Fabrication of Selective and Sensitive Hydrazine Sensor Using Sol-Gel Synthesized MoSe2 as Efficient Electrode Modifier

Abstract

Due to its hazardous nature, the determination of hydrazine is of great significance. This study designed and fabricated a hydrazine electrochemical sensor. Two-dimensional (2-D) molybdenum diselenide (MoSe₂) has been synthesized by using the sol-gel method. The phase purity and formation of MoSe₂ was determined by a powder X-ray diffractometer. The surface morphological characteristics of the MoSe₂ were studied by scanning electron microscopy. The presence of Mo and Se elements in the synthesized MoSe₂ was checked by energy dispersive X-ray spectroscopy. The glassy carbon (GC) electrode (3 mm) was modified with the prepared MoSe₂ via a drop-cast approach. This MoSe₂-glassy carbon (MoSe₂-GC) electrode was used as the working electrode for the hydrazine sensing application. The electrochemical sensing properties of the fabricated MoSe₂-GC were analyzed by linear sweep voltammetry and cyclic voltammetry. The MoSe₂-GC has shown a sensitivity of 0.68 µA/µMcm² and a detection limit of 0.091 µM. In addition, MoSe₂-GC also has good selectivity toward hydrazine determination in the presence of various interfering compounds. The excellent electro-catalytic behavior of MoSe₂ is solely responsible for this enhanced sensing performance of MoSe₂-GC.

1. Introduction

Hydrazine (N₂H₄) is a well-known carcinogen and has been widely used in many industrial applications such as antioxidants, catalysis, photographic developers, rocket propellants, plant growth regulators, reducing agents, fuel cells, blowing agents, corrosion inhibitors, and pharmaceutical inhibitors [1,2,3,4]. Exposure to hydrazine may cause various health-related problems such as liver damage, skin damage, respiratory problems, eye irritation, kidney damage, and dermal deterioration [5,6,7,8]. In addition, hydrazine may also. influence the central nervous system, potentially causing genetic diseases, through prolonged exposure [9,10]. The United States Environmental Protection Agency (USEPA) limited the allowed hydrazine concentration to less than 10 ppb in drinking water to avoid the negative impacts on human health and the environment [11,12,13]. The unfavorable aspects of toxic hydrazine have led the scientific community to work on the safe use and detection of hydrazine [12]. Conventional methods such as fluorescent probe detection, electrochemical technology, liquid chromatography, and spectrophotometry have been used for hydrazine sensing application [14,15,16]. Unfortunately, these methods have some limitations such as being time consuming, expensive, and requiring large installations [17,18]. Thus, it is necessary to develop methods and sensors for the determination of low levels of hydrazine.

In this context, electrochemical technology may be an effective approach for hydrazine sensing application compared to conventional methods [19]. The electrochemical method has several advantages such as good selectivity, rapidity, high sensitivity, simplicity, and stability [20,21,22,23,24]. In previous years, various hydrazine sensors have been developed using transition metal oxides, polymers, hybrid composite materials, and perovskite or metal-organic-frameworks-based materials [25,26,27,28]. The electrochemical sensing performance largely depends on the properties of the electrode materials, which act as electro-catalysts during electrochemical reactions [24]. Recently, two-dimensional (2-D) ultrafine nanomaterials consisting of a few layers of atoms have drawn the interest of researchers in a wide variety of applications due to their exceptional mechanical, electronic, and optical properties [29]. Transition metal dichalcogenides, also known as 2-D layered materials, have excellent optoelectronic properties [30]. Particularly, molybdenum diselenide (MoSe₂) has been widely explored in developing solar cells, sensors, water treatment, catalysis, hydrogen production, fuel cells, photo-catalysis, and electro-catalysis [31,32,33,34]. MoSe₂ possesses good electro-catalytic features, with makes it a suitable sensing material for electrochemical sensing applications [31]. Previously, many synthetic methods such as hydrothermal, sol-gel, microwave, chemical vapor deposition, etc., have been utilized for the preparation of nanostructured materials. Among them, sol-gel has attracted the scientific community because of the high chemical reactivity of the metal precursor in the solution phase, synthesis of uniform compounds, ability to control the chemical composition, simplicity of the process, and very high production efficiency. Therefore, we have adopted the sol-gel method for the preparation of electrode material for electrochemical sensing application.

