Ca2Ge7O16 nanowires grown from CaO and GeO2

Ca₂Ge₇O₁₆ nanowires grown from CaO and GeO₂

Li-Zhai Pei^*; Yin-Qiang Pei; Yong Yang; Qian-Feng Zhang

Key Lab of Materials Science and Processing of Anhui Province, Institute of Molecular Engineering and Applied Chemistry, School of Materials Science and Engineering, Anhui University of Technology, Ma'anshan, Anhui 243002, P. R. China

ABSTRACT

Single crystalline Ca₂Ge₇O₁₆ nanowires have been synthesized using CaO and GeO₂ as the raw materials. Various factors that affect the formation and size of the Ca₂Ge₇O₁₆ nanowires have been analyzed. The obtained products are characterized by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The results show that the calcium germanate nanowires are composed of orthorhombic Ca₂Ge₇O₁₆ phase with the length and diameter of several dozens of micrometers and about 50 nm, respectively. Hydrothermal temperature plays an important role on the formation and growth of the Ca₂Ge₇O₁₆ nanowires. The formation process of the Ca₂Ge₇O₁₆ nanowires is initially interpreted according to the shape evolution of the products obtained from different growth conditions.

Keywords: Ca₂Ge₇O₁₆ nanowires, CaO, GeO₂, electron microscopy

1. Introduction

Ternary germanate nanowires have been attracted great attention as important one-dimensional (1D) nanomaterials for the application of electrochemical sensors, optical devices, electron devices and catalysts^1-4. Several kinds of germanate nanowires, such as Zn₂GeO₄^[5,6], PbGeO₃^[7] and strontium germanate⁸ have been reported previously. Among these germanates, calcium germanate is a kind of excellent optical material exhibiting strong fluorescence emission at 620, 700 and 800 nm⁹. Perng et al.¹⁰ reported the synthesis and photoluminescence of amorphous Ca₅Ge₂O₉ nanowires by dehydrating the hydrated Ca₅Ge₂O₉ nanowires originated by submersing Ge nanoparticles into calcium hydroxide aqueous solution. However, Ge nanoparticles with the size ranging from 10-50 nm need be firstly prepared by a vapor condensation technique taking the complexity and expensive apparatus for the synthesis of the calcium germanate nanowires. Very recently, different from the amorphous calcium germanate nanowires, crystalline calcium germanate nanowires have been synthesized by a simple hydrothermal process using GeO₂ and Ca(CH₃COO)₂·H₂O as the raw materials¹¹. However, the obtained crystalline calcium germanate nanowires consist of a mixed germanate phases with tetragonal Ca₂GeO₄, orthorhombic Ca₂Ge₇O₁₆ and triclinic CaGe₂O₅ phases. It is difficult to gain single phase using GeO₂ and Ca(CH₃COO)₂·H₂O as the raw materials. In addition, the cost of Ca(CH₃COO)₂·H₂O is high which may confine the possible application of calcium germanate nanowires. Therefore, it is of important significance for the low-cost synthesizing calcium germanate nanowires with single phase.

CaO, as a kind of cheap Ca raw material, is slightly soluble to form Ca(OH)₂ in water becoming a proper Ca source material for the synthesis of 1D Ca-based nanoscale materials^12,13. In the paper, single crystalline Ca₂Ge₇O₁₆ nanowires have been synthesized using GeO₂ and CaO as the raw materials. Using CaO instead of Ca(CH₃COO)₂ as the Ca raw material also saves the cost of the calcium germanate nanowires. The effects of hydrothermal temperature and time on the formation and size of the Ca₂Ge₇O₁₆ nanowires are demonstrated. And the growth process of the Ca₂Ge₇O₁₆ nanowires is discussed.

2. Experimental

High pure GeO₂ (purity: >99.99 wt. (%)) and CaO (purity: >99.9 wt. (%)) were purchased from Sinopharm Chemical Reagent Co., Ltd. of China. All source materials were used without further purification. In a typical procedure, 0.16 g GeO₂ and 0.22 g CaO were dissolved in 60 mL deionized water under vigorous stirring. Then, the mixture was placed in a 100 mL autoclave with a Teflon liner. The autoclave was maintained at 180ºC for 24 hours. Subsequently the autoclave was cooled naturally in air. The resulting white precipitates were filtered, washed with deionized water for several times and dried at 60ºC in air. Finally, the white powders were obtained.

The obtained products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). XRD pattern was carried out on a Bruker AXS D8 X-ray diffractometer equipped with a graphite monochromatized Cu-Kα radiation (λ = 1.5406 Å). The samples were scanned at a scanning rate of 0.05º/s in the 2θ range of 20º ~ 80º. SEM observation was performed using JEOL JSM-6490LV SEM with a 15-kV accelerating voltage. TEM and HRTEM samples were prepared by putting several drops of solution with calcium germanate nanowires onto a standard copper grid with a porous carbon film after the nanowire samples were dispersed into distilled water and treated for about 10 minutes using supersonic wave apparatus. TEM and HRTEM observations were performed using JEOL JEM-2100 TEM operating with 1.9Å point-to-point resolution operating at 200-kV accelerating voltage with a GATAN digital photography system.

