COBALT SULFIDE/REDUCED GRAPHITE OXIDE COMPOSITE AND APPLICATION THEREOF IN GAS SENSORS

Abstract

A method for preparing a cobalt sulfide/reduced graphite oxide composite includes: preparing a glycerol-cobalt precursor by taking a water-soluble cobalt salt, a micromolecular alcohol solvent, and glycerol as raw materials; mixing the glycerol-cobalt precursor with an alkali liquor to prepare a Co(OH).sub.2 nanoflower; calcining the Co(OH).sub.2 nanoflower to obtain a Co.sub.3O.sub.4 nanoflower; subjecting the Co.sub.3O.sub.4 nanoflower to a reaction with a water-soluble sulfur salt to obtain a COS nanoflower, and mixing the COS nanoflower with graphite oxide and carrying out a heat treatment to obtain the composite. The response characteristics of a gas sensor to NO.sub.2 gas are studied at room temperature, and the graphite is complexed with a transition metal sulfide with unique morphology to construct a unique heterostructure. While expanding the specific surface area to increase the number of adsorption sites, the heterostructure of a contact surface is used to greatly enhance the charge-transfer efficiency.

Claims

1. A cobalt sulfide/reduced graphite oxide composite, comprising CoS nanoflowers and graphite covered on the CoS nanoflowers.

2. The cobalt sulfide/reduced graphite oxide composite according claim 1, wherein the graphite is a reduced graphite oxide.

3. A method for preparing the cobalt sulfide/reduced graphite oxide composite according claim 1, comprising mixing the CoS nanoflowers with a graphite oxide and conducting a heat treatment to obtain the cobalt sulfide/reduced graphite oxide composite.

4. The method for preparing the cobalt sulfide/reduced graphite oxide composite according claim 3, comprising a CoS nanoflower dispersion with a graphite oxide dispersion, subjecting to centrifugation treatment to obtain a precipitate, and heating the precipitate at 150-250 C. for 60-120 minutes to obtain the cobalt sulfide/reduced graphite oxide composite.

5. The method for preparing the cobalt sulfide/reduced graphite oxide composite according claim 3, wherein a mass ratio of the CoS nanoflowers to the graphite oxide is 0.2-20:1.

6. The method for preparing the cobalt sulfide/reduced graphite oxide composite according to claim 5, wherein the mass ratio of the CoS nanoflowers to the graphite oxide is 2-10:1.

7. The method for preparing the cobalt sulfide/reduced graphite oxide composite according to claim 3, wherein a water-soluble cobalt salt, a small molecule alcohol solvent, and glycerin are used as starting materials to prepare a glycerol cobalt precursor; the glycerol cobalt precursor is mixed with an alkaline solution to prepare Co(OH).sub.2 nanoflowers; and the Co(OH).sub.2 nanoflowers are calcined to obtain Co.sub.3O.sub.4 nanoflowers; and the Co.sub.3O.sub.4 nanoflowers reacts with a water-soluble sulfur salt to obtain the CoS nanoflowers.

8. A gas sensor, comprising a conductive base, an interdigital electrode, and a gas sensitive material on a surface of the interdigital electrode, wherein the gas sensitive material is the cobalt sulfide/reduced graphite oxide composite of claim 1.

9. A method for detecting nitrogen oxides in an environment, wherein the gas sensor according to claim 8 is placed in the environment to be detected to complete the detection of the nitrogen oxides in the environment.

10. An application of the cobalt sulfide/reduced graphite oxide composite according to claim 1 in the preparation of gas sensors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 shows the SEM pattern of CoS nanoflowers.

[0014] FIG. 2 shows an SEM pattern of CoS/rGO composite.

[0015] FIG. 3 shows the physical image of the sensor with the composite material.

[0016] FIG. 4 shows the performance comparison tests of Co.sub.3O.sub.4, CoS, rGO, and CoS/rGO.

[0017] FIG. 5 shows the performance testing of CoS/rGO composite materials with different graphite contents: CoS/rGO-1, CoS/rGO-2, CoS/rGO-3, CoS/rGO-4, CoS/rGO-5, and CoS/rGO-6.

[0018] FIG. 6 shows the gas sensing response curves of the CoS/rGO-4 composite material sensor to different concentrations of NO.sub.2.

