Crystal Growth of RuS 2 Using a Chemical Vapor Transport Technique and Its Properties

: In this work, we study the effect of increasing temperature on the structure parameters (lattice, sulfur–sulfur distance, and ruthenium–sulfur distance) and the energy gap of RuS 2 . However, it was very challenging to obtain a sample of RuS 2 due to many factors, some of which are discussed in the introduction. To prepare the crystal growth of RuS 2 , we have used the chemical vapor transport technique. The crystals obtained show a pyrite structure, of which we studied its crystallographic structure, including the structure of crystals in surface (100). The sample was then characterized by X-ray diffraction and by microprobe analysis. We determine the relationship between the energy gap and the sulfur–sulfur distance. We analyzed the S-S bond compared with the S 2 molecule.


Introduction
The aim of this work is the study of the effect of the sulfur-sulfur distance on the electronic and optical properties of the RuS 2 pyrite. Over the past few years much attention has been given to the study of sulfur-containing compounds. This tendency is due to the increasing environmental issues, as well as academic interests [1]. Ruthenium Sulfide, RuS 2 is one of the interesting sulfur compounds from both fundamental and technological points of view. It is one of the semiconducting transition-metal dichalcogenide (TMDC) materials, with a reported band gap of 1.8 eV [2] and has a pyrite structure [3]. Ruthenium Sulfide, RuS 2 has several possible uses, including its use as a catalyst [4] and as a photoelectrode [5][6][7][8]. However, it is difficult to obtain the crystalline RuS 2 due to several facts, for instance we can obtain RuS 2 only at temperatures greater than 1000 • C. Therefore, obtaining its crystalline structure at low temperatures is practically impossible. Moreover, the physical vapor transport method is difficult to use because the vapor pressure of RuS 2 is very low, at temperatures between 800 and 1050 • C.
Our work is structured as follows. First, we provide a detailed description of the experimental procedure used to obtain RuS 2 by the chemical vapor transport technique. Next, we provide a brief description of the techniques and tools used to analyze the obtained sample, such as X-ray diffraction and microprobe analysis. In addition, we provide a detailed analysis of our findings; that is, the influence of increasing temperature on the stoichiometry shift of sulfur, S and how the different values of the energy gap helped us to understand and analyze the effect of other parameters, such us temperature, sulfur-sulfur distance, and ruthenium-sulfur distance on the energy gap. Moreover, and as one of our results demonstrates, we show the correlation between the sulfur-sulfur bond and energy gap. 2 of 9 The study of the surface is an important key toward understanding the effect of distribution of sulfur nanoparticles on the value of band gap. Finally, to further understand the interaction between the RuS 2 nanoparticles and surface bonding, we clarified the electronic processes that relate to the bonding in the surface of RuS 2 nanoparticles.

Experimental Section
In this work we carry out the chemical vapor transport (CVT) growth in a closed quartz ampule. The phase vapor transport is carried out using silica ampules containing RuS 2 powder and a very low percentage of sulfur. The ampule is 200 mm in length and 25 mm in diameter. The ampule is sealed under chlorine atmosphere (100 mm of Hg). We started crystallization of RuS 2 ; we used lCl 3 and S 2 Cl 2 as transport agents. After, we introduced a small quantity of the oxygen form RuO 2 . In the end, the quantity of chlorine and RuO 2 determined 2 atmospheres of RuOCl 2 in total at 900 • C. The RuOCl 2 was then annealed in a dynamical vacuum of 2 atmospheric pressures at a temperature of 900 • C. The crystal growth took place in a graphite-covered quartz ampule.
The mixture RuS 2 + RuO 2 + Cl 2 was used as the chemical agent to transport the material from the warm to the cool zone. The temperature of the source materials was between 900 • C and 1025 • C. The crystallization took place in the ampule where the temperature was reduced by 50 • C after each crystallization. The growth region was situated in a hot zone, 925-1050 • C. The duration of transport was between 7 and 15 days. Mono-crystalline RuS 2 was formed in the cool zone of the ampule. By this method, we obtained a polycrystalline structure that consists of mono-crystal grains where the dimensions can decrease from 4 mm × 4 mm × 4 mm to 0.5 mm × 0.2 mm × 0.2 mm. The color of the obtained crystal varies based on the temperature from dark gray to shiny light gray; our result is listed in Table 1. Table 1. The change of the color of grain crystal. This change is associated with a percentage of concentration of sulfur and the temperature. Additionally, the color of crystals related to the dimension of the grain. However, we succeeded in obtaining RuS 2 by the chemical vapor transport technique. We have noticed that the better quality of RuS 2 was obtained at the lowest temperature, which makes this technique of chemical vapor transport very interesting. The importance of this technique is the ability to lower the crystallization temperature of the most refractory materials.