3. Results and Discussion

Figure 1 shows the XRD pattern of the products obtained from the hydrothermal conditions of 180ºC for 24 hours. All of the diffraction peaks for the products can be assigned to the orthorhombic phase of Ca₂Ge₇O₁₆ (JCPDS card No. 34-0286). No characteristic peaks from impurities are observed demonstrating that the high pure Ca₂Ge₇O₁₆ nanowires can be synthesized using CaO as the Ca raw material.

The morphology and size of the Ca₂Ge₇O₁₆ nanowires are investigated by SEM, TEM and HRTEM. Figure 2 is the SEM images of the Ca₂Ge₇O₁₆ nanowires with different magnifications synthesized at 180ºC for 12 hours. A large amount of uniform Ca₂Ge₇O₁₆ nanowires are achieved according to Figure 2. No other morphologies are observed showing the highly pure nanowire-shaped structure of the products. The length of the nanowires is several dozens of micrometers, even longer than 100 micrometers. The nanowires with smooth surface are straight. The magnified SEM image (Figure 2b) further shows that the Ca₂Ge₇O₁₆ nanowires appear as uniform nanowire-shaped structure. The Ca₂Ge₇O₁₆ nanowires have a uniform diameter distribution with average diameter of about 50 nm. The morphology and size of the Ca₂Ge₇O₁₆ nanowires are similar to those synthesized using GeO₂ and Ca(CH₃COO)₂·H₂O as the raw materials¹¹.

The detailed structure of the Ca₂Ge₇O₁₆ nanowires is further analyzed by TEM and HRTEM which are shown in Figure 3. Figure 3a shows the typical TEM image of the as-synthesized calcium germanate nanowires with the diameter of about 50 nm which is similar to those of the SEM observations. The nanowires are straight and have smooth surface. The diameter of the nanowires is uniform throughout the length. It is noticed that the Ca₂Ge₇O₁₆ nanowires have flat tips. The flat tips are similar to those of the germanate nanowires prepared by other methods^4,7,8,11. Figure 3b displays the HRTEM image of single calcium germanate nanowire. The lattice fringes distinguished exhibit good single crystalline in nature demonstrating that the Ca₂Ge₇O₁₆ nanowires are composed of single crystalline structure. The interplanar spacing of the crystalline is about 0.796 nm according to the HRTEM measurement and the subsequent calculation by the software of Digital Micrograph (Gatan Inc., Pleasanton, CA) applied in the HRTEM, which is the same as the interplanar spacing for the {110} plane of orthorhombic Ca₂Ge₇O₁₆. Combined the XRD pattern with the HRTEM image of the nanowires, the strong diffraction peak of (320) indicates that the main growth orientation of the calcium germanate nanowires is <320> crystallographic direction. In addition, some nanowires are also observed to originate from <322> and <330> growth direction, respectively.

The growth conditions, such as hydrothermal temperature and time on the formation and growth of the calcium germanate nanowires are analyzed so as to research the possible formation process of the calcium germanate nanowires. Figure 4 is the SEM images of the calcium germanate nanowires obtained at 180ºC for different time. Calcium germanate nanowires with similar morphology can still be obtained when the reaction time is 0.5, 6, 12 and 48 hours, respectively. The length of the calcium germanate nanowires obtained from different time is similar with several dozens of micrometers. However, the diameter of the calcium germanate nanowires decreases obviously with the decrease of the reaction time at the same hydrothermal temperature. The average diameter of the calcium germanate nanowires is about 30 nm when the reaction time is 0.5 hours (Figure 4a). But the diameter of the calcium germanate nanowires also increases to about 300 nm with the reaction time increasing to 48 hours at 180ºC . The time dependence results show that the reaction time plays an important role on the size of the calcium germanate nanowires. The diameter and length of the calcium germanate nanowires can be adjusted by controlling the reaction time.

 

Figure 5 shows the effect of the hydrothermal temperature on the formation and growth of the calcium germanate nanowires. Obviously, only nanorods with the length of about 1 µm and a small amount of nanoparticles exist in the products (Figure 5a) with the hydrothermal temperature decreases to 80ºC for 24 hours. When the hydrothermal temperature increases to 120ºC for 24 hours, the products are mainly composed of nanowires with the length of several dozens of micrometers (Figure 5b) which is similar to those prepared from 180ºC for 24 hours. However, it is interesting that some nanoparticles attach at the tips of the nanowires which is shown in Figure 5b and 5c. With the hydrothermal temperature further increases to 160ºC for 24 hours, the nanoparticles in the nanowires disappear and pure calcium germanate nanowires are obtained (Figure 5d). The results suggest that the calcium germanate nanowires originate from the short nanorods and nanoparticles. Therefore, hydrothermal temperature plays an essential role on the formation and growth of the calcium germanate nanowires.

 

The diameter of the calcium germanate nanowires decreases obviously with the decrease of the reaction time, which illustrates that the growth direction is along horizontal direction. However, the length of the nanowires increases with increasing the hydrothermal temperature, which means that the growth direction for these nanowires is along vertical direction. The growth velocity of the nanowires along the horizontal direction is far slower than that of the nanowires along the vertical direction.

