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Low temperature synthesis of Zn2GeO4 nanorods and their photoluminescence
Journal of Luminescence 136 (2013) 322–327
Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Low temperature synthesis of Zn2GeO4 nanorods and their photoluminescence Meng-Yen Tsai a,b, Sheng-Hsin Huang a, Tsong-Pyng Perng a,c,n a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan c Department of Chemical Engineering and Materials Science, Yuan Ze University, Chungli 320, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 September 2012 Received in revised form 23 November 2012 Accepted 4 December 2012 Available online 12 December 2012
Zn2GeO4 nanorods were synthesized using a simple reflux method. The product with 0.05 M Zn2GeO4 is an aggregation of short nanorods with the diameter ranging from 30 to 50 nm. If the Zn2GeO4 molarity was increased, the nanorods became longer and aggregated as bundles. An intense white-bluish photoluminescence (PL) was observed from these nanorods, and the PL band can be dissolved into four Gaussian peaks that are associated with the native defects. Since the PL intensity of the nanorods is comparable to that of sintered particles, this reflux method provides a time- and energy-efficient route to prepare Zn2GeO4 phosphor. & 2012 Elsevier B.V. All rights reserved.
Keywords: Zinc germinate (Zn2GeO4) Reflux method Photoluminescence
1. Introduction Since the discovery of carbon nanotubes, extensive research on one-dimensional (1-D) nanomaterials, including nanowires, nanotubes, nanobelts, and nanorods, has been performed because of their unique physical and chemical properties and potential application in nanoscaled devices [1–3]. Among them, 1-D oxide nanomaterials, such as functional metal oxides like ZnO, In2O3, SnO2, and Ga2O3 [4–6] and structrural ceramics like SiO2, GeO2, and MgO, [7–9] have attracted a lot of attention because of their unique properties. Although the investigation of 1-D binary oxide nanomaterials has become a quite popular research field, the study on 1-D ternary oxide nanomaterials remains limited. Nevertheless, in recent years there has been increasing interest in 1-D ternary oxide nanomaterials, such as Zn2SnO4, [10–12] ZnGa2O4, [13,14] CoFe2O4, [15] and MgAl2O4, [16] because they exhibit specific functions in comparison to binary oxides. Zn2GeO4, as a ternary oxide, has multiple properties. It exhibits negative thermal expansion below room temperature [17]. RuO2dispersed Zn2GeO4 was found to be a stable photocatalyst for decomposition of water [18] Zn2GeO4:Mn showed green luminescence, [19] and there have been extensive studies focusing on corresponding alternating-current thin-film electroluminescence devices [20,21] Recently, Liu et al. reported that Zn2GeO4 presented a white-bluish emission, and its photoluminescence (PL)
was approximately 40% brighter than that of commercial ZnO phosphor [22]. In our previous work, luminescent Zn2GeO4 nanorods were formed for the first time, simply by submersing Zn-containing Ge nanoparticles (ZCGNs) in water at room temperature [23]. Due to the surface instability in water, these crystalline ZCGNs first undergo a structural transformation into amorphous membranes from which single-crystalline Zn2GeO4 nanorods directly grow out after further incubation. In general, Zn2GeO4 is prepared by a solid state reaction in which the stoichiometric mixture of GeO2 and ZnO powders is heated at T41000 1C [22,24,25]. In order to obtain Zn2GeO4 phosphor with high brightness, the reaction temperature even has to be over 1200 1C. Therefore, it is rather energy-consuming. As for our previous study on the formation of Zn2GeO4 nanorods, the processing temperature is at room temperature. However, the incubation time for obtaining a respectable amount of nanorods generally takes more than one month. Such a time-consuming route is unsuitable for practical applications. Considering the disadvantages of the aforementioned processes, we therefore tried to prepare Zn2GeO4 by a simpler method which was both time- and energy-efficient. In this study, Zn2GeO4 nanorods were synthesized using a simple reflux method. Since Zn2GeO4 is a typical self-activated phosphor, the luminescence from these nanorods was also characterized.
