Method for obtaining transition metal dichalcogenides, product obtained and uses of same

Abstract

The present invention relates to a method for obtaining transition metal dichalcogenides (TMDs) in the form of a powder of overlapping sheets with a high number of edges. The powdered product obtained is especially used in chemical sensors, in particular gas sensors, and may have other uses; for example, in inks. The method of the invention solves most of the problems of synthesising TMDs using other existing techniques.

Description

[0001] PROCEDURE FOR OBTAINING TRANSITION METAL DICALCOGENIDES, PRODUCT OBTAINED AND USES GIVEN TO IT

[0002] DESCRIPTION

[0003] The present invention relates to a process for obtaining transition metal dichalcogenides in the form of a powder of overlapping sheets with a high number of edges. The resulting powder is used especially in chemical sensors, particularly gas sensors.

[0004] Background of the invention

[0005] Transition metal dichalcogenides (DMTs) have generated widespread research interest for physical and chemical detection applications due to their crystalline structure and large effective surface area.

[0006] Transition metal dichalcogenides have the chemical formula MX2, where M is a transition metal and X is a chalcogen (such as sulfur (S), selenium (Se) or tellurium (Te)).

[0007] The synthesis of these compounds has been the subject of study. For example, US patent 10643842 describes a method for fabricating two-dimensional transition metal dichalcogenide (DMT) nanomaterials. The method includes synthesis on a substrate.

[0008] However, it is necessary to develop new high-performance procedures that avoid substrate synthesis to improve the applications of DMTs obtained in various applications, especially in chemical sensors.

[0009] Description of the invention

[0010] An objective of the present invention is a process for obtaining transition metal dichalcogenides in the form of a powder of overlapping sheets with a high number of edges. The objective is to develop a process that is high-yield, reproducible, and low-cost.

[0011] The invention process obtains DMTs from the direct reaction of powdered transition metal oxides with sulfur, selenium, and tellurium, using a chemical vapor deposition reactor at atmospheric pressure.

[0012] The reactor is a tube-within-a-tube design: a larger-radius tube with open ends and a smaller-radius tube with one open and one closed end. The closed end is the key component of this technique. This configuration provides better confinement of the chalcogen vapor around the metal oxide powder and eliminates the need for hydrogen or alkali halide salts. Neither hydrogen nor alkali halides are used in the reaction atmosphere. Sulfur, selenium, or tellurium is deposited on two different carriers at different temperatures and positions to create an atmosphere rich in the vapor of these compounds around the oxide carrier.

[0013] The invented procedure solves most of the problems associated with DMT synthesis using other existing techniques, such as manufacturing cost. The advantages of the procedure are: high yield, good material quality, reproducibility of synthesis, and a suitable synthesis time.

[0014] Therefore, a first aspect of the invention relates to a process for obtaining transition metal dichalcogenides in powder form comprising the steps of: a) positioning the reagents in a tube-in-tube reactor, a larger radius tube with open ends and a smaller radius tube with one open end and one closed end, as follows; first, positioning inside the smaller radius tube, two carrier elements, a first carrier with transition metal oxide powder located at the closed end and a second carrier with sulfur, selenium or tellurium powder next to the first carrier, second, placing this tube inside the larger radius tube, third, placing at the entrance of the larger radius tube on the side where the smaller radius tube has the open end, a third carrier with sulfur, selenium or tellurium powder;b) removing oxygen from the reactor by means of a noble gas flow, where the flow enters through the end closest to the third carrier and exits through the opposite end; c) heating the reactor so as to create a temperature profile with a central zone of higher temperature and two zones of lower temperature, relative to the central zone, at the ends where the first and second carrier elements are located in step a); d) reducing the noble gas flow of step b); e) when the reaction temperature is reached, pushing the third carrier to the lower temperature zone and pushing the first and second carriers to the central zone; wherein the sulfidation of the transition metal oxide powder occurs throughout steps c), d) and e) by chemical vapor deposition at atmospheric pressure.

[0015] As detailed, the noble gas flow is reduced from stage b) to d), this decrease in flow rate is one of the key factors affecting the success of the invention's procedure.