Herein, we report the synthesis of MoSe₂ via the sol-gel method. The glassy carbon (GC) electrode was modified with MoSe₂ as hydrazine sensing material via the drop-casting method. The sensing performance of MoSe₂-GC for hydrazine determination was studied by employing cyclic voltammetry and linear sweep voltammetry methods. The modified GC electrode (MoSe₂-GC) revealed the presence of good electro-catalytic features for the oxidation of hydrazine. The MoSe₂ provides active sites for the electro-oxidation of hydrazine at the MoSe₂-GC surface. The reasonable limit of detection of 0.091 µM, including sensitivity of 0.68 µA/µMcm², was obtained via MoSe₂-GC. This type of electrochemical sensor can be used in the agricultural and pharmacological industries to monitor the presence of hydrazine.

So far, there have been no reports on the fabrication of a hydrazine sensor using MoSe₂. According to our literature survey, this is the first report which demonstrates the electro-catalytic role of sol-gel-prepared MoSe₂ for hydrazine detection by employing the voltametric method.

2. Experimental Section

2.1. Chemicals

Molybdenum (V) chloride and diphenyl diselenide were purchased from Sigma. Phenol and catechol were bought from Alfa Aesar. Urea, ascorbic acid, and glucose were purchased from Merck. Dopamine and hydrogen peroxide were bought from Sigma. Sodium chloride (NaCl) and uric acid were bought from Fischer Scientific. All the chemicals and reagents were used without any further purification.

2.2. Synthesis of MoSe₂

MoSe₂ was synthesized according to a previous report with minor modifications [35]. First, 550 mg of molybdenum (V) chloride (MoCl₅) and 1.20 g of diphenyl diselenide (C₁₂H₁₀Se₂; DDS) were mixed in the beaker. Further, 30 mL of ethanol was slowly added to the mixture and ultrasonicated for 30 min to obtain the clear solution. After drying, a gel-like precursor was obtained which was transferred to the ceramic boat and heated in a quartz tube at 700 °C for 2 h under Ar/H₂ flow. After cooling down the temperature, a MoSe₂ sample was collected which was further characterized by various physiochemical methods. The calcinated product (MoSe₂) was used as obtained without any further purification.

2.3. Characterization

The powder X-ray diffraction (XRD) of the MoSe₂ sample was obtained on Rigaku (Tokyo, Japan; RINT 2500 V; powder X-ray diffractometer). The surface morphological images of the MoSe₂ were obtained using scanning electron microscope (SEM; Supra 55 Zeiss). The energy-dispersive X-ray spectroscopic (EDS) data were collected on X-max, Aztec spectroscope. The X-ray photoelectron spectroscopic (XPS) results were obtained on PHI 5000 VersaProbe III Instrument. Electrochemical investigations were carried out on a 3-electrode system (CH Instrument; GC = working, silver/silver chloride = reference and platinum wire = counter electrode), which was connected to a computer.

2.4. Electrode Preparation (MoSe₂-GC)

A glassy carbon (GC) electrode with a 3 mm diameter was cleaned with 0.5 µm alumina slurry and velvet pad. Further, 2 mg MoSe₂ was dispersed in 1 mL of ethanol (5 µL nafion) and sonicated for 1 h. Nafion acted as a binder to enhance the adhesiveness of the MoSe₂ film on the GC surface. Later, 8 µL of the prepared MoSe₂ dispersion was carefully drop-casted on the GC surface (Scheme 1). The film thickness of MoSe₂ was ~2.0 µm.

3. Results and Discussion

3.1. MoSe₂ Characterization

The XRD data of the sol-gel synthesized MoSe₂ were obtained at a two-theta range of 20–80°. The obtained XRD pattern of the sol-gel synthesized MoSe₂ is presented in Figure 1.

The XRD exhibited the presence of four major diffraction peaks at 13.57°, 27.23°, 32.61°, and 54.81°. These diffraction peaks of 13.57°, 27.23°, 32.61° and 54.81° can be assigned to the (002), (100), (103), and (110) diffraction planes, respectively. The XRD pattern of the sol-gel synthesized MoSe₂ is consistent with the reported JCPDS number 29-0914. The XRD pattern of the sol-gel synthesized MoSe₂ did not show the presence of any other diffraction peak related to the impurity. Thus, the XRD of the sol-gel synthesized MoSe₂ indicated the formation of MoSe₂ with good phase purity. The Raman spectrum of MoSe₂ was also recorded and has been provided in Figure S1. The Raman spectrum of MoSe₂ shows that the well-known three peaks appeared. These peaks can be assigned to A_1g, E¹_2g, and B¹_2g (Figure S1).