2. Experimental n Corresponding author at: Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan. Tel.: þ886 3 574 2634; fax: þ 886 3 572 3857. E-mail addresses: [email protected]. [email protected] (T.-P. Perng).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.12.018
2.1. The reflux method In the present study, Zn2GeO4 nanorods were synthesized using a reflux method where aqueous solutions of zinc acetate
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dihydrate (Zn(CH3COO)2 2 H2O, Sigma-Aldrich, purity 99.999%) and GeO2 (Sigma-Aldrich, purity 99.999%) were used as the precursors. Stoichiometric amounts of GeO2 (1 mmole) and zinc acetate dihydrate (2 mmole) were pre-dissolved in deionized water separately, and the solutions were then mixed with a total solution volume of 20 ml. The solution mixture was subsequently refluxed in a silicon oil bath at 160 1C for 3 h with the aid of magnetic stirring. After the reflux process was completed, the product was collected by centrifugation followed by low temperature drying at 50 1C. The Zn2GeO4 molarity of this specimen is 0.05 M. To investigate the effect of higher reactant concentration, another sample with a molar concentration of 0.25 M of Zn2GeO4 was also prepared. The experimental condition of refluxing for this sample was completely the same as that for the previous one. 2.2. Solid state reaction A solid state reaction was also employed to prepare Zn2GeO4 for comparison. Following the method frequently used in the literature, powders of ZnO and GeO2 were chosen as the starting materials. The starting materials weighed at the stoichiometric ratio were mixed by manual grinding. An evacuated quartz furnace was then utilized to fire the powder mixture at 1000 1C. The heating rate was set at 10 1C/min, and the heating process was sustained for 12 h. 2.3. Characterization and analysis Both Zn2GeO4 products prepared by reflux and solid state reaction methods were characterized and analyzed by fieldemission scanning electron microscopy (FESEM) (JEOL 6500F) and transmission electron microscopy (TEM, JEOL 2010) equipped with energy-dispersive X-ray (EDX) spectroscopy. The structural analysis of the products was performed using an X-ray diffractometer (XRD, Shimadzu 6000) operated at 40 kV and 30 mA, with a Cu Ka radiation. The PL was measured using a PL system equipped with a 325 nm He–Cd laser (Kimmon, IK series) with an excitation power of 35 mW.
3. Results and discussion 3.1. Formation of Zn2GeO4 nanorods In the very beginning of this study, the reflux method was not considered as the first step. Instead, the mixture of zinc acetate dihydrate and GeO2 solutions was placed at room temperature for 12 h without any disturbance. With increasing of aging time, some white precipitates gradually formed (denoted as sample A). As shown by the XRD patterns in Fig. 1(a), the peak positions do not correspond to any compound of zinc germinates, and are supposed to comprise multiple phases of compounds related to Zn, Ge and oxygen with a nearly amorphous structure. The product is an aggregation of wrinkled porous particles interconnected with each other, as displayed by the SEM micrographs in Fig. 2(a). The morphology of these particles is irregular, and their physical dimension is several hundreds of nanometer. From the TEM observation, these wrinkled particles lack of a long-term ordered crystalline structure, in accordance with the XRD result. Further, from the EDX analysis, though the particles are composed of Zn, Ge and oxygen, the relative ratio of these three elements varies a lot with the location. If the solution mixture was kept at 80 1C, the crystallinity of the product after aging for 12 h would be very much improved, with Zn2GeO4 as the main crystalline phase, and a small amount of second phase was also present in the product.
Fig. 1. XRD patterns of (a) sample A, (b) sample B, (c) sample C and (d) sample D. The XRD peak indices in (b) correspond to those from JCPDS card no. 11-0687.
Based on the above experimental tests, a reflux method was adopted to synthesize the Zn2GeO4 compound. Fig. 1(b) and (c) displays the XRD patterns of the refluxed samples with Zn2GeO4 molarity of 0.05 M (denoted as sample B) and 0.25 M (denoted as sample C), respectively. As a comparison, the XRD pattern of Zn2GeO4 product prepared by the solid state reaction (denoted as sample D) is also shown in Fig. 1(d). These three XRD patterns correspond well to pure Zn2GeO4 phase with a phenacite structure [26] and the space group number of 148 (JCPDS card no. 11-0687). Nevertheless, the full widths at half maximum (FWHM) of XRD peaks are different, indicating various grain sizes. As illustrated in Fig. 2(b), the product of sample B is an aggregation of short nanorods. The diameter of nanorods ranges from 30 to 50 nm, and the length is approximately 300 nm. Estimated from the FWHM of (4 1 0) peak in Fig. 1(b), the grain size is 21.3 nm which is reasonable considering the rod dimension. As for sample C with a higher Zn2GeO4 molarity, the nanorod feature remains. However, it seems that these nanorods tend to agglomerate to form nanorod bundles or larger nanorods, resulting in a larger physical dimension (Fig. 2(c)). The estimated grain size of this specimen is 32.3 nm, larger than that of sample B. According to the results of Figs. 1 and 2(a), Zn2GeO4 compound cannot be formed by simply adding the ions of Zn and Ge together in the aqueous solution at room temperature, although an amorphous structure consisting of Zn, Ge, and oxygen is observed. To obtain crystalline Zn2GeO4, additional energy seems necessary to facilitate nucleation and growth. It is seen that the reflux