[0016] The inventive process yields DMTs in powder form, consisting of overlapping sheets with a high number of edges. These edges are defects that present active sites, allowing the material, when used as a chemical sensor, to react with the molecules to be detected. The sheets have a very large lateral surface area with a vertical growth direction, which enriches the material with active sites and significantly improves its chemical and electrical properties.

[0017] Therefore, a second aspect of the invention relates to powdered transition metal dichalcogenides obtained according to the procedure of the first aspect of the invention with a sheet thickness in the range of 24 nm to 40 nm.

[0018] The synthesized TMD powders can be used to develop ink formulations in solvents of choice for screen printing or inkjet printing, spin coating, drop pouring, or spraying onto a wide range of substrates such as ceramics, paper, silicon-based materials, or flexible polymers, for various applications, including electronics, photonics, sensors, and energy devices. Therefore, another aspect of the invention relates to an ink comprising the transition metal dichalcogenides of the second aspect of the invention.

[0019] As mentioned, the presence of a high number of edges on the DMT sheets obtained by the process of the invention can provide additional active sites for chemical reactions, improving the sensitivity of the sensors. Therefore, another aspect of the invention is the use of the DMTs obtained by the process according to the first aspect of the invention in chemical sensors. Also an aspect of the invention is the sensor comprising the DMTs according to the second aspect of the invention deposited on a substrate with electrodes. The sensors provide a stable response and perform well even in humid environments. Finally, another aspect of the invention is the method of gas detection in the sensor defined herein, where detection is carried out at a temperature between 20°C and 150°C.

[0020] Brief description of the drawings

[0021] Figures 1A and 1B show a schematic of the reaction procedure.

[0022] Figure 2A shows the sensor responses to 5 ppm of NH3 at different temperatures. The response (%) is plotted against the temperature (°C). Figure 2B shows the actual changes in film resistance over time at 5 ppm of NH3 at 150°C. The resistance (megaohms) is plotted against time (s).

[0023] Figure 3A shows the response of the WS2 sensor as a function of NH3 concentration at 150°C, with the response (%) plotted against NH3 (ppm). Figure 3B shows the changes in resistance of the gas-sensing film as a function of time at different NH3 concentrations at 150°C. Resistance (ohms) is plotted on a vertical axis, and NH3 (ppm) on a second vertical axis, versus time in seconds.

[0024] Figure 4 shows a response graph of WS2 to NO2 (800 ppb), H2 (100 ppm), carbon monoxide (80 ppm), ammonia (5 ppm), and benzene (5 ppm) at 150°C. Figure 5 shows the sensor's sensitivity in dry and humid conditions to 5 ppm of NH3 at 150°C. The sensor resistance changes are normalized to [0,1] and are plotted in ohms versus time in seconds.

[0025] Description of a preferred embodiment

[0026] As stated, the first aspect of the invention relates to a process for obtaining transition metal dichalcogenides in powder form comprising the steps of: a) positioning the reagents in a tube-in-tube reactor, a larger radius tube (1) with open ends and a smaller radius tube (2) with one open end (2.1) and one closed end (2.2), as follows; first, positioning inside the smaller radius tube (2) two carrier elements, a first carrier (3) with transition metal oxide powder located at the closed end (2.1) and a second carrier (4) with sulfur, selenium or tellurium powder next to the first carrier (3), second, placing this tube inside the larger radius tube (1), third, placing at the entrance of the larger radius tube (1) on the side where the smaller radius tube (2) has the open end (2).1) a third carrier (5) with powdered sulfur, selenium, or tellurium; b) removing oxygen from the reactor by means of a noble gas flow, wherein the flow enters at the end nearest to the third carrier and exits at the opposite end; c) heating the reactor so as to create a temperature profile with a central zone (6) of higher temperature and two zones (7) of lower temperature, relative to the central zone, at the ends where the first and second carrier elements are located in step a); d) reducing the noble gas flow of step b); e) when the reaction temperature is reached, pushing the third carrier to the lower temperature zone (7) and pushing the first (3) and second carrier (4) into the central zone (7); wherein the sulfidation of the transition metal oxide powder occurs along steps c), d), and e) by atmospheric pressure chemical vapor deposition.