In some cases, the morphological properties of the sensing material can affect the sensing behavior of the developed sensor. In this regard, it is necessary to study the surface structural property of the sol-gel synthesized MoSe₂. The top view surface morphological images of the sol-gel synthesized MoSe₂ have been presented in Figure 2. The SEM results of the sol-gel synthesized MoSe₂ indicated that MoSe₂ has uniform morphological characteristics (Figure 2a,b).

The elemental composition of the sol-gel synthesized MoSe₂ should be checked to ascertain the presence of impurity. This can be studied by utilizing EDS analysis.

Therefore, we have recorded the EDS spectrum of the sol-gel synthesized MoSe₂. The obtained EDS spectrum of the sol-gel synthesized MoSe₂ is shown in Figure 3. The obtained EDS results suggested the presence of peaks related to Mo and Se elements only. No peak for any other element appeared in the EDS spectrum of the sol-gel synthesized MoSe_2, which indicated the good phase purity of the sol-gel synthesized MoSe₂.

The XPS method is a more useful technique to study the presence of oxidation states or elemental compositions of the prepared materials. Hence, the XPS technique was also applied to ascertain the oxidation states of the Mo and Se elements in the sol-gel synthesized MoSe₂. The obtained XPS results for the sol-gel-prepared MoSe₂ are shown in Figure 4a,b.

The Mo3d spectrum of the MoSe₂ is displayed in Figure 4a. The Mo3d spectrum of MoSe₂ showed the binding energy of 231.42 eV and 228.08 eV, which can be ascribed to the presence of Mo3d_3/2 and Mo3d_5/2, respectively (Figure 4a). The Se3d spectrum of MoSe₂ has also been displayed in Figure 4b. The Se3d spectrum of MoSe₂ deconvoluted into the two peaks at binding energy of 55.44 eV and 54.53 eV and can be assigned to the presence of Se3d_3/2 and Se3d_5/2, respectively (Figure 4b). The XPS data of sol-gel synthesized MoSe₂ were consistent with previous reports and authenticated the formation of MoSe₂ using the sol-gel approach [36].

3.2. Electrochemical Sensing Properties

Initially, we investigated the electrochemical sensing behavior of the GC in 350 µM hydrazine using cyclic voltammetry (CV). The CV of the GC was obtained in 350 µM hydrazine under applied potential at a scan rate of 50 mVs⁻¹ (Figure 5). The 350 µM hydrazine was prepared in 0.1 M PBS of pH 7.0, which was used for all electrochemical investigations. The obtained CV data of the GC revealed a very low current response of 3.47 µA for the electro-oxidation of hydrazine (Figure 5). Further, the electrochemical sensing ability of the MoSe₂-GC was examined under similar conditions and environment. The CV result for MoSe₂-GC is provided in Figure 5. The CV results show the enhanced current of 9.48 µA for MoSe₂-GC towards the electro-oxidation of 350 µM hydrazine under an applied potential scan rate of 50 mVs⁻¹. The presence of MoSe₂ on the bare GC surface acted as an electro-catalytic agent and improved the electro-oxidation of hydrazine.

The applied potential scan rate was also varied to examine the effect of the scan rate on the electro-oxidation of 350 µM hydrazine using MoSe₂-GC. The scan rate was varied in the range of 50 mVs⁻¹ to 500 mVs⁻¹. The obtained CV results for the electro-oxidation of 350 µM hydrazine using MoSe₂-GC at scan rates of 50–500 mVs⁻¹ are displayed in Figure 6a. The obtained CV trend shows that the current for the electro-oxidation of hydrazine increases when the scan rate changes from 50 mVs⁻¹ to 500 mVs⁻¹. This increment in the current for the electro-oxidation of hydrazine was found to be linear as shown in the calibration plot between the electro-oxidation peak current and applied scan rate (Figure 6b).

