were strongly dependent on La content doping. The ZnWO4 without La doping as shown in Fig. 4a shows the uniform rod-shaped crystals and a majority of the crystals had exceeded 1500 nm in length. Fig. 4b–d shows the TEM image of La-doped ZnWO4 for La = 0.4%, 1% and 2% by weight. Increasing of La content in ZnWO4, the products are still one-dimensional ZnWO4 nanostructures but not uniform in length. The products are mixture of short and long nanorod-shaped crystals can be seen. For 2% La-doped ZnWO4 with two length ranges of 100–200 nm and 400–800 nm were observed. To understand the preferential orientation growth, ZnWO4 nanorods and 2% La-doped ZnWO4 nanorods have been further studied using high resolution transmission electron microscope (HRTEM) and selected area electron diffraction (SAED) techniques and the results were shown in Figs. 5 and 6. The pure ZnWO4 sample consists of unifrom nanorods with average width and length of 20 nm and >1500 nm (aspect ratio of >75). Each nanorod is straight and perfect over their entire lengths. The electron diffraction pattern of a single ZnWO4 nanorod (Fig. 5c) obtained by focusing the electron beam perpendicular to the longitudinal axis presented in Fig. 5b. The SAED pattern revealed the product is a single-crystalline nature of the nanorods which can be index to (0 2 1), (1 2 1) and (1 0 0) planes with [0 1–2] as zone axis. So the nanorods grew preferentially along the [0 2 1] direction, which was parallel to the (1 0 0) face. The HRTEM image showed only one periodic fringe spacing which
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Fig. 5. TEM, HRTEM images and SAED pattern of ZnWO4 sample.
confirmed that nanorod is a nanocrystalline structure. The lattice spacing of nanorod is along the longitudinal axis of the growth direction of nanorods which lattice spacing is 0.4674 nm which corresponded to the (1 0 0) plane of monoclinic ZnWO4, as marked by an arrow in Fig. 5d. It confirms that the single ZnWO4 nanorod is grow along the [0 2 1] direction. Fig. 6 presents TEM and HRTEM images and SAED pattern of as-synthesized 2% La-doped ZnWO4 sample. TEM image show that the product is rod-like in shape, which indicates the symmetrical growth of nanorods along a certain direction. It can be seen that the nanorods are about 10–25 nm in diameter and 60–600 nm in length with aspect ratio of 6–24. SAED pattern is essentially identified over the entire rod from individual ZnWO4 nanorod in Fig. 6b, indicating the brightness sport pattern. It presents that product is a single-crystalline nature. SAED pattern revealed the (0 2 1), (1 2 1) and (1 0 0) plans in zone axis of [0 1–2] in accordance with the XRD result. Fig. 6d reveals the HRTEM image of 2% La-doped ZnWO4. A lattice spacing of about 0.4673 nm can be observed, corresponding to the separation between the (1 0 0) planes of 2% La-doped ZnWO4. It indicates that the growing plane and the growing direction of the nanorods are in the [0 2 1] direction, as marked by an arrow in Fig. 6d. Thus, both ZnWO4 and 2% La-doped ZnWO4 have the same growth direction along [0 2 1] which were confirmed by SAED and HRTEM results. The optical absorption of the as-synthesized ZnWO4 and La-doped ZnWO4 nanorods was measured by UV–visible spectroscopy in wavelength of 200–800 nm in ethanol solution as shown in Fig. 7a. All samples show the spectra with two shoulder absorptions at around 240–260 nm and 280–290 nm
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Fig. 6. TEM, HRTEM images and SAED pattern of as-synthesized 2% La-doped ZnWO4 sample.
which assigned to be the intrinsic band gap transition. The band gaps of the as-synthesized ZnWO4 nanorods were calculated by the below equation.
ahm ¼ hm Eg
n
ð1Þ
where a is the absorbance, h the Planck constant, m the photon frequency, Eg the energy gap, and n the pure numbers associated with the different types of electronic transitions [1,14,17,18]. For n = 1/2, 2, 3/2 and 3, the transitions are the direct allowed, indirect allowed, direct forbidden, and indirect forbidden, respectively. The value of n for ZnWO4 equals to 1/2 and its band gap was estimated to be 3.59, 3.53, 3.36, 3.24 and 3.11 eV for ZnWO4, 0.2% La-doped ZnWO4, 0.6% La-doped ZnWO4, 1.0% Ladoped ZnWO4 and 2.0% La-doped ZnWO4, respectively. It is worth to note that the La content in ZnWO4 plays the role in decreasing the energy band gap. 4. Conclusions In this research, 0–2% La doped ZnWO4 nanorods have been synthesized by hydrothermal method. XRD patterns of pure ZnWO4 and La doped ZnWO4 samples are a wolframite monoclinic ZnWO4 structure. Raman and FTIR spectra show a vibration of Zn–O, W–O and Zn–O–W in WO6 and ZnO6 octahedra. HRTEM image and SAED pattern confirmed that the nanorods grew preferentially along the [0 2 1]
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Fig. 7. (a) UV–visible spectra and (b) (ahm)2 vs hv curves of products.
direction. The UV–visible spectra show the great absorbance in UV region assigned to be the intrinsic band gap transition. Acknowledgements We wish to thank the National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), for providing financial support through the project P-10-11345, and the Thailand’s Office of the Higher Education Commission through the National Research University Project (NRU) for Chiang Mai University (CMU), and the Center for Innovation in Chemistry (PERCH-CIC), including the Graduate School of CMU through a general support.
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