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concentrations. With additional concentrations 0.025 M, 0.10 M, 0.15 M, and 0.20 M studied, it was observed that the final products were all nanorods, although the nanorods tended to agglomerate to form bundles or larger nanorods as the concentration increased. As a comparison, a solid state reaction at as high as 1000 1C would also lead to formation of crystalline Zn2GeO4 (sample D), as evidenced by the XRD pattern in Fig. 1(d). The product of sample D shown in Fig. 2(d), however, is particle-like, with an average physical dimension of micrometer scale which is relatively lager than those of reflux samples. The grain size estimated from the corresponding XRD pattern is 51.6 nm. The growth process of Zn2GeO4 with a molarity of 0.25 M is shown in Fig. 3. In the very beginning, the Zn and Ge precursors were mixed together to form wrinkled particles which exhibit an amorphous structure composed of Zn, Ge, and oxygen according to the XRD pattern shown in Fig. 4. This amorphous nature transformed to a clear phenacite structure of Zn2GeO4 after 30 min of refluxing reaction, although the wrinkled morphology did not change much. Interestingly, the wrinkled particles gradually merged together after 40 min of refluxing, and subsequently transformed into a nanorod aggregation (Fig. 3(e)–(g)). It seems that the wrinkled structure of phenacite Zn2GeO4 during the reflux reaction is not stable so that eventually a more stable nanorod geometry with the same crystalline phase was formed. It is also believed that the refluxing at 160 1C provides sufficient thermal energy for interdiffusion of Zn, Ge and oxygen atoms, enabling the geometric reconstruction of this crystalline phase. The formation of Zn2GeO4 nanorods was basically completed after 60 min of refluxing reaction since the nanorod feature along with its crystallinity did not change afterward (Figs. 3(g)–(i) and 4). In our previous work, [23] Zn2GeO4 nanorods were derived by submerging Zn-containing Ge nanoparticles in water. When the aging time is increased, the nanoparticles will be completely oxidized, and then amorphous membranes are formed by agglomeration of the oxidized product. Zn2GeO4 nanorods subsequently nucleate from the membranes, and continue to grow in the aqueous environment either by the diffusion of Zn, Ge, and oxygen through the membranes or by the precipitation of wandering ions of Zn, Ge, and hydroxide in the solution onto the nanorods. The whole process is very slow, and generally takes more than one month. 3.2. PL of Zn2GeO4
Fig. 2. SEM micrographs of (a) sample A, (b) sample B, (c) sample C and (d) sample D.
at 160 1C is sufficient for this purpose. For a lower Zn2GeO4 molarity (sample B), short Zn2GeO4 nanorods can be obtained by this simple reflux method. In the case of higher Zn2GeO4 molarity (sample C), the increase of physical dimension and grain size may be considered as a consequence of higher reactant
Since Zn2GeO4 is a kind of self-activated phosphor, light emissions from aforementioned specimens are also studied. Fig. 5(a)–(d) shows the PL spectra measured from these specimens. A bright white luminescence with a little bit blue was observed from sample A (Fig. 5(a)), with the peak maximum at 480 nm (2.58 eV). The emission band covers from 350 to more than 600 nm and the FWHM of PL peak is as large as 168 nm (0.93 eV), resulting in a white-like luminescence. By applying Gaussian fitting, four peaks are needed to fit the broad band. These peak positions are located at 396 nm, 432 nm, 481 nm, and 533 nm. For sample B, it exhibits an intense white-bluish luminescence peaked at 475 nm (2.61 eV), as shown in Fig. 5(b). The emission band is not as broad as that of sample A, and the FWHM of PL peak is 131 nm (0.69 eV). This PL band can be fitted with three Gaussian peaks at 448 nm, 499 nm, and 545 nm. The light emission observed from sample C is similar to that of reflux sample B, though with a little bit green (Fig. 5(c)). Compared with sample B, the PL peak position is red-shifted to 490 nm (2.53 eV), and the FWHM of PL peak is 140 nm (0.71 eV). The three dissolved Gaussian peaks are located at 448 nm, 497 nm, and 534 nm, very similar to those of sample B.
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Fig. 3. Zn2GeO4 naonrods grown at different stages observed by SEM. (a) 0 min, (b) 5 min, (c) 15 min, (d) 30 min, (e) 40 min, (f) 50 min, (g) 60 min, (h) 120 min and (i) 180 min.