Further, we explored linear sweep voltammetry (LSV) as an electrochemical sensing technique towards the sensing of hydrazine. The LSV of the GC was obtained in 350 µM hydrazine at a scan rate of 50 mVs⁻¹ (Figure 7). The collected LSV data of the GC revealed the current response of 8.14 µA for the electro-oxidation of hydrazine (Figure 7). Subsequently, electrochemical sensing behavior of the MoSe₂-GC was also examined under similar conditions (350 µM hydrazine; scan rate = 50 mVs⁻¹). The collected LSV result for MoSe₂-GC is presented in Figure 7. The observations indicated that MoSe₂-GC has an improved current of 15.86 µA for the electro-oxidation of 350 µM hydrazine under an applied scan rate of 50 mVs⁻¹ (Figure 7). Therefore, the MoSe₂-GC has good electro-catalytic behavior towards the determination of hydrazine. Hence, our research group selected this MoSe₂-GC for further hydrazine sensing examinations.

Hydrazine concentration may play a significant role in the electro-oxidation process using MoSe₂-GC. Therefore, it would be beneficial to study the effect of the concentration of hydrazine on the electro-catalytic behavior of the MoSe₂-GC. In this regard, we have used the LSV technique to study the influence of the hydrazine’s various concentrations (0.05 µM to 350 µM).

The LSV response of the MoSe₂-GC was recorded in the presence of various concentrations (0.05 µM to 350 µM) of hydrazine. The recorded LSV responses of the MoSe₂-GC in various concentrations (0.05 µM to 350 µM) of hydrazine are shown in Figure 8a. The observations revealed that the current for the electro-oxidation of hydrazine increased with respect to the concentration of hydrazine. The electro-catalytic current response was found to be directly proportional to the concentration of hydrazine. The current for the electro-oxidation of hydrazine increased linearly with respect to the concentration of hydrazine, as shown in the calibration curve between the current versus the concentration of hydrazine (Figure 8b).

The sensing of hydrazine at the MoSe₂-GC surface may involve the electro-oxidation of hydrazine. The electrochemical sensing of hydrazine at the MoSe₂-GC surface has been described in Scheme 1. It can be assumed according to previous studies that the electro-oxidation process of the hydrazine is an irreversible oxidation process wherein hydrazine hydrate is converted to nitrogen (N₂) and hydronium ions (H₃O⁺) with four electrons. The overall sensing mechanism can be further explained according to the reactions given below:

N₂H₄ + H₂O → N₂H₃ + H₃O⁺ + e⁻ (1)

N₂H₃ + 3H₂O → N₂ + 3H₃O⁺ + 3e⁻ (2)

In Equation (1), it can be clearly seen that first step is the rate-determining step, which is a slow process and involves the transfer of one electron. In Equation (2), it is a fast step that involves the release of three electrons, and it is suggested that the electro-oxidation of hydrazine is an irreversible process (Scheme 1).

The limit of detection (LoD) and sensitivity of the MoSe₂-GC for hydrazine detection were calculated by using the equations given below:

LoD = 3 × σ/S(3a)

Sensitivity = S/A(3b)

(S = slope, σ = standard deviation and A = area of GC).

The MoSe₂-GC demonstrated the LoD of 0.091 µM and sensitivity of 0.68 µA/µMcm² which are compared with reported sensors in Table 1 [9,37,38,39,40,41,42,43,44,45,46].