Sample
Given the fact that the thermal stability of RuS 2 (Pdissociation = 5 mbar at T = 1100 • C) which is a high temperature. It is impossible to transport halides between 800 and 850 • C and that is since the vapor pressure of ruthenium containing species is low. So far, the CVT of RuS 2 has not been a successful growth method; for example, see [9]. However, for us we succeed in forming RuO x Cl y when it is transported at a low temperature, and that is due the fact that we used oxygen from RuO 2 . Figure 1, shows the growth of monocrystal of RuS 2 by CVT. us we succeed in forming when it is transported at a low temperature, and that is due the fact that we used oxygen from . Figure 1, shows the growth of monocrystal of by CVT.

Analysis
Several different single crystals of RuS2 grown by the above technique have been analyzed by microprobe and X-ray diffraction.

Analysis by Microprobe
We used microprobe casting (camera MS 46-CNRS de Bellevue), which provides a specific chemical analysis using an accelerated and focused electron beam on the sample (Φ < 1 μm at the surface of the sample). Under the effect of electron bombardment, the single crystal produces an X emission of lines characteristic of the elements present. The main reason for using microprobe casting is to observe the influence of both the chemical vapor transport method and the temperature increase in the stoichiometry shift of the sulfur (S) rich atmosphere. As expected, a significant influence on the obtained concentration of RuS2 was observed. In Table 2, we can see the heavy influence of temperature on the stoichiometry shift of sulfur, S. When the temperature increases, we do not obtain exactly RuS2, instead the quantity of sulfide slightly decreases, and therefore obtaining a sample of is challenging, as mentioned in the introduction. In these preliminary results we have not observed the micro-weight of oxygen or anything chloric. It has been observed that as the temperature increases, sulfur concentration decreases, and the material becomes more nonstoichiometric: see Figure 2.

Analysis
Several different single crystals of RuS 2 grown by the above technique have been analyzed by microprobe and X-ray diffraction.

Analysis by Microprobe
We used microprobe casting (camera MS 46-CNRS de Bellevue), which provides a specific chemical analysis using an accelerated and focused electron beam on the sample (Φ < 1 µm at the surface of the sample). Under the effect of electron bombardment, the single crystal produces an X emission of lines characteristic of the elements present. The main reason for using microprobe casting is to observe the influence of both the chemical vapor transport method and the temperature increase in the stoichiometry shift of the sulfur (S) rich atmosphere. As expected, a significant influence on the obtained concentration of RuS 2 was observed. In Table 2, we can see the heavy influence of temperature on the stoichiometry shift of sulfur, S. When the temperature increases, we do not obtain exactly RuS 2 , instead the quantity of sulfide slightly decreases, and therefore obtaining a sample of RuS 2 is challenging, as mentioned in the introduction. In these preliminary results we have not observed the micro-weight of oxygen or anything chloric. It has been observed that as the temperature increases, sulfur concentration decreases, and the material becomes more nonstoichiometric: see Figure 2.
As can be seen, where the temperature increases, the stoichiometry shift of sulfur decreases. As can be seen, where the temperature increases, the stoichiometry shift of sulfur decreases.

Analysis by X-ray
Powder X-ray diffraction measurements were made on crushed crystals using a Philips diffractometer with CuK radiation. Cell parameters were calculated, with the aid of a computer, using a least-squares refinement program. Selected crystals were also examined by microprobe casting. Powder X-ray diffraction patterns showed the cubic, with lattice parameters close to the literature value of 5.609Å 5.635Å [10], where RuS2 crystallizes as laurite in a pyrite type structure, in which disulfide ions are octahedrally coordinated to the Ru metal ion; having the space group symmetry Th 6 (Pa3), the lattice parameters are the same as in ASTM file. The calculated experimental values for constant lattice a, parameter of structure , sulfur-sulfur distance and Ruthenium-sulfur distance are listed in Table 3.  Figures 3 and 4, which parameters of structure define the atomic position of sulfur. Both the and bonds increase with temperature. Thus, they conclude the influence of temperature to parameter structure , it is presented in Figure 4 increases when temperature increases. The precise bond for the structure (Ru-S and S-S distances) comes from the balance between the temperature and the method to prepare . Hence, we deduce that the structure of . depends heavily on the temperature.