Fig. 4. XRD patterns of Zn2GeO4 nanorods formed by refluxing reaction for 0–180 min.
Concerning with the PL property of Zn2GeO4, Lee [25] has studied the PL from Zn2GeO4 sintered at 1200 1C and 1300 1C, and two main PL peaks at 453 nm and 526 nm were observed. These light emission bands are correlated to the oxygen defects. A white-bluish luminescence was also observed by Liu et al. from sintered Zn2GeO4. [22] In their study, the peak maximum is at 519 nm, with the FWHM of 137 nm. The PL band can be fitted with three Gaussian peaks at 454 nm, 521 nm and 544 nm. This emission is ascribed to the donor-acceptor recombination, where V O and Zni are donors, while VGe and V 00Zn serve as acceptors. As a comparison, for Zn2GeO4 particles (sample D) prepared in the present study by a solid state reaction, the PL color becomes white green, as displayed in Fig. 5(d). The peak maximum is at 530 nm, with a narrower FWHM of 55 nm (0.24 eV), compared to the other three specimens. For this PL band, there are only two dissolved Gaussian peaks at 525 nm and 544 nm. Interestingly, similar Gaussian peak positions of these two can also be found in sample B and those reported by Lee [25] and Liu et al. [22] Except for the dissolved peak at around 499 nm, one can notice the high similarity between the PL band positions along with dissolved Gaussian peaks reported from the literature and those identified in our study. Therefore, the light emissions observed for samples B, C, and D are quite likely associated with the native defects present in Zn2GeO4 nanorods and particles, though further study is needed for a clearer luminescence mechanism. Based on this analysis, the red shift of PL peak of sample C can be explained by the variation of relative intensity of dissolved Gaussian peaks. One can clearly see from Fig. 5(b) that the dissolved Gaussian peaks of sample B have higher intensities toward shorter wavelengths, whereas sample C exhibits an inverse trend, resulting in the red-shift of PL peak position. Due to the smaller physical dimension, sample B would have a higher
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Gaussian peaks cannot be closely correlated to those of the other three specimens because this sample does not contain the crystalline Zn2GeO4 phase. However, the PL intensity remains strong. The origin of light emission may be quite different and can be attributed to some specific compositions within the wrinkled particles which also exhibit luminescence.
4. Conclusion In summary, Zn2GeO4 nanorods were synthesized using a simple reflux method where aqueous solutions of GeO2 and zinc acetate dihydrate were used as the precursors. The product with lower Zn2GeO4 molarity (sample B) is an aggregation of short nanorods with the diameter ranging from 30 to 50 nm, exhibiting a crystalline Zn2GeO4 phase of phenacite structure. If the Zn2GeO4 molarity in the solution was increased by 5 times, sample C, the nanorods became longer and tended to aggregate together, resulting in a larger physical dimension. An intense white-bluish light emission peaked at 475 nm was observed from sample B, and the emission band distributed from 350 to more than 600 nm. For sample C, the PL peak position red-shifted to 490 nm though the band profile remained similar. As a comparison, the PL of sintered Zn2GeO4 particles, sample D, exhibited a green luminescence. The light emission from Zn2GeO4 nanorods was associated with the native defects, and the red shift of PL peak could be interpreted by the variation of relative amount of different luminescent centers. Since the PL intensity of Zn2GeO4 nanorods is comparable to that of sintered particles, the reflux method provides not only a time-efficient but also a power-saving route to prepare luminescent Zn2GeO4 phosphor.