Ni et al. [37] adopted the hydrothermal approach for the fabrication of flowers such as ZnO and hierarchical ZnO. The fabricated flowers were used for the construction of the hydrazine sensor. The surface of the gold electrode was modified with the prepared flowers as hydrazine sensing materials [37]. Furthermore, the authors explored CV and amperometric techniques for the sensing of hydrazine, and an LoD of 0.25 µM was obtained for ZnO flowers/Au electrode [37]. Although ZnO has excellent electro-catalytic ability for electrochemical oxidation or reduction reactions, its poor conductivity remains a challenge for its application in electrochemical devices or sensors. The conductivity of the ZnO could be improved by introducing conductive support to the ZnO nanostructures. In this regard, Zhang et al. [38] fabricated Au nanoparticles (nano Au) and mixed them with ZnO-MWCANTs films. Further, the authors modified GCE with the fabricated Au/ZnO/MWCNT as a hydrazine sensing material. The developed hydrazine sensor showed an LoD of 0.15 µM [38]. This indicated that the presence of conductive support enhanced the sensing behavior of the ZnO. In other study, Ghanbari et al. [39] developed a hydrazine sensor by employing a polymeric composite. A silver nanoparticle/polypyrrole (PPy) composite matrix was fabricated, and its electrochemical sensing property for hydrazine sensing was studied by using the CV technique. An LoD of 0.2 µM was obtained by using this fabricated electrode (AgNPs/PPy/GCE) [39]. In another report, ZnO was incorporated with poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) to develop a highly sensitive hydrazine sensor. In this regard, the authors developed an inkjet-printed sensor which was comprised of ZnO/PEDOT:PSS encapsulated with nafion. Amperometric investigations were performed and an LoD of 5 µM was obtained [40]. The surface area of the sensing materials can also play a significant role, and it is really interesting to explore the high surface area electrode materials towards the determination of hydrazine. In this connection, Wang et al. [41] prepared novel electrode material for hydrazine sensing application. Poly(styrene sulfonate)/graphene was prepared and deposited on the surface of the GCE. This fabricated electrode (poly(styrene sulfonate)/graphene/GCE) was further used as a hydrazine sensor, and the observed results exhibited the presence of good electro-catalytic ability for the electro-oxidation of hydrazine, and an LoD of 1 µM was achieved [41]. The hydrazine sensing ability of the modified GCE can be easily tuned by the presence of sensing materials on the GCE surface. Majumder et al. [42] obtained three-dimensional (3D) α-Fe₂O₃ micro-snowflake architectures via the hydrothermal route. The fabricated ITO substrate with 3D-α-Fe₂O₃ (3D-α-Fe₂O₃/ITO) was used as a hydrazine sensor which demonstrated a good LoD of 5 µM [42]. Afshari et al. [43] reported the fabrication of a hydrazine sensor using a ternary composite. A polyaniline/graphitic carbon nitride/Ag composite (PANI/g-CN/AgNPs) was fabricated on an FTO electrode. This sensor exhibited an LoD of 300 µM [43]. Perovskite materials have good properties and have been explored in the development of electrochemical sensing devices. Another report by Faisal et al. [44] showed the sensing role of SrTiO₃/PANI composite for the determination of hydrazine. The SrTiO₃/PANI composite-based hydrazine sensor exhibited an LoD of 1.09 µM, which is due to the synergistic interaction between PANI and SrTiO₃ [44]. Manganese dioxide (MnO₂) is a widely used sensing material, and Wu et al. [45] fabricated a flower-shaped MnO₂ using the hydrothermal route. The hydrazine sensor was developed using a flower-shaped MnO₂ as a hydrazine sensing material, and an LoD of 2.06 µM was achieved [45]. Tungsten trioxide (WO₃) was also fabricated on an Au electrode by Shukla et al. [46] and used as a hydrazine sensor. This fabricated electrode (WO₃/Au) exhibited an LoD of 144 µM [46]. In another report, Ding et al. [9] prepared porous Mn₂O₃ nanofibers via the electrospinning approach and used it as a hydrazine sensing material. The fabricated hydrazine sensor demonstrated an LoD of 0.3 µM [9]. In the present study, we have demonstrated the hydrazine sensing behavior of MoSe₂-GC, which exhibits a reasonably good LoD of 0.091 µM and can be compared with previous reports shown in Table 1.

Cyclic stability and repeatability are the most important parameters for the practical use of any sensor. Therefore, we have also studied the cyclic repeatability and stability of the MoSe₂-GC for hydrazine sensing. The LSV curves of the MoSe₂-GC were obtained in 100 µM hydrazine under an applied potential scan rate of 50 mVs⁻¹. The 1st, 20th, 50th, and 100th LSV curve of the MoSe₂-GC in 100 µM hydrazine are displayed in Figure 9. These studies showed that MoSe₂-GC retained good cyclic stability or cyclic repeatability in the 100th cycles.

