Obtaining of Nitride on the Surface of Single-Crystalline Germanium for Photocatalytic Research

1. Introduction

Germanium nitride Ge₃N₄ finds application in micro- and nanoelectronics, photoluminescence, energy storage, photocatalysis [1,2,3,4,5,6,7,8,9,10] and others. Usually Ge₃N₄ is obtained by interaction of ammonia with elemental germanium at (650-700) ^oC or its dioxide (GeO₂) at (700-750) ^oC:

3Ge+4NH₃⟶ Ge₃N₄+6H₂,

(1)

3GeO₂+4NH₃⟶ Ge₃N₄+6H₂O.

(2)

According to reaction (1), the α-modification of nitride is formed, and according to reaction (2), the β-modification ^(*).

An original method is also the use of hydrazine vapors [16]:

3Ge+2N₂H₄⟶Ge₃N₄+4H₂.

(3)

^(*) Germanium nitride exists in the form of several crystal modifications: α-, β-, γ-, δ- Ge₃N₄ [11,12,13]. The t-, m-, o-modifications of nitride are also theoretically discussed [14,15] (α, β, δ – hexagonal, γ – cubic, t – tetragonal, m – monoclinic, o – orthorhombic syngony). At normal temperatures and pressures, only the α- and β-modifications are stable.

Bot modifications consist of Ge(N₄) tetrahedra and crystallize in hexagonal syngonia. The difference between them lies in the arrangement of Ge(N₄) tetrahedra along the “c” axis (Figure 1). The values of elementary cell parameters from various literature data are given in Table 1. Figure 2 shows their elementary cells.

Below it will be shown that at 650 °C hydrazine in the presence of germanium decomposes according to the scheme:

2N₂H₄→2NH₃+N₂+H₂.

(4)

It is known that at experimentally achievable temperatures and pressures, nitrogen does not interact with germanium, and the Ge-H bond is broken below 650 °C. Therefore, in hydrazine vapors, nitride is actually formed according to scheme (1). Since germanium is an active catalyst for the decomposition reaction of hydrazine, this issue will be discussed in detail.

2. Materials & Reagents

Сommertial hydrazine-hydrate containing 50 mol.% (36 wt.%) water was distilled using the Raschig’s method with improvement. In particular, before distillation, it was boiled with NaOH in an inert atmosphere of nitrogen at a temperature of 120 °C for two hours. Hydrazine purified in this way had a density of $\mathsf{\rho }\cong$1.0024 g/cm³ and a refractive index of${\mathrm{n}}_{\mathrm{D}}^{20}\cong$ 1.4705. According to the literature, this latter value corresponds to 100% N₂H₄. However, this can be considered not entirely correct.

Plates of single-crystalline germanium doped with antimony (charge carrier concentration $\mathrm{n}\cong$2$·$10¹⁴cm^-3) had a resistivity of $\cong$35 Ohm$·$cm. The crystallographic orientation of Ge plates are (111) or (100). They were previously degreased in boiling toluene, etched in liquid etchant HF:HNO₃:CH₃COOH=1:15:1 for 4-5 minutes and washed in running distilled water.

3. Results and Discussion

3.1. Decomposition of Hydrazine on the Surface of Single-Crystalline Germanium

Hydrazine is one of the most chemically active substances - a strong reducing agent. He has wide application in various fields of industry, technology, medicine, etc. and has been intensively studied both previously and currently [14,15,16,17,18,19,20,21,22,23,24,25]. Liquid N₂H₄ is very hygroscopicand has a noticeable ability to absorb oxygen and carbon dioxide from the air. It is called “high purity” when the water content does not exceed 1 wt.% and “ultra-pure” - with a maximum of 0.5 wt.% H₂O. The concentration of water in hydrazine is estimated by the density, melting point, or refractive index of the mixture. However, literature data on these parameters are different, due to the difficulty of accurately determining the physical characteristics of pure hydrazine ^(*).

Hydrazine is easily decomposed by heat and radiation, especially in the presence of catalysts [35,36,37,38]. The general form of this reaction is given by the equation:

3N₂H₄→4(1-x) NH₃+(1+2x) N₂+6xH₂.