EXAMPLES OF THE PRESENT INVENTION

[0019] Graphite is a strong candidate for sensing materials due to its large specific surface area. However, in the prior art, gas sensors constructed with graphite sheet films have issues of low sensitivity and slow adsorption/desorption; the construction of graphite heterostructure in the present invention is an effective way to improve its performance. Transition metal sulfide has a unique stratified structure and good conductivity at room temperature, making it a research focus in gas sensing materials; the nanocomposite materials can not only compensate for the defect of the single material, but also provide more adsorption sites for gas adsorption, improving sensing performance. The rational design of the micro-nano structure is the key and challenge for high-performance gas sensors of the graphite heterostructure. The present invention combines graphite with transition metal sulfide of unique morphology to construct a unique heterostructure to increase the specific surface area and increase adsorption sites, and the heterostructure of the contact surface is utilized to significantly enhance charge transfer efficiency, thereby effectively improving gas sensitivity.

[0020] The raw materials of the present invention are all existing available products, and the specific preparation and testing methods are existing conventional methods.

Example 1 Preparation of Glycerol Cobalt Precursor

[0021] The glycerol cobalt precursor was prepared by hydrothermal synthesis under closed high-temperature conditions. The specific preparation process is as follows: [0022] (1) 0.291 g of cobalt nitrate hexahydrate was weighed and placed in a beaker. 23 ml of isopropyl alcohol and 4 ml of glycerin were added to the beaker with a graduated cylinder respectively, and stirred magnetically at room temperature for 30 minutes to form a red transparent mixed solution. [0023] (2) The mixed solution was transferred to a 50 ml reactor and placed in an oven to react at 160 C. for 6 hours. [0024] (3) After the reaction, the solution was naturally cooled to room temperature. The supernatant was sucked away with a dropper, and then the solution was collected by centrifugation, washed several times with anhydrous ethanol, and dried in an oven at 60 C. for 2 hours for later use.

Example 2 Preparation of Tricobalt Tetraoxide Nanoflowers

[0025] The glycerol cobalt precursor in Example 1 reacted under alkaline conditions to form cobalt hydroxide nanoflowers, which were further annealed at high temperature to obtain tricobalt tetraoxide nanoflowers. The specific preparation process is as follows. [0026] (1) 0.5 g of NaOH solid was weighed and dissolved in 50 ml of deionized water, and 0.25M of NaOH aqueous solution was prepared and stirred magnetically at room temperature for 30 minutes. [0027] (2) 0.1 g of glycerol cobalt precursor was weighed and dissolved in 5 ml of deionized water, ultrasonically dispersed for 10 minutes to obtain a uniform suspension. [0028] (3) The two solutions from (1) and (2) were mixed and magnetically stirred at room temperature for 30 minutes, and the products after reaction were centrifugally collected and washed several times with anhydrous ethanol, and dried in an oven under 60 C. for 10 hours to obtain Co(OH).sub.2 nanoflowers. [0029] (4) 0.1 g of Co(OH).sub.2 powder was weighed and placed in a clean quartz boat. And the quartz boat was placed in a tube furnace (air) and the temperature was raised to 350 C. at a rate of 5 C./min for 30 minutes, and the powder changed from brownish green to black to obtain Co.sub.3O.sub.4 nanoflowers.

Example 3 Preparation of Cobalt Sulfide (CoS) Nanoflowers

[0030] Cobalt sulfide nanoflowers were prepared through ion exchange reaction. 1.2 g of sodium sulfide nonahydrate was weighed and dissolved in 50 ml of deionized water and magnetically stirred for 30 minutes to form a uniform solution of sodium sulfide nonahydrate with a concentration of 0.1 M; 0.1 g of Co.sub.3O.sub.4 nanoflowers prepared in the Example 2 was weighed and added to the above solution for ultrasonic dispersion for 10 minutes; and the mixed solution was transferred to a 100 ml reactor and placed in an oven to react at 160 C. for 8 hours; after the reaction, the solution was naturally cooled to room temperature, the reaction products were collected by centrifugation, washed several times with deionized water, and dried in an oven at 60 C. for 3 hours to prepare CoS nanoflowers. FIG. 1 shows the SEM pattern of CoS nanoflowers.