[0027] Preferably, the larger tube (1) has a length between 1 m and 1.5 m and a diameter between 30 mm and 40 mm. Preferably, the smaller tube (2) has a length between 0.1 m and 0.5 m and a diameter between 20 mm and 29 mm. In fact, the dimensions, as well as the geometry, play an important role in the success of the TMD synthesis. With these small dimensions (volume), oxygen can be easily removed from the tube during the cleaning phase, before the reaction begins, without the need to install a vacuum pump; therefore, the reaction can be carried out at atmospheric pressure.

[0028] Preferably, step b) is performed with an inert gas flow rate between 90 ml / min and 110 ml / min for 45 min to 90 min. Preferably, in step d), the inert gas flow rate is between 30 ml / min and 4 ml / min. Specifically, the inert gas is argon.

[0029] Preferably, the distance between the third carrier and the end of the tube with the smallest radius in stage a) is between 10 and 15 centimeters. This distance is very important, as it directly affects the growth rate, the rate of sulfidation, selenization or theorization, and the size of the lamellae.

[0030] In one particular case, tungsten oxide, WO3, reacts with S to obtain WS2. In this particular case, in step c), the first carrier (3) and the second carrier (4) are in a temperature range of 300° to 400°C, while the third carrier, located at the entrance of the larger radius tube, is in a temperature range of 40°C to 90°C. In step e), the first (3) and second (4) carriers are located in the central zone (6) at a temperature between 800°C and 950°C.

[0031] Specifically, the reaction time is between 45 minutes and 90 minutes.

[0032] Preferably the temperature in stages c) to e) is reached by ramp heating in a range between 35°C / min and 40°C / min.

[0033] As mentioned, another aspect of the invention is a sensor comprising the DMTs according to the second aspect of the invention deposited on a substrate with electrodes. Preferably, the substrate is selected from alumina, polymethyl methacrylate (PMMA), silicon, and glass. Preferably, the sensor is a gas sensor where the DMT is WS2; in particular, it is a sensor for a gas selected from: NH3, CO, CeHe, H2, and NO2. Specifically, NO2 and NH3. The sensor detects NO2 at room temperature. The sensors are stable and reproducible at NO2 levels of 100 ppb. The sensor detects NH3 at levels below 1 ppm, demonstrating its remarkable sensitivity even in low-concentration environments.

[0034] EXAMPLES

[0035] 1. Obtaining WS2

[0036] Commercially available tungsten trioxide powder purchased from Sigma Aldrich (CAS: 1314-35-8) was used and sulfided to obtain WS2 powder nanostructures in the form of sheets with a high number of edges.

[0037] The sulfidation reaction was carried out at 900°C for 60 minutes using sulfur powder (Sigma Aldrich, CAS: 7704-34-9) in a temperature gradient tube-in-tube reactor. The schematic of the setup is shown in Fig. 1. The middle zone of the furnace reaches the set temperature of 900°C; in Fig. 1, this zone is shown with the red coil. The zones adjacent to the middle zone are at lower temperatures (approximately 400°C) compared to the reactor's middle zone. Three corundum carriers were placed within different temperature zones of the furnace; one carrier contained 100 mg of WO3 precursor powder, while the other two carriers each contained 1 g of sulfur. The carrier containing WO3 was placed next to a can containing sulfur inside a semi-sealed secondary quartz tube, both located in the 900°C temperature zone.The carrier outside the secondary quartz tube is placed upstream of the argon flow inside the larger quartz tube. Before the sulfidation process, the quartz tube was purged with 100 ml / min of argon to remove any traces of oxygen present in the reactor. The argon flow was maintained at 30 ml / min during the reaction. The furnace was programmed to heat from ambient temperature to the set temperature of 900°C at a rate of 40°C / min. As soon as the furnace reached 900°C, the outer quartz tube was carefully positioned so that the sulfur container, initially located outside the furnace, was within the 400°C temperature zone of the furnace (Fig. 1b). This strategic placement of the carriers facilitates the creation of a sulfur-rich environment that ensures complete sulfidation of the WO3. This configuration guarantees double sulfidation. The furnace was allowed to cool naturally after the reaction was complete.