The quantitative analysis of the chemical composition of the product obtained from the hydrothermal conditions of 120ºC for 24 hours (Figure 5b) using SEM-EDS experiment was performed by the software applied in the Link ISIS300 EDS. The EDS spectra of the nanowires and nanoparticles attached in the nanowires are shown in Figure 6. Elements Ca, Ge and O originate from the calcium germanate product and Cu arises from the copper sheet substrate. The atomic ratio of Ca:Ge:O of the nanowires is about 2:6.9:16 showing the composition of the calcium germanate nanowries is very similar to Ca₂Ge₇O₁₆. However, the atomic ratio of Ca:Ge:O of the nanoparticles attached in the nanowires is about 2:6.8:22.6. Obviously the O content of the nanoparticles attached in the nanowires is more than that of the nanowires. It is considered that Ca₂Ge_6.8O_22.6 in the nanoparticles further reacts with H₃GeO₃ to form Ca₂Ge₇O₁₆.

Figure 7 shows the XRD patterns of the products synthesized from 80 and 120ºC , respectively for 24 hours. Obviously, the intensity of the diffraction peaks increase with the increase of the reaction temperature. The XRD patterns of the products are still similar which is mainly composed of orthorhombic Ca₂Ge₇O₁₆. However, besides the orthorhombic Ca₂Ge₇O₁₆, some diffraction peaks of the monoclinic CaGe₂O₅ (JCPDS card No. 21-0797) occur in the XRD patterns of the product at the initial reaction stage (Figure 7a). With the increase of the hydrothermal temperature, the most of the diffraction peaks of the product disappears obviously. Only the diffraction peaks of the orthorhombic Ca₂Ge₇O₁₆ exist in the product with the hydrothermal temperature further increasing to 180ºC . The results demonstrate that the two phases with orthorhombic Ca₂Ge₇O₁₆ and monoclinic CaGe₂O₅ structures form at the initial formation stage of the calcium germanate nanowires, and temperature plays an essential role on the formation of pure Ca₂Ge₇O₁₆ nanowires.

It is clear Ca₂Ge₇O₁₆ nanowires are formed by the hydrothermal treatment of CaO and GeO₂, but the reasons for formation Ca₂Ge₇O₁₆ nanowires with single phase is still puzzling. It is noticed that the CaO and the phases formed at the initial reaction stage play essential role on the formation and growth of Ca₂Ge₇O₁₆ nanowires. Only orthorhombic Ca₂Ge₇O₁₆ and a small amount of monoclinic CaGe₂O₅ structures form at the initial reaction stage of CaO and GeO₂. The monoclinic CaGe₂O₅ phase disappears with the increase of the hydrothermal temperature. Therefore, based on the experimental results, the possible formation process of the calcium germanate nanowires are proposed, which is depicted in Figure 8. At the initial reaction stage, H₂GeO₃ forms from the reaction of GeO₂ and H₂O. Ca(OH)₂ originates from CaO and H₂O. Therefore, many nanoscale spherical particles spontaneously appear in the supersaturated solution through the hydrothermal reaction of H₂GeO₃ and Ca(OH)₂ forming Ca₂Ge₇O₁₆ and CaGe₂O₅. Then the nanoparticles serve as the crystalline nuclei for the anisotropic growth of the calcium germanate nanocrystals. The linear growth is attributed to the preferential adsorption of nanoparticles to special crystal facets, which directs the growth of the nanoparticles into nanorods by controlling the growth rates along different crystal axes^14,15. With the reaction going on, the smaller nanoparticles vanish at the site of the longer nanorods through an "Ostwald ripening" process due to their higher surface free energy compared with that of the longer nanorods^16,17 which is confirmed by the SEM images of Figure 5b and 5c. With the increase of hydrothermal temperature and reaction time, the nanorods grow continuously and CaGe₂O₅ phase disappears resulting in the final formation of the Ca₂Ge₇O₁₆ nanowires.

4. Conclusions

In summary, Ca₂Ge₇O₁₆ nanowires have been synthesized using CaO as the Ca source material by a simple hydrothermal process. The nanowires are composed of orthorhombic Ca₂Ge₇O₁₆ phase with average diameter of about 50 nm and length of several dozens of micrometers, even longer than 100 µm. Hydrothermal temperature plays an essential role on the formation and growth of the calcium germanate nanowires. Using cheap CaO instead of Ca(CH₃COO)₂ saves the cost of the calcium germanate nanowires. The experimental results suggest that the calcium germanate nanowires are formed via an "Ostwald ripening" growth process.

Acknowledgements

This work was supported by the Natural Science Foundation of the Education Bureau of Anhui Province of China (KJ2011A042), Innovative Research Fundation of Postgraduate of Anhui University of Technology (2011005) and National Basic Research Program of China (863 Program, 2009AA03Z529).

Received: June 27, 2011; Revised: September 7, 2011

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Publication Dates

• Publication in this collection
  29 Nov 2011
• Date of issue
  Feb 2012

History

• Received
  27 June 2011
• Reviewed
  07 Sept 2011