Fast and safe synthesis of micron germanium in an ammonia atmosphere using Mo₂N as catalyst

Abstract

Here, we reported a new method for fast and safe synthesis of a micron germanium (Ge) semiconductor. The Ge was successfully prepared from mixed GeO₂ with a low amount of MoO₃ by the NH₃ reduction method at 800 °C for an ultra-short time of 10 min. XRD patterns show that the Ge has a tetragonal structure. SEM images show that the size of the Ge particles is from 5 μm to 10 μm, and so it is on the micron scale. UV-visible diffuse reflectance spectroscopy shows that the Ge has good light absorption both in the ultraviolet and visible regions. The formation of Ge mainly goes through a two-step conversion in the NH₃ flow. Firstly, GeO₂ is converted to Ge₃N₄, and then Ge₃N₄ is decomposed to generate Ge. The comparison experiments of MoO₃ and Mo₂N demonstrate that Mo₂N is the catalyst for the Ge synthesis which improves the Ge₃N₄ decomposition. The presented fast and safe synthesis method of Ge has great potential for industrialization and the proposed Mo₂N boosting the Ge₃N₄ decomposition has provided significant guidance for other nitride decomposition systems.

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1 Introduction

With the development of advanced electronic information industries, semiconductor materials have attracted widespread attention due to their unique optical and electrical properties.^1–6 Ge as an important semiconductor material with stable chemical properties and obvious non-metallic properties has many advantages, such as non-toxicity, biocompatibility, electrochemical stability, current microelectronics compatibility, etc.^7–13 At present, Ge has extensive and important applications in fields such as semiconductors, aerospace measurement and control, nuclear physics detection, optical fiber communication, infrared optics, solar cells, chemical catalysts, and biomedicine.^14–20 Ge is one of the most dispersed elements and there is almost no concentrated Ge deposit in the Earth's crust. Thus, it is particularly important to find suitable methods to enrich, prepare and purify Ge.

The synthetic method of Ge can be divided into physical and chemical methods. The physical methods mainly include chemical vapor deposition,²¹ gas phase pyrolysis,²² plasma technology,^23–25 sputtering,^26,27 etching,^28,29 laser ablation³⁰ and so on. However, these technological processes need extreme temperatures and pressures and are expensive. Chemical methods mainly include room temperature reduction of a GeCl₄/Br₄ precursor,^31,32 high temperature reduction of a GeI₂/I₄ precursor,^33,34 a one-step synthesis method,^35,36 an electrodeposition method^37–41 and so on. In the industry, rough Ge is commonly prepared by the reduction of GeO₂ in hydrogen flow at 650–680 °C, and then Ge with high purity is obtained by chemical gasification and decomposition of rough Ge. However, in the process of preparing rough Ge, the produced hydrogen is a dangerous gas due to its large range of the explosion limit. In addition, a leak of hydrogen is not easy to discover due to it being colorless and tasteless. So, it is necessary to develop and improve a safe and low-cost method to produce rough Ge.

Ammonia is an important chemical raw material, which has important applications in industrial and agricultural production. At the same time, ammonia is also a reducing gas capable of reducing various metal oxides.^42–45 Here, we successfully prepared micron Ge material in ammonia atmosphere using GeO₂ as raw material with some MoO₃ in 10 min. The transformation process from GeO₂ to Ge and the molybdenum based catalyst were determined. The synthesis method of Ge has the advantages of safe, fast and low cost, which is beneficial to industrial production.

2 Experimental

2.1 Synthesis of Ge

All the chemical agents are from China National Pharmaceutical Group Corporation. GeO₂ as raw material and a small amount of MoO₃ are mixed and milled in a mortar for 30 minutes. The mixed samples are the calcined in an ammonia flow of 100 mL min⁻¹ at different temperature and for different time. The synthesis of Ge in H₂ flow is same as that in NH₃ flow except H₂ instead of NH₃.

Also, in this paper A/B means A mixed with B. For example, Mo₂N/GeO₂ means Mo₂N mixed with GeO₂.