The electrochemical sensors should possess a high anti-interference nature. In this regard, our research group investigated the anti-interference nature of the MoSe₂-GC using LSV. The LSV curves of the MoSe₂-GC were obtained in 100 µM hydrazine and 100 µM hydrazine + 500 µM interfering compounds (phenol, catechol, urea, ascorbic acid, glucose, dopamine, hydrogen peroxide, NaCl, and uric acid). The obtained LSV curves are displayed in Figure 10. According to Figure 10, the presence of various interfering compounds could not affect the current response of the MoSe₂-GC for hydrazine determination. Thus, it can be easily said that MoSe₂-GC has a good anti-interference nature. We have also obtained the Raman spectrum of MoSe₂ after electrochemical investigations (Figure S1). There was no significant change observed.

4. Conclusions

In this study, we have adopted sol-gel as a synthetic approach for the preparation of MoSe₂. This fabricated MoSe₂ was characterized by various advanced physiochemical techniques. Furthermore, a hydrazine sensor was developed by utilizing synthesized MoSe₂ as the electrode material. The glassy carbon electrode was modified with synthesized MoSe₂ and employed as a working electrode towards the determination of hydrazine via linear sweep voltammetry (LSV). The fabricated hydrazine sensor exhibited a good limit of detection and sensitivity. Moreover, excellent selectivity, repeatability, and stability were also observed for hydrazine sensing.

Author Contributions

Conceptualization, A.A.; and H.A.; methodology, A.A.; formal analysis, H.A.; investigation, H.A.; resources, H.A.; writing—original draft preparation, H.A.; writing—review and editing, A.A.; supervision, A.A.; funding acquisition, A.A.; All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not available.

Acknowledgments

The authors extend their appreciation to the Deputyship for Research & Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project no. IFKSURG2-1261.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures, Scheme and Table

Scheme 1. Schematic representation of the surface modification of GC.

Figure 1. XRD data of MoSe2.

Figure 2. SEM images of MoSe2 (a,b).

Figure 3. EDS spectrum of MoSe2.

Figure 4. Mo3d (a) and Se3d (b) XPS scan of MoSe2.

Figure 5. CV responses of the GC and MoSe2-GC for 350 µM hydrazine (scan rate = 50 mVs−1).

Figure 6. CV responses (a) of MoSe2-GC for 350 µM hydrazine at varied scan rates (50–500 mVs−1) and calibration curve (b) of the current response against scan rate.

Figure 7. LSV responses of the GC and MoSe2-GC for 350 µM hydrazine (scan rate = 50 mVs−1).

Figure 8. LSV responses of MoSe2-GC in different concentrations of hydrazine (0.05 µM, 5 µM, 15 µM, 25 µM, 35 µM, 45 µM, 55 µM, 65 µM, 75 µM, 85 µM, 100 µM, 120 µM, 140 µM, 160 µM, 180 µM, 200 µM, 225 µM, 250 µM, 275 µM, 300 µM, 325 µM, 350 µM) under applied potential scan rate of 50 mVs−1 (a) and calibration plot (b) between current response and concentrations.

Figure 9. LSV response of MoSe2-GC (1st, 20th, 50th and 100th) for 100 µM hydrazine (scan rate = 50 mVs−1).

Figure 10. LSV responses of MoSe2-GC for 100 µM hydrazine and 100 µM hydrazine + 500 µM interfering compounds (phenol, catechol, urea, ascorbic acid, glucose, dopamine, hydrogen peroxide, NaCl, and uric acid) at scan rate = 50 mVs−1.

Table 1

Comparison of the electrochemical sensing performance of MoSe₂-GC with reported sensors.

Sensing Material	Detection Limit (µM)	Sensitivity (µA/µMcm2)	References
MoSe2-GCZnO/nafion/Au	0.0910.25	0.68-	This study [37]
Au/ZnO/MWCNT/GCE	0.15	0.0428	[38]
AgNPs/PPy/GCE	0.2	0.0114	[39]
ZnO NPs/PEDOT:PSS	5	0.14	[40]
Flower-like ZnO	2.1	0.095	[37]
poly(styrene sulfonate)/graphene/GCE	1	-	[41]
3D-α-Fe2O3	5	0.024	[42]
PANI/g-C3N4/AgNPs	300	-	[43]
PANI/SrTiO3/GCE	1.09	0.21	[44]
MnO2 nanostructureWO3 NPsPorous Mn2O3	2.061440.3	0.1090.1850.474	[45] [46] [9]

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst13020161/s1, Figure S1: Raman of MoSe₂.

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