Analysis by X-ray
Powder X-ray diffraction measurements were made on crushed crystals using a Philips diffractometer with CuKα radiation. Cell parameters were calculated, with the aid of a computer, using a least-squares refinement program. Selected crystals were also examined by microprobe casting. Powder X-ray diffraction patterns showed the cubic, with lattice parameters close to the literature value of 5.609-5.635 [10], where RuS 2 crystallizes as laurite in a pyrite type structure, in which disulfide ions are octahedrally coordinated to the Ru metal ion; having the space group symmetry T h 6 (Pa3), the lattice parameters are the same as in ASTM file. The calculated experimental values for constant lattice a, parameter of structure ν, sulfur-sulfur distance d s−s and Ruthenium-sulfur distance d Ru−s are listed in Table 3. In terms of bond distances, we can see that the Ru − S bond decreases from 2.373 back to 2.367 . The S 2 pair is a weakening of the S − S bond, as the calculated bond length increases when the temperature increases from 900 • C to 1050 • C. To recover the wellknown bond character within the S 2 molecules. The S − S bond increases from 1.990 A. The effect of the temperature to bonding in the RuS 2 . is shown in Figures 3 and 4, which parameters of structure ν define the atomic position of sulfur. Both the S − S and Ru − S bonds increase with temperature. Thus, they conclude the influence of temperature to parameter structure ν, it is presented in Figure 4 ν increases when temperature increases. The precise bond for the structure (Ru-S and S-S distances) comes from the balance between the temperature and the method to prepare RuS 2 . Hence, we deduce that the structure of RuS 2 . depends heavily on the temperature.
Next, by using the results of our experimental work we provide more details about the relationship between the structure of RuS 2 and the temperature, along with the effect of the temperature and the S − S bond on the gap energy.
We have determined that there are different values of the energy gap of RuS 2 . These values are listed in Table 4 below. In Figure 5, we plotted (αhν) 1 2 versus hν photon energy.   Next, by using the results of our experimental work we provide more details about the relationship between the structure of RuS2 and the temperature, along with the effect of the temperature and the bond on the gap energy. We have determined that there are different values of the energy gap of . These values are listed in Table 4 below. In Figure 5, we plotted ( ℎ ) versus ℎ photon energy. From this graph we conclude that pyrite is a semiconductor, has an indirect band gap, and different values of band gap.   Next, by using the results of our experimental work we provide more details about the relationship between the structure of RuS2 and the temperature, along with the effect of the temperature and the bond on the gap energy. We have determined that there are different values of the energy gap of . These values are listed in Table 4 below. In Figure 5, we plotted ( ℎ ) versus ℎ photon energy. From this graph we conclude that pyrite is a semiconductor, has an indirect band gap, and different values of band gap.  The results show a clear and strong dependency of the energy gap on temperature (Figure 6a) and show an extraordinary decrease in the energy gap when the crystallization of the samples is carried out at high temperatures. It is also clear that when the excess of sulfur increases, the energy gap decreases (Figure 6b). However, we have determined the relationship between the growth parameters (temperature, lattice, and distance) and the energy gap. Table 5 shows that the S-S bond is in good agreement with crystallographic data and the elongation of the S-S bond compared with the S 2 molecule. As the resulting energy gap decreases (from 1.68 to 1.25 eV), the sulfur-sulfur distance increases (from 1.990 to 2.118 . A). This clearly shows that the energy gap is strongly dependent on the S-S distance. However, all founded values of the energy gap have the same type of S-S bond in RuS 2 . Moreover, the interesting property of RuS 2 is that, regardless of the change of the value of energy gap, it keeps pairs of sulfur S 2 and not an individual S atom. In our study, the sample CS9 has smallest dimension, it was prepared at the lowest temperature of 900 • C and it has the highest band gap 1.68 eV, which shows that the morphology of crystal RuS 2 strongly depends on the band gap and temperature, especially the distribution of sulfur, similar to the work carried out by Aqueel et al. in [11] where they showed the temperature effect on the morphology of CuCo 2 S 4 .  