[0038] Example 2. Obtaining sensors with the product obtained in Example 1

[0039] The WS2 sheets were deposited, using an airbrush system, onto commercial alumina transducer substrates (Ceram Tech GmbH, Germany), which have interdigitated Pt electrodes with a separation of 300 pm on the front and a resistive Pt heater.

[0040] Before coating with WS2, the substrates were cleaned by sequential sonication in acetone, ethanol, and deionized water, followed by air drying. After that, the cleaned substrates were placed on the heating plate in the airbrush system.

[0041] Ten milliliters of WS2 powder were sonicated in 10 ml of ethanol (Scharlab, CAS: 64-17-5) for 1 hour to produce a brown suspension. The resulting solution was then transferred to the airbrush container, the substrate transducer electrodes were connected to a multimeter to monitor the resistance of the deposited films, and the hot plate was turned on and set to 55°C. Finally, the solution was airbrushed onto the alumina substrates using N2 gas as a carrier.

[0042] Example 3. Gas measurements with the sensor from Example 2

[0043] Gas detection measurements using the sensors obtained in Example 2 were performed using a gas detection system with a 35 ml Teflon chamber. The chamber could accommodate four sensors simultaneously. It has an inlet and an outlet connected to the gas supply and exhaust systems, respectively. The chamber was connected to a fully automated gas flow measurement setup capable of supplying dilute gas mixtures via mass flow controllers (Bronkhorst High-Tech BV). Calibrated and balanced gas cylinders filled with dry synthetic air (Air Premier purity: 99.999%) were used for the gas detection measurements. Sensor operating temperatures were controlled by connecting the sensor heaters to an external power source (Agilent LI8002A).Sensor responses were recorded using an Agilent-34972A data acquisition system, monitoring the resistance of the sensor material after exposure to different concentrations of target gases, including NH3, NO2, H2, CO, and benzene. The sensors operated at room temperature, 100°C, and 150°C. To ensure initial stabilization, a continuous dry airflow of 100 ml / min was maintained in the chamber for 3 hours before gas detection measurements began. The sensors were exposed to the target gases for 10 minutes, followed by exposure to dry air to recover and stabilize the baseline. The initial recovery time was adjusted according to the sensor's operating temperature: 60 minutes for 100°C and 150°C, and 120 minutes for room temperature operation. Throughout the gas detection measurements, a general airflow of 100 ml / min was maintained.

[0044] For reducing species such as NH3, the sensor response was calculated using equation 1,

[0045] R = (Rgas-Rair * 100) / Rair while for oxidizing species such as NO2, the relative response was calculated using equation 2.

[0046] R = (Rair-Rgas * 100 ) / Rair

[0047] Rair and Rgas are the real-time resistances of the sensors when exposed to air and the target gas, respectively.

[0048] The gas detection properties of the manufactured WS2 sensors were tested for ammonia gas and evaluated using a gas monitoring system. To determine the optimal operating temperature, the sensors were tested at 5 ppm NH3, at room temperature, 100°C, and 150°C.

[0049] Figure 2A shows the sensor responses to 5 ppm NH3 at different operating temperatures ranging from 25°C to 150°C. As shown in Figure 2A, the sensor responses increase with increasing operating temperature. The standard deviation of the sensor responses is negligible, indicating stable and highly reproducible detection characteristics.

[0050] Furthermore, the sensor's response at room temperature to 5 ppm NH3 was calculated to be 0.89%, increasing to 80% at 100 °C and to 100% at the optimum operating temperature (150 °C). Figure 2B shows the actual changes in film resistance over time at 5 ppm NH3 at 150 °C. When exposed to ammonia, a reducing gas, the WS2 sensor responds as a p-type semiconductor, exhibiting an increase in resistance. The sensors show stable and reproducible responses to the target gas. The sensor's baseline recovers well after each exposure cycle.

[0051] Furthermore, the sensors were tested with a wide range of NH3 gas concentrations, from 1, 2.5, 5, 7.5, and 10 ppm, at the optimum operating temperature. The responses were calculated to be 24.45%, 53.44%, 100%, 135%, and 168%, respectively.