Example 4 Preparation of CoS/rGO Composite Material

[0031] The graphite oxide of the present invention was prepared by the optimized Hummers method. The general principle is to use strong acid and strong oxidant to make oxidization intercalation of natural graphite flakes to increase the interlayer spacing of graphite flakes, and then use thermal expansion method to further increase the interlayer spacing. During the process of increasing the interlayer spacing, mechanical stirring and ultrasonic peeling are carried out to separate the graphite flakes to finally prepare the graphite oxide (GO), and the preparation process is as follows. [0032] 1) 2 g of natural graphite (500 mesh) was weighed and mixed with 50 ml of concentrated sulfuric acid in a 250 ml beaker, and stirred regularly for 30 minutes, and the concentrated sulfuric acid was used as an intercalator and solvent to complete the preliminary oxidation intercalation. [0033] 2) 1 g of sodium nitrate was added to the reaction solution to generate nitric acid in situ and stirred regularly for 2 hours in an ice bath to prevent rapid oxidation from affecting intercalation. [0034] 3) 7.3 g of potassium permanganate was added in three batches to the mixed solution and stirred in a 35 C. water bath for 2 hours to further complete the oxidation intercalation. [0035] 4) 150 ml of deionized water was added to the mixed solution and stirred for 30 minutes for heat release. The thermal expansion was used to further increase the interlayer spacing. Then 55 ml of 4% hydrogen peroxide solution was added dropwise to the mixed solution and stirred for 30 minutes to further oxidize and consume excessive potassium permanganate. After stirring, a brownish yellow GO suspension was obtained. [0036] 5) The brownish yellow supernatant solution was filtered and washed three times with dilute hydrochloric acid (3%, 100 ml), centrifuged three times, then dissolved in deionized water and dialyzed in a dialysis bag for one week. After dialysis, it was dried in an oven at 40 C. to obtain graphite oxide.

[0037] 0.01 g of CoS nanoflowers (prepared in Example 3) was weighed and ultrasonically dispersed in 2 ml of anhydrous ethanol to form a dispersion of 5 mg/ml; 0.2 ml of GO aqueous solution with a concentration of 10 mg/ml was taken and ultrasonically dispersed in 50 ml of deionized water to form a 0.04 mg/ml GO solution; CoS was added dropwise dispersively into the GO solution under stirring state and then magnetically stirred for 3 hours. The solution was collected by centrifugation and the bottom precipitate was taken and dried in an oven at 60 C. to obtain the CoS/GO composite material; the obtained CoS/GO composite material was insulated in an oven at 200 C. for 2 hours to produce CoS/rGO composite material. The mass ratio of CoS to GO was 5:1, and it was named CoS/rGO-4. FIG. 2 shows an SEM pattern of CoS/rGO composite material and the graphite nanosheets can fully cover the surface of CoS nanoflowers.

[0038] With the above preparation method, the mass ratio of CoS to GO was adjusted to 1:5, 1:2, 2:1, 10:1, and 20:1. After reduction by annealing, CoS/rGO composite materials with different graphite contents were named CoS/rGO-1, CoS/rGO-2, CoS/rGO-3, CoS/rGO-5, and CoS/rGO-6, respectively.

Example 5 Preparation of Gas Sensor

[0039] The interdigital electrode of the gas sensor used in this experiment was manufactured based on silicon technology and traditional micromachining technology. The preparation process is as follows: The cleaned silicon wafer was placed in a prepared mixed solution of concentrated H.sub.2SO.sub.4 and H.sub.2O.sub.2, and treated at 90 C. for half an hour to obtain a surface hydrophilic silicon wafer substrate. After washing and drying, the surface was spin coated with photoresist, and a conventional interdigital photomask was placed for exposure and development. Then, gold was sputtered on the substrate, and finally, ultrasonic peeling was carried out for the photoresist was to obtain the interdigital electrode. The interdigital electrode was prepared with a spacing of 10 microns, a width of 10 microns, and a length of 600 microns.

[0040] The prepared interdigital electrode was washed with acetone and deionized water several times, fixed on a metal base with glue, and the two ends of the interdigital electrode were connected to the metal base with gold wire. The prepared CoS/rGO composite material was dispersed in an ethanol solution and prepared as a 0.1 mg/ml solution. A micropipette was used to take 2 microliters of the solution and evenly drop it onto the interdigital electrode, which was dried in an oven for later use. The physical picture is shown in FIG. 3.

[0041] The methods of using other materials as gas sensitive layers to prepare the device are the same as this, except for the replacement of gas sensitive materials.