(5)

^(*) According to various authors, the density of liquid hydrazine at 25^oC is 1.0045, 1.0036 and 1.0024, 1.008 g/cm³ at 23^oC. The melting point of the system N₂H₄/H₂O: 1, 1.4, 1.53, 1.6-1.7, 1.8, 1.85 and 2^oC [26,27,28].

Depending on external conditions (temperature, pressure, catalyst, electromagnetic radiation, electric discharge, etc.) 0$\le$х$\le$1^(*). The catalytic decomposition of hydrazine on the surface of germanium has been studied relatively littleand there is data when carrying out the reaction up to 80 °C. In early work [32], powders of Ge of n- and p-type conductivity were used. It was found that the decomposition products were ammonia and nitrogen:

3N₂H₄→NH₃+N₂.

(6)

The type of conductivity did not affect the catalytic properties.

Figure 3a shows the kinetic curves of the accumulation of hydrogen and ammonia at 650 °C. It can be seen that the amount of ammonia is constant in the absence of germanium, and in its presence gradually decreases. The hydrogen content in the presence of Ge increases sharply, and in its absence it first increases and then decreases. The resulting ammonia corresponds to an equimolar amount of chemisorbed hydrazine. As a result, the total change of pressure (Figure 3b) is determined only by the decomposition reaction.

Thermodynamic calculation of the change of free energy showed that reaction (5) at x = 0.25 (i.e. reaction (4)) has almost the same probability as reaction (6): change in Gibbs free energy ∆G$\cong$220.5 and $\cong$222.6 kJ/mol respectively ^(**). However, the discovered fact of hydrogen evolution gives preference to reaction (4).

^(*) On alkaline catalysts x=1, on some semiconductor catalysts (Ga, Ga₂Se₃ and others), as well as on some metals (Te, Pt) x=0, on some semiconductors (V₂O₅, Ga₂Te₃ and others), as well as on acid catalysts 0$<$x$<$1, during decomposition using a spark x = 0.38, and during bombardment with α-particles x = 0.12-0.22 [29,30,31].

^(**) The estimate of ∆G should be considered approximate since a change in pressure occurs in the reaction area.

A sharp increase of the amount of hydrogen and a decrease of the amount of ammonia in the presence of germanium can be associated with a heterogeneous reaction (1).

The study of high-temperature decomposition of hydrazine was also carried out using IR absorption spectra. Figure 4 shows the IR spectra of N₂H₄ vapor, demonstrating the dynamics of its decomposition at 650 °C. Curve 1 corresponds to hydrazine vapor, curves 2 and 3 to hydrazine heated for 15 and 30 minutes, and curve 4 to pure ammonia. These spectra indicate that the decomposition of hydrazine at 650 °C occurs mainly during the first 15 minutes and is completely completed within 30 minutes.

3.2. Formation of Nitride on Germanium Surface

At temperatures ˃650 °C, nitride Ge₃N₄ is formed in hydrazine vapor on the surface of germanium, and by registration mass change of the sample using the microgravimetric method, the following processes are observed¹⁶: first, an increase of mass occurs due to the accumulation of hydrazine and its decomposition products on the surface, then the mass of the sample decreases due to etching of Ge with contained in hydrazine water vapors, and then observes its gradual increase due to formation of Ge₃N₄.

It should be noted that when freshly distilled hydrazine was stored in a special ampoule under vacuum, over fairly long periods of time (two weeks, a month), we did not detect any change in determining of the refractive index within the measurement accuracy. However, a significant difference in the nitridation kinetics was observed (Figure 5). From this figure, in particular, one can see the difference in the etching rates of the germanium surface at the same temperature³³. This can be attributed to the gradual humidization of hydrazine, despite precautions. Really, under the above conditions, the following occurs: first, β-Ge₃N₄ is formed, and then traces of α-modification are observed in the nitride. When hydrazine is specially hydrated, the amount of α-Ge₃N₄ increases and it is finally possible to obtain it in pure form³⁴.