Acknowledgments This work was supported by the National Science Council of Taiwan under the Contract no. NSC 96–2221-E-007-006. We are grateful to Prof. Shangjr Guo and Mr. Chen-Ying Wu of the Department of Physics and Institute of MEMS, National Tsing Hua University for the PL measurements. References
Fig. 5. PL spectra of (a) sample A, (b) sample B, (c) sample C, and (d) sample D. The black curves are measured spectra, and the red ones are obtained by Gaussian fitting. The dissolved Gaussian peaks are illustrated in blue dashed lines. The optical photographs for light emissions were taken by a digital camera after the samples had been irradiated by the laser. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
specific surface area. Thus, the ratio of native defects on the surface to those within the bulk may be different from that of sample C, leading to different amount of luminescent centers. Furthermore, the dissolved peak at around 499 nm may be caused by the specific reflux solution process, since it is hardly found in Zn2GeO4 obtained by solid state reactions (i.e., sample D and those reported by Lee [25] and Liu et al.[22]). As for the luminescence from sample A, it is reasonable that the dissolved
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Contents lists available at SciVerse ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Low temperature synthesis of Zn2GeO4 nanorods and their photoluminescence Meng-Yen Tsai a,b, Sheng-Hsin Huang a, Tsong-Pyng Perng a,c,n a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan c Department of Chemical Engineering and Materials Science, Yuan Ze University, Chungli 320, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 September 2012 Received in revised form 23 November 2012 Accepted 4 December 2012 Available online 12 December 2012
Zn2GeO4 nanorods were synthesized using a simple reflux method. The product with 0.05 M Zn2GeO4 is an aggregation of short nanorods with the diameter ranging from 30 to 50 nm. If the Zn2GeO4 molarity was increased, the nanorods became longer and aggregated as bundles. An intense white-bluish photoluminescence (PL) was observed from these nanorods, and the PL band can be dissolved into four Gaussian peaks that are associated with the native defects. Since the PL intensity of the nanorods is comparable to that of sintered particles, this reflux method provides a time- and energy-efficient route to prepare Zn2GeO4 phosphor. & 2012 Elsevier B.V. All rights reserved.
Keywords: Zinc germinate (Zn2GeO4) Reflux method Photoluminescence
1. Introduction Since the discovery of carbon nanotubes, extensive research on one-dimensional (1-D) nanomaterials, including nanowires, nanotubes, nanobelts, and nanorods, has been performed because of their unique physical and chemical properties and potential application in nanoscaled devices [1–3]. Among them, 1-D oxide nanomaterials, such as functional metal oxides like ZnO, In2O3, SnO2, and Ga2O3 [4–6] and structrural ceramics like SiO2, GeO2, and MgO, [7–9] have attracted a lot of attention because of their unique properties. Although the investigation of 1-D binary oxide nanomaterials has become a quite popular research field, the study on 1-D ternary oxide nanomaterials remains limited. Nevertheless, in recent years there has been increasing interest in 1-D ternary oxide nanomaterials, such as Zn2SnO4, [10–12] ZnGa2O4, [13,14] CoFe2O4, [15] and MgAl2O4, [16] because they exhibit specific functions in comparison to binary oxides. Zn2GeO4, as a ternary oxide, has multiple properties. It exhibits negative thermal expansion below room temperature [17]. RuO2dispersed Zn2GeO4 was found to be a stable photocatalyst for decomposition of water [18] Zn2GeO4:Mn showed green luminescence, [19] and there have been extensive studies focusing on corresponding alternating-current thin-film electroluminescence devices [20,21] Recently, Liu et al. reported that Zn2GeO4 presented a white-bluish emission, and its photoluminescence (PL)
was approximately 40% brighter than that of commercial ZnO phosphor [22]. In our previous work, luminescent Zn2GeO4 nanorods were formed for the first time, simply by submersing Zn-containing Ge nanoparticles (ZCGNs) in water at room temperature [23]. Due to the surface instability in water, these crystalline ZCGNs first undergo a structural transformation into amorphous membranes from which single-crystalline Zn2GeO4 nanorods directly grow out after further incubation. In general, Zn2GeO4 is prepared by a solid state reaction in which the stoichiometric mixture of GeO2 and ZnO powders is heated at T41000 1C [22,24,25]. In order to obtain Zn2GeO4 phosphor with high brightness, the reaction temperature even has to be over 1200 1C. Therefore, it is rather energy-consuming. As for our previous study on the formation of Zn2GeO4 nanorods, the processing temperature is at room temperature. However, the incubation time for obtaining a respectable amount of nanorods generally takes more than one month. Such a time-consuming route is unsuitable for practical applications. Considering the disadvantages of the aforementioned processes, we therefore tried to prepare Zn2GeO4 by a simpler method which was both time- and energy-efficient. In this study, Zn2GeO4 nanorods were synthesized using a simple reflux method. Since Zn2GeO4 is a typical self-activated phosphor, the luminescence from these nanorods was also characterized.
2. Experimental n Corresponding author at: Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan. Tel.: þ886 3 574 2634; fax: þ 886 3 572 3857. E-mail addresses: [email protected]. [email protected] (T.-P. Perng).