[0042] 0.1 mg/ml aqueous solution of GO was taken, and 2 microliters of the solution were uniformly dropped onto the interdigital electrode with a micropipette. It was annealed in an annealing furnace at 250 C. for 2 hours to obtain the rGO sensor component.

Example 6 Testing System for Gas Sensor

[0043] The testing system for gas sensor was completed through a conventional gas path. The entire testing system consisted of dry compressed air, the gas to be tested, a gas pipe, a flow controller, a switch, a gas mixing chamber, a testing chamber, a filter, and an Agilent semiconductor device analyzer. The testing system had two gas paths that can pass through the testing chamber, namely the background gas and the test gas. The background gas was dry compressed air, and the test gas was diluted with NO.sub.2 and dry compressed air. The background gas and the test gas can alternately pass through the testing chamber through the adjustment of the switch. During testing, the gas concentration can be adjusted through a flow controller to prepare the required test gas concentration. The calculation formula for NO.sub.2 gas concentration C.sub.N is:

[00001]$C_N=\frac{1000F_N}{F_N+F_A}.$

[0044] Wherein, the concentration of NO.sub.2 gas coming out of the NO.sub.2 gas cylinder is 1000 ppm, F.sub.N (sccm) is the flow rate of NO.sub.2 gas, and F.sub.A (slm) is the flow rate of compressed air used to dilute NO.sub.2 gas.

[0045] The gas sensing test was conducted at room temperature and was completed by the Agilent B1500A semiconductor device analyzer in conjunction with the testing gas circuit system. When testing, the voltage was set to 5 mV. Before starting the test, air was introduced to stabilize the system. The I-V mode of the semiconductor analyzer was used to determine whether the device was conducting, and the initial resistance R.sub.0 of the device was calculated accordingly.

[0046] FIG. 4 shows the performance comparison tests of Co.sub.3O.sub.4, CoS, rGO, and CoS/rGO. The prepared Co.sub.3O.sub.4, CoS, rGO or CoS/rGO composite materials were added dropwise onto the interdigital electrode to obtain a gas sensor. At room temperature, the gas sensitivity tests were conducted on Co.sub.3O.sub.4, CoS, rGO, and CoS/rGO composite gas sensors with a concentration of 1 ppm NO.sub.2. As shown in FIG. 4, the response and recovery time of the device are both 150 s. From the figure, it can be seen that Co.sub.3O.sub.4, CoS, rGO, and CoS/rGO composite material all exhibit p-type response characteristic in the oxidizing gas NO.sub.2 and the response values of Co.sub.3O.sub.4, CoS, rGO, and CoS/rGO composite gas sensors were 4.5%, 9.5%, 12.7%, and 39.7%, respectively, indicating good response values, which indicates that Co.sub.3O.sub.4, CoS, and rGO have the potential to be used as sensitive materials for detecting NO.sub.2 gas, while the response of CoS/rGO composite material is greatly increased, which ensures that CoS/rGO composite material can be used as a gas sensitive material for detecting NO.sub.2 gas, and the response characteristic curve of the device can be recovered to the initial value, indicating its good recoverability.

[0047] In order to investigate the effect of additive amount of graphite on the gas sensing performance of CoS/rGO composite material, composite materials with a mass ratio of CoS to GO of 1:5, 1:2, 2:1, 5:1, 10:1, and 20:1 were prepared, respectively. After reduction by high-temperature annealing, CoS/rGO composite materials with different graphite contents were named CoS/rGO-1, CoS/rGO-2, CoS/rGO-3, CoS/rGO-5, and CoS/rGO-6, respectively, and the corresponding gas sensing characteristics were studied. The resistances from CoS/rGO-1 to CoS/rGO-6 were 2.4 K, 4.2 K, 5.5 K, 6.5 K, 8.5 K, and 12.0 K, respectively. The gas sensing performance of CoS/rGO-1 to CoS/rGO-6 with different graphite contents was tested at a concentration of 1 ppm of NO.sub.2. As shown in FIG. 5, the gas sensing responses of devices CoS/rGO-1, CoS/rGO-2, CoS/rGO-3, CoS/rGO-4, CoS/rGO-5, and CoS/rGO-6 were 13.2%, 17.3%, 26.1%, 39.7%, 22.0%, and 16.2%, respectively. And it can be seen that when the mass ratio of CoS to GO was 5:1, the CoS/rGO composite material had the highest gas sensing response to 1 ppm of NO.sub.2, reaching 39.7%. When the content of graphite was too high, the overall response of CoS/rGO composite material was lower. Graphite reduced the specific surface area of the composite material and reduced the effective gas adsorption sites, thus affecting the gas sensing performance.