[0052] As shown in Fig. 3A, there is a linear increase in response with increasing ammonia concentration. The standard deviation error of the responses is so small that it is barely visible in the figure, indicating stable and reproducible detection responses from the sensors. As anticipated, increasing the NH3 concentration leads to larger resistance changes in the WS2 sensor, resulting in improved responses. The linear trend of the sensor responses reveals that the sensors do not saturate with exposure to ammonia concentrations below 10 ppm. The dynamics of the change in resistance of the gas sensor film towards different ammonia concentrations are shown in Fig. 3B. The sensors can detect ammonia concentrations as low as 1 ppm.

[0053] Selectivity is one of the most important criteria for determining sensor performance. The selectivity of WS2 sensors was evaluated with respect to fixed concentrations of various interfering gases, such as benzene, carbon monoxide (CO), nitrogen dioxide (NO2), and hydrogen (H2), at a temperature of 150°C. These analyte gases are particularly important for testing selectivity due to their potential health and environmental risks. The radar chart shown in Fig. 4 illustrates the sensor responses to the tested gases at 150°C. The results show that, among all the gases, the sensors respond only to NH3 and CeHe, with the highest response to NH3. These results suggest high selectivity for NH3 gas.

[0054] Finally, the performance of the gas sensor in the presence of moisture was evaluated to verify its suitability for real-world applications. Figure 5 illustrates the responses of the WS2 sensor to 5 ppm of NH3 in dry and humid environments (50% relative humidity at 25°C). It was observed that the sensor response decreased slightly from 100% to 93%. The initial resistance decreased from approximately 60 MΩ in a dry environment to approximately 50 MΩ in a humid environment. Therefore, the sensors of the invention exhibited resistance to the presence of moisture. This characteristic makes them well-suited for real-world applications.

Claims

CLAIMS 1. A process for obtaining transition metal dichalcogenides in the form of a powder of overlapping sheets with a high number of edges, characterized in that it comprises the steps of: a) positioning the reagents in a tube-in-tube reactor, a larger radius tube (1) with open ends and a smaller radius tube (2) with one open end (2.1) and one closed end (2.2), as follows; first, positioning inside the smaller radius tube (2) two carrier elements, a first carrier (3) with transition metal oxide powder located at the closed end (2.1) and a second carrier (4) with sulfur, selenium or tellurium powder next to the first carrier (3), second, placing this tube inside the larger radius tube (1), third, placing at the entrance of the larger radius tube (1) on the side where the smaller radius tube (2) has the open end (2.1) a third carrier (5) with powdered sulfur, selenium, or tellurium; b) removing oxygen from the reactor by means of a noble gas flow; c) heating the reactor so as to create a temperature profile with a central zone (6) with a higher temperature and two zones (7) with a lower temperature, relative to the central zone, at the ends where the first carrier element and the second carrier are located in step a); d) reducing the noble gas flow of step b); e) when the reaction temperature is reached, pushing the third carrier to the lower temperature zone (7) and pushing the first (3) and second carrier (4) into the central zone (7); wherein the sulfidation of the transition metal oxide powder occurs along steps c), d), and e) by atmospheric pressure chemical vapor deposition.

2. A method according to claim 1 characterized in that step b) is performed with an inert gas flow between 90 ml / min and 110 ml / min.

3. A method according to any of claims 1 to 2 characterized in that in step d) the inert gas flow is between 30 ml / min and 4 ml / min.

4. A process according to any of claims 1 to 3, characterized in that the inert gas is argon.

5. The obtaining process according to any of claims 1 to 4 characterized in that the reaction temperature is reached by ramp heating in a range between 35°C / min and 40°C / min.

6. Transition metal dichalcogenides in powder form of overlapping sheets, obtained according to any of claims 1 to 5, having a sheet thickness in the range of 24 nm to 40 nm.

7. Ink comprising the transition metal dichalcogenides defined in claim 6.

8. Use of the transition metal dichalcogenides defined in claim 6 in chemical sensors.

9. Sensor comprising the transition metal dichalcogenides defined in claim 6.

10. Method of gas detection in the sensor defined in claim 6 characterized in that the detection is carried out at a temperature between 20°C and 150°C.

Images