2.2 Characterization

The structure of the as-prepared Ge was determined by X-ray diffraction (D/MAX2500, Rigaku, Japan) with a Cu-K_α radiation at a voltage of 4 kV at room temperature. The ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) were obtained on a spectrometer (U-4100), and BaSO₄ was used for the corrected base line. The size and morphology were examined by scanning electron microscopy (SEM, JSM7500F) with the accelerating voltage of 0.5–30 kV and transmission electron microscopy (TEM, F20) with the accelerating voltage of 120 kV.

3 Results and discussion

3.1 Synthesis of Ge in NH₃ flow using Mo based catalyst

Fig. 1 shows the XRD patterns of GeO₂ calcined at different temperatures in an ammonia atmosphere. The XRD of the GeO₂ is also present for comparison. When the reaction temperature is lower than 800 °C, GeO₂ (PDF # 36-1463) still remain unchanged. At 800 °C, all GeO₂ changed into Ge₃N₄ (PDF # 38-1374). At 900 °C, a part of Ge (PDF # 04-0545, 2θ = 27.5°) generates which indicates a small part of Ge₃N₄ decomposes at overhigh temperature.

Fig. 1 XRD patterns of commercial GeO₂ and GeO₂ calcined in NH₃ flow at different temperatures for 2 h.

Fig. 2 shows the XRD patterns of 5.0 wt% MoO₃/GeO₂ calcined in NH₃ flow at different temperatures for 2 h. It can be seen that when the reaction temperature is at or above 800 °C, almost all the GeO₂ are changed into Ge (PDF # 04-0545). Comparing with Fig. 1, the introduction of the Mo element decreases the reaction temperature of Ge generation over 100 °C.

Fig. 2 XRD patterns of commercial GeO₂ and samples of 5.0 wt% MoO₃/GeO₂ calcined in NH₃ flow at different temperatures for 2 h.

Fig. 3 shows XRD patterns of 5.0 wt% MoO₃/GeO₂ calcined in NH₃ flow at 800 °C for different time. The process of Ge generation is ultra fast and almost all the GeO₂ can be reduced to Ge in 10 min. Fig. 4 shows the XRD pattern of MoO₃/GeO₂ with different mass ratio calcined in ammonia flow. When the amount of MoO₃ is less than 5.0%, the reaction product is Ge and Ge₃N₄ (e.g. the sample of 2.0 wt% MoO₃/GeO₂). When the amount of MoO₃ is 5.0%, almost all the GeO₂ is converted to Ge. In view of the fact of that GeO₂ changed into Ge₃N₄ (Fig. 1, 800 °C) without any catalyst, we inferred the process Ge generation is that first the GeO₂ transfers into Ge₃N₄, and then the Ge₃N₄ decomposes into Ge with the Mo base catalyst. This ultra fast method of Ge generation using Mo based catalyst has major advantages of low cost and high efficiency.

Fig. 3 XRD patterns of commercial GeO₂, 5.0 wt% MoO₃/GeO₂ and 5.0 wt% MoO₃/GeO₂ calcined in NH₃ flow at 800 °C for different time.

Fig. 4 XRD patterns of GeO₂ and MoO₃/GeO₂ with different MoO₃ loading mass ratio calcined in NH₃ flow at 800 °C for 0.5 h.

3.2 Determination of the catalyst of Ge generation

It is well known the molybdenum oxide will change into molybdenum nitride in NH₃ atmosphere at high temperature. So, it is necessary to judge which Mo species play the catalytic role in the Ge generation. Fig. 5 shows XRD patterns of MoO₃ calcined in the NH₃ flow at different temperatures. At 600 °C, some MoO₃ (PDF # 35-0609) is reduced into MoO₂ (PDF # 32-0671). When the temperatures reach at and above 700 °C, all MoO₃ is converted to Mo₂N (PDF # 25-1366). In Fig. 2, Ge is generated at 800 °C from MoO₃/GeO₂. At this moment (800 °C), the MoO₃ had been reduced into Mo₂N (Fig. 5, 800 °C), and the GeO₂ had been reduced into Ge₃N₄ (Fig. 1, 800 °C). These results indicate that the Mo₂N generated by MoO₃ acts as a catalyst and improves the decomposition of the Ge₃N₄ into Ge.