[0048] Nanocomposite materials not only possess the physical and chemical properties of a single material, but also may exhibit more outstanding properties than the single material due to the synergistic effects between materials. The present invention solved the problem that the composition of transition metal sulfides with other materials in the prior art changes the conductivity of the composite material, thereby affecting the gas sensing performance. When the mass ratio of CoS to GO was 5:1, CoS/rGO-4 composite material had good gas sensing performance. In order to better test the gas sensitivity response performance of CoS/rGO composite material, the gas sensitivity tests were conducted for CoS/rGO-4 composite material at different concentrations of NO.sub.2. As shown in FIG. 6, with the increase of NO.sub.2 concentration, the response of CoS/rGO-4 composite gas sensor also increased. The sensor's response to 50 ppb, 100 ppb, and 200 ppb of NO.sub.2 gas was 10.5%, 16.7%, and 23.6%, respectively. It can be seen that the CoS/rGO-4 composite gas sensor still exhibited good response recovery characteristic at a lower concentration of NO.sub.2 at 50 ppb, indicating that the sensor had a lower detection limit.

[0049] The present invention mainly uses the hydrothermal method to prepare the glycerol cobalt precursor, prepares Co(OH).sub.2 nanoflowers under alkaline conditions, obtains the Co.sub.3O.sub.4 nanoflowers after high-temperature annealing, uses sodium sulfide nonahydrate for ion exchange to prepare the CoS nanoflowers and adds different contents of graphite to prepare the CoS/rGO composite material through high-temperature annealing. SEM was used to make characterization observation of the surface morphology and structure of the composite material and FTIR, Raman, XRD, and XPS were used for the composition and crystal phase characterization of the composite material to demonstrate the successful preparation of CoS/rGO heterostructure. The CoS/rGO composite materials with different contents of graphite were added to a solution for ultrasonic dispersion, and added dropwise on the interdigital electrode to prepare a gas sensor, which was used to tested for gas sensitivity with NO.sub.2 gas. And the results showed that the CoS/rGO composite gas sensor had good response to NO.sub.2 gas, as well as excellent stability, repeatability, and selectivity. The specific results are as follows: (1) Cobalt nitrate hexahydrate was used as raw material to prepare the glycerol cobalt precursor by the hydrothermal method, prepares Co.sub.3O.sub.4 nanoflowers by alkaline conditions and high-temperature annealing, uses the ion exchange to prepare the CoS nanoflowers and adds different contents of graphite to change the ratio of CoS to GO to prepare the CoS/rGO composite materials with different ratios, which were further used to prepare the CoS/rGO composite material gas sensors with different graphite contents. [0050] (2) The gas sensitivity test results for NO.sub.2 gas showed that when the ratio of CoS to GO was 5:1, the CoS/rGO composite material had the highest response value to 1 ppm NO.sub.2 gas, reaching 39.7%; Compared with the response values of pure graphite and CoS, the response value of CoS/rGO composite material was greatly improved. [0051] (3) The CoS/rGO composite material gas sensor was subjected to gas sensitivity testing on different concentrations of NO.sub.2 gas. The results showed that the composite material sensor had a response value of 62.6% to 100 ppm high concentration of NO.sub.2 gas. At a lower concentration of 50 ppb NO.sub.2, it still had good response recovery characteristics, indicating that the sensor had a lower detection limit.

[0052] Air pollution has become a thorny issue that must be faced in the process of rapid socio-economic development. As one of the sources of air pollution, NO.sub.2 mainly comes from automobile exhaust and industrial chemical combustion, so the monitoring of NO.sub.2 is crucial. So, it is particularly important to develop a highly sensitive and reusable gas sensor. Graphite and transition metal sulfide form composite materials that can form heterostructures at the interface. The composition of the two materials can effectively prevent the stacking and aggregation of graphite sheets, increase the contact area between the material and gas, provide more adsorption sites, effectively improve electron transfer and enhance gas sensing performance. So, it has a large specific surface area and high charge carrier mobility, making it an ideal gas sensing material.