The results show a clear and strong dependency of the energy gap on temperature ( Figure 6a) and show an extraordinary decrease in the energy gap when the crystallization of the samples is carried out at high temperatures. It is also clear that when the excess of sulfur increases, the energy gap decreases (Figure 6b). However, we have determined the relationship between the growth parameters (temperature, lattice, and distance) and the energy gap. Table 5 shows that the S-S bond is in good agreement with crystallographic data and the elongation of the S-S bond compared with the S2 molecule. As the resulting energy gap decreases (from 1.68 to 1.25 eV), the sulfur-sulfur distance increases (from 1.990 to 2.118 A ). This clearly shows that the energy gap is strongly dependent on the S-S distance. However, all founded values of the energy gap have the same type of S-S bond in RuS2. Moreover, the interesting property of RuS2 is that, regardless of the change of the value of energy gap, it keeps pairs of sulfur S2 and not an individual S atom. In our study, the sample CS9 has smallest dimension, it was prepared at the lowest temperature of 900 ℃ and it has the highest band gap 1.68 , which shows that the morphology of crystal strongly depends on the band gap and temperature, especially the distribution of sulfur, similar to the work carried out by Aqueel et al. in [11] where they showed the temperature effect on the morphology of .  The pyrite structure of RuS2 is schematically shown in Figure 7a,b. From the two figures, we can see that the pyrite RuS2 presents as a face-centered cube of Ru. Figure 7c presents molecules, it is clear there are molecules similar to , which we have at the cube center, and in the middle of the cube edges (in molecules) molecules are located. However, RuS2 appears as S2 pairs coordinating a metal center. Each Ru atom is in an octahedral arrangement surrounded by six S2 molecules. Moreover, only one S of each pair is bonded to the Ru atom and the bonds from the metal atom are arranged in a distorted octahedron. Each S atom has three Ru neighbors, and a S2 pair has six metal neighbors in a pseudo-octahedral coordination. Figure 7d shows that has only the S-S bond type in this structure. Even that can show significant variation of energy gap values of RuS2, and it is therefore important to understand the S-S bond in the and how it affects the energy gap. This is why it is important to study the surface (100) of pyrite . The Figure 8 results show the structure of surface (100) and surface (110). It proved that small nanoparticles of sulfur are responsible for most properties. It shows the active sites that can react with surface and affect electron transmission. Ru atom has a electronic configuration [12] with low spin where S atom has 3 state with up spin * . This motivated us to confirmed that band gap depends only the position of Sulfur  The pyrite structure of RuS 2 is schematically shown in Figure 7a,b. From the two figures, we can see that the pyrite RuS 2 presents as a face-centered cube of Ru. Figure 7c Crystals 2022, 12, 994 7 of 9 presents S 2 molecules, it is clear there are molecules similar to RuS 2 , which we have at the cube center, and in the middle of the cube edges (in RuS 2 molecules) S 2 molecules are located. However, RuS 2 appears as S 2 pairs coordinating a metal center. Each Ru atom is in an octahedral arrangement surrounded by six S 2 molecules. Moreover, only one S of each pair is bonded to the Ru atom and the bonds from the metal atom are arranged in a distorted octahedron. Each S atom has three Ru neighbors, and a S 2 pair has six metal neighbors in a pseudo-octahedral coordination. Figure 7d shows that RuS 2 has only the S-S bond type in this structure. Even that can show significant variation of energy gap values of RuS 2, and it is therefore important to understand the S-S bond in the RuS 2 and how it affects the energy gap. This is why it is important to study the surface (100) of pyrite RuS 2 . The Figure 8 results show the structure of surface (100) and surface (110). It proved that small nanoparticles of sulfur are responsible for most properties. It shows the active sites that can react with surface and affect electron transmission. Ru atom has a d electronic configuration [12] with low spin t 2g where S atom has S 3p state with up spin PPσ * . This motivated us to confirmed that band gap depends only the position of Sulfur (parameter of structure ν) and S-S distance.