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.12.018
2.1. The reflux method In the present study, Zn2GeO4 nanorods were synthesized using a reflux method where aqueous solutions of zinc acetate
M.-Y. Tsai et al. / Journal of Luminescence 136 (2013) 322–327
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dihydrate (Zn(CH3COO)2 2 H2O, Sigma-Aldrich, purity 99.999%) and GeO2 (Sigma-Aldrich, purity 99.999%) were used as the precursors. Stoichiometric amounts of GeO2 (1 mmole) and zinc acetate dihydrate (2 mmole) were pre-dissolved in deionized water separately, and the solutions were then mixed with a total solution volume of 20 ml. The solution mixture was subsequently refluxed in a silicon oil bath at 160 1C for 3 h with the aid of magnetic stirring. After the reflux process was completed, the product was collected by centrifugation followed by low temperature drying at 50 1C. The Zn2GeO4 molarity of this specimen is 0.05 M. To investigate the effect of higher reactant concentration, another sample with a molar concentration of 0.25 M of Zn2GeO4 was also prepared. The experimental condition of refluxing for this sample was completely the same as that for the previous one. 2.2. Solid state reaction A solid state reaction was also employed to prepare Zn2GeO4 for comparison. Following the method frequently used in the literature, powders of ZnO and GeO2 were chosen as the starting materials. The starting materials weighed at the stoichiometric ratio were mixed by manual grinding. An evacuated quartz furnace was then utilized to fire the powder mixture at 1000 1C. The heating rate was set at 10 1C/min, and the heating process was sustained for 12 h. 2.3. Characterization and analysis Both Zn2GeO4 products prepared by reflux and solid state reaction methods were characterized and analyzed by fieldemission scanning electron microscopy (FESEM) (JEOL 6500F) and transmission electron microscopy (TEM, JEOL 2010) equipped with energy-dispersive X-ray (EDX) spectroscopy. The structural analysis of the products was performed using an X-ray diffractometer (XRD, Shimadzu 6000) operated at 40 kV and 30 mA, with a Cu Ka radiation. The PL was measured using a PL system equipped with a 325 nm He–Cd laser (Kimmon, IK series) with an excitation power of 35 mW.
3. Results and discussion 3.1. Formation of Zn2GeO4 nanorods In the very beginning of this study, the reflux method was not considered as the first step. Instead, the mixture of zinc acetate dihydrate and GeO2 solutions was placed at room temperature for 12 h without any disturbance. With increasing of aging time, some white precipitates gradually formed (denoted as sample A). As shown by the XRD patterns in Fig. 1(a), the peak positions do not correspond to any compound of zinc germinates, and are supposed to comprise multiple phases of compounds related to Zn, Ge and oxygen with a nearly amorphous structure. The product is an aggregation of wrinkled porous particles interconnected with each other, as displayed by the SEM micrographs in Fig. 2(a). The morphology of these particles is irregular, and their physical dimension is several hundreds of nanometer. From the TEM observation, these wrinkled particles lack of a long-term ordered crystalline structure, in accordance with the XRD result. Further, from the EDX analysis, though the particles are composed of Zn, Ge and oxygen, the relative ratio of these three elements varies a lot with the location. If the solution mixture was kept at 80 1C, the crystallinity of the product after aging for 12 h would be very much improved, with Zn2GeO4 as the main crystalline phase, and a small amount of second phase was also present in the product.
Fig. 1. XRD patterns of (a) sample A, (b) sample B, (c) sample C and (d) sample D. The XRD peak indices in (b) correspond to those from JCPDS card no. 11-0687.
Based on the above experimental tests, a reflux method was adopted to synthesize the Zn2GeO4 compound. Fig. 1(b) and (c) displays the XRD patterns of the refluxed samples with Zn2GeO4 molarity of 0.05 M (denoted as sample B) and 0.25 M (denoted as sample C), respectively. As a comparison, the XRD pattern of Zn2GeO4 product prepared by the solid state reaction (denoted as sample D) is also shown in Fig. 1(d). These three XRD patterns correspond well to pure Zn2GeO4 phase with a phenacite structure [26] and the space group number of 148 (JCPDS card no. 11-0687). Nevertheless, the full widths at half maximum (FWHM) of XRD peaks are different, indicating various grain sizes. As illustrated in Fig. 2(b), the product of sample B is an aggregation of short nanorods. The diameter of nanorods ranges from 30 to 50 nm, and the length is approximately 300 nm. Estimated from the FWHM of (4 1 0) peak in Fig. 1(b), the grain size is 21.3 nm which is reasonable considering the rod dimension. As for sample C with a higher Zn2GeO4 molarity, the nanorod feature remains. However, it seems that these nanorods tend to agglomerate to form nanorod bundles or larger nanorods, resulting in a larger physical dimension (Fig. 2(c)). The estimated grain size of this specimen is 32.3 nm, larger than that of sample B. According to the results of Figs. 1 and 2(a), Zn2GeO4 compound cannot be formed by simply adding the ions of Zn and Ge together in the aqueous solution at room temperature, although an amorphous structure consisting of Zn, Ge, and oxygen is observed. To obtain crystalline Zn2GeO4, additional energy seems necessary to facilitate nucleation and growth. It is seen that the reflux