It should also be noted that the initial increase of mass (Figure 5) is 2-3 orders of magnitude greater than is typical for physical adsorption. This can be associated with the accumulation of polar molecules of hydrazine and water with high dipole moments ($~$2 D^35,36) on the germanium surface.

One can also take into account the existence of hydrazine in the imide tautomeric form:

$${\mathrm{H}}_2\mathrm{N}\text{-}{\mathrm{NH}}_2\text{↔}{\mathrm{N}}^{-}\mathrm{H}\text{-}{\mathrm{N}}^{+}{\mathrm{H}}_3.$$

The bipolar imide form of hydrazine is characterized by a pronounced ability to associate molecules and a strong donor property to atoms with unfilled d- and f-shells, especially in substances with a small band gap (for example, germanium).

The above can be confirmed by the results of supplementary experiments on the interaction of germanium with ammonia, as with a molecule of the amine form. At the initial stage of this reaction at (500-700) °C, we observed an increase in the sample by (2-4) μg/cm², which is characteristic of the process of physical adsorption of neutral molecules.

3.3. The Possibility of Using of Germanium Nitride as a Photocatalyst in the Conversion of Carbon Monoxide to Dioxide

The role of photocatalysis in natural photosynthesis, energy, biotechnology, ecology, other fields of science and technology, or in solving household problems is widely known. Among the compounds that are studied to achieve the catalytic effect by visible or ultraviolet radiation, non-oxide materials occupy an important place. Among them are simple (binary) nitrides: C₃N₄ [37,38,39,40,41], GaN [42,43,44], TiN [45,46], Ta₃N₅ [47,48], HfN [49,50], Si₃N₄ [51,52], Ge₃N₄.

The essence of photocatalysis is to increase of the rate or excitation of chemical reactions under the influence of light in the presence of substances that absorb light quanta and participate in the chemical transformations of these substances, repeatedly entering into intermediate interactions with them and regenerating their chemical composition after each cycle. (A simplified diagram of the process is shown in Figure 6.) All this became possible after the fundamental works of A. Fujishima [53,54,55,56,57].

As noted in the introduction, germanium nitride was successfully tested using photoradiation in the process of water splitting. The authors of the cited works used β-Ge₃N₄ doped with RuO₂.

The authors of this paper are currently conducting experiments to determine the photocatalytic activity of Ge₃N₄ for converting CO into CO₂. We use α-Ge₃N₄ and mixtures of α- and β-modifications doped with platinum or palladium. It is evident from the Figure 2 that this modification of the nitride is capable of dissolving the dopant in itself more effectively ^(*).

4. Conclusions

At $\ge$650 °C hydrazine decomposes on the surface of single-crystalline germanium according to the scheme: 2N₂H₄→2NH₃+N₂+H₂. A sharp increase in the amount of hydrogen and a decrease in the amount of ammonia in the presence of germanium is observed. This is due to a heterogeneous reaction: 3Ge+4NH₃→Ge₃N₄+6H₂. The phase composition of solid product of this reaction is an indicator of the degree of humidity of hydrazine: in pure hydrazine vapors, β-Ge₃N₄ is formed on the surface of germanium, and as water is added, a mixture of α- and β-modifications is formed until pure α-Ge₃N₄ is formed. The question of the possibility of using α-Ge₃N₄ and mixtures of α- and β-Ge₃N₄ as a photocatalyst was considered.

^(*) The problem of converting toxic CO into harmless CO₂ is a very urgent task^58-66. Purification of atmospheric air from harmful substances is of great importance for human health. One of the main sources of air pollution are internal combustion engines, namely cars. The most toxic component of their exhaust gases is precisely carbon monoxide. CO is especially dangerous because, due to its physical properties, it enters people’s homes or workplaces more easily than other toxic exhaust gas components. It is odorless and cannot be detected by the senses. The most effective means of protecting residential and workplaces of people from carbon dioxide are cleaning devices containing photocatalysts, which, under the conditions of the use of an appropriate catalyst and natural air convection, will effectively purify them from CO.