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Approach for Selection of a Synthesis Procedure of GeO₂ Ultra-small Nano Particles and Its Characterization

Mrinal Seal, Sampad Mukherjee*

Department of physics, Indian Institute of Engineering Science and Technology, Howrah, India

Abstract: A rigorous study of germanium oxide nano-particle synthesis is done by hydrothermal method. An optimum synthesis condition to obtain the stable and ultra-small size as of 10 nm of the material is determined by characterizing the prepared samples with X-Ray diffraction pattern analysis and TEM. The sample with smallest particle size is also characterized with HRTEM, PL and FTIR spectroscopy. The characterization results are analyzed accordingly. The most remarkable feature of the ultra small sized sample as observed is the current-voltage characteristic, which has been explained with oxygen vacancy phenomenon.

Keywords: hydrothermal synthesis, nanostructures, crystal growth, oxides, strain

Received April 02, 2015; Revised April 21, 2015; Accepted May 04, 2015

Citation: Mrinal Seal, Sampad Mukherjee, Approach for Selection of a Synthesis Procedure of GeO₂ Ultra-small Nano Particles and Its Characterization. International Journal of Physics, 3 (3). 133-138. January 2015. doi:10.12691/ijp-3-3-7

1. Introduction

The unique properties of Germanium oxide (GeO₂), particularly in nano form attract great interest of scientists, which involve them to synthesize and characterize this material. These studies are very much devoted in applications like electro optical modulators, piezoelectric glass materials, optical fibre materials, non-linear optical applications [1,2] etc. Regarding the application, a great attention is paid to synthesis a particular stable nano structure of GeO₂ [3,4]. Among various well-established synthesis procedures (physical evaporation, electro-spinning process, chemical precipitation), hydrothermal root is easier comparably and advantageous regarding control over the optimization of shape, size as well as developing of particular phase of the material. Besides optical properties [1,2], GeO₂ in nano-structured form can exhibit typical property like charge storing effect [5] as contradictory to its bulk behaviour. Resistance switching effect is also a typical property, which is still unexplored for GeO2 in nano-structured form. Although some insulating oxides having similar chemical structure of GeO2, like titanium oxide (TiO₂) [6], cerium oxide (CeO₂) [7] shows the above-mentioned behaviour, which has drawn extensive interest to the research community. Such resistance-switching phenomenon has been explained with the help of several theoretical models, like Mott transition [7], Schottky barrier behaviour at the interface [8,9], charge trapping and detrapping [10], polaron melting and ordering [11] and Oxygen vacancy diffusion [12,13]. Apart from those theoretical models, an experimental evidence of resistance switching effect due to such oxygen vacancies in CeO₂ is reported by P. Gao et.al [14]. A few earlier reports show that there exist a correlation of lattice distortion and electrical properties for few materials [15,16].

However, it is found that, researchers are not engaged with rigorous studies on the preparation and several types of characterization to make a correlation between the synthesis procedures and their different types of physical properties so that a standard method can be established, which will be helpful to the predecessor. Therefore, in this communication, we report the synthesis of GeO₂ nanoparticles of different particle sizes by hydrothermal technique at different synthesis pressure / temperature, Characterization of these particles with X-ray diffraction (XRD) and Transmission electron microscopy (TEM). From these, a correlation of particle size of synthesized materials with the pressure of synthesis is tried to develop. However, the reason for choosing GeO₂ for the present work is to see whether we are able to change its insulating behavior and any other physical properties in nano ordered particle size. The correlation that we have obtained from TEM study allows us developing especially small sized nanoparticles, which differ by physical properties very much from their relatively larger sized counterpart. Different types of characterization like, High-resolution transmission electron microscopy (HRTEM), Selected Area Electron Diffraction (SAED), Photoluminescence spectroscopy (PL), Fourier transform infrared spectroscopy (FTIR) and measurement of Current –Voltage (I-V) characteristics have been made. The results are explained with the help of established model. Such current-voltage characteristics of the synthesized GeO₂ nanoparticles support that the material can be used for generation of nonvolatile memory devices (NVM) [17].

2. Experimental Procedure

Analytical grade GeO₂ powder (Alfa Aesar, 99.99%) is used to prepare a stock solution with moderately hot double distilled water. Twenty milliliter of this solution is taken in an in-house designed autoclave attached with a pressure gauge. After sealing properly, the autoclave is heated in a furnace at different required temperatures corresponding to various pressures measured with pressure gauge attached with the autoclave as mentioned in Table 1 for twenty-four hours. After that, the autoclave is cooled naturally to ambient temperature and the samples are collected from the product solution by evaporating water.