Fig. 5 XRD patterns of MoO₃ and MoO₃ calcined in NH₃ flow at different temperatures for 2 h.

In order further determine the origination (Mo₂N or MoO₃) of the catalysis role in Ge generation, the calcination experiments in nitrogen flow using GeO₂ and Ge₃N₄ as raw materials have been designed. Fig. 6 shows the XRD patterns of Mo₂N/GeO₂, MoO₃/GeO₂, Mo₂N/Ge₃N₄ and MoO₃/Ge₃N₄ calcined in N₂ atmosphere at 800 °C for 2 h. It can be seen that there are all a small amount of Ge generated by the addition of Mo₂N whether the raw material is GeO₂ or Ge₃N₄. However, by the addition of MoO₃, there is no Ge generation for both MoO₃/GeO₂ and MoO₃/Ge₃N₄. These again demonstrate that Mo₂N plays the catalysis role in Ge generation, whereas MoO₃ is not the catalyst.

Fig. 6 XRD patterns of GeO₂, 5.0% Mo₂N/GeO₂, 5.0% MoO₃/GeO₂, 5.0% Mo₂N/Ge₃N₄ and 5.0% MoO₃/Ge₃N₄ calcined in N₂ atmosphere at 800 °C for 2 h.

3.3 Comparison with the industrial preparation method of Ge in H₂ flow

In order to compare our preparation method with industrial method, GeO₂ is reduced into Ge in hydrogen atmosphere has been carried out. Fig. 7 shows the XRD patterns of GeO₂ calcined in hydrogen flow at different temperatures for 4 hours. At or over 500 °C, Ge is generated in H₂ flow, and 650 °C is chosen generally in the industry. Fig. 8 shows the XRD pattern of GeO₂ calcined in hydrogen flow at 500 °C for different time. Ge can be produced in the H₂ flow in 2 hours. So, compared with the industrial method in the H₂ flow, our preparation method in the NH₃ flow with Mo₂N catalyst has the advantages of ultra short reaction time (10 min vs. 2 h) and more safety (NH₃ vs. H₂), though the reaction temperature is higher (800 °C vs. 650 °C). We also performed the experiment which the GeO₂ was calcined in H₂ flow at 800 °C for the further contrast. The Fig. 9 shows the XRD pattern of GeO₂ calcined in hydrogen flow at 800 °C for different time. The GeO₂ was converted into Ge in 10 min under the hydrogen atmosphere at 800 °C. It is similar to that of in the NH₃ flow reduction. However, calcining GeO₂ in an ammonia atmosphere means more secure.

Fig. 7 XRD patterns of GeO₂ calcined in hydrogen atmosphere at different temperatures for 4 h.

Fig. 8 XRD patterns of GeO₂ calcined in H₂ flow at 500 °C for different time.

Fig. 9 XRD patterns of GeO₂ calcined in H₂ flow at 800 °C for different time.

3.4 The physical properties of prepared Ge

Fig. 10 shows the morphologies of the raw material GeO₂ and the prepared Ge in NH₃ flow with different magnifications. The GeO₂ particles (Fig. 10a–c) are uniform and show sharp angular shapes. The prepared Ge in NH₃ flow (Fig. 10d–f) shows a smooth surface and some agglomeration, indicating the structure changes from GeO₂ to Ge. The size of Ge is between 5 μm and 10 μm. Fig. 11 shows the HRTEM of prepared Ge in NH₃ flow, the lattice fringes of Ge with interplanar distances of 0.237 nm is indexed to the (111) planes of Ge.

Fig. 10 SEM images of the raw material of GeO₂ (a–c) and the Ge (d–f) prepared from 5% MoO₃/GeO₂ in NH₃ flow at 800 °C for 2 h.

Fig. 11 HRTEM of Ge prepared in the NH₃ flow from GeO₂ mixed with 5.0% MoO₃ at 800 °C for 2 h.