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concentrations. With additional concentrations 0.025 M, 0.10 M, 0.15 M, and 0.20 M studied, it was observed that the final products were all nanorods, although the nanorods tended to agglomerate to form bundles or larger nanorods as the concentration increased. As a comparison, a solid state reaction at as high as 1000 1C would also lead to formation of crystalline Zn2GeO4 (sample D), as evidenced by the XRD pattern in Fig. 1(d). The product of sample D shown in Fig. 2(d), however, is particle-like, with an average physical dimension of micrometer scale which is relatively lager than those of reflux samples. The grain size estimated from the corresponding XRD pattern is 51.6 nm. The growth process of Zn2GeO4 with a molarity of 0.25 M is shown in Fig. 3. In the very beginning, the Zn and Ge precursors were mixed together to form wrinkled particles which exhibit an amorphous structure composed of Zn, Ge, and oxygen according to the XRD pattern shown in Fig. 4. This amorphous nature transformed to a clear phenacite structure of Zn2GeO4 after 30 min of refluxing reaction, although the wrinkled morphology did not change much. Interestingly, the wrinkled particles gradually merged together after 40 min of refluxing, and subsequently transformed into a nanorod aggregation (Fig. 3(e)–(g)). It seems that the wrinkled structure of phenacite Zn2GeO4 during the reflux reaction is not stable so that eventually a more stable nanorod geometry with the same crystalline phase was formed. It is also believed that the refluxing at 160 1C provides sufficient thermal energy for interdiffusion of Zn, Ge and oxygen atoms, enabling the geometric reconstruction of this crystalline phase. The formation of Zn2GeO4 nanorods was basically completed after 60 min of refluxing reaction since the nanorod feature along with its crystallinity did not change afterward (Figs. 3(g)–(i) and 4). In our previous work, [23] Zn2GeO4 nanorods were derived by submerging Zn-containing Ge nanoparticles in water. When the aging time is increased, the nanoparticles will be completely oxidized, and then amorphous membranes are formed by agglomeration of the oxidized product. Zn2GeO4 nanorods subsequently nucleate from the membranes, and continue to grow in the aqueous environment either by the diffusion of Zn, Ge, and oxygen through the membranes or by the precipitation of wandering ions of Zn, Ge, and hydroxide in the solution onto the nanorods. The whole process is very slow, and generally takes more than one month. 3.2. PL of Zn2GeO4
Fig. 2. SEM micrographs of (a) sample A, (b) sample B, (c) sample C and (d) sample D.
at 160 1C is sufficient for this purpose. For a lower Zn2GeO4 molarity (sample B), short Zn2GeO4 nanorods can be obtained by this simple reflux method. In the case of higher Zn2GeO4 molarity (sample C), the increase of physical dimension and grain size may be considered as a consequence of higher reactant
Since Zn2GeO4 is a kind of self-activated phosphor, light emissions from aforementioned specimens are also studied. Fig. 5(a)–(d) shows the PL spectra measured from these specimens. A bright white luminescence with a little bit blue was observed from sample A (Fig. 5(a)), with the peak maximum at 480 nm (2.58 eV). The emission band covers from 350 to more than 600 nm and the FWHM of PL peak is as large as 168 nm (0.93 eV), resulting in a white-like luminescence. By applying Gaussian fitting, four peaks are needed to fit the broad band. These peak positions are located at 396 nm, 432 nm, 481 nm, and 533 nm. For sample B, it exhibits an intense white-bluish luminescence peaked at 475 nm (2.61 eV), as shown in Fig. 5(b). The emission band is not as broad as that of sample A, and the FWHM of PL peak is 131 nm (0.69 eV). This PL band can be fitted with three Gaussian peaks at 448 nm, 499 nm, and 545 nm. The light emission observed from sample C is similar to that of reflux sample B, though with a little bit green (Fig. 5(c)). Compared with sample B, the PL peak position is red-shifted to 490 nm (2.53 eV), and the FWHM of PL peak is 140 nm (0.71 eV). The three dissolved Gaussian peaks are located at 448 nm, 497 nm, and 534 nm, very similar to those of sample B.
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Fig. 3. Zn2GeO4 naonrods grown at different stages observed by SEM. (a) 0 min, (b) 5 min, (c) 15 min, (d) 30 min, (e) 40 min, (f) 50 min, (g) 60 min, (h) 120 min and (i) 180 min.