Collected samples were grinded with mortar and pestle and prepared for different types of characterization. TEM (TECNEI G2) and HRTEM (TECNEI20 with EBS) photographs are obtained after preparing the grids following conventional methods. SAED has been performed on a chosen area of the sample. The XRD pattern is obtained using Mini Flex2 X-ray powder diffractometer with Cukα (wave length-1.54Ǻ) radiation and operating in a step scan mode with 0.01⁰ in 2θ per step. PL spectroscopy is performed with Spectrophotometer (Perkin Elmer Model: Lambda45) at different excitation energies. FTIR (Spectrum-RX1 of Parkin Elmer) spectroscopy has been done by making pellet of the sample. After the hydrothermal treatment at 150 bar pressure, 2ml of the product solution is taken and poured on a piece of glass slide drop wise with a moderate heating at 40⁰C to prepare a film of the sample. Ohomic contact is made with double-sided carbon tape and copper wire. A data acquisition system of Hewlett-Packard and a variable voltage source is connected across the film to get voltage-resistance data of the sample. Current across the sample is obtained by taking the ratio of voltage and resistance.

Table 1. Samples as synthesized hydrothermally at different pressure corresponding to temperature

3. Results and Discussions

Figure 1 Shows the XRD patterns of the samples Ge1-Ge5 as well as Ge7. This substantiate that the nature of the synthesized samples are crystalline having α-quartz type hexagonal structure and of phase P3₂21 (as mentioned in JCPDS data, PDF-85-1519). The peaks at each XRD patterns marked as (100), (011), (110), (102), (111), (200), (112), (202) and (121) are quite similar in broad view. However, the magnified view of these peaks differs in broadening, which increase with the synthesis temperature, similar as reported earlier [18,19].

Figure 1. XRD pattern of samples Ge1-Ge5 and Ge7

Figure 2. TEM images of samples Ge1-Ge3.

Figure 3. TEM images of samples Ge4 and Ge5.

The TEM images as observed for the samples Ge1- Ge5 are shown in Figure 2 and Figure 3. These images shows that the particles developed within each samples are uniform and identical. This arouses us to calculate average particle size from each TEM image with the help of Image-J software. The obtained particle sizes respects to the samples are presented in Table 1. Taking into account the particles of all the samples as observed in the TEM images are spherical in nature we have calculated the volume (V) of particles for each samples by considering the particle size as diameter. Considering the particle size of sample Ge1as initial form of the material, we have calculated the differences in volume (ΔV) of the respective samples (Ge2-Ge5). This allows us to get the volume strain by taking the ratios of ΔV and V (volume of the sample Ge1) for the samples Ge2-Ge5. The volume strains are plotted against the synthesis pressures as shown in Figure 4 and we have recognized this plot similar to well establish stress-strain plot.

Figure 4. Variation of relative volume strain (ΔV/V) corresponding to synthesis pressure. (The solid line guides the eye.)

After finding this plot, we deliberately go for a synthesis at such pressure that is well above the synthesis pressure of sample Ge5. Figure 5 shows the TEM image of this sample (Ge6).

Figure 5. TEM image of sample Ge6

From this image, it is observed that a single grain of the material bursts into several small fragments and we have calculated the average grain size of these smaller fragments from Figure 6 (TEM image of Ge6 magnifying the smaller fragments only) with the above-mentioned process as used for the grain size calculations of sample Ge1- Ge5.

Figure 6. TEM image of sample Ge6 (magnifying smaller fragments)

It is observed that the particle size of sample Ge7 as shown in Table 1 is changed abruptly relative to the previous. Therefore, we suggest the importance of the stress-strain like plot (suggested first as our knowledge is concerned) in synthesis of nano particles, which is for not only GeO₂, but can also be generalized for nano particle synthesis of other materials. However, our aim is to find a standard process to synthesis smallest ever grain of GeO₂. That is why we have prepared a sample (Ge7) at a synthesis pressure far beyond the synthesis pressure of Ge6. We observe from the HRTEM image that the grain size of this sample lowers to the order of 10 nm as shown in Figure 7.

Figure 7. HRTEM image of sample Ge7 with SAED pattern inset

The SAED pattern as shown inset of Figure 7 confirms the crystalline nature of the sample. The spacing between two successive atomic lines (dhkl) is calculated from the SAED pattern as 0.224 nm, which is very much comparable to the value of dhkl as observed in HRTEM image.

The PL characteristics of the samples, Ge1 to Ge5 have already reported [20]; therefore, we are interested for photoluminescence characteristics of the sample ‘Ge7’ under different excitation wavelengths (viz. 325nm, 300 nm, 275 nm and 250nm). Figure 8 shows the respective PL spectra for the sample Ge7.

Figure 8. PL Spectra of the sample Ge7at different excitation energies

From Figure 8, it is observed that the peak at 427nm remains unchanged with the increase of excitation energy. This indicates blue light emission due to the oxygen-vacancies and oxygen-germanium vacancy centre [21-24]. Another peak is observed at 371 nm for the excitation wavelength 325nm, which shifts towards smaller value of wavelength regions as the excitation wavelength decreases. This phenomenon is being observed probably due to the Raman scattering [20]. However, it is to be mentioned here that sample Ge6 is in critical point for transition so we are least interested to study the physical properties of this particular one.

The FTIR spectrum as observed for the sample (Ge7) is shown in Figure 9. A broad peak is observed at 556 cm⁻¹ instead of three separate peaks in this region as reported due to hexagonal structure of GeO₂ [25,26,27,28]. The broadness of these three peaks is possibly due to the very small particle sizes of the particles. There is no peak observed at 880 cm⁻¹. Therefore, the sample does not maintain any layered structure as reported before²¹. The peak observed at 857 cm⁻¹ supports the existence of oxygen vacancy in the sample [29]. The peak at 1645 cm⁻¹ as well as one of its harmonics at the vicinity of 3450 cm⁻¹ confirms the presence of hydroxyl (–OH) group [27], which is probably responsible for the water absorption properties of the sample as contradictory with the properties of bulk GeO₂.

Figure 9. FTIR Spectrum of sample Ge7

Figure 10. I-V characteristic of sample Ge7

A typical voltage-current (V-I) characteristic as obtained across the sample at room temperature is shown in Figure 10. It shows that the current reaches at a peak value of 18.75mA at 0.03 volt and decreases with the voltage and then shows the bulk behaviour. This phenomenon repeats as the polarity of the voltage changes. Presence of oxygen vacancy in the sample is responsible for such incident. An evidence of similar type of phenomenon in Cerium Oxide (CeO₂) has been observed [14]. Due to a fair structural similarity of CeO₂ and GeO2, we propose that the similar phenomena are responsible for the I-V characteristic as mentioned for CeO₂. However, in GeO₂ molecule, Ge atom shares two electrons from ‘4p’ band to oxygen ‘2p’ band. Due to oxygen vacancy, if an oxygen atom leaves its lattice position an electron at ‘4p’ band of Ge atom becomes unpaired, which increase the possibility of electron hopping. Besides this, as proposed by A.V. Shaposhnikov et.al. [17] oxygen vacancies remaining in GeO₂ is a type of defect and defects generate a disturbance in the lattice periodicity through the crystal. Such irregularity is responsible for the conducting path between the electrodes [30]. Therefore, the carriers produced due to the formation of oxygen vacancy conduct as the voltage is applied. The number of carrier increases with the increasing voltage and reaches at the peak value and subsequently the number of carriers available for conduction drops. Therefore, current is found negligible as comparable to the bulk behaviour, which is shown in Figure 11. The concentration of the oxygen vacancy has been estimated as shown in Table 2. The result is quite similar to the earlier report [31].

Figure 11. I -V characteristic of bulk GeO₂

Table 2. Oxygen vacancy concentration in sample ‘Ge8’ as obtained from I-V curve

4. Conclusion

An extensive study on the process of synthesis of GeO₂ nano materials has been done by treating the material hydrothermally at different successive temperatures and pressures. From this, we are trying to establish a stress-strain like plot that is most useful to synthesis the ultra small size. The TEM images of the samples reveals that the particles produced for each sample are uniform, which indicates that particles of equal size distribution can be produced with hydrothermal technique. The HRTEM image and SAED pattern of the ultra small sample shows crystalline nature. The distance between the atomic planes as observed from HRTEM micrograph and calculated from SAED pattern are in fair agreement. The PL spectroscopy of the sample reveals that the material contains oxygen-germanium vacancies. The FTIR spectroscopy analysis indicates the presence of water, which is also a typical property of this particular sample in contradiction with bulk GeO₂ along with the nanoparticles formed in comparatively lower synthetic pressure. The FTIR spectrum also reveals the eradication of traditional layered structure as observed in the bulk sample. Moreover, a current-voltage characteristic of this sample has been obtained, which is not observed in any other GeO₂ samples (bulk along with the nanoparticles formed in comparatively lower synthetic pressure) as prepared before. The reason behind this phenomenon is probably due to the increased number of oxygen vacancies (which increases the available carrier for transport) for decrement of particle size.

Acknowledgement

We acknowledge Indian Institute of Chemical Biology for helping us for performing experiments with TEM. Authors are thankful to Department of Atomic Energy, Government of India for providing partial financial support.

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