In order to further explore the morphology changes in the formation process of Ge, SEM as shown in Fig. 12 has been carried out for the samples prepared from MoO₃/GeO₂ for different times and with different MoO₃ mass ratios. Compared the Fig. 12b with Fig. 12a, it is found the rod-shaped material (Fig. 12a) decreases and the large smooth-surfaced Ge (Fig. 12b) increases with the reaction time prolonging. Associate with XRD patterns in Fig. 4, the rod-shaped material can be inferred as Ge₃N₄. Compared the Fig. 12c with Fig. 12b, it can be seen that there is hardly any Ge₃N₄ particles in the Fig. 12c, indicating that almost all Ge₃N₄ transforms into Ge with the increase of MoO₃. Here, the MoO₃ had been converted into Mo₂N due to in the NH₃ flow at 800 °C (Fig. 5, 800 °C). Fig. 12 again demonstrates the process of Ge generation goes through two stages. Firstly, GeO₂ is reduced to Ge₃N₄, and then Ge₃N₄ decomposed into Ge by the catalysis of Mo₂N.

Fig. 12 SEM images of samples prepared in the NH₃ flow from 2.0% MoO₃/GeO₂ at 800 °C for 0.5 h (a), 2.0% MoO₃/GeO₂ at 800 °C for 2 h (b) 5% MoO₃/GeO₂ at 800 °C for 2 h (c).

Fig. 13 shows the UV-vis diffuse reflectance spectra of MoO₃/GeO₂ calcined in the NH₃ flow with different mass ratios of MoO₃. The raw material of GeO₂ show a strong light absorption at ultraviolet region with the absorption edge of 218 nm, which means it is a large gap semiconductor. With the amount of MoO₃ increasing from 0 to 5.0%, the light absorption region gradually extends from ultraviolet region to visible region and the absorption strength gradually increases. The Ge prepared from 5.0% MoO₃/GeO₂ shows the most strongest light absorption in the range from 200 nm to 800 nm. The light absorption curve of the Ge prepared in NH₃ flow is same as that prepared in H₂ flow except the absorption strength. The difference of absorption strength can be attributed to the small amount of Mo₂N catalyst.

Fig. 13 The UV-visible diffuse reflectance spectra of MoO₃/GeO₂ with different mass ratios of MoO₃ calcined in NH₃ flow at 800 °C for 2 h. The spectra of GeO₂ and Ge prepared in the H₂ flow at 650 °C for 4 h are presented for comparison.

4 Conclusions

A fast, low cost and safe method for the Ge synthesis has been reported by the NH₃ reduction of GeO₂ using a Mo₂N catalyst. When GeO₂ and 5.0% MoO₃ is mixed and nitrided in NH₃ flow at 800 °C, the Ge is formed in 10 min. The prepared Ge shows micron scale particles size and good light absorption both in the ultraviolet and visible region. The reaction mechanism of Ge generation carries out a two-stage process. GeO₂ is converted into Ge₃N₄ firstly, and then Ge₃N₄ is decomposed into Ge with the Mo₂N catalyst. Mo₂N instead of MoO₃ is demonstrated the catalyst for the Ge generation by two proofs. One is that MoO₃ is converted into Mo₂N at 800 °C in the NH₃ flow. Another is that a small part of Ge is produced from both the mixed Mo₂N/GeO₂ and the Mo₂N/Ge₃N₄ calcined in N₂ flow, whereas there is no Ge generated from both MoO₃/GeO₂ and the MoO₃/Ge₃N₄. Finally, the method for Ge preparation in NH₃ flow has been compared with the industrial method in H₂ flow. Our presented method has the advantages of fast and safety and has great potential for industrialization. The proposed Mo₂N boosting the Ge₃N₄ decomposition has guiding significance to other nitride decomposition system.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is funded by the Natural Science Foundation of Ningxia Province of China (NZ17038), National First-rate Discipline Construction Project of Ningxia (Chemical Engineering and Technology NXYLXK2017A04), Major Innovation Projects for Building First-class Universities in China's Western Region (ZKZD2017003) and the National Natural Science Foundation of China (NSFC, 21263018).

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