Fig. 4. XRD patterns of Zn2GeO4 nanorods formed by refluxing reaction for 0–180 min.
Concerning with the PL property of Zn2GeO4, Lee [25] has studied the PL from Zn2GeO4 sintered at 1200 1C and 1300 1C, and two main PL peaks at 453 nm and 526 nm were observed. These light emission bands are correlated to the oxygen defects. A white-bluish luminescence was also observed by Liu et al. from sintered Zn2GeO4. [22] In their study, the peak maximum is at 519 nm, with the FWHM of 137 nm. The PL band can be fitted with three Gaussian peaks at 454 nm, 521 nm and 544 nm. This emission is ascribed to the donor-acceptor recombination, where V O and Zni are donors, while VGe and V 00Zn serve as acceptors. As a comparison, for Zn2GeO4 particles (sample D) prepared in the present study by a solid state reaction, the PL color becomes white green, as displayed in Fig. 5(d). The peak maximum is at 530 nm, with a narrower FWHM of 55 nm (0.24 eV), compared to the other three specimens. For this PL band, there are only two dissolved Gaussian peaks at 525 nm and 544 nm. Interestingly, similar Gaussian peak positions of these two can also be found in sample B and those reported by Lee [25] and Liu et al. [22] Except for the dissolved peak at around 499 nm, one can notice the high similarity between the PL band positions along with dissolved Gaussian peaks reported from the literature and those identified in our study. Therefore, the light emissions observed for samples B, C, and D are quite likely associated with the native defects present in Zn2GeO4 nanorods and particles, though further study is needed for a clearer luminescence mechanism. Based on this analysis, the red shift of PL peak of sample C can be explained by the variation of relative intensity of dissolved Gaussian peaks. One can clearly see from Fig. 5(b) that the dissolved Gaussian peaks of sample B have higher intensities toward shorter wavelengths, whereas sample C exhibits an inverse trend, resulting in the red-shift of PL peak position. Due to the smaller physical dimension, sample B would have a higher
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Gaussian peaks cannot be closely correlated to those of the other three specimens because this sample does not contain the crystalline Zn2GeO4 phase. However, the PL intensity remains strong. The origin of light emission may be quite different and can be attributed to some specific compositions within the wrinkled particles which also exhibit luminescence.
4. Conclusion In summary, Zn2GeO4 nanorods were synthesized using a simple reflux method where aqueous solutions of GeO2 and zinc acetate dihydrate were used as the precursors. The product with lower Zn2GeO4 molarity (sample B) is an aggregation of short nanorods with the diameter ranging from 30 to 50 nm, exhibiting a crystalline Zn2GeO4 phase of phenacite structure. If the Zn2GeO4 molarity in the solution was increased by 5 times, sample C, the nanorods became longer and tended to aggregate together, resulting in a larger physical dimension. An intense white-bluish light emission peaked at 475 nm was observed from sample B, and the emission band distributed from 350 to more than 600 nm. For sample C, the PL peak position red-shifted to 490 nm though the band profile remained similar. As a comparison, the PL of sintered Zn2GeO4 particles, sample D, exhibited a green luminescence. The light emission from Zn2GeO4 nanorods was associated with the native defects, and the red shift of PL peak could be interpreted by the variation of relative amount of different luminescent centers. Since the PL intensity of Zn2GeO4 nanorods is comparable to that of sintered particles, the reflux method provides not only a time-efficient but also a power-saving route to prepare luminescent Zn2GeO4 phosphor.
Acknowledgments This work was supported by the National Science Council of Taiwan under the Contract no. NSC 96–2221-E-007-006. We are grateful to Prof. Shangjr Guo and Mr. Chen-Ying Wu of the Department of Physics and Institute of MEMS, National Tsing Hua University for the PL measurements. References
Fig. 5. PL spectra of (a) sample A, (b) sample B, (c) sample C, and (d) sample D. The black curves are measured spectra, and the red ones are obtained by Gaussian fitting. The dissolved Gaussian peaks are illustrated in blue dashed lines. The optical photographs for light emissions were taken by a digital camera after the samples had been irradiated by the laser. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
specific surface area. Thus, the ratio of native defects on the surface to those within the bulk may be different from that of sample C, leading to different amount of luminescent centers. Furthermore, the dissolved peak at around 499 nm may be caused by the specific reflux solution process, since it is hardly found in Zn2GeO4 obtained by solid state reactions (i.e., sample D and those reported by Lee [25] and Liu et al.[22]). As for the luminescence from sample A, it is reasonable